Soil Carbon Conservation in Anoxic Microsites in 'Natural Vegetation Land' Was Higher Than in 'Artificially Managed Land'

Abstract

Anoxic microsites in soil may lead to oxygen limitation even in well-aerated upland soils, which consequently impedes the rate of soil carbon loss. Nonetheless, the influence of various land use types on the carbon conservation potential within anoxic microsites, which is referred to here as 'anoxic protection', remains poorly understood. This investigation categorizes four land use types on upland soil into two groups based on the level of human influence, with natural shrubland and natural grassland classified as 'natural vegetation land' and farmland and planted forest designated as 'artificially managed land'. The extent of anoxic protection (EAP), which quantifies the contribution of anoxic microsites to soil organic carbon (SOC) preservation, was determined by assessing carbon dioxide (CO₂) efflux rates before and after aeration during soil incubation assays, with gas chromatography serving as the measurement technique. The EAP was 33.5% and 36% of natural shrubland and natural grassland, respectively. Planted forest exhibited a lower protection value at 15.9%, while farmland exhibited the most negligible anoxic protection at −8.9%, which is presumed to be due to agricultural practice-induced soil disruptions. In upland soils, the EAP was positively associated with anoxic bacterial activity, while methanogen DNA abundance was inversely correlated with the oxygen diffusion capacity. The findings indicate that a stable physical soil structure is essential for strong anoxic protection. Even in natural grasslands, where oxygen availability is ample, anoxic microsites enhance the activity of anoxic bacteria and reduce CO₂ emissions by over one-third compared to a fully aerobic environment, thereby offering increased protection for organic matter susceptible to decomposition under aerobic conditions.