Dep. of Soil Science, Univ. of Wisconsin-Madison, 1525 Observatory Dr., Madison, WI, 53706, USA.
Dep. of Biological Systems Engineering, Univ. of Wisconsin-Madison, 460 Henry Mall, Madison, WI, 53706, USA.
J Environ Qual. 2020 Mar;49(2):314-323. doi: 10.1002/jeq2.20053. Epub 2020 Mar 9.
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 N O 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 N O 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.
土壤冻融循环（FTCs）深刻影响着许多北半球和高山生态系统的生物物理条件，并改变了生物地球化学过程。随着气候变化的推进和冬季积雪量的减少，未来 FTCs 将如何促进季节性积雪覆盖的生态系统中的全球过程尤为重要。由于我们对 FTCs 特征的年内和年际变化的了解有限，因此我们对这些贡献的理解有限，部分原因是我们对这些特征中哪些与特定的生物地球化学过程最相关的认识不足。在这里，我们引入了有效 FTCs 的概念：与关键土壤过程变化最相关的 FTCs。我们还提出了一组参数，使用标准的田间土壤温度数据来量化和描述有效 FTCs。为了将这些建议的参数付诸实践，我们提出了 FTCQuant，这是一个基于一组用户定义的参数标准来量化 FTCs 的 R 包，并且重要的是，它总结了每个 FTC 的单独特征。为了展示这些新概念和工具的实用性，我们应用 FTCQuant 包重新分析了两个已发表研究的数据，以帮助解释冬季 N O 排放和湿团聚体稳定性的变化。我们发现，对于每个响应变量，有效 FTCs 的定义都不同，并且与传统的聚焦于 0°C 左右波动的 FTC 量化方法相比，有效 FTCs 分别使湿团聚体稳定性和累积 N O 排放的模型拟合度提高了 76%和 33%。这些结果表明，在将土壤过程扩展到区域或全球水平时，确定有效 FTCs 非常重要。我们希望我们的贡献将为未来关于全球冻融循环变化的推断、假设生成和实验提供信息。
