Above-ground litter decomposition experiments: moving beyond mass loss (Invited)

Experimental studies of litter decomposition have largely focused on quantifying litter mass loss rates and on determining rate controls, namely litter quality and climate. Litter bags have been extensively used in these studies, and we currently have a good understanding of the factors controlling above-ground litter mass loss rates. This detailed conceptual and quantitative knowledge has been used to develop and parameterize controls of litter decay in widely-used soil C models, such as CENTURY and DayCent. However little experimental work has been carried out to quantify the three component fluxes that contribute to above-ground litter mass loss: i) CO2 losses to the atmosphere, ii) losses of dissolved organic matter belowground and iii) losses of litter fragments to the mineral soil. On the other hand, the DayCent model uses these processes to describe surface litter decay, yet lacks experimental data to parameterize them. In addition, we have a very limited knowledge of the factors that may control the relative contribution of those processes to total litter mass loss and how, in turn, above-ground litter-derived organic matter may or not contribute to the stable forms of soil organic matter. We present here quantitative evidence of the limits of the litter bag method to determine litter decay rates and a novel conceptual framework to conduct litter decomposition experiments. Results from past and on-going litter decomposition experiments using isotope labeled litter will be shown that provide conceptual insights and quantitative information on C and N flows from the leaf litter to the atmosphere and to different soil organic matter pools.