How deep can ectomycorrhizas go? A case study on Pisolithus down to 4 meters in a Brazilian eucalypt plantation.

Despite the strong ecological importance of ectomycorrhizal (ECM) fungi, their vertical distribution remains poorly understood. To our knowledge, ECM structures associated with trees have never been reported in depths below 2 meters. In this study, fine roots and ECM root tips were sampled down to 4-m depth during the digging of two independent pits differing by their water availability. A meta-barcoding approach based on Illumina sequencing of internal transcribed spacers (ITS1 and ITS2) was carried out on DNA extracted from root samples (fine roots and ECM root tips separately). ECM fungi dominated the root-associated fungal community, with more than 90% of sequences assigned to the genus Pisolithus. The morphological and barcoding results demonstrated, for the first time, the presence of ECM symbiosis down to 4-m. The molecular diversity of Pisolithus spp. was strongly dependent on depth, with soil pH and soil water content as primary drivers of the Pisolithus spp. structure. Altogether, our results highlight the importance to consider the ECM symbiosis in deep soil layers to improve our understanding of fine roots functioning in tropical soils.

Pearl Millet Genetic Traits Shape Rhizobacterial Diversity and Modulate Rhizosphere Aggregation.

Root exudation contributes to soil carbon allocation and also to microbial C and energy supply, which subsequently impacts soil aggregation around roots. Biologically-driven soil structural formation is an important driver of soil fertility. Plant genetic determinants of exudation and more generally of factors promoting rhizosphere soil aggregation are largely unknown. Here, we characterized rhizosphere aggregation in a panel of 86 pearl millet inbred lines using a ratio of root-adhering soil dry mass per root tissue dry mass (RAS/RT). This ratio showed significant variations between lines, with a roughly 2-fold amplitude between lowest and highest average values. For 9 lines with contrasting aggregation properties, we then compared the bacterial diversity and composition in root-adhering soil. Bacterial α-diversity metrics increased with the "RAS/RT ratio." Regarding taxonomic composition, the Rhizobiales were stimulated in lines showing high aggregation level whereas Bacillales were more abundant in lines with low ratio. 184 strains of cultivable exopolysaccharides-producing bacteria have been isolated from the rhizosphere of some lines, including members from Rhizobiales and Bacillales. However, at this stage, we could not find a correlation between abundance of EPS-producing species in bacterial communities and the ratio RAS/RT. These results illustrated the impact of cereals genetic trait variation on soil physical properties and microbial diversity. This opens the possibility of considering plant breeding to help management of soil carbon content and physical characteristics through carbon rhizodeposition in soil.

Nitrous Oxide (N2O) Emissions by Termites: Does the Feeding Guild Matter?

In the tropics, termites are major players in the mineralization of organic matter leading to the production of greenhouse gases including nitrous oxide (N2O). Termites have a wide trophic diversity and their N-metabolism depends on the feeding guild. This study assessed the extent to which N2O emission levels were determined by termite feeding guild and tested the hypothesis that termite species feeding on a diet rich in N emit higher levels of N2O than those feeding on a diet low in N. An in-vitro incubation approach was used to determine the levels of N2O production in 14 termite species belonging to different feeding guilds, collected from a wide range of biomes. Fungus-growing and soil-feeding termites emit N2O. The N2O production levels varied considerably, ranging from 13.14 to 117.62 ng N2O-N d(-1) (g dry wt.)(-1) for soil-feeding species, with Cubitermes spp. having the highest production levels, and from 39.61 to 65.61 ng N2O-N d(-1) (g dry wt.)(-1) for fungus-growing species. Wood-feeding termites were net N2O consumers rather than N2O producers with a consumption ranging from 16.09 to 45.22 ng N2O-N d(-1) (g dry wt.)(-1). Incubating live termites together with their mound increased the levels of N2O production by between 6 and 13 fold for soil-feeders, with the highest increase in Capritermes capricornis, and between 14 and 34 fold for fungus-growers, with the highest increase in Macrotermes muelleri. Ammonia-oxidizing (amoA-AOB and amoA-AOA) and denitrifying (nirK, nirS, nosZ) gene markers were detected in the guts of all termite species studied. No correlation was found between the abundance of these marker genes and the levels of N2O production from different feeding guilds. Overall, these results support the hypothesis that N2O production rates were higher in termites feeding on substrates with higher N content, such as soil and fungi, compared to those feeding on N-poor wood.

Plant Roots Increase Bacterivorous Nematode Dispersion through Nonuniform Glass-bead Media.

Dispersion of bacterivorous nematodes in soil is a crucial ecological process that permits settlement and exploitation of new bacterial-rich patches. Although plant roots, by modifying soil structure, are likely to influence this process, they have so far been neglected. In this study, using an original three-compartment microcosm experimental design and polyvinyl chloride (PVC) bars to mimic plant roots, we tested the ability of roots to improve the dispersion of bacterivorous nematode populations through two wet, nonuniform granular (glass bead) media imitating contrasting soil textures. We showed that artificial roots increased migration time of bacterivorous nematode populations in the small-bead medium, suggesting that plant roots may play an important role in nematode dispersion in fine-textured soils or when soil compaction is high.

Endogeic earthworms shape bacterial functional communities and affect organic matter mineralization in a tropical soil.

Priming effect (PE) is defined as a stimulation of the mineralization of soil organic matter (SOM) following a supply of fresh organic matter. This process can have important consequences on the fate of SOM and on the management of residues in agricultural soils, especially in tropical regions where soil fertility is essentially based on the management of organic matter. Earthworms are ecosystem engineers known to affect the dynamics of SOM. Endogeic earthworms ingest large amounts of soil and assimilate a part of organic matter it contains. During gut transit, microorganisms are transported to new substrates and their activity is stimulated by (i) the production of readily assimilable organic matter (mucus) and (ii) the possible presence of fresh organic residues in the ingested soil. The objective of our study was to see (i) whether earthworms impact the PE intensity when a fresh residue is added to a tropical soil and (ii) whether this impact is linked to a stimulation/inhibition of bacterial taxa, and which taxa are affected. A tropical soil from Madagascar was incubated in the laboratory, with a (13)C wheat straw residue, in the presence or absence of a peregrine endogeic tropical earthworm, Pontoscolex corethrurus. Emissions of (12)CO(2) and (13)CO(2) were followed during 16 days. The coupling between DNA-SIP (stable isotope probing) and pyrosequencing showed that stimulation of both the mineralization of wheat residues and the PE can be linked to the stimulation of several groups especially belonging to the Bacteroidetes phylum.