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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.