Interaction between two decomposer basidiomycetes and a collembolan under Sitka spruce: Grazing and its potential effects on fungal distribution and litter decomposition

The hypothesis that selective grazing by Onychiurus latus Gisin altered the outcome of competition between two basidiomycetes, Mycena galopus (Pers. ex Fr.) Kummer and Marasmius androsaceus (L. ex Fr.) Fr. on Sitka spruce litter was tested. Without grazing M. androsaceus had the higher colonizing ability of litter in the laboratory whereas with grazing the position was reversed. Field exclusion experiments also suggested that higher densities of O. latus resulted in a reduction in the activity of M. androsaceus and an increase in that of M. galopus and vice versa. It is suggested that selective grazing may be an important factor determining the field distribution of these fungi, since M. androsaceus, the preferred food source, was restricted to the L litter horizon where the density of O. latus was not high enough to alter the outcome of competition whereas M. galopus predominated in the F₁ horizon. The rate of decomposition of Sitka spruce litter in the laboratory caused by M. androsaceus (17% dry weight loss over 6 months at 11°C) increased by 4–14% in the presence of grazing by O. latus whereas that of M. galopus (11.5%) decreased by 12–16%.     

Development of an immunological technique for estimating mycelial biomass of Mycena galopus in leaf litter

Antisera were produced in rabbits against cell wall and cytoplasmic components of the mycelium of a litter-decomposing basidiomycete, Mycena galopus (Pers. ex Fr.) Kummer. The antisera were conjugated with fluorescein isothiocyanate, and positive reactions were obtained in direct and indirect fluorescent antibody (FA) tests with homologous antigens and with heterologous antigens of M. galopus from pure cultures and field material. Cross reactions occurred with fungal associates of M. galopus on Quercus litter from a deciduous woodland, but these were mostly removed by absorption with a preparation of Cladosporium herbarum Link ex Fr. Membrane filtration combined with FA staining and rhodamine-gelatin counter-staining was found to be a feasible means of estimating the biomass of FA-reactive mycelium of M. galopus in leaf litter.     

Saprotrophic fungi transform organic phosphorus from spruce needle litter

Fungal decomposition of and phosphorus transformation from spruce litter needles (Picea abies) were simulated in systems containing litter needles inoculated with individual saprotrophic fungal strains and their mixtures. Fungal strains of Setulipes androsaceus (L.) Antonín, Chalara longipes (Preus) Cooke, Ceuthospora pinastri (Fr.) Höhn., Mollisia minutella (Sacc.) Rehm, Scleroconidioma sphagnicola Tsuneda, Currah & Thormann and an unknown strain NK11 were used as representatives of autochthonous mycoflora. Systems were incubated for 5.5 months in laboratory conditions. Fungal colonization in systems and competition among strains were assessed using the reisolation of fungi from individual needles. After incubation, needles were extracted with NaOH and extracts were analysed using ³¹P nuclear magnetic resonance spectroscopy (NMR). Needle decomposition was determined based on the decrease in C:N ratio. Systems inoculated with the basidiomycete S. androsaceus revealed substantial decrease in C:N ratio (from 25.8 to 11.3) while the effect of ascomycetes on the C:N ratio was negligible. We suppose that tested strains of saprotrophic ascomycetes did not participate substantially in litter decomposition, but were directly involved in phosphorus transformation and together with S. androsaceus could transform orthophosphate monoesters and diesters from spruce litter needles into diphosphates, polyphosphates and phosphonates. These transformations seem to be typical for saprotrophic fungi involved in litter needle decomposition, although the proportion of individual phosphorus forms differed among studied fungal strains. Phosphonate presence in needles after fungal inoculation is of special interest because no previous investigation recorded phosphonate synthesis and accumulation by fungi. Our results confirmed that the ³¹P NMR spectroscopy is an excellent instrumental method for studying transformations of soil organic phosphorus during plant litter decomposition. We suggest that polyphosphate production by S. androsaceus may contribute to the phosphorus cycle in forest ecosystems because this fungus is a frequent litter colonizer that substantially participates in decomposition.     

Potential for monoterpenes to affect ectomycorrhizal and saprotrophic fungal activity in coniferous forests is revealed by novel experimental system

The fungal community in coniferous forest soils plays a pivotal role in ecosystem processes such as decomposition and carbon and nutrient cycling. Both saprotrophic (SP) and ectomycorrhizal (ECM) fungi occur throughout the upper soil horizons in coniferous forests and could therefore be exposed to high concentrations of monoterpenes occurring in the needle litter and roots of some tree species. Previous work has noted the differential effects of monoterpenes on the mycelial growth of a range of both SP and ECM fungi when grown in artificial nutrient media. This study used a novel experimental system to assess the effect of environmentally relevant concentrations of monoterpenes on the activity of ECM and SP fungi grown on more natural substrata. Exposure of the ECM fungus Paxillus involutus (Batsch) Fr. to vapours of either α-pinene or β-pinene resulted in a significantly greater proportion of root tips being colonised by the fungus when it was grown with seedlings of Picea abies (L.) Karst. Exposure to monoterpenes resulted in a significant decrease in respiration rate of two species of litter degrading SP fungi, Mycena galopus var. candida J. E. Lange and Collybia butyracea (Bull.) P. Kumm. There was no difference in response between the two SP species, despite the fact that previous tests in liquid nutrient media, with monoterpenes at higher concentrations, indicated that one species was sensitive and one was not. The high volatility and low solubility of monoterpenes in water make them challenging to work with. The experimental system developed here, although still artificial, provides a bridge between pure culture studies in defined media and all the complexities of forest soils in the field, by allowing the exposure of fungi to environmentally relevant monoterpene concentrations in more natural substrata.     

Effects of fungal inocula on the decomposition of lignin and structural polysaccharides in Pinus sylvestris litter

 Litter bags containing sterile Scots pine (Pinus sylvestris) needles (19.8% lignin, 26.5% cellulose and 0.34% N) were inoculated with two species of fungi in the laboratory and then placed in the litter layer of a pine plantation. Marasmius androsaceus, which can degrade lignocellulose, was initially displaced by other fungal colonisers and was not detected in the litter after 2–3 months; but was re-isolated from the needles after 12 months. Trichoderma viride, which is a cellulolytic species and also antagonistic to other fungi, dominated the litter throughout the experiment. The control litter was naturally colonised by litter fungi. After 12 months, mass losses were similar at 52% for M. androsaceus and 48% for T. viride, compared with 36% for the control litter colonised by a more complex fungal community. Lignin concentrations increased with time in control litter and with T. viride because mass losses of carbohydrates were greater than those of lignin. Litter inoculated with M. androsaceus showed significant lignin decomposition throughout the experiment but cellulose concentrations showed a proportional increase in the first 6 months, suggesting that the fungus was preferentially exploiting hemicellulose and non-structural carbohydrates. Analysis of TFA-extractable sugars (mainly from hemicellulose) and CuO-derived phenylpropanoid moieties from lignin confirmed the differential patterns of resource decomposition which were not evident from total mass losses. During the initial stages of decomposition, T. viride was as effective in utilising structural polysaccharides as the complex fungal community in the control litter. Furthermore, M. androsaceus not only exhibited unexpectedly low cellulolytic activity but also facilitated lignin depolymerisation after the fungus was no longer detectable in the litter. The pre-inoculation of litter with these two fungal species therefore affected the overall dynamics of decomposition at a biochemical level. This study illustrates the importance of understanding the effects and interactions of specific fungi, rather than assumptions about the functional competence of diverse communities, on the processes of litter decomposition. Received: 5 July 2000     

Effects of collembolan grazing on nutrient release and respiration of a leaf litter inhabiting fungus

An attempt was made to assess the effect of grazing by Onychiurus subtenuis Folsom on nutrient release from and respiration of a sterile dark fungus using simple microcosms held under controlled laboratory conditions.The results showed that the fungus was very efficient in taking up all available soluble NO₃^? and PO₄^2? and that collembolan grazing had no effects in releasing these nutrients during the 10 day experiment. Significant increase in respiration of litter colonized by the fungus was observed following collembolan grazing, but this increase was attributed to the activity of bacteria and fungi tracked into the leaf litter systems by the Collembola.     

Effects of aluminum on mineral content of mycorrhizal fungi in vitro

The influence of Al³⁺ on the mineral content of mycorrhizal fungi was studied in vitro. Amanita muscaria (L. ex Fr.) Pers. ex Hooker whose growth is reduced and Lactarius piperatus (L. ex Fr.) S.F. Gray whose growth is stimulated by Al³⁺ where chosen for the experiments. 0, 0.1, 1, and 10mM A³⁺ were added as Al₂(SO₄)₃ · 18H₂O to modified M-40 medium. Al³⁺ drastically affected the mineral contents of the mycelia of both fungi: the Al, Ca, and Na contents increased while P, Mg, and K decreased with the increasing Al³⁺ concentrations in the media. On the other hand, some important differences between the two mycelia were detected. In the mycelia of Lacterius piperatus grown on the control media, there was about twice as much Ca content and a three times lower P content than in the Amanita muscaria mycelia. The Al³⁺ content in the mycelia of Lactarius piperatus grown on 10mM Al³⁺ was six times higher than in Amanita muscaria at the same concentration.     

Changes in humus properties and collembolan communities following the replanting of beech forests with spruce

Differences of collembolan communities within the organo-mineral A layer were studied in relation to physico-chemical changes in humus at nine sites of beech forests (Fagus sylvatica L.) and first generation spruce stands (Picea abies (L.) Karst.), planted on former beech forest 30 years ago (Central Pyrenees, France). Changes in humus form were caused by the spruce plantation and occurred mainly within the fermentation horizon where acidifying litter accumulation increased the horizon depth. The recent replacement of beech by spruce induced a shift from mull towards moder humus forms, which is explained by the decreasing organic matter turnover rate. A significant decrease in the three exchangeable cations and pH under spruce was also observed. Collembolan species diversity within the A horizon was significantly lower under spruce at this early stage of the silvogenetic cycle. Differences between species composition of Collembola between the two forest stands is related to changes in environmental conditions (e.g. nutrient availability, soil porosity, soil moisture). This study shows how forest management practices are susceptible to modify biological activity within the A horizon under exotic conifer plantations.     

Mycorrhizal fungi of Pseudotsuga menziesii in the North Island of New Zealand

Attempts were made to isolate mycorrhizal fungi of a timber species, Pseudotsuga menziesii (Mirb.) Franco which is exotic to New ZeaLond. Over 2000 pieces of mycorrhizal roots were plated out on Hagem medium and modified Melin-Norkrans medium; 14% of these yielded Rhizopogon vinicolor A. H. Smith, 6% gave rise to Amanita muscaria (L. ex Fr.) S.F. Gray, and 7% to unidentified basidiomycetes, 21% yielded dark sterile mycelia, 13% were sporing Fungi Imperfecti presumed to be contaminants, and 39% remained free of fungal growth.Eleven basidomycetous fungi were tested for their mycorrhiza-forming ability on P. menziesii seedlings. Under the test conditions R. vinicolor, Hebeloma crustuliniforme (Bull, ex St. Am) Quel., A. muscaria, Scleroderma bovista Fr., Laccaria laccata (Scop, ex Fr.) Berk. & Br., Inocybe corydalina (?), I. jurana (?), and I. maculata (?) formed mycorrhizas.     

Litter decomposition and faunal activity in Mediterranean forest soils: effects of N content and the moss layer

Accumulation of soil carbon is mainly controlled by the balance between litter production and litter decomposition. Usually In Mediterranean forests there are contrasting conditions in the distribution of faunal activity and the moss layer that may have different effects on litter decomposition. Decomposition and faunal activity were studied by exposing litter of contrasting quality (Pinus halepensis Mill. and Quercus ilex L.) for 3.5 yr in three Mediterranean pine forests of the eastern Iberian Peninsula. The effects of mosses on decomposition and on faunal activity were studied by exposing P. halepensis litter either on moss patches or directly on the forest floor. Faecal pellet production was used as an indication of faunal activity. Water availability or soil characteristics seem to limit faunal activities in the drier sites. Faecal pellets were not found during the first stages of decomposition and in all sites they appeared when about a 30% of the initial litter had decomposed. Under wet conditions faecal pellet production was very high and a mass balance suggested that soil faunal activity may result in a net flow of organic matter from the lower organic horizons to the surface Oi horizon. Mosses slightly increased mass loss of pine litter probably as a consequence of high potentially mineralizable nitrogen in the Oa horizon of moss patches and also, perhaps, as a consequence of the higher moisture content measured in the Oi horizon needles sampled among the mosses. In contrast, moss patches reduced faunal activity. The effect of litter quality on mass loss was not always significant, suggesting an interaction between litter quality and site conditions. During the first stages of decomposition there was N immobilisation in P. halepensis litter (poorer in N) and N release from Q. ilex litter (richer in N). In conclusion, in these forests soil microclimate and/or N availability appear to be more important controlling litter decomposition than the distribution of faunal activity.     

Effects of litter (beech and stinging nettle) and earthworms (Octolasion lacteum) on carbon and nutrient cycling in beech forests on a basalt-limestone gradient: A laboratory experiment

Effects of leaf litter of beech (Fagus sylvatica L.) and stinging nettles (Urtica dioica L.) and of the endogeic earthworm species Octolasion lacteum (Örley) on carbon turnover and nutrient dynamics in soil of three beechwood sites on a basalt hill (Hesse, Germany) were investigated in a laboratory experiment lasting for about 1 year. The sites were located along a gradient from basalt (upper part of the hill) to limestone (lower part of the hill) with an intermediate site in between (transition zone). At the intermediate site U. dioica dominated in the understory whereas at the other sites Mercurialis perennis L. was most abundant. The amount and composition of organic matter was similar in soil of the basalt (carbon content 5.9%, C/N ratio 13.8) and intermediate site (carbon content 5.6%, C/N ratio 14.3) but the soil of the intermediate site produced more CO₂ (in total +17.5%) and more nitrogen (as nitrate) was leached from this soil (in total +55.6%). It is concluded that the soil of the intermediate site contains a large mobile carbon and nitrogen pool and the formation of this pool is ascribed to the input of U. dioica litter. Leaf litter of U. dioica strongly increased NO₃ ^–-N leaching immediately after the litter had been added, whereas nitrogen was immobilized due to addition of beech litter. Despite the very fast initial decomposition of nettle litter, the increase in CO₂ production due to this litter material was only equivalent to 20.1% of the amount of carbon added with the nettle litter; the respective value for beech litter was 34.8%. Earthworms altered the time course of carbon and nitrogen mineralization in each of the treatments. In general, earthworms strongly increased mineralization of nitrogen but this effect was less pronounced in soil of the intermediate site (treatments without litter), which is ascribed to a depleted physically protected nitrogen and carbon pool. In contrast, their effect on the total amount of nitrogen mobilized from nettle litter was small. Earthworms significantly reduced CO₂ production from soil of the intermediate site (treatments without litter) and it is concluded that earthworm activity contributes to the restoration of the depleted physically protected carbon pool at this site.     

Microsite assessment of forest soil nitrogen,phosphorus, and potassium supply rates in‐field using ion exchange membranes

Abstract

A new method for microsite assessment of soil nutrient supply in forest soil was developed. The method involves the use of ion exchange membranes to assess differences in soil nitrogen (N), phosphorus (P), and potassium (K) supply rates in‐field over small depth increments in the forest floor (i.e., the L, F, and H horizons). Ion exchange membranes were buried and retrieved from the forest floor in an aspen forest stand in Saskatchewan, Canada. Small (6 mm diameter) sections of the membrane were cut out and ion concentration on the sections measured to provide a nutrient supply rate at that location. Soil nutrient supply rates at the site ranged from 4.6–6.0, 7.3–8.5, 11.6–21.5, and 122–196μg 10 cm²#lb2 h^‐1 for NH₄ ⁺‐N, NC₃ ^‐‐N, P, and K, respectively. On average, the highly humified H horizon had the highest N and P supply rates, followed by the F horizon, with the surface litter (L horizon) having the lowest N supply rates. The simplicity and sensitivity of the procedure make this method appropriate for in‐field assessment of differences in soil nutrient supply over small vertical and horizontal distance and was especially appropriate for the forest floor horizons in forest soils.     

Do oribatid mites (Acari: Oribatida) show a higher preference for ubiquitous vs. specialized saprotrophic fungi from pine litter?

Previous studies of oribatid mite feeding preferences for different saprotrophic fungi were limited to ubiquitous fungal species, whereas saprophytes specialized to decompose particular substrates have been neglected. We examined the preference of seven oribatid mite species (Adoristes ovatus, Eniochthonius minutissimus, Eueremaeus silvestris, Nothrus silvestris, Oppiella subpectinata, Porobelba spinosa and Spatiodamaeus verticillipes) for nine autochthonous saprotrophic fungi from Scots pine litter (Pinus sylvestris). Among the fungal species offered were specific coniferous litter colonizers (Allantophomopsis lycopodina, Ceuthospora pinastri, Hormonema dematioides, Scleroconidioma sphagnicola, Verticicladium trifidum, Marasmius androsaceus and Sympodiella acicola) and two ubiquitous species (Cladosporium herbarum and Oidiodendron griseum). The fungi were inoculated on fragments of pine needles and offered simultaneously and separately to the mites. Our main hypothesis, that oribatid mites (usually occurring in more than one type of ecosystems) would prefer the ubiquitous fungal species rather than those specific to pine litter, was supported only partly. The ubiquitous C. herbarum was highly preferred by all studied mites, but most of them preferred one or more of the specialized fungi with similar intensity. The basidiomycete M. androsaceus along with sterile needles were consistently avoided by all mites in all experiments. Our results do not support the hypothesis, that the “true” fungivorous oribatid mites in traditional sense are more selective fungal feeders than are the “unspecialized” panphytophagous ones. We observed no gradation in preference of fungi for oribatid mites as a group, but rather a discontinuous and dynamic mosaic with particular mites preferring particular fungal species. This heterogeneous mosaic shapes the feeding niches occupied by particular oribatid mite species and probably reduces competition for food source among numerous species coexisting in a given habitat and time.     

Microfungal communities in soil,litter and casts of Lumbricus terrestris L. (Lumbricidae): a laboratory experiment

The anecic earthworm Lumbricus terrestris L. was kept in laboratory microcosms containing beech forest soil without litter, with beech leaf litter or with lime leaf litter. The structure of microfungal communities in soil, litter and fresh and aged (100 days) earthworm faeces was analysed using the washing and plating technique. The passage of mineral soil through the gut of L. terrestris affected the structure of the fungal community only little. In contrast, in the litter treatments the structure of the fungal community in fresh earthworm casts significantly differed from that in soil and litter. The majority of soil and litter inhabiting fungi survived passage through the gut of L. terrestris and the fungal community in casts consisted of a mixture of soil and litter inhabiting fungi. However, the frequency of Cladosporium spp., Alternaria spp., Absidia spp., and other taxa was strongly reduced in fresh casts. The degree of colonization of litter particles (number of isolates per number of plated particles) also decreased, but some fungi (mainly Trichoderma spp.) benefited from gut passage and flourished in fresh casts. During ageing of cast material the dominance structure of the fungal community changed. Both the degree of colonization of organic particles and the species diversity increased and approached that in soil. However, the structure of the fungal community in casts remained cast specific even after 100 days of incubation. It is concluded that the feeding and burrowing activity of L. terrestris accelerates the colonization of litter by the edaphic mycoflora but also extends the range of occurrence of litter-associated fungi into mineral soil layers.     

Translocation of surface litter carbon into soil by Collembola

Soil invertebrates are important in nutrient cycling in soils, but the degree to which mesofauna such as Collembola are responsible for the direct movement of carbon (C) from the litter layer into soil has not yet been ascertained. We used naturally occurring stable C isotopic differences between a C₄ soil and alder leaves (C₃) to examine the effect of the collembolan Folsomia candida on C translocation into soil in laboratory microcosms. Collembolan numbers greatly increased in the presence of alder, but despite large collembolan populations there were no changes in decomposition rate (measured as litter mass loss, cumulative respired CO₂ and alder C:N ratios). Small changes in the δ¹³C values of bulk soil organic matter were detected, but could not be assigned to collembolan activity. However, mean δ¹³C values of soil microbial phospholipid fatty acids (PLFAs) were significantly lower in the presence of alder and Collembola together, demonstrating that collembolan activities resulted in greater availability of litter-derived C to the soil microbial community. Additionally, the presence of Collembola resulted in the translocation of alder-derived compounds (chlorophyll and its breakdown product pheophytin) into soil, demonstrating that Collembola modify soil organic matter at the molecular level. These results are consistent with deposition of collembolan faeces in underlying soil and demonstrate that despite their small size, Collembola contribute directly to C transport in the litter-soil environment.     

Soil compaction and poultry litter effects on factors affecting nitrogen availability in a claypan soil

Soil compaction may affect N mineralization and the subsequent fate of N in agroecosystems. Laboratory incubation and field experiments were conducted to determine the effects of surface soil compaction on soil N mineralization in a claypan soil amended with poultry litter (i.e., Turkey excrement mixed with pine shavings as bedding). In a laboratory study, soil from the surface horizon of a Mexico silt loam soil was compacted to four bulk density levels (1.2, 1.4, 1.6 and 1.8 Mg m⁻³) with and without poultry litter and incubated at 25 °C for 42 days. A field trial planted to corn (Zea mays L.) was also conducted in 2002 on a Mexico silt loam claypan soil in North Central Missouri. Soil was amended with litter (0 and 19 Mg ha⁻¹) and left uncompacted or uniformly compacted. Soil compaction decreased soil inorganic N by a maximum of 1.8 times in the laboratory study; this effect was also observed at all depths of the field trial. Compacted soil with a litter amendment accumulated NH₄⁺-N up to 7.2 times higher than the noncompacted, litter-amended soil until Day 28 of the laboratory incubation and in the beginning of the growing season of the field study. Ammonium accumulation may have been due to decreased soil aeration under compacted conditions. Application of litter increased soil N mineralization throughout the growing season. In the laboratory study, soil inorganic N in unamended soil was negatively correlated with soil bulk density and the proportion of soil micropores, but was positively related with soil total porosity and the proportion of soil macropores. These results indicate that soil compaction, litter application and climate are interrelated in their influences on soil N mineralization in agroecosystems.     

Effects of air drying on the respiration of forest soil microbial populations

Microbial populations from dried, remoistened and undried forest organic horizons were allowed to thoroughly recolonize sterilized forest organic horizons; oxygen uptake was followed during recolonization. Microbial damage due to air drying was in the order litter (L) > fermentation (F) > humus (H). Species diversity, as indicated by diminished O₂ uptake, was reduced in all horizons which had been air dried, with greatest damage in the litter horizon. Samples of these forest organic soil materials, designated for the study of microbially driven processes, should not be air dried.     

In vitro salinity resistance of three ectomycorrhizal fungi

The growth models, diameter growth rates, biomass yield and Na⁺ contents of three ectomycorrhizal (ECM) fungi, Suillus bovinus (L. ex Fr.) O. Kuntze, Suillus luteus (L. ex Fr.) Gray, Boletus luridus Schaeff ex Fr., were investigated at nine NaCl levels (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 mol/L). The results showed that the growth models of the three ECM fungal species were not affected by the NaCl concentration, but the growth rates reduced with the increasing NaCl concentration. The growth rates of B. luridus and S. bovinus were significantly higher than that of S. luteus at the same NaCl level; the biomass yields of three ECM fungal species were different, S. bovinus < S. luteus < B. luridus. Of the three species, B. luridus exhibited the highest growth rates, best biomass yield, and greatest Na⁺ concentration in the mycelia over the NaCl gradient tested, indicating B. luridus has the most tolerance to NaCl stress and assimilation to Na⁺ under salt stress. The growth rate of S. luteus was the lowest, but the biomass yield and Na⁺ concentration in the mycelia were only lower than those of B. luridus. S. bovinus was the most sensitive to NaCl stress and its growth rate was faster than that of S. luteus, but the biomass yield and Na⁺ concentration in the mycelia were the lowest.     

Soil resource heterogeneity in terms of litter aggregation promotes nitrous oxide fluxes and slows decomposition

To better understand the role of resource heterogeneity in decomposition and nitrous oxide (N₂O) flux we systematically altered the degree of plant litter aggregation in soil, from uniformly distributed to highly aggregated. In laboratory incubations, we distributed 4.5 g of dried clover shoots (Trifolium pratense L.) in two particle sizes (1 or >5 mm) into 1, 3, or 9 patches versus uniformly distributed. Soil moisture content was also varied to manipulate soil oxygen (O₂) concentrations. In moist soil (50% water-filled pore space, WFPS), litter aggregation delayed the peak litter decomposition rate by 3-5 days compared to uniformly distributed litter regardless of the litter particle size. In contrast, under near-saturated soil conditions (80% WFPS) litter aggregation suppressed decomposition throughout the 26-day incubation period. This significant interaction between litter aggregation and soil moisture treatments suggests that diffusion of soil resources (likely O₂) plays an important role in the influence of litter aggregation on decomposition. Specifically, O₂ diffusion may more adequately meet O₂ consumption rates when litter is distributed than when aggregated. In contrast to the temporary influence of aggregation on litter decomposition, N₂O fluxes under 50% WFPS conditions were consistently greater and on average 7.9, 7.2, and 4.7-fold greater than fine aggregated litter (1, 3, and 9 patches, respectively) than when uniformly distributed. Coarse litter aggregation also stimulated N₂O emissions, but not as much as fine litter. Under field conditions with growing maize (Zea mays L.), litter aggregation also stimulated N₂O emissions. The results suggest that litter aggregation plays a role in N₂O flux from agricultural soils and it might be manipulated to provide an additional N₂O mitigation strategy.     

CO2 efflux from deciduous forest litter and soil in response to simulated acid rain treatment

Using both field and laboratory measurements of CO₂ evolution as an index of decomposer activity, forest microcosms were used to evaluate the impact of simulated acidic precipitation on decomposition. The following pH treatments: 5.7, 4.5, 4.0, and 3.5 annual average were applied for a 30 mo period. No statistically significant effect of treatment on decomposition could be found in the field measurements. When the microcosm was partitioned into 01 and 02 litter, mineral soil (A and B horizons), and roots within the mineral soil horizons for laboratory determination of CO₂ efflux, only the 02 litter exhibited a statistically significant decrease as a function of treatment. The data collected do not allow a complete evaluation of the potential impact of this decrease. However, efflux of CO₂ from the 02 layer was small compared to the other layers, and this may account for the failure to detect a significant response in the field measurements. Although the field data did not exhibit a significant response, there is sufficient question concerning the 02 response to warrant additional investigation, especially since many plants derive a major portion of their nutritional requirements directly from the 02 litter layer.