Reduced snow cover alters root‐microbe interactions and decreases nitrification rates in a northern hardwood forest

Abstract

Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils are expected to become colder and experience more winter soil freeze‐thaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root‐microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snow‐ and/or soil frost‐mediated) vs. indirect (e.g., root‐mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2 mm pore size) or excluded root ingrowth (50 μm pore size) for up to 29 months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P < 0.05; R2 = 0.25–0.46) than to root biomass, snow or soil frost, or winter soil temperature (R2 < 0.10). Root ingrowth was positively related to soil frost (P < 0.01; R2 = 0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re‐allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30% (P < 0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plant‐microbe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.

Declines in northern forest tree growth following snowpack decline and soil freezing

“…While we did not explicitly quantify rates of root turnover in our experiment, Comerford et al (2013) found root electrolyte leakage in our snow-removal plots to be 37% higher than in our reference plots, indicating elevated rates of root damage. These findings may suggest a shift in tree C allocation away from long-lived C pools in aboveground woody biomass and toward short-lived pools belowground to regrow fine roots, which is consistent with studies showing increases in compensatory root growth following soil freezing (Sorensen, Templer, Christenson et al, 2016;Tierney et al, 2001). Increased root damage and mortality are commonly observed responses to soil freezing, and our findings of reduced tree growth and forest C uptake may therefore extend to other systems where root damage is observed, such as Norway spruce (Picea abies (L.) H. Karst) forests in Germany (Gaul et al, 2008).…”

Soil Microbes Trade-Off Biogeochemical Cycling for Stress Tolerance Traits in Response to Year-Round Climate Change

“…Warming decreases soil temperatures in winter, as warmer air temperatures reduce snowpack and increase the extent and frequency of soil freeze/thaw events (Campbell et al, 2010;Brown and DeGaetano, 2011). Although warming during the snow-free times of year can initially increase C and nitrogen (N) fluxes as decomposition of soil organic matter increases (Rustad et al, 2001;Knorr et al, 2005;Brzostek and Finzi, 2011;Romero-Olivares et al, 2017), these effects can be reversed over the short term by increased frequency of soil freeze/thaw cycles in winter (Durán et al, 2014;Sorensen et al, 2016Sorensen et al, , 2018. Additional storage or loss of soil C and nutrients over the longer term will be determined by how the biogeochemical cycling activities of soil microorganisms ("effect traits") are linked to traits that allow microbial species to persist under environmental stressors ("response traits") (Allison and Martiny, 2008;Shade et al, 2012;Koide et al, 2014).…”

Soil carbon and nitrogen responses to snow removal and concrete frost in a northern coniferous forest

“…Unlike findings from previous snow manipulation studies in the region (e.g. Pilon, Côté, and Fyles 1994;Boutin and Robitaille 1995;Fitzhugh et al 2001;Reinmann et al 2012;Campbell et al 2014b;Sorensen et al 2016 decline in the microbial biomass (Schmidt et al 2007). It is likely that this turnover occurred around 22 April in 2015 and 8 March in 2016, as evidenced by the low SUVA 254 values (Fig.…”

“…Most snow manipulation experiments in northeastern North America have been conducted in hardwood stands (e.g. Boutin and Robitaille 1995;Fitzhugh et al 2001;Groffman et al 2001Groffman et al , 2011Campbell et al 2014b;Reinmann and Templer 2016;Sorensen et al 2016), while similar studies in coniferous forests have mostly been conducted in Europe (Sulkava and Huhta 2003;Hentschel et al 2008Hentschel et al , 2009Öquist and Laudon 2008;Haei et al 2012). Conifers account for approximately 50% of total forest cover in eastern Canada (Canada's National Forest Inventory 2013) and approximately 16% of total forest cover in the northeastern United States (Oswalt et al 2018) -more dominant in some regions like Maine where 33% of the forest cover is coniferous (Huff and McWilliams 2016).…”