Evidence for denitrification as main source of N2O emission from residue-amended soil

Catch crops, especially leguminous catch crops, may increase crop nitrogen (N) supply and decrease environmental impacts in cropping systems, but they may also stimulate nitrous oxide (N2O) emissions following spring incorporation. In this 28-day laboratory incubation study, we examined the carbon (C) and N dynamics and N2O evolution after simulated incorporation of residues from three catch crop species into a loamy sand soil, with variable soil moisture (40, 50 or 60% water-filled pore space (WFPS)). The catch crops include two leguminous (red clover and winter vetch) and one non-leguminous species (ryegrass). Plant material was placed in a discrete layer surrounded by soil in which the nitrate (NO3−) pool was enriched with 15N to distinguish N2O derived from denitrification and nitrification. Net N mineralisation from leguminous catch crops was significant (30-48 mg N kg−1 soil, accounting for 41-56% of the added residue-N), whereas ryegrass incorporation resulted in net N immobilisation. The evolution of N2O was probably enhanced by N release from the residues, especially during the second week, which can explain the lower N2O evolution after application of ryegrass. Emission of N2O occurred at all moisture levels, but was higher at 50 and 60% WFPS than at 40% in soil with leguminous residues. The 15N enrichment of N2O indicated that denitrification was the dominant source independent of moisture level and residue type. We conclude that catch crop residues will stimulate N2O emissions via denitrification over a wide range of soil moisture conditions, but that emission levels may depend significantly on residue quality and soil moisture.