Towards improved modeling of SOC decomposition: soil water potential beyond the wilting point

Abstract

Soils are important carbon (C) reservoirs and play a critical role in regulating the global C cycle. Soil water potential (SWP) measures the energy with which water is retained in the soil and is one of the most vital factors that constrain the decomposition of soil organic C (SOC). The measurements for soil water retention curve (SWRC), on which the estimation of SWP depends, are usually carried out above −1.5 MPa (i.e., the wilting point for many plants). However, the average moisture threshold at which soil microbial activity ceases is usually below −10 MPa in mineral soils. Beyond the measurement range, the SWP estimation has to be derived from extrapolating the SWRC, which violates the statistical principle, resulting in possibly inaccurate SWP estimations. To date, it is unclear to what extent the extrapolated SWP estimation deviates from the "true value" and how it impacts the modeling of SOC decomposition. This study combined SWRC measurements down to −43.7 MPa, a 72-day soil incubation experiment with four moisture levels, and an SOC decomposition model. In addition to the complete SWRC (SWRC all ), we fitted two more SWRCs by using measurements above −0.5 MPa (SWRC 0.5 ) and −1.7 MPa (SWRC 1.7 ), respectively, to quantify the deviations of extrapolated SWPs from the complete SWRC. Results showed that extrapolating the SWRC beyond its measurement range significantly underestimated the SWP. Incorporating the extrapolated SWP in the model significantly underestimated the SOC decomposition under relatively dry conditions. With the extrapolated SWP, the model predicted no SOC decomposition in the driest treatment, while the experiment observed a significant CO 2 emission. The results emphasize that accurate SWP estimations beyond the wilting point are critically needed to improve the modeling of SOC decomposition.

Simulating net ecosystem exchange under seasonal snow cover at an Arctic tundra site

“…Values for Q10 sampled a wide range of measured Q10 from Arctic soils (based on Chen et al, 2020;Elberling, 2007;Elberling and Brandt, 2003;Grogan and Jonasson, 2005;Mikan et al, 2002;Schmidt et al, 2008), and values of Ψmin were based on Tao et al (2021). We note that the most negative values (< -200 MPa) of Ψmin used by Tao et al (2021) and herein are unlikely to be physically representative (Liang et al, 2022).…”

“…Even in frozen soils, liquid water can be present within the soil matrix, enabling respiration at temperatures as low as -18 °C (Elberling and Brandt, 2003), whereas when Ψmin exceeds Ψ simulated soil respiration ceases at warmer temperatures than -18 °C. Recent findings from Liang et al (2022) suggest that mineral soils should be able to respire below a Ψ of -10 MPa, suggesting a Ψmin below -10 MPa would be more physically representative than the current CLM5.0 default of -2 MPa. Tao et al (2021) also highlighted the unsuitability of such a high Ψmin, the default Ψmin of -10 MPa in E3SM, a five times larger negative Ψmin than the CLM5.0 default, prevented simulation of respiration when soil temperatures were sub-zero and failed to allow the accurate simulation of wintertime respiration in permafrost tundra environments.…”

Soil Respiration Response to Simulated Precipitation Change Depends on Ecosystem Type and Study Duration

“…Therefore, our second hypothesis (that manipulation intensity would be an important modifier for both +P and −P) was only partially supported. This may be due to water‐holding limitations of soil: water can run off of saturated soil or pass through soil into groundwater, but there is a lower bound for biologically available water that cannot be circumvented (Liang et al., 2022). Intuitively, we would expect that water‐limited ecosystems such as deserts and shrublands would have C cycling responses proportional to increased precipitation intensity, and this is supported by research (Ahlström et al., 2015; Parton et al., 2012).…”

Increasing global land surface soil organic carbon under climate change

“…Due to insufficient understanding on global SOC mechanism and the huge uncertainty of models, the projection of future SOC variation keeps out even with both negative and positive feedbacks. Quite a lot studies focus on microscopic mechanism of microbial decomposition, biological carbon and sequestration effect [20][21][22][23] , which are mainly based on steady state assumption of SOC for regional scales. The steady state assumption is apparently inapplicable to global scale.…”