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. (C) 2013 Elsevier B.V. All rights reserved.