Soil mineral N retention and N2O emissions following combined application of 15N-labelled fertiliser and weed residues

RATIONALE The combination of plant residues with inorganic fertiliser-N provides the potential to increase N-use efficiency in agricultural fruit production systems, such as olive orchards. The development of weeds in the inter-canopy area of olive orchards is encouraged as a novel strategy to reduce soil erosion. However, little is known about soil N retention or N2O production following the combined application of inorganic-N with the mulched weed residues. METHODS Emissions of 15N-N2O and soil mineral 15N retention were measured following combined applications of 15N-labelled fertiliser and a range of olive crop weed residues to a silty loam soil under controlled conditions. These plant residues differed in their C:N ratios, lignin and polyphenol contents. RESULTS The magnitude of soil 15N-NO3 retention from combining plant residues and fertiliser-N was highly dependent on potential N mineralisation (r?=?-0.96) and the (lignin?+?polyphenol)-to-N ratio (r?=?0.98) of the residues. Fertiliser-N-derived retention was zero for a legume-based mulch but up to 80% in the treatment containing plant residues with a high (lignin?+?polyphenol)-to-N ratio. N2O emissions increased after the addition of residues, and increased further (up to 128%) following the combined application of inorganic fertiliser and residues. Fertiliser-derived 15N-N2O was <1.4% of the total 14+15N-N2O emission and <0.01% of the applied 15N-NO3. Enhanced N2O emissions following the application of residues and the fertiliser-N values were positively correlated with the C:N ratio of the residue. Thus, combining organic- and inorganic-N immobilised a significant proportion of the inorganic N with little increase in N2O, especially in low C:N ratio residues. CONCLUSIONS The results demonstrate that whilst there is potential for N2O emissions to be controlled by combining weed residues and inorganic fertilisers, this is not easy to achieve as the magnitude and direction of interactions vary between different species due to their varying substrate qualities. Copyright (c) 2012 John Wiley & Sons, Ltd.