Cover Crop Types Determine Soil Organic Carbon Fractions and Rhizosphere Biological Indicators in Citrus Orchard Soils

ABSTRACT

Cover crops are increasingly used to improve soil quality in orchards, but species‐specific associations among cover crop functional type, soil organic carbon (SOC) fractions and rhizosphere biological indicators remain insufficiently resolved. A field experiment was conducted in a citrus orchard to compare clean tillage (CK), spontaneous vegetation (NG), Lolium perenne (LP) and three legumes ( Trifolium repens (TR), Vicia villosa (VV) and Pisum sativum (PS)). SOC fractions, rhizosphere enzyme activities and cbbL ‐harbouring microbial communities were examined in rhizosphere soil and bulk soils at 0–20 and 20–40 cm during the vigorous growth and mature stages. SOC, particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC) showed clear spatial differentiation, with high concentrations in rhizosphere soil and lower concentrations in deeper bulk soil. Cover crop treatments were associated with higher SOC concentrations than CK, with the largest differences under VV and PS. At maturity, differences relative to CK were 1.44–2.90 g kg ⁻¹ in the 0–20 cm layer and 1.21–2.95 g kg ⁻¹ in the 20–40 cm layer. MAOC accounted for more than 50% of the physically fractionated carbon pool (POC + MAOC) across treatments. Sown cover crops enhanced rhizosphere β‐D‐cellobiohydrolase and β‐1,4‐glucosidase activities relative to NG, and both enzymes were positively correlated with SOC and MAOC. Cover crops were also associated with shifts in cbbL ‐harbouring microbial communities, with biomarker genera including Variovorax , Haliangium , Rhodovastum , Aeromonas and Cereibacter . Mantel tests and PLS‐SEM indicated close statistical associations among carbon‐acquiring enzymes, cbbL community attributes and SOC pools. These findings indicate that leguminous cover crops, particularly VV and PS, are associated with greater SOC stabilization through coordinated changes in rhizosphere enzymatic carbon processing and cbbL ‐harbouring microbial communities.