Growth and yield of rice (Oryza sativa L.) under resource conservation technologies in the irrigated drylands of Central Asia
Introduction
More than 75% of the world's rice production is grown using irrigation, with prolonged periods of flooding. As a result, irrigated rice production is one of the largest consumers of water, using 30% of the total freshwater diverted for all uses (Barker et al., 1999). However, only 15–30% of the applied water is actually consumed by a rice crop for transpiration and growth. This reflects the fact that prolonged ponding leads to very high water losses: 10–30% through deep percolation, 30–50% through evaporation, and 10–25% through surface runoff (Falkenmark and Rockstrom, 2004). Given the increasing competition for fresh water by urban and industrial water users, and the predicted adverse impacts of climate change on water availability, for instance, in Central Asia, the present practice of rice production under flood-irrigated conditions has to change to adjust to future water scarcity. Tuong and Bouman (2003) estimated that by 2025, about 15 out of 75 million ha of Asia's flood-irrigated rice area will experience water shortage.
In Central Asia, rice is grown in rotation with wheat on 0.18 million ha (FAOSTAT, 2010), and most of the rice-growing area is located in the irrigated lowlands of the Amu Darya and Syr Darya river basins (Christmann et al., 2009). Water seeded rice (WSR) is the most widespread cultivation method in the region. With this method, farmers flood the field prior to seeding with pre-germinated seed, and maintain a water depth of 15–20 cm throughout the rice-growing period, with an irrigation input of more than 4200 mm (Ferrero and Tinarelli, 2008). Thus, although rice is the third major cereal and one of the most remunerative crops (Bobojonov, 2008) in Central Asia, its area is decreasing (FAOSTAT, 2010) due to diminishing water availability and mismanagement of irrigation water (Christmann et al., 2009).
The decline in water availability for flood-irrigated rice production has triggered worldwide research on increasing water productivity and water use efficiency using water-wise tillage and crop establishment practices. Many studies in Asia have shown that continuous flooding is not necessary to maintain rice yields at reasonable levels while saving substantial amounts of irrigation water (20–40%) (Humphreys et al., 2010). Further, rice can withstand soil water tensions of up to 10–20 kPa (Belder et al., 2005, Kukal et al., 2005, Sudhir-Yadav et al., 2011). These characteristics offer an opportunity to cultivate rice as an upland crop in non-puddled soil with proper irrigation management. Also, several studies in South Asia (Gupta et al., 2002, Malik and Yadav, 2008), Australia (Beecher et al., 2006), and China (Yan et al., 2010) have shown that rice can be successfully dry seeded. However, some studies have shown also a yield penalty while changing to dry seeding with non-flooded (alternate wetting and drying; AWD) water management (Bhushan et al., 2007, Choudhury et al., 2007). The causes of such yield reductions are numerous, and depend on climate and management (Belder et al., 2004, Gathala et al., 2006, Kato et al., 2009, Singh et al., 2011).
Alternative establishment methods for irrigated rice have been developed in recent years, primarily in sub-tropical regions of South Asia. These are based on innovative resource conservation technologies and management practices to reduce human labor requirement and other inputs while maintaining or increasing economic productivity. For example, zero tillage dry seeded rice (DSR) on raised beds (DSRB) and the flat (DSRF) are being evaluated for their effects on water and nutrient use efficiency (Humphreys and Roth, 2008) and to avoid manual transplanting labor requirement and alleviate soil degradation problems (Ladha et al., 2009, Farooq et al., 2011). Dry seeding without puddling avoids the deleterious effects of intensive soil tillage on soil structure and fertility for upland crops in the rotation (e.g., wheat), and can improve irrigation water productivity (Jat et al., 2009, Saharawat et al., 2010). Studies that address a systematic comparison of WSR and DSR are lacking, especially in arid regions. Several studies in South Asia showed similar or lower yields with dry seeded rice (DSR) than puddled transplanted rice (PTR) with the same water management. For example, yields of DSR were lower by 13% (Bhushan et al., 2007); 32–42% (Choudhury et al., 2007); 40% (Yadvinder-Singh et al., 2008); and similar yields (>6.5 t/ha) of DSR and PTR (Sharma et al., 2005). Sudhir-Yadav et al. (2011) showed similar yields of DSR and PTR with daily irrigation, or with an irrigation threshold of 20 kPa, however at higher irrigation thresholds, yield of DSR declined more than yield of PTR. There were irrigation water savings of 12–53% (Jat et al., 2009, Sudhir-Yadav et al., 2011) with DSR on the flat compared with PTR. Studies in India showed that DSR yield on permanent beds (3- and 8-year-old beds) was lower by 46% and 23%, respectively than yield of PTR, but with reductions in irrigation input of 9–24% (Jat et al., 2009).
With the introduction of combine harvesting, small to large amounts of rice and wheat residues are left in the field. Surface residue retention increases soil water content through increased infiltration and suppression of soil evaporation, and this can lead to significant improvements in growth and yield of wheat where water is limiting (Balwinder-Singh et al., 2011). Surface residue retention also offers the potential benefits of reduced runoff, suppression of weeds, increased soil organic carbon, and improved soil structure (Yadvinder-Singh et al., 2005). Surface residue retention also reduces soil temperature (Balwinder-Singh et al., 2011) which may be a benefit or disadvantage depending on the situation. Lowering soil temperature below the optimum retards wheat establishment and growth (Chen et al., 2007), impairs the development of the root system and thus the ability to absorb water (Cochran et al., 1982) and nutrients (Chen et al., 2002) and reduces soil nitrogen availability (Smika and Ellis, 1971). The effects of standing crop residues on rice growth and yield and irrigation water saving have not been studied in arid regions. Therefore, the objectives of this study were to investigate the effects of AWD water management, crop establishment method and residue retention on rice growth and yield and irrigation water savings in a rice–wheat cropping system in the Khorezm region of Uzbekistan.
Section snippets
Site description
Field experiments were conducted in the 2008 and 2009 rice-growing seasons at the Cotton Research Institute in the Khorezm region (41°32′12″ N, 60°40′44″ E) in north-western Uzbekistan. The site is on the left bank of the Amu Darya River, and within the transition zone of the Karakum and Kyzalkum deserts. The climate of the area is arid, with a long-term average annual rainfall of approximately 100 mm, but neither summer nor winter precipitation plays a significant role in the water balance of
Weather
Mean daily temperature and mean solar radiation throughout the rice season in 2008 were 1 °C and 1 MJ m−2 d−1 higher, respectively, than in 2009 (Fig. 3). Total rainfall during the rice growing period was 70 mm in 2009 (which all fell over 4 days, September 15–18) during the flowering to grain filling stage, and only 5 mm in 2008.
Crop phenology
Physiological maturity in 2009 was delayed in all treatements by 5–13 days compared to 2008 (Table 3). In the absence of residues, dry seeding delayed emergence by 1–2 days
Effect of irrigation method
Changing from continuous flooding to alternate wet and drying (AWD) water management in rice reduced irrigation input greatly, consistent with the results of many other studies with puddled transplanted rice in the literature (Belder et al., 2005, Humphreys et al., 2010, Sudhir-Yadav et al., 2011). However, the reduction of 68–73% was much larger than that normally found, because of the low clay content of the soil and the fact that the soil is not puddled for water and dry seeded rice. In dry
Conclusions
In this study, the combination of AWD with alternative establishment methods has shown large irrigation water savings (68–73%) compared to the present continuously flooded wet seeded rice cultivation practices in Uzbekistan. However, AWD resulted in considerable losses in rice yield, which appeared to be due to water deficit stress in the first year, and which was primarily due to cold damage in the second year. With the light textured soils and dry climate of the region, it is difficult to
Acknowledgements
The German Ministry for Education and Research (BMBF) funded this study. This paper includes research results made possible by the ZEF/UNESCO project entitled: Economic and Ecological Restructuring of Land- and Water Use in Khorezm Region (Uzbekistan): A Pilot Project in Development Research. Comments and suggestions from two anonymous reviewers substantially improved the clarity of this manuscript.
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