Tree-scale spatial responses of extracellular enzyme activities and stoichiometry to different types of fertilization and cover crop in an apple orchard

Abstract

The effects of cover crops and fertilization on the soil properties of apple orchards have been studied widely, but our knowledge of the variations in the soil enzyme activity and stoichiometry remains limited, particularly the tree-scale spatial responses. In this study, field experiments were conducted in two main plots comprising cover crop (CC) and no cover crop (NC) treatments with four sub-plots comprising no fertilization, manure (M), N, P, and K fertilizer (NPK), and NPK fertilizer combined with manure (NPKM). We determined the activities of C, N, and P acquisition enzymes comprising β-1,4-glucosidase, (BG), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and alkaline phosphatase (PHOS), the soil nutrient contents, and microbial biomass. Cover crop and fertilization significantly affected the soil nutrient contents, microbial biomass, and enzyme activities. Cover crop growth decreased the BG:NAG + LAP and BG:PHOS enzyme activity ratios (before mowing and mulching the cover crop). Fertilization also influenced the BG:NAG + LAP, BG:PHOS, and NAG + LAP:PHOS ratios. The soil enzyme activities were affected by the soil nutrient contents and microbial biomass, especially at distances 90 and 120 cm from the trees, as well as by the manure application treatments (M and NPKM). The soil enzyme ratios and microbial biomass ratio were influenced by the soil properties, especially at distances of 90 and 120 cm form the trees. The relationships among the soil properties, microbial biomass ratio, and soil enzyme ratios were affected by the growth of the cover crop and apple trees. The chemical fertilizer treatments (NPK and NPKM) significantly increased the negative relationships between the soil nutrient ratios and enzyme ratios. Our findings demonstrate that the C:N:P stoichiometry of the soil microbes and enzyme activities were affected by the cover crop and fertilization, and the spatial effects were detected at the tree-scale in an apple orchard.