Overwinter Changes in Denitrification and N2 O-Producing Microorganisms in an Agricultural Soil with Different N Sources.

Soil nitrogen (N) processes and changes in related microbial communities have been extensively studied during the growing season. However, there is a scarcity of information on N cycle processes and microbial communities in agricultural soils during winter. It is possible that different N sources such as inorganic fertilizer or manure could influence soil denitrifier and nitrifier abundance and structure thus changing N cycle processes during winter. We evaluated changes in nitrifiers and denitrifiers abundance and composition and in denitrification and N₂O emissions during the winter period in soils with spring application of NO₃NH₄, cattle manure or poultry manure. Soil samples were collected from a sandy loam (Saint-Augustin-de-Desmaures, Qc, Canada) during two consecutive winters (2009-2010 and 2010-2011) in November (end of cropping season), December (freeze-thaw cycles), January (moderate snow cover, frozen soil), March (deep snow cover, progressive soil warming) and April (early spring after thaw period).  Seasonal variations, but not different N fertilizer sources, had the largest impact on the greenhouse gas N₂O emission, and denitrification rates that were higher during January and March for two consecutive winters. Nitrifier and denitrifier abundances responded to seasonal changes and N fertilizer sources only in the winter of 2010-2011. Abundance of nitrifiers (nxrA, archaeal amoA) and denitrifiers (nirK, nirS and nosZ) increased in March and April compared with other sampling dates. Nitrifier abundance (nxrA) was greater in soil with cattle manure application, while denitrifiers (nirK and nosZ) were more abundant in poultry manure treated soil compared with other N fertilizer treatments. Bacterial and archaeal amoA, nxrA, nirK, nirS and nosZ showed different composition between sampling times and different N treatments in the two monitored winters, suggesting that changes in environmental conditions overtime and N fertilizer sources both influenced these communities. Our findings demonstrate that N₂O emission and denitrification rates and denitrifier and nitrifier abundance and composition are controlled by different factors in agricultural field during winter. The decoupling between N₂O emissions and denitrification and the community composition of N₂O-producing microorganisms suggests a functional redundancy of nitrifiers and denitrifiers during the winter months.