Purpose

Explicitly implement soil carbon saturation theory in SIPNET to prevent infinite soil carbon sequestration. Based on soil mineralogical, physical, and chemical properties, only a finite amount of carbon entering the soil can be stabilized for long-term storage. Explicitly incorporating this limit into a process-based model can enable analysis of model handling of soil carbon storage dynamics.

Methods

• Create carbonSaturation flag to turn functionality off/on
• Create new site parameters for clay + silt fractions
  • Can the parameters be optional if carbonSaturation = 0 and required if carbonSaturation = 1?
• Calculate maximum carbon saturation threshold using three models to show range of possible limits from literature
  • $C_\mathrm{sat} = 21.1 + 0.0375 * (Particles <2 \mu m)$ - Hassink and Whitmore 1997
  • $C_\mathrm{sat} = 14.76 + 0.21 * (Particles <50 \mu m)$ - Six et al 2002 (for 2:1 clay type)
  • $C_\mathrm{sat} = 7.18 + 0.2 * (Particles <50 \mu m)$ - Six et al 2002 (for cultivated ecosystem)
• Create asymptotic approach to soil carbon saturation threshold
  • $C_t = \frac{I}{k + \frac{I}{C_\mathrm{sat}}}$ - Stewart et al 2007
  • $I$ is inputs (fluxes currently used to calculate envi.soilC), $k$ is a decay constant, $C_\mathrm{sat}$ is the saturation threshold calculated above, and $C_t$ is envi.soilC
  • This equation would be used in place of the current envi.soilC calculation. I'm not sure what the value of $k$ would be at this time, but in the paper it includes loss to respiration and assumes leaching/erosion is zero. This assumption aligns with SIPNET.
• Feedback requested: Deal with unstabilized carbon
  • This all assumes the soil C pool is analogous to mineral-associated organic carbon, the long-term C that builds in the soil over time. However, there is also the fast turnover pool of soil C that isn't stabilized and could potentially surpass saturation limits at certain times (due to specific management practices or fresh inputs). There are a few possible options to deal with leftover soil carbon that is not being stabilized into envi.soilC:
    • Option 1: loss of all excess C to respiration (would increase short-term soil respiration flux, in reality this wouldn't be an immediate loss to respiration)
    • Option 2: create flux of C back to litter C pool (this treats litter more like a fast turnover pool, what are the implications of a new two directional flow?)
    • Option 3: create a new pool for unstabilized soil C (this adds complexity. Would it just be used for tracking purposes? Would it interact with the rest of the model in any way? Where would the C in this pool eventually end up to prevent it from growing too large, or would it function like a slush fund of soil C where we just say that it is the amount of unstabilized C lost to various sources over time?)

Analyses

1. SIPNET representation of soil organic carbon compared to observations: carbon saturation off versus on
2. SIPNET representation of soil organic carbon compared to observations with carbon saturation on: nitrogen cycle off versus on
3. SIPNET with carbon saturation on and nitrogen cycle on: sensitivity analysis of impact of management practices on soil organic carbon

Known Limitations and Explanations

• Carbon saturation models are least square linear regressions. Some soils may be considered over-saturated by definition. Complexity of these models fit with SIPNET.
• There are newer models (boundary line analysis) and other widely used models, but they are based on $20 \mu m$ threshold. This is not a common measurement, but $2 \mu m$ and $50 \mu m$ measurements are standard, so the selected models above are more appropriate.
• Management greatly impacts soil carbon dynamics. A possible outcome is that none of these models can accurately represent soil carbon observations. Sensitivity analysis of impact of management on soil carbon should provide some insight.

References

Hassink and Whitmore 1997.pdf
Six et al 2002.pdf
Stewart et al 2007.pdf