Grazing intensity drives above-belowground productivity trade-offs and reveals belowground dominance in temperate herbaceous marsh wetlands.

Introduction: Livestock grazing, as a major anthropogenic disturbance, significantly affects plant species diversity and net primary productivity (NPP) in wetland ecosystems and their relationships with soil properties. However, how grazing intensity interacts with environmental factors to regulate the trade-off between aboveground and belowground productivity remains unclear. Methods: Field experiments were conducted in natural herbaceous marshes in the lower Tumen River basin of northeastern China. Three grazing intensity treatments were established: non-grazed (NG), moderately grazed (MG), and heavily grazed (HG). Aboveground net primary productivity (ANPP), belowground net primary productivity (BNPP), plant diversity indices, and soil physicochemical properties were measured. Structural equation modeling (SEM) was used to evaluate the direct and indirect pathways through which grazing intensity influences productivity, plant diversity, and soil properties. Results: Grazing altered plant community structure, shifting dominance from Cyperaceae to forb species, and moderate grazing significantly increased plant species diversity. With increasing grazing intensity, ANPP increased while BNPP decreased, indicating a clear aboveground-belowground productivity trade-off. Non-grazed plots exhibited the highest total NPP, suggesting that reductions in BNPP under grazing constrain ecosystem carbon accumulation. Grazing also reduced soil organic carbon, nutrient availability, moisture, and electrical conductivity while increasing bulk density and redox potential. SEM analysis showed significant direct effects of grazing on ANPP and BNPP, whereas indirect effects mediated by species diversity or soil properties were weak. Discussion: Although BNPP declined with increasing grazing intensity, it exerted the strongest control over total NPP, highlighting the critical role of belowground processes in regulating ecosystem carbon cycling under grazing disturbance. Moderate grazing can promote aboveground growth and diversity, whereas heavy grazing damages root systems and threatens ecosystem stability. These findings provide new insights into grazing-environment interactions and offer a scientific basis for the conservation and restoration of herbaceous marsh wetlands.