Assessing Soil Organic Carbon Change and Nutrient Loads in Surface Water in the Suwannee River Basin, Florida.

Terrestrial cycles of carbon, nitrogen, and phosphorus are key components of ecosystem services that have been manipulated by human activities over decades. Research gaps exist to interface and compare carbon and nutrients in soils and surface waters considering space and time dimensions. Our three objectives are to: i) assess spatially-explicit relationships between changes in soil organic carbon (SOC) stocks and unit area total organic carbon (TOC) loads in the lotic system, ii) analyze temporal trends of TOC, total nitrogen (TN), and total phosphorus (TP) loads in surface waters, and iii) identify cross-linkages between environmental factors and change in area-standardized SOC stocks and TOC loads. We collected 138 soil samples (0-20 cm) in 2009 and measured SOC and bulk densities (BD). We retrieved 169 historic SOC and BD data from the Florida Soil Characterization Database (1965-1996), harmonized and standardized current and historic SOC stocks (0-20 cm), used block kriging to estimate historic and current SOC, and then derived SOC change. We retrieved water discharge and nutrient concentration from twenty-three water gauging stations and used the Kruskal-Wallis test to analyze the seasonal variations of TOC, TN, and TP monthly loads in surface water and the Mann-Kendall test to detect the trend in loads between 2000 and 2010. Results suggested that SOC stocks have increased by 1.0 kg C m^-2 due to overall increased wetland and decreased agricultural coverage. The average unit area TOC loads also increased with 0.001 kg C m^-2 within the basin. While most sub-basins within the study area have shown no significant increase in monthly unit area TOC, TN and TP loads, increased agricultural activity and population growth along the Suwannee River explained increases in TOC, TN and TP loads in surface water within the Upper and Lower Suwannee and some areas in the Santa Fe River sub-basins.