The influence of soil depth and tree age on soil enzyme activities and stoichiometry in apple orchards

Abstract

The indispensable role of soil microorganisms in apple orchards, impacting soil quality, nutrient cycling, and crop productivity, cannot be overstated. Nonetheless, as apple trees mature, the potential for soil nutrient imbalances arises, affecting apple growth and yield. Research on how deep soil properties evolve with tree age is limited. Soil samples were gathered from apple orchards of various ages, utilizing the microplate fluorescence method to examine soil enzyme activity, microbial stoichiometry, and their correlation with soil nutrients. Results reveal that soil properties, microbial biomass, and enzymes are influenced by both tree age and depth. Microbial biomass decreases with depth, peaking in young orchards with the highest MBN:MBP ratio. Initially declining, microbial entropy nitrogen rises with tree age. Redundancy analysis identifies OP as pivotal in explaining extracellular enzyme activity. The enzyme C:N ratio exhibits a negative correlation with the MBN:MBP ratio. Additionally, ratios of ln(NAG) to ln(BG), ln(AKP) to ln(BG), and ln(AKP) to ln(NAG) demonstrate positive correlations. Enzyme activity ratios for C, N, and P transformation deviate from the global ecosystem ratio, indicating nuanced nutrient dynamics. Soil vector angles are influenced by tree age, suggesting limitations on microbial metabolism by carbon and phosphorus. Structural equation modeling unveils that soil depth negatively impacts pH, AKP, and Enzyme N:P, while tree age adversely affects Enzyme N:P. Furthermore, MBN:MBP is directly influenced by AKP and soil depth, indirectly by tree age and pH. This study underscores the significance of microbial stoichiometry in evaluating soil resource ecology relations, offering insights into soil microbe-orchard ecosystem dynamics.