Wandering spiders limit densities of a major microbi-detritivore in the forest-floor food web

In a long-term field experiment, densities of wandering spiders (i.e. species that do not build webs to capture prey) were reduced in order to determine whether or not a major group of microbi-detritivores, the Collembola (springtails), would increase in response to lowered spider predation. Thirty 4-m² fenced plots (15 spider-removal, 15 control plots) and 15 unfenced, undisturbed reference areas were established in a deciduous forest. Spiders were taken from the removal-treatment after being collected by periodic trapping with pitfall traps that contained no preservative, and by searching the litter surface. Judging from a comparison of the numbers captured during periodic censuses in spider-removal and control plots (one census period was two consecutive days of pitfall trapping), wandering spiders were rapidly reduced by ca. 50% and were maintained at levels lower than control plots for 1.3 years. The reduction in numbers was greatest for wolf spiders (Lycosidae; ≈90%). Of six families of Collembola abundant in the leaf litter during the experiment, one family, the Tomoceridae, exhibited a significant release from spider predation. Densities of tomocerids, which are the largest and most active springtails on the research site, gradually increased until they were consistently 2× higher in the spider-removal treatment than in either fenced control plots or unfenced, open reference areas.     

Unexpected indirect effect of spiders on the rate of litter disappearance in a deciduous forest

Previous experiments in meadow and forest-floor communities have uncovered a negative indirect effect of spiders and other large epigeic predators on decomposition, presumably through depression of densities of microbi-detritivores. We report the results of a 17-month field experiment in a deciduous forest in which spiders exerted the opposite effect on decomposition. In autumn we collected falling leaves of oak (Quercus), maple (Acer) and hickory (Carya), and placed three bags of mixed litter, and three groups of tethered leaves, in 1-m² fenced plots. We determined the disappearance rate of these leaves through April of the second year. Spider densities were reduced by >50% in the spider-removal treatment. The rate of leaf-litter disappearance was not higher in spider-removal plots, but instead was ca. 20% lower (P=0.057). Thus, spider predation clearly was not decreasing the rates of litter decomposition in our experiment; on the contrary, the results suggest that spiders were indirectly enhancing decomposition. Possible hypotheses to explain this unexpected result are discussed.     

Effects of altered precipitation and wolf spiders on the density and activity of forest-floor Collembola

A 2×2 factorial field experiment was conducted in a deciduous forest to determine how changes in moisture and numbers of a common wolf spider, Schizocosa, affect Collembola abundance and activity. Each of four 4-m² fenced, roofed plots was divided into eight 0.5-m² fenced subplots, and each subplot was assigned a rainfall treatment (drought or high-rainfall) and a Schizocosa treatment (Schizocosa-removal or Schizocosa-addition). Water was sprayed on the high-rainfall subplots weekly from 7 August to 1 October 2003 at a rate of roughly 2× the long-term mean, while the drought subplots received no water. In order to increase the probability of uncovering possible effects of Schizocosa, predatory arthropods (Schizocosa, other spiders, and centipedes) were first removed from all subplots; Schizocosa-addition subplots were then stocked with five juvenile Schizocosa on 19 August, with additional Schizocosa added throughout the experiment. An index of relative Collembola activity was calculated by comparing the number captured in pitfall traps (activity-density) with absolute density (no./m²) measured by extracting Collembola from litter samples. Decreased moisture reduced overall Collembola density, but led to increased Collembola activity. This response to rainfall was exhibited by the families Entomobryidae, Tomoceridae, and Hypogastruridae. The presence of Schizocosa had no impact on overall Collembola density, nor did spider presence affect Collembola activity, with the possible exception of the Hypogastruridae, for which there was an interaction between rainfall and Schizocosa presence. Hypogastrurid Collembola were more active in the drought subplots and exhibited the same level of activity in the presence or absence of Schizocosa under drought conditions. However, in the high-rainfall treatment, the presence of Schizocosa increased hypogastrurid activity.     

Long-term effects of experimental forest harvesting on abundance and reproductive demography of terrestrial salamanders

Both observational and experimental studies have documented drastic reductions in salamanders after forest harvesting. Yet, the amount of time until salamander populations rebound and the factors limiting recovery after harvesting are unknown. We compared the effects of six oak regeneration practices to a control, representing a disturbance gradient from no treatment to silvicultural clearcut, on the relative abundance and reproductive demography of terrestrial salamanders through 13-years post-harvest. Following the experimental disturbance, relative abundance of terrestrial salamanders in treatments that opened the canopy were significantly and persistently lower than in either untreated control stands or midstory herbicide treatments. In general, this trend persisted through 9-13 years after treatment. Changes to demography of commonly captured salamanders varied by species, but Plethodon cinereus had a greater proportion of juveniles in unharvested treatments 7-13-years post-harvest, and Desmognathus ochrophaeus had a greater proportion of juveniles and a greater number of eggs/female in unharvested treatments 1-6-years post-harvest. Population modeling of P. cinereus indicated that adult survival had the greatest elasticity of the vital rates. Further, >60 years may be needed before P. cinereus reduced by the observed decline could reach pre-harvest levels of abundance. Of the treatments with canopy disturbance, the group selection harvest had the greatest abundances of salamanders 7-13-years post-harvest, but when coupled with future stand entries, the volume of wood fiber extracted, costs of harvesting, reduced sprouting of oaks, and soil disturbances, this method may not have the best balance of ecological and economic sustainability in central Appalachian hardwood forest.     

Litter mixture effects on decomposition in tropical montane rainforests vary strongly with time and turn negative at later stages of decay

In a litterbag study in a tropical montane rainforest in Ecuador we assessed the impact of leaf litter species identity and richness on decomposition. We incubated leaf litter of six native tree species in monocultures and all possible two and four species combinations and analysed mass loss over a period of 24 months. Mass loss in monocultures averaged 30.7% after 6 month and differed significantly between species with variations being closely related to initial concentrations of lignin, Mg and P. At later harvests mass loss in monocultures averaged 54.5% but did not vary among leaf litter species and, unexpectedly, did not increase between 12 and 24 months suggesting that litter converged towards an extremely poor common quality retarding decomposition. After 6 months mass loss of leaf litter species was significantly faster in mixtures than in monocultures, resulting in synergistic non-additive mixture effects on decomposition, whereas at later harvests mass loss of component litter species was more variable and leaf litter mixture effects differed with species richness. Mass loss in the two species mixtures did not deviate from those predicted from monocultures, while we found antagonistic non-additive mixture effects in the four species mixtures. This suggests that litter species shared a poor common quality but different chemistry resulting in negative interactions in chemically diverse litter mixtures at later stages of decomposition. Overall, the results suggest that interspecific variations in diversity and composition of structural and secondary litter compounds rather than concentrations of individual litter compounds per se, control long term leaf litter decomposition in tropical montane rainforests. Plant species diversity thus appears to act as a major driver for decomposition processes in tropical montane rainforest ecosystems, highlighting the need for increasing plant conservation efforts to protect ecosystem functioning of this threatened biodiversity hotspot.     

Nonadditive effects of litter mixtures on decomposition and correlation with initial litter N and P concentrations in grassland plant species of northern China

We studied the occurrence of nonadditive effects of litter mixtures on the decomposition (the deviation of decomposition rate of litter mixtures from the expected values based on the arithmetic means of individual litter types) of litters from three plant species (i.e., Stipa krylovii Roshev., Artemisia frigida Willd., and Allium bidentatum Fisch. ex Prokh. & Ikonn.-Gal.) endemic to the grassland ecosystems of Inner Mongolia, northern China and the possible role of initial litter N and P on such effects. We mixed litters of the same plant species that differed in N and P concentrations (four gradients for each species) in litterbags and measured mass losses of these paired mixtures after 30 and 80 days under field conditions. We found the occurrence of positive, nonadditive effects of litter mixtures and showed that the magnitude of the nonadditive effects were related to the relative difference in the initial litter N and P concentrations of the paired litters.     

Organic matter and nutrient dynamics of the litter layer on a forest Rendzina under beech

Summary The decomposition of beech (Fagus sylvatica L.) leaf litter was investigated in a calcareous beech forest using mesh cages containing two layers, fresh leaf litter (O layer), and partly decomposed leaf litter (F layer). C loss was monitored, together with the changes in the contents of total N, hexosamines, ash, Na, K, Mg, Ca, Fe, Mn, Al, Cl, Sulphate, and Phosphate.In 1-mm mesh cages, which excluded access to the macrofauna, the mean annual loss rates for C were 28% in the O leaf litter and 17% in the F leaf litter, totalling approximately 23% for the two layers. The mean loss rates from the 12-mm mesh cages were 54% in the O leaf litter and 58% in the F leaf litter. Degradation processes and feeding activities caused increased contents of ash, total N, and hexosamines in the O layer of both treatments. This increase was greater for the ash and smaller for N, glucosamine, and galactosamine in the 12-mm mesh cages. The sum of ions (Na+K+Mg+Ca+Fe+Mn+Al+Cl+SO₄+PO₄) and also the contents of most single ions were not markedly affected, despite the much higher ash content in the O leaf litter of the 12-mm mesh cages. The ash content increased mainly as a consequence of contamination by soil, which increased the contents of Fe and Al in the ash.     

Photodegradation effects on CO2 emissions from litter and SOM and photo-facilitation of microbial decomposition in a California grassland

Decomposition of soil organic matter (SOM) and plant litter has been shown to be affected by high solar radiation; this could partly explain why biogeochemical models underestimate decomposition in arid and semi-arid ecosystems. We set out to test the effect of using traditional PVC chambers for measuring soil gas fluxes versus quartz chambers that allowed passage of light during field measurements in a dry-land field in Davis, CA. Results showed that fluxes from quartz-top chambers were on average 29% higher than from opaque chambers. We also studied the effect of solar light exposure on decomposition of native grass litter and SOM in a field experiment where plots were shaded or left exposed for 157 days during summer; litter did not seem to be affected by exposure to light. However, we concluded that SOM decomposition was affected by light exposure since shaded soil had similar respiration to sunlight-exposed soil indicating that microbial respiration occurred under the shade while photo-degradation likely occurred under the sun. Additionally, ¹⁵N-labeled grass was placed in litter bags in the field with either clear filters to allow light or aluminum covers to block light; 3-month exposure caused a change in lignin degradability as indicated by the change in the Ad/Al ratio. Incubation of that litter showed 9.3% more CO₂ produced from litter in clear and aluminum bags than unexposed litter. This showed that photo-facilitation occurred although to a small degree and was a result of light exposure and/or heat degradation. We attributed the similar respiration from clear- and aluminum-exposed litter to heat degradation of the aluminum-exposed litter. In conclusion, our results show that in hot dry ecosystems conventional PVC chambers underestimate measured CO₂ flux rates; sunlight exposure changes litter chemistry and appears to affect the degradation of soil organic matter, but the magnitude of degradation depends on an interaction of factors such as soil temperature and moisture.     

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.     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

An analysis of food-web structure and function in a shortgrass prairie,a mountain meadow,and a lodgepole pine forest

Summary The structure of the below-ground detrital food web was similar in three different semiarid vegetation types: lodgepole pine (Pinus contorta subsp. latifolia), mountain meadow (Agropyron smithii), and shortgrass prairie (Bouteloua gracilis). The densities of component food-web functional groups and the response to removal of component groups, differed however. As measured by biomass, bacteria were dominant in the meadow and prairie, while fungi were dominant in the forest. Resourde-base dominance was reflected in consumer dominance, and both directly correlated with the form of inorganic N present. Bacterial-feeding nematodes were numerically dominant in the meadow and prairie, while microarthropods were dominant in the forest. Ammonium-N was the dominant form in the forest, while nitrate —nitrite-N was the more important form in both bacterial-dominated grasslands.Addition of a biocide solution containing carbofuran and dimethoate reduced the numbers of both microarthropods and nematodes. In the bacterial-dominated grasslands, these reductions resulted in no apparent effect on bacterial densities because one group of bacterial consumers (protozoa) increased following the decrease in bacteria-feeding nematodes, in increased fungal biomass, and in increased soil inorganic N. Conversely, in the forest, following the biocide-induced reduction in consumers, the total fungal biomass decreased, but inorganic-N levels increased. The meadow appeared to be the most resilient of the three ecosystems to biocide disturbance, as both nematode and arthropod numbers returned to control levels more rapidly in the meadow than in the prairie or the forest.     

Trophic structure of a macroarthropod litter food web in managed coniferous forest stands: a stable isotope analysis with δN and δC

We studied the composition of a litter detrital community in a temperate coniferous forest using stable isotopes of nitrogen and carbon. Samples of mineral soil, bulk litter material, macroarthropods and understory plants were collected from ten experimental forest stands. Half of the stands were previously thinned 17–42 years ago, the other half served as controls. Values of δ¹⁵N and δ¹³C were based on the analysis of almost 500 individuals of at least 22 species in 11 arthropod families. The isotopic analysis showed a significant increase in δ¹⁵N and δ¹³C values with soil depth. Isotopic signatures of macroarthropods ranged from −26.51‰ to −20.52‰ for δ¹³C and −2.85‰ to 5.10‰ for δ¹⁵N. All consumers showed levels of ¹³C enrichment substantially higher than those of primary producers and litter. Predators were generally significantly more ¹⁵N enriched than detritivores and herbivores, but their δ¹³C levels were similar to those of primary consumers. Our data indicate that this community consists of at least 2–3 trophic levels with a considerable amount of variation in the ¹⁵N enrichment among detritivores and predators. We suggest that the spread of δ¹⁵N values of predators likely reflects the diversity of potential prey among detritivores and a varying degree of intraguild predation among different species. Our findings generally agree closely with the results of similar studies from other forest litter communities. Thinning did not appear to influence the overall isotopic composition of the detrital food web. Extensive omnivory and intraguild predation among litter consumers may buffer long-term effects of thinning on the trophic structure of these species-rich communities.     

Effect of earthworm addition on soil nitrogen availability,microbial biomass and litter decomposition in mesocosms

The aim of the study was to determine the effect of adding two tropical earthworm species, Rhinodrilus contortus and Pontoscolex corethrurus, to mesocosms on the availability of mineral N (NH₄ ⁺ and NO₃ ^– concentrations), soil microbial biomass (bio-N), and the decomposition rates of three contrasting leaf litter species, in a glasshouse experiment. The mesocosms were filled with forest soil and covered with a layer of leaf litter differing in nutritional quality: (1) Hevea brasiliensis (C/N=27); (2) Carapa guianensis (C/N=32); (3) Vismia sp., the dominant tree species in the second growth forest (control, C/N= 42); and, (4) a mixture of the former three leaf species, in equal proportions (C/N=34). At the end of the 97-day experiment, the soil mineral N concentrations, bio-N, and leaf litter weight loss were determined. Both earthworm species showed significant effects on the concentrations of soil NO₃ ^– (p<0.01) and NH₄ ⁺ (p<0.05). Bio-N was always greater in the mesocosms with earthworms (especially with R. contortus) and in the mesocosms with leaf litter of H. brasiliensis (6 µg N g^–1 soil), the faster decomposing species, than in the other treatments (0.1–1.6 µg N g^–1). Thus, earthworm activity increased soil mineral-N concentrations, possibly due to the consumption of soil microbial biomass, which can speed turnover and mineralization of microbial tissues. No significant differences in decomposition rate were found between the mesocosms with and without earthworms, suggesting that experiments lasting longer are needed to determine the effect of earthworms on litter decomposition rates.     

Effects of different land use on soil chemical properties, decomposition rate and earthworm communities in tropical Mexico

The effects of land use on soil chemical properties were evaluated, and earthworm communities and the decomposition rate of three typical land use systems in tropical Mexico, namely banana plantations (B), agroforestry systems (AF) and a successional forest (S) were compared.The study was carried out from November 2005 to April 2006. A completely randomized sampling design was established in six sites (B1, B2, AF1, AF2, S1 and S2). Soil properties and chemical characteristics (texture, pH, organic carbon (Corg), nutrients, and available Zn and Mn), earthworm communities and the decomposition of Bravaisia integerrima and Musa acuminata litter were analyzed over a period of 8 weeks.All soils were loamy clays with a medium to high content of nutrients. Three principal clusters were generated with the soil chemical properties: a first cluster for forest soils with high Corg and Ntot and low available Zn content, a second cluster for AF1 and a third cluster for B1, B2 and A2.The decomposition of B. integerrima litter was significantly faster (half-life time: 1.8 (AF2)–3.1 (B1) weeks) than that of M. acuminata (4.1 (AF2)–5.8 (S2) weeks). However, the decomposition rates did not differ significantly among the different sites.The greatest earthworm diversities were observed in AF2 and B1. Native species were dominant in the forest soils, whereas exotic species dominated in AF and in the banana plantations. The abundance and biomass of certain earthworm species were correlated to physical and chemical soil parameters. However, litter decomposition rates were not correlated with any of the soil physical–chemical parameters.While none of the land use systems studied led to a decrease in nutrient status, earthworm biodiversity and abundance, or in litter decomposition rate, they did result in a change in earthworm species composition.     

Effects of addition of maize litter and earthworms on C mineralization and aggregate formation in single and mixed soils differing in soil organic carbon and clay content

C mineralization and aggregate stability directly depend upon organic matter and clay content, and both processes are influenced by the activity of microorganisms and soil fauna. However, quantitative data are scarce. To achieve a gradient in C and clay content, a topsoil was mixed with a subsoil. Single soils and the soil mixture were amended with 1.0 mg maize litter C g soil⁻¹ with and without endogeic earthworms (Aporrectodea caliginosa). The differently treated soils were incubated for 49 days at 15 °C and 40% water holding capacity. Cumulative C mineralization, microbial biomass, ergosterol content and aggregate fractions were investigated and litter derived C in bulk soil and aggregates were determined using isotope analyses. Results from the soil mixture were compared with the calculated mean values of the two single soils. Mixing of soil horizons differing in carbon and clay content stimulated C mineralization of added maize residues as well as of soil organic matter. Mixing also increased contents of macro-aggregate C and decreased contents of micro-aggregate C. Although A. caliginosa had a stimulating effect on C mineralization in all soils, decomposition of added litter by A. caliginosa was higher in the subsoil, whereas A. caliginosa decreased litter decomposition in the soil mixture and the topsoil. Litter derived C in macro-aggregates was higher with A. caliginosa than with litter only. In the C poor subsoil amended with litter, A. caliginosa stimulated the microbial community as indicated by the increase in microbial biomass. Furthermore, the decrease of ergosterol in the earthworm treated soils showed the influence of A. caliginosa on the microbial community, by reducing saprotrophic fungi. Overall, our data suggest both a decrease of saprotrophic fungi by selective grazing, burrowing and casting activity as well as a stimulation of the microbial community by A. caliginosa.     

Effect of nitrogen and phosphorus application on agricultural soil food webs

Aim of the present research is to investigate the effect of nitrogen (N) and phosphorus (P) on soil food webs (microbes, nematodes and microarthropods) trophic interactions in agriculture ecosystems. A complete randomized block design experiment of N and P fertilization was initiated in 2010 with four treatments: (1) P-addition, (2) N-addition, (3) NP-addition and (4) control. After 4 years of fertilization, compared with control, N-addition had a negative effect on microarthropods and clarify indirectly by significantly (P < 0.05) increasing soil total nitrogen (0.37 g kg^?1) and available nitrogen (20.03 mg kg^?1). The reduction in microarthropods resulted significant (P < 0.05) increase in bacterivores and fungivores feeding on bacteria and fungi, an example of top-down control. P-addition had indirect negative effects on microarthropods by means of significantly (P < 0.05) increasing soil total phosphorus (0.62 g kg^?1) and available phosphorus (24.17 mg kg^?1), aggravated fungivores feeding on fungi and strengthened top-down control. NP-addition significantly (P < 0.05) increased total microbial biomass, nematodes and microarthropods and resulted in bottom-up control. These results suggested that top-down effects were the dominant force in N- or P-addition treatments. NP-addition strengthened bottom-up control by enriching food resource. Unbalance fertilization could pose adverse on agricultural soil ecosystem and yield potential of crops.     

Assessing the effects of plant protection products on organic matter breakdown in arable fields—litter decomposition test systems

There is a need for plant protection products (PPPs) to be assessed for their effects on the breakdown of organic matter (OM), which is an important functional process in terrestrial ecosystems. Little information is available on to how to assess effects of PPPs on this complex system and formal guidelines for a standardised test method are lacking. We critically reviewed the literature to determine appropriate methods to investigate OM breakdown for the risk assessment of PPPs. Five methods appeared to be potentially suitable: namely the use of mini-containers or litter-bags to enclose OM, cotton-strip and bait-lamina assays which provide an artificial OM substrate, and stable isotopes to track the chemical decomposition of OM. These methods were compared on the basis of 10 suitability criteria, which included ecological relevance, ease of use and relevance to risk assessors. Each test method has limitations but the use of litter-bags, which is the most frequently used method, has distinct advantages over the other approaches. Accordingly, literature describing OM breakdown in litter-bags when applying PPPs are reviewed, gaps in the methodology are highlighted and recommendations for the development of a standardised and validated test method are proposed.