Soil carbon cycling proxies: Understanding their critical role in predicting climate change feedbacks

Abstract

The complexity of processes and interactions that drive soil C dynamics necessitate the use of proxy variables to represent soil characteristics that cannot be directly measured (correlative proxies), or that aggregate information about multiple soil characteristics into one variable (integrative proxies). These proxies have proven useful for understanding the soil C cycle, which is highly variable in both space and time, and are now being used to make predictions of the fate and persistence of C under future climate scenarios. However, the C pools and processes that proxies represent must be thoughtfully considered in order to minimize uncertainties in empirical understanding. This is necessary to capture the full value of a proxy in model parameters and in model outcomes. Here, we provide specific examples of proxy variables that could improve decision‐making, and modeling skill, while also encouraging continued work on their mechanistic underpinnings. We explore the use of three common soil proxies used to study soil C cycling: metabolic quotient, clay content, and physical fractionation. We also consider how emerging data types, such as genome‐sequence data, can serve as proxies for microbial community activities. By examining some broad assumptions in soil C cycling with the proxies already in use, we can develop new hypotheses and specify criteria for new and needed proxies.

Arctic Soil Governs Whether Climate Change Drives Global Losses or Gains in Soil Carbon

“…MIMICS and CORPSE also differ in how they represent factors controlling the formation and turnover of physicochemically protected C (Text S1 in the supporting information). Despite these differences, models in our testbed all use clay content as a proxy to determine the capacity of soils to physicochemically protect soil C (Bailey et al, ). Although generalizability of this assumption is being questioned (Rasmussen et al, ; Rowley et al, ), the protected soil C simulated here broadly corresponds to mineral associated organic matter (MAOM) that could be isolated by particle size or density fractionation (Sohi et al, ).…”

A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils

“…The relatively low cost of the Rock-Eval 6 technique and the robustness of the thermal analysis regression model make it possible to apply it to soil monitoring networks across a continuum of scales as a reliable proxy for SOC persistence. This may be part of the framework proposed by O'Rourke et al (2015) to better understand SOC processes at the biosphere to biome scale and should be added to the soil carbon cycling proxies recently listed by Bailey et al (2018). Mapping persistent SOC at large scales may allow for the identification of regional hotspots of centennially persistent SOC that may contribute little to climate change by 2100.…”

“…Even the well-established radiocarbon ( 14 C) analytical technique cannot precisely determine the size of the centennially persistent SOC pool (Schrumpf and Kaiser, 2015;Menichetti et al, 2016). The importance of better information on the size of the centennially persistent SOC pool has been emphasized recently (Soil Carbon Initiative, 2011;Bispo et al, 2017;Bailey et al, 2018;Harden et al, 2018), stressing the need for operational and standardized metrics or proxies to accurately quantify SOC persistent at the centennial timescale. The general lack of information on the size and turnover rate of measurable SOC pools hampers the initialization of SOC pools in dynamic models, questioning their predictions of the evolution of the global SOC reservoir (Falloon and Smith, 2000;Luo et al, 2014;Feng et al, 2016;He et al, 2016;Sanderman et al, 2016).…”

Variation in the Molecular Structure and Radiocarbon Abundance of Mineral-Associated Organic Matter across a Lithosequence of Forest Soils

“…In the dataset examined here, heavy fraction MRT was explained almost equally by Fe D and SSA (Table 3 and Table S1). The production of surface area (SSA) and pedogenic Fe go hand-in-hand as weathering progresses, and both may act as a proxy for the general developmental stage of the soil [58,59] (absolute abundance of clay and pedogenic Fe also varying with parent material composition). However, mounting evidence suggests that Fe-rich phases play a unique and highly influential role in the stabilization and preservation of SOM.…”