Combined role of no-tillage and cropping systems in soil carbon stocks and stabilization

Increases in carbon (C) input and stabilization are key processes to turn soils into sinks of atmospheric CO2–C and help mitigating global warming. We hypothesized (i) that C sequestration in no-tillage soil is further enhanced by high input cropping systems and (ii) that the sequestered C is stored mainly in the mineral associated fraction. The objective of this study was to assess, in 2003, the C sequestration and stabilization in a subtropical Acrisol (Eldorado do Sul, Brazil) subjected to 18-year conventional tillage [CT] and no-tillage [NT] combined with two cropping systems: black oat (Avena strigosa Schreb) as winter cover crop – maize (Zea mays L.) as summer grain crop [Ot/M]; and black oat plus vetch (Vicia villosa Roth) as winter cover crops – maize in summer intercropped with cowpea (Vigna unguiculata (L.) Walp) cover crop [Ot+V/M+C]. Soil C stock in the 0–20 cm layer was higher in NT than in CT, either in Ot/M (31.1 vs. 27.8 Mg ha−1, P < 0.05) or Ot+V/M+C (37.3 vs. 32.8 Mg ha−1, P < 0.05). Annual C sequestration rate in NT relative to CT was 0.25 Mg ha−1 in Ot+V/M+C and 0.18 Mg ha−1 in Ot/M, in agreement to the higher biomass-C addition and legume cover crops inclusion in Ot+V/M+C (7.6 vs 4.0 Mg ha−1 year−1) and to our first hypothesis. Increase in the proportion of large macroaggregates (9.51–4.76 mm) and of mean weight diameter occurred in NT soil up to 10 cm depth, both in Ot/M and Ot+V/M+C. In NT, most of the C accumulation relative to CT occurred in the mineral-associated fraction, showing the importance of organo-mineral interaction in C stabilization and supporting our second hypothesis. However, the physical protection by aggregates played equally important role by stabilizing the occluded particulate organic matter (occluded-POM) before it was further stabilized by organo-mineral interaction. No-tillage is recommended as a sustainable soil management, but to increase soil C accumulation, the potential of cropping systems such those based on legume cover crops must be concurrently explored.