On-farm greenhouse gas emissions and soil carbon stocks of a soybean‒maize system

The agriculture sector is responsible for approximately one-quarter of Brazilian greenhouse gas (GHG) emissions. In the present study, we aimed to quantify the CO₂ equivalent (CO₂eq) balance on annual basis of three farms with soybean‒maize crop systems in the state of Mato Grosso. Each farm had an areas with native vegetation and two other areas cultivated with soybean under no-till practices with different conversion times [2–5 years (S1) and more than 10 years (S2)]. Soil samples were taken to determine soil C stocks. GHG emissions were estimated for the 2020/2021 crop season according to the scope, GHG type, and source. The results showed that, on average, the soil total carbon (C) stocks under S1 and S2 underwent a 12% reduction relative to native vegetation (< 15 Mg ha⁻¹), but they increased by 30% (> 22 Mg ha⁻¹) with increasing time since conversion (S2 vs. S1). The scope 1 (direct emissions from the use of fuels in mobile and stationary implements when applying lime and nitrogen fertilizers, and from decomposition of crop residues) represented 67% of the total emissions, whereas the scope 3 (indirect emissions associated with the production of synthetic fertilizers, lime, fuels and seeds) were responsible 33% of the total GHG emissions on the three farms. On average, for the soybean‒maize farms, the observed GHG emissions (losses) were approximately 2.1 Mg CO₂eq ha⁻¹ yr⁻¹, while soil C sequestration amounted to 4 Mg CO₂eq ha⁻¹ yr⁻¹, resulting in a mean CO₂eq balance of approximately -1.9 Mg CO₂eq ha⁻¹ yr⁻¹. Our study showed that the sequestration of C in soil can be adopted in initiatives as a carbon dioxide removal strategy in agriculture systems.