Soil carbon sequestration under N fertilized or mixed legume-grass pastures depends on soil type and prior land-use

Grass-fed beef operations with well-managed pastures mixed with stoloniferous forage legumes are an alternative to increase animal productivity and sequester carbon (C) in the soil. However, using the same system, differences in response time and C sequestration may occur when established in different regions. The objective of this study was to evaluate the impact on C and nitrogen (N) stocks in the soil due to the implementation in the same year, of pasture systems with and without N fertilization or mixed with stoloniferous forage legumes in two regions under different edaphoclimatic conditions. The two experiments were established at the same time and carried out in two areas contrasting in soil type and edaphoclimatic features. The first area was in a transition between the Cerrado and the Atlantic Forest biomes with a Ferralsol with a uniform clayey texture (Lavras site). The second area was in an Atlantic Forest biome with an Acrisol with low natural fertility and sandy texture (Itabela site). Marandu grass (Urochloa brizantha cv. Marandu) fertilized or not with N or mixed with forage legumes for a shorter and longer times were evaluated. Soil samples were taken at the start of the study and after 8 and 15–38 years of grazing and analyzed for total C and N, ¹³C abundance and fractionation of soil organic matter. After the first eight years at Lavras, the soil indicated that C and N stocks were starting to increase under this management after a long period of plough tillage for maize production. The longer-term pastures (after 15 and 20 years of establishment) mixed with forage legume or fertilized with N, respectively, showed that C stocks recovered to their original status under the native vegetation at the Lavras site. At Itabela, the soil C and N stocks were similar to the stocks under the native vegetation even after 35 or 38 years of mixed pasture, but the stocks were far lower than at Lavras, which can be attributed to the much coarser soil texture. Pasture systems of similar productivity may promote the accumulation of very different C stocks depending on the land use history prior to their installation and soil clay content.