Tradeoffs to manage sustainable systems of grain production in tropical soils

Long-term sustainability of intensive cropping systems in tropical soils depends on strategies to reconcile grain output, cover crops to maximize biomass yield for soil mulching, and maintaining soil organic carbon (SOC). In this study, we evaluated spatiotemporal constraints underpinning the diversification from soybean (Glycine max (L.) Merr.) monoculture under conventional tillage (CT) or no-tillage (NT) and soybean in succession with maize (Zea mays L.) double cropping (NTS3) to the use of millet (Pennisetum glaucum L., NTS1) and brachiaria (Brachiaria ruziziensis (R.Germ. & Evrard) Crins, NTS2) in successions or millet, crotalaria (Crotalaria spectabilis Roth.), and maize+brachiaria in rotations in a 12-year field experiment in an Oxisol. We quantified total grain output, total biomass production, and SOC contents and stocks (0–40 cm depth). Over 12 years, cumulative grain output under the double cropping (NTS3) was 135.0 Mg ha⁻¹, with soybean-maize proportions of 35–65 %. Thus, the successions NTS1 and NTS2 would reduce maize grain output relative to NTS3 (7.3 Mg ha⁻¹ year⁻¹). Under the rotations, reduction in soybean and maize grain output relative to NTS3 was about 1.4 and 4.1 Mg ha⁻¹ year⁻¹, respectively. Biomass production under NTS3 was 12.0 Mg ha⁻¹ year⁻¹ but reached 15.7 Mg ha⁻¹ year⁻¹ under the rotation including summer soybean (1st and 2nd years) and off-season crotalaria (1st year) or maize+brachiria (2nd year), and only brachiaria (3rd year). Under this rotation, changes in SOC stocks were +2.0 Mg ha⁻¹ year⁻¹ for the 0–40 cm depth layer, whereas under NTS3 it was +1.4 Mg ha⁻¹ year⁻¹, relative to the native Cerrrado. For soybean monoculture under CT or NT changes in SOC were about −0.5 Mg ha⁻¹ year⁻¹ relative to the native Cerrado. Overall, despite the reduced total grain output, the rotation system proposed may be critical to reconcile sustainable food security, soil protection by mulching and SOC maintenance in agricultural Cerrado soils.