Synthetic fertilizer and livestock manure differently affect δ15N in the agricultural landscape: A review

Synthetic fertilizers and raw or composted livestock manure are typical nitrogen (N) sources in intensive cropping and livestock-farming systems. The distinct N isotope ratios (15N/14N, expressed as δ15N) of the N sources make it possible to use the δ15N of soil, plant and groundwater samples to trace the N derived from the two N sources in the agricultural landscape. However, N isotope fractionation during N cycling may hamper the usefulness of the δ15N technique for tracing N. This paper reviews the changes in the δ15N of soil, plant and groundwater samples in the agricultural landscape as affected by synthetic fertilizer and raw or composted manure applications with consideration of the effect of N source and N fractionation during N cycling on variations in δ15N. First, this review summarizes the fundamental N isotope fractionation theory with an emphasis on the critical role of nitrification in changing δ15N through N loss. Second, the differences in the δ15N of synthetic fertilizer and raw or composted manure are discussed with an emphasis on mechanisms that increase δ15N in raw or composted manure. Third, the effects of synthetic fertilizer and raw or composted manure applications on the variations of δ15N in soil, plant and groundwater samples across different scales of experiments from laboratory to watershed are discussed. We conclude that in spite of N isotope fractionation, the feasibility of the δ15N technique in tracing N originated from synthetic fertilizer and raw or composted manure in soil, plant and groundwater in the agricultural landscape can be strengthened when site-specific information on the δ15N of N sources as well as the dominant N processes is available.