Carbon sequestration in mixed pasture with forage peanut or in nitrogen-fertilized monoculture

Global warming and its effects on our planet have been widely debated. Brazil, as a country with vast livestock areas, is part of this discussion due to the greenhouse gas emissions associated with these systems. This study assessed whether a mixed pasture is effective in carbon (C) sequestration as a grass pasture system in monoculture fertilized with nitrogen (N). The research was carried out at the Livestock Research Station (ESSUL-CEPLAC; 16º 39′ S, 39º 30′W, 92 m a.s.l.). The area is in the Atlantic Forest biome and the climate is a transition between Af and Am. The soil at this site is an Acrisol acidic, with low natural fertility. An area of eight ha of secondary forest was cleared of vegetation in 1995. The area was divided into four blocks with two treatments randomly distributed in each block, named: Marandu grass (Urochloa brizantha cv. Marandu) and the other elephant grass (Cenchrus purpureus cv. Cameroom), both amended with 160 kg of N ha^-1 yr^-1 until 2003. In 2003, four plots of elephant grass were ploughed and planted with Marandu grass and forage peanut (Arachis pintoi cv. Belomonte). Marandu grass monoculture was fertilized twice a year with 60 kg N ha^-1 as urea. Soil samples were originally taken from the deforested area in 1995, and after in 2003 and 2019. Soil samples were collected at a depth of 0-30 cm to determine soil bulk density and C concentration. Over eight years, C stocks increased compared to native vegetation (38.4 Mg ha^-1) by 4.8 Mg ha^-1 under elephant grass and 9.9 Mg ha^-1 under Marandu grass, reaching C stocks of 43.2 and 48.3 Mg ha^-1, respectively. Thus, the Marandu grass monoculture was more efficient at storing C than elephant grass, sequestering approximately 5 Mg ha^-1 more, with an annual C sequestration rate of 1.23 Mg ha^-1 year^-1, compared to 0.60 Mg ha^-1 year^-1 for elephant grass. After 16 years, the C stock in the mixed pastures (52.4 Mg ha^-1) and the fertilized pasture (51.8 Mg ha^-1) were similar statistically and indicate a decline in sequestration rates, at 0.57 and 0.22 Mg ha^-1 year^-1, respectively. Therefore, mixed pasture with forage peanut was effective in accumulating SOM as such as annual N fertilization. Over time, there was a reduction in the C accumulation rate. This highlights the grass-legume mixed pasture is a viable alternative to replace N fertilization in pastures when aiming for C sequestration.