Do enchytraeid worms and habitat corridors facilitate the colonisation of habitat patches by soil microbes?

Due to their high abundance and ubiquitous existence, microbes are considered to be efficient colonisers of newly established habitats. To shed light on the dispersal mechanisms of soil microbes, a controlled microcosm experiment was established. In these microcosms, the dispersal of microbes from a source humus patch to originally sterile humus patches (embedded in a mineral soil matrix) was followed for 16 months, applying 16S and 18S ribosomal DNA-based PCR-DGGE molecular methods. Specifically, the role of enchytraeid worms and habitat (humus) corridors as possible facilitators of microbe dispersal was studied. The results showed that enchytraeid worms function efficiently as vectors for horizontal dispersal of saprophytic fungi in soil. Some of the fungi also proved to disperse through the corridors by vegetative growth, although this dispersal was inefficient as compared to dispersal with the enchytraeids. Virtually no saprophytic fungi were able to disperse through the mineral soil matrix in the absence of both enchytraeid worms and corridors. Unlike soil fungi, the dispersal of soil bacteria was not affected by any of the studied factors. The results of the present experiment provide direct evidence of the crucial role of soil fauna in aiding the horizontal dispersal of soil fungi. The role of enchytraeids as a functionally important species in boreal forest soils is further emphasized, since bringing microbes into contact with new resources is likely to enhance the rate of decomposition in soils.     

Do same-aged but different height Norway spruce (Picea abies) clones affect soil microbial community?

The influence of individual trees in monocrop forests on soil microbial communities is poorly understood. We measured basal respiration, substrate-induced respiration and phospholipid fatty acids (PLFA), bacterial growth rate with the ³H-thymidine incorporation technique and fungal growth rate as ¹⁴C-acetate incorporation into ergosterol to investigate whether slow- and fast-growing 12-year-old Norway spruce (Picea abies) clones have affected differently on their associated soil microbial communities. Understorey vegetation, soil chemical properties and elemental concentrations of needles were also determined. The slow- and fast-growing spruce clones differed in PLFA profiles, understorey vegetation and elemental concentrations in needles suggesting that spruce clones have directly or indirectly affected soil microbes.     

Microbial responses of forest soil to moderate anthropogenic air pollution

There is a need to introduce soil microbiological methods into long term ecological monitoring programs. For this purpose we studied the impact of moderate anthropogenic air pollution in polluted and less polluted area districts, forest site types Calluna (CT), Vaccinium (VT) and Myrtillus (MT) and the amount of organic matter, measured as carbon content on the soil respiration activity and the ATP content. The main sources of local air pollutants (SO₂ and NO_x) in the polluted area district were from the capital' region and an oil refinery. Humus (F/H-layer) and the underlying 0 to 5 cm mineral soil samples were collected from 193 study plots located in the 5300 km² study area. We found that the soil respiration rate in humus layer samples was lower in the polluted area district compared to the less polluted one (16.0 and 19.5 μL CO₂ h^?1g^?1 dw, respectively), but the difference occurred only in the dry, coarse-textured CT forest site type. The mineral soil respiration rate and the mineral soil and humus layer ATP content were not affected, by the air pollution. Most of the variations of the biological variables were explained primarily by the soil carbon content, secondly by the forest site type and thirdly by the area division.     

Characterization of dissolved organic matter in decomposing Norway spruce and silver birch litter

Dissolved organic matter (DOM) plays an important role in transport, storage and cycling of carbon (C) and nitrogen (N) in forest soils where litter is one of the main sources. The aim was to study the amount and characteristics of DOM leached from freshly fallen litters of silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) and their mixture during decomposition. DOM was collected after irrigation on eight occasions during 252 days incubation in the laboratory at about 18°C, including one freeze‐thaw cycle. During the incubation about 33–35% of C from birch and spruce litter and 40% of C from their mixture was lost. The total cumulative flux of dissolved organic carbon (DOC) from the mixture of litters was approximately 40% larger than that from single litters. The flux of DOC, DON, phenolic compounds and proteins followed a two‐stage pattern during decomposition. In the first stage the initially large fluxes decreased gradually. In the second stage, after freezing and thawing, the fluxes tended to increase again. Mixing birch and spruce litters and a freeze‐thaw cycle seems to increase the decomposition of litter and result in the increased flux of DOC, DON and phenolic compounds. The flux of hemicelluloses and the degradability of DOM were large at the first leaching occasion and decreased during the incubation. Birch had a 40% larger total flux of easily degradable DOM than spruce, supporting the previous consistent signs of greater microbial biomass and activities related to C and N cycling in soil under birch than under spruce. It is known that recalcitrant DOM might be stabilized whereas labile DOM may promote microbial activity and nutrient cycling. We conclude that the storage and cycling of C and N is affected by both tree species and degradation stage of litter in forest soils.     

Properties of dissolved organic matter derived from silver birch and Norway spruce stands: Degradability combined with chemical characteristics

Dissolved organic matter (DOM) derived from the humus layer under silver birch (Betula pendula Roth), Norway spruce (Picea abies (L.) Karst.), and mixed stands, and from senescent birch leaves and from spruce needles of the four oldest year-growth were characterized microbiologically and chemically. Samples were collected in the autumn and the solutions were obtained by centrifugation-drainage technique. The degradability of DOM, the availability of DOM to bacteria and fungi, concentrations of phenolic compounds and carbohydrates, and the distribution of carbon and nitrogen into fractions according to the chemical nature and the molecular size were studied. DOM derived from leaves and needles was clearly more labile than DOM derived from the humus layer indicating the importance of studying the DOM originating from fresh litter when assessing the turnover of DOM.DOM derived from spruce needles appeared to differ chemically greatly from all other samples. It had very high concentrations of carbohydrates, probably due to the sampling time, and phenolic compounds. The chemical composition of DOM derived from humus layer did not reflect the composition of DOM derived from needles and leaves. DOM derived from birch leaves degraded more than DOM derived from spruce needles and DOM derived from humus layer collected at the birch sites degraded more than DOM derived from humus layer collected at the spruce sites. The degradability of different compound groups of DOC and DON was studied in a short-term incubation (20 d) of DOM solutions by characterizing the solutions initially and after the incubation. Almost all compound groups appeared to degrade but weak hydrophobic acids, bases, hydrophilic neutrals, the smallest molecular size compounds, carbohydrates, and phenolic compounds degraded the most.     

Dissolved soil organic matter from surface organic horizons under birch and conifers: Degradation in relation to chemical characteristics

The degradability and chemical characteristics of dissolved organic carbon (DOC) and nitrogen (DON) from the litter, F and H layers of silver birch (Betula pendula Roth), Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.) stands were studied in an incubation experiment. Soil dissolved organic matter (DOM) was collected by centrifugation. Degradability was assessed in an incubation experiment by measuring the loss of DOC and DON, the mineralization rate of DOC and the availability of DOM to both bacteria and fungi, and by estimating the proportion of labile DOC of the total DOC. The degradability of DOC was highest in the litter layer and in that layer under birch. In the F and H layers, however, the degradability was highest under spruce. The most degradable fractions were the hydrophilic neutral fraction of DOC, the hydrophilic base fraction of DON, and the phenol fraction, as well as the smallest (<1 kDa) and largest (>100 kDa) molecular size classes of both DOC and DON. The degradability of these fractions seemed to be related to their relatively low C-to-N ratios. The hydrophilic acid fraction and the molecular size class of 1-10 kDa were more abundant in the H layer than in the litter layer, and thus apparently indicating a more decomposed DOM. In general, the effect of tree species on DOM was more obvious in the litter layer than in the lower organic layers.     

Exploring the mechanisms behind elevated microbial activity after wood ash application

Wood ash fertilization increases the pH and concentration of dissolved organic carbon (DOC) in the soil solution and enhances the activity of soil microorganisms. However, it is unknown whether DOC or pH is primarily responsible for the increase in microbial activity. We designed an experiment to separate the effects of DOC and/or pH on soil microbial activity using suspensions of humus extracts and bacteria that had not previously been exposed to wood ash fertilization. After a 3-week incubation, DOC extracts were obtained from control (DOC_C) and ash (DOC_A) treatments with carbon concentrations of 9.1 and 32.5 mg C l⁻¹, respectively. These extracts were supplied to bacterial suspensions at concentrations of 0 and 5 mg C l⁻¹. We controlled for pH by matching adjustments, i.e. the original pH of the DOC_C extract was 4.5 and its adjusted pH was 6.9, whereas the DOC_A extract was pH 6.9 originally and pH 4.5 adjusted. The relative bacterial growth rate (RBGR), as measured by ³H-thymidine incorporation, increased in suspensions of 5 mg C l⁻¹ DOC as compared to control suspensions of 0 mg C l⁻¹. At pH 6.9, RBGR was higher for both DOC extracts than at pH 4.5. These results suggest that both DOC and pH influence microbial activity. As the growth rate at pH 6.9 with DOC_A was higher than with DOC_C, the quality of the DOC extract must also play a role since the carbon concentration was controlled for. The decrease in relative abundance of hydrophobic and hydrophilic acids in DOC_A compared to DOC_C indicates a quality shift. As measured by DGGE banding patterns, the bacterial community structure changed over the course of the 24-h experiment in the following three trials, all of which received 5 mg C l⁻¹: DOC_C at pH 6.9 and DOC_A at pH 4.5 and 6.9. These results demonstrate that both the DOC origin (control vs. ash) and the pH influence a subset of the bacterial community.