Forest Floor Samples Research Articles

Evidence of species interactions within an ectomycorrhizal fungal community

Ectomycorrhizal fungal communities can be structured by abiotic and biotic factors. Here, we present evidence for community structuring by species interactions. We sampled ectomycorrhizas and forest floor seven times during a 13-month period. The presence of various ectomycorrhizal fungal species was determined for each sample, and species co-occurrence analyses were performed. For both ectomycorrhizas and forest floor samples there was significantly less co-occurrence among species within the community than expected by chance, mostly because of negative associations involving Cenococcum geophilum or Clavulina cinerea. For some species pairs, there was significantly more co-occurrence than expected by chance. Both nitrogen and tannin additions to the forest floor altered some interactions among species. The causes of these nonrandom distributions are currently unknown. Future investigations on competition, antibiosis, parasitism and facilitation among ectomycorrhizal fungal species appear to be warranted.

Temperature sensitivity of mineral N transformation rates, and heterotrophic nitrification: possible factors controlling the post-disturbance mineral N flush in forest floors

A major forest disturbance such as clearcutting may bring on a flush of mineral N in organic forest floor horizons, but the magnitude of this flush can vary markedly from one ecosystem to another. For example, it was previously established that clearcutting in a high elevation Engelmann spruce-subalpine fir (ESSF) ecosystem results in significantly higher NH 4 + and NO 3 − concentrations, whereas clearcutting in an old-growth coastal western hemlock (CWH) ecosystem has little effect on mineral N dynamics. We hypothesized that the higher mineral N flush observed in the ESSF ecosystem is due to a greater temperature sensitivity of mineral N transformation rates, and to a lower proportion of heterotrophic nitrifiers, compared to the CWH ecosystem. To test these two hypotheses, we sampled forest floors several times over the growing season from clearcut and old-growth plots in both ecosystems, and measured gross mineral N transformation rates at field temperatures and at 10 °C above field temperatures, as well as with and without acetylene to inhibit autotrophic nitrifiers. Gross NH 4 + transformations rates ranged between 20 and 120 μg N (g forest floor) −1 day −1 at the ESSF site, and between 15 and 40 μg N (g forest floor) −1 day −1 at the CWH site. Higher temperature increased gross NH 4 + transformation rates in forest floor samples at both sites, but the average Q 10 value was higher at the ESSF site (3.15) than at the CWH site (1.25). Temperature sensitivity at the ESSF site was greater in clearcut plots ( Q 10=4.31) than in old-growth plots ( Q 10=1.98). Gross NO 3 − transformation rates ranged between 10 and 32 μg N (g forest floor) −1 day −1 at the ESSF site, and between 10 and 24 μg N (g forest floor) −1 day −1 at the CWH site, but there were no significant effects of temperature or clearcutting on gross NO 3 − transformation rates at either site. Likewise, there were no significant differences in the proportion of heterotrophic nitrifiers between sites. Overall, our results support the view that the temperature sensitivity of microbial processes may explain the magnitude of the NH 4 + flush in some coniferous ecosystems, but we lack the evidence relating the magnitude of the NO 3 − flush to the proportion of heterotrophic nitrifiers.

Soil parent material may control forest floor properties more than stand type or stand age in mixedwood boreal forests

:Soil quality in the boreal mixedwood forest depends to a certain extent on the chemical, biochemical, and microbial properties of the forest floor. Given that forest floors are composed mainly of decomposing plant detritus, forest composition (i.e., stand type) should play a key role in determining soil quality. Stand age is also expected to control certain forest floor properties, due to the balance in the amount and quality of substrates introduced into, and lost from, the forest floor during stand development. Finally, mineral parent material should also be related to some forest floor properties, by controlling site nutrient capital which may affect litter quality. In order to compare the relative effects of these three factors, forest floor material was collected in the Abitibi region of northwestern Québec, from replicated (n = 3) treatment plots comprising a factorial array of three stand types (spruce/fir, aspen, and paper birch), three stand ages (57, 78-85, and 131 y-old), and two parent materials (clay and till). Forest floor samples were compared on the basis of 18 variables depicting i) chemical properties, ii) base cation status, iii) nitrogen cycling, and iv) microbial properties. All three sampling factors exerted a significant effect on some of the measured forest floor properties, but parent material had a significant effect on twice the number of forest floor properties as did stand type or stand age. Besides exerting an effect on nutrient capital, we propose that parent material also affects cation exchange capacity and unsaturated hydraulic conductivity, which in turn influence litter quality and ecosystem productivity. Our results suggest, therefore, that a detailed classification of mineral parent materials might be a useful basis for interpreting and predicting soil quality in the mixedwood boreal forest.

Effect of gap size on litter decomposition and soil nitrate concentrations in a high-elevation spruce–fir forest

Possible mitigation of nitrate losses associated with clearcuts through harvesting smaller gaps was tested in a high-elevation forest of Engelmann spruce (Picea engelmannii Parry ex Engelm.) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.). We measured concentrations of ammonium and nitrate after 6-week buried bag incubations of forest floor and mineral soil samples in replicated plots of uncut forest and gaps of 10, 1.0, and 0.1 ha and single-tree removal for 7 years after harvest. Nitrate concentrations in forest floor and mineral soil were elevated 3–7 years after harvesting in gaps of 0.1 ha and larger. Removal of the same proportion of trees as single trees did not result in increased nitrate concentrations, suggesting that nitrate losses could be reduced by harvesting single trees rather than creating gaps. Greater N availability was not associated with faster rates of decomposition of litter and forest floor, which were similar in gaps of all sizes (0–10 ha). Reciprocal transplant of forest floor and soil from the 10-ha gaps and the uncut forests indicated that changes in the nature of the forest floor or soil following harvest had a greater influence on nitrate concentrations than the changes in environmental conditions in the gaps.

Microbial biomass, nitrogen and phosphorus mineralization, and mesofauna in boreal conifer and deciduous forest floors following partial and clear-cut harvesting

The effects of partial and clear-cut harvesting on forest floor physical, chemical, and biological properties, forest floor mesofauna, and nutrient cycling were investigated in conifer- and deciduous-dominated stands of Alberta's mixedwood boreal forest. Forest floor samples were collected 2.5 years after harvest from clearcuts, strip-cut corridors in a partial cut, green tree retention patches in a partial cut, and uncut control sites. Partial cuts showed intermediate decreases in annual litter input and NH4-N between uncut and clear-cut sites of both the deciduous and conifer stands, as did microbial biomass, PO4-P, mesofauna abundance (total, Acari, and Collembola), and fine root biomass in the conifer stands. In the deciduous stands, microbial biomass and fine root biomass in partial- and clear-cut treatments were not significantly different, but were significantly reduced compared with the uncut controls. Mesofauna abundance was reduced in the corridors of the partial-cut treatment compared with partial-cut patch, clear-cut, and uncut treatments. In both deciduous and conifer stands, NO3-N was elevated in the partial-cut corridors and clearcuts compared with partial-cut patch and uncut treatments. Findings from this study show that negative impact to forest floor properties associated with clear-cut harvesting may be reduced in partial-cut harvesting systems.

Responses of available soil nitrogen and litter decomposition to openings of different sizes in dry interior Douglas-fir forests in British Columbia

Selection of an appropriate silvicultural system that maintains productivity requires an understanding of the effects of harvest type on post-harvest changes in nitrogen availability. The effects on inorganic nitrogen availability and litter decomposition of various harvesting systems, including partial canopy removal and a range of clearcut opening sizes, were investigated in a dry Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) forest in southern British Columbia. We measured concentrations of ammonium and nitrate after 6-week buried bag incubations of forest floor and mineral soil samples in replicated plots within uncut forest, within partially harvested forest, and in openings of 1.7, 0.4, and 0.1 ha, from two to seven growing seasons after harvest. Post-incubation nitrate and the proportion of inorganic N as nitrate in forest floor and mineral soil were elevated from 2 to 7 years after harvesting in the 1.7 ha openings. There was no consistent trend of higher ammonium concentration in either uncut forest or 1.7 ha openings. The proportion of inorganic N as nitrate was consistently the lowest in the uncut forest, and often significantly different from any opening treatment, including partial cutting. The increased proportion of inorganic N as nitrate occurred within 11 m of the uncut forest edge. At very low moisture contents, nitrate concentrations tended to zero only in the uncut forest. Leaching of the increased available nitrate is unlikely because of the large increase in grass biomass that occurs after harvesting, and the dry soil moisture conditions during the growing season. Decomposition of Douglas-fir, lodgepole pine and trembling aspen litters, or forest floor was inconsistently, or not affected by any harvest treatment or by distance from, or aspect of, the edge of the forest. The post-harvest increases in nitrate concentrations cannot be attributed to faster decomposition.

Short-term increases in relative nitrification rates due to trenching in forest floor and mineral soil horizons of different forest types

The representation of NO3− dynamics within forest growth simulation models could improve forest management. An extensive literature review revealed an 88% probability of measuring a higher relative nitrification index (i.e. RNI = [NO3−] ÷ [NO3− + NH4+]) in mineral soil horizons than in forest floors, across a wide range of conifer and hardwood ecosystems. We then hypothesised that humus form and fine root density could be used as two crude variables to predict changes in in situ, potential and relative nitrification rates. Twenty-seven trench plots were established in 1999, across nine contrasting hardwood and coniferous stands in the Eastern Townships of Quebec. Forest floor and mineral soil samples were collected from each plot, and from a 1 m radius surrounding each plot, on three dates during summer 2000. In situRNI values increased significantly in trench plots as the season progressed. Potential nitrification rates (i.e. NO3− concentrations following incubation) were two orders of magnitude higher in forest floor than in mineral soil samples. RNI was significantly higher in mineral soil than in forest floor samples after incubations, but the relative increase in RNI due to trenching was higher in forest floor samples. Indices of available C were significantly higher in forest floor than mineral soil samples, and decreased only in forest floor samples during incubations. Likewise, trenching significantly reduced available C in forest floor samples only. Seasonal changes in soil temperature and fine root growth were the most plausible explanations for seasonal changes in NO3− dynamics, whereas other factors such soil acidity and moisture appeared less important in determining NO3− dynamics in this study. We conclude that crude assessments of humus form and fine root density have the potential to be used as calibration parameters for the simulation of NO3− dynamics in forest growth and yield models.

Immobilisation of molybdate by iron oxides: effects of organic coatings

It has been suggested that molybdate penetrates into micropores and interdomains of iron oxides. This process will cause immobilisation of molybdate. The influence of organic matter that may occlude the pores by adsorptive cover has not yet been examined. Thus, the aim of our study is to elucidate the role of organic coatings around iron oxides for intraparticular molybdate diffusion. We used two synthetic goethites of different crystallinity (specific surface G13: 13 m 2 g −1 and G83: 83 m 2 g −1), in pure form and equilibrated with dissolved organic matter (DOM), that has been extracted from forest floor samples. The iron oxide samples that were characterised by N 2 adsorption, X-ray diffraction analysis and scanning electron microscopy were preincubated with molybdate solution (5 g iron oxide l −1, 0.2 mM molybdate, pH 4) for 12, 24 and 48 h. To follow the molybdate immobilisation, molybdate desorption kinetics (desorption periods 0.5–48 h) were determined with ion exchange resins in batch systems after the different incubation times. In addition, the preincubated iron oxides were examined by XPS. Fractional coverage of DOM-treated iron oxides estimated according to the enthalpy of N 2 adsorption was 0.33 m 2 m −2 for G83 and 1.19 m 2 m −2 for G13. The pore volume of G13 decreased after DOM treatment. Furthermore, SEM images show that DOM treatment results in microaggregation of the iron oxides. A combination of the first-order equation and a diffusion term was applicable to the Mo desorption data of both, the pure and the DOM-treated iron oxides. Desorbability and apparent diffusion constants of molybdate decreased with increasing residence time. However, the decrease was less distinct for the DOM treated than for the pure goethites. The Mo/Fe XPS ratios of the iron oxides indicate that in the presence of organic matter a higher percentage of molybdate is sorbed to outer surfaces. The results confirm the hypothesis that molybdate diffuses into the pores of iron oxides. Organic coatings slow down the molybdate immobilisation probably by decreasing the accessibility of diffusion pathways. This mechanism may be relevant even at low molybdate and C concentrations, where no competition effect of sorbed organic molecules can be observed.

COMPARING LOSS-ON-IGNITION WITH DRY COMBUSTION AS A METHOD FOR DETERMINING CARBON CONTENT IN UPLAND AND LOWLAND FOREST ECOSYSTEMS

Peat and forest floor samples (n=307) from sites across continental western Canada are analyzed to determine (a) Carbon/organic matter (C/OM) ratios for different peat and forest floor types, (b) variability in C/OM ratios, and (c) the influence of vegetation type and ash content on C/OM ratios. Measured C/OM ratios for different peat and forest floor types are between 0.50 and 0.56, with an average of 0.52 over all samples. C/OM ratios change with ash content, with a positive relationship between the two variables if ash content is <20%. In samples with ash contents >20%, C/OM ratios are highly variable and our data suggest a slight decrease in average C/OM ratios with increased mineral content. Measurement error due to loss of structural water from clay minerals and the presence of inorganic C increasingly influence observed C/OM ratios as mineral content increases. While these problems can be partially overcome by use of appropriate analytical technique, LOI may not be a reliable measure of OM content in samples with very high mineral content.

Abundance, diversity, and species composition of fungal communities in a temperate forest affected by excreta of the Great Cormorant Phalacrocorax carbo

The possible effects of excreta of the Great Cormorant Phalacrocorax carbo on abundance, diversity, and species composition of fungal communities were investigated in a temperate evergreen coniferous forest near Lake Biwa in central Japan. Samples were collected at three study sites that had the same vegetation composition, but which had been influenced by different stages of breeding colony establishment: Site C (control site), Site 2 (colonizing site), and Site 3 (post-colony site). In forest floor samples and in needles and twigs of Chamaecyparis obtusa, total hyphal length was lowest at Site 3, and clamp-bearing hyphal length (biomass of basidiomycetous fungi) was lower at Sites 2 and 3 than at Site C. Dark-pigmented hyphal length was highest at Site 2. Dilution plating of forest floor samples and mineral soil revealed: (i) species richness was higher at Sites 2 and 3 than at Site C, (ii) diversity was higher at Site 3 than at Sites C and 2, and that (iii) species composition differed among the sites. Surface sterilization of needles and twigs of C. obtusa revealed (i) with the exception of species richness in twigs, species richness and diversity were higher at Site 3 than at Sites C and 2, and that (ii) species composition differed markedly among the sites. In twig samples white rot Marasmius-like fungus and Geniculosporium sp. 1 were dominant at Site C and reduced at Sites 2 and 3. A coprophilous species, Sordaria sp. 1, showed a marked increase at Site 2 in needle and twig samples.

Carbon sequestration in trees and regrowth vegetation as affected by clearcut and partial cut harvesting in a second-growth boreal mixedwood

Ecosystem biomass and C sequestration and cycling were measured in a mature, budworm-ravaged, second-growth boreal mixedwood stand subjected to clearcut and partial cut harvest treatments. Ninety permanent sample plots, distributed among fifteen 10 ha blocks, were established in 1993 and remeasured shortly after harvest and, again, 5 years later to ascertain C removals, as well as post-harvest ingrowth and upgrowth. In addition, litter bags and litter traps were installed and ground vegetation and forest floor samples were collected to monitor changes in ecosystem C pools and fluxes as affected by harvesting. The harvested blocks were quickly reoccupied by a thriving ground species regrowth, followed by a prolific trembling aspen thicket. Despite this, 5-year growth per area on control plots exceeded that on either partial cut or clearcut plots. Annual C assimilation rates in the post-harvest period were similarly significantly higher on control (3.1 Mg ha −1 per year) than on partial cut (1.8 Mg ha −1 per year) or clearcut (0.3 Mg ha −1 per year) plots. When growth was expressed as a function of biomass, the order reversed indicating the young growth to be the more vigorous. The forest floor lost little mass during the post-harvest period. Further, leaf litter decomposition was slower on clearcut plots than in uncut forest. Results suggest that ecosystem C pools and fluxes are rapidly reconstituted following harvest. If harvesting were to occur, ecosystem C assimilation in boreal mixedwood forest would be maximized under partial cutting.

Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils

The feedbacks between plant and soil processes play an important role in driving forest succession. One poorly understood feedback mechanism is the interaction between plant secondary chemicals and soil microbes. In the Alaskan taiga, changes in nutrient cycling caused by balsam poplar ( Populus balsamifera) secondary chemicals may affect the transition from alder ( Alnus tenuifolia) to balsam poplar on river floodplains. We examined the effects of four poplar condensed tannin fractions on N cycling in alder and poplar soils. Tannins were added to forest floor samples from both poplar and alder sites. Samples were incubated for 1 month in the laboratory with soil respiration rates measured over the course of the incubation. At the end of the incubation we measured both net and gross nitrogen mineralization and nitrification, microbial biomass C and N, and the activity of various exoenzymes. In all soils, tannin additions reduced N availability, however, the mechanisms differed depending on the molecular weight of the tannin and the native soil microbial community. Low molecular weight tannin fractions served as a labile C source in poplar Oi, poplar Oe, and alder Oe horizons but were toxic to microbes in alder Oi. High molecular weight tannin fractions appeared to act primarily by binding extracellular substrates and thus limiting C and N mineralization, with the strongest effects observed in the alder soils.

The effect of organic acids on base cation leaching from the forest floor under six North American tree species

SummaryOrganic acidity and its degree of neutralization in the forest floor can have large consequences for base cation leaching under different tree species. We investigated the effect of organic acids on base cation leaching from the forest floor under six common North American tree species. Forest floor samples were analysed for exchangeable cations and forest floor solutions for cations, anions, simple organic acids and acidic properties. Citric and lactic acid were the most common of the acids under all species. Malonic acid was found mainly under Tsuga canadensis (hemlock) and Fagus grandifolia (beech). The organic acids were positively correlated with dissolved organic carbon and contributed significantly to the organic acidity of the solution (up to 26%). Forest floor solutions under Tsuga canadensis contained the most dissolved C and the most weak acidity among the six tree species. Under Tsuga canadensis we also found significant amounts of strong acidity caused by deposition of sulphuric acid from the atmosphere and by strong organic acids. Base cation exchange was the most important mechanism by which acidity was neutralized. Organic acids in solution from Tsuga canadensis, Fagus grandifolia, Acer rubrum (red maple) and Quercus rubra (red oak) were hardly neutralized while much more organic acidity was neutralized for Acer saccharum (sugar maple) and Fraxinus americana (white ash). We conclude that quantity, nature and degree of neutralization of organic acids differ among the different tree species. While the potential for base cation leaching with organic acids from the forest floor is greatest under Tsuga canadensis, actual leaching with organic anions is greatest under Acer saccharum and Fraxinus americana under which the forest floor contains more exchangeable cations than does the strongly acidified forest floor under Tsuga canadensis.

Quantitative characterization of nutrient regimes of high-elevation forest soils in the southern coastal region of British Columbia, Canada

To quantify the gradient of plant-available nutrients on high-elevation coastal sites that support growth of Pacific silver fir ( Abies amabilis (Dougl. ex Loud.) Forbes) in southern British Columbia, we collected samples of forest floor and mineral soil (0–30 cm) from montane and subalpine sites within maritime-windward, maritime-leeward, submaritime, and subcontinental climatic strata. The samples were analyzed for acidity, total C, total N, mineralizable-N, and extractable Ca, Mg, K, P, and SO4−S. Relationships between field-identified soil nutrient regimes (SNRs) and measured soil nutrient properties were examined using principal component analysis, discriminant function analysis, and covariance analysis. The nutrient gradient appeared to be heterogeneous as indicated by a relatively low percentage (29%) of variation explained by the first principal component. Within the space of chemical soil properties resulting from principal component analysis, the grouping of samples was weak and the relationship to SNRs also appeared weak. However, using discriminant analysis (DA) 70% agreement was found between field-identified SNRs and the classification based on measured soil properties. Nitrogen-related variables (total N, mineralizable-N, and C/N ratio) and the sum of extractable Ca, Mg, and K were the properties most strongly related to the field-identified SNRs. Mineral soil min-N concentrations appeared to be unrelated to changes in continentality within the study area. These findings agree with the results of previous studies suggesting that mineralizable-N of the mineral soil is the best property for a quantitative characterization of a soil nutrient gradient, and, hence for quantification of field-estimated SNRs. Field-estimated SNRs were also good predictors of site index as they explained the variation in site index of Pacific silver fir similarly well as total N and C/N ratio. However, the absence of a relationship between mineralizable-N and the Pacific silver fir site index requires a further study of plant–soil relationships in the area.

An inverse modeling approach for the characterization of unsaturated water flow in an organic forest floor

Although the retention of rain-water by a forest floor consisting of vegetation litter has large hydrological and geomorphological effects in a forested catchment, its physical processes have not yet been clarified. For the purpose of examining the applicability of Richards equation to unsaturated water flow in forest floor, results of artificial rainfall experiments were analyzed by the inverse method. Forest floor samples were collected from a beech-stand, an oak-stand, a cedar-stand and a mixed-stand of coniferous and broad-leaved trees. The samples were piled up to make long columns of 20.6 cm in inner diameter and about 50 cm in height, with which step-wise drainage experiments and random-rainfall experiments were conducted. Parameters in water retention and hydraulic conductivity functions were optimized by comparing the observed versus computed discharge rates for the step-wise drainage experiments. The derived retention and conductivity functions succeeded to reproduce the observed drainage hydrographs during the random-rainfall experiments, indicating that Richards equation can describe the unsaturated water flow in all the forest floor studied. The derived retention functions suggested that the forest floors have a small water capacity except for the very wet range where the matric pressure head, ψ, is greater than −5 cm. The conductivity functions of all the forest floors exhibited a sharp drop in the range of ψ>−5 cm, and decreased gradually as ψ decreased further. The forest floors at the beech-stand and the oak-stand had larger water capacities and smaller conductivity values than the forest floors at the cedar-stand and the mixed-stand. Consequently, the discharge hydrographs of the forest floors at the beech-stand and the oak-stand were characterized by more gradual responses to rainfall than the forest floors at the cedar-stand and the mixed-stand. Overall, the proposed inverse technique was effective to characterize the unsaturated water flow in forest floor.

Carbon Control on Nitrogen Dynamics in the Forest Floor of an N-Enriched Deciduous Forest Ecosystem

Recent evidence from nitrogen (N) saturation studies indicates that forest floors in moderately impacted forests are the primary sink for atmospheric N inputs. Some researchers have suggested that the sink capacity of organic horizons is dependent on the amount of available carbon (C), which can be used for microbial N assimilation. To test the hypothesis that C limitation in forest floors exposed to chronic N deposition leads to an enhanced N leaching, a field C input manipulation experiment is under way in a deciduous forest. Since September 1999 aboveground C input has been doubled (by doubling litter input or by amending glucose) or excluded in replicated plots. Here we report the short-term response of concentrations of dissolved inorganic N (DIN: NO3−-N and NH4+-N) in forest floor percolate to the C input manipulation. In autumn following the C input manipulation, DIN concentrations in forest floor percolate decreased in all plots except the No Litter plots compared to the pre-treatment summer concentrations. In contrast, the concentrations of DIN in the No Litter plots remained high. A different seasonal pattern of DIN leaching among treatments, along with measurements of microbial biomass C and potential nitrification rates of forest floor samples, indicates that seasonal N dynamics in the forest floor are largely regulated by C availability changes assoicated with litterfall C input.

Substrate limitations to microbial activity in taiga forest floors

A combination of laboratory and field experiments showed substantial differences in microbial activity, substrate processing, and N cycling in forest floor samples from different Alaskan boreal forest ecosystems. In soils from black spruce ( Picea mariana [Mill.] B.S.P.) communities, low organic matter quality (e.g. low %N, high C:N, high lignin, high lignin:N, low pH, low extractable inorganic N) and cold soils were associated with low rates of soil respiration, microbial turnover and gross microbial N uptake in both laboratory and field measurements. Soils from aspen ( Populus tremuloides Michx.) communities had attributes of low organic-matter quality (high lignin and high lignin:N) but also attributes favorable to decomposition (high pH, high %N, high soil temperature) and exhibited much higher relative microbial activity in the field than in the laboratory, probably because warmer field conditions or other processes that occurred only in the field (e.g. root exudation) offset the effects of low organic matter quality. Field soils from birch ( Betula papyrifera Marsh.) communities on warm sites also exhibited higher in situ rates of microbial activity than expected from their performance in the laboratory. Microbial activity was more important than microbial biomass in explaining community differences in soil respiration and N cycling. Addition of labile carbon (C) and nitrogen (N) substrates to soils in the field and in the laboratory permitted microbial resource limitations to be evaluated. Microbial response to added N was greatest when labile C was abundant. Microbial demand for available soil N was greatest in soils with the highest organic C concentrations and the lowest rates of N mineralization. These observations support the conventional concept that microbial activity responds to a balanced supply of C and N. However, microbial respiration responded more strongly to sucrose (field) and cellobiose (laboratory) than to cellulose addition, indicating that the degree of defined C limitation depends on the nature of the substrate added and the ability of microbial populations to utilize it. Respiration and N immobilization responded more strongly to substrate additions than did microbial biomass, suggesting that the nature of resource limitation depends on the particular microbial parameter considered. The response of microbial respiration to added C and N also depended on the quality of native soil organic matter.

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.

Indices of forest floor nitrogen status along a climate gradient in Maine, USA

Nitrogen has long been recognized as the most commonly limiting nutrient for plant production throughout the world. Yet, air pollution has created a modern chemical climate that has sometimes resulted in excess ecosystem N due to N deposition. In addition, climate warming could accelerate N cycling and N export from forested ecosystems. The result is increasing interest in understanding forest ecosystem N dynamics. This study used recently delineated climatic regions in Maine to investigate the possible influences of forest species composition, and energy and moisture gradients, on laboratory indices of forest floor N cycling. Concentrations of N and C, and potential net nitrification, potential net ammonification, and potential net N mineralization, were measured on forest floor samples from 20 sites distributed across Maine in both hardwood and softwood stands. Both forest types had nearly identical concentrations of N in the forest floor (∼1.6%), but the mean C/N ratio (28) under softwoods was significantly higher than that under hardwoods (24) due to higher concentrations of total C in soils under conifers. Forest floor N concentration was a better predictor of potential net N mineralization than was total C or C/N ratio. Although the most northerly region in this study was predictably the coldest, it was also the region with the highest values for total N and N cycling indices. Wet N deposition for the region indicates N deposition differences are not responsible for this spatial pattern, and further work is warranted to explain these results. Laboratory incubation measures of potential net N mineralization were significantly correlated with in situ annual net N mineralization, which supports the use of these techniques for forest soil N status evaluations. Most site measures of mean temperatures were negatively correlated with soil N indices indicating that warmer sites had lower rates of N cycling. Although differences existed in forest floor N characteristics between climate regions, they could not be predicted by simple relationships with temperature.

Forest floor microbial biomass across a northern hardwood successional sequence

Microbial dynamics partially control forest productivity, yet are poorly understood in the northern hardwood forest. Following forest cutting, microbial biomass may reflect changes in the forest floor environment. Using direct counts, forest floor microbial biomass was studied in a sequence of northern hardwood forest stands ranging in age since clearcutting from 3 to more than 120 yr. The organic horizon was collected 3 times in each of 1994 and 1995, in June, August and October. Samples were analyzed for active fungi, active bacteria, pH, forest floor moisture content and organic fraction. Forest floor temperature was measured in the field with each collection; forest floor mass was measured once in each stand. In 1995, forest floor samples were also analyzed for total N, nitrate and ammonium. Amounts of active microbial biomass (fungi plus bacteria) ranged from 38 to 103 μg g −1 organic matter; active microbial biomass per unit area ranged from 40 to 800 mg m −2 forest floor organic matter. Microbial biomass varied significantly among sampling periods in 1994, but not in 1995, and was greater in early- and late-successional stands than in mid-successional stands. Microbial biomass was not very responsive to the environmental factors measured in this study. Moisture content was the environmental variable that most often contributed to variation in microbial biomass, and together with additional factors explained only 23% of the variation in 1994, and only 27% in 1995. Lower microbial biomass in mid-successional stands suggests that microbial dynamics in this forest are not controlled by factors directly related to forest harvesting.