Assessment of Nitrogen Leaching of Cropping Pattern by Soil Nitrogen Balance Equation (Case Study: Varamin Irrigation and Drainage Network)

Nitrogen is often one of the most important limiting factors for biomass production. Usually few soils have proper amount of nitrogen, so it is usually added by fertilizers. In cropping systems, nitrogen fertilization practices can provide a sufficient nitrogen supply for plants to achieve the potential yield. However, to ensure reaching to this potential yield, farmers often apply more nitrogen fertilizers than the required nitrogen for achieving maximum yield. Nitrogen fertilizer should be given to soil according to nitrogen content of soil, water and crop nitrogen demand during the growth periods. Especially in the lands that apply wastewater and groundwater for irrigation and irrigation sources have large quantities of nitrogen, therefore we Should reduce the amount of fertilizer used in agricultural lands, In other words for planning the plant fertilizer demand, nitrogen balance in soil should be considered. Nitrogen overuse increases the risk of nitrogen leaching to groundwater, contaminating groundwater and threatening the human health. In recent years, critical plant nitrogen concentration equation is used to determine crop nitrogen demand during the different growth stages of plants. The purpose of this study is to determine the nitrogen demand of Varamin network’s cropping pattern by using the critical plant nitrogen concentration equation and comparing the result with the amount of nitrogen fertilizer commonly applied in the study area. In this study, monthly biomass production for growing period was determined based on normalized water productivity index and plant canopy development. Coefficient of critical nitrogen concentration equation for each plant was determined by previous researches. The result of this study showed that for barley, the amount of nitrogen applied in Varamin network is equal to nitrogen demand of the cropsand for wheat, maize and tomato are 25%, 61% and 18%, respectively, higher than the amount obtained from critical plant nitrogen concentration equation.also according to the results of soil Nitrogen balance in lands covered by AU canal of Varamin network, 707 ton of nitrogen entered to soil and groundwater by leaching and we should considered appropriate solutions to reduce leaching, such as using high-yielding crops that remove a significant amount of N in the harvested portion, synchronizing fertilizer application with crop demand, conjunctive use of wastewater and groundwater with proper nitrogen concentration and etc.


Introduction
Due to the great expansion of agriculture in the arid regions and the use of nitrogen fertilizers to meet crop nutrient demand, significant amounts of this ion entered into aquifers causing serious health and environmental hazards.The amount of nitrogen fertilizer given to the land should be applied based on its soiland water availability and plant demand during the growing season.
Excessive application of nitrogen fertilizers increases nitrate leaching below the root zone.As a result, it can be harmful for human health due to contaminating the groundwater (Doltra & Munoz 2010).for each plant was determined by previous researches represented in Table 1.(Yao & Zhao 2014).Because the value of these coefficients was not available from field studies in Iran, so we applied the average amount of a and b coefficient for each crop according to various studies of other countries.Note that these values are depending on physiological characteristics of plants, so they do not change much more by different climates.(Justes et al. 1994).
For computing the DM in equation 1, the AquaCrop 4 software was employed which follows the FAO bulletin No. 33.The AquaCrop is practical and useful software.Its basic concepts were developed and also, it uses the algorithm proposed by (Raes et al. 2009).AquaCrop is the FAO crop-model to simulate yield response to water of several herbaceous crops.The model is applied for different agricultural areas throughout the world.
The average annual rainfall in the area is about 145 mm.There is no unique pattern for the cultivation and also for the annual cultivated area.Several parameters including weather condition, cost of crop and its marketing status can affect the cultivation area and pattern.The cropping pattern of the area is presented in Table 2.

Estimation of Water Demand in Varamin Irrigation and Drainage Network
In order to estimate the monthly water demand and planning for irrigation in Varamin irrigation and drainage network, the CROPWAT 8 software was employed.The software applies FAO Penmann-Monteith equation for computing and obtaining the potential evapotranspiration.The model uses a set of input data such as minimum and maximum daily temperature, relative humidity, wind speed and sunshine hours to estimate potential evapotranspiration.The crop evapotranspiration and irrigation planning are computed by inserting the information related to the soil and crop.Figure 2 illustrates the irrigation planning menu in the software.Computed values attained by the equation of critical plant nitrogen concentration and they were compared with those applied fertilizer in the area (Table 7).Table 7 indicates that the amount of applied fertilizer in wheat and barley was roughly equivalent to the crop demand to nitrogen.Also, barley is a kind of gramineous crops in which it has some N fixing glands in its root.In this way, it supplies part of its demand to nitrogen by rhizobium bacteria on its roots.Therefore, its demand to nitrogen fertilizer decreases.The amount of fertilizer consumed for corn and tomato was about 34 and 18 per cent more than the crop demand, respectively.Therefore, the loss may transport in the form of nitrate leaching to the groundwater resources.In this regard, the consumption values of nitrogen fertilizer for these crops should be adjusted.For alfalfa, there is no need to nitrate fertilizer in real condition due to its nitrogen fixing properties.The crop provides its nitrogen demand by rhizobium bacteria on its root.

Soil Tests
In order to determine nitrate, nitrogen and organic matter in the soil of study area, Random sampling of cropping pattern's soil in two stages in the beginning and end of the growing season was conducted, thus in area covered by AU canal, wheat, barley, maize, tomatoes and alfalfa farm, were selected and 3 soil samples were taken randomly from three points for each plant and the mentioned parameters were measured, results are presented in Table 8 and also Soil total nitrogen at the end of the cultivation season, were presented in table 9.   Due to the small amount of ammonium in wastewater, Ammonia losses is ignored.The denitrification can be calculated by equation (3): By solving equation 1, the amounts of nitrogen leaching into the soil and groundwater resources annually in lands covered by the AU canal of Varamin network, was obtained 707 ton in the area that is quite an alarming amount.

Conclusions
Nitrate fertilizer consumption in farm-lands should be considered with special care and accurate planning as well.As excessive consumption of fertilizer can lead to serious environmental hazards, therefore, a good knowledge of the amount and time of Appling fertilizers, play an important role in controlling the problems and has great importance.One of the common methods is to apply critical nitrogen concentration equation in order to determine the crop nitrogen demand during the growing season.This equation determines the nitrogen demand of crop with an acceptable accuracy.The result of this study showed that for barley the amount of nitrogen applying in Varamin network is equal to nitrogen demand of the crops and for wheat, maize and tomato are 25%, 61% and 18%, respectively higher than the amount obtain from critical plant nitrogen concentration equation.Therefore, optimization for fertilizer consumption in these farm lands should be considered to avoid nitrogen leaching into groundwater.Also by solving the soil nitrogen balance equation, the amounts of nitrogen leaching to soil and groundwater resources Annually in lands covered by the AU canal of Varamin network, was obtained 707 ton that is a quite alarming amount.Therefore we should considered appropriate solutions to reduce leaching, such as using high-yielding crops that remove a significant amount of nitrogen in the harvested portion, synchronizing fertilizer application with crop demand, conjunctive use of wastewater and groundwater with proper nitrogen concentration and etc.

Table 7 .
Crop nitrogen demand in comparison of applied fertilizer in the area

Table 8 .
Total nitrogen, nitrate and Organic matter* of soil samples of AU canal