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.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

Plant growth and microbial activity in a tropical soil amended with faecal pellets from millipedes and woodlice

The effect of soil fauna-mediated leaf litter (faecal pellets) versus mechanically fragmented (finely ground) leaf litter on biomass production of rice (Oryza sativa, var. Primavera) was assessed in pot tests. Rice seedlings were either grown in soil samples amended with faecal pellets of diplopods and isopods fed on leaf litter of a legume cover crop (Pueraria phaseoloides (Roxb.) Benth) and a peach palm (Bactris gasipaes) or in soil amended with finely ground leaf litter. The addition of faecal pellets caused a significant and dose-related increase in plant biomass compared to pure soil. Ground leaf litter induced a significantly smaller positive effect on plant biomass development with Pueraria litter > Bactris litter > mixed primary forest litter. In contrast, soil microbial biomass development during the 4 weeks plant test was higher in the soil amended with ground litter as compared to soil amended with feacal pellets. The results show a clear positive effect of the soil fauna on soil fertility and indicate differences in the availability of nutrients from the organic substrates to higher plants and soil microorganisms.     

Relative importance of substrate type and previous soil management in synthesis of microbial biomass and substrate mineralization

Our aim was to determine whether the soil microbial biomass, which has developed naturally over many years in a given ecosystem, is specially adapted to metabolize the plant‐derived substrate C of the ecosystem within which it developed or whether the nature of recently added substrate is the more important factor. To examine this, soils from three sites in close proximity (woodland, grassland and arable from the Broadbalk Experiment at Rothamsted Research, Harpenden, UK) were each amended with air‐dried wheat straw (Triticum aestivum), ryegrass leaves (Lolium perenne) or woodland leaf litter (mainly Quercus robur and Fagus sylvatica) in a fully replicated 3 × 3 factorial laboratory experiment. The initial mineralization rates (evolved CO₂‐C) were determined during the first 6.5 hours and again, together with the amount of microbial biomass synthesized (microbial biomass C), at 7, 14, 21, 30 and 49 days of incubation. The hourly rate of CO₂‐C production during the first 6.5 hours was slowest following leaf litter addition, while the added grass gave the fastest rates of CO₂‐C evolution both within and between soils. Ryegrass addition to the arable soil led to approximately four times more CO₂‐C being evolved than when it was added to the woodland soil, at an overall rate in the arable soils of 41 μg C g^?1 soil hour^?1. In each soil, the net amounts of CO₂‐C produced were in the order grass > straw > leaf litter. In each case, the amount produced by the added leaf litter was significantly less (P < 0.05) than either the added grass or straw. Overall, the trend was for much slower rates of mineralization of all substrates in the woodland soil than in either the arable or grassland soils. During 49 days of incubation in the woodland and grassland soils, the net total amounts of CO₂‐C evolved differed significantly (P < 0.01), with grass > straw > leaf litter, respectively. In the arable soil, the amounts of CO₂‐C evolved from added grass and straw were significantly larger (P < 0.01) than from the leaf litter treatment. Our findings indicated that the amounts of CO₂‐C evolved were not related to soil management or to the size of the original biomass but to the substrate type. The amount of biomass C synthesized was also in the order grass > straw > leaf litter, at all stages of incubation in the woodland and grassland soil. In the arable soil, the same effect was observed up to 14 days, and for the rest of the incubation the biomass C synthesized was in the order grass > straw > leaf litter. Up to three times more biomass C was synthesized from the added grass than from the other substrates in all soils throughout the incubation. The maximum biomass synthesis efficiency was obtained with grass (7% of added C). Overall, the woodland soil was most efficient at synthesizing biomass C and the arable soil the least. We conclude that substrate type was the overriding factor that determined the amount of new soil microbial biomass synthesized. Mineralization of substrate C by soil microorganisms was also influenced mainly by substrate type and less by soil management or size of original biomass.     

Microbial biomass and activity in soil and litter underPinus sylvestris, Picea abies andBetula pendula at originally similar field afforestation sites

Microbial biomass C and N, and activities related to C and N cycles, were compared in needle and leaf litter, and in the uppermost 10 cm of soil under the litter layer in Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.) and silver birch (Betula pendula L.) stands, planted on originally similar field afforestation sites 23–24 years ago. The ground vegetation was differentiated under different tree species, consisting of grasses and herbs under birch and pine, and mosses or no vegetation with a thick layer of needles under spruce. The C:N ratio of the soils was 13–21 and the soil pH_{CaCl 2} 3.8–5.2. Both showed little variation under different tree species. Microbial biomass C and N, C mineralization, net ammonification, reduction) did not differ significantly in soil under different tree species either. Birch leaf litter had a higher pH_{CaCl 2} (5.9) than spruce and pine needle litter (pH 5.0 and 4.8, respectively). The C:N ratio of spruce needles was 30, and was considerably higher in pine needles (69) and birch leaves (54). Birch leaves tended to have the highest microbial biomass C and C mineralization. Spruce needles appeared to have the highest microbial biomass N and net formation of mineral N, whereas formation of mineral N in pine needles and birch leaves was negligible. Microbial biomass C and N were of the same order of magnitude in the soil and litter samples but C mineralization was tenfold higher in the litter samples.     

Seasonal dynamics of leaf- and root-derived C in arctic tundra mesocosms

We investigated contributions of leaf litter, root litter and root-derived organic material to tundra soil carbon (C) storage and transformations. ¹⁴C-labeled materials were incubated for 32 weeks in moist tussock tundra soil cores under controlled climate conditions in growth chambers, which simulated arctic fall, winter, spring and summer temperatures and photoperiods. In addition, we tested whether the presence of living plants altered litter and soil organic matter (SOM) decomposition by planting shoots of the sedge Eriophorum vaginatum in half of the cores. Our results suggest that root litter accounted for the greatest C input and storage in these tundra soils, while leaf litter was rapidly decomposed and much of the C lost to respiration. We observed transformations of ¹⁴C between fractions even when total C appeared unchanged, allowing us to elucidate sources and sinks of C used by soil microorganisms. Initial sources of C included both water soluble (WS) and acid-soluble (AS) fractions, primarily comprised of carbohydrates and cellulose, respectively. The acid-insoluble (AIS) fraction appeared to be a sink for C when conditions were favorable for plant growth. However, decreases in ¹⁴C activity from the AIS fraction between the fall and spring harvests in all treatments indicated that microorganisms consumed recalcitrant C compounds when soil temperatures were below 0 °C. In planted leaf litter cores and in both planted and unplanted SOM cores, the greatest amounts of ¹⁴C at the end of the experiment were found in the AIS fraction, suggesting a high rate of humification or accumulation of decay-resistant plant tissues. In unplanted leaf litter cores and planted and unplanted root litter cores most of the ¹⁴C remaining at the end of the experiment was in the AS fraction suggesting less extensive humification of leaf and root detritus. Overall, the presence of living plants stimulated decomposition of leaf litter by creating favorable conditions for microbial activity at the soil surface. In contrast, plants appeared to inhibit decomposition of root litter and SOM, perhaps because of microbial preferences for newer, more labile inputs from live roots.     

Decomposition and substrate quality of leaf litters and fine roots from three exotic plantations and a native forest in the southwestern highlands of Ethiopia

Substrate quality and decomposition (measured as CO₂ release in laboratory microcosms) of fresh leaf litter and fine roots of Cupressus lusitanica, Pinus patula, Eucalyptus grandis and native forest trees were studied. Changes in litter chemistry in each forest stand were analysed by comparing fresh leaf litter (collected from trees) and decomposed litter from the forest floor. Elemental concentrations, proximate fractions including monomeric sugars, and cross polarisation magic-angle spinning (CPMAS) ¹³C NMR spectra were analysed in leaf litters, decomposed litter and fine roots. Leaf litters and fine roots varied in their initial substrate chemistry with Ca concentration in leaf litters being higher than that in fine roots. In each stand, fine roots had a higher acid unhydrolysable residue (AUR) (except for the Pinus stand), higher holocellulose concentration and lower concentration of water-soluble extractives (WSE) and dichloromethane extractives (NPE) than fresh leaf litter. Likewise, ¹³C NMR spectra of fine roots showed lower alkyl and carboxyl C, and higher phenolic (except P. patula), aromatic and O-alkyl C proportions than leaf litters. Compared with fresh leaf litter, decomposed litter had lower concentrations of potassium, holocellulose, WSE, NPE, arabinose and galactose, similar or higher concentrations of Mg, Ca, S and P, and higher concentrations of N and AUR. CPMAS ¹³C NMR spectra of decomposed litter showed a higher relative increase in signal intensity due to methoxyl C, aromatic C, phenolic C and carboxylic C compared with alkyl C. In a microcosm decomposition study, the proportion of initial C remaining in leaf litter and fine roots significantly fitted an exponential regression model. The decomposition constants (k) ranged between 0.0013 and 0.0030 d⁻¹ for leaf litters and 0.0010-0.0017 d⁻¹ for fine roots. In leaf litters there was a positive correlation between the k value and the initial Ca concentration, and in fine roots there was an analogous positive correlation with initial WSE. Leaf litters decomposed in the order Cupressus>native forest>Eucalyptus∼Pinus, and fine roots in the order Pinus>native forest>Cupressus∼Eucalyptus. In each stand the fine root decomposition was significantly lower than the leaf litter decomposition, except for the P. patula stand where the order was reversed.     

Growth response and extracellular enzyme activity of Ulocladium botrytis LPSC 813 cultured on carboxy-methylcellulose under a pH range

The fungus Ulocladium botrytis was isolated from Scutia buxifolia leaf litter and its growth was evaluated on both liquid and solid medium with sodium-carboxy-methylcellulose (CMC, 0.5%) as sole C source at a pH range between 4.0 and 10.0 and the synthesis of cellulose-degrading enzymes on litter. Growth on CMC-agar medium was maximum at pH 6.0, while in liquid CMC cultures, the highest biomass levels were found at pH 8.0 in both cases after 7 days of incubation. Cellulose-degrading enzyme activities such as β-glucosidase (2.40 U dry leaf g⁻¹), cellobiohydrolase (3.92 10⁻³ U dry leaf g⁻¹), and endoglucanase (2.01 U dry leaf g⁻¹) activities were detected in water-soluble fractions of inoculated leaves after 30 days of incubation. Endoglucanase activity was maximum at pH 6.0 and relatively stable as the pH increase, being 100 and 60% stable at pH 7 and 8, respectively. As a consequence of these enzyme activities, leaf mass was reduced by 5.8%. Our findings suggest that U. botrytis contains a cellulose-degrading enzyme complex that, unlike other cellulolytic systems, can degrade recalcitrant plant litter under alkaline conditions.     

Effects of plant species on microbial biomass phosphorus and phosphatase activity in a range of grassland soils

Soil P transformations are primarily mediated by plant root and soil microbial activity. A short-term (40 weeks) glasshouse experiment with 15 grassland soils collected from around New Zealand was conducted to examine the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil microbial properties and microbiological processes involved in P dynamics. Results showed that the effect of plant species on soil microbial parameters varied greatly with soil type. Concentrations of microbial biomass C and soil respiration were significantly greater in six out of 15 soils under radiata pine compared with ryegrass, while there were no significant effects of plant species on these parameters in the remaining soils. However, microbial biomass P (MBP) was significantly lower in six soils under radiata pine, while there were no significant effects of plant species on MBP in the remaining soils. The latter indicated that P was released from the microbial biomass in response to greater P demand by radiata pine. Levels of water soluble organic C were significantly greater in most soils under radiata pine, compared with ryegrass, which suggested that greater root exudation might have occurred under radiata pine. Activities of acid and alkaline phosphatase and phosphodiesterase were generally lower in most soils under radiata pine, compared with ryegrass. The findings of this study indicate that root exudation plays an important role in increased soil microbial activities, solubility of organic P and mineralization of organic P in soils under radiata pine.     

Changes in soil organic matter and net nitrogen mineralization in heathland soils,after removal,addition or replacement of litter from Erica tetralix or Molinia caerulea

Summary The effects of different litter input rates and of different types of litter on soil organic matter accumulation and net N mineralization were investigated in plant communities dominated by Erica tetralix L. or Molinia caerulea (L.) Moench. Plots in which the litter on the soil had repeatedly been removed were compared with plots in the same plant community in which litter had been added to the soil. In another treatment, litter was removed and replaced by litter from the other plant community. Net N mineralization was measured in situ after 5 years. Less soil organic matter and soil N was found in plots in which litter had been removed, compared with control plots, or plots to which litter had been added, but these differences were significant for the Erica sp. soils only. Plots in which litter had been replaced and control plots did not differ significantly in the amount of soil organic matter. However, in both plant communities, the differences agreed with the faster decomposition rate of Molinia sp. litter compared with Erica sp. litter. The gravimetric soil moisture content was correlated positively with the amount of soil organic matter, both in the Erica sp. soils and the Molinia sp. soils. Net N mineralization rates (g N m^-2) differed significantly between treatments for Erica sp. soils but no for Molinia sp. soils. For Erica sp. soils, net N mineralization rates increased with increasing amounts of soil organic matter and soil N. Replacing the litter with Molinia sp. litter (which differs in chemical composition) had no clear additional effect on the net N mineralization rate.     

Effect of Different Leaf Litters on Carbon,Nitrogen and Microbial Activities of Sodic Soils

This study investigates the effect of single leaf litter of Terminalia arjuna (Ta) and Prosopis juliflora (Pj), mixed leaf litters [Ta, Pj, Azadirachta indica (Ai) and Albizia procera (Ap)] and paddy straw (Ps; Oryza sativa) on chemical properties and microbial activities of slightly sodic (SS), moderately sodic (MS) and highly sodic (HS) soils during 1 year in vitro decomposition process. For this purpose, equal amount (60 g) of single leaf litter [Ta (C : N = 43) and Pj (C : N = 38)], mixed leaf litters [1/4 of Ta, Pj, Ai and Ap (C : N = 30)] and Ps (C : N = 107) was added to equal amount (600 g) of SS, MS and HS soils. After addition of litters, changes in soil organic carbon (SOC), available nitrogen (N_av), microbial biomass carbon, nitrogen, soil respiration, microbial quotient (C_mic : C_org) and metabolic quotient (qCO₂) were observed at 2 months intervals for the whole year in greenhouse at constant soil moisture. The respective annual increase, at the end of the experiment, in SOC and N_av was highest in MS soil (40% and 45%), whereas soil microbial biomass and soil respiration showed decreasing trend from HS soil (39% and 29%) to SS soil (28% and 21%). The highest SOC was mineralized in the MS (42%) and HS (32%) soils containing litter of Ta; although greater (20%) accumulation of SOC in SS soil was noticed with mixed leaf litters. The study reveals that MS and HS soils comparatively showed fast decomposition of litters and significant increase in carbon, nitrogen and microbial activities. Copyright © 2014 John Wiley & Sons, Ltd.     

紫茎泽兰和黄顶菊入侵对土壤微生物群落结构和旱稻生长的影响

为比较入侵植物与本地植物对土壤微生态影响的差异, 探索外来植物入侵的土壤微生物学机制, 本研究通过同质园试验, 比较分析了2种入侵菊科植物(紫茎泽兰、黄顶菊)和2种本地植物(马唐、猪毛菜)对土壤肥力和微生物群落的影响, 并通过盆栽反馈试验验证入侵植物改变后的土壤微生物对本地植物旱稻生长的反馈作用。同质园试验结果表明: 2种入侵植物和2种本地植物分别对土壤微生态产生了不同的影响, 尤其是紫茎泽兰显著提高了土壤有效氮、有效磷和有效钾含量,紫茎泽兰根际土壤中有效氮含量为39.80 mg·kg^-1,有效磷含量为48.52 mg·kg^-1。磷脂脂肪酸指纹图谱结果表明, 2种入侵植物与2种本地植物相比, 较显著增加了土壤中放线菌数量, 而紫茎泽兰比其他3种植物显著增加了细菌和真菌数量。盆栽结果表明: 黄顶菊生长过的土壤灭菌后比灭菌前旱稻株高增加113%, 紫茎泽兰也使旱稻的株高增加17%。由以上结果可知, 紫茎泽兰和黄顶菊可能通过改变入侵地土壤的微环境, 形成利于其自身生长扩散的微生态环境从而实现其成功入侵。     

Trees on farms and their contribution to soil fertility parameters in Badessa, eastern Ethiopia

Surface (0–15 cm) and subsurface (30–45 cm) soil samples from under canopy, edge of canopy and away from canopy of isolated Cordia africana Lam. and Croton macrostachyus Del. trees and their leaves were examined to investigate leaf nutrient content, root biomass and the contribution of trees on farms to soil fertility parameters in Badessa area, eastern Ethiopia. Leaves of C. macrostachyus had 20% higher P and 25% lower K contents than those of C. africana. The studied species had comparable leaf N content. Both species produced shallow lateral roots that extended beyond the canopy zone. Typically, higher fine root biomass was observed in the surface soils than the subsurface soils. Both species did not affect soil organic C, pH and cation exchange capacity. Surface and subsurface soils under tree canopies had 22–26 and 12–17% higher N, respectively, than the corresponding soils away from tree canopies. Surface soil available P under tree canopies was 34–50% higher than the corresponding soil away from canopies. Available P content of subsurface soil was improved only under C. africana canopy. The available P of surface soil under C. macrostachyus canopy was more than double that for C. africana. Trees of both species increased underneath surface and subsurface exchangeable K by 18–46% compared with the corresponding controls. In conclusion, C. macrostachyus and C. africana trees on farms keep soil nutrient high via protection against leaching, translocation of nutrients from deeper to the surface layer and accumulation of litter, which create a temporary nutrient pool in the surface soils under their canopies.     

C and N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is ¹³C, and especially ¹⁵N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was ¹⁵N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and ¹³C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also ¹³C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but ¹³C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.     

不同林龄樟子松人工林针叶-凋落叶-土壤生态化学计量特征

为明确科尔沁沙地引种樟子松人工林生态系统的C、N、P含量及化学计量特征,采用时空互代的方法,在章古台地区选取4种不同林龄(15,25,35,45年)、立地条件基本一致的樟子松人工林作为研究对象,比较针叶-凋落叶-土壤的C、N、P含量及化学计量比的差异,探讨它们随林龄的变化及其相互间的关系。结果表明:(1)C、N、P含量表现为针叶凋落叶土壤,C/N、C/P、N/P表现为凋落叶针叶土壤,且在3个库之间都有显著差异;(2)林龄对针叶-凋落叶-土壤的C、N、P及C/N、C/P有显著影响,均在35年生樟子松林中针叶-凋落叶-土壤的C、N、P含量最高;(3)相较于其他地区,针叶和凋落叶均表现出高C、P和低N的特征,具有较高的C/N、C/P和较低的N/P;(4)各林龄针叶N/P均小于14,表明该地区樟子松林整个生长过程始终受N的限制,但不同林龄间差异不显著;(5)针叶-凋落叶-土壤的C、N、P含量及其C/N、N/P之间存在显著的相关性,说明该樟子松林生态系统的C、N、P元素在针叶、凋落叶和土壤3个库之间存在运输转换,但其内在维持机制需要进一步深入研究。     

Leaf litter selection by detritivore and geophagous earthworms

Summary Litterbag experiments with 10 different kinds of leaf litter showed that detritivore (Lumbricus species) and geophagous (Aporrectodea species) earthworms prefer certain litter types over others, since different numbers of worms were found below the litter after 50–52 days of exposure in a pasture. The detritivores preferred Fraxinus, Tilia, and predecomposed Ulmus and Fagus litter to Fagus litter and paper, while geophages preferred Tilia litter to Alnus and Ulmus litter, so that the two groups of earthworms showed different preferences. The detritivores seemed to be more selective than the geophages. The palatability of the litter was examined in relation to the C: N ratio, the lignin concentration and the initial and final polyphenol concentration. The numbers of detritivores were significantly correlated with the C: N ratio and the final polyphenol concentration, so that selection of litter seems to be related to palatability. The numbers of geophages were not significantly correlated with any of the parameters for palatability. The disappearance of litter after 50–52 days appeared to be due to detritivore activity, since the numbers found below the litter were positively and significantly correlated with the litter disappearance. There was no significant correlation with geophage activity. This indicates that detritivores use litter as food, and therefore influence the composition of the litter layer.     

Control of pitch pine seed germination and initial growth exerted by leaf litters and polyphenolic compounds

We investigated the influence of three concentrations of water extracts of three leaf litter species (pitch pine, huckleberry and white oak) and a mixture of all litters on the germination of pitch pine seeds and initial seedling growth in a microcosm experiment. All three plant species are important components of the pine barrens ecosystems in New Jersey, where it has been seen that pine seedling recruitment occurs only after stand replacing fire or in disturbed sites, where surface organic soil horizons and leaf litter have been removed. Leaf litter extracts did not influence seed germination, but significantly reduced seedling growth at high concentrations. There was little difference between the leaf litter species in growth suppression. As charcoal is a natural residue on the forest floor following fire, its influence on growth suppression was examined; it has been shown to immobilize polyphenols. Charcoal removed the inhibitory effect of leaf litter extracts and allowed the fertilizer effect of nutrients leached from the leaves to enhance seedling growth, particularly at the higher concentration of litter extract used. Responses to litter extracts were compared to four pure phenolic compounds, catchecol, p-coumaric acid, p-hydroxybenzoic acid and tannic acid. None of these compounds suppressed pine seedling growth, suggesting that these phenolics are not allelopathic to pine seedlings. The results are discussed in the context of fire as a driving factor in these oligotrophic and seasonally dry ecosystems and the interactions between nutrient supply and allelopathic chemistry of different leaf litters.     

Recent (<4 year old) leaf litter is not a major source of microbial carbon in a temperate forest mineral soil

Microbial communities in soil A horizons derive their carbon from several potential sources: organic carbon (C) transported down from overlying litter and organic horizons, root-derived C, or soil organic matter. We took advantage of a multi-year experiment that manipulated the ¹⁴C isotope signature of surface leaf litter inputs in a temperate forest at the Oak Ridge Reservation, Tennessee, USA, to quantify the contribution of recent leaf litter C to microbial respiration and biomarkers in the underlying mineral soil. We observed no measurable difference (<∼40‰ given our current analytical methods) in the radiocarbon signatures of microbial phospholipid fatty acids (PLFA) isolated from the top 10 cm of mineral soil in plots that experienced 3 years of litterfall that differed in each year by ∼750‰ between high-¹⁴C and low-¹⁴C treatments. Assuming any difference in ¹⁴C between the high- and low-¹⁴C plots would reflect C derived from these manipulated litter additions, we estimate that <∼6% of the microbial C after 4 years was derived from the added 1-4-year-old surface litter. Large contributions of C from litter < 1 year (or >4 years) old (which fell after (or prior to) the manipulation and therefore did not differ between plots) are not supported because the ¹⁴C signatures of the PLFA compounds (averaging 200-220‰) is much higher that of the 2004-5 leaf litter (115‰) or pre-2000 litter. A mesocosm experiment further demonstrated that C leached from ¹⁴C-enriched surface litter or the O horizon was not a detectable C source in underlying mineral soil microbes during the first eight months after litter addition. Instead a decline in the ¹⁴C of PLFA over the mesocosm experiment likely reflected the loss of a pre-existing substrate not associated with added leaf litter. Measured PLFA Δ¹⁴C signatures were higher than those measured in bulk mineral soil organic matter in our experiments, but fell within the range of ¹⁴C values measured in mineral soil roots. Together, our experiments suggest that root-derived C is the major (>60%) source of C for microbes in these temperate deciduous forest soils.     

Microbial community utilization of added carbon substrates in response to long-term carbon input manipulation

The chemical composition and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. Little is known about how changes in the composition of the soil microbial community affect decomposition rates and other ecosystem functions. This study examined the degradation of universally ¹³C-labeled glucose, glutamate, oxalate, and phenol in soil from an old-growth Douglas-fir (Pseudotsuga menziesii)—western hemlock (Tsuga heterophylla) forest in the Oregon Cascades that has experienced 7 y of chronic C input manipulation. The soils used in this experiment were part of a larger Detritus Input and Removal Treatment experiment and have received normal C inputs (control), doubled wood inputs, or root and litter input exclusion (no inputs). Soil from the doubled wood treatment had a higher fungal:bacterial ratio, and soil from the no inputs treatment had a lower fungal:bacterial ratio, than the control soil. Differences in the utilization of the compounds added to the field-manipulated soils were assessed by following the ¹³C tracer into microbial biomass and respiration. In addition, ¹³C-phospholipid fatty acids (PLFA) analysis was used to examine differential microbial utilization of the added substrates. Glucose and glutamate were metabolized similarly in soils of all three litter treatments. In contrast, the microbial community in the double wood soil respired more added phenol and oxalate, whereas microbes in the no inputs soil respired less added phenol and oxalate, than the control soil. Phenol was incorporated primarily into fungal PLFA, especially in soil of the double wood treatment. The addition of all four substrates led to enhanced degradation of soil organic matter (priming) in soils of all three litter treatments, and was greater following the addition of phenol and oxalate as compared to glucose and glutamate. Priming was greater in the no inputs soil as compared to the control or doubled wood soils. These results demonstrate that altering plant inputs to soil can lead to changes in microbial utilization of C compounds. It appears that many of these changes are the result of alteration in the size and composition of the microbial community.     

Responses of co-occurring populations of Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) to soil pH, moisture and resource addition

Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) are the two most dominating soil invertebrates in terms of biomass in boreal coniferous forest soils. A microcosm experiment was set up in order to study the influence of pH, moisture and resource addition on D. octaedra and C. sphagnetorum when both species are simultaneously present. Two kinds of coniferous forest humus were used as substrate, pine stand humus (pH 4.2), and spruce stand humus (pH 4.6); in the third treatment the pine stand humus was adjusted with slaked lime (CaOH₂) to the same initial pH as the spruce stand humus. Each substrate was adjusted to water contents of 25%, 42.5% and 60% of WHC (referred to as ‘dry’, ‘moist’ and ‘wet’). In the second part of the experiment, spruce needle litter and birch leaf litter were separately added into the pine stand humus (‘moist’, unlimed) and compared with a control without litter. The microcosms were plastic jars with 75 g (d.m.) of humus, into which 4 specimens of D. octaedra and 70 specimens of C. sphagnetorum were added. D. octaedra showed the highest biomass and C. sphagnetorum the lowest biomass in the spruce stand humus with higher pH. Moisture did not affect earthworms, while C. sphagnetorum thrived best at the highest moisture. Addition of both kinds of litter increased the numbers and biomass of D. octaedra, while on C. sphagnetorum resource addition had little effect. The results help to explain the abundance of these two species in coniferous forests differing in soil acidity, moisture and fertility.