Straw Residual Retention on Wheat Photosynthetic Characteristics, Utilization of Water and Nitrogen, and Reactive Nitrogen Losses

Abstract

Straw residual retention is an emerging and promoted practice in rain-fed northwest China, but its effect on wheat photosynthetic characteristics, the utilization of water and nitrogen, and reactive nitrogen losses is poorly understood. A two-year consecutive field experiment was conducted to investigate the impacts of residual incorporation into soil and nitrogen application on wheat nitrogen and water utilization, yield and nitrogen losses during 2018–2020. The split-plot design of two tillage systems [conventional tillage (CT), and straw residue incorporated into soil (SR)] and three nitrogen rates [0 kg ha⁻¹ (N0), 144 kg ha⁻¹ (N144), 180 kg ha⁻¹ (N180)] was implemented. Our results demonstrated that compared to CT, SR significantly influenced several key metrics. Compared with CT, SR increased the wheat photosynthetic rate (Pn), transpiration rate (Tr), leaf area index (LAI), leaf total chlorophyll (Chl-total), glutamine synthetase (GS) and nitrate reductase (NR) by an average of 5.38%, 12.75%, 8.21%, 5.79%, 16.21% and 20.08%, respectively (p < 0.05). In addition, SR increased the wheat grain yield and nitrogen uptake accumulation (NUA), evapotranspiration (ET), precipitation storage efficiency (PSE), and mineral nitrogen residual after harvest (except for SR-N180 in 2019–2020), but decreased the apparent nitrogen recovery when compared with CT. However, there was an insignificant difference in the ammonia (NH₃) volatilization and nitrous oxide (N₂O) emissions of SR and CT. With an increase in the N-fertilization rate, the Pn and Tr, NH₃ volatilization, N₂O emission, mineral nitrogen residual (except for SR-N180 in 2019–2020), LAI, Chl-total (except for SR-N180 and CT-N180 in 2018–2019), GS, NR, grain yield, WUE, and NUA increased significantly; however, the ET, PSE, apparent nitrogen recovery (ANR), and nitrogen harvest index (NHI) decreased significantly. Furthermore, the differences between N144 and N180 in terms of the photosynthetic characteristics of wheat, the utilization of water and nitrogen, and yield were not significant. Overall, straw retention with N144 could be recommended as a resource-saving and environment-friendly management practice in a rain-fed winter wheat–fallow cropping system in northwest China.

1. Introduction

The incorporation of straw into soils is one of the most useful conservation tillage methods, and can alleviate soil degradation and improve soil fertility [1,2,3,4]. Previous studies have indicated that straw incorporation combined with nitrogen fertilizer application is conducive to increasing the utilization of nitrogen, crop yield and sustainable intensification (defined as a system in which yields are increased without adverse environmental impacts) of wheat production in this region [5,6]. Tan et al. [7] found that wheat straw is a slowly available nutrient source that can release carbon, nitrogen and other elements with soil microbial decomposition abilities. The long-term addition of organic materials can improve the biophysical properties of soil (such as microbial biomass carbon in soil aggregate) and the availability of water to plants [8,9]. Straw return can both increase the total porosity of soil and decrease its bulk density, leading to improved nutrient leaching, better soil aeration, and enhanced root penetration deeper into the soil [10,11]. Accordingly, residual retention with reasonable nitrogen application affects the utilization of water and nitrogen and crop production.

Straw return is vital for maintaining the temperature and moisture of soil, which increases its water use efficiency and photosynthetic capacity [12]. A limited water content in soil reduces the spring maize photosynthetic (Pn) and transpiration rate (Tr) in water deficit conditions [13]. Meanwhile, nitrogen is a constitute element of chlorophyll (Chl-total), and more chlorophyll results in an increased photosynthetic active leaf area [14,15]. Photosynthetic capacities (leaf area index, Chl-total, Pn and Tr) are positively correlated with grain yield [16]. In addition, nitrogen is also an important component of plant enzymes. Nitrate reductase (NR) and glutamine synthetase (GS) play important roles in nitrogen assimilation, which can assimilate ammonium nitrogen and nitrate nitrogen into amino acids and proteins to supply the growth and physiological activities of crops [17].

To achieve the goal of sustainable agricultural development, the rational use of chemical fertilizers and improving tillage systems are two key practices in cropping systems [18,19,20,21]. However, using excess fertilizer to obtain high yields is ubiquitous in China, causing serious non-point-source pollution [22,23]. Excessive nitrogen (N) fertilizer can be lost to the agroecosystem environment through ammonia (NH₃) volatilization, nitrous oxide (N₂O) emission and leaching, which can reduce nitrogen recovery efficiency and aggravate haze (PM_2.5), climate warming and groundwater pollution [24,25,26,27]. Approximately4 billion metric tons of crop straw was produced annually at the beginning of the 21st century worldwide [28]. While this straw serves as a nutrient resource, it also poses a potential environmental threat. The recycling of crop residues is pivotal for agricultural sustainable development and environmental protection.

Wheat (Triticum aestivum L.) is one of the staple crops in rain-fed land, such as the Loess Plateau, China [18]. N leaching is the main path of nitrogen loss, and its main form is nitrate N, which has high mobility in soil [25,29]. N fertilizer-combined straw retention could increase crop yield and nutrient utilization, and reduce N losses [30]. Meanwhile, N immobilization during straw decomposition consumes mineral N, which reduces the risk of N leaching [31]. However, some research has indicated that straw treatment has no influence on N leaching [32]. Therefore, it is necessary to further understand the influence of straw on nitrogen leaching. The gaseous loss of nitrogen, such as through ammonia volatilization and nitrous oxide emissions, is another pathway of field N loss. Li et al. [25] showed that straw incorporated into soil has no pronounced impacts on the emission of soil ammonia. Meanwhile, Sun et al. [33] showed that straw retention could increase NH₃ volatilization. Furthermore, there is no consensus on nitrous oxide emissions regarding fertilizer mixed with straw [34,35]. Understanding the potential impacts of reactive nitrogen releases from wheat straw retention and their relationship with crop nitrogen utilization is essential.

In this study, winter wheat photosynthesis, water and nitrogen utilization and gaseous nitrogen emissions was assessed under the straw residual retention in two growing seasons in 2018–2020. The objectives of this study were to (1) characterize the wheat photosynthetic characteristics, utilization of water and nitrogen, and reactive nitrogen losses under straw returning and nitrogen fertilization; and (2) investigate the relationship between the wheat growth and nitrogen gaseous emission under residual returning condition. The hypothesis were: (1) straw retention and nitrogen fertilization increased wheat photosynthetic characteristics, utilization of water and nitrogen, and reactive nitrogen losses, (2) straw retention and nitrogen fertilization increased the grain yield and nitrogen gaseous emission.

2. Materials and Methods

2.1. Experiment Site

The field experiment was conducted at Cao Xinzhuang Experimental Station (34°18′40.78″ N, 108°06′14.48″ E, 513ma.s.l.) in Shaanxi Province, and the cropping system is winter wheat (Triticum aestivum L. cv. Xiaoyan 22) and summer fallow. The climate of the study area is a continental semi-arid and warm temperate zone, with a daily mean air temperature of 13.06 °C and a mean annual precipitation of 563.13 mm. Figure 1 shows the daily mean temperature and precipitation for two wheat seasons from 1 June 2018 to 1 June 2020. The soil is classified as Terric Anthrosols according to the FAO-UNESCO soil map [36]. The topsoil (0–20 cm) has a pH of 8.03 (H₂O). The bulk density is 1.31 g cm⁻³, the organic matter is 13.28 g kg⁻¹, the total N is 0.91 g kg⁻¹, the available phosphorus (P) is 6.21mg kg⁻¹, and the available potassium is 124.55 mg kg⁻¹.

2.2. Experiment Design and Field Management

The field experiment included two tillage systems as the main factor (conventional tillage with residue removed, CT; straw residue incorporated into the soil, SR) and three N application rates as sub-factor (0 kg ha⁻¹, N0; 144 kg ha⁻¹, N144; 180 kg ha⁻¹, N180). All treatments were laid out in a split-plot design with three replicates, and each plot was 40 m² with a 0.3 m ridge enclosed in the center (for distinguishing the cultivation treatments). The field management method was the same as Li et al.’s method [25].

2.3. Sampling and Measurement

2.3.1. Physiological Characteristics

Wheat Pn and Tr were measured using a LI-Cor LI-6400XT portable photosynthesis system (LI-Cor, Lincoln, NE, USA). The fully expanded flag leaves were measured in vivo on sunny days between 9:00 and 11:00 am at the flowering stage (186 days after planting). While in operation, the leaf chamber temperature of 25 °C was maintained using an open gas circuit, and the photosynthetic active radiation and relative humidity were set at 1100 μmol m⁻² s⁻¹ and 56–60% in the leaf chamber, respectively. Nine leaves in each of the three replicates of each treatment were randomly determined.

The Chl-total content was quantified using the method of Wu et al. [37] at the same time as Pn and Tr determination. NR and GS activities were measured with the method of Geng et al. [17] in fully expanded flag leaves (at the flowering stage). The leaf area index (LAI) was determined according to the method of Wu et al. [16] at the flowering stage.

2.3.2. Measurements of Soil NH₃ Volatilization, N₂O Emission, and N Leaching

Soil NH₃ volatilization was quantified using a modified semi-open static chamber method (Figure S3). Each plot had one chamber system, which consisted of one polyvinyl chloride cylinder and two polyurethane foam disks. Both foam disks were soaked with 60 mL of acid reagent before use; the lower one was taken back to the lab for determination in a zip-lock bag. The trapped ammonia was extracted with potassium chloride, and the extracts were analyzed via flow injection AutoAnalyzer3—AA3 (SEAL Analytical GmbH, Norderstedt, Germany). A recovery test verified the reliability of this extraction method (recovery rate was 95%). Ammonia samples were collected every 2–6 days, and the chamber system was rotated each time. The NH₃ volatilization flux and cumulation of NH₃ volatilization were calculated using Equations (1) and (2) [25]:

$${\mathrm{N}\mathrm{H}}_3 \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{f}\mathrm{l}\mathrm{u}\mathrm{x}\left(\mathrm{k}\mathrm{g} {\mathrm{N}\mathrm{H}}_3 - \mathrm{N} {\mathrm{h}\mathrm{a}}^{-1}{\mathrm{d}}^{-1}\right)=\left(\mathrm{m}/\mathsf{\rho }\times \mathrm{A}\right)\times 0.00001/\mathrm{S}\times {\mathrm{T}}_{\mathrm{i}}\times 0.95$$

(1)

$$\mathrm{C}\mathrm{u}\mathrm{m}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{o}\mathrm{f} {\mathrm{N}\mathrm{H}}_3 \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{k}\mathrm{g} {\mathrm{N}\mathrm{H}}_3 - \mathrm{N} {\mathrm{h}\mathrm{a}}^{-1}\right)=\sum \left(\mathrm{m}/\mathsf{\rho }\times \mathrm{A}\right)\times 0.00001/\mathrm{S}\times {\mathrm{T}}_{\mathrm{i}}\times 0.95$$

(2)

where m is the weight of extracting solution (g), ρ is the density of liquid ammonia extracts (g ml⁻¹), A is the ammonia concentration of ammonia extracts (μg ml⁻¹), S is the trapped area of the chamber (S = π × 0.0752 m²), T_i is trapped time (d) for the ith sampling, and 0.95 means the ammonia recovery rate.

Soil N₂O emission was measured using the static closed chamber–gas chromatography method (Figure S3). Each plot has one closed box, and a sampling port is arranged in the middle of the side of the box. A thermometer is inserted into the top of the box to record the temperature during sampling. After sowing and fertilizing, the lower base is inserted into the measured area, and the box on the base is covered when testing the sample, sealed with water, and removed once complete. The gas collection time is fixed from 8:00 am to 11:00 am, and 20 mL of gas inside the box is extracted with a syringe 0, 15, and 30 min after sealing. The monitoring frequency was once every 2~3 d in the first two weeks after seeding, and then every 7~10 d until crop harvest. The sampling interval was extended during the overwintering period of wheat, and the sampling was again measured after rainfall. There are no crops in the box, so the N₂O measured is the N₂O emission in the soil. The N₂O content of the gas samples was analyzed using agas chromatograph (Agilent Technologies 7890B, Shanghai, China) within two days of sampling. The carrier gas was high-purity nitrogen (99.999%), the flow rate was 21 mL min⁻¹, and the temperature of the detector and column chamber were 300 °C and 60 °C, respectively. The N₂O emission and cumulation of N₂Oemission were calculated using Equations (3) and (4) [35]:

$$\begin{array}{cc}\hfill {\mathrm{N}}_2\mathrm{O} \mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}& \mathrm{f}\mathrm{l}\mathrm{u}\mathrm{x}\left(\mathrm{k}\mathrm{g} {\mathrm{N}}_2\mathrm{O} - \mathrm{N} {\mathrm{h}\mathrm{a}}^{-1}{\mathrm{d}}^{-1}\right)\hfill \\ & =\mathrm{P}/1013\times 28/22.4\times 273/\left(273+\mathrm{T}\right)\times \mathrm{H}\times \mathrm{d}\mathrm{c}/\mathrm{d}\mathrm{t}\times 60\times 24\times 0.00001\hfill \end{array}$$

(3)

$$\begin{array}{cc}\hfill \mathrm{C}\mathrm{u}\mathrm{m}\mathrm{u}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{o}\mathrm{f}& {\mathrm{N}\mathrm{H}}_3 \mathrm{v}\mathrm{o}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{z}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \left(\mathrm{k}\mathrm{g} {\mathrm{N}\mathrm{H}}_3 - \mathrm{N} {\mathrm{h}\mathrm{a}}^{-1}\right)\hfill \\ & =\sum \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{a}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e} {\mathrm{N}}_2\mathrm{O} \mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{f}\mathrm{l}\mathrm{u}\mathrm{x} \mathrm{b}\mathrm{e}\mathrm{t}\mathrm{w}\mathrm{e}\mathrm{e}\mathrm{n} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g} \times \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{d}\mathrm{a}\mathrm{y} \mathrm{g}\mathrm{a}\mathrm{p} \mathrm{b}\mathrm{e}\mathrm{t}\mathrm{w}\mathrm{e}\mathrm{e}\mathrm{n} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}\hfill \end{array}$$

(4)

where P is the atmospheric pressure during gas production (h Pa), T is the average temperature in the chamber during gas production (°C), H is the height of the sampling chamber (m), and dc/dt is the change rate of N₂O concentration (μL L⁻¹ min⁻¹, i.e., the slope of gas content measured in 0, 15, 30 min and corresponding time).

Moreover, soil NH₃ volatilization and N₂O emission tests were quantified during the 2018–2019 season, while the N₂O emission values in the spring of 2020 were missing because of COVID-19.

Soil N leaching was assessed according to the residue mineral N in soil profiles because of the deep soil in the Loess Plateau. However, no leachate was obtained. The residual mineral nitrogen (ammonium nitrogen, NH₄⁺-N; and nitrate nitrogen, NO₃⁻-N) in 0–200 cm profile (2018–2019) or 0–300 cm profile (2019–2020) was determined after wheat harvest for every 20 cm depth. Additionally, we sampled one 0–500 cm soil profile in each treatment and quantified their NO₃⁻-N content in 2020. The mineral nitrogen was extracted and quantified using a flow injection AutoAnalyzer3—AA3 (SEAL Analytical GmbH, Norderstedt, Germany). Moreover, soil samples of 0–500 cm soil profiles were also taken before sowing in 2020 to assess the N movement during the fallow stage.

2.3.3. Soil Water Storage, Evapotranspiration, and Plant N

Soil gravimetric water content (SWC, %) was determined before sowing and after harvest, and three replications were taken from the 0–200 cm soil layer for every 20 cm depth. Soil water storage (SWS, mm) was calculated using Equation (5). Evapotranspiration (ET, mm) was calculated using Equation (6) [38].

$$\mathrm{S}\mathrm{W}\mathrm{S}=\sum \left({\mathrm{S}\mathrm{W}\mathrm{C}}_{\mathrm{i}}\times {\mathrm{H}}_{\mathrm{i}}\times {\mathrm{B}\mathrm{D}}_{\mathrm{i}}\right)$$

(5)

$$\mathrm{E}\mathrm{T}=\mathrm{P}+∆\mathrm{S}\mathrm{W}\mathrm{S}$$

(6)

where the i, H, and BD represent the ith soil layer, soil thickness (H, cm), and the soil bulk density (BD, g cm⁻³), respectively. P is precipitation (mm); ΔSWS is the difference between SWS at the beginning and the end of a wheat growth season (mm).

At harvest, a 1 m² area of crops was randomly selected in each plot. Then, we separated the organs (stem, leaf, spike, and grain) of the crops and dried them in an oven for three days at 70 °C. Afterward, each sample was weighed (kg m²), multiplied by 10,000, in which the weight was M (kg ha⁻¹), and ground with a blade mill for the total N content (C, g kg⁻¹) digest using the Kjeldahl method [39]. Therefore, the nitrogen uptake accumulation (NUA, kg ha⁻¹) was calculated using Equation (7) [38].

$$\mathrm{N}\mathrm{U}\mathrm{A}=\left[{(\mathrm{M}}_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}\times {\mathrm{C}}_{\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}})+{(\mathrm{M}}_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{f}}\times {\mathrm{C}}_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{f}})+{(\mathrm{M}}_{\mathrm{s}\mathrm{p}\mathrm{i}\mathrm{k}\mathrm{e}}\times {\mathrm{C}}_{\mathrm{s}\mathrm{p}\mathrm{i}\mathrm{k}\mathrm{e}})+{(\mathrm{M}}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}}\times {\mathrm{C}}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}})\right]\times 0.001$$

(7)

2.3.4. Grain Yield

At harvest, a 1 m² area of crops was selected in the middle of each plot; this process was repeated three times. Then, the ears were air-dried and threshed to determine grain moisture content and grain yield (14% water content was standardized) [40].

2.4. Calculations and Statistical Analysis

The water use efficiency (WUE, kg ha⁻¹ mm⁻¹), precipitation storage efficiency (PSE, %), apparent nitrogen recovery (ANR, %), and nitrogen harvest index (NHI, %) [24,25] were calculated as follows:

$$\mathrm{W}\mathrm{U}\mathrm{E}=\mathrm{G}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}/\mathrm{E}\mathrm{T}$$

(8)

$$\mathrm{P}\mathrm{S}\mathrm{E}=\left(\mathrm{S}\mathrm{W}\mathrm{P}-\mathrm{S}\mathrm{W}\mathrm{H}\right)/{\mathrm{P}}_{\mathrm{f}}$$

(9)

where SWP, SWH, and P_f represent the soil water at wheat sowing, the soil water at the wheat harvest of the previous year, and total precipitation during the fallow period, separately.

$$\mathrm{A}\mathrm{N}\mathrm{R}=\left({\mathrm{N}\mathrm{U}\mathrm{A}}_{\mathrm{f}}-{\mathrm{N}\mathrm{U}\mathrm{A}}_{\mathrm{u}\mathrm{f}}\right)/\mathrm{N} \mathrm{a}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}$$

(10)

where NUA_f and NUA_uf represent the N uptake accumulation in fertilized plot and N uptake in unfertilized plot, respectively.

$$\mathrm{N}\mathrm{H}\mathrm{I}=\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{N} \mathrm{o}\mathrm{f} \mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}/\mathrm{N}\mathrm{U}\mathrm{A}$$

(11)

The data, such as LAI, Pn, Tr, total Chl content, NR, GS, grain yield, ET, WUE, SWS, PSE, NUA, ANR, NHI, cumulative ammonia volatilization, and cumulative nitrous oxide emission were analyzed using the split-plot design analysis of variance (ANOVA). The significant differences among the means for treatments effect on the index were detected using the least significant difference (LSD) test at the 5% level. The Kolmogorov–Smirnov test was used to test for normality, while Levene’s test was used to test for equality. The relationship between photosynthetic characteristics, the utilization of water and nitrogen, reactive nitrogen losses, and grain yields were calculated using Pearson correlation method. All figures were created via Origin 2015 (OriginLab, Northampton, MA, USA), and statistical analysis was conducted using the ANOVA procedure in SPSS Statistics 19 (IBM, Armonk, NY, USA).

3. Results

3.1. Physiological Characteristics

3.1.1. Wheat Leaf Area Index

The tillage, nitrogen, and tillage × nitrogen significantly influenced the LAI in wheat anthesis stage, respectively (p < 0.05) (

Table 1. Effect of tillage and nitrogen on the leaf area index (LAI), flag leaf total chlorophyll (Chl-total), glutamine synthetase (GS) and nitrate reductase (NR) activity in anthesis.

Year	Treatment	N Rate	LAI	Chl-Total (mg g−1 FW)	GS (A540 mg−1 Protein h−1)	NR (μg NO2− g−1 FW h−1)
2018–2019	CT	N0	2.19 ± 0.36 Bb	3.36 ± 0.21 Bb	0.110 ± 0.02 Ab	2.79 ± 0.17 Ab
	CT	N144	3.47 ± 0.14 Aa	3.96 ± 0.08 Aa	0.114 ± 0.02 Ba	3.05 ± 0.03 Aa
	CT	N180	3.61 ± 0.08 Aa	3.82 ± 0.18 Aa	0.115 ± 0.01 Ba	3.03 ± 0.11 Ba
	SR	N0	2.65 ± 0.43 Ab	3.62 ± 0.19 Ab	0.114 ± 0.03 Ac	2.82 ± 0.07 Ab
	SR	N144	3.39 ± 0.44 Aa	3.96 ± 0.28 Aa	0.121 ± 0.02 Ab	3.04 ± 0.06 Aa
	SR	N180	3.64 ± 0.12 Aa	3.83 ± 0.24 Aa	0.133 ± 0.01 Aa	3.65 ± 0.10 Aa
2019–2020	CT	N0	2.41 ± 0.05 Ab	3.41 ± 0.32 Bb	0.129 ± 0.01 Bb	1.79 ± 0.20 Bc
	CT	N144	2.85 ± 0.17 Ba	3.96 ± 0.07 Ba	0.147 ± 0.02 Ba	3.75 ± 0.20 Bb
	CT	N180	2.89 ± 0.19 Ba	3.97 ± 0.25 Ba	0.150 ± 0.01 Ba	5.81 ± 0.14 Ba
	SR	N0	2.05 ± 0.11 Bb	3.82 ± 0.06 Ab	0.161 ± 0.03 Ab	2.77 ± 0.30 Ab
	SR	N144	3.54 ± 0.11 Aa	4.24 ± 0.26 Aa	0.178 ± 0.02 Aa	5.61 ± 0.07 Aa
	SR	N180	3.58 ± 0.06 Aa	4.28 ± 0.23 Aa	0.182 ± 0.01 Aa	6.39 ± 0.34 Aa
ANOVA	Tillage		*	*	*	*
	Nitrogen		**	*	*	*
	Year		n.s.	n.s.	*	*
	Tillage × Nitrogen		*	n.s.	*	*
	Tillage × Year		n.s.	n.s.	n.s.	n.s.
	Year × Nitrogen		n.s.	n.s.	n.s.	n.s.
	Tillage × Nitrogen × Year		n.s.	n.s.	n.s.	n.s.

Note: CT, SR represent conventional cultivation and straw residue incorporated into soil separately. The N0, N144 and N180 were fertilizer at 0 kg ha⁻¹, 144 kg ha⁻¹ and 180 kg ha⁻¹. Values within a column and for the same year and treatment followed by different lowercase letters are significantly different at p < 0.05 level. Different capital letters indicated that there was significant difference between different tillage methods at the same nitrogen application rate (p < 0.05). ** significant at p < 0.01 level; * significant at p < 0.05 level; n.s. means no significant at p < 0.05 level.

). The LAI increased with the increase in nitrogen application. Moreover, LAI in SR was significantly higher than that in CT treatment separately in the 2019–2020 period (p < 0.05).

3.1.2. Total Chlorophyll Content

The SR management and N rate significantly influenced total Chl (p < 0.05) (Table 1). In the 2019–2020 wheat season, the total Chl content in SR treatments was 3.82–4.28 mg g⁻¹ FW, which was significantly higher than that in CT treatments (p < 0.05). The total Chl content of fertilizer treatment was significantly higher than those unfertilized (p < 0.05).

3.1.3. Nitrate Reductase and Glutamine Synthetase Activity

The GS and NR were significantly influenced by tillage, nitrogen, year, and tillage × nitrogen (p < 0.05) (Table 1). In both years, the addition of N could significantly promote GS and NR activities (p < 0.05). Moreover, GS and NR activities in SR treatments were significantly higher than that in CT (p < 0.05).

3.1.4. Photosynthesis and Transpiration

Both Pn and Tr were significantly influenced by the SR management and N rate at the anthesis stage (p < 0.05) (Figure 2 and Figure 3). Pn in SR treatment (13.79–15.98 μmol CO₂ m⁻² s⁻¹) was significantly higher than that in CT treatment (12.96–14.35 μmol CO₂ m⁻² s⁻¹) (p < 0.05) in 2019. In both years, the Pn of N144 and N180 treated with CT was significantly higher than that of N0, and the Pn of N180 treated with SR was significantly higher than that of N0.

The Tr of SR were significantly higher than those of CT treatment, respectively (p < 0.05), in 2020. Only the Tr of N180 treated with SR (8.30 mmol H₂O m⁻² s⁻¹) was higher than that of N180 treated with CT (9.17 mmol H₂O m⁻² s⁻¹) in 2019 (p < 0.05).

Figure 3 Effect of tillage practices and nitrogen on transpiration rate (Tr) of flag leaves in anthesis. CT and SR represent conventional cultivation and straw residue incorporated into soil separately. The N0, N144 and N180 were fertilizer at 0 kg ha⁻¹, 144 kg ha⁻¹ and 180 kg ha⁻¹. Bars within the same year and treatment with different lowercase letters indicate significant difference at p < 0.05 level. Bars within the same year and nitrogen rate with * indicate significant difference at p < 0.05 level.

3.2. Soil Water Storage, Consumption and Water Use Efficiency

CT and SR affected soil water content soil profiles at winter wheat before planting and harvesting during the 2018–2020 growing seasons (Figure S1). The tillage, N fertilization, year, and tillage × nitrogen significantly influenced the SWS, respectively(p < 0.05) (Table S1). The main difference occurs in the 0–1.2 m soil layer and 0.6–1.4 m, respectively. Regardless of the N rate, the soil water storage of SR was significantly higher than that of CT (p < 0.05). The SWS decreased in CT and SR treatments, while the N rate increasedduring2018–2019. However, it was the opposite in 2019–2020. Furthermore, the ET of SR treatment (247–273 mm) was higher than CT (242–266 mm) in 2019–2020 (

Table 2. Grain yield, evapotranspiration (ET), water use efficiency (WUE), precipitation-storage efficiency (PSE), nitrogen uptake accumulation (NUA), apparent nitrogen recovery (ANR),nitrogen harvest index (NHI) for wheat during the growing seasons of 2018–2019 and 2019–2020.

Year	Treatment	N Rate	Yield (kg ha−1)	ET (mm)	WUE (kg ha−1 mm−1)	PSE (%)	NUA (kg ha−1)	ANR (%)	NHI (%)
2018–2019	CT	N0	4856 ± 451 Ab	266 ± 10 Aa	18 ± 2 Ab	19 ± 0 Ba	101 ± 10 Bb		86 ± 0 Aa
	CT	N144	6860 ± 498 Bab	254 ± 15 Aab	27 ± 2 Aa	16 ± 0 Bb	190 ± 14 Aa	62 ± 1 Aa	85 ± 1 Aa
	CT	N180	7585 ± 323 Ba	242 ± 9 Ab	31 ± 1 Aa	14 ± 1 Bb	175 ± 21 Ba	42 ± 2 Bb	85 ± 0 Aa
	SR	N0	5502 ± 567 Ab	273 ± 14 Aa	20 ± 2 Ab	20 ± 1 Aa	120 ± 16 Ab		87 ± 1 Aa
	SR	N144	7922 ± 211 Aa	263 ± 17 Aab	30 ± 1 Aa	17 ± 0 Ab	191 ± 18 Aa	49 ± 1 Ba	86 ± 0 Aa
	SR	N180	8454 ± 657 Aa	247 ± 16 Ab	34 ± 3 Aa	15 ± 1 Ab	209 ± 14 Aa	48 ± 1 Aa	86 ± 0 Aa
2019–2020	CT	N0	4611 ± 754 Bb	247 ± 11 Ba	18 ± 3 Ab	18 ± 0 Aa	89 ± 17 Bb		89 ± 0 Aa
	CT	N144	7023 ± 732 Aa	247 ± 17 Ba	28 ± 3 Aa	17 ± 1 Bb	215 ± 20 Aa	87 ± 3 Aa	84 ± 1 Ab
	CT	N180	6919 ± 848 Aa	249 ± 15 Ba	28 ± 3 Aa	15 ± 0 Bb	216 ± 16 Aa	71 ± 1 Ab	83 ± 1 Ab
	SR	N0	5485 ± 624 Ab	256 ± 10 Aa	21 ± 2 Ab	18 ± 1 Ab	129 ± 7 Ab		89 ± 0 Aa
	SR	N144	7178 ± 328 Aa	269 ± 12 Aa	27 ± 1 Aa	20 ± 0 Aa	221 ± 12 Aa	64 ± 1 Ba	84 ± 1 Ab
	SR	N180	7770 ± 870 Aa	264 ± 13 Aa	29 ± 3 Aa	17 ± 0 Ab	228 ± 10 Aa	55 ± 2 Bb	83 ± 2 Ab
ANOVA	Tillage		*	*	n.s.	*	*	*	n.s.
	Nitrogen		**	*	*	**	*	*	*
	Year		*	n.s.	n.s.	*	*	*	n.s.
	Tillage × Nitrogen		*	n.s.	n.s.	n.s.	*	n.s.	n.s.
	Tillage × Year		n.s.	n.s.	n.s.	*	n.s.	n.s.	n.s.
	Year × Nitrogen		n.s.	n.s.	n.s.	*	n.s.	n.s.	n.s.
	Tillage × Nitrogen × Year		n.s.	n.s.	n.s.	n.s.	n.s.	n.s.	n.s.

Note: CT and SR represent conventional cultivation and straw residue incorporated into soil separately. The N0, N144 and N180 were fertilizer at 0 kg ha⁻¹, 144 kg ha⁻¹ and 180 kg ha⁻¹. Values within a column and for the same year and treatment followed by different lowercase letters are significantly different at p < 0.05 level. Different capital letters indicated that there was significant difference between different tillage methods at the same nitrogen application rate (p < 0.05). ** significant at p < 0.01 level; * significant at p < 0.05 level; n.s. means no significant at p < 0.05 level.

). The ET declined with the N fertilizer rate increased in 2018–2019.

The N input significantly influenced the WUE; the tillage, nitrogen, year, tillage × year, and year × nitrogen significantly influenced the PSE, respectively (p < 0.05) (Table 2). N application could significantly increase the WUE and decrease the PSE. SR (15–20%) increased the PSE, which was significantly higher than CT (14–19%) with the same nitrogen rate in both years (p < 0.05).

3.3. Wheat Yield, Plant N Uptake and N Use Efficiency

The tillage, nitrogen, year, and tillage × nitrogen significantly influenced the yield separately (Table 2) (p < 0.05). Fertilization and SR would significantly increase the grain yield, respectively, especially in the 2018–2019 wheat season (p < 0.05). There was no yield difference between the two N rates regardless of CT or SR.

The tillage, nitrogen, year, and tillage × nitrogen significantly influenced the NUA, respectively (p < 0.05) (Table 2). N fertilization enhanced the NUA regardless of tillage practices. The NUA of SR was higher than that of CT, especially in 2018–2019 with the N180 nitrogen rate.

The tillage, nitrogen, and year significantly influenced the ANR (p < 0.05) (Table 2). The ANR of N144 treatments were higher than that of N180 significantly (p < 0.05). In 2019–2020, the ANR of SR (55–64%) was significantly lower than CT (71–87%) (p < 0.05). The N fertilization significantly influenced the NHI, and NHI declined as N input increased.

3.4. Soil Gaseous N (NH₃ and N₂O) Emissions

3.4.1. Ammonia Volatilization

NH₃ volatilization peaked at about 2 weeks post-sowing and then declined and stabilized at a low value (Figure 4).The ammonia flux increased as the N input increased, especially at the NH₃ volatilization peak. Moreover, the cumulative NH₃ volatilization was 7.8–16.5 and 8.4–16.6 kg NH₃-N ha⁻¹ and 3.9–11.2 and 3.9–9.1 kg NH₃-N ha⁻¹ of CT and SR in 2018–2019 and 2019–2020, respectively.

3.4.2. Nitrous Oxide Emission

N₂O emission mainly peaked at about 3 days post-sowing and then declined and stabilized at a low value (Figure 5). In 2018–2019, the average N₂O emission flux of two nitrogen application rates was 9.07 × 10⁻⁴–10.94 × 10⁻⁴ kg N₂O-N ha⁻¹d⁻¹, while it was 4.68 × 10⁻⁴–5.04 × 10⁻⁴ kg N₂O-N ha⁻¹d⁻¹ in the unfertilized treatment. The average N₂O emission flux values in 2019–2020 were higher than in 2018–2019, which were 25.61 × 10⁻⁴–43.46 × 10⁻⁴ kg N₂O-N ha⁻¹d⁻¹ (fertilized) and 4.61 × 10⁻⁴–6.01 × 10⁻⁴ kg N₂O-N ha⁻¹d⁻¹ (unfertilized). Under the same tillage practice, the N₂O emission flux of soil declined as follows: N180 > N144 > N0. Under the same nitrogen application rate, the average emission flux of SR was higher than that of CT. In both years, the cumulative N₂O emission in fertilized treatment (0.2–0.34 kg N₂O-N ha⁻¹) was significantly higher than in unfertilized treatment (0.09–0.14 kg N₂O-N ha⁻¹) (p < 0.05).

3.5. Soil Residual Mineral N

In 2018–2019, the residual mineral nitrogen in fertilized treatment had two peaks in 0–2 m soil layers: one occurred at 0.2–0.4 m, and the other occurred at 1.2–1.4 m (Figure 6B,C). Moreover, the residual nitrogen of SR fertilized treatment was higher than CT in 0–2 m soil layers, and the accumulation of residual nitrogen in SR-N144 (258.23 kg ha⁻¹) was significantly higher than in CT-N144 (203.68 kg ha⁻¹) (p < 0.05). In 2019–2020, the residual mineral nitrogen in the fertilized treatment had three peaks in 0–3 m soil layers, which occurred at 0–0.2 m, 1.2–1.4 m, and 2.0–2.8 m, respectively (Figure 6E,F). Furthermore, the CT-N180 treatment was significantly higher than SR-N180 in 0.6–1.4 m and 1.6–2.4 m soil layers (p < 0.05), while the residual nitrogen of SR-N144 was significantly higher than CT-N144 in 1.8–3.0 m soil layers (p < 0.05). It was worth noting that the accumulation of residual nitrogen in SR-N144 (307.27 kg ha⁻¹) was significantly higher than CT-N144 (276.92 kg ha⁻¹) (p< 0.05), and the residual mineral nitrogen of SR-N144 mostly occurred in the 2–3 m soil layer. However, the residual nitrogen accumulation of the SR-N180 treatment (265.59 kg ha⁻¹) was significantly lower than that of CT-N180 (367.43 kg ha⁻¹) (p < 0.05). Moreover, the accumulation of residual nitrogen in SR-N0 was higher than in CT-N0 (Figure 6D), significantly in the 0–0.2 m soil layer (p < 0.05). Moreover, the nitrogen residual of CT-N180 and SR-N180 at the 2–5 m soil layers accounted for 93.16% and 93.07%of the 0–5 m soil layers, respectively, before cultivation in 2020 (Figure S2). There was a high peak at 2.8–3.0 m in SR-N180, and three peaks at 2.8–3.0 m, 3.4–3.6 m, and 3.8–4.0 m in CT-N180 profiles, respectively.

4. Discussion

Nitrogen and straw management are well known to affect water and nutrient use efficiency, crop yield, and nutrient loss [4,23]. In our study, we focused on how nitrogen and straw retention affect the N uptake by plants, gaseous N emission, nitrate movement, precipitation-storage and use efficiency, soil evaporation and transpiration, N and water use efficiency, photosynthesis, and crop yield. The results indicated that the N rate and straw significantly affected wheat physiological characteristics, nitrate leaching, nitrogen use efficiency, water use efficiency, and crop yield.

4.1. Photosynthesis, N Assimilation, WUE, ANR and Yield

Being an essential nutrient, N plays a vital role in photosynthetic activity, nitrogen assimilation enzyme, and crop growth [16,17]. Therefore, N application improved all growth and physiological parameters of wheat, such as Pn, Tr, total Chl content, NR and GS activities, and LAI in this study (Table 1), which might be attributed to its involvement in delaying leaf senescence and boosting leaf nitrogen assimilation and leaf area [14]. Furthermore, the higher leaf area index and Pn provided a greater carbohydrate supply to roots and improved their growth and uptake capability. Hence, SR increased the N storage capacity within leaves [41], resulting in proportionality between the LAI and N intake for most species [42].

The straw return treatment promoted the grain yield and the fraction of the root system in the subsurface layers and might be the result of better LAI along with higher chlorophyll contents, leading to more accrual of photosynthates [11,14]. Furthermore, Zhai et al. [43] pointed out that deep straw rotary (25–40 cm) tillage improved the nitrogen assimilation and enzyme activities associated with starch synthesis and photosynthetic capacity. Soil with straw retention improved Pn wheat, reduced Tr, and increased leaf WUE [44]. However, it increased both Pn and Tr in the present study (Figure 3). This may be because straw returning to the field under different soil types and climate types has different effects on crop growth and development and further affects leaf area and photosynthetic performance.

Grain yield is a key indicator of crop productivity and is influenced by climatic conditions, soil environment, and practice [16,45,46,47]. Wang et al. [48] found that straw retention combined with a 150–225 kg ha⁻¹nitrogen fertilizer could ensure a certain amount of grain yield (6480–6660 kg ha⁻¹), which agreed with the present results (Table 2). Therefore, the resource characteristics of nutrient return measures, such as straw returning used in grain crop production in China, can achieve fertilizer reduction in wheat production. However, if the supply of nitrogen fertilizers does not increase in a timely and effective manner, the yield will be negatively affected.

Both straw return and nitrogen application significantly reduced the nitrogen harvest index (Table 2), which could be lower N translocation from functional leaves owing to the delayed senescence during the late growth stage. This is in good agreement with the findings of previous studies [24,49]. In this study, the ANR of the straw return treatment was lower than that of the conventional cultivation treatment (Table 2), probably because straw incorporation changed the soil mineralization–immobilization turnover [50], which resulted in lower plant uptake of nitrogen fertilizers.

Furthermore, ET and PSE with straw incorporation were significantly higher compared with CK (Table 2), which agreed with the results of Wang et al. [48]. The WUE of SR treatment was higher than CT. Like straw retention, a high N-fertilization rate decreased soil water storage and increased ET, which is in line with the findings of Fang et al. [24]. The reason for this is that under the straw return condition, water conditions are improved, which increases the effectiveness of soil nutrients and facilitates the growth and development of the root system of winter wheat and the uptake of soil nutrients and water by the plants. The residual mineral nitrogen in the 0–100 cm soil layer is unstable from year to year under long-term nitrogen application.

4.2. Soil Gaseous N Emission and Nitrate Leaching Potential

Previous research found that tillage management and urea administration had indefinite effects on ammonia volatilization [51,52]. Sun et al. [33] found that straw additions might boost soil microorganism activity and urea hydrolysis, yielding higher soil NH₄⁺-N concentrations. Tian et al. [53] discovered that straw interacted with nitrogen to minimize ammonia volatilization throughout the wheat season of the wheat–rice system. As a result, straw residual retention’s effect on ammonia volatilization was related to the straw C/N ratio and soil type.

The N fertilizer input is the most direct factor influencing soil N₂O emissions [35]. Xu et al. [34] showed that the increased soil N₂O emissions of straw retention is mainly due to the carbon and nitrogen content generated via the decomposition process of straw, which provides a sufficient substrate for the nitrification and denitrification of microorganisms. Nevertheless, the high amount of straw with a high carbon-to-nitrogen ratio returning to the field caused the soil mineral nitrogen to be immobilized by microorganisms and reduced the emission of soil N₂O [54]. The mineral N content of the soil was significantly reduced under straw return conditions, thus reducing the nitrification–denitrification substrate. Furthermore, Peng et al. [35] pointed out that under the N fertilization condition, soil water, nitrification, and denitrification contributed to the N₂O emissions in the rain-fed winter wheat fields.

N residuals in soil are one of the main effects of nitrogen fertilizers. The residual N of SR-N144 was significantly higher than CT-N144 (Figure 6), which was similar to the results of Liang et al. [55], Xia et al. [56], and Sun et al. [48]. Furthermore, nitrate is mobile in the soil, which makes it easy to transport and leach in the soil profiles [26]. It can be seen that the residual content of mineral nitrogen in the 0–200 cm soil layer of winter wheat in dry land is relatively large after harvesting, and the residual content of mineral nitrogen in the 200–300 cm soil layer also tends to increase, and the mineral nitrogen may leach to the soil layer below 200 cm after fertilization.

The inter-annual instability of mineral N residues in the soil profiles straw retention with 180 kg ha⁻¹ application was shown in Figure 1 and Figure 6. It could be interpreted that the vertical soil distribution of mineral nitrogen was mainly influenced by rainfall and its distribution [57]. Alternatively, it could be explained by the trade-off effect in different carbon and nitrogen ratios under different fertilizer applications and straw residual management [30,31,49]. In addition, the local winter wheat harvest is followed by a summer fallow of about four months, which resulted in the continued leaching of higher mineral N residues to deeper soil layers causing losses (Figure S2). It supported the results of residual nitrate–nitrogen peaks in the 7.5–8.5 m loess profile [58].

4.3. Reduced Nitrogen Application with Straw Retention

In this study, a reduced 20% nitrogen application of conventional nitrogen rate (180 kg ha⁻¹) [59] did not affect physiological characteristics, such as the total Chl content, NR and GS activities, and LAI (Table 1). In addition, there was no significant grain yield. Moreover, reduced nitrogen application significantly reduced reactive nitrogen emission and nitrate leaching potential. Furthermore, Xu et al. [12] showed that a proper proportion of straw returned to the field, along with a modest nitrogen shortage, might be a more eco-friendly and sustainable agricultural approach in field production. A possible mechanism by which SR can influence grain yield is shown in Figure 7. First, the straw retention improved yield via LAI, N assimilation, and the PSE directly. Then, it affected the yield through the Pn, Tr, and WUE indirectly. NR activity would stimulate the GS activity and increase the N assimilation and total Chl content. Therefore, LAI, NR, and Chl total had a direct positive influence on NUA. Meanwhile, Tr and LAI would positively affect WUE, which had an indirect positive impact on NUA and grain yield. Furthermore, WUE and NUA increased the grain yield, while ANR and PSE had significantly opposite influences on it. It was worth mentioning that plant growth needed to absorb the subsoil water content, which indicated that the high grain yield came at the expense of soil water, i.e., it caused PSE to decrease with the yield.

5. Conclusions

Compared with CT, straw residual retention improved wheat leaf physiological characteristics (Pn, LAI, Chl-total) increased plant N uptake and N assimilation enzymes (GS and NR), and soil precipitation-storage efficiency, and ultimately get higher grain yield. Furthermore, it did not acerbate soil gaseous N emissions. Wheat Pn and Tr, LAI, Chl-total, GS, NR, grain yield, WUE, and NUA, soil NH₃ volatilization, N₂O emission, mineral nitrogen residual, all increased with N rate increased. Nevertheless, ET, PSE, ANR, and NHI decreased with the N rate increased. In addition, reduce 20% nitrogen application of conventional nitrogen rate (180 kg ha⁻¹) could maintain yield stability, increase ANR and reduce environmental pollution risk in rain-fed winter wheat-fallow cropping system northwest China.