Carbon Mineralization and Labile Organic Carbon Pools in the Sandy Soils of a North Florida Watershed

The large pool of actively cycling carbon (C) held in soils is susceptible to release due to changes in landuse, management, or climate. Yet, the amount and distribution of potentially mineralizable C present in soils of various types and the method by which this soil C fraction can best be quantified, are not well established. The distribution of total organic C (TOC), extractable C pools (hot-water-extractable and acid-hydrolyzable), and in vitro mineralizable C in 138 surface soils across a north Florida watershed was found to be quite heterogeneous. Thus, these C quality parameters could not statistically distinguish the eight landuses or four major soil orders represented. Only wetland and upland forest soils, with the largest and smallest C pool size, respectively, were consistently different from the soils of other landuse types. Variations in potential C mineralization were best explained by TOC (62%) and hot-water-extractable C (59%), whereas acid-hydrolyzable C (32%) and clay content (35%) were generally not adequate indicators of C bioavailability. Within certain landuse and soil orders (Alfisol, Wetland and Rangeland, all with > 3% clay content), however, C mineralization and clay content were directly linearly correlated, indicating a possible stimulatory effect of clay on microbial processing of C. Generally, the sandy nature of these surface soils imparted a lack of protection against C mineralization and likely resulted in the lack of landuse/soil order differences in the soil C pools. If a single parameter is to be chosen to quantify the potential for soil C mineralization in southeastern U.S. coastal plain soils, we recommend TOC as the most efficient soil variable to measure.