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Increased soil stable nitrogen isotopic ratio following phosphorus enrichment: historical patterns and tests of two hypotheses in a phosphorus-limited wetland
Authors:P W Inglett  K R Reddy  S Newman  B Lorenzen
Institution:(1) Wetland Biogeochemistry Laboratory, Soil and Water Science Department, University of Florida, Gainesville, FL 32611-0510, USA;(2) South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, FL 33416-4680, USA;(3) Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, 8240 Risskov, Denmark
Abstract:We used a P enrichment gradient in the Everglades to investigate patterns of the stable N isotopic ratio (δ15N) in peat profiles as an indicator of historic eutrophication of this wetland. We also tested two hypotheses to explain the effects of P on increased δ15N of organic matter including: (1) increased N mineralization/N loss, and (2) reduced isotopic discrimination during macrophyte N uptake. Spatial patterns of δ15N in surface litter and soil (0–10 cm) mimic those of the aboveground macrophytes (Typha domingensis Pers. and Cladium jamaicense Crantz). Peat profiles also show increased δ15N in the peat accumulated in areas near the historic P discharges since the early 1960s. The increased δ15N of bulk peat correlated well with both measured increases in soil total P and the historical beginning of nutrient discharges into this wetland. In 15-day bottle incubations of soil, added P had no effect on the δ15N of NH4+ and significantly increased the δ15N of water-extractable organic N. Measurements of surface soils collected during a field mesocosm experiment also revealed no significant effect of P on δ15N even after 5 years of P addition. In contrast, δ15N of leaf and root tissues of hydroponically grown Typha and Cladium were shown to increase up to 12‰ when grown at elevated levels of P and fixed levels of N (as NH4+). The magnitude of changes in δ15N resulting from altered discrimination during N uptake is significant compared with other mechanisms affecting plant δ15N, and suggests that this may be the dominant mechanism affecting δ15N of organic matter following P enrichment. The results of this study have implications for the interpretation of δ15N as an indicator of shifts in relative N limitation in wetland ecosystems, and also stress the importance of experimental validation in interpreting δ15N patterns.
Keywords:Peat profile  Wetland eutrophication  Nitrogen isotopes  Biogeochemistry  Macrophytes
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