沙棘人工林土壤微生物群落结构及酶活性的季节变化

采用时空互代法,研究了林龄和季节对沙棘人工林地土壤微生物群落结构和酶活性的影响.结果表明: 沙棘人工林磷脂脂肪酸总量、细菌脂肪酸总量、真菌脂肪酸总量均随着林龄的增长而增加,且在成熟林时达到最大值.土壤β-葡糖苷酶、纤维素酶、脲酶、蛋白酶、磷酸酶活性均随着林龄的增长逐渐增加,并于成熟林时达到最大值.雨季的土壤酶活性均显著高于旱季.土壤微生物群落结构及酶活性均与土壤pH、全氮、速效磷含量呈显著正相关.表明林龄和季节对沙棘人工林地土壤微生物群落结构和酶活性有显著影响.在黄土高原,营造沙棘人工林能有效提高土壤肥力.     

科尔沁沙地草地营造樟子松人工林对土壤化学和生物学性状的影响

以科尔沁沙地东南缘沙质草地和不同年龄樟子松（Pinus sylvestris var. mongolica）人工林（15、24和30年生）为对象,研究草地造林对土壤pH,土壤C、N、P含量,无机N（铵态氮、硝态氮）含量,C、N矿化速率,微生物生物量C含量以及土壤酶（脲酶、转化酶和过氧化氢酶）活性的影响.结果表明：草地造林初期,林地土壤C、N、P含量逐渐降低,随着林龄增加而逐渐恢复;与草地相比,24年生樟子松人工林土壤C、N、P含量最低,分别下降29%、34%和33%,而30年生樟子松人工林土壤C和N含量与草地差异不显著.草地造林能够影响土壤无机N存在形式,使土壤铵态氮含量逐渐增加,硝态氮含量下降.草地造林对土壤潜在N矿化速率和硝化速率影响不显著,但能够改变土壤C矿化速率,不同林龄樟子松人工林土壤C矿化速率依次为：24年生>30年生>草地>15年生.草地造林初期,土壤微生物生物量C含量和土壤转化酶活性明显降低,随着林龄的增加又逐渐增加;草地造林对土壤脲酶活性影响不显著,而使土壤过氧化氢酶活性逐渐增加.科尔沁沙地草地造林能够显著改变土壤化学和生物学性状,且随着林龄的变化而有所差异.     

Interactive Effects of Nitrogen and Phosphorus on Soil Microbial Communities in a Tropical Forest

Elevated nitrogen (N) deposition in humid tropical regions may exacerbate phosphorus (P) deficiency in forests on highly weathered soils. However, it is not clear how P availability affects soil microbes and soil carbon (C), or how P processes interact with N deposition in tropical forests. We examined the effects of N and P additions on soil microbes and soil C pools in a N-saturated old-growth tropical forest in southern China to test the hypotheses that (1) N and P addition will have opposing effects on soil microbial biomass and activity, (2) N and P addition will alter the composition of the microbial community, (3) the addition of N and P will have interactive effects on soil microbes and (4) addition-mediated changes in microbial communities would feed back on soil C pools. Phospholipid fatty acid (PLFA) analysis was used to quantify the soil microbial community following four treatments: Control, N addition (15 g N m⁻² yr⁻¹), P addition (15 g P m⁻² yr⁻¹), and N&P addition (15 g N m⁻² yr⁻¹ plus 15 g P m⁻² yr⁻¹). These were applied from 2007 to 2011. Whereas additions of P increased soil microbial biomass, additions of N reduced soil microbial biomass. These effects, however, were transient, disappearing over longer periods. Moreover, N additions significantly increased relative abundance of fungal PLFAs and P additions significantly increased relative abundance of arbuscular mycorrhizal (AM) fungi PLFAs. Nitrogen addition had a negative effect on light fraction C, but no effect on heavy fraction C and total soil C. In contrast, P addition significantly decreased both light fraction C and total soil C. However, there were no interactions between N addition and P addition on soil microbes. Our results suggest that these nutrients are not co-limiting, and that P rather than N is limiting in this tropical forest.     

Forest understory plant and soil microbial response to an experimentally induced drought and heat‐pulse event: the importance of maintaining the continuum

Drought duration and intensity are expected to increase with global climate change. How changes in water availability and temperature affect the combined plant–soil–microorganism response remains uncertain. We excavated soil monoliths from a beech (Fagus sylvatica L.) forest, thus keeping the understory plant–microbe communities intact, imposed an extreme climate event, consisting of drought and/or a single heat‐pulse event, and followed microbial community dynamics over a time period of 28 days. During the treatment, we labeled the canopy with ¹³CO₂ with the goal of (i) determining the strength of plant–microbe carbon linkages under control, drought, heat and heat–drought treatments and (ii) characterizing microbial groups that are tightly linked to the plant–soil carbon continuum based on ¹³C‐labeled PLFAs. Additionally, we used 16S rRNA sequencing of bacteria from the Ah horizon to determine the short‐term changes in the active microbial community. The treatments did not sever within‐plant transport over the experiment, and carbon sinks belowground were still active. Based on the relative distribution of labeled carbon to roots and microbial PLFAs, we determined that soil microbes appear to have a stronger carbon sink strength during environmental stress. High‐throughput sequencing of the 16S rRNA revealed multiple trajectories in microbial community shifts within the different treatments. Heat in combination with drought had a clear negative effect on microbial diversity and resulted in a distinct shift in the microbial community structure that also corresponded to the lowest level of label found in the PLFAs. Hence, the strongest changes in microbial abundances occurred in the heat–drought treatment where plants were most severely affected. Our study suggests that many of the shifts in the microbial communities that we might expect from extreme environmental stress will result from the plant–soil–microbial dynamics rather than from direct effects of drought and heat on soil microbes alone.     

Asynchronous responses of soil microbial community and understory plant community to simulated nitrogen deposition in a subtropical forest

Atmospheric nitrogen (N) deposition greatly affects ecosystem processes and properties. However, few studies have simultaneously examined the responses of both the above- and belowground communities to N deposition. Here, we investigated the effects of 8 years of simulated N deposition on soil microbial communities and plant diversity in a subtropical forest. The quantities of experimental N added (g of N m⁻² year⁻¹) and treatment codes were 0 (N0, control), 6 (N1), 12 (N2), and 24 (N3). Phospholipid fatty acids (PLFAs) analysis was used to characterize the soil microbial community while plant diversity and coverage were determined in the permanent field plots. Microbial abundance was reduced by the N3 treatment, and plant species richness and coverage were reduced by both N2 and N3 treatments. Declines in plant species richness were associated with decreased abundance of arbuscular mycorrhizal fungi, increased bacterial stress index, and reduced soil pH. The plasticity of soil microbial community would be more related to the different responses among treatments when compared with plant community. These results indicate that long-term N deposition has greater effects on the understory plant community than on the soil microbial community and different conservation strategies should be considered.     

Impacts of nitrogen-fixing and non-nitrogen-fixing tree species on soil respiration and microbial community composition during forest management in subtropical China

Forest management with N-fixing trees can improve soil fertility and tree productivity, but have little information regarding belowground carbon processes and microbial properties. We aimed to evaluate the effects of three forest management regimes, which were Erythrophleum fordii (N-fixing tree), Pinus massoniana (non-N-fixing tree), and their mixed forest, on soil respiration and microbial community composition in subtropical China, using Barometric Process Separation and phospholipid fatty acid profiles, respectively. We found that the inclusions of N-fixing species in forests significantly increased the soil respiration, but have no effects on SOC and ecosystem total C stock. In addition, soil microbial communities were obviously different among the three forest management regimes. For instance, total and bacterial PLFAs were higher in the E. fordii and mixed forest than in the P. massoniana forest. Conversely, fungal PLFAs in the P. massoniana forest were elevated versus the other two forests. Soil total N, nitrate-N and pH were the key determinants shaping the microbial community composition. Our study suggests that variations in soil respiration in the studied forests could be primarily explained by the differences of root biomass and soil microbial biomass, but not soil organic carbon. Although soil fertility and microbial biomass were promoted, N-fixing plantings also brought on increased CO₂ emissions in laboratory assays. The future decision of tree species selection for forest management in subtropical China therefore needs to consider the potential influences of tree species on CO₂ emissions.     

Contribution of litter layer to soil greenhouse gas emissions in a temperate beech forest

Background and aims

The litter layer is a major source of CO₂, and it also influences soil-atmosphere exchange of N₂O and CH₄. So far, it is not clear how much of soil greenhouse gas (GHG) emission derives from the litter layer itself or is litter-induced. The present study investigates how the litter layer controls soil GHG fluxes and microbial decomposer communities in a temperate beech forest.

Methods

We removed the litter layer in an Austrian beech forest and studied responses of soil CO₂, CH₄ and N₂O fluxes and the microbial community via phospholipid fatty acids (PLFA). Soil GHG fluxes were determined with static chambers on 22 occasions from July 2012 to February 2013, and soil samples collected at 8 sampling events.

Results

Litter removal reduced CO₂ emissions by 30 % and increased temperature sensitivity (Q₁₀) of CO₂ fluxes. Diffusion of CH₄ into soil was facilitated by litter removal and CH₄ uptake increased by 16 %. This effect was strongest in autumn and winter when soil moisture was high. Soils without litter turned from net N₂O sources to slight N₂O sinks because N₂O emissions peaked after rain events in summer and autumn, which was not the case in litter-removal plots. Microbial composition was only transiently affected by litter removal but strongly influenced by seasonality.

Conclusions

Litter layers must be considered in calculating forest GHG budgets, and their influence on temperature sensitivity of soil GHG fluxes taken into account for future climate scenarios.

     

Responses of soil microbial communities to prescribed burning in two paired vegetation sites in southern China

Prescribed burning is a common site preparation practice for forest plantation in southern China. However, the effects of prescribed burning on soil microbial communities are poorly understood. This study examined changes in microbial community structure, measured by phospholipid fatty acids (PLFAs), after a single prescribed burning in two paired vegetation sites in southern China. The results showed that the total amount of PLFA (totPLFA) was similar under two vegetation types in the wet season but differed among vegetation type in the dry season, and was affected significantly by burning treatment only in the wet season. Bacterial PLFA (bactPLFA) and fungal PLFA (fungPLFA) in burned plots all decreased compared to the unburned plots in both seasons (P = 0.059). Fungi appeared more sensitive to prescribed burning than bacteria. Both G⁺ bacterial PLFA and G⁻ bacterial PLFA were decreased by the burning treatment in both dry and wet seasons. Principal component analysis of PLFAs showed that the burning treatment induced a shift in soil microbial community structure. The variation in soil microbial community structure was correlated significantly to soil organic carbon, total nitrogen, available phosphorus and exchangeable potassium. Our results suggest that prescribed burning results in short-term changes in soil microbial communities but the long-term effects of prescribed burning on soil microbial community remain unknown and merit further investigation.     

Microbial assimilation of new photosynthate is altered by plant species richness and nitrogen deposition

To determine how plant species richness impacts microbial assimilation of new photosynthate, and how this may be modified by atmospheric N deposition, we analyzed the microbial assimilation of recent photosynthate in a 6-year-long field experiment in which plant species richness, atmospheric N deposition, and atmospheric CO₂ concentration were manipulated in concert. The depleted δ¹³C of fumigation CO₂ enabled us to investigate the effect of plant species richness and atmospheric N deposition on the metabolism of soil microbial communities in the elevated CO₂ treatment. To accomplish this, we determined the δ¹³C of bacterial, actinobacterial, and fungal phospholipid fatty acids (PLFAs). In the elevated CO₂ conditions of this study, the δ¹³C of bacterial PLFAs (i15:0, i16:0, 16:1ω7c, 16:1ω9c, 10Me16:0, and 10Me18:0) and the fungal PLFA 18:1ω9c was significantly lower in species-rich plant communities than in species-poor plant communities, indicating that microbial incorporation of new C increased with plant species richness. Despite an increase in plant production, total PLFA decreased under N deposition. Moreover, N deposition also decreased fungal relative abundance in species-rich plant communities. In our study, plant species richness directly increased microbial incorporation of new photosynthate, providing a mechanistic link between greater plant detritus production in species-rich plant communities and larger and more active soil microbial community.     

Variability of soil N cycling and N2O emission in a mixed deciduous forest with different abundance of beech

The mixture of other broadleaf species into beech forests in Central Europe leads to an increase of tree species diversity, which may alter soil biochemical processes. This study was aimed at 1) assessing differences in gross rates of soil N cycling among deciduous stands of different beech (Fagus sylvatica L.) abundance in a limestone area, 2) analyzing the relationships between gross rates of soil N cycling and forest stand N cycling, and 3) quantifying N₂O emission and determining its relationship with gross rates of soil N cycling. We used ¹⁵N pool dilution techniques for soil N transformation measurement and chamber method for N₂O flux measurement. Gross rates of mineral N production in the 0–5 cm mineral soil increased across stands of decreasing beech abundance and increasing soil clay content. These rates were correlated with microbial biomass which, in turn, was influenced by substrate quantity, quality and soil fertility. Leaf litter-N, C:N ratio and base saturation in the mineral soil increased with decreasing beech abundance. Soil mineral N production and assimilation by microbes were tightly coupled, resulting in low N₂O emissions. Annual N₂O emissions were largely contributed by the freeze-thaw event emissions, which were correlated with the amount of soil microbial biomass. Our results suggest that soil N availability may increase through the mixture of broadleaf species into beech forests.     

氮肥添加对高寒藏嵩草(Kobresia tibetica)沼泽化草甸和土壤微生物群落的影响

以藏嵩草沼泽化草甸为研究对象,利用磷脂脂肪酸(PLFA)技术,研究连续6年N素添加对地上植被群落数量特征、土壤微生物群落结构的影响。结果表明:①藏嵩草沼泽化草甸群落生物量、枯枝落叶对施肥处理无明显响应,且莎草科植物对土壤氮素的吸收和利用率较低。②施肥增加了0-10 cm土壤微生物类群PLFAs丰富度尤其细菌和革兰氏阳性菌PLFAs,降低了10-20 cm PLFAs丰富度;③磷脂脂肪酸饱和脂肪酸/单烯不饱和脂肪酸、细菌PLFAs/真菌PLFAs的比值随土壤层次增加而增加;④0-10 cm土层革兰氏阳性菌、真菌PLFAs含量与pH、土壤速效磷、速效氮、土壤有机质显著正相关(P0.05或P0.01);10-20 cm土层,细菌、革兰氏阳性菌、真菌和总PLFAs含量与土壤有机质含量显著正相关(P0.05或P0.01)。表明藏嵩草沼泽化草甸微生物PLFAs含量和丰富度对施肥的响应存在明显的土层梯度效应,土壤微生物PLFAs含量和丰富度主要受表层土壤初始养分含量的影响。     

A closeup study of early beech litter decomposition: potential drivers and microbial interactions on a changing substrate

Aims

Litter decomposition and subsequent nutrient release play a major role in forest carbon and nutrient cycling. To elucidate how soluble or bulk nutrient ratios affect the decomposition process of beech (Fagus sylvatica L.) litter, we conducted a microcosm experiment over an 8 week period. Specifically, we investigated leaf-litter from four Austrian forested sites, which varied in elemental composition (C:N:P ratio). Our aim was to gain a mechanistic understanding of early decomposition processes and to determine microbial community changes.

Methods

We measured initial litter chemistry, microbial activity in terms of respiration (CO₂), litter mass loss, microbial biomass C and N (C_mic and N_mic), non purgeable organic carbon (NPOC), total dissolved nitrogen (TDN), NH₄ ⁺, NO₃ ^- and microbial community composition (phospholipid fatty acids – PLFAs).

Results

At the beginning of the experiment microbial biomass increased and pools of inorganic nitrogen (N) decreased, followed by an increase in fungal PLFAs. Sites higher in NPOC:TDN (C:N of non purgeable organic C and total dissolved N), K and Mn showed higher respiration.

Conclusions

The C:N ratio of the dissolved pool, rather than the quantity of N, was the major driver of decomposition rates. We saw dynamic changes in the microbial community from the beginning through the termination of the experiment.     

南亚热带红椎和格木人工幼龄林土壤微生物群落结构特征

采用氯仿熏蒸浸提法和磷脂脂肪酸法(Phospholipids fatty acid,PLFA)研究了我国南亚热带地区非固氮树种红椎(Castanopsis hystrix)和固氮树种格木(Erythrophleum fordii)人工幼龄林土壤微生物生物量与微生物群落结构特征。结果表明,在旱季和雨季,红椎幼龄林土壤微生物总PLFAs量,细菌PLFAs量、放线菌PLFAs量及丛枝菌根真菌PLFAs量均大于格木幼龄林。红椎幼龄林土壤PLFA Shannon多样性指数(H_(PLFA))在旱季和雨季均大于格木幼龄林。主成分分析表明,土壤微生物群落结构组成受到林分类型和季节的双重影响。冗余分析表明,土壤硝态氮(NO_3~--N)含量、土壤含水量、p H及土壤微生物生物量氮(MBN)与特征磷脂脂肪酸之间呈显著相关关系。以上结果表明固氮树种格木与非固氮树种红椎人工幼龄林对土壤微生物生物量和群落结构的影响存在显著差异。     

Minor contribution of leaf litter to N nutrition of beech (Fagus sylvatica) seedlings in a mountainous beech forest of Southern Germany

Aims

Our aims were to characterize the fate of leaf-litter-derived nitrogen in the plant-soil-microbe system of a temperate beech forest of Southern Germany and to identify its importance for N nutrition of beech seedlings.

Methods

¹⁵N-labelled leaf litter was traced in situ into abiotic and biotic N pools in mineral soil as well as into beech seedlings and mycorrhizal root tips over three growing seasons.

Results

There was a rapid transfer of ¹⁵N into the mineral soil already 21 days after tracer application with soil microbial biomass initially representing the dominant litter-N sink. However, ¹⁵N recovery in non-extractable soil N pools strongly increased over time and subsequently became the dominant ¹⁵N sink. Recovery in plant biomass accounted for only 0.025 % of ¹⁵N excess after 876 days. After three growing seasons, ¹⁵N excess recovery was characterized by the following sequence: non-extractable soil N?>>?extractable soil N including microbial biomass?>>?plant biomass?>?ectomycorrhizal root tips.

Conclusions

After quick vertical dislocation and cycling through microbial N pools, there was a rapid stabilization of leaf-litter-derived N in non-extractable N pools of the mineral soil. Very low ¹⁵N recovery in beech seedlings suggests a high importance of other N sources such as root litter for N nutrition of beech understorey.     

Growth of ectomycorrhizal mycelia and composition of soil microbial communities in oak forest soils along a nitrogen deposition gradient

Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi, especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2ω6,9 was used as an indicator to estimate the total fungal biomass. Higher N deposition (20 kg N ha⁻¹ y⁻¹ compared with 10 kg N ha⁻¹ y⁻¹) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the PLFA 16:1ω5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1ω9, a17:0 and 18:1ω7, while some were negatively affected by pH, such as i15:0, 16:1ω7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect on the PLFAs 16:1ω7c and 16:1ω9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2ω6,9. Low amounts of EM mycelia covaried with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at increased N input.     

Altered microbial community structure and organic matter composition under elevated CO2 and N fertilization in the duke forest

The dynamics and fate of terrestrial organic matter (OM) under elevated atmospheric CO₂ and nitrogen (N) fertilization are important aspects of long‐term carbon sequestration. Despite numerous studies, questions still remain as to whether the chemical composition of OM may alter with these environmental changes. In this study, we employed molecular‐level methods to investigate the composition and degradation of various OM components in the forest floor (O horizon) and mineral soil (0–15 cm) from the Duke forest free air CO₂ enrichment (FACE) experiment. We measured microbial responses to elevated CO₂ and N fertilization in the mineral soil using phospholipid fatty acid (PLFA) profiles. Increased fresh carbon inputs into the forest floor under elevated CO₂ were observed at the molecular‐level by two degradation parameters of plant‐derived steroids and cutin‐derived compounds. The ratios of fungal to bacterial PLFAs and Gram‐negative to Gram‐positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin‐derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. ¹H nuclear magnetic resonance (NMR) spectra of soil humic substances revealed an enrichment of leaf‐derived alkyl structures with both elevated CO₂ and N fertilization. We suggest that microbial decomposition of SOM constituents such as lignin and hydrolysable lipids was promoted under both elevated CO₂ and N fertilization, which led to the enrichment of plant‐derived recalcitrant structures (such as alkyl carbon) in the soil.     

Redefining the dose of the entomopathogenic fungus Metarhizium brunneum (Ascomycota,Hypocreales) to increase Fe bioavailability and promote plant growth in calcareous and sandy soils

Background and aims

Plants differ in their ability to use different nitrogen (N) chemical forms, these differences can be related to their ecology and drive community structure. The capacity to uptake intact organic N has been observed in plants of several ecosystems. However, soil organic N uptake by Mediterranean plants is unknown despite organic N being abundant in Mediterranean ecosystems. We compare the uptake of different N forms in two widespread coexisting Mediterranean forest trees with contrasting ecophysiological characteristics: Quercus ilex and Pinus halepensis.

Methods

To estimate root uptake rate of each N form we used equimolar solutions (1 mM N) of ¹⁵NO₃ ^?, ¹⁵NH₄ ⁺ and ¹⁵N-¹³C glycine.

Results

NH₄ ⁺ and glycine were taken up at a similar rate, but faster than NO₃ ^? in both species. Intact dual labeled glycine was found in both species, demonstrating that both species can absorb intact organic N.

Conclusions

Despite their ecological differences, both species had similar preference for N forms suggesting no fundamental niche complementarity for N uptake. The higher preference for NH₄ ⁺ and glycine over NO₃ ^? possibly reflects adaptation to the differing proportions of N forms in Mediterranean soils.

     

氮添加对亚热带毛竹林土壤微生物群落结构的影响

氮沉降会影响森林生态系统地上(如植物生产力和组成)和地下特性(如土壤养分循环),进而影响土壤微生物群落结构和功能。本研究以亚热带戴云山毛竹林为对象,设置N0(0 kg N·hm^-2·a^-1)、N20(20 kg N·hm^-2·a^-1)、N80(80 kg N·hm^-2·a^-1) 3个施氮水平,进行3年的氮沉降模拟实验。通过测定土壤基本理化性质、腐殖化指数和微生物磷脂脂肪酸等指标,研究氮添加对毛竹林土壤养分、腐殖化指数和微生物群落结构的影响。结果显示,N20显著增加土壤腐殖化指数,降低土壤中碱性阳离子总量(K⁺,Na⁺,Ca²⁺,Mg²⁺)、革兰氏阳性菌(G⁺)、革兰氏阴性菌(G^-)、总磷脂脂肪酸含量和G⁺/G^-。与N20相比,N80处理土壤NO₃^-     

Decomposition of <Superscript>15</Superscript>N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

The decomposition and the fate of ¹⁵N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, ¹⁵N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and ¹⁵N concentration. Mass loss of the ¹⁵N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released ¹⁵N accumulated essentially at the soil surface. ¹⁵N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. ¹⁵N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of ¹⁵N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales. Received: 3 May 1999 / Accepted: 3 January 2000     

Plant‐facilitated effects of exotic earthworm Pontoscolex corethrurus on the soil carbon and nitrogen dynamics and soil microbial community in a subtropical field ecosystem

Earthworms and plants greatly affect belowground properties; however, their combined effects are more attractive based on the ecosystem scale in the field condition. To address this point, we manipulated earthworms (exotic endogeic species Pontoscolex corethrurus) and plants (living plants [native tree species Evodia lepta] and artificial plants) to investigate their combined effects on soil microorganisms, soil nutrients, and soil respiration in a subtropical forest. The manipulation of artificial plants aimed to simulate the physical effects of plants (e.g., shading and interception of water) such that the biological effects of plants could be evaluated separately. We found that relative to the controls, living plants but not artificial plants significantly increased the ratio of fungal to bacterial phospholipid fatty acids (PLFAs) and fungal PLFAs. Furthermore, earthworms plus living plants significantly increased the soil respiration and decreased the soil NH₄⁺‐N, which indicates that the earthworm effects on the associated carbon, and nitrogen processes were greatly affected by living plants. The permutational multivariate analysis of variance results also indicated that living plants but not earthworms or artificial plants significantly changed the soil microbial community. Our results suggest that the effects of plants on soil microbes and associated soil properties in this study were largely explained by their biological rather than their physical effects.