Kwame Agyei Frimpong, Elizabeth M. Baggs:
Does Combined Appliction of Crop Residues and Inorganic Fertiliser Lower Emisiosn of N₂O from Soil?

KWAME AGYEI FRIMPONG, ELIZABETH M. BAGGS
University of Aberdeen, Institute of Biological and Environmental Sciences, United Kingdom

Emissions of N₂O were measured following addition of ¹⁵N-labelled residues of Vigna unguiculata (cowpea), Mucuna pruriens and Leucaena leucocephala) to a Ferric Luvisol from Ghana at a rate of 100 mg N kg^-1 soil under controlled environment conditions. Residues were also applied in different ratio combinations with inorganic N fertiliser, each combination applied at a total rate of 100 mg N kg^-1 soil. N₂O emissions were increased after addition of residues, and further increased with combined applications of residues and inorganic N fertiliser. However, ¹⁵N-N₂O production was low and short-lived in all treatments, suggesting that most of the measured N₂O-N was derived from the applied fertiliser or native soil mineral N pools. There was no consistent trend in magnitude of emissions with increasing proportion of inorganic fertiliser in the application. The positive interactive effect between residue- and fertiliser-N sources was most pronounced in the 25:75 Leucena:fertiliser treatment where 22.5 g N₂O-N m^-2 kg biomass^-1 was emitted over 30 days. N₂O (loge) emission over the 30 day experiment from all residue amended treatments was positively correlated with residue C:N ratio (r=0.63; p < 0.05), and negatively correlated with residue polyphenol content (r=-0.59; p < 0.05), polyphenol:N ratio (r=-0.61; p < 0.05) and (lignin+polyphenol):N ratio (r=-0.61; p < 0.05), indicating the role of residue chemical composition, or quality, in regulating emissions even when combined with inorganic fertiliser. Under our controlled experimental conditions the 75:25 residue: fertiliser ratio appeared to offer the best compromise between release of N and management of N₂O emission, but the positive interactive effect even in this treatment means that it is unlikely that combined applications of residues and inorganic fertiliser can lower N₂O emissions, unless the residue is of very low quality promoting strong immobilisation of soil mineral N.

Keywords: Crop residues, inorganic fertiliser, nitrous oxide, stable isotopes, tropical soil

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Contact Address: Kwame Agyei Frimpong, University of Aberdeen, Institute of Biological and Environmental SciencesCruickshank Building, AB24 3UU Aberdeen, United Kingdom, e-mail: k.frimpongabdn.ac.uk