Nitrate leaching in soil: Tracing the NO 3− sources with the help of stable N and O isotopes

Abstract

Legumes increase the plant-available N pool in soil, but might also increase NO 3 − leaching to groundwater. To minimize NO 3 − leaching, N-release processes and the contribution of legumes to NO 3 − concentrations in soil must be known. Our objectives were (1) to quantify NO 3 −-N export to >0.3 m soil depth from three legume monocultures ( Medicago x varia Martyn, Onobrychis viciifolia Scop., Lathyrus pratensis L.) and from three bare ground plots. Furthermore, we (2) tested if it is possible to apply a mixing model for NO 3 − in soil solution based on its dual isotope signals, and (3) estimated the contribution of legume mineralization to NO 3 − concentrations in soil solution under field conditions. We collected rainfall and soil solution at 0.3 m soil depth during 1 year, and determined NO 3 − concentrations and δ 15N and δ 18O of NO 3 − for >11.5 mg NO 3 −-N l −1. We incubated soil samples to assess potential N release by mineralization and determined δ 15N and δ 18O signals of NO 3 − derived from mineralization of non-leguminous and leguminous organic matter. Mean annual N export to >0.3 m soil depth was highest in bare ground plots (9.7 g NO 3 −-N m −2; the SD reflects the spatial variation) followed by Medicago x varia monoculture (6.0 g NO 3 −-N m −2). The O. viciifolia and L. pratensis monocultures had a much lower mean annual N export (0.5 and 0.3 g NO 3 −-N m −2). The averaged NO 3 −-N leaching during 70 days was not significantly different between field estimates and incubation for the Medicago x varia Martyn monoculture. The δ 15N and δ 18O values in NO 3 − of rainfall ( δ 15N: 3.3±0.8‰; δ 18O: 30.8±4.7‰), mineralization of non-leguminous SOM (9.3±0.9‰; 6.7±0.8‰), and mineralization of leguminous SOM (1.5±0.6‰; 5.1±0.9‰) were markedly different. Applying a linear mixing model based on these three sources to δ 15N and δ 18O values in NO 3 − of soil solution during winter 2003, we calculated 18–41% to originate from rainfall, 38–57% from mineralization of non-leguminous SOM, and 18–40% from mineralization of leguminous SOM. Our results demonstrate that (1) even under legumes NO 3 −-N leaching was reduced compared to bare ground, (2) the application of a three-end-member mixing model for NO 3 − based on its dual isotope signals produced plausible results and suggests that under particular circumstances such models can be used to estimate the contributions of different NO 3 − sources in soil solution, and (3) in the 2nd year after establishment of legumes, they contributed approximately one-fourth to NO 3 −-N loss.