Vetch cover crops reduce the magnitude of citrus rhizosphere effect on plant-derived carbon by promoting suberin and lignin phenol accumulation in citrus orchard

Abstract

The rhizosphere is an important interface for plant carbon (C) input. The rhizosphere effect (RE) plays a substantial role in soil C maintenance and stabilisation. However, it remains unclear how the RE interacts with various residual C sources, such as plant and microbial residues from green manure planting in orchards. We examined the plant- and microbial-derived C contents in the rhizosphere and bulk soil under two management practices of smooth vetch (Vicia villosa var. glabresens) cover and clean tillage. The citrus RE on plant-derived C shifted from positive to negative due to the increase in total root biomass and the carbon/nitrogen ratio of vetch roots and leaves. This shift led to a significantly higher accumulation of lignin phenols (40.64 %, 31.60 %) and suberin (95.71 %, 59.02 %) in the rhizosphere compared to bulk soil. The magnitude of the citrus RE on microbial-derived C was enhanced, primarily driven by increases in soil carbon/phosphorus and nitrogen/phosphorus ratios, and the fungi-to-bacteria ratio (41 % in rhizosphere soil). Additionally, a decrease in the gram-positive-to-gram-negative bacteria ratio in the rhizosphere soil contributed to this effect. Finally, vetch decreased fungal (-22.77 %, −24.52 %) and microbial necromass C (-16.10 %, −13.74 %) in rhizosphere and bulk soil respectively. Partial least squares path modelling showed that vetch characteristics (total effect −1.01) were key contributors to citrus RE on plant-derived C, whereas edaphic variables (total effect 1.86) played an essential role in citrus RE on microbial-derived C in orchards under vetch cover. The results of this study contribute to a comprehensive understanding of the mechanisms underlying the RE on soil C accumulation in orchards managed with green manure.