Assessing the impact of pasture management on mineral-associated organic matter and soil health

Pastures managed under extensive grazing are susceptible to degradation, leading to negative effects on soil organic carbon (SOC) stocks and soil health, including microbial stress and reduced cation exchange capacity (CEC). Understanding how SOC storage can be enhanced through management practices and their effects on soil health is crucial for developing sustainable agricultural practices. This study investigates SOC stocks in pastures under different management and their relationship with microbial composition and CEC. The impact of different management practices on the recovery of degraded pastures was assessed in a long-term experiment initiated in 1996 in São Carlos, São Paulo, Brazil, by Embrapa Pecuária Sudeste. The experiment aimed to intensify productivity under various systems: irrigated pasture with high stocking rate (IHS, 600 kg N ha^-1 year^-1), rainfed pasture with high stocking rate (RHS, 400 kg N ha^-1 year^-1), and rainfed pasture with moderate stocking rate (RMS, 200 kg N ha^-1 year^-1). Degraded pasture (DP) under extensive grazing and adjacent native forest (FO) were used as references. All pastures, except DP, were limed, fertilized, and subjected to rotational grazing. Soil samples were collected to a depth of 1 meter in a Rhodic Haplic Ferralsol, and SOC pools were differentiated using physical fractionation to separate particulate organic matter (POM, <1.8 g cm^-3) from mineral-associated organic matter (MAOM, >1.8 g cm^-3). Clay-sized particles in MAOM were analyzed with nanoSIMS, and phospholipid fatty acids (PLFA) were quantified by GC-FID. CEC was measured in bulk soil and SOC pools. The combination of a moderate stocking rate and low nitrogen fertilizer input in the RMS system resulted in higher carbon stocks compared to the DP system, with an increase of 63 Mg C ha^-1. SOC was primarily stored in clay-sized particles, and nanoSIMS revealed a heterogeneous distribution of SOC on mineral surfaces, with SOC associated with Al and Fe. PLFA analysis showed that microorganisms in the DP system adapted their membranes, likely due to microbial stress, as indicated by the 16:1n7/cy17:0 ratio. In contrast, the RMS system improved MAOM, alleviated microbial stress, and increased CEC to 108 mmolc kg^-1, compared to 58 mmolc kg^-1 in DP. The MAOM fraction plays a critical role in carbon storage and CEC improvement. These pasture management practices not only increased SOC storage in Brazilian Ferralsols but also enhanced CEC, providing additional co-benefits.