Regional modelling of soil carbon at multiple depths within a subtropical watershed

Environmental factors that exert control over fine-scale spatial patterns of soil organic carbon (SOC) within profiles and across large regions differ by geographic location and landscape setting. Regions with large SOC storage and high variability can serve as natural laboratories to investigate how environmental factors generate vertical and horizontal SOC patterns across the landscape. This was investigated in the Santa Fe River watershed (SFRW), Florida, where we modelled the spatial distribution of total C (TC) at four depths to 180 cm (0–30, 30–60, 60–120, and 120–180 cm) and at an aggregated depth of 0–100 cm. A total of 554 samples from 141 sampling sites distributed along land use and soil order combinations were analyzed for TC by high-temperature combustion. A vertical trend of TC stocks decreasing with depth was identified. Horizontal trends of TC were modelled to identify the environmental determinants of TC in the SFRW. We used analysis of variance (ANOVA) and compared regression block kriging with lognormal block kriging to scale up TC across the SFRW. Total soil C was influenced by soil depth, land use, soil type, soil drainage class, and geologic unit. Regression kriging performed better than block kriging to scale up TC at three out of five depth intervals. This indicates that in the majority of cases environmental factors were the major determinants of the spatial distribution of TC relative to its spatial autocorrelation. At 60–120 and 120–180 cm, the local spatial dependence of TC was more important than environmental factors to explain its variation across the watershed. Our models show that 54.0 Tg (teragrams) of C is held in the upper 1 m of soils in the SFRW, and significant amounts are stored in deeper layers. They also identified the major factors responsible for regional spatial patterns of TC in this subtropical region, providing information to support current efforts of conservation of soil resources in Florida, and under similar environmental conditions in the southeastern U.S. and elsewhere.