ORCHIMIC (v1.0), a microbe-mediated model for soil organic matter decomposition

Abstract. The role of soil microorganisms in regulating soil\r\norganic matter (SOM) decomposition is of primary importance in the carbon\r\ncycle, in particular in the context of global change. Modeling soil\r\nmicrobial community dynamics to simulate its impact on soil gaseous carbon\r\n(C) emissions and nitrogen (N) mineralization at large spatial scales is a\r\nrecent research field with the potential to improve predictions of SOM\r\nresponses to global climate change. In this study we present a SOM model called\r\nORCHIMIC, which utilizes input data that are consistent with those of global\r\nvegetation models. ORCHIMIC simulates the decomposition of SOM by explicitly\r\naccounting for enzyme production and distinguishing three different\r\nmicrobial functional groups: fresh organic matter (FOM) specialists, SOM\r\nspecialists, and generalists, while also implicitly accounting for microbes\r\nthat do not produce extracellular enzymes, i.e., cheaters. ORCHIMIC and two other organic matter decomposition models,\r\nCENTURY (based on first-order kinetics and representative of the structure of most current global soil carbon models)\r\nand PRIM (with FOM accelerating the decomposition rate of SOM), were calibrated\r\nto reproduce the observed respiration fluxes of FOM and SOM from the incubation experiments of\r\nBlagodatskaya et al. (2014).\r\nAmong the three models, ORCHIMIC was the only\r\none that effectively captured both the temporal dynamics of the respiratory fluxes\r\nand the magnitude of the priming effect observed during the incubation\r\nexperiment. ORCHIMIC also effectively reproduced the temporal dynamics of microbial\r\nbiomass. We then applied different idealized changes to the model input\r\ndata, i.e., a 5 K stepwise increase of temperature and/or a doubling of plant\r\nlitter inputs. Under 5 K warming conditions, ORCHIMIC predicted a 0.002 K−1\r\ndecrease in the C use efficiency (defined as the ratio of C allocated to\r\nmicrobial growth to the sum of C allocated to growth and respiration) and a\r\n3 % loss of SOC. Under the double litter input scenario, ORCHIMIC\r\npredicted a doubling of microbial biomass, while SOC stock increased by less\r\nthan 1 % due to the priming effect. This limited increase in SOC stock\r\ncontrasted with the proportional increase in SOC stock as modeled by the\r\nconventional SOC decomposition model (CENTURY), which can not reproduce the\r\npriming effect. If temperature increased by 5 K and litter input was doubled,\r\nORCHIMIC predicted almost the same loss of SOC as when only temperature\r\nwas increased. These tests suggest that the responses of SOC stock to\r\nwarming and increasing input may differ considerably from those simulated by\r\nconventional SOC decomposition models when microbial dynamics are included.\r\nThe next step is to incorporate the ORCHIMIC model into a global vegetation\r\nmodel to perform simulations for representative sites and future scenarios.