Aboveground-belowground relationships in temperate forests: Plant litter composes and microbiota orchestrates

The effects of plant diversity on ecosystem functioning have been mainly studied for fast growing systems such as grassland. These studies have stressed the context dependence of aboveground and belowground relationships (e.g. soil fertility, field vegetation or soil biota). Over the last few years, with the growing need for indicators for sustainable forest management, there has been a considerable increase in studies on AG-BG relationships within forest ecosystems. Nevertheless, given the specific characteristics of forest systems (long-term dynamics, the single tree effect) our mechanistic understanding of these relationships remains poor. We thus assessed the effects of litter composition on litter disappearance rates and nitrogen release with a field litterbag experiment within a pure forest stand and a mixed one, over 1 year. We aimed at (i) characterising the magnitude and the direction of the litter composition effects, (ii) determining which soil biota group mediates the litter manipulation effects and (iii) quantifying the influence of field vegetation on both litter manipulation effects and soil biota activity. Litter mixtures were made with two groups of temperate tree species with contrasting litter quality: Fagus sylvatica and Quercus petraea with poor litter quality and Carpinus betulus, Prunus avium, Acer pseudoplatanus with good litter quality. Three types of litterbag with different mesh sizes (0.175 mm, 2 mm and 10 mm), filled with 10 g of five different leaf litters, were used to assess the contribution of the microbiota (microflora + microfauna), mesofauna and macrofauna to the processes measured. We found that litter disappearance rates and nitrogen release depend more on the functional characteristics of the component litter species (litter quality) than on their number, and on the functional characteristics of the dominant species rather than on their number. These results were mainly due to microbiota activity. Field vegetation only influenced nitrogen release for bags filled with 100% hombeam. In this study we thus examine our results in the light of contradictory outcomes from the literature, and discuss methodological aspects of litter diversity experiments with regard to forest ecosystem specificity. (C) 2009 Elsevier B.V. All rights reserved.