Devkota, Krishna Prasad: Resource utilization and sustainability of conservation-based rice-wheat cropping systems in Central Asia. - Bonn, 2011. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc:
author = {{Krishna Prasad Devkota}},
title = {Resource utilization and sustainability of conservation-based rice-wheat cropping systems in Central Asia},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2011,
month = aug,

note = {Excessive and inefficient water use, intensive soil tillage and decreasing soil fertility have caused land degradation and desertification, which are threatening the sustainability of rice-wheat systems in the irrigated lowlands of Central Asia. Water-saving and conservation agriculture (CA) practices such as alternate wet and dry irrigation (WAD), direct seeding on raised beds, zero tillage flat lands, and residue retention can help counterbalancing these threats. A randomized complete block design experiment with four replications was conducted from 2008-2010 in rice-wheat rotation systems in Khorezm region, Uzbekistan. Objectives were to (1) investigate the growth and yield formation of rice, (2) examine mineral-nitrogen (N) dynamics in a rice-wheat system, and (3) evaluate the sustainability of a rice-wheat system under water-saving irrigation and CA practices. Next, the rice growth model ORYZA2000 was parameterized and evaluated to assess the impact of increased temperature on phenology and grain yield of rice at various emergence dates under IPCC (2007) projected A1F1 and B1 climate change scenarios. The rice variety used was a puddle rice variety subjected to dryland conditions.
The field experiment centered on six WAD rice treatments involving dry-direct seeded rice (DSR) under raised bed planting (BP) and zero tillage (ZT) planting on flat land combined with three levels of residue retention, i.e., residue harvested (RH), 50% residue retention (R50), and 100% residue retention (R100). These were compared with wet-DSR grown under conventional tillage (CT) with continuous flood irrigation (CT-FI) and with intermittent irrigation (CT-II). The WAD and CT-II treatments were irrigated only when the soil water potential in the top 20 cm soil reached around -20 kPa. CT-FI was irrigated as practiced by the farmers in the region. Wheat was surface seeded into the standing rice field in all treatments. To assess the impact of climate change with ORYZA2000, field experiments with six seeding dates and three varieties were conducted in 2008-2009.
WAD irrigation led to a 68-73% water-saving potential but reduced rice yield by 30-56%; however, the yield of surface seeded wheat was higher by more than 1.5 t ha-1 than the irrigated wheat yield in Khorezm in both years. With WAD, this yield penalty was caused by water stress, higher amount of standing residue retention, and N stress, which reduced biomass accumulation and root growth, delayed phenological development, reduced number of spikelets, and led to a high percentage of unfilled grains. WAD plus residue retention (R50 and R100) in rice led to N losses. Combined over two seasons of rice and one season of wheat, the N loss was highest (>350 kg ha-1) with R100 and lowest with CT-FI (261 kg ha-1) and could have been caused by leaching and denitrification. No mineral N was lost in any of the wheat treatments.
Rice-wheat system productivity, gross margin, and benefit to cost (B:C) ratio were highest in CT-FI followed by RH, and lowest with R100 in BP and ZT. The discharge of irrigation water through seepage and percolation from the rice fields amounted to about 90% with CT. Yields can be increased with water-saving under CA practices with proper residue management, optimization of irrigation, N drilling (nitrogen placement 5-10 cm deep into the soil), strip-till transplanting (seed/seedling placement in a shallow-tilled strip), and the use of suitable aerobic rice varieties. With the present anaerobic varieties used under aerobic conditions, yield penalties are discouraging despite the high savings in water use. As long as farmers do not pay for irrigation water, the incentives may be insufficient to provoke any change.
The parameterized ORYZA2000 simulated phenology and grain yield of rice with a high accuracy (RMSE<15%). An increased temperature caused a decline in yields by 20% (6% °C-1 or 393 kg ha-1°C-1) in 2079 compared to 2000. Under current conditions, the best seeding dates are ~July 5 for short-, ~June 15 for medium-, and ~ June 5 for long-duration rice varieties (varieties maturing in <100, 100-110 and >110 days, respectively). Seeding dates were not changed under the B1 climate change scenario but could be delayed by 10 days under the A1F1 scenario. Development of heat-tolerant, medium- and long-duration rice varieties is crucial under climate change scenarios.},

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