Mechanisms leading to enhanced soil nitrous oxide fluxes induced by freeze–thaw cycles

Abstract

Risk, N., Snider, D. and Wagner-Riddle, C. 2013. Mechanisms leading to enhanced soil nitrous oxide fluxes induced by freeze–thaw cycles. Can. J. Soil Sci. 93: 401–414. The freezing and thawing of soil in cold climates often produces large emissions of nitrous oxide (N2O) that may contribute significantly to a soil's annual greenhouse gas emission budget. This review summarizes the state of knowledge of the physical and biological mechanisms that drive heightened N2O emissions at spring melt. Most studies of freeze–thaw N2O emissions have concluded that denitrification is the dominant process responsible for the large thaw fluxes. Soil moisture, availability of carbon and nitrogen substrates, and freeze temperature and duration are the major factors identified as controlling freeze–thaw cycle (FTC) N2O emissions. Two mechanisms are proposed to lead to enhanced N2O emissions at thaw: (1) the physical release of N2O that is produced throughout the winter and trapped under frozen surface layers and/or within nutrient-rich water films in the frozen layers, and (2) the emission of newly produced (de novo) N2O at the onset of thaw, which is stimulated by increased biological activity and changes in physical and chemical soil conditions. Early studies implicated the physical release of N2O from subsurface soil layers as the main mechanism contributing to spring thaw emissions, but most current studies do not support this hypothesis. Mounting evidence suggests that most of the emitted N2O is produced de novo. This may be fueled by newly available denitrification substrates that are liberated from dead microbes, fine roots, and/or the disintegration of soil aggregates. The release of N2O trapped in shallow surface layers may represent a small, but important contribution of the total emissions. Application of new techniques to study microbial communities in their natural environments, such as metagenomics and stable isotope studies, have the potential to enhance our understanding of the soil N cycle and its linkages to FTC N2O emissions. Future field studies of N2O emissions ought to quantify both overwinter accumulation/release and the de novo production of N2O so that the contribution of each mechanism to the annual emission budget is known.

Non‐Legume Cover Crops Can Increase Non‐Growing Season Nitrous Oxide Emissions

“…This is consistent with other studies from southern Alberta, Canada, which showed peak N 2 O fluxes during winter (Hao, 2015) or the late-winter to spring period (Chang et al, 1998;Ellert and Janzen, 2008) during the non-growing season. Typically, N 2 O fluxes are greatest during the spring snowmelt in temperate regions (WagnerRiddle and Thurtell, 1998;Johnson et al, 2012;Risk et al, 2013;Congreves et al, 2016). However, our study location typically does not accumulate much snow over-winter.…”

“…Furthermore, the solubility of N 2 O is inversely related to temperature, water at 0°C will contain about twice the N 2 O as water at 20°C (Weiss and Price, 1980). This may partly contribute to soils near 0°C emitting unexpectedly large quantities of N 2 O, as diffusion gradients between the dissolved pool of N 2 O and the atmosphere may drive degassing of N 2 O during warming events (Goodroad and Keeney, 1984;Risk et al, 2013). However, N 2 O fluxes persisted even when soil temperatures did not rise above the freezing point.…”

“…Thus, the surface soil is poorly insulated from air temperature fluctuations and is prone to freeze-thaw cycles during the winter. As freeze-thaw cycles are considered the principal mechanism affecting the physical release of N 2 O from below the thawing surface layer and from the increased biological activity at the onset of thaw (Goodroad and Keeney, 1984;Chang and Hao, 2001;Risk et al, 2013), these cycles exert significant control over the magnitude of N 2 O fluxes in the nongrowing season in southern Alberta.…”

“…As the freeze temperature modulates the potential N 2 O flux at thawing (Risk et al, 2013), the amplitude of the temperature change from frozen to thawed (Wertz et al, 2016), the 'transition effect' (Butterbach-Bahl et al, 2013), controls the magnitude of the N 2 O flux during a thaw event. Furthermore, the solubility of N 2 O is inversely related to temperature, water at 0°C will contain about twice the N 2 O as water at 20°C (Weiss and Price, 1980).…”

“…The rationale was, that for cover cropped soil in semiarid regions, a good time to make these measurements would be immediately before winter, and that the concentration of NO 3 and WEOC at this time is critical because N 2 O may accumulate in soil solution near 0°C, as N 2 O is twofold more soluble in water at 0°C than 20°C (Weiss and Price, 1980). Thus, the NO 3 and WEOC at this time would represent the N 2 O emission potential, representing what could enter the water soluble pool and later be released with a thaw as temperatures increase and the solubility of N 2 O decreases (Goodroad and Keeney, 1984;Risk et al, 2013), and the N 2 O that could be emitted by facultative anaerobes during thaw events as minimal leaching and no plant N uptake would occur over-winter.…”