Groundwater contribution to N fertilization in irrigated cotton and winter wheat in the Khorezm Region, Uzbekistan

Abstract

In the irrigated areas of Uzbekistan the nitrogen (N) fertilizer efficiency in crop production is low, as N is partially leached to the groundwater. The N-fertilizer use is still based on recommendations from Soviet times when fertilizer supply was subsidized to maximize production at all costs. Also irrigation water is applied sub-optimally, and groundwater levels have been reported to be of less than 1 m during the vegetation period. As substantial upward movement of salts from the groundwater is frequently observed due to high evapotranspiration rates, it can be expected that nitrate from leached N fertilizer may also move in the soil profile thus influencing the N balance of the soil. In this study we therefore estimated the groundwater contribution to N-fertilization to improve the N management while sustaining yields and quality and reducing negative environmental effects of groundwater nitrate. Nitrate in irrigation and groundwater was measured during spring and summer. Data were complemented with field measurements of groundwater levels, irrigation and N-fertilizer amounts. With the CropSyst model, upward fluxes of groundwater and evapotranspiration rates were derived, as we could not measure these in the field. We calculated the contribution from the upward flux of nitrate-containing groundwater to the N content in the rooting zone. The difference between the simulated actual evapotranspiration and the irrigated water amount was 335 mm. The average nitrate content in the groundwater was low under summer crops (2 mg nitrate L-1) and higher under the spring crop (24 mg nitrate L-1). However, the temporal dynamics were very much linked to the irrigation and fertilization practices, and corresponded to the changes in groundwater table depth: Almost immediately after fertilization, the nitrate content increased to up to 75 mg nitrate L-1 in spring. At the end of the growing period, the nitrate amounts had reached levels similar to those prior to fertilization. A groundwater contribution of 355 mm and an average nitrate concentration of up to 75 mg nitrate L-1 would enhance the N stocks in the soil by up to 5-61 kg N ha-1. This is equivalent to one single fertilizer application event. However, in case farmers would rely on the input of N through the groundwater to satisfy crop demand and consequently reduce N fertilizer application levels, the N concentrations in the groundwater would reduce and become an unreliable source.