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Article

Soil Phosphorus Forms in Saline Soil after the Application of Biomass Materials

by
Xuewei Guan
1,2,
Jinlin Chen
2,
Guangming Liu
1,* and
Xiuping Wang
3
1
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
2
Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
3
Institute of Coast Agriculture, Hebei Academy of Agriculture and Forestry Sciences, Tangshan 063200, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(2), 255; https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020255
Submission received: 18 December 2023 / Revised: 17 January 2024 / Accepted: 22 January 2024 / Published: 25 January 2024
(This article belongs to the Special Issue Sustainable Water-Fertilizer Management for Soil Salinization Amendment and Crop Production in Salt-Affected Agroecosystems)
Browse Figures
Versions Notes

Abstract

:
Although the application of biological materials has shown potential in improving the environment of salinized soil, the impact on the transformation of soil phosphorus forms in salinized soil, especially when combined with B. mucilaginosus (BM), has rarely been studied. Therefore, this study investigated soil’s properties and phosphorus forms, as well as their relationship, in salinized soil after the application of different biological materials, including rice straw (RS), biochar (B), acidified biochar (AB), BM, RS+BM, B+BM, and AB+BM. A sequential extraction procedure was employed to determine the soil organic/inorganic phosphorus forms (Po/Pi). In our study, the contents of the soil’s resin-P, NaHCO3-Pi, NaOH-Pi, and HCl-P increased by 50–341.66%, 4.08–184.63%, 37.45–163.45%, and 10.19–38.29%, respectively, with the application of the biological materials compared to with conventional fertilization (CK1). However, the contents of the soil’s NaHCO3-Po, NaOH-Po, and residual-P significantly decreased in comparison to with CK1, particularly in the soils that received a combined application with biological materials and BM. Furthermore, the results from the statistical analysis revealed that the application of biological materials could increase the level of soil stable phosphorus, facilitate the transformation from soil stable phosphorus and soil unstable Po to soil unstable Pi, and enhance the effectiveness of soil phosphorus, especially when combined with BM, particularly in soil with AB+BM.

1. Introduction

Soil salinization is one of the main forms of land degradation, which occurs when salt accumulates in the surface layer mostly after the rise of soil water or groundwater with capillary pores [1]. This process is influenced by natural and human factors, such as the high groundwater level and the large evaporation intensity mainly found in hot desert and semi-desert climates [2]. The consequences of soil salinization include poor soil structure, low soil nutrients, limited nutrient availability, and reduced crop yields [3]. These factors severely restrict the sustainable development of farmland agriculture, having a negative impact on the regional economy and ecology [4]. Therefore, it is crucial to improve the salinized soil and the overall soil environment to increase crop yield and to effectively utilize the land resources of saline soils. Despite the efforts made by many scholars who have studied the characteristics of the nutrients, the structure, and the physical properties of saline–alkali soils using various chemical, physical, and biological improvement measures [2,5], the effectiveness and security of these methods cannot be guaranteed. For example, Wang et al. found that compost using sludge as a raw material can effectively improve soil salinization and increase crop yields [6], but whether there are toxic substances in the sludge has been ignored. Therefore, solving and controlling soil salinization while achieving the sustainable development of farmland agriculture remains a significant challenge.
Phosphorus is an essential nutrient element for vegetation, playing an important role in crop growth and development [7]. It is involved in physiological processes such as cell metabolism, photosynthetic phosphorylation, and the tricarboxylic acid cycle [8]. The phosphorus in soil can be divided into two categories: organic (Po) and inorganic (Pi). Soil organic phosphorus generally accounts for 30% to 65% of the total phosphorus in soil, which can be divided into phytate acid (stable) and nucleic acid and ribose (unstable) [9]. Inorganic phosphorus, which generally accounts for 35% to 70% of the total phosphorus in soil, provides most of the phosphorus required for plant growth [10]. In terms of whether it can be directly absorbed and utilized by plants, the inorganic phosphorus in soil can be categorized into three types: water-soluble phosphorus, adsorbed phosphorus, and mineral phosphorus [11]. Water-soluble phosphorus is mineralized from minerals, adsorbed phosphorus, or organic phosphorus, and it can be directly absorbed and utilized by crops [9]. Therefore, the effectiveness of the phosphorus is determined by the mutual transformation of the phosphorus forms in the soil [12].
In saline–alkali soil, a higher salinity increases the fixation of phosphorus, leading to a decrease in the soil available phosphorus content, which in turn hampers crop growth [13]. To enhance the yield, a large amount of phosphate fertilizer is typically utilized, but this practice causes the leaching and fixation of an excessive amount of phosphorus [4]. Consequently, it results in the deterioration of the soil quality and water environment pollution [5]. An effective method to address this issue is the addition of exogenous biomass materials, which helps to increase the availability of soil phosphorus [14]. The decomposition of the biomass materials releases phosphorus, thus increasing the soil’s active phosphorus content [15]. The organic anions produced through the biomass decomposition can form stable complexes with Fe/Al oxides, reducing phosphorus adsorption and enhancing phosphorus effectiveness [16]. Meanwhile, the application of biomass materials could enhance soil alkaline phosphatase activity, promoting the mineralization of potential organic phosphorus [17]. Furthermore, phosphate-solubilizing microorganisms play an important role in the transformation of the soil phosphorus [18]. They can reduce the soil’s pH and increase the availability of the soil phosphorus through organic acid hydrolysis, enzymatic hydrolysis, chelating complexation, proton exchange, and NH4+ assimilation [19,20]. Although many scholars have studied the effects of the application of biomass materials to soil’s physical, chemical, and biological properties [5,21,22], knowledge regarding the incorporation of different biomass materials and phosphate-solubilizing microbial agents on the behavior of the phosphorus in saline–alkali soil, as well as the relationship between the phosphorus forms and properties of soil, remains limited.
As a result, we selected the saline–alkali soil in a coastal saline area as the research object and investigated the impact of incorporating rice straw, biochar, acidified biochar, and phosphate-solubilizing microbial agents on the behavior of the phosphorus in the saline–alkali soil. In our study, we provided the chemical properties of saline–alkali soil and a soil leaching liquor. Then, we used the Hedley et al. [23] and Sui et al. [24] phosphorus fractionation method to determine the content of different phosphorus forms in the saline–alkali soil. Subsequently, we examined the effect of different biomass materials on the form transformation and the cumulative loss of the soil phosphorus. This study aims to contribute valuable data and a theoretical basis that can help improve the saline–alkali soil environment and increase the utilization rate of phosphorus in saline soil.

2. Materials and Methods

2.1. Soil Profile

The test soil was taken from 0–20 cm and 20–40 cm soil layers in Tiaozini, Jianggang Town, Dongtai City, Yancheng City, Jiangsu Province (32°43′ N–32°53′ N, 120°52′ E–120°58′ E). It belongs to the coastal saline soil with the properties in Table 1.

2.2. Biomass Materials

In our study, the application amount of each biomass material is shown in Table 2 under the condition of conventional fertilization (according to the local fertilization standards). The local conventional fertilization materials are monoammonium phosphate (375 kg ha−1) and urea (525 kg ha−1). It is worth noting that all treatments in our experiment were based on these conventional fertilization conditions. Due to the fact that organic matter is the main source of the soil’s organic phosphorus pool, which is closely related to the long-term transformation of phosphorus [14], the application amount of each biomass material follows the principle of equal carbon content. The contents of the total phosphorus of RS, B, and AB are 1.23 g kg−1, 5.96 g kg−1, and 6.44 g kg−1, respectively. The eight treatments in our study are as follows: conventional fertilization (CK1) (control), rice straw (RS) (Yutai County Agricultural Products Co., Ltd., Jining, China), biochar (B) (Jiangsu Huafeng Bioengineering Co., Ltd., Yangzhong, China), acidified biochar (AB) (Jiangsu Huafeng Bioengineering Co. Ltd., Yangzhong, China), B. mucilaginosus (BM) (Beihai Yeshengwang Biotechnological Co., Ltd., Beihai, China), rice straw+B. mucilaginosus (RS+BM), biochar+B. mucilaginosus (B+BM), and acidified biochar+B. mucilaginosus (AB+BM).

2.3. Experimental Design and Soil Sampling

Soil column leaching experiments were conducted to investigate the impact of different biomass materials on the form transformation and the cumulative loss of the soil phosphorus. Before the experiment, soil from the depths of 0–20 cm and 20–40 cm was air-dried and passed through a 2-mm mesh. Cylindrical soil columns, with a diameter of 20 cm and height of 50 cm, were used in the experiment (Figure 1). The experiment comprised eight treatments (Table 2), with each treatment repeated 3 times. Therefore, a total of 24 cylindrical soil columns were used in the experiment. To prepare the soil columns, two layers of acid-washed quartz sand with a diameter of 2–3 mm and a layer of nylon mesh were evenly placed at the bottom of each column. It is worth noting that the soil’s bulk density (1.45 g cm−3) of the soil is large because the experiment was conducted on uncultivated coastal saline soil. Next, the amount of soil required for each soil column was 18.21 kg, which was calculated according to the volume of the soil column and the soil’s bulk density. Then, the water-insoluble biomass materials were mixed uniformly into the 9.11 kg of coastal saline soil (without any fertilizers) of 0–20 cm and 20–40 cm (Figure 1). The bulk density for each soil column was ensured to be 1.45 g cm−3. Following this, the soil columns were left to stand for 7 days. Subsequently, the soil columns were saturated by adding deionized water. Based on the treatments (Table 2), the water-soluble biomass materials and fertilizers were dissolved in 500 mL of deionized water and slowly introduced into the soil columns. For the next 95 days, 500 mL of deionized water was added to each soil column daily, and they were incubated at room temperature.
During the experiment, the physical and chemical properties of leaching liquid samples were measured after collecting the leaching solution on the 1st, 3rd, 7th, 14th, 28th, 42nd, 63rd, 84th, and 98th days. Additionally, soil samples were collected at depths of 0–20 cm and 20–40 cm after the end of the experiment. These soil samples were air-dried and passed through a 2-mm mesh before the physical and chemical properties were measured.

2.4. Analytical Methods

2.4.1. Analysis of the Soil’s Properties

The soil’s pH and electrical conductivity (EC) were determined with the electrometric method and the EC method, respectively. The soil organic matter (SOM) was determined via the potassium dichromate volumetric method—an external heating method. The soil available phosphorus (SAP) was determined through the 0.5 mol L−1 NaHCO3 method. The content of the soil available iron (Fe) and calcium ion (Ca) were determined with the DTPA extraction–atomic absorption spectrophotometry and the EDTA titrimetric method, respectively [25]. The soil total nitrogen (STN) and the soil total phosphorus (STA) were determined with the sulfuric acid digestion and sodium salicylate methods and the sulfuric acid digestion and molybdenum antimony resistance methods, respectively, using Cleverchem 380 Auto Discrete Analyzers (Dechem-Tech. GmbH, Hamburg, Germany).

2.4.2. Leaching Liquid Sample Analysis

The total phosphorus was determined through the nitric acid–sulfuric acid digestion method and the molybdenum antimony anti-spectrophotometric method. The leaching liquid samples were filtered through a 0.45 μm filter membrane used for the determination of the total dissolved phosphorus and dissolved inorganic phosphorus, and the methods of measurement were the nitric acid–sulfuric acid digestion method, the molybdenum antimony anti-spectrophotometric method, and the molybdenum antimony anti-spectrophotometric method, respectively [26]. Furthermore, the dissolved organic phosphorus = the total dissolved phosphorus – the dissolved inorganic phosphorus; and the particle phosphorus = the total phosphorus – the total dissolved phosphorus.

2.4.3. Sequential Extraction Procedure for Phosphorus

The sequential extraction procedure for the phosphorus was determined by Hedley et al. and Sui et al. [23,24]. The specific steps are shown in Figure 2. Meanwhile, according to the activity in the soil, it can be divided into (1) active phosphorus: resin-P, NaHCO3-Pi, and NaHCO3-Po (NaHCO3-P − NaHCO3-Pi); (2) medium active phosphorus: NaOH-Pi and NaOH-Po (NaOH-P − NaOH-Pi); and (3) stable phosphorus: HCl-P and residual-P.

2.5. Fertilizer Amount Conversion

The conversion formula between the amount of fertilizer applied in field and the amount of fertilizer applied in soil column:
W = F A / 10000 × 0.4 × 1.45 × 1000 × w
where W is the fertilization amount of the soil column (kg); F A is the fertilization amount of the field (kg ha−1); 10,000 is the land area of 1 ha2 (m2); 0.4 is the thickness of the soil layer (m); 1.45 is the soil’s bulk density (g cm−3); 1000 is the conversion coefficient (t—kg), and w is the weight of the soil in the soil column (kg).

2.6. Statistical Analyses

The data were summarized by calculating the average and the standard deviation. The difference between the different soil layers of the same biological materials and the difference between the different biological materials in the same soil layer were analyzed through a one-way ANOVA analysis (with the SPSS V24 software) (IBM, Armonk, NY, USA), followed with a Waller–Duncan multiple comparisons post-test. Correlation analysis was used to calculate the Pearson correlation coefficient and to measure the relationship between the soil’s properties and the different forms of the soil phosphorus (with the SPSS V24 software) (IBM, Armonk, NY, USA). The models were then screened. Then, a regression model (with the SPSS V24 software) (IBM, Armonk, NY, USA) and redundancy analysis (with the Canoco 5 program) (Microcomputer Power, Ithaca, NY, USA) were applied to confirm the influence of the soil’s properties on the different forms of the soil phosphorus.

3. Results

3.1. Properties of the Soil

Table 3 presents the results of the soil’s chemical properties from the application of the different biological materials. The soil’s pH values varied among the treatments, with the CK1 having a higher pH (8.59) compared to the soils treated with the application of biological materials, while the soil treated with AB+BM (8.30) was the lowest. The single application of biomass materials resulted in a 0.14–0.24 decrease in the soil’s pH compared to with CK1 but the difference is not significant. Furthermore, the soil’s pH was lower in the soils treated with a combination of biological materials and B. mucilaginosus compared to the soils with a single material application. Additionally, the incorporation of RS+BM and AB resulted in a significant increase in the soil’s electrical conductivity (EC) (p < 0.05). Combined with the results of the soil’s pH, the application of biological materials (except RS and RS+BM) could lead to a 3.51% (BM)–96.49% (AB+BM) increase in the soil’s water-soluble calcium (Ca) compared to with CK1, although there were significant differences between the B+BM, AB+BM, and CK1 (p < 0.05). Conversely, the application of biological materials led to a significant decrease in the soil available iron (Fe), with a decrease of 11.77% (AB+BM)–30.07% (AB) compared to with CK1 (p < 0.05). Among all the treatments, the soil treated with the AB had the lowest Fe value (48.84 mg kg−1), likely due to the adsorption of the biochar on the Fe. Moreover, for the different soil layers with the application of the same biological materials, the soil’s pH significantly increased with the soil depth (p < 0.05), whereas the soil’s EC significantly decreased with an increasing soil depth (p < 0.05). The Ca and Fe decreased with the increasing soil depth but the differences are not significant (except for RS, B, and AB+BM), which may be caused by the leaching of soil salt through irrigation.
In Table 3, the application of the biological materials, whether alone or combined with B. mucilaginosus, significantly increased the contents of soil organic matter (SOM), soil total P (STP), soil total N (STN), and soil available P (SAP). The single application of biomass materials significantly increased the SOM contents by 24.46% (RS)–53.69% (AB) compared to with CK1 (p < 0.05). However, the combined application of the biological materials and B. mucilaginosus significantly increased the SOM contents by 22.29% (B+BM)–30.39% (AB+BM) compared to with CK1 but significantly decreased by 0.35–15.16% compared to the single application (p < 0.05). This decrease may be caused by the SOM consumed by the increase in microorganisms. Additionally, the single application of biomass materials significantly increased the STP, STN, and SAP contents by 2.27% (B)–19.70% (AB), 2.97% (BM)–44.55% (RS), and 65.76% (AB)–110.60% (BM), respectively, compared to with CK1 (p < 0.05). The combined application with B. mucilaginosus was more effective in increasing the SAP compared to the single application of biological materials (p < 0.05). However, the single application of biological materials showed higher STN contents compared to the combined application with B. mucilaginosus. Furthermore, for the different soil layers with the application of the same biological materials, the incorporation of different biomass materials into the soil resulted in a significant decrease in the SOM, STP, STN, and SAP as the depth of the soil increased (p < 0.05).

3.2. Phosphorus Forms Determined by Sequential Extraction

3.2.1. Soil Active Phosphorus Component

Figure 3 provides the contents of the different soils’ active phosphorus forms in the applications of the different biological material. The application of biological materials effectively increases the contents of the soil’s resin-P and NaHCO3-Pi, with increases of 50% (AB)–341.66% (AB+BM) and 4.08% (AB)–184.63% (AB+BM), respectively, compared to with CK1 (p < 0.05). The contents of the soil’s resin-P and NaHCO3-Pi with a single application of the biomass materials were as follows: BM > B > RS > AB > CK1 and RS > BM > B > AB > CK1, respectively. Meanwhile, with the combined application of the biological materials and B. mucilaginosus, the contents of the soil’s resin-P and NaHCO3-Pi were expressed as follows: AB+BM > B+BM > RS+BM > CK1 and AB+BM > RS+BM > B+BM > CK1, respectively, with significant increases of 17.34% (RS+BM)–194.79% (AB+BM) and 41.34% (B+BM)–173.47% (AB+BM), compared to the single application of the biomass materials (p < 0.05), whereas the contents of the B+BM decreased compared to the single application, possibly due to the higher pH of the biochar-inhibiting microbial activity. However, compared to the soil treated with the BM, the contents of the soil’s resin-P and NaHCO3-Pi were significantly increased in the AB+BM and RS+BM, respectively (p < 0.05). Additionally, the content of the soil with the NaHCO3-Po application of biological material significantly decreased compared to the CK1 (p < 0.05) and significantly increased compared to the BM. This suggests that the biological material application effectively promotes the conversion of organic phosphorus to inorganic phosphorus and further increases the contents of the soil’s resin-P and NaHCO3-Pi. It is worth noting that the combined application of biological materials and B. mucilaginosus exhibited greater effectiveness than a single application. Furthermore, for the different soil layers with the application of the same biological materials, the contents of the soil’s resin-P and NaHCO3-Pi were significantly decreased as the depth of the soil increased (p < 0.05), whereas the decrease in the content of the NaHCO3-Po is not significant. This suggested the different soil layers still follow the law of surface accumulation of the soil’s active Pi in terms of the contents of the soil’s active phosphorus with different treatments.

3.2.2. Soil Medium Active Phosphorus Component

Figure 4 depicts the contents of different forms of the soil’s medium active phosphorus with various applications of biological materials. Compared with the CK1, the content of the soil’s NaOH-Pi significantly increased by 37.45% (AB)–163.45% (RS+BM) with the application of different biological materials (p < 0.05), while the content of the soil’s NaOH-Po significantly decreased by 3.02% (AB+BM)–64.86% (RS) (p < 0.05). The content of the soil’s NaOH-Pi increased by 37.45% (AB)–146.59% (RS) with a single application of biological materials, and soils with RS have the highest content at 32.92 mg kg−1. In addition, compared to the single application, the content of the soil’s NaOH-Pi significantly increased by 6.83% (AB+BM)–116.74% (RS+BM) with combined applications of biological materials and B. mucilaginosus (p < 0.05), while the content of the soil’s NaOH-Po significantly decreased by 45.88% (B+BM)–53.44% (AB+BM) (p < 0.05). Furthermore, compared to the soil treated with the BM, the contents of the soil’s NaOH-Pi and NaOH-Po with combined applications of biological materials and B. mucilaginosus were significantly increased and decreased to 18.20% (B)–91.66% (RS+BM) and 21.35% (B+BM)–48.31% (AB+BM), respectively, (p < 0.05). Moreover, for different soil layers with the application of the same biological materials, compared with soil at a depth of 20–40 cm, the contents of the soil’s NaOH-Pi and NaOH-Po with the application of biological materials in a depth of 0–20cm soil were significantly increased 2.80% (AB)–436.35% (RS+BM) and 7.96% (AB+BM)–138.21% (RS), respectively (p < 0.05).

3.2.3. Soil Stable Phosphorus Component

In Figure 5, the contents of different soils’ stable phosphorus forms with the application of different biological materials are shown. The application of biological materials significantly increased the contents of the soil’s HCl-P by 10.19% (RS)–38.29% (BM) (p < 0.05) but decreased the content of the soil’s residual-P by 8.72% (AB)–47.27% (AB+BM), compared to with CK1, although the difference is not significant. The single application of biological materials resulted in a significant increase of 10.19% (RS)–38.29% (BM) in the soil’s HCl-P (p < 0.05), and a decrease of 6.54% (AB)–13.20% (B) in the soil’s residual-P compared to with CK1, respectively. In contrast, the combined application of biological materials and B. mucilaginosus led to a significant increase/decrease of 4.23% (B+BM)–12.55% (RS+BM) and 5.56% (B+BM)–43.59% (AB+BM) in the soil’s HCl-P and residual-P, respectively, compared to the single application (p < 0.05). Additionally, compared to the soil treated with the BM, the contents of the soil’s HCl-P and residual-P were decreased in the RS+BM, AB+BM, B+BM, and AB+BM, but the differences are not significant. Furthermore, for the different soil layers with the application of the same biological materials, the contents of the soil’s HCl-P and residual-P were higher in the soil layer of a depth of 0–20 cm compared to the 20–40 cm depth, but the difference in the HCl-P is not significant.

3.3. Relationships between the Different Forms of Phosphorus Fractions and the Soil’s Properties

Table 4 summarizes the correlation between the soil’s properties and phosphorus forms. The soil’s resin-P showed a negative relationship with the pH and Fe (p < 0.05, p < 0.01), but showed a significantly positive relationship with the Ca and SAP (p < 0.01). The soil’s NaHCO3-Pi was significantly positive correlated with the Fe, SAP, and STP (p < 0.01, p < 0.05). The soil’s NaHCO3-Po showed positive relationships with the Fe (p < 0.05) but showed a significant negative relationship with the SAP (p < 0.01). Additionally, the soil’s NaOH-Pi showed a significant positive relationship with the Fe, STP, and SAP (p < 0.01, p < 0.05), while the soil’s NaOH-Po exhibited significant negative relationships with the Ca, SOM, STP, and SAP (p < 0.01, p < 0.05). Furthermore, the soil’s HCl-P demonstrated positive relationships with the Ca, SOM, and SAP (p < 0.01). Moreover, the relationship between different phosphorus forms in the soil is shown in Table 4. The soil’s residual-P demonstrated significant positive relationships with the pH, Fe, and SOM (p < 0.01, p < 0.05). The resin-P demonstrated a significant positive relationship with the NaHCO3-Pi and HCl-P (p < 0.01). The NaOH-Pi and NaHCO3-Pi showed negative relationships with the NaHCO3-Po and NaOH-Po (p < 0.05), and NaOH-Po had a significant positive relationship with the NaHCO3-Po and residual-P (p < 0.001).
The redundancy analysis results for the different forms of the soil phosphorus and the soil’s properties are presented in Figure 6. It was found that factors associated with the soil’s properties explained 63.09% of the variation in the different forms of the soil phosphorus. The interpretation rate for the first axis was 47.34%, while the second axis had an interpretation rate of 15.75%. Specifically, the Ca and SOM had a significant impact on the resin-P and HCl-P, whereas the STP, SNP, and STN notably affected the NaHCO3-Pi and NaOH-Pi. The most pronounced effect of pH was observed on residual-P, NaHCO3-Po, and NaOH-Po. These findings, combined with the results of the Regression model (Table 5), indicate that the SAP is the main controlling factor for resin-P, NaHCO3-Pi, and NaOH-Pi. On the other hand, the SOM, STP, and Ca are the main controlling factors for the HCl-P and residual-P, while the Fe and SOM are the main controlling factors for the NaHCO3-Po and NaOH-Po.
The application of the biological materials was found to decrease the soil’s pH and to increase the SOM, STP, SAP and STN of the soil, resulting in improved levels of the soil stable phosphorus content. This increase in the content of the soil stable phosphorus could increase the organic P levels and promote the conversion of organic P to inorganic P, thereby improving the effectiveness of the soil phosphorus.

3.4. Cumulative Loss of Soil Phosphorus

In our study, the cumulative loss of the soil phosphorus in the AB and AB+BM treatment decreased by 29.49% and 7.79%, respectively, compared to with CK1. This indicates that these treatments resulted in a lower cumulative phosphorus loss in the soil (Figure 7). On the other hand, the remaining treatments led to an increase in the cumulative loss of the soil phosphorus. It is worth noting that the increase in the cumulative loss of the soil total phosphorus was primarily attributed to the increase in the soil’s dissolved total phosphorus loss. The accumulation loss of the soil’s dissolved total phosphorus resulting from the application of different biological materials ranged from 51.89% (RE+BM) to 81.56% (AB) of the cumulative loss of the soil total phosphorus. Furthermore, the rise in the cumulative loss of the soil’s dissolved total phosphorus was primarily a result of the increased organic phosphorus loss in the soil, accounting for 26.75% (B) to 86.35% (AB) of the cumulative loss of the soil’s dissolved total phosphorus.

4. Discussion

In our study, the application of biological materials, which contain various amounts of ash alkalinity [27], significantly increased the soil’s EC with RS, B, B+BM, AB, and AB+BM compared to with CK1, especially with the AB+BM incorporation, which is consistent with similar findings by Yi et al. [28], who reported that the addition of straw and biochar induced an increase in the soil’s EC. This increase also led to an increase in the soil’s Ca. The soil’s Ca with the application of biological materials increased by 3.51%(BM)–96.49%(AB+BM) compared to with CK1, particularly with the AB+BM-treated soil (Table 3). The soil’s Ca of AB+BM increased by 96.49% compared to with CK1, due to the large amount of Ca2+ carried by the biochar [29]. Moreover, the soil’s available Fe of RS and RS+BM showed a significant increase of 54.83% and 68.47%, respectively, compared to with CK1 (Table 3). On the other hand, due to the larger specific surface area and amount of negative charge carried by the biochar, which can adsorb positively charged inorganic metal ions in the soil [30], there was a significant decrease (11.77–30.07%) in the soil’s available Fe of B, B+BM, AB, and AB+BM compared to with CK1. Furthermore, the application of biological materials significantly increased the soil organic matter (SOM) by 24.46% (RS)–53.69% (AB) compared to with CK1, especially with the AB (10.62 g kg−1), due to the significant amount of organic carbon carried by the biochar, which is consistent with the similar results of Guan et al. [31], who reported that the different straw returning methods, such as straw or straw biochar, resulted in the accumulation of SOM. Additionally, the application of biological materials significantly increased the STP and SAP of the soil by 2.27% (B)–19.69% (AB) and 65.76% (AB)–110.60% (BM), respectively, compared to with CK1, particularly with the combined application of biological materials and B. mucilaginosus. Moreover, the content of STN with the application of different biological materials significantly increased by 2.97–57.43% compared to with CK1, indicating that incorporation of biological materials resulted in the soil’s N accumulation, as N is a constituent of biological materials. These results are consistent with similar findings by Metawee et al. [30] and Nie et al. [32], who reported that the application of organic materials could lead to an increase in SAP, STP, and STN.
Resin-P and NaHCO3-Pi were considered as available phosphorus sources in the soil, serving as direct supplements to the soil’s effective P [33]. In our study, the application of biological materials alone increased the contents of resin-P and NaHCO3-Pi in the soil by 43.89% (BM) to 301.84% (RS) compared to with CK1. Moreover, the soil treated with biological materials exhibited the highest content of these elements. These findings are consistent with those of Parvage et al. [34] and Liu et al. [35], who found that the addition of biomass materials could cause the content of the soil’s resin-P and NaHCO3-Pi to increase by 126% and 30.04%. This indicates that the application of biological materials, particularly the treatment with BM, significantly increases the effectiveness of the soil’s P. This enhancement in effectiveness can be attributed to various factors, including (1) organic acid hydrolysis, enzymatic hydrolysis, chelation, and complexation brought about by microorganisms, promoting the decomposition of insoluble phosphorus in the soil [23]; (2) the decomposition of straw, which releases organic acids and available phosphorus [36]; (3) the adsorption of biochar on metal ions, which promotes the dissolution of insoluble phosphate compounds [37]. In contrast, the content of NaHCO3-Po, which constitutes a component of the active Pi in the soil, decreased by 39.09% to 61.52% with the application of the BM and B compared to with CK1. The findings in this study coincide with Sui et al. [38], suggesting that the application of biomass materials can lead to a decrease in the soil’s NaHCO3-Po content. In contrast, Gao et al. [39] reported an increase in the soil’s NaHCO3-Po content due to the application of biochar under flooding conditions. This divergence is attributed to the inhibitory effect of flooding conditions on soil’s microbial activity, consequently hindering the mutual transformation of the soil’s organic phosphorus and inorganic phosphorus, and further caused the increased of the soil’s NaHCO3-Po. It is noteworthy that the combined application of biological materials and B. mucilaginosus had a more pronounced effect on the availability of the soil’s P. Compared to single applications, the contents of resin-P and NaHCO3-Pi increased by 17.34% (B+BM) to 194.79% (AB+BM), while the content of NaHCO3-Po decreased by 3.21% (B+BM) to 30.76% (RS+BM), which were consistent with the similar results of Wei et al. [40]. Meanwhile, the contents of the soil’s resin-P and NaHCO3-Pi were significantly increased in the AB+BM and RS+BM, respectively, compared to the BM (p < 0.05). This may be attributed to the promotion of the decomposition of biomass materials and the accelerated mineralization of the organic phosphorus brought about by the application of B. mucilaginosus [20], especially in soil treated with AB+BM.
NaOH-Pi and NaOH-Po are potential phosphorus sources in soil, and they easily combine with iron and aluminum oxides to form insoluble phosphates, requiring long-term geochemistry for absorption and utilization by vegetation [4]. In our study, the content of the soil’s NaOH-Pi increased by 37.45% (AB)–163.45% (RS+BM) with the application of different biological materials (Figure 4), which was consistent with similar results from Sui et al. [38]. Gao et al. [39] also found that biochar application could increase the content of the soil’s NaOH-Pi but decrease the content of the soil’s NaOH-Po. Meanwhile, the content of the soil’s NaOH-Po decreased by 3.02% (AB+BM)–64.86% (RS) (Figure 4), due to the application of biological materials promoting the conversion of the soil’s Po to the soil’s Pi. The reasons for the increase in the soil’s NaOH-Pi content are as follows: (1) the application of the biological materials significantly increased the content of the soil organic matter (SOM), leading to a decrease in the adsorption capacity of the iron oxides on the soil phosphorus; (2) the adsorption of biochar to metal cations also reduced the adsorption capacity of the iron oxides on the soil phosphorus [16].
The soil’s HCl-P and residual-P are the stable phosphorus forms, which are difficult to be absorbed and utilized by vegetation [40]. In our study, we found that the application of biological materials could increase the contents of the soil’s HCl-P compared to with CK1, increasing it to 10.19% (RS)–38.29% (BM) (Figure 5). This is consistent with similar results from Metawee et al. [32] (Figure 7), who found that the incorporation of different straws, especially medium-quality organic straw, could increase the content of the soil’s HCl-P. Moreover, previous studies have shown that HCl can extract apatite-type minerals [38] (Figure 7); therefore, the result of the soil’s Ca (Table 3) supports this finding. Conversely, the incorporation of biological materials resulted in a decrease in the content of the soil’s residual-P compared to with CK1, decreasing it to 8.72% (AB)–47.27% (AB+BM). Wu et al. [41] found a continuous application of phosphate-solubilizing bacterial fertilizer can effectively reduce the content of the soil’s residual-P by 9.93%. Meanwhile, Metawee et al. [32] reported an increase in the soil’s residual-P content by the application of different straws, which could support the results of the RS and RS+BM in our study. Therefore, it can be concluded that the application of biological materials promotes the conversion of the soil’s residual-P to other forms of the soil’s P.
Soil phosphorus transformation is affected by many factors, such as the soil’s properties and structure, and management methods [11]. The transformation greatly affects the availability of the soil phosphorus [16]. In our study, the main controlling factors for the resin-P, NaHCO3-Pi, and NaOH-Pi were found to be the SAP and pH, as determined by the redundancy analysis and regression modeling (Figure 6, Table 5). In contrast, the HCl-P and residual-P were primarily controlled by the SOM, STP, and Ca, while NaHCO3-Po and NaOH-Po were influenced by the Fe and SOM. These findings supported the results of a previous study by Metawee et al. [32], which indicated that the application of biological materials could significantly increase the SOM, STP, and Ca of the soil (Table 3). Consequently, this increase in the soil’s nutrients led to a higher content of stable P, particularly in soils with a high abundance of AB. In other words, the inclusion of biological materials promoted the transformation of fertilized P into the stable P pool [42]. Furthermore, a negative relationship was observed between the soil’s resin-P, NaHCO3-Pi, and NaOH-Pi, and the soil’s NaHCO3-Po, NaOH-Po, and residual-P (p < 0.01, p < 0.05), particularly in soils that received combined biological materials and the application of B. mucilaginosus. This suggests that the incorporation of biological materials could enhance the conversion of unstable Po to available Pi. Notably, the addition of B. mucilaginosus can accelerate this conversion.

5. Conclusions

The application of biological materials could significantly decrease the soil’s pH, leading to an increase in the contents of the soil organic matter (SOM) and nutrients. Moreover, the biomass materials that were applied alone or in combination with B. mucilaginosus could both increase the contents of the soil’s active and medium active inorganic phosphorus and decrease the content of the soil’s organic phosphorus. The combined application with biological materials and B. mucilaginosus demonstrated a better effect than the single application, especially in soils with AB+BM. Furthermore, results from the redundancy analysis, the relative analysis, and regression models revealed that the application of biological materials could increase the level of the soil stable phosphorus, facilitate the transformation from soil stable phosphorus and soil unstable Po to soil unstable Pi, and enhance the effectiveness of the soil phosphorus, especially when combined with B. mucilaginosus, particularly in soil with AB+BM.

Author Contributions

Conceptualization, X.G.; data curation, X.G.; funding acquisition, G.L. and J.C.; investigation, X.G.; methodology, X.G.; supervision, G.L. and J.C.; validation, X.G.; writing—original draft, X.G.; and writing—review and editing, X.W. All authors have read and agreed to the published version of this manuscript.

Funding

This research was supported by the National Key Research and Development Program (2022YFD1900104, Guangming Liu), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, Jinlin Chen). Special thanks are given to the referees and the editors for their instructive comments, suggestions, and editing of this manuscript.

Data Availability Statement

The data are available from the corresponding author on reasonable request.

Acknowledgments

We acknowledge the valuable comments provided by the editors and anonymous reviewers.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Abbreviations

Organic phosphorus (Po); inorganic phosphorus: (Pi); soil organic matter (SOM); soil total phosphorus (STP); soil total nitrogen (STN); soil available phosphorus (SAP); soil calcium ion (Ca); and soil available iron (Fe).

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Figure 1. (a) Soil column specifications; (b) Soil column leaching experiments.
Figure 1. (a) Soil column specifications; (b) Soil column leaching experiments.
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Figure 2. Sequential extraction procedure of Hedley for phosphorus.
Figure 2. Sequential extraction procedure of Hedley for phosphorus.
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Figure 3. The contents of the soil’s active phosphorus with the application of different biological materials in different soil layers. Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). For example, 0–20 cm and 20–40 cm of the CK1 in NaHCO3-Pi have the same capital letter “A”, which means that there was no significant difference between 0–20 cm and 20–40 cm of the CK1. Different small letters denote significant differences among different biological materials in the same soil layer (p < 0.05). For example, 0–20 cm of the CK1 and BM in NaHCO3-Pi have different small letters “b” and “e”, which means that there was a significant difference between the CK1 and BM. CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar.
Figure 3. The contents of the soil’s active phosphorus with the application of different biological materials in different soil layers. Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). For example, 0–20 cm and 20–40 cm of the CK1 in NaHCO3-Pi have the same capital letter “A”, which means that there was no significant difference between 0–20 cm and 20–40 cm of the CK1. Different small letters denote significant differences among different biological materials in the same soil layer (p < 0.05). For example, 0–20 cm of the CK1 and BM in NaHCO3-Pi have different small letters “b” and “e”, which means that there was a significant difference between the CK1 and BM. CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar.
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Figure 4. The contents of the soil’s medium active phosphorus with the application of different biological materials in different soil layers. Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). For example, 0–20 cm and 20–40 cm of CK1 in NaOH-Pi have the same capital letter “A”, which means that there was no significant difference between 0–20 cm and 20–40 cm of the CK1. Different small letters denote significant differences among the different biological materials in the same soil layer (p < 0.05). For example, 0–20 cm of CK1 and BM in NaOH-Pi have different small letters “d” and “c”, which means that there was a significant difference between the CK1 and BM. CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar.
Figure 4. The contents of the soil’s medium active phosphorus with the application of different biological materials in different soil layers. Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). For example, 0–20 cm and 20–40 cm of CK1 in NaOH-Pi have the same capital letter “A”, which means that there was no significant difference between 0–20 cm and 20–40 cm of the CK1. Different small letters denote significant differences among the different biological materials in the same soil layer (p < 0.05). For example, 0–20 cm of CK1 and BM in NaOH-Pi have different small letters “d” and “c”, which means that there was a significant difference between the CK1 and BM. CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar.
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Figure 5. The contents of the soil stable phosphorus with the application of different biological materials in different soil layers. Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). For example, 0–20 cm and 20–40 cm of CK1 in HCl-P have the same capital letter “A”, which means that there was no significant difference between 0–20 cm and 20–40 cm of the CK1. Different small letters denote significant differences among different biological materials in the same soil layer (p < 0.05). For example, 0–20 cm of CK1 and BM in HCl-P have the same small letters “a”, which means there was no significant difference between CK1 and BM, but there was a significant difference between CK1 and RS, because they have different small letters “a” and “bc”. CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar.
Figure 5. The contents of the soil stable phosphorus with the application of different biological materials in different soil layers. Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). For example, 0–20 cm and 20–40 cm of CK1 in HCl-P have the same capital letter “A”, which means that there was no significant difference between 0–20 cm and 20–40 cm of the CK1. Different small letters denote significant differences among different biological materials in the same soil layer (p < 0.05). For example, 0–20 cm of CK1 and BM in HCl-P have the same small letters “a”, which means there was no significant difference between CK1 and BM, but there was a significant difference between CK1 and RS, because they have different small letters “a” and “bc”. CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar.
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Figure 6. Redundancy analysis of the soil’s properties and phosphorus forms (RDA).
Figure 6. Redundancy analysis of the soil’s properties and phosphorus forms (RDA).
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Figure 7. Cumulative loss of the soil phosphorus with the application of different biological materials. Po: organic phosphorus; Pi: inorganic phosphorus; PP: particulate phosphorus; TDP: total dissolved phosphorus.
Figure 7. Cumulative loss of the soil phosphorus with the application of different biological materials. Po: organic phosphorus; Pi: inorganic phosphorus; PP: particulate phosphorus; TDP: total dissolved phosphorus.
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Table 1. The properties of the original soil.
Table 1. The properties of the original soil.
Soil
Layer		pHEC
μs cm−1		SOM
g kg−1		STN
g kg−1		STP
g kg−1		SAP
mg kg−1
0–20 cm8.552696.505.150.380.325.45
20–40 cm8.442503.504.320.500.682.11
pH: soil’s acidity and alkalinity; EC: soil electrical conductivity; SOM: soil organic matter; STN: soil total nitrogen; STP: soil total phosphorus; SAP: soil available phosphorus.
Table 2. The different application rates of the biological material.
Table 2. The different application rates of the biological material.
AbbreviationTreatmentApplication Rate (kg ha−1)
1CK1conventional fertilization\
2BMB. mucilaginosus120
3RSrice straw14,800
4Bbiochar13,200
5ABacidified biochar13,200
6RS+BMrice straw+B. mucilaginosus14,800 + 120
7B+BMbiochar+B. mucilaginosus13,200 + 120
8AB+BMacidified biochar+B. mucilaginosus13,200 + 120
Table 3. The soil’s properties characteristic of the application of the different biological material.
Table 3. The soil’s properties characteristic of the application of the different biological material.
Soil Layer TreatmentpHECSOMSTPSTNSAPCaFe
μs cm−1g kg−1g kg−1g kg−1mg kg−1mg kg−1mg kg−1
0–20 cmCK18.59
Aa 		254.60
Aab 		6.91
Ac 		1.32
Aa		1.01
Aa 		10.28
Ac 		76.00
Ac		69.84
Ac
BM8.36
Aa		228.80
Ab 		8.61
Ab 		1.46
Aa 		1.04
Aa 		18.95
Aab 		78.67
Abc		57.31
Ade
RS8.38
Aa		255.53
Aab 		8.60
Ab 		1.39
Aa 		1.46
Aa		18.24
Aab 		93.33
Aabc		108.13
Ab
RS+BM8.52
Aa		228.73
Ab 		8.57
Ab		1.48
Aa		1.59
Aa 		21.65
Aa		66.00
Ac		117.66
Aa
B8.45
Aa 		280.43
Aab 		9.63
Aab 		1.35
Aa 		1.17
Aa 		17.05
Aab 		116.00
Aabc		52.00
Aef
B+BM8.44
Aa		298.17
Aab 		8.45
Ab 		1.56
Aa		1.26
Aa 		19.11
Ab 		128.67
Aab		49.82
Aef
AB8.35
Aa 		299.35
Aa		10.62
Aa 		1.47
Aa 		1.27
Aa		17.04
Ab 		122.67
Aabc		48.84
Af
AB+BM8.30
Aa 		308.93
Aab 		9.01
Aab		1.58
Aa 		1.35
Aa 		21.53
Aa 		149.33
Aa		61.62
Ad
20–40 cmCK18.65 A 217.63 B 6.45 B 1.05 B 0.83 B 2.29 B 77.00 A 52.93 A
BM8.64 B 203.55 B 5.98 B 1.31 B 1.39 B 4.48 B 55.50 B 55.36 A
RS8.62 B 258.90 A 6.83 B 1.06 B 1.25 A 1.90 B 76.67 B 86.99 B
RS+BM8.62 A 248.00 A 6.36 B 1.05 B 2.33 B 3.10 B 77.33 A 94.11 A
B8.64 B 222.27 B 7.84 B 1.17 B 0.73 B 3.72 B 78.67 B 49.55 B
B+BM8.62 B 236.75 B 7.32 B 1.26 B 0.60 B 2.64 B 113.00 A 39.83 B
AB8.45 B 243.40 B 9.00 B 1.44 A 0.91 B 4.84 B 107.00 A 68.11 A
AB+BM8.49 B 281.90 A 8.62 A 1.15 B 0.68 B 3.64 B 114.00 B 71.40 B
Different capital letters denote significant differences between different soil layers of the same biological material (p < 0.05). Different small letters denote significant differences among different biological materials in the same soil layer (p < 0.05). CK1: conventional fertilization; BM: B. mucilaginosus; RS: rice straw; B: biochar; AB: acidified biochar; pH: soil’s acidity and alkalinity; EC: electric conductivity; SOM: soil organic matter; STP: soil total phosphorus; STN: soil total nitrogen; SAP: soil available phosphorus; Ca: soil’s water-soluble calcium; Fe: soil available iron.
Table 4. Relationship between the soil’s properties and the forms of soil phosphorus.
Table 4. Relationship between the soil’s properties and the forms of soil phosphorus.
Resin-PNaHCO3-PiNaHCO3-PoNaOH-PiNaOH-PoHCl-PResidual-P
Resin-P1
NaHCO3-Pi0.415 **1
NaHCO3-Po−0.522 **−0.104 *1
NaOH-Pi0.1150.645 **−0.0291
NaOH-Po−0.682 **−0.505 **0.705 **−0.236 *1
HCl-P0.779 **0.148−0.606 **0.058−0.670 **1
Residual-P−0.452 **−0.1220.230.140.621 **−0.293 *1
pH−0.234 *−0.036−0.0540.0370.212−0.2060.326 **
EC0.056−0.0150.173−0.231−0.1770.191−0.270 *
SOM0.1620.029−0.057−0.032−0.357 **0.474 **0.239 *
STP0.0040.231 *−0.1610.203 *−0.270 *0.077−0.185
STN−0.1510.084−0.2150.227−0.179−0.073−0.034
Ca0.402 **0.091−0.0830.025−0.482 **0.305 **−0.540 **
Fe−0.412 **0.456 **0.258 *0.726 **0.249 *−0.492 **0.448 **
* Significant at the 0.05 level. ** Significant at the 0.01 level. pH: soil’s acidity and alkalinity; EC: electric conductivity; SOM: soil organic matter; STP: soil total phosphorus; STN: soil total nitrogen; SAP: soil available phosphorus; Ca: soil’s water-soluble calcium; Fe: soil available iron.
Table 5. Regression model of the forms of soil phosphorus and the soil’s properties.
Table 5. Regression model of the forms of soil phosphorus and the soil’s properties.
Regression ModelR2p
Resin-P = 370.35 + 4.31SAP − 0.069EC − 0.57Fe − 8.70SOM − 33.69pH0.88<0.001
NaHCO3-Pi = 477.25 + 10.10SAP − 16.72SOM − 50.07pH0.70<0.001
NaOH-Pi = −9.19 + 0.21Fe + 1.10SAP + 0.07Ca − 1.13SOM0.79<0.001
HCl-P = 228.57 + 12.64STP + 6.66STN + 1.34SOM + 1.92Ca0.78<0.001
NaHCO3-Po = −10.36 + 0.47Fe + 0.10EC + 4.51SOM − 2.72SAP − 8.14STP − 3.84STN0.60<0.001
NaOH-Po = 37.22 + 0.17Fe + 2.01SOM − 1.68SAP − 2.37STN − 0.07Ca − 4.01STP0.87<0.001
Residual-P = 860.25 + 58.62STP + 1.13pH + 1.21SOM − 13.83STN0.46<0.001
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Guan, X.; Chen, J.; Liu, G.; Wang, X. Soil Phosphorus Forms in Saline Soil after the Application of Biomass Materials. Agronomy 2024, 14, 255. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020255

AMA Style

Guan X, Chen J, Liu G, Wang X. Soil Phosphorus Forms in Saline Soil after the Application of Biomass Materials. Agronomy. 2024; 14(2):255. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020255

Chicago/Turabian Style

Guan, Xuewei, Jinlin Chen, Guangming Liu, and Xiuping Wang. 2024. "Soil Phosphorus Forms in Saline Soil after the Application of Biomass Materials" Agronomy 14, no. 2: 255. https://0-doi-org.brum.beds.ac.uk/10.3390/agronomy14020255

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