Biosphere-atmosphere-exchange of C and N trace gases and microbial N turnover processes in irrigated agricultural systems of the Aral Sea Basin, Uzbekistan

Land-use and agricultural practices affect the soil microbial carbon (C) and nitrogen (N) turnover and hence the biosphere-atmosphere exchange of greenhouse gasses (GHG), namely N2O, CH4 and CO2. In view of the global importance of irrigated agriculture, it is crucial to understand how and to which extent this land-use system interferes with the terrestrial N and C cycles and contributes to the global source strength of atmospheric GHG. Up to now, knowledge of trace gas exchange and N turnover from irrigated agriculture in arid and semiarid regions is much less developed than in other climate zones. Therefore, this study aims at providing more detailed insights into the biosphereatmosphere exchange of trace gases and the underlying soil microbial transformation processes of the irrigated agricultural systems in the Aral Sea Basin (ASB), Uzbekistan. A two-year field study was carried out to quantify and compare emissions of N2O and CH4 in various annual and perennial land-use systems dominating in the study region Khorezm in western Uzbekistan: irrigated cotton, winter wheat and rice crops, a poplar plantation as well as a natural Tugai (floodplain) forest. Irrigated agricultural production in the ASB was shown to be a relevant source of GHG. Seasonal variations in N2O emissions during the annual cropping of wheat and cotton were principally controlled by fertilization and irrigation management. Very high N2O emissions (> 3000 μg N2O-N m h) were measured in periods directly following N fertilizer application in combination with irrigation events. These “emission pulses” accounted for 80-95% of the total N2O emissions over the cropping season for cotton and wheat. Cumulated emissions over one season varied from 0.5 to 6.5 kg N2O-N ha. The unfertilized poplar plantation showed high N2O emissions over the entire study period (30μg N2O-N mh), whereas only negligible fluxes of N2O (< 2μg N2O-N mh) occurred in the natural Tugai forest. Observations of significant CH4 fluxes were restricted to the flooded rice fields, with mean flux rates of 32 mg CH4 md and a seasonal total of 35.2 kg CH4 ha. The global warming potential (GWP) of the N2O and CH4 fluxes was highest under rice and cotton, with seasonal changes between 500 and 3000 kg CO2 eq.ha. The biennial cotton-wheat-rice crop rotation commonly practiced in the region averaged a GWP of 2500 kg CO2 eq.ha year. In addition, laboratory incubation studies were conducted to assess the aggregated gaseous N losses composed of NO, N2O, and N2 from fertilized and irrigated agricultural fields in the ASB. NO3 fertilizer and irrigation water were applied to the incubation vessels to assess its influence on the gaseous N emissions. Under the soil conditions, naturally found after concomitant irrigation and fertilization, denitrification was the dominant process and N2 the main gaseous product of denitrification. Based on the results of these laboratory incubation studies, the magnitude of N2 emissions for the different field research sites of irrigated cotton could be estimated to be in the range of 24±9 to 175±65 kg-N haseason, while emissions of NO were only of minor importance (between 0.1 and 0.7 kg-N ha season). The findings demonstrate that under the current agricultural practices in the irrigated dryland soils of the ASB, denitrification is a major pathway of N losses and that beside N2O extensive amounts of N fertilizer are lost as N2 to the atmosphere. Moreover, the experimental design of this study allows assessing the potential for reducing GHG emissions from these land-use systems. It is argued that there is wide scope for reducing the GWP of this agroecosystem by (i) optimization of fertilization and irrigation practices and (ii) conversion of annual cropping systems into perennial forest plantations, especially on less profitable, marginal lands.