The relative importance of decomposition and transport mechanisms in accounting for soil organic carbon profiles

[摘要] Soil is the major terrestrial reservoir of carbon and a substantial part ofthis carbon is stored in deep layers, typically deeper than 50 cm below thesurface. Several studies underlined the quantitative importance of this deepsoil organic carbon (SOC) pool and models are needed to better understandthis stock and its evolution under climate and land-uses changes. In thisstudy, we tested and compared three simple theoretical models of verticaltransport for SOC against SOC profiles measurements from a long-term barefallow experiment carried out by the Central-Chernozem State NaturalBiosphere Reserve in the Kursk Region of Russia. The transport schemestested are diffusion, advection and both diffusion and advection. They arecoupled to three different formulations of soil carbon decompositionkinetics. The first formulation is a first order kinetics widely used inglobal SOC decomposition models; the second one, so-called "priming"model, links SOC decomposition rate to the amount of fresh organic matter,representing the substrate interactions. The last one is also a first orderkinetics, but SOC is split into two pools. Field data are from a set ofthree bare fallow plots where soil received no input during the past 20, 26and 58 yr, respectively. Parameters of the models were optimised using aBayesian method. The best results are obtained when SOC decomposition isassumed to be controlled by fresh organic matter (i.e., the priming model).In comparison to the first-order kinetic model, the priming model reducesthe overestimation in the deep layers. We also observed that the transportscheme that improved the fit with the data depended on the soil carbonmineralisation formulation chosen. When soil carbon decomposition wasmodelled to depend on the fresh organic matter amount, the transportmechanism which improved best the fit to the SOC profile data was the modelrepresenting both advection and diffusion. Interestingly, the older the barefallow is, the lesser the need for diffusion is, suggesting that stabilisedcarbon may not be transported within the profile by the same mechanisms thanmore labile carbon.

[学科分类] 地球化学与岩石