Nitrogen fixation in mixed <Emphasis Type="Italic">Hylocomium splendens</Emphasis> moss communities

The pleurocarpus feather moss, Hylocomium splendens, is one of two co-dominant moss species in boreal forest ecosystems and one of the most common mosses on earth, yet little is known regarding its capacity to host cyanobacterial associates and thus contribute total ecosystem N. In these studies, we evaluated the N-fixation potential of the H. splendens–cyanobacteria association and contrasted the N-fixation activity with that of the putative N-fixing moss–cyanobacteria association of Pleurozium schreberi. Studies were conducted to: quantify N-fixation in H. splendens and P. schreberi in sites ranging from southern to northern Fennoscandia; assess N and P availability as drivers of N-fixation rates; contrast season-long N-fixation rates for both mosses; and characterize the cyanobacteria that colonize shoots of H. splendens. Nitrogen-fixation rates were generally low at southern latitudes and higher at northern latitudes (64–69°N) potentially related to anthropogenic N deposition across this gradient. Nitrogen fixation in H. splendens appeared to be less sensitive to N deposition than P. schreberi. The season-long assessment of N-fixation rates at a mixed feather moss site in northern Sweden showed that H. splendens fixed a substantial quantity of N, but about 50% less total N compared to the contribution from P. schreberi. In total, both species provided 1.6 kg fixed N ha⁻¹ year⁻¹. Interestingly, H. splendens demonstrated somewhat higher N-fixation rates at high fertility sites compared to P. schreberi. Nostoc spp. and Stigonema spp. were the primary cyanobacteria found to colonize H. splendens and P. schreberi. These results suggest that H. splendens with associated Nostoc or Stigonema communities contributes a significant quantity of N to boreal forest ecosystems, but the contribution is subordinate to that of P. schreberi at northern latitudes. Epiphytic cyanobacteria are likely a key factor determining the co-dominant presence of these two feather mosses across the boreal biome.     

Substrate stoichiometry determines nitrogen fixation throughout succession in southern Chinese forests

The traditional view holds that biological nitrogen (N) fixation often peaks in early‐ or mid‐successional ecosystems and declines throughout succession based on the hypothesis that soil N richness and/or phosphorus (P) depletion become disadvantageous to N fixers. This view, however, fails to support the observation that N fixers can remain active in many old‐growth forests despite the presence of N‐rich and/or P‐limiting soils. Here, we found unexpected increases in N fixation rates in the soil, forest floor, and moss throughout three successional forests and along six age‐gradient forests in southern China. We further found that the variation in N fixation was controlled by substrate carbon(C) : N and C : (N : P) stoichiometry rather than by substrate N or P. Our findings highlight the utility of ecological stoichiometry in illuminating the mechanisms that couple forest succession and N cycling.     

Response of feather moss associated N2 fixation and litter decomposition to variations in simulated rainfall intensity and frequency

Feather mosses in boreal forests form a dense ground‐cover that is an important driver of both nutrient and carbon cycling. While moss growth is highly sensitive to moisture availability, little is known about how moss effects on nutrient and carbon cycling are affected by the dynamics of moisture input to the ecosystem. We experimentally investigated how rainfall regimes affected ecosystem processes driven by the dominant boreal feather moss Pleurozium schreberi by manipulating total moisture amount, frequency of moisture addition and moss presence/absence. Moisture treatments represented the range of rainfall conditions that occur in Swedish boreal forests as well as shifts in rainfall expected through climate change. We found that nitrogen (N) fixation by cyanobacteria in feather mosses (the main biological N input to boreal forests) was strongly influenced by both moisture amount and frequency, and their interaction; increased frequency had greater effects when amounts were higher. Within a given moisture amount, N fixation varied up to seven‐fold depending on how that amount was distributed temporally. We also found that mosses promoted vascular litter decomposition rates, concentrations of litter nutrients, and active soil microbial biomass, and reduced N release into soil solution. These effects were usually strongest under low moisture amount and/or frequency, and revealed a buffering effect of mosses on the decomposer subsystem under moisture limitation. These results highlight that both the amount and temporal distribution of rainfall, determine the effect of feather mosses on ecosystem N input and the decomposer subsystem. They also emphasize the role of feather mosses in mediating moisture effects on decomposer processes. Finally, our results suggest that projected shifts in precipitation in the Swedish boreal forest through climate change will result in increased moss growth and N₂ fixation but a reduced dependency of the decomposer subsystem on feather moss cover for moisture retention.     

The Cyanobacterial Role in the Resistance of Feather Mosses to Decomposition—Toward a New Hypothesis

Cyanobacteria-plant symbioses play an important role in many ecosystems due to the fixation of atmospheric nitrogen (N) by the cyanobacterial symbiont. The ubiquitous feather moss Pleurozium schreberi (Brid.) Mitt. is colonized by cyanobacteria in boreal systems with low N deposition. Here, cyanobacteria fix substantial amounts of N₂ and represent a potential N source. The feather moss appears to be resistant to decomposition, which could be partly a result of toxins produced by cyanobacteria. To assess how cyanobacteria modulated the toxicity of moss, we measured inhibition of bacterial growth. Moss with varying numbers of cyanobacteria was added to soil bacteria to test the inhibition of their growth using the thymidine incorporation technique. Moss could universally inhibit bacterial growth, but moss toxicity did not increase with N₂ fixation rates (numbers of cyanobacteria). Instead, we see evidence for a negative relationship between moss toxicity to bacteria and N₂ fixation, which could be related to the ecological mechanisms that govern the cyanobacteria – moss relationship. We conclude that cyanobacteria associated with moss do not contribute to the resistance to decomposition of moss, and from our results emerges the question as to what type of relationship the moss and cyanobacteria share.     

N2 Fixation in Feather Mosses is a Sensitive Indicator of N Deposition in Boreal Forests

Nitrogen (N) fixation in the feather moss–cyanobacteria association represents a major N source in boreal forests which experience low levels of N deposition; however, little is known about the effects of anthropogenic N inputs on the rate of fixation of atmospheric N₂ in mosses and the succeeding effects on soil nutrient concentrations and microbial community composition. We collected soil samples and moss shoots of Pleurozium schreberi at six distances along busy and remote roads in northern Sweden to assess the influence of road-derived N inputs on N₂ fixation in moss, soil nutrient concentrations and microbial communities. Soil nutrients were similar between busy and remote roads; N₂ fixation was higher in mosses along the remote roads than along the busy roads and increased with increasing distance from busy roads up to rates of N₂ fixation similar to remote roads. Throughfall N was higher in sites adjacent to the busy roads but showed no distance effect. Soil microbial phospholipid fatty acid (PLFA) composition exhibited a weak pattern regarding road type. Concentrations of bacterial and total PLFAs decreased with increasing distance from busy roads, whereas fungal PLFAs showed no distance effect. Our results show that N₂ fixation in feather mosses is highly affected by N deposition, here derived from roads in northern Sweden. Moreover, as other measured factors showed only weak differences between the road types, atmospheric N₂ fixation in feather mosses represents a highly sensitive indicator for increased N loads to natural systems.     

Amino acid uptake in deciduous and coniferous taiga ecosystems

We measured in situ uptake of amino acids and ammonium across deciduous and coniferous taiga forest ecosystems in interior Alaska to examine the idea that late successional (coniferous) forests rely more heavily on dissolved organic nitrogen (DON), than do early successional (deciduous) ecosystems. We traced ¹⁵N-NH₄⁺ and ¹³C-¹⁵N-amino acids from the soil solution into plant roots and soil pools over a 24 h period in stands of early successional willow and late successional black spruce. Late successional soils have much higher concentrations of amino acid in soil solution and a greater ratio of DON to dissolved inorganic N (DIN) (ammonium plus nitrate) than do early successional soils. Moreover, late successional coniferous forests exhibit higher rates of soil proteolytic activity, but lower rates of inorganic N turnover. Differences in ammonium and amino acid uptake by early successional willow stands were insignificant. By contrast, the in situ uptake of amino acid by late successional black spruce forests were approximately 4-fold greater than ammonium uptake. The relative difference in uptake of ammonium and amino acids in these forests was approximately proportional to the relative difference of these N forms in the soil solution. Thus, we suggest that differences in uptake of different N forms across succession in these boreal forests largely reflect edaphic variation in available soil N (composition), rather than any apparent physiological specialization to absorb particular forms of N. These finding are relevant to our understanding of how taiga ecosystems may respond to increases in temperature, fire frequency, N deposition, and other potential consequences of global change.     

Physiological and molecular diversity of feather moss associative N2-fixing cyanobacteria

Cyanobacteria colonizing the feather moss Pleurozium schreberi were isolated from moss samples collected in northern Sweden and subjected to physiological and molecular characterization. Morphological studies of isolated and moss-associated cyanobacteria were carried out by light microscopy. Molecular tools were used for cyanobacteria identification, and a reconstitution experiment of the association between non-associative mosses and cyanobacteria was conducted. The influence of temperature on N2 fixation in the different cyanobacterial isolates and the influence of light and temperature on N2-fixation rates in the moss were studied using the acetylene reduction assay. Two different cyanobacteria were effectively isolated from P. schreberi: Nostoc sp. and Calothrix sp. A third genus, Stigonema sp. was identified by microscopy, but could not be isolated. The Nostoc sp. was found to fix N2 at lower temperatures than Calothrix sp. Nostoc sp. and Stigonema sp. were the predominant cyanobacteria colonizing the moss. The attempt to reconstitute the association between the moss and cyanobacteria was successful. The two isolated genera of cyanobacteria in feather moss samples collected in northern Sweden differ in their temperature optima, which may have important ecological implications.     

Composition and diversity of nifH genes of nitrogen-fixing cyanobacteria associated with boreal forest feather mosses

Recent studies have revealed that nitrogen fixation by cyanobacteria living in association with feather mosses is a major input of nitrogen to boreal forests. We characterized the community composition and diversity of cyanobacterial nifH phylotypes associated with each of two feather moss species (Pleurozium schreberi and Hylocomium splendens) on each of 30 lake islands varying in ecosystem properties in northern Sweden. Nitrogen fixation was measured using acetylene reduction, and nifH sequences were amplified using general and cyanobacterial selective primers, separated and analyzed using density gradient gel electrophoresis (DGGE) or cloning, and further sequenced for phylogenetic analyses. Analyses of DGGE fingerprinting patterns revealed two host-specific clusters (one for each moss species), and sequence analysis showed five clusters of nifH phylotypes originating from heterocystous cyanobacteria. For H. splendens only, N(2) fixation was related to both nifH composition and diversity among islands. We demonstrated that the cyanobacterial communities associated with feather mosses show a high degree of host specificity. However, phylotype composition and diversity, and nitrogen fixation, did not differ among groups of islands that varied greatly in their availability of resources. These results suggest that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses.     

Exotic grass invasion alters potential rates of N fixation in Hawaiian woodlands

Exotic grass invasion promotes fire which drives the conversion of native woodlands to exotic grasslands in the seasonally dry submontane forests of the island of Hawai'i. We compared potential rates of N fixation in an unburned forest site and a converted grassland site using the acetylene reduction assay. In addition to measuring rates of N fixation on separate and mixed substrates in each site, we tested the effect of abiotic factors on rates of N fixation of specific substrates. We hypothesized that rates of N fixation would be higher in the converted grassland site. N fixation estimates were 4.9 kg N ha⁻¹ year⁻¹ for the unburned forest, and 0.10 kg N ha⁻¹ year⁻¹ for the grassland site, so our hypothesis was rejected. The N fixation in the unburned forest occurs mostly on the leaf litter of native woody species. These substrates are absent from the grassland site, except for wood debris which was not consumed during the fires. No nitrogenase activity was detected in the rhizosphere and litter of grasses, the rhizospheres of shrubs or in soil. Although wood debris is not a significant contributor to the N fixed in the unburned forest, it contributes the majority of N fixed in the grassland. The response of nitrogenase activity to varying conditions of moisture and temperature suggests that microclimatic differences between sites do not control differences in N fixation activity; rather, these differences are due to the abundance of N-fixing substrates. The substantial decrease in N fixation activity after the conversion from woodland to grassland implies that ecosystem-level rates of N accretion are decreased by fire in these sites so much that the N lost during volatilization due to fire is not replenished over the long term by N fixation. Received: 10 January 1997 / Accepted: 7 August 1997     

Phyllostomid bat assemblages in different successional stages of tropical rain forest in Chiapas,Mexico

Due to their role in seed dispersal, changes in the community of phyllostomid bats have direct consequences on ecological succession. The objective of this work was to document changes in the structure of bat assemblages among secondary successional stages of tropical rain forest in Chiapas, Mexico. Bats were mist-netted at ground level during 18 months in 10 sites belonging to 3 successional stages: four sites represented early succession (2–8 years of abandonment), four intermediate succession (10–20 years of abandonment), and two late succession (mature old-growth forest).We captured 1,179 phyllostomids comprising 29 species. Phyllostomid species richness was 17 (58% of all species) in the early stage, 18 (62%) in the intermediate stage and 24 (83%) in the late stage. The late successional mature forest possessed nine species that were exclusively found there, whereas early and intermediate successional stages contained only one exclusive species. Sturnira lilium, Artibeus lituratus, Carollia perpicillata, Artibeus jamaicensis and Glossophaga soricina represented 88% of all captured phyllostomid bats. Frugivores made up more than 90% of the species captured in early and intermediate successional stages and 84% in late successional forest. The Bray–Curtis index of dissimilarity showed a replacement of species through successional stages with the largest dissimilarity between early and late stages, followed by intermediate and late, and the lowest dissimilarity between early and intermediate stages. The number of gleaning insectivore species increased during succession. The carnivorous guild was exclusively found in the late stage (three species). We conclude that the late successional mature forest was the main reservoir for the gleaning insectivore and carnivore guilds; however, early and intermediate successional stages possessed a great diversity of species including many frugivores.     

Persistent and pervasive compositional shifts of western boreal forest plots in Canada

Species compositional shifts have important consequences to biodiversity and ecosystem function and services to humanity. In boreal forests, compositional shifts from late‐successional conifers to early‐successional conifers and deciduous broadleaves have been postulated based on increased fire frequency associated with climate change truncating stand age‐dependent succession. However, little is known about how climate change has affected forest composition in the background between successive catastrophic fires in boreal forests. Using 1797 permanent sample plots from western boreal forests of Canada measured from 1958 to 2013, we show that after accounting for stand age‐dependent succession, the relative abundances of early‐successional deciduous broadleaves and early‐successional conifers have increased at the expense of late‐successional conifers with climate change. These background compositional shifts are persistent temporally, consistent across all forest stand ages and pervasive spatially across the region. Rising atmospheric CO₂ promoted early‐successional conifers and deciduous broadleaves, and warming increased early‐successional conifers at the expense of late‐successional conifers, but compositional shifts were not associated with climate moisture index. Our results emphasize the importance of climate change on background compositional shifts in the boreal forest and suggest further compositional shifts as rising CO₂ and warming will continue in the 21st century.     

The effect of altered macroclimate on N-fixation by boreal feather mosses

Plant productivity is predicted to increase in boreal forests owing to climate change, but this may depend on whether N inputs from biological N-fixation also increases. We evaluated how alteration of climatic factors affects N input from a widespread boreal N-fixer, i.e. cyanobacteria associated with the feather moss Pleurozium schreberi. In each of 10 forest stands in northern Sweden, we established climate-change plots, including a control (ambient climate) plot and three plots experiencing a +2°C temperature increase, an approximately threefold reduction in precipitation frequency, and either 0.07, 0.29 or 1.16 times normal summer precipitation. We monitored N-fixation in these plots five times between 2007 and 2009, and three times in 2010 after climate treatments ended to assess their recovery. Warmer temperatures combined with less frequent precipitation reduced feather moss moisture content and N-fixation rates regardless of total precipitation. After climate treatments ended, recovery of N-fixation rates occurred on the scale of weeks to months, suggesting resilience of N-fixation to changes in climatic conditions. These results suggest that modelling of biological N-inputs in boreal forests should emphasize precipitation frequency and evaporative water loss in conjunction with elevated temperature rather than absolute changes in mean precipitation.     

The relationship between fire history and an exotic fungal disease in a deciduous forest

Exotic diseases have fundamentally altered the structure and function of forest ecosystems. Controlling exotic diseases across large expanses of forest has proven difficult, but fire may reduce the levels of diseases that are sensitive to environmental conditions. We examined Cornus florida populations in burned and unburned Quercus–Carya stands to determine if burning prior to anthracnose infection has reduced the impacts of an exotic fungal disease, dogwood anthracnose, caused by Discula destructiva. We hypothesized that fire has altered stand structure and created open conditions less conducive to dogwood anthracnose. We compared C. florida density, C. florida health, and species composition and density among four sampling categories: unburned stands, and stands that had burned once, twice, and 3 times over a 20-year period (late 1960s to late 1980s). Double burn stands contained the greatest density of C. florida stems (770 stems ha⁻¹) followed by triple burn stands (233 stems ha⁻¹), single burn stands (225 stems ha⁻¹) and unburned stands (70 stems ha⁻¹; P < 0.01). We observed less crown dieback in small C. florida trees (<5 cm diameter at breast height) in burned stands than in unburned stands (P < 0.05). Indicator species analysis showed that burning favored species historically associated with Quercus–Carya forests and excluded species associated with secondary succession following nearly a century of fire suppression. Our results suggest that fire may mitigate the decline of C. florida populations under attack by an exotic pathogen by altering forest structure and composition. Further, our results suggest that the burns we sampled have had an overall restorative effect on forest communities and were within the fire return interval of the historic fire regime. Consequently, prescribed fire may offer a management tool to reduce the impacts of fungal disease in forest ecosystems that developed under historic burning regimes.     

Consequence of salinity and excess boron on growth, evapotranspiration and ion uptake in date palm (Phoenix dactylifera L., cv. Medjool)

Patches of common juniper (Juniperus communis L.) shrubs potentially facilitate the formation of fertile islands in heath tundra ecosystems thereby influencing the long-term resilience of these ecosystems. Although the role of juniper in the formation of such ‘islands of fertility’ has been studied in semiarid landscapes, there has been little attention paid to the importance of juniper in other ecosystems. In this study we contrast the soil fertility and rates of N fixation under juniper shrubs with that in open heath tundra in northern Sweden. Plots were established at several individual sites in alpine heath tundra in Northern Sweden and mineral soils to a depth of 10 cm were characterized for available N and P and total C, N, P, Ca, Mg, K, Fe, Mn, Zn, and Cu. Nitrogen fixation rates were measured by acetylene reduction in feather mosses under juniper canopies and contrasted with N fixation in both feather mosses and surface soils in the open heath. Soils under juniper had concentrations of total P greatly in excess of P in open heath, furthermore, juniper islands had the highest concentrations of bioavailable P. Nitrogen fixation rates in the feather moss Pleurozium schreberi (Bird.) Mitt were approximately 150 μmol acetylene reduced m⁻² d⁻¹ under the juniper canopy compared to less than 10 μmol acetylene reduced m⁻² d⁻¹ in the open heath. Feather mosses under the juniper canopy also fixed N at a significantly higher rate (on an aerial basis) than that of surface cores from the open heath that included lichen, mosses, and soil crusts. Juniper facilitates the formation of islands of soil fertility that may in turn facilitate the growth of other plants and positively influence the long term recovery of heath tundra ecosystems following disturbance.     

Successional changes in soil nitrogen availability,non-symbiotic nitrogen fixation and carbon/nitrogen ratios in southern Chilean forest ecosystems

Vast areas of southern Chile are now covered by second-growth forests because of fire and logging. To study successional patterns after moderate-intensity, anthropogenic fire disturbance, we assessed differences in soil properties and N fluxes across a chronosequence of seven successional stands (2–130 years old). We examined current predictions of successional theory concerning changes in the N cycle in forest ecosystems. Seasonal fluctuations of net N mineralization (N_min) in surface soil and N availability (N_a; N_a=NH ₄ ⁺ –N+NO ₃ ^– –N) in upper and deep soil horizons were positively correlated with monthly precipitation. In accordance with theoretical predictions, stand age was positively, but weakly related to both N_a (r ²=0.282, P<0.001) and total N (N_tot; r ²=0.192, P<0.01), and negatively related to soil C/N ratios (r ²=0.187, P<0.01) in surface soils. A weak linear increase in soil N_min (upper plus deep soil horizons) was found across the chronosequence (r ²=0.124, P<0.022). N_min occurred at modest rates in early successional stands, suggesting that soil disturbance did not impair microbial processes. The relationship between N fixation (N_fix) in the litter layer and stand age best fitted a quadratic model (r ²=0.228, P<0.01). In contrast to documented successional trends for most temperate, tropical and Mediterranean forests, non-symbiotic N_fix in the litter layer is a steady N input to unpolluted southern temperate forests during mid and late succession, which may compensate for hydrological losses of organic N from old-growth ecosystems.     

Slow understory redevelopment after clearcutting in high mountain forests

Besides natural tree regeneration itself, the development of the forest understory community is highly indicative of the ecological recovery of forest stands post-harvesting, and therefore of the sustainability of forest management. High mountain forests might show particularly slow recovery of the understory plant community because of harsh environmental conditions. We compared understory community richness and composition among three age classes of forest stands in the subalpine Engelmann Spruce–Subalpine Fir zone in the interior of British Columbia, Canada. Species composition was found to differ significantly between mature stands (>110 years old and never harvested) and both recent clearcuts (5–8 years old) and the oldest clearcuts present in the study area (second growth: 24–28 years old). A non-metric multidimensional scaling (NMDS) ordination revealed no unidirectional return of species composition in harvested stands towards that of mature forest; indeed, plots in recent clearcuts and second growth stands were similar to one another and clearly separated from the mature stands. Indicator Species Analysis revealed that moss species were particularly indicative of mature forest, with four moss species being common in mature stands but absent from both younger stages. Compared to what has been reported for lower elevation coniferous forests, e.g. in the U.S. Pacific Northwest, redevelopment of the understory appears to be slow after harvesting in these high elevation mountain forests. Rotation intervals that consider the natural temporal pattern of species turnover and the occurrence interval of major natural disturbances (here: fire) should provide effective approaches to sustainable forest management of these forests.     

Stability in the patterns of long‐term development and growth of the Canadian spruce–moss forest

Aim The spruce–moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long‐term development and growth of the spruce–moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re‐establishment of black spruce (Picea mariana) trees. Location The study area occurs in Québec, Canada, and extends from 70°00′ to 72°00′ W and 47°30′ to 56°00′ N. Methods A spatial inventory of spruce–moss forest stands was performed along 34 transects. Nineteen spruce–moss forests were selected. A 500 m² quadrat at each site was used for radiocarbon and tree‐ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co‐dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp . Forests < 325 cal. yr bp are dominated by trees of the first post‐fire cohort and are not yet at equilibrium, whereas older forests show a reverse‐J diameter distribution typical of mature, old‐growth stands. The younger forests display faster height and radial growth‐rate patterns than the older forests, due to factors associated with long‐term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce–moss forests are able to self‐regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325–4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce–moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.     

Impact of Ecosystem Management on Microbial Community Level Physiological Profiles of Postmining Forest Rehabilitation

We investigated the impacts of forest thinning, prescribed fire, and contour ripping on community level physiological profiles (CLPP) of the soil microbial population in postmining forest rehabilitation. We hypothesized that these management practices would affect CLPP via an influence on the quality and quantity of soil organic matter. The study site was an area of Jarrah (Eucalyptus marginata Donn ex Sm.) forest rehabilitation that had been mined for bauxite 12 years previously. Three replicate plots (20 × 20 m) were established in nontreated forest and in forest thinned from 3,000–8,000 stems ha⁻¹ to 600–800 stems ha⁻¹ in April (autumn) of 2003, followed either by a prescribed fire in September (spring) of 2003 or left nonburned. Soil samples were collected in August 2004 from two soil depths (0–5 cm and 5–10 cm) and from within mounds and furrows caused by postmining contour ripping. CLPP were not affected by prescribed fire, although the soil pH and organic carbon (C), total C and total nitrogen (N) contents were greater in burned compared with nonburned plots, and the coarse and fine litter mass lower. However, CLPP were affected by forest thinning, as were fine litter mass, soil C/N ratio, and soil pH, which were all higher in thinned than nonthinned plots. Furrow soil had greater coarse and fine litter mass, and inorganic phosphorous (P), organic P, organic C, total C, total N, ammonium, microbial biomass C contents, but lower soil pH and soil C/N ratio than mound soil. Soil pH, inorganic P, organic P, organic C, total C and N, ammonium, and microbial biomass C contents also decreased with depth, whereas soil C/N ratio increased. Differences in CLPP were largely (94%) associated with the relative utilization of gluconic, malic (greater in nonthinned than thinned soil and mound than furrow soil), l-tartaric, succinic, and uric acids (greater in thinned than nonthinned, mound than furrow, and 5–10 cm than 0–5 cm soil). The relative utilization of amino acids also tended to increase with increasing soil total C and organic C contents but decreased with increasing nitrate content, whereas the opposite was true for carboxylic acids. Only 45% of the variance in CLPP was explained using a multivariate multiple regression model, but soil C and N pools and litter mass were significant predictors of CLPP. Differences in soil textural components between treatments were also correlated with CLPP; likely causes of these differences are discussed. Our results suggest that 1 year after treatment, CLPP from this mined forest ecosystem are resilient to a spring prescribed fire but not forest thinning. We conclude that differences in CLPP are likely to result from complex interactions among soil properties that mediate substrate availability, microbial nutrient demand, and microbial community composition.     

N2 fixing alder (Alnus viridis spp. fruticosa) effects on soil properties across a secondary successional chronosequence in interior Alaska

Green alder (Alnus viridis ssp. fruticosa) is a dominant understory shrub during secondary successional development of upland forests throughout interior Alaska, where it contributes substantially to the nitrogen (N) economy through atmospheric N₂ fixation. Across a replicated 200+ year old vegetation chronosequence, we tested the hypotheses that green alder has strong effects on soil chemical properties, and that ecosystem-level N inputs via N₂ fixation decrease with secondary successional stand development. Across early-, mid-, and late-successional stands, alder created islands of elevated soil N and carbon (C), depleted soil phosphorus (P), and more acidic soils. These effects translated to the stand-level in response to alder stem density. Although neither N₂ fixation nor nodule biomass differed among stand types, increases in alder densities with successional time translated to increasing N inputs. Estimates of annual N inputs by A. viridis averaged across the upland chronosequence (6.6 ± 1.2 kg N ha^?1 year^?1) are substantially less than inputs during early succession by Alnus tenuifolia growing along Alaskan floodplains. However, late-succession upland forests, where densities of A. viridis are highest, may persist for centuries, depending on fire return interval. This pattern of prolonged N inputs to late successional forests contradicts established theory predicting declines in N₂-fixation rates and N₂-fixer abundance as stands age.     

Nitrogen cycling in an Antarctic ecosystem 2. Estimation of the amount of nitrogen fixation in a moss community on East Ongul Island

Nitrogen fixation by cyanobacteria in a moss community on East Ongul Island (69°00'S 39°35'E), Antarctica was investigated using the acetylene reduction method. The mean acetylene reduction rate at 10°C and 200 μE·m⁻²·s⁻¹ photosynthetically active radiation was 7.12 nmol C₂H₄ per square centimeter of moss community per hour. The effects of temperature, radiation, desiccation and rehydration on the acetylene reduction rates were examined. A simple predictive model was constructed in order to estimate the amount of nitrogen fixed in the field. Using this model, the daily amount of nitrogen fixation was calculated from microclimatic data (temperature and radiation) measured in the experimental field at Syowa Station on East Ongul Island between 1983 and 1984. The cumulative amount of nitrogen fixation in the growing season during this period was estimated to be 329 mg N per square meter of moss community. It is suggested that nitrogen fixation by cyanobacteria in the moss community is important as a nitrogen source for the community growth on East Ongul Island.