Soil N2O emissions following cover-crop residues application under two soil moisture conditions

Cover crops in rotation or intercropped with annual crops are important strategies to increase C and N input in agricultural soils. However, these practices may also enhance soil N2O emissions. The effect on N2O emissions may be dependent upon the biochemical composition of cover crop residues. A 47-d incubation study was conducted to determine soil N2O emissions following the addition of residues from three summer legume species [pigeon pea (Cajanus cajan L. Millsp.), cowpea (Vigna unguiculata L. Walp.), lablab bean (Lablab purpureus)], one winter legume [vetch (Vicia sativa L.)], one winter monocotyledon [black oat (Avena strigosa Schreb.)], and maize (Zea mays L.) under two water-filled pore space levels (40 and 70% WFPS). Short-term peaks of N2O fluxes were observed after the addition of all crop residues, but were much greater under 70 than 40% WFPS (5.2 and 5133 mu g N-N2O kg(-1) soil for 40 and 70% WFPS, respectively). Under both WFPS, significantly higher peaks were detected after the application of N-rich legume residues (7.7 and 3,356 mu g N-N2O kg(-1) soil under 40 and 70% WFPS, respectively) than after the application of grass residues (2.8 and 1,777 mu g N-N2O kg(-1) soil under 40 and 70% WFPS, respectively). Cumulative soil N2O produced under 70% WFPS was approx. 110 times greater than under 40% WFPS. Soil N2O emissions increased linearly as residue N content increased. Soil N2O emissions also increased linearly as the content of the recalcitrant compounds lignin and polyphenols increased, because residues with high N content also had high lignin and polyphenols content. When the content of C and recalcitrant compounds were expressed on an N basis, soil N2O emission decreased linearly as residue C/N, lignin/N, polyphenol/N, and (lignin + polyphenols)/N ratios increased. Indices that include recalcitrant compounds on N basis may beuseful for the selection of cover-crop species withthe least impact on soil N2O emissions. However, our results show that these ratio indices had similar correlation coefficients compared to C/N ratio, indicating that C/N ratio is an efficient index to predict soil N2O emission following cover crop application.