Isabelle Barois

The study of elevational diversity gradients is a central topic in biodiversity research. In this study, we tested for the effect of climate, resource quality and habitat heterogeneity on earthworm communities along an altitudinal gradient and around the treeline in the French Alps. Earthworm communities and environmental properties (i.e. climate, soil properties and vegetation structure and composition) were sampled in six altitudinal stages from 1400 to 2400 m. Results were analysed through multi-table factorial analyses and structural equation modelling. We found average density, biomass and species richness in the range of what is usually reported in comparable ecosystems. We found no monotonic decrease in species richness along the altitudinal gradient, which we explain by the species pool being dominated by taxa with high environmental tolerance and dispersal capacities. Instead, we highlighted the ecotone associated with the treeline as the primary driving factor of earthworm communities: at 1800-2000m altitude, communities were more abundant and diverse, and had a greater variability in body mass. This result was largely explained by the structure and composition of the vegetation, whereas soil and climate appeared to have only indirect effects. Therefore, the treeline effect on earthworm communities can be explained both by the effect of environmental heterogeneity and of trophic resource quality which increases at the ecotone level.

A good bioindicator of soil health is one that perceives minor changes due to changes in soil management and leads to changes in the processes of the soil ecosystem. The aim was to evaluate the use of soil bioindicators in the determination of the utility of two different Cedrela odorata forestry management systems. The litter and soil samples were collected from the rhizosphere of a monoculture (with no fertilization nor irrigation), and a co-culture (with fertilization and irrigation) stand with Citrus latifolia. The soil and litter bioindicators measured were the activity of acetylesterase (FDA), acid phosphatase (AcPh), alkaline phosphatase (AlkPh), laccase, and the estimation of the potential net rate of carbon mineralization (PNRCM). The results indicated that soil enzyme activities (FDA and phosphatases) in the litter were sensitive to the different management system treatments. Enzymatic activity, per unit dry weight, was higher in litter of co-culture (FDA = 1.05 nkat g-1 AcPh = 1.33 nkat g-1; AlkPh = 03.0 nkat g-1) than in litter of monoculture (FDA = 0.617nkat g-1; AcPh = 0.40 nkat g-1; AlkPh = 0. 983 nkat g-1). Differences in soil total nitrogen content, organic matter (OM) content, and micronutrients were found. In general, the co-culture stand (with fertilization and irrigation) with C. latifolia showed the best results.

Objectives: Altitude integrates changes in environmental conditions that determine shifts in vegetation, including temperature, precipitation, solar radiation and edaphogenetic processes. In turn, vegetation alters soil biophysical properties through litter input, root growth, microbial and macrofaunal interactions. The belowground traits of plant communities modify soil processes in different ways, but it is not known how root traits influence soil biota at the community level. We collected data to investigate how elevation affects belowground community traits and soil microbial and faunal communities. This dataset comprises data from a temperate climate in France and a twin study was performed in a tropical zone in Mexico.Data description: The paper describes soil physical and chemical properties, climatic variables, plant community composition and species abundance, plant community traits, soil microbial functional diversity and macrofaunal abundance and diversity. Data are provided for six elevations (1400–2400 m) ranging from montane forest to alpine prairie. We focused on soil biophysical properties beneath three dominant plant species that structure local vegetation. These data are useful for understanding how shifts in vegetation communities affect belowground processes, such as water infiltration, soil aggregation and carbon storage. Data will also help researchers understand how plant communities adjust to a changing climate/environment.

The positive effect of earthworms on soil processes and plant growth has been extensively documented. The capacity of earthworms to decompose organic matter has been attributed to the microbial communities that inhabit their digestive track or the structures they build, which in turn contribute to make up the drilosphere, a hotspot for microbial activity. However, how earthworms modify the structure of soil microbial communities and how these changes affect soil microbial processes is still unclear. Do earthworms reduce microbial abundance and activity because they feed on microorganisms or do they select and stimulate specific microbial groups? We hypothesise that “the effect of earthworms on nutrient cycling and plant growth is not only a direct effect but is mainly mediated indirectly, via modifications of the microbial community.” The objective of this review is to synthesize the existing literature concerning the influence of earthworms on the structure and function of soil microbial communities, as well as to understand how earthworm-induced changes in the soil microbiota would in turn impact soil processes, particularly those occurring in the rhizosphere and involved in plant growth and health. Recent reports have shown that specific bacterial groups consistently increase in soils where earthworms are present, regardless of the earthworm functional group. The extent of this increase seems to be dependent upon the type of substrate under study. Our synthesis also reveals that endogeic and anecic earthworms regularly induce an increase in soil nutrients, whilst this positive effect is not as evident in the presence of epigeic earthworms. The effect of earthworms on nutrient cycling has been further investigated with microbial functional genes, although existing reports largely focus on nitrogen cycling. Earthworms seem to enhance denitrification, most likely through the increase in organic compounds due to organic matter decomposition. By enhancing soil nutrient availability, earthworms indirectly promote plant growth, which has also been attributed to the induction of signal molecules. However, no experiment to date has been able to prove a direct causal relationship between specific signal molecules, earthworms and plant growth promotion. Finally, we propose a framework for earthworm-microbiota interactions and recommend further research.

Tropical rainforest soils harbor a considerable diversity of soil fauna that contributes to emissions of N₂O. Despite their ecological dominance, there is limited information available about the contribution of epigeal ant mounds to N₂O emissions in these tropical soils. This study aimed to determine whether ant mounds contribute to local soil N emissions in the tropical humid rainforest. N₂O emission was determined in vitro from individual live ants, ant-processed mound soils, and surrounding reference soils for two trophically distinct and abundant ant species: the leaf-cutting Atta mexicana and omnivorous Solenopsis geminata. The abundance of total bacteria, nitrifiers (AOA and AOB), and denitrifiers (nirK, nirS, and nosZ) was estimated in these soils using quantitative PCR, and their respective mineral N contents determined. There was negligible N₂O emission detected from live ant individuals. However, the mound soils of both species emitted significantly greater (3-fold) amount of N₂O than their respective surrounding reference soils. This emission increased significantly up to 6-fold in the presence of acetylene, indicating that, in addition to N₂O, dinitrogen (N₂) is also produced from these mound soils at an equivalent rate (N₂O/N₂ = 0.57). Functional gene abundance (nitrifiers and denitrifiers) and mineral N pools (ammonium and nitrate) were significantly greater in mound soils than in their respective reference soils. Furthermore, in the light of the measured parameters and their correlation trends, nitrification and denitrification appeared to represent the major N₂O-producing microbial processes in ant mound soils. The ant mounds were estimated to contribute from 0.1 to 3.7% of the total N₂O emissions of tropical rainforest soils.