Frequent drought in winter and spring as well as excessive application of nitrogen limited wheat yield and sustainable development in rainfed farming areas of dryland in southwest China. The objective of this study was to clarify the effects of straw mulching from autumn fallow and reducing nitrogen application on root distribution, wheat yield, water and nitrogen utilization of dryland winter wheat, so as to provide a basis for optimizing wheat tillage system and green, high quality and efficient production of winter wheat in dryland of Sichuan province.

The experiment was carried out at Renshou experiment station of Sichuan Agricultural University in Sichuan province from 2016 to 2018. The spilt plot design was employed with main plot of maize straw mulching (mulching from autumn fallow to wheat harvesting: SM) and no mulching (NM), and sub-plot of different nitrogen application rates (N₀: 0; reducing nitrogen rate (RN): 120 kg N·hm^-2, and conventional nitrogen rate (CN): 180 kg N·hm^-2). Soil moisture, root length, root distribution, wheat yield, water consumption (ET), water and nitrogen use efficiencies were investigated.

Compared with NM, SM significantly increased the soil water content in 0-10 cm and 10-20 cm layers from sowing to booting stage and the soil water storage in 0-100 cm layer at sowing and jointing, and the soil water retention effect of straw mulching lasted until booting and anthesis stage. SM also promoted the wheat root growth at jointing and anthesis stage with higher root diameter and significantly higher root length density in 0-10 cm layer. The mean values of total ET, WUE (water use efficiency), nitrogen accumulation amount of wheat plant, nitrogen accumulation rate from sowing to jointing, nitrogen accumulation rate from jointing to anthesis, nitrogen use efficiency for grain production (NUEg), agronomic efficiency of nitrogen fertilizer (AEN), nitrogen fertilizer partial factor productivity (NPFP) and wheat yield under SM increased by 11.4%, 71.8%, 73.1%, 119.0%, 100.0%, 3.6%, 264.7%, 78.2% than that under NM, respectively, and the recovery efficiency of nitrogen fertilizer (REN) under SM increased by 44.4 percentage points compared with NM. The wheat yield under SM increased by 92.9% because of the significantly increased average spike number (31.8%) and grains per spike (44.4%). The effect of straw mulching was greater than that of nitrogen application. Compared with CN, RN did not significantly decrease the wheat root length density, ET, WUE and wheat yield. Combining with SM, RN significantly improved the NUEg, physiological efficiency of nitrogen (PEN), AEN, NPFP, and REN.

Straw mulching from autumn fallow improved soil water content and storage and optimized root distribution in 0-10 cm layer with higher root length density and root diameter, thus to increase the nitrogen absorption and utilization, wheat yield, and the utilization of water and fertilizer. Straw mulching combined with 120 kg N·hm^-2 was a high yield cultivation mode with reducing nitrogen application and high water and nitrogen use efficiency for dryland winter wheat in Sichuan province, China.