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Urea and legume residues as 15N-N2O sources in a subtropical soil

Gomes, J., Brüggemann, N., Dick, D. P., Pedroso, G. M., Veloso, M., Bayer, C.
Soil research 2019 v.57 no.3 pp. 287-293
Acrisols, ammonium nitrogen, climate-smart agriculture, cover crops, cowpeas, crop residues, fertilizer application, greenhouse gas emissions, mass spectrometry, nitrate nitrogen, nitrogen, nitrogen fertilizers, nitrous oxide, stable isotopes, subtropical soils, urea
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.