Dissolved organic carbon (DOC) and organic nitrogen (DON) in surface waters represent a loss of sequestered C and N from terrestrial to aquatic ecosystems (allochthonous inputs). Recently, aquatic DOC and DON concentrations have been increasing. Multiple studies have suggested that increased sodium inputs in urban centers contribute to this increasing DOC and DON. To determine the effect of sodium exposure to urban soils, this study took samples from 3 different cities exposed to three different sodium types: Bryan-College Station, TX (sodic irrigation water), Frederick, MD (deicing salts), and Galveston, TX (sea salt deposition). Multiple regression models found that the best independent variables for predicting DOC were sodium type, pH, EC, ammonium-N, phosphate-P, DON, magnesium, boron, iron, zinc, and manganese. The best independent variables for DON were time of sodium exposure, sodium type, nitrate-N, phosphate-P, magnesium, sodium, zinc, manganese, and DOC. The difference in independent variables for estimating water extractable DOC and DON concentrations in urban soils suggests that the mechanisms controlling DOC and DON desorption are extremely complex. It is likely that the mechanisms controlling DOC are directly or indirectly related to pH while the mechanisms controlling DON are controlled indirectly by pH as well as microbial respiration rates.
The second part of this study aimed to look at the effect of 4- and 2- hour shakes on measured concentrations of DOC, DON, TDN, NH4, NO3, and PO4. As there is no definitive extraction method for DOC and DON, cross comparison across studies is unreliable. Using different methods can result in overestimation through nutrient release through microbial release during extraction or underestimation through organic nutrient degradation. To look at the impact of shake time on measured analyte concentrations, results of 2 and 4 hour shakes were compared across 4 cities: Chicago, IL, Frederick, MD, Galveston, TX, and Bryan-College Station, TX. The results of 2 and 4 hour shakes for DOC, DON, or nutrients (except NO3) were highly variable across cities, suggesting that each city's distinct soil properties were responsible for the differences in analyte response to shake time.
- Aitkenhead, Jacqueline Associate Professor