Coupled effects of mulching and nitrogen fertilization on crop yield, residual soil nitrate, and water use efficiency of summer maize in the Chinese Loess Plateau

Appropriate water-saving and nitrogen management strategies are critical for achieving sustainable agricultural development in dry sub-humid areas of the Chinese Loess Plateau. The present study was conducted in 2004, 2005, 2008, and 2012 based on a long-term field experiment and aimed to investigate the coupled impacts of mulching and N fertilization on maize yield, water use efficiency (WUE), and residual soil nitrate (RSN) accumulated in the soil profile (0–200 cm). The results demonstrated that mulch is conducive to increasing summer maize yield. The plastic film-mulched ridge and straw-mulched furrow (RF) treatment significantly increased maize yield across the studied period, while the straw mulch (SM) treatment did not significantly increase maize yield until the third experimental year. Compared with SM, the RF treatment showed more significant and positive effects on maize yield, WUE, and RSN accumulated in the 0–200 cm soil depth. N fertilization significantly increased maize yield and WUE, but no significant differences were observed when 120 and 240 kg N ha⁻¹ were applied. The N240 treatment was characterized by relatively high NO₃ ⁻-N accumulation in 0–200 cm soil depth and low ratios of soil nitrate in the upper to the lower soil layers, indicating a considerable potential for NO₃ ⁻-N leaching. Averaged across years, economic optimum N fertilizer rates (Nops) were 154, 148, and 150 kg N ha⁻¹ for the no mulch, RF, and SM treatments, respectively. This suggested that 25.8–51.2% of N rate can be reduced while maintaining an acceptably high maize yield. Additionally, understanding NO₃ ⁻-N depth distribution in 0–100 cm soil profile can adequately predict and represent the characteristics of NO₃ ⁻-N accumulated in the 100–200 cm and 0–200 cm soil layers because of their significant correlations, thus saving time and money. In conclusion, the practice of RF combined with properly reduced farmers’ N rate (~ 150 kg N ha⁻¹) is the preferred option for maize production in the Chinese Loess Plateau, and further research is required to investigate the effects of mulching on summer maize under Nop conditions.