Forest Floor Samples Research Articles

Methane and carbon dioxide flux in the profile of wood ant (Formica aquilonia) nests and the surrounding forest floor during a laboratory incubation.

We compared methane (CH4) and carbon dioxide (CO2) fluxes in samples collected from the aboveground parts of wood ant nests and in the organic and mineral layer of the surrounding forest floor. Gas fluxes were measured during a laboratory incubation, and microbial properties (abundance of fungi, bacteria and methanotrophic bacteria) and nutrient contents (total and available carbon and nitrogen) were also determined. Both CO2 and CH4 were produced from ant nest samples, indicating that the aboveground parts of wood ant nests act as sources of both gases; in comparison, the forest floor produced about four times less CO2 and consumed rather than produced CH4 Fluxes of CH4 and CO2 were positively correlated with contents of available carbon and nitrogen. The methanotrophic community was represented by type II methanotrophic bacteria, but their abundance did not explain CH4 flux. Fungal abundance was greater in ant nest samples than in forest floor samples, but bacterial abundance was similar in both kinds of samples, suggesting that the organic materials in the nests may have been too recalcitrant for bacteria to decompose. The results indicate that the aboveground parts of wood ant nests are hot spots of CO2 and CH4 production in the forest floor.

Forest floor chemical transformations in a boreal forest fire and their correlations with temperature and heating duration

Boreal soils account for ~30% of the global soil organic carbon (C) stock. Wildfires are an important perturbation of this C pool, particularly affecting the top organic soil layer, which constitutes the forest floor. Alterations to the forest floor by fire are relevant to the soil C balance and have profound implications for soil properties. However, relationships between forest floor transformations and actual wildfire characteristics have not been established to date due to the logistical challenges of obtaining the necessary fire behaviour data, together with associated pre- and post-fire sample material. We used a high-intensity experimental wildfire to address this research gap, which enabled us to determine forest floor chemical transformations in a Canadian boreal forest in relation to temperature–time profiles for 18 sampling points during the fire. Forest floor samples taken pre- and post-fire were characterized using elemental and δ13C analysis, differential scanning calorimetry and 13C nuclear magnetic resonance.During this typical boreal crown fire average maximum temperature (Tmax) at the forest floor was 745°C (550<Tmax<976°C) with the average heating duration (t) >300°C being 176s (65<t<364s). Significant correlations were detected between the chemical characteristics of the pyrogenic (charred) forest floor layer and the temperature–time profiles at the corresponding sampling points. Higher Tmax and associated prolonged heating durations correlated with greater C enrichments, increased thermal recalcitrance and degree of aromaticity of the pyrogenic organic matter. These changes were particularly pronounced for Tmax>600–700°C, which is higher than the range of 300–500°C for aromaticity development previously reported from laboratory experiments. One reason for this discrepancy could be the generally much longer heating durations used in laboratory studies, and we therefore advise caution when extrapolating findings from laboratory studies to wildfire conditions.Almost half of the initial total C stock in the forest floor (20MgCha−1) was affected by fire, with ~24% of this fire-affected C transformed to pyrogenic organic matter. This pyrogenic material possessed variable, yet distinct, chemical characteristics when compared to unburnt forest floor, including higher recalcitrance and associated resistance to biological degradation. As some boreal regions already show a rise in fire severity and area burned linked to climate change, our findings suggest a potential accompanying increase in the more stable organic carbon stock, with important implications for the functioning and turnover of organic matter in boreal soils.

Nutrient-cycling microbes in coastal Douglas-fir forests: regional-scale correlation between communities, in situ climate, and other factors.

Microbes such as fungi and bacteria play fundamental roles in litter-decay and nutrient-cycling; however, their communities may respond differently than plants to climate change. The structure (diversity, richness, and evenness) and composition of microbial communities in climate transects of mature Douglas-fir stands of coastal British Columbia rainshadow forests was analyzed, in order to assess in situ variability due to different temperature and moisture regimes. We compared denaturing gradient gel electrophoresis profiles of fungi (18S-FF390/FR1), nitrogen-fixing bacteria (NifH-universal) and ammonia-oxidizing bacteria (AmoA) polymerase chain reaction amplicons in forest floor and mineral soil samples from three transects located at different latitudes, each transect spanning the Coastal Western Hemlock and Douglas-fir biogeoclimatic zones. Composition of microbial communities in both soil layers was related to degree days above 0°C (2725–3489), while pH (3.8–5.5) best explained shifts in community structure. At this spatial scale, climatic conditions were likely to directly or indirectly select for different microbial species while local site heterogeneity influenced community structure. Significant changes in microbial community composition and structure were related to differences as small as 2.47% and 2.55°C in mean annual moisture and temperature variables, respectively. The climatic variables best describing microbial composition changed from one functional group to the next; in general they did not alter community structure. Spatial distance, especially associated with latitude, was also important in accounting for community variability (4–23%); but to a lesser extent than the combined influence of climate and soil characteristics (14–25%). Results suggest that in situ climate can independently account for some patterns of microbial biogeography in coastal Douglas-fir forests. The distribution of up to 43% of nutrient-cycling microorganisms detected in forest soils responded to smaller abiotic gradients than host trees.

Using foliar and forest floor mercury concentrations to assess spatial patterns of mercury deposition

We evaluated spatial patterns of mercury (Hg) deposition through analysis of foliage and forest floor samples from 45 sites across Adirondack Park, NY. Species-specific differences in foliar Hg were evident with the lowest concentrations found in first-year conifer needles and highest concentrations found in black cherry (Prunus serotina). For foliage and forest floor samples, latitude and longitude were negatively correlated with Hg concentrations, likely because of proximity to emission sources, while elevation was positively correlated with Hg concentrations. Elemental analysis showed moderately strong, positive correlations between Hg and nitrogen concentrations. The spatial pattern of Hg deposition across the Adirondacks is similar to patterns of other contaminants that originate largely from combustion sources such as nitrogen and sulfur. The results of this study suggest foliage can be used to assess spatial patterns of Hg deposition in small regions or areas of varied topography where current Hg deposition models are too coarse to predict deposition accurately.

Total and pyrogenic carbon stocks in black spruce forest floors from eastern Canada

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

Soil N and C Response to Mid-Rotation Vegetation Management in Intensively Managed Pine Stands

Long-term sustainability of intensively managed forests depends on historical management and inherent soil productivity. However, long-term effects of prescribed fire and herbicide on forest floor and soil C and N pools are poorly understood, whereas benefits to biodiversity are well supported. Therefore, we investigated forest floor and mineral soil C and N responses to factorial combinations of mid-rotation dormant season prescribed fire and imazapyr herbicide (Arsenal®) in intensively managed mid-rotation pine (Pinus species) stands in east-central Mississippi. We used a randomized complete block design of six pine stands, each containing four 10-ha treatment plots receiving one of three treatments (herbicide only, prescribed fire only, both herbicide and fire) and a control at random. We applied herbicide via skidder in fall 1999 and prescribed fires using drip torches in winter, 2000, 2003, and 2006. During winter 2009, we sampled substrate using soil cores, a hammer core, and a fixed-area sampling frame for forest floor samples. Prescribed fire, with or without herbicide, reduced forest floor total N. However, C:N was only greater in burn + herbicide plots than controls, with independent treatments intermediate to treatment extremes. Considering the minimal effects of fire and herbicide use on soil nutrients and observed biodiversity benefits from these practices, prescribed fire with or without imazapyr herbicide does not seem to negatively impact long-term sustainability within similar managed pine landscapes of the southeastern United States.

Biogeochemical nitrogen properties of forest soils in the Japanese archipelago

AbstractThis data paper provides some biogeochemical nitrogen (N) properties and related chemical properties of forest soils from 39 sites throughout the Japanese archipelago. The data set was collected and analyzed under the GRENE (Green Network of Excellence) environmental information project and the ReSIN (Regional and comparative Soil Incubation study on Nitrogen dynamics in forest ecosystems) project. The sites cover 44°20′N to 26°50′N and the climate ranges from cool‐temperate zone to subtropical zone. At each site, litter on forest floor and soil samples (three or four layers to 50 cm depth) were collected between August and November in 2010–2013 from five soil profiles. From the litter layer samples, the stocks and concentrations of total carbon (C) and N were measured. From the mineral soil samples, bulk density, pH (H2O), total C and N concentrations, net and gross rates of N mineralization, nitrification and concentrations of water‐soluble substances were measured. The measurements are relevant for other biogeochemical N studies in forest ecosystems and the data set provides basic information on the N pool and fluxes with related chemical properties of forest soils across the Japanese archipelago. The average rates of net and gross N transformation at 20 °C across the sites were 0.26 ± 0.47 mgN kg−1 soil d−1 for net N mineralization, 0.25 ± 0.45 mgN kg−1 soil d−1 for net nitrification, 4.06 ± 0.47 mgN kg−1 soil d−1 for gross N mineralization, and 1.03 ± 1.29 mgN kg−1 soil d−1 for gross nitrification (average ± SD).

인공강우실험에 의한 활엽수 부후낙엽층의 강우차단손실량 추정

Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 151-921, Korea(Received August 16, 2014; Revised September 29, 2014; Accepted September 29, 2014)ABSTRACTSForest floor is one of most distinctive features of forest ecosystem, which provides plants and soil microbeswith nutrients, and controls hydrologic condition within the floor by intercepting water during a rainfallevent and evaporates back into the atmosphere. In this study rainfall interception loss by decomposed forestfloor of a deciduous forest has been experimentally estimated using rainfall simulation experiments. Litter-decomposing fungi were incubated on deciduous forest floor samples for the experiment purposes. On adeciduous floor, a 4.22 mm·kg

Deep Soil Carbon: Quantification and Modeling in Subsurface Layers

Soil is the primary sink for C in forest ecosystems but is often overlooked in ecosystem C budgets. Efforts to quantify C pools often sample soils to a depth of 0.2 m despite observations that deep soil C is neither scarce nor entirely stable. This study examined the systematic sampling depth for ecosystem C analyses in the Pacific Northwest and compared best‐fit models of C in deep soil layers with laboratory measurements. Forest floor samples and mineral soil bulk density samples were collected at regular intervals from the soil surface to depths of 2.5 m from 22 sites across the coastal Pacific Northwest Douglas‐fir [Pseudotsuga menziesii (Mirb.) Franco] zone. Soil samples were screened to 4.7 mm and analyzed for C content. We found that systematic soil sampling shallower than 1.5 m significantly underestimated total soil C. On average, sampling to 2.5 m compared with 0.5 m increased total C by 156% (85.3–132.7 Mg ha−1), and 21% of total C within the depth range sampled was below 1.0 m. A nonlinear mixed model using an inverse polynomial curve form and predicting total C to 2.5 m given only data to 1.0 m was reliable for 20 of 22 sites; the sites that could not be accurately modeled carried the greatest C at depth and contained noncrystalline minerals. Shallow soil sampling at best provides a biased estimate and at worst leads to misleading conclusions regarding soil C. Researchers seeking to quantify soil C or measure change with time should sample deep soil to create a more complete picture of soil pools and fluxes.

Prescribed Fire and Timber Harvesting Effects on Soil Carbon and Nitrogen in a Pine Forest

Thinning and prescribed fire are common management tools used to eliminate thick fuel loads that could otherwise facilitate and encourage a more severe catastrophic wildfire. The objective of this study was to quantify the lasting effects of prescribed fire on forest floor and soil nutrients approximately 9 yr after a burn occurred near Truckee, CA. The study site includes a prescribed fire following various harvest and understory removal treatments: whole‐tree thinning, cut‐to‐length thinning, and no thinning. Data were collected before, immediately after, and 9 yr later following a prescribed burn. All forest floor and soil samples were analyzed for nutrients (O horizon: total N; mineral soil: total N, total C, mineral N). Fuel reductions were evident 9 yr after the fire in the burned plots. No significant changes in total C or total N in surface (0–20‐cm) mineral soils occurred during the 9‐yr period. Mineral N concentrations in surface soils were greater in unburned than in burned cut‐to‐length thinning treatments after 9 yr. These differences were attributed to N inputs from decomposing slash and to the reduction in the biomass of N2 fixers by burning.

Comparing hydraulic properties of different forest floors

AbstractThe forest floor plays an important role in runoff rate, soil erosion and soil infiltration capacity by protecting mineral soils from the direct impact of falling raindrops. Forest floor consists of different kinds of litter with different hydraulic properties. In this study, the inverse method was used to estimate the hydraulic properties of three kinds of forest floor (broad‐leaved, needle‐leaved and mixed‐stand) at three replications in a completely random design. Forest floor samples were collected from the Gilan Province, Iran. The samples were piled up to make long columns 40.88 cm high with an inner diameter of 18.1 cm. Artificial rainfall experiments were conducted on top of the columns, and free drainage from the bottom of the columns was measured in the laboratory. Saturated hydraulic conductivity (Ks), saturated water content and water retention curve parameters (van Genuchten equation) were estimated by the inverse method. The results showed that the Ks of needle‐leaved samples differed significantly (p &lt; 0.05) from those of broad‐leaved and mixed‐stand samples, whereas the latter two did not differ in this regard. No significant differences emerged in the water retention function parameters of van Genuchten (θr, β and α) in the three forest floor samples. The saturated water content of mixed‐stand samples was significantly different (p &lt; 0.05) from that of broad‐leaved and needle‐leaved treatments with the latter two samples showing no significant difference. The good agreement between simulated and observed free drainage for all forest floor samples in the validation period indicates that the estimated hydraulic properties efficiently characterize the unsaturated water flow in the forest floor. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Carbon and nitrogen accumulation in forest floor and surface soil under different geographic origins of Maritime pine (Pinus pinaster Aiton.) plantations

Aim of study : To determine if plantations consisting of different geographic origins of the Maritime pine (Pinus pinaster Aiton.) could have altered C and N stocks in the forest floor and surface soils.Area of study : Forest floor and mineral soil C and N stocks were measured in four adjacent plantations of different geographic origins of Maritime pine (Gironde, Toulon, Corsica and Spain) and adjacent primary native Sessile oak (Quercus petraea L.) at Burunsuz region in Belgrad Forest where is located in the Istanbul province in the Marmara geographical region between 41°09' -41°12' N latitude and 28°54' - 29°00' E longitude in Turkey.Material and Methods : Plots were compared as common garden experiments without replications. 15 surface soil (0-10 cm) and 15 forest floor samples were taken from each Maritime pine origins and adjacent native Sessile oak forest. C and N contents were determined on LECO Truspec 2000 CN analyzer. The statistical significance of the results was evaluated by one-way Analysis of Variance (ANOVA).Research highlights : Forest floor carbon mass, nitrogen concentration and nitrogen mass of forest floor showed a significant difference among origins. Soil carbon mass and nitrogen mass did not significantly differ among investigated plots.Keywords: carbon sequestration; C/N ratio; decomposition; exotic; tree provenance.

Mercury isotopes in a forested ecosystem: Implications for air‐surface exchange dynamics and the global mercury cycle

Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10‐year‐old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had δ202Hg values of −0.74 to 0.06‰ and ∆199Hg values of 0.16 to 0.82‰. Atmospheric THg(g) had δ202Hg values of 0.48 to 0.93‰ and ∆199Hg values of −0.21 to −0.15‰. Uptake of THg(g) by foliage resulted in a large (−2.89‰) shift in δ202Hg values; foliage displayed δ202Hg values of −2.53 to −1.89‰ and ∆199Hg values of −0.37 to −0.23‰. Forest floor samples had δ202Hg values of −1.88 to −1.22‰ and ∆199Hg values of −0.22 to −0.14‰. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for ~16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in δ202Hg values and MIF of even isotopes (∆200Hg and ∆204Hg). Mercury evasion from the forest floor may have arisen from air‐surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re‐emission of wet‐deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopes during foliar uptake and air‐surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive δ202Hg values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.

Moisture effects on microbial communities in boreal forest floors are stand-dependent

Landscape level factors such as overstory canopy composition can have a profound effect on the ecology of microbial communities in boreal forest floors. However, factors influencing community composition at the microsite scale are still poorly described and understood. Here we explored moisture effects on microbial communities in forest floor derived from undisturbed trembling aspen and white spruce stands, two of the dominant trees in the boreal forest of western Canada. Forest floor samples were incubated in a laboratory experiment for a period of one month under a moisture regime ranging from moist to dry (field capacity, 60% of field capacity and wilting point). As in previous studies we found that the origin of the forest floor material had a strong effect on the microbial community. Additionally, we found that moisture manipulation did not alter the microbial communities of the white spruce forest floor. On the other hand, the moisture had a profound effect on the aspen forest floor, and resulted in structurally and functionally distinct microbial communities. This different response to moisture could be linked to the adaptation of microbial groups to the physical environment inherent to the aspen and spruce forest floors and provides an avenue to further work into microbial mediated biogeochemical processes in the boreal forest.

Dynamics of soil and root C stocks following afforestation of croplands with poplars in a semi-arid region in northeast China

Background and aims Afforestation on croplands can help sequester atmospheric CO2 through increased carbon (C) storage in the soil and vegetation. However, the dynamics of soil organic C (SOC) and rootC stocks, particularlythose inthedeepersoillayers, following afforestation are not well documented for semi-arid regions. The aim of this study was to investigate the dynamics of soil and root C stocks to 1 m depth following afforestation with poplar (Populus×xiaozhuanica W. Y. Hsu & Liang) on croplands at the Keerqin Sandy Lands in northeast China. Methods Forest floor, root and mineral soil samples were collected from 23 paired plots of poplar plantations with different stand basal areas (SBA, ranging from 0.2 m 2 ha �1 to 32.6 m 2 ha �1 ) and reference croplands using a paired-site design. Changes of SOC concentration and content, and root C content were analyzed using paired t tests, and the relationships between forest floor C content, soil and root ΔC (ΔC refers to the difference in C stocks between a poplar plantation and the paired cropland) and SBA were tested with a polynomial regression analysis. Results Afforestation resulted in linear increases of ΔC in the forest floor and 0–10 cm mineral soil with SBA (R 2 =0.67, p<0.001 and R 2 =0.34, p=0.003, respectively), but there were no clear relationships between SOC stocks in the soil deeper than 10 cm and SBA. The fine root C stock increased by afforestation across all the soil layers (p<0.05), and root ΔC had a quadratic curve (the first two mineral soil layers) or linear (the other mineral soil layers) relationship with SBA. About 73 % of the variance of ΔC in the top soil was explained by changes in the forest floor C stock, but changes in plant derived C stocks did not explain the variance of soil ΔC in the deeper layers very well. Conclusions Our study suggest that afforestation increased C sequestration in the forest floor and surface mineral soil, and C stocks in the forest floor and surface mineral soil and roots were strongly controlled by the SBA, which changes with stand development, in the studied semiarid region in northeast China.

Drying-induced changes in phosphorus status of soils with contrasting soil organic matter contents – Implications for laboratory approaches

Phosphorus (P) fractions are commonly assessed on dried soils in environmental and long-term field research, because this is the most convenient method of storing large quantities of samples before analysis. However sample pre-treatment may seriously affect the results, especially those regarding the organic P fractions. The objective of the present study was to assess the effect of soil drying under laboratory conditions on the P status of soils with contrasting physico-chemical and microbial properties. We used soils from a (Pinus pinaster) forest in southwest France sampled from different soil depths and different site classes featuring a gradient of soil organic matter (SOM), and aluminium (Al) and iron (Fe) oxides. Total P, total organic and inorganic P, water soluble phosphate ions (iP), microbial P and the potentially mineralizable P in ‘dead’ organic matter were determined on fresh, moist soils maintained at 4°C and on the same soils, oven-dried at 60°C. Laboratory-drying resulted in a general decrease in total organic P, which was positively correlated with microbial P and SOM content. These drying-induced changes in total organic P were associated with a concomitant increase in total inorganic P and water soluble iP, which were dramatic in the organic forest floor samples (on average +2965% (×31 increase) in total inorganic P, and +11880% (×123 increase) in water soluble iP). The changes in water soluble iP were correlated with the SOM: Al and Fe oxides ratio and showed little effect of soil drying in deeper soil layers with a low SOM and high oxide contents. Our results indicated that soil drying mainly resulted in the lysis of the microbial pool, and to a much lesser extent, in the physical disruption and mineralization of the SOM. We conclude that soil drying before analyses can generate a significant bias in the evaluation of soil P stocks and fluxes, especially in SOM-rich layers. We recommend that fresh soil samples should be used as far as possible to determine soil P status.

Five new species of Ceratoppia (Acari: Oribatida: Peloppiidae) from western North America

I present the systematics and distribution of five new species of oribatid mites in the genus Ceratoppia (Oribatida: Peloppiidae) from western North America. The species are described on the basis of adult morphology using the following character states: number of hypostomal setae, number, length and expression of posterior notogastral setae, length of lamellae and lamellar cusp, length of interlamellar setae, and the shape and dentition of the rostrum. Ceratoppia indentata n. sp. is described from forest floor habitats, while Ceratoppia longicuspis n. sp. and Ceratoppia tofinoensis n. sp. are described from arboreal bryosphere habitats; Ceratoppia offarostrata n. sp. is associated with bark habitats. Ceratoppia valerieae n. sp. was collected from both arboreal and forest floor samples. Distributions of all species are provided based on museum and collection records; C. indentata, C. longicuspis, C. tofinoensis are recorded from coastal temperate coniferous rainforests of the Pacific Northwest of North America, while C. valerieae was found in coastal temperate rainforests and extending along the southern border of British Columbia into eastern Alberta. Ceratoppia offarostrata is collected only from a small number of locations on the west coast of Canada. Comments on other North American Ceratoppia species is given. A morphological key is presented to the described adult species for the genus Ceratoppia in North America.

Calcium isotope constraints on the uptake and sources of Ca 2+ in a base-poor forest: A new concept of combining stable (δ 44/42Ca) and radiogenic ( εCa) signals

Plant-available reserves of major base cations, Ca 2+ and Mg 2+, decreased markedly in soils over the past century, thus posing a potential threat to forest ecosystem health. Trees are thought to obtain dissolved Ca 2+ ions mainly from an easily accessible soil–water reservoir also termed the ‘exchangeable cation pool’. The status of Ca reserves in this soil pool is sensitive to anthropogenic perturbations such as soil acidification induced by acid rain and/or excessive timber harvesting. Here we show that in a base-poor forest of the northeastern USA (i.e. Wachusett Mountain, Massachusetts) the ‘exchangeable Ca pool’ of deeper mineral soils has a unique isotope signature that is significantly enriched in the radiogenic 40Ca, due to the dissolution of K-rich silicate minerals such as biotite. Using a simple isotope mass balance, and assuming that the input of Ca from biotite has a ε Ca signature of ∼16, the results of our calculation indicate that the weathering of biotite may supply a sizeable fraction, up to 25%, of Ca 2+ ions into the ‘exchangeable cation pool’ of deeper mineral soils. Importantly, samples of local vegetation (i.e. woody tissues of red oak) show no detectable excess of the radiogenic 40Ca, and based on our model the upper limit of a possible biotite-derived Ca contribution in vegetation is estimated at ∼5%. We also found no evidence of the radiogenic 40Ca signal in the samples of forest floor and the uppermost organic-rich soils (0–15 cm depth), which in turn suggest that over the long-term development of the forest and its organic matter accumulation, the vegetation growth must have also relied primarily on the non-radiogenic Ca sources. Based on our experimental data, such sources may include (i) wet atmospheric deposition, (ii) the organically-complexed Ca in topsoil horizons, and (iii) chemical weathering and/or fungal-mediated dissolution of apatite and Ca-rich plagioclase. Hence, our stable and radiogenic Ca isotope data indicate that the studied base-poor forest is able to bypass the ‘exchangeable cation pool’ of deeper (i.e. below 15 cm) mineral soils, and still manages to meet its nutritional requirements with respect to Ca. Another important implication of this study is that the organically-complexed Ca in the topsoil horizon (0–15 cm depth) has to be tightly bound to the ion exchange sites, otherwise the large radiogenic 40Ca signatures present in the ‘exchangeable cation pool’ of deep mineral soils would be swamped by the downward gravitational flux of non-radiogenic Ca from the decaying organic matter and litterfall. Hence, the limited mobility of the organically-complexed Ca in soils and its tight biological cycling could explain the lack of a significant impact of vegetation on the Ca isotope systematics observed in large rivers.

Chloride and Organic Chlorine in Forest Soils: Storage, Residence Times, And Influence of Ecological Conditions

Recent studies have shown that extensive chlorination of natural organic matter significantly affects chlorine (Cl) residence time in soils. This natural biogeochemical process must be considered when developing the conceptual models used as the basis for safety assessments regarding the potential health impacts of 36-chlorine released from present and planned radioactive waste disposal facilities. In this study, we surveyed 51 French forested areas to determine the variability in chlorine speciation and storage in soils. Concentrations of total chlorine (Cl(tot)) and organic chlorine (Cl(org)) were determined in litterfall, forest floor and mineral soil samples. Cl(org) constituted 11-100% of Cl(tot), with the highest concentrations being found in the humus layer (34-689 mg Cl(org) kg(-1)). In terms of areal storage (53 - 400 kg Cl(org) ha(-1)) the mineral soil dominated due to its greater thickness (40 cm). Cl(org) concentrations and estimated retention of organochlorine in the humus layer were correlated with Cl input, total Cl concentration, organic carbon content, soil pH and the dominant tree species. Cl(org) concentration in mineral soil was not significantly influenced by the studied environmental factors, however increasing Cl:C ratios with depth could indicate selective preservation of chlorinated organic molecules. Litterfall contributions of Cl were significant but generally minor compared to other fluxes and stocks. Assuming steady-state conditions, known annual wet deposition and measured inventories in soil, the theoretical average residence time calculated for total chlorine (inorganic (Cl(in)) and organic) was 5-fold higher than that estimated for Cl(in) alone. Consideration of the Cl(org) pool is therefore clearly important in studies of overall Cl cycling in terrestrial ecosystems.

Forest floor development and biochemical properties in reconstructed boreal forest soils

Following resource extraction by surface mining in the oil sands region of northeastern Alberta, sites are reclaimed by reconstructing soils using a variety of salvaged organic and mineral materials, and planted to native tree species. This study assessed the influence of three distinct stand types ( Populus tremuloides Michx., Pinus banksiana Lamb., and Picea glauca (Moench) Voss) on forest floor development (thickness, morphology, total carbon and nitrogen contents), soil organic matter composition, and associated soil microbial communities. Forest floor and top mineral soil (0–5 cm) samples were collected from 32 sites reclaimed 16–33 years ago. Soil organic matter composition was measured using ramped-cross-polarization 13C nuclear magnetic resonance, and microbial communities were characterized using phospholipid fatty acid analysis. Morphological characteristics indicated little mesofaunal or fungal activities within the forest floors. Stands dominated by P. tremuloides fostered more rapid forest floor development than the coniferous ( P. banksiana and P. glauca) stands, and showed a significant increase in forest floor thickness with time since reclamation. Within the P. tremuloides stands, forest floor development was accompanied by temporal changes in soil organic matter composition that reflected inputs from the canopy. Soil microbial community composition differed among reclamation treatments of the reconstructed soils, specifically as a function of their subsoil mineral textures, when canopy cover was below 30%. Above 30%, significant differences became apparent among stand types. Taken together, our results document how canopy cover and stand type were both important factors for the reestablishment of plant–soil relationships at these sites. Furthermore, achieving a canopy cover of 30% emerged as a critical threshold point during soil reclamation.