Newly assimilated carbon allocation in grassland communities under different grazing enclosure times

Abstract

The flow of photosynthesized carbon (C) from plants to soil and its loss from the respiration in the soil-plant system were measured via in situ 13CO2 pulse labeling pasture in overgrazed and long-term grazing exclosures (where grazing was not permitted for 5, 10, and 30 years). During a 30-day chase period, the allocation of newly assimilated C in the shoots, roots, and soil of grazing-excluded grasslands was higher than that in overgrazed grasslands. But the newly assimilated C allocated to shoots and soil respiration in overgrazed grasslands was significantly higher than that in grazing-excluded grasslands. Thirty days after labeling, 35% and 40% of the newly assimilated C in overgrazed grasslands were lost through shoots and soil respiration, respectively. The shoot 13C recovery was 32% in the 30 years 30 days after labeling, which was significantly higher than that obtained at the 5 and 10 years exclosure sites. Root and soil 13C recoveries in the 5- and 10-year exclosures were higher than those at the 30-year sites. In the grazing-excluded grasslands, the amount of 13C in soil organic C peaked 3 days after 13CO2 exposure and became relatively stable after 19 days of assimilation. The mean C residence time for non-structural C used in the soil respiration under overgrazing (10 days) was shorter than that under 5- (20 days) and 10- (17 days) or 30-year exclosures (14 days). Compared to overgrazing, grazing exclosure, especially moderate grazing exclosure, led to more allocation of newly assimilated C in the plant-soil system but fewer allocation in shoots and soil respiration.