Nitrogen forms in runoff export from St. Augustinegrass

Nitrogen exports from landscapes can contribute to the eutrophication of surface waters. The most common form of N studied in prior runoff research has been nitrate, although ammoniacal and organic forms of N are also considered important causes of eutrophication. The relative abundance of each N form in runoff from turfgrasses has not been widely reported. The objective of this study was to quantify dissolved inorganic N (DIN) and dissolved organic N (DON) exports in runoff from St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] turf over a 2-yr period in College Station, TX. Treatments were arranged as a randomized complete block design having eight combinations of irrigation (100, 75, or 50% of estimated turfgrass water requirements) and fertility level (0, 88, and 176 kg N ha(-1) yr(-1)). Runoff from 31 rainfall events and one excess irrigation event were used to estimate annual and seasonal DIN and DON exports. Nitrate-N concentration was typically below 5 mg L-1, but peaks in excess of 20 mg L-1 were measured during late winter in one year. Aside from this atypical peak, the largest portion of total dissolved N (TDN) was DON, whereas NH4-N was typically the smallest portion. Increasing fertilizer rates typically amplified each form of N in runoff, but NO3-N was the most responsive to N rate, with corresponding reductions in the DON/TDN ratio and increases in the nitrate/ammonium ratio. Nitrate-N may be the best indicator for fate of anthropogenic N inputs, but studies focusing only on NO3-N are likely underreporting N exports.