Seasonal effects of drip irrigation on soil quality index, microbial stoichiometry, and carbon use efficiency in an apple orchard

The soil quality index (SQI) is a comprehensive indicator that reflects the agricultural productivity of soil, as well as playing important roles in understanding microbial nutrient metabolism and carbon use efficiency (CUE). However, it is unclear how drip irrigation treatments in apple orchards affect the SQI, eco-enzyme stoichiometry, and soil microbial CUE. Thus, in the present study, we tested three different treatments in orchard plots: T1 (50-60 % field water capacity (theta f)), T2 (65-75 % theta f), and T3 (80-90 % theta f), as well as control with no drip irrigation (CK). The study focused on the effects of these treatments during two key stages: bud breaking and fruit maturity. During the bud breaking stage, we observed that water availability had a more pronounced influence on the SQI when soil moisture was limited. Specifically, in the 0-20 cm soil layer, the T2 treatment showed a significantly lower SQI value compared to T3, with a decrease of 31.89 %. On the other hand, there were no significant differences among all the irrigated treatments during the maturity stage. Both vector length and angle were significantly affected by water availability during the bud breaking stage, while only the vector angle was impacted during the maturity stage. The vector length and angle were both influenced by SQI (Mantel's test: p < 0.01). During the bud breaking stage, the CUE values in 0-20 cm layer under T1, T2, and T3 were 30.27 %, 21.79 %, and 85.47 % lower, respectively, compared with CK. By contrast, in the fruit maturity stage, CUE was 27.39 % higher under T1 compared with CK. SQI and CUE had a negative correlation in the bud breaking stage (p < 0.001, R-2 = 0.26), but a positive correlation in the fruit maturity stage (p < 0.001, R-2 = 0.51). Our findings suggest that the T3 treatment consistently yields the highest Soil Quality Index (SQI) across most soil layers during the bud breaking and maturity stages. Moreover, the T3 treatment effectively alleviates early spring drought in the Weibei region and encourages deep-root development, enabling fruit trees to absorb nutrients from deeper soil levels. Overall, these findings enhance our understanding of how the SQI and enzyme stoichiometry under drip irrigation affect phosphorus and carbon metabolism in soils, and they suggest that SQI should be considered a key factor that limits microbial metabolic restrictions and microbial CUE.