Abstract Nitrous oxide emissions are usually increased following incorporation of N-rich plant residues, but the effects of residue soluble C and N contents on emissions have still to be determined. Here we report a controlled environment experiment in which emissions of N 2O were measured following addition of 15N-labelled (2.5–4.2 atom % excess 15N) agroforestry residues ( Sesbania sesban, Macroptilium atropurpureum and Crotalaria grahamiana) to an Oxisol. Exposure of these trees to different irradiance during growth resulted in differences in water-soluble C and N contents. The highest emissions were generally measured from the lower water-soluble C and N (LS) residues with 7 mg N 2O–N m −2 emitted over 29 d after addition of Crotalaria LS residues (4.9% soluble C, 0.7% soluble N). Emissions were negatively correlated with the residue soluble C-to-N ratio ( r=−0.68 to −0.89; P<0.05) at the time of main flux activity during the first 8 d after residue addition, indicating that under controlled environmental conditions substrates with a high soluble C-to-N ratio may result in low N 2O emissions during the early stages of residue decomposition. This relationship has still to be verified under field conditions.