Lysimeter and field studies were conducted for two years on three soils (Alfisols) to quantify the leaching losses of nitrogen due to rainfall. Lysimeters of 20 cm diameter and 60 cm depth were filled with three soils (S1, S2 and S3), varying in texture and installed in the compound outside of the laboratory building. Each lysimeter was provided with an outlet at the bottom which was connected to the plastic containers (6L capacity) via rubber tubing. Soils were filled in the lysimeters and compacted to match the weight of soil contained in similar volume under field condition (Figure 1). Leachate due to rainfall was collected in plastic containers, measured at measured at weekly intervals and analyzed for N concentration. Total N leached from lysimeters was determined on the basis of N concentration and total amount of leachate collected from a particular container. The physico-chemical characteristics of the soils are given in Table 1. The nitrogen treatment were; T1, no nitrogen; T2, 100 kg N/ha; T3, 200 kg N/ha and; T4, 200 kg N/ha +
|Available N (kg ha-1)||182||202||232|
|EC (dS m-1)||0.32||0.25||0.18|
The field study was conducted at sites from where the soils were taken for lysimeter investigation. The soil characteristics and N treatments were the same as in lysimeter investigation. The potato tubers of var. Kufri Jyoti, were planted at inter and intra spacing of 60 cm and 20 cm, respectively, in plots measuring 3m x 4m. The soil NO3-N was also determined for the soil samples collected after potato harvest from three depths such as; 0-20 cm, 21-40 cm and 41-60 cm. The applied FYM contained 0.379% N, 0.151% P2O5 and 0.372% K2O.
Samples of tubers, haulms and roots of potato plant were collected from lysimeter and field to analyze for N content. These samples were washed in 0.1 HCl, followed by distilled water and then dried in oven at 65°C. Analysis of soil, plant and water samples was done as per Jackson (1973).
To quantify leached N (kg/ha), a simple model was developed with three independent variables based on a partial regression equation of the form:
N leached (kg/ha) = a + b1x1 + b2x2 + b3x3
where b1, b2, b3, are the partial regression coefficients for the rainfallin cm (x1) soil clay % (x2) and, N (both applied and native) in kg/ha (x3). The equation was developed using the Doolittle method as described by Goulden (1960), and it gave a R2 value of 0.7056.
The rainfall during the potato growing period was 1134 mm during the first year and 879 mm during the second year of study (Figure 2). There was a great variation in intensity and amount of rainfall during two years as well as within the year of study.
In lysimeter experiment, N concentration in the leachate was influenced by the soil texture, treatments and time of sampling (Figure 3a to 3f) and showed a decreasing trend over time in all the soils. The possible reason was that half of N was applied initially before planting and remainder after 30 days of planting. Application of FYM did not show any significant effect on the N concentration of the leachate. Average N concentrations in the leachate due to N treatments varied from 2.6 to 6.6 mg/lin S1, 3.2 to 7.0 mg/l in S2 and 5.5 to 11.7 mg/lin S3 during the first year and; 4.2 to 9.2 mg/l in S1, 5.2 to 11.5 mg/l in S2 and 8.1 to 14.5 mg/l in S3 during the second year of study (Fig. 3). Average N concentrations due to T1, T2, T3 and T4 during first year were; 3.8, 6.1, 8.0 and 8.5 mg/l of leachate and, during second year of study were; 5.7, 8.8, 11.6 and 11.7 mg/l, respectively. Mean N concentration was 6.6 mg/l during first year and 9.4 mg/l during second year of investigation.
N leached from soil S1 was 21.0, 56.4, 71.9 and 79.5 mg per lysimeter, 21.9, 63.6, 77.8 and 88.3 mg per lysimeter from S2 and, 44.5, 123.3, 164.4 and 169.5 mg per lysimeter from S3, at N levels of 0, 100 kg, 200 kg and 200 kg + 10 t/ha of FYM, respectively, during first year (Figure 4a). The corresponding values of mg N leached per lysimeter during second year, respectively, were; 17.4, 51.1, 67.1 and 71.7 from S1, 20.2, 48.5, 70.5 and 76.2 from S2 and, 36.3, 84.7, 120.6 and 121.7 from S3 (Fig. 4b). When converted to N leached kg/ha, maximum loss of N through leaching was 53.9 and 38.7 kg/ha during first and second year, respectively, in S3T4. However, mean loss of N from all lysimeters was 26.0 kg/ha during first year when the rainfall was 113.4 cm and 20.8 kg N/ha in second year when rainfall received was 87.9 cm during the potato growing period. Weekly leachate collected (Figure 5) and amount of N leached at various sampling periods increased markedly with increase in the amount of rainfall. Leaching losses of N varied significantly according to N applied, soil characteristics and the period of sampling. Variations in N leaching were found within the soils. Relatively more of N was found in coarse textured soil. The mean loss of N increased by 7.1% with decrease in the clay content of the soil. Leaching loss of N increased by 166.2, 256.8 and 278.4 per cent with T2, T3 and T4 treatments, respectively, over T1 (control).
Significant interactions between soil and N treatments were found during both the years of study (Figure 6). A minimum value of 19.2 mg N leached per lysimeter was found under S1T1 treatment and a maximum value of 145.5 mg N in S3T3. Both soil and rate of applied N significantly affected total N leached from the lysimeters during the potato growing season. The application of FYM enhanced the quantity of leached N in S1 and S2, while no significant effect was found in S3.
Mean values of N leaching were 16.1, 18.8 and 32.8 kg/ha from S1, S2 and S3 during first year and, 12.2, 14.3 and 27.8 kg/ha during second year of study from S1, S2 and S3, respectively (Figure 7). The applied N had a tremendous impact on N leaching and the mean values of both the years were; 6.5, 19.6, 26.3 and 29.1 kg/ha of N from control, 100 kg N, 200 kg N and 200 kg N + 10 t FYM/ha, of applied nutrients, respectively.
More than 84% (R = 0,847) of the variability in N leaching was attributed to the amount of rainfall, clay content of the soil and N (available native + applied). Doolittle method was used for developing a partial regression equation simple model, based on the rainfall, soil texture and the applied + native N (Goulden 1960). The loss of leached N can be estimate as: N leached (kg/ha) = 2.41 + 0.184 (rainfall in cm) – 0.174 (clay % of soil) + 0.128 (native available N + N applied).
Application of N to potato significantly increased the dry matter yield of tuber, haulms and roots during both the years (Figure 8). Application of FYM significantly improved the dry matter yield in all the soils studied. The dry matter yield was significantly higher in S3 than S2 and S1. It was found in the field study that the tuber yield was significantly higher in light textured soil, S3, compared to S1 and S2 while no significant difference was observed in latter two soils (Table 2). Rainfall variation during two years had a significant effect on the tuber yield.
|Treatments||NO3-N content (kg ha-1)||Tuber yield
|0-20 cm||21-40 cm||41-60 cm|
|C.D. (p = 0.05)||1.9||1.7||1.7||1.9||7.8|
|N (kg ha-1)|
|200 + FYM (10 t ha-1)||48.7||42.1||32.4||24.9||108.4|
|C.D. (p = 0.05)||2.1||1.9||1.9||2.2||9.2|
|C.D. (p = 0.05)||1.7||NS||NS||1.5||6.4|
The nitrate-N content of all the soils, measured prior and subsequent to leaching, varied significantly at different depths, both in field and lysimeter studies. The soil N variation was linked to the nitrogen treatments (Table 2). Increase in N supply significantly enhanced the NO3-N in the soils at different depths and application of FYM further improved it. The results show that application of N and FYM significantly enhanced the supply of N to the crop because of improved soil physical condition. The NO3-N content decreased in all the three soils with depth. The mean NO3-N content was found 33.4, 28.8 and 23.3 kg/ha at 0-20 cm, 21-40 cm and41-60 cm, respectively. A non-significant variation was found in NO3-N content in the top soil (0-20 cm) at 30 and 45 days after planting thereby, indicating that the top soil maintained almost the same level of NO3-N up to about 45 days after planting of potato crop, however, it declined at later dates (Figure 9). The NO3-N content within different soils varied significantly and S3 maintained significantly higher levels than S1 and S2 at 0-20 cm depth. Amount of rainfall significantly affected the nitrate-N content of the top soil (0-20 cm), however, no significant effect was found at lower depths, indicating that a sizeable amount of nitrate might have escaped beyond 60 cm depth with percolating water due to heavy rainfall in the region.
In lysimeter study, maximum N uptake was found in S3, which was, on an average, 29.2 and 36.1% higher than S1 and S2, respectively (Fig. 10). Uptake of N was significantly correlated (r = 0.877**) with the dry matter yield (tubers, haulms and roots). More recovery of N was found in S3 and application of FYM significantly enhanced the amount of recovery per unit of N application. Nitrogen influences vegetative growth and the uptake of other nutrients such as potassium and phosphorus. Even under best management practices, approximately 30-50% of applied nitrogen is lost through different processes and, hence, the farmer is compelled to apply more than what the crop needs to compensate for losses through leaching, volatilization, and denitrification making the nutrient unavailable during the critical stages of crop growth (Maidl., et al. 2002, Hyatt., et al, 2010). Split application of nitrogen is one of the strategies of improving nitrogen use by the crops. In field study, highest mean N uptake in all soils was 108.4 kg/ha when 200 kg N/ha was applied along with 10 t/ha of FYM.
After harvest of potato crop (field study), it was found that the mean residual N balance in soil was -54.7, 10.3, 83.4 and 68.8 kg/ha in the plots where N was applied @ 0, 100, 200 and 200 kg/ha (including FYM), respectively (Figure 11). There was negative balance of N in control plots which was due to mining of native available N by the potato crop as there was no outside supply. There was build-up of N in the plots where nitrogen was applied through fertilizers alone or with FYM. The mean N balance due to soils was 42.2, 34.7 and 3.8 kg/ha in S1, S2 and S3, respectively. In control (no N) plots, the N balance was -46.7, -51.0 and -66.4 kg/ha in S1, S2 and S3 plots, respectively. In T2S1, T2S2 and T2S3, N balance was 24.5, 17.6 and -11.2 kg N/ha, in T3S1, T3S2 and T3S3, N balance was 101.7, 92.9 and 55.6 kg N/ha and, in T4S1, T4S2 and T4S3, balance was 89.5, 79.5 and 37.4 kg N/ha, respectively.
Highest concentration of N in the leachate was found in S3 because of higher available N in this soil and due to its coarse texture. The concentration of N in the leachate increased with increase in N application during both the years. It corroborates the findings of Singh & Singh (1987) and Sharma (1990). The higher N concentration in the leachate during second year was due to the reason that amount of leachate collected was less compared to first year. Higher leachate during first year diluted the N concentration, though total N leached was more. The soil concentration of N increased initially up to third sampling date but, decreased subsequently in all the soils and at all applied N levels, possibly due to plant uptake and leaching losses.
Leaching of N increased significantly with increase in N application in three soils during both the years of study (Figures 3a and 3b). The loss of N was slightly more in FYM applied lysimeters than N applied alone, more so in heavier soils. In lysimeter study, a highly significant relationship was observed between rainfall (Figure 3, r = 0.962**), clay content (r = 0.826**), available and applied N (r = 0.761**) and leaching of N. The leaching of plant nutrients from agricultural fields is affected by the nature of soil, climatic factors and soil-plant system management. Havlin., et al. (1999) reported that under certain conditions, NH4+ is transformed to NO3- through a biological process called nitrification. Whereas NH4+ is readily immobilized by soil microbes and absorbed on negatively charged soil particles, NO3- is a highly mobile anion that may be leached out of the upper soil horizons under conditions of excessive precipitation and high N application rates. Nitrification is also a potential gateway to N losses through denitrification, whereby NO3- can be biologically reduced to NO, N2O or N2 under saturated soil conditions. As a result of suboptimal field management practices, when much of nitrogen has not been utilized by crop and has accumulated in the root zone in the form of nitrate, is subjected to leaching along with precipitation and irrigation water (Michael., et al. 2009). In light textured soils, nutrient retention is generally poor and soluble nutrients like nitrogen (N) are prone to leaching (MacKown & Tucker 1985).
Nitrogen leaching during the first year was 54.7% higher than second year (Figure 6). This was due to higher rainfall during the potato growing season in the first year (1134 mm) than second year (879 mm). Significantly very high coefficient of correlation between the amount of rainfall and leaching of N was observed (r = 0.912). Loss of N through leaching in S3 was 133.8% and 83.6% higher than S1 and S2. This may be attributed to the reason that light texture of S3 provided more pore space for easy percolation of N during potato growth period. Our results corroborate the finding of Sharma (1990) and Mack and Ruark 2015). Nitrate is present in almost all soils, except flooded soils. Since nitrate is susceptible to many transformations and loss pathways, nitrate concentrations should ideally be no more than is required to meet plant nutritional needs. Soil nitrate should be depleted as much as possible by the time harvest occurs to minimize loss between crops.
Increase in applied N increased the dry matter yield of potato crop significantly over control plots and application of FYM @ 10 t/ha further improved it by 10.1% over N applied alone (Fig. 7). This may be due to additional N present in the FYM. Similar results were reported by Sharma & Arora (1987). Both dry matter yield and N uptake were significantly influenced by the amount of rainfall received during the crop growing period. The tuber yield (r = 0.882**) and N uptake (r = 0.791**) were significantly correlated with the amount of rainfall water received. The weather conditions during potato growth is generally considered as the decisive factor for plant growth and tuber production (N’Dayegamiye., et al. 2013). It has been reported that split application of nitrogen is one of the strategies of improving nitrogen use by the crops (Kelling., et al. 2015; Tetsuhisa., et al 2015). The decrease in nitrate content at lower depths may be due to denitrification by facultative organisms in the absence of aerobic condition and volatilization of N as nitrous oxide and elemental N (Kamp., et al. 2015, Wrage., et al. 2004). The time of soil sampling after the planting of the potato crop had a significant effect on the NO3-N content in all the soils and at all the depths.
Nitrogen uptake was significantly related to potato yield (Fig. 8). N balance in FYM plots (200 kg/ha applied N) was low compared to the plots where 200 kg N/ha was applied through fertilizer only. Besides N uptake, there was more N loss through leaching and runoff resulting in low soil N balance. Our results are in agreement with Munoz., et al. (2003).
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