Estimating soil labile organic carbon and potential turnover rates using a sequential fumigation–incubation procedure

Labile carbon is the fraction of soil organic carbon with most rapid turnover times and its oxidation drives the flux of CO 2 between soils and atmosphere. Available chemical and physical fractionation methods for estimating soil labile organic carbon are indirect and lack a clear biological definition. We have modified the well-established Jenkinson and Powlson's fumigation–incubation technique to estimate soil labile organic carbon using a sequential fumigation–incubation procedure. We define soil labile organic carbon as the fraction of soil organic carbon degradable during microbial growth, assuming that labile organic carbon oxidizes according to a simple negative exponential model. We used five mineral soils and a forest Oa horizon to represent a wide range of organic carbon levels. Soil labile organic carbon varied from 0.8 mg/g in an Entisol to 17.3 mg/g in the Oa materials. Potential turnover time ranged from 24 days in an Alfisol to 102 days in an Ultisol. Soil labile organic carbon contributed from 4.8% in the Alfisol to 11.1% in the Ultisol to the total organic carbon. This new procedure is a relatively easy and simple method for obtaining indices for both the pool sizes and potential turnover rates of soil labile organic carbon and provides a new approach to studying soil organic carbon.