Long-term effects of legume mulching on soil chemical properties and bacterial community composition and structure

Applying legumes as green manure has been reported to improve soil fertility and alter soil bacterial community structure. Few studies have been conducted to determine the effects of Chamaecrista rotundifolia and Arachis pintoi mulching on soil bacterial communities in orchards. In this study, we described the bacterial community in the 0-20 cm soil profile under long-term (approximate 20 years) C. rotundifolia and A. pintoi mulching of persimmon orchards in subtropical and tropical China. The experiment included three treatments: (1) terraced persimmon orchard with A. pintoi mulching (TPA), (2) terraced persimmon orchard with C. rotundifolia mulching (TPC), and (3) terraced persimmon orchard with no vegetation mulching (TPN, CK). Soil chemical properties were significantly altered by long-term A. pintoi and C. rotundifolia mulching. Concentrations of total phosphorus (TP), available nitrogen (AN), soluble organic carbon (SOC), total soluble carbon (TSC), and organic matter (OM) were significantly higher in TPA and TPC soils than in TPN soils; TN and NH4+-N were highest in TPA soils and NO3−-N was highest in TPC soils. Soluble organic nitrogen (SON) and total soluble nitrogen (TSN) concentrations were significantly lower in mulched soils than in TPN soil. Illumina-based sequencing showed that soil bacterial community composition and structure were significantly altered by C. rotundifolia and A. pintoi mulching (TPC and TPA), and the changes in the dominant soil bacterial communities were demonstrated as reductions in the Simpson and Shannon indices. Proteobacteria (relative abundance 37.55%-50.94%) and Acidobacteria (relative abundance 9.47%-12.02%) were enriched in TPA and TPC soils, while Firmicutes (relative abundance 15.74%-22.00%) were higher in TPN soil. Further taxonomic analyses of Proteobacteria revealed that the relative abundance of Gammaproteobacteria and Deltaproteobacteria in PTA and TPC were significantly higher than in TPN, while the Betaproteobacteria in TPA and TPC were significantly lower than in TPN. Genus Phenylobacterium and Escherichia were enriched in TPC, Rhodoplanes and Escherichia were enriched in TPA, while, Kaistobacter, Sphingomonas, Burkholderia, and Enterobacter were enriched in TPN. Family Koribacteraceae classified as Acidobacteria was significantly higher in TPA than in TPN and TPC, and the genus Candidatus Koribacer of Acidobacteria was significantly higher in TPA and TPC than in TPN. In-depth taxonomic analyses of Firmicutes showed the relative abundance of family Bacillaceae, and Streptococcaceae and genus Bacillus and Lactococcus were lower in TPA and TPC than in TPN, while those of family Leuconostocaceae and Lactobacillaceae and genus Pediococcus were higher in TPA and TPC than in TPN. The genus Weissella was only observed in TPC and TPA. Our results indicate that long-term C. rotundifolia and A. pintoi mulching significantly influences the soil chemical properties and bacterial communities of persimmon orchards in subtropical and tropical China.