Nitrogen mineralization in soil depends on carbon input of plant materials, site environmental conditions and soil physical properties. Although soil clay content has been negatively correlated with nitrogen mineralization rates in some studies, others have found no such relationship. This could result from complexes interactions among carbon input rate, carbon quality and clay content, which may obscure the relationship between nitrogen mineralization and soil texture. Here we build a multiple regression model which related carbon input rates, organic matter quality (leaf nitrogen) and soil texture on nitrogen mineralization in Chilean temperate rain forest soils. Nitrogen mineralization increased with both carbon input rate and size of the clay-silt fraction. However, organic matter quality had no effect on mineralization. We explained our results within the framework of soil organic matter saturation concept by examining the influence of soil texture on organic matter levels and nitrogen mineralization. Soil organic carbon associated with clay and silt sized particles (< 50 µm) of soils has been proposed to have a maximum concentration that is referred to as the soil protective capacity (the upper limit of carbon saturation of agricultural soils). We compared carbon storage of forest soils with the protective capacity. In this comparison the carbon levels of long-term cropped soils is also presented. Total soil organic C was significantly correlated to the clay and silt content of soils. The correlation between clay and silt sized particles of forest soils and the amounts of soil organic carbon associated with this fraction was significantly greater than the soil protective capacity. In contrast, carbon content of cropping soils was below of this limit. This was consistent with the finding that nitrogen mineralization of forest soils increased with the amount organic matter associated with the sand size fraction, indicating that this soils are still accumulating organic matter in their clay and silt sized particles.

Key words: Carbon sequestration; protective capacity; particle size fraction; nitrogen mineralization; forest soils.