Soil Ecosystem Processes Research Articles

Short-term impact of soybean management on ammonia oxidizers in a Brazilian savanna under restoration as revealed by coupling different techniques

Interactions between soil characteristics and soil microbiota influence soil ecosystem processes such as nitrification however, their complexity makes interpretation difficult. Furthermore, the impact of soil management systems on abundance and activity of soil microbial community is poorly understood, especially in the Neotropics. To investigate these interactions, the effects of tillage, inorganic fertilization, and plant cover on the abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) were assessed by quantification of the marker gene (amoA) during different stages of soybean cultivation in a site under restoration from gravel extraction in the Central Brazilian Savanna (Cerrado). Results of molecular analysis and classic and isotope techniques showed that levels of organic C and NH4 +–N were higher in the soybean field during fallow than in an adjacent undisturbed field (Campo sujo). Ammonia oxidizer abundance and nitrification rates were also higher in the agricultural soil than in the undisturbed site, with the lowest ammonium/nitrate ratio in tilled soil. Soil δ15N was lower in the undisturbed soil than the agricultural soil. Both AOA and AOB were more abundant during soybean crop transitional stages, and this increase positively correlated with soil pH, particularly for AOB abundance, in tilled soil and within the soybean rhizosphere. The results suggest that AOB have more copiotrophic characteristics than AOA and are better able to change available ammonium in the soil. The combination of standard soil ecological methods and modern molecular analysis show the short-term modification of ammonia oxidizer abundance and soil N dynamics in a managed system within the Cerrado biome.

The effect of zeolite and some plant residues on soil organic carbon changes in density and soluble fractions: Incubation study

Organic carbon (OC) fractions play an important role in soil and many ecosystem processes. This study focuses on changing of OC in different fractions in a soil treated with different levels of zeolite and plant residue that incubated for 90 days. The results showed that the amounts of light fraction (LF) and heavy fraction (HF) increased with increasing the percentage of zeolite and plant residues in the soil. The highest amounts of LF (22.7 g LF. Kg -1 Soil ) and HF (26.7 g. Kg -1 Soil ) were found when 30% zeolite, 5% wheat and alfalfa straws was added to the soil respectively. Accordingly, wheat straw and alfalfa straw were effective for increasing the LF and HF respectively. However they declined with decreasing the OC from the 1 st day of experiment until the 90 th day of experiment. Soluble OC in hot (2.80 g. Kg -1 Soil ) and cool (2.25 g. Kg -1 Soil ) water fractions increased with the addition of 30% zeolite and 5% plant residues particularly alfalfa straw in comparison with control. Although they increased after 30 days of starting incubation but, then they decreased in the continuation of the experiment. In fact, OC contents in density and soluble fractions increased with application and addition of 30% zeolite and 5% plant residues into the soil; however they decreased after 30 days of incubation with decreasing the OC. The findings of this research revealed the application of zeolite and plant residues improved carbon pools in density and soluble fractions and carbon sequestration increase by increasing the OC contents in soil.

Assessment of changes in different fractions of the organic carbon in a soil amended by nanozeolite and some plant residues: incubation study

Organic carbon (OC) fractions play important roles in soil and many ecosystem processes. This study focuses on changing of OC in density and soluble fractions in a soil amended by nanozeolite and plant residues that incubated in lab condition for 90 days. The results showed that amounts of OC in light fraction (LF) and heavy fraction (HF) increased with the increasing percentage of nanozeolite and plant residues in the soil. The highest amounts of LF (7.54 g LF. kg-1 Soil) and HF (11.10 g kg-1 Soil) were found when 30 % nanozeolite and 5 % wheat and alfalfa straws were added to the soil. Accordingly, wheat and alfalfa straws were effective on increasing the LF and HF, respectively. However, they decreased with declining the OC from the 1st day of experiment until the 90th day of experiment. Soluble OC in hot (2.22 g kg-1 Soil) and cool (1.54 g kg-1 Soil) water fractions increased by addition of 30 % nanozeolite and 5 % plant residues particularly alfalfa straw in comparison with control. Although these soluble fractions increased after initial 30 days of incubation, they decreased in the continuation of the experiment. In fact, OC contents in density and soluble fractions increased by addition of 30 % nanozeolite and 5 % plant residues into the soil; however, they decreased in initial 30 days of incubation with declining the OC. The findings of this research revealed the application of nanozeolite and plant residues improved carbon pools in density and soluble fractions and carbon sequestration increased by increasing OC contents in soil.

Labile, recalcitrant, microbial carbon and nitrogen and the microbial community composition at two Abies faxoniana forest elevations under elevated temperatures

We investigated the interactions of altitude and artificial warming on the soil microbial community structure in a subalpine Abies faxoniana forest in southwestern China after four years of warming. Open top chambers (OTCs) at two elevations (3000 m and 3500 m) were established, and their soil microbial characteristics, organic carbon (C) and nitrogen (N) were measured. The microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis. A two-step sulfuric acid hydrolysis was used to quantify the labile and recalcitrant C fractions in the soil organic matter. The results showed that bacterial PLFAs and gram-negative bacterial PLFAs increased and the fungal PLFAs and the fungi/bacteria ratio decreased with warming at the high altitude. By contrast, the warming effects on those parameters at low altitude were small. The higher proportion of labile easily decomposable soil C may explain the different responses of the microbial community composition at the two altitudes. An RDA analysis confirmed that the variations in the soil community structure were significantly associated with soil organic matter properties such as the sizes of the soil labile N pool (LP-N), the recalcitrant N pool (RP-N), and the labile C pool as well as dissolved organic C (DOC) and dissolved organic N concentrations (DON). Our results also showed that labile C and N pools increased with the altitude, but the microbial biomass C as measured with chloroform fumigation techniques decreased. Warming increased only the recalcitrant C pools at the high altitude. Given the longer mean residence time for recalcitrant C and the much greater size of this soil organic carbon pool, the results indicated that a rise in temperature in our case increased soil C pools at higher altitudes, at least during the early stages of experimental soil warming. Warming could also cause changes in the composition of the microbial community and enzyme activities, consequently leading to functional changes in soil ecosystem processes at the high altitude.

Subordinate plants mitigate drought effects on soil ecosystem processes by stimulating fungi

Summary The subordinate insurance hypothesis suggests that highly diverse communities contain greater numbers of subordinate species than less diverse communities. It has previously been reported that subordinate species can improve grassland productivity during drought, but the underlying mechanisms remain undetermined. Using a combination of subordinate species removal and summer drought, we show that soil processes play a critical role in community resistance to drought. Interestingly, subordinate species drive soil microbial community structure and largely mitigate the effect of drought on grassland soil functioning. Our results highlight subordinate species in shifting the balance within the phospholipid fatty acid (PLFA) microbial community towards more fungal dominance. Fungal communities promoted by subordinate species were more resistant to drought and maintained higher rates of litter decomposition and soil respiration. These results emphasize the important role of subordinate species in mitigating drought effects on soil ecosystem functions. Reciprocal effects between fungi and subordinate species explain also how subordinate species better resisted to drought conditions. Our results point to a delayed plant–soil feedback following environmental perturbation. Additionally, they extend the diversity insurance hypothesis by showing that more diverse communities not only contain species well adapted to perturbations, but also species with higher impacts on soil microbial communities and related ecosystem functions.

Overland flow directs soil moisture and ecosystem processes at patch scale in Mediterranean restored hillslopes

Semiarid and arid environments are frequently structured in vegetation patches that heterogeneously distribute water resources (water runoff and soil moisture). This redistribution is interrelated with episodes of rainfall triggering pulses of plant growth according to the Trigger–Transfer–Reserve–Pulse (TTRP) model. Spatial heterogeneity in the hydrological behaviour of surface patches has been described in Mediterranean mining restored hillslopes. Nevertheless studies describing the interactions of this hydrological heterogeneity with ecological processes on restored environments are lacking. This study investigates the relationships between overland flow running at hillslope scale and ecosystem processes at patch scale in restored hillslopes. We selected three approximately 20year old restored hillslopes along a gradient of overland flow (hillslope runoff coefficients are 15.9%, 2.2% and 0.3% for the three experimental hillslopes). We studied environmental conditions describing the ecohydrological interactions under the TTRP approach for arid and semiarid environments. Our results indicate that in restored hillslopes: 1) soil moisture content was associated to the type of vegetation patches; 2) higher soil water content enhanced vegetation diversity and soil properties, improving vegetation performance and colonization opportunities; and 3) there was an inverse relationship between the volume of overland flow and soil moisture at the hillslope scale, influencing, in turn, ecohydrological processes at the patch scale. Overall our results highlight the importance of overland flow modifying soil moisture distribution at patch scale and hence, influencing vegetation dynamics and ecological succession in these novel ecosystems.

Ant-mediated ecosystem functions on a warmer planet: effects on soil movement, decomposition and nutrient cycling.

1. Direct and indirect consequences of global warming on ecosystem functions and processes mediated by invertebrates remain understudied but are likely to have major impacts on ecosystems in the future. Among animals, invertebrates are taxonomically diverse, responsive to temperature changes, and play major ecological roles which also respond to temperature changes. 2. We used a mesocosm experiment to evaluate impacts of two warming treatments (+3·5 and +5 °C, set-points) and the presence and absence of the ant Formica subsericea (a major mediator of processes in north temperate ecosystems) on decomposition rate, soil movement, soil respiration and nitrogen availability. 3, Replicate 19-L mesocosms were placed outdoors in lathe houses and continuously warmed for 30 days in 2011 and 85 days in 2012. Warming treatments mimicked expected temperature increases for future climates in eastern North America. 4. In both years, the amount of soil displaced and soil respiration increased in the warming and ant presence treatments (soil movement: 73-119%; soil respiration: 37-48% relative to the control treatments without ants). 5. Decomposition rate and nitrogen availability tended to decrease in the warmest treatments (decomposition rate: -26 to -30%; nitrate availability: -11 to -42%). 6. Path analyses indicated that ants had significant short-term direct and indirect effects on the studied ecosystem processes. These results suggest that ants may be moving more soil and building deeper nests to escape increasing temperatures, but warming may also influence their direct and indirect effects on soil ecosystem processes.

Differentiating the mineralization dynamics of the originally present and newly synthesized amino acids in soil amended with available carbon and nitrogen substrates

Newly synthesized amino acids are the principle compounds created after inorganic nitrogen (N) is rapidly immobilized into microbial tissues. However, little is known about the mineralization kinetics of these newly synthesized amino acids compared to the amino acids originally present in the soil, and how substrate availability controls their mineralization. With 15N isotope tracing, the newly synthesized (15N-labeled) amino acids can be differentiated from the amino acids originally present (unlabeled) in soil, making it possible to evaluate the mineralization of the newly synthesized amino acids in tandem with the original amino acids. As amino acids can serve as both N and carbon (C) sources for microorganisms, the mineralization dynamics of amino acids may be manipulated by the availability of extraneous C and N. In this study, an aerobic 30-week intermittent leaching experiment was conducted, using glucose as C source and (14NH4)2SO4 as N source, following separate additions to soil. The newly synthesized amino acids were determined by an isotope-based high performance liquid chromatography/mass spectrometry (HPLC/MS). The newly synthesized soil amino acids mineralized faster than the original ones, which indicated more rapid cycling of N in the newly synthesized soil amino acids pool. Glucose addition significantly decreased the mineralization of both the newly synthesized and the original amino acids. However, when inorganic N was abundant, the newly synthesized amino acids decomposed rapidly, and preferentially as a C source and energy, while N addition inhibited the mineralization of the original amino acids in the soil. We conclude that the presence of readily degradable C (e.g. glucose) and inorganic N controls the mineralization of newly synthesized and original amino acid pools in soil differently, which is a crucial mechanism in adjusting the N supply and sequestration processes in soil ecosystems.

Seasonal Changes and Vertical Distributions of Soil Organic Carbon Pools under Conventional and No-Till Practices on Loess Plateau in China

Tillage practices affect soil organic carbon (SOC) pools, which in turn influence soil ecosystem processes. In this study we measured the effects of long-term conventional tillage (CT) and no-till (NT) practices on SOC and its fraction over the winter wheat growing season in surface and subsurface soils. Soil samples were taken during five physiological stages of winter wheat growth to a depth of 60 cm from the long-term (19 yr) experimental station on Loess Plateau in China. While the SOC content increased slowly in the surface soils during winter wheat growth with the NT treatment, it showed less fluctuation with the CT treatment. On average, NT treatment resulted in 82 and 53% higher SOC content in depth of 0 to 5 and 5 to 10 cm than CT treatment (P < 0.05). However, seasonal variations in microbial biomass carbon (MBC) and particulate organic carbon (POC) were similar under NT and CT, and showed maximum values in before-winter anthesis stage. The dissolved organic carbon (DOC) trend was highest before sowing, decreased before the winter and jointed stages, and increased again during the anthesis stage. Particulate organic carbon, MBC, and DOC were all significantly higher with NT than with CT in the upper 10 cm. Soil depth affected SOC and its fraction which decreased from surface to subsurface soil. The POC, MBC, and DOC were highly correlated with the SOC. This study demonstrated that measurements of the effect of tillage practices on SOC based on SOC fractions should include both seasonal changes and profile distribution.

Are there links between responses of soil microbes and ecosystem functioning to elevated CO2, N deposition and warming? A global perspective.

In recent years, there has been an increase in research to understand how global changes' impacts on soil biota translate into altered ecosystem functioning. However, results vary between global change effects, soil taxa, and ecosystem processes studied, and a synthesis of relationships is lacking. Therefore, here we initiate such a synthesis to assess whether the effect size of global change drivers (elevated CO2, N deposition, and warming) on soil microbial abundance is related with the effect size of these drivers on ecosystem functioning (plant biomass, soil C cycle, and soil N cycle) using meta-analysis and structural equation modeling. For N deposition and warming, the global change effect size on soil microbes was positively associated with the global change effect size on ecosystem functioning, and these relationships were consistent across taxa and ecosystem processes. However, for elevated CO2, such links were more taxon and ecosystem process specific. For example, fungal abundance responses to elevated CO2 were positively correlated with those of plant biomass but negatively with those of the N cycle. Our results go beyond previous assessments of the sensitivity of soil microbes and ecosystem processes to global change, and demonstrate the existence of general links between the responses of soil microbial abundance and ecosystem functioning. Further we identify critical areas for future research, specifically altered precipitation, soil fauna, soil community composition, and litter decomposition, that are need to better quantify the ecosystem consequences of global change impacts on soil biodiversity.

Dust effects on snowpack melt and related ecosystem processes are secondary to those of forest canopy structure and interannual snowpack variability

AbstractDust deposition lowers the albedo of snow and can significantly alter snowpack energy balance. Investigation of aeolian dust deposition in the mountains of the western U.S. has shown that these effects advance the timing of snowpack melt and spring runoff across much of the region. These studies have primarily focused on alpine snowpacks with little to no overstory vegetation. To evaluate the impacts of aeolian dust on ecohydrological processes in forests, we conducted a manipulative experiment in a subalpine conifer forest in Utah's Wasatch Mountains. During the spring of 2010–2012, we added dust to the snow surface in forested plots every 1 to 2 weeks, roughly doubling the natural dust loading. We then measured snowpack ablation in control and dust addition plots, along with below‐snowpack and warm season soil temperature (Tsoil), soil water content (θ), litter decomposition rate (D), soil respiration rate (Rs), and tree xylem water potential (ψ). Differences in ablation between control and dust addition plots were similar in magnitude to differences associated with the canopy structure of the forest. Seasonal patterns in Tsoil and θ were similar between dust treatments and canopy structure groups. D, Rs, and ψ varied little between dust treatments, but there were significant differences between years. During our 3‐year study, an unusual level of interannual variability in snowfall had the greatest effect on the soil environment and ecosystem processes. The effects of aeolian dust on snowpack mass and energy balance in our forest were slightly smaller than those associated with canopy structure. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Saltcedar (Tamarix ramosissima) Invasion Alters Decomposer Fauna and Plant Litter Decomposition in a Temperate Xerophytic Deciduous Forest

Plant invasions may alter the soil system by changing litter quality and quantity, thereby affecting soil community and ecosystem processes. We investigated the effect of Tamarix ramosissima invasion on the decomposer fauna and litter decomposition process, as well as the importance of litter quality in decomposition. Litter decomposition and decomposer communities were evaluated in two monospecific saltcedar forests and two native forests in Argentina, in litterbags containing either local litter (saltcedar or dominant native species) or a control litter. Saltcedar invasion produced an increase in Collembola, Acari, and total mesofauna abundance, regardless of the litter type. Control litter decomposition was higher in the native forest than in the saltcedar forest, showing that increased abundance of decomposer fauna does not necessarily accelerate decomposition processes. Local litter decomposition was not different between forests, suggesting that decomposer fauna of both ecosystems is adapted to efficiently decompose the autochthonous litter. Our results suggest that the introduction of a resource with higher quality than the local one has a negative effect on decomposition in both ecosystems, which is more pronounced in the invaded forest than in the native forest. This finding stresses the low plasticity of saltcedar decomposer community to adapt to short-term environmental changes.

Soil food web properties explain ecosystem services across European land use systems

Intensive land use reduces the diversity and abundance of many soil biota, with consequences for the processes that they govern and the ecosystem services that these processes underpin. Relationships between soil biota and ecosystem processes have mostly been found in laboratory experiments and rarely are found in the field. Here, we quantified, across four countries of contrasting climatic and soil conditions in Europe, how differences in soil food web composition resulting from land use systems (intensive wheat rotation, extensive rotation, and permanent grassland) influence the functioning of soils and the ecosystem services that they deliver. Intensive wheat rotation consistently reduced the biomass of all components of the soil food web across all countries. Soil food web properties strongly and consistently predicted processes of C and N cycling across land use systems and geographic locations, and they were a better predictor of these processes than land use. Processes of carbon loss increased with soil food web properties that correlated with soil C content, such as earthworm biomass and fungal/bacterial energy channel ratio, and were greatest in permanent grassland. In contrast, processes of N cycling were explained by soil food web properties independent of land use, such as arbuscular mycorrhizal fungi and bacterial channel biomass. Our quantification of the contribution of soil organisms to processes of C and N cycling across land use systems and geographic locations shows that soil biota need to be included in C and N cycling models and highlights the need to map and conserve soil biodiversity across the world.

The PAMPA datasets: a metagenomic survey of microbial communities in Argentinean pampean soils

BackgroundSoil is among the most diverse and complex environments in the world. Soil microorganisms play an essential role in biogeochemical cycles and affect plant growth and crop production. However, our knowledge of the relationship between species-assemblies and soil ecosystem processes is still very limited. The aim of this study was to generate a comprehensive metagenomic survey to evaluate the effect of high-input agricultural practices on soil microbial communities.ResultsWe collected soil samples from three different areas in the Argentinean Pampean region under three different types of land uses and two soil sources (bulk and rhizospheric). We extracted total DNA from all samples and also synthetized cDNA from rhizospheric samples. Using 454-FLX technology, we generated 112 16S ribosomal DNA and 14 16S ribosomal RNA amplicon libraries totaling 1.3 M reads and 36 shotgun metagenome libraries totaling 17.8 million reads (7.7 GB). Our preliminary results suggested that water availability could be the primary driver that defined microbial assemblages over land use and soil source. However, when water was not a limiting resource (annual precipitation >800 mm) land use was a primary driver.ConclusionThis was the first metagenomic study of soil conducted in Argentina and our datasets are among the few large soil datasets publicly available. The detailed analysis of these data will provide a step forward in our understanding of how soil microbiomes respond to high-input agricultural systems, and they will serve as a useful comparison with other soil metagenomic studies worldwide.

Effects of spatial heterogeneity and subsample pooling on the measurement of abiotic and biotic soil properties in rainforest, pasture and reforested sites

AbstractSpatial variability presents a challenge for effective measurement of ecological processes in soil ecosystems. This study characterized the heterogeneity of seven different biotic and abiotic soil properties at three ecological scales: land‐use type (vegetation type), site (scale of km to tens of km, and subplot (10–30 m). We addressed three questions about soil properties: (i) Do they differ more at any ecological scale when compared with other scales? (ii) Does physically combining samples from replicate subplots affect their measurement? (iii) What are the implications of this variation for the detection of differences caused by changing land‐use? There were five land‐use types: remnant rainforest, pasture, ecological restoration plantings 10–15 yrs old, 40–50 yrs regrowth dominated by the non‐native tree camphor laurel (Cinnamomum camphora) and rainforest regrowth 3–6 yrs after poisoning of camphor trees within older regrowth. Sites were interspersed within a 750 km2 region of subtropical Eastern Australia. In general, variability was greatest at the site and subplot levels. Soil water content, soil organic matter and pH were less variable than total organic carbon, microbial biomass, nitrate and nitrification. Power analyses using total organic carbon and nitrate showed that adequate replication at both site and subplot levels was important for detecting effects of land‐use type, although increased replication was slightly more effective for subplots than for sites. Subplot spacing was relatively unimportant. Physically combining subplot samples to yield one aggregate measurement per site was successful in reducing analytical effort without sacrificing accuracy for all soil properties except total organic carbon and nitrification.

Phylogenetic Distribution of Potential Cellulases in Bacteria

Many microorganisms contain cellulases that are important for plant cell wall degradation and overall soil ecosystem functioning. At present, we have extensive biochemical knowledge of cellulases but little is known about the phylogenetic distribution of these enzymes. To address this, we analyzed the distribution of 21,985 genes encoding proteins related to cellulose utilization in 5,123 sequenced bacterial genomes. First, we identified the distribution of glycoside hydrolases involved in cellulose utilization and synthesis at different taxonomic levels, from the phylum to the strain. Cellulose degradation/utilization capabilities appeared in nearly all major groups and resulted in strains displaying various enzyme gene combinations. Potential cellulose degraders, having both cellulases and β-glucosidases, constituted 24% of all genomes whereas potential opportunistic strains, having β-glucosidases only, accounted for 56%. Finally, 20% of the bacteria have no relevant enzymes and do not rely on cellulose utilization. The latter group was primarily connected to specific bacterial lifestyles like autotrophy and parasitism. Cellulose degraders, as well as opportunists, have multiple enzymes with similar functions. However, the potential degraders systematically harbor about twice more β-glucosidases than their potential opportunistic relatives. Although scattered, the distribution of functional types, in bacterial lineages, is not random but mostly follows a Brownian motion evolution model. Degraders form clusters of relatives at the species level, whereas opportunists are clustered at the genus level. This information can form a mechanistic basis for the linking of changes in microbial community composition to soil ecosystem processes.

The effects of arbuscular mycorrhizal hyphal networks on soil aggregations of purple soil in southwest China

Soil aggregation is a crucial soil property that affects a wide range of physical and chemical processes in soil ecosystems. Arbuscular mycorrhizal (AM) association is recognised as an important promoter of soil aggregation through the action of individual roots, mycelia and an insoluble, glue-like and hydrophobic proteinaceous substance, which is (at least partly) of AM fungi origin, named glomalin-related soil protein. Considering the increasing application of commercial AM inoculants, we addressed how the soil aggregates respond to the hyphal functions of AM inoculation in the field with a resident AM community.To this end, we introduced a new system in which the hyphae were separated by mesh and regular rotation to break the ingrowing hyphae as a control and to demonstrate the causal link between the hyphae and soil aggregates under conditions simulating natural parameters. The results showed the following: (i) the hyphal length was positively correlated with the mean weight diameter (r = 0.384, P < 0.05), geometric mean diameter (r = 0.257, 0.05 < P < 0.10) and easily extractable glomalin (r = 0.296, P < 0.05); (ii) the colonisation rate of the roots in the cores was increased by constantly severing the extraradical mycelium and (iii) the colonisation rate of the control plant roots (approx. 10%) was significantly lower compared to those inoculated with AM fungi (ranging from 34% to 54%). It was concluded that the hyphal networks of AM inoculations can promote the formation and stability of soil aggregates under conditions that closely simulate those occurring in nature.

Fungal communities influence root exudation rates in pine seedlings

Root exudates are hypothesized to play a central role in belowground food webs, nutrient turnover, and soil C dynamics in forests, but little is known about the extent to which root-associated microbial communities influence exudation rates in trees. We used a novel experimental technique to inoculate loblolly pine (Pinus taeda L.) seedlings with indigenous forest fungi to examine how diverse fungal communities influence exudation. Surface-sterilized seeds were sown in intact, unsieved soil cores for 14weeks to promote root colonization by fungi. After 14weeks, we transferred seedlings and root-associated fungi into cuvettes and measured exudate accumulation in trap solutions. Both the abundance and identity of root-associated fungi influenced exudation. Exudation rates were greatest in root systems least colonized by ectomycorrhizal (ECM) fungi and most colonized by putative pathogenic and saprotrophic fungi. However, the ECM community composition was not a strong determinant of exudation rates. These results suggest that environmental conditions that influence the degree to which tree roots are colonized by pathogenic and saprotrophic vs. mutualistic fungi are likely to mediate fluxes of labile C in forest soils, with consequences for soil biogeochemistry and ecosystem processes.

Greenhouse gas soil production and surface fluxes at a high arctic polar oasis

Arctic vegetation and soil biological communities interact with a range of biotic and abiotic factors to produce or consume the greenhouse gases (GHG) carbon dioxide, methane, and nitrous oxide. In Arctic environments the parameters controlling these processes are not well understood. We measured soil GHG concentrations and surface fluxes from six vegetation communities at a High Arctic polar oasis and adjacent polar deserts in order to identify regions within the soil profile of production and consumption of CO2, CH4, and N2O. Examined communities included two polar deserts differing in parent material and soil pH, and four lowland tundra communities: prostrate dwarf-shrub, herb tundra, prostrate/hemiprostrate dwarf-shrub tundra, nontussock sedge, dwarf-shrub, moss tundra and a sedge/grass, moss wetland, representative of large areas at lower Arctic latitudes. Polar desert soils were net producers of greenhouse gases during the brief High Arctic growing season, including at depths close to the permafrost layer, and effluxes from the surface were of a similar magnitude to nearby mesic and hydric tundra soils including for CO2, indicative of soil respiration in desert soils with few roots. Differences in water content, rather than calculated diffusivity, appear to drive gas transport in at least some soils, with all three GHG appearing to move rapidly through, for example, the soil at 10 cm above permafrost in the Prostrate (dominated by Dryas integrifolia) plant community. Such physical processes may obscure or falsely suggest biological processes in soil ecosystems.

The impact of garlic mustard on sandy forest soils

Garlic mustard (Alliaria petiolata), an exotic biennial herb invasive to North American forests, has potential to affect resource availability in invaded soils. Garlic mustard produces a suite of toxic chemicals that impact diversity both above and belowground and thus likely alter ecosystem processes. To examine the effects of garlic mustard on soil biota and ecosystem processes, we sampled soil from invaded and uninvaded stands at a pine plantation on sandy soils in central Illinois. Several of the pine stands were also planted with the N2-fixing tree black locust (Robinia pseudoacacia). Results indicate that pine soils underlying garlic mustard have higher pH, and exhibit higher rates of N mineralization, relative nitrification and soil respiration compared to soils without garlic mustard. Nitrogen turnover on pine invaded stands is more similar to pine soils with the black locust trees present than to uninvaded pine stands without black locust. Garlic mustard cover was much greater in stands with black locust trees planted, suggesting that garlic mustard may be attracted to high N sites. Catabolic response profiles, which provide a measure of soil microbial function, indicated a shift in substrate use for one of the substrates tested in the presence of garlic mustard. While many of the soil characteristics measured did not differ between invaded and uninvaded stands, the differences with regard to N turnover were striking and will likely have long term effects on soil nutrient status, with potential to feedback to forest health.