Nitrogen cycling in forest and grass ecosystems irrigated with N-15-enriched wastewater

Nitrogen cycling was studied during the first 2 yr of spray irrigation of vegetation by treated wastewater in Falmouth (Cape God), Massachusetts. We attempted to lake advantage of an unexpected natural N-15 enrichment during wastewater treatment to trace the fate of wastewater N following irrigation. Wastewater N was enriched by 6-20 parts per thousand relative to soil and by 13-25 parts per thousand relative to vegetation. We were able to estimate retention of wastewater N by soil using a mass balance of N-15. However, under conditions of N saturation delta(15)N values for plants were lower than expected, possibly due to discrimination against N-15 during uptake of NH4+ and NO3- by plant roots. This is a potential weakness of trying to use low-level natural N-15 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 mu mol/L in irrigation in year 1 and 1400 mu mol/L in year 2. In contrast, soil pore water NO3- levels in the oak forest did not consistently exceed 100 mu mol/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.