Nitrogen (N) is the essential macro-nutrient for plants and is thereby added to agricultural soil to enhance crop production. Soil contains nitrogen in the form of organic and inorganic compounds which are usually in minute amounts with respect to crop demand, thus the application of N fertilizers is considered as a key factor to increase crop yield throughout the world. Globally, N is added in larger quantities, so more N losses transpire to the environment in the form of nitrate leaching (NL) and nitrous oxide emission (Cameron et al., 2013; Yousaf et al., 2016). Nitrate and ammonium ions are the two major forms of N that are consumed by the plants and the preference of one form over another depends upon plant species and environmental factors. These two inorganic pools describe the major portion of plant-available nitrogen (PAN) in soil for crop production and contribute to nitrogen losses (Britto and Kronzucker, 2013; Shah et al., 2017).

Every soil contains nitrate and ammonium concentrations that are the product of soil mineralization of organic matter and applied N fertilizers (Zhang et al., 2013). The mineralization rate in agricultural soil is directly linked with soil pH, temperature, and soil electrical conductivity (EC) (Curtin et al., 1998; Yan et al., 2015; Liu et al., 2016; Gubry-Rangin et al., 2017). Soil nitrate and ammonium are present mostly in lower amounts, hence they are supplemented as N fertilizer and their residence time in the soil root zone mainly depends on the soil texture (Bimüller et al., 2014). Furthermore, N addition more than plant demands may build a peak concentration layer of inorganic N in the soil profile and depends on the fertilizer application and move down with time along with water (Wang et al., 2014; Dai et al., 2015; Van Meter et al., 2016). The gradual leaching of residual nitrate into the layers below the root zone is a major N loss in cropping systems (Chen et al., 2014; Li et al., 2016). Total organic N and inorganic N present in the different horizons of soil from the topsoil to bedrock is a function of depth from the upper surface, time, drainage quantity, and quality passing through the soil profile (Johnsson et al., 1987; Negm et al., 2014). After N fertilization, ammonium ions in upper soil layers increase up to a threshold and then start to reduce with an associated increase in nitrate concentration (Kraft et al., 2014). According to Sullivan et al. (2014), the amount of biologically fixed atmospheric nitrogen added to soil is smaller as compared to the amount of fertilizer added for a specific period of time. Therefore, fertilizer addition is the real cause of NL in many regions of the world.

Nitrate is highly soluble in water and does not bind well with the soil particles because of its negative charge, similar to clay particles. Thus, water movement carries nitrate from the soil medium to surface water (runoff) or groundwater via NL (Zhou and Butterbach-Bahl, 2014). Therefore, nitrate is the most important ion related to groundwater pollution due to its high solubility in water and potential for NL (Basso et al., 2013). Higher N application favors the higher mineralization with an ultimate abundance of nitrate ions in the soil causes higher nitrate accumulation in the different parts of the plant body, particularly in vegetables (Colla et al., 2018). Subsequently, this connection may generate peroxynitrite (ONOO^–) through nitrate reductase in portions of the plant, which is highly toxic to human beings and animals (Islam et al., 2015; Schwertz et al., 2016).

Riley and Decker (2000) found significant variations in the soil nitrate and ammonium levels in the soil profile from 0 to 60 cm under the different types of fertilizers during the wheat cropping season. In addition, the most commonly used fertilizer rate of 250 kg N/ha with irrigation had the potential to raise the nitrate level in soil layers and subsequently cause NL to the deeper layers in the soil profile. Moreover, they concluded that the most commonly used fertilization practices (irrigation and 250 kg N/ha) had the potential to increase the N accumulation in soil layers as well as NL. Zhongjiang et al. (2014) found a significant decrease in ammonium levels from the top 10–15 cm soil with an increase in the nitrate level under different fertilization. Similarly, Yang et al. (2013) studied dry land soil in Shanxi, China and found that the peak nitrate concentration layer is developed at 80 cm in soil profile after longtime straw application. In fact, PAN accumulation in the soil profile below the root zone is the main cause of groundwater nitrate contamination in the Chinese semi-humid croplands because heavy rainfall in the monsoon season or irrigation water moves mineral N deeper in soil layers (Yang et al., 2015; Zhou et al., 2016). A recent study on higher nitrate concentration in the filtered groundwater of Shaanxi and Shandong provinces showed the adverse effects on the resident population (Zhai et al., 2017).

Increased food demand increased chemical fertilization throughout the world, especially in the large populated Asian countries, such as China and India (Liu et al., 2014). According to the results of the Rapid Diagnostic Appraisal survey, vegetables in the Shunyi District were heavily fertilized approximately 2,250 kg N/ha (150 kg N/μ) of organic manure and sometimes combined use with potassium fertilizers (Quan et al., 2015). Poultry industries have been increased multi-fold times since 1980, especially by small farmers as the family-run farming system in China (Fan et al., 2010). These farmers are incorporating the poultry manures abundantly without being concerned about the crop requirements, resulting in the accumulation of inorganic N in the soil profile and ultimately causing to increase NL (Lu et al., 2012; Zheng et al., 2017). Different types of seedbeds are prepared in the crop rotation system, such as a row cropping to cultivate the vegetable and the hill drop or broadcasting for cereal crops. These cultivation practices may leave few places fallow in the agricultural fields. In a similar way, fields are left fallow without crop cultivation in the crop rotation to maintain soil fertility or for any other purpose. These fallow soils in agricultural land have higher N mineralization and nitrate in the upper soil layers. Eventually, this increase in the nitrate level contributes to building a peak concentration deeper in sub-soil or causes to contaminate the groundwater. During seedbed preparation, a bare land section may construct when cereal cropping fields are prepared to grow row cropping vegetables. Similarly, during crop rotation seasons without plantation and fertilization (fallow period) are adjusted to maintain fertility and for any other purpose. According to Chen et al. (2016), bare land may also contribute to significant NL in the agricultural soil in China because of the previous fertilization and rainfall effects (Zheng et al., 2017).

This study was planned to analyze the variations in the PAN with 25–27%, pH, and EC in the soil profile under different fertilization during and after the cropping season of Chinese cabbage (Brassica rapa subsp. pekinensis) in the experimental fields. The variations in soil parameters were observed with respect to time (cropping season) and layers (0–100 cm) under fertilizer treatments including chicken manure, potassium nitrate and urea, CMP, KNP, and URP, non-fertilization with crop cultivation, and bare land without cropping. Moreover, the comparative yield analyses were also made among the treatments in Shunyi District, China.

A cabbage root zone was spread over 100 cm of soil and rapidly decreased with the increasing soil depth that was able to effectively absorb the part of soil nutrients in the root zone. As a result, we selected soil depths from 0 to 40 cm in this study. The results showed that the soil nitrogen content increased with an increasing soil depth from 0 to 30 cm in 25 days after planting. In extremely well-drained soils, the content was mainly distributed in the topsoil (0-20 cm) 43 days after planting, and the soil nitrogen content was reached to 71.98 mg kg^–1. The same result was obtained when the soil nitrogen concentration was reduced from 30 cm. Most parts of the soil nitrogen content remained in the topsoil (0–50 cm), which was associated with several root parameters (Yousaf et al., 2016). Soil pH temporal variation in a particular cropping system depends upon the plant species or crop grown (Curtin and Trolove, 2013), nitrogen fertilization source (Bird and Rapport, 1986), and the buffering capacity of the soil (Min et al., 2016). In many regions of the world, it was observed that NL after inorganic N fertilizers may cause considerable acidification in the soil such as in Atlantic Canada, 74% acidic effects came from N fertilizers and 26% from acidic rain (Kemmitt et al., 2008). An increase in the nitrate ions may decrease pH in a soil profile exponentially. Hence, nitrate accumulation in the soil profile is a substantial factor affecting soil acidification (Duruigbo et al., 2007). Soil pH is the main regulating factor of soil microbial activities and the rate of soil carbon and N cycling that may be disturbed by urea acidification (Eigenberg et al., 2002). In this study, the same results were found in the urea plot as pH declined in the first month and thereafter, returned to the initial values. In previous results, the poultry manure has the ability to increase the pH level of soil when it was treated as 15 ton/ha (300 kg N/ha with 2% N) (Ruter, 1992).

Statistically, overall temporal EC variations in the soil profile before, during, and after plantation and fertilization were observed significantly in all plots but the maximum increase was observed in KNP. Potassium nitrate is considered to be the salt fertilizer and has the ability to change the soil EC (Djurhuus and Olsen, 1997). Thus, significant variation in soil EC might be occurred due to the potassium nitrate fertilization. Organic and inorganic fertilization in agricultural soil raises its EC (Sims et al., 1995; Sainju et al., 2019) which might be the reason for high EC in KNP, URP, and CMP as compared to CTP and NVP.

The nitrate levels in the first 2 months were increasing in NVP and thereafter declined to initial values in the soil profile which might be due to the increasing mineralization of bare tilled soil. The same results were noted by Sainju et al. (2019) in Piarco Series Soil (West Indies) while analyzing the temporal differences between nitrate concentrations in the soil profile from 0 to 90 cm in the early and late rainfall seasons. Nitrate in CTP increased in the first month but declined up to the harvest period, such as the lowest concentrations were measured in all layers of the soil profile at the end of the cropping season. It might be happened due to the plant growth which might increase the nitrate uptake from the root zone as well as decreased the total N budget and mineralization. It was recorded that nitrate concentration was increased in all the layers of soil profile as compared with the initial values in CMP, particularly after 2 months of chicken manure application. It was observed that chicken manure contained a high rate of available N and a larger fraction in organic form which needed further mineralization to release ammonium or/and nitrate ions. The time rate of poultry manure mineralization was not fixed, such as Azeez and Van Averbeke (2010) found that 48% of the organic N in poultry manure was released within 10 weeks during the laboratory experiment, whereas Bitzer and Sims (1988) reported 40% during 18 weeks. Fan et al. (2003) reported maximum nitrate available in the soil about 10–15 weeks after broiler manure application (Rong and Xuefeng, 2011). In this study, higher nitrate concentration after cropping season might be happened due to the slow mineralization rate. In KNP soil layers from 0 to 60 cm showed that the nitrate level increased to a certain limit in the first 2 months and thereafter decreased up to nearly initial concentration whereas, in soil layers 60–100 cm, the nitrate level increased up to the end of the cropping season. Few researchers reported that the balance amount of potassium in the soil might withhold nitrate ions, increase nitrate uptake in the plant body, and decrease NL (Gabriel et al., 2012; Zhou et al., 2016). Sainju et al. (2019) worked out the adsorption effects of potassium ions with nitrate ions and found minimum adsorption in the soil surface layers from 0 to 15 cm as compared with subsoil layers. Furthermore, subsoil layers from 0 to 90 cm showed maximum adsorption effects as withholding the maximum amount of nitrate in the field experiment. Therefore, it might be the reason why subsoil layers from 60 to 100 cm presented nitrate in raising trend up to the harvesting time. In soil profile from 0 to 40 cm in URP treatment, the nitrate level increased after the first 2 months and turned back to the initial level thereafter, in the soil layers from 40 to 60 cm inclined up to the harvesting period as shown in Figure 5. Rong and Xuefeng (2011) affirmed in their experiment that urea fertilization had a positive relation with nitrate accumulation in soil profiles from 0 to 100 cm. In this study, nitrate incremented in soil layers of 0–40 cm and lasting up to 2 months, was previously found, such as Sainju et al. (2019) reported non-significant variation in soil profile within 4 weeks subsequent to urea fertilization but thereafter, found a significant increase in the nitrate level.

It was obvious from the previous studies that the maximum mineralization of organic matters and the nitrification of residual nitrogen occurred in the fallow period or bare soil without vegetation after harvesting (Bishop and Manning, 2010), which might be the cause of ammonium level raised in CTP and NVP at t₁ in the soil profile. In Shunyi District, clay level was found in increasing order with the soil depth as presented in Table 4, which might be constructed the plow pan in the subsoil and raised the ammonium level in lower layers. The temporal variations in ammonium levels before and after cropping season in CTP showed that the final values were three times lesser than the initial values in the 0–60 cm soil profile which might be happened due to lower soil mineralization rate as compared with ammonium utilization (nitrate conversion, volatilization, and plant uptake). In KNP, it was recorded that the ammonium level was decreased in 0–60 cm soil layers before and after cropping season. Meanwhile, a little increment was noted in 60–100 cm soil layers that might be due to the potassium nitrate which promotes the nitrate concentration in soil, not ammonium. Thus, the overall ammonium in soil decreased during the cropping season due to the plant consumption and utilization as nitrate conversion and volatilization. Soil layers 0–60 cm in CMP made available higher ammonium concentration throughout the cropping season as compared with initial values (before fertilization) whereas, soil layers from 60 to 100 cm showed negligible variation. Similar results were observed in the study of Perego et al. (2012). Temporal increment in ammonium level in URP treatment was disappeared in soil profile from 0 to 60 cm but lasted in 60–100 cm after harvesting. As urea addition might be the cause of significantly increased ammonium (Xiong et al., 2012) up to the maximum limit within 4 weeks after fertilization in the presence of moisture, whereas subsequent rainfall or irrigation might drag down the urea concentration in the soil profile (Gai et al., 2016). In this study, it was observed that the total PAN in the soil profile (0–100 cm) increased to maximum level after 1 month (September) of fertilization, not only in fertilized plot but also in without fertilized (NVP and CTP) plots which might be due to the heavy rainfall occurred during this month as shown in Figure 1. It was proved that heavy rainfall events increased nitrate and ammonium in the soil profile (0–100 cm) (Zhang et al., 2013).

It was observed that the maximum Chinese cabbage yield was produced in fertilized plots than control plots but significant variations were also observed among the fertilizer treated plots as well. Maximum yield was observed in URP than KNP but both have significantly higher values as compared to CMP. It might be possible that because poultry manure had the slow N release at the time of high plant requirement as previously reported in a similar study (Li et al., 2021).

Chinese cabbage produced maximum yields in fertilizer treated plot because of optimum PAN level during the cropping season. Chinese cabbage had shallow root depth, required frequent irrigation, and optimum PAN in the upper soil layer 0–40 cm for optimum production. Therefore, total PAN concentration and PAN lasting time in a root zone (0–30 cm) are important while selecting fertilizer type and crop type. In our experiment, the variations in soil pH, EC, nitrate, ammonia, and PAN in soil profile 0–60 cm with respect to initial values for all treatments were larger as compared to variations in soil profile 60–100 cm. PAN level as ammonium was found maximum in CMP as compared to the other treatments. CMP showed a higher ammonium concentration in the upper soil layer during the cropping season and increased the potential use of PAN and decreased the potential NL as compared to KNP and URP. Bare land without plantation also caused to increase in PAN concentration in lower soil profile 0–60 cm. Higher PAN levels as compared to initial values in soil profile 60–100 cm showed the N losses occurred after all treatments and had the potential to contaminate the groundwater resources except for the CTP. Furthermore, Chinese cabbage productions without fertilization significantly dragged down the PAN level in soil layers 0–60 cm as well as reduced crop yield. For a single cropping season, pH variations in the soil profile of 0–100 cm were significant during and after cropping season with respect to time and treatments, although these were very minute as compared to the initial values. Similar results were observed for EC as well. Overall, the soil EC and pH values were in the decreasing trend for KNP and URP, whereas in increasing trend in NVP, CMP, and CTP. Therefore, a single cropping season may vary notable differences in nitrate, ammonium, and PAN levels in the soil (Shunyi District, Beijing, China) profile with smaller differences in soil pH and EC.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

RA designed the experiment methodology and wrote the manuscript. YL was the supervisor and project administrator, assisted to correct the methodology, and validate the design. JD, WL, and SA supported data curation and laboratory work. LM helped in reviewing and editing. RA made the software analysis with the assistance of LM, SA, and WL. WA and AA have done the data analysis and curated and helped in manuscript writing. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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