What Does Data Tell Us about Decomposition Model of Soil Organic Matter?

Abstract:

Traditionally, soil carbon decomposition has been modeled by first-order, donor pool-dominated carbon transfers (i.e., linear model) as modified primarily by temperature and soil moisture. How the decomposition of soil organic matter (SOM) is regulated by various other biotic and abiotic processes, such as microbial community and activities, OM-mineral interactions, priming by input C, and SOM chemistry, has not been rigorously evaluated. As a consequence, those processes have not been well represented in Earth system models.

In this presentation, we use three studies to illustrate how the data assimilation approach is used to examine influences of microbial functional genomics, soil properties, and priming on SOM decomposition. First, we optimally fitted a traditional three-pool (active, slow, and passive) C-cycling model with hundreds of data sets from soil incubation studies. Our results showed that decomposition rates of the active and the slow C pools significantly decrease with clay content, field water holding capacity, and C:N ratio. Multifactor regression and path analyses showed that clay content was the most important variable in regulating decomposition of SOC. Second, we used information from functional genomics study to constrain a soil C dynamic model. With an assumption that a specific community of microbes is associated with each substrate pool (i.e., quality), we linked the relative change in these microbes to the relative change in the substrate pool. Quantification of functional genes, combined with traditional CO2 flux and SOC measurements, substantially improves the transfer coefficient estimation in traditional decomposition models. Third, we estimated priming effects in a two-pool decomposition model from isotope data in soil incubation experiments. Our results showed that around half of the newly added labile carbon was stabilized into recalcitrant carbon in all the forest, grassland and cropland soils. As a result, despite that more old recalcitrant carbon was respired via priming, labile carbon input increased recalcitrant carbon. Thus, stabilization of new labile carbon input may overcompensate the primed carbon loss, leading to recalcitrant carbon increase in terrestrial soils.