Wrage, N; Lauf, J; del Prado, A; Pinto, M; Pietrzak, S; Yamulki, S; Oenema, O; Gebauer, G: Distinguishing sources of N2O in European grasslands by stable isotope analysis, Rapid Commun. Mass Spectrom., 18, 1201-1207 (2004), doi:10.1002/rcm.1461
Nitrifiers and denitrifiers are the main producers of the greenhouse gas nitrous oxide (N2O). Knowledge of the respective contributions of each of these microbial groups to N2O production is a prerequisite for the development of effective mitigation strategies for N2O. Often, the differentiation is made by the use of inhibitors. Measurements of the natural abundance of the stable isotopes of N and O in N2O have been suggested as an alternative for the often unreliable inhibition studies. Here, we tested the natural abundance incubation method developed by Tilsner et al.(1) with soils from four European grasslands differing in long-term management practices. Emission rates of N2O and stable isotope natural abundance of N2O and mineral N were measured in four different soil incubations: a control with 60% water-filled pore space (WFPS), a treatment with 60% WFPS and added ammonium (NH4+) to support nitrifiers, a control with 80% WFPS and a treatment with 80% WFPS and added nitrate (NO3-) to support denitrifiers. Decreases in NH4+ concentrations, linked with relative 15N-enrichment of residual NH4+ and production of 15N-depleted NO3-, showed that nitrification was the main process for mineral N conversions. The N2O production, however, was generally dominated by reduction processes, as indicated by the up to 20 times larger N2O production under conditions favouring denitrification than under conditions favouring nitrification. Interestingly, the N2O concentration in the incubation atmospheres often levelled off or even decreased, accompanied by increases in delta15N and delta18O values of N2O. This points to uptake and further reduction of N2O to N2, even under conditions with small concentrations of N2O in the atmosphere. The measurements of the natural abundances of 15N and 18O proved to be a valuable integral part of the natural abundance incubation method. Without these measurements, nitrification would not have been identified as essential for mineral N conversions and N2O consumption could not have been detected.
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