Changes in soil organic matter fractions under subtropical no-till cropping systems

Conservation management systems increase soil C and N pools. However, their effects on particulate (> 53 mum) and mineral-associated (< 53 mum) soil organic matter (SOM) fractions are less understood under subtropical climatic conditions. This study evaluated the long-term (12-yr) effects of three no-till cropping systems {bare soil, BS; oat (Avena strigosa Schreb.) + vetch (Vicia sativa L.)/maize (Zea mays L.) + cowpea (Vigna unguiculata L. Walp.), O + VIM + C; and maize + Cajanus [Cajanus cajan (L.) Millsp.], M + C} on C and N pools in particulate and mineral-associated SOM. The study was performed in southern Brazil, on a sandy clay loam Acrisol. Cropping systems that included cover crops increased C and N pools in both particulate and mineral-associated SOM when compared with BS. Mineral-associated SOM had five to nine times more C and 13 to 26 times more N than particulate organic matter and was responsible for 69 to 80% of total atmospheric CO2 sequestred by soil in O + V/M + C (38 Mg ha(-1)) and M + C (51 Mg ha(-1)). The higher C and N pools were associated with greater recalcitrance of mineral-associated SOM to biological decomposition, resulting from its interaction with variable charge minerals. The negative relationship between decay rates of SOM and the concentrations of Fe oxides and kaolinite demonstrated the physical stability of SOM caused by interaction with variable charge minerals. Power saturation curves of electron spin resonance (ESR) spectroscopy in the 20- to 53-, 2- to 20-, and <2-mum granulometric fractions also reinforced this hypothesis. The SOM interaction with variable charge minerals plays an important role in preserving SOM storage, enhancing the potential of tropical and subtropical soils to act as an atmospheric CO2 sink.