Dual Isotope Measurements to Identify and Quantify N Transformations in Soil.

Ratios of N and O isotopes in nitrate can be used to trace N cycling in soil, because isotope fractionation occurs in incomplete (bio)chemical reactions. This results in distinct differences of the isotopic composition of nitrate among different sources. However, the quantification of different nitrate sources can be complicated by simultaneous reactions in soil with opposed fractionation effects. We used N and O isotopes in nitrate to quantify the contribution to nitrate in soil solution of (1) mineral fertilizers applied to a vineyard, and (2) rainfall, mineralization of leguminous (M-LOM) and non-leguminous organic matter (M-NLOM) in a legume monoculture.
In the first study, the vineyard was characterized by high nitrate concentrations in soil solution compared to an adjacent forest. The forest site showed lower δ¹⁵N values (2.7 ± SE 0.6 ‰) and higher δ¹⁸O values (9.2 ±0.5 ‰) than the vineyard (δ¹⁵N 6.2 ±0.4 ‰; δ¹⁸O 5.2 ±0.5 ‰). However, the application of mineral fertilizers in the vineyard should have resulted in an opposite pattern. The reason was a change in the type of nitrogen fertilizer used in the vineyard from mineral to organic in the last ca. 10 yrs. In the second study, nitrate originating from rainfall, M-LOM and M-NLOM significantly differed with respect to δ¹⁵N and δ¹⁸O values (δ¹⁵N: 3.3 ± 0.8‰, 1.5 ± 0.6‰, 9.3 ± 0.9‰; δ¹⁸O: 30.8 ± 4.7‰, 5.1 ± 0.9‰, 6.7 ± 0.8‰). Rainfall, M-LOM and M-NLOM contributed 18-41%, 18-40% and 38-57%, respectively, to nitrate concentrations in soil solution under a legume monoculture.
We conclude that under certain circumstances, nitrate sources can be identified by using dual isotope measurements combined with the knowledge of agricultural management history. In these cases, dual isotope signals of nitrate can be used to optimize agricultural management practices.