Tracing 15N through landscapes: potential uses and precautions

Abstract

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 ( δ 15N) method uses natural 15N differences between N sources and sinks. The δ 15N 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 δ 15N 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 δ 15N between lower and upper slope positions. However, if the difference between source- δ 15N and sink- δ 15N 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, δ 15N 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 15N-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 δ 15N. 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.