Justine Lejoly

Non-native earthworms have been invading North America since European settlement. Compared to temperate forests, their presence in the boreal forest is much more recent and thus remains understudied, despite the potential threat they represent for soil carbon (C) stocks. Here we compared earthworm-invaded and earthworm- free zones in soil types representative of the boreal forest, including Luvisols, Podzols, and Brunisols (Cambisols). We observed that the forest floor (surface organic layer, or LFH) decreased in thickness after invasion in most cases and developed into a Vermimull, with the loss of the most humified layer (humic or H horizon). Simul- taneously, the surface mineral horizon was reworked by earthworms into a novel Ahu horizon, characterized by higher organic matter and enriched in earthworm casts. Forest floor C stocks decreased by 94% and 59% for Luvisols and Brunisols respectively, while those of Podzols remained apparently unaffected. Mineral soil C stocks in Brunisols increased after invasion, while no changes were observed in Luvisols and Podzols. Our results demonstrated the substantial impact that invading earthworms are having on soil morphological features and C stocks in boreal forests. Effects were similar to what has been reported for temperate forests, although the degree of impact depended on soil type. While C stocks were less affected in the mineral soil compared to the forest floor, the development of a novel surface horizon reworked by earthworms could alter microbial dynamics and impact mineral C persistence. Further research is needed to quantify long-term implications of earthworm presence for boreal soil C stocks.

Earthworm invasion in North American forests has the potential to greatly impact soil microbial communities by altering soil physicochemical properties, including structure, pH, nutrient availability, and soil organic matter (SOM) dynamics. While most research on the topic has been carried out in northern temperate forests, little is known about the impact of invasive earthworms on soil microbial communities in hemiboreal and boreal forests, characterized by a slower decay of organic matter (OM). Earthworm activities can increase OM mineralization, altering nutrient cycling and biological activity in a biome where low carbon (C) and nitrogen (N) availability typically limits microbial and plant growth. Here, we characterized and compared microbial communities of earthworm-invaded and non-invaded soils in previously described sites across three major soil types found in the Canadian (hemi)boreal forest using a space-for-time approach. Microbial communities of forest floors and surface mineral soils were characterized using phospholipid fatty acid (PLFA) analysis and metabarcoding of the 16S rRNA gene for bacteria and archaea and of the internal-transcriber-spacer-2 (ITS2) region for fungi. In forest floors, the effects of earthworm invasion were minor. In mineral soil horizons, earthworm invasion was associated with higher fungal biomass and greater relative abundance of ectomycorrhizal fungi. Oligotrophic bacteria (Acidobacteriota and Chloroflexi) were less abundant in invaded mineral soils, where Gram(+) : Gram(−) ratios were also lower, while the opposite was observed for the copiotrophic Bacteroidota. Additionally, earthworm-invaded mineral soils harboured higher fungal and bacterial species diversity and richness. Considering the important role of soil microbial communities for ecosystem functioning, such earthworm-induced shifts in their community composition are likely to impact nutrient cycling, as well as vegetation development and forest productivity at a large scale, as the invasion progresses in these (hemi)boreal systems.

Soil organic matter (SOM) dynamics and termite activity have now been widely accepted as key players for improving soil properties in tropical agro-ecosystems. Numerous studies have described environmental impacts of aboveground termite mounds, while few data are available on temporary structures built for food foraging, called termite sheetings. The effects of termite activity on soil properties resulting from organic matter (OM) amendment under two contrasting management practices were studied in similar pedological and climatic conditions in Southern India (Auroville). Our results showed an increase in bio-available nutrients (K, Mg and P), organic carbon (OC) content, cationic exchange capacity (CEC), exchangeable base cations and water pH in the termite sheetings compared to the underlying and reference soils, in the organic tilled field. On the other hand, only bio-available K increased in the permanent raised beds. Aggregation processes were improved in termite sheetings for the organic tilled field, as the amounts of macroaggregates (250 μm– 2 mm) and protected mi- croaggregates increased, whereas the amount of free microaggregates (50–250 μm) decreased. Moreover, termite activity favoured SOM storage in termite sheetings by increasing OC content in each aggregate fraction, while no differences were observed in the permanent raised beds. Our study demonstrates that termite activity can im- prove nutrient availability, carbon storage and pH conditions in agro-ecosystems but that the magnitude of the effect likely depends on the agronomic practices in use.

Two types of organic-matter-rich coversoils are used during reclamation in the oil sands region of Alberta: forest floor material (FFM) salvaged from upland forests, and peat material (PM) salvaged from boreal wetlands. In this study, we tested the hypothesis that carbon (C) and nutrient availability may limit microbial activity in these reclamation materials by measuring their response to either 13C-labeled glucose or NPKS addition. Coversoil materials were compared with two natural forest soils corresponding to target sites for reclamation. A shift in microbial community structure (determined using phospholipid fatty acid analysis) was detected after both additions, but it was stronger with glucose than NPKS, especially for the two reclamation materials. For all soils, the increase in microbial respiration was stronger after glucose than after NPKS addition. The majority of CO2 originated from soil organic matter (SOM) for the natural forest soils but from glucose for the reclamation materials. In PM, glucose addition triggered SOM mineralization, as shown by a positive priming effect. Despite the absence of a priming effect for FFM, microbial communities incorporated higher rates of glucose into their biomass and respired double the amount of glucose compared with the other materials. Furthermore, the overall microbial community structure in the FFM became more similar to that of the natural forest soil materials following glucose addition. These findings indicate that C and NPKS limitations were stronger for the two reclamation materials than for the two natural forest soils. Furthermore, microbial communities in the two reclamation materials responded more readily to labile C than to NPKS addition.

With respect to the pedosphere, human activities in the last 100 years have been the major driver of soil change. Despite human activities being one of the main soil forming factors recognized by soil scientists (in addition to climate, organisms, parent material, relief, groundwater, and time), the Canadian System of Soil Classification (CSSC) emphasizes soil as a natural body. We argue human agricultural activities are direct and indirect drivers of significant changes to the carbon balance and cycling in A horizons of Gray Luvisolic soils in western Canada, resulting in changes to A horizon carbon stocks, structure, and micromorphology. Evidence from scientific literature, in-field soil profile observations, and the National Pedon Database are presented in support of our argument. We propose a polygenetic, two-stage model of Gray Luvisol soil formation. The first stage is dominated by the climate forcing of the Holocene, resulting in a relatively stable boreal forest ecosystem including perturbations from natural and human-induced wildfire and other disturbances. The second stage is dominated by direct, human-driven disturbances such as cultivation, release of exotic fauna (earthworms), and indirect human-driven disturbances associated with anthropogenic climate change. Further, we propose modest amendments to the CSSC to reflect a polygenetic model of soil genesis in Gray Luvisolic soils that preserve the balance between observation and interpretation inherent in the system.