Catch Crop Residues Stimulate N2O Emissions During Spring, Without Affecting the Genetic Potential for Nitrite and N2O Reduction

Agricultural soils are a significant source of anthropogenic nitrous oxide (N 2 O) emissions, because of fertilizer application and decomposition of crop residues. We studied interactions between nitrogen (N) amendments and soil conditions in a 2-year field experiment with or without catch crop incorporation before seeding of spring barley, and with or without application of N in the form of digested liquid manure or mineral N fertilizer. Weather conditions, soil inorganic N dynamics, and N 2 O emissions were monitored during spring, and soil samples were analyzed for abundances of nitrite reduction ( nirK and nirS ) and N 2 O reduction genes ( nosZ clade I and II), and structure of nitrite- and N 2 O-reducing communities. Fertilization significantly enhanced soil mineral N accumulation compared to treatments with catch crop residues as the only N source. Nitrous oxide emissions, in contrast, were stimulated in rotations with catch crop residue incorporation, probably as a result of concurrent net N mineralization, and O 2 depletion associated with residue degradation in organic hotspots. Emissions of N 2 O from digested manure were low in both years, while emissions from mineral N fertilizer were nearly absent in the first year, but comparable to emissions from catch crop residues in the second year with higher precipitation and delayed plant N uptake. Higher gene abundances, as well as shifts in community structure, were also observed in the second year, which were significantly correlated to NO 3 - availability. Both the size and structure of the nitrite- and N 2 O-reducing communities correlated to the difference in N 2 O emissions between years, while there were no consistent effects of management as represented by catch crops or fertilization. It is concluded that N 2 O emissions were constrained by environmental, rather than the genetic potential for nitrite and N 2 O reduction.