Grazing exclusion facilitates more rapid ecosystem carbon sequestration of degraded grasslands in humid than in arid regions

Grazing exclusion has been proposed as an effective practice to enhance ecosystem carbon stocks in arid-semiarid degraded grasslands. However, in humid regions, the effects of grazing exclusion on ecosystem carbon stocks and their components in degraded grasslands remain largely unknown. Here, the effects of enclosure on ecosystem carbon stocks and their components were explored using four enclosure ages, each paired with adjacent grazed areas. Moreover, the results were synthesized with previous studies to test if enclosure's effects in semihumid-humid regions differed from those in arid-semiarid regions. The field experiment found that six months to 1 year of grazing exclusion sequestrated 14.6 % higher carbon in the enclosed grasslands than in paired-grazed grasslands as a consequence of increased carbon storage of plants and topsoils. However, compared to grazed areas, enclosures over 3–6 years contained 14.4 % less ecosystem carbon stocks, primarily due to markedly decreased soil carbon sequestration, despite increased plant carbon stocks. Structural equation models further suggested that the decrease in soil carbon stock might be owing to the decomposition of organic matter induced by low phosphorus. Additionally, compared with the northern slope of the mountain, ecosystem carbon stocks on the southern slope showed a lagged and weak response to grazing exclusion. The results, combined with a meta-analysis, revealed that plant and soil carbon stocks in semihumid-humid grasslands were more sensitive to enclosure than in arid-semiarid grasslands. These results highlight that short-term fencing (e.g., six months to 1 year), rather than long-term, is a practical strategy to foster degraded grassland ecosystem carbon sequestration in semihumid-humid regions, thus contributing to climate change mitigation.