SOIL CARBON STORAGE IN SOYBEAN SYSTEMS: A PATHWAY TO SUSTAINABLE TROPICAL AGRICULTURE

To ensure the sustainability of tropical agriculture while maintaining profitability, food security, and resilience to climate change, no-tillage systems incorporating diverse cover crop strategies are implemented. This study aimed to assess the effects of different soybean cropping systems on soil carbon (C) fractions and C storage capacity in tropical agricultural systems. The experimental area belongs to the MT Foundation and is located in Itiquira, MT, Brazil. The soil at this site was classified as Oxisol (clay content of 664 g kg^-1). The experiment followed a randomized block design with four replications and eight cultivation systems: conventional tillage soybean–fallow (CT), conventional tillage without soil disturbance soybean–fallow (NT), no-tillage in succession with the following crops: soybean–millet (NTS1), soybean–brachiaria (NTS2), and soybean–maize (NTS3); and  no-tillage in crop rotation with soybean–millet / soybean–crotalaria / maize+brachiaria (NTR1), soybean–crotalaria / maize+brachiaria / soybean–crotalaria (NTR2), and soybean–crotalaria / soybean–maize+brachiaria / brachiaria (NTR3). After 12 years of cultivation, soil samples were collected from the 0–40 cm depth layer and the physical fractionation of soil organic matter into particulate organic matter (POM) and mineral-associated organic matter (MAOM) was carried out. Subsequently, the total C stocks (C_TOTAL stock) and within fractions (C_POM and C_MAOM stocks) were determined and corrected based on the equivalent soil mass using NT as the reference soil. In the 0–40 cm layer, the C_TOTAL stock was higher in NTS2 (104.04 Mg ha^-1), NTR2 (100.04 Mg ha^-1), and NTR3 (106.06 Mg ha^-1) compared to CT (79.27 Mg ha^-1) and NT (80.24 Mg ha^-1). The C_POM stock was greater in the no-tillage treatments (15.72 Mg ha^-1 on average) than in CT (4.18 Mg ha^-1) and NT (6.47 Mg ha^-1), except for NTS1 (8.33 Mg ha^-1). NTS2 (21.19 Mg ha^-1) exhibited the highest C_POM stock, which was higher than in CT and NT. NTR3 had the highest C_MAOM stock (90.42 Mg ha^-1), which was 1.20 times greater compared to NTS1 (77.24 Mg ha^-1), CT (75.09 Mg ha^-1), and NT (73.77 Mg ha^-1). No-tillage systems increase C_TOTAL, C_POM, and C_MAOM stocks compared to CT and NT, indicating that the isolated removal of soil tillage processes is not a sufficient management practice to promote an increase in soil C stocks. Brachiaria demonstrates greater efficiency in maintaining and increasing soil C stocks compared to millet and maize. No-tillage systems are a pathway to sustainable soybean production.