Wheat responses to novel rice cultural practices and soil moisture conditions in the rice–wheat rotation of Nepal

Abstract

Degraded soil physical conditions from puddled rice ( Oryza sativa L.) have been associated with low wheat ( Triticum aestivum L.) yields in the rice–wheat rotation. To assess wheat productivity responses to rice management alternatives at a site in Nepal, we compared the impact of six rice tillage (10 cm surface tillage – T 1, 50 cm deep chisel – T 2, deep chisel + moldboard plough – T 3) and establishment method (soil puddling + transplanting – TPR, direct seeding – DSR) combinations over two seasons (Y 1, Y 2). While significant soil physical differences were documented among the treatments during the rice season, we found little evidence that deep tillage or direct seeding for rice improves wheat performance. Rice cultural practices had no influence on the dynamics of soil water acquisition or inferred patterns of wheat root development. Although aboveground biomass production was similar, water acquisition, plant morphology, and yield were notably different between the first and second years. The Y 1 crop had superior grain size (500-grain weight: 19.2 g versus 17.8 g) and tiller densities (382 m −2 versus 320 m −2), but grain productivity was 1.4 t ha −1 lower (3.0 t ha −1 versus 4.4 t ha −1) than in Y 2. Comparatively poor performance in Y 1 was reflected in the harvest index (0.27 versus 0.41) and density of spike-bearing tillers (188 m −2 versus 247 m −2). As an apparent consequence of pre-emergence irrigation, wheat in Y 2 had higher root activities at depth with 11% of total water uptake derived from soil regions below 54 cm under non-limiting moisture conditions. In contrast, the Y 1 crop was first irrigated at 42 DAE and uptake below 54 cm accounted for only 3% of the total under similar non-limiting conditions. Deeper rooting and a second irrigation at the end of tillering growth phase enabled the crop in Y 2 to sustain exponential development for 25 d during the floral initiation and spike growth stages, whereas the Y 1 crop sustained exponential growth for only 14 d. This difference was reflected in greater canopy height (94 cm versus 80 cm) and maximum leaf area (3.1 versus 2.7) in Y 2. For allocating limited water resources, these findings suggest irrigation at planting to facilitate deep rooting followed by a second application at the end of the tillering growth phase to minimize water stress during grain sink formation. Optimized water management rather than modified rice cultural practices appears to be the best route for maximizing resource capture and wheat productivity at this site.