Mowing in place of conventional grazing increased soil organic carbon stability and altered depth-dependent protection mechanisms

Abstract

Conventional grazing with high grazing pressure can decrease soil organic carbon (SOC) stability by disrupting its protective mechanisms, eliciting soil depth-dependent responses. Grazing exclusion for hay-making by mowing is usually adopted to restore grassland. However, there is still a lack of systematic evaluation of SOC stability and its impact factors in different soil layers when mowing replaces conventional grazing. Here, based on an investigation of 15 paired sites from mowing vs. conventional grazing in a temperate grassland of northeast China, we found that mowing increased mean weight diameter of soil aggregates (MWD, 5.14 %), Fe/Al associated organic carbon (Fe/Al-OC, 12.20 %), and SOC stability (11.46 %) at topmost soil layer (0–10 cm) but only increased MWD (8.50 %) at subsurface soil (10–30 cm). Mowing increased root biomass of plant variables, soil bulk density and pH of soil properties, and microbial biomass carbon (MBC) of microbial properties at the topmost layer, collectively contributing to SOC protection. However, the reduction in soil nitrate nitrogen and increasing MBC induced by mowing drove the subsurface SOC protection. The protection mechanism for SOC stability shifted from a single physical protection dominance in mowing areas to joint physical and mineral protection in conventional grazing grasslands at the topmost depth. In contrast, subsurface SOC stability was consistently governed by mineral protection regardless of grazing. Our results imply that although grassland ecosystems can initiate more protection mechanisms to cope with disturbances, mowing induced the increase of physical and mineral protection resulting from the substantial promotion in plant C input quantity and microbial biomass, combined with alteration in soil properties, finally stabilized topmost SOC. The limited impact of management practices on the subsurface SOC stability indicates that the interaction of subsurface soil and microbial properties with SOC protection should be fully considered to forecast soil C dynamics and its resistance to disturbance accurately.