This study emphasizes the critical role of soil porosity as an environmental variable influencing infiltration, compaction, runoff, and erosion, which are inversely related to bulk density. An analysis of topsoil porosity across Europe (0-20 cm) was conducted using data from the LUCAS monitoring program, focusing on the fine earth fraction of soils. The conversion from bulk density to porosity-more intuitive for hydrological studies-requires knowledge of the particle density of both mineral and organic components, which is often lacking. A novel method was developed to estimate the particle density of organic matter using stoichiometric datasets from various land use types, resulting in an EU LUCAS average soil particle density of 2.53 g cm-3. The generated fine earth porosity map aligns with high porosity areas in Northern Europe's peatlands and Central Europe's forests, providing insights into soil densification processes linked to compaction from traffic or organic matter depletion due to land use changes. This understanding is crucial for assessing compaction and erosion risk. (c) 2026 International Research and Training Center on Erosion and Sedimentation and China Water and Power Press. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

ABSTRACT International commitments advocate large‐scale forest restoration as a nature‐based solution to climate change mitigation through carbon (C) sequestration. Mounting evidence suggests that mixed compared to monospecific planted forests may sequester more C, exhibit lower susceptibility to climate extremes and offer a broader range of ecosystem services. However, experimental studies comprehensively examining the control of tree diversity on multiple C stocks and fluxes above‐ and belowground are lacking. To address this gap, we leverage data from the Sardinilla experiment in Panama, the oldest tropical tree diversity experiment, which features a gradient of one‐, two‐, three‐ and five‐species mixtures of native tree species. Over 16 years, we measured multiple above‐ and belowground C stocks and fluxes, ranging from tree aboveground C, over leaf litter C production, to soil organic carbon (SOC). We show that tree diversity significantly increased aboveground C stocks and fluxes, with a 57% higher gain in aboveground tree C in five‐species mixtures compared to monocultures (35.7 ± 1.8 vs. 22.8 ± 3.4 Mg C ha −1 ) 16 years after planting. In contrast, we observed a net reduction in SOC (on average −11.2 ± 1.1 Mg C ha −1 across diversity levels) and no significant difference in SOC 3 stocks (the predominantly tree‐derived, i.e., C 3 plant‐derived SOC fraction) between five‐species mixtures and monocultures (13.0 ± 0.9 vs. 15.1 ± 1.3 Mg C ha −1 ). Positive tree diversity effects persisted despite repeated climate extremes and strengthened over time for aboveground tree growth. Structural equation models showed that higher tree growth in mixtures enhanced leaf litter and coarse woody debris C fluxes to the soil, resulting in a tightly linked C cycle aboveground. However, we did not observe significant links between above‐ and belowground C stocks and fluxes. Our study elucidates the mechanisms through which higher tree diversity bolsters the climate mitigation potential of tropical forest restoration. Restoration schemes should prioritize mixed over monospecific planted forests.

To study long-term impacts of nutrient addition on carbon sequestration capacity, we investigated changes in vegetation and ecosystem CO2 exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) and in non-fertilized control plots for 13-18 years. The vegetation structure and species composition were measured in all treatments mid July 2001-2018 (14 measurement years) using a point intercept method. Gross photosynthesis, ecosystem respiration, and net CO2 exchange were measured weekly during June–August 2001-2016 (7 measurement years, usually every two years) using climate-controlled chambers. Using Bayesian approach, we analyzed whether there were changes over time in vegetation and ecosystem CO2 exchange and whether those trends differed between treatments. We found that shrubs had become taller and more abundant at the unfertilized plots during the 18 study years likely owing to warmer summers and a drying trend that favor shrubs. At the fertilized plots, the increase in shrub height was greater and faster than in unfertilized plots, and the addition of PK with N further accelerated growth of the shrub canopy. Among the dwarf shrubs, only Chamaedaphne calyculata benefitted from the fertilization. No change towards more gramineous vegetation was observed. Because the plants at the bog are N-P co-limited rather than N-limited, PK addition alleviated growth limitation. Sphagnum cover decreased with the increasing nutrient load. Ecosystem respiration increased in all treatments, but it increased faster and more in fertilized plots than in unfertilized plots. In all treatments, increases in ecosystem respiration resulted in less net CO2 uptake during the recent ten years (since 2008), because gross photosynthesis rates did not compensate for increases in ecosystem respiration. In general, the magnitude of this trend of reduced net C sink potential did not differ markedly in unfertilized from fertilized plots. These CO2 flux trends could be explained by changes in nutrient availability, a larger proportion of nongreen biomass in dense stands and enhanced peat decomposition. Our long-term field experiment revealed that ecosystem responses to the combination of nutrient addition and drying must be considered when evaluating the impact of climate change on the carbon sink potential of peatlands.

Peatlands store about one third of the global soil organic carbon. This carbon storage is partly controlled by the availability of nitrogen (N) and phosphorus (P) in peat, which affects primary productivity, decomposition, plant community composition, and microbial community composition in these ecosystems. While extensive research has been conducted on the N cycle in peatlands, much less is known about the biogeochemistry of P. To date, little is known about how an increase of atmospheric N deposition affects the availability and biogeochemistry of P in peat. To fill this gap of knowledge, we studied the effect of increased N additions on soil P pools in an ombrotrophic bog in Canada. For this purpose, soil samples were taken from a 20 year old fertilization trial at Mer Bleue Bog in south-eastern Ontario and subjected to Hedley sequential fractionation. In unfertilized peat, P concentrations were highest in the available and highly recalcitrant pools, with little between them. This U-shaped distribution of P along the gradient of availability contrasts with established patterns in mineral soils. In plots which received PK and NPK fertilizers, concentrations of both available P and highly recalcitrant P doubled. In plots receiving N fertilization alone, available and total P concentrations decreased, which may indicate increased demand for P by plants and microorganisms when N status is high. In all plots receiving fertilizer, concentrations of highly recalcitrant P increased, which may indicate increased decomposition of peat. In addition, fertilization led to changes aboveground. Chamaedaphne calyculata leaves in plots receiving PK and NPK were enriched in P compared to Chamaedaphne calyculata leaves in unfertilized plots and plots receiving N alone. These findings indicate, that formerly N limited peatlands may become P limited due to anthropogenically enhanced atmospheric nitrogen depositions which may impact their potential to store soil organic carbon in the future.

Predicting the carbon dynamics of northern peatlands requires adequate representation of the vegetation phenology in terrestrial biosphere models. In this study, we analyzed the relative importance of various environmental controls to explain the start of the growing season through photosynthetic CO2 uptake for a temperate continental bog; accordingly, we used a multiyear measured dataset comprising eddy covariance (EC), supporting environmental measurements, and a digital image archive obtained using repeat photography. The vegetation in the studied bog is dominated by “evergreen” shrubs and mosses. The vegetation phenological indices data, including enhanced vegetation index, normalized difference vegetation index, and green chromatic coordinate, showed high correlations with the gross primary productivity (GPP) of the ecosystem obtained from EC measurements, near-surface soil temperatures, and the growing degree-day sum (∑GDD). We developed a new phenology scheme in the process-based CoupModel using ∑GDD to represent the gradual greening of the evergreen shrubs that regulate spring photosynthesis turn-on and increase. The new model simulates the earlier photosynthesis turn-on of the mosses and photosynthesis onset of the shrubs from days with ∑GDD = 50°C. Model simulations incorporating the new phenology subroutine for two vegetation layers (shrubs and mosses) show improved agreement with the daily EC-derived GPP. Our results show that when the spring phenology is not explicitly factored in, the CoupModel overestimates GPP by 24% and MODIS GPP by 45% at the end of the spring season. The results from this study are expected to advance our understanding of ecosystem dynamics and provide a foundation for refining ecosystem models to better capture the intricate interplay between phenology, carbon dynamics, and environmental conditions.

Peat is used as the chief ingredient of growing media in horticulture. The high cation exchange capacity, water retention capacity, low bulk density, and appropriate physical properties make peat-based growing media desirable for horticulture. Peat in its natural form is acidic and low in nutrient composition. Therefore, for suitability as a growing media, peat is mixed with liming agents, nutrients, surfactants, perlite among several other possible additives. Using lab incubations, we assessed the change in soil biogeochemistry and CO2 fluxes because of horticultural additives. We obtained samples of raw peat and additive mixed growing media (n=52) from four different peat extraction companies in Canada. Our analysis shows that the key soil biogeochemical parameters C: N ratio, pH, dissolved organic carbon, bulk density, C content differs significantly (p<0.01) between raw peat and growing media. There is a more than a two-fold increase in CO2 from growing media as compared to raw peat. Further experiment showed the longer-term contribution of carbonates borne CO2 to the total flux. IPCC (2007) calculates that all C from harvested peat is lost in the atmosphere in the first year. However, our initial results estimate less than 10% of peat C loss in the first year from growing media. Although the influence of horticultural additives in C loss from peat is significant, the current accounting from IPCC is an overestimation.

We examined how changes in plant-fungal relationships induced by atmospheric nitrogen (N) deposition alter nutrient limitation and carbon sequestration in two main types of peatlands, bogs and fens. The study was carried out at three of the longest running nutrient addition experiments on peatlands: Whim Bog, United Kingdom, Mer Bleue Bog, Canada, and Degerö Stormyr Fen, Sweden. The treatments receive an additional load of 1.6-6.4 N g m-2 y-1 either as ammonium, nitrate, or ammonium nitrate with or without phosphorus (P) and potassium, alongside with unfertilized controls. We determined the peak season aboveground biomass production and coverage of vascular plants using the point intercept method and measured fine roots production rates using the ingrowth core method. The ingrowth roots were also studied for amount of the root-associated fungi based on ergosterol and chitin concentrations (living and dead fungal mass indicators). In addition, we sampled fine roots from ericoid mycorrhizal shrubs and microscopically quantified them for abundance of fungal colonization as well as measured their potential to produce a set of hydrolytic enzymes degrading organic matter. The leaves of dominant vascular plants were analyzed their isotopic δ15N patterns and nutrient contents under different nutrient addition treatments. Long-term nutrient addition increased foliar δ15N of shrubs, suggesting that ericoid mycorrhizal fungi were less important for plant N supply with increasing N load. Under high inorganic N availability, the plant biomass allocation shifted from belowground to aboveground at the two shrub-dominated bog sites: Mer Bleue and Whim, but not at the wet sedge dominated Degerö Stormyr. Unexpectedly, mycorrhizal colonization rates did not change significantly, but the presence of endophytic fungal mycelia in ericoid roots as well as ergosterol and chitin content in all fine roots generally increased under nutrient load. Interestingly, high doses of ammonium alleviated N deficiency in ericoid shrubs, whereas low doses of ammonium and nitrate improved plant P nutrition, indicated by the lowered foliar N:P ratios. Shrub root acid phosphatase activities correlated positively with foliar N:P ratios, suggesting enhanced P uptake as a result of improved N nutrition. Collectively, altered biomass allocation to roots and fungi, altered functionality of root associated fungi and altered plant reliance on nutrient uptake systems as well as altered function of roots and their associated fungi in degrading organic matter suggest changes in the quantity and quality of carbon input to peat soils under nitrogen load. The study revealed that the responses depend on the dose and form of N added and interestingly may interact with uptake of other nutrients. The plant-fungal feedbacks also seem to differ between the two functionally and structurally distinct peatland types.

Northern peatlands are globally important carbon (C) and nitrogen (N) sinks due to slow decomposition rates resulting in long‐term organic matter accumulation. Despite their large N storage, peatlands depend on sources of bio‐available N to sustain their biomass production. Di‐nitrogen (N2) fixation represents an important biological N source in ombrotrophic bogs, but its environmental controls are still poorly understood. We examined seasonal and spatial variability of Sphagnum‐associated N2 fixation across a hydrological transect (hummock‐hollow‐beaver pond edge) in a temperate ombrotrophic bog. We measured N2 fixation in live Sphagnum plants by acetylene reduction assay calibrated with a 15N2 tracer method, bi‐weekly, from May to November over two growing seasons. We found that N2 fixation increased with soil temperature at 5 cm in the living Sphagnum mat explaining the seasonal variability in N2 fixation. Peak N2 fixation rates occur in mid‐August, when N2 fixation rates are about 10 times larger than during the shoulder seasons (May and November). Spatially, N2 fixation was larger in wetter Sphagnum with larger gravimetric water content in Sphagnum. This relationship was most pronounced in the peak growing season when N2 fixation rates were the highest. Finally, we estimated that the Mer Bleue bog receives around 0.3 g N m−2 annually through Sphagnum‐associated N2 fixation, which accounts for about a fourth of the N accumulated annually into Sphagnum. Future contributions from Sphagnum‐associated N2 fixation to N budgets in peatlands will depend on temperature and moisture changes which have contrasting effects on N2 fixation rates.

A Correction to this paper has been published: https://doi.org/10.1038/s41558-021-00991-1.

Atmospheric nitrogen (N) deposition is increasing owing to fossil fuel burning and agriculture. In nutrient limited peatland ecosystems, the excess of reactive N has been found to increase vascular plant growth, but decrease Sphagnum growth. Higher vascular plant abundance and higher nutrient content alter decomposability of plant litter. These changes are likely to affect net imbalance of production and decomposition and thus carbon (C) accumulation in peatlands, which store about a third of global soil C. We studied whether the vegetation feedbacks of N deposition lead to stronger or weaker C sink in nutrient-poor peatlands. We investigated vegetation and ecosystem CO2 exchange at two of the longest-running nutrient addition experiments on peatlands, Mer Bleue Bog, Canada and Degerö Stormyr poor fen, Sweden that have been fertilized with NH4NO3 (2-15 times ambient annual wet deposition) for 12-23 years. Gross photosynthesis, ecosystem respiration and net CO2 exchange were measured weekly during June-August using chambers. To examine vegetation changes with increasing N influx, we determined the peak growing season aboveground biomass and coverage of vascular plants using the point intercept method. After 12-23 years of nutrient addition, the two sites revealed contrasting patterns: At Mer Bleue the highest nutrient additions were associated with up to 3-fold net CO2 uptake potential than in the control, whereas N addition treatments at Degerö Stormyr showed close to zero net CO2 uptake potential, only 0.3 fold compared to the control. The stronger C sink potential at Mer Bleue was mainly due to up to 50% increase in the gross photosynthesis and a diminished C sink potential at Degerö Stormyr due to down to 40 % lower gross photosynthesis. Ecosystem respiration showed similar trends at both peatlands: the rates were unaltered or increased to a lesser extent under N load. At both sites, the vegetation structure had changed remarkably. Most of the N addition treatments showed an increase of up to 90% in total vascular aboveground plant abundance and a concomitant loss of Sphagnum. At Mer Bleue along with the decrease in Sphagnum cover, the plots under highest N additions had become wetter, counterbalancing the impact of dry summer conditions in the study year whereas at Degerö Stormyr long term treatments did not alter wetness of the site. Thus, the contrasting C sink responses to long term N load may be explained by the type of vegetation and the water table depth. Shrubs were strong competitors at the dry Mer Bleue Bog while sedges had gained in abundance under N load at the wetter Degerö Stormyr. Our bog-fen comparison emphasizes the value of the long-term experiments in examining the ecosystem response of peatlands to N deposition, possible nonlinear responses and whether the key feedback mechanisms to ecosystem C sink potential differ in two main types of peatlands.

Along the southern limit of permafrost in northwestern Canada rising air temperatures have caused widespread land cover changes at unprecedented rates. A prominent change includes thermokarst wetland expansion at the expense of black spruce-dominated boreal forest stands due to the permafrost thaw-induced collapse of peat plateaus. We present a multi-year (2013 – 2017) net ecosystem carbon (C) balance (NECB, g C m-2year-1) at Scotty Creek near Fort Simpson, NT. The highly fragmented study site is dominated by permafrost-free wetlands and forested permafrost peat plateaus. Eddy covariance measurements of net ecosystem carbon dioxide (CO2) and methane (CH4) exchanges (2013 – 2017) are complemented by discharge (2014 – 2016) and water chemistry monitoring (2015 and 2016) at the outlets of three small headwater catchments (<0.5 km2) draining the eddy covariance footprint area. In addition to net ecosystem CO2and CH4exchanges, the NECB includes the export of dissolved C (DC) as the sum of inorganic and organic C (DIC and DOC), free CO2and CH4through runoff, and the estimated import of DOC through precipitation. We use absorbance spectroscopy for dissolved organic matter (DOM) characterization to distinguish different DOM sources among catchments and characteristic land cover types. Between 2013 and 2017, the NECB varied between a weak net C source (~16 ±5 g C m-2year-1) and sink (~-22 ±5 g C m-2year-1) in 2015 and 2013, respectively, with a mean value of -1 ±7 g C m-2year-1. The net C sink-source strength was largely controlled by variations in net CO2exchange, ranging between a weak net CO2 sink (~-29 ±3 g C m-2year-1) and source (~8 ±4 g C m-2year-1) in 2015 and 2013, respectively. In contrast, our study site was a persistent annual net CH4source (~8 ±1 g C m-2year-1). Compensated by the import of DOC through precipitation, DC exported from the three catchments was a negligible component of the NECB. There were no significant differences in DOC concentrations and absorbance indices among catchments, and thawed and frozen land cover types, overall illustrating high DOM aromaticity (SUVA254= 3.3 ± 0.6 L mg-1m-1) and high molecular weight (a254:a365 = 4.3 ± 0.3) characteristic for peatlands and peat-dominated landscapes outside the circumpolar permafrost region. We conclude that a rapidly thawing boreal peat landscape along the southern limit of permafrost presently appears to be C neutral.

(1) Department of Forest Sciences, University of Helsinki, Helsinki, Finland (sari.juutinen@helsinki.fi; sini.arnkil@helsinki.fi), (2) Department of Environmental Sciences, University of Helsinki, Finland (sari.juutinen@helsinki.fi), (3) Environmetal Studies Department, Mount Holyoke College, South Hadley, USA (jbubier@mtholyoke.edu), (4) Natural Resources Institute Finland, Vantaa, Finland (tuula.larmola@luke.fi), (5) Department of Geography and Environmetal Studies, Carleton University, Ottawa, Canada (elyn.humphreys@carleton.ca), (6) Department of Geography, McGill University, Montreal, Canada (cameron.roy@mail.mcgill.ca; tim.moore@mcgill.ca)

Nitrogen (N) pollution of peatlands alters their carbon (C) balances, yet long-term effects and controls are poorly understood. We applied the model PEATBOG to explore impacts of long-term nitrogen (N) fertilization on C cycling in an ombrotrophic bog. Simulations of summer gross ecosystem production (GEP), ecosystem respiration (ER) and net ecosystem exchange (NEE) were evaluated against 8 years of observations and extrapolated for 80 years to identify potential effects of N fertilization and factors influencing model behaviour. The model successfully simulated moss decline and raised GEP, ER and NEE on fertilized plots. GEP was systematically overestimated in the model compared to the field data due to factors that can be related to differences in vegetation distribution (e.g. shrubs vs. graminoid vegetation) and to high tolerance of vascular plants to N deposition in the model. Model performance regarding the 8-year response of GEP and NEE to N input was improved by introducing an N content threshold shifting the response of photosynthetic capacity (GEPmax) to N content in shrubs and graminoids from positive to negative at high N contents. Such changes also eliminated the competitive advantages of vascular species and led to resilience of mosses in the long-term. Regardless of the large changes of C fluxes over the short-term, the simulated GEP, ER and NEE after 80 years depended on whether a graminoid- or shrub-dominated system evolved. When the peatland remained shrub–Sphagnum-dominated, it shifted to a C source after only 10 years of fertilization at 6.4 g N m−2 yr−1, whereas this was not the case when it became graminoid-dominated. The modelling results thus highlight the importance of ecosystem adaptation and reaction of plant functional types to N deposition, when predicting the future C balance of N-polluted cool temperate bogs.

The relationship between spectral reflectance and foliar chlorophyll (Chl) and nitrogen (N) was examined for 19 species over a six-month growing period at Mer Bleue, an ombrotrophic bog located near Ottawa, Ontario, Canada. The goal of this study was to model total Chl and N concentration at the landscape-scale from remotely sensed data utilizing a model insensitive to plant functional type (PFT), species and season. To date the relationship between spectral reflectance and foliar properties is poorly understood in peatlands owing to the scarcity of studies examining the spectral variability between mosses and vascular plants. A model that comprised a continuous wavelet transform coupled with a neural network was constructed to predict Chl and N content from selected wavelet features (coefficients) at both leaf and airborne image scales. The model was compared to thirteen common spectral indices used to determine vegetation properties in forest environments. The heterogeneity of the vascular plant/moss cover over small spatial scales (< 1 m) and the spectral complexity of the vegetation cover, precluded a regression model to be derived from the spectral indices for all species across seasons; the best model for all species combined was R2 = 0.3. The final continuous wavelet model resulted in a noticeable improvement with R2 values ranging from 0.8 to 0.9 (for both Chl and N content). We scaled up our predictive model from the leaf/capitulum level data to 40 cm spatial resolution 72-band airborne imagery (CASI-2 429.6–968.8 nm) to create surfaces of Chl and N content for the study area.