Forest Nutrient Cycling Research Articles

What shapes cerambycid beetle communities in a tropical forest mosaic? Assessing the effects of host tree identity, forest structure, and vertical stratification

Due to anthropogenic activities, tropical rain forests face many challenges in sustaining biodiversity and maintaining global climates. This study explores how forest successional stage, tree composition, and stratum affect communities of saproxylic cerambycid beetles—concealed feeders that play important roles in forest nutrient cycling. Forty trees in five families (Fabaceae, Lecythidaceae, Malvaceae, Moraceae, and Sapotaceae) were sampled in a mosaic of old-growth and secondary forest on the Osa Peninsula, Costa Rica. Bait branches yielded 3549 cerambycid individuals in 49 species. Species richness was almost identical in old-growth and secondary forest, and both yielded specialists, but abundance was higher in old-growth forest. Overall community structure was most strongly influenced by host plant species; within most plant families it was also impacted by forest successional status. Moraceae was the exception, presumably because the focal tree species was abundant in both old-growth and secondary forest. Several host and old-growth specialist species reached high densities within patches of old-growth forest, but seldom colonized apparently suitable trees within secondary forest. This suggests that even small areas of old-growth forest can act as refuges, but that secondary forest may act as a barrier to dispersal. The vulnerability of specialized saproxylic insects to land use change will be linked to the ability of their preferred hosts to disperse to and persist in successional habitats; rearing studies may provide the most accurate method to monitor community changes over time.

Ecological Predictors and Consequences of Non-Native Earthworms in Kennebec County, Maine

Abstract Non-native earthworms are found throughout much of the United States and southern Canada in areas glaciated during the most recent glaciation. Following invasion, these earthworms altered nutrient cycling, soil structure, and diversity in forests throughout the northern United States. There are no comprehensive studies of earthworm distributions in forested areas of Maine. We surveyed earthworms in forested recreation areas in Kennebec County, ME, and investigated ecological and landscape attributes that may predict their presence. To examine whether the presence of worms modifies forests, we measured environmental variables known from other studies to be affected by worms. We found earthworms at 12 out of 23 sites. Sample sites near roads, in deciduous forests, and in small forests were more likely to have earthworms. We also found that locations with worms have less surface litter and more soil phosphorous, suggesting that earthworms modify soils in Maine forests. Our study is the first to expl...

Distinct Growth and Secretome Strategies for Two Taxonomically Divergent Brown Rot Fungi.

Brown rot fungi are wood-degrading fungi that employ both oxidative and hydrolytic mechanisms to degrade wood. Hydroxyl radicals that facilitate the oxidative component are powerful nonselective oxidants and are incompatible with hydrolytic enzymes unless they are spatially segregated in wood. Differential gene expression has been implicated in the segregation of these reactions in Postia placenta, but it is unclear if this two-step mechanism varies in other brown rot fungi with different traits and life history strategies that occupy different niches in nature. We employed proteomics to analyze a progression of wood decay on thin wafers, using brown rot fungi with significant taxonomic and niche distances: Serpula lacrymans (Boletales; "dry rot" lumber decay) and Gloeophyllum trabeum (order Gloeophyllales; slash, downed wood). Both fungi produced greater oxidoreductase diversity upon wood colonization and greater glycoside hydrolase activity later, consistent with a two-step mechanism. The two fungi invested very differently, however, in terms of growth (infrastructure) versus protein secretion (resource capture), with the ergosterol/extracted protein ratio being 7-fold higher with S. lacrymans than with G. trabeum In line with the native substrate associations of these fungi, hemicellulase-specific activities were dominated by mannanase in S. lacrymans and by xylanase in G. trabeum Consistent with previous observations, S. lacrymans did not produce glycoside hydrolase 6 (GH6) cellobiohydrolases (CBHs) in this study, despite taxonomically belonging to the order Boletales, which is distinguished among brown rot fungi by having CBH genes. This work suggests that distantly related brown rot fungi employ staggered mechanisms to degrade wood, but the underlying strategies vary among taxa.IMPORTANCE Wood-degrading fungi are important in forest nutrient cycling and offer promise in biotechnological applications. Brown rot fungi are unique among these fungi in that they use a nonenzymatic oxidative pretreatment before enzymatic carbohydrate hydrolysis, enabling selective removal of carbohydrates from lignin. This capacity has independently evolved multiple times, but it is unclear if different mechanisms underpin similar outcomes. Here, we grew fungi directionally on wood wafers and we found similar two-step mechanisms in taxonomically divergent brown rot fungi. The results, however, revealed strikingly different growth strategies, with S. lacrymans investing more in biomass production than secretion of proteins and G. trabeum showing the opposite pattern, with a high diversity of uncharacterized proteins. The "simplified" S. lacrymans secretomic system could help narrow gene targets central to oxidative brown rot pretreatments, and a comparison of its distinctions with G. trabeum and other brown rot fungi (e.g., Postia placenta) might offer similar traction in noncatabolic genes.

Do storm synoptic patterns affect biogeochemical fluxes from temperate deciduous forest canopies

The volumetric quantity and biogeochemical quality of throughfall and stemflow in forested ecosystems are influenced by biological characteristics as well environmental and storm meteorological conditions. Previous attempts at connecting forest water and nutrient cycles to storm characteristics have focused on individual meteorological variables, but we propose a unified approach by examining the storm system in its entirety. In this study, we use methods from synoptic climatology to distinguish sub-canopy biogeochemical fluxes between storm events to understand the response of forest ecosystems to daily weather patterns. For solute inputs tied to atmospheric deposition (NH4+, NO3−, SO42−, Na+, Cl−), stagnant air masses resulted in high inputs in rainfall (273.42, 81.81, 52.30, 156.99, 128.70 μmol L−1), throughfall (355.05, 130.66, 83.24, 239.55, 261.32 μmol L−1), and stemflow (338.34, 182.75, 153.74, 125.75, 272.88 μmol L−1). For inputs tied to canopy exchange (DOC, K+, Ca2+, Mg2+), a clear distinction was observed between throughfall and stemflow pathways. The largest throughfall concentrations were in the Great Lakes Low (1794.80, 352.96, 72.75, 74.37 μmol L−1) while the largest stemflow concentrations were in the Weak Upper Trough (3681.78, 497.34, 82.36, 72.46 μmol L−1). Stemflow leaching is likely derived from a larger reservoir of leachable cations in the tree canopy than throughfall, with stemflow fluxes maximized during synoptic types with greater rainfall amounts and throughfall fluxes diluted. For flux-based enrichment ratios, water volume, storm magnitude, antecedent dry period, and seasonality were important factors, further illustrating the influence of synoptic characteristics on wash-off, leaching and, ultimately, dilution processes within the canopy.

Do the rich get richer? Varying effects of tree species identity and diversity on the richness of understory taxa.

Understory herbs and soil invertebrates play key roles in soil formation and nutrient cycling in forests. Studies suggest that diversity in the canopy and in the understory are positively associated, but these studies often confound the effects of tree species diversity with those of tree species identity and abiotic conditions. We combined extensive field sampling with structural equation modeling to evaluate the simultaneous effects of tree diversity on the species diversity of understory herbs, beetles, and earthworms. The diversity of earthworms and saproxylic beetles was directly and positively associated with tree diversity, presumably because species of both these taxa specialize on certain species of trees. Tree identity also strongly affected diversity in the understory, especially for herbs, likely as a result of interspecific differences in canopy light transmittance or litter decomposition rates. Our results suggest that changes in forest management will disproportionately affect certain understory taxa. For instance, changes in canopy diversity will affect the diversity of earthworms and saproxylic beetles more than changes in tree species composition, whereas the converse would be expected for understory herbs and detritivorous beetles. We conclude that the effects of tree diversity on understory taxa can vary from positive to negative and may affect biogeochemical cycling in temperate forests. Thus, maintaining high diversity in temperate forests can promote the diversity of multiple taxa in the understory.

Leaf Litter Translocation and Consumption in Mangrove Ecosystems: the Key Role Played by the Sesarmid Crab <i>Neosarmatium malabaricum</i>

Nutrient cycling in mangrove forests is strongly linked to detrital processing of leaf litter, as compared to direct herbivorous consumption. Sesarmid crabs play a key role in detrital pathways in mangrove forests by processing a large amount of leaf litter produced in the ecosystem. We studied the rate of leaf litter translocation and consumption by a sesarmid crab, Neosarmatium malabaricum , through an ex situ experiment simulating field conditions. We supplied weighed senescent leaves of Aegiceras corniculatum, Avicennia officinalis , Excoecaria agallocha and Rhizophora mucronata to the crab. When provided separately, the crab translocated leaf litter of E. agallocha the maximum, and that of Rhizophora mucronata the minimum to the burrows. When litter mixed together was provided, the rate of translocation was the highest. The crabs consumed up to 80.24% of the litter that they translocated. We found this species capable of translocating 4.39 ±1.68 g of leaf litter m -2 per day and that its population had the potential to translocate 1.81 times more leaf litter than the ecosystem produced, based on comparisons of translocation rate, density of N. malabaricum and leaf litter production in the study area. Our experiments emphasize the key role played by this crab in detrital pathways of mangrove ecosystem.

Revisiting Burton and Likens (1975): Nutrient Standing Stock and Biomass of a Terrestrial Salamander in the Midwestern United States

Animals found in high densities can have significant influence in nutrient cycles of ecosystems. For example, frogs have been known to influence nutrient cycles in tropical forests. However, research understanding the influence of vertebrates in nutrient cycles of North American forest is limited. It has been found that the biomass of terrestrial salamanders (family Plethodontidae) is higher than that of birds, small mammals, and deer in a New Hampshire forest, and recent studies have found prior estimates of terrestrial salamander densities are likely lower than current estimates using sampling and analysis frameworks to account for imperfect detection. A re-evaluation of the impact plethodontid salamanders could have on forest nutrient cycles is therefore justified. We quantified the degree to which a completely terrestrial, lungless salamander (Plethodon albagula; Western Slimy Salamander) constitutes a standing stock of limiting nutrients in a Missouri, USA forest ecosystem. We utilized values of whol...

Relationships of wood density and wood chemical traits between stems and coarse roots across 53 Bornean tropical tree species

Abstract:Wood density and wood chemical traits are strong predictors of tree performance, carbon stock, and wood decomposition, which play important roles in ecosystem processes and carbon and nutrient cycling in forests. However, it remains unknown how root wood traits are related to stem wood traits. We examined the relationships of wood density and wood chemical traits (lignin and nitrogen concentrations, carbon-to-nitrogen ratio) between the stems and coarse roots of 90 individuals representing 53 tropical tree species in Malaysian Borneo. We developed regression equations of each wood trait using the standardized major axis method. Each root wood trait was highly correlated with the corresponding stem wood trait, and most regression equations fitted well (R2 > 0.5). The lignin concentration of roots was significantly greater than that of stems. We conclude that root wood traits can be estimated from the corresponding stem wood traits in South-East Asian tropical trees. Further analysis of coarse root decomposability will provide more accurate estimates of carbon and nutrient fluxes in tropical forest ecosystems.

Nutrient distribution and cycling in a subtropical rainforest in New South Wales

Subtropical rainforests in New South Wales (NSW) are distributed on the more fertile forest soils and are nutritionally distinct from the Eucalyptus forests in the same areas. The distribution, cycling of organic matter and nutrients and nutrient use efficiency in an Australian subtropical rainforest were studied and aspects were compared with reported Eucalyptus studies. The available nutrients were greatly in excess of the stand uptake or requirement. A single undisturbed plot within a research trial in mature forest was selected for the study. At the beginning of the study, the aboveground forest biomass was ~334 t ha–1 of organic matter and, 22 years later, there was 357 t ha–1, giving a net accumulation rate of 1.03 t ha–1 year–1, and net primary productivity of 13.0–14.6 t ha–1 year–1. Litterfall and forest-floor analyses indicated a very rapid turnover of organic matter, with an estimated half-life of ~0.5 years. The quantity of nutrients in the stand was high relative to other forest types in the area, with 1109.2 kg N ha–1, 62 kg P ha–1, 1999 kg Ca ha–1, 591 kg Mg ha–1 and 901 kg K ha–1. Nutrient requirement estimated as nutrient content of the current tissue was estimated to be 107, 5.3, 99, 26 and 61 kg ha–1 year–1 for N, P, Ca, Mg and K, respectively, and uptake defined as removal from the soil was estimated to be 112, 4.7, 128, 37 and 49 kg ha–1 year–1 for the same nutrients, the difference between these being net nutrient redistribution. Nutrient use efficiency (NUE), defined as net primary productivity (NPP) per requirement (t kg–1), was calculated to be 0.12, 2.43, 0.13, 0.50 and 0.21 for N, P, Ca, Mg and K, respectively; these values were low, for example, compared with mature E. pilularis, for which NPP was 0.20, 6.5, 0.43, 1.04 and 0.52 t kg–1 for N, P, Ca, Mg and K, respectively. Using NUE defined as NPP per uptake provided comparable estimates. The rainforest represents a forest growing with basically no nutrient limitations, and, as such, is a benchmark for forest nutrient distribution, cycling and NUE.

森林での窒素分などの肥料・栄養素の循環利用((共通)若手研究者勉強会)

森林での窒素分などの肥料・栄養素の循環利用((共通)若手研究者勉強会)

Mistletoes and epiphytic lichens contribute to litter input in Nothofagus antarctica forests

Litter input is one of the key components that define nutrient cycling in forests and the majority of studies only consider the tree components of litterfall. However, epiphytic species can play a crucial role in litter input throughout the growing season. This work evaluates changes in litter production by mistletoe (Misodendrum sp.) and epiphytic lichen (Usnea sp.), related to crown cover in mature unmanaged, second-growth and managed (thinned for silvopastoral use) forests in Tierra del Fuego (Argentina). We used plastic traps to collect litterfall biomass from trees, lichens and mistletoes on a monthly basis over three consecutive years. Tree litter was considerable during autumn (March to May), which is typical of Nothofagus deciduous species in the Southern hemisphere. In contrast, peak litterfall from mistletoes and lichens occurred during spring and summer seasons. Tree litter (1954–3398 kg dry matter ha−1 year−1) was correlated with crown cover gradient being highest in second-growth forests and lowest in thinned sites. While litter input from mistletoes did not vary among forest types (307–333 kg dry matter ha−1 year−1), lichen litter (11–40 kg dry matter ha−1 year−1) was higher in unmanaged and thinned mature forests despite differences in tree crown cover. Contrary to what we expected, the management practices investigated here did not affect the biomass of canopy communities compared to unmanaged mature forests. Mistletoes and lichens significantly increased the spatial (forest type) and temporal complexity (extended period of falling) of litterfall in Nothofagus antarctica forests. This study provides a starting point to understand the ecological relevance of canopy communities in the Patagonian forests of southern Argentina.

Modelling fine root biomass of boreal tree stands using site and stand variables

Quantification of fine root biomass is needed for estimating the role of roots as carbon (C) stores and sources of C input into the soil, as well as for modelling of forest nutrient cycling. Due to the laborious nature of root biomass determinations, there is a need to develop indirect methods that would allow fine root biomass to be estimated using data on easily measurable stand and site variables. We developed models for estimating tree fine root (diameter⩽2mm) biomass of boreal forests by compiling data from 95 Finnish forest stands (55 on upland soils and 40 on drained peatlands). Stand basal area predicted fine root biomass (R2-adj.=0.35, p<0.001) better than any other stand variable alone. A model that included stand basal area, dominant tree species group [birch/other], soil type [upland soil/drained peatland], temperature sum and the interaction of soil type and temperature sum accounted for 46% of the variation in fine root biomass. Stand basal area, the C:N ratio of the organic layer or upper 0–20cm peat layer and the dominant tree species group together explained 45% of the variation in fine root biomass. Temperature sum correlated with fine root biomass in opposite ways in upland soils and in drained peatlands: fine root biomass increased with decreasing temperature sum in upland soil sites, whereas in peatlands fine root biomass decreased as the temperature sum decreased. Although the models were based on a large dataset representing well the variation in stand age, basal area, site fertility and climatic conditions in northern European boreal forests, half of the variation in fine root biomass remained unexplained. More comprehensive fine root biomass sampling in relation to spatial and temporal variation in the forthcoming studies, for example in connection with monitoring campaigns, and additional predictors such as soil texture, successional stage and the fine root biomass of ground vegetation might improve the predictive power of fine root biomass models.

Identification of Eastern United States Reticulitermes Termite Species via PCR-RFLP, Assessed Using Training and Test Data.

Reticulitermes termites play key roles in dead wood decomposition and nutrient cycling in forests. They also damage man-made structures, resulting in considerable economic loss. In the eastern United States, five species (R. flavipes, R. virginicus, R. nelsonae, R. hageni and R. malletei) have overlapping ranges and are difficult to distinguish morphologically. Here we present a molecular tool for species identification. It is based on polymerase chain reaction (PCR) amplification of a section of the mitochondrial cytochrome oxidase subunit II gene, followed by a three-enzyme restriction fragment length polymorphism (RFLP) assay, with banding patterns resolved via agarose gel electrophoresis. The assay was designed using a large set of training data obtained from a public DNA sequence database, then evaluated using an independent test panel of Reticulitermes from the Southern Appalachian Mountains, for which species assignments were determined via phylogenetic comparison to reference sequences. After refining the interpretive framework, the PCR-RFLP assay was shown to provide accurate identification of four co-occurring species (the fifth species, R. hageni, was absent from the test panel, so accuracy cannot yet be extended to training data). The assay is cost- and time-efficient, and will help improve knowledge of Reticulitermes species distributions.

Insect‐mediated nitrogen dynamics in decomposing wood

1. Wood decomposition is characterised by complex and poorly understood nitrogen (N) dynamics with unclear implications for forest nutrient cycling and productivity. Wood‐dwelling microbes have developed unique strategies for coping with theNlimitations imposed by their substrate, including the translocation ofNinto wood by cord‐forming fungi and the fixation of atmospheric nitrogen (N2) by bacteria andArchaea.2. By accelerating the release of nutrients immobilised in fungal tissues and promotingN2fixation by free‐living and endosymbiotic prokaryotes, saproxylic insects have the potential to influenceNdynamics in forests.3. Prokaryotes capable of fixingN2appear to be commonplace among wood‐feeding insects, with published records from three orders (Blattodea,Coleoptera andHymenoptera), 13 families, 33 genera and at least 60 species. These organisms appear to play a significant role in theNeconomies of their hosts and represent a widespread solution to surviving on a diet of wood.4. While agricultural research has demonstrated the role that termites and other insects can play in enhancing crop yields, the importance of saproxylic insects to forest productivity remains unexplored.

PASSIFOR: A reference library of DNA barcodes for French saproxylic beetles (Insecta, Coleoptera).

Saproxylic beetles – associated with dead wood or with other insects, fungi and microorganisms that decompose it – play a major role in forest nutrient cycling. They are important ecosystem service providers and are used as key bio-indicators of old-growth forests. In France alone, where the present study took place, there are about 2500 species distributed within 71 families. This high diversity represents a major challenge for specimen sorting and identification.The PASSIFOR project aims at developing a DNA metabarcoding approach to facilitate and enhance the monitoring of saproxylic beetles as indicators in ecological studies. As a first step toward that goal we assembled a library of DNA barcodes using the standard genetic marker for animals, i.e. a portion of the COI mitochondrial gene. In the present contribution, we release a library including 656 records representing 410 species in 40 different families. Species were identified by expert taxonomists, and each record is linked to a voucher specimen to enable future morphological examination. We also highlight and briefly discuss cases of low interspecific divergences, as well as cases of high intraspecific divergences that might represent cases of overlooked or cryptic diversity.

Arbuscular mycorrhizal fungi reduce decomposition of woody plant litter while increasing soil aggregation

The decomposition of plant organic matter and the stability of soil aggregates are important components of soil carbon cycling, and the relationship between decomposition rate and arbuscular mycorrhizal fungi (AMF) has recently received considerable attention. The interaction of AMF with their associated microorganisms and the consequences for litter decomposition and soil aggregation still remain fairly unclear. In a laboratory pot experiment we simultaneously tested the single and combined effects of one AMF species (Rhizophagus irregularis) and a natural non-AMF microbial community on the decomposition of small wooden sticks and on soil aggregation. To disentangle effects of hyphae and roots we placed mesh bags as root exclusion compartments in the soil. The decomposition of the wooden sticks in this compartment was significantly reduced in the presence of AMF, but not with the non-AMF microbial community only, compared to the control, while aggregation was increased in all treatments compared to the control. We suggest that AMF directly (via localized nutrient removal or altered moisture conditions) or indirectly (by providing an alternative carbon source) inhibited the activity of decomposers, leading to different levels of plant litter degradation under our experimental settings. Reduced decomposition of woody litter in presence of AMF can be important for nutrient cycling in AMF-dominated forests and in the case of woody plants and perennials that develop lignified roots in grasslands.

Soil microbial communities identify organic amendments for use during oil sands reclamation

Most of the soil microbial community responsible for nutrient cycling in boreal forests is found in surficial organic horizons, often referred to as forest floor layers. In the Athabasca oil sands region of northern Alberta, Canada, reclamation of upland forests utilizes peat as an organic amendment due to shortages in forest floor materials. However, differences in physical and chemical properties between peat and forest floor raise the concern that the use of peat will fail to promote a soil microbial community consistent with sustainable forest ecosystems. As a strategy to maximize the limited supply of forest floor, this study investigated mixing both materials. Specifically, we determined the differences in microbial community composition and activity between fresh and stockpiled forest floor, peat and mixtures of both using basal respiration and phospholipid fatty acid (PLFA) analysis. Basal respiration increased with forest floor addition, but only responded to water content within the pure forest floor treatment. Mixing fresh peat and forest floor produced greater than additive respiration, and the overall PLFA profiles illustrated a microbial community more similar to the forest floor material than to the peat. This suggests a proportionally greater activity in the microbial community associated with the forest floor that is dominating the mixed treatment. Individual phospholipid indicators further demonstrated a distinct shift in their abundance going from the pure peat to the pure forest floor. Taken together, results provide evidence that mixing forest floor and peat provides benefits by increasing the water-holding capacity of the mixed material as compared to pure forest floor, while producing a microbial community more analogous to an upland forest community than to peat alone.

Free-living nitrogen fixation responds to elevated nutrient inputs in tropical montane forest floor and canopy soils of southern Ecuador

Although often overlooked in forest research, the canopy can play an important role in forest nutrient cycling. Since the canopy is spatially isolated from the forest floor, nutrient cycling in the two areas may differ as terrestrial nutrients accumulate. We measured rates of free-living N2 fixation along an elevation gradient (1,000, 2,000 and 3,000 m) of tropical montane canopy soils, compared these to rates measured in the top 5 cm of forest floor soils (excluding fresh litter), and assessed the effects of elevated nutrient inputs to the forest floor. N2 fixation was measured using the acetylene reduction assay. Measurements occurred in the field, in the wet and dry seasons, using intact cores of soil. The forest floor had been fertilized biannually with moderate amounts of nitrogen (N) and phosphorus (P) for 4 years; treatments included control, N, P and N + P. N2 fixation rates exhibited little variation with elevation but were higher in the dry season than the wet season. Fixation was inhibited in forest floor N plots compared to control and P plots, and stimulated in canopy P plots compared to control. At 2,000 m, the canopy contributed 12 % of measured canopy and forest floor N2 fixation (1.2 kg N ha−1 year−1). Results suggest that N2 fixation is an active process in canopy soils, which is variable across seasons and sensitive to changes in terrestrial nutrient availability. Long-term terrestrial accumulation of N and/or P has the potential to significantly change the dynamics of soil N cycling in these canopies.

Effects of a simulated hurricane disturbance on forest floor microbial communities

Forest floor microbial communities play a critical role in the processes of decomposition and nutrient cycling. The impact of cultivation, contamination, fire, and land management on soil microbial communities have been studied but there are few studies of microbial responses to the effects of tropical storms. The Canopy Trimming Experiment was executed in the Luquillo Experimental Forest of Puerto Rico to decouple two prominent effects of a hurricane—canopy opening and debris deposition on the forest floor—on forest biota and processes. We studied the independent and interactive hurricane effects of canopy openness and debris deposition on the relative abundance and diversity of microorganisms in soil and leaf litter using ester link fatty acids methyl esters (EL-FAME) analysis, and terminal restriction fragment length polymorphism (TRFLP) profile. Non-metric multi-dimensional scaling analysis of soil FAME showed soil microbial community composition was significantly different between pre- and post-hurricane periods including in the unmanipulated plots and among blocks, but there was no significant separation among treatments. This shows that there are strong spatiotemporal dynamics in the structure of soil microbial communities which masked hurricane effects (canopy opening and deposition of green debris). The degree of difference among treatments decreased with time in soil which suggests that our study may have started too late after the manipulations and therefore missed the effects of canopy opening and debris addition. This reflects the resilience of the soil microbial communities. The richness of soil bacterial TRF’s however, showed a significant positive response to added debris. Neither fungal nor bacterial NMDS clusters for leaf microbial communities showed significant grouping by treatment, time or litter cohorts. Significant differences were observed through time for fungal diversity in green leaves and for both bacterial and fungal diversity in senesced leaves. Senesced leaves microbial succession apparently stopped when both the canopy and debris were removed, and there was a suggestive trimming by time interaction which reflects the susceptibility of the leaf litter microbial community. Our findings contribute to the understanding of how microbial community structures can be affected by hurricane disturbances and forestry management practices that remove canopy and debris from the forest floor, and shows the need to analyze the microbial community immediately after the disturbance. Short-term changes in microbial communities due to forest disturbances can have significant implications for litter decomposition, soil organic matter accumulation, nutrient cycling, and food web dynamics in tropical forests. All of these factors should be taken into consideration when selecting the appropriate forest management practice.

Root-induced changes in nutrient cycling in forests depend on exudation rates

(1) While it is well-known that trees release carbon (C) to soils as root exudates, the factors that control the magnitude and biogeochemical impacts of this flux are poorly understood.(2) We quantified root exudation and microbially-mediated nutrient fluxes in the rhizosphere for four ∼80 year-old tree species in a deciduous hardwood forest, Indiana, USA. We hypothesized that trees that exuded the most carbon (C) would induce the strongest rhizosphere effects (i.e., the relative difference in nutrient fluxes between rhizosphere and bulk soil). Further, we hypothesized that tree species that associate with ectomycorrhizal (ECM) fungi would exude more C than tree species that associate with arbuscular mycorrhizal (AM) fungi, resulting in a greater enhancement of nutrient cycling in ECM rhizospheres.(3) Mass-specific exudation rates and rhizosphere effects on C, N and P cycling were nearly two-fold greater for the two ECM tree species compared to the two AM tree species (P < 0.05). Moreover, across all species, exudation rates were positively correlated with multiple indices of nutrient cycling and organic matter decomposition in the rhizosphere (P < 0.05). Annually, we estimate that root exudation represents 2.5% of NPP in this forest, and that the exudate-induced changes in microbial N cycling may contribute ∼18% of total net N mineralization.(4) Collectively, our results indicate that the effects of roots on nutrient cycling are consequential, particularly in forests where the C cost of mining nutrients from decomposing soil organic matter may be greatest (e.g., ECM-dominated stands). Further, our results suggest that small C fluxes from exudation may have disproportionate impacts on ecosystem N cycling in nutrient-limited forests.