Soil N 2 O 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 N 2 O emissions. The effect on N 2 O emissions may be dependent upon the biochemical composition of cover crop residues. A 47‐d incubation study was conducted to determine soil N 2 O 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 N 2 O fluxes were observed after the addition of all crop residues, but were much greater under 70 than 40% WFPS (5.2 and 5133 µg N‐N 2 O 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 µg N‐N 2 O kg −1 soil under 40 and 70% WFPS, respectively) than after the application of grass residues (2.8 and 1,777 µg N‐N 2 O kg −1 soil under 40 and 70% WFPS, respectively). Cumulative soil N 2 O produced under 70% WFPS was approx. 110 times greater than under 40% WFPS. Soil N 2 O emissions increased linearly as residue N content increased. Soil N 2 O 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 N 2 O 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 be useful for the selection of cover‐crop species with the least impact on soil N 2 O 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 N 2 O emission following cover crop application.