Coniferous Forest Floor Research Articles

Phenotypic plasticity and thermal efficiency of temperature responses in two conspecific springtail populations from contrasting climates

Temperature drives adaptation in life-history traits through direct effects on physiological processes. However, multiple life-history traits co-evolve as a life-history strategy. Therefore, physiological limitations constraining the evolution of trait means and phenotypic plasticity can be larger for some traits than the others. Comparisons of thermal responses across life-history traits can improve our understanding of the mechanisms determining the life-history strategies. In the present study, we focused on a soil microarthropod species abundant across the Northern Hemisphere, Folsomia quadrioculata (Collembola), with previously known effects of macroclimate. We selected an arctic and a temperate population from areas with highly contrasting climates — the arctic tundra and a coniferous forest floor, respectively — and compared them for thermal plasticity and thermal efficiency in growth, development, fecundity, and survival across four temperatures for a major part of their life cycle. We intended to understand the mechanisms by which temperature drives the evolution of life-history strategies. We found that the temperate population maximized performance at 10–15 °C, whereas the arctic population maintained its thermal efficiency across a wider temperature range (10–20 °C). Thermal plasticity varied in a trait-specific manner, and when considered together with differences in thermal efficiency, indicated that stochasticity in temperature conditions may be important in shaping the life-history strategies. Our study suggests that adopting a whole-organism approach and including physiological time considerations while analysing thermal adaptation will markedly improve our understanding of plausible links between thermal adaptation and responses to global climate change.

Dosimetric depth distribution of Fukushima-derived 137Cs in coniferous forest soil

The ambient dose rate (ADR) in a coniferous forest floor contaminated by Fukushima-derived 137Cs was calculated for small temporal scale as a function of depth. The calculated total ADR was statistically evaluated along the soil vertical strata and among the sampling time series. Moreover, the contribution of the forest floor components to the total radiation dose were examined. Accordingly, the litter and fragmented organic layers accounted for about 58 to 96% (mean 77 ± 14%) of the ambient dose, indicating the organic layer is a chief sources of external radiation exposure, and an important radiological soil section.

Impact of forest thinning on the dynamics of litterfall derived 137Cs deposits in coniferous forest floor after Fukushima accident

The effects of a 50% forest thinning intensity on Fukushima-derived 137Cs deposition by litterfall and its discharge by runoff in hillslope coniferous forest were monitored using four litterfall traps and a hillslope erosion plot. The observation was underway during the pre-and post-thinning periods. Results demonstrated that during the pre-thinning period a total 150 ± 13 g m−2 of litterfall deposited about 924 ± 69 Bq m−2 of 137Cs. This accounts for 11% of the local 137Cs fallout recorded for the study site in the aftermath of the accident. After thinning, both litterfall and 137Cs increased by more than six- and two-fold, respectively. This is possibly owing to the slow individual tree recovery rate assisted by the change on the running space provided by canopy openings, which can accelerate even the normal gust wind to gain damaging power on the unshielded mechanically injured parts of the contaminated residual trees. In both cases, litterfall generally transferred about 37% (3 ± 0.2 kBq m−2) of the local 137Cs fallout onto the forest floor over the observation period. The eroded litter-associated 137Cs increased by about a factor of two after thinning, which only accounted for less than 1% of 137Cs deposited by litterfall. This implies that the forest floor retains 137Cs and remains contaminated regardless of the size of the eroded litter material. But this could become a potential secondary contamination source for the downstream resources such as water bodies and villages, especially at the time of flooding, which in turn calls a serious attention in designing decontamination schemes.

Soil organic matter composition and quality across fire severity gradients in coniferous and deciduous forests of the southern boreal region

AbstractRecent patterns of prolonged regional drought in southern boreal forests of the Great Lakes region, USA, suggest that the ecological effects of disturbance by wildfire may become increasingly severe. Losses of forest soil organic matter (SOM) during fire can limit soil nutrient availability and forest regeneration. These processes are also influenced by the composition of postfire SOM. We sampled the forest floor layer (i.e., full organic horizon) and 0–10 cm mineral soil from stands dominated by coniferous (Pinus banksiana Lamb.) or deciduous (Populus tremuloides Michx.) species 1–2 months after the 2011 Pagami Creek wildfire in northern Minnesota. We used solid‐state 13C NMR to characterize SOM composition across a gradient of fire severity in both forest cover types. SOM composition was affected by fire, even when no statistically significant losses of total C stocks were evident. The most pronounced differences in SOM composition between burned and unburned reference areas occurred in the forest floor for both cover types. Carbohydrate stocks in forest floor and mineral horizons decreased with severity level in both cover types, whereas pyrogenic C stocks increased with severity in the coniferous forest floor and decreased in only the highest severity level in the deciduous forest floor. Loss of carbohydrate and lignin pools contributed to a decreased SOM stability index and increased decomposition index. Our results suggest that increases in fire severity expected to occur under future climate scenarios may lead to changes in SOM composition and dynamics with consequences for postfire forest recovery and C uptake.

The role of litterfall in transferring Fukushima-derived radiocesium to a coniferous forest floor

The deposition of Fukushima-derived radiocesium via falling litter in a coniferous forest 180km downwind immediately following the nuclear power plant accident was investigated. The litterfall contribution to the transfer of radiocesium from the forest canopy to the forest floor was determined, and this pathway was compared with hydrological pathways. The results demonstrated that during the observation period, a total of approximately 5.5kBqm−2 of Fukushima-derived radiocesium was deposited on the forest floor through throughfall (53%), stemflow (2.3%) and litterfall (45%) routes. The data revealed that the contributions of hydrological pathways became less important as time passed. However, the litterfall route, which transferred approximately 31% (2.5±0.6kBqm−2) of the local fallout within the observation period, continued depositing radiocesium onto the forest floor.

Utility of information in photographs taken upwards from the floor of closed-canopy deciduous broadleaved and closed-canopy evergreen coniferous forests for continuous observation of canopy phenology

Hemispherical photographs taken on forest floors are used to monitor seasonal changes in canopy openness or leaf area index in ecological studies. Those analyses usually use black and white images converted from the original colour images. Photographs taken by downwards-facing cameras installed on towers are used to provide detailed information on leaf expansion, maturation and senescence of various tree species through the analysis of red, green and blue ‘digital numbers’ (DNRGB) extracted from those images. To examine the usefulness of colour information encoded in upwards hemispherical photographs in monitoring canopy phenological characteristics, we examined the consistency of DNRGB values between downwards and upwards images in deciduous broadleaved and evergreen coniferous forests in Japan. In the deciduous broadleaved forest, the DNRGB values in the upwards images were able to detect canopy phenology almost as well as those in the downwards images. However, we found the effects on DNRGB of (1) the spatial heterogeneity among observed points, (2) low-vegetation (before the beginning of leaf-expansion and after the end of leaf-fall period) and (3) white balance settings. In the evergreen coniferous forest, in contrast, the DNRGB values in the upwards images did not capture canopy phenology. These different results may be attributable to the light attenuation characteristics in the canopies due to the geometries of leaves and branches. Thus, the DNRGB values obtained from upwards images are almost as good as those of downwards images for monitoring detailed canopy phenology in deciduous broadleaved forests with a closed canopy.

Influence of tree species on carbon and nitrogen transformation patterns in forest floor profiles

Among forest management practices, forest tree species substitution influences biogeochemical cycles and soil interactions rapidly (decades) and significantly. We studied the impact of tree species on forest floor properties in 27-year-old monospecific stands of oak, beech, spruce and Douglas-fir, grown in common garden. In spite of similar litterfall C-inputs (1.3–1.8 Mg ha-1 yr-1), C-stocks strongly differed between coniferous (16–21 Mg ha-1) and broadleaved (7–9 Mg ha-1) forest floors. The large C-mineralization under oak (196 C-μg Ctotal-g-1 d-1) relatively to other stands (125–147 C-μg Ctotal-g-1 d-1) and the large dissolved organic carbon release under broadleaved forest floors may explain this difference. Spruce forest floor exhibited the lowest net nitrification and the largest contribution of organic anions to the anionic charge of aqueous leachate (62.5 % ± 1.7). Conversely, Douglas-fir forest floor exhibited the largest relative nitrification and the largest contribution of nitrate to the anionic charge (70.8 % ± 0.7). We conclude that tree species induce a specific signature in term of proton donors at the forest floor level (organic vs nitric acid), which involves potential impacts on pedogenetic processes in the mineral horizon beneath.

Does Soil Organic Layer Thickness Affect Climate–Growth Relationships in the Black Spruce Boreal Ecosystem?

The observed long-term decrease in the regional fire activity of Eastern Canada results in excessive accumulation of organic layer on the forest floor of coniferous forests, which may affect climate–growth relationships in canopy trees. To test this hypothesis, we related tree-ring chronologies of black spruce (Picea mariana (Mill.) B.S.P.) to soil organic layer (SOL) depth at the stand scale in the lowland forests of Quebec’s Clay Belt. Late-winter and early-spring temperatures and temperature at the end of the previous year’s growing season were the major monthly level environmental controls of spruce growth. The effect of SOL on climate–growth relationships was moderate and reversed the association between tree growth and summer aridity from a negative to a positive relationship: trees growing on thin organic layers were thus negatively affected by drought, whereas it was the opposite for sites with deep (>20–30 cm) organic layers. This indicates the development of wetter conditions on sites with thicker SOL. Deep SOL were also associated with an increased frequency of negative growth anomalies (pointer years) in tree-ring chronologies. Our results emphasize the presence of nonlinear growth responses to SOL accumulation, suggesting 20–30 cm as a provisional threshold with respect to the effects of SOL on the climate–growth relationship. Given the current climatic conditions characterized by generally low-fire activity and a trend toward accumulation of SOL, the importance of SOL effects in the black spruce ecosystem is expected to increase in the future.

Microbial community structure and function: The effect of silvicultural burning and topographic variability in northern Alberta

Forest floor chemistry and microbial communities can be influenced by forest land management, such as harvesting and prescribed burning. Here, we used phospholipid fatty acid (PLFA) and multiple carbon-source substrate-induced respiration (MSIR) analyses to characterize microbial communities of deciduous, mixedwood and coniferous boreal forest floors with different silvicultural treatments. The sites were stem-only harvested with 10% retention, and silvicultural treatments consisting of slash being evenly distributed on the site and then burned, or not burned. The burned sites exhibited lower microbial biomass and greater NO 3 − concentrations than the unburned sites. However, burning appeared to have no effect on forest floor microbial community structure or function. On the other hand, during drier months (August sampling), the composition of forest floor microbial communities appeared to be strongly influenced by topographic position rather than stand related differences. Harvested sites located at higher elevations had similar microbial communities, regardless of the overstory composition, while coniferous and mixedwood sites located at lower elevations had similarly structured microbial communities that were distinct from deciduous sites. Differences in microclimatic conditions of the forest floor between higher elevation sites and lower elevations sites may select for some microbial groups over others. Indicator analysis found a strong association of a fungal PLFA biomarker (20:1ω9c), with sites at higher elevation, while a biomarker for actinomycetes (10Me19:0) was strongly associated with deciduous sites at lower elevation. Structural differences in microbial communities observed between sites at higher and lower elevations appear to be linked to seasonal patterns in moisture, as previous studies in this region found no apparent effect of elevation during times of higher monthly precipitation.

Oribatid mites (Acari: Oribatida) in boreal forest floor and decaying wood

We studied the oribatid mite communities in forest floor and in decaying wood in pine (Pinus sylvestris), spruce (Picea abies) and deciduous (mainly Betula pendula and B. pubescens) forests at different latitudes in Finland. The study sites were either in mature managed forests or in old-growth forests in nature reserves. Altogether 78 sites were sampled in 2004 and 2005, yielding a total of 38,145 oribatid mites belonging to 133 species, of which four were new to Finland. Oribatid mite communities differed in terms of total number and community structure between forest types and latitudes within the boreal forest zone. The most abundant and diverse communities were in spruce forests and at central and southern latitudes. Total oribatid number in deciduous forests was three-fold lower compared to that in coniferous forests; still the number of species was roughly equal in all forests. The greatest differences in communities were observed between forest floor and decaying wood microhabitats. Total densities in decaying wood were three-fold lower than in coniferous forest floor. Strong differences were also observed in community structure (indicated by non-metric multidimensional scaling (NMDS)), although the majority of the species encountered were shared. Our results suggest that decaying wood maintains diverse sub-communities of mites which vary little between different forests and latitudes. Coarse woody debris (CWD) provides favorable habitat for many forest floor species and arboreal habitat species.

The contribution of fresh litter to dissolved organic carbon leached from a coniferous forest floor

SummaryThe relative contributions of litter and humified organic matter as the source of dissolved organic carbon (DOC) leached from organic layers of forest soils are poorly understood. In the present investigation, 13C labelled spruce litter was used to study the role of recent litter in the leaching of DOC from a coniferous forest floor in southern Sweden, while litterbags were used to quantify the total loss of C from the labelled litter. The labelled litter applied on bare lysimeters released considerable amounts of DOC during the first weeks, but the concentration of DOC originating from labelled litter decreased gradually from 176 mg litre−1 during the first sampling period in May to 5 mg litre−1 in the last sampling period in October. Only a moderate flush of DOC from the labelled litter occurred under the Oe and Oa horizons, with concentrations of 20 and 6 mg litre−1 from labelled litter, equal to 19 and 9% of the total DOC flux, respectively, during the first sampling period. Total flux of DOC from labelled litter from May to September was 16 g m−2, whereas only 2.2 and 0.9 g m−2 were captured under the Oe and Oa horizons, respectively. The almost complete loss of new DOC implies that DOC leached from the Oe and Oa horizons consists not of recent litter‐derived carbon, but of DOC produced in these two horizons themselves. Water‐extractable organic carbon from labelled litter left in litterbags in the field for 4 months consisted of about one‐third native carbon from external sources at the experimental site and two‐thirds of the labelled litter. In contrast, the 13C content of the bulk litter from the litterbags was not changed by the incubation in the field. We suggest that the soluble native carbon in water extracts originated from throughfall DOC that had been assimilated by microorganisms in the litterbags.

Greenhouse gas fluxes of coniferous forest floors as affected by wood ash addition

Wood ash has been used to alleviate nutrient deficiencies of peat forests and to combat acidification of forest soils. Ash may change the activities of soil microbes, including those producing or consuming greenhouse gases, such as methane (CH 4), nitrous oxide (N 2O) and carbon dioxide (CO 2). We studied the effects of wood ash (loose wood ash originating from pulp mill or power plants) on the fluxes of CH 4, N 2O and CO 2 in forests with mineral or peat soils in northern Finland. The ash doses were from 3 to 8 t ha −1. Gas fluxes were measured with a closed chamber method from five recently fertilized experiments for 1 year after application of ash and from five long-term trials 14–50 years after application. Wood ash did not affect N 2O gas fluxes. In the long-term experiments, wood ash increased the soil CO 2 production and the CH 4 uptake and lowered the CH 4 emissions.

Dissolved Organic Carbon Leaching from a Coniferous Forest Floor -- A Field Manipulation Experiment

Leaching of dissolved organic carbon (DOC) from the O layer is important for the carbon cycling of forest soils. Here we study the role of the Oi, Oe and Oa horizons in DOC leaching from the forest floor in field manipulations carried out in a Norway spruce forest stand in southern Sweden. The manipulations involved the addition and removal of litter and the removal of Oi, Oe and Oa horizons. Our data suggest that both recent litter and humified organic matter contribute significantly to the leaching of dissolved organic matter from the O layer. An addition of litter corresponding to four times annual litterfall resulted in a 35% increase in DOC concentrations and fluxes although the specific UV absorbance remained unchanged. The removal of litterfall and the Oi horizon resulted in a decreased DOC concentration and in a significant increase in the molar UV absorptivity. The DOC concentration under the Oa horizon was not significantly different from that under the Oe horizon and there were no increase in DOC flux, but rather a decrease, from the bottom of the Oe horizon to the bottom of the Oa horizon, suggesting that there is no net release of DOC in the Oa horizon. However, significant leaching of DOC occurred from the Oa horizon when litterfall and the Oi and Oe horizons were removed. This indicates that there is both a removal of DOC from the Oi and Oe horizons and a substantial production of DOC in the Oa horizon. Quantitatively, we suggest that the Oi, Oe and Oa horizons contributed approximately 20, 30 and 50%, respectively, to the overall leaching of DOC from the O layer.

DOC leaching from a coniferous forest floor: modeling a manipulation experiment

AbstractThe DyDOC model simulates the C dynamics of forest soils, including the production and transport of dissolved organic matter (DOM), on the basis of soil hydrology, metabolic processes, and sorption reactions. The model recognizes three main pools of soil C: litter, substrate (an intermediate transformation product), and humic substances. The model was used to simulate the behavior of C in the O horizon of soil under a Norway spruce stand at Asa, Sweden, that had been subjected to experimental manipulations (addition and removal) of above‐ground litter inputs and to removal of the Oi and Oe layers. Initially, the model was calibrated using results for the control plots and was able to reproduce the observed total soil C pool and 14C content, DOC flux and DO14C content, and the pool of litter C, together with the assumed content of C in humic substances (20% of the total soil C), and the assumed distribution of DOC between hydrophilic and hydrophobic fractions. The constant describing DOC exchange between micro‐ and macropores was estimated from short‐term variations in DOC concentration. When the calibrated model was used to predict the effects of litter and soil manipulations, it underestimated the additional DOC export (up to 33%) caused by litter addition, and underestimated the 22% reduction in DOC export caused by litter withdrawal. Therefore, an additional metabolic process, the direct conversion of litter to DOC, was added to the model. The addition of this process permitted reasonably accurate simulation of the results of the manipulation experiments, without affecting the goodness‐of‐fit in the model calibration. The results suggest that, under normal conditions, DOC exported from the Asa forest floor is a mixture of compounds derived from soil C pools with a range of residence times. Approximately equal amounts come from the litter pool (turnover time 4.6 yr), the substrate pool (26 yr), and the humic‐substances pool (36 yr).

Distribution of 36 Element Deposition Rates in a Historic Mining and Smelting Area as Determined Through Fine-Scale Biomonitoring Techniques. Part II: Relative Long-Term Accumulated Atmospheric Deposition Levels

The content of 36 elements in the H horizon of coniferous forest floor humus was determined for 56 sampling plots situated in linear transects running out radially to a distance of 14 km from a lead smelter. Fine-scale contour maps of element content in humus revealed five types of basic patterns for the positions of hot spots. Using PCA and contour maps the following four dominant pollution sources were identified that had influenced the accumulation of some elements in humus in the area (615 km2): operation of a lead smelter (Ag, Bi, Cd, Cu, Fe, Hg, In, Pb, S, Sb, Zn), gravel production from stones from uranium pit heaps (Al, Be, Co, Cr, Ga, Mn, Sc, Se, Th, U, V), operation of works processing heat resistant steel (Mo) and anthropogenic or local geogenic factors (Ce, Co, La, Nd, Pr, Y). Except in the case of Pb, Cd, Zn, (As, Hg), high current and long-term accumulated loads of the other elements named above have not previously been indicated in the literature for this area. In addition, no patterns of accumulated elements in humus in details of registers and allotments are known for this area. The interpolated hot spot area for heavy forest humus contamination was found to be 5–80 km2, however, U, Cd and Pb were highly accumulated in humus at 100, 320 and more than 630 km2, respectively. At a distance of 200 m and 22 km windward and 600 m and 22 km leeward from the lead smelter, the Pb content in the forest floor humus was 78 500 and 131 μg g-1 windward and 81 050 and 175 μg g-1 leeward, respectively. The normal Pb content in forest floor humus is less than 100 μg g-1 in the Czech Republic. Also patterns of accumulated elements in humus in details of registers and allotments have not been known for the area up to now. The distributions of the long-term accumulated elements in humus related to the distance from the source could be described nearly perfectly by exponential equations, while elevation, topography, leeward/windward exposure, and the geology of the sampling plots have not significantly influenced accumulation ofmost elements. Results of the forest floor humus analysis were fully supported by the parallel moss monitoring results. Humus monitoringtechniques can reliably determine the long-term effects of pollutionsources and reveal the hidden accumulated loads of elements in details in the landscape.

Distillation in a boreal mossy forest floor

The feathermoss-dominated floor of coniferous boreal forests can experience midsummer drought. From ecophysiological studies, based on single shoots, it is unclear how the live moss carpet can survive such stress. External capillary wicking from the lowest, moist organic layers is one possibility. Another is evaporation from the same source followed by condensation on the upper, live moss shoots (distillation). A laboratory wicking experiment showed that, under ideal conditions, much of the organic forest floor profile can be supplied with moisture by capillarity from below. However, the uppermost live moss shoots could not be hydrated by this mechanism. In contrast, a gravimetric field experiment indicated nocturnal mass gain by turves of live moss shoots, placed in situ on the forest floor, during dry-down conditions. For turf treatments with an underlying vapour barrier, no such mass gain was evident. Turf treatments with a vapour barrier on top were little different from controls. It is concluded that nocturnal distillation occurs during all summer dry-downs and that this is likely to ensure moss shoot survival during diurnal periods of drought stress. Limited microclimate monitoring indicated that nocturnal cooling at the forest floor surface was sufficient to bring the moss shoot surfaces to the dew point and to reverse the daytime temperature gradient through the organic forest floor profile. This appears to be most noticeable late in the growing season when the lowermost organic layers have progressively warmed throughout the summer.

Distribution of Sulphur and Heavy Metals in Forest Floor Humus of the Czech Republic

Forest humus, which has a large cation binding capacity (up to 550 meq/100 g), and provides relatively very stable bindings with elements and their compounds, can effectively adsorb elements from atmospheric deposition and retain them overa long period. In order to test the monitoring potential of usingforest humus to retain the recent and long-term accumulated loadsof 14 elements (Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mo, Ni, Pb, S, V,and Zn), samples of coniferous forest floor humus (H or Oh horizon) were collected over an area of 78 864 km2 using a 20 × 20 km scale grid of sampling points overlaid on theentire territory of the Czech Republic (CZ). Data from 192 pointsand the distribution of the concentrations of individual elementsin forest floor humus are presented here. The extent of relatively very high loading covers 18% of CZ and includes thefollowing four hot spots: an area in the western part of centralBohemia, northwestern Bohemia in the lignite coal mining area, the cross-border area in the Sudeten mountains in northern Bohemia, and the black coal mining area and industrial centre innorthern Moravia. Relatively high, moderate and low loads of theelements were observed in 42, 30, and 28%, respectively, of CZ.Coniferous forest floor humus is evenly spread in the Central European territory and has proved to be a suitable and sensitiveindicator of long-term relative atmospheric deposition loads.

Lignin demethylation and polysaccharide decomposition in spruce sapwood degraded by brown rot fungi

The organic residues produced in the brown-rot (BR) of wood by many basidiomycetes fungi are ubiquitous on most coniferous forest floors. This degraded wood tissue is characterized by low levels of polysaccharides and a high proportion of demethylated lignin with minor glycerol side chain oxidation. Because of the selective enrichment in an aromatic dihydroxy-rich lignin residue, the chemical and biological reactivity of BR degraded wood will be distinctly different from white rot, the other primary class of fungal wood decay, which typically produces oxidized, lignin-depleted residues. The biochemical mechanism by which BR fungi perform this distinctive degradative chemistry is only starting to become known, and molecular studies which examine the chemical changes imparted to lignin over the long-term decay process are lacking. Using 13C-labeled tetramethylammonium hydroxide thermochemolysis (13C-TMAH) and solid state 13C NMR, we investigated the relationship between lignin oxidation/demethylation and polysaccharide metabolism in a 32-week time series study of spruce sapwood inoculated with either of two BR fungi (Postia placenta and Gloeophyllum trabeum). Our findings demonstrate a close relationship between lignin demethylation and polysaccharide loss and suggest demethylation may play a mechanistic role in polysaccharide loss, possibly by assisting in Fenton reactions where catechol/quinone oxidation and cycling aids in iron reduction. The residue remaining after 16 weeks of decay is devoid of polysaccharides, in contrast to the 68% polysaccharide carbon present in the initial spruce, and exhibits an increased aromatic dihydroxy content (resulting from demethylation of the 3-methoxyl carbon) of up to 22% of the lignin, as determined by 13C-TMAH thermochemolysis. In a typical soil or porewater environment these chemical changes would make BR residues highly reactive toward redox sensitive polyvalent metals (e.g. ferric iron) and likely to adsorb to metal hydroxide surfaces.

Nitrogen availability in forest floors of three tree species on the same site: the role of litter quality

Forest floor samples from a 25-year-old plantation of three tree species (Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.), and paper birch (Betula papyrifera Marsh.)) growing on the same site were incubated (aerobically) in the laboratory for 29 days. Rates of N mineralization in the forest floors of Douglas-fir (165.1 µg/g) was significantly greater than either birch (72.9 µg/g) or lodgepole pine (51.2 µg/g). Douglas-fir forest floors also had the highest N concentration, lowest C/N ratio, and highest NH4-N concentrations, followed by paper birch and lodgepole pine. Douglas-fir forest floors also mineralized more N per unit of either N or C than the other species. There were no differences in rates of CO2-C mineralization in forest floors among the three species. Nitrogen mineralization rates were positively correlated with the N concentration of the forest floor (r2 = 0.81) and also with the C/N and NH4-N concentration of the forest floor. Nitrogen concentration, C/N, and lignin/N of foliar litter were poor predictors of N mineralization rates resulting from Douglas-fir litter having the lowest N concentrations in litter but the highest rates of net N mineralization in the forest floor. Nitrogen mineralization in the forest floor was negatively correlated (r2 = 0.67) with the lignin concentration in foliar litter. Douglas-fir litter had low lignin concentrations, which may allow more of the mineralized N to remain in inorganic forms rather than being bound in humus. Our results suggest that a component of Douglas-fir might improve N availability in coniferous forest floors.

Microbial C and N transformations during drying and rewetting of coniferous forest floor material

Microbial C and N transformation rates in air-dried and subsequently rewetted coniferous forest floor material were examined in a laboratory incubation study. Gross N transformation rates were determined through parallel 15NH 4 + and 15NO 3 − enrichment experiments. After drying of the litter for 12 d to a water content of 10% of dry weight, CO 2 respiration, net N mineralization and nitrification rates were strongly restricted. Microbial biomass C was reduced to 67% of the amount in the continuously moist material. Remoistening of the dry litter to a water content of 340% resulted in a flush in C and net N mineralization within a few hours after rewetting. The increase in net N mineralization could be attributed to a larger increase in gross N mineralization relative to the increase in gross N immobilization. Gross N immobilization had increased to the same rate as gross N mineralization after 26 h, and a small secondary peak in respiration and microbial C was observed 48 h after rewetting. It was concluded that both biomass-derived substrate with a low C-to-N ratio and `nonbiomass'-derived substrate with a high C-to-N ratio have been released, and metabolized, as a result of the drying–rewetting treatment. Despite the very extreme drying treatment, the mineralization flush after rewetting could not compensate for the large reduction in CO 2 and mineral N production during dry conditions due to its short duration. Since there was no increase in nitrification rate after drying and rewetting, the NO 3 − concentration at the end of the incubation was strongly reduced due to the extremely slow net nitrification rates during desiccation.