Aspen Litter Research Articles

An invasive grass and litter impact tree encroachment into a native grassland

AbstractQuestionsWoody plant encroachment and invasive plants are two critical factors that negatively impact grass‐dominated ecosystems. While studies have extensively investigated these factors individually, research on how invasive species impact the susceptibility of grasslands to encroachment is less common. Using the aspen parkland, an endangered savannah‐type ecosystem, we asked how the presence of a widespread invasive grass in North America, smooth brome (Bromus inermis), impacted the growth and survival of one‐year old trembling aspen (Populus tremuloides) seedlings. We also asked if plant litter was a potential mechanism of inhibition (as has been proposed for brome litter) or coexistence (as has been hypothesized for aspen litter) in this system.LocationAlberta, Canada.MethodsWe used a manipulative experimental approach to determine if the survival and subsequent growth of planted aspen seedlings was impacted by the presence of smooth brome and manipulation of litter amount and type.ResultsWe found that the presence of smooth brome reduced aspen survival by 57% compared to uninvaded habitats, likely mediated by reduced soil moisture, while litter manipulation had no effect on survival. For surviving seedlings, local context had complex impacts on growth; the addition of aspen litter to brome‐invaded communities increased seedling growth while aspen litter additions to native communities resulted in decreased growth.ConclusionThese results suggest that invasion by smooth brome will alter the dynamics of aspen establishment in this system, potentially leading to significant changes to this already endangered landscape. Though smooth brome may serve as a barrier to aspen establishment, accumulation of aspen litter from nearby stands to brome patches could lead to faster growth of seedlings in invaded areas at the edge of existing aspen stands. Our results also suggest more generally that the impact of invasive plants on the establishment of native woody plants can be dependent on litter inputs.

Microbial Substrate Utilization and Vegetation Shifts in Boreal Forest Floors of Western Canada

Boreal forest soils are highly susceptible to global warming, and in the next few decades, are expected to face large increases in temperature and transformative vegetation shifts. The entire boreal biome will migrate northward, and within the main boreal forest of Western Canada, deciduous trees will replace conifers. The main objective of our research was to assess how these vegetation shifts will affect functioning of soil microbial communities and ultimately the overall persistence of boreal soil carbon. In this study, aspen and spruce forest floors from the boreal mixedwood forest of Alberta were incubated in the laboratory for 67 days without (control) and with the addition of three distinct 13C labeled substrates (glucose, aspen leaves, and aspen roots). Our first objective was to compare aspen and spruce substrate utilization efficiency (SUE) in the case of a labile C source (13C-glucose). For our second objective, addition of aspen litter to spruce forest floor mimicked future vegetation shifts, and we tested how this would alter substrate use efficiency in the spruce forest floor compared to the aspen. Tracking of carbon utilization by microbial communities was accomplished using 13C-PLFA analysis, and 13C-CO2 measurements allowed quantification of the relative contribution of each added substrate to microbial respiration. Following glucose addition, the aspen community showed a greater 13C-PLFA enrichment than the spruce throughout the 67-day incubation. The spruce community respired a greater amount of 13C glucose, and it also had a much lower glucose utilization efficiency compared to the aspen. Following addition of aspen litter, in particular aspen leaves, the aspen community originally showed greater total 13C-PLFA enrichment, although gram positive phospholipid fatty acids (PLFAs) were significantly more enriched in the spruce community. While the spruce community respired a greater amount of the added 13C-leaves, both forest floor types showed comparable substrate utilization efficiencies by Day 67. These results indicate that a shift from spruce to aspen may lead to a greater loss of the aspen litter through microbial respiration, but that incorporation into microbial biomass and eventually into the more persistent soil carbon pool may not be affected.

The fate of leaf-litter N under contrasting pedo-climatic conditions in south-western Siberia

Nitrogen (N) made available through the decomposition of organic matter is a major source for plants in terrestrial ecosystems. N cycling in Siberia is however poorly documented despite the region representing a substantial surface area of the globe.We studied the influence of pedo-climate (using two forest-steppe and two southern taiga sites) and vegetation type (aspen forest and grassland) on the redistribution of N released from decomposing 15N-labelled leaf-litter in south-western (SW) Siberia. A model of N dynamics was fit to field measurements that yielded estimates of N mean residence time (MRT) within litter and soil layers, as well as the proportion of N transferred from one layer to another.The release of N from the aspen litter was slower in the forest-steppe (MRT in litter: 2.9–4.6 years) than in the southern taiga (0.9–1.5 years), likely because winter soil freezing and summer drought slowed decomposition in the forest-steppe. In contrast, no difference between the bioclimatic zones was observed for the grass litter (MRT 1.2–1.6 years), suggesting litter chemistry outweighs pedo-climate in these zones. While most of the vertical transfer of N down the soil profile occurred during the vegetative season, important losses were observed after snow-melt. Over three years, the transfer of N within the soil profile was deeper in the southern taiga sites than in the forest-steppe, and in forest than in grassland. In the topsoil, the MRT of N was longer in grassland (4.9–9.4 years) than in forest (1.5–2.1 years) but there were no pronounced differences between bioclimatic zones.The detailed and quantitative view of current N cycling in SW Siberia provided in this study may serve as the basis for informing ecosystem models that anticipate future climate and land-use changes.

Assessing structural and functional indicators of soil nitrogen availability in reclaimed forest ecosystems using15N-labelled aspen litter

Landscape-level disturbance is a reality in many parts of the world including the Athabasca oil sands region, Canada, and soils play an essential part in the overall reclamation process. Soils are ...

Effects of substrate availability and competing vegetation on natural regeneration of white spruce on logged boreal mixedwood sites

Given a seed source, the quality of available substrates is a key factor in determining the success of white spruce (Picea glauca (Moench) Voss) natural regeneration. We examined the influence of substrate and competing vegetation on survival and growth of natural regeneration of white spruce up to 4 years following harvesting in deciduous-dominated upland boreal mixedwood sites. Feather moss, thick soil surface organic layers, litter, and solid wood were poor substrates for establishment. Early successional mosses establishing on mineral soil, thin organics, and rotten wood were generally favourable microsites but were not highly available on postharvest sites. Mineral soil substrates were not as suitable as expected, likely because on a postlogged site, they are associated with unfavourable environmental characteristics (e.g., low nutrient availability, exposure). There was some evidence that survival and growth of seedlings were improved by surrounding vegetation in the first years, but heavy competing vegetation had a negative impact on older seedlings. Burial by aspen litter greatly increased seedling mortality, especially when combined with a brief period of submergence due to heavy spring snowmelt. The results provide insight into conditions under which natural regeneration could be an option for establishing white spruce following harvesting of deciduous-dominated boreal mixedwood forests.

Microbial utilization of double-labeled aspen litter in boreal aspen and spruce soils

Aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) are archetypal species of the boreal forest of western Canada. With the many new and varied environmental pressures (e.g., fire, harvesting, climate change) on, this boreal forest biome, we were interested in understanding the importance of potential changes in organic matter inputs to its soil microbial community structure and function. Double -labeled (15N and 13C) aspen leaf litter was utilized as an amendment to the forest floor of two stands, an aspen and a spruce, in a 16-month paired field experiment, where amended mesocosms were compared to control mesocosms with no litter added. Nitrogen mineralization from the aspen leaf litter occurred within both stand types, as evidenced by higher δ15N values in roots and live aboveground vegetation. In term of carbon fluxes, stable isotope 13C analysis of the forest floor microbial phospholipid fatty acid (PLFAs) indicated a slight initial enrichment within all amended mesocosms, likely due to leaching of 13C enriched solubles from the labeled aspen litter. Within the aspen stand, continued 13C enrichment of most PLFAs demonstrated that incorporation of the litter-derived carbon by a wide range of microorganisms was maintained through both growing seasons. Response of the spruce microbial community differed from that of aspen. Its overall structural composition did not alter in response to aspen litter addition; yet, δ13C values of several PLFA biomarkers decreased at the end of the first growing season, suggesting that priming stimulated mineralization of a 13C-depleted pool. These results indicate that while nitrogen mineralization was maintained, the deep forest floor carbon stocks of spruce stands may be vulnerable to increased decomposition rates with changing organic matter input.

Assessing the Effect of Leaf Litter Diversity on the Decomposition and Associated Diversity of Fungal Assemblages

Although the effect of litter mixture on decomposition has been well documented, few studies have examined the relationships between richness and relative abundance of leaf species in litter mixture and changes in universal fungal communities during the decomposition process in temperate forests. In this study, we used the litterbag method and included three leaf litter species, i.e., aspen (Populus davidiana Dode), birch (Betula platyphylla Sukaczev) and oak (Quercus mongolica Fischer ex Ledebour), to investigate the mass loss rate and diversity of universal fungal communities in each litter treatment, which were sampled in situ after 180, 240, 300 and 360 days of decomposition (between 2012 and 2013) in broadleaved mixed forests in Chinese temperate forests. Eight mixture proportions were examined: pure aspen litter (10A), pure birch litter (10B), pure oak litter (10O), 50% aspen litter mixed with 50% birch litter (5A:5B), 50% aspen litter mixed with 50% oak (5A:5O), 50% birch litter mixed with 50% oak litter (5B:5O), 10% birch litter mixed with 80% aspen litter and 10% oak litter (1B:8A:1O), 30% birch litter mixed with 40% aspen litter and 30% oak litter (3B:4A:3O). Over 360 days of decomposition, approximately 46.6%, 43.6%, 28.0%, 54.4%, 40.2%, 39.5%, 54.5% and 49.46% of litter mass was lost from 10A, 10B, 10O, 5A:5B, 5A:5O, 5B:5O, 1B:8A:1O and 3B:4A:3O, respectively. In addition, the number of fungal denaturing gradient gel electrophoresis (DGGE) bands showed a positive correlation with mass loss rate, indicating a positive feedback between leaf litter decomposition and universal fungal communities in the leaf litter. The results revealed that the 5A:5B, 1B:8A:1O and 3B:4A:3O litter mixtures had a synergistic effect on the litter mixture, while the 5A:5O and 5B:5O litter mixtures had a nearly neutral effect on the litter mixture. Thus, leaf litter species composition and relative abundance seem to be more important than leaf litter richness in driving the direction and magnitude of litter mixture decomposition.

Chronic nitrogen deposition alters the structure and function of detrital food webs in a northern hardwood ecosystem

During the next century, atmospheric nitrogen (N) deposition is projected to more than double, potentially slowing litter decomposition by altering microbial community composition and function. If the flow of energy though detrital food webs is diminished by the slowing of decay under higher rates of atmospheric N deposition, this agent of global change could also negatively impact the abundance and composition of soil fauna. To test this hypothesis, we studied soil faunal communities in four sugar-maple-dominated forests that comprise a long-term N deposition experiment. To examine whether changes in soil faunal communities could then feed back to influence litter decay, litterbags with 13C-enriched aspen litter were placed in the forest floor in one study site. The microbial community within the litterbags was characterized using PLFA analysis. Overall, long-term experimental N deposition reduced the abundance of microarthropods (ambient vs. experimental N deposition: 7844 vs. 4357 individuals/m2, respectively; P = 0.004). We attribute this overall decline partly to the reduced energy flow entering the detrital food web, which has been documented in previous work in our system. Although there was no difference in microarthropod species richness between N deposition treatments, there was a shift in community composition within the most abundant group (Oribatida), indicating species-specific responses to N deposition. Experimental N deposition reduced the number of microarthropods colonizing litterbags by 41% (P = 0.014). This was associated with a reduction in 13C mobilization from leaf litter into microbial biomass. Overall, this study demonstrates that chronic N deposition has a detrimental effect on the soil detritus food web, and that the negative effect may feed back to influence litter decay and ecosystem functioning.

Soil–litter nitrogen transfer and changes in δ13C and δ15N values in decomposing leaf litter during laboratory incubation

Abtract Leaf litter is the main source of nutrient and energy input into the soil. Therefore, detailed knowledge on the short-term variations in the isotopic composition of plant litter is needed for correctly estimating the “isotopic baseline” in stable isotope-based studies on detrital foodwebs. In a laboratory experiment, standardized fragments of freshly fallen leaves of Quercus robur, Ulmus glabra, and Populus tremula were incubated on the surface of natural or 15N-labeled soil during 260 days. At the end of the experiment, the remaining mass represented 62, 53 and 50% of the initial mass for oak, elm and aspen litter, respectively. There was a small decrease in the mean δ13C values during the initial stages of decomposition, although it was inconsistent among the three litter species tested and did not exceed 1.0‰ during the decomposition period. Calculations based on the total N content, as well as the isotope mixing model suggested that up to 50% of the total litter N was incorporated from the underlying soil. The rates of N transfer from the soil to litter were not affected by the disruption of mycelial connections between the soil and litter. As indicated by δ15N values of filter paper placed on the soil surface, labile soil N was depleted by 6–8‰ in 15N relative to bulk soil organic matter. However, in the given experimental settings the input of 15N-depleted labile N from mineral soil was likely counterbalanced by an increase in the 15N-enriched microbial biomass and bulk litter δ15N values changed little.

Ciliate dependent production of microbial anthranilic acid occurring within aspen litter

Terrestrial protozoa and their role in the soil microbial loop are intricately linked to the functioning of forest soils. Yet, in spite of their recognition as vital components of soil ecosystems, protozoa remain understudied when compared to other soil microorganisms. In addition to directly stimulating soil nutrient levels by releasing bacterial nutrients, soil protozoa may affect plant growth indirectly by promoting bacteria that produce plant auxins. We conducted a four week incubation study using defaunated trembling aspen leaves that were selectively inoculated with ciliates. Ciliates modified microbial community structure, as assessed with phospholipid fatty acid (PLFA) analysis. Using High Performance Liquid Chromatography (HPLC), we found that the presence of ciliates did not favor bacteria that produce the plant auxin, indole-3-acetic acid (IAA). Instead, ciliates were associated with the production of anthranilic acid, which indicates that they were responsible for maintaining populations of r-selected bacteria within a relatively stable and nutrient poor environment. Additionally, while ciliates were observed using phase contrast microscopy, the commonly used PLFA indicator for soil protozoa (20:4) was not detected therefore it was shown to be a poor and potentially invalid biomarker.

Genotype and soil nutrient environment influence aspen litter chemistry and in-stream decomposition

A growing body of genes-to-ecosystems research has documented the ecosystem-level consequences of intraspecific variation in plants caused in large part by variation in chemical composition. Understanding how this genetic variation in trees might interact with elevated nutrients resulting from increases in anthropogenic deposition may give us insight into how future riparian forests might influence adjacent streams through leaf-litter deposition. We examined the effects of tree genotype, soil nutrient environment, and their interaction on aspen (Populus tremuloides) litter chemistry and aquatic decomposition. We used litter collected from 5 aspen genotypes grown in a common garden under low and high nutrient availability and monitored decomposition over 112 d in a woodland stream. Genotype, environment, and genotype × environment interactions influenced litter chemistry and decomposition dynamics. Genotype and environment both strongly influenced initial litter chemistry, with significant genotype × environment interactions for bound condensed tannins and C∶N. Consistent with effects on litter chemistry, decomposition rates were significantly affected by genotype, environment, and genotype × environment interactions. These results suggest that future changes in intraspecific genetic variation of tree species and deposition of nutrients because of shifts in climate, land use, and related factors may influence decomposition processes in terrestrial systems and in the aquatic systems with which they are coupled via material transfer.

The importance of mycelial connection at the soil–litter interface for nutrient translocation, enzyme activity and litter decomposition

The translocation of nutrients by networks of fungal hyphae is an important mechanism controlling decomposition rates of plant residues. We performed a 15-month long field experiment aiming to assess the effects of the C and N availability and the role of the mycelial connection between litter and underlying mineral soil on the decomposition rates and levels of enzyme activities in plant litter. Litterbags containing either N-rich litter of alder ( Alnus glutinosa) or N-poor litter of aspen ( Populus tremula) were exposed on top of A-horizon of forest soil. The experiment was set up in a full-factorial design with litter species, C and N availability, and presence of mycelial connection as factors. Labile C and N were added as sucrose and ammonium nitrate solutions in four combinations: −C−N, +C−N, −C+N and +C+N. To disrupt the mycelial connection between litter and soil half of the litterbags were slightly moved, whereas others were left untouched. All experimental manipulations were performed every ten days during snow-free period. At the natural levels of C and N availability, N-poor aspen litter, but not N-rich alder litter, accumulated significant amounts of N. The addition of easily available C decreased the activity of xylanase and phenoloxidase and reduced litter decomposition rates of aspen litter; in alder litter similar but less pronounced trends were also observed. The severance of the mycelial connection between plant litter and underlying soil generally decreased xylanase activity and decomposition rates of the N-limited litter, likely due to the reduced presence of basidiomycete fungi in the litterbags. This effect was not observed, if the N-limitation was artificially relieved by the addition of easily available N, likely due to the increased activity of bacteria and microfungi.

Soil microbial communities adapt to genetic variation in leaf litter inputs

Plant genotypes can have important community‐ and ecosystem‐level effects. However, whether the extended phenotypes of plants feed back to influence the fitness of causal genotypes through soil processes remains unknown. We investigated whether aspen genotypes create distinct soil microbial communities that could potentially affect plant fitness. Using naturally occurring aspen stands in an old‐field system, we set up reciprocal litter transplants among ten genetically distinct aspen clones and tracked decomposition and changes in belowground nutrients and microbial communities for three years. We found that belowground microbial communities became adapted to process specific genotypes of aspen litter to the extent allowable by environment and litter chemistry. Belowground processes were driven by a combination of little quality and prior exposure to specific genotypes of litter. In general, litter from aspen genotypes native to the soil community decomposed more rapidly than did litter from foreign aspen genotypes (i.e. a home‐field advantage existed). While home‐field advantages have been documented to occur among litters of different species, we show that intraspecific variation can elicit similar, albeit weak, effects within a single species. Because rapid decomposition and nutrient cycling is likely to benefit fast‐growing, early‐successional species such as aspen, genotype‐mediated selection for soil microbial communities may feed back to positively affect plant fitness. In addition, belowground communities exhibited significant shifts in response to leaf litter inputs. When exposed to foreign litter, microbial communities changed to become more similar to the microbial community beneath the foreign litter's origin, indicating that belowground microbial communities are predictable given the genotype of the aboveground aspen clone.

How does a tree species influence litter decomposition? Separating the relative contribution of litter quality, litter mixing, and forest floor conditions

Litter quality is often considered the main driver of decomposition rate. The objective of this study was to investigate the relative contribution of two other tree-driven mechanisms, litter mixing and forest floor conditions, to foliar litter decomposition and nutrient dynamics for trembling aspen ( Populus tremuloides Michx.) and black spruce ( Picea mariana (Mill.) BSP) using a microcosm approach. Results based on mixed linear models show that the greater influence over these processes was obtained through litter quality followed by forest floor conditions and litter mixing. Specifically, the results indicate that significantly more C and nutrients were mineralized (i) from aspen than from spruce litter, (ii) from spruce litter in mixture with aspen litter than from spruce litter applied singly, and (iii) from litter incubated on forest floor from the aspen stand rather than from the spruce stand, except for nutrients in the spruce litter. Collectively, our results show that the litter and forest floor material from aspen both favour decomposition and nutrient mineralization processes. Hence, we provide evidence that the effect of tree species on litter decomposition may not only be caused by the properties of its litter but also, indirectly, by the specific conditions and the decomposer community that tree species develop in their forest floor.

Biodiversity at the plant–soil interface: microbial abundance and community structure respond to litter mixing

The interactive effects of diversity in plants and microbial communities at the litter interface are not well understood. Mixtures of plant litter from different species often decompose differently than when individual species decompose alone. Previously, we found that litter mixtures of multiple conifers decomposed more rapidly than expected, but litter mixtures that included conifer and aspen litter did not. Understanding the mechanisms underlying these diversity effects may help explain existing anomalous decay dynamics and provide a glimpse into the elusive linkage between plant diversity and the fungi and bacteria that carry out decomposition. We examined the microbial communities on litter from individual plant species decomposing both in mixture and alone. We assessed two main hypotheses to explain how the decomposer community could stimulate mixed-litter decomposition above predicted rates: either by being more abundant, or having a different or more diverse community structure than when microbes decompose a single species of litter. Fungal, bacterial and total phospholipid fatty acid microbial biomass increased by over 40% on both conifer and aspen litter types in mixture, and microbial community composition changed significantly when plant litter types were mixed. Microbial diversity also increased with increasing plant litter diversity. While our data provide support for both the increased abundance hypothesis and the altered microbial community hypothesis, microbial changes do not translate to predictably altered litter decomposition and may only produce synergisms when mixed litters are functionally similar.

Effects of open-top chambers and substrate type on biogeochemical processes at disturbed boreal forest sites in northwestern Quebec

Open-top chamber (OTC) climate change simulation was used to predict the potential effects of climate change on biogeochemistry, including: 1) soil decomposition of three litter types (trembling aspen (Populus tremuloides Michx.), black spruce (Picea mariana (Miller) BSP), and Sphagnum); 2) soil nutrient supply rates, and; 3) soil acidity. We assessed the effects of OTCs on these biogeochemical factors in the presence or absence of Sphagnum moss substrate at post-fire and logging sites, in the transitional mixedwood-boreal zone of northwestern Quebec. Higher air temperatures and cooler, drier soils created by the OTC treatment resulted in lower decomposition rates and a higher C:N ratio of aspen litter, and lower Ca concentrations of the Sphagnum litter after 14 months of incubation, as well as lower K concentrations of spruce litter after 24 month incubation. There were no effects of the OTC treatment on decomposition rates for Sphagnum and spruce litter. The nutrient supply rates of Ca and Mg were consistently lower in the OTCs. The supply rates of N were significantly higher in the control plots at the logging site than at any other combination of the OTC treatment and disturbance site. Soil pH was lower in the OTCs by the end of the growing seasons in 2006 and 2007. The results suggest that the impact of climatic changes, as simulated by the OTC treatment, on the soil system of mixedwood-boreal post-disturbance sites is likely to affect biogeochemical processes such as nutrient supply rates and the soil pH, but the effects on decomposition may be minimal.

Tree species traits influence soil physical, chemical, and biological properties in high elevation forests.

BackgroundPrevious studies have shown that plants often have species-specific effects on soil properties. In high elevation forests in the Southern Rocky Mountains, North America, areas that are dominated by a single tree species are often adjacent to areas dominated by another tree species. Here, we assessed soil properties beneath adjacent stands of trembling aspen, lodgepole pine, and Engelmann spruce, which are dominant tree species in this region and are distributed widely in North America. We hypothesized that soil properties would differ among stands dominated by different tree species and expected that aspen stands would have higher soil temperatures due to their open structure, which, combined with higher quality litter, would result in increased soil respiration rates, nitrogen availability, and microbial biomass, and differences in soil faunal community composition.Methodology/Principal FindingsWe assessed soil physical, chemical, and biological properties at four sites where stands of aspen, pine, and spruce occurred in close proximity to one-another in the San Juan Mountains, Colorado. Leaf litter quality differed among the tree species, with the highest nitrogen (N) concentration and lowest lignin∶N in aspen litter. Nitrogen concentration was similar in pine and spruce litter, but lignin∶N was highest in pine litter. Soil temperature and moisture were highest in aspen stands, which, in combination with higher litter quality, probably contributed to faster soil respiration rates from stands of aspen. Soil carbon and N content, ammonium concentration, and microbial biomass did not differ among tree species, but nitrate concentration was highest in aspen soil and lowest in spruce soil. In addition, soil fungal, bacterial, and nematode community composition and rotifer, collembolan, and mesostigmatid mite abundance differed among the tree species, while the total abundance of nematodes, tardigrades, oribatid mites, and prostigmatid mites did not.Conclusions/SignificanceAlthough some soil characteristics were unaffected by tree species identity, our results clearly demonstrate that these dominant tree species are associated with soils that differ in several physical, chemical, and biotic properties. Ongoing environmental changes in this region, e.g. changes in fire regime, frequency of insect outbreaks, changes in precipitation patterns and snowpack, and land-use change, may alter the relative abundance of these tree species over coming decades, which in turn will likely alter the soils.

The Influence of Chemistry, Production and Community Composition on Leaf Litter Decomposition Under Elevated Atmospheric CO2 and Tropospheric O3 in a Northern Hardwood Ecosystem

We examined the effects of elevated CO2 and O3 and their interaction on leaf litter chemistry and decomposition in pure stands of aspen (Populus tremuloides) and mixed stands of birch (Betula papyrifera) and aspen at the Aspen Free Air CO2 Enrichment (FACE) experiment. A 935-day in situ incubation study was performed using litterbags filled with naturally senesced leaf litter. We found that elevated CO2 had no overall effects on litter decomposition rates, whereas elevated O3 reduced litter mass loss (−13%) in the first year. The effect of O3 on mass loss disappeared in the second year. For aspen litter but not mixed birch-aspen litter, decomposition rates were negatively correlated with initial concentrations of condensed tannins and phenolics. Most soluble components (94% of soluble sugars, 99% of condensed tannins, and 91% of soluble phenolics) and any treatment effects on their initial concentrations disappeared rapidly. However, the mean residence time (MRT) of birch-aspen litter (3.1 years) was significantly lower than that of aspen litter (4.8 years). Further, because of variation in total litterfall, total litter mass, C, lignin and N remaining in the ecosystem was highest under elevated CO2 and lowest under elevated O3 during the incubation period. Our results indicate that elevated CO2 and O3 can alter short-term litter decomposition dynamics, but longer-term effects will depend more on indirect effects mediated through changes in forest community composition. Treatment effects on soluble components are likely to influence cyclical microbial processes and carbon pulses in the ecosystem only when coupled with increased (CO2) or decreased (O3) litter inputs.

N-transfer through aspen litter and feather moss layers after fertilization with ammonium nitrate and urea

When fertilizer is broadcast in boreal forest stands, the applied nutrients must pass through a thick layer of either feather moss or leaf litter which covers the forest floor. In a growth chamber experiment we tested the transfer of N through living feather moss or aspen litter when fertilized with urea ((NH2)2CO) or NH4NO3 at a rate of 100 kg ha−1 and under different watering regimes. When these organic substrates were frequently watered to excess they allowed the highest transfer of nutrients through, although 72% of the applied fertilizer was captured in the substrates. In a field experiment we also fertilized moss and aspen litter with urea ((NH2)2CO) or NH4NO3 at a more operationally relevant rate of 330 kg ha−1. We captured the NO3 − or NH4 + by ion exchange resin at the substrate–mineral soil interface. In contrast to the growth chamber experiment, this fertilizer rate killed the moss and there was no detectable increase in nutrient levels in the aspen litter or feather moss layers. Instead, the urea was more likely transferred into the mineral soil; mineral soil of the urea treatment had 1.6 times as much extractable N compared to the NH4NO3 treatment. This difference between the growth chamber and field studies was attributed to observed fertilizer-damage to the living moss and possibly damage to the litter microflora due to the higher rate of fertilization in the field. In addition, the early and substantial rainfall after fertilization in the field experiment produced conditions for rapid leaching of N through the organic layers into the mineral soil. In the field, only 8% of the urea-N that was applied was captured by the ion exchange resin, while 34% was captured in for the NH4NO3 fertilization. Thus, the conditions for rapid leaching in the field moved much of the N in the form of urea through the organic layers and into the mineral soil before it was hydrolyzed.

Independent, Interactive, and Species-Specific Responses of Leaf Litter Decomposition to Elevated CO2 and O3 in a Northern Hardwood Forest

The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO2-mediated shifts in nutrient cycling through changes in litter chemistry, and by interactions with pollutants like O3. We evaluated the independent and interactive effects of elevated CO2 (560 l ll )1 ) and O3 (55 nl l l )1 )o n leaf litter decomposition in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) at the Aspen free air CO2 enrichment (FACE) site (Wisconsin, USA). Fumigation treatments consisted of replicated ambient, +CO2, +O3, and +CO2 +O 3 FACE rings. We followed mass loss and litter chemistry over 23 months, using reciprocally transplanted litterbags to separate substrate quality from environment effects. Aspen decayed more slowly than birch across all treatment conditions, and changes in decomposition dynamics of both species were driven by shifts in substrate quality rather than by fumigation environment. Aspen litter produced under elevated CO2 decayed more slowly than litter produced under ambient CO2, and this effect was exacerbated by elevated O3. Similarly, birch litter produced under elevated CO2 also decayed more slowly than litter produced under ambient CO2. In contrast to results for aspen, however, elevated O3 accelerated birch decay under ambient CO2, but decelerated decay under enriched CO2. Changes in decomposition rates (k-values) were due to CO2- and O3-mediated shifts in litter quality, particularly levels of carbohydrates, nitrogen, and tannins. These results suggest that in early-successional forests of the future, elevated concentrations of CO2 will likely reduce leaf litter decomposition, although the magnitude of effect will vary among species and in response to interactions with tropospheric O3.