A microbial-explicit soil organic carbon decomposition model (MESDM): development and testing at a semiarid grassland site

Authors: ZHANG, X. - Chinese Academy Of Sciences; MA, Z. - Chinese Academy Of Sciences; BARRON-GAFFORD, G - University Of Arizona; Scott, Russell - Russ; NIU, G. - University Of Arizona

Submitted to: Journal of Advances in Modeling Earth Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2021
Publication Date: 1/9/2022
Citation: Zhang, X., Ma, Z., Barron-Gafford, G., Scott, R.L., Niu, G. 2022. A microbial-explicit soil organic carbon decomposition model (MESDM): development and testing at a semiarid grassland site. Journal of Advances in Modeling Earth Systems. 14(1). Article e2021MS002485. https://doi.org/10.1029/2021MS002485.
DOI: https://doi.org/10.1029/2021MS002485

Interpretive Summary: Soil carbon dioxide efflux (Fsoil) represents the flow of carbon dioxide out of the soil due to belowground plant and microbial respiration and biogeochemical processes, and is a major component of the total amount of carbon moving from the land surface to the atmosphere. Current mathematical representations, or models, of Fsoil are incapable of adequately reproducing field measurements in the semiarid, southwest United States, especially after isolated rainfall. This study developed a new model that addressed this limitation. The results show that the model can reproduce the observed Fsoil in response to rainfall events of various sizes. This study improves our understanding of and our ability to simulate complex soil carbon dynamics found in semiarid regions.

Technical Abstract: Explicit representations of microbial processes in soil organic carbon (SOC) decomposition models have received increased attention, because soil heterotrophic respiration remains one of the greatest uncertainties in climate–carbon feedbacks projected by earth system models (ESMs). Microbial-explicit models have been developed and applied in site- and global-scale studies. These models, however, lack the ability to represent microbial respiration responses to drying-wetting cycles, and few of them have been incorporated in land surface models (LSMs) and validated against field observations. In this study, we developed a multi-layer, microbial-explicit SOC decomposition model (MESDM), based on two main assumptions that 1) extracellular enzymes remain active at dry reaction microsites, and 2) microbes at the wet microsites are active or potentially actives while microbes at the dry microsites are dormant, by dividing the soil volume into wet and dry zones. MESDM with O2 and CO2 gas transport models was coupled with the Noah-MP LSM and tested against half-hourly field observations at Santa Rita semiarid grassland site in the southwest of USA characterized by pulsed precipitation. The results show MESDM can reproduce the observed soil respiration pulses of various sizes in response to precipitation (Birch effect) and thus improve the simulation of net ecosystem exchange. Here, both microbial accessibility to accumulated dissolved organic carbon and reactivation of dormant microbes at the dry microsites upon rewetting are critical to reproducing the Birch effect. This study improves our understanding of and our ability to simulate complex soil carbon dynamics that experience drying-wetting cycle in climate–carbon feedbacks.