Quantifying the legacy of snowmelt timing on soil greenhouse gas emissions in a seasonally dry montane forest

Abstract

The release of water during snowmelt orchestrates a variety of important belowground biogeochemical processes in seasonally snow‐covered ecosystems, including the production and consumption of greenhouse gases (GHGs) by soil microorganisms. Snowmelt timing is advancing rapidly in these ecosystems, but there is still a need to isolate the effects of earlier snowmelt on soil GHG fluxes. For an improved mechanistic understanding of the biogeochemical effects of snowmelt timing during the snow‐free period, we manipulated a high‐elevation forest that typically receives over two meters of snowfall but little summer precipitation to influence legacy effects of snowmelt timing. We altered snowmelt rates for two years using black sand to accelerate snowmelt and white fabric to postpone snowmelt, thus creating a two‐ to three‐week disparity in snowmelt timing. Soil microclimate and fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) were monitored weekly to monthly during the snow‐free period. Microbial abundances were estimated by potential assays near the end of each snow‐free period. Although earlier snowmelt caused soil drying, we found no statistically significant effects (p < 0.05) of altered snowmelt timing on fluxes of CO2 or N2O, or soil microbial abundances. Soil CH4 fluxes, however, did respond to snowmelt timing, with 18% lower rates of CH4 uptake in the earlier snowmelt treatment, but only after a dry winter. Cumulative CO2 emission and CH4 uptake were 43% and 88% greater, respectively, after the dry winter. We conclude that soil GHG fluxes can be surprisingly resistant to hydrological changes associated with earlier snowmelt, likely because of persistent moisture and microbial activities in deeper mineral soils. As a result, a drier California in the future may cause seasonally snow‐covered soils in the Sierra Nevada to emit more GHGs, not less.

Unraveling the Controls on Snow Disappearance in Montane Conifer Forests Using Multi‐Site Lidar

“…Snow and ice melt provide about 20% of the global water supply, with snow water supplies concentrated in northern hemisphere forests with complex topography (Barnett et al., 2005). Snowmelt timing and disappearance date have substantial impacts on montane forests, by affecting soil moisture and deeper recharge (Bales et al., 2011; Conner et al., 2015; Harpold, Marshall, et al., 2015; Huntington & Niswonger, 2012; Pavlovskii et al., 2019), ecosystem water availability and streamflow timing (Harpold, 2016; Kormos et al., 2017; Stewart et al., 2004), growing season length (Harpold, 2016; O’Leary et al., 2018), spring phenology (O’Leary et al., 2018; Pederseng et al., 2018), soil greenhouse gas emission (Blankinship et al., 2018), and land surface‐atmosphere energy fluxes (Knowles et al., 2015; Peichl et al., 2013; Slater et al., 2001). A later snow disappearance delays soil water inputs, resulting in a longer recession in soil moisture (Harpold & Molotch, 2015), delays peak forest transpiration (A. E. Cooper et al., 2020), and limits the duration of soil moisture stress for vegetation if summer rains are not present (Harpold, 2016).…”

Impacts of climate and disturbance on nutrient fluxes and stoichiometry in mixed-conifer forests

“…These estimated rates were approximately 30% higher than atmospheric N deposition at our sites (8.0 and 7.7 kg N ha -1 year -1 at the transition and snow-dominated site, respectively), suggesting that biological N fixation is an important N input to mixed-conifer forests in the Sierra Nevada. Soil emission of N 2 O was measured to be approximately 0.02 kg N ha -1 year -1 at the two sites (Blankinship et al 2018). However, total N losses from soil emissions (i.e., N 2 and N x O) were not measured at our sites.…”

Forest-floor greenhouse gas fluxes in a subalpine spruce forest: Continuous multi-year measurements, drivers, and budgets

“…Generally, in temperate forests, CH4 uptake increases in warmer and drier soils (Borken et al, 2006;Ni and Groffman, 2018). Winter dynamics further impact CH4 fluxes, with frozen soil and snow cover affecting microbial activity and gas transport (Blankinship et al, 2018;Borken et al, 2006). Understanding the drivers of forest-floor CH4 fluxes, including the complex interplay between biotic and abiotic factors, is vital for accurately modeling and predicting the role of forest ecosystems in the global CH4 cycle.…”