Clay Type Rather Than Clay Amount Affects Wood Decomposition in Soil – A Controlled Mesocosms Experiment.

 Large amount of wood material is found either buried in soils or at the soil surface in forest ecosystems and it is unclear whether differences in wood decomposition relate to soil texture and clay mineralogy. In situ detection of clay mineral effects on decomposition is obscured by a number of biotic and abiotic factors that also affect wood decomposition. We conducted a rigorous laboratory experiment in which we manipulated the type (montmorillonite versus kaolinite) and amount (8%, 16%, and 24% clay) of clay in soils that were incubated in mesocosms at three different temperatures (10°C, 20°C, and 30°C) for 420-days. Aspen wood stakes were placed at three locations with respect to soil: buried in the soil, at the soil surface, and on top of a layer of aspen leaves in each of the clay-amended soils mesocosms. Wood decomposition was assessed at the end of the incubation as wood weight loss and we investigated for the effect of clay type, clay amount and incubation temperature on wood decomposition at each location. Results show that clay type has a significant effect on wood decomposition at all three locations, while there is no significant effect of clay amount. Wood stakes buried in montmorillonite-amended soils showed on average a 24.3% (± 2.7) weight loss, while kaolinite-amended soils lost on average 36.3% (± 2.9) of their initial weight during the 420-d incubation. Clay with elevated SOM bonding capacity slowed wood decomposition and resulted in more SOC accumulating in soils at the end of the incubation. Our study indicates that the large amount of C stored in wood material in soil or on the soil surface will show varying decomposition rates as a result not only of climatic effects on decomposition and on decomposers community, but also depending on clay mineral assemblages.