Moderate pasture intensification enhances soil organic carbon stocks in a degraded Brazilian Ferralsol

Abstract

Sustainable intensification presents a unique opportunity to enhance carbon stocks in degraded pastures by optimizing forage production and consumption, thereby increasing soil organic matter (SOM) inputs. This study aimed to (i) compare soil carbon stocks to a depth of 1 m across various intensively managed pastures subjected to nitrogen fertilization, adjustments in animal stocking rates, and rotational grazing on a previously degraded pasture (DP) in a long-term field experiment in Brazilian Ferralsols; and (ii) evaluate changes in SOM composition. We compared irrigated pastures with a high animal stocking rate (IHS), a rainfed pasture with a high animal stocking rate (RHS), and a rainfed pasture with a moderate animal stocking rate (RMS) to improve DP. The origin of SOM was assessed using the natural abundance of ¹³C and its composition through the H/C atomic ratio and the aromaticity index (H_LIFS) obtained by laser-induced fluorescence spectroscopy. A comparison between the degraded pasture and the adjacent native vegetation showed that unmanaged pasture reduced carbon stocks by 45 Mg C ha⁻¹. However, intensification in degraded pasture enhanced the organic carbon stocks from 102 to 139 Mg C ha⁻¹ for RHS and 162 Mg C ha⁻¹ for RMS in the top 1 m. These values were comparable to those of the adjacent native forest (148 Mg C ha⁻¹). In contrast, high intensification in IHS with irrigation and high nitrogen fertilization did not increase organic carbon stocks compared to unmanaged DP, which remained at 111 Mg C ha⁻¹. Both RHS and RMS promoted the accumulation of aliphatic compounds, as evidenced by the high H/C atomic ratio and low H_LIFS index. When comparing RHS and RMS, we observed that a moderate animal stocking rate resulted in a greater accumulation of pasture-derived carbon (52 Mg C ha⁻¹) than a high animal stocking rate (37 Mg C ha⁻¹) in the top 30 cm. This difference is likely due to faster pasture regrowth under moderate stocking rates, leading to increased incorporation of pasture-derived SOM. Overall, moderate pasture intensification promoted carbon sequestration (2 Mg C ha⁻¹ year⁻¹) compared to DP by introducing carbon derived from forage, reducing nitrogen fertilizer use, and preserving carbon from native vegetation.