Contents lists available at ScienceDirect Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil Short communication Forest humus forms as a playground for studying aboveground-belowground relationships: Part 1, theoretical backgrounds Michaël Aubert ⁎ , Fabrice Bureau Normandie Univ, UNIROUEN, IRSTEA, ECODIV, 76000 Rouen, France ARTICLE INFO Keywords: Soil fauna Biodiversity Functional traits Soil functioning Community ecology ABSTRACT Within the context of soil biodiversity erosion and of soil recognition as a non-renewable resource i.e., not recoverable within a human lifespan, we mix theoretical backgrounds from community ecology and functional ecology to address links between aboveground diversity and belowground diversity and their functional con- sequences for soil. We develop a working hypothesis, for future research, stating that the best performance of the soil-plant interface (i.e., high organic matter recycling, low losses of biogenic elements, and plant productivity) is achieved when all communities (plant and soil biota) have reached a similar organizational pattern based on species assemblages, limiting functional traits similarity, and maximizing complementary traits. We conclude that the humus system is the best candidate for testing this hypothesis. 1. Introduction Plant–soil feedback is becoming an important concept for explaining how the global performance of terrestrial ecosystems respond to global changes and has been the subject of numerous studies in the last several years (van der Putten et al., 2013). The study of aboveground-below- ground relationships and their consequences for ecosystems functioning are not new but are an extension of an older, central issue in ecological research that began in the early 1990′s: the search for relationships between biodiversity and ecosystem functioning (Hooper et al., 2005). Mainly fundamental at the beginning, the research efforts have turned toward applied finalities, such as conservation ecology or ecosystem management, thus highlighting the way biodiversity changes (in species composition or in species functional identity rather than only in species richness) could impede the provision of goods and services by ecosys- tems (Lavelle et al., 2006; Schwartz et al., 2000). Subsequent studies focused on potential direct links between vegetation and ecological processes such as primary productivity, organic matter decomposition, or nutrient cycling. The first experiments in manipulating plant species richness did not lead to a consensus about positive, negative, or idio- syncratic links between plant species richness and a given process (Hooper et al., 2005). However, they did highlight two main points: (1) the importance of considering species functional traits (e.g., growth rate, litter quality, symbiotic N fixation) as the facilitators between biological communities and a given ecosystem process, and (2) the importance of considering the community the most proximal to the studied process (plant for primary productivity or soil biota for organic matter recycling are examples). These works led to the development of functional trait databases for plants (e.g., the LEDA database (Kleyer et al., 2008); the TRY database (Kattge et al., 2011; Kleyer et al., 2008)) and for soil fauna (the BETSI database (Pey et al., 2014a)). It also led to the use of multitaxonomic approaches for biodiversity manipulation experiments (Coulis et al., 2015; Hättenschwiler and Gasser, 2005; Hedde et al., 2010). Unfortunately, these multitaxonomic approaches remain relatively scarce to date, while ecological processes associated with organic matter recycling within humus systems require numerous taxa from bacteria to moles or rabbits, including nematodes, mites, collembola, isopods, and earthworms. Moreover, despite the increasing number of studies of soil biodiversity over these last decades, the bio- logical trait framework proposed to improve the mechanistic under- standing of biodiversity-functioning relationships has rarely been tested for organisms other than plants (Gagic et al., 2015). The result is that if plant diversity influences a wide range of ecosystem processes, (1) the underlying mechanisms remain poorly understood and (2) the links between plant diversity and belowground processes remain only frag- mentarily explained (Lange et al., 2014). Thus, the need is growing for an improved understanding of the mechanisms that structure soil biodiversity and for construction of a novel ecological theoretical framework to understand the relationships between soil biodiversity and soil functioning (Bardgett and van der Putten, 2014). Therefore, the aim of this chapter is twofold. We first summarize theories from both community ecology and functional ecology. Based on these theories, we thus propose a new working hy- pothesis linking aboveground diversity – belowground diversity – http://dx.doi.org/10.1016/j.apsoil.2017.09.004 Received 11 December 2016; Received in revised form 20 July 2017; Accepted 3 September 2017 ⁎ Corresponding author at: Laboratoire d’Ecologie, URA IRSTEA/EA 1293 ECODIV, Bât Blondel, UFR Sciences et Techniques, Université de Rouen, F 76821 Mont Saint Aignan, France. E-mail address: michael.aubert@univ-rouen.fr (M. Aubert). Applied Soil Ecology xxx (xxxx) xxx–xxx 0929-1393/ © 2017 Elsevier B.V. All rights reserved. Please cite this article as: AUBERT, M., Applied Soil Ecology (2017), http://dx.doi.org/10.1016/j.apsoil.2017.09.004