Modeling water−salt−nitrogen dynamics and crop growth of saline maize farmland in Northwest China: Searching for appropriate irrigation and N fertilization strategies

Soil salinization, severe NO3--N leaching and low water-fertilizer use efficiency are constraints to sustainable maize production in the upper Yellow River basin (YRB) of Northwest China. Agro-hydrological models have been proven to be a promising decision tool for water and fertilizer management of farmland. In this paper, the AHC (Agro-Hydrological & Chemical and Crop Systems Simulator) model was calibrated and validated by measured soil water, salt, nitrogen (N) and crop growth data and used for scenario analysis to search for the appropriate irrigation and N fertilization strategies for drip-irrigated saline maize farmland. The scenarios were designed with different soil matric potential thresholds (SMPT) for irrigation, different saline degrees (i.e., slightly and moderately saline soils), and with or without additional salt leaching (ASL) in different hydrological years (wet, normal and dry). The results indicated that increasing SMPT and ASL increased the frequency and total amount of irrigation, thus favoring soil salt leaching. NO3--N was leached out of the root zone soil as SMPT exceeded –20 kPa, especially for the cases with ASL. For the case of –20 kPa SMPT, compared with the irrigation scenario without ASL, the irrigation scenario with ASL could significantly increase maize yield and water use efficiency (WUE) in moderately saline farmland. However, for the case of –15 kPa SMPT, the irrigation scenario with ASL caused a significant decrease in WUE for both saline farmlands. Maize yield increased as the N application rate increased from 150 kg ha–1 to 250 kg ha–1 and remained stable as the N application rate continuously increased, resulting in high NO3--N leaching and low partial factor productivity of N fertilizer (PFPn). Comprehensively considering crop yield, WUE, PFPn, soil desalting and environmental impacts, in wet and normal years, the appropriate irrigation and N fertilization strategy is irrigation under –20 kPa SMPT and a 250 kg ha–1 N application rate without ASL for slightly saline maize farmland and with ASL for moderately saline maize farmland, respectively. In dry years, the appropriate irrigation and N fertilization strategy is irrigation without ASL under –15 kPa SMPT and a 250 kg ha–1 N application rate for both slightly and moderately saline maize farmlands in the study and relevant areas.