Optimizing nitrogen fertilizer and straw management promote root extension and nitrogen uptake to improve grain yield and nitrogen use efficiency of winter wheat (Triticum aestivum L.)

ABSTRACT

Straw returning is an efficient straw usage strategy in rice-wheat rotation, but nitrogen (N) use efficiency (NUE) was decreased due to incorrect straw and N fertilizer managements. To investigate the effects of straw and N fertilizer management on root growth, N fertilizer fates, grain yield and NUE of wheat, a two-year field and micro-plot ¹⁵N-labelled experiment under three levels of N application rate (0, 180 and 240 kg N ha⁻¹) with two basal N application stages [seeding (BN), and 3-leaf stage (TN)] and three straw treatments [no straw return (NS), straw return by rotary tillage (SR) and straw return by ploughing (SP)] was conducted. The results indicated that SP increased grain yield and NUE, and the increase was highest under TN180. SP increased N uptake by enhancing root extension and soil N supply capacity, and TN decreased ¹⁵N residual in 60–100 cm soil layer. SP and TN180 both decreased ¹⁵N fertilizer loss and increased ¹⁵N recovery. Reducing basal N and applied at third-leaf stage (TN180) under SP had the same grain yield level as conventional N management (BN240) under NS, while highly improved NUE due to more root extension in deep soil layer and less N fertilizer loss.

KEYWORDS:

• Wheat
• straw return
• nitrogen
• grain yield
• nitrogen use efficiency

Introduction

In recent years, there has been a growing recognition of the comprehensive utilization of straw, commonly referred to as the ‘Five Utilizations’, which include fertilizer utilization, feed utilization, fuel utilization, material utilization and raw material utilization. The adoption of these strategies promotes sustainable and efficient resource utilization in agriculture (Wang et al. 2018; Yin et al. 2018). The practice of returning crop straw to the field as organic fertilizer is a crucial and highly relevant method of straw utilization. It provides essential nutrients for plants, enhances soil fertility, improves soil structure and reduces reliance on chemical fertilizers (Li et al. 2018). In the Yangtze River Basin under the rice-wheat rotation system, due to the long growth period of rice, returning rice straw to fields after rice harvest is considered a common strategy to save time and economic costs before wheat sowing. Although a few results showed that the grain yield of wheat increased after rice straw return, more results indicated that the grain yield of wheat was not affected or significantly declined under different straw return methods (Tian et al. 2012). In fact, the amount of nitrogen (N) fertilizer application is often increased in actual production after straw returned to alleviate the reduction in yield under rice-wheat rotation (Zhang et al. 2016; Wang et al. 2018). However, it may result in a huge loss of N fertilizer, lower N fertilizer utilization and environmental pollution. Therefore, it is urgent to explore the appropriate straw return method and N fertilizer management to improve crop yield and N use efficiency (NUE) under straw return in rice-wheat rotation.

Different straw return methods will affect crop yield and NUE by changing soil property as well as the root extension of crops. However, recent studies have shown that straw returned by rotary is most widely applied among straw returning methods. However, there is little time for straw biodegradation, therefore the straw was mainly distributed in the surface soil layer, which is detrimental to root extension in soil. Meanwhile, because most of the crop straws have high C/N (65–85:1), microbes for straw breakdown compete with crops for nutrients lending to the low N accumulation in the upper soil layer at seeding stage (Zhao et al. 2017). Yang’s study showed that ditch-buried straw return could significantly increase the retention of the N fertilizer in straw soil layer in a rice-wheat rotation system, thus increasing crop uptake and minimizing leaching N loss (Yang et al. 2015). Similar to the ditch-buried straw return, straw returning by ploughing tillage (SP) could return straw in deeper depth compared to straw returned by rotary (SR), which may also reduce N loss by leaching. What is more, it could improve the quality of soil more evenly. Compared to SR, SP not only improves the soil properties at the topsoil layer but also promotes the root growth in these layers without negative effects at the seeding stage of wheat (Alcántara et al. 2016, 2017). Due to the complex soil environment under the rice-wheat rotation in the Yangtze River Basin, few investigations focused on the different effects of SP and SR on N uptake and grain yield, so that it is difficult to reduce N fertilizer application to improve NUE.

N fertilizer plays a crucial role in promoting wheat growth and increasing grain yield (Jing et al. 2009). However, the N application for wheat grown is more than 240 kg ha⁻¹ in the Yangtze River Basin, and NUE was decreased with the increasing of the N fertilizer application rate (Tian et al. 2016), especially the increase of basal N fertilizer after rice straw returned, which may greatly decrease NUE (Tian et al. 2018). Reducing N fertilizer application and maintaining high crop yield is a difficult problem all over the world, and many studies have found that directly decreasing N application always reduces grain yield under straw return due to the worse soil N supply and shallower root system expansion (DUNBABIN et al. 2003). Because wheat accumulates only 6% of N in the seedling stage, 72% in the vegetable period and 22% in the reproductive growth period (Chen et al. 2014), it is beneficial to improve soil N supply after jointing and anthesis stages. Previous studies found that only a moderate reduction in basal N fertilizer application and delaying application to the seedling stage of wheat did not significantly reduce yield, but significantly increased N uptake and reduced the N loss in wheat (Tian et al. 2020). In the Yangtze River Basin, soil in fields is always in alternating wet and dry conditions during the wheat growing season (Jing et al. 2009), if optimizing N fertilizer management under straw return could increase NUE and wheat yield is not clear.

The root system plays a vital role in nutrient and moisture absorption in wheat. The distribution of the root system in the soil and its morphological characteristics have a significant correlation with N uptake and utilization (Wang et al. 2014; Liu et al. 2020). Different straw return methods have a great influence on wheat root expansion (Chen et al. 2020). Sui et al. (2020) discovered that straw returned by deep ploughing significantly decreases the soil density in soil layers and increases N uptake in wheat by promoting root expansion in soil layers. However, some studies have shown that SR reduced the root weight density of wheat in the tillage layer and decreased the N uptake of wheat (Smith et al. 2004). The root system of wheat seedlings was mainly distributed in the 0–40 cm soil layer, and a large amount of basal fertilizer N was lost through leaching to the deeper soil layers. The percentage of N loss was significantly negatively correlated with the amount of applied N fertilizer (Palta et al. 2007). Studies have shown that increasing root growth and root weight density of wheat can reduce the loss of N fertilizer (Atkinson et al. 2020). By appropriately reducing the N application, the root weight density of the root system was significantly increased, leading to improved N fertilizer utilization efficiency in wheat (Xu et al. 2018). However, most studies on the effects of straw return methods and N fertilizer management on wheat root expansion and N uptake have primarily focused on wheat-maize rotations. The effects of different straw return methods and N fertilizer management on wheat root growth and N uptake under the rice-wheat rotation in the Yangtze River Basin remain unclear.

The purpose of the current study was to determine the root extension and N fertilizer distribution in soil under different straw return and N fertilizer managements to increase NUE under straw return in rice-wheat rotation. Previous studies indicated that SP increased grain yield of wheat and NUE compared to SR (Chen et al. 2017). In our previous study, optimizing basal N fertilizer application did not decrease the grain yield but significantly increased NUE (Tian et al. 2018). Therefore, it was hypothesized that (1) optimizing N fertilizer management under SP would be more effective than under SR by increasing root extension and soil N supply; (2) SP accompanies with optimizing N fertilizer reduction would increase NUE and increase grain yield.

Materials and methods

Experimental site

Field experiments were conducted at the experimental station of Nanjing Agricultural University, Nanjing, China (32^◦32’N and 118^◦82’E), from 2020 to 2022. The weather data were recorded by an automatic weather station (CM10, Campbell Scientific, U.S.A.) at the experimental site. The climate is humid and warm, total rainfall during the wheat season was 419 mm in 2020–2021 and 311 mm in 2021–2022 (Figure 1).

Figure 1. Monthly mean rainfall, mean maximum and minimum temperatures during the wheat-growing season from 2020 to 2022 in Nanjing, China.

Field experimental design

Experiments were performed in two adjacent fields for two wheat seasons at the same site in 2020–2021 and 2021–2022. The previous crop was rice with the same N fertilization in both adjacent fields. The soil type is paddy soil which can retain water and is suitable for rice cultivation. The soil texture of the test field was loam, and other properties of soil (0 − 100 cm) before sowing are shown in Table 1.

Table 1. The soil properties at the experimental site at the beginning of the experiment in two wheat growth seasons.

Soil layer	Organic matter	Total N	Nitrite N	Olsen-P	Exchangeable K	pH	Bulk density
cm	g kg^−1	g kg^−1	mg kg^−1	mg kg^−1	mg kg^−1		g cm^−3
2020–2021
0–10	22.81	1.19	13.08	13.13	88.37	6.62	1.25
10–20	21.37	1.13	12.65	12.33	81.13	6.73	1.28
20–40	20.11	1.03	10.84	11.06	72.88	6.88	1.37
40–60	15.62	0.87	7.78	10.25	66.71	6.95	1.44
60–100	13.65	0.72	6.24	10.12	61.88	7.05	1.35
2021–2022
0–10	23.56	1.24	13.85	13.71	95.13	6.74	1.22
10–20	22.16	1.15	12.74	12.56	84.62	6.77	1.25
20–40	21.08	0.96	10.65	11.24	75.61	6.93	1.33
40–60	14.77	0.85	7.73	10.18	59.54	6.89	1.41
60–100	13.18	0.68	6.28	10.14	57.49	7.12	1.29

Field plot experiment

The field plot experiment was arranged in a split-plot design with three replicates, includes three straw management methods: (1) no straw return (NS), (2) straw return by rotary tillage (SR) and (3) straw return by ploughing (SP), with four N fertilizer managements: (1) conventional N management (BN240), (2) basal N applied at 3-leaf stage (TN240), (3) reduce half of basal N (BN180), (4) reduce half of basal N and applied at 3-leaf stage (TN180). The main plots were straw return methods and subplots were N managements. The details of N fertilizer application time and application amount are shown in Table 2. In the two wheat growth seasons, 150 kg P₂O₅ ha⁻¹ and 150 kg K₂O ha⁻¹ were supplied in all plots before sowing. According to local tradition, the amount of straw returned to the field is 9000 kg ha⁻¹. The rice straw was cut into 2–3 cm length, then returned to the field by ploughing (SP), and rotary tillage (SR) at depths of 0–10 cm and 10–20 cm, respectively. Rice straw was removed, with the soil rototilling in the no straw return (NS) plots. The plot area was 9 m⁻² (3 m × 3 m), at a seeding rate of 225 plants m⁻², with a 0.25 m row spacing. The showing dates were 4 November in 2020 and 30 October in 2021. Weeds were controlled by hand, and fungicides and pesticides were used at the jointing, booting and grain-filling stages to prevent disease and pests. Because of the abundant rainfall during the study years, no irrigation was applied during the growing seasons.

Table 2. Nitrogen application rates and basal nitrogen application stage under different nitrogen fertilizer managements.

			N application stage and rate (kg N ha^−1)
		N rate	Basal nitrogen		Top-dressed nitrogen
Nitrogen	Straw management	(kg N ha^−1)	Seeding	3-leaf	Jointing	Booting
BN240	NS	240	120	–	72	48
	SR	240	120	–	72	48
	SP	240	120	–	72	48
TN240	NS	240	–	120	72	48
	SR	240	–	120	72	48
	SP	240	–	120	72	48
BN180	NS	180	60	–	72	48
	SR	180	60	–	72	48
	SP	180	60	–	72	48
TN180	NS	180	–	60	72	48
	SR	180	–	60	72	48
	SP	180	–	60	72	48
N0	NS	0	–	–	–	–
	SR	0	–	–	–	–
	SP	0	–	–	–	–

¹⁵N micro-plot experiment

The micro-plots were set by polyvinyl chloride (PVC) tubes with 25 cm in diameter and 105 cm high to monitor the fates of ¹⁵N-labeled fertilizer. This method was also used in our previous studies (Shi et al. 2012; Hu et al. 2018; Tian et al. 2020). The ¹⁵N micro-plot experiment designed same as the field plot experiment. To keep the micro plots with the similar soil conditions to field plots, soil was dug out and separated into four layers: 0–20, 20–40, 40–60 and 60–100 cm, 20–100 cm soil layer was backfilled into the PVC tube in the correct order, followed by water to consolidate the layers, then the PVC tubes were buried into plot with the top edge at 5 cm above the ground. The rice straw was mixed in 0–10 cm and 10–20 cm soil layer to emulate the straw return methods as SR and SP. N fertilizer managements were identical to that of the field plots. All ¹⁵N micro-plots were supplied¹⁵N-labeled fertilizer with all treatments. N fertilizers were used with¹⁵N-enriched (10.16 at% excess ¹⁵N) ammonium sulfate (Shanghai Chemical Industry Institute) and normal ammonium sulfate. 0–20 cm soil was backfilled into the PVC tube after mixing with basal fertilizer. Top-dressed N fertilizers were applied equally by dissolving in 100 ml water at the beginning of jointing and booting stage. Every micro-plot planted 11 seedlings. To reduce edge effects, around the micro-plots, wheat seedlings were planted. The crop and nutrient management in micro-plots were referred to the field plots.

Plant and soil sampling collection

The samples for treatments of straw return methods and N management were taken in the macro-plots and micro-plots at jointing (133 DAS), anthesis (171 DAS) and maturity (204 DAS). The wheat in each macro-plot was harvested to determine the grain yield at maturity. The wheat was collected at jointing, anthesis and maturity to determine the N uptake. The soil samples in macro-plots were collected in 0–20 cm soil layer at tillering (3-leaf), jointing and anthesis to determine soil inorganic N, soil microbial biomass carbon (C) and soil microbial biomass N.

Soil sampling and analysis

Fresh soil samples were thoroughly mixed and representative sub-samples were extracted immediately using 2 M potassium chloride (KCL) solution (soil solution ratio: 1:5) by shaking for 1 h on a rotary shaker (180 rev min⁻¹), followed by filtration. The extracts were directly analyzed for the content of NO₃⁻-N and NH₄⁺-N using an automated continuous flow analyzer (Seal Autoanalyzer 3, Seal Analytical, Inc., Germany). The sum of NH₄⁺-N and NO₃⁻-N was considered as soil inorganic N.

Soil microbial biomass C and N were determined referring to Vance’s and Joergensen’s methods (Vance et al. 1987; Joergensen 1996). Microbial biomass C: the soil was extracted with K₂SO₄ after chloroform fumigation and non-fumigation, respectively, and the organic C content in the extract was determined by potassium dichromate oxidation method. Microbial biomass C content in soil samples = (C content of fumigated soil samples – C content of non-fumigated soil samples) × 0.45. Microbial biomass N: after the above extracts were completely dissolved by the decoction method, the N content in the liquid was determined by the semi-micro Kjeldahl method. Microbial biomass N content in soil samples = (N content of fumigated soil samples – N content of unfumigated soil samples) × 0.45.

Plant sampling and ¹⁵N analysis

The root, wheat and soil samples were taken in the micro-plots to determine the root extension and the fates of ¹⁵N fertilizer. Three replications were sampled at jointing, anthesis and maturity. Roots of each micro-plot were collected and washed in five soil layers: 0–10, 10–20, 20–40, 40–60 and 60–100 cm. Then, the root and plant shoots were dried at 70°C to constant weight for determination of dry weight. Soil samples were taken at five layers: 0–10, 10–20, 20–40, 40–60 and 60–100 cm. Each soil sample was separated into two parts. One part was oven-dried at 105°C for determination of water content. Another part was dried under natural conditions for determination of ¹⁵N enrichment. Plants and soil were taken outside the micro-plot (more than 1 m away) for determination of the natural ¹⁵N enrichment. The dry samples of soil and plants were finely ground to 100 µm and analyzed for total N and ¹⁵N enrichment by an automated continuous flow Isotope Cube (Elementar, Germany) coupled with a continuous flow mass spectrometer (Isoprime, United Kingdom) using Dumas flash combustion.

Calculation methods

The plant N accumulation and plant N accumulation rates were calculated according to (Wang et al. 2015):

Plant N accumulation (kg ha⁻¹) = NC × DM

Where, NC is N concentration (mg N g⁻¹ DM), and DM is dry matter (kg ha⁻¹).

Plant N accumulation rate (kg ha⁻¹ d⁻¹) = $\frac{\mathrm{plant}\mathrm{N}\mathrm{accumulation}}{\mathrm{growth}\mathrm{duration}}$

$\mathit{NRE}\left(\mathrm{\%}\right)=\frac{{\mathit{U}}_{\mathit{WN}}-U_{\mathit{WO}}}{A_{\mathit{WN}}}\times 100$

Where, U_WN (kg N ha⁻¹) is total shoot N uptake in N treatments, U_W0 (kg N ha⁻¹) is total shoot N uptake in no N treatment and A_WN (kg N ha⁻¹) is the amount of fertilizer applied.

Root weight density (g m⁻³) = $\frac{\mathrm{root}\mathrm{weight}\mathrm{in}\mathrm{soil}\mathrm{layer}}{\mathrm{volume}\mathrm{of}\mathrm{soil}\mathrm{layer}}$

The percentage of N derived from fertilizer were calculated by the following equation (Malhi et al. 2004).

Ndff (%) = $\frac{\mathrm{c}-\mathrm{b}}{\mathrm{a}-\mathrm{b}}$

where a is the atom% ¹⁵N in the labeled fertilizer, b is the atom% ¹⁵N in the plant or soil receiving no ¹⁵N and c is the atom% ¹⁵N in the plant or soil receiving¹⁵N.

Plant N uptake and the fates of the N fertilizer were calculated by the following equations:

(1) $\mathit{Plant}\mathit{total}\mathrm{N}\mathit{uptake}\left(\mathrm{g}{\mathrm{m}}^{-2}\right)=\frac{\mathrm{plant}\mathrm{dry}\mathrm{matter}\times \mathrm{plant}\mathrm{N}\mathrm{concentration}}{100}$(1)

(2) $\mathit{Plant}\mathrm{N}\mathit{uptake}\mathit{from}\mathit{fertilizer}\left(\mathrm{g}{\mathrm{m}}^{-2}\right)=\left(1\right)\times \mathit{Ndf}{f}_{\mathit{plant}}$(2)

Plant N uptake from soil (g m⁻²) = (1) − (2)

(3) $\mathit{Fertilizer}\mathrm{N}\mathit{residual}\left(\mathrm{g}{\mathrm{m}}^{-2}\right)=\mathit{soil}\mathit{thickness}\left(\mathit{cm}\right)\times \mathit{soil}\mathit{bulk}\mathit{density}\left(\mathrm{g}\mathit{c}{m}^{-3}\right)\times \mathit{soil}\mathrm{N}\mathit{concentration}\times 10\times \mathit{Ndf}{f}_{\mathit{soil}}$(3)

(4) $\mathit{Fertilizer}\mathrm{N}\mathit{loss}\left(\mathrm{g}{\mathrm{m}}^{-2}\right)=\mathrm{N}\mathit{fertilizer}\mathit{application}\mathit{amount}-\left(2\right)-\left(3\right)$(4)

Fertilizer N recovery percentage (%) = $\frac{\left(2\right)}{\mathrm{N}\mathrm{fertilizer}\mathrm{application}\mathrm{amount}}$ × 100

N residual percentage (%) = $\frac{\left(3\right)}{\mathrm{N}\mathrm{fertilizer}\mathrm{application}\mathrm{amount}}$ × 100

N loss percentage (%) = $\frac{\left(4\right)}{\mathrm{N}\mathrm{fertilizer}\mathrm{application}\mathrm{amount}}$ × 100

Statistical analysis

The experimental data were analyzed using SPSS version 21.0 (SPSS, Chicago, IL, U.S.A.). The results of root growth, N fertilizer distribution and soil properties were analyzed using two-way ANOVA under three straw return methods and four N fertilizer managements, with straw return method, N management, year and their interaction serving as fixed effects and field serving as a random effect. The mean values of the three replicates for each treatment. The least significant difference (LSD) was used to separate the means and interactions, and the statistical significance was evaluated at p ≤ 0.05. Graphics were drawn by using Origin 2019 software.

Results

Grain yield, N accumulation and N recovery efficiency

The trends of grain yield, N accumulation and N recovery efficiency (NRE) in treatments were approximately the same across two growing seasons (Table 3). SP increased the grain yield of wheat by enhancing grain number and 1000 grains weight, and the increase was most significant under TN180, which increased by 11.17% and 10.61% compared to NS and SR, respectively. Though SR also increased the grain number and 1000 grains weight (except BN180), SR decreased the spikes and had reduced grain yield under BN180. Compared to BN180 and BN240, TN180 and TN240 increased the grain yield, respectively, and the increase in yield of TN180 was higher than TN240. Furthermore, the yield of SP under TN180 had no difference compared to NS under BN180 (conventional straw and N management).

Table 3. Effects of straw and nitrogen fertilizer managements on grain yield, N accumulation and N recovery efficiency (NRE).

		Spikes		1000-grain weight	Grain yield	N accumulation	NRE
Nitrogen	Straw	(×10^4 ha^−1)	Grain number	(g)	(kg ha^−1)	(kg ha^−1)	(%)
2020–2021
BN240	NS	363.33ab	45.64c	42.37c	6346.13c	159.8 cd	37.37e
	SR	356.33b	46.38bc	44.32ab	6601.60bc	169.05bc	39.45d
	SP	369.67ab	47.67b	45.25a	6831.43b	177.76b	40.22d
TN240	NS	376.33a	47.69b	42.47c	6480.17c	163.06c	38.73de
	SR	370.33ab	48.38ab	44.48ab	6904.30ab	172.91b	41.06 cd
	SP	383.33a	49.20a	45.86a	7134.17a	183.69a	42.69c
BN180	NS	329.67d	43.33d	43.18bc	5501.20e	142.26e	40.08d
	SR	302.33e	42.96e	43.58b	5230.10f	147.16de	40.44d
	SP	322.33d	43.62d	45.02ab	5975.43d	158.22 cd	42.77c
TN180	NS	350.67bc	45.09c	43.65b	5810.43d	147.28de	42.87c
	SR	327.67d	46.36bc	44.80ab	5840.20d	156.98d	45.90b
	SP	342.33c	47.22b	46.01a	6459.57c	166.38c	47.30a
2021–2022
BN240	NS	385.33b	43.91b	41.71d	6621.74c	165.85c	37.39e
	SR	374.33bc	44.59ab	43.38c	7063.29b	172.42bc	40.86d
	SP	385.67b	44.82ab	44.26bc	7248.74ab	176.32b	41.28d
TN240	NSR	397.33ab	44.31ab	41.02d	6821.75bc	171.65bc	39.81de
	SR	388.67b	45.47a	44.15bc	7210.58ab	178.84b	43.53c
	SP	411.67a	45.95a	45.61b	7523.67a	188.24a	46.25b
BN180	NS	346.33d	40.73d	41.27d	6035.78de	146.26e	38.97de
	SR	316.67e	40.48d	43.19c	5532.48e	148.66e	41.28d
	SP	348.33d	41.25c	44.72b	6209.73d	156.74d	44.17c
TN180	NS	365.67c	42.18bc	42.67c	6073.41de	154.45d	43.52c
	SR	343.67d	43.57b	45.69b	6229.45d	158.23 cd	46.59b
	SP	369.67c	44.48ab	47.27a	6770.21bc	164.71c	48.59a
Year		*	*	ns	*	*	ns
Nitrogen		**	*	*	**	*	**
Straw		*	*	**	*	*	*
N × S		*	ns	*	*	*	*
Y × N		*	ns	*	ns	ns	ns
Y × S		ns	ns	ns	ns	ns	ns
Y × N × S		ns	ns	ns	ns	ns	ns

NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing respectively.

NRE refer N recovery efficiency, respectively. Lower case letters refer to significant difference between treatments (p < 0.05), values are means (n = 3). *, ** and ns are significant at p < 0.05, p < 0.01 and not significant, respectively.

SP increased the N accumulation, leading to higher NRE, and the increased range was highest under TN180 (Table 3). Furthermore, the NRE of SP under TN180 was 26.57%, 7.63% and 14.72% higher than the NRE of NS under BN240, SP under BN240 and NS under TN180, respectively, indicated that moderately optimizing the straw and N management could effectively increase the NRE.

Nitrogen accumulation and N accumulation rate

SP increased the N accumulation and the N accumulation rate mainly from jointing to anthesis and anthesis to maturity, and the increase was most significant under TN180 (Table 4). Meanwhile, SR increased the N accumulation and the N accumulation rate only under BN240 and TN240. Delaying the application of basal N fertilizer did not significantly decrease the N accumulation and the N accumulation rate from sowing to the third-leaf stage, but increased these from jointing to anthesis and anthesis to maturity, especially under SP and SR, indicating that optimizing N management under straw return could meet the N requirement of wheat.

Table 4. Effects of straw and nitrogen fertilizer managements on N accumulation and N accumulation rate during different growth periods.

		N accumulation（kg ha^−1）				N accumulation rate（kg ha^−1d^−1）
Treatment		S-T	T-J	J-A	A-M	S-T	T-J	J-A	A-M
2020–2021
BN240	NS	20.56a	28.07ab	84.71bc	26.46c	0.82ab	0.29a	2.07d	0.63c
	SR	20.29ab	26.30bc	94.17ab	28.29b	0.81b	0.27ab	2.30c	0.66b
	SP	21.05a	27.46b	100.25a	28.99b	0.84a	0.28a	2.45b	0.67b
TN240	NS	19.92ab	27.86ab	87.77b	27.48bc	0.80b	0.28a	2.19c	0.64bc
	SR	19.39b	27.63ab	95.90ab	29.96b	0.78b	0.28a	2.46b	0.68b
	SP	20.02ab	29.02a	102.37a	32.27a	0.80b	0.30a	2.62a	0.73a
BN180	NS	20.65a	25.25c	75.45d	20.91e	0.83a	0.26b	1.80e	0.52e
	SR	19.83b	23.22e	80.42c	23.69d	0.79b	0.24b	1.87de	0.58d
	SP	20.21ab	25.69c	87.04bc	25.29c	0.81b	0.26b	2.02d	0.62c
TN180	NS	19.92ab	25.84c	78.09c	23.43d	0.80b	0.26b	1.90de	0.57d
	SR	19.39b	24.28d	87.61bc	25.70c	0.78b	0.25b	2.09d	0.60 cd
	SP	20.02ab	26.34bc	92.02b	28.00b	0.80b	0.27ab	2.18c	0.65b
2021–2022
BN240	NS	20.35a	18.41c	100.68b	26.40c	0.75a	0.18b	2.65b	0.60 cd
	SR	19.66ab	17.64 cd	106.60ab	28.53bc	0.73a	0.17bc	2.73ab	0.63c
	SP	20.33a	18.26c	107.35a	30.38b	0.75a	0.18b	2.75ab	0.68b
TN240	NS	19.20b	21.54b	103.05b	27.97c	0.71b	0.21ab	2.71ab	0.64bc
	SR	18.53c	23.52a	105.91ab	30.95b	0.68c	0.23a	2.72ab	0.69b
	SP	19.24b	24.07a	110.65a	34.44a	0.71b	0.24a	2.84a	0.77a
BN180	NS	19.59ab	15.31e	89.01c	22.35e	0.73ab	0.15c	2.23d	0.55de
	SR	19.41ab	13.88f	86.83c	21.54f	0.72b	0.15c	2.22d	0.56d
	SP	19.80a	15.18e	95.37bc	26.39c	0.73ab	0.15c	2.33 cd	0.63c
TN180	NS	19.20b	16.28d	95.17bc	23.80d	0.71b	0.16bc	2.44c	0.57d
	SR	18.53c	17.61 cd	96.43bc	25.66 cd	0.69c	0.17bc	2.41c	0.58d
	SP	19.24b	17.99c	100.79b	26.69c	0.71b	0.18b	2.52bc	0.61c
Year		ns	*	*	ns	*	*	*	ns
Nitrogen		ns	ns	*	*	ns	ns	**	*
Straw		*	*	**	**	*	*	*	**
N×S		ns	ns	*	*	*	ns	*	*
Y×N		ns	ns	ns	ns	ns	ns	ns	ns
Y×S		ns	ns	ns	ns	ns	*	ns	ns
Y×N×S		ns	ns	ns	ns	ns	ns	ns	ns

NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing respectively.

S-T, T-J, J-A and A-M refer to the growth period of sowing to 3-leaf stage, 3-leaf stage to jointing stage, jointing stage to anthesis stage and anthesis stage to maturity stage respectively. Lower case letters refer to significant difference between treatments (p < 0.05), values are means (n = 3). *, ** and ns are significant at p < 0.05, p < 0.01 and not significant, respectively.

Root biomass and root weight density

The trend of root biomass of wheat was consistent in both years, and the effect of straw return methods on the root biomass of wheat was different under different N fertilizer managements (Figure 2). Higher N application did not increase the root biomass of wheat, while it increased the root by applying basal N at the third-leaf stage. Compared to NS, the root biomass of SP under TN180 and TN240 was significantly increased, and increased by 14.98%, 16.68% (2020–2021) and 15.88%, 19.36% (2021–2022) at the anthesis stage, reducing basal N application decreased the root biomass of wheat in all periods under SR, and the decrease was the greatest at the anthesis stage, but the reduction in root biomass was mitigated by applying basal N at the third-leaf stage.

Figure 2. Effects of straw and nitrogen fertilizer managements on root biomass at jointing (A, B), anthesis (C, D) and maturity (E, F) in 2020–2022. NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing, respectively. * and ** indicate significant differences at the 0.05 and 0.01 levels, respectively; ns indicates no significant difference. Whiskers on the top of the bars indicate standard error.

The root weight density of wheat decreased with the depth of soil layer (Figure 3). The effect of root weight density with changes in N fertilizer managements was less at the jointing stage, and root weight density increased with increasing N application in the 0–40 cm soil layer at the anthesis stage and decreased with increasing N application in the soil layer below 60 cm, but the extension of the lower root system could be improved by delaying the application of basal N. SP mainly increased the root weight density at the anthesis and maturity stage, and the increase was more significant in the 0–10 cm and below 40 cm layers under BN180 and TN180. SR reduced the root weight density in the 0–20 cm soil layer under BN180 and TN180 at the jointing and anthesis, and the most significant decrease in root weight density occurred in the 0–10 cm soil layer.

Figure 3. Effects of straw and nitrogen fertilizer managements on root weight density at jointing, anthesis and maturity in 2021–2022. NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing, respectively. The four rows from top to bottom show the root weight density under N management for BN240 (A, B, C), TN240 (D, E, F), BN180 (G, H, I) and TN180 (J, K, L), respectively. * and ** indicate significant differences at the 0.05 and 0.01 levels, respectively; ns indicates no significant difference. Whiskers on the top of the bars indicate standard error.

Distributions of residual ¹⁵N fertilizer and soil properties

The ¹⁵N residue showed a decreasing trend with the increase in soil depth; the differences were not consistent among soil layers (Figure 4), and the differences in ¹⁵N residue showed that the topsoil layer > subsoil layer > middle soil layer, and the¹⁵N residue increased with the increase of N application. What’s more, differences in ¹⁵N residues were not consistent among soil layers in different periods, with the largest differences at the jointing stage, followed by the anthesis stage, while the differences at the maturity stage showed fewer differences, and the difference was only in the 0–20 cm soil layer under N180. SR and SP significantly increased the residual ¹⁵N content in the 0–40 cm soil layer at the jointing and anthesis stages, while they decreased the residual ¹⁵N content in the soil layer below 40 cm, and the variation was greater for SP.

Figure 4. Effects of straw and nitrogen fertilizer managements on ¹⁵N residue content in different soil layers at jointing, anthesis and maturity stages in 2021–2022. NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing, respectively. The four rows from top to bottom show the root weight density under N management for BN240 (A, B, C), TN240 (D, E, F), BN180 (G, H, I) and TN180 (J, K, L), respectively. * and ** indicate significant differences at the 0.05 and 0.01 levels, respectively; ns indicates no significant difference. Whiskers on the top of the bars indicate standard error.

Soil inorganic N content increased with the increase in total N application and the delay of basal N fertilizer application (Figure 5). The difference in soil inorganic N content between different straw return methods and basal N application periods at low basal N fertilizer application was more significant than that at high N fertilizer application. Compared with NS, SP and SR both increased soil inorganic N content at the jointing and anthesis stage，which was more significant under the N managements of TN180 and TN240. SR decreased soil inorganic N content at the tillering stage compared to NS, especially under BN180, while SP decreased slightly more than SR.

Figure 5. Effects of straw and nitrogen fertilizer managements on inorganic activity, microbial biomass C and microbial biomass N in the 0–20 cm soil layer at tillering, jointing and anthesis in 2021–2022. NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing, respectively. * and ** indicate significant differences at the 0.05 and 0.01 levels, respectively; ns indicates no significant difference. Whiskers on the top of the bars indicate standard error.

Soil microbial biomass C and N increased with the increase of N application and the delaying of basal N (Figure 5), and the difference was greater at the jointing stage than at the anthesis stage among treatments. Compared with NS, SP and SR both increased soil microbial biomass C and N content at the jointing and anthesis stages, and the increase was the greatest at TN240, suggesting that the basal N delayed after straw return might be more fixed in the tillage layer by microorganisms and released by mineralization at the peak of N uptake in wheat to improve N supply capacity.

The fates of ¹⁵N fertilizer

The percentage of ¹⁵N fertilizer recovery and residual decreased with the increase in total N application but increased by applying basal N fertilizer at the third-leaf stage (Figure 6). The percentage of ¹⁵N fertilizer loss increased with the increase in total N application but decreased by applying basal N fertilizer at the third-leaf stage. Straw return increased the percentage of ¹⁵N recovery and residual, thereby decreasing the percentage of ¹⁵N loss, especially under TN180 and TN240. In addition, the trend was more significant under SP than SR.

Figure 6. Effects of straw and nitrogen fertilizer managements on the fates of ¹⁵N fertilizer in 2021–2022. NS, SR and SP refer to no straw return, straw return by rotary tillage and straw return by ploughing, respectively. Whiskers on the top of the bars indicate standard error.

Discussion

The use of ¹⁵N isotope-labeled fertilizer allows for the direct quantification of the fates of N fertilizer in the wheat-soil system. N fertilizer applied to the soil has three fates: uptake by the crop, residual in the soil and loss from the soil-wheat system (Ruisi et al. 2016). In the present study, rice straw return increased ¹⁵N recovery, ¹⁵N residual and decreased ¹⁵N loss compared to NS, and the improvements of SP were greater than SR. Compared to NS and SR, SP increased grain yield by 7.65%–14.25% (2020–2021) and 9.47%–12.24% (2021–2022) under the same N fertilizer management. SP reached the highest NRE under TN180, which was 3.05%, 10.33% (2020–2021) and 4.29%, 11.65% (2021–2022) higher than SR and NS, respectively (Table 3), which showed a similar trend to previous findings (Chen et al. 2016; Islam et al. 2022). The current results were consistent with the hypothesis that optimizing N fertilizer under different straw return methods showed different effects. Although SP and SR both increased ¹⁵N residual, SR appeared to have a reduction in ¹⁵N recovery under BN180 and TN180. Recent studies have indicated that SR tends to strongly increase microbial activities at the seeding stage and priming effect would occur (Chen et al. 2022). Reducing basal N fertilizer under SR tended to decrease soil N availability and reduce the tiller number before the jointing stage (Table 3) leading to less root extension in the soil layer, while reducing basal N fertilizer did not significantly have negative effects on wheat before jointing.

Some studies have found through multi-year field location trials that long-term straw return improves the physicochemical and biological properties of the soil (Ma et al. 2019). Zhang’s study showed that long-term straw return treatment improved N uptake and rice yield stability, especially in years of low photosynthetically active radiation (PAR), which is mainly caused by the improvement of soil nutrient supply capacity (Su et al. 2020; Zhang et al. 2021). In this study, through the determination of soil microbiological properties in the 0–20 cm soil layer, it was found that both urease and microbial biomass C and N in the soil were significantly increased after straw return, which may be due to the release of soluble C and N from straw enhancing microbial activity and increased microbial biomass (Ma et al. 2021), thus promoting microbial fixation of applied N fertilizer, which was mostly retained as organic N in the 0–20 cm soil layer before jointing stage and mineralized after jointing to improve soil N supply capacity with the increase in temperature. The 1000-grain weight is usually positively related to the soil N supply capacity after flowering (Shan et al. 2021). In our study, straw return treatments also increased the soil N supply after anthesis, which may be the reason that straw return treatments increased grain yield. The results of Yang’s et al. (2015) study also confirmed that ditch-buried straw return (DBSR) resulted in more¹⁵N fertilizer residues in the upper soil layer and reduced fertilizer N residues in the subsoil layer compared with no straw return. It is indicated that straw returned to deeper soil layers, such as SP, could reduce part of the N fertilizer application and meet the needs of wheat for N in different periods.

In this study, delaying the application of basal N fertilizer had no effect on the N uptake before jointing, it was mainly because wheat did not have a strong root network to intercept the basal fertilizer N applied to the soil before jointing stage (Ma et al. 2021). Basal fertilizer N applied to the soil is transformed into nitrate available for wheat uptake through ammonification and nitrification. While the heavy rainfall always occurs after wheat sowing in the Yangtze River Basin, nitrate is easily leached to the subsoil layer. In our study, it was found that straw return (SP, SR) treatments significantly increased the residual of N fertilizer in the soil, especially in the 0–20 cm soil layer, but also significantly reduced the residue of N fertilizer in the soil layer below 40 cm (Figure 4). This experiment reduced the application of basal N fertilizer only by delaying the application until the 3-leaf stage of wheat, which not only ensured the growth of wheat but also further reduced the loss of basal fertilizer in the system.

A strong wheat root network can effectively intercept and capture mineral N in the soil, thereby improving N uptake and utilization by wheat and reducing N losses through downward transport (Zhang et al. 2020). Some studies have shown that the straw return method and N fertilizer management can significantly affect the root expansion of wheat, and the straw return significantly reduced the soil bulk density, especially in the middle and lower soil layers, and significantly increased the root weight density of the lower root layer of wheat (Xu et al. 2018). In this study, it was found that both SP and SR significantly increased the root density in the lower soil layer. However, SR significantly reduced the root density in the 0–40 cm soil layer, and decreased even more significantly in the 0–20 cm soil layer. This may be due to the fact that the straw is mostly distributed in the topsoil layer under rotary tillage, inhibiting the absorption of water and nutrients by the root system. The analysis of root morphology showed that (Figure 1) SP increased both root length and root surface area in the 0–40 cm and below 60 cm soil layers, especially under TN180. The improvement in root morphology could effectively intercept the N in the soil, which was also the reason why SP decreased the N fertilizer residual in the deep soil layer.

Conclusion

Rice straw return promoted root extension and increased N fertilizer retention in the plough soil layer to increase grain yield and NUE, and the improvements of straw return by ploughing (SP) were greater than the straw return by rotary tillage (SR). Reducing half of basal N and postponing it at 3-leaf stage (TN180) increased grain yield and N uptake by promoting deep root. The grain yield of SP accompanies with TN180 equivalent to that of no straw return (NS) with conventional N fertilizer management (BN240), while significantly improving NUE. Overall, optimizing N fertilizer and straw returned by ploughing both could increase grain yield and NUE. To maintain the high grain yield and increase NUE, it is suggested that basal N fertilizer could be reduced appropriately and delayed under SP.

Acknowledgments

The authors sincerely thank the Collaborative Innovation Center for Modern Crop Production by Province and Ministry (CIC-MCP), Nanjing Agricultural University.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/03650340.2024.2316232

Additional information

Funding

The work was supported by the National Natural Science Foundation of China under Grant (32272215); National Key R&D Program of Jiangsu under Grant (BE2021361-1).