Carbon storage in labile fractions of soil organic matter in a tropical no-tillage Oxisol

Carbon storage in the soil organic matter (SOM) is an important strategy to mitigate carbon dioxide concentration in the atmosphere. The objective of this study was to evaluate the effect of the no-tillage (NT) use for six years, under four autumn (dry-season) cover crops based maize and soybean crop rotations (pigeon pea-pearl millet, bengan hemp-sorghum, sunflower-black oat, and wild radish-maize), on C storage in the particulate (>53 mum) and mineral-associated (<53 mum) SOM fractions of a clayey Oxisol from Brazilian Cerrado region. In comparison to conventional tillage (CT) under summer cash-crops, NT increased the total organic carbon (TOC) stocks in the surface soil layer (0-5 cm). In the top 20 cm layer, NT soil under wild radish-maize had 9% (4.66 Mg ha(-1)) more TOC than the conventionally tilled soil. On the other hand, C storage in NT soil under other cropping systems was statiscally similar to the CT soil. The C stocks in the particulate SOM fraction increased by 37 to 52% in NT soil (0-20 cm) in comparison to CT. The higher sensitivity to soil management changes made the particulate organic matter a more adequate soil quality index compared to the TOC stocks. The C stock in the mineral-associated SOM fraction was not affected by soil management systems, which can be related to the short-term under NT and or to the highly stable soil microaggregates in this clayey Oxisol. The preferential C storage in the labile SOM fraction is an environmental benefit, which is expected to occur only under continuous no-tillage and crop residues addition.