From Fertilizer to Food: Tracing Nitrogen Dynamics in Conventional and Organic Farming Systems Using ¹⁵N Natural Abundance

Synthetic nitrogen (N) fertilizers differ markedly from organic N fertilizer sources in relative isotopic composition at natural abundance levels (δ¹⁵N). The objective of this paper is to provide an overview of the applications of δ¹⁵N techniques to study the dynamics of synthetic fertilizers, animal excreta and composts in the soil-plant-atmosphere continuum. However, isotopic fractionation processes often complicate the interpretation of results. These fractionation processes and the factors affecting the δ¹⁵N signatures of organic N fertilizers are reviewed. Published data from short-, medium- and long-term experiments with annual crop rotations and in pastures subject to organic N inputs are also examined and analyzed with respect to changes in delta nitrogen-15 (δ¹⁵N) signatures of the soil, the crop or pasture, the soil biota and leachates. The use of δ¹⁵N to differentiate organic and conventional plant products is briefly covered. There are few data on the dynamics of N during the storage of animal excreta or the composting of agricultural wastes as shown by δ¹⁵N values in the organic, inorganic or gaseous N phases. The major N loss process is ammonia (NH₃) volatilization. Reviewed data show significant relationships between bulk δ¹⁵N signatures of stored manure and cumulative NH₃ loss or bulk δ¹⁵N of livestock manure composts and N concentration. These significant relationships suggest that δ¹⁵N may have wider applications in estimating the efficiency of N conservation during storage or composting. In addition, the combined use of bulk δ¹⁵N and delta oxygen-18 (δ¹⁸O) signatures of nitrous oxide (N₂O) evolved during storage and composting, together with the isotopomer- derived site preference of N₂O, are emerging technologies for identifying N₂O production pathways. δ¹⁵N in combination with appropriate statistical analysis is a promising diagnostic tool for differentiating organic and conventional plant products. (author)