C and N stocks and the role of molecular recalcitrance and organomineral interaction in stabilizing soil organic matter in a subtropical Acrisol managed under no-tillage

Molecular recalcitrance and organomineral interaction are factors that determine soil organic matter (SOM) stability. This study was based on a medium-term (5 years) experiment located in southern Brazil and aimed (i) at investigating the influence of native grassland and two no-till cropping systems [bare soil and fallow/maize + mucuna (Stizolobium cinereum)-F/M + Mu] over the C and N contents in the whole soil down to 17.5 cm depth and in the particle-size fractions (> 150, 150-53, 53-20, 202, and < 2 mu m) of the 0-2.5 cm layer of a sandy loam Acrisol (87 g kg(-1) clay) and (ii) at assessing the role of molecular recalcitrance and organomineral interaction in stabilizing SOM in this subtropical soil. Molecular recalcitrance of SOM was inferred through the concentration of semiquinone free radicals, as assessed by ESR (electron spin resonance) spectroscopy. Semiquinone radicals are supposed to derive from and be stabilized by recalcitrant aromatic structures and are also believed to relate to the recalcitrance imparted by disordered structural conformation of humic substances. Organomineral interaction was assessed through power saturation experiments of ESR spectroscopy. Compared to grassland, the legume-based F/M+Mu system increased the C and N stocks of the 0-17.5 cm layer at rates of 0.40 Mg C ha(-1) year(-1) and 0.06 Mg N ha(-1) year(-1). Losses of 0.80 Mg C ha(-1) year(-1) and 0.02 Mg N ha(-1) year(-1) were observed in bare soil. Only the fine silt (20-2 mu m) and clay (< 2 mu m) fractions had their C and N contents affected by management. The clay fraction contained the highest C (33.2 to 50.5 g C kg(-1)) and N (2.95 to 4.34 g N kg(-1)) concentrations, and the lowest C:N ratios (10.1 to 11.6). The concentration of semiquinone free radicals in physical fractions increased in the order F/M+Mu < grassland < bare soil. Among the < 53 mu m fractions, the highest concentration of semiquinone free radicals was found in fine silt (20-2 mu m). These results suggest that organic matter in bare soil or in fine silt fraction is more recalcitrant than in the grassland and F/M + Mu systems or in the coarse silt and clay fractions. Saturation of the electron spin in the ESR power saturation experiments easily occurred in the coarse silt fraction, required more power to occur in fine silt and was not observed, within the employed power range, in the clay fraction. This result evidenced the existence of stronger organomineral interactions in the clay-size fraction, which contains the highest iron oxides and kaolinite concentrations, than in fine and coarse silt-size fractions. (c) 2005 Elsevier B.V. All rights reserved.