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 15N-enriched method applies an artificially enriched source of 15N and the 15N natural abundance (delta15N) method uses natural 15N differences between N sources and sinks. The delta15N 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 delta15N 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 delta15N between lower and upper slope positions. However, if the difference between source-delta15N and sink-delta15N 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, delta15N will not work as a tracer. With the 15N-enriched method, the isotopic signature of the enriched tracer can be pre-determined to ensure a significant difference in atom%15N between source and background levels, even when fractionation occurs. In most situations, the 15Nenriched method can be successfully used as a tracer to test hypotheses and to quantify N cycling through the landscape, regardless of background variability in delta15N. Limitations of the 15N-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.