Ryegrass intercropping with residue incorporation enhances apple yield and reduces nitrogen loss despite minimal water-nitrogen competition on the Loess Plateau, China

Cover crops are widely intercropped in apple orchards on the Loess Plateau, Northwest China. Generally, there can be important benefits for soil water and nitrogen conditions, while these benefits depend upon how the farmers manage the cover crops, such as the type of cover crops, the method used to terminate their growth, and residue management practices employed. Through a two-year field experiment (2022-2023), this study assessed the effects of different cover crop intercropping systems combined with residue returning on soil water and nitrogen dynamics and apple tree growth. Three intercropping systems, i.e., apple-ryegrass (AR), apple-oilseed rape (AO), and apple-alfalfa (AA) were conducted against clean tillage as a control. In terms of soil water dynamics, three intercropping systems led to an average increase in evapotranspiration of 3.5 % (AR), 3.4 % (AO), and 2.2 % (AA) respectively. Moreover, among three intercropping systems, the frequency of occurrence where the soil water difference ratio was negative and significantly differed from the control was once (AR), twice (AO), and three times (AA) respectively. The contributions of soil water from different depths to ryegrass growth were 41 % (0-20 cm), 37.1 % (20-60 cm), 21.9 % (60-100 cm), and the corresponding values were 34.4 %, 16 %, and 49.7 % for apple trees. Regarding soil nitrogen dynamics, three intercropping systems promoted apple trees N-15 use efficiency ((NUE)-N-15) by 40.9 % (AR), 16.1 % (AO), and 14.1 % (AA), respectively. The AR exhibited the lowest cover crop (NUE)-N-15 (19.7 %), while AA had the highest (39.4 %). All intercropping systems significantly reduced N-15 loss ratio (p < 0.05), with total nitrogen in 0-60 cm soil layer increasing by 16.9 % (AR), 9.4 % (AO), and 7.7 % (AA). As for apple tree growth, the AR increased apple yield and crop water productivity (WPc) by 11.7 % and 5.4 % in 2022, and by 9.8 % and 9.1 % in 2023 respectively; while the AO increased apple yield and WPcby 5.7 % and 0.5 % in 2022, it reduced them by 2.4 % and 3.6 % in 2023; and the AA decreased them in both years. Thus, ryegrass intercropping with residue returning is recommended for apple orchard management on the Loess Plateau due to its negligible water-nitrogen competition, decreased soil nitrogen loss, and enhanced fruit yield.