Abstract
Abstract. Accurate representation of soil organic matter (SOM) dynamics in Earth system models is critical for future climate prediction, yet large uncertainties exist regarding how, and to what extent, the suite of proposed relevant mechanisms should be included. To investigate how various mechanisms interact to influence SOM storage and dynamics, we developed an SOM reaction network integrated in a one-dimensional, multi-phase, and multi-component reactive transport solver. The model includes representations of bacterial and fungal activity, multiple archetypal polymeric and monomeric carbon substrate groups, aqueous chemistry, aqueous advection and diffusion, gaseous diffusion, and adsorption (and protection) and desorption from the soil mineral phase. The model predictions reasonably matched observed depth-resolved SOM and dissolved organic matter (DOM) stocks and fluxes, lignin content, and fungi to aerobic bacteria ratios. We performed a suite of sensitivity analyses under equilibrium and dynamic conditions to examine the role of dynamic sorption, microbial assimilation rates, and carbon inputs. To our knowledge, observations do not exist to fully test such a complicated model structure or to test the hypotheses used to explain observations of substantial storage of very old SOM below the rooting depth. Nevertheless, we demonstrated that a reasonable combination of sorption parameters, microbial biomass and necromass dynamics, and advective transport can match observations without resorting to an arbitrary depth-dependent decline in SOM turnover rates, as is often done. We conclude that, contrary to assertions derived from existing turnover time based model formulations, observed carbon content and Δ14C vertical profiles are consistent with a representation of SOM consisting of carbon compounds with relatively fast reaction rates, vertical aqueous transport, and dynamic protection on mineral surfaces.
Highlights
Soil organic matter (SOM) represents a large stock of carbon that can be exchanged with the atmosphere over short, intermediate, and long time frames
The model predicts that there are significant interactions between soil organic matter (SOM) and dissolved organic matter (DOM) beyond those expected from simple non-reactive transport of DOM from surficial layers (Sanderman et al, 2008), and that these interactions are responsible for the predicted older SOM and DOM 14C values (Fig. 6) and long turnover times inferred from discrete pulse experiments and www.geosci-model-dev.net/7/1335/2014/
The field of soil biogeochemistry faces pressing questions about how the many mechanisms known to be important for SOM cycling interact, which processes are most important for explaining current patterns, and which will be important for predicting future dynamics and transient responses
Summary
Soil organic matter (SOM) represents a large stock of carbon that can be exchanged with the atmosphere over short (seconds to weeks; Trumbore, 2000), intermediate (months to annual; Baldocchi et al, 2001), and long time frames (decades to centuries; Baisden et al, 2002; Ciais et al, 2012; Hsieh, 1993). Current estimates are that more than three times as much carbon is stored in terrestrial soils (2344 Pg C) than is in the atmosphere (Jobbagy and Jackson, 2000), recent estimates for high-latitude systems indicate that value for soil carbon may be an underestimate (Schuur et al, 2009; Tarnocai et al, 2009). Riley et al.: Long residence times of rapidly decomposable soil organic matter
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