Abstract
Abstract. Production and reduction of nitrous oxide (N2O) by soil denitrifiers influence atmospheric concentrations of this potent greenhouse gas. Accurate projections of the net N2O flux have three key uncertainties: (1) short- vs. long-term responses to warming, (2) interactions among soil horizons, and (3) temperature responses of different steps in the denitrification pathway. We addressed these uncertainties by sampling soil from a boreal forest climate transect encompassing a 5.2 ∘C difference in the mean annual temperature and incubating the soil horizons in isolation and together at three ecologically relevant temperatures in conditions that promote denitrification. Both short-term exposure to warmer temperatures and long-term exposure to a warmer climate increased N2O emissions from organic and mineral soils; an isotopic tracer suggested that an increase in N2O production was more important than a decline in N2O reduction. Short-term warming promoted the reduction of organic horizon-derived N2O by mineral soil when these horizons were incubated together. The abundance of nirS (a precursor gene for N2O production) was not sensitive to temperature, whereas that of nosZ clade I (a gene for N2O reduction) decreased with short-term warming in both horizons and was higher from a warmer climate. These results suggest a decoupling of gene abundance and process rates in these soils that differs across horizons and timescales. In spite of these variations, our results suggest a consistent, positive response of denitrifier-mediated net N2O efflux rates to temperature across timescales in these boreal forests. Our work also highlights the importance of understanding cross-horizon N2O fluxes for developing a predictive understanding of net N2O efflux from soils.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas with ∼ 300 times the global warming potential of carbon dioxide on a 100-year timescale and uncertain climate feedback effects (Ciais et al, 2013; Portmann et al, 2012)
Net N2O production was tempered by more mineral soil N2O reduction at warmer incubation temperatures (Figs. 4, 5), indicating that soil horizon interactions may be critical to rates of net N2O efflux to the aboveground atmosphere
Our results suggest a decoupling of process rates and denitrifier genetic controls or that the long-term temperature-related increase in genetic potential for N2O reduction did not translate to rates as effectively as the short-term temperature-related decrease in genetic potential for N2O reduction
Summary
Nitrous oxide (N2O) is a potent greenhouse gas with ∼ 300 times the global warming potential of carbon dioxide on a 100-year timescale and uncertain climate feedback effects (Ciais et al, 2013; Portmann et al, 2012). Increases in atmospheric N2O are attributed to nitrogen (N) fertilizer use (Mosier et al, 1998), emissions from natural systems dominate terrestrial fluxes (Ciais et al, 2013) and experimental manipulations indicate that warming may enhance these fluxes (Benoit et al, 2015; Billings and Tiemann, 2014; Kurganova and Lopes de Gerenyu, 2010; Szukics et al, 2010; Wang et al, 2014). One of the most important biogeochemical pathways of N2O formation in natural systems is denitrification – the stepwise reduction of NO−3 to N2. In this pathway, soil denitrifiers can both produce and reduce. Buckeridge et al.: Short- and long-term temperature responses of soil denitrifiers
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