Seasonal Variability of Soil Organic Carbon Fractions Under Arable Land

Carbon fractions in soils apparently vary not only in space, but also over time. A lack of knowledge on the seasonal variability of labile carbon fractions under arable land hampers the reliability and comparability of soil organic carbon (SOC) surveys from different studies. Therefore, we studied the seasonal variability of two SOC fractions, particulate organic matter (POM) and dissolved organic carbon (DOC), under maize cropping: POM was determined as the SOC content in particle-size fractions, and DOC was measured as the water-extractable SOC (WESOC) of air-dried soil. Ammonium, nitrate, and water-extractable nitrogen were measured as potential regulating factors of WESOC formation because carbon and nitrogen cycles in soils are strongly connected. There was a significant annual variation of WESOC (coefficient of variation (CV) = 30%). Temporal variations of SOC in particle-size fractions were smaller than those of WESOC. The stocks of SOC in particle-size fractions decreased with decreasing particle sizes, exhibiting a CV of 20% for the coarse sand-size fraction (250-2 000 μm), of 9% for the fine sand-size fraction (50-250 μm), and of 5% for the silt-size fraction (20-50 μm). The WESOC and SOC in particle-size fractions both peaked in March and reached the minimum in May/June and August, respectively. These results indicate the importance of the time of soil sampling during the course of a year, especially when investigating WESOC.