Potential long-term impacts of livestock introduction on carbon and nitrogen cycling in grasslands of Southern South America
2006
PIÑEIRO, GERVASIO | Paruelo, José María | OESTERHELD, MARTIN
Empirical evidence based on grazing exclusion at the scale of years to decades shows that grazing modifies carbon (C) and nitrogen (N) cycling. However, long-term effects at the scale of centuries are less known, yet highly relevant to understand local and global impacts of grazing. Additionally, most studies have focused on the isolated response of C and N, with little understanding of their interactions. Using CENTURY, a process-based biogeochemical model, we analyzed the impacts of 370 years of livestock grazing (i.e. long term, from early European colonization to present) in 11 sites across the Río de la Plata grasslands and compared them with those resulting from two decades of grazing (i.e. mid-term, typical exclosure experiment). In the long term, livestock grazing primarily altered the N cycle through faster N returns to the soil via urine and dung, which were offset by uninterrupted N outputs by volatilization and leaching. As a result, soil organic N decreased by -880 kg ha⁻¹ or -19%. Higher N outputs (mainly as NH₃) opened the N cycle, potentially decreasing N₂O and NOx emissions and increasing N depositions over the region. These greater outputs of N constrained C accumulation in soils, reducing soil organic C by -21 200 kg ha⁻¹ (-22%, a reduction of -1.5 Pg of C for the whole region) and net primary production by -2192 kg ha⁻¹ yr⁻¹ (-24%). Mid-term simulations showed that the effects of livestock introduction in a decadal time scale were substantially different both in magnitude and direction from long-term responses. Long-term results were not substantially affected when atmospheric CO₂ content, species composition and fire regime were changed within plausible ranges, but highlighted fire-grazing interactions as a major constraint of long-term C and N dynamics in these grasslands.
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