Soil Nutrient Balance and Soil Fertility Status under the Influence of Fertilization in Maize-Wheat Cropping System in Nepal

Soil nutrient balance is affected by nutrient management in crops. A poor nutrient management technique results in an imbalance in the soil nutrient status which could have a long-term negative impact on crop production. The current study was carried out to assess the effect of different rates of nitrogen (N), phosphorus (P), and potassium (K) on soil nutrient balance in a maize-wheat cropping system in Cambisols of Khumaltar, Lalitpur, Nepal during 2019/20 and 2020/21. The experiment included three-factor randomized complete block design with three doses of each N, P, and K which was replicated three times. There was a remarkable change in soil pH, soil organic carbon (SOC), and total N, P, and K contents of soil over a period of time with the application of different doses of NPK. Soil pH changed from 5.98 to 5.53, SOC increased from 11.7 to 16.8g · kg − 1 , total N decreased from 1264 to 1177 mg · kg − 1, available P 2 O 5 declined from 214 to 63.6mg · kg − 1, and available K 2 O decreased from 71.7 to 24.8mg · kg − 1 with varying rates of NPK after four cropping seasons. Furthermore, partial, apparent, and net N, P, K balance were predominantly negative in all the fertilizer treatments, but the magnitude was lower under higher nutrient rates and positive partial N balance was noticed in higher N levels. The depletion of native P and K pools even at higher application rates was attributed to higher crop removal over time as compared to inputs. Therefore, continuous application of balanced fertilizers is crucial in maintaining the fertility of soil and productivity of crops.


Introduction
Maize (Zea mays)-wheat (Triticum aestivum) is the second most important cropping system in Nepal after rice-wheat and the most prominent cropping system under the midhills of Nepal. Crop response to nutrients varies greatly from location to location, depending on soil fertility and environmental factors. If fertilizer management is not adequately addressed, a continuous maize-wheat cropping system can deplete soil fertility. Chemical fertilizers are necessary inputs in modern crop production to meet the crop's nutrient demand. ese are important parts of modern crop production systems [1], with chemical fertilizer use contributing to almost half of the global crop production [2]. Chemical fertilizers are well known for their contribution to increased agricultural production in farming systems. Several studies have shown that commercial fertilizers can increase 30-50 percent crop production [3].
In Nepal, there is limited land, the proportion of cultivated land to population is low, and there is a lot of pressure to raise crop yields to ful ll the demands of a growing population; fertilizers are increasingly being applied to crops to improve their quality and yield. Changes in soil fertility owing to imbalanced fertilization may be identi ed as one of the key factors limiting crop yields under continuous and intensive cropping. In intensive cropping systems, the production of improved and hybrid varieties of crops increases fertilizer requirement [4]. Continuous cropping and long-term fertilization are likely to in uence the properties of soil and crop yield depending on the type of crop and nutrient management techniques. is creates high nutrient demand and a negative nutrient balance if the mined nutrients are not substituted with the application of fertilizers. Chemical fertilizer inputs are essential to maintain a positive nutrient balance by replacing nutrients that are withdrawn and depleted during cropping [5].
Having a quantitative knowledge of soil nutrient depletion may aid in the application of optimal nutrient management measures. A negative balance in the soil can be minimized by applying the appropriate rate of fertilizer according to the crop's demand. Increasing the application of nutrients (NPK) using inorganic fertilizers increased the available nitrogen, phosphorus, and potassium contents in soil for wheat [6]. e use of NPK fertilizers is critical for restoring soil nutrients and closing the yield gap [7]. e drop in soil fertility in the hills is increased by a decrease in organic matter content, with nitrogen being the most important nutrient in Nepalese soils [8]. Similarly, applying an inadequate amount of K fertilizer over multiple years may lead to K deficit and reduction in crop yield [9].
In Nepal, current fertilizer recommendations for various crops are generalized and do not consider other aspects such as soil, climate, season, or geography, all of which needed revision. So far, there was little information about the effects of fertilizer application on nutrient balance in the maizewheat cropping system in the mid-hills of Nepal. Multiple nutrient deficiencies caused by the overmining of nutrients from the soil and an imbalance in fertilizer application are causing diminishing yield trends in the maize-wheat system. erefore, the current study was carried out to see how different rates of N, P, and K affected soil nutrient balance and to quantify changes in the status of soil nutrients, depletion, and accumulation in the soil after four successive cropping sequences.

Location and Characteristics of the Experiment Site.
Field experiments were conducted in Agronomy Farm, Khumaltar, while laboratory work was carried out at National Soil Science Research Centre, Khumaltar Lalitpur, for two years in 2019/20 and 2020/21. e location is situated at 27°39′ N Latitude and 85°19′E longitudes with an elevation of 1285 m from the mean sea level in the mid-hills valley of the Bagmati province of Nepal. e experimental site was located in the sub-tropical climatic belt of Nepal. In general, the site received ample rainfall during the rainy season, which started in May and continued up to September. e total annual rainfall received was 324.4 mm and 1304.1 mm in the experiment years 2019/20 and 2020/21, respectively. e area enjoyed weather conditions with cold winters, mild summers, and distinct rainy seasons. e minimum temperature was up to 0°C during the coldest month (December-March) and the range of minimum temperature was 3.5 to 21.3°C. e maximum winter temperature was 21.2°C in February 2021. In the hottest months of the year, the highest mean maximum temperature was 29.8°C in June 2019. Monthly average data on different weather parameters, i.e., maximum and minimum temperatures, and total rainfall, recorded during the maize and wheat growing seasons at National Agronomy Research Centre, Khumaltar, Lalitpur are presented in Figure 1. e soil of the experimental field had slightly acidic pH, low organic matter, medium total N, high available P 2 O 5 , and medium available K 2 O as rated using the soil value chart [10]. e soil texture was silty clay loam, with a bulk density of 1.39 gm·cm −3 . e detailed status of the macro and micronutrients of the soil before the field experiment is presented in Table 1.

Experimental Design and Treatments.
e experiment was conducted for four consecutive cropping periods under the maize-wheat cropping system. ere were 27 treatments consisting of N, P, and K levels that were laid out in a threefactorial randomized complete block design (RCBD) with three replications in a fixed plot of 10.5 m 2 (4.2 m × 2.5 m). e treatments' details for maize and wheat are presented in Table 2.

Crop Management.
e hybrid maize variety Khumal Hybrid-2 was sown on 9 May 2019 and 15 May 2020 with a planting geometry of 60 cm × 25 cm as summer crop. Similarly, the wheat variety Surma was sown with continuous sowing in a row spacing of 25 cm at a recommended seed rate of 120 kg/ha on 14 November 2019 and 10 November 2020 in well-prepared plots as winter crop. e fertilizers applied to supply nitrogen, phosphorus, and potassium were urea (46% N), single superphosphate (16% P 2 O 5 ), and muriate of potash (60% K 2 O), respectively. A whole amount of phosphorous and potassium was applied as basal at the time of final land preparation while nitrogen was applied in three equal splits. For maize, nitrogen was applied at the planting, knee height, and tasseling stages while for wheat, it was applied at the planting, tillering, and spike initiation stages. Hoeing was done twice in the maize season at the knee height and tasseling stages. Furthermore, during different stages of maize and wheat growth, all essential agronomic practices were carried out equitably and properly. Irrigation, weed control, pest control, and other cultural operations were performed as needed. Maize was manually harvested on September 22, 2019, and September 28, 2020, while wheat was harvested on May 12, 2020, and May 17, 2021.

Plant Sampling and Analysis.
Five soil samples were collected at a depth of 0-20 cm from each plot after the harvest of each crop and these samples were analyzed for pH, organic carbon, total nitrogen, and available P and K contents. e details of the methods applied are presented in Table 1. After crop maturity, plant samples (straw and grain) from each plot were collected and cleaned to remove the adherent soil and dust. e stem and leaf samples from each plot were combined to produce a composite straw sample, and the seeds sample from the net plot harvest was taken randomly. Both grain and straw samples were oven-dried at 70°C until constant weight and grinded and sieved through a 0.2 mm sieve for total N, P, and K concentrations' estimation. Total N was analyzed by Kjeldahl's digestion-distillation method [21], total P was determined by the Vanado-Molybdo-phosphoric acid yellow colour method [13], and total K content of the plant samples was estimated with the help of ame photometer from the digested extract prepared for P [22]. e total nutrient uptake was calculated using the formulae: Nutrient Grainuptake Yield grain × Nutrient content grain , Nutrient strawuptake Yield straw × Nutrient content straw , Nutrient totaluptake Nutrient Grainuptake + Nutrient strawuptake . (1)

Calculation of Soil Nutrient
Balance. Nutrient balance was assessed using three ways: partial nutrient balance (PNB), apparent gain or loss of nutrients (ANB), and net gain or loss of nutrients (NNB). Partial nutrient balance is the total nutrient (NPK) pro t or loss amount in the soil of each treatment every year.
Nutrient input (N I ) is the amount of nutrients applied from the fertilizer and nutrient output (N O ) is the total amount of nutrients harvested from the maize and wheat crops [23].
where F N � final status of nutrients in soil, I N � initial status of nutrients in soil, N I � Nutrient input through fertilizers, and N O � Nutrient output through above-ground plant uptake.
2.6. Data Analysis. All the data were subjected to statistical analysis of variance using GenStat. Various parameters were analyzed at p < 0.05 level using the LSD (least significant difference) test as described for factorial randomized block design [25]. For mean separation, data were analyzed using Duncan's Multiple Range Test (DMRT).

Crop Yield and Nutrient Uptake by Crops.
Maize and wheat grain yields were significantly affected by the N and K rates, whereas P rates had no significant effect in all four seasons ( Figure 2). Yields were high at higher rates compared to the lower fertilizer rates. e two-year data revealed that N@210 kg·ha −1 and K 2 O@80 kg·ha −1 were significant for grain yield of maize, whereas N@125 kg·ha −1 and K 2 O@ 50 kg·ha −1 were found considerable for grain yield of wheat, which was at par when the levels increased further. However, there was no considerable effect of P beyond 40 and 25 kg·ha −1 in maize and wheat, respectively, in both the years. In the second year, mean grain yield was very low, especially for maize. e uptake of nutrients by crops was also affected by different nutrient doses (Tables 3, 4, and 5). With an increase in nutritional supply, the uptake of nutrients was increased. Significantly higher uptake of N, P, and K was obtained with the higher rates of N, P, and K applied. e total uptake of N, P, and K from the application of their higher rates was 31.9, 13.8, and 19.0 percent higher than the lower rates.

Nitrogen Balance.
e annual partial nitrogen balance (PNB), apparent nitrogen loss or gain (ANB), and net gain or loss (NNB) for all soil treatments in the maizewheat cropping system are shown (Table 3). e results showed that partial nitrogen balance was significantly affected by the NPK rates, whereas ANB and NNB were found to be nonsignificantly affected by different fertilizer rates. With the increase in N levels, PNB was increased, whereas higher P and K inputs indicate N deficit to the soil. Crops remove a considerable portion of the nitrogen from the soil as a result of the large volume of grain and straw removed for consumption. PNB showed that 16.3 kg·N/ha/ year was added to the soil with the application of 210 kg·N for maize and 150 kg·N for wheat per hectare which was higher as compared to other lower N rates. e partial N deficit was higher (19.5 kg·N/ha/year) with the application of higher levels of K and vice-versa. Similarly, the results showed that ANB and NNB in the soil remained negative for all the treatments, though ANB and NNB in the soil were higher with higher levels of nitrogen and lower levels of P and K.

Phosphorus Balance.
e annual partial P balance (PPB), apparent P loss or gain (APB), and net gain or loss (NPB) for all soil treatments in the maize-wheat cropping system are shown (Table 4). e data revealed that PPB and APB were considerably affected by the NPK rates, while NPB was significantly influenced by P rates but N and K rates did not vary significantly. Furthermore, the results showed that PPB, APB, and NNB in the soil remained negative for all the treatments. With the increase in P levels, PNB and NPB increased and APB decreased, whereas higher N and K inputs indicate more P deficit to the soil. e APB, APB, and NPB ranged from −19.4 to −88.2 kg P 2 O 5 /ha/year, −97.9 to −159.2 kg P 2 O 5 /ha/year, and −177.6 to −186.2 kg P 2 O 5 /ha/ year, respectively. e partial and net P deficit were higher with the application of lower rates of P and were lower with higher rates of P.

Potassium Balance.
e annual partial K balance (PKB), apparent K loss or gain (AKB), and net gain or loss (NKB) for NPK rates in the maize-wheat cropping system are shown in Table 5. e data showed that PKB was significantly influenced by the NPK rates, while AKB was remarkably affected by NK rates but it was not considerably influenced by P rates. Similarly, the effect of NPK rates on NKB was not notable. Furthermore, the results revealed that PPB and NNB in the soil remained negative for all the treatments. With higher use of N and P levels, K deficit to the soil increased, whereas higher K inputs indicate decrement in K deficit to the soil. Likewise, after four seasons of successive cropping, annual PKB in the soil ranged from −157.7 kg·K 2 O/ha in lower K rates to −110.0 kg·K 2 O/ha in higher K rates. A positive balance in apparent K was obtained in the soil under all the fertilizer application rates with the maximum gain in higher N and K rates, whereas a negative balance in net K was obtained in the soil for the applied treatments.

Status of the Soil after Harvesting
Although there was a slight decrement in soil pH in the first year (Table 6), there was no significant variation due to the different levels of fertilizers. However, in the second year, after maize cropping, there was a considerable effect on phosphorus application rates, whereas there was a significant effect of different rates of nitrogen on soil pH after the second-year experiment. Application of chemical fertilizer resulted in a reduction in soil pH with more decline seen under urea application. Despite some variations in all treatments, the pH values indicated a clear downward trend over time. On average, the soil pH decreased from 5.98 to 5.53 after four successive chemical fertilizer applications in the soil.

Soil Organic Carbon.
e fertilizer application rates had no signi cant e ects on the soil organic carbon content (Table 6). However, the result showed that the organic carbon content of the soil was slightly increased as a result of continuous cropping and fertilizer use over years. e OC content was more after harvesting the wheat crop than after  harvesting maize. Organic carbon content varied from 11.2 g/kg after the first maize crop to 16.8 g/kg after harvesting the final wheat crop.

Total Nitrogen.
e application of fertilizer rates had a nonsignificant effect on the total nitrogen content of the soil in the first maize experiment (Table 6). In the second crop, the total N content was considerably affected by the N levels but the influence of P and K was found to be nonremarkable. Significantly higher content of N was found from the application of higher nitrogen rates (N 3 ). Likewise, there was a nonsignificant effect of different rates of N, P, and K in soil nitrogen content in the third and fourth seasons, though there was slight increment in the soil's total nitrogen with increased nitrogen levels. e total nitrogen content varied from 1145 mg/kg after the first maize crop harvest to 1261 mg/kg after harvesting the second maize crop. At the end of the experiment, the total nitrogen content in the soil declined to 1177 mg/kg. e data revealed that higher the rates of P and K, the lower the nitrogen content in the soil. e interaction of N, P, and K is shown in Table 6.

Available
Phosphorus. e effect of application of N, P, and K at different rates was nonsignificant on the available phosphorus at harvest of the first maize crop (Table 6). However, P rates had a considerable effect on the available P at the end of the first-year experiment in which higher available P content was observed from the application of 75 kg P 2 O 5 per ha. Similar to the second crop, after the harvest of the third (maize) and fourth (wheat) crops, the available P content was significantly higher from the application of 75 kg P 2 O 5 per ha, but the effect of N and K application rates was nonsignificant. e data revealed that higher the rates of N and K, lower the P content in the soil. Over the years, a declining trend of P was observed in almost all the treatments in comparison to its initial status.
ere was a heavy reduction in the P content of soil during four successive cropping seasons even after fertilization through SSP.

Available
Potassium. Different levels of nitrogen did not significantly affect the available potassium content in the first years of experiment, whereas the residual soil K was considerably affected by the N levels in the second year of the experiment (Table 6). e data revealed that higher the rates of N, lower the available K content in the soil. However, the application of P rates at different rates had a nonsignificant effect on the available K at harvest of each crop during the two years. Likewise, K application rates had a nonremarkable effect on the K content of soil after the harvest of each maize crop, whereas K rates had a considerable effect on the available K after the harvest of each wheat crop during  Fa � fertilizer applied in 2 years, M1 � maize crop K uptake in 1 st year, W1 � wheat crop K uptake in 1 st year, M2 � maize crop K uptake in 2 nd year, M1 � wheat crop K uptake in 2 nd year, PKB � partial potassium balance, AKB � apparent potassium loss or gain, NKB � net potassium loss or gain, and NS � nonsignificant (p > − 0.05); * * * � significant at 0.1%; * * � significant at 1%; and * � significant at 5%, GM � Grand Mean.  the two years. e data showed that in the first year higher available K content was observed from the application of 75 kg P 2 O 5 per ha, whereas the K content was higher from 25 kg P 2 O 5 per ha. Even after fertilizing with potassic fertilizers, the P content of the soil decreased dramatically during four subsequent cropping seasons.

Crop Yield and Nutrient
Uptake. Low indigenous supply of N and K from the soil may be the cause of the significant response of N and K rates. Similarly, the uptake of N and K by high nutrients demanding crops: hybrid maize and improved wheat, were comparatively higher with respect to supply through fertilization, which may be another factor for the response. e results are in agreement with [26] who obtained significant increase in grain yield of wheat with increased N levels and [27] who observed an increase in grain yield of maize with increased N rates. Likewise, increased grain yield of maize was reported with higher K levels [28] and enhanced yield of wheat was found with K fertilization rates [29]. With the progress of time, there was successive decrement in the yield of both maize and wheat crops. Similar to our findings, fertilization and fertilization years had a considerable effect on crop yield [30]. e main reason for the reduced yield in maize in the second year was water stress conditions that occurred due to waterlogging in the maize crop, which reduced its capacity to absorb and uptake nutrients. e other factor for reduced yield and uptake with the progress of time was the mining of nutrients, especially N and K in the soil. ere was high uptake of nutrients with an increase in fertilization rate which may be due to the increased availability of applied fertilizers in addition to the nutrients present. e initial vigorous growth with high photosynthetic rate enhanced the uptake of nutrients in higher fertilization. e findings are in line with [31,32].

Nitrogen
Balance. An increased nitrogen rate from N 1 to N 3 increased the balance of nitrogen. More N under higher doses of N increased the total nitrogen content in the soil which ultimately helped increase the N balance of the soil. Similar findings were obtained by [24,33] who confirmed that more N at higher N levels enhanced the available N in the soil, which worked to enhance the net N balance in the soil. Increased cropping intensity and the introduction of highyielding improved variety of wheat and hybrid maize have resulted in significant N depletion and positive crop response to additional N in the soil which is in line with [34]. Higher uptake of N in biomass of maize and wheat with higher levels of P and K resulted in lower PNB, ANB, and NNB in the soil.

Phosphorus
Balance. With higher use of N and K levels, P deficit to the soil increased, whereas higher P inputs indicate decrement in the P deficit to the soil. More P under higher doses of P increased the available P content in the soil which ultimately helped increase the P balance of the soil. More uptake of P by high-yielding improved wheat and hybrid maize varieties has resulted in significant P depletion and negative P balance in the soil after two years of the maize-wheat cropping system. e negative P balance is obviously due to low P fertilization as compared to the excess uptake by the crops [35].

Potassium Balance.
e partial K deficit was higher with the application of lower rates of K and the deficit was lower with higher rates of P, suggesting less fertility exhaustion under high P rates. Crop K removal considerably exceeds the K applied through fertilizer, resulting in a negative K balance in the soil. Furthermore, intensification of the cropping systems, as well as the adoption of highyielding varieties, has led to soil mining, resulting in K deficit. is findings is in support of the findings of [36] who concluded that crop residues remove around five times the amount of potassium that fertilizers applied to the soil. Similarly, there was a negative K balance in today's intensive and high yield-oriented agriculture, and soils were being mined for this vital element [37].

Soil pH.
At the completion of the experiment, there was a 7.5% decrement in soil pH, with the application of chemical fertilizers which is in line with [38] who observed a decrease in soil pH due to the acidifying effects of nitrogen fertilizers. ere were no noticeable changes in pH between any treatments in the beginning years after fertilization, but gradually the soil pH declined sharply to 5.53 from its initial pH value of 5.98. Similarly, the pH of the soil gradually decreased as a result of continuous cropping and fertilizer use over the years [39]. During the hydrolysis of NH 4 -based N fertilizers, H + ions are released, which may enhance soil acidity [40]. Infertile soils that do not respond well to additional N fertilizer application result from increased soil acidity caused by N fertilizer application [41,42], which may cause the inefficient utilization of fertilizer [43].

Soil Organic Carbon.
e finding showed that using fertilizers can assist in increasing the soil organic carbon content. It was revealed that as fertilizer levels increased, organic carbon content increased over time [44]. Similarly, the application of fertilizers had a remarkable increment in soil organic carbon as compared to the control [45]. is could be due to the fact that the use of higher rates of fertilizers enhanced the increase in crop biomass supporting additional root residues in the soil which consequently increased soil organic matter [46]. Furthermore, the results are in line with the findings of [47,48]. An increase in organic carbon content in treatments receiving inorganic fertilizer can be attributed to a higher contribution of biomass to the soil in the form of crop stubbles and residues over years. A similar conclusion was drawn by [49] who indicated that the use of nitrogenous fertilizer improved organic carbon content in the soil as a result of increased crop biomass and residue returned to the soil. e nitrogen content decreased from 1264.2 mg/kg at initial to 1177 mg/kg at the final harvest after two years. e huge drop in nitrogen content even after the application of higher amounts of nitrogen might be due to higher uptake of nitrogen during the harvest of maize and wheat crops. e overall drop in the available nitrogen content could be attributable to leaching losses of nitrogen under extremely high rainfall conditions, as well as its application schedule not matching crop requirements [38,50]. At harvest, the higher the rates of nitrogen, the higher the nitrogen content in the soil which is in line with [51] who found that adding nitrogen to the wheat crop improved soil N status. In the same way, a considerable impact of nitrogen fertilizer was observed on soil fertility [52]. With increased nitrogen rates, there was a slight increment in soil nitrogen after the harvest of wheat which is in agreement with [53] who found that the soil total nitrogen at 10-20 cm depth increased with an increase in N rates.

Available Phosphorus.
Phosphorus content in soil increased as P levels increased, most likely due to the mobilization of native soil phosphorus, resulting in enhanced P availability. is result is in line with [54] who observed that the amount of available P in the soil samples after harvest increased significantly as the rate of P increased. Similarly, when the amount of P application in the soil increased, so did the amount of available P [55] and the highest residual soil P value of 10.95 mg P·kg −1 was noticed with higher P levels applied [56]. After two years of continuous cropping, the available P value dropped from 213.7 mg/kg at the start of the experiment to 63.6 mg/kg at the end of the experiment. e large decline (70.2%) in P content even after applying P fertilizer in each crop could be associated with greater P uptake during maize and wheat crop harvest.

Available Potassium.
e lower available K content in the soil was seen with higher N rates, which could be attributable to greater K uptake by maize and wheat crops from the soil with higher grain yield and biomass from higher N application. K content in the soil increased as K levels increased in the first year, most likely due to the mobilization of native soil K, resulting in enhanced K availability. However, at the end of the experiment, due to heavy mining of K by continuous cropping resulting in higher K uptake by crops, the K content was lower. e average available K value declined from 71.7 mg/kg at the start of the experiment to 24.8 mg/kg at the end of the experiment after two years of continuous cropping. e heavy drop (65.4%) in K content even after the application of higher amounts of K might be due to higher uptake of K through the harvest of huge amount of biomass of hybrid maize and improved wheat crops.
is finding is in accordance with [57] who conclude that increased cropping intensity, biomass removal from the field, and introduction of higher yielding hybrid varieties have all resulted in significant soil K exhaustion. Likewise, low K application rates in crops have resulted in an over-dependance on the native K reserve in the soil [58] which caused heavy mining of K. Similarly, a declining trend in the available K from its initial status was observed, indicating significant mining of the available K as a result of continuous cropping [59][60][61].

Conclusions
In conclusion, there was a decline in soil pH and an increase in soil organic carbon over the period with the application of fertilizers. Similarly, even after the application of fertilizers, there was a substantial decrease in the total N, P, and K contents in the soil over time. ere was heavy mining of nutrients, especially P and K after intensive cropping and heavy feeding of hybrid maize and improved wheat. In this study, we reported that the nutrient balance for all the treatments was negative which indicated that removable nutrients exceeded the quantity added to the soil. is means chemical fertilizer replenishment was insufficient to compensate for crop P and K removal.
Data Availability e datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest
e authors declare no conflicts of interest.