Assessment of Nitrogen Accumulation and Movement in Soil Profile under Different Irrigation and Fertilization Regime

MATERIALS AND METHODS

This experiment was conducted in 2007 at the research farm of Amir Kabir sugarcane research center that district lies between 31° 15’ and 31° 40’ N latitude and 48° 12’ and 48° 30’ E longitude. In growth season the mean of maximum and minimum temperatures were 38.7 and 18.8°C, respectively and the mean rainfall was 36.6 mm in during study time.

Subsamples of the soil samples that taken on 21 May 2007 were air dried and then passed through a 2 mm sieve and analyzed for particle size distribution (Table 1).

The experiments were carried out on 50 m furrows and each experiment included five irrigated furrows with 9.15 m width. The layout of the field plot design consists of 27 plots. The experimental design was a split plot with a randomized complete block arrangement with three replications. Three irrigation regimes were allocated to the main plots and three amount of N to the subplots.

Table 1:	Soil characteristics of the study area for different soil depth increments

Irrigation treatments consisted of full irrigation (I1) that equal to evaporation from a standard US Class A pan and a crop-coefficient curve and other treatments were 80 (I2) and 75 (I3) percent of I1 treatment. Nitrogen fertilizer were applied as granular urea and added with irrigation water. Urea application accrued in the sugarcane third ratoon and consist of (N1) 150, (N2) 250 and (N3) 350 kg ha^-1. On 21 May 2007 the first phase of fertilization accrued and 50, 100 and 150 kg fertilizer urea per hectare applied for plots and the second fertilization occurred on 23 June 2007 in 100, 150 and 200 kg ha^-1 levels for N1, N2 and N3 treatments, respectively. Soil was sampled with augers in 30 m increments to a depth of 120 cm. In each field, soil was sampled in triplicate and the samples bulked for each depth. Sub-samples were taken to the laboratory in polyethylene bags and stored at 4°C prior to extraction, usually within 2 days of collection. Twenty grams of soil were extracted in 100 of 2 M KCl by shaking on a rotary shaker for one hour, followed by filtering through Whatman No.1 filter paper into polyethylene containers. Nitrate nitrogen (NO₃¯-N) and ammonium nitrogen (NH₄⁺-N) in KCl extracts were determined by the steam distillation method. N was determined by the micro-Kjeldahl method (Bremner, 1996).

Data was statistically analyzed separately for each depth for each treatment for each date and for the total of all depths to 120 cm with regard to NH₄⁺-N and NO₃¯-N. Data were considered as GLM and analyzed statistically using SAS package.

RESULTS AND DISCUSSION

In Fig. 1a-d the effects of treatments on NO₃¯-N in the soil layers at different dates is shown. Increasing of NO₃^--N value was shown when the amount of fertilizer increased. A peak level, 18 mg kg^-1, accrued in the I3N3 treatment at 30-60 cm depth, was recorded at the 7 July 2007 sampling. With passing time the NO₃¯-N concentration in deeper depth increased. At end time, NO₃¯-N was low, especially in the top layers, as soil-N was partly taken up by the crop and partly leached to deeper soil layers. Approximately at all sampling dates there was more NO₃¯-N in the soil profile of the high N treatments, (N3), compare with the others. Soil N leaching rate was most sensitive to precipitation and fertilizer application rate. This result is basically in agreement with observations reported by other researchers (Bakhsh et al., 2000, 2001).

The NH₄⁺-N values in soil layers at different dates are shown in Fig. 2a-d. In all treatments decrease in NH₄⁺-N from the surface to 120 cm accrued. All NH₄⁺-N values, at 0-120 cm depth, were lower than the corresponding NO₃¯-N. The ammonium concentration in the soil profile was much lower than that of NO₃¯-N. This meant that the differences between the surface and lowest layers were greater than for NO₃¯-N. The NH₄⁺-N decreased with increasing depth, as it easily nitrified and was converted to nitrate in the upper layers.

Fig. 1:	NO3¯-N concentrations at different soil depths (a) 30-60, (b) 0-30, (c) 90-120 and (d) 60-90 cm for sampling dates

Fig. 2:	NH4+-N concentrations at different soil depths (a) 30-60, (b) 0-30, (c) 90-120 and (d) 60-90 cm for sampling dates

Nitrate, produced through nitrification processes in the upper layers, subsequently moves downward and accumulates in deeper layers (Miller and Gardiner, 2001; Pierzynski et al., 2005).

The negative charge on the clay particles retains ammonium ions (NH₄⁺-N). Retention of ammonium ions on clay particles protects them (ammonium ions) from leaching. Nitrate ions (NO₃¯-N) are negatively charged and are not retained by clay particles. Water does not pass easily through clay soils so nitrates, which only move with water, do not leach to groundwater. Pore space in clay soils is often filled with water. Water-filled pores of clay soils lack oxygen. Lacking oxygen, a group of soil bacteria, called facultative anaerobes, substitute nitrates for oxygen for respiration. When bacteria use nitrates as a substitute for oxygen, they convert nitrates to nitrogen gas through a process called denitrification. More nitrates are lost by denitrification in clay soils. Nitrate losses through denitrification in I1 treatment comparison other irrigation treatments reduced the amount of nitrates that could potentially leach to groundwater. Denitrfication and leaching were the main reasons for low concentration of nitrate in soil profile under I1 treatment. In a study on the fate of N fertilizer applied to a calcareous sandy loam soil in the north China plain (Cai et al., 2002), ammonia volatilization was found an important pathway of N loss, e.g., 30-39% of the applied N in rice, 11-48% in maize and less than 20% in wheat.

This situation may have facilitated the adsorption of NH₄⁺-N ions on the clay complex, transport of NH₄⁺-N through mass flow was controlled and ammonium leaching and movement to deeper depth had delay.

The influence of subsurface drainage in field experiment helped to improving the soil properties and probably enhanced the urea activity. Also, the pH of soil water solution during the observation period varied between 7.7 and 7.9 which are ideal for the release of NH₄⁺-N after the hydrolysis of urea. This led to higher losses of NH₄⁺-N. Due to soil salinity in Khuzestan field the exchange complex of the clay was so much saturated with Na⁺ ion that it did not allow NH₄⁺-N ion to get adsorbed on the clay complex. In the specific situation like this where the exchange complex of the clay is saturated with Na⁺ and also the salinity of soil water is relatively high, NH₄⁺-N ion may have reminded in diffused layer and moved slowly downward along with continuously percolating water.

Figure 3 and 4 show the NO₃¯-N and NH₄⁺-N concentration in total depth. These were averaged for overall depths to obtained values for each treatment. In Fig. 3 are shown, I3N3 and I1N1 have a maximum and minimum NO₃¯-N concentration. For sampling date on 7 July all treatments had maximum NO₃¯-N because the second fertilization accrued on 23 June. With passing the time the nitrate concentration in soil profile decreased and NO₃¯-N leached under root zone, uptake with sugarcane or denitrified.

Fig. 3:	Total NO3¯-N concentration in 0-120 cm depth in the soil profile

Fig. 4:	Total NH4+-N concentration in 0-120 cm depth in the soil profile

Fig. 5:	Means of (a) NH4+-N and (b) NO3¯-N accumulation in sampling dates within soil profile (0-120 cm), different letters indicated of significant differently with Duncan’s test at 5% level

In dry land areas in the US, due to inefficient use or over-application of fertilizer, after harvest up to 90 kg ha^-1 NO₃¯-N had leached below the root zone to between 1.5 and 2.5 m (Fuentes et al., 2003). It appears that with increasing amount of N levels, the NO₃¯-N concentrations enhanced. Maximum and minimum NH₄⁺-N value accrued in I2N3 on 7th July and I1N1 on 7th September. The ammonium concentration in the soil profile was much lower than that of nitrate (Fig. 4). The NH₄⁺-N values were lowest in the N1 treatment on all sampling date compare with the other treatments. With the passing the time was seen that NO₃¯-N and NH₄⁺-N concentration in soil depth decreased, because nitrate is the mobile ion and it leached with irrigation water under root zone and also some of it uptake with sugarcane or denitrified.

Time and N level application affected on NO₃¯-N and NH₄⁺-N concentration in soil profile. The different between sampling dates was significant during time study, for NO₃¯-N and NH₄⁺-N concentration in first measurement (11 June) compare with second measurement (7 July) were significant at 5% level, with passing the time in end sampling different was not significant because some of the nitrate and ammonium exited from the soil (Fig. 5a, b).

Figure 6a and b show that different levels of N treatments were the important factor for nitrate and ammonium accumulation in soil profile and the N levels application must be managed because with the increasing of amount of N fertilizer it remained and accumulated in soil and didn’t uptake with sugarcane and lost by leaching or denitrification.

Fig. 6:	Means of (a) NH4+-N and (b) NO3¯-N accumulation in different N treatments during study period within soil profile (0-120 cm), Different letters indicated of significant differently with Duncan’s test at 5% level

CONCLUSION

Land use appeared the most influential factor for the spatial distribution of N concentrations, because it determines the water balance components, such as runoff, evapotranspiration and percolation. The fertilizer and irrigation regimes, in interaction with crop growth, also strongly affected the magnitude of N leaching and, thus, N concentrations in groundwater. Concentrations of NH₄⁺-N were low but NO₃¯-N were relatively high, associated with favorable conditions for nitrification and the relatively high mobility of the ion.

The results of this fertilizer study indicated that the environmental loss potential of N is strongly influenced by amount of fertilizer and water application and timing. The associated processes are quite complex, but management guidelines can be established to help reduce N-leaching risk. The method aims at determining uncertainty in other areas, where the determining factors are strongly influenced by environmental conditions and management. Moreover we can use models usually take into account dynamic processes, such as crop growth with actual transpiration and nutrient uptake, nitrogen application and its transformation in soil and water.

It concluded that improvements in farm management are required to lower the accumulation of nitrate in soils and groundwater while maintaining or improving agricultural productivity. The key factor behind N overuse is that the majority of farmers do not take account of N inputs from manure and irrigation water when they decide how much fertilizer N to apply and extension workers do not recommend that they should make such adjustments.

This should be achieved by long term studies to determine more accurately crop N requirements and by enhancing the local extension service to persuade farmers to minimize environmental degradation while maintaining high crop yields. Continued research is needed to assess more factors influencing the potential for NO₃¯-N leaching in soils under deficit irrigation in production agriculture and to improved irrigation scheduling techniques.