Why is Mineral-Associated Organic Matter Enriched in 15N? Evidence from Grazed Pasture Soil

Tuesday, 16 December 2014: 9:15 AM
W Troy Baisden1, Naomi S Wells2, Paul L Mudge3, Timothy J Clough4, Louis A Schipper5, Anwar Ghani6 and Bryan Stevenson3, (1)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand, (2)Helmholtz Centre for Environmental Research UFZ Halle, Halle, Germany, (3)Landcare Research, Hamilton, New Zealand, (4)Lincoln University, Lincoln, New Zealand, (5)University of Waikato, Hamilton, New Zealand, (6)AgResearch, Hamilton, New Zealand
Throughout the scientific literature, measurements across soil depth and density fractions suggest that, with few exceptions, mineral-associated organic matter (OM) has higher δ15N than non-mineral-associated OM. This implies that the δ15N difference between N inputs and mineral-stabilized OM may characterize the microbial processes involved in stabilization and mineral association. Yet current understanding of observed N isotope fractionation in terrestrial ecosystems suggests the large isotope effects are expressed during inorganic N transformations from NH4 to gaseous loss pathways of NH3 volatilization and denitrification. How can the relative importance of N isotope fractionation during OM stabilization versus loss pathways be resolved?

We recently examined N isofluxes when a temporary nitrogen excess is created by urine deposition in a New Zealand dairy pasture.

We found that the N isotopic composition of volatilized NH3, and NO3 available for leaching or denitrification could not be linked back to the added N using Rayleigh distillation models. Instead, the results imply that the added N was immobilized, and the N available for losses was increasingly derived from mineralization of organic matter during the course of the experiment. These results are consistent with recent evidence of enhanced OM mineralization in urine patches, understanding of N isotope mass balances and long-standing evidence that gross mineralization and immobilization fluxes greatly exceed net mineralization and nitrification, except at very high N saturation.

These results suggest that where 15N enrichment occurs due to fractionating loss pathways, the isotope effects are primarily transmitted to immobilized N, forming 15N enriched stabilized OM. This further explains earlier findings that the δ15N of soil OM represents an integrated indicator of losses, reflecting the intensity and duration of pastoral agriculture. We suggest that development of an indicator based on δ15N in mineral-associated OM might relate mineralization rates to the δ15N of stabilized or immobilized N.