Boreal Forest Soils Research Articles

Spatial patterns of nitrogen isotope ratios in forest soils are related to latitude and soil phosphorus concentration

The aim of this study was to identify the parameters that affect the nitrogen (N) isotope ratio (δ15N) in soils of temperate and boreal forests. We measured the δ15N in 30 soil profiles of temperate and boreal forests in Sweden and analyzed the relationships between δ15N in the soils and soil chemical properties as well as site characteristics. In addition, we conducted a meta-analyses of δ15N in the organic layer of European forests. We identified two types of δ15N patterns; in type D soils, the δ15N in the mineral soil decreases with depth, whereas in type C soil, the δ15N in the soil profile is almost constant. Type D soils had a significantly higher δ15N in the organic layer and upper mineral soil than type C soils, which is likely due to N isotope fractionation by ectomycorrhizal fungi in the topsoil. Type D soils were found in boreal forests, but not in temperate forests. They had a significantly lower atmospheric N deposition rate than type C soils, by a factor of 2.3, and a significantly higher phosphorus (P) concentration of the organic layer than type C soils, by a factor of 1.5. We also found that the δ15N was negatively correlated with the N:P ratio of the organic layer (R2 = 0.21, p < 0.001). Across Europe, the δ15N of the organic layer was positively correlated with latitude (R2 = 0.58, p < 0.001), and negatively with mean annual temperature (R2 = 0.52, p < 0.001) and atmospheric N deposition (R2 = 0.42, p < 0.001). In conclusion, our results show that the δ15N of the organic layer and microbial N (re-)cycling in forest soils is positively related with latitude and the P concentration of the organic layer.

Seasonal controls override forest harvesting effects on the composition of dissolved organic matter mobilized from boreal forest soil organic horizons

Abstract. Dissolved organic matter (DOM) mobilized from the organic (O) horizons of forest soils is a temporally dynamic flux of carbon (C) and nutrients, and the fate of this DOM in downstream pools is dependent on the rate and pathways of water flow as well as its chemical composition. Here, we present observations of the composition of DOM mobilized weekly to monthly from O horizons in mature forest and adjacent harvested treatment plots. The study site was experimentally harvested, without replanting, 10-years prior to this study. Thus, the treatments differ significantly in terms of forest stand and soil properties, and they interact differently with the regional hydrometeorological conditions. This presented an opportunity to investigate the role of forest structure relative to environmental variation on soil DOM mobilization. On an annual basis, fluxes of total dissolved nitrogen (TDN) and dissolved organic nitrogen (DON) were largest from the warmer and thinner O horizons of the harvested (H) treatment compared to the forest (F) treatment; however, neither phosphate nor ammonium fluxes differed by treatment type. On a short-term basis in both H and F treatments, all fluxes were positively correlated to water input, and all concentrations were positively correlated to soil temperature and negatively correlated to water input. Soil moisture was negatively correlated to the C : N of DOM. These results suggest common seasonal controls on DOM mobilization regardless of harvesting treatment. Optical characterization of seasonally representative samples additionally supported a stronger control of season over harvesting. The chemical character of DOM mobilized during winter and snowmelt: lower C : N, higher specific ultraviolet absorbance and lower molecular weight of chromophoric DOM (CDOM; higher spectral slope ratio) were representative of relatively more decomposed DOM compared to that mobilized in summer and autumn. This shows that the decomposition of soil organic matter underneath a consistently deep snowpack is a key determinant of the composition of DOM mobilized from O horizons during winter and the hydrologically significant snowmelt period regardless of harvesting impact. Despite the higher proportion of aromatic DOM in the snowmelt samples, its lower molecular weight and rapid delivery from O to mineral horizons suggests that the snowmelt period is not likely to be a significant period of DOM sequestration by mineral soil. Rather, the higher-molecular-weight, high-C : N DOM mobilized during slow and relatively infrequent delivery during summer and rapid, frequent delivery during autumn are more likely to support periods of mineral soil sequestration and increased export of fresher terrestrial DOM, respectively. These observed seasonal dynamics in O horizon DOM suggest the predicted decreases in winter and spring snowfall and increasing autumn and winter rainfall with climate warming in this region will enhance mobilization of DOM that is more reactive to mineral interactions in deeper soil, but also more biological and photoreactive in the aquatic environment. Understanding the downstream consequences of this mobilized DOM in response to these shifts in precipitation timing and form can improve our ability to predict and manage forest C balance but requires understanding the response of landscape hydrology to these changing precipitation regimes.

Post-fire and harvest legacy on soil carbon and microbial communities in boreal forest soils

The boreal forest is currently facing multiple disturbances, which are increasing in extent, frequency and severity. Of particular concern in the western boreal forest of Canada are wildfires and harvesting, which, in the short-term, may impact the forest floor, a storehouse of organic matter and site of highest soil microbial activity. However, how these disturbances may individually and cumulatively affect soil properties during ecosystem recovery is not well documented. Here we compared the separate and compound (salvage-logging) effects of wildfire and harvesting in the mixedwood boreal forest of northern Alberta. Ten years following disturbance, carbon chemical composition of the harvested forest floors was comparable to the control plots as was indicated by solid-state nuclear magnetic resonance analysis. We report a legacy effect of fire in recovering forest floors, with the continued presence of black carbon. Further, as determined by phospholipid fatty acid analysis (PLFA), both wildfire and salvage-logging disturbance resulted in distinct microbial composition from the control and harvest treatments. A shift in organic matter composition indicative of fresh plant inputs was determined to be a key driver of community differences. Despite the presence of black carbon, and shallower depth, the fire-disturbed forest floor harbored the greatest microbial biomass. However, this greater microbial biomass was not present following salvage-logging of the fire disturbed stands. Instead, the salvage-logged stands had the shallowest forest floor, low microbial biomass, and differed the most from the control forest floors based on PLFA results. Taken together, these results indicate that, ten years after disturbance, the compound disturbance of salvage-logging had a greater impact on the recovery of forest floor microbial communities than harvest or wildfire alone.

Factors of soil CO2 emission in boreal forests: evidence from Central Siberia

Soils of boreal forests are crucial carbon reserves. The response of soil carbon emission to climate change significantly affects the concentration of carbon dioxide in the atmosphere. Soil carbon emission models frequently show a nonlinear response to temperature, but soil moisture is an important limiting factor, often overlooked in energy limited ecosystems. We suggest a statistical model of soil CO2 emission constrained by soil moisture and temperature for different ecosystems in the boreal zone. We tested this modelling strategy using direct measurements of seasonal soil CO2 emission near the research observatory ZOTTO near the Bor settlement, Central Siberia, Russia, in 2012-2017. Soil moisture explained a significant amount of variability of soil emission: the adjusted R2 was twice higher than in the baseline model. Although the temperature-only model describes the annual variability of carbon dioxide emissions quite well, the addition of moisture measurement significantly refines the quality of prediction of the seasonal component dynamics. Models including both temperature and soil moisture could serve as a promising tool to analyze the carbon cycle in boreal forest ecosystems.

Climate and forest properties explain wildfire impact on microbial community and nutrient mobilization in boreal soil

The boreal landscape stores an estimated 40% of the earth's carbon (C) found in terrestrial vegetation and soils, with a large portion collected in thick organic soil layers. These ground stores are subject to substantial removals due to the centurial return of wildfire, which has strong impacts on the soil microbial community and nutrient cycling, which in turn can control ecosystem recovery patterns and process rates, such as C turnover. Currently, predictive knowledge used in assessing fire impacts is largely focused on ecosystems that experience only superficial burning and few robust observations exist regarding the effect that smoldering combustion in deeper active soil layers has on post-fire soil activity. This study provided a highly replicated and regionally extensive survey of wildfire impact on microbial community structure (using fatty acid biomarkers) and nutrient cycling (using in situ ionic resin capsules) across broad gradients of climate, forest properties and fire conditions within 50 separate burn scars and 50 additional matched unburnt boreal forest soils. The results suggest a strong metabolic shift in burnt soils due to heat impact on their structure and a decoupling from aboveground processes, releasing ecosystem N limitation and increasing mobilization of N, P, K, and S as excess in conjunction with an altered, C-starved microbial community structure and reduced root uptake due to vegetation mortality. An additional observed climatic control over burnt soil properties has implications for altered boreal forest function in future climate and fire regimes deserving of further attention.

Peribolosporomycetes class. nov.: description of a new heat resistant and osmotolerant basidiomycete lineage, represented by Peribolospora gen. nov., P. kevripleyi sp. nov., and P. baueri sp. nov.

Heat resistance is the ability to survive short, extreme temperature stresses, exceeding the own growth temperature by far. Despite their occurrence in natural substrates and their relevance for the food and healthcare industry, the diversity of fungi with heat resistance abilities is poorly studied. Sampling of boreal forest soils in Canada in combination with a heat-shock treatment (75 °C, 30 min) yielded, among others, four heat resistant, mesophilic fungal isolates. Based on rDNA barcode sequences, the novel isolates were assigned to Basidiomycota. In this study, we use macromorphological and micromorphological observations, cultivation assays and comparative genomics for physiological characterization, interspecific differentiation, and phylogenetic placement of these isolates. A phylogeny of 38 single-copy orthologous genes, an orthology analysis, and septal pore type analysis revealed the isolates as representatives of two new basidiomycetous species of the novel class Peribolosporomycetes, a sister lineage to all other members of Ustilaginomycotina. Further genomic and phenotypic data support two different species (Peribolospora kevripleyi, Peribolospora baueri), which are heat resistant and osmotolerant.

Generation and properties of organic colloids extracted by water from the organic horizon of a boreal forest soil

Organic colloids are an important part of dissolved organic matter (DOM) yet many of their properties remain elusive. The main aims of this study were to assess how the colloidal properties of DOM extracted with water from an organic boreal soil horizon varied with the extraction protocol, and thereby provide insight into the nature of the DOM colloids and develop a mechanistic understanding of how the colloids were generated from the parent soil aggregates. This was accomplished by systematic variations of extraction temperature (4 °C–100 °C), time, mechanical agitation, and pH, together with a combination of chemical analyses, and light and X-ray scattering. Our results agreed with the previous identification of two main colloidal DOM species, one fractal cluster and a second, smaller colloidal DOM species described as chains or coils. Fractal clusters completely dominated the colloidal DOM in extracts from our soil at room temperature and below. Colloidal coils only existed in DOM extracted above room temperature, and their amount increased significantly between 50 °C–100 °C. Moreover, the temperature variation indicated that the fractal clusters partly dissolved into colloidal coils at elevated temperatures. Mechanical agitation at 4 °C significantly increased the amount of DOM extracted, increasing the concentrations of both fractal clusters and low-molecular weight organic compounds. While the clusters were extracted from agitated and non-agitated soil suspensions, the low molecular weight organics were mainly released by agitation. Based on the experimental observations, we propose a conceptual model where parent soil aggregates contain the fractal clusters in mobile and occluded forms, and that the occluded clusters co-exist with occluded low molecular weight organics. These occluded forms may be released by mechanical forces, increasing pH and temperature. At higher temperatures, the soil aggregates and the fractal clusters start to break up, and subsequently individual colloidal coils, presumably carbohydrates, disperse in the water phase. The model explains the origin and properties of the fractal clusters that completely dominate the colloidal DOM extracted from our soil at room temperature and below.

Linking soil microbial community to the chemical composition of dissolved organic matter in a boreal forest during freeze–thaw cycles

Soil dissolved organic matter (DOM) is mainly derived from the microbial degradation of organic matter. Intense freeze–thaw cycles (FTCs) elicited by warming alter the production and consumption of DOM in boreal forest soils, but little is known about the interactions between soil microbial community and DOM chemical composition. In this study, a FTCs incubation experiment was conducted to investigate the association between DOM dynamics and microbial community. DOM fingerprints were characterized using pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) technology, and bacterial and fungal community composition was analyzed using high-throughput sequencing and bioinformatics. Both two and six freeze–thaw cycles (2FTCs and 6FTCs) significantly increased soil dissolved organic carbon (DOC) and NH4+-N contents at 0–10 cm and 50–60 cm depths. FTCs significantly decreased the relative abundance of aromatic components in DOM, whereas increasing the relative abundance of polysaccharides and fatty acid methyl esters (FAMEs) in DOM. The degradation of lignin-derived aromatic compounds was accelerated and microbial-derived components accumulated. Also, bacterial community responded more sensitively to FTCs than fungal community. Significant changes in bacterial community network was observed during FTCs with increased linkages, shortened average path lengths, reduced modularity and relative abundance of dominant phyla, and decreased α diversity. However, FTCs only increased the average path length of soil fungal community. The higher connectivity indicated that FTCs enhanced the interactions between microbial community and DOM chemical components. FTCs led to a shift in bacterial keystone taxa from being present in specific DOM molecules to competing for common substrates. Overall, FTCs tended to activate DOM and increase availability, which could promote DOM turnover and increase soil C stabilization. These findings provide new insights into the molecular mechanisms of DOM dynamics in the boreal forest soils under increased FTCs.

Enhanced loss but limited mobility of pyrogenic and organic matter in continuous permafrost-affected forest soils

Pyrogenic organic matter (PyOM) is a product of incomplete combustion during wildfires and an important pool of soil organic carbon (SOC). The dynamics of PyOM and SOC in boreal and permafrost-affected soils are largely unknown, while storing large amounts of global carbon and being vulnerable to climate change. Here, we traced the vertical mobility, allocation in soil fractions and decomposition losses of highly 13C-labeled PyOM and its precursor ryegrass organic matter (grass OM) after two years of in-situ incubation in soil cores installed in the upper 10 cm of continuous (northern sites) and discontinuous to sporadic (southern sites) permafrost-affected forest soils in Northern Canada. At the northern sites, up to three times more PyOM was lost by decomposition (39% of initial) compared to the southern sites (11% of initial). Losses of grass OM were substantial (69–84% of initial) and larger in southern soils. The vertical incorporation was limited and >90% of recovered PyOM and grass OM were found at the applied depth (0–3 cm). The PyOM strongly interacted with mineral surfaces, as indicated by around 40% recovered in the mineral-associated heavy density fractions (<63 μm). Microscale analyses by SEM and NanoSIMS showed that PyOM was mainly allocated towards the fine fraction in a particulate and aggregated form, highlighting the importance of abiotic processes for the incorporation of PyOM in permafrost-affected soils. The grass OM was mainly recovered in the mineral fractions at southern soils with enhanced allocation towards mineral surfaces as well as increased distribution at the microscale after initial decomposition, while it remained as particulate OM in northern soils. Our results highlight that permafrost-affected boreal forest soils are sensitive to fresh PyOM and OM inputs with substantial losses. Especially PyOM persistence depended on site and soil specific properties and not solely on its physico-chemical persistence. The responses are decoupled for PyOM and non-pyrolyzed OM and require a better understanding to evaluate carbon feedbacks of high-latitude soils with global warming and associated shifts in vegetation and wildfire regimes.

Linkages between the molecular composition of dissolved organic matter and soil microbial community in a boreal forest during freeze-thaw cycles.

Soil dissolved organic matter (DOM) plays a vital role in biogeochemical processes. Global warming leads to increased freeze-thaw cycles (FTCs) in boreal forest soils, which can change DOM production and consumption. However, the interactions between the chemical composition of DOM molecules and the microbial communities that drive C decomposition in the context of freeze-thaw are poorly understood. Here, a FTCs incubation experiment was conducted. Combined with pyrolysis gas chromatography-mass spectrometry and high-throughput sequencing techniques, the relationships between DOM chemodiversity and microbial community structure were assessed. Results indicated that both low-frequency (2FTCs) and high-frequency freeze-thaw cycles (6FTCs) significantly increased soil dissolved organic carbon (DOC) contents in the surface (0-10 cm) and subsurface (50-60 cm) soil layers. In the topsoil, FTCs significantly reduced the relative abundance of aromatic compounds, but increased the relative proportions of alkanes, phenols, fatty acid methyl esters (Me) and polysaccharides in the DOM. In the subsuface soil layer, only the relative abundance of Me in the 6FTCs treatment increased significantly. The response of bacterial communities to FTCs was more sensitive than that of fungi, among which only the relative abundance of Gammaproteobacteria increased by FTCs. Moreover, the relative abundance of these taxa was positively correlated with the increment of DOC. Co-occurrence networks confirmed DOM-bacterial interactions, implying that specific microorganisms degrade specific substrates. At class level, Gammaproteobacteria were significantly positively correlated with labile C (polysaccharides and alkanes), whereas other bacterial classes such as Actinobacteria, Alphaproteobacteria, and Thermoleophilia were significantly positively correlated with aromatic compounds in the topsoil. Collectively, FTCs tended to activate DOM and enhance its biodegradability of DOM, potentially hampering DOC accumulation and C sequestration. These findings highlight the potential of DOM molecular mechanisms to regulate the functional states of soil bacterial communities under increased FTCs.

Weathering rates of Ca and Mg related to granitic and gabbro mineralogy in boreal forest soils and the effect of mechanical soil disturbance on weathering release

This study’s aim was to calculate the weathering rates of Ca and Mg for five boreal forest soils in southern Finland on granitic and gabbro containing bedrock. The effect of mineralogy on the total concentrations of Ca and Mg in soil and weathering rates was evaluated. The aim was also to estimate the effect of mechanical soil disturbance related to ploughing on the weathering in the gabbro area. The total concentrations of SiO2, CaO, MgO, and Zr were determined by XRF, and weathering rates of Ca and Mg were determined based on the changes in the CaO, MgO, and Zr concentrations. The weathering rates of Ca+Mg varied 5–38 mmolc m–2 year–1 in the E+B/BC horizons among the plots. Soil disturbance related to ploughing increased the weathering of Ca and Mg largely in the disturbed part of the topmost mineral soil as indicated by the decreasing concentrations of Ca and Mg after mechanical soil disturbance. The weathering input of Ca in the undisturbed soil did not fully replace the Ca output in final whole-tree cutting. The weathering input of Mg in the undisturbed soil was sufficient to replace the lost Mg in stemwood harvesting but not on all the plots the lost Mg in whole-tree harvesting. Weathering rates were higher in the gabbro than the granitic areas.

Strong Responses of Soil Greenhouse Gas Fluxes to Litter Manipulation in a Boreal Larch Forest, Northeastern China

Alterations in plant litter inputs into the soil are expected to significantly affect soil greenhouse gas (GHG) emissions. However, the influence on boreal forest soils is not clear, given the large amount of accumulated soil organic matter that may buffer the impacts from the input of fresh litter. In this study, we conducted a litter manipulation experiment to explore the effects of the litter layer on soil GHG fluxes in a Dahurian larch (Larix gmelinii) forest ecosystem in northeastern China. Three litter treatments were implemented, namely aboveground litter removal (LR), litter double (LD), and unchanged litter input (CK). The associated microclimate, litter characteristics, and soil properties were also measured. The results showed that this larch forest soil acts as a source of CO2 and N2O but acts as a sink for CH4 for all litter manipulation treatments. LD increased the soil CO2 and N2O fluxes by 15% and 34%, while LR decreased them by 8% and 21%, respectively. However, soil CH4 uptake decreased by 34% in LD treatment and increased by 22% in LR treatment, respectively. Litter manipulation treatments can not only affect soil GHG fluxes directly but also, via their effects on soil MBC, NH4+−N, and NO3−−N content, indirectly affect variations in soil CO2, CH4 and N2O fluxes, respectively. Our study highlights the importance of the plant litter layer in regulating soil GHG between the atmosphere and soil in a Dahurian larch forest ecosystem, especially for litter addition. Considering the natural increase in litter quantity over time, this important regulatory function is essential for an accurate estimation of the role of boreal forests in mitigating future climate change.

Interactions between soil organic matter chemical structure and microbial communities determine the spatial variation of soil basal respiration in boreal forests

Interactions between soil organic matter (SOM) composition and microbial communities determine soil basal respiration (BR, SOM-derived CO2). However, few studies have investigated the relative importance of SOM chemical structure, microbial community composition, and soil environmental factors to the spatial variability of soil BR on a large scale. Here, organic and mineral layer soils were collected along a 650-km boreal forest transect in Northeast China. Combining 13C-nuclear magnetic resonance spectroscopy and high-throughput sequencing techniques, SOM chemical structure, microbial community composition, and soil physicochemical properties were determined to investigate the biotic and abiotic drivers of soil BR. Soil BR, microbial communities, and soil physicochemical properties exhibited significant spatial heterogeneity across sites, but the variations were independent of latitude regardless of soil layer. Soil BR rates were significantly higher in the organic layer than in the mineral layer (0.82 vs. 0.31 CO2-C kg−1soil−1 h−1, P < 0.001). Multi-model averaging and partial regression showed that SOM chemical structure exhibited the strongest control on the spatial variation of soil BR (contributing 23.0 %), followed by the organic layer soil C/N ratio (contributing 17 %). However, only soil C/N ratio was the most important predictor of soil BR variation in the mineral layer. Copiotrophic bacteria had a higher impact on soil BR than oligotrophic bacteria. In addition, soil physicochemical properties (i.e. pH, mechanical composition) indirectly affected soil BR through regulating soil microbial composition, especially bacteria. Overall, these results highlighted that SOM chemical structure directly or indirectly affected the microbial decomposition of SOM, and contributed more relative to SOM quality. These findings should be incorporated into ecosystem process models to better predict the response of boreal forest soil C emissions to climate change.

Boreal forest soil biotic communities are affected by harvesting, site preparation with no additional effects of higher biomass removal 5 years post-harvest

The market for forest products has diversified to include biomass energy products sourced from materials that were typically left on-site after harvest. In this study we examined whether intensified biomass removal following harvest will effect site soil biological diversity and metabolic functioning, and how decisions surrounding sampling, in sites prepared with disc-trenching soil scarification, influence the effect assessment of harvesting activities. We compared the influence of harvesting, below-ground biomass removal, stem only versus full tree harvesting, micro-topographic features and seasonal effects on soil biota. We assessed soil biotic changes using potential enzyme activity assays and metabarcoding using targeted primers for four groups (arthropods, bacteria, fungi and broad eukaryotes). We found that harvesting and forest floor removal had strong influences on soil biotic communities, with changes that generally favoured more general soil saprotrophy and reductions in plant-associated organisms. Disc-trenching created differences between micro-topographic features where organic soil was redistributed and trenches, where it was removed. We found no significant difference to how seasonal variation effected communities in harvested sites, compared to unharvested sites. These results show that there is significant site heterogeneity due to disc scarification, and that the sampling micro-topographic features should be considered during study design. Additional studies are needed to assess the effects of higher biomass removal on these features as these sites age.

Water Retention Characteristics of Mineral Forest Soils in Finland: Impacts for Modeling Soil Moisture

Soil hydraulic properties are central for soil quality and affect forest productivity and the impacts of climate change on forests. The water retention characteristics (WRC) of mineral forest soils in Finland are not well known, and practical tools to predict them for hydrological, biogeochemical and forest models are lacking. We statistically analyzed mineral forest soils WRC from over 130 sites in Finland, focusing on the humus layer and main root zone (0–19 cm depth). We showed that mineral forest soils can be grouped into five WRC classes that are well predictable from soil bulk density, organic matter content and clay fraction. However, the majority of the forest soils are hydrologically rather similar. We found that neither topsoil maps nor any combination of open geospatial data were able to predict WRC. Thus, in the absence of site-specific soil data, parameterizing WRC as a function of forest site fertility type was proposed. We demonstrated the approach in soil moisture modeling at a small forest headwater catchment and showed that the soil moisture response to weather conditions is jointly affected by WRC, stand attributes and topography. We showed that drought risks are highest for dense mature forests at nutrient-poor, coarse-textured sites and lower for young stands on peatlands and lowland herb-rich sites with groundwater influence. The results improve hydrological predictions for Finnish forests, and the open dataset can contribute to the larger synthesis and development of boreal forest soil pedo-transfer functions.

Fire decreases gross mineralization rate but does not alter gross nitrification rate in boreal forest soils

Wildfire has several environmental and ecological effects on soil biogeochemistry, including the nitrogen (N) cycle in forest ecosystems. In this study with 30 days of incubation, we examined the effect of a fire event on changes in soil properties, especially gross N mineralization and nitrification processes in boreal forest soils. We also tested how the addition and removal of fire-derived natural pyrogenic carbon (PyC, 1.5%) affected N transformation processes. After incubation, the concentrations of NH4+-N, NO3−-N, total N, and dissolved organic N were largely enhanced in the burnt soils compared to the unburnt soils. Fire significantly decreased gross N mineralization rates from 6.88 μg N g−1 d−1 to 0.87 μg N g−1 d−1 and NH4+ immobilization rates from 6.04 μg N g−1 d−1 to 0.55 μg N g−1 d−1 but did not significantly affect gross N nitrification rates or NO3− immobilization rates on day 30. The removal from the burnt soil or addition to the unburnt soil of PyC had no significant effect on N transformation. The abundances of nirK, nosZ I, narG, and nifH in the burnt soils were the highest across the different treatments. Our study revealed that fire decreased gross N mineralization processes, which was associated with enhanced abundances of N-cycling genes (nirK, nosZ I and narG) in the boreal forest soils.

Modelling the Rhizosphere Priming Effect in Combination with Soil Food Webs to Quantify Interaction between Living Plant, Soil Biota and Soil Organic Matter.

A model of rhizosphere priming effect under impact of root exudate input into rhizosphere soil was developed as an important process of the plant-soil interaction. The model was based on the concept of nitrogen (N) mining, compensating for the N scarcity in exudates for microbial growth by accelerating SOM mineralisation. In the model, N deficiency for microbial growth is covered (“mined”) by the increased SOM mineralisation depending on the C:N ratio of the soil and exudates. The new aspect in the model is a food web procedure, which calculates soil fauna feeding on microorganisms, the return of faunal by-products to SOM and mineral N production for root uptake. The model verification demonstrated similar magnitude of the priming effect in simulations as in the published experimental data. Model testing revealed high sensitivity of the simulation results to N content in exudates. Simulated CO2 emission from the priming can reach 10–40% of CO2 emission from the whole Ah horizon of boreal forest soil depending on root exudation rates. This modeling approach with including food web activity allows quantifying wider aspects of the priming effect functioning including ecologically important available N production.

Genotypes exhibit no variation in precision foraging in mycorrhizal Norway spruce seedlings

AimsFine roots, that comprise the adjustable part of the root system, are important in spatially heterogeneous boreal forest soils. We investigated the soil exploring patterns of Norway spruce (Picea abies) seedlings of equal height belonging to families representing two contrasting growth phenotypes that have shown fast and slow growth rates in long-term experiments. We hypothesised that seedlings of the fast-growing phenotype would show a more explorative root growth strategy, intense branching, and root proliferation in response to the nutrient patch, and that slow-growing phenotypes would be more tolerant to drought stress.MethodsSeedlings were grown in flat Perspex microcosms with a clod of humus placed in the mid-bottom part of each microcosm for eight months. The order-based and functional classification, branching topology, and size of seedling root systems were studied with WinRHIZO™ image-analysis software and root exploration patterns with LIGNUM-model simulations. In addition, transpiration, stomatal conductance, net assimilation rate responses were measured.ResultsNo differences were found in the early foraging of roots for the humus clod nor net assimilation rate and transpiration between the phenotype groups. Seedlings were favouring exploitation over exploration in the early phases of development regardless of growth phenotype group. However, in fast-growing phenotypes, the main roots were longer, and the lateral root pool favoured long and bifurcated laterals that formed larger absorptive root area.ConclusionsOur results indicate that in nutrient-poor conditions, better growth of lateral roots precedes future differences in the aboveground growth rate of Norway spruce.

Mapping the relative abundance of soil microbiome biodiversity from eDNA and remote sensing

Although an enormous number of plant and animal species have been directly observed and recorded in online databases, the spatial variation in the composition of the microbiome remains relatively largely unknown. In this study, for the first time, we demonstrate mapping of the relative abundance of the soil microbiome for three terrestrial ecosystems across North America (savanna, boreal and tundra) using airborne image spectroscopy and environmental DNA (eDNA) data. We identified field plots of publicly available eDNA data co-occurring with AVIRIS-NG hyperspectral imagery. An eDNA processing pipeline was developed to generate a consistent profile of the relative abundance for thousands of microbiome operational taxonomic units (OTU) and 225 microbiome families. Using Linear Discriminate Analysis (LDA) scores for the eDNA data, we identified 81 families with the greatest explanatory power based on the community composition between the three ecosystems. A case study example demonstrates our conceptual approach by selecting a dominant and functionally important bacterial family for each ecosystem, with each family representing a specific biomarker. A partial least squares regression (PLSR) was applied to spatially predict the relative abundance of each bacteria family (in the boreal, tundra, and savanna ecosystems) from the hyperspectral imagery. For the boreal, Pseudomonadaceae is shown to be a dominant family taxon, as it is a saprophytic family specialized in decomposing the dense organic matter of boreal forest soils. Members of an understudied family of Acidobacteria, so far indicated as AKIW659, are abundant in acidic Arctic soils and peat bogs. Finally, the Micromonosporaceae are dominant and functionally important in drier regions with grass-tree dominated woodlands, being a member of Actinobacteria with a high relative abundance in soils with high carbon content and nitrate leaching. We demonstrate for the first time how the spatial prediction of relative abundance of these bacteria taxa based on remote sensing, showing patterns of the soil microbiome biodiversity and ecosystem function within and across the three ecosystems.

Niche differentiation of atmospheric methane-oxidizing bacteria and their community assembly in subsurface karst caves.

Karst caves are recently proposed as atmospheric methane sinks in terrestrial ecosystems. Despite of the detection of atmospheric methane-oxidizing bacteria (atmMOB) in caves, we still know little about their ecology and potential ability of methane oxidation in this ecosystem. To understand atmMOB ecology and their potential in methane consumption, we collected weathered rocks and sediments from three different caves in southwestern China. We determined the potential methane oxidization rates in the range of 1.25 ± 0.08 to 1.87 ± 0.41 ng CH4 g-1 DW h-1 , which are comparable to those reported in forest and grassland soils. Results showed that alkaline oligotrophic caves harbour high numbers of atmMOB, particularly upland soil cluster (USC), which significantly correlated with temperature, CH4 and CO2 concentrations. The absolute abundance of USCγ was higher than that of USCα. USCγ-OPS (open patch soil) and USCγ-SS (subsurface soil) dominated in most samples, whereas USCα-BFS (boreal forest soil) only predominated in the sediments near cave entrances, indicating niche differentiation of atmMOB in caves. Overwhelming dominance of homogenous selection in community assembly resulted in convergence of atmMOB communities. Collectively, our results demonstrated the niche differentiation of USC in subsurface alkaline caves and their non-negligible methane-oxidizing potential, providing brand-new knowledge about atmMOB ecology in subsurface biosphere.