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

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.

In-situ  ¹⁵NH₄⁺ / ¹⁵NO₃⁻ and ¹³C-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 ¹³C-labeling. Experiment had a completely randomized design (n = 3), and every plot included Leymus chinensis, Carex korshinskyi, Cleistogenes squarrosa, and Stipa grandis.

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 µg N m⁻²; belowground: 0.04–9.92 vs. 0.01–0.49 µg N m⁻²), irrespective of grazing intensity. C. korshinskyi showed the highest N uptake (3–21 µg N m⁻²) under the three grazing intensities. ¹³C-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. ¹³C-CO₂ assimilation by L. chinensis and C. squarrosa decreased 3–5 times with grazing intensity. Grazing increased ¹³C-SOC irrespective to soil depth compared to no grazing.

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.