Abstract. Reliable soil biogeochemical modeling is a prerequisite
for credible projections of climate change and associated ecosystem
feedbacks. This recognition has called for frameworks that can support
flexible and efficient development and application of new or alternative
soil biogeochemical modules in Earth system models (ESMs). The the Biogeochemical Transport and Reaction model version 1 (BeTR-v1) code
(i.e., CLM4-BeTR) is one such framework designed to accelerate the
development and integration of new soil biogeochemistry formulations into
ESMs and to analyze structural uncertainty in ESM simulations. With a
generic reactive transport capability, BeTR-v1 can represent multiphase
(e.g., gaseous, aqueous, and solid), multi-tracer (e.g., nitrate and organic
carbon), and multi-organism (e.g., plants, bacteria, and fungi) dynamics.
Here, we describe the new version, Biogeochemical Transport and Reaction model version 2 (BeTR-v2), which adopts more robust
numerical solvers for multiphase diffusion and advection and coupling
between biogeochemical reactions and improves code modularization over
BeTR-v1. BeTR-v2 better supports different mathematical formulations in a
hierarchical manner by allowing the resultant model be run for a
single topsoil layer or a vertically resolved soil column, and it allows the model to be fully coupled
with the land component of the Energy Exascale Earth System Model (E3SM). We
demonstrate the capability of BeTR-v2 with benchmark cases and example soil biogeochemical (BGC)
implementations. By taking advantage of BeTR-v2's generic structure
integrated in E3SM, we then found that calibration could not resolve biases
introduced by different numerical coupling strategies of plant–soil
biogeochemistry. These results highlight the importance of numerically
robust implementation of soil biogeochemistry and coupling with hydrology,
thermal dynamics, and plants – capabilities that the open-source BeTR-v2
provides. We contend that Earth system models should strive to minimize this uncertainty by applying better numerical solvers.