Managed pastures enhance soil carbon stocks from degraded pasture in Ferralsol of Brazilian Cerrado

The conversion of native vegetation into agricultural lands is often associated with a decrease in soil C. The soils from the Brazilian savannah (named Cerrado), with 200 million hectares, are rich in Fe and Al (hydr)oxides, which could result in more organo-mineral associations and lead to particularly high C storage. The changes in the C stocks from the conversion of native forest into degraded pasture (DP), and the adoption of proper management to recover DP and increase C stocks in such Ferralsols are not well understood. To provide insights into the drivers of C storage, this study compared the C stocks across depth in the top 1m and the distribution of C in the soil fractions 24 years after the adoption of different management systems in degraded pastures in the Brazilian Cerrado.

A DP area located in São Carlos, São Paulo, Brazil was converted into different management systems: (i) RMS: rainfed pasture with moderate animal stocking rate, (ii) RHS: rainfed pasture with higher animal stocking rate, and (iii) IHS:  irrigated pasture with higher stocking rate. As a control, the adjacent native vegetation (FO) was also evaluated. The adoption of management started in 1996 with RMS and in 2002 for RHS and IHS. Except for the DP, all areas were limed and N-fertilized. RMS with 200 kg N ha, RHS 400 kg N ha, and IHS with 600 kg N ha. Soil sampling was carried out in 2020 and the C stocks were evaluated up to 1 m deep. To state vegetation change from C3 (native forest) to C4 (introduced pasture) the isotopic natural abundance of ¹³C was analyzed. To evaluate the contribution of mineral-associated and particulate organic matter forms to C storage, we performed a physical fractionation by size and density with SPT 1.8 g cm^-3, respectively.

Our results showed that the conversion of FO into DP decreased soil C stocks.  Otherwise, the adoption of management in DP with RMS and RHS increased C stocks achieving levels similar to FO. RMS showed the highest C stocks with the lower dosage of N-fertilizer and animal stocking rate. IHS area did not increase their C stocks compared to DP, which may be related to limited root growth after irrigation decreasing the C input. Around 50% of the C stocks in RHS and RMS systems are pasture-derived (C4 plants) according to the ¹³C abundance. This shows that half of C stocks from rainfed pastures is of preserved organic matter from previous FO. While in the IHS and DP systems, the organic matter composition is mainly pasture-derived. Our preliminary data showed that the RMS topsoil contained more free particulate organic matter than the FO, suggesting that the C stocks were enhanced mainly by pasture-derived biomass input. The contribution of mineral-associated organic matter still will be evaluated.

Our study shows that the recovery of degraded pasture soils by management leads to increased OC stocks derived from fertilized pasture but also higher maintenance of OC from FO.