Irrigation Combines with Nitrogen Application to Optimize Soil Carbon and Nitrogen, Increase Maize Yield, and Nitrogen Use Efficiency

doi:10.21203/rs.3.rs-3343343/v1

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Research Article

Irrigation Combines with Nitrogen Application to Optimize Soil Carbon and Nitrogen, Increase Maize Yield, and Nitrogen Use Efficiency

https://doi.org/10.21203/rs.3.rs-3343343/v1

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published 20 Jan, 2024

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Aim and methods Staged water shortages and excessive nitrogen application lead to a decline in crop yield, and nitrogen loss, waste of resources and environmental pollution. In order to explore the appropriate water management in humid areas and to determine an efficient nitrogen rates for maize, we conducted a field experiment (2020-2021), with five nitrogen rates N0, N150, N200, N250, N300 (0, 150, 200, 250, 300 kg ha^-1) under different water conditions (rainfed and irrigated).

Conclusion Consequently, under the irrigated conditions, the soil organic matter (SOM), microbial biomass carbon (MBC), dry matter accumulation and grain yield were significantly (P < 0.05) increased by 8.06%, 15.50%, 11.43% and 13.86%, respectively, compared with the rainfed conditions. And the total nitrogen (TN), nitrate nitrogen (NO₃^--N), ammonia nitrogen (NH₄⁺-N) decreased significantly by 4.38%, 8.28%, 13.21%, respectively. Furthermore, compared with other nitrogen rates, N250 and N300 combined with irrigated significantly (P < 0.05) increased soil carbon and nitrogen content, dry matter accumulation and grain yield. However, N250 and N300 displayed no significant difference (P > 0.05) in TN, NO₃^--N, NH₄⁺-N, SOM, MBC, dry matter accumulation and grain yield. Moreover, under the irrigated conditions, when the nitrogen rates increased more than 250 kg ha^-1, the nitrogen use efficiency decreased. Hence, application of nitrogen rates of 250 kg ha^-1 under irrigated conditions is the best choice to increase grain yield, improve nitrogen use efficiency, and ensuring safe and efficient production in maize.

irrigation

nitrogen

soil properties

nitrogen use efficiency

grain yield

The rapid increase in population has leads to a significant rise for the food demand. It is projected that by 2050, the combination of urbanization intensification and global population growth will lead to a 60% surge in food demand (De Souza et al., 2020). Water is a crucial requirement for crop growth, and drought is now recognized as the primary factor that limits crop yield globally (Campos et al., 2004). Population growth, increased industrial water consumption, and deteriorating water quality lead to water resource depletion (Suyker and Verma, 2009), in addition, the uneven distribution of rainfall in time and space can easily lead to drought or periodic drought. Drought stress can cause various irreversible damages to plants, including stunted growth, reduced aboveground biomass, hindered root development, and decreased photosynthetic rate leading to reduction in crops yield (Li et al., 2022, Zhao et al 2018). The critical period for crop water demand is crucial for crop growth. (Alae-Carew et al., 2020). Drought during the jointing stage of maize reduces the number of rows per ear and grains per row (Campos et al., 2004). Similarly, drought during the silking stage delays maize growth, decreases pollination seed setting rate, and leads to a significant reduction in yield. Drought during grain filling also reduces the photosynthetic rate of maize. Despite Guangxi's abundant rainfall, drought stress can still occur during critical growth and development stages, resulting in yield loss (Gao et al., 2023). Therefore, it is crucial to implement scientific irrigation practices during the crop growth period.

Nitrogen is a crucial nutrient for plant growth (Sharma et al., 2022). Soil nitrogen primarily originates from exogenous nitrogen application and the nitrogen present in the soil. The application of nitrogen can enhance soil fertility, microbial activity, and maize yield (Yang et al. 2022). However, application of nitrogen fertilizer at lower rates may result in decline of soil nutrient content and availability. On the other hand, excessive nitrogen application can lead to environmental pollution and greenhouse gas emissions (Zhang et al., 2022). Unfortunately, in Chinese agricultural production, there is a prevalent issue of unreasonable nitrogen fertilizer application and low utilization rates. Leaching, volatilization, and surface runoff of nitrogen leads to water, soil, and atmosphere pollution (Sun et al., 2018; Cui et al., 2023). It can be seen that reducing ineffective nitrogen application is very necessary. Corn is the second largest food crop in Guangxi, however seasonal drought negatively affects the germination and seedlings emergence rate of corn seeds leads to a reduction in corn production.

Application of nitrogen at a rate of 250 kg ha^− 1 is a sustainable and economical approach to enhance maize yield (Hu et al., 2023). Previous studies have demonstrated that neither irrigation nor nitrogen application alone can attain the maximum crop yield. On the other hand, excessive nitrogen application and irrigation can result in resource wastage and environmental pollution. Consequently, in order to achieve the objectives of high and stable crop yield, as well as environmentally-friendly production, it becomes imperative to investigate an optimal combination of water and nitrogen application. Furthermore, previous research has primarily focused on the effects of supplementary irrigation and N fertilization on soil N residues and/or single aspects of crop N fertilization, with little attention paid to the role of both on soil-plant interactions for N utilization and economic benefit. As a result, this study investigated the effects of supplemental irrigation and varying nitrogen fertilizer on soil carbon pool, nitrogen pool, crop yield and nitrogen use efficiency to determine the optimal nitrogen application level, to enhance maize production in subtropical regions.

2.1 Experiment site, experimental design and field Management

A two-year field experiment was carried out during 2020–2021 in Guangxi University (22°50'N, 108°17'E), Nanning, China, with the same experiment being conducted continuously since 2017. The experimental site has a subtropical monsoon climate, with an average annual temperature of 21.6°C and an average annual rainfall of 1243.7 mm. From August to November, the irrigated amounts in 2020 and 2021 were 120.5 mm and 259.4 mm, respectively (Fig. 1A). The rainfall from August to November was 438.2 mm (2020) and 406.6 mm (2021) (Fig. 1B). The soil was classified as loam. The basic soil properties of experimental site (0–20 cm soil layer) such as soil pH, available N, available P and available K contents were 5.6, 126.2 mg kg^− 1, 40 mg kg^− 1 and 124.5 mg kg^− 1, respectively. Furthermore, the soil bulk density, organic matter content and maximum field capacity were 1.36 g cm^− 3, 17.5 g kg^− 1 and 37.2%, respectively.

The maize variety used was Wanchuan-1306. The experiment employed a split-plot design, encompassing two factors: water conditions and nitrogen rates. Water conditions included rainfed (relying solely on rainfall during maize growth) and irrigated (drip irrigation when the soil moisture content was lower than 60% of the maximum field capacity). Five nitrogen rates were used such as N0, N150, N200, N250, and N300 (0, 150, 200, 250, and 300 kg ha^− 1 pure nitrogen content). Ten treatments were established, each replicated thrice, totaling 30 plots. Each plot size of 4.2 m×3.9 m, with a row spacing of 0.6 m, plant spacing of 0.3 m, and planting density of 56,000 plants ha^− 1. Urea was used as nitrogen fertilizer (N = 46.4%), phosphate fertilizer was calcium magnesium phosphate fertilizer with P₂O₅ content of 18%, and potassium fertilizer was potassium sulfate with K₂O content of 60%. P₂O₅ and K₂O were applied as base fertilizer, 100 kg ha^− 1 each. Nitrogen fertilizer was divided into 2/3 as base fertilizer and 1/3 as topdressing (topdressing at large bell stage). The maize was sown on August 1, 2020, and harvested on November 2, 2020; the maize in 2021 was sown on August 1 and harvested on November 26.

2.2 Collection of soil samples

In the R6 period, 0–20 cm soil samples were selected by five-point sampling method, and quickly passed with 2 mm soil sieve after sampling. After fully mixed, it was divided into two parts. One was naturally dried, and used to determine TN and SOM through a 100-mesh sieve. The other was immediately stored in a 4°C refrigerator for the determination of NO₃⁻-N, NH₄⁺-N and MBC.

2.3 Soil total nitrogen (TN)

The semi-automatic Kjeldahl method (Anantakrishnan and Srinivasa, 1952) was used to determined soil total nitrogen. 1.00 g of air-dried soil was weighed and added with 5 mL of concentrated sulfuric acid, and then boiled in a digestion furnace. After the sample in the test tube became grayish white and slightly green, it was boiled for 1h. After the digestion, the sample was cooled and finally distilled.

2.4 Soil nitrate nitrogen, ammonia nitrogen (NO₃⁻-N, NH₄⁺-N)

To determine soil NO₃⁻-N and NH₄⁺-N, previously described method of Liu was used (Liu et al., 2022b). In details 5.00 g fresh soil was weighed, 25 mL of 1 M L^− 1 KCl solution was added, and shaken at 200 r min^− 1 for 30 min at 28°C with ZWYR-4912 shaker (Shanghai Zhicheng Analytical Instrument Manufacturing Co., Ltd., Shanghai, China). The filtrate was filtered, and the filtrate was determined by AutoAnalyzer3 continuous flow chemical analyzer (Bran + Luebbe, Norderstedt, German).

2.5 Soil organic matter (SOM)

Potassium dichromate volumetric-external heating method (Cheshire 1987) was used for SOM determination. The 0.20 g air-dried soil sample was weighed and placed at the bottom of the test tube, and 5 mL 0.8000 M L^− 1 (K₂Cr₂O₇) standard solution and 5 mL concentrated sulfuric acid were added for digestion. The sample in the test tube was transferred into a 250 mL triangular flask for titration.

2.6 Soil microbial biomass carbon (MBC)

For (MBC) chloroform fumigation-potassium sulfate extraction method (Vance et al., 1987) was employed. Briefly, a 15.00 g fresh soil was weighed and put into a petri dish, 6 parts for each treatment. After 7 days of closed dark culture in a dry tank, 3 parts were extracted and titrated, and the rest were fumigated with chloroform for 24 h, finally titrated of all samples.

2.7 Dry matter accumulation

The previous established method of (Ren et al., 2021) was used for the determination of dry matter accumulation. Samples were taken at R1 and R6, and the stems, leaves, bract + cob, and grains were bagged separately. They were deactivated at 105°C for 1 h in an oven, and then dried at 80°C to constant weight and weighed.

2.8 Yield and yield component, nitrogen use efficient (NUE)

In the R6 period, 2 m² maize ears were randomly taken from each plot, and the number of grains per row, the number of rows per ear, the 1000-grain weight and the grain yield were measured after natural air drying, and the nitrogen use efficiency was calculated according to the following formula (Ji et al., 2021).

NUE (%) = (NU-NU₀)/N

NU is aboveground biomass total nitrogen uptake quantity of different nitrogen treatment; NU₀ is aboveground biomass total nitrogen uptake quantity of N0; N is application nitrogen rates.

2.9 Statistical analyses

All statistical analyses were performed using SPSS version 25 (IBM SPSS Statistics software). The difference between the means were determined using an LSD test at P ≤ .05. The correlation analysis was performed in R v.3.63 to evaluate the relationship among the TN, NO₃⁻-N, NH₄⁺-N, SOM, SMBC, dry matter accumulation, yield, NUE. All graphs were processed using OriginPro2021 (Origin Lab Corporation, Northampton, MA, USA).

3.1 Soil TN content

At maize maturity stage (R6), the soil TN content under rainfed increased by 6% in 2020 and 3% in 2021 (P < 0.05) than those of irrigated (Fig. 2). The soil TN content increased with increasing nitrogen rates in two study years. The highest soil TN content was found under the N300 treatment, which is 1.96 g kg^− 1 (2020) and 1.87 g kg^− 1 (2021), respectively. The soil TN content of N250 and N300 treatments was significantly higher than other nitrogen rates, but there was no significant difference (P > 0.05) between N250 and N300 treatments. The results showed that water conditions and nitrogen rates had significant effects (P < 0.01) on soil TN content during both years.

3.2 Soil nitrate nitrogen (NO₃⁻-N) content

At maize maturity stage (R6), the soil NO₃⁻-N content was affected by water conditions and nitrogen rates (Fig. 3). Compared to rainfed, the soil NO₃⁻-N content of irrigated was significant decreased 8.40% in 2020 and 7.65% in 2021 (P < 0.05). The soil NO₃⁻-N content of all treatments were ranged of 5.78–15.71 mg kg^− 1 and 4.68–15.94 mg kg^− 1 in 2020 and 2021, respectively. The soil NO₃⁻-N content of N300 and N250 treatments were significantly higher than other nitrogen rates under the same water conditions. However, there was no significant difference (P > 0.05) between N250 and N300 treatments.

3.3 The soil ammonia nitrogen (NH₄⁺-N) content

The results showed that the water conditions and nitrogen rates significantly affected the soil NH₄⁺-N content at maize maturity stage (R6) during 2020–2021 (P < 0.05). The N150, N200, N250 and N300 treatments increased by 121.66% (2020), 159.89% (2021), 138.44% (mean) than N0, respectively. The maximum of NH₄⁺-N content was found under the N300 treatments, and was 19.18 and 17.38 mg kg^− 1 in 2020 and 2021, respectively. The N300 treatment was higher than N250 treatment by 5.07% in 2020 and 0.84% in 2021, but there was no significant difference (P > 0.05). The rainfed were 18.58% (2020) and 7.87% (2021) higher than those of irrigated (Fig. 4).

3.4 Soil organic matter

The results shown that soil organic matter content increased and growth rate decreased gradually with increasing nitrogen rates under the same water conditions. At maturity stage (R6), the irrigated increased by 8.45% in 2020 and 9.10% in 2021 than those of rainfed (Fig. 5). In 2020, the highest soil organic matter content was found at N250 treatments; in 2021, the soil organic matter content of N300 treatments was higher than other nitrogen rates under same water conditions. There was no significant difference (P > 0.05) between N250 and N300 treatment. Water conditions and nitrogen rates had significant effects (P < 0.05) on soil organic matter content.

3.5 Soil microbial biomass carbon content

At maturity stage (R6), the SMBC content change of different nitrogen rates and water conditions were shown in Fig. 6. The highest SMBC content was observed at N250 treatment under irrigated, which was 144.43 mg kg^− 1 (2020) and 144.08 mg kg^− 1 (2021), respectively. The order of SMBC were N0 < N150 < N200 < N300 < N250, and N250 was significantly higher (P < 0.05) than other nitrogen rates, except for N300 treatment. Two years’ study results showed that the content of SMBC of irrigated as higher than those of rainfed by 19.05% in 2020 and 17.65% in 2021.

3.6 Dry matter accumulation

During 2020–2021, water conditions and application of different nitrogen rates had significant effects (P < 0.05) on dry matter weight of maize. Under the same water conditions, the order of dry matter weight was N300 > N250 > N200 > N150 > N0. The total dry matter weight of ear, bract and cob at R6 were significantly higher (P < 0.05) than R1 (Fig. 7). The total dry matter weight of irrigated was significantly higher (P < 0.05) than rainfed (Fig. 8).

In 2020, at R1, the dry matter weight of stem, leaf, bract and cob under irrigated increased by 15.86%, 17.70% and 24.70% compared with those under the rainfed, respectively; at R6, the dry matter weight of grain under irrigated was 90.56 g plant^− 1, which was 8.43% higher than that under the rainfed. The total dry matter weight of N250 was 0.18% higher than that of N300 under rainfed and 4.51% higher under irrigated (P > 0.05).

In 2021, at R1, the dry matter weight of stem, leaf, bract and cob was 56.88, 30.40, 17.46 g plant^− 1 (irrigated) and 50.07, 26.78, 15.55 g plant^− 1 (rainfed), respectively; at R6, the dry matter weight of grain under irrigated was significantly increased (P > 0.05) by 19.35% compared with rainfed. Under the irrigated, the total dry matter weight at R6, there was no significant difference (P > 0.05) between N300 and N250 treatments.

3.7 Nitrogen use efficient (NUE), yield and yield component

The results of the 2020–2021 experiment showed that water conditions affected GN, TSW and yield (Table 1). Nitrogen rates had a significant effect on GN, TSW, yield; water conditions and nitrogen rates interaction significant affected (P < 0.05) TSW and yield. During 2020–2021, the GN, TSW and yield under the irrigated were increase 7.19%, 6.84%, 16.56% (2020), 7.19%, 6.84%, 16.56% (2021) respectively compared to rainfed. The GN, TSW and yield under irrigated was significantly higher (P > 0.05) for N300 and N250 as compared to other N levels, which no significant difference between the two nitrogen rates.

Table 1

Nitrogen use efficient, yield and yield component of maize under different water conditions and nitrogen rates during 2020–2021.
Treatments		Grain number per ear (grain ear^− 1)			Thousand seed weight (kg)			Yield (kg ha^− 1)			Nitrogen use efficiency (%)
Treatments		2020	2021	Mean	2020	2021	Mean	2020	2021	Mean	2020	2021	Mean
Rainfed	N0	284 ± 7.67c	212 ± 16.27c	248 ± 4.49c	195.8 ± 3.21c	187.5 ± 7.38c	191.6 ± 5.30d	3067 ± 60.09e	2917 ± 83.33c	2992 ± 71.20d	-	-	-
	N150	338 ± 7.51b	333 ± 4.37b	335 ± 5.64b	249.9 ± 7.07b	250.5 ± 2.80b	250.2 ± 3.88c	4400 ± 57.74d	4500 ± 144.34b	4450 ± 62.92c	32.45 ± 0.04b	29.24 ± 0.03b	30.84 ± 0.02c
	N200	406 ± 8.08a	412 ± 4.33a	409 ± 6.02a	256.8 ± 3.73ab	259.4 ± 2.02ab	258.1 ± 1.17ab	5517 ± 60.09c	5833 ± 83.33a	5675 ± 66.14b	35.60 ± 0.04ab	29.85 ± 0.00b	32.72 ± 0.02bc
	N250	406 ± 8.08a	415 ± 4.67a	411 ± 2.33a	263.4 ± 6.25a	267.7 ± 0.88a	265.6 ± 2.94a	5983 ± 16.67b	6133 ± 116.67a	6058 ± 54.65a	42.64 ± 0.02a	30.69 ± 0.01b	36.67 ± 0.01ab
	N300	420 ± 8.37a	422 ± 6.00a	421 ± 5.10a	260.9 ± 2.68a	272 ± 5.05a	266.5 ± 3.86a	6233 ± 16.67a	6283 ± 33.33a	6258 ± 22.05a	40.90 ± 0.02a	41.66 ± 0.01a	41.28 ± 0.01a
Irrigated	N0	329 ± 11.46d	295 ± 16.86d	312 ± 2.74d	229.4 ± 0.78c	225.1 ± 3.67c	227.3 ± 2.20d	4183 ± 83.33d	3667 ± 166.67c	3925 ± 72.17d	-	-	-
	N150	383 ± 4.67c	391 ± 15.33c	387 ± 9.06c	258 ± 4.10b	261.7 ± 8.50b	259.9 ± 4.50c	5417 ± 44.10c	6167 ± 220.48b	5792 ± 109.29c	37.78 ± 0.07a	30.25 ± 0.03b	34.02 ± 0.04b
	N200	411 ± 4.67b	434 ± 8.08b	422 ± 2.33b	259.8 ± 3.64b	275.7 ± 0.78b	267.8 ± 2.19b	6050 ± 76.38b	6583 ± 300.46a	6317 ± 137.18b	40.90 ± 0.05a	36.76 ± 0.02a	38.83 ± 0.02a
	N250	430 ± 10.00ab	459 ± 10.67ab	444 ± 5.17a	279.2 ± 6.50a	282.2 ± 0.79a	280.7 ± 3.52a	6733 ± 16.67a	6750 ± 86.60a	6742 ± 36.32a	45.43 ± 0.06a	37.13 ± 0.01a	41.28 ± 0.03a
	N300	435 ± 8.67a	464 ± 6.17a	450 ± 4.31a	284.3 ± 2.12a	282.1 ± 1.04a	283.2 ± 1.46a	6750 ± 28.87a	6750 ± 250.00a	6750 ± 125.83a	43.62 ± 0.01a	35.08 ± 0.01a	39.35 ± 0.00a
		P values
Water		0.0028	0.0028	0.0165	0.0105	0.0105	0.0081	0.0009	0.0050	0.0015	0.4073	0.0955	0.2778
Nitrogen		0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0893	0.0002	0.0064
Water × Nitrogen		0.1241	0.1241	0.0008	0.0245	0.0245	0.0084	0.0001	0.0279	0.0019	0.9824	0.0006	0.2280
Note: Data were the means ± SE. Different lowercase letters within each column indicate significant difference at P < 0.05.

Compare with rainfed conditions, the NUE of irrigated conditions was higher by 7.79%. Two years’ experiments have found that the order of nitrogen use efficient with nitrogen rates was N250 > N300 > N200 > N150 (under the irrigated conditions).

3.8 Correlation analysis

During 2020–2021, there was positive correlation between TN, NO₃⁻-N, NH₄⁺-N, SOM, MBC, DM (stems, leaves, bract + cob and grain), yield and NUE (Fig. 9). The correlation coefficient (R) of TN, NO₃⁻-N, NH₄⁺-N, SOM, MBC were significant positive correlation (P < 0.001) with N, and R value was 0.82, 0.93, 0.88, 0.78, 0.70, respectively. The NO₃⁻-N, NH₄⁺-N, SOM, MBC were significantly positively correlative (P < 0.01) with TN, and the SOM, MBC were positively correlated with NH₄⁺-N. The stems, leaves, bract + cob DM (average of R1 and R6) and yield were positively correlated with TN, NO₃⁻-N, NH₄⁺-N, SOM, MBC (P < 0.05). NUE has significant correlation (P < 0.05) with N, TN, NO₃⁻-N and other measurement.

4.1 Respond of soil total nitrogen

Nitrogen is an essential element for plant growth and a constituent of many organic compounds (Zhu et al., 2021). Plants rely greatly on available N stock in the root zone. Despite comparatively substantial soil N stores, soil N supply for crop productivity is quite low in warm and humid locations. Furthermore, plant uptake may decrease soil N stocks, which can be compensated for by N fertilizer (Nendel et al., 2019; Losacco et al., 2021).Soil TN is the sum of various forms of nitrogen in soil, including organic nitrogen and inorganic nitrogen, which is an important index to measure soil fertility (Qin et al., 2022). The results of this study showed that, with the increase of nitrogen rates, the content of soil TN also increased, but the soil TN of N250 and N300 treatment were no significant difference (P > 0.05; Fig. 2). This might be due to the nitrogen application can promote plant growth and significantly increase the biomass of aboveground and underground parts (Xu and Takahashi, 2020). However, when nitrogen rates increase to a certain extent, nitrogen fertilizer will be lost through volatilization, leaching, runoff and other forms, resulting in no significant increase with nitrogen rates (Abbaraju et al., 2022). Due to heavy fertilization and high soil inorganic N content, N runoff loss, which accounted for 88.3–90.7% of dissolved inorganic N from the surface, was mostly caused (Li et al., 2021; Liu et al., 2021b). Our research showed that under the same water conditions, the soil TN content of rainfed were higher than that of irrigated conditions. Suitable water and fertilization management strategies improve soil properties and govern the structure of the microbial population, encourage N uptake by crop metabolism and reduce runoff. (Qi et al., 2020; Ye et al., 2021). The findings of this study showed that soil TN contents increased and positively correlated with high N fertilization compared to low N treatment. This is likely because low N fertilization rates were insufficient to meet the plant growth's need for N, which in turn depleted the soil's initial stock of N and resulted in a decrease in soil N content.

4.2 Soil total nitrate nitrogen and ammonia nitrogen change

Although most nitrogen in soil is in the form of organic matter, plants prefer to absorb inorganic nitrogen (NO₃⁻-N and NH₄⁺-N) more readily than organic nitrogen. Soil must undergo constant nitrification in order to change organic nitrogen into inorganic form. According to Liu et al. (2019) nitrification in soil is most active when the soil's water content is between 50 and 60 percent of its maximum field capacity, and it can be somewhat impeded by excessively high or low soil moisture levels. In addition to hindering nitrification, excess water and fertilizer also significantly increase the amount of NO₃⁻-N leaching and groundwater pollution. In the two-year study, the content of NH₄⁺-N was higher than that of NO₃⁻-N, which may be due to the fact that nitrate ions mainly exist in soils with high moisture content (Fig. 3) (Muhammad et al., 2022). It may also be because soil water content affects soil redox reaction, ventilation status and soil microbial growth and development (Furtak et al., 2022). When the soil moisture content is high, the soil microorganisms are not active, which increases the NH₄⁺-N content (Liu et al., 2021a). Furthermore, Nitrate ion is extremely soluble in water and moves with it, and it is not easily adsorbed by soil colloids as an anionic oxidized nitrogen, which will leach into deeper soil layers, while soil NH₄⁺-N is the opposite of NO₃⁻-N (Gurmesa et al., 2022). In this study, the soil inorganic nitrogen increased with the increasing of nitrogen rates, but with the increased of nitrogen rates, the improvement effect of soil inorganic nitrogen was getting smaller and smaller, and there was no significant difference between N250 and N300 (P > 0.05). Nitrogen concentration will directly affect the concentration of soil inorganic nitrogen, and nitrogen application can significantly increase the content of soil inorganic nitrogen (Chang et al., 2023). Excessive nitrogen application will be leached to the deep soil by rainfall and other ways because the plants cannot be absorbed completely, resulting in nitrogen loss and environmental pollution (Meng et al., 2021).

4.3 Respond of soil organic matter

SOM is an important part of soil and one of the important sources of nutrients for plants (Kafesu et al., 2018). Appropriate irrigated conditions can promote the mineralization rate of soil carbon and nitrogen, thus increasing the content of SOM (Chen et al., 2019). In this experiment, the SOM under rainfed was lower than that of irrigated, which was consistent with this conclusion. It may be due to the strong hydrophilicity of SOM under irrigated conditions, the condensation degree of lignin in the soil surface layer is lower than that of rainfed, and the degree of microbial degradation is reduced, which is conducive to the accumulation of stable carbon and the increase of organic matter content (Sánchez–González et al., 2017). In this experiment, with the increase of nitrogen rates, the content of SOM increased (Fig. 5). This may be due to the increase of nitrogen rates, promoting the increase of crop biomass and stubble, thus promoting the increase of SOM content (López-Bellido et al., 2010). It may also be because the important stable source of SOM pool is soil microbial residues. Increasing the nitrogen rates also increases the contribution of bacterial residual carbon to soil organic carbon and soil total nitrogen (Hu et al., 2023).

4.4 Microbial biomass carbon

MBC is the most active part of SOM and a key factor affecting soil fertility (Campbell et al., 2022). The amount of MBC can be impacted by cultivation practices and climatic circumstances (Ma et al., 2020). The results of this experiment showed that there was a significantly positive correlation between the MBC and yield was found (Fig. 9). When 250 kg ha^− 1 of nitrogen was applied, the MBC peaked, and the presence of water had impact on the MBC. Under irrigated conditions, the soil water is relatively sufficient, which can avoid the persecution of soil microorganisms during stage drought and is conducive to the increase of MBC. The current study revealed that under nitrogen application treatment MBC was significantly higher, comparatively to without N application treatment.It may be because the appropriate soil nitrogen rates promote the utilization of carbon by microorganisms in the soil. On the contrary, the metabolism and life activity of microorganisms will be inhibited (Liu et al., 2019).

4.5 Dry matter accumulation

The material basis of maize yield is the accumulation of dry matter, and increasing the accumulation of dry matter and its transfer to the grain is the fundamental way to obtain high yield (Wang et al., 2020). Wan et al. (2022) showed that water and nitrogen are the main factors that limit the accumulation of dry matter. This study showed that irrigation and nitrogen application can significantly promote dry matter accumulation. Dry matter accumulation of N250 and N300 after anthesis was significantly higher than that of other treatments (P < 0.05), and there was no significant difference (P > 0.05) between them. Perhaps the excessive application of nitrogen led to a high nutrient growth of the plants in the early stage, resulting the leaves are highly assimilative, but the plant has insufficient storage capacity. So, the accumulation of dry matter in the ear no longer significantly increasing with the increase of nitrogen application. Additionally, boosting N fertilization to a specific range can make it easier for dry matter to be transported to the grains. This may be the result of fertilizing with high N rates after the flowering stage (Yu et al., 2021).

4.6 Yield, yield components and NUE

The control of water-N consumption efficiency is a factor of plant growth and development, resulting in high yield (Yue et al., 2021), which are highly interconnected. The selection of an appropriate water management benefits crop output, and boosts N use efficiency. According to the findings of Lu et al., (Lu et al., 2021) irrigation, together with N rates, had a substantial impact on grain production, water productivity, and N fertilization efficiency in summer maize. In line with that, our study discovered that the gain yield and yield component of optimal N fertilization (N250) was no significant difference between those of high dose of N (N300) treatment under supplementary irrigation. Implying that excessive N inputs did not result in significant grain yield increases but rather increased N fertilization expenditure. According to Li et al. (2017), the Yellow River Basin's cotton production can reap significant ecological and financial rewards from using the right amount of N fertilizer during the sowing and flowering stages.

Supplemental irrigation is one of the most successful water management methods for increasing agricultural output and reducing water-N use (Brar and Vashist, 2020; Liu et al., 2022a). In this study, we discovered that N250 fertilization significantly increased N uptake and utilization under supplementary irrigation versus rainfed conditions (Table 1). This could be due to supplementary irrigation promoting N transport in soil, which increases N availability to plants, resulting in increased NUE. However, excessive N fertilization (N300) promoted robust maize growth and prolonged the vegetative growth duration, which disrupted the reproductive growth duration, decreased grain filling time, and did not boost NUE. (Hammad et al., 2017; Kassie and Fanataye, 2019). In this study we found N250 combined with supplimentry irrigation has maximum yield NUE and net benefits.

4.7 Correlation analysis between soil and yield

Soil moisture and nutrients play a crucial role in the growth and production of maize. Irrigation can supplement soil moisture, enhancing water absorption by maize and mitigating yield reduction caused by intermittent water scarcity (Fang and Su, 2019). Additionally, nitrogen application can boost soil nutrient content, improve the soil environment, meet the nutrient requirements during crop growth, and promote crop growth and development (Zhao et al., 2019). Maize gain yield exhibited significant correlations with TN, NH₄⁺-N, NO₃⁻-N, SOC, MBC, dry matter accumulation (Fig. 9) Wang et al. (2021) demonstrated that water-nitrogen interaction stimulates the uptake of soil carbon and nitrogen by plants, thereby facilitating dry matter accumulation in maize and ultimately increasing grain yield. In this study, the soil nutrients, plant dry matter accumulation, grain yield, and nitrogen use efficiency were superior under water-nitrogen interaction compared to rainfed conditions and N0 treatment, resulting in higher yields under water-nitrogen interaction than under rainfed conditions with nitrogen treatment (P < 0.05).

According to the aforementioned findings, irrigation combined with nitrogen application enhanced MBC content, aboveground biomass, yield and yield component. Irrigation promote plant uptake of soil TN and NO₃⁻-N, NH₄⁺-N, as evidence by the soil TN and NO₃⁻-N, NH₄⁺-N content under supplementary irrigation were low compared to rainfed condition. The soil TN, NH₄⁺-N, NO₃⁻-N, MBC, yield, yield component and dry matter accumulation were no significant difference (P > 0.05) between N250 and N300 treatment. Furthermore, the NUE under the N250 treatment was higher than the N300 treatment in irrigated conditions. Therefore, a fertilization at the rate of 250 kg N ha^− 1 is advised to produce high maize grain yield with maximum grain N accumulation and economic gain, bearing in mind the goal of optimizing N rates and optimum usage of soil available N and high grain yield. This work can offer a theoretical foundation for lowering nitrogen levels, boosting nitrogen usage effectiveness, and raising yield in Southwest China's humid subtropical region.

Acknowledgments

The research was funded by the National Natural Science Foundation of China (31760354). The authors sincerely thank Yu Xin Hu for field assistance.

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Irrigation Combines with Nitrogen Application to Optimize Soil Carbon and Nitrogen, Increase Maize Yield, and Nitrogen Use Efficiency

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Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods

2.1 Experiment site, experimental design and field Management

2.2 Collection of soil samples

2.3 Soil total nitrogen (TN)

2.4 Soil nitrate nitrogen, ammonia nitrogen (NO3−-N, NH4+-N)

2.5 Soil organic matter (SOM)

2.6 Soil microbial biomass carbon (MBC)

2.7 Dry matter accumulation

2.8 Yield and yield component, nitrogen use efficient (NUE)

2.9 Statistical analyses

3. Results

3.1 Soil TN content

3.2 Soil nitrate nitrogen (NO3−-N) content

3.3 The soil ammonia nitrogen (NH4+-N) content

3.4 Soil organic matter

3.5 Soil microbial biomass carbon content

3.6 Dry matter accumulation

3.7 Nitrogen use efficient (NUE), yield and yield component

3.8 Correlation analysis

4. Discussion

4.1 Respond of soil total nitrogen

4.2 Soil total nitrate nitrogen and ammonia nitrogen change

4.3 Respond of soil organic matter

4.4 Microbial biomass carbon

4.5 Dry matter accumulation

4.6 Yield, yield components and NUE

4.7 Correlation analysis between soil and yield

5. Conclusion

Declarations

Acknowledgments

References

Status:

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2.4 Soil nitrate nitrogen, ammonia nitrogen (NO₃⁻-N, NH₄⁺-N)

3.2 Soil nitrate nitrogen (NO₃⁻-N) content

3.3 The soil ammonia nitrogen (NH₄⁺-N) content