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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.

This paper is part of a Special Issue entitled “University of Alberta Contributions to Soil Science: Past, Present, and Future”. Soil quality (SQ) indicators such as plant available water (PAW), soil organic carbon (SOC), and microbial biomass carbon (MBC) can reveal agroecological functions; however, their spatial variabilities across contrasting land uses need to be better understood. This study examined the spatial variation of these key SQ indicators as a function of two land-use systems and using topography covariates. We sampled a total of 116 point locations in a native grassland (NG) site and an irrigated cultivated (IC) site located near Brooks, Alberta. Compared with NG, cultivation altered soil pore-size distribution by sharply reducing macroporosity by 25%. However, conditions in the IC soil supported greater accrual of microbial growth (MBC of 601 vs. 812 nmol phospholipid fatty acids g−1 soil) probably due to more availability of water and nutrients. Focusing on the effects of topography on SQ indicators, terrain elevation (by light detection and ranging) and estimated depth-to-water were found to be key controllers of SQ at the two land-use systems. Also, there were gradual increases in both SOC and MBC where estimated water table was deeper, and higher SOC also associated with lower elevation. A comparison of ordinary kriging and cokriging (coK) geostatistical mapping indicated that the coK method performed better as demonstrated by improvements in the accuracies of spatial estimations of PAW, SOC concentration, and MBC. Thus, implementing coK using the aforementioned topography covariates enhances the capability for predictive mapping of SQ, which is particularly useful when spatial data for key SQ indicators are sparse and challenging to measure.

Stable isotope probing of phospholipid fatty acids (PLFA-SIP) is useful when studying bacterial contributions to soil processes, and it is an effective way to separate fungal and bacterial activity by linking 13C enrichment to specific PLFAs. Distinguishing bacterial contributions to soil processes often employs selective inhibitors; however, studies demonstrating their efficacy when using PLFA-SIP are less common. Here, we determined the effect of the fungal inhibitor cycloheximide (4.8 mg g−1 dry soil) and the bacterial inhibitor bronopol (0.48 mg g−1 dry soil) on microbial communities white spruce [Picea glauca (Moench) Voss] forest floor by measuring the uptake of 13C-enriched glucose (2 mg g−1 dry soil) in microbial PLFAs. We targeted [13C]glucose uptake by the bacterial community conditioned to a stable soil environment of 23 °C for over 2 wk rather than new bacteria generated from active colony growth caused by glucose addition. Nearly all bacterial PLFAs exhibited pronounced inhibition of 13C enrichment in the presence of bronopol. Limited inhibition of 13C enrichment in the presence of cycloheximide was observed as bacterial PLFA affected by cycloheximide had roughly one third less 13C enrichment than samples emended with [13C]glucose alone. Inhibitory effects only reduced 13C enrichment and did not affect total PLFA concentrations, implying that the inhibitors in the concentrations applied were impeding bacterial activity without causing cell death. Based on this work, we conclude that bronopol is an effective inhibitor for bacteria. Additionally, non-targeted effects of cycloheximide on soil bacteria must be accounted for when it is used in soil incubations.

A 12 wk laboratory incubation examined the effects of application of various nitrogen (N) and sulfur (S) fertilizers on soil plant-available nutrient levels and nitrous oxide (N2O) gas emissions with respect to soil fertilization history using soils sampled from the University of Alberta Breton Classical Plots. Fertilization history and added fertilizer treatments showed significant effects on N2O emissions and NO3−-N and SO4−-S recovered on ion-exchange resins over the 12 wk. Mean cumulative N2O emissions ranged from 0.43 to 1.18 kg N2O-N ha−1. The relationship between observed total resin-recovered NO3−-N and N2O emissions was not consistent for soils receiving long-term applications of various combinations of N, phosphorus, potassium, and S fertilizers. The N2O emission from two soils with a history of long-term N fertilizer applications but different S fertilization histories was significantly different even though resin-recovered NO3−-N levels were similar. When grouped according to added fertilizer treatments, mean cumulative N2O emissions showed a strong linear relationship with mean resin-adsorbed NO3−-N production. We hypothesize that the differences in the relationship between NO3−-N production and N2O-N emissions for soils with different long-term fertilization histories may be a result of the interaction of N and S oxidation processes. Further, soil fertilization history may significantly influence soil N2O emissions in response to N fertilizers added within the growing season of observation but isn’t often considered in short-term experiments, and this may be a significant source of uncertainty in the estimation of greenhouse gases inventories from agricultural soils.

Soil stockpiles are essential to the reclamation of large‐ and small‐scale mining and other industrial sites. However, stockpiling soils can lead to the degradation of seed banks. This study examines the diversity, composition, depth of seed storage, and relationships between the aboveground and seed bank plant communities in stockpiles and compares them to the nearby boreal forest. The seed bank and aboveground vegetation sampled at eight stockpiles and six mature forest sites were near Fort McMurray (57.337°N, 111.755°W) and Cold Lake (54.695°N, 110.730°W), Alberta, Canada. Seed bank samples were taken from the forest floor (LFH) and depths of 0–5, 5–10, 10–20, 20–30, >50 cm. Aboveground vegetation cover was also estimated at these locations. The seed bank composition was determined using the seedling emergence method in a greenhouse. Stockpile seed banks had higher seedling abundance and species richness than nearby forested sites but were dominated by grasses and non‐native forbs. Most seeds germinated from the surface layer, with 92% of seeds germinating from the LFH layers in the forested sites, and 68% from the 0 to 5 cm layer in the stockpiles. Mature forest sites had more similar aboveground and seed bank communities than the stockpiles. Overall, integrating information on seed bank and aboveground plant communities would improve reclamation decisions, rather than relying on aboveground vegetation alone.

Early forest re-establishment in landforms constructed from materials such as overburden or mine waste is partly determined by nitrogen (N) availability in reclamation covers. Here we examined whether the ecosystem model ecosys which simulates key processes governing N availability such as mineralization, plant N uptake and N return to soil through litterfall could be used to forecast potential N limitations for forest re-establishment in these constructed landforms. In this study, N cycling was simulated and tested against measured soil, foliar and surface litter N concentrations with three soil covers differing in thickness (35, 50, and 100 cm) in a 17-year-old forest reclamation site and in an analogue natural forested site in northern Alberta. Overall, results from this study demonstrated the applicability of the ecosys model in predicting nutrient cycling in reclaimed upland forestlands. Results of this study highlight the importance of optimum cover depth to ensure sufficient N is available for plant growth. Even though the modelled net N mineralization, N uptake and thereby plant productivity increased with cover depth, the foliar and surface litter N concentrations did not. A non-linear relationship between total soil nitrogen (TN) stocks and modelled net N mineralization indicated that cover depth, which determines TN stock, had little effect on net primary productivity beyond a threshold TN. This threshold was 17 Mg N ha−1, similar to TN for the 100 cm cover, giving a net N mineralization rate of ~ 3.5 g N m−2 year−1, and this was attributed to reduced microbial activity in deeper soil layers.

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.

Boreal forest soils are highly susceptible to global warming, and in the next few decades, are expected to face large increases in temperature and transformative vegetation shifts. The entire boreal biome will migrate northward, and within the main boreal forest of Western Canada, deciduous trees will replace conifers. The main objective of our research was to assess how these vegetation shifts will affect functioning of soil microbial communities and ultimately the overall persistence of boreal soil carbon. In this study, aspen and spruce forest floors from the boreal mixedwood forest of Alberta were incubated in the laboratory for 67 days without (control) and with the addition of three distinct 13C labeled substrates (glucose, aspen leaves, and aspen roots). Our first objective was to compare aspen and spruce substrate utilization efficiency (SUE) in the case of a labile C source (13C-glucose). For our second objective, addition of aspen litter to spruce forest floor mimicked future vegetation shifts, and we tested how this would alter substrate use efficiency in the spruce forest floor compared to the aspen. Tracking of carbon utilization by microbial communities was accomplished using 13C-PLFA analysis, and 13C-CO2 measurements allowed quantification of the relative contribution of each added substrate to microbial respiration. Following glucose addition, the aspen community showed a greater 13C-PLFA enrichment than the spruce throughout the 67-day incubation. The spruce community respired a greater amount of 13C glucose, and it also had a much lower glucose utilization efficiency compared to the aspen. Following addition of aspen litter, in particular aspen leaves, the aspen community originally showed greater total 13C-PLFA enrichment, although gram positive phospholipid fatty acids (PLFAs) were significantly more enriched in the spruce community. While the spruce community respired a greater amount of the added 13C-leaves, both forest floor types showed comparable substrate utilization efficiencies by Day 67. These results indicate that a shift from spruce to aspen may lead to a greater loss of the aspen litter through microbial respiration, but that incorporation into microbial biomass and eventually into the more persistent soil carbon pool may not be affected.

Success in establishing upland forests on landforms constructed from overburden is determined by the characteristics of the reclamation soil covers and depth. We explored whether an ecosystem model that uses water potential gradients to simulate soil-plant-atmosphere water transfers can be used to forecast effects of water availability on plant water relations and net primary productivity (NPP) with different cover depths in these constructed landforms. Plant water relations and growth were simulated with ecosys and tested against measured soil moisture content, rooting depth, transpiration, leaf area and biomass production in three soil cover depths over 5 years. Shallow reclamation soil cover depth caused greater water potential gradients and less soil water content, tree water uptake and growth to be modelled, consistent with measured data. Modelled transpiration increased nonlinearly with increasing cover depth, indicating a threshold depth above which additional gains in transpiration and hence NPP would be limited. This study highlights the importance of sufficient cover depth on forest community end-land use re-establishment. It also demonstrated that a terrestrial ecosystem model such as ecosys can be a useful tool in forecasting land capability for reclamation soil covers of different depths and properties under diverse climates.

To address concerns among members of the Canadian Society of Soil Science (CSSS) regarding the discipline’s capacity to train new soil professionals, specifically in pedology and field skills, members of the CSSS’s soil education and pedology committees have proposed to develop a pedology field school. To aid in the selection of learning outcomes that are relevant to professional practice, an online survey was sent to Canadian soil professionals within private industry and governmental organizations. Professional feedback was also requested regarding the creation of a web-based national soil education resource and the certification of soil pedological skills. According to the survey results, the quality of new graduates’ pedology and field skills was perceived as poor. Certain soil field skills and knowledge were thought to be either completely absent from the current Canadian curriculum (e.g., spatial variability of soil processes), or not well mastered by graduates (e.g., interpreting soil survey reports). Important learning outcomes were identified, such as interpreting soil survey information, soil mapping, and soil-landscape classification with soil description–classification and soil genesis content needed as a refresher. Taking into consideration existing regional field schools, we recommend that the CSSS co-create, where needed, and coordinate, where they already exist, regional pedology field schools throughout Canada. We also propose that the CSSS develop a national pedology certification and a web-based soil education resource. Also, further study is necessary to shed light on the contribution of non-disciplinary graduates to the professional practice and the impact this has on the perception of soil education in Canada.

Under food security concerns and accelerated global warming, northern regions are becoming new agricultural frontiers. While diverse regional, national, and local policies support northern agricultural intensification and expansion through land use conversion, the scope and environmental consequences of northern agriculture are yet to be fully understood. As northern agriculture is poised to increase its role in both the local and global food production, its sustainable growth depends on a comprehensive understanding of opportunities and challenges. To evaluate the current perception of the status of northern agricultural research and the extent to which there is a need for a coordinated approach to its growth, we developed a targeted survey delivered online. Questions are aimed at revealing similarities and discrepancies in the awareness of northern agriculture and obtaining feedback on the need for a global synchronization of research and development. Out of 309 respondents, a curated dataset of 238 respondents was employed for analysis. This included respondents with knowledge and expertise in boreal and/or Arctic agriculture (41%), of temperate and/or alpine agriculture (37%), and other respondents (22%). Most are involved in agricultural (60%) or applied environmental (23.5%) sciences. Results revealed that scientists working in the northern regions are cautiously optimistic about climate change–driven expansion of northern agriculture while also realistic about the need for environmentally sustainable agricultural systems. Respondents with limited exposure to northern agriculture were more pessimistic about its expansion. Surprisingly, there was a limited and inconsistent knowledge of extant networks and research entities focusing on northern agriculture research. Nevertheless, there was a strong consensus that a concerted approach among multiple disciplines and across global regions would benefit northern agriculture and its ongoing growth. The survey’s results illuminate for the very first time the need for better coordination and a greater emphasis on northern agriculture under climate change.

The application of soil proximal sensors on reclaimed sites presents a novel method for assessing the quality of reclaimed landscapes. This method improves assessment reliability, information management, and environmental assurance. One proximal sensing system that could be used to provide high spatial resolution measurements of soil parameters is an on-the-go optical sensor that collects data at two wavelengths: 660 and 940 nm. Proximal soil sensing data were collected at 27 sites, where organic matter, cation exchange capacity (CEC), and soil water content were collected from 221 soil samples from 0 to 15 cm. The proximal soil sensor data were then automatically clustered using a combination of self-organizing maps and random uniform forests. Overall, the proximal sensor data combined with this data analysis approach created maps with either three or four soil zones. On average, soil zones had statistically significant differences in organic matter, CEC, and water content. This system could be used to map out zones with significant soil variation as part of reclamation monitoring and then used to guide laboratory analytical sampling. Future work should focus on development of on-the-go reflectance spectroscopy systems to provide quantitative soil data with high spatial resolution.

Bitumen extraction via surface mining has affected nearly 1000 km2 of boreal forest habitat in the Athabasca oil sands region of Alberta, Canada, pushing entire ecosystems back to the primary stages of forest and soil succession. Previous work in the Athabasca oil sands region has investigated the influence of vegetation, mineral reclamation material, and peat cover soil on microbial communities and the quality of soil carbon. No research has been conducted to assess the bioindicator potential of soil fauna, specifically oribatid mites, in these reclaimed soils. This study investigated the influence of time-since-reclamation (8–31 yrs.) and canopy type (white spruce (Picea glauca (Moench) Voss), trembling aspen (Populus tremuloides, Michx)) on the abundance and diversity of oribatid mite adults ≥300 μm in size after land reclamation in the Athabasca Oil Sands Region. Thickness of the forest floor accumulating on top of the peat cover soil at the reclaimed sites was the most powerful predictor of oribatid mite richness, and was a better predictor than time-since-reclamation. A forest floor with a thickness ≥2 cm strongly increased oribatid mite abundance to levels higher than those found in the forest floors of natural, undisturbed stands in the area. Species richness in the (≥2 cm thick) forest floor within reclaimed stands was slightly lower than within natural stands but was notably higher than in the peat cover soil. Assemblage diversity followed the same trend as species richness, and mite diversity in the forest floor with a thickness ≥2 cm in reclaimed stands was the most similar to natural stands. Four of the six reclaimed stands with a forest floor thickness ≥2 cm were aspen stands, and increased aspen density may aid in faster reestablishment of oribatid mite communities after oil sands mining.

Brunisolic soils developed on sandy deposits comprise a significant portion of the land disturbed by surface mining in the Athabasca Oil Sands Region. These soils support forest communities ranging from nutrient limited jack pine to more productive aspen stands. The objective of this study was to determine if and how the physical properties of these sandy soils influence the accumulation and distribution of soil nutrients. Sixteen sites, selected to capture the natural range in forest productivity of the area, were characterized to assess particle size distribution, total and available nutrient stocks in forest floors, total C and N, and extractable nutrients in B horizons. Under jack pine, relatively small decreases in sand content of the upper soil profiles, from 97% to 88%, were associated with greater total nutrient stocks and lower C/N ratios in the forest floors. In soils under aspen, forest floor nutrient stocks related most strongly with texture of the B horizons, with finer B horizons (from 94% t...

Landscape-level disturbance is a reality in many parts of the world including the Athabasca oil sands region, Canada, and soils play an essential part in the overall reclamation process. Soils are ...

Soil is a critical component of global biogeochemical cycles, and there is an increasing need for cost effective tools to measure soil carbon stocks and determine soil nitrogen contents. Reflectance spectroscopy can deliver large volumes of soil carbon data. However, as soil carbon concentrations can be spatially heterogeneous, imaging spectroscopy presents the best potential to provide high resolution measurements and accurately characterize soil carbon heterogeneity. For this study, discrete, intact and unground soil samples were collected and analyzed using a SisuROCK automated hyperspectral imaging system in a laboratory setting, focused on the shortwave infrared portion of the electromagnetic spectrum. Samples were also analyzed for soil organic carbon and total nitrogen concentrations by dry combustion to prepare a training data set. Predictive models were built using continuous wavelet processing along with partial least squares regression and CUBIST models. Spatial variation of carbon and nitrogen was determined using Moran's i and comparisons of spatial variations among soil types and horizons were made using a spatial generalized least squares model. Overall, soil organic carbon was more aggregated in Chernozemic soils and in B and C horizons compared to A horizons. Nitrogen in turn showed more aggregation for all soil types and horizons compared to soil organic carbon. Results indicated that imaging spectroscopy can be successfully used to measure and characterize the spatial variability of soil carbon and nitrogen at the soil aggregate scale.

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). Simultaneously, 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.

Managing the land properly can help conserve the soil which is critical for sustaining our life and the global society. However, measuring the quality of the soil explicitly is still a challenge. Therefore, this study was conducted to identify suitable soil quality indicators among contrasting land managements (i.e., simple vs. complex crop rotations; manure vs. balanced fertilization) at two long-term experimental fields. The fractal structure of soils was documented by the mass-diameter relationship of soil aggregates using 3D laser scanning. Hydraulic conductivity (K), pore size fractions, and soil physical S parameter (S-index) were determined from moisture retention curves using a HYPROP system. Soil microbial community structure was characterized using phospholipid fatty acid analysis. Our results demonstrated an improved hierarchical fractal aggregation in soils under perennial legumes and grasses (Dm = 0.97) compared to nonfractal aggregation under fallow phases (Dm = 0.99). In addition, across nutrient managements, only balanced fertilization exhibited significantly enhanced fractal aggregation. Moreover, complex crop rotations and balanced fertilization also improved S-index, saturated water content (sat. WC), and plant available water (PAW) compared to their counterparts (Ps

In forests undergoing reclamation, monitoring spatial patterns and relationships may be one way to decide if reclamation goals have been met. As a first attempt, linkages between soil nutrients and the canopy were assessed based on spatial patterns of trees, forest floor biomass, and seasonal nutrient availability in reclaimed forest sites in northern Alberta, Canada using a spatially explicit protocol (0.5 minimum resolution). Three different stand types, including aspen ( Populus tremuloides Michx.), jack pine ( Pinus banksiana Lamb.), and white spruce ( Picea glauca (Moench) Voss) were assessed. Nutrient supply rate patterns were spatially dependent at scales of 3–20 m and varied among seasons suggesting that controls shift seasonally between aboveground and belowground processes. Increased canopy cover and forest floor mass led to increased soil nutrient supply rates for all macronutrients in the aspen stand. Only forest floor mass was associated with increased sulfur and calcium in the pine stand, and canopy cover with increased potassium in the spruce stand. Linkages between aboveground and belowground processes appear to be reestablishing, however further investigation is needed to determine missing parameters controlling nutrient supply, to explicitly compare with natural background sites and account variation in reclamation material.

Glacial retreat areas offer an excellent opportunity for microbial ecologists to study ecological theories. The place of this study was the glacier retreat area of Iztaccihuatl volcano in Mexico (19.02°N, 98.62°W). Sampling took place at eleven different sites along the retreat area in 2008 and 2009. Soil samples (200 g each) were frozen, and air-shipped to the laboratory and analysed for organic C and N, ammonia, nitrate and DNA content as a proxy for microbial biomass. The composition of the soil microbial community was analysed by ester-linked phospholipid fatty acid analysis and by PCR-DGGE. From the youngest sites to the oldest ones the pH increased from 2.5 to 5.1, electrical conductivity decreased. We found an increase of C and N along the chronosequence, and also an increase in microbial biomass. The C-to-N-ratio increased from around 4 at 5000 m a.s.l. to 14 at 3800 m a.s.l., indicating soil development. According to PLFA analysis the microbial community showed a clear shift from bacteria to fungi during the initial phase, while in the later phase the trend was reverse. From PCR-DGGE profiles it may be inferred that the diversity of the microbial community increased with time. Results confirm earlier studies on microbial succession in the Alps, North American Rocky Mountains and the Andes. The initial increase of fungi, and their subsequent decrease might be explained by the resource heterogeneity hypothesis. The results also indicate that in the future much larger sample sets should be used to make best use of the retreat continuum at this type of subtropical glaciers, and to perform a metagenomic analysis to deepen the understanding of microbial succcession at a genus or species level.

Assessing the success of soil reclamation programs can be costly and time-consuming due to the cost of traditional soil analytical techniques. One cost-effective tool that has been successfully use...

Better management of synthetic nitrogen (N) fertilizers in conventional agricultural systems laid the foundation for feeding the increasing world's population since the Green Revolution. However, excessive reliance on inorganic fertilizer has resulted in environmental degradation issues. Difficulties in soil nutrition management in organic cropping systems often results in lower and variable yields, also raising questions of sustainability. Improving nitrogen use efficiency (NUE) is thus of key importance to overcome environmental concerns in conventional systems and production limitations in organic systems. The differences in the two farming systems have impacts on crop traits and N cycles, making it difficult to enhance NUE with a single strategy. Different approaches need to be adopted to improve NUE in each system. Extensive efforts have been made to better understand mechanisms to potentially improve NUE in cereal crops under both systems. This review suggests that NUE may be improved through a combination of management practices and breeding strategies specific to the management system. Diversified crop rotations with legumes are effective practices to optimize the N cycle in both conventional and organic systems. Best Management Practices coupled with nitrification inhibitors, controlled release products and split-application practices can reduce N loss in conventional systems. In organic systems, we need to take advantage of available N sources and adapt practices such as no-tillage, cover crops, and catch crops. Utilization of beneficial soil microorganisms is fundamental to optimizing availability of soil N. Estimation of soil organic matter mineralization using prediction models may be useful to enhance NUE if models are calibrated for target environments. Cereal crops are often bred under optimum N conditions and may not perform well under low N conditions. Thus, breeders can integrate genetic and phenotypic information to develop cultivars adapted to specific environments and cultivation practices. The proper choice and integration of strategies can synchronize N demand and supply within a system, resulting in reduced risk of N loss while improving NUE in both conventional and organic systems.

International audience; Post-mining land reclamation of Athabasca Oil Sands (AB, Canada) involves the reconstruction of soil profiles able to support a mosaic of boreal forest communities. However, the use of coarse-textured reclamation materials to recreate forest ecosystems represents a challenge in terms of soil water and nutrient availability. This work aimed to quantify nutrient leaching in reclaimed coarse-textured soils constructed with two coversoils (peat mineral mix and forest floor mineral mix) underlain by mineral materials, including a blended B/C subsoil reclamation material, lean oil sand overburden substrate, and tailing sand. Water retention and conductivity curves were estimated for each material, and their retention capacity for inorganic N and P was measured in sorption isotherm experiments. The redistribution of water, inorganic N and P five days after an intense rain event was evaluated in six different reclaimed soil profiles using a laboratory-controlled leaching experiment in 1.2-m deep columns. The redistribution of fertilizer nutrients was also measured following the addition of 15N-labelled ammonium and phosphate over the top 10 cm of the columns. In addition, a 25-day incubation experiment with the two coversoils enabled us to estimate the timing of N immobilization and nitrification processes. Our results show that, depending on the combination of materials used for land reclamation, the soil profiles may provide equal or higher amounts of inorganic N and P in the rooting zone compared to natural, coarse-textured soils of the region. Following the simulated intense rainfall, the peat-mineral mix was able to retain 44% of its initial inorganic N within the top 20 cm of the reclaimed soil profiles, while 84% of the inorganic N present in the forest floor mineral mix was leached down. Compared to the movement of water, the leaching of N down the soil profiles was slower and partly restricted by the presence of lean oil sand, and to a lesser degree tailing sand. Most of the introduced fertilizer-N remained in the first 20 cm of the soil profiles under the form of nitrate, although the incubation experiment suggested that nitrification only occurred after the simulated rainfall event. Based on our experimental data and on additional simulations of water and nutrient transport, we conclude that nutrient leaching in reclaimed soils can be significant if specific materials such as forest floor mineral material and coarse-textured subsoil are combined and when an intense rainfall occurs at a period coinciding with a high concentration of nitrate-N in the topsoil.

Ecosystem reconstruction after mining disturbance is a challenge considering the multitude of factors that affect soil formation and revegetation. In the boreal forest of western Canada, peat material is often used as the organic amendment for land reclamation to upland forest. Carbon and water dynamics of peat-dominated ecosystems differ from natural upland forest soils. The objective of this work was to evaluate the evolution of soils reconstructed after mining disturbance using 13 C and 2 H analyses of n-alkane tracers. Ten soils from natural ecosystems were sampled (0-10 cm) and compared with 11 soils from novel ecosystems ranging in age from 0 to 30 yr, as well as a fresh peat sample. Soils supported different vegetation, including pine (Pinus spp.), aspen (Populus spp.), and white spruce [Picea glauca (Moench) Voss]. Despite overlaps for some individual n-alkanes, we found a dominance of n-C25 in reconstructed soils, also dominant in the peat material, and a dominance of n-C27 in natural soils, one of the dominant n-alkanes in natural forest vegetation. In addition, there was a significant difference in odd n-alkane δ2 H and δ13 C values between natural and reconstructed soils (p

The boreal forest is the single largest terrestrial store of carbon on Earth, and approximately 25% of these carbon stocks are in the forest floor. Climate change is expected to alter boreal vegetation, and aspen-dominated stands will replace conifers in Western Canada. We investigated how these vegetation shifts could affect the composition and function of soil microbial communities, using forest floor samples from the Ecosystem Management Emulating Natural Disturbance (EMEND) project in northwestern Alberta, Canada. Soil microbial communities were surveyed in rhizosphere and bulk forest floor of 17-year-old spruce clear-cuts where aspen was naturally regenerating, mature stands of aspen and spruce, and 17-year-old clear-cuts of aspen. Phospholipid fatty acid (PLFA) analysis was used to characterize microbial community composition and multiple substrate induced respiration (MSIR) to quantify microbial community functional capacity. Carbon source utilization by microorganisms was investigated through natural abundance isotope analysis of individual PLFAs. Rhizosphere samples had a significantly higher proportion of fungi and a higher gram-negative to gram-positive bacteria ratio compared to bulk soil. Fungi and gram-negative bacteria biomarkers in the rhizosphere showed 13C depletion compared to bulk forest floor, indicating that they had assimilated more newly-photosynthesized carbon than bulk forest floor microbes. Aspen trees exerted a greater influence over their rhizospheres than spruce trees in terms of microbial community composition and functional capacity, and aspen rhizospheres showed the highest basal respiration. In less than two decades, aspen regeneration in former spruce stands shifted the composition of microbial communities towards that of native aspen stands, with the rhizosphere microbiome responding more quickly than bulk forest floor. This study suggests that predicted vegetation shifts in the boreal have the potential to cause more immediate and profound changes in the rhizosphere, and emphasizes the need to include the rhizosphere in future studies.

The circumpolar boreal forest biome contains a significant portion of the global forest carbon (C) stocks, and is generally considered to be a sink for atmospheric C. However, there is a concern that disturbances occurring concurrently with fibre utilization may cause these boreal forest ecosystems to become net C sources resulting from cumulative effects of natural and anthropogenic disturbances, and altered C cycling processes at stand- and landscape scales. In this study, we have synthesized soil C data (forest floor + upper 20 cm of mineral soil) from six long-term, forest management-based manipulative experiments situated in both the Boreal Plain and Boreal Shield ecozones to examine the collective effects of disturbance type and intensity, and dominant tree cover type on soil C pools. The disturbances of clearcut harvest, burn, burn + salvage logging, harvest + forest floor removal resulted in rapid and significant declines in measured soil C following disturbance. In contrast, partial stand removals (i.e., light and heavy thinnings through partial harvest treatments) conducted at the Ecosystem Management Emulating Natural Disturbance (EMEND) trial resulted in slight increases in total measured soil C by year 6, with the exception of conifer dominated stands. The observed declines in total soil C, particularly in the high intensity treatments, were largely a function of declines within the forest floor. In most cases, the rate of change in measured soil C converged over time with baseline levels (clearcut and partial harvests by year 6). In contrast, the data suggests that longer periods of time (>15 years) will be required for the total soil C pool to fully recover to pre-disturbance levels for the higher intensity disturbances. Dominant tree species cover type also influenced the rate of change in total measured soil C. There was a clear difference in the rate of change in the soil C pool between conifer-dominated stands compared to mixed or aspen-dominated stands, going from a positive rate of change (accumulation) under deciduous cover to a negative rate of change under conifers. Our results illustrate that the complex and interactive relationships among species effects, inherent soil and site properties, and management practices should collectively be considered in the development of options related to boreal forest soil C management and climate change mitigation strategies.

Aspen ( Populus tremuloides Michx.) and white spruce ( Picea glauca (Moench) Voss) are archetypal species of the boreal forest of western Canada. With the many new and varied environmental pressures (e.g., fire, harvesting, climate change) on, this boreal forest biome, we were interested in understanding the importance of potential changes in organic matter inputs to its soil microbial community structure and function. Double -labeled ( 15 N and 13 C) aspen leaf litter was utilized as an amendment to the forest floor of two stands, an aspen and a spruce, in a 16-month paired field experiment, where amended mesocosms were compared to control mesocosms with no litter added. Nitrogen mineralization from the aspen leaf litter occurred within both stand types, as evidenced by higher δ 15 N values in roots and live aboveground vegetation. In term of carbon fluxes, stable isotope 13 C analysis of the forest floor microbial phospholipid fatty acid (PLFAs) indicated a slight initial enrichment within all amended mesocosms, likely due to leaching of 13 C enriched solubles from the labeled aspen litter. Within the aspen stand, continued 13 C enrichment of most PLFAs demonstrated that incorporation of the litter-derived carbon by a wide range of microorganisms was maintained through both growing seasons. Response of the spruce microbial community differed from that of aspen. Its overall structural composition did not alter in response to aspen litter addition; yet, δ 13 C values of several PLFA biomarkers decreased at the end of the first growing season, suggesting that priming stimulated mineralization of a 13 C-depleted pool. These results indicate that while nitrogen mineralization was maintained, the deep forest floor carbon stocks of spruce stands may be vulnerable to increased decomposition rates with changing organic matter input.

Migration of salts from overburden/mine spoil with elevated salinity and sodicity into placed soil reclamation cover materials can adversely affect forest growth. In this study, we explored whether the ecosystem model ecosys which uses water potential and salt concentration gradients to simulate lateral and vertical water and solute fluxes can be used to determine the effects of salt redistribution on plant productivity in reclaimed landforms. In this study, salt transport and consequent effects on plant growth were simulated and tested against electrical conductivity (EC) and plant growth measured with soil cover depths of 35 cm, 50 cm and 100 cm in a 17-year-old forest reclamation site on a slope constructed on saline-sodic overburden in Northern Alberta. Overall, results from this study demonstrated the applicability of ecosys in predicting salinity effects on forest productivity in reclaimed landscapes. The modelled results showed that the upward salt movement from overburden to coversoil was influenced by the cover depth as observed in the measured values. Upward salt movement into the covers equilibrated with modelled downslope salt discharge after nine years from reclamation, maintaining a stable EC profile in the root zone. Even though modelled aspen vs. white spruce growth was adversely affected by salinity in shallow covers (≤50 cm depth) during dry years, long-term plant productivity in all the covers did not change markedly under the current climate. A clear topographical effect on salinity in the reclamation covers was not found during the study period, although the modelled salt discharge indicated a risk of salinization in downstream areas.

The development of a functional soil is fundamental to restoring forest ecosystems following major disturbances such as surface mining. Given that it is much easier to revive soil than it is to create soil, salvaging and replacing topsoil is the simplest and most effective means of restoring a functional soil following disturbance. Soil hydrological function must be restored at both the site level (having soil water retention to support plant growth and soil biota), and landscape level (limiting or enhancing runoff or deep percolation). Practices for restoring water-related functions include avoiding compaction during soil reconstruction, creating rough surfaces to increase infiltration and reduce runoff and designing structures such as drainage swales to convey surface runoff to desired receiving areas with minimal damage. The rates at which organic matter and nutrients accumulate in soils are influenced by the vegetation; nitrogen-fixing plants, broadleaf tree species and grasses build stocks faster than conifer trees. Practices that enhance soil water-holding capacity, aeration and accumulation of organic matter and nutrients also make soils more conducive for soil microbial and faunal communities. The development of the soil biota is largely determined by the rate at which the plant community develops, but can be hastened by applying native soil inoculum to restored sites. Biodiversity can be enhanced at the landscape level by retaining or creating a variety of habitat types within the post-mining landscape, rather than applying a single prescription to the entire site. Practices that promote the hydrological and nutritional conditions that support plant, microbial and faunal communities will also assist in the development of soil structure in rehabilitated mine soils. The development of structure may provide the best indication that a soil has been rehabilitated.

Storing a significant portion of the global carbon (C) stocks, soils of the boreal forest display a high degree of spatial heterogeneity across the landscape, arising from variation in forest structure and landscape morphology, as well as natural and anthropogenic disturbances. Because of this high degree of variability, accurately quantifying C storage in this ecosystem poses a challenge. Forestry is an important feature of Canada’s natural resource-based economy, but there is still considerable uncertainty on how management practices will affect boreal soil C sequestration in the long-term. With increasing pressures due to a changing climate and intensified forest management, developing better tools and techniques to quantify soil C dynamics is of paramount importance. In this study, we measured soil C stocks and associated properties in the forest floors and mineral soils (0–7 cm) of conifer-dominated and deciduous (broadleaf)-dominated boreal forest stands, 17 years following variable retention harvest. We investigated if the Wet Areas Mapping-based depth-to-water (DTW) index, derived from remotely sensed Light Detection and Ranging (LiDAR) data, was related to soil properties of unharvested stands. In addition, relationships between harvest-induced changes in soil properties and the DTW index were examined. In unharvested stands, several forest floor properties were related to DTW. However, these relationships were stand-specific. In unharvested conifer-dominated stands, forest floor C stocks were positively related to site wetness, while in harvested stands there was no relationship; this suggests that C was differentially lost from wetter sites following harvesting. Conversely, in unharvested deciduous-dominated stands, there was no relationship between forest floor C stocks and site wetness, but in harvested stands, wetter sites had higher C stocks. The DTW index was more strongly related to soil properties in the mineral soil (0–7 cm) than in the forest floor. In both forest cover types, mineral soil C and N concentrations, and C stocks, increased with increasing wetness. Relationships between mineral soil properties and DTW were not stand-specific, and were of similar magnitude under deciduous-dominated and conifer-dominated cover. In addition, harvest and forest regeneration had limited impact on the relationships between DTW and mineral soil properties. This study highlights the potential of using remote sensing and the DTW index to model forest floor and mineral soil properties in the boreal forest. In turn, this approach may be utilized in effective forest management strategies that aim to conserve boreal C stocks.

Conversion of arable cropland to forage crops has been proposed as a potential method to increase soil organic carbon (SOC) stocks to sequester carbon and improve soil quality. In this study, intact soil cores were collected from long-term boreal forest soil research plots established in 1980 consisting of: a mixed arable crop and forage agroecological rotation (AE), continuous forage (CF), and continuous grain (CG) rotations. These cores were analyzed using a SisuROCK automated hyperspectral imaging system in a laboratory setting collecting shortwave infrared reflectance data. Samples were then analyzed for SOC and total nitrogen (TN) contents by dry combustion to prepare a training data set. Predictive models were successfully built for SOC and TN using a combination of wavelet analysis and Bayesian Regularized Neural Nets. The CF rotation was found to have the highest SOC and TN contents compared to AE rotation for only the top 3 and 4 cm, respectively. These two rotations had comparable contents for both parameters for the rest of the topsoil, which was greater than the SOC and TN contents in the CG rotation to depths of approximately 12 cm. Increases in both SOC and TN were associated with increased spatial aggregation at fine spatial scales. These results indicate that adding forages to rotations in boreal forest soils increases SOC and TN, however these changes were concentrated in the surface depths.

Emulating the variability that exists in the natural landscape prior to disturbance should be a goal of soil reconstruction and land reclamation efforts following resource extraction. Long-term ecosystem sustainability within reclaimed landscapes can only be achieved with the re-establishment of biogeochemical processes between reconstructed soils and plants. In this study, we assessed key soil biogeochemical attributes (nutrient availability, organic matter composition, and microbial communities) in reconstructed, novel, anthropogenic ecosystems covering different reclamation treatments following open-cast mining for oil extraction. We compared the attributes to those present in a range of natural soils representative of mature boreal forest ecosystems in the same area of northern Alberta. Soil nutrient availability was determined in situ with resin probes, organic matter composition was described with 13C nuclear magnetic resonance spectroscopy and soil microbial community structure was characterized using phospholipid fatty acid analysis. Significant differences among natural ecosystems were apparent in nutrient availability and seemed more related to the dominant tree cover than to soil type. When analyzed together, all natural forests differed significantly from the novel ecosystems, in particular with respect to soil organic matter composition. However, there was some overlap between the reconstructed soils and some of the natural ecosystems in nutrient availability and microbial communities, but not in organic matter characteristics. Hence, our results illustrate the importance of considering the range of natural landscape variability, and including several soil biogeochemical attributes when comparing novel, anthropogenic ecosystems to the mature ecosystems that constitute ecological targets.

Soil organic matter (SOM) is a major driver of key agroecosystem functions. Our objective was to examine the dynamics of organic matter in whole soil, particulate (POM; > 53 mu m size), and mineral-associated (MAOM) fractions under varying crop rotations and nutrient managements at two long-term experimental sites (Breton and Lethbridge). Soil samples were collected from simple (2 yr) and complex (5 or 6 yr) crop rotations at the 5 - 10 cm depth. We found associations between SOM pools versus microbial community and soil aggregation. Compared to cropped soils, an adjacent forest exhibited a significantly higher soil total organic carbon (TOC) and a shift in SOM fractions with substantially higher POM. However, the forest soil had the lowest microbial biomass C among all the assessed land use systems (P

Examining the spatial interactions between above and belowground components of terrestrial ecosystems can give meaningful insight into the ecological processes happening at different scales. Understanding spatial dependence in these processes may help to evaluate reclamation success which is crucial for future management of such areas. The aim of this study was to measure the spatial patterns of soil biogeochemical properties in a young aspen stand reclaimed after oil sands extraction and to evaluate how the patterns were related to nutrient availability. Samples were collected from a 14-year old reclaimed site using a spatially explicit protocol with a minimum resolution of 0.5 m. Field-measured variables included forest floor depth and mass, tree cluster (canopy overlap), distance to nearest tree, and resin available nutrients. Soil microbial properties including microbial biomass C and N, basal respiration, and extracellular enzyme activity were measured during an eight-week laboratory incubation experiment. Geo-statistics were applied to examine the spatial patterns and model the space effect. A fine scale (

Kubota, H., Quideau, S. A., Hucl, P. J. and Spaner, D. M. 2015. The effect of weeds on soil arbuscular mycorrhizal fungi and agronomic traits in spring wheat (Triticum aestivum L.) under organic management in Canada. Can. J. Plant Sci. 95: 615–627. Understanding the influence of weeds in agroecosystems may aid in developing efficient and sustainable organic wheat production systems. We examined the effect of weeds on soil microbial communities and the performance of spring wheat (Triticum aestivum L.) under organic management in Edmonton, AB, Canada. We grew 13 Canadian spring wheat cultivars in organically managed hand-weeded less-weedy and weedy treatments in 2010 and 2011. The less-weedy treatment exhibited greater grain yield and tillers per square meter, while kernel weight, test weight, days to maturity, plant height, grain P and protein content were not altered by weed treatment. Canada Western Red Spring (CWRS) wheat cultivars CDC Go and CDC Kernen were the most yield-stable because they minimized fertile tiller reduction in response to weed pressure (10 and 13% reduction, respectively, compared with the average reduction of 20%). Other cultivars exhibited yield stability through increased kernel weight. The contribution of arbuscular mycorrhizal fungi (AMF) to the total phospholipid fatty acid increased in both treatments; however, the rate of this increase was greater in the weedy treatment than the less-weedy treatment (from 2.9 to 3.9%, from 2.8 to 3.1%, respectively). Weed dry biomass was positively correlated with AMF% in the less-weedy treatment only. Organic systems tend to be weedier than conventional systems. We found that weeds are important determinants of AMF proliferation in soil. In addition, choosing wheat cultivars that maintain important yield components under severe weed stress is one strategy to maximize yields in organic systems.

Pyrogenic carbon (PyC), a by-product of recurrent boreal wildfires, is an important component of the global soil C pool, although precise assessment of boreal PyC stock is scarce. The overall objective of this study was to estimate total C stock and PyC stock in forest floors of Eastern Canada boreal forests. We also investigated the environmental conditions controlling the stocks and characterized the composition of the forest floor layers. Forest floor samples were collected from mesic black spruce sites recently affected by fire (3–5 yr) and analyzed using elemental analysis and solid state 13C nuclear magnetic resonance (NMR) spectroscopy. PyC content was further estimated using a molecular mixing model. Total C stock in forest floors averaged 5.7 ± 2.9 kg C/m2 and PyC stock 0.6 ± 0.3 kg C/m2. Total stock varied with position in the landscape, with a greater accumulation of organic material on northern aspects and lower slope positions. In addition, total stock was significantly higher in spruce-dominated forest floors than stands where jack pine was present. The PyC stock was significantly related to the atomic H/C ratio (R2 0.84) of the different organic layers. 13C NMR spectroscopy revealed a large increase in aromatic carbon in the deepest forest floor layer (humified H horizon) at the organic-mineral soil interface. The majority of the PyC stock was located in this horizon and had been formed during past high severity fires rather than during the most recent fire event.

Community level physiological profiles (CLPPs) have been widely used to assess microbial community diversity in soils. Refinement of techniques to determine soil CLPP eventually led to the development of the MicroResp technique. This technique avoids many of the pitfalls of earlier methods by using a whole-soil approach coupled with the convenience of microplates. However, issues related to soil pretreatment (primarily sieving) can arise when using the standard MicroResp method to determine the CLPPs of forest floors. Here, we developed a modified multiple substrate induced respiration (multi-SIR) method that lessens the effects of pretreatment by using a larger soil volume in custom 24-well deep-well plates. Microbial community indices including catabolic evenness (E) and CLPP were determined on a range of forest soils using both the standard MicroResp and our modified multi-SIR method. The modified method reduced the variation among the triplicate substrate wells and displayed a wider range of E among the soils measured. Additionally, using multivariate nonmetric multidimensional scaling (NMDS) as well as cluster analysis, we found that the modified method was able to better detect differences in soil CLPPs. The standard MicroResp method remains a valuable technique for many soils; however, our modified multi-SIR method is more suitable for organic soils, such as forest floors, that have low bulk density.

Most of the soil microbial community responsible for nutrient cycling in boreal forests is found in surficial organic horizons, often referred to as forest floor layers. In the Athabasca oil sands region of northern Alberta, Canada, reclamation of upland forests utilizes peat as an organic amendment due to shortages in forest floor materials. However, differences in physical and chemical properties between peat and forest floor raise the concern that the use of peat will fail to promote a soil microbial community consistent with sustainable forest ecosystems. As a strategy to maximize the limited supply of forest floor, this study investigated mixing both materials. Specifically, we determined the differences in microbial community composition and activity between fresh and stockpiled forest floor, peat and mixtures of both using basal respiration and phospholipid fatty acid (PLFA) analysis. Basal respiration increased with forest floor addition, but only responded to water content within the pure forest floor treatment. Mixing fresh peat and forest floor produced greater than additive respiration, and the overall PLFA profiles illustrated a microbial community more similar to the forest floor material than to the peat. This suggests a proportionally greater activity in the microbial community associated with the forest floor that is dominating the mixed treatment. Individual phospholipid indicators further demonstrated a distinct shift in their abundance going from the pure peat to the pure forest floor. Taken together, results provide evidence that mixing forest floor and peat provides benefits by increasing the water-holding capacity of the mixed material as compared to pure forest floor, while producing a microbial community more analogous to an upland forest community than to peat alone.

Over a period of 2 yr, the effects of dehydrated septic tank sludge application on the chemical properties of a severely disturbed forest clayey soil were assessed and compared with application of native forest floor (i.e., from neighboring forest). Six treatments [fresh and mature sludges × two depths (15 and 25 cm), forest floor, and a control] were replicated three times according to a complete random design. Total organic C and N concentrations of amendments and their chemical structure, based on 13C nuclear magnetic resonance (NMR) spectroscopy, were determined. Mineral soil C and N concentrations and C mineralization rates were monitored as well as nutrient supply rates using Plant Root Simulator™ probes. White spruce [Picea glauca (Moench) Voss] seedling foliar nutrition and growth were also monitored. NMR spectroscopy revealed differences among amendments, with the forest floor spectra displaying lower O-alkyl C and higher alkyl C and carbonyl C proportions relative to sludge. Neither soil C conce...

Significant areas of the mixedgrass prairie (MGP) in Alberta have experienced heavy equipment traffic due to industrial infrastructure development, which may change edaphic conditions that influence soil microbial communities. Direct wheeled traffic effects may be alleviated by using access mats that spread traffic impacts over a larger area; however, no scientific assessments of access mat efficacy have been conducted on soil microbial responses. Our objectives were to assess soil properties and the diversity, community structure, and size of soil microbial communities, as impacted by wheeled traffic occurring with and without access mat use on grassland. Two representative sites with differing soil texture (sandy versus loamy) were selected from a set of MGP sites established at the University of Alberta Mattheis Research Ranch in SE Alberta, Canada. Treatments at each site included non-disturbed vegetation controls, areas affected by direct traffic, and grassland with access mats placed for 12 weeks with traffic applied. Soil was sampled in July 2015 (immediately after access mat/traffic treatments ended) and in 2016 (12 months after access mat/traffic treatments) from both sites. Soil DNA was extracted and used to target N-cycling genes via qPCR and for next-generation sequencing (Illumina Miseq) of bacterial, archaeal, and fungal communities. Direct wheeled traffic without access matting increased the size of the N2O-reducing community in the sandy soil and conversely deceased the size of this community in the loamy soil. Non-metric multidimensional scaling, perMANOVA, and indicator species analyses indicated a large effect of traffic without access mat use on fungal community structure, as well as distinct bacterial and fungal indicator OTUs associated with disturbed soils compared to control soils. Fungal community structure remained affected by traffic impacts 12 months later, highlighting the extended impacts of traffic on fungal communities. Access matting mitigated some of the impacts direct traffic had on fungal community structure, but in the loamy site increased soil N and decreased the size of the N2O-reducing bacterial community. The latter indicates that ecosystem functioning of these grasslands may not only be impacted by direct wheeled traffic but also by using matting as a mitigation technique.

Wildfire, a recurrent disturbance in the boreal, converts part of the forest floor into pyrogenic carbon (PyC). The latter is an important component of the global soil carbon pool, yet knowledge of its stocks and storage mechanisms in these boreal ecosystems is scarce. Podzolization processes, which are frequent under boreal vegetation, result in distinctive patterns of soil organic carbon (SOC) accumulation in the mineral subsoil; how this may affect PyC storage remains largely unknown. The objectives of this study were to estimate SOC and PyC stocks in podzolic soils from fire-affected black spruce forests, and to explore the storage mechanisms taking place in their mineral horizons. We also compared PyC stocks in mineral soils to forest floor stocks. Samples were collected from 23 soil profiles under black spruce forests located throughout the province of Quebec. To further explore the relationship between podzolization and PyC storage mechanisms, we measured SOC and PyC contents in size and density fractions of a subset of 11 podzolic B horizons. Total SOC stocks in the mineral horizons and forest floors were comparable. Pyrogenic carbon stocks in the mineral soils, estimated by a H 2 O 2 /dilute HNO 3 digestion, averaged 0.2 (± 0.1) kg C m − 2 . This was significantly lower than forest floor stocks, which ranged from 0.2 to 1.2 kg C m − 2 . Consequently, PyC constituted a smaller fraction of total SOC (2–15%) in mineral soils than in forest floors, where it was as high as 68% (± 5) in some horizons. In the mineral soils, SOC and PyC concentrations were strongly correlated. While some PyC was found in unprotected particulate organic matter (POM), the rest was associated with organo-mineral and organo-metallic complexes in the micro-aggregate protected POM and fine fraction. Patterns of PyC accumulation in mineral soils were similar to SOC, and the greater PyC stocks were found in podzolic B horizons.