Humification decrease of soil organic matter under no-tillage

The less oxidative environment in soils of no-tillage systems decreases the humification degree of organic matter. To test this hypothesis, the concentration of semiquinone free radicals (SFR) of organic matter from the surface layer (0 to 25 mm) of a Humic Cambisol (Lages, Santa Catarina State) was evaluated. The soil had been cultivated for 8 yrs under conventional tillage (CT), reduced tillage (RT), and no-tillage (NT). The SFR concentration in organic matter was assessed by electron spin resonance (ESR) in the granulometric fractions (> 53,53-20,20-2, and < 2 mum). Among the tillage systems, the lowest concentration of SFR in the whole soil organic matter was observed in no-tilled soil (15.83 x 10(17) 'spins' g(-1) C), in comparison with CT (18.33 x 10(17) 'spins' g(-1) C) and RT (18.39 x 10(17) 'spins' g(-1) C). Organic matter of the 20-2 Am size fraction showed a higher concentration of SFR and a smaller line width, which is consistent with a higher humification degree and/or higher interaction with soil minerals, in comparison with the size fractions > 53 and < 2 mum. The ESR signal of SFR was not detected in the 53-20 mum granulometric fraction due to the low organic C contents. In addition to the effect on soil C and N stocks, no-tillage decreases the humification degree of organic matter which may have an important influence on the soil quality.