Soil organic matter pools and their associations with carbon mineralization kinetics

The labile component of soil organic matter (SOM) plays an important role in short-term nutrient turnover. Our objectives were (i) to establish the relationships between carbon in soil density fractions with carbon mineralization and the microbial biomass under contrasting conditions, (ii) to compare the goodness of fit of different mathematical models to describe carbon mineralization, and (iii) to evaluate the relationships of the SOM pools and the mineralization parameters estimated by the best kinetic model. Twenty-eight soil samples were collected from a fine, illitic, thermic Typic Argiudoll localized in Argentina. These samples differed in the soil management (pasture and agriculture), tillage systems (chisel tillage, plough tillage, and no-tillage), crop rotation, or depths. Microbial biomass was highly correlated with total carbon (r(2) = 0.777, P < 0.001) and carbon in the SOM light density fraction (density < 1.59 g mL(-1)) but less strongly correlated to medium (density 1.59-2.0 g mL(-1)) and heavy (density > 2.0 g mL(-1)) soil fractions. Carbon in the soil light fraction was strongly related to the carbon mineralized at 10 and 160 d of incubation. The exponential and hyperbolic models showed a good description of the mineralization data (r(2) > 0.982), The application of models which considered two organic matter pools could not describe the mineralization of some samples. The hyperbolic model estimated higher potentially carbon mineralizable pools (C-0) and semidecomposition time periods than the exponential one, The C-0 estimated by the exponential model were similar to the carbon content in the soil light fraction, This soil organic component seemed to be the driving variable of microbial activity and a good predictor of soil potential carbon mineralization.