To Örebro University

The effectiveness of biochar amendment for remediation purposes depends on many factors related to the biochar and the contaminated site. Therefore, each application must be evaluated site-specifically. To facilitate full-scale implementation, more information from field studies on biochar-amended contaminated sites, as well as cost-effective approaches to evaluate the remediation efficacy of specific biochar materials are needed. We studied the effects of biochar and peat on metal solubility and bioavailability in a contaminated soil in a fully factorial field trial. The biochar was produced from wood via gasification in a floating fixed-bed reactor at 750 degrees C. Soil solutions from field-installed lysimeters, grass (Lolium perenne L), and earthworms (Eisenia fetida) were analyzed. In addition, a standardized batch leaching test (ISO 21268-2:2019) was performed to evaluate its feasibility to mimic soil solution concentrations. The results showed that biochar generally reduced the solubility and uptake of cationic metals. In situ solubility of Cu and Hg was reduced more than 80%, and Zn up to 70%. Soil solution concentrations of Cr increased in biochar-amended soils, but this effect was reduced by peat. Peat had small effects on in situ solubility of other metals. For cations, the batch test showed the same trends as the soil solution, with biochar decreasing solubility. However, mobilization of colloids during shaking in the batch test induced artefacts, leading to an overestimation of the solubility of some metals, especially Pb and Hg, an effect that was enhanced by peat applications.

Ericaceous understory shrubs and ericoid mycorrhizal fungal communities are ubiquitous in boreal forests, and their interactions with ectomycorrhizal and saprotrophic fungi may determine organic matter dynamics in forest soils. We followed decomposition of pine needle litter and mor-layer humus over 3 yr in a factorial shrub removal- and pine root exclusion experiment in an old-growth Scots pine (Pinus sylvestris) forest, to evaluate effects of fungal guilds on mass loss. Litter mass loss was 23% greater when ectomycorrhizal fungi were excluded suggesting increased saprotrophic activity, independently of ericoid shrub presence. However, this 'Gadgil effect' was only found after 17 months following a summer drought. By contrast, humus mass loss was overall stimulated by ectomycorrhizal fungi, while ericoid mycorrhizal shrubs appeared to counteract this effect, potentially caused by simultaneous addition of recalcitrant organic matter and inhibition of ectomycorrhizal decomposers. We conclude that competitive saprotrophic-ectomycorrhizal fungal interactions may slow early-stage litter decomposition, but this effect was small and inconsistent. Furthermore, interactions between ecto- and ericoid mycorrhizal guild members appear to determine the late-stage organic matter balance of boreal forest humus.

Mycorrhizal fungi play an integral role in nutrient and carbon cycling in soils, which may be especially important in the Arctic, one of the world's most soil carbon-rich regions. Large mammalian herbivores can influence these fungi through their impacts on vegetation and soil conditions, however the strength and prevalence of these interactions in the Arctic is still uncertain. We collected soils from 15 large mammal exclusion experiments across the Arctic. We sequenced both ITS regions and partial SSU regions using two sets of amplicons to determine the composition of soil mycorrhizal fungal communities. This allowed us to assess how these communities are impacted by exclusion of large mammalian herbivores, plant communities, and climate and soil properties. Large mammalian herbivore exclusion had a significant impact on the arbuscular mycorrhizal (AM) community dissimilarity between sites. The AM community was also influenced by growing season temperature and pH, which may indicate that conditions are becoming more favourable for these species in some Arctic communities. Large herbivore exclusion did not have a coherent impact on ectomycorrhizal and ericoid mycorrhizal fungal community dissimilarity, which were primarily correlated with delta 15N signature in the soil, rather than herbivory, climate, or plant functional types. The consistent detection of arbuscular mycorrhizal fungi identified highlights the need for more thorough evaluations of these communities and their role in Arctic carbon and nutrient dynamics, as these fungi are currently understudied in the Arctic.

Soil contaminants may restrict soil functions. A promising soil remediation method is amendment with biochar, which has the potential to both adsorb contaminants and improve soil health. However, effects of biochar amendment on soil-plant nitrogen (N) dynamics and N cycling microbial guilds in contaminated soils are still poorly understood. Here, a metal- and polycyclic aromatic hydrocarbon (PAH) contaminated soil was amended with either biochar (0, 3, 6 % w/w) and/or peat (0, 1.5, 3 % w/w) in a full-factorial design and sown with perennial ryegrass in an outdoor field trial. After three months, N and the stable isotopic ratio δ15N was measured in soil, roots and leaves, along with microbial responses. Aboveground grass biomass decreased by 30 % and leaf N content by 20 % with biochar, while peat alone had no effect. Peat in particular, but also biochar, stimulated the abundance of microorganisms (measured as 16S rRNA gene copy number) and basal respiration. Microbial substrate utilization (MicroResp™) was altered differentially, as peat increased respiration of all carbon sources, while for biochar, respiration of carboxylic acids increased, sugars decreased, and was unaffected for amino acids. Biochar increased the abundance of ammonia oxidizing archaea, while peat stimulated ammonia oxidizing bacteria, Nitrobacter-type nitrite oxidizers and comB-type complete ammonia oxidizers. Biochar and peat also increased nitrous oxide reducing communities (nosZI and nosZII), while peat alone or combined with biochar also increased abundance of nirK-type denitrifiers. However, biochar and peat lowered leaf δ15N by 2-4 ‰, indicating that processes causing gaseous N losses, like denitrification and ammonia volatilization, were reduced compared to the untreated contaminated soil, probably an effect of biotic N immobilization. Overall, this study shows that in addition to contaminant stabilization, amendment with biochar and peat can increase N retention while improving microbial capacity to perform important soil functions.

Tree growth in boreal forests is driven by ectomycorrhizal fungal mobilisation of organic nitrogen and mineral nutrients in soils with discrete organic and mineral horizons. However, there are no studies of how ectomycorrhizal mineral weathering and organic nitrogen mobilisation processes are integrated across the soil profile. We studied effects of organic matter (OM) availability on ectomycorrhizal functioning by altering the proportions of natural organic and mineral soil in reconstructed podzol profiles containing Pinus sylvestris plants, using 13 CO2 pulse labelling, patterns of naturally occurring stable isotopes (26 Mg and 15 N) and high-throughput DNA sequencing of fungal amplicons. Reduction in OM resulted in nitrogen limitation of plant growth and decreased allocation of photosynthetically derived carbon and mycelial growth in mineral horizons. Fractionation patterns of 26 Mg indicated that magnesium mobilisation and uptake occurred primarily in the deeper mineral horizon and was driven by carbon allocation to ectomycorrhizal mycelium. In this horizon, relative abundance of ectomycorrhizal fungi, carbon allocation and base cation mobilisation all increased with increased OM availability. Allocation of carbon through ectomycorrhizal fungi integrates organic nitrogen mobilisation and mineral weathering across soil horizons, improving the efficiency of plant nutrient acquisition. Our findings have fundamental implications for sustainable forest management and belowground carbon sequestration.

Societal Impact Statement:

Efficient mitigation of climate change requires predictive models of forest ecosystems as sinks for atmospheric carbon. Mycorrhizal fungi are drivers of soil carbon storage in boreal forests, yet they are typically excluded from ecosystem models, because of a lack of information about their growth and turnover. Closing this knowledge gap could help us better predict future responses to climate change and guide policy decisions for sustainable management of forest ecosystems. This study provides new estimates of the production and turnover of mycorrhizal mycelial biomass and necromass. This information can facilitate the integration of mycorrhizal fungi into new predictive models of boreal forest soils.

Summary:

• In boreal forests, turnover of biomass and necromass of ectomycorrhizal extraradical mycelia (ERM) are important for mediating long-term carbon storage. However, ectomycorrhizal fungi are usually not considered in ecosystem models, because data for parameterization of ERM dynamics is lacking.
• Here, we estimated the production and turnover of ERM biomass and necromass across a hemiboreal Pinus sylvestris chronosequence aged 12 to 100 years. Biomass and necromass were quantified in sequentially harvested in-growth bags, and incubated in the soil for 1-24 month, and Bayesian calibration of mathematical models was applied to arrive at parametric estimates of ERM production and turnover rates of biomass and necromass.
• Steady states were predicted to be nearly reached after 160 and 390 growing season days, respectively, for biomass and necromass. The related turnover rates varied with 95% credible intervals of 1.7-6.5 and 0.3-2.5 times yr-1, with mode values of 2.9 and 0.9 times yr-1, corresponding to mean residence times of 62 and 205 growing season days.
• Our results highlight that turnover of necromass is one-third of biomass. This together with the variability in the estimates can be used to parameterize ecosystem models, to explicitly include ERM dynamics and its impact on mycorrhizal-derived soil carbon accumulation in boreal forests.

Understanding the environmental and climate influence on the Holocene stable isotope record in peat is essential for applying the proxies in the paleoenvironmental reconstruction of lake and bog ecosystems. Here, we report total organic carbon (TOC), TOC to total nitrogen (N) ratio (C/N), and stable isotope ratios of organic carbon (S13C) and nitrogen (S15N) of bulk sediment and peat organic matter (OM) from a radiocarbon-dated core collected from Stavs & aring;kra bog in southern Sweden. Changes in the TOC, C/N, S13C and S15N between 12 ka and 10.4 ka BP represent shifts in the OM source from aquatic primary producers to terrestrial swamp vegetation and pinpoint the transition of a highly productive water body into a reed swamp. The variation in S13C values and increase in peat accumulation rate (AR) from 10.8 ka BP imply a rapid transition of the aquatic body into a peatforming swamp and a shift from aquatic to atmospheric CO2 as the source of carbon (C) to the vegetation at the site. A sharp drop in N and S15N in the reed swamp peat may indicate fixed-N in the soil as the source of N to the growing vegetation. Heavier S13C and higher C/N ratio at the bottom of wood carr peat may reflect fire events. The rapid increase in peat AR between 7 ka and 6.5 ka BP suggests increased peat deposition under warmer and drier conditions. Lighter S15N and S13C, lower N, and TOC concentrations between 1.1 ka and 0.8 ka BP likely suggest warmer conditions related to the 'Medieval Climate Anomaly'. The research demonstrates the successful application of stable C and N isotope ratios of bulk peat in palaeoenvironment and palaeoecological interpretations.

Although coal ashes (CA) can be converted into an eco-friendly product through vermicomposting, the utility of vermiconverted CA in agriculture still needs to be explored. Therefore, the feasibility of vermicomposted tea industry coal ash (VCA) as an alternative nutrient source for cabbage (Brassica oleracea, var. Capitata) production was evaluated through an on-field experiment in alluvial soil. Two types of vermicomposts were prepared using Eisenia fetida (VCAE) and Lampito mauritii (VCAL) and were applied in different combinations with chemical fertilizers. The results revealed a significant increase in nutrient availability (nitrogen, phosphorus, and potassium) in the soil treated with VCA, alongside a concurrent build-up of soil organic carbon stocks, activation of microbial growth, and enhanced soil enzyme activity. Additionally, VCA application substantially reduced toxic metals in the soil, thereby improving soil health and promoting the uptake of essential nutrients (nitrogen, phosphorus, potassium, iron, manganese, copper, and zinc) in cabbage. Correspondingly, VCA application reduced the bioaccumulation of potentially toxic metals (chromium, lead, and cadmium) from coal ash, ensuring safer food production. Notably, a 25 % substitution of chemical fertilizers with VCA and farmyard manure (FYM) led to a two-fold increase in the growth and productivity of cabbage. The economic assessment also indicated that large-scale and sustainable recycling of toxic tea industry coal ash in agriculture is feasible. Hence, by integrating VCA-based nutrient management into agricultural practices, developing nations can take significant strides toward achieving circular economy objectives while addressing environmental challenges associated with CA disposal.

Heavy metal pollution has important effects on ecosystem nitrogen (N) cycling, but factors driving differences between the direction, onset and intensity of responses are poorly understood. We used two contrasting grassland soils to examine the effects of copper (Cu) on the abundance and activity of N cycling microbial guilds and plant responses, including plant delta 15N as an integrator of the N cycle. A low pH sandy soil and a high pH sandy loam soil were aged two years in outdoor mesocosms with copper (Cu) treatments of background, 200, 400 or 1000 mg kg-1 Cu. After two years, increased Cu treatments resulted in significantly lower abundances of ammonia oxidizing archaea, Nitrospira nitrite oxidizing bacteria (NOB), potential ammonia oxidation rates and plant biomass in both soils. Plants possessed significantly increased N content and enriched shoot delta 15N in with higher Cu in both soils. While abundances of ammonia oxidizing bacteria were unaffected by Cu, the response among Nitrobacter NOB and denitrifiers and plant delta 13C differed between the two soils. In contrast to plants, differences in the intensity and direction of microbial guild responses were not explained by increasing soluble Cu but rather shaped by soil type. This indicates that the two soils differed in metal bioavailability to plants, as well as harbored microbial communities with inherent differences in metal sensitivity. Furthermore, effects of increasing Cu on microbial N-cycling guilds became more apparent with longer incubation time, emphasizing the importance of long-term studies to assess important ecosystem effects of Cu contamination. Taken together, we conclude that a combination of plant and microbial responses can give better insights on how Cu is affecting the N cycle in polluted soils.

Ectomycorrhizal fungi (EMF) are important components of soil microbial communities, and EMF biomass can potentially increase carbon (C) stocks by accumulating in the soils as necromass and producing recalcitrant structures. EMF growth depends on the C allocated belowground by the host trees, and the nutrient limitation on tree growth is expected to influence this allocation. Therefore, studying EMF production and understanding the factors that regulates it in natural soils are important to understand C cycling in forests.

Fungal mycelium collected from ingrowth mesh bags is commonly used to estimate EMF biomass, but these measurements might not reflect the total EMF production since turnover rates of the hyphae are not considered. Here we estimated EMF production and turnover in response to P fertilization (applied as superphosphate) in a Norway spruce forest where nitrogen (N) deposition has resulted in phosphorus (P) limitation of plant production by using a combination of mesh bags with different incubation periods and with Bayesian inferences. To test how localized patches of N and P influence EMF production and turnover we amended some bags with a nitrogen source (methylene urea) or P source (apatite). Additionally, the Bayesian model tested the effect of seasonality (time of mesh-bag harvesting) on EMF production and turnover.

We found that turnover of EMF was not affected by P fertilization or mesh-bag amendment. P fertilization had a negative effect on EMF production in all the mesh-bag amendments, suggesting a reduced belowground C allocation to the EMF when P limitation is alleviated. Apatite amendment significantly increased EMF biomass production in comparison with the pure quartz bags in the control plots but not in the P-fertilized plots. This indicates that P-rich patches enhance EMF production in P-limited forests, but not when P is not limiting. Urea amendment had a generally positive effect on EMF production, but this was significantly reduced by P fertilization, suggesting that a decrease in EMF production due to the alleviated P limitation will affect N foraging. Seasonality had a significant effect on EMF production, and the differences registered between the treatments were higher during the warmer months and disappeared at the end of the growing season.

Many studies highlight the importance of N for regulating belowground C allocation to EMF in northern coniferous forests, but here we show that the P status of the forest can be equally important for belowground carbon allocation to EMF production in areas with high N deposition.