Effects of intercropping grasses on soil organic carbon and microbial community functional diversity under Chinese hickory (Carya cathayensis Sarg.) stands

Chinese hickory (Carya cathayensis Sarg.) is a woody nut and oil tree from China. Intensive management including heavy application of chemical fertiliser and long-term application of herbicides has resulted in serious soil loss and degradation. This study aimed to test the hypothesis that intercropping in the soil under Chinese hickory stands may improve soil fertility and microbial community functional diversity. A field experiment consisting of four treatments (clean tillage; intercropping rape (Brassica rapa L.), ryegrass (Lolium perenne L.) or Chinese milk vetch (Astragalus sinicus L.) was conducted to study the effects of intercropping on soil organic carbon (SOC) structure and microbial community functional diversity under C. cathayensis stand, by means of 13C-nuclear magnetic resonance (NMR), and EcoPlates incubated at 25°C. After 4 years of treatment, intercropping increased available nitrogen (N), phosphorus and potassium in the soil by 25.1–54.2, 4.2–6.0 and 0–22.5 mg kg–1, respectively, relative to the clean tillage treatment; intercropping rape, ryegrass and Chinese milk vetch increased SOC, microbial biomass C (MBC), and water-soluble organic C (WOC) by 23.1–24.7, 138.6–159.7 and 56.2–69.5% (P < 0.05), respectively. The structure of SOC was also greatly changed by intercropping treatments. Intercropping increased carbonyl C by 29.9–36.9% (P < 0.05) and decreased alkyl C, O-alkyl C and aromatic C by 10.0–16.4, 18.9–20.9 and 10.5–16.6% (P < 0.05), respectively. Intercropping markedly improved microbial community functional diversity, which is characterised by increases in average well-colour development (AWCD), Shannon index and evenness index. Correlation analysis showed significant positive correlations among microbial biomass N, water-soluble organic N, SOC, WOC, MBC and AWCD (P < 0.05 or P < 0.01). The results demonstrate that sod cultivation is an effective soil management practice that improves soil quality and eliminates detrimental effects of clean tillage in Chinese hickory production.