Intercropping grasses improve soil organic carbon content and microbial community functional diversities in Chinese hickory stands

Abstract: Chinese hickory (Carya cathayensis Sarg) is a unique tree species with seeds used for high-grade oil production. It is mainly distributed in northeastern China and is in high abundance on Tianmu Mountain, located at the junction of Zhejiang and Anhui provinces. Intensive management, including heavy application of chemical fertilizer and long-term application of herbicides, has resulted in serious soil loss and degradation. To evaluate the potential of sod-culture to improve soil fertility and microbial activities of Carya cathayensis forest land, we conducted a field intercropping experiment using four treatments: Chinese milk vetch (Astragalus sinicus L.), rape (Brassica campestris L.), ryegrass (Lolium perenne L.), and a no-tillage control. We compared the response of various components of total organic carbon (TOC) and microbial community functional diversity in each treatment. We found that interplanting rape, ryegrass, and Chinese milk vetch increased TOC contents by 23.12%, 26.61%, and 24.74% (P<0.05) compared with clean tillage, while there were no significant differences in TOC contents among the grass intercropping plots. After intercropping grasses for 4 years, the concentrations of microbial biomass carbon (MBC) in the plots of intercropping rape, ryegrass, and Chinese milk vetch were increased by 138.61%, 159.68%, and 144.24% respectively. The concentrations of water-soluble organic carbon (WSOC) in the plots of intercropping rape, ryegrass, and Chinese milk vetch were increased by 56.24%, 69.47%, and 66.05% respectively. There were no significant differences in MBC or WSOC concentrations among the intercropping treatments. The three interplantings increased soil 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%, as compared with clean tillage (P<0.05). However, there were no significant differences in the increase in soil carbonyl C and the decreases in alkyl C, O-alkyl C, and aromatic C among the treatments of intercropping grass. The ratios of aliphatic C/aromatic C, hydrophilic C/hydrophobic C, and aromaticity in soil under Chinese hickory were not affected by intercropping grasses. Interplanting grasses markedly improved microbial community functional diversity. The soil microbial activity (AWCD) values of the three intercropping treatments were much higher than the no-tillage treatment (P<0.05), while there were no significant differences in the soil AWCD values among intercropping grasses treatments. The microbial diversity indexes (H) and evenness index（E） in the treatments of rape and Chinese milk vetch were much greater than the treatment of no-tillage (P<0.05), but there was no significant difference between the ryegrass and no-tillage treatments. The results of this study demonstrated that sod cultivation is an effective soil management practice that improves soil quality and eliminates the detrimental effects of clean tillage in Chinese hickory production.