A Reactive Transport Module of Land Biogeochemistry in the Community Land Model: Description, Evaluation, and Application.

The recognition of the important roles played by various biogenic greenhouse gases (CO₂, CH₄, N₂O, NO, etc.) in modulating the global climate system calls for a land biogeochemical model that can consistently represent the relevant reactions, phase changes, transport, and soil-atmosphere exchanges within an earth system modeling framework. We developed such a module—the bigeochemical transport and reaction (BeTR) module—for CLM (the community land model), which is the land process simulator of the community earth system model. CLM-BeTR represents the transport, interactions, and biotic and abiotic transformations of an arbitrary number of tracers (chemical species, isotopologues) in an arbitrary number of phases (e.g., dissolved, gaseous, sorbed, aggregate). It obtains the numerical solutions using an operator splitting approach and gives accurate tracer transport at a half hourly time step using the CLM soil vertical discretization. Simulations for the Harvard Forest site indicate the model simulates soil-gas phase CO₂ profiles accurately compared to gas sampling observations. The results challenge the assumption that soil respiration equals the soil surface CO₂ efflux on various times scales that is typically made in ‘bucket’-type climate scale land biogeochemical models. Predictions also indicate that atmospheric intrusion is small compared with bulk soil respiration, but can significantly impact the surface isotopic CO₂ signal. We will also discuss new insights on soil radiocarbon dynamics and the budget of soil CO₂ through different pathways including losses through surface gas efflux, plant transpiration, and hydrological runoff and describe relevant implications on simulating carbon-climate feedbacks.