Soil organic carbon in clay and silt sized particles in Chinese mollisols: Relationship to the predicted capacity

Soil organic carbon (SOC) associated with clay plus silt sized particles (< 20 μm) in uncultivated grassland soils has been proposed to have a maximum concentration that is referred to as the soil protective capacity. For cultivated soils, SOC is lost from the soil during various operations such as tillage, crop removal, lack of a crop cover between cropping seasons and as a result this SOC is typically lower than the maximum concentration. In this study, 28 surface soil samples (12 Black soils (Udolls) and 16 Baijiang soils (Albolls)) from long-term monoculture maize (Zea mays L.) fields in Jilin Province, China, were analysed for SOC associated with the clay and silt particle fractions. Total SOC was significantly related to the clay plus silt content of soils. The correlation between the proportion of clay and silt sized particles and the amounts of SOC associated with this fraction was significantly greater in Black soils compared to Baijiang soils. Under the monoculture maize production, the SOC associated with clay plus silt fraction for Black soils was about half of the protective capacity estimated using the predictive model of Hassink (1997) [Hassink, J., 1997. The capacity of soils to preserve organic C and N by their association with clay and silt particles. Plant and Soil 191, 77–87.]. However, the SOC in clay plus silt fraction of Baijiang soils varied greatly, from ∼40% to 120% of the calculated protective capacity. The SOC contents in the fraction > 20 μm were relatively low and similar amongst all the Black soils. In contrast, SOC in the fraction > 20 μm varied considerably amongst the Baijiang soils. Soils which have the SOC associated with clay plus silt size fraction in amounts approaching or exceeding the protective capacity in general were those which had C accumulation in the > 20 μm size fraction. Since the SOC in Black soils has already decreased to about 50% of the protective capacity, there may be a greater potential for sequestering C in these Black soils than in the Baijiang soils.