Exploring grazing intensity effects: nitrogen uptake in grassland species and soil carbon allocation

Background and aims Grazing drives carbon (C) and nitrogen (N) dynamics of grasslands through livestock trampling, defoliation, and excretion. Still, the responses of N uptake by plant species and simultaneous C allocation into the soil to grazing intensity remain unclear. Methods In-situ (NH4+)-N- 15 / (NO3-)-N-15 and C-13-CO2 labeling experiment was conducted in Inner Mongolia grasslands under 5 years of grazing with no, light (4 sheep 1.33 ha(-1)) and heavy (12 sheep 1.33 ha(-1)) intensity to reveal the contribution of plant-derived C into the soil and the fate of N on day one and three after C-13-labeling. Experiment had a completely randomized design (n = 3), and every plot included Leymus chinensis, Carex korshinskyi, Cleistogenes squarrosa, and Stipa grandis. Results Grazing increased plants' total N uptake compared to control (no grazing); higher NO3- uptake was found compared to NH4+ (aboveground: 0.40-20.78 vs. 0.32-6.58 mu g N m(-2); belowground: 0.04-9.92 vs. 0.01-0.49 mu g N m(-2)), irrespective of grazing intensity. C. korshinskyi showed the highest N uptake (3-21 mu g N m(-2)) under the three grazing intensities. C-13-CO2 assimilation was the lowest under heavy grazing (aboveground: 1.06-10.67 mg C m(-2); belowground: 0.25-1.53 mg C m(-2)) regardless of plant species. C-13-CO2 assimilation by L. chinensis and C. squarrosa decreased 3-5 times with grazing intensity. Grazing increased C-13-SOC irrespective to soil depth compared to no grazing. Conclusions Grazing patterns affected the plants' total assimilation C capacity and N uptake and the response varies among plant species, as well as the allocation of plant-C transfer into the soil.