Date Published: February 6, 2017
Publisher: Public Library of Science
Author(s): Lamiae El Moujahid, Xavier Le Roux, Serge Michalet, Florian Bellvert, Alexandra Weigelt, Franck Poly, Shiping Wang.
The effect of plant diversity on aboveground organisms and processes was largely studied but there is still a lack of knowledge regarding the link between plant diversity and soil characteristics. Here, we analyzed the effect of plant identity and diversity on the diversity of extractible soil organic compounds (ESOC) using 87 experimental grassland plots with different levels of plant diversity and based on a pool of over 50 plant species. Two pools of low molecular weight organic compounds, LMW1 and LMW2, were characterized by GC-MS and HPLC-DAD, respectively. These pools include specific organic acids, fatty acids and phenolics, with more organic acids in LMW1 and more phenolics in LMW2. Plant effect on the diversity of LMW1 and LMW2 compounds was strong and weak, respectively. LMW1 richness observed for bare soil was lower than that observed for all planted soils; and the richness of these soil compounds increased twofold when dominant plant species richness increased from 1 to 6. Comparing the richness of LMW1 compounds observed for a range of plant mixtures and for plant monocultures of species present in these mixtures, we showed that plant species richness increases the richness of these ESOC mainly through complementarity effects among plant species associated with contrasted spectra of soil compounds. This could explain previously reported effects of plant diversity on the diversity of soil heterotrophic microorganisms.
During the past decades, many studies have analysed the role of biodiversity on ecosystems, demonstrating that a loss in biological diversity can affect ecosystem characteristics and functioning [1–3]. Key mechanisms underlying the effects of plant diversity on ecosystems have been debated. In particular, several authors distinguished causal mechanisms such as facilitation or complementarity involving interactions between several species which potentially results in overyielding, i.e. better performance for assemblages of species than the best one observed for monoculture of each of the species ; and probabilistic mechanisms through a sampling or selection effect, in which the probability of finding a species with high performance is higher in more diverse communities, the presence of such species explaining the high mean level of functioning of diverse mixtures. Williams and McCarthy  gave a general definition of complementarity as “a property of set of objects that exists when at least some of the objects in one set differ from the objects in another set”. In many ecological studies, these ‘objects’ refer to plant species and ‘property’ to the level of a process like primary productivity, complementarity being assessed by comparing the performance of monocultures as compared to mixtures [6–7]. Plant diversity effects and underlying mechanisms have also been studied focusing on other ecosystem functions including nutrient retention and partitioning , soil microbial activities , and emissions of chemical compounds [10–11], these effects having great ecological importance .
In the present work, the relationship between soil organic compound diversity and plant diversity was explored by applying up-to-date chromatographic profiling to characterize the diversity of extractible soil organic compounds for experimental grassland plots with contrasted plant species richness. This places our study at the crossroad of biochemistry, soil science and general ecology, which is the case for few studies [34–35] and even more rarely for studies based on field trials inspired by state-of-the-art ecological concepts.
Our study provides new insights to the analysis of biodiversity effect on ecosystem characteristics. By assessing the effect of plant diversity manipulated in situ on the diversity of extractible soil organic compounds, this study connects plant and general ecology with biochemistry and soil science, accounting for a broad range of compounds and a high number of plant species and plant mixtures rather than focusing on particular compounds for a few plant species. This is needed to further understand the mechanisms underlying the effect of plant diversity on ecosystem processes, particularly belowground characteristics and functions that are less studied .