Stocks and humification degree of organic matter fractions as affected by no-tillage on a subtropical soil

Assessment of particulate (>53-mum) and mineral-associated (<53-mum) soil organic matter (SOM) fractions is a useful approach to understand the dynamic of organic matter in soils. This study aimed to compare the long-term (9-yr) effects of no-tillage (NT) and conventional tillage (CT) on C and N stocks in the two above mentioned organic fractions in a Brazilian Acrisol. The degree of SOM humification, which has been associated with the concentration of semiquinone-type free radicals ('spin') determined by electron spin resonance (ESR), was also evaluated. Soil under no-tillage had 7.55 Mg ha(-1) (25%) more C and 741 kg ha(-1) (29%) more N than conventionally tilled soil in the 0-175-mm depth. Both particulate and mineral-associated SOM increased in the no-tilled soil. The increase of C and N stocks in the mineral-associated SOM accounted for 75% and 91% of the difference in total soil C and N stocks between NT and CT, respectively. Averaged across tillage systems, C and N stocks were respectively 4.6 and 16.8 times higher in the mineral-associated SOM than in particulate SOM. The higher C and N stocks were associated with greater recalcitrance of mineral-associated SOM to biological decomposition, resulting, probably, from its interaction with variable charge minerals. This is corroborated by a positive relationship between concentrations of C and iron oxides and kaolinite in the 53-20, 20-2 and <2-mum particle size classes, of the 0-25-mm soil layer. The degree of SOM humification, assessed by ESR, decreased in both the 53-20 and 20-2-mum fractions under NT. However, it was unaffected by tillage in the <2-mum fraction, which normally presented the lowest 'spin' concentration. Since quality as well as quantity of SOM improved in the no-tillage soil, adoption of this system is highly recommended for amelioration of degraded tropical and subtropical soils.