Novel determination of effective freeze–thaw cycles as drivers of ecosystem change

Abstract

Soil freeze–thaw cycles (FTCs) profoundly influence biophysical conditions and modify biogeochemical processes across many northern‐hemisphere and alpine ecosystems. How FTCs will contribute to global processes in seasonally snow‐covered ecosystems in the future is of particular importance as climate change progresses and winter snowpacks decline. Our understanding of these contributions is limited because there has been little consideration of inter‐ and intrayear variability in the characteristics of FTCs, in part due to a limited appreciation for which of these characteristics matters most with respect to a given biogeochemical process. Here, we introduce the concept of effective FTCs: those that are most likely linked to changes in key soil processes. We also propose a set of parameters to quantify and characterize effective FTCs using standard field soil temperature data. To put these proposed parameters into effective practice, we present FTCQuant, an R package of functions that quantifies FTCs based on a set of user‐defined parameter criteria and, importantly, summarizes the individual characteristics of each FTC counted. To demonstrate the utility of these new concepts and tools, we applied the FTCQuant package to re‐analyze data from two published studies to help explain over‐winter changes to N2O emissions and wet‐aggregate stability. We found that effective FTCs would be defined differently for each of these response variables and that effective FTCs provided a 76 and 33% increase in model fit for wet‐aggregate stability and cumulative N2O emission, respectively, relative to conventional FTC quantification methods focusing on fluctuations around 0 °C. These results demonstrate the importance of identifying effective FTCs when scaling soil processes to regional or global levels. We hope our contributions will inform future deductions, hypothesis generation, and experimentation with respect to expected changes in freeze–thaw cycling globally.

Cold spots and cold moments: The potential for sediment freezing to depress denitrification in wetland sediments

“…Work on FTCs and winter biogeochemical cycling in wetland ecosystems is in its infancy, as is work on winter conditions in lakes and rivers (e.g., Hampton et al., 2016). Further assessment of factors such as freezing temperatures, duration of freezing, and time for recovery after freezing is important to better understand effects of single incidents of freezing and the effects of repeated FTCs in situ (Boswell et al., 2020; Libby et al., 2020; Wagner‐Riddle et al., 2017). Although FTCs can, in some wetlands, lead to suppressed denitrification rates, the dominant control on denitrification rates may often be NO 3 – availability, as is commonly the case in high‐C, low‐NO 3 – ecosystems (Hume et al., 2002).…”

“…Dissolved inorganic N can increase with FTCs in wetlands (Yu et al., 2011) and upland soils (Ruan & Robertson, 2017), although complex changes can also be mediated by multiple FTCs (Sang et al., 2021), including physical changes. Although the first FTC may have the greatest impact on soil/sediment porosity (Congreves et al., 2018), multiple FTCs can be important in breaking up soil/sediment aggregates and increasing sediment surface area (Boswell et al., 2020; Liao et al., 2019; Ruan & Roberston, 2017).…”

Topography and canopy cover influence soil organic carbon composition and distribution across a forested hillslope in the discontinuous permafrost zone

“…Soil temperatures at the surface (5 cm) and subsurface (75 cm) were recorded at the backslope position via a borehole from 2020 to 2021. Soil temperature sensors (TMC‐HD, Onset Computer Inc., USA) were placed at 5 and 75 cm, with temperatures recorded every 6 h. Freeze–thaw cycles were quantified using the method adapted and modified from Rooney et al (2022) 36 using the FTCQuant R Package 37 . The total number of freeze–thaw cycles was calculated for the period of May 2020 to September 2021 when data sets were complete for both open and closed cover soils at depths of 5 and 75 cm.…”

The Impact of Freeze‐Thaw History on Soil Carbon Response to Experimental Freeze‐Thaw Cycles

“…Freeze‐thaw cycles are a deformation process that shape soil function in warming permafrost landscapes and represent an intrinsic component of thaw (Henry, 2008; Ping et al., 2015; Yi et al., 2015). Freeze‐thaw occurs in the surface or subsurface of the active layer during any season depending on air temperature, snowpack, and organic mat thickness (Boswell, Thompson, et al., 2020; Ping et al., 2015; Sharratt et al., 1992). Disturbance from freeze‐thaw alters the soil environment through the reorganization of the physical soil structure (Konrad & McCammon, 1990; Ma et al., 2021; Rooney et al., 2022), increases in extractable carbon and nitrogen as well as higher solute concentrations (Bing et al., 2015; Patel et al., 2021), and microbial death and stimulated activity following cell lysis (Blackwell et al., 2010; Schimel & Clein, 1996), with potential implications for permafrost soil carbon loss.…”

“… Visual hypothesis: Left soil profile is a high freeze‐thaw frequency site (Healy), and the right profile is a low freeze‐thaw frequency site (Toolik). Freeze‐thaw cycles across both sites were measured via the FTCQuant package in R (Boswell, Thompson, et al., 2020) using temperature data from the National Ecological Observatory Network (National Ecological Observatory Network., 2021). …”