Correlation of Dissolved Organic Carbon with Water Soluble Nutrients and Metals In Soils From Lake Okeechobee Watershed, South Florida.

Eutrophication of Lake Okeechobee has been a major environmental concern for many years.  Transport of dissolved organic matter (DOM) in runoff water from watershed contributes to the increased inputs of nutrients (N and P) and metals (Cu and Zn). In this study, 124 soil samples were collected with varying soil types, land uses, and soil depths in Lake Okeechobee watershed and analyzed for water extractable C, N, P, and metals to examine  the relationship between dissolved organic carbon (DOC)  and water soluble nutrients (N and P) and metals in the soils.  DOC in the soils ranged from 27.6 to 400, averaging 69.3 mg kg^-1, varying with soil types, land uses, and soil depth. The highest DOC (150 mg kg^-1) was measured in Histosol and under the land use of field crop or sugar cane (244 and 277 mg kg^-1, respectively). The concentration of DOC was generally higher in the surface layer than in deeper layers. 

Water soluble concentrations of N and P were in the range of 6.46-129 and 0.015-60.8 mg kg^-1, respectively. The ratios of water extractable C/N and C/P in soils, ranging in 0.676-12.5 and 3.19-2329 with their averages of 3.23 and 216, respectively, were affected by land uses.  The lowest water extractable C/N was measured in the soils under dairy (1.66), resident (1.79), and coniferous forest (4.49), whereas the lowest water extractable C/P was observed with land uses of dairy (13.1) and citrus (33.7).  Nitrogen and P in the soils under these land uses may have high availability and leaching potential. The concentrations of water extractable Co, Cr, Cu, Ni, and Zn were in the ranges of < detection limit (MDL)-0.331, <MDL-0.528, 0.035-2.42, <MDL-0.707, and 0.090-1.13 mg kg^-1, respectively, with corresponding mean values of 0.022, 0.014, 0.504, 0.067, and 0.370 mg kg^-1.  The highest water soluble Co (0.099 mg kg^-1), Cr (0.263 mg kg^-1), Ni (0.307 mg kg^-1), and Zn (0.805 mg kg^-1) were observed in soils under the land use of sugar cane, whereas, the highest Cu (1.50 mg kg^-1) was with field crop.  

The concentration of DOC was positively correlated with TOC (P <0.01), water soluble N (P <0.01), electrical conductivity (EC, P <0.01) and water extractable concentrations of Co, Cr, Ni, and Zn (P <0.01), and Cu (P <0.05), whereas water soluble N was positively related to water extractable P, Cu and Zn (P <0.01) in soils. These results indicate that the transport of DOC from land to water bodies may enhance the export of macro-nutrients (N, P), micro-nutrients (Cu, Zn, and Ni), and contaminants (Cr, and Co).

 Key words: Land use; Water extractable C/N; Water extractable C/P; Correlations