Catch Crop Residues Stimulate NO Emissions During Spring, Without Affecting the Genetic Potential for Nitrite and NO Reduction.

Agricultural soils are a significant source of anthropogenic nitrous oxide (NO) 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 NO emissions were monitored during spring, and soil samples were analyzed for abundances of nitrite reduction ( and ) and NO reduction genes ( clade I and II), and structure of nitrite- and NO-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 depletion associated with residue degradation in organic hotspots. Emissions of NO 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 availability. Both the size and structure of the nitrite- and NO-reducing communities correlated to the difference in NO emissions between years, while there were no consistent effects of management as represented by catch crops or fertilization. It is concluded that NO emissions were constrained by environmental, rather than the genetic potential for nitrite and NO reduction.