N Fertilizer in Combination with Straw Improves Soil Physicochemical Properties and Crop Productivity in Sub-Humid, Drought-Prone Areas

Straw returning may be an efficient strategy to maintain agricultural sustainability. However, which straw returning strategy can effectively improve soil properties and crop yield remain unclear. A five-year (2011–2016) field experiment in sub-humid, drought-prone areas of northwestern China with uneven rainfall distribution and irrigation was conducted to evaluate the effects of nitrogen fertilizer without straw mulching (CK), with regular straw mulching (LSM), and with ammoniated straw plowing (ALSP) on soil water, soil aggregates, soil organic carbon (SOC), total nitrogen (TN), and water use efficiency (WUE) in an annual winter wheat (Triticum aestivum L.)–summer maize (Zea mays L.) rotation system. The results demonstrate that ALSP had a greater soil water content than CK in the 0–60 cm soil layer. ALSP also had substantially more soil water than LSM in the 0–100 cm layer during the wet year (2011–2012) and two dry years (2014–2015 and 2015–2016). In the normal years (2012–2013 and 2013–2014), the soil water content in ALSP was significantly lower than in LSM in the 0–20 cm soil layer. ALSP was better able to alleviate soil drought in dry years and excessive humidity in wet years. Compared to CK, SOC in the 0–20 cm soil layer in 2016 increased by 8.3% in LSM and 11.7% in ALSP, and TN in the upper soil increased by 6.6% in LSM and 10.1% in ALSP. The equivalent wheat yield and WUE increased in ALSP by 15.6% and 17.5%, respectively, relative to CK, and by 6.79% and 5.97%, respectively, relative to LSM. Thus, we concluded that plowing ammoniated straw with N fertilization is a promising strategy for improving soil fertility and crop productivity in winter wheat–summer maize rotation systems in the sub-humid, drought-prone areas of northwestern China.