Land Use and Water Quality in Urbanizing Post Oak Savannah Watersheds: Controls on DOC and DON.

Dissolved organic carbon (DOC) and nitrogen (DON) concentrations and exports in freshwaters are controlled by watershed land use, soil C:N ratio or carbon pools. This study investigated DOC and DON in nine watersheds in an urbanizing moist sub-tropical post oak savannah biome in SE Texas. Urban land use was sub-divided into a) urban open (parks and golf courses), b) urban low density, c) urban medium density and d) urban high density. Range within each urban land use was 5.5-29.9% (open), 0.5-27.0% (low density), 0.2-31.0% (medium density) and 0-13.6% (high density). Mean annual concentrations of DOC ranged from 20.4 to 52.5 ppm and DON ranged from 0.6 to 1.0 ppm. Sixtry-eight percent of the variance in DOC was explained by urban open land use (p < 0.01). No land use classification was able to describe any significant variance in DON. Much of the urban open land use is irrigated with domestic water supplies derived from high sodium groundwater. Indeed sodium concentrations in domestic water were 244.5 ppm with a sodium adsorption ratio (SAR) of 52.2. Sodium concentrations and SAR were also significantly related to urban open land use (r² = 0.69 p = 0.006; r²  = 0.65 p = 0.008 respectively). The relationship between DOC and SAR was strong and significant (r²  = 0.94; p < 0.0001).  We suggest that due to high losses of DOC, sequestration of carbon in urban soils under turfgrass is compromised when irrigated with domestic water high in sodium.