Tracing 15N through landscapes:: potential uses and precautions

Stable N isotopes are used to examine the source, flow and fate of N at scales ranging from greenhouse pots to landscapes. There are two main approaches: the N-15-enriched method applies an artificially enriched source of N-15 and the N-15 natural abundance (delta N-15) method uses natural N-15 differences between N sources and sinks. The delta N-15 method is good for semi-quantitative estimates of N flow in undisturbed ecosystems, for analyzing patterns, and for developing new hypotheses, particularly when spatial variability across a landscape or watershed can be explained. The spatial variability of delta N-15 across a landscape is often non-random, following predictable spatial patterns. Topographic features control the rate of various hydrological and biological processes, resulting in significantly different. N between lower and upper slope positions. However, if the difference between source-delta N-15 and sink-delta N-15 is small due to inherent background variability and/or if fractionating processes have a large effect on the isotopic signature of the N to be traced, delta N-15 will not work as a tracer. With the N-15-enriched method, the isotopic signature of the enriched tracer can be pre-determined to ensure a significant difference in atom% (15) N between source and background levels, even when fractionation occurs. In most situations, the N-15-enriched method can be successfully used as a tracer to test hypotheses and to quantify N cycling through the landscape, regardless of background variability in delta N-15. Limitations of the N-15-enriched method include the cost associated with applying an enriched tracer, especially at the landscape scale, and the potentially confounding effects of applying N to a previously undisturbed landscape. (C) 2002 Elsevier Science B.V. All rights reserved.