Replacing nitrogen in mineral fertilizers with nitrogen in maize straw increases soil water-holding capacity

Soil water-holding capacity decreases due to long-term mineral fertilizer application. The objective of this study was to determine how replacing mineral fertilizer with maize straw affected the soil water retention curve, soil water content, soil water availability, and soil equivalent pore size. Replacement treatments in which 25% (S25), 50% (S50), 75% (S75), and 100% (S100) of 225 kg ha−1 nitrogen from mineral fertilizer (CK) was replaced with equivalent nitrogen from maize straw were conducted for five years in the Loess Plateau of China. The Gardner model was used to fit the soil water retention curve and calculate the soil water constant and equivalent pore size distribution. The results indicated that the Gardner model fitted well. Replacing nitrogen from mineral fertilizer with nitrogen from straw increased soil specific water capacity, soil readily available water, soil delayed available water, soil available water, soil capillary porosity, and soil available water porosity over time. S25 increased field capacity and wilting point from the fourth fertilization year. S50 enhanced soil readily available water, soil delayed available water, soil available water, and soil available water porosity from the fifth fertilization year, whereas S25 and S75 increased these from the third fertilization year or earlier. Soil specific water capacity, soil readily available water, soil delayed available water, soil available water, soil capillary porosity, and soil available water porosity could better reflect soil water-holding capacity and soil water supply capacity compared with field capacity and wilting point.


Site description and experimental design
A five-year field experiment was performed using loam (sand 39.8%, silt 31.1%, and clay 29.1%) 18 under maize cultivation in 2016-2020 at the Dongyang Research Station of Shanxi Agricultural University, Jinzhong, Shanxi, China (37° 56′ N, 112° 69′ E; 800 m altitude).The mean annual air temperature was 9.8 °C.The mean minimum air temperature of the coldest month (January) was − 6.1 °C, and the mean maximum air temperature of the hottest month (July) was 28.1 °C.The experimental site was characterized by low and erratic rainfall with droughts occurring at different stages of maize growth.The long-term mean annual rainfall at the site was 430.2 mm and the mean annual evaporation was 1860.1 mm.The rainfall was 352.4,308.0 and 572.1 mm during 2018, 2019 and 2020, respectively.Analysis of soil samples taken from the same experimental area in April 2016 showed that the top 20 cm of soil was characterized as follows: pH 8.4, soil organic matter 13.0 g kg −1 , total nitrogen 1.3 g kg −1 , total phosphorus 0.9 g kg −1 , total potassium 27.1 g kg −1 , available nitrogen 51.2 mg kg −1 , available phosphorus 7.7 mg kg −1 , and available potassium 176.4 mg kg −1 .
The field experiment used a completely randomized block design with five treatments and three replicates in a 5 × 6 m plot.Nitrogen provided by maize straw instead of 0%, 25%, 50%, 75%, and 100% of 225 kg ha −1 nitrogen provided by mineral fertilizer were conducted in 2016-2020.The five treatments were as follows: (i) application of 100% of 225 kg ha −1 nitrogen provided by mineral fertilizer only (CK); (ii) application of 25% (56.25 kg ha −1 ) of 225 kg ha −1 nitrogen provided by maize straw in combination with 75% (168.75 kg ha −1 ) of 225 kg ha −1 nitrogen provided by mineral fertilizer (S 25 ); (iii) application of 50% (112.50kg ha −1 ) of 225 kg ha −1 nitrogen provided by maize straw in combination with 50% of 225 kg ha −1 nitrogen provided by mineral fertilizer (S 50 ); (iv) application of 75% of 225 kg ha −1 nitrogen provided by maize straw in combination with 25% of 225 kg ha −1 nitrogen provided by mineral fertilizer (S 75 ); and (v) application of 100% of 225 kg ha −1 nitrogen provided by maize straw only (S 100 ).Maize straw was incorporated at a 0-15 cm soil depth in each experimental year in late October.The 105 kg ha −1 phosphorus provided by mineral fertilizer was applied to CK. Replacement treatments applied phosphorus provided by mineral fertilizer with 105 kg ha −1 minus the phosphorus content of maize straw incorporated into soil.The mineral nitrogen and phosphorus fertilizers were applied separately as basal fertilizers before sowing maize.Urea and monoammonium phosphate were also applied.In each experimental year, the Dafeng 30 maize variety was planted at a rate of 49,500 plants ha -1 in late April or early May and harvested in late September.

Sampling and analysis methods
Soil samples used for measuring the soil water retention curve were collected with a cutting ring at the plow layer after maize harvest.The soil samples were saturated slowly (> 24 h), weighed, and finally put into the CR22N high-speed refrigerated centrifuge (Hitachi Co.) to perform the soil water retention curve measurements starting from full saturation at 20 °C.The soil sample weight was measured at 10, 30, 50, 80, 100, 300, 500, 800, 1000, and 1500 kPa.Subsequently, the soil samples were oven-dried at 105 °C for 24 h.The volumetric water content at different suction levels was calculated using the equation: where θ is the soil volumetric water content at a certain suction (cm 3 cm −3 ), V W is the volume of water of the soil sample at a certain suction (cm 3 ), V is the volume of the soil sample with 100 cm 3 (cm 3 ), W S is the soil sample weight under a certain suction (g), W o is the soil sample weight after oven-drying (g), and ρ is the water density with 1 g cm −3 (g cm −3 ) 19 .
The Gardner model was used to fit the acquired data in Microsoft Excel 2016 (Microsoft Corp., Redmond, WA, USA) as follows 20 : Soil specific water capacity was derived from formula (2), and it was defined as where θ is the soil volumetric water content (cm 3 cm −3 ), S is the soil water suction (kPa), A and B are dimensionless parameters related to the curve shape, and C is the soil specific water capacity (kPa −1 ) 21 .
The field capacity, soil volumetric water content at 600 kPa, and wilting point were calculated by the Gardner model at 33, 600, and 1,500 kPa, respectively 22 .
Soil readily available water, soil delayed available water, and soil available water content were defined as where θ r is the soil readily available water content (cm 3 cm −3 ), θ f (cm 3 cm −3 ) is the field capacity, θ 600 (cm 3 cm −3 ) is the soil volumetric water content at 600 kPa, θ d (cm 3 cm −3 ) is the soil delayed available water content, θ w (cm 3 cm −3 ) is the wilting point, and θ a (cm 3 cm −3 ) is the soil available water content 22 .
The pore size range of soil capillary porosity was 0.03-0.1 mm and that of soil available water porosity was 0.002-0.06mm 19 ).The ranges of water suction of soil capillary porosity and soil available water porosity were 3-10 kPa and 5-150 kPa, respectively 22 .The soil volumetric water content at 3, 5, 10, and 150 kPa was calculated using formula (2).Soil capillary porosity was the soil volumetric water content at 3 kPa minus the soil volumetric water content at 10 kPa, multiplied by 100%.Soil available water porosity was the soil volumetric water content at 5 kPa minus the soil volumetric water content at 150 kPa, multiplied by 100% 22 .

Statistical analysis
Analysis of variance (ANOVA) was performed using SAS 6.2 for Windows.The significance of treatment effects in each year was determined using the F-test.Multiple comparisons of means were performed using Duncan's multiple range test 23 at P ≤ 0.05.IBM SPSS statistics 27 and R language were used for principal component analysis (PCA) analysis.

Plant materials statement
The experiment complied with relevant institutional, national, and international guidelines and legislation.

Soil water retention curve
The Gardner model 20 was used to fit the measured data of the soil water retention curve.The points were the measured values, whereas the lines were the fitted values in Fig. 1.The soil water content of each treatment showed a rapid decreasing trend when the water suction was lower than 100 kPa but a slowly decreasing trend when the water suction was greater than 100 kPa.
The fitting coefficient R 2 of the soil water retention curve of each treatment was above 0.950 (Table 1).The fitting effect was good.Parameter A determined the height of the curve and the level of the water-holding capacity.The larger the value of A, the stronger the water-holding capacity 24 .S 100 had the highest value of parameter A, followed by S 75 , whereas parameter A with S 25 and S 50 was lower than that with CK in 2018.S 25 had the highest value of parameter A, followed by S 75 , whereas parameter A with S 50 and S 100 was lower than that with CK in 2019.S 75 had the highest value of parameter A, followed by S 25 , S 50 , S 100 , and CK in 2020.Thus, replacement treatments could increase soil water-holding capacity over time.

Soil specific water capacity
The soil specific water capacity at 100 kPa soil water suction reflected soil water supply capacity well 25 .S 50 decreased soil specific water capacity by 5.93% compared with CK, whereas S 75 and S 100 increased it by 19.16% and 24.30%; by 14.28% and 19.21%; and by 26.67% and 32.14% compared with CK, S 25 , and S 50 , respectively, in 2018 (Fig. 2).Soil specific water capacity with S 25 was 10.84% higher than that with S 50 in 2018.
S 50 decreased soil specific water capacity by 21.64% compared with CK.However, S 25 and S 75 increased soil specific water capacity by 14.90% and 8.50%, respectively, compared with CK; and by 46.63% and 38.46%, respectively, compared with S 50 ; and by 16.95% and 10.43%, respectively, compared with S 100 in 2019.Soil specific water capacity with S 100 was 25.38% higher than that with S 50 in 2019.
Replacement treatments increased soil specific water capacity in 2020.S 50 , S 75 , and S 100 increased soil specific water capacity by 13.87%, 16.81%, and 7.60%, compared with CK in 2020.Soil specific water capacity with S 50 and S 75 was 10.41% and 13.27% higher than that with S 25 , and 5.83% and 8.57% higher than that with S 100 , in 2020.
( www.nature.com/scientificreports/Thus, the replacement of nitrogen from mineral fertilizer with equivalent nitrogen from maize straw increased soil water supply capacity over time.

Soil water constant
Compared with CK, S 25 slightly decreased field capacity, whereas S 50 , S 75 , and S 100 decreased it by 22.17%, 12.33%, and 6.41%, respectively, in 2018 (Fig. 3).S 25 increased field capacity by 14.23% and 7.78%, whereas S 75 slightly decreased it, and S 50 decreased it by 14.06% and 7.20% in 2019 and 2020, respectively.S 100 slightly decreased field capacity in 2019 and decreased it by 7.78% in 2020.
S 25 , S 50 , S 75 , and S 100 decreased the wilting point by 7.49%, 29.25%, 25.86%, and 19.66%, respectively, compared with CK in 2018 (Fig. 4).The wilting point with S 25 was 13.79% and 10.31% higher than that with CK in 2019 and 2020, respectively.Relative to CK, S 50 and S 75 decreased the wilting point by 9.02% and 6.48%, respectively, in 2019, and by 18.15% and 12.90%, respectively, in 2020.S 100 slightly decreased the wilting point in 2019, and decreased it by 15.86% in 2020 compared with CK.
S 100 had the highest soil readily available water, soil delayed available water, and soil available water, followed by S 75 in 2018 (Figs.www.nature.com/scientificreports/

Principal component analysis
The values of soil water constant, soil available water content, and soil equivalent pore size reflect soil waterholding capacity and soil water supply capacity.To comprehensively measure the indices of soil water constant, soil available water content, and soil equivalent pore size, principal component analysis (PCA) was performed with the evaluation variables of soil specific water capacity (X 1 ), field capacity (X 2 ), wilting point (X 3 ), soil readily available water content (X 4 ), soil delayed available water content (X 5 ), soil available water content (X 6 ), soil capillary porosity (X 7 ), and soil available water porosity (X 8 ).Principal components (PCs) were extracted according to the criteria of characteristic values greater than 1 (Table 2).The first two PCs had a cumulative contribution rate of 99.97%.Thus, the original eight indices could be replaced by the two PCs for comprehensive evaluation.The PC1 contribution rate reached 80.08%, which mainly reflected the influence of soil specific water capacity (X 1 ), soil readily available water content (X 4 ), soil delayed available water content (X 5 ), soil available water content (X 6 ), soil capillary porosity (X 7 ), and soil available water porosity (X 8 ).The PC2 contribution rate reached 19.88%, which mainly reflected the influence of field capacity (X 2 ) and wilting point (X 3 ) (Table 3).Thus, soil specific water capacity, soil readily available water, soil delayed available water, soil available water, soil capillary porosity, and soil available water porosity could better reflect soil water-holding capacity and soil water supply capacity compared with field capacity and wilting point.www.nature.com/scientificreports/According to principal component analysis based on three-year average data, S 25 , S 75 and S 100 positively correlated with the first principal component (Fig. 10).CK and S 50 negatively correlated with the first principal component.S 75 and S 100 had a close distance, both located in quadrant IV, belonging to the same category.S 25 , CK and S 50 were far apart, located in quadrants I, II, and III respectively, belonging to different categories.

Discussion
The soil water characteristic curve can reflect the water holding and releasing characteristics of different soils, and can also be used to understand some soil water constants and characteristic indexes.Therefore, it is an important tool for studying soil water movement, regulating and utilizing soil water, and soil improvement.Fan et al. (2020) Fan 26 reported that incorporating straw of rape, maize, potato, oats, and buckwheat increased field capacity compared with non-cultivated and without fertilization in the 0-60-cm soil layer.Ren 27 demonstrated that different straw returning depths enhanced field capacity compared with no fertilization.Du 28 showed that the wilting coefficient of finely cut straw was higher than long cut straw.In this study, S 25 increased field capacity and wilting point from the fourth fertilization year, whereas the rest of the replacement treatments had lower field capacity and wilting point compared with CK.This might be because S 25 had lower straw incorporation rate, which resulted in complete straw decomposition, whereas the rest of the replacement treatments had a higher straw incorporation rate, which resulted in incomplete straw decomposition.In addition, the replacement of mineral fertilization with straw might have immobilized soil inorganic N in the initial stages of maize.Meanwhile, because the decomposition degree of straw was affected by soil moisture, the nitrogen, phosphorus, potassium and other trace elements released during the decomposition of straw would affect the soil moisture and nutrient status and improve the water retention performance of the soil.
Yang 29 found that the returning of straw of rice, maize, and wheat significantly increased soil water retention capacity at the matric potential of − 0.033 and − 1.5 MPa, and consequently, enhanced soil available water content.Fan 30 reported that straw returning improved the effects of potassium fertilizer on soil porosity.This study showed that S 50 decreased soil specific water capacity, soil readily available water, soil delayed available water, soil available water, and soil available water porosity in 2018 and 2019, whereas S 50 increased these in 2020 compared with single application of mineral fertilizer.These indicated that S 50 could increase soil water-holding capacity and soil water supply capacity over time.Because the crop root system is the absorption organ of soil moisture and nutrients, it can respond to soil moisture.It was speculated that the reason was that the effective nitrogen content in the soil was sufficient when the content of straw nitrogen and inorganic nitrogen was 50% respectively, which might increase the root density by stimulating the growth of maize roots near the nitrogenrich area, thereby increase the hydraulic conductivity of maize and improve the absorption of soil moisture by maize.This study also presented that replacement treatments increased soil readily available water, soil delayed available water, soil available water, soil capillary porosity, and soil available water porosity relative to single application of mineral fertilizer in the fifth fertilization year.This might be because continuous straw incorporation promoted the formation of larger soil macroaggregates 31 , which resulted in the improvement of soil structure.
Soil organic carbon is an important chemical component in soil, which can characterize the change in soil quality.Studies had shown that different proportions of straw returning had different effects on the change of soil organic carbon content, but less or too much straw returning would hinder the decomposition of straw and slow down the increased rate of soil organic carbon 32 .At the same time, the application of nitrogen fertilizer could change the number of microbial populations in the soil, promote their activity, build a healthy soil microecological environment, further promote the growth of crop roots, and strengthen the efficient use of water in the soil by crops.Straw returning would also increase the content of soil organic carbon 33 .Soil organic carbon is also the main precursor of soil humus and aggregates, which is of great significance in improving soil fertility.This shows that the application of appropriate straw nitrogen substitution for inorganic fertilizer may be to maintain soil moisture and nutrient characteristics by increasing the content of organic carbon in the soil.Besides, the results showed that soil specific water capacity, soil readily available water, soil delayed available water, soil available water, soil capillary porosity, and soil available water porosity could better reflect soil water-holding capacity and soil water supply capacity compared with field capacity and wilting point.It was indicated that in this experimental area when the straw was used to replace part of chemical fertilizer under the condition of equal nitrogen amount, more attention should be paid to the changes of soil specific water capacity, soil readily available water, soil delayed available water, soil available water, soil capillary porosity and soil available water porosity.Fertilization measures to increase soil specific water capacity, soil readily available water, soil delayed available water, soil available water, soil capillary porosity and soil available water porosity had a beneficial effect on improving soil water status and promoting soil health, thereby promoting crop growth.It could also be speculated that the appropriate adjustment of the proportion of straw nitrogen instead of chemical fertilizer nitrogen according to rainfall conditions would help to better play the benefits of straw returning to the field, promote the water and nitrogen cycle of agricultural ecosystems, and maintain ecological balance.
The significance of replacing part of mineral nitrogen with organic nitrogen in straw is also to reduce the application of mineral nitrogen without damaging the nutritional status of the plant.In this experiment, the effects of straw organic nitrogen instead of mineral nitrogen on soil moisture characteristics in different experimental years were mainly explored.Efficient use of water is one of the important conditions to ensure the normal growth and development of crops.Under the condition of equal nitrogen amount of straw instead of chemical fertilizer application, it can not only ensure the normal metabolic activity in the process of crop vegetative growth and reproductive growth, but also promote the improvement of soil physical and chemical properties, and finally achieve the effect of saving chemical fertilizer and reducing environmental pollution, which is the focus of this experimental study in the future.

Conclusions
Replacing nitrogen from mineral fertilizer with nitrogen from maize straw gradually increased soil water-holding capacity and soil water supply capacity relative to applying mineral fertilizer only over time.The 25% nitrogen provided by maize straw combined with the 75% nitrogen provided by mineral fertilizer increased field capacity and wilting point compared with applying mineral fertilizer only at the same nitrogen content.Applying equal proportions of nitrogen from maize straw and mineral fertilizer increased soil water availability and soil available water porosity relative to applying mineral fertilizer only from the fifth fertilization year.

Figure 2 .Figure 3 .
Figure 2. Specific water capacity of soil at 100 kPa as a function of the different replacement treatments in 2018-2020.

Figure 7 .Figure 8 .
Figure 7. Available water content as a function of the different replacement treatments in 2018-2020.

Figure 9 .
Figure 9. Available water porosity of soil as a function of the different replacement treatments in 2018-2020.

Figure 10 .
Figure 10.Principal component analysis based on three-year average data.
5, 6, 7).S 100 increased soil readily available water, soil delayed available water, and soil available water by23.08%,13.51%, and 21.11%, respectively, compared with CK; by 31.67%,27.92%, and 30.80%, respectively, compared with S 50 ; and by 18.25%, 11.50%, and 16.89%, respectively, compared with S 25 in 2018.S 75 increased soil readily available water, soil delayed available water, and soil available water by 17.83%, 8.11%, and 15.83%, respectively, compared with CK; by 26.06%, 21.83%, and 25.10%, respectively, compared with S 50 ; and by 13.21%, 6.20%, and 11.80%, respectively, compared with S 25 in 2018.S 25 increased soil readily available water, soil delayed available water, and soil available water by 11.35%, 14.72%, and 11.90%, respectively, compared with S 50 , and slightly increased these compared with CK, whereas S 50 decreased these by 6.53%, 11.26%, and 7.41%, respectively, compared with CK in 2018.S 25 had the highest soil readily available water, soil delayed available water, and soil available water, followed by S 75 in 2019.S 25 increased soil readily available water, soil delayed available water, and soil available water by 14.91%, 14.52%, and 14.83%; by 46.05%, 41.54%, and 45.16%; by 6.30%, 9.52%, and 6.90%; and by 16.98%, 16.46%, and 16.88% compared with CK, S 50 , S 75 , and S 100 , respectively, in 2019.S 75 increased soil readily available water, soil delayed available water, and soil available water by 8.09%, 4.56%, and 7.42%, respectively, compared CK; by 37.39%, 29.23%, and 35.79%, respectively, compared with S 50 ; and by 10.05%, 6.33%, 9.33%, respectively, compared with S 100 in 2019.S 50 decreased soil readily available water, soil delayed available water, and soil available water by 21.32%, 19.09%, and 20.89%, respectively, whereas S 100 slightly decreased these compared with CK in 2019.Replacement treatments could increase soil readily available water, soil delayed available water, and soil available water in 2020.S 75 had the highest soil readily available water, soil delayed available water, and soil available water, followed by S 50 in 2020.S 75 increased soil readily available water, soil delayed available water, and soil available water by 16.03%, 9.47%, and 14.72%, respectively, compared with CK; by 12.34%, 4.52%, and 10.76%, respectively, compared with S 25 ; and by 8.48%, 7.22%, and 8.23%, respectively, compared with S 100 in 2020.S 50 increased soil readily available water, soil delayed available water, and soil available water by 13.01%, 5.79%, and Vol:.(1234567890) Scientific Reports | (2024) 14:9337 | https://doi.org/10.1038/s41598-024-59974-9www.nature.com/scientificreports/with 11.46%, respectively, compared with CK.The soil readily available water, soil delayed available water, and soil available water first increased and then decreased with the increase in nitrogen from straw instead of nitrogen from chemical fertilizer.Compared with CK, S 50 slightly increased soil capillary porosity, whereas S 25 , S 75 , and S 100 increased it by 7.35%, 35.49%, and 39.70%, respectively, in 2018 (Fig.8).S 100 increased soil capillary porosity by 30.13% and 38.06%, whereas S 75 increased it by 26.21% and 33.9% compared with S 25 and S 50 , respectively, in 2018.Soil capillary porosity with S 25 was 6.09% higher than that with S 50 in 2018.S 100 slightly decreased soil capillary porosity, whereas S 50 decreased it by 25.00% relative to CK in 2019.S 25 and S 75 increased soil capillary porosity by 15.23% and 13.69%; by 53.64% and 51.59%; and by 17.29% and 15.72% compared with CK, S 50 , and S 100 , respectively, in 2019.Soil capillary porosity with S 100 was 31.00%higherthan that with S 50 in 2019.S 25 slightly increased soil capillary porosity compared with CK, whereas S 50 and S 75 increased it by 25.39% and 27.14%; by 23.73% and 25.46%; and by 8.65% and 10.17% relative to CK, S 25 , and S 100 , respectively, in 2020.Soil capillary porosity with S 100 was 15.41% and 13.88% higher than that with CK and S 50 , respectively, in 2020.S 50 slightly decreased soil available water porosity compared with CK, whereas S 25 , S 75 , and S 100 increased it by 5.73%, 26.81%, and 31.53%, and by 8.52%, 30.15%, and 34.99% compared with CK and S 50 , respectively, in 2018.S 75 and S 100 increased soil available water porosity by 19.94% and 24.40%, respectively, relative to S 25 in 2018 (Fig.9).Soil available water porosity with S 25 was 8.52% higher than that with S 50 in 2018.S 100 slightly decreased soil available water porosity, whereas S 50 decreased it by 23.30% relative to CK in 2019.S 25 and S 75 increased soil available water porosity by 15.06% and 11.03%; by 50.01%and 44.76%; and by 17.12% and 13.01%compared with CK, S 50 , and S 100 , respectively, in 2019.Soil available water porosity with S 100 was 28.09% higher than that with S 50 in 2019.S 25 slightly increased soil available water porosity compared with CK, whereas S 50 and S 75 increased it by 19.36% and 21.74%; by 16.72% and 19.05%; and by 7.21% and 9.35% compared with CK, S 25 , and S 100 , respectively, in 2020.Soil available water porosity with S 100 was 11.33% and 8.87% higher than that with CK and S 25 , respectively, in 2020.

Table 2 .
Explanation of total variance across principal component analysis.

Table 3 .
Component matrix of principal component.