Work

Moderate grazing optimizes the allocation and microbial fate of recently assimilated carbon in an alpine grassland

Year: 2026

Type: article

Abstract: Alpine grasslands are crucial carbon (C) reservoirs, facing strong disruptions from intensified grazing. However, how grazing intensity regulates the allocation of recently assimilated C belowground and its utilization by soil microorganisms remains uncertain. Using in-situ 13CO2 pulse labeling in an alpine steppe, we compared the effects of non-grazing (NG), moderate grazing (MG), and heavy grazing (HG) on the allocation and cycling of recently assimilated C in shoots, roots, soil, and soil microorganisms. The initially assimilated 13C in shoots were highest under MG (245 mg C m–2). Root δ13C peaks were also highest under MG (28.5 mg C m–2), yet their mean residence time (MRT) was shortest (3.1 days), indicating rapid uptake and downward transport of newly fixed C through roots under MG. In contrast, HG delayed shoot δ13C peaks (to day 14) and produced the only pronounced soil δ13C peak (4‰), followed by a rapid decline, suggesting accelerated but short-lived belowground C allocation after severe defoliation. Despite phospholipid fatty acid (PLFA) profiles remaining stable, the share of 13C-PLFA held by Gram-positive bacteria increased from 30% to 40% under HG, indicating they became the dominant consumers of rhizodeposits. Besides, increasing grazing intensity exponentially shortened MRT across plant, soil, and microbial C pools. These results demonstrate that MG optimizes C allocation belowground and the microbial fate of plant-derived C, but HG accelerates plant–soil C fluxes and alters microbial consumer of rhizodeposits. Our study emphasizes the need to track where C flows and how long it resides in grazing grasslands.

Source: Journal of Plant Ecology

DOI: 10.1093/jpe/rtag165