Water and Nitrogen Coupling on the Regulation of Soil Nutrient–Microbial Biomass Balance and Its Effect on the Yield of Wolfberry (<i>Lycium barbarum</i> L.)

Due to the problems of relatively fragile stability, the quality of soil in the drip-irrigated agricultural ecosystem has high spatial heterogeneity and experiences significant degradation. We conducted a two-year field plot study (2021–2022) in a typical region of the arid zone with the “wolfberry” crop as the research object, with three irrigation and three nitrogen application levels, and the local conventional management as the control (CK). Soil quality under experimental conditioning was comprehensively evaluated based on Principal Component Analysis (PCA) and Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS), and regression analyses were carried out between the soil quality evaluation results and wolfberry yield. The results showed that short-term water and nitrogen regulation enhanced the soil nutrient content in the root zone of wolfberry to some extent, but it did not significantly affect soil carbon:soil nitrogen (C_soil:N_soil), soil carbon:soil phosphorus (C_soil:P_soil), and soil nitrogen:soil phosphorus (N_soil:P_soil). When the irrigation quota was increased from I₁ to I₂, the soil microbial biomass carbon, nitrogen, and phosphorus (C_mic, N_mic, and P_mic) tended to increase with the increase in N application, but the microbial biomass carbon:nitrogen (C_mic:N_mic), microbial biomass carbon:phosphorus (C_mic:P_mic), and microbial biomass nitrogen:phosphorus (N_mic:P_mic) did not change significantly. The comprehensive evaluation of the principal components and TOPSIS showed that the combined soil nutrient–microbial biomass and its ecological stoichiometry characteristics were better under the coupled treatments of I₂, I₃, N₂, and N₃, and the overall soil quality under these treatment conditions was significantly better than that under the CK treatment. Under I₁ irrigation, nitrogen application significantly increased the yield of wolfberry, while under I₂ and I₃ irrigation, the wolfberry yield showed a parabolic trend with the increase in nitrogen application. The highest yield was recorded in the I₂N₂ treatment in the first and second years, with yields of 9967 kg hm⁻² and 10,604 kg hm⁻², respectively. The coefficient of determination (explained quantity) of the soil quality based on soil nutrient–microbial biomass and the characteristics of its ecological stoichiometry for wolfberry yield ranged from 0.295 to 0.573. These findings indicated a limited positive effect of these indicators of soil on wolfberry yield. The short-term water and nitrogen regulation partly influenced the soil and soil microbial biomass in agroecosystems, but the effect on elemental balance was not significant. Our findings might provide theoretical support for managing the health of agricultural ecosystems.