Improved management increases soil mineral-protected organic carbon storage via plant-microbial-nutrient mediation in semi-arid grasslands

Abstract. Soil organic carbon (SOC) storage in semi-arid grasslands is threatened by both climate change and land degradation, impacting food production and climate regulation. Improved management has been proposed to increase SOC stocks and overcome these challenges. However, the benefits of improved management practices in semi-arid regions are in question. Little is known about the effects of management on the functional components of SOC, particulate (POC) and mineral-associated organic carbon (MAOC), which are expected to respond differently, and about the pathways that mediate these responses, such as changes in vegetation and soil microbial communities. This work analyses the effect of rotational grazing, legumes sowing and grazing exclusion on topsoil SOC, POC and MAOC stocks in Mediterranean wooded grasslands compared to continuous conventional grazing. Changes in plant diversity and morpho-chemical traits, soil fertility and microbial composition were also evaluated. A total of 188 plots were sampled in 9 farms across a wide environmental gradient. More resource-acquisitive, nitrogen-rich and less lignified plant community, higher soil microbial biomass with lower Gram+/Gram- ratio, and higher soil fertility were associated with higher SOC storage, with similar impacts on POC and MAOC. Rotational grazing increased MAOC and total SOC stocks by 11 % compared to continuous grazing. This effect was mediated by an increase in soil fertility in the rotationally grazed paddocks. On the other hand, grazing exclusion reduced POC stocks by 12 % compared to continuous grazing. This depletion was mainly due to a reduction in microbial biomass and an increase in the C/N ratio of vegetation in non-grazed paddocks. Both POC and MAOC stocks tended to be lower at the warmer sites. We conclude that rotational grazing can enhance long-term SOC storage in semi-arid grasslands, thereby increasing their resilience and climate mitigation capacity, whereas abandoning grazing could lead to SOC losses.