Renée-Claire Le Bayon

Root, mycorrhiza and earthworm interactions: their effects on soil structuring processes, plant and soil nutrient concentration and plant biomass

Earthworms, arbuscular mycorrhiza fungi (AMF) and roots are important components of the belowground part of terrestrial ecosystem. However, their interacting effects on soil properties and plant growth are still poorly understood. A compartmental experimental design was used in a climate chamber in order to investigate, without phosphorus (P) addition, the single and combined effects of earthworms (Allolobophora chlorotica), AMF (Glomus intraradices) and roots (Allium porrum) on soil structure, nutrient concentration and plant growth. In our experimental conditions, plant roots improved soil structure stability (at the level of macroaggregates) whereas earthworms decreased it. AMF had no effect on soil structure stability but increased P transfer from the soil to the plant and significantly increased plant biomass. Earthworms had no direct influence on P uptake or plant biomass, and the N/P ratio measured in the shoots indicated that P was limiting. Interactions between AMF and earthworms were also observed on total C and N content in the soil and on total root biomass. Their effects varied temporally and between the different soil compartments (bulk soil, rhizosphere and drilosphere). After comparison with other similar studies, we suggest that effects of earthworms and AMF on plant production may depend on the limiting factors in the soil, mainly N or P. Our experiment highlights the importance of measuring physical and chemical soil parameters when studying soil organism interactions and their influence on plant performance.

Earthworm surface casts affect soil erosion by runoff water and phosphorus transfer in a temperate maize crop

Summary To test the hypothesis that earthworm surface casts contribute to soil erosion and nutrient transfers in a temperate maize crop, two rainfall experiments were set up. One was focused on the erodibility of earthworm casts, the second examined in how casts affect water runoff and nutrient transfers. Casts produced from anecic and endogeic earthworm species were both analyzed. Visual observations in the field showed no cast transport but only cast disintegration and transfers of particles. Erodibility of newly deposited casts was high and differed significantly between age groups. Cast erosion was significantly positively related to initial mass when young but not when old. The paradox is that despite a high cast abundance (25% of the area) and obvious cast erosion, amounts of sediment and nutrient losses (C, N and P) in the runoff were at least twice as high without, than in the presence of, surface casts. Earthworm activities were shown to act as a physical brake for soil erosion by (i) creating a surface roughness with the deposition of surface casts and (ii) reducing water runoff by associated enhanced water percolation. Once the breaking-down point of the physical resistance of casts was reached, all surface casts were quickly disintegrated and finally completely washed away. The amount of particulate phosphorus recovered in water runoff was 34.7 mg P m − 2 , while 128.5 mg P m − 2 was estimated to have been released from casts. The transfers were found to occur over a short-distance through successive deposition/suspension of soil particles in the water runoff.

Impact of two root systems, earthworms and mycorrhizae on the physical properties of an unstable silt loam Luvisol and plant production

Background and aimsSoil organisms are known to engineer the soil physical properties, but their impact is difficult to assess and poorly documented. Shrinkage analysis has a good potential for such assessment. This study analyses the effects of mycorrhizae (Glomus intraradices), earthworms (Allolobophora chlorotica) and two plants, Allium porrum (leek) and Petunia hybrida (petunia), on the physical properties of an unstable loamy Luvisol, as well as the biological interactions between the soil organisms.MethodsIn addition to soil organism biomass, shrinkage analysis and soil aggregate stability analysis were used to characterize the soil physical properties.ResultsThe soil aggregate stability, specific volume and structural pores volumes were increased with plant roots compared to control. The drilling effect of roots could not explain the pore volume increase, which was several orders of magnitude larger than the volume of the roots. Leek had larger impact on volumes while petunia mostly increased soil aggregate stability. Mycorrhizae increased the soil stability and the soil volume. Earthworms alone decreased the pore volumes at any pore size, and plant roots mitigated this.ConclusionsOur results highlight (1) the large impact of soil biota on soil physical properties, (2) that their separated effects can either combine or mitigate each other and (3) that the observed changes are varying in intensity according to soil type and plant type.

Functional responses of multitaxa communities to disturbance and stress gradients in a restored floodplain

Trait-based approaches can reveal the mechanisms through which disturbances or stress impact communities, allowing comparisons of the role of different mechanisms in shaping communities among taxonomic groups. Such information can lead to higher comparability, transferability and predictability of the outcome of restoration projects. However, multitaxa trait-based approaches were rarely used in the context of ecosystem restoration. We investigated the responses to environmental gradients of seven taxa (vascular plants, staphylinid and carabid beetles, spiders, isopods, diplopods and earthworms) in a restored floodplain using a species traits approach. We assessed the impact of flood disturbances and soil hydric stress on the functional diversity (FD) and community-weighted mean (CWM) response of traits for each taxon. Ordination of hydrological variables revealed two main gradients. The first was related to the spatiotemporal dynamics of flood disturbances and the second to the average changes in soil hydric conditions. The analysis of CWM revealed that larger, poorly mobile species with narrow ecological tolerances were filtered by regular floods and/or changes in soil hydric conditions. Functional diversity patterns differed between the two gradients: decreasing with increasing flood disturbance, but increasing along the soil hydric stress gradient. This suggests that the mechanisms shaping community composition differ between the two gradients with environmental filtering being dominant with increasing flood disturbances and competition decreasing with more soil hydric stress.Synthesis and applications. Our study shows that the impact of restored flood disturbances and soil hydric stress on plant and invertebrate functional diversity and community-weighted mean can be positive, negative or more complex depending on the taxonomic group and environmental gradient considered. The patterns can to some extent be explained by the specific characteristics of each group. Larger, poorly mobile species with narrow ecological tolerances were particularly vulnerable to changes in disturbance and stress regime following floodplain restoration. These species may therefore be lost in the initial phases of restoration projects, but other more characteristic species of dynamic floodplains will be favoured. Understanding the consequences of these contrasted responses for biodiversity conservation and ecosystem functioning constitutes the next challenge for ecosystem restoration. Our study shows that the impact of restored flood disturbances and soil hydric stress on plant and invertebrate functional diversity and community-weighted mean can be positive, negative or more complex depending on the taxonomic group and environmental gradient considered. The patterns can to some extent be explained by the specific characteristics of each group. Larger, poorly mobile species with narrow ecological tolerances were particularly vulnerable to changes in disturbance and stress regime following floodplain restoration. These species may therefore be lost in the initial phases of restoration projects, but other more characteristic species of dynamic floodplains will be favoured. Understanding the consequences of these contrasted responses for biodiversity conservation and ecosystem functioning constitutes the next challenge for ecosystem restoration.

Are urban soils similar to natural soils of river valleys

PurposeUrban soils and soils of river valleys are constituted of heterogeneous materials that have been manipulated, disturbed or transported at different spatial and temporal scales. Despite these similarities, little is known about soil evolution in urban soils and their comparison with natural soils remains therefore highly useful. We hypothesized that, according to their degree of perturbation, urban soils and natural soils of river valleys have similar soil processes related to their structure, physical and chemical characteristics.Materials and methodsUsing a synchronic approach, we compared two soil gradients, one located in the natural reserve of the Allondon River (canton of Geneva, Switzerland) and the other in and around the city of Neuchâtel, Switzerland. A total of five alluvial and 18 urban soil profiles were described according to vegetation type and alluvial terraces formed at different distances from the river for the river valley ecosystem and to soil age for the urban ecosystem. Correlations between soil gradients and classical physical (soil depth, particle-size distribution, coarse fraction) and chemical (Corg, pHH2O, Ptot, Ntot, CaCO3, CEC and C/N ratio) parameters of soils were first tested in order to identify similarities and differences among soil gradients. Data of soil properties were then clustered hierarchically in order to identify soil group classification.Results and discussionOur results showed similarities and differences between soil gradients. In the urban context, soil thickness was positively correlated to soil age, while the coarse fraction, sand content and C/N were negatively correlated to soil age gradient. In soils of the river valley, most of the chemical variables were either negatively (pHH2O and CaCO3) or positively (CEC, Corg and Ntot) correlated to soil distance from the river. These differences between gradients can be mainly explained by parent material, depositional conditions and land use which can influence soil processes. However, alluvial soils were well clustered with two identified urban soil groups according to soil maturity. Evolved alluvial soils far from the river were grouped with natural and near natural urban soils. Conversely, “young” perturbed alluvial soils were most clustered with human-made soils.ConclusionsFrom the two selected soil gradients, soils on alluvial sediments are similar to urban soils in some characteristics. However, parent material, depositional conditions and soil and vegetation interactions on soil processes (e.g. matter cycle, energy flux) still need more investigation. This study contributes to the development of a natural soil reference for urban soils.

White lupin leads to increased maize yield through a soil fertility-independent mechanism: a new candidate for fighting Striga hermonthica infestation?

Nitrogen (N)-deficiency and lack of phosphorus (P) availability are major constraints to maize yields in Western Kenya. In a two-season field study in the lake Victoria basin, we tested the capacity of white lupin (Lupinus albus (L.), cv. Ultra), as a nitrogen-fixing crop with a highly efficient P-acquisition capacity, to increase maize yields when used as a companion or cover crop, or as a source of organic matter. Each experiment was performed on three different fields (Vertisols) differing in N/P availability, previous cropping history and in levels of infestation by the parasitic weed Striga hermonthica (Del.) Benth. Our results show that white lupin led to significantly higher yields of maize when used as a cover crop. When lupin was grown as a companion crop, it also slightly enhanced the yield of the co-cultivated maize. When lupin shoots were incorporated to the soil, the positive effect of lupin on maize growth was field-dependent and only occurred in the field most heavily infested with S. hermonthica. Despite the beneficial impact on maize yield, no clear effect of lupin on soil N and P availability or on maize N/P uptake were observed. In contrast, lupin significantly inhibited infestation of maize by S. hermonthica: when lupin was grown together with maize in pots inoculated with S. hermonthica, the emergence of the weed was strongly reduced compared to the pots with maize only. This work opens a new range of questions for further research on white lupin and its potential beneficial impact as a S. hermonthica-inhibiting crop.

A Gardener's Influence on Urban Soil Quality

Gardens are hot spots for urban biodiversity and provide habitats for many plant and animal spe- cies, both above- and below-ground. Furthermore, gardens provide a wide range of ecosystem services, including carbon (C) storage and nutrient cycling. Although the soil is the foundation of sustainable gardens providing those ecosystem services, very little is known about the conseque- nces of garden management on soil quality. Here we present a comprehensive assessment of urban garden soil quality, including biotic and abiotic site characteristics combined with land-use history and garden management information in a multivariate evaluation. A set of 44 soil quality indicators was measured at 170 sites of 85 gardens in the city of Zurich, Switzerland, comprising contrastingly managed garden habitats along a gradient of urban density. Taken together, our results show that garden management was the driving factor that influenced soil quality and soil functions. Eco-physiological soil quality indices were useful to identify differences in disturbance and intensity of soil use, showing highest microbial (microbial biomass (Cmic)/soil organic carbon (SOC)) and lowest metabolic (qCO2) quotients in perennial grass sites compared to annual vegetable sites. Despite the intensity of soil disturbance in annual vegetable and flower beds, the highest endogeic earthworm biomass and diversity were found in those habitats. Whereas decomposition of green tea bags was higher in grass sites. Soil heavy metal contents varied considerably and could not be linked with garden management practices, but with spatial patterns of industry and traffic. We conclude that understanding soil quality in urban ecosystems needs multi-indicator frameworks to capture the complexity of soil characteristics and the influencing factors in space and time. This study contributes to a better understanding of urban gardens and enhances the development of sustainable soil management strategies aimed at long-term improvement of soil quality and related ecosystem services in cities.

Earthworms affect plant growth and resistance against herbivores: A meta-analysis

Subterranean detritivores such as earthworms can increase soil nutrient availability through their burrowing and casting activities. A number of recent studies have explored whether these changes caused by earthworms may in turn affect plant performance and resistance to herbivores, but no formal synthesis of this literature has been conducted to date. We tested for the effects of earthworms on plant growth, resistance and chemical defences against insect herbivores by performing a meta-analysis of the existing literature up to 2016. We also explored ecological factors that might explain among-studies variation in the magnitude of the earthworm effects on plant growth and resistance. We found that earthworm presence increases plant growth (by 20%) and nitrogen content (by 11%). Overall, earthworms did not affect plant resistance against chewing herbivores (caterpillars, slugs and rootworms), and even led to a 22% decrease in plant resistance against phloem-feeding herbivores (aphids). However, earthworm presence increased production of chemical defences by 31% when plants where attacked by cell-feeders (thrips), and resulted in an 81% increase in resistance against thrips. The magnitude of earthworm effects was stronger when earthworm inoculations consisted of a mix of species and ecological types, and when densities of earthworms were high. These results suggest that earthworm presence is an important factor underlying natural variation in plant defences against herbivores, and call for a better integration of the soil fauna in the studies of plant-herbivore interaction, both for applied and fundamental research. A plain language summary is available for this article.