Mineral Soil Layers Research Articles

Soil chemical fertility change over four decades in the Morvan Mountains and influence of tree species (Burgundy, France)

Intensive silvicultural practices and the planting of monospecific forests of coniferous, that are more productive compared to hardwoods may threaten over the mid to long-term the sustainability of soil chemical fertility of forest ecosystems, and are a major concern for forest managers and policy. We investigated the tree species effect (Quercus sessiliflora Smith, Fagus sylvatica L., Picea abies Karst., Pseudotsuga menziesii Mirb. Franco., Abies nordmanniana Spach. and Pinus nigra Arn. ssp. laricio Poiret var. corsicana) on the change over time of soil chemical properties and nutrient pool sizes in the mineral and organic layers of the soil during the 45 years after the plantation of the Breuil-Chenue common garden experiment (Burgundy, France). The organic and mineral soil layers down to 70-cm depth were sampled in the different monospecific plots in 1974, 2001 and 2019. Exchangeable Ca and Mg pools and soil pH increased on average over time in the 0–70 ​cm soil profile in most stands. However, in the topsoil layers (0–15 ​cm), the decrease of pH, the increase of exchangeable acidity over time under the coniferous stands and the decrease of exchangeable K pools in most stands highlighted that soil acidification is still on-going at Breuil-Chenue site but the intensity of this process depends on the tree species. Indeed, three groups of species could be distinguished: i) Nordmann fir (Abies nordmanniana Spach.)/Norway spruce (Picea abies Karst.) where acidolysis and chelation occurred, resulting in the most pronounced pH decrease in the topsoil, ii) Douglas fir (Pseudotsuga menziesii Mirb. Franco.)/Laricio pine (Pinus nigra Arn. ssp. laricio Poiret var. corsicana) where acidification caused by elevated nitrification rates is probably currently compensated by larger weathering and/or atmospheric depositions fluxes, and iii) oak (Quercus sessiliflora Smith)/beech (Fagus sylvatica L.) where soil acidification was less intense. Counterintuitively, soil acidification at Breuil-Chenue site resulted in an increase in soil CEC which limited the loss of nutrient cations. This change in soil CEC was most likely explained by the precipitation/dissolution dynamics of aluminium (Al) (hydr)oxides in the interfoliar space of phyllosilicates and/or the increase in soil carbon (C) content in the topsoil layers. After 45 years, tree species continue to exert influence on the chemical fertility of the soil and the pedogenetic processes which in turn may impact forest ecosystem functions and services.

Quantifying the influence of different biocrust community states and their responses to warming temperatures on soil biogeochemistry in field and mesocosm studies

Biocrusts influence soil biogeochemistry by fixing carbon (C) and nitrogen (N) and through leachate inputs to soils. Functional rates can vary among biocrust community states and in response to edaphic properties, heterotrophic microbial activity, and global change. Using soils and biocrusts from the Colorado Plateau, Utah, USA, we aimed to quantify the influence of early-successional (ES) and late-successional (LS) biocrust community states on soil biogeochemistry. In a field setting, we found soil was less “fertile” under ES than LS biocrust, but ES biocrust had a relative influence 1.3 times greater than LS biocrust on soil fertility. Leachate collected from LS biocrust had, on average, 6 times more organic C and 1.7 times more dissolved N than ES biocrust, but concentrations of phosphorus (P) and inorganic N did not differ among the two biocrust types. To disentangle influences of biocrusts and soil properties on biogeochemical pools, we constructed mesocosms from homogenized soil and left the surface bare or covered with ES or LS biocrust, before assignment to ambient or warmed (+5 °C) temperature treatments for 3 months. Multivariate biogeochemical properties differed among cover types, yet all exhibited losses of P, N, and organic C and nearly half of the biogeochemical variables considered did not differ among cover types. Mesocosms with LS biocrust retained more dissolved N, supported 8 additional, significant correlations among biogeochemical pools of the biocrust and mineral soil layer on average, and lost fewer of these correlations under warming. Overall, while soils under LS biocrusts were more fertile (i.e., had higher nutrient concentrations) than under ES, we did not find evidence implicating leachate as the primary driver of this difference. Biocrust influences on soil fertility were greater when mineral soil nutrients were in lower concentrations, highlighting the value of even incipient biocrusts for dryland functioning.

Evaluation of contribution rate of the infiltrated water collected using zero-tension lysimeter to the downward migration of 137Cs derived from the FDNPP accident in a cedar forest soil

The vertical distribution of 137Cs in forest soil is important for predicting air dose rates and future cycling in forest ecosystems. However, there are many unexplained questions about the mechanisms of its downward migration. In this study, the 137Cs flux by rainfall infiltration was observed for three years from August 2017 using zero-tension lysimeters in a mature cedar forest where monitoring of the vertical distribution of 137Cs has been conducted since 2011. As a result, the 137Cs concentration in infiltrated water through the litter layer, 5 cm and 10 cm showed a tendency to be high in summer, but no such seasonal variation was found at 20 cm. Although the 137Cs inventory in the litter layer has been exponentially decreasing, the annual 137Cs fluxes in infiltrated water through the litter layer were almost the same in three years, and about 0.14–0.17% of the deposition density of 137Cs. Comparing these 137Cs fluxes with the apparent amounts of downward migration of 137Cs estimated from the change in the vertical distribution of 137Cs, the contribution rate of the infiltrated water to downward migration of 137Cs from litter to soil was calculated to be 8.5–17.7%. Similarly, the contribution rate in mineral soil layers was calculated to be 0.6–0.8% on a measured basis and estimated to be 3.0 ± 0.2% after correcting the amount of collected water, which is a problem with zero-tension lysimeter. It indicates that rainfall infiltration can explain a small part of the downward migration of 137Cs, thus further studies are required to clarify the contribution rate of remaining mechanisms such as advection-diffusion, colloidal transport, physical mixing, bioturbation, and growth and death of plant roots.

Carbon and nitrogen dynamics in tropical ecosystems following fire

AbstractAimTropical ecosystems have grown increasingly prone to fire over the last century. However, no consensus has yet emerged regarding the effects of fire disturbances on tropical biogeochemical cycles.LocationTropics.Time period1960–2018.Major taxa studiedTropical ecosystems: Above‐ and below‐ground carbon (C) and nitrogen (N) dynamics.MethodsWe analysed the impacts of fire on C and N dynamics in tropical ecosystems through a meta‐analysis of 1,420 observations from 87 studies.ResultsFire reduced both above‐ and below‐ground C and N pools, with greater reductions above‐ than below‐ground. Fire decreased soil total carbon (TC), total nitrogen (TN) and nitrate nitrogen () and increased ammonium nitrogen () in surface mineral soil layers but did not affect those in deep layers. Fire decreased TC and TN in savanna but did not affect those in tropical dry and moist forests. Fire did not affect and in savanna because of non‐significant responses of N mineralization rate (Nmin) to fire. Conversely, fire increased and decreased in tropical dry forest, but did not affect and increased in tropical moist forest owing to thermal decomposition of soil organic N and increased soil nitrification, respectively. Moreover, declined and increased initially and then decreased with time after fire. Above‐ and below‐ground response variables to prescribed fire were mediated largely by fire frequency and experimental duration, respectively.Main conclusionsOur results suggest a high vulnerability of the above‐ground C and N pools to fire, whereas the biogeochemical cycles below‐ground are of high complexity. Fire effects on below‐ground C and N pools, which are highly uncertain and vegetation specific, should be investigated further.

Boreal tree species change as a climate mitigation strategy: impact on ecosystem C and N stocks and soil nutrient levels

AbstractTo increase the annual uptake of CO2 as well as the long‐term storage of carbon (C) in forests, the Norwegian government consider large‐scale replacements of native, deciduous forests with faster‐growing species like Norway spruce. To assess the effects of tree species change on ecosystem C and nitrogen (N) stocks and soil chemistry, we used a paired plot approach including stands of native downy birch and planted 45‐ to 60‐yr‐old Norway spruce. The birch stands were used as reference for the assessment of differences following the tree species change. We found significantly higher C and N stocks in living tree biomass in the spruce stands, whereas no significant differences were found for dead wood. The cover of understory species groups, and the C and N stocks of the aboveground understory vegetation were significantly higher in the birch stands. The tree species change did not affect the soil organic carbon (SOC) stock down to 1 m soil depth; however, the significantly higher stock in the forest floor of the spruce stands suggested a re‐distribution of SOC within the profile. There was a significant positive correlation between the SOC stock down to 30 cm soil depth and the total ecosystem C stock for the birch stands, and a negative correlation for the spruce stands. Significant effects of tree species change were found for C and N concentrations, C/N, exchangeable acidity, base saturation, and exchangeable Ca, K, Mg, Na, S, and Fe in the organic horizon or the upper mineral soil layer. The total ecosystem C stock ranged between 197 and 277 Mg/ha for the birch stands, and 297 and 387 Mg/ha for the spruce stands. The ecosystem C accumulation varied between 32 and 142 Mg/ha over the past 45–60 yr, whereas the net ecosystem C capture was considerably lower and potentially negative. Our results suggest that the potential to meet the governments' targets to increase C sequestration depend on the C debt incurred from the removed birch stands, the rotation length, and potentially also the susceptibility of the different stand types to future risk factors related to climate change.

Evaluation of the Terrestrial Ecosystem Model Biome-BGCMuSo for Modelling Soil Organic Carbon under Different Land Uses

Soil organic carbon (SOC) is a mandatory pool in national inventory reports on greenhouse gas (GHG) emissions and removals to the UNFCCC. Hence, its accurate assessment is important. Modelling SOC changes for national GHG reports is encouraged, but the uncertainty related to this pool still presents a significant challenge; thus, verifying modelling results with field observations is essential. We used the process-based model Biome-BGCMuSo and assessed its suitability for use in Croatia’s GHG reporting. We modelled SOC stocks in the top 30 cm of the mineral soil layer (SOC30) for four different land-use (LU) categories (Deciduous/Coniferous Forest, Grassland and Annual Cropland) distributed in three biogeographical regions (Alpine, Continental and Mediterranean) and compared them with results of a national soil survey. A total of 573 plot level simulations were undertaken and results were evaluated at three stratification levels (LU, LU × biogeographical region, and plot). The model reproduced the overall country mean of SOC30 with no overall bias, and showed good performance at the LU level with no significant (p < 0.05) difference for all LUs except Deciduous Forest (11% overestimation). At finer stratifications, the model performance considerably worsened. Further model calibration, improvement and testing, as well as repeated soil survey are needed in order to assess the changes in SOC30 and to evaluate the potential of the Biome-BGCMuSo model for use in GHG reporting.

Soil Microbial Biomass Carbon and Freeze‐Thaw Cycles Drive Seasonal Changes in Soil Microbial Quotient Along a Steep Altitudinal Gradient

AbstractUnderstanding seasonal changes in soil microbial quotient (qMB, i.e., the MBC‐to‐SOC ratio) along altitudinal gradients and their dominant drivers is essential for determining the effects of environmental changes on the soil organic carbon pool. A 3‐year in situ soil core incubation experiment was thus conducted in a dry valley shrub, a valley‐mountain ecotone forest, a subalpine coniferous forest, an alpine coniferous forest, and an alpine meadow across a 2,431‐m altitudinal gradient on eastern the Qinghai‐Tibetan Plateau. Soil qMB in the soil organic layer (OL) and mineral soil layer (ML) was measured at five critical periods. The average soil qMB was significantly higher in the alpine meadow than at other sites, with no significant difference among the other sites. The average soil qMB in this region was less than the global average value (1.2%), indicating that the soil organic carbon pool of the local ecosystem has higher stability and a lower mineralization potential than that of the other ecosystems globally. Seasonal variations in soil qMB differed along the altitudinal gradient. Significant increases in soil qMB during the 3‐year incubation were mainly observed in the OL at higher altitudes, implying increased sensitivity of the soil organic carbon pool to environmental changes. Microbial biomass carbon and soil freeze‐thaw cycles were critical factors determining the altitudinal pattern and seasonal changes in soil qMB. Soil microbial biomass and freeze‐thaw regime changes under global climate change scenarios will alter the balance between SOC microbial assimilation and maintenance respiration demands, affecting soil carbon biogeochemical cycling in mountainous ecosystems.

Spatial variations in radiocesium deposition and litter–soil distribution in a mountainous forest catchment affected by the Fukushima nuclear accident

The Fukushima Dai-ichi Nuclear Power Plant accident caused serious 137Cs contamination in mountainous forest areas. To understand the spatial variation in soil 137Cs inventory in complex mountainous topography and the influencing factors, a whole-area investigation of 137Cs deposition in a broad-leaved forest catchment of a mountain stream was conducted using grid sampling. Across the catchment, organic and surface mineral soil layers were collected at 42 locations in 2013 and 6 locations in 2015. Cesium-137 deposition on the forest floor exhibited high spatial heterogeneity and altitude-dependent distribution over the catchment. The 137Cs retention ratio in the organic layer, determined as the inventory in the organic layer divided by the soil (organic and mineral soil layers) inventory, ranged from 6% to 82% in 2013, and the coefficient of variation was 0.6. The 137Cs retention ratios had positive correlations with the material inventory in the organic layer and the elevation. The 137Cs retention ratios in the organic layer were less than 20% in 2015, even at the locations where the retention ratio was higher than 55% in 2013. Although there was spatial variation in the migration speed, 137Cs migration from the organic layer to mineral soil was almost completed within 4 y of the deposition, suggesting a decrease in 137Cs circulation within the forest ecosystem. This study also examined a relationship between the 137Cs inventory and the air dose rate to assess the potential of using the air dose rate to estimate soil 137Cs inventory. Soil 137Cs inventories and air dose rates were highly positively correlated, indicating that measurement of air dose rate can provide an easier and quicker alternative to measurement of soil 137Cs inventory in forest ecosystems.

Effects of mixing tree species and water availability on soil organic carbon stocks are depth dependent in a temperate podzol

AbstractMixing tree species is a forest management strategy put forward to increase and stabilize primary productivity. Yet, little is known about soil organic carbon (SOC) storage in mixed species forests, particularly under water shortage. In this study, we used a tree diversity experiment in southwestern France to assess the interactive effects of water availability (via irrigation) and mixing tree species (monocultures of pine (Pinus pinaster) and birch (Betula pendula) versus mixed plots of pine‐birch and pine‐birch‐oak (Quercus robur)) on SOC stocks in the forest floor and across five mineral soil layers (0–5, 5–15, 15–30, 30–60 and 60–90 cm deep). We found that SOC stocks were negatively affected by irrigation in the forest floor due to an increase in decomposition rates. However, SOC stocks were positively influenced by both the mixing of tree species and irrigation in the mineral soil, particularly at the 15–30‐cm soil depth. This is because root niche partitioning in mixed plots and an increase in tree biomass in irrigated plots may have resulted in greater organic matter inputs through rhizodeposition and the incorporation of microbially‐derived compounds. These preliminary results indicate that SOC dynamics and its response to biotic and abiotic factors are strongly dependent on soil depth. Our data further highlight that the positive effect of mixing trees on SOC stocks was higher in irrigated plots, thereby contradicting the idea that tree mixture effects are expected to be greater when environmental conditions are harsher. We conclude that mixing tree species can increase SOC stocks in the short term in temperate forests, even if the exact mechanisms remain to be identified.Highlights Effects of mixing trees and increasing water availability on SOC are depth dependent. Increase in SOC stocks occurred mainly at the mid‐soil depth (15–30 cm). Effects of tree mixture on SOC were greater when soil water availability was high.

Effects of Fire on the Organic and Chemical Properties of Soil in a Pinus halepensis Mill. Forest in Rocallaura, NE Spain

The present study was conducted following a fire that took place in Rocallaura (NE Iberian Peninsula) in 23 June 2016 with the aim of analyzing the chemical properties of soil in burnt and unburnt areas in order to determine the short-term effects of fire on an ecosystem dominated by Pinus halepensis Mill. Transects were laid out in a control area and an adjacent burnt area. Laboratory analyses were performed to determine the chemical properties of the organic and mineral soil layers at 5 cm and 10 cm (total carbon, total nitrogen, organic and inorganic carbon, carbonates, pH, electrical conductivity, and major cations). The results show partial combustion of the organic substrate, with a significant increase in TC, OC, TN, EC and K+, and a slight decrease in pH in the post-fire area compared to the control area, demonstrating that the fire was of low intensity and did not generate significant short-term negative impacts on the soil. Soils of this type, with high organic matter content, tend to maintain their structure after a low-intensity fire and retain the nutrients necessary for ecosystem recovery and resilience.

Nitrogen cycling in tropical dry forests is sensitive to changes in rainfall regime and nitrogen deposition

The impacts of changes in rainfall regimes and atmospheric nitrogen (N) deposition on the biogeochemical cycles of tropical dry forests (TDF) remain unclear. To analyze whether amounts of mean annual rainfall (1240 mm year−1 in subhumid vs. 642 mm year−1 in semiarid forests) and rates of atmospheric N deposition affect the N cycle in TDF, we examined N concentrations in (i) green leaves, (ii) forest floor and mineral soil layers, and (iii) soil microbial biomass, as well as (iv) δ15N natural abundance in green leaves and (v) rates of soil enzyme activity and (vi) N losses in two sites (one subhumid and another semiarid) of Yucatan. The semiarid site had higher N concentrations in leaves and both soil layers than its subhumid counterpart. Natural abundance δ15N in the vegetation suggests a more open N cycle with lower rates of rainfall, which was corroborated by N losses as nitrous oxide (N2O) being twice as high in the semiarid compared to subhumid site. A more open N cycle subject to greater losses in response to N addition in the semiarid site, but not in the subhumid site, reinforce that rainfall amounts strongly affect N cycling in TDF.

The molecular composition of extractable soil microbial compounds and their contribution to soil organic matter vary with soil depth and tree species

Recent findings on soil organic matter (SOM) revealed that soil microorganisms are not only crucial for SOM formation through plant litter degradation but soil microbial biomass (SMB) may also directly contribute to SOM and its composition. However, the role and interactions of litter quality, microbial turnover and composition of SMB and SOM remain unclear. Hence, soil profiles (organic forest floor and mineral soil layers) at a recultivated and afforested post-mining site were investigated for the influence of litter quality from different tree species (Fagus sylvatica, Pseudotsuga menziesii, Pinus nigra) and soil depth – representing different degrees of organic matter (OM) turnover – on the molecular composition of chloroform fumigation extracted SMB-derived compounds in comparison with easily extractable (non-fumigated) SOM-derived compounds. The SMB extracts were analyzed for microbial biomass carbon (MBC), nitrogen (MBN) and phosphorus (MBP). The molecular composition of SMB and SOM compounds were determined by electrospray ionization Fourier transformation ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) complemented by the determination of nine monosaccharides representing microbial or plant origin.Van Krevelen diagrams obtained from the ESI-FT-ICR mass spectra revealed the substantial contribution of microbial-derived compounds to extractable SOM that further increased with soil depth. Analysis of the easily extractable monosaccharides implied that >99% were of microbial origin. Microbial sugars as well as MBC, MBN and MBP explained best depth-related variations of extractable SMB compounds indicating that supply and availability of C-rich OM drive these parameters. Furthermore, the contribution of microbial sugar C to MBC increased with depth, suggesting that recycling of carbohydrates is an adaptation strategy of microorganisms in C-limited environments. The supply of tree species-specific substrates resulted in different chemical composition of SMB with largest differences between deciduous and coniferous stands and vice versa, microorganisms contributed to SOM resulting in large similarity in the composition of extractable SOM and SMB.

Spatial–temporal variations of absorptive fine roots in the organic and soil layers of a Larix gmelinii forest

In the organic layer, absorptive fine roots of Larix gmelinii represent a significant proportion of the total fine-root biomass and can morphologically adjust during the growing season to optimize resource acquisition. Larix gmelinii is a foundational tree species in forests of northern China and Far-East Russia. A better understanding of the species’ belowground resource acquisition strategy is important for soil nutrient and carbon estimates. Here, we investigated seven traits of absorptive fine-roots at four depth levels, from the organic layer (organic, decomposed litter from dead plants) down to a soil depth of 30 cm in May, July and September of 2016. Biomass and functional traits of absorptive fine roots varied significantly, indicating the species ability to adjust its resource acquisition across the four depth layers. From the organic layer to deeper mineral soil layers, root diameter and tissue density increased, while specific root length decreased, in other words, roots become longer and thinner with increasing values of soil water and nutrient concentrations. At the same time, the average biomass and length of absorptive fine roots in the organic layer accounted for 14% and 16% of the total, respectively. With increasing soil depth, the averaged proportion of biomass and length density of absorptive fine roots to the total fine roots decreased significantly from almost 57 to 32%, and 80–62%, respectively. Finally, total soil nitrogen concentration showed the strongest link with absorptive fine-root traits, such as root diameter, specific root length, root standing biomass and length density. Our results indicate that absorptive fine roots in the organic layer play a crucial role in belowground resource acquisition and carbon cycle in a common and economical important species.

Long-term litter type treatments alter soil carbon composition but not microbial carbon utilization in a mixed pine-oak forest

Changes in litter and nutrient inputs into soil could have significant consequences on forest carbon (C) dynamics via controls on the structure and microbial utilization of soil organic C (SOC). In this study, we assessed changes in physical fractions (250–2000 μm, 53–250 μm, and < 53 μm soil aggregates) and chemical fractions (labile, intermediate and recalcitrant pools) of SOC in the top 20 cm mineral soil layer and their influences on microbial substrate utilization after eight years of experiment in a mixed pine-oak forest. The litter treatments included: control (Lcon), litter removal (Lnil), fine woody litter addition (Lwoody), leaf litter addition (Lleaf) and a mix of leaf and fine woody litter (Lmix). Nitrogen (N) addition (application rates of 0, 5 and 10 g N m−2 year−1, respectively) was also applied. We found that complete removal of forest-floor litter (Lnil) significantly reduced the pool sizes of all SOC fractions in both the physical and chemical fractions compared with treatments that retained either leaf litter (Lleaf) or mixture of leaves and fine woody materials (Lmix). The type of litter was more important in affecting SOC fractions than the quantity of inputs; neither the level of N addition rate nor its interaction with litter treatment had significant effects on both physical and chemical SOC fractions. Microbial respiration differed significantly among the treatments of varying litter types. However, the effectiveness of microbial C utilization inferred by microbial C use efficiency and biomass-specific respiration was not affected by either the litter treatments or N addition. These results suggest that despite significant changes in SOC composition due to long-term treatments of forest-floor litter and N addition in this mixed pine-oak forest of temperate climate, microbial C utilization strategies remain unaffected.

Alternative afforestation options on sandy heathland result in minimal long-term changes in mineral soil layers

Extensive afforestation is currently being widely promoted as a key nature-based solution for climate change mitigation. Fundamental to this strategy is the sequestration of carbon into long-term stable storage, either in wood products or the soil. However, the long-term effects of tree planting on soil carbon, or other soil properties, has rarely been examined. Importantly, afforestation can take many different forms, with differing effects on soil properties. Here, we evaluate how the historical afforestation of sandy heathland adopting a range of management options – including different combinations of conifers and broadleaves in monocultures and mixtures – have affected soil pH, total carbon and nitrogen concentrations, the C:N ratio, and carbon and nitrogen stocks almost a century later. We analyse these properties at a range of soil depths through the organic (litter, F and grass layers) and upper mineral (0–5 cm, 5–10 cm and 10–20 cm depth) soil layers. In comparison to the historical heathland sites, afforestation decreased soil pH, most dramatically under conifers, and increased the C:N ratio. However, there was overall little difference in carbon and nitrogen concentrations between alternative management options. While the total carbon and nitrogen concentrations were much higher in the organic layers of the forest options compared to the open sites, this did not translate into differences in the mineral layers. Furthermore, although we found some evidence of the transferral of carbon and nitrogen into the uppermost soil mineral layers, this was minimal in comparison to the concentrations of the organic layers. The soils at our study site are low quality and sandy, and are therefore unfavourable for incorporating organic matter, but it is still notable how little was incorporated after nearly a century of afforestation. Given the current emphasis on tree planting as a means to tackle climate change, these results demonstrate the fundamental importance of the appropriate consideration of both the afforestation management option and underlying soil type.

Long‐term, amplified responses of soil organic carbon to nitrogen addition worldwide

Soil organic carbon (SOC) is the largest carbon sink in terrestrial ecosystems and plays a critical role in mitigating climate change. Increasing reactive nitrogen (N) in ecosystems caused by anthropogenic N input substantially affects SOC dynamics. However, uncertainties remain concerning the effects of N addition on SOC in both organic and mineral soil layers over time at the global scale. Here, we analysed a large empirical data set spanning 60years across 369 sites worldwide to explore the temporal dynamics of SOC to N addition. We found that N addition significantly increased SOC across the globe by 4.2% (2.7%-5.8%). SOC increases were amplified from short- to long-term N addition durations in both organic and mineral soil layers. The positive effects of N addition on SOC were independent of ecosystem types, mean annual temperature and precipitation. Our findings suggest that SOC increases largely resulted from the enhanced plant C input to soils coupled with reduced C loss from decomposition and amplification was associated with reduced microbial biomass and respiration under long-term N addition. Our study suggests that N addition will enhance SOC sequestration over time and contribute to future climate change mitigation.

How to build a firebreak to stop smouldering peat fire: insights from a laboratory-scale study

Smouldering wildfire is an important disturbance to peatlands worldwide; it contributes significantly to global carbon emissions and provides positive feedback to climate change. Herein, we explore the feasibility of firebreaks to control smouldering peat fires through laboratory-scale experiments. The dry-mass moisture content (MC) of peat soil was varied from 10% (air-dried) to 125%. We found that smouldering peat fire may be successfully extinguished above the mineral soil layer, even if the peat layer is not entirely removed. There are two criteria for an effective peat firebreak: (I) adding water to make the peat layer sufficiently wet (&gt;115% MC in the present work); and (II) ensuring that the peat layer is thinner than the quenching thickness (&lt; 5 cm). Criterion I may fail if the water table declines or the peat layer is dried by surface fires and hot weather; thus, satisfying Criterion II is more attainable. A sloping trench-shaped firebreak is recommended to guide water flow and help maintain high peat moisture content. This work provides a scientific foundation for fighting and mitigating smouldering wildfires and provides guidance about protective measures for field-scale peat fire experiments.

Spatial distribution of mercury accumulation in the surface soil of Japanese forests

ABSTRACT Mercury (Hg) is a pollutant that can affect human and ecosystem health. The transport and fate of Hg in the environment are dynamic and complex, but our understanding remains unclear for Japanese forest ecosystems. Here, we report the first country-wide survey of Hg concentrations in litter and surface mineral soil layers in 42 forest stands across Japan. The median concentrations of Hg in the litter layer and 0–10 cm of mineral soils were 99 (range: 56.7–297) and 145 (range: 22.8–294) μg kg−1, respectively, and tended to decrease down to 50 cm. There was a positive relationship between the Hg and total organic carbon concentrations in soil, suggesting that organic carbon strongly binds with Hg in soil. There was a significant positive correlation between the Hg and lead concentrations in the litter layer. Partial least square regression analysis indicated that the soil and litter properties as well as the atmospheric and geological Hg concentrations and some soil characteristics such as soil pH, organic matter properties and soil physical factors were effective explanatory variables of the country-wide spatial patterns of Hg concentrations in litter and mineral soil. In addition, we found the influence of atmospheric Hg is stronger in litter layer, whereas the influences of geological and soil factors are stronger in mineral soil. These results suggest that air pollution partly affects the spatial patterns of Hg concentrations in litter and soil of Japanese forests under the given geological and soil conditions at the country scale.

Modeling the temperature and humidity conditions of mineral soils in an active layer model taking into account in depth changes in the thermodynamic properties of the soil

The thermal regimes of peat and mineral soils differ significantly. Peat soils are characterized by the presence of a surface peat horizon, which is replaced by organogenic rock. The total thickness of the peat layer can reach several meters. The peat strata is a complex organomineral system with specific properties: high water content and porosity, the content of a large amount of little decomposed organic. For the correct modeling of this type of soil, a transition was made from the approach of modeling heat and moisture transfer in a porous medium with averaged thermodynamic characteristics (porosity, density, hydraulic conductivity …), to modeling heat and moisture transfer for each of the identified soil types with individual properties. In addition, in the modified model, all constants describing the thermodynamic properties of the soil are functions of depth. This made it possible to model soils with a complex soil profile, for example, bog ecosystems (peat deposit and a layer of mineral soil under it). To calculate the thermophysical characteristics of soils, the measured volumetric content of organic matter, mineral particles of different sizes, and soil porosity at all depths were used. Calculations based on the model of the active layer showed that within the framework of the approach used it is possible to correctly reproduce heat and moisture transfer in both mineral and organic soils without adding any runoff-related parameterizations, but using relevant thermophysical characteristics of the soil.

Vertical distributions of radiocesium in Japanese forest soils following the Fukushima Daiichi Nuclear Power Plant accident: A meta-analysis

This study investigated the temporal change in vertical distributions of radiocesium inventories in Japanese forest soils during the early phase (from 2011 to 2017) following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using three simple parameters. We calculated the fraction in the organic layer (Fl/t), the migration center (Xc) and the relaxation depth (α) using 99 soil inventory data sets. Fl/t decreased significantly from 2011 to 2017 (logistic analysis, p < 0.001). In addition, Fl/t in the FDNPP zone rapidly decreased compared to that in the Chernobyl Nuclear Power Plant (ChNPP) zone from the first year to the second year. Different migration rates from organic to mineral soil layers between previous studies in the ChNPP and this study have several possible causes such as organic litter features, climate and physico-chemical forms of initial deposition. In mineral soil layers in the FDNPP zone, only Xc increased significantly with time according to generalized mixed model analysis (p < 0.01). However, Xc and α in the ChNPP zone decreased from two to five years after the accident in 1986, which shows a high 137Cs retention in the organic layer even in the fifth year after the accident. The vertical migration of 137Cs in the mineral soil layer in the FDNPP zone appears to be due to low input of 137Cs from organic to surface mineral soil layer after the second year. These results indicate that 137Cs retention capacity of the organic layer can affect the apparent vertical migration of 137Cs in the underlying mineral soil layer.