Environmental Services of Soil: Anion Competition In Urban Soils.

High concentrations of dissolved organic carbon (DOC) in urban streams have been related to urban turfgrass open areas such as parks and neighborhood gardens. A strong relationship between orthophosphate, DOC and bicarbonate led to an investigation of the environmental service of urban soil in terms of adsorption of PO43- and DOC and further, if their sorption was being compromised by high bicarbonate in municipal tap irrigation water. Soil cores (2 cm diameter and 15 cm depth) were taken from irrigated and non-irrigated parks with differing turfgrass species, from urban wetland forests and urban riparian soils with and without natural vegetation. A mass isotherm approach was used. Stock solution (20:1 DDW:vegetation ratio) for the experiment was prepared from the vegetation within a 30 cm area of the soil core taken. From the stock solution a range (6) of DOC concentrations were made ranging from 0 to just below the concentration of the stock solution (approximately 180 mg/L). Three grams of soil were combined with 30 mL of one of the stock standards in 40 mL centrifuge tubes and refrigerated to minimize microbial activity. The soil:solution units were gently inverted every 15 min for 1 h prior to centrifuging at 19,600 g-force at 4° C for 20 min. The supernatant was removed and pH and electrical conductivity recorded, the samples were then filtered through ashed Whatman GF/F filters and analyzed for DOC, total nitrogen, ammonium-N, nitrate-N, alkalinity (as CaCO3) and anions (Cl-,F-,PO43- and SO42-). Carbonate equilbria (CO2, HCO3- and CO32-) were calculated for each sample using AqQA. Output mass of each ion or compound was deducted from its input mass to give a value for release or retention (RE). Input mass was used in regression analysis with RE to produce an equation that would describe the adsorption co-efficient and a release term. Bicarbonate adsorped significantly better to mineral soil than did DOC and orthophosphate in some urban soils but not others suggesting there was some anion competition for adsorption sites in those soils that had been exposed to irrigation with Na-HCO3 municipal tap water or its runoff. The implications of our findings include enhanced DOC and orthophosphate-P mobilization to surface waters from irrigated urban soils.