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Article

Improving the Key Enzyme Activity, Conversion Intensity, and Nitrogen Supply Capacity of Soil through Optimization of Long-Term Oilseed Flax Rotation Planting Patterns in Dry Areas of the Loess Plateau of China

by
Yuhong Gao
1,2,*,
Yong Zhang
3,
Haidi Wang
2,
Bing Wu
1,4,
Yue Li
5,
Bin Yan
1,2,
Yifan Wang
1,
Peina Lu
1,
Ruijun Wang
2,
Ming Wen
2,
Xingkang Ma
2,
Peng Xu
2,
Wenfang Xue
2,
Changyan Chao
2 and
Zedong Wen
2
1
State Key Laboratory of Arid Land Crop Science, Lanzhou 730070, China
2
College of Agronomy, Gansu Agricultural University, Lanzhou 730070, China
3
State Key Laboratory of Herbage Improvement and Grassland Agro-Ecosystems, College of Ecology, Lanzhou University, Lanzhou 730070, China
4
College of Life Science and Technology, Gansu Agricultural University, Lanzhou 730070, China
5
College of Information Science and Technology, Gansu Agricultural University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(2), 262; https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020262
Submission received: 7 December 2023 / Revised: 13 January 2024 / Accepted: 16 January 2024 / Published: 25 January 2024
(This article belongs to the Section Innovative Cropping Systems)
Browse Figures
Versions Notes

Abstract

:
Various crop rotation patterns can result in differences in nutrient consumption and the accumulation of toxic substances in the soil, indirectly impacting the soil environment and its nutrient supply capacity. Implementing optimized crop planting practices is beneficial for maintaining the favorable physical and chemical properties of farmland soil in the arid area of northwestern China. This study aimed to establish a crop rotation pattern to improve key enzyme activities and soil nitrogen conversion efficiency, as well as understand the underlying mechanism for enhancing nitrogen supply capacity. A field experiment was conducted to study the effect of four flax planting patterns, which included 13 crop rotation patterns with different crop frequencies: 100% Flax (Cont F), 50% Flax (I) (WFPF, FPFW, PFWF, FWFP), 50% Flax (II) (FWPF, WPFF, PFFW, FFWP), 25% Flax (WPWF, PWFW, WFWP, FWPW), on the key enzyme activities and the rate of soil nitrogen conversion, as well as the nitrogen supply capacity. Here, F, P, and W represent oilseed flax, potato, and wheat, respectively. The results indicated that the wheat stubble significantly increased the intensity of soil ammonification and denitrification before planting. Additionally, the activity levels of soil nitrate reductase and nitrite reductase under wheat stubble were significantly increased by 66.67% to 104.55%, while soil urease activity significantly decreased by 27.27–133.33% under wheat stubble compared to other stubbles. After harvest, the activities of soil nitrate reductase and nitrite reductase under the wheat stubble decreased significantly, and the intensity of soil ammonification, nitrification, and denitrification reduced significantly by 7.83–27.72%. The WFWP and FWFP treatments led to a significant increase in soil nitrogen fixation intensity under various crop rotations after harvest and significantly increased the levels of inorganic nitrogen in the soil before the planting of the next crop. This study suggests that the long-term rotation planting patterns WFWP and FWFP can significantly enhance the key enzyme activities of soil nitrogen conversion and significantly improve soil nitrogen conversion before crop sowing. This may increase the rate of soil nitrogen transfer and raise the available nitrogen content of the soil. These findings are crucial for reducing soil nitrogen loss and improving soil nitrogen nutrient supply capacity in dry areas of the Loess Plateau of China.

1. Introduction

Soil nitrogen is an essential elemental nutrient for the growth and development of crops. Mineralization, nitrification, and nitrogen fixation in clay loam minerals play a crucial role in the process of material conversion and energy flow [1]. Implementing appropriate agronomic measures can influence the conversion of ammonium nitrogen to nitrate nitrogen through nitrification, promote the movement of nitrogen in the soil, decrease nitrification and nitrogen loss, and significantly enhance the retention rate of nitrogen in the soil system, as well as nitrogen use efficiency [2]. Therefore, nitrogen uptake and utilization in agricultural ecosystems could be improved by further optimizing planting patterns and agronomic measures in production. Soil enzymatic activity has been widely utilized to assess nitrogen cycling processes and to evaluate soil quality changes resulting from soil management practices [3]. Some studies have indicated that soil enzymes are involved in the depolymerization process of nitrogen-containing polymers. In the process of soil nitrogen transformation, the total nitrogen mineralization rate is significantly positively correlated with the activities of soil urease and protease, indirectly reflecting the supply capacity of soil inorganic nitrogen [4], while soil nitrate reductase and nitrite reductase activities reflect the capacity for soil denitrification [5]. An appropriate crop rotation system can increase soil enzyme activity and microbial diversity and promote soil nitrogen transformation. Prior research indicated that an appropriate crop rotation system reduced the accumulation of mineral nitrogen in soil and reduced the loss of gaseous nitrogen or the leaching of nitrate nitrogen during the denitrification process [6,7]. Significant differences were observed in soil nitrogen components, microbial biomass, and soil enzyme activities under different crop rotation patterns [8,9,10]. After crop rotation, soil organic matter, microbial biomass carbon, and nitrogen content were significantly increased, and soil enzyme activities were enhanced, which was beneficial to maintaining soil fertility and productivity [11,12,13]. Studies have found that sorghum–alfalfa rotation and continuous cropping of sorghum significantly increased the soil urease and invertase activities in farmland. Compared with continuous cropping, rotation planting could improve soil enzyme activity and increase the number of microorganisms [14,15]. When legumes and wheat crops were added to a potato rotation system, the soil urease activity changed significantly, and the soil urease activity of legume rotation was significantly higher than that of wheat crops during the potato tuber expansion period by 14.73% [16]; furthermore, the soil nitrogen conversion process was significantly promoted through rational crop rotation with changes in stubble management [17,18]. The ammoniation, nitrification rate and soil ammonium nitrate nitrogen content of different soil microflora all changed with different planting systems and crop growth and development stages. A significant correlation was found between soil quick-acting nitrogen content and the nitrogen conversion rate in a tobacco rotation system [19]. Therefore, through reasonable crop rotation and stubble planting, the soil environment can maintain a stable microbial abundance and soil enzyme activity level, and it is possible to increase soil fertility, maintain the nutrient balance, and promote sustainable productivity. The appropriate planting system can promote crop growth and development [20,21].
Flax (Linum usitatissimum L.), as the main rotation crop, is a key economic crop in dry areas of the Loess Plateau in China [22]. Prior research demonstrated that the rotation of forage grass and crops could effectively reduce early soil nitrogen loss, significantly enhance the nitrogen cycle process, improve the farmland ecological environment, and reduce nitrogen pollution without reducing the crop yield [23]. To date, there have been numerous studies on the nitrogen use efficiency of oilseed flax based on nitrogen fertilizer and water–nitrogen coupling management [24,25], but there has been limited research on the impact of oilseed flax rotation systems on soil nitrogen conversion and utilization. Therefore, we hypothesis that optimization of long-term oilseed flax rotation planting patterns can increase the activities of key soil nitrogen conversion enzymes and their conversion efficiency, leading to improved soil nitrogen conversion, enhanced nitrogen utilization, and reduced nitrogen leaching in the case of limited nitrogen input. The purpose of this study was to (i) elucidate the effects of long-term oilseed flax rotation planting patterns on the soil enzyme activity, nitrogen conversion efficiency, and quick-acting nitrogen content; (ii) explore the interaction between soil enzymes and nitrogen conversion under different crop rotation patterns; and (iii) unveil the mechanisms enhancing the nitrogen supply capacity of soil in the dry areas of the Loess Plateau of China.

2. Materials and Methods

2.1. Study Site Description

The study site was established as part of a long-term field trial initiated in 2013. The soil samples were collected before sowing and after harvest in 2021. The experiment was conducted at the Dingxi Academy of Agricultural Science Experimental Station (34.26° N, 103.52° E, altitude 2020 m) in Anding District, Gansu Province, located in the dry areas of the Loess Plateau of China (Figure 1). This region is characterized as a typical mid-temperate semi-arid area, and the soil type is Ustorthent. The frost-free period lasts approximately 140 days, with an annual average of 2476.6 h of sunshine, an average annual temperature of 6.43 °C, and annual average precipitation of 390.9 mm. The organic matter content of the test site was 17.51 g·kg−1, the total nitrogen was 1.05 g·kg−1, the total phosphorus was 0.81 g·kg−1, the available phosphorus was 26.43 mg·kg−1, the alkali-hydrolyzable nitrogen was 47.91 mg·kg−1, the available potassium was 108.32 mg·kg−1, and the pH was 8.13 (Table 1).

2.2. Experimental Design

Based on an experiment on oilseed flax planting frequency, four basic patterns (100% Oilseed flax, 50% Flax (I), 50% Flax (II), and 25% Flax) were observed in the same year, resulting in 13 rotation modes over a four-year crop cycle. These modes included 100% Flax in a pattern of Cont F; 50% Flax (I) in patterns of WFPF, FPFW, PFWF, and FWFP; 50% Flax (II) in patterns of FWPF, WPFF, PFFW, and FFWP; and 25% Flax in patterns of WPWF, PWFW, WFWP, and FWPW. In these designations, F, P, and W represent oilseed flax, potato, and wheat, respectively. The control (CK) refers to the fallow treatment (Table 2). The experiment consisted of 14 treatments, each repeated three times, resulting in a total of 42 plots. The plot area measured 3 m × 5 m and were arranged in a random block group. The oilseed flax variety used was ‘Long Ya 10’, with a planting density of 7.5 × 106 plants·hm−2. The potato variety was ‘Xin Daping’, with a planting density of 5.25 × 104 plants·hm−2. The wheat variety was ‘Gan Chun 27’, with a planting density of 3.75 × 106 plants·hm−2. The nitrogen and phosphorus application rates were as follows: 112.5 kgN·hm−2 nitrogen and 75 kgP2O5·hm−2 phosphorus for oilseed flax, 225 kgN·hm−2 nitrogen and 150 kg P2O5·hm−2 phosphorus for potato, and 150 kgN·hm−2 nitrogen and 112.5 kg P2O5·hm−2 phosphorus for wheat. Urea (containing N 46%) was used as nitrogen fertilizer, superphosphate (containing P2O5 16%) as phosphate fertilizer, and both nitrogen and phosphorus fertilizers were applied as base fertilizers. During the experiment, the oilseed flax was sown on 24 March and harvested on 13 August, the potato crop was sown on 7 April and harvested on 29 September, and the wheat was sown on 24 March and harvested on 31 July in 2021. Other management measures were similar to those used in the general field conditions.

2.3. Soil Sample Collection

In 2021, soil samples were collected before the planting of different crops in the experiment. The sample were collected on 23 March in the oilseed flax field, 7 April in the potato field, and 24 March in the wheat field. The post-harvest soil samples from different crop fields were collected on 13th August in the oilseed flax field, 29 September in the potato field, and 31 July in the wheat field. For each plot, five soil samples were randomly collected from a range of 0–30 cm soil depth using the ring-knife method and then mixed. Some of the samples were allowed to dry naturally, while 300 g of the samples were stored in a 4 °C refrigerator for soil nutrient and enzyme activity determination.

2.4. Test Methods

2.4.1. Soil Enzyme Activity Determination

Before crop sowing and after harvesting, soil samples from the 0–30 cm soil layer in each plot were collected, with three repetitions from each plot. The collected soil samples were then mixed and passed through a 1 mm sieve. Urease activity was measured using the indophenol blue colorimetric method, and the soil urease activity was expressed by the NH4+-N content in 1 g soil after 24 h, with the unit being mg·g−1·24 h−1. The nitrate reductase activity and nitrite reductase activity were measured using the soil culture method. The soil nitrate reductase activity was represented by the NO3−-N content generated by a unit mass of soil in a unit of time, and the soil nitrite reductase activity was represented by the NO2−-N content of a unit mass of soil in a unit of time, with units of mg·g−1·48 h−1 [26]. Before and after culture, the soil samples were combined with 50 mL of 2 mol·L−1 KCl solution; next, the soil samples were leached by shaking for 1 h and then filtered. After the samples were clarified, a certain amount of supernatant was filtered and collected to measure nitrogen (NO3−-N), ammonium nitrogen (NH4+-N), total nitrogen (TN) and nitrous nitrogen (NO2−-N) [27]. These measures can reflect the status of soil at pre-planting. Soil nitrification, denitrification, ammoniation, and nitrogen fixation were measured by the soil culture method [19]. The ammoniation, nitrogen fixation, nitrification and nitrogen fixation intensities of the soil were calculated using these formulas:
Ammonization intensity = (NH4+-N content before soil cultivation − NH4−-N content after soil culture)/7 day
Nitrogen fixation intensity = (TN content before soil cultivation − TN content after soil culture)/30 day)
Nitrification intensity = (NH4+-N, NO3−-N and NO2−-N content before soil cultivation − NH4+-N, NO3−-N and NO2−-N content after soil culture)/30 day
Denitrification intensity = (NO3−-N content before soil cultivation − NO3−-N content after soil culture)/5 day

2.4.2. Soil Quick-Acting Nitrogen Content Determination

The determination of nitrate and ammonium nitrogen content in soil involved weighing out a 5 g soil sample with a particle size of 0.5–1.0 mm after dry sieving; next, the sample was extracted with potassium chloride solution and then shaken on a shaker for 1 h, after which it was removed and allowed to stand [28,29]. After the sample was clarified, it was filtered, and a certain amount of supernatant was collected. The supernatant was used to measure the soil nitrate and ammonium nitrogen content with a Smartchem 450 (AMS Alliance, Rome, Italy) discontinuous chemical analyzer.

2.5. Statistical Analysis

Statistical analysis was performed using SPSS (version 21.0; IBM, Armonk, NY, USA). The normality and homogeneity of variance of the data were tested before analyses. The significant difference (p < 0.05) in the soil’s physical and chemical properties under different crop rotation treatments was examined using one-factor analysis of variance and Duncan’s multiple range test. Additionally, Excel 2018 was used for statistics and graphing of the data.

3. Results

3.1. Soil Key Enzyme Activity in Nitrogen Conversion

The study revealed that different crop rotation patterns had a significant effect on the activities of key enzymes involved in soil nitrogen conversion (Figure 2). The activities of soil enzymes were founded to be related to the type and planting sequence of previous crops. At pre-planting, soil urease activities were significantly higher under FPFW, PWFW, PFFW and FWPW treatments than under other treatments, ranging from 13.5% to 418.4% higher. Similarly, the soil nitrate reductase activities under WFWP and FFWP treatments showed a significant increase ranging from 16.78% to 135.09% compared to other treatments (except WPFF, PWFW and FWFP). Additionally, soil nitrite reductase activities under FWPF, WFWP, FFWP and FWFP treatments were significantly higher by 19.08% to 131.34% than under other treatments. At post-harvest, soil urease activities under WFPF, WPWF and WPFF treatments were higher by 20.0–135.9% than other treatments (except WFWP treatment). Furthermore, they were significantly higher by 67.42–148.91% under PWFW, PFFW and PFWF treatments than under other treatments. The study also indicated that planting wheat in the previous stubble and the current season significantly increased the soil urease activity under different oilseed flax rotation patterns at post-harvest, thereby promoting the decomposition of nitrogen-containing organic matter in the soil. The study also found that nitrate reductase and nitrous acid under WFWP and FFWP treatments at pre-planting, as well as under PWFW, PFFW and PFWF treatments after the harvest, were significantly increased. This indicated that the higher reductase activity could promote the reduction of nitrate nitrogen to ammonia nitrogen in the soil.
The effects of different crop rotation stubble on the enzyme activities of farmland soil were observed to be significantly different (Table 3). Specially, the soil urease activities under potato stubble at pre-planting and wheat stubble after harvest were significantly increased, ranging from 23.08% to 275.00% and 27.27% to 133.33%, respectively, compared to other stubbles. The fallow period (CK) was found to significantly decreased the soil nitrate reductase and nitrite reductase activities at pre-planting. At post-harvest, these activities were significantly increased, ranging from 19.87% to 29.86% and 66.67% to 104.55% under the potato stubble, respectively, compared to other stubbles. In summary, the study concluded that oilseed flax and wheat stubbles at pre-planting, and potato stubble at post-harvest significantly increased the soil nitrate reductase and nitrite reductase activities.
Different oilseed flax planting frequencies were founded to have a significant effect on the soil enzyme activity (Table 4). Specifically, the soil urease activity under 100% Flax frequency (continuous cropping) was significantly increased by 200% to 225% at pre-planting compared with other treatments. Additionally, the soil nitrate reductase in the fallow period (CK) was significantly reduced by 90.79% to 102.63%, and the nitrite reductase activities under the 100% Flax and CK treatments were significantly reduced by 4.35% to 34.78% compared with the other planting frequencies. After the harvest, the urease activity of CK was significantly reduced by 66.67% to 83.33% compared with other treatments. This indicated that continuous oilseed flax and fallow treatments significantly reduced soil nitrogen transformation compared with other oilseed flax planting frequencies. The finding suggested that oilseed flax rotation planting could significantly improve soil enzyme activity and nitrogen migration, reduce the intensity of soil denitrification, and improve the soil nitrogen use efficiency.

3.2. Intensity of Nitrogen Conversion

The intensity of nitrogen transformation indirectly reflects the soil’s physical, chemical properties and biological activity, and directly affects the nitrogen absorption and utilization efficiency of crops. The study found that significant difference in the soil nitrogen transformation intensity under different crop rotation patterns (Figure 3). The pre-planting soil nitrogen fixation intensity of the FPFW, PFFW, and FWPW treatments were significantly higher by 19.18% to 36.89% than those of the Cont F, PFWF, WFWP, and FWFP treatments. At post-harvest, those of the WFWP, FFWP, and FWFP treatments were significantly higher by 64.25% to 145.99% than those of other treatments. These results indicated that the FPFW, PFFW, and FWPW treatments significantly promoted the fixation of inorganic nitrogen and free nitrogen in the soil. The ammoniation intensity of PFFW and PFWF treatments at pre-planting were significantly decreased by 31.09–74.84% compared with those of other treatments, except WPFF, FFWP and fallow treatment, and they were significantly higher by 27.68% to 73.58% under the PWFW and PFWF treatments at post-harvest than under the Cont F, FWPF, WFPF, WPWF, FFWP, and FWFP treatments. This indicated that PWFW and PFWF treatments significantly increased the decomposition and transformation rates of organic nitrogen compounds in the soil at post-harvest. In contrast to the WFWP treatment, the nitrification under the FFWP and FWFP treatments at pre-planting were significantly higher by 11.24% to 71.07% compared with other treatments. At post-harvest, the nitrification under the treatments of PWFW, PFFW and PFWF were significantly increased 12.97% to 81.82% compared with other treatments. This indicated that FWFP treatment was useful for improving the soil nitrogen supply capacity.
The study found significant differences in the soil nitrogen transformation intensity under different crop rotation patterns (Table 5). At pre-planting, the fallow (CK) treatment significantly reduced the soil nitrogen fixation and nitrification intensity compared with all rotation patterns. Additionally, wheat stubble was founded to significantly increase the soil ammoniation and denitrification intensity. Potato stubble and CK were observed to significantly reduce the soil nitrogen fixation intensity. Moreover, the soil ammoniation and nitrification intensity under wheat and oilseed flax stubbles were significantly reduced by 16.67% to 27.72%, and the soil denitrification intensity under oilseed flax stubble was significantly reduced by 7.83% to 12.90%. These findings showed that the wheat and oilseed flax stubbles significantly reduced the soil nitrogen conversion intensity and reduced nitrogen loss at post-harvest.
The study revealed significant differences in the soil nitrogen transformation intensity under different oilseed flax planting frequencies (Table 6). Specifically, compared with CK and the 50% Flax (II) planting frequency at pre-planting, the planting frequency of 25% Flax was found to significantly increase the intensity of soil ammoniation by 17.21% to 21.48%. Additionally, the soil nitrification intensity under the treatments of 100% Flax and CK was significantly lowered by 21.43% to 50.00% compared to other treatments. The soil denitrification intensity of CK was also significantly higher by 4.34% than that of 25% Flax frequency. At post-harvest, the soil nitrogen fixation intensity under the 50% Flax (I) and 25% Flax treatments was significantly increased by 5.56% to 23.53% compared with other frequencies. The soil nitrification intensity of the 100% Flax and CK treatments wase significantly decreased by 18.75% to 72.73%, and was significantly reduced by 7.98% to 15.02% under a frequency of 50% Flax (I) compared with other treatments. In summary, the 100% Flax frequency and CK were found to significantly reduce the soil nitrogen conversion rate, while the 50% Flax (I) and 25% Flax planting frequencies significantly increased the soil nitrogen fixation at post-harvest and the supply capacity of inorganic nitrogen at pre-planting.

3.3. Soil Available Nitrogen Content

The available nitrogen content of the soil was affected by different oilseed flax rotation patterns (Figure 4). This difference indirectly reflected the soil nitrogen fixation and migration state at pre-planting and post-harvest. In contrast to FWPF, the FFWP, FWFP, and WFWP treatments had significantly increased soil available nitrogen content at pre-planting, ranging from 12.33% to 49.07% higher than the other treatments. Additionally, the WFPF, WPFF, and PFWF treatments reduced the soil available nitrogen content, which was significantly lower by 14.17% to 91.69 than under other treatments, except for the Cont F, PWFW, and PFFW treatments. After the harvest, the PWFW, PFFW, and PFWF treatments increased the soil available nitrogen content significantly, raising it 14.75% to 60.37% higher than the other treatments, except for the WFPF, WWWF, and FWFP treatments. The soil available nitrogen content under the Cont F, WPFF, FWPW, and CK treatments was significantly reduced by 14.79% to 37.65% compared with the other treatments, except for the FPFW and FFWP treatments. This indicated that the soil available nitrogen following a potato crop in the current season remained high after the harvest. It can be inferred that the soil nitrogen was surplus and that the oilseed flax and wheat stubbles could enhance soil nitrogen utilization.
The soil available nitrogen under potato stubble (P) was higher by 26.52%, 18.64%, and 18.20% at pre-planting and higher by 24.49%, 19.04%, and 66.17% at post-harvest, respectively, than under oilseed flax stubble (F), wheat stubble (W), or fallow (CK) conditions (Figure 4). The result showed that the nitrogen surplus under the potato stubble was significantly higher than that of other stubbles, and the oilseed flax stubble significantly reduced the soil available nitrogen content, which is useful for increasing the nitrogen use efficiency.
Under different oilseed flax planting frequencies, the content of soil available nitrogen under 25% Flax frequency at pre-planting was significantly higher by 7.64% to11.32% than that of 100% Flax and 50% Flax (I) (Figure 5). Additionally, 100% Flax, 50% Flax (I), and 50% Flax (II) planting frequency had no significant difference compared with CK. At post-harvest, the content of soil available nitrogen under the planting frequency of 50% Flax (I) was significantly increased 23.23%, 8.62% and 41.57%, respectively, compared with 100% Flax, 50% Flax (II) and CK. It showed that the 100% Flax frequency and CK significantly reduced the soil available nitrogen content at post-harvest and significantly increased the nutrient requirement for soil available nitrogen after the winter fallow period.

3.4. Correlation Analysis

In the correlation analysis results, it was found that nitrite reductase and available nitrogen had a significant positive correlation with each other and a significant negative correlation with urease before planting (Figure 6). There was also a significant positive correlation between nitrate reductase and nitrite reductase. Overall, increased nitrate reductase and nitrite reductase content could significantly increase the nitrification intensity. After the harvest, the content of soil available nitrogen had significant positive correlations with nitrate reductase and nitrification intensity. Urease had significant negative correlations with nitrate reductase, nitrite reductase, and ammoniation. Additionally, nitrate reductase had a significant positive correlation with nitrite reductase. There were significant positive correlations between nitrite reductase, ammonification, and nitrification. This showed that enzyme activity related to soil nitrogen conversion could indirectly reflect the intensity of soil nitrogen conversion, and improve the soil nitrogen migration and supply under different rotation patterns.

4. Discussion

4.1. Soil Enzyme Activity

Enzyme activity serves as a sensor of soil stress, reflecting the degradation of soil quality and influencing soil biological health. Changes in enzyme activity can be influenced by abiotic factors, such as temperature and humidity, as well as biotic factors, including plant growth, residue, and microbial properties [30]. These factors have different effects on soil enzyme activity as well as nitrogen conversion and utilization in the soil [31]. Crop rotation can lead to significant changes in enzyme activity [32], which, in turn, affects soil nutrient conversion and utilization. The addition of legumes and organic fertilizers (farmyard manure/crop residues) to crop rotation can help maintain the optimal biological functions of the soil [33,34]. Hydrolase activity in the soil can be used as a diagnostic indicator of fertilizer effectiveness, as increasing hydrolase activity can improve oilseed flax grain yield. In this study, it was found that the 100% Flax frequency and the fallow (CK) treatment significantly reduced soil nitrite reductase activity and urease activity at both pre-planting and post-harvest. However, soil enzyme activity increased to varying degrees under different rotation sequences. For example, the activities of nitrate reductase and nitrite reductase were significantly increased under oilseed flax stubble at pre-planting and under wheat and potato stubbles at post-harvest. The role of diverse substrate supply for different crop residues was demonstrated well by Sainju UM et al. [35].
Among different crop rotation patterns, it was observed that the FPFW, PFFW, and PWFW treatments significantly increased soil urease activity at pre-planting, while the PWFW, PFFW, PFWF treatments significantly increased soil urease activity at post-harvest. Meanwhile, the WFWP and FFWP treatments at pre-planting and the WFPF, WPWF treatments at post-harvest significantly increased the soil nitrate reductase and nitrous acid reductase activity. Additionally, the soil active C pools and microbial biomass carbon and nitrogen all increased as planting diversity increased. The crop diversity affected the handling of newly added residues, microbial dynamics, and nutrient cycling over time. It suggested that crop rotation diversification has the potential to enhance the soil ecosystem function [36]. The correlation analysis showed that increasing the content of nitrite reductase and nitrate reductase could significantly increase the content of available nitrogen in the soil and the soil nitrification intensity. An increase in soil urease activity at post-harvest could significantly reduce soil ammoniation, while an increase in soil nitrite reductase activity could significantly increase the soil ammoniation intensity. The content of crop residues and microbial biomass can significantly vary under different crop rotations, leading to differences in the uptake and fixation of soil nitrogen among crops. Additionally, the quality and quantity of crop residues vary. Crop residues can also provide different polyphenolic compounds, thereby changing the quality and quantity of soil microorganisms and affecting the nitrogen cycle in the soil [37]. As a result, long-term crop rotation may produce relative stability for the soil ecological environment, and it can help maintain a relatively high microbial activity, improve the soil enzyme activity and nitrogen transformation intensity, increase the field holding rate of nitrogen, and achieve sustainable agricultural development.

4.2. Soil Nitrogen Transformation

Research indicates that changes in tobacco planting systems and growth stages can significantly impact soil ammoniation, nitrification rates and ammonium nitrate nitrogen content under different soil microflora. The nitrogen conversion rate is closely related to the soil nitrogen content [19], and there is a significant correlation between the content of ammonium nitrate nitrogen. A reasonable crop rotation pattern can lead to a significantly increase the abundance of microbes and activity of enzymes related to soil nitrogen transformation, therefore significantly promoting soil nitrogen transformation and maintaining high levels of soil inorganic nitrogen content [6,8]. This study also emphasizes the importance of adding one or more crops to the nitrogen cycle, as they provide carbon and nitrogen inputs and affect nitrogen transformation and migration in the soil through the decomposition and mineralization processes of different nitrogen-containing organics [38]. In particular, it was observed in this study that the CK treatment significantly reduced the soil nitrogen fixation and nitrification intensity at pre-planting, while wheat stubble significantly increased soil ammoniation and denitrification intensity. Wheat stubble accelerated the conversion of soil organic nitrogen to inorganic nitrogen and reduced the oilseed flax planting years in different crop rotation sequences. Additionally, under a 25% Flax frequency, there was a significantly increase in ammonification, a reduction in organic nitrogen levels, and an acceleration of nitrogen loss.
The different crop rotation sequences, such as FPFW, PFFW, FWPW, WFWP, FFWP, and FWFP, were found to significantly increase soil nitrogen fixation, accelerate the nitrogen conversion process, and promote the conversion of free nitrogen from inorganic nitrogen to organic nitrogen. It was also observed that potato stubble significantly reduced the soil nitrogen fixation intensity at post-harvest, while wheat and oilseed flax stubbles reduced the soil ammoniation and nitrification intensity, and oilseed flax stubble significantly reduced the soil denitrification intensity. On the whole, the influence of both biotic and abiotic factors on the soil nitrogen transformation process could be observed. For example, the post-harvest nitrogen holding rate under potato stubble was significantly higher than that of other stubble, leading to a reduction in soil nitrogen fixation intensity and the conversion of organic nitrogen to available nitrogen, ultimately contributing to stabilizing the overall soil nitrogen level. Conversely, the holding rate of available nitrogen was lowest in the oilseed flax stubble, as it reduced the intensity of ammoniation, nitrification and denitrification, causing an overall decline in soil nitrogen conversion process and a reduction in the soil nitrogen migration rates well as the loss of nitrogen as denitrification products (NH3, NO, etc.). This study also found that the different planting frequencies of oilseed flax, such as 50% (I) Flax and 25% Flax, significantly increased the soil intensity of nitrogen fixation, with 50% (I) Flax significantly decreased the intensity of soil denitrification. Moreover, the PFFW and PFWF treatments as measured at pre-planting and PWFW, PFWF treatments as measured at post-harvest significantly reduced soil ammoniation and the conversion of inorganic nitrogen to available nitrogen, thereby increasing the soil nitrogen holding rate.

4.3. Soil Quick-Acting Nitrogen Content

Planting patterns and field management measures have a significant impact on the soil nitrogen cycle process [39]. The soil nitrogen transformation process is mainly affected by soil enzyme activities and the differences in soil microbial communities [37]. The depolymerization process of nitrogen-containing polymers through the involvement of soil enzymes indirectly reflects the soil inorganic nitrogen supply capacity, regulates the soil nitrogen transformation intensity, and increases the soil nitrogen holding rate [13], which is of great significance for maintaining soil fertility and plant growth and development [40]. Optimizing nitrogen management under different rotation conditions is key to improving grain crop yields. For example, in the rice–rape and cotton–rape rotation systems, there was still a significant amount of soil available nitrogen after the cotton was harvested, indicating different nitrogen storage capacities and increasing the supply of available nitrogen to subsequent crops at the seedling stage [6]. In this study, potato stubble could significantly increase soil available nitrogen content at post-harvest compared to other stubbles, while oilseed flax stubble significantly reduced the soil available nitrogen content. Different crop stubbles have different nitrogen storage abilities. The residual available nitrogen content in the soil at pre-planting under potato stubble remained at a high level, indicating that oilseed flax stubble significantly increased the utilization of soil available nitrogen, while the compensation after-effect of the potato stubble was more prominent. These findings suggested that different crop stubbles can impact nitrogen utilization in the early growth stage of subsequent crops.
The analysis from the perspective of different oilseed flax planting frequencies revealed that the planting frequency of 25% Flax significantly increased the content of soil available nitrogen at pre-planting compared with 100% Flax, 50% Flax (I), 50% Flax (II) and fallow treatments (CK). There was no significant difference among 100% Flax, 50% Flax (I), 50% Flax (II) and fallow (CK) at pre-planting, but a significant difference was observed at post-harvest. This indicated that the 100% Flax planting frequency and fallow (CK) significantly increased the content of soil available nitrogen after the fallow period in winter, and accelerated the soil nitrogen conversion process. Potato stubble has a strong capacity for nitrogen fixation and inorganic nitrogen storage after harvest in the following year. It significantly improved the mineral nitrogen retention ability of the soil.

5. Conclusions

This study showed that a reasonable crop rotation mode was beneficial for regulating soil enzyme activities, leading to a significant optimization of soil nitrogen conversion in agro-ecosystems. This was able to improve the nitrogen supply and ecological environment of farmland and increase the productivity of agro-ecosystems. Specifically, the study observed significant increases in soil nitrate reductase and nitrite reductase activities under wheat stubble, ranging from 66.67% to 104.55% at pre-planting. However, at post-harvest, the soil nitrate reductase and nitrite reductase activities under wheat stubble were significantly decreased, leading to a reduction in the intensity of soil ammonification, nitrification and denitrification by 7.83% to 27.72%. The content of soil available nitrogen under a planting frequency of 50% Flax (I) was increased significantly by 8.62% to 41.57% compared to other treatments at post-harvest. In conclusion, the WFWP and FWFP rotation patterns were found to significantly increase the activity of key soil nitrogen conversion enzymes at pre-planting, while significantly reducing soil nitrogen conversion enzyme activity and nitrogen conversion intensity at post-harvest. We conclude that the optimization of long-term oilseed flax rotation planting patterns can improve the key enzyme activity, conversion intensity, and nitrogen supply capacity of soil in dry areas of the Loess Plateau of China.

Author Contributions

Methodology, Y.G.; software, Y.L.; validation, R.W., C.C., W.X. and Z.W.; formal analysis, Y.Z.; investigation, Y.Z., H.W., R.W., M.W., X.M., P.X., W.X., C.C. and Z.W.; resources, B.W., Y.W. and P.L.; data curation, H.W., M.W. and P.X.; writing—original draft preparation, Y.G. and Y.Z.; writing—review and editing, Y.G., B.W. and Y.L.; visualization, X.M.; supervision, Y.G.; project administration, B.Y., Y.W. and P.L.; funding acquisition, B.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Natural Science Foundation of China (32260551), the Gansu Education Science and Technology Innovation Industry Support Program (2021CYZC-38), the Technology Innovation Guidance Program–Science and Technology Mission Topic of Gansu Province (23CXNA0035), the China Agriculture Research System of MOF and MARA (CARS-14-1-16), the State Key Laboratory of Arid Land Crop Science, Gansu Agricultural University (GSCS-2020-Z6), and the Fuxi Outstanding Talent Cultivation Plan of Gansu Agriculture University (Gaufx-02J05).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Tang, Y.F.; Xu, Y.B.; Zheng, Y.; Lei, B.K. Effects of wheat and faba bean intercropping on microorganism involved in nitrogen transformation in the rhizosphere soils. J. Food Agric. Environ. 2016, 33, 482–490. [Google Scholar]
  2. Ding, S.J.; Xiong, S.P.; Ma, X.M. Effects of tillage and nitrogen application rate on soil nitrogen transformation and yield in a winter wheat/summer maize multiple cropping system. J. Appl. Ecol. 2017, 28, 142–150. [Google Scholar]
  3. Chang, W.H.; Ma, W.W.; Song, L.C.; Tang, Y.M.; Long, Y.C.; Xu, G.R.; Yuan, J.Y. Responses of soil N-cycle enzyme activities to vegetation degradation in a wet meadow on the Qinghai-Tibet Plateau. Front. Ecol. Evol. 2023, 11, 1210643. [Google Scholar] [CrossRef]
  4. Song, M.; He, T.; Chen, H.; Wang, K.; Li, D. Dynamics of soil gross nitrogen transformations during post-agricultural succession in a subtropical karst region. Geoderma 2019, 341, 1–9. [Google Scholar] [CrossRef]
  5. Wang, J.H.; Wang, X.L.; Wang, K. Effects of Replacing Chemical Fertilizers with Bio-organic Fertilizers on Microenvironment of Wheat Rhizosphere Soil. Acta Agric. Boreali-Sin. 2021, 36, 155–162. [Google Scholar]
  6. Koeslin-Findeklee, F.; Becker, M.A.; Graaff, E.V.D.; Roitsch, T.; Horst, W.J. Differences between winter oilseed rape (Brassica napus L.) cultivars in nitrogen starvation-induced leaf senescence are governed by leaf-inherent rather than root-derived signals. J. Exp. Bot 2015, 66, 3669–3681. [Google Scholar] [CrossRef]
  7. Yang, W.; Feng, G.; Adeli, A.; Tewolde, H.; Qu, Z.Y. Simulated long-term effect of wheat cover crop on soil nitrogen losses from no-till corn-soybean rotation under different rainfall patterns. J. Clean. Prod. 2021, 280, 124255. [Google Scholar] [CrossRef]
  8. Bian, X.B.; Yang, X.H.; Li, Q.; Sun, X. Effects of planting of two common crops, Allium fistulosum and Brassica napus, on soil properties and microbial communities of ginseng cultivation in northeast China. BMC Microbiol. 2022, 22, 182. [Google Scholar]
  9. Liu, X.L.; Liu, X.L.; Liu, X. Impacts of continuous and rotational cropping practices on soil chemical properties and microbial communities during peanut cultivation. Sci. Rep. 2022, 12, 2758. [Google Scholar] [CrossRef]
  10. Groffman, P.M.; Eagan, P.; Sullivan, W.M.; Lemunyon, J.L. Grass species and soil type effects on microbial biomass and activity. Plant Soil 1996, 183, 61–67. [Google Scholar] [CrossRef]
  11. Groffman, P.M.; Rice, C.W.; Tiedje, J.M. Denitrification in a Tallgrass Prairie Landscape. Ecology 1993, 74, 855–862. [Google Scholar]
  12. Xie, Y.; Dong, C.; Chen, Z.; Liu, Y.; Guo, T. Successive biochar amendment affected crop yield by regulating soil nitrogen functional microbes in wheat-maize rotation farmland. Environ. Res. 2020, 194, 110671. [Google Scholar] [CrossRef] [PubMed]
  13. Lynn, T.M.; Zhran, M.; Wang, L.F.; Ge, T.; Yu, S.S.; Kyaw, E.P. Effect of land use on soil properties, microbial abundance and diversity of four different crop lands in central myanmar. 3Biotech 2021, 11, 154. [Google Scholar] [CrossRef] [PubMed]
  14. Wang, J.S.; Fan, F.F.; Guo, J. Effects of different crop rotations on growth of continuous cropping sorghum and its rhizosphere soil micro-environment. Chin. J. Appl. Ecol. 2016, 27, 2283–2291. [Google Scholar]
  15. Liu, Q.; Zhao, Y.X.; Li, T.; Chen, L.; Chen, Y.Q.; Sui, P. Changes in soil microbial biomass, diversity, and activity with crop rotation in crop systems: A global synthesis. Appl. Soil Ecol. 2023, 186, 104815. [Google Scholar] [CrossRef]
  16. Wang, L.H.; Guo, X.D.; Tan, X.L.; Guo, T.W.; University, G.A. Effects of different crop rotations on enzyme activities and microbial quantities in potato soil. Agric. Res. Arid. Areas 2016, 34, 109–113. [Google Scholar]
  17. Wang, L.G.; Ye, C.L.; Chen, J.; Li, J.J.; Luo, J.J. The effect of rotation wheat on reducing continuous cropping obstacle of oil flax. Agric. Res. Arid. Areas. 2020, 38, 158–163. [Google Scholar]
  18. Nayak, D.R.; Babu, Y.J.; Adhya, T.K. Long-term application of compost influences microbial biomass and enzyme activities in a tropical Aeric Endoaquept planted to rice under flooded condition. Soil Biol. Biochem. 2007, 39, 1897–1906. [Google Scholar] [CrossRef]
  19. Zhang, J.G.; Zhang, Y.Z.; Zheng, L.L.; Shi, Y.; Kong, F.Y.; Ma, X.H.; Tian, L.; Zhang, Z.F. Effects of tobacco planting systems on rates of soil N transformation and soil microbial community. Int. J. Agric. Biol. 2017, 19, 992–998. [Google Scholar]
  20. Wang, P.X.; Nie, J.W.; Yang, L.; Zhao, J.; Wang, X.Q.; Zhang, Y.D.; Zang, H.D.; Yang, Y.D.; Zeng, Z.H. Plant growth stages covered the legacy effect of rotation systems on microbial community structure and function in wheat rhizosphere. Environ. Sci. Pollut. Res. 2023, 30, 59632–59644. [Google Scholar] [CrossRef]
  21. Ghani, M.I.; Ali, A.; Atif, M.J.; Ali, M.; Ahanger, M.A.; Chen, X.Y.L.; Cheng, Z.H. Different leafy vegetable cropping systems regulate growth, photosynthesis, and PSII functioning in mono-cropped eggplant by altering chemical properties and upregulating the antioxidant system. Front. Plant Sci. 2023, 14, 1132861. [Google Scholar] [CrossRef] [PubMed]
  22. Cui, Z.J.; Yan, B.; Gao, Y.H.; Wu, B.; Wang, Y.F. Agronomic cultivation measures on productivity of oilseed flax: A review. Oil Crop Sci. 2022, 7, 10. [Google Scholar] [CrossRef]
  23. Walkup, J.; Freedman, Z.; Kotcon, J.; Morrissey, E.M. Pasture in crop rotations influences microbial biodiversity and function reducing the potential for nitrogen loss from compost. Agric. Ecosyst. Environ. 2020, 304, 107122. [Google Scholar] [CrossRef]
  24. Wu, B.; Gao, Y.H.; Li, P.H.; Yan, B.; Niu, J.Y. Effect of density and nitrogen on water and nitrogen utilization and seed yield of oilseed flax. ARAA 2020, 38, 163–171. [Google Scholar]
  25. Cui, Z.J.; Liu, D.; Wu, B.; Yan, B.; Niu, J.Y. Effects of water and nitrogen coupling on grain yield formation and nitrogen accumulation, transportation of oilseed flax in dryland. J. Appl. Ecol. 2020, 31, 909–918. [Google Scholar]
  26. Guan, S.M. Soil Enzymes and Their Research Methods; Agriculture Press: Beijing, China, 1986. [Google Scholar]
  27. Shan, J.; Zhao, X.; Sheng, R.; Xia, Y.Q.; Ti, C.P.; Quan, X.F.; Wang, S.W.; Wei, W.X.; Yan, X.Y. Dissimilatory nitrate reduction processes in typical Chinese paddy soils: Rates, relative contributions, and influencing factors. Environ. Sci. Technol. 2016, 50, 9972–9980. [Google Scholar]
  28. Lu, R.K. Analytical Method of Soil Agro-Chemistry; Chinese Agriculture Science and Technology Press: Beijing, China, 2000. [Google Scholar]
  29. Bao, S.D. Soil Agrochemical Analysis, 3rd ed.; China Agriculture Press: Beijing, China, 2000. [Google Scholar]
  30. Ren, C.J.; Wu, J.P.; Guang, X. Temporal variation in soil enzyme activities after afforestation in the loess plateau, China. Geoderma Int. J. Soil Sci. 2016, 282, 103–111. [Google Scholar] [CrossRef]
  31. Liu, S.B.; He, F.K.; Kuzyakov, Y.; Xiao, H.X.; Hang, D.T.T.; Pu, S.Y.; Razavi, B.S. Nutrients in the rhizosphere: A meta-analysis of content, availability, and influencing factors. Sci. Total Environ. 2022, 826, 153908. [Google Scholar]
  32. Gong, H.; Li, J.; Ma, J.; Hou, R.; Ouyang, Z. Effects of straw incorporation combined with inorganic-organic fertilization on soil water and nitrogen changes and microbial community structure in winter wheat. Acta Ecol. Sin. 2019, 39, 2203–2214. [Google Scholar]
  33. Sun, Q.; Wu, H.L.; Chen, F.; Kang, J.H. Effects of Soil Enzyme Activity and Bacterial Community under Different Crop Rotations. Energy Environ. Ecol. Environ. 2020, 29, 2385–2393. [Google Scholar]
  34. Kolbe, H. Comparative Analysis of Soil Fertility, Productivity and Sustainability of Organic Farming in Central Europe—Part 1: Effect of Medium Manifestations on Conversion, Fertilizer Types and Crop Systems. Agronomy 2022, 12, 2001. [Google Scholar]
  35. Sainju, U.M.; Lenssen, A.W.; Allen, B.L.; Stevens, W.B.; Jabro, J.D. Soil residual nitrogen under various crop rotations and cultural practices. J. Plant Nutr. Soil Sci. 2017, 180, 187–198. [Google Scholar] [CrossRef]
  36. Cadel, M.; Cousin, I.; Therond, O. Relationships between soil ecosystem services in temperate annual field crops: A systematic review. Sci. Total Environ. 2023, 902, 165930. [Google Scholar] [PubMed]
  37. Fan, D.M.; Zhao, Z.M.; Wang, Y.; Ma, J.H.; Wang, X.C. Crop-type-driven changes in polyphenols regulate soil nutrient availability and soil microbiota. Front. Microbiol. 2022, 13, 964039. [Google Scholar] [CrossRef]
  38. Clivot, H.; Mary, B.; Vale, M.; Cohan, J.P.; Champolivier, L.; Piraux, F.; Laurent, F.; Justes, E. Quantifying in situ and modeling net nitrogen mineralization from soil organic matter in arable crop systems. Soil Biol. Biochem. 2017, 111, 44–59. [Google Scholar]
  39. Ma, T.T.; Yang, K.; Yang, L.; Zhu, Y.; Jiang, B.Q.; Xiao, Z.P.; Shuai, K.F.; Fang, M.; Gong, J.; Gu, Z.M.; et al. Different rotation years change the structure and diversity of microorganisms in the nitrogen cycle, affecting crop yield. Appl. Soil Ecol. 2024, 193, 105123. [Google Scholar] [CrossRef]
  40. Bhattacharyya, S.S.; Furtak, K. Soil-plant-microbe interactions determine soil biological fertility by altering Rhizospheric nutrient cycling and biocrust formation. Sustainability 2022, 15, 625. [Google Scholar]
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Figure 1. Google Earth image of the Dingxi Test Station on the Loess Plateau (a) and photographs of the experimental plots (b).
Figure 1. Google Earth image of the Dingxi Test Station on the Loess Plateau (a) and photographs of the experimental plots (b).
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Figure 2. Effects of different crop rotation patterns on the activity of various soil enzymes: (A) Urease, (B) Nitrate reductase, (C) Nitrite reductase. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
Figure 2. Effects of different crop rotation patterns on the activity of various soil enzymes: (A) Urease, (B) Nitrate reductase, (C) Nitrite reductase. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
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Figure 3. Effect of different rotation patterns on soil nitrogen conversion intensity: (A) Nitrogen fixation, (B) Ammonification, (C) Nitrite reductase, (D) Denitrification. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
Figure 3. Effect of different rotation patterns on soil nitrogen conversion intensity: (A) Nitrogen fixation, (B) Ammonification, (C) Nitrite reductase, (D) Denitrification. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
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Figure 4. Effects of different crop rotation sequences on soil available nitrogen. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
Figure 4. Effects of different crop rotation sequences on soil available nitrogen. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
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Figure 5. Effects of different crop rotation stubble and planting frequency on soil available nitrogen. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
Figure 5. Effects of different crop rotation stubble and planting frequency on soil available nitrogen. Bars show mean standard error, and different lowercase letters above the bars indicate that the difference was significant at p < 0.05 under the LSD test.
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Figure 6. Correlation analysis of soil available nitrogen content, nitrogen transformation intensity, and soil enzyme activity at pre-sowing (A) and post-harvest (B). *, significant at p < 0.05; **, significant at p < 0.01; ***, significant at p < 0.001. AN: available nitrogen; U: urease; NR: nitrate reductase; NIR: nitrite reductase; NF: nitrogen fixation; A: ammonification; N: nitrification; D: denitrification.
Figure 6. Correlation analysis of soil available nitrogen content, nitrogen transformation intensity, and soil enzyme activity at pre-sowing (A) and post-harvest (B). *, significant at p < 0.05; **, significant at p < 0.01; ***, significant at p < 0.001. AN: available nitrogen; U: urease; NR: nitrate reductase; NIR: nitrite reductase; NF: nitrogen fixation; A: ammonification; N: nitrification; D: denitrification.
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Table 1. Physical and chemical characteristics of soil before the experiment began in 2013.
Table 1. Physical and chemical characteristics of soil before the experiment began in 2013.
Organic MatterTotal NitrogenTotal PhosphorusAvailable
Phosphorus		Alkali-Hydrolyzable NitrogenAvailable PotassiumpH
g kg−1mg kg−1
17.511.050.8126.4347.91108.328.13
Table 2. The sequence of different crop rotations.
Table 2. The sequence of different crop rotations.
Frequency of Oilseed FlaxCodeCrop Rotation SequenceCrop in 2021
100% FlaxCont FFlax→Flax→Flax→FlaxFlax
50% Flax (I)WFPFWheat→Flax→Potato→FlaxWheat
FPFWFlax→Potato→Flax→WheatFlax
PFWFPotato→Flax→Wheat→FlaxPotato
FWFPFlax→Wheat→Flax→PotatoFlax
50% Flax (II)FWPFFlax→Wheat→Potato→FlaxFlax
WPFFWheat→Potato→Flax→FlaxWheat
PFFWPotato→Flax→Flax→WheatPotato
FFWPFlax→Flax→Wheat→PotatoFlax
25% FlaxWPWFWheat→Potato→Wheat→FlaxWheat
PWFWPotato→Wheat→Flax→WheatPotato
WFWPWheat→Flax→Wheat→PotatoWheat
FWPWFlax→Wheat→Potato→WheatFlax
0CKFallowFallow
Table 3. Effects of different rotation stubble on soil nitrogen conversion strength.
Table 3. Effects of different rotation stubble on soil nitrogen conversion strength.
Determination PeriodSoil Enzyme
Activities		Type of Stubble in Crop Rotation System
FWPCK
Pre-sowingUrease (mg·g−1·24 h−1)1.24 ± 0.01 b1.04 ± 0.03 c1.47 ± 0.08 a0.38 ± 0.02 d
Nitrate reductase (mg·g−1·48 h−1)1.48 ± 0.04 a1.47 ± 0.03 a1.53 ± 0.03 a0.76 ± 0.16 b
Nitrite reductase (mg·g−1·24 h−1)0.32 ± 0.00 a0.32 ± 0.01 a0.23 ± 0.01 b0.23 ± 0.033 b
Post-harvestUrease (mg·g−1·24 h−1)1.08 ± 0.04 b1.38 ± 0.04 a0.67 ± 0.06 c0.65 ± 0.07 c
Nitrate reductase (mg·g−1·48 h−1)1.45 ± 0.02 b1.44 ± 0.11 b1.87 ± 0.01 a1.56 ± 0.06 b
Nitrite reductase (mg·g−1·24 h−1)0.27 ± 0.00 b0.22 ± 0.02 c0.45 ± 0.02 a0.24 ± 0.01 bc
Note: Data in the table are mean ± standard error, and different lowercase letters indicate a significant difference among treatments at 0.05 level; the same is true below.
Table 4. Effects of different oilseed flax planting frequencies on nitrogen transformation.
Table 4. Effects of different oilseed flax planting frequencies on nitrogen transformation.
Determination PeriodSoil Enzyme ActivitiesOilseed Flax Frequency
100% Flax50% (I) Flax50% (II) Flax25% FlaxCK
Pre-sowingUrease (mg·g−1·24 h−1)1.33 ± 0.08 a1.25 ± 0.01 ab1.19 ± 0.05 b1.22 ± 0.05 ab0.38 ± 0.02 c
Nitrate reductase (mg·g−1·48 h−1)1.50 ± 0.14 a1.54 ± 0.05 a1.45 ± 0.05 a1.47 ± 0.04 a0.76 ± 0.16 b
Nitrite reductase (mg·g−1·24 h−1)0.24 ± 0.02 b0.29 ± 0.01 a0.31 ± 0.00 a0.30 ± 0.01 a0.23 ± 0.03 b
Post-harvestUrease (mg·g−1·24 h−1)1.04 ± 0.06 a1.06 ± 0.06 a1.05 ± 0.03 a1.12 ± 0.07 a0.65 ± 0.07 b
Nitrate reductase (mg·g−1·48 h−1)1.62 ± 0.06 a1.51 ± 0.11 a1.55 ± 0.02 a1.55 ± 0.10 a1.56 ± 0.06 a
Nitrite reductase (mg·g−1·24 h−1)0.20 ± 0.02 c0.32 ± 0.01 a0.30 ± 0.00 a0.31 ± 0.01 a0.24 ± 0.01 b
Note: Data in the table are mean ± standard error; different lowercase letters indicate a significant difference among treatments at 0.05 level.
Table 5. Effects of different stubbles on nitrogen transformation.
Table 5. Effects of different stubbles on nitrogen transformation.
Determination PeriodNitrogen Conversion IndexesType of Stubble in Crop Rotation System
FWPCK
Pre-sowingNitrogen fixation (g·kg−1·30 d−1)0.27 ± 0.01 a0.26 ± 0.00 a0.27 ± 0.01 a0.16 ± 0.017 b
Ammonification (mg·kg−1·7 d−1)8.76 ± 0.27 ab9.32 ± 0.07 a7.13 ± 0.57 c7.96 ± 0.91 ab
Nitrification (mg·kg−1·30 d−1)0.17 ± 0.00 a0.17 ± 0.00 a0.17 ± 0.00 a0.12 ± 0.01 b
Denitrification (mg·5 d−1)2.24 ± 0.02 c2.31 ± 0.03 ab2.25 ± 0.03 bc2.33 ± 0.03 a
Post-harvestNitrogen fixation (g·kg−1·30 d−1)0.22 ± 0.00 a0.21 ± 0.01 a0.16 ± 0.01 c0.18 ± 0.01 b
Ammonification (mg·kg−1·7 d−1)6.57 ± 0.34 b6.35 ± 0.31 b8.11 ± 0.29 a7.02 ± 1.14 ab
Nitrification (mg·kg−1·30 d−1)0.18 ± 0.01 b0.18 ± 0.01 b0.21 ± 0.01 a0.11 ± 0.01 c
Denitrification (mg·5 d−1)2.17 ± 0.02 b2.39 ± 0.11 a2.34 ± 0.04 a2.45 ± 0.08 a
Note: Data in the table are mean ± standard error; different lowercase letters indicate a significant difference among treatments at 0.05 level.
Table 6. Effects of different planting frequencies on nitrogen conversion of oilseed flax.
Table 6. Effects of different planting frequencies on nitrogen conversion of oilseed flax.
Determination PeriodNitrogen
Conversion Indexes		Oilseed Flax Frequency
100% Flax50% (I) Flax50% (II) Flax25% FlaxCK
Pre-sowingNitrogen fixation (g·kg−1·30 d−1)0.23 ± 0.04 a0.26 ± 0.01 a0.27 ± 0.01 a0.27 ± 0.01 a0.16 ± 0.02 b
Ammonification (mg·kg−1·7 d−1)8.57 ± 0.12 ab8.64 ± 0.45 ab7.68 ± 0.48 b9.33 ± 0.11 a7.96 ± 0.91 b
Nitrification (mg·kg−1·30 d−1)0.14 ± 0.01 b0.17 ± 0.00 a0.17 ± 0.00 a0.18 ± 0.00 a0.12 ± 0.01 c
Denitrification (mg·5 d−1)2.30 ± 0.01 ab2.31 ± 0.01 ab2.25 ± 0.06 ab2.23 ± 0.03 b2.33 ± 0.03 a
Post-harvestNitrogen fixation (g·kg−1·30 d−1)0.17 ± 0.01 b0.21 ± 0.00 a0.19 ± 0.01 b0.21 ± 0.01 a0.18 ± 0.01 b
Ammonification (mg·kg−1·7 d−1)6.46 ± 0.36 a6.65 ± 0.13 a6.91 ± 0.30 a7.12 ± 0.36 a7.02 ± 1.14 a
Nitrification (mg·kg−1·30 d−1)0.16 ± 0.015 b0.19 ± 0.00 a0.19 ± 0.00 a0.19 ± 0.00 a0.11 ± 0.01 c
Denitrification (mg·5 d−1)2.31 ± 0.03 a2.13 ± 0.05 b2.39 ± 0.025 a2.30 ± 0.11 a2.45 ± 0.08 a
Note: Data in the table are mean ± standard error; different lowercase letters indicate a significant difference among treatments at 0.05 level.
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Gao, Y.; Zhang, Y.; Wang, H.; Wu, B.; Li, Y.; Yan, B.; Wang, Y.; Lu, P.; Wang, R.; Wen, M.; et al. Improving the Key Enzyme Activity, Conversion Intensity, and Nitrogen Supply Capacity of Soil through Optimization of Long-Term Oilseed Flax Rotation Planting Patterns in Dry Areas of the Loess Plateau of China. Agronomy 2024, 14, 262. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020262

AMA Style

Gao Y, Zhang Y, Wang H, Wu B, Li Y, Yan B, Wang Y, Lu P, Wang R, Wen M, et al. Improving the Key Enzyme Activity, Conversion Intensity, and Nitrogen Supply Capacity of Soil through Optimization of Long-Term Oilseed Flax Rotation Planting Patterns in Dry Areas of the Loess Plateau of China. Agronomy. 2024; 14(2):262. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020262

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Gao, Yuhong, Yong Zhang, Haidi Wang, Bing Wu, Yue Li, Bin Yan, Yifan Wang, Peina Lu, Ruijun Wang, Ming Wen, and et al. 2024. "Improving the Key Enzyme Activity, Conversion Intensity, and Nitrogen Supply Capacity of Soil through Optimization of Long-Term Oilseed Flax Rotation Planting Patterns in Dry Areas of the Loess Plateau of China" Agronomy 14, no. 2: 262. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020262

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