Nitrogen Cycling in Forest and Grass Ecosystems Irrigated With15n-Enriched Wastewater

Nitrogen cycling was studied during the first 2 yr of spray irrigation of vegetation by treated wastewater in Falmouth (Cape Cod), Massachusetts. We attempted to take advantage of an unexpected natural 15N enrichment during wastewater treatment to trace the fate of wastewater N following irrigation. Wastewater N was enriched by 6–20‰ relative to soil and by 13–25‰ relative to vegetation. We were able to estimate retention of wastewater N by soil using a mass balance of 15N. However, under conditions of N saturation δ15N values for plants were lower than expected, possibly due to discrimination against 15N during uptake of NH4+ and NO3− by plant roots. This is a potential weakness of trying to use low-level natural 15N enrichment in a tracer study. This problem is not likely to occur in N-limited ecosystems, or at high levels of isotopic enrichment. Three different ecosystem types were irrigated: a successional pitch pine woodland 26 yr in age; a mixed oak–pitch pine forest >70 yr in age; and cleared areas revegetated with grasses and old-field weeds. Areas near the spray heads were overloaded with N at deposition rates of 370–480 kg·ha−1·yr−1. Pine woods and grass areas rapidly became N saturated; soil pore water NO3− levels reached 800 μmol/L in irrigation in year 1 and 1400 μmol/L in year 2. In contrast, soil pore water NO3− levels in the oak forest did not consistently exceed 100 μmol/L until late in year 2. Soil was a major sink for wastewater N in year 1, but in year 2 soil N retention fell to near zero, and N leaching losses greatly increased. At the onset of N saturation, irrigated forests were invaded by weedy species including Phytolacca americana, Polygonum convolvulus, and Solanum dulcamera, later followed by Celastrus orbiculatus and Lonicera tartarica.