Responses of soil reactive nitrogen pools and enzyme activities to water and nitrogen levels and their relationship with apple yield and quality under drip fertigation

Improper water and nitrogen supply can lead to resource loss, soil quality deterioration, low nutrient effec-tiveness, and decreased fruit yield and quality in apple orchards. To investigate the soil nitrogen characteristics and enzyme activities in drip-fertigated apple orchards on the Loess Plateau and their relationship with fruit yield and quality formation, field experiments were conducted on dwarf Fuji apple from 2017 to 2022. The experiment consisted of two factors, including two irrigation levels (W) and four nitrogen rates (N) applied. Irrigation was set to two upper irrigation limit levels based on soil moisture, 85 % (W1) and 100 % (W2) of field capacity (FC), and the nitrogen rates were 0 (N1), 120 (N2), 240 (N3) and 360 (N4) kg ha-1. The results showed that irrigation and nitrogen rates had significant effects on soil mineral nitrogen, soluble organic nitrogen (SON), microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), enzyme activity, and apple yield and quality. At W1, the proportion of soil NO3 �-N to the total soluble nitrogen (TSN) pools increased with increasing nitrogen rates, and NO3 �-N gradually replaced SON as the main form of TSN. The responses of MBC and MBN to nitrogen rates were different at different irrigation levels. Apple yield and total soluble solids (TSS), soluble sugar (SS) and vitamin C (VC) contents first increased and then decreased with increasing nitrogen application. Ridge regression analysis showed that SON was the most important factor affecting apple yield, followed by MBN. The VC and TSS con-tents were most affected by soil urease and sucrase, respectively. The comprehensive evaluation by the TOPSIS method ranked W1N3 as the first and optimal treatment, which can achieve high fruit yield and quality while improving soil nitrogen nutrition and biological characteristics to attain sustainable apple farming.