Legume cover crops under no-tillage favor organomineral association in microaggregates and soil C accumulation

Both no-tillage and legume cover crops have been shown to increase soil organic carbon (SOC) in subtropical soils. However, the mechanisms underpinning management system effects on SOC accumulation are still not well understood. We used a combination of aggregate size and density fractionation to elucidate these mechanisms at a 30-year old experiment on an Acrisol in southern Brazil. The effects of two tillage systems [conventional system (CT) and no-tillage (NT)] combined with three cropping systems [oat/maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cowpea (OV/MC)] were evaluated in the top 20 cm soil layer. Overall, macro-aggregation (> 0.25 mm) was significantly influenced by tillage with NT showing values 14% greater than CT in the 0-5 cm soil depth. On average, the occluded light fraction-C content in macroaggregates was more than twice as high under NT compared to CT (4.4 vs. 1.8 g kg(-1)). This effect was more pronounced when legume cover crops were grown. However, the most significant effect of cover crops was observed in the organomineral fraction of microaggregates, especially under NT (12.1 under O/M and 19.8 g kg(-1) under OV/MC). Our results suggest that, although NT increased the occluded light fraction-C compared to CT, this effect was smaller than the gains that legume cover crops offered in organomineral association.