Effects of nitrogen fertilizer and water management practices on pH and nitrogen distributions in the wetted-soil volume using drip irrigation

With the development of intensive farming practices, agricultural land is becoming more acidic, which poses harm to agricultural production and the ecological environment (Guo et al. 2010; Yan et al. 2020). According to statistics, the average farmland soil pH decreased by 0.5 units in China from the 1980s to 2010s, which was significantly greater than the soil acidification rate in Europe and the United States (Wang et al. 2020; Qiu & Sun 2021). Furthermore, the acidified area of farmland in China has reached 2.04 × 10⁸ ha, accounting for 22.7% of the total land area (Qiu et al. 2023). Soil acidification increased the concentration of H⁺ and Al³⁺ in soil, activated heavy metal ions such as Mn²⁺, Pb²⁺, Cu²⁺, and Cd²⁺, and intensified the leaching loss of base cations such as Ca²⁺, K⁺, Mg²⁺, and Na⁺, thereby resulted in fertility decreasing and reducing crop yield (Xu et al. 2018). When compared to rainfalls, anthropogenic actualities such as the excessive application of nitrogen fertilizer, irrational irrigation with large infiltration loss (furrow and basin irrigations), and planting of acid-inducing crops have accelerated the acidification process of farmland (Xu et al. 2018; Qiu et al. 2023).

Extensive studies showed that more than 70–90% of soil acidification occurred in cereal and vegetable fields attributed to the excessive application of nitrogen fertilizers (Zhang 2017; Xu et al. 2018; Yan et al. 2020). Furthermore, the immediate cause for excessive fertilization is low nitrogen use efficiency. In a study conducted by Cai et al. (2011) in Hunan Province, the influence of long-term fertilization on soil pH and nitrogen uptake of crops was evaluated. Their results indicated that the soil pH and nitrogen uptake of crops decreased significantly with long-term fertilization. The decreasing rate and the magnitude of soil pH were positively correlated with the nitrogen application rate. Han et al. (2015) conducted a greenhouse pot experiment to investigate the influence of nitrogen fertilization on soil acidity and indicated that soil acidity increased significantly as nitrogen input increased with a pH decrease ranging from 0.45 to 1.06 units. Additionally, irrational irrigation in agricultural practices is another important factor in intensifying soil acidification due to the acceleration of the migration and leaching of base cations. A field experiment using basin irrigation conducted in the eastern Shandong Province of China confirmed that the irrigation water washed rapidly into the subsoil and resulted in a lot of exchangeable calcium, magnesium, and other salt base ions leaching in a short irrigation period (Wang et al. 2010; Qiu & Sun 2021). Similar results were also supported by other studies conducted in China (Fan et al. 2013; Hao et al. 2020). Therefore, water and nitrogen input are crucial for the mitigation and prevention of soil acidification in intensive agriculture.

Drip irrigation is an irrigation method that saves water and fertilizer by allowing water to drip slowly to the roots of plants, either onto the soil surface or directly onto the root zone through emitters at a small operating pressure, resulting in partial wetting of the soil surface (Dasberg & Or 1999; Lamm & Ayars 2007; Elnesr et al. 2018; Li 2020). Compared with other irrigation methods, the potential advantages of drip irrigation include increasing water use efficiency, improving fertilizer application, reducing deep percolation, improving crop yields and quality, and decreasing energy requirements (Lamm & Ayars 2007; Li 2020). These preponderances make drip irrigation possible for reducing soil acidification. Field experiments were conducted by Li et al. (2012) to evaluate the influences of furrow irrigation, subsurface irrigation, and drip irrigation on soil acidification in China. Their results confirmed that drip irrigation was beneficial to the mitigation of soil acidification attributed to a lower concentration of Al³⁺, exchangeable acid, and active acidity in the soil after 13 years of irrigation. Nevertheless, the effects of drip fertigation strategies that dominate water and nitrogen migration and transformation on soil acidification are not well documented, and the spatial distribution of pH in the soil under drip irrigation is still unclear. Thus, the soil column study was planned with objectives (1) to examine the influences of fertigation strategies and water and nitrogen application rates on distributions of pH and nitrogen in the wetted soil volume and (2) to give suggestions for the application of drip irrigation in acidifying soil. Nevertheless, field experiments best approximate the actual production conditions when compared to laboratory experiments. Additionally, the effect of crops on soil acidification must be considered in agricultural practices. Therefore, an additional field experiment with maize (ongoing) to back up the findings of the column experiment (current study) is needed for this study.

The experiment soil was silt loam soil comprised of 29.0 ± 2.4% sand, 9.4 ± 1.5% clay, and 61.6 ± 0.9% silt. It was collected from 15 to 30 cm depth of field. The average background values of NO₃-N, NH₄-N, and pH for the silt loam soil were 35.8 ± 5.1 mg kg⁻¹, 3.6 ± 0.28 mg kg⁻¹, and 6.09 ± 0.25, respectively. The air-dried soil was passed through a 1 mm sieve and packed in the containers with 5 cm increments to obtain a constant bulk density of 1.2 g cm⁻³. The surface (15–30 cm) soil properties that existed at the site prior to the experiment are shown in Table 1.

To obtain a uniform initial soil water distribution, the packed soil in containers was allowed to equilibrate for 24 h. The nitrogen solution was added into the soil through a No. 7 needle connected to a flexible tube pump (YZ-2515, TIANLI Fluid Technology Co., Ltd, China) with a discharge rate from 0.1 to 400 ml min⁻¹ (Figure 1). Urea (CAS [57-13-6], XILONG Scientific Co., Ltd, China) as a nitrogen fertilizer was used in all experiments.

Three variables, including irrigation amount, nitrogen application rate, and fertigation strategies, which may impact the water flow and solute transport, were considered. In total, nine experiments were conducted with irrigation amounts from 4.8 to 12 L, nitrogen application rates varied from 80 to 360 kg ha⁻¹, and fertigation strategies applied from 1/2N–1/2W to 1/2W–1/2N. While 1/2N–1/2W represents the first applying fertilizer solution for 1/2 of the total irrigation time, then water for 1/2 of the total irrigation time is applied (Table 2). In a laboratory experiment conducted by Li et al. (2003) in Beijing, fertigation strategies for nitrogen distribution in soil under drip irrigation were evaluated. Their results indicated that the soil nitrogen distribution was highly dependent on drip fertigation strategies. The variables are presented on the basis of a completely cylindrical system in the article based on the verified assumptions that the 30° wedge-shaped container could represent 1/12 of the complete cylinder, and that the shape of the test device had a negligible impact on the flow patterns (Wen et al. 2016; Qiu et al. 2023). The apparent application rate and apparent irrigation volume applied were obtained by multiplying the actual variables by 12.

In each experiment, three containers were irrigated simultaneously, and the soil samples were collected at different times of 0, 5, and 10 days after the fertigation in three containers. The sampling layout of pH and EC were both 50 mm for radial and vertical intervals, starting 2.5 cm from the emitter and moving outward to the edge of the wetted surface (Figure 1). The soil samples of NO₃-N and NH₄-N were collected immediately below the emitter and at a distance of 15 cm below the emitter. Soil cores were divided into 0–10, 10–20, 20–30, and 30–40 cm depths.

For each soil sample, 5 g of field-moist soil was transferred into 25 ml of deionized water (UPT-I-5T, ULUPURE Water Equipment Co., Ltd, China), thoroughly shaken for 15 min, and stood for 30 min (Bao 2000; Carter & Gregorich 2008). Then the soil pH and EC were measured with a glass electrode (PHB-4 (pH), INESA Scientific Instrument Co., Ltd, China; DDB-303A (EC), INESA Scientific Instrument Co., Ltd, China).

For each soil sample, 5 g of field-moist soil was extracted with 50 ml of 1 mol L⁻¹ KCl (CAS[7447-40-4], XILONG Scientific Co., Ltd, China), and the NO₃-N and NH₄-N contents were determined using a water quality analyzer (DGB-480, INESA Scientific Instrument Co., Ltd, China).

The accuracy of NH₄-N and NO₃-N was verified by calibrating the equipment with a certified reference material (ammonium standard solution and nitrate nitrogen standard solution, respectively). The pH meter was calibrated by two standard buffer solutions at 25 °C: (phthalate) and (borate), both of them traceable to SRM from NIST and PTB. The EC meter was calibrated by conductivity/TDS standard solutions at 25 °C, which is accredited by the CNCA. The precision of the analytical methods was obtained by repeating the samples thrice and expressed as the standard deviation. Furthermore, all the research analyses were subjected to a laboratory control sample for validation and were checked by the quality control charts.

The contents of mineral nitrogen at different soil depths for each treatment are summarized in Table 3. As shown in the table, the content of soil NH₄-N was minimal after fertigation and decreased with the soil depth for each treatment. As an example, the average concentration of soil NH₄-N for A2 treatment was 3.85, 29.93, and 18 mg kg⁻¹ at times of 0, 5, and 10 days after the irrigation ended, respectively. Additionally, a decreasing trend of soil NH₄-N content after first increasing with the extended fertigation interval and reaching the maximum value at 5 days after the irrigation was observed in all treatments. Those may be put down to the urea in the soil profile completely hydrolyzes at 5 days after the fertigation (Xi & Zhou 2003; Qiu et al. 2023). Table 3 also shows that irrigation amount, nitrogen application rate, and fertigation strategies exerted a substantial effect on NH₄-N in the soil. The influences of irrigation amount on soil NH₄-N content showed that the NH₄-N content in soil at a depth of 0–10 cm decreased with irrigation amount, while the opposite phenomenon was found at a depth of 20–40 cm. For example, the soil NH₄-N content at a depth of 20–40 cm for A3 treatment was 174.3 and 114.9% higher than that for A1 and A2 at 5 days after the fertigation ended. These results indicated that an increasing irrigation amount promoted the migration of NH₄-N into deep soil under drip irrigation. Moreover, the NH₄-N content in the soil profile was increased with the nitrogen application rate. As an example, the average soil NH₄-N content at 10 days after irrigation ended for A6 treatment was 51.83 mg kg⁻¹ and higher than that for A4 and A5 by 324.1 and 136.7%, respectively. The result indicated that a greater nitrogen application input led to the accumulation of NH₄-N in the soil and enhanced the risk of soil pH declining. Furthermore, when the fertigation strategy altered, the distributions of NH₄-N in the soil profile were distinct. For instance, the content of soil NH₄-N for A7 and A9 treatments was mainly distributed at a depth of 0–30 cm, while the content of NH₄-N for A8 treatment was distributed at a depth of 0–10 cm soil at 5 and 10 days after the irrigation ended.

Compared with the distribution of soil NH₄-N, the NO₃-N content for each treatment in the soil profile increased with the extended fertigation interval and peaked at 10 days after the fertigation ended. For instance, the average concentration of soil NO₃-N for A4 treatment was 46.83, 85.61, and 148.42 mg kg⁻¹ at periods of 0, 5, and 10 days after irrigation, respectively. The continuous nitrification of NH₄-N in the soil could be the main reason for this result (Xi & Zhou 2003; Qiu et al. 2023). The soil NO₃-N content at a depth of 0–30 cm significantly increased with an irrigation amount. As an example, the concentrations of soil NO₃-N at a depth of 0–30 cm at 10 days after the irrigation ended for A1, A2, and A3 treatment were 277.92, 296.64, and 425.34 mg kg⁻¹, respectively. These results indicated that a greater irrigation amount facilitated the accumulation of NO₃-N in the soil. As shown in the table, the NO₃-N content in the soil profile increased with a nitrogen application rate and accumulated in deep soil that imposed a threat to soil and groundwater under conditions of high irrigation and rainfall. For instance, the concentration of soil NO₃-N at a depth of 20–40 cm at 5 days after the irrigation ended for A6 treatment was 387.91 mg kg⁻¹ and higher than that for A4 and A5 by 242.5 and 113.9%, respectively. Furthermore, the effects of nitrogen strategies on soil NO₃-N content in Table 3 showed that the soil NO₃-N content for A8 treatment was mainly distributed at a depth 0–20 cm, while the soil NO₃-N content at a depth of 0–30 cm for A7 and A9 treatments accounted for the vast majority of NO₃-N content in the soil profile. These results may suggest that the fertigation strategy 1/4W–1/2N–1/4W (A8) was beneficial in reducing nitrogen leaching loss and the risk of soil acidification under equal conditions.

Urea is a polar, highly water soluble, and charge-neutral molecule and moves rapidly with irrigation water (Zhao et al. 2022). The non-ionic urea is weakly absorbed by soil particles through the formation of complexes with soil organic matter (Haynes 1990; Perin et al. 2020), and that objectively enhances the transfer of urea in the soil. Urea applied to the soil reacts with water and the soil urease and is rapidly converted to NH₄-N (Diaz 2021). Compared with urea, the soil particles are liable to absorb NH₄-N, especially in soil with more clay and silt (Wu et al. 2020). Nonetheless, the urea and NH₄-N in the soil are eventually converted to mobile NO₃-N (Qiu et al. 2023).

The various nitrogen fertilizer and water management practices of drip irrigation differentially impact the migration of urea in the soil, which substantially affects the distribution of soil pH and nitrogen. The increase in the surface wetted radius and vertical wetted depth with the water volume applied under drip irrigation, resulted in a migration and accumulation of urea in deep soil, and followed by urea hydrolysis and ammonium nitrification. This could be a reason for the pH decrease in a broad range of the soil profile and NO₃-N accumulation in deep soil for an irrigation amount of 12 L (A3). Similar variation trends of soil nitrogen accumulation and pH decrease were reported in high nitrogen input treatment. As an example, the NO₃-N content at a soil depth of 30–40 cm for A6 (surface drip irrigation) was 359.2% higher than that for A3 (later depth of 10 cm). Moreover, the decreasing magnitude and rate of soil pH were obviously positively correlated with nitrogen input. A field experiment was conducted in China to evaluate the effects of long-term nitrogen fertilization on soil acidification (Yang et al. 2018). Their results demonstrated that a high nitrogen application rate (569 kg ha⁻¹) significantly decreased the soil pH from 3.32 to 3.15 in the 0–40 cm soil depth, while the low nitrogen input (119 kg ha⁻¹) showed non-significant influences on soil pH. Furthermore, the high irrigation and nitrogen inputs can easily transport nitrogen to the deeper soil causing NO₃-N leaching and soil pH further decrease under conditions of high rainfall and frequent irrigation (Li 2020; Qiu & Sun 2021). Additionally, the fertigation strategies also impact the distributions of NO₃-N and pH in the soil; for instance, the content of NO₃-N was uniformly distributed at a depth of 0–20 cm when fertigated through 1/4W–1/2N–1/4W (A8), which avoided a pH decrease in a broad range of the soil profile due to its hard to produce NO₃-N leaching. The abovementioned facts distinctly indicate that minimizing the downward movement of applied fertilizer (urea) based on nitrogen fertilizer and water management practices of drip irrigation is a way to minimize the generation of soil acidification.

At the initial stage of the experiment, quite a few treatments increased the soil pH. This observation has been attributed to the hydrolysis of urea. As shown in the urea hydrolysis chemical reaction, ⁠, hydrogen ions (H⁺) are consumed and hydroxide ions (OH⁻¹) generate, causing the soil pH to rise (Diaz 2021). Field experiments also confirmed that about 29% of urea was converted to ammonium at 3–7 h after fertigation, and its conversion was 64–77%, 80%, and 100% at 24 h, 48 h, and 5 days after the fertigation, respectively (Nkrumah et al. 1989; Qiu et al. 2023). Nonetheless, the urea residence time in the soil will also depend upon the soil texture, moisture, pH, temperature, microorganisms, and urease activity (Perin et al. 2020). For instance, soil with large amounts of clay and organic matter and low pH in the wetted soil volume may well increase the detention time of urea since the optimum edaphic condition for urea hydrolysis is sandy soil with low organic matter and high pH (Diaz 2021).

Urea hydrolysis is converted to ammonium, which may be the principal reason for the great content of ammonium in the soil for each treatment 5 days after the fertigation ended. Subsequently, the nitrification of ammonium in the soil led to a substantial decrease in pH (Raza et al. 2020). However, the nitrification rate of NH₄-N is governed by the activity of nitrobacteria in the soil as well as by soil temperature and moisture content (Haynes 1990; Yang et al. 2018). Field experiments confirmed that the increase in water content in the soil led to a decrease in soil aeration after fertigation, which restrained the nitrification of NH₄-N and increased the residence period of ammonium in the soil (Haynes 1990; Perin et al. 2020). These results could have been the main reason for the pH in the soil profile for the given irrigation amount 12 L treatment (A3, A6) did not distinctly reduce at 10 days after the irrigation ended when compared with that prior to fertigation. However, the high concentrations of ammonium and nitrate in the soil profile mean that the soil pH may further decrease in the future.

Leaching loss of NO₃-N from ammonium nitrification is another important reason for increasing soil acidification (Hao et al. 2020; Raza et al. 2020). As shown in the nitrification reaction of ammonium, ⁠, one mole of ammonium nitrifies to generate two moles of hydrogen ions and one mole of nitrate. If all nitration products leached out, the two moles of hydrogen ions would completely contribute to soil acidification. For all treatments in the experiment, the average concentration of NO₃-N in the soil profile peaked 10 days after the fertigation ended. In the case of heavy rainfall and frequent irrigation at this moment, the NO₃-N would leach into deep soil and lead to a great decrease in soil pH (Qiu et al. 2017; Li 2020). Furthermore, the deep soil acidification would be hard to relieve by applying lime attributed to its poor mobility in soil when compared with topsoil. Therefore, the suitable nitrogen fertilizer and water management practices of drip irrigation to reduce nitrogen distribution in deep soil are a way to decrease underground soil acidification. For a given irrigation amount of 12 L, a nitrogen application rate of 80 kg ha⁻¹, an emitter discharge rate of 3.2 L h⁻¹, and a fertigation strategy of 1/4W–1/2N–1/4W, the soil NO₃-N concentrations at 10 days after the irrigation ended for A4 treatment (lateral depth 0 cm) were higher than that for A3 treatment (lateral depth 10 cm) by 72, 3, 21, and 7% at depths of 0–10, 10–20, 20–30, and 30–40 cm, respectively. These results documented that a lateral depth of 10 cm can be conducive to decrease NO₃-N leaching and deep soil acidification under equivalent irrigation conditions.

The results of the present study have shown that drip fertigation strategies play a crucial role in the distribution of pH and nitrogen in the soil. An increase in water input resulted in an increase in the area of pH decrease in the soil 10 days after the irrigation ended. The nitrogen content in the soil was significantly increased with the nitrogen application rate. The strategy of 1/4W–1/2N–1/4W made a minimal soil pH decreasing area and a homogeneous nitrate distribution at 0–20 cm depth. Prior to its hydrolysis to NH₄-N, fertigated urea can move down the soil profile in the mobile urea form and subsurface soil acidification can be ensured. Furthermore, the NH₄-N nitration and NO₃-N leaching have been the main reasons for the soil pH decrease in the laboratory experiment. To reduce NO₃-N leaching and avoid deep soil acidification, minor water and nitrogen inputs with shallow lateral depth through the 1/4W–1/2N–1/4W fertigation were recommended practices. To further understand the influence of drip irrigation on soil acidification, a field experiment with maize is conducted to confirm and improve the results of this soil column experiment.

Z.Q. conceived the idea, designed the study, and drafted the manuscript. M.S., Y.L., R.L., and W.L. collected and analyzed the data. All the authors read and approved the final manuscript for publication.

This study was financially supported by the Hunan Provincial Natural Science Foundation of China (grant no. 2020JJ5022) and the Excellent Youth Funding of Hunan Provincial Education Department (grant no. 22B0781).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.