Abstract
Abstract. Spatial and temporal variations in atmospheric carbon dioxide (CO2) reflect large-scale net carbon exchange between the atmosphere and terrestrial ecosystems. Soil heterotrophic respiration (HR) is one of the component fluxes that drive this net exchange, but, given observational limitations, it is difficult to quantify this flux or to evaluate global-scale model simulations thereof. Here, we show that atmospheric CO2 can provide a useful constraint on large-scale patterns of soil heterotrophic respiration. We analyze three soil model configurations (CASA-CNP, MIMICS, and CORPSE) that simulate HR fluxes within a biogeochemical test bed that provides each model with identical net primary productivity (NPP) and climate forcings. We subsequently quantify the effects of variation in simulated terrestrial carbon fluxes (NPP and HR from the three soil test-bed models) on atmospheric CO2 distributions using a three-dimensional atmospheric tracer transport model. Our results show that atmospheric CO2 observations can be used to identify deficiencies in model simulations of the seasonal cycle and interannual variability in HR relative to NPP. In particular, the two models that explicitly simulated microbial processes (MIMICS and CORPSE) were more variable than observations at interannual timescales and showed a stronger-than-observed temperature sensitivity. Our results prompt future research directions to use atmospheric CO2, in combination with additional constraints on terrestrial productivity or soil carbon stocks, for evaluating HR fluxes.
Highlights
Atmospheric CO2 observations reflect net exchange of carbon between the land and oceans with the atmosphere
Fluxes from the soil test-bed ensemble with three representations of soil biogeochemistry (CASA-CNP, CORPSE, MicrobialMineral Carbon Stabilization model (MIMICS))
Results show that the phasing of heterotrophic respiration fluxes relative to net primary productivity fluxes is an important source of bias in evaluating simulated CO2 against atmospheric observations at both seasonal and interannual timescales
Summary
Atmospheric CO2 observations reflect net exchange of carbon between the land and oceans with the atmosphere. Heterotrophic respiration has been represented as a first-order decay process based on precipitation, temperature, and a linear relationship with available substrate (Jenkinson et al, 1990; Parton, 1996; Randerson et al, 1996) Such representations may neglect key processes for the formation of soil and persistence of soil organic carbon (SOC) stocks (Lehmann and Kleber, 2015; Schmidt et al, 2011; Rasmussen et al, 2018). Models have begun to explicitly represent microbial processes in global-scale simulations of the formation and turnover of litter and SOC (Sulman et al, 2014; Wieder et al, 2013) as well as to evaluate microbial trait-based signatures on SOC dynamics (Wieder et al, 2015) These advances in the representation of SOC formation and turnover increase capacities to test emerging ideas about soil C persistence and vulnerabilities, but they increase the uncertainties in how to implement and parameterize these theories in models (Bradford et al, 2016; Sulman et al, 2018; Wieder et al, 2018).
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