Optimum water and nitrogen supply regulates root distribution and produces high grain yields in spring wheat (Triticum aestivum L.) under permanent raised bed tillage in arid northwest China

Shortages in water, energy and human resources, low water productivity, soil degradation, and environmental pollution in northwestern China are compelling farmers to develop and adopt sustainable conservation technologies. While the effect of permanent raised beds with furrow irrigation (PRBF) on soil fertility, crop yields and water use efficiency under different soil and climatic conditions has been studied extensively, few experiments have investigated the regulation of irrigation and nitrogen (N). A two-year field experiment, conducted in arid northwestern China, assessed the effects of nitrogen and irrigation rates on grain yield, root growth, root distribution, soil nitrate-nitrogen (NO3--N) distribution, and water and N use efficiencies in spring wheat under PRBF tillage. The experiment followed a completely randomized split-plot design with three irrigation amounts-one-third conventional irrigated [1200 m3 ha-1 (I0.33)], two-thirds conventional irrigated [2400 m3 ha-1 (I0.67)], and conventional irrigated [3600 m3 ha-1 (I1.0)]-as the main-plot treatments, and four nitrogen levels-unfertilized [0 kg ha-1 N (N0)], one-third conventional unfertilized [90 kg ha-1 N (N0.33)], two-thirds conventional unfertilized [180 kg ha-1 N (N0.67)], and conventional fertilized [270 kg ha-1 N (N1.0)]-as the sub-plot treatments.We identified a significant interaction between irrigation and N application rate. The response of root length density (RLD) and its distribution to the N treatments also depended on the irrigation amount. When conventionally irrigated (3600 m3 ha-1), N fertilizer was the main factor limiting spring wheat root growth; N application at 270 kg ha-1 produced the highest RLD values (1.273 and 1.374 cm cm-3) in 2014 and 2015, respectively. At lesser irrigation amounts (1200 and 2400 m3 ha-1), N fertilizer had little effect, with the contribution of water more important to spring wheat root growth; N application at 180 kg ha-1 produced maximum RLD values (1.335 and 1.545 cm cm-3) in the 2400 m3 ha-1 irrigation treatment in 2014 and 2015, respectively, and N application at 90 kg ha-1 produced maximum RLD values (1.042 and 1.163 cm cm-3) in the 1200 m3 ha-1 irrigation treatment in 2014 and 2015, respectively. The I0.67N0.67 treatment produced the highest yields (5022 and 6124 kg ha-1) and RLD values (1.223 and 1.554 cm cm-3) in 2014 and 2015, respectively, and the same treatment had a relatively wide distribution of RLD at 20-60 cm. The NO3--N concentration from 0 to 120 cm increased with N fertilization and decreased with irrigation amount. The results showed that the I0.67N0.67 treatment, with one-third less N and irrigation water than traditional rates (I1.0N1.0), produced equivalent grain yields, had relatively low N losses, and had the highest 20-60 cm root distribution. In the PRBF irrigation system, the I0.67N0.67 treatment appears to be the most promising strategy for producing a high percentage of roots in deep soil layers and increasing grain yields, and water and N use efficiencies.