Large variation in nitrogen isotopic composition of a fertilized legume

Plant nitrogen isotopic compositions are highly variable and are influenced by a diversity of environmental and anthropogenic factors, including the application of animal-derived fertilizers. Legumes that acquire most of their nitrogen from atmospheric N2 (rather than mineralized soil nitrogen) tend to have relatively low δ15N values (consistently around 0‰), and it has been presumed that their δ15N values are largely or wholly unaffected by fertilization. This study presents nitrogen isotopic data from leguminous (garden bean, Phaseolus vulgaris) and non-leguminous (summer squash, Cucurbita pepo) plants subjected to seabird guano fertilization while growing under controlled conditions. Both bean and squash tissue δ15N values were substantially increased by seabird guano fertilization: +16.3 to +19.2‰ for bean and +19.6 to +24.5‰ for squash. The results of this study demonstrate that the enrichment in plant 15N resulting from seabird guano fertilization occurs consistently in non-maize species. Moreover, it demonstrates that under conditions of high soil nitrogen availability, leguminous plants may obtain a substantial portion of their nitrogen through the uptake of inorganic soil nitrogen (ammonium and nitrate), rather than atmospheric N2. In general, where the δ15N values of fertilizers differ substantially from that of endogenous soil nitrogen and mineralized nitrogen derived from the fertilizer is readily available, a significant manuring effect can be expected in leguminous plants.