Modeling microsite processes for gigatons of soil carbon
Enzyme kinetics, microbial biomass growth, physical and chemical stabilization, and the temperature sensitivities of all of the above are important processes affecting soil-C stocks, but there is little agreement on how to integrate this understanding in useful numerical models. When current soil-C maps are used to validate Earth System Model (ESM) simulations, the test is really how well stabilized soil organic matter (SOM), which comprises most of the soil-C, is simulated. But do these models really simulate the microsite processes of: (1) accumulation in wetlands due to anaerobiosis; (2) locking up previously accumulated C in permafrost; (3) physical and chemical protection of SOM on clay-sized minerals and in aggregates; and (4) persistence of low concentrations of C in deep soils that add up to large stocks due to their sheer volume? Most ESMs focus largely on photosynthesis and respiration, yet those are not the processes that are the best predictors of distributions of soil-C stocks. ESMs are used to project potential future feedbacks to climate change, which is where soil enzymatic and microbial processes could fit in, but they, too, are poorly represented in current generation ESMs. The challenge is to identify the key processes of substrate availability to enzymatic degradation at microsite scales that can be represented numerically for larger scale simulations.
If the objective is:
A. to simulate spatial distributions of soil-C stocks, focus more on the processes of SOM stabilization, and less on the fast responses of respiration. The latter is a large flux, but primarily important for annual C balances rather than long-term stock changes.
B. to simulate changes in the largest and most vulnerable soil-C stocks under climate change, focus on permafrost and wetlands. Terra firma in non-polar regions is where we do agriculture and forestry, but it is probably less important regarding changing global soil-C stocks in the coming century.
C. to debate about Q10s and thermal acclimation of microbial decomposition, that can be fun and intellectually fascinating, but it is probably less important regarding present and future soil-C stocks than are items A and B.
D. to develop skillful soil-C models across spatial and temporal scales, include explicit numerical representations of: temperature sensitive enzymatic processes; organismal responses to environmental cues; community changes in response to changing environments; substrate availability, nutrient interactions, and stabilization/destabilization processes that have small annual rates but large long-term effects.