Devkota-Wasti, Mina Kumari: Nitrogen management in irrigated cotton-based systems under conservation agriculture on salt-affected lands of Uzbekistan. - Bonn, 2011. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Mina Kumari Devkota-Wasti}},
title = {Nitrogen management in irrigated cotton-based systems under conservation agriculture on salt-affected lands of Uzbekistan},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2011,
month = nov,

volume = 82,
note = {Intensive soil tillage and mismanagement of irrigation water and fertilizers reduce soil organic matter and increase secondary soil salinization. These processes are increasing production costs, reducing soil fertility and threatening the sustainability of crop production systems in the irrigated drylands of Uzbekistan, Central Asia. These adverse effects can be counterbalanced by conservation agriculture (CA) practices combined with optimum nitrogen (N) management. This has been demonstrated in rainfed areas, but only sparse findings exist for irrigated crop production. Therefore, the effects of tillage, crop residue management and N rates were examined on growth, yield, water and N use efficiency (NUE), and the N balance of crops as well as the soil salinity dynamics in two cotton-based systems, (i) cotton/wheat/maize and (ii) cotton/cover-crop/cotton, in Khorezm, a region in northwest Uzbekistan. Also, on smaller subplots the effect of three different furrow irrigation techniques on the distribution and management of soil salinity on raised beds was studied. These techniques were every-furrow (EFI), alternating skip furrow (ASFI), and permanent skip furrow irrigation (PSFI).
The split-plot experiments with four replications were conducted from 2008-2009 in an area covering 3 ha. They included two tillage methods (permanent raised bed, BP; and conventional tillage, CT); two residue levels (retaining the maximum possible amount, RR; and removing residues according to farmers’ practices, RH); and three N levels: no application (N- 0); low-N (125 kg N ha-1 for cotton and 100 kg N ha-1 for wheat and maize); and high-N (250 kg N ha-1 for cotton, and 200 kg N ha-1 for wheat and maize). These treatments were evaluated on land previously cropped using conventional means (CT). The official N recommendation for the study region is 160-180, 180 and 150 kg N ha-1 for cotton, wheat and maize, respectively.
Raw cotton yield and its components were not affected by tillage methods in both cotton-based rotation systems in the first season after transformation from CT to CA practices. However, already one cropping cycle later, wheat and maize under BP produced, respectively, 12 and 42% higher grain yields than under CT. Under BP, water productivity increased in wheat by 27% and in maize by 84%, whilst 12% less water was applied during wheat and 23% during maize production compared to CT. Nitrogen applications significantly increased the growth and yield of all crops under both tillage practices. However, the response to N applied was higher under BP than CT. Increased boll density and boll weight in cotton, number of spikes m-2 and grains per spike in wheat, and cob density and number of grains per cob in maize predominantly caused higher yields. Total NUE in BP was higher by 42% in cotton, 12% in wheat, and 82% in maize crops compared to CT. With high N applications, the apparent positive N balance (N loss) in BP was lower by 71% in the cotton/wheat/maize system and by 53% in the cotton/cover-crop/cotton system than under CT.
Residue retention in BP increased grain yield of wheat and maize in the absence of N applications, but had an insignificant effect on crop yield at low-N and high-N application rates. Residue retention had no effect at all N levels under CT. In BP, it minimized the rate of soil salinity increase by 45% in the top 10 cm and by 18% in the top 90 cm soil profile compared to RH. The inclusion of a winter cover crop in the cotton-cotton rotation reduced the groundwater nitrate contamination considerably, and increased the NUE under both BP and CT.
Soil salinity on top of the beds increased significantly with EFI and ASFI compared to PSFI. The latter practice of salinity management provided the less saline area towards the irrigated furrow, as salts accumulated on the dry furrows. These accumulated salts can be leached, which reduced the salinity level in the center of the beds two-fold compared to EFI and ASFI.
For cotton, wheat and maize, grown in rotation, BP and residue retention with application of the recommended N for maize and ~15% less than recommended N for cotton and wheat were in many aspects superior to CT practices. Permanent bed cotton cultivation with a winter cover crop is a suitable alternative for cotton-cotton based systems in irrigated drylands of Uzbekistan. Should residues not be available, PSFI is a suitable alternative for salt management in raised bed planting in salt-affected irrigated lands.},

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