Soil organic matter dynamics and soil carbon stocks from tropical and sub-tropical areas under no-till: An on-farm research

Understanding the mechanisms of soil organic matter (SOM) stabilization is fundamental to soil carbon sequestration and reduction of greenhouse gas emission (GEE), playing a pivotal role in the soil carbon cycle. Soil samples were collected from 1-meter trenches in no-till systems and compared to native vegetation, totalizing 256 samples from commercial farms in the Cerrado, Atlantic Forest, and Pampa Brazilian biomes. Soil fractionation (>53 µm and < 53 µm), laser-induced fluorescence spectroscopy (LIFS), and δ¹³C isotopic analysis were employed to investigate SOM dynamics and their relationships with soil carbon stocks. SOM presents chemical compounds more susceptible to decomposition in the surface, increasing its recalcitrance in deeper layers. Over 90 % of soil carbon is in the < 53 μm fraction, indicating organo-mineral association as the major SOM stabilization mechanism. However, in a minor extension physical protection by soil aggregation, can play a role in SOM stabilization. H_LIFS from < 53 μm fraction demonstrated that SOM in these agricultural areas contains more labile compounds than in native vegetation areas, indicating recently incorporated less transformed organic material. Farms 2 and 4 (Cerrado) showed increases in carbon stocks in agricultural areas compared to native vegetation of 89 Mg C ha^-1 and 24 Mg C ha^-1, respectively, with distinct δ¹³C patterns. Farm 1 (Atlantic Forest) presented a reduction of 59 Mg C ha^-1, primarily due to land use change, from native vegetation to agriculture. Farm 3 (Pampa) maintained its carbon stock, with no differences in δ¹³C patterns. These findings provide valuable insights into SOM stabilization for carbon sequestration, and the benefits of conservationist management practices, emphasizing the importance of generate data from on-farm research experiments. Measurements from agricultural and native vegetation areas in on-farm research across commercial farms provide real-field data to optimize protocols and support good management practices for enhancing soil carbon sequestration in tropical regions.