Effects of Water and Nitrogen Coupling on Apple Yield, Soil Water and Nitrogen Distribution, and Enzyme Activity Under Surge-Root Irrigation in Loess Plateau, China

The objective of this study was to explore the effects of surge-root irrigation and nitrogen coupling on apple yield, soil water and nitrogen distribution, and enzyme activity in the Loess Plateau of China. The differences in the effects of four irrigation levels (I 1 : 85% ~ 95% θ f , θ f is the field capacity; I 2 : 75% ~ 85% θ f ; I 3 : 65% ~ 75% θ f ; and I 4 : 55% ~ 65% θ f ) and four nitrogen application levels (N 1 : 600 kg ha −1 ; N 2 : 450 kg ha −1 , N 3 : 300 kg ha −1 ; and N 4 : 150 kg ha −1 ) on apple yield, water use efficiency (WUE), nitrogen fertilizer partial productivity (NPFP), and soil environment were analyzed through a 2-year (2020 and 2021) field experiment. The results showed that the distribution of soil water and nitrogen were wider with increased irrigation and nitrogen level. With the increase of irrigation level, soil water content and catalase activity raised, and sucrase activity, apple yield, and NPFP firstly increased and then decreased, but WUE decreased. With the increase of nitrogen level, soil NO 3 − -N and NH 4 + -N content increased, urease, sucrase activity, and yield first increased and then decreased, but NPFP increased. The soil comprehensive index method indicated that a decrease in irrigation level led to a decrease in the soil quality comprehensive index (SQI), while SQI firstly increased and then decreased as nitrogen level decreased, and the yield first increased and then decreased as the SQI increased. Compared with the I 1 N 1 treatment, the I 2 N 2 treatment had the highest apple yield, increasing by 5.88% and 8.29% in 2 years. Moreover, the WUE of I 4 N 1 treatment and the PFP of I 2 N 4 treatment achieved maximum values. According to regression analysis, the maximum apple yield was obtained at 93 mm and 548 kg ha −1 of irrigation and nitrogen amounts in 2020 and 92 mm and 538 kg ha −1 in 2021. Based on the high yield of apples and improved of soil quality, the recommended irrigation amount of apples in the Loess Plateau was 92–96 mm, and the nitrogen amount was 500–577 kg ha −1 . The results obtained can provide a scientific basis for apple water and nitrogen management in the Loess Plateau of China.