In Dassonville et al. (2008), the Japanese knotweed (Fallopia japonica) appeared as the species with the strongest impacts on soil nutrients (especially cations and P), probably after a nutrient uplift from deeper horizons. On the other hand, surprisingly, N is not concerned by such a nutrient uplift process as total N concentration in the Ah horizon tended to be unchanged (Dassonville et al., 2007). Nitrogen concentrated in the topsoil as soil organic matter presents a large reserve of nitrogen. Higher biomass and litter production should have an impact on N and organic matter dynamic. Therefore, we examine in more details litter decomposition and nitrogen cycling in monospecific stands of Fallopia japonica as well as the impacts of the invasive on soil fauna involved in decomposition processes. Native vegetation was a rough grassland dominated by Calamagrostis epigejos and Eupatorium cannabinum (details on the study site (Ghi) in Dassonville et al., 2007).
Litter decomposition is controlled by three main factors: litter quality, structure of decomposing organism communities and microclimate (Couteaux et al., 1995). The invasion of a plant community by a novel species can potentially modify these three factors. A litterbag experiment with replicates at random was set up in the field to measure the differences of mass loss rate and N dynamics between indigenous (50-50 mixture of Calamagrostis epigejos and Eupatorium cannabinum) and Fallopia litter, and to assess the impact of the microclimate and the role of soil fauna community (invaded versus uninvaded environment) in the litter decomposition process.
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