Are soil carbon and nitrogen stocks at steady state despite introducing grass and legumes to soybean and maize production system?

Abstract

Land use change from Brazilian savanna (Cerrado) to agrosystems has resulted in the depletion of soil carbon (C) and nitrogen (N) pools, despite the adoption of conservation management practices in soybean and maize production systems (SMPSs). This study aimed to investigate a nine-year experiment under SMPSs in continuous succession (DC) and in rotation (CR) with Congo grass, rattlepod, and pigeon pea using a gradient of N fertilizer levels. Soil sampling was conducted down to a 1.0 m depth to measure total and dissolved C and N stocks; topsoil was further evaluated for CO2 evolution in a 500-d incubation assay. Additional soil samples from native Cerrado vegetation were assessed as a reference. Land use change led to 35% and 28% soil C and N stock depletion in the 0–0.2 m layer, respectively, and to a reduction in dissolved forms throughout the soil profile. Although no differences in total C and N stocks between cropping treatments were observed, higher C and N inputs were detected under crop rotation (15% and 28%) and at high N fertilizer levels (14% and 20%). The soil organic matter mineralization assay revealed slower kinetics under CR than under DC, suggesting that further effects on total C and N stocks may occur over longer periods. The stimulated decay of all C pools by 25–175% with increasing N supply reflects the impact that its deficiency has on the cycling of C under these systems. Overall, nine years of soybean–maize succession has not caused a depletion of soil C and N stocks compared with more diverse crop production while providing two annual harvests when N demand is met by fertilization supply. Graphical abstract: [Figure not available: see fulltext.]