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NITROGEN CYCLING IN FOREST AND GRASS ECOSYSTEMS IRRIGATED WITH 15 N‐ENRICHED WASTEWATER
Author(s) -
Jordan Marilyn J.,
Nadelhoffer Knute J.,
Fry Brian
Publication year - 1997
Publication title -
ecological applications
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.864
H-Index - 213
eISSN - 1939-5582
pISSN - 1051-0761
DOI - 10.1890/1051-0761(1997)007[0864:ncifag]2.0.co;2
Subject(s) - environmental science , cycling , wastewater , irrigation , ecosystem , agronomy , ecology , biology , forestry , environmental engineering , geography
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 15 N 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 15 N. However, under conditions of N saturation δ 15 N values for plants were lower than expected, possibly due to discrimination against 15 N during uptake of NH 4 + and NO 3 − by plant roots. This is a potential weakness of trying to use low‐level natural 15 N 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 NO 3 − levels reached 800 μmol/L in irrigation in year 1 and 1400 μmol/L in year 2. In contrast, soil pore water NO 3 − 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.