N2O emission from soil following combined application of fertiliser-N and ground weed residues

Emissions of N2O and CO2 were measured following combined applications of N-15-labelled fertiliser (100 mu g N g(-1); 10 atom % excess N-15) and organic olive crop weed residues (Avena sativa, Ononis viscosa, Ridolfia segetum and Olea europea; 100 mu g N g(-1)) to a silt loam soil under controlled environment conditions. The objective was to determine the effect of varying combinations of inorganic fertiliser and plant residues on these emissions and soil mineral N dynamics. Emissions were generally increased following application of residues alone, with 23 ng N2O-N g(-1) soil (2 ng N2O-N g(-1) soil mg(-1) biomass) and 389 mu g CO2-C g(-1) soil (39 mu g CO2-C g(-1) soil mg(-1) biomass) emitted over 28 days after addition of the Ridolfia residues in the absence of fertiliser-N. N2O emissions from these residue-only treatments were strongly negatively correlated with residue lignin content (r = -0.91; P < 0.05), total carbon content (r = -0.90; P < 0.05) and (lignin + polyphenol)-to-N ratio (r = -0.70; P < 0.1). However, changes in the net input of these compounds through application of 25:75, 50:50 and 75:25 proportional mixtures of Avena and Ononis residues had no effect on emissions compared to their single (0:100 or 100:0) applications. Addition of fertiliser-N increased emissions (by up to 30 ng N2O-N g(-1) 28 days(-1); 123%), particularly from the low residue-N treatments (Avena and Ridolfia) where a greater quantity of biomass was applied, resulting in emissions above that of the sum from the unfertilised residue and fertilised control treatments. In contrast, fertiliser application had no impact on emissions from the Olea treatment with the highest polyphenol (2%) and lignin (11%) contents due to strong immobilisation of soil N, and the N-15-N2O data indicated that residue quality had no effect on the denitrification of applied fertiliser-N. Such apparent inconsistencies mean that before the potential for manipulating N input (organic + inorganic) to lower gaseous N losses can be realised, first the nature and extent of interactions between the different N sources and any interactions with other compounds released from the residues need to be better understood.