Increased soil stable nitrogen isotopic ratio following phosphorus enrichment: historical patterns and tests of two hypotheses in a phosphorus-limited wetland

We used a P enrichment gradient in the Everglades to investigate patterns of the stable N isotopic ratio (delta N-15) 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 delta N-15 of organic matter including: (1) increased N mineralization/N loss, and (2) reduced isotopic discrimination during macrophyte N uptake. Spatial patterns of delta N-15 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 delta N-15 in the peat accumulated in areas near the historic P discharges since the early 1960s. The increased delta N-15 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 delta N-15 of NH4+ and significantly increased the delta N-15 of water-extractable organic N. Measurements of surface soils collected during a field mesocosm experiment also revealed no significant effect of P on delta N-15 even after 5 years of P addition. In contrast, delta N-15 of leaf and root tissues of hydroponically grown Typha and Cladium were shown to increase up to 12 parts per thousand when grown at elevated levels of P and fixed levels of N (as NH4+). The magnitude of changes in delta N-15 resulting from altered discrimination during N uptake is significant compared with other mechanisms affecting plant delta N-15, and suggests that this may be the dominant mechanism affecting delta N-15 of organic matter following P enrichment. The results of this study have implications for the interpretation of delta N-15 as an indicator of shifts in relative N limitation in wetland ecosystems, and also stress the importance of experimental validation in interpreting delta N-15 patterns.