Vetch cover crops increase particulate organic carbon in citrus orchard by increasing lignin phenols

Abstract

Green manure has long been used to increase soil organic carbon (SOC) and as an amendment to promote soil health in agricultural soils. Its use in orchard crops has a long history, but its role in SOC maintenance and accrual has received less attention than in row crop agriculture. In this study, we compared citrus orchards under clean tillage (control) with those under a 3-year cover of smooth vetch (Vicia villosa var. glabrescens) cover. The distribution of carbon content in SOC and its fractions, including free and occluded particulate organic carbon (fPOC and oPOC) content, mineral-associated organic carbon (MAOC), and changes in lignin phenols, amino sugars, and glomalin-related soil proteins (GRSP), were examined in the topsoil (0–10 cm) and subsurface soil (10–30 cm) and subsoil (30–50 cm). At topsoil under vetch cover, the contents of SOC, oPOC, and lignin phenols increased by 19.23 %, 65.49 %, and 52.44 %, respectively. The topsoil cover crop treatment also enhanced microbial necromass C turnover and decreased lignin phenol degradation, without affecting GRSP. The SOC and MAOC did not change significantly deeper soil layers. The increased available nitrogen (AN) content, fungi-to-bacteria ratio (F/B ratio), and AMF-to-saprophytic fungi ratio contributed to the accumulation of plant-derived C, while dissolved organic nitrogen, AN, F/B ratio, and exchangeable calcium positively influenced microbial-derived C. SOC accrual was mainly driven by the accumulation of oPOC in topsoil resulting in the increase in lignin phenols under vetch cover. These findings support the use of vetch cover as a green manure to improve SOC and promote soil health in orchard systems.