Soil carbon stock and sources in pastures systems in monoculture or mixed with Desmodium

 Soil management systems influence carbon (C) dynamics. Inadequate pasture management in Brazil can lead to soil C loss, contributing to CO2 emissions and global warming. This study assessed the impact of pasture systems on soil C stocks using the isotopic 13C technique to identify the contribution of C3 and C4 vegetation. The research was carried out at the Livestock Research Station during the year 2022 (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, with low natural fertility. The study analyzed the soil under five different areas: NV: native vegetation; BMNF: monoculture of Marandu grass [Urochloa brizantha cv. Marandu] planted in 2014 with no additional N fertilizer. BM+N: monoculture of Marandu grass planted in 2014 with annual N fertilizer addition of 150 kg N ha-1 yr-1. BMDO: mixed pasture of Marandu grass with desmodium [Grona heterocarpa subsp. Ovalifolia cv. Itabela] planted in 2014 with no N fertilizer. BMDO33: mixed pasture of Marandu grass with desmodium planted in 1989. Soil samples were collected for the analysis of bulk density, C content, and isotopic abundance (¹³C). The total C stock (0-40cm) and the C derived from C3 plants and C4 plants were calculated. Statistical differences were analyzed using the Tukey's test at a 5% probability level. The BMNF (54.7 Mg C ha-1), BM+N (58.6 Mg C ha-1), and BMDO (62.6 Mg C ha-1) exhibited a greater C stock than NV (50.5 Mg C ha-1). BMDO had a C stock of 4.0 Mg C ha-1 larger than BM+N showing that legume forage can replace mineral N fertilizers enabling more mitigation of greenhouse gas emissions and improving carbon accumulation and 12.1 Mg C ha-1 greater than VN. Regarding the C stock origin, the mixed pasture systems showed a greater C amount derived from C3 plants due to the legume contribution, with BMDO and BMDO33 having 34.6 and 34.9 Mg C ha-1, respectively. For the pasture systems in monoculture, approximately 50% of the C originated from NV, with 26.4 and 28.7 Mg C ha-1 in BMNF and BM+N, respectively. The grass-legume mixed pasture system showed considerably greater C levels compared to the monoculture without fertilization (approximately 8 Mg C ha-1), indicating that it may be a sustainable alternative for C sequestration and mitigation of greenhouse gas emissions.