Activity of carbohydrases in the gut of Bibionidae (Diptera) larvae

Dipteran larvae play an important role in the soil of some deciduous forests. They can consume a considerable part of the annual litter fall and produce a large amount of faecal pellets, which are forming an important part of the fermentation horizon of forest soils. We measured the pH changes and assayed the activities and pH optima of saccharolytic enzymes in the digestive tracts of the larvae of Bibio pomonae and Penthetria holosericea (Bibionidae). The pH of the litter offered as a food was about 5. The gut content became highly alkaline (pH about 10) in the anterior part of the midgut and the pH decreased posteriorly and in the hindgut. Excrements were neutral or had slightly alkaline pH (7.5–8.0) in both species. The alkaline pH optima of amylase (9.5–10.0) and maltase (8.0), trehalase and cellobiase (7.0) were the same in both species. Saccharolytic activity showed an optimum at pH 7 in B. pomonae and at pH 8 in P. holosericea. Though there are previously published reports of high assimilation efficiency of bibionid larvae fed in litter (46–76% in B. pomonae), activities of both exo and endocellulases were below the detection limit of saccharolytic and chromolytic assays along a wide pH range (5–10). Possible explanations are discussed.     

Tree species effects on microbial respiration from decomposing leaf and fine root litter

Tree species have an impact on decomposition processes of woody litter, but the effects of different tree species on microbial heterotrophic respiration derived from decomposing litter are still unclear. Here we used leaf and fine root litter of six tree species differing in chemical and morphological traits in a temperate forest and elucidated the effects of tree species on the relationships between litter-derived microbial respiration rates and decomposition rates and morphological traits, including specific leaf area (cm² g⁻¹) and specific root length (m g⁻¹) of litter at the same site. Litterbags set in forest soil were sequentially collected five times over the course of 18 months. During litter decomposition, microbial respiration from leaf and fine root litter differed among the six tree species. Temporal changes in the remaining mass and morphology (specific leaf area and specific root length) were observed, and the magnitude of these changes differed among species. Positive correlations were observed between respiration and mass loss or morphology across species. These results revealed that litter mass loss and morphological dynamics during decomposition jointly enhanced microbial respiration, and these carbon-based litter traits explained species differences in decomposition of leaves and fine roots. In conclusion, tree species influenced the magnitude and direction of microbial respiration during leaf/fine root litter decomposition. Tree species also affected the relationship between microbial respiration and litter decomposition through direct effects of litter traits and indirect effects mediated by regulation of heterotroph requirements.     

Ant-mediated effects on spruce litter decomposition, solution chemistry, and microbial activity

Forest management practices often generate clear-cut patches, which may be colonized by ants not present in the same densities in mature forests. In addition to the associated changes in abiotic conditions ants can initiate processes, which do not occur in old-growth stands. Here, we analyse the effects of ants and aphid honeydew on litter solution of Norway spruce, microbial enzyme activities, and needle decomposition in a field and greenhouse experiment during summer 2003. In the field, low ant densities had relatively little effects on litter solution 30 cm away from a tree trunk, but significantly increased organic carbon concentrations and decreased inorganic nitrogen concentrations next to a trunk where ants tend to build their nests. In a greenhouse experiment, the addition of ants to lysimeters containing spruce litter significantly increased dissolved organic carbon (DOC), dissolved organic nitrogen (DON), NH₄-N, NO₃-N and K concentrations in litter solutions compared to the control treatment, while the simulation of aphid infestation (addition of honeydew) significantly increased DOC as a direct result of honeydew leaching, and decreased inorganic N concentrations in leachates. The presence of ants resulted in a changed composition of dissolved organic matter (DOM) with more aromatic and complex compounds, and microbial enzyme activity was significantly higher in litter extracts from the ant treatment compared to the honeydew and control treatment. However, mass loss, litter %C and %N were not affected by ants or honeydew. Our results suggest that ants have a distinct and immediate effect on solution composition and microbial activity in the litter layer indicating accelerated litter decay whereas the effect of honeydew was insignificant.     

Soil amendment with seed meals: Short term effects on soil respiration and biochemical properties

In the last decades, worldwide biofuel production increased up to 105 billion liters in 2010; in this year, the world's first biodiesel producer was the European Union. Biodiesel is produced from a variety of oils, mainly soybean, Brassicaceae and sunflower. One of the most important biodiesel production byproducts are seed meals. The most common use of seed meals is for animal nutrition, but another potential use is agricultural soil organic amendments. Soil biological and biochemical properties have been widely used as soil quality indicators, due to their quick response to changes in soil management. Although there are many studies on the effects of regular amendments on respiration and enzymatic activities of the soil, very few papers refer to the effect of seed meals. Therefore, the aim of this study was to investigate the short-term effect of seed meal amendment on soil respiration and enzymatic activities (alkaline phosphatase, dehydrogenase, fluorescein diacetate hydrolase, arylsulphatase and β-glucosidase). The two seed meals used were obtained from Brassica carinata (A.) Braun (Ethiopian mustard) and sunflower (Helianthus annuus L.). The study was carried out, from June 2007 to October 2007, at an experimental farm of CRA-ORT, Battipaglia, in the Sele River (Plain Salerno, Campania Region, Italy). The results of this study support an alternative use of seed meals as organic amendments. Generally, a significant positive response of enzymatic activities, evaluated in this study, to the addition of seed meals, indicates a beneficial effect on soil quality as regards microbial activity.     

Rainfall manipulation effects on litter decomposition and the microbial biomass of the forest floor

Simple structures aimed at regulating the amount of rain water dropping into the forest floor were installed to determine the impact of rainfall on leaf litter mass loss, respiration rates, microbial biomass C (C_mic) and metabolic quotient (qCO₂). The rainfall manipulation treatments were (I) fully covered (100% reduction); (II) partially covered (50% reduction) and (III) control (fully exposed). Using the litterbag technique, the mass losses of covered Quercus serrata, Quercus acutissima, Acer rufinerve and Pinus densiflora leaf litter were reduced (P<0.01) by 19–26% compared to fully exposed litter. A positive linear relationship (r=0.90; P<0.0001) between litter C_mic and mass loss was noted across all litter types and covering regimes. The mass losses in fully exposed litter were attributed to the leaching effect of rainfall coupled with the synergistic actions of microbes and soil fauna, as suggested by their respiration and microbial biomass. In the covered litter, C_mic was generally reduced (P<0.01) while fully and partially exposed litter were comparable (P>0.05). On the other hand, respiration rates and qCO₂ were variable and showed no consistent treatment effect except for respiration rates at 3 months. Similarly, soil respiration rates and C_mic were not consistently affected by cover treatments. Evidently, the zero-rainfall condition negatively affected some biological processes in the litter layer but sporadically affected soil processes. The absence of rainfall, even if the soil moisture content was maintained, could affect organic matter turnover in the forest floor.     

Interactive effects of warming, soil humidity and plant diversity on litter decomposition and microbial activity

Human activity has induced a multitude of global changes that are likely to affect the functioning of ecosystems. Although these changes act in concert, studies on interactive effects are scarce. Here, we conducted a laboratory microcosm experiment to explore the impacts of temperature (9, 12 and 15 °C), changes in soil humidity (moist, dry) and plant diversity (1, 4, 16 species) on soil microbial activity and litter decomposition.We found that changes in litter decomposition did not mirror impacts on microbial measures indicating that the duration of the experiment (22 weeks) may not have been sufficient to determine the full magnitude of global change effects. However and notably, changes in temperature, humidity and plant litter diversity/composition affected in a non-additive way the microbial parameters investigated. For instance, microbial metabolic efficiency increased with plant diversity in the high moisture treatment but remained unaffected in low moisture treatment suggesting that climate changes may mask beneficial effects of biodiversity on ecosystem functioning. Moreover, litter decomposition was unaffected by plant litter diversity/composition but increased with increasing temperature in the high moisture treatment, and decreased with increasing temperature in the low moisture treatment.We conclude that it is inevitable to perform complex experiments considering multiple global change agents in order to realistically predict future changes in ecosystem functioning. Non-additive interactions highlight the context-dependency of impacts of single global change agents.     

Effect of N addition,moisture, and temperature on soil microbial respiration and microbial biomass in forest soil at different stages of litter decomposition

Purpose

The objective of the present study was to investigate the interactive effects of nitrogen (N) addition, temperature, and moisture on soil microbial respiration, microbial biomass, and metabolic quotient (qCO₂) at different decomposition stages of different tree leaf litters.

Materials and methods

A laboratory incubation experiment with and without litter addition was conducted for 80 days at two temperatures (15 and 25 °C), two wetting intensities (35 and 50 % water-filled porosity space (WFPS)) and two doses of N addition (0 and 4.5 g N m^?2, as NH₄NO₃). The tree leaf litters included three types of broadleaf litters, a needle litter, and a mixed litter of them. Soil microbial respiration, microbial biomass, and qCO₂ along with other soil properties were measured at two decomposition stages of tree leaf litters.

Results and discussion

The increase in soil cumulative carbon dioxide (CO₂) flux and microbial biomass during the incubation depended on types of tree leaf litters, N addition, and hydrothermal conditions. Soil microbial biomass carbon (C) and N and qCO₂ were significantly greater in all litter-amended than in non-amended soils. However, the difference in the qCO₂ became smaller during the late period of incubation, especially at 25 °C. The interactive effect of temperature with soil moisture and N addition was significant for affecting the cumulative litter-derived CO₂-C flux at the early and late stages of litter decomposition. Furthermore, the interactive effect of soil moisture and N addition was significant for affecting the cumulative CO₂ flux at the late stage of litter decomposition but not early in the experiment.

Conclusions

This present study indicated that the effects of addition of N and hydrothermal conditions on soil microbial respiration, qCO₂, and concentrations of labile C and N depended on types of tree leaf litters and the development of litter decomposition. The results highlight the importance of N availability and hydrothermal conditions in interactively regulating soil microbial respiration and microbial C utilization during litter decomposition under forest ecosystems.

     

Direct and indirect effects of nitrogen deposition on litter decomposition

Elevated nitrogen (N) deposition can affect litter decomposition directly, by raising soil N availability and the quantity and quality of litter inputs, and indirectly by altering plant community composition. We investigated the importance of these controls on litter decomposition using litter bags placed in annual herb based microcosm ecosystems that had been subject to two rates of N deposition (which raised soil inorganic N availability and stimulated litter inputs) and two planting regimes, namely the plant species compositions of low and high N deposition environments. In each microcosm, we harvested litter bags of 10 annual plant species, over an 8-week period, to determine mass loss from decomposition. Our data showed that species differed greatly in their decomposability, but that these differences were unlikely to affect decomposition at the ecosystem level because there was no correlation between a species’ decomposability and its response to N deposition (measured as population seed production under high N, relative to low N, deposition). Litter mass loss was ～2% greater in high N deposition microcosms. Using a comprehensive set of measurements of the microcosm soil environments, we found that the most statistically likely explanation for this effect was increased soil enzyme activity (cellobiosidase, β-glucosidase and β-xylosidase), which appears to have occurred in response to a combination of raised soil inorganic N availability and stimulated litter inputs. Our data indicate that direct effects of N deposition on litter input and soil N availability significantly affected decomposition but indirect effects did not. We argue that indirect effects of changes to plant species composition could be stronger in natural ecosystems, which often contain a greater diversity of plant functional types than those considered here.     

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.     

Effect of paclobutrazol on microbial biomass,respiration and cellulose decomposition in soil

Paclobutrazol is a plant growth regulator largely utilized in mango cultivation and usually applied directly to soil. The aim of this study was to examine the effect of paclobutrazol on soil microbial biomass, soil respiration and cellulose decomposition in Brazilian soils under laboratory conditions. Soil samples were collected from fields with and without a reported history of paclobutrazol application. A solution of paclobutrazol (8 mg of active ingredient kg^?1 of soil) was added to soils, which were then incubated at 28 °C for 30 days. Paclobutrazol decreased soil microbial biomass, soil respiration and cellulose decomposition in soil with and without a report of paclobutrazol application, while significant increase was observed in the respiratory quotient (qCO₂). Our results show that the soil microbiological attributes were negatively affected by paclobutrazol in short-term experiment.     

Plant litter decomposition and microbial characteristics in volcanic soils (Mt Etna,Sicily) at different stages of development

Soils at different developmental stages were sampled from eight sites on the slopes of Mt Etna, Sicily (Italy) and characterized for total C, microbial biomass and microbial respiration. The values of these parameters were greatest for the most developed soils, but differences in recent management and site characteristics limited analysis of trends with soil development across the eight sites. The decomposition kinetics of both intact leaf litter and the water-insoluble fraction of leaf litter from three common species on Etna [Etnean broom (Genista aetnensis), European chestnut (Castanea sativa), and Corsican pine (Pinus nigra)] were determined in four of the soils (the two with the smallest and the two with the largest organic C contents) in a laboratory experiment over 168 days to test two hypotheses. First, that the readily mineralized fraction of added plant C is greater when the plant material decomposes in well-developed soils compared to less developed soils, and second, that the microbial communities in less developed soils are less efficient at mineralizing C from low quality plant residues. The first hypothesis held for Genista and Pinus litter, but not Castanea litter. The second hypothesis was supported for the Castanea and Pinus litter, but not for the Genista litter. Thus, the general applicability of the hypotheses was dependent on the precise source and characteristics of the litter.     

Salinity and sodicity effects on respiration and microbial biomass of soil

An understanding of the effects of salinity and sodicity on soil carbon (C) stocks and fluxes is critical in environmental management, as the areal extents of salinity and sodicity are predicted to increase. The effects of salinity and sodicity on the soil microbial biomass (SMB) and soil respiration were assessed over 12weeks under controlled conditions by subjecting disturbed soil samples from a vegetated soil profile to leaching with one of six salt solutions; a combination of low-salinity (0.5dSm⁻¹), mid-salinity (10dSm⁻¹), or high-salinity (30dSm⁻¹), with either low-sodicity (sodium adsorption ratio, SAR, 1), or high-sodicity (SAR 30) to give six treatments: control (low-salinity low-sodicity); low-salinity high-sodicity; mid-salinity low-sodicity; mid-salinity high-sodicity; high-salinity low-sodicity; and high-salinity high-sodicity. Soil respiration rate was highest (56–80mg CO₂-C kg⁻¹ soil) in the low-salinity treatments and lowest (1–5mg CO₂-C kg⁻¹ soil) in the mid-salinity treatments, while the SMB was highest in the high-salinity treatments (459–565mg kg⁻¹ soil) and lowest in the low-salinity treatments (158–172mg kg⁻¹ soil). This was attributed to increased substrate availability with high salt concentrations through either increased dispersion of soil aggregates or dissolution or hydrolysis of soil organic matter, which may offset some of the stresses placed on the microbial population from high salt concentrations. The apparent disparity in trends in respiration and the SMB may be due to an induced shift in the microbial population, from one dominated by more active microorganisms to one dominated by less active microorganisms.     

硝化抑制剂和秸秆对潮棕壤碳氮转化和微生物群落特征的短期影响

氮是植物和微生物生长繁殖的必需营养元素,而氮矿化表征了土壤供氮能力。通过盆栽实验,采用同位素稀释法和磷脂脂肪酸(PLFA)法,研究了添加硝化抑制剂和秸秆条件下,潮棕壤碳氮矿化和微生物群落组成变化特征。结果表明,与施氮量N 0.1 g·kg~(-1)的单施氮肥处理(NF)相比,氮肥配施1%硝化抑制剂(NFI)的土壤铵态氮提高32%,而硝态氮降低53%。氮肥与施用量为5 g·kg~(-1)的秸秆配施(NS),土壤氮素总矿化速率增加36%,微生物生物量碳提高51%,β-葡萄糖苷酶活性提高36%,同时显著增加了土壤总PLFA以及细菌、真菌、真菌/细菌和革兰式阴性菌(P0.05),土壤呼吸熵降低50%。与氮肥配施秸秆处理(NS)相比,氮肥、秸秆和硝化抑制剂配施处理(NSI),土壤铵态氮提高33%,硝态氮下降47%。综上所述,氮肥和秸秆配施可以提高土壤微生物生物量,改变土壤微生物群落组成,配施1%(N)硝化抑制剂后降低土壤硝化速率,增加土壤供氮能力。     

Differential effects of lichens and mosses on soil enzyme activity and litter decomposition

In the New Jersey Pinelands, canopy gaps in the pine-dominated forest support patches of lichens, mosses, and caespitose grasses. We tested the hypotheses that non-vascular plants and lichens can affect nutrient cycling processes and that mosses and lichens would differ from each other. We predicted that (1) lichen tissues would decompose more slowly than pine or moss tissues, (2) all plant materials would decompose more slowly beneath lichens than beneath mosses, and (3) soil enzyme activities would be higher under lichens than under mosses or grasses, reflecting greater nutrient limitation. We compared rates of decomposition of the litter of Pinus rigida and moss and lichen tissues, and measured soil enzyme activities responsible for nutrient mineralization from litter (acid and alkaline phosphatases, chitinase, β-glucosidase, aminopeptidase, and phenol oxidase) under three types of groundcover (lichens, mosses, and grasses) and unvegetated soil at two sites. While groundcover affected enzyme activities, the patterns of enzyme activities differed markedly between the two sites. In general, the enzyme activities were uniformly low. Decomposition rates were more strongly affected by the groundcover than by litter materials. While all litters tended to decompose more slowly under lichens than under mosses, supporting one of our initial hypotheses, the rates of decomposition were markedly different between the two sites. These results suggest that while mosses and lichens create patches of different soil function in both sites, the differences between the sites in unknown factors cause the enzyme activities and decomposition rates to differ.     

Soil moisture and soil-litter mixing effects on surface litter decomposition: A controlled environment assessment

Recent studies suggest the long-standing discrepancy between measured and modeled leaf litter decomposition in drylands is, in part, the result of a unique combination of abiotic drivers that include high soil surface temperature and radiant energy levels and soil-litter mixing. Temperature and radiant energy effects on litter decomposition have been widely documented. However, under field conditions in drylands where soil-litter mixing occurs and accelerates decomposition, the mechanisms involved with soil-litter mixing effects are ambiguous. Potential mechanisms may include some combination of enhanced microbial colonization of litter, physical abrasion of litter surfaces, and buffering of litter and its associated decomposers from high temperatures and low moisture conditions. Here, we tested how soil-litter mixing and soil moisture interact to influence rates of litter decomposition in a controlled environment. Foliar litter of two plant species (a grass [Eragrostis lehmanniana] and a shrub [Prosopis velutina]) was incubated for 32 weeks in a factorial combination of soil-litter mixing (none, light, and complete) and soil water content (2, 4, 12% water-filled porosity) treatments. Phospholipid fatty acids (PLFAs) were quantified one week into the experiment to evaluate initial microbial colonization. A complementary incubation experiment with simulated rainfall pulses tested the buffering effects of soil-litter mixing on decomposition.Under the laboratory conditions of our experiments, the influence of soil-litter mixing was minimal and primarily confined to changes in PLFAs during the initial stages of decomposition in the constant soil moisture experiment and the oscillating soil moisture conditions of the rainfall pulse experiment. Soil-litter mixing effects on CO₂ production, total phospholipid concentrations, and bacterial to total PLFA ratios were observed within the first week, but responses were fairly weak and varied with litter type and soil moisture treatment. Across the entire 32-week incubation experiment, soil moisture had a significant positive effect on mass loss, but soil-litter mixing did not. The lack of strong soil-litter mixing effects on decomposition under the moderate and relatively constant environmental conditions of this study is in contrast to results from field studies and suggests the importance of soil-litter mixing may be magnified when the fluctuations and extremes in temperature, radiant energy and moisture regimes common dryland field settings are in play.     

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.     

The influence of slug (Arion rufus) mucus and cast material addition on microbial biomass,respiration, and nutrient cycling in beech leaf litter

We investigated the effects of slug (Arion rufus L.) mucus and cast material on litter decomposition, nutrient mobilization, and microbial activity in two laboratory experiments: (1) Slug mucus and cast material was added to beech leaf litter (Fagus sylvatica L.), and leaching of N and P and CO₂ production in microcosm systems were measured during 77 days of incubation; (2) mucus was added to beech leaf litter, and basal respiration, microbial biomass (substrate-induced respiration), specific respiration (qO₂), microbial growth ability after C, CN, CP, and CNP amendment, and lag time (time between CNP addition and start of exponential increase in respiration rate) were measured during 120 days of incubation. Leaching of N and P from beech leaf litter was significantly increased in treatments with mucus or faecal material of A. rufus. Following day 3, slug mucus increased nitrification processes. Mucus addition to beech leaf litter also increased basal respiration and microbial biomass significantly. In contrast, specific respiration was not significantly affected by mucus addition, and generally declined until day 60 but then increased until day 120. Nutrient amendments indicated that between days 1 and 30, N was available for microbial growth in litter with mucus but not in control litter. Generally, the lag time in beech leaf litter with added mucus was shorter than in control litter. Lag times generally increased with age, indicating dominance of slow-growing microbial populations at later stages as a consequence of depletion of easily available C resources and nutrients. We conclude that C, N, and P cycling is accelerated by slug activity.