Urea and legume residues as 15N-N2O sources in a subtropical soil
J. Gomes A , N. Brüggemann B , D. P. Dick C , G. M. Pedroso A , M. Veloso A and C. Bayer A DA Department of Soil Science and Graduate Program on Soil Science, Faculty of Agricultural and Life Sciences, Federal University of Rio Grande do Sul, 91540-000, Porto Alegre/RS, Brazil.
B Forschungszentrum Jülich, Institute of Bio- and Geosciences – Agrosphere (IBG-3), Wilhelm-Johnen-Strasse, 52428 Jülich, Germany.
C Department of Physical Chemistry, Institute of Chemistry, Federal University of Rio Grande do Sul. 91501-970, Porto Alegre/RS, Brazil.
D Corresponding author. Email: cimelio.bayer@ufrgs.br
Soil Research 57(3) 287-293 https://doi.org/10.1071/SR18300
Submitted: 6 October 2018 Accepted: 11 February 2019 Published: 19 March 2019
Abstract
In this work, we used the 15N labelling technique to identify the sources of N2O emitted by a subtropical soil following application of mineral nitrogen (N) fertiliser (urea) and residues of a legume cover crop (cowpea). For this purpose, a 45-day incubation experiment was conducted by subjecting undisturbed soil cores from a subtropical Acrisol to five different treatments: (1) control (no crop residue or fertiliser-N application); (2) 15N-labelled cowpea residue (200 μg N g–1 soil); (3) 15N-labelled urea (200 μg N g–1 soil); (4) 15N-labelled cowpea residue (100 μg N g–1 soil) + unlabelled urea (100 μg N g–1 soil); and (5) unlabelled cowpea residue (100 μg N g–1 soil) + 15N-labelled urea (100 μg N g–1 soil). Cores were analysed for total N2O formation, δ15N-N2O and δ18O-N2O by continuous flow isotope ratio mass spectrometry, as well as for total NO3–-N and NH4+-N. Legume crop residues and mineral fertiliser increased N2O emissions from soil to 10.5 and 9.7 µg N2O-N cm–2 respectively, which was roughly six times the value for control (1.5 µg N2O-N cm–2). The amount of 15N2O emitted from labelled 15N-urea (0.40–0.45% of 15N applied) was greater than from 15N-cowpea residues (0.013–0.015% of 15N applied). Unlike N-poor crop residues, urea in combination with N-rich residues (cowpea) failed to reduce N2O emissions relative to urea alone. Legume cover crops thus provide an effective mitigation strategy for N2O emissions in relation to mineral N fertilisation in climate-smart agriculture. Judging by our inconclusive results, however, using urea in combination with N-rich residues provides no clear-cut environmental advantage.
Additional keywords: 15N, cover crops, nitrous oxide, urea.
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