A new conceptual model for the fate of lignin in decomposing plant litter

Lignin is a main component of plant litter. Its degradation is thought to be critical for litter decomposition rates and the build-up of soil organic matter. We studied the relationships between lignin degradation and the production of dissolved organic carbon (DOC) and of CO2 during litter decomposition. Needle or leaf litter of five species (Norway spruce, Scots pine, mountain ash, European beech, sycamore maple) and of different decomposition stage (freshly fallen and up to 27 months of field exposure) was incubated in the laboratory for two years. Lignin degradation was followed with the CuO method. Strong lignin degradation occurred during the first 200 incubation days, as revealed by decreasing yields of lignin-derived phenols. Thereafter lignin degradation leveled off. This pattern was similar for fresh and decomposed litter, and it stands in contrast to the common view of limited lignin degradation in fresh litter. Dissolved organic carbon and CO2 also peaked in the first period of the incubation but were not interrelated. In the later phase of incubation, CO2 production was positively correlated with DOC amounts, suggesting that bioavailable, soluble compounds became a limiting factor for CO2 production. Lignin degradation occurred only when CO2 production was high, and not limited by bioavailable carbon. Thus carbon availability was the most important control on lignin degradation. In turn, lignin degradation could not explain differences in DOC and CO2 production over the study period. Our results challenge the traditional view regarding the fate and role of lignin during litter decomposition. Lignin degradation is controlled by the availability of easily decomposable carbon sources. Consequently, it occurs particularly in the initial phase of litter decomposition and is hampered at later stages if easily decomposable resources decline.     

森林凋落物分解研究进展

森林凋落物是指森林生态系统内由生物组分产生,然后归还到林地表面的所有有机物质的总称。森林凋落物在促进森林生态系统正常的物质循环和养分平衡,维持生态系统功能中具有重要作用,其分解受多因素影响,且各因素之间相互交错。不同情况下,各因子的重要性可能不同。温度和湿度被认为是影响凋落物分解主要的气候因子。凋落物随着温度升高分解速率加快,增加土壤湿度对凋落物分解有积极作用。凋落物的化学性质中,C、N比和木质素含量被认为是最重要的指标。凋落物分解前期的分解速率受到养分含量、水溶性碳化合物和结构碳化合物含量的强烈影响,而后期则更多地受到木质素及纤维素/木质素比值的支配。土壤动物可以粉碎凋落物,土壤微生物也是促进凋落物分解的重要因素,人为活动也影响凋落物分解。N沉降、全球变暖和臭氧层破坏等全球变化对森林凋落物分解的影响已逐渐成为研究热点。未来凋落物分解的研究方向是统一研究方法,开展长期定位监测,加强对分解过程中有机碳含量和释放量的研究,以及N沉降对凋落物分解作用机理的研究。     

Interactions among lignin, cellulose, and nitrogen drive litter chemistry-decay relationships

Litter decay rates often correlate with the initial ratios of lignin:nitrogen (N) or lignin:cellulose in litter. However, the chemical and microbial mechanisms that give rise to these patterns are still unclear. To identify these mechanisms, we studied the decomposition of a model plant system, Arabidopsis thaliana, in which plants were manipulated to have low levels of lignin, cellulose, or litter N. Nitrogen fertilizer often increases the loss of cellulose, but it suppresses the breakdown of lignin in plant litter. To understand the mechanisms driving these patterns, we decomposed plants in litterbags for one year in control and N-fertilized plots in an Alaskan boreal forest. We found that litter N had a positive effect on total mass loss because it increased the loss of lignin, N, and soluble C. Lignin had a negative effect on rates of total litter mass loss due to decreases in the loss of cellulose and hemicellulose. Cellulose had a positive effect on lignin loss, supporting the concept of a "priming effect" for lignin breakdown. However, the low-cellulose plants also lost more of their original cellulose compared to the other plant types, indicating that decomposers mined the litter for cellulose despite the presence of lignin. Low-lignin litter had higher fungal biomass and N-acetyl glucosaminidase (NAG, a chitinase) activity, suggesting that lignin restricted fungal growth and may have influenced competitive interactions between decomposers. Nitrogen fertilization increased NAG activity in the early stages of decay. In the later stages, N fertilization led to increased cellulase activity on the litters and tended to reduce lignin losses. The transition over time from competition among decomposers to high cellulase activity and suppressed lignin loss under N fertilization suggests that, in N-limited systems, N fertilization may alter decomposer community structure by favoring a shift toward cellulose- and mineral-N users.     

Tree species effects on decomposition and forest floor dynamics in a common garden

We studied the effects of tree species on leaf litter decomposition and forest floor dynamics in a common garden experiment of 14 tree species (Abies alba, Acer platanoides, Acer pseudoplatanus, Betula pendula, Carpinus betulus, Fagus sylvatica, Larix decidua, Picea abies, Pinus nigra, Pinus sylvestris, Pseudotsuga menziesii, Quercus robur, Quercus rubra, and Tilia cordata) in southwestern Poland. We used three simultaneous litter bag experiments to tease apart species effects on decomposition via leaf litter chemistry vs. effects on the decomposition environment. Decomposition rates of litter in its plot of origin were negatively correlated with litter lignin and positively correlated with mean annual soil temperature (MAT(soil)) across species. Likewise, decomposition of a common litter type across all plots was positively associated with MAT(soil), and decomposition of litter from all plots in a common plot was negatively related to litter lignin but positively related to litter Ca. Taken together, these results indicate that tree species influenced microbial decomposition primarily via differences in litter lignin (and secondarily, via differences in litter Ca), with high-lignin (and low-Ca) species decomposing most slowly, and by affecting MAT(soil), with warmer plots exhibiting more rapid decomposition. In addition to litter bag experiments, we examined forest floor dynamics in each plot by mass balance, since earthworms were a known component of these forest stands and their access to litter in litter bags was limited. Forest floor removal rates estimated from mass balance were positively related to leaf litter Ca (and unrelated to decay rates obtained using litter bags). Litter Ca, in turn, was positively related to the abundance of earthworms, particularly Lumbricus terrestris. Thus, while species influence microbially mediated decomposition primarily through differences in litter lignin, differences among species in litter Ca are most important in determining species effects on forest floor leaf litter dynamics among these 14 tree species, apparently because of the influence of litter Ca on earthworm activity. The overall influence of these tree species on leaf litter decomposition via effects on both microbial and faunal processing will only become clear when we can quantify the decay dynamics of litter that is translocated belowground by earthworms.     

Leaf herbivory and decomposability in a Malaysian tropical rain forest

There is accumulating evidence that similar suites of plant traits may affect leaf palatability and leaf litter decomposability. However, the possible association between leaf herbivory and litter decomposition rates across species in species-diverse natural ecosystems such as tropical rain forests remains unexplored, despite its importance in estimating the herbivory effects on carbon and nutrient cycling of ecosystems. We found no strong association between leaf herbivory and litter decomposition rates across 40 tree species in a Malaysian tropical rain forest, even though the leaf and litter traits were tightly correlated. This is because the leaf and litter traits related to herbivory and decomposition rates in the field were inconsistent. Leaf toughness accounted for only a small part of the variation in the herbivory rate, whereas a number of litter traits (the leaf mass per area, lignin to nitrogen ratio, and condensed tannin concentration) accurately predicted the decomposition rate across species. These results suggest that herbivory rate across species may not be strongly related to single leaf traits, probably because plant-herbivore interactions in tropical rain forests are highly diverse; on the other hand, plant-decomposer interactions are less specific and can be governed by litter chemicals. We also investigated two factors, phylogeny and tree functional types, that could affect the relationship between herbivory and decomposition across species. Phylogenetic relatedness among the species did not affect the relationship between herbivory and decomposition. In contrast, when the plants were segregated according to their leaf emergence pattern, we found a significant positive relationship between herbivory and decomposition rates for continuous-leafing species. In these species, the condensed tannin to N ratios in leaves and litter were related to herbivory and decomposition rates, respectively. However, we did not observe a similar trend for synchronous-leafing species. These results suggest that the relationship between herbivory and decomposition may be more greatly affected by functional types than by phylogenetic relatedness among species. In conclusion, our results suggest that well-defended leaves are not necessarily less decomposable litter in a tropical rain forest community, implying that herbivory may not generate positive feedback for carbon and nutrient cycling in this type of ecosystem.     

大兴安岭南部温带山杨天然次生林不同生长阶段生物量及碳储量

基于内蒙古赛罕乌拉森林生态系统定位研究站山杨（Populus davidiana Dode）天然次生林幼龄林、中龄林、近熟林、成熟林及过熟林生物量调查,探讨了不同龄组山杨天然次生林单株木、林分、林下植被和枯落物的生物量及群落碳储量的时空变化规律。结果表明：随林龄的增大,山杨天然次生林木和各器官生物量总体呈增加趋势,树干所占比例增加,中龄林增加尤为明显;林下植被层、枯落物层生物量随林龄增大呈增加趋势。群落总碳储量的空间分布序列是：乔木层〉枯落物层〉林下植被层。幼龄林、中龄林、近熟林、成熟林和过熟林群落的碳储量分别为27.146 6、53.545 1、60.889 8、77.915 8、79.135 3t.hm-2,乔木层碳储量分别为22.206 5、47.215 7、52.056 3、68.445 3、68.773 1 t.hm-2,枯落物层和林下植被层碳储量平均值分别为5.814 4、2.172 7 t.hm-2。乔木层、枯落物层和林下植被层碳储量占总量的平均率分别为86.05%、10.39%和3.57%。研究认为山杨天然次生林群落碳储量随林龄增加的变化规律明显,碳汇潜力巨大;中龄林为碳储量增长迅速期,且持续较长一段时间,是林分管理的关键阶段;自然稀疏有利于促进林木生长,林分碳储量并未随林分密度下降而减小。     

Litter decomposition and nitrogen mineralization from an annual to a monthly model

The forest litter decomposition model (FLDM) described in this paper provides an important basis for assessing the impacts of forest management on seasonal stream water quality and export of dissolved organic carbon (DOC). By definition, models with annual time steps are unable to capture seasonal, within-year variation. In order to simulate seasonal variation in litter decomposition and DOC production and export, we have modified an existing annual FLDM to account for monthly dynamics of decomposition and residual mass in experimental litterbags placed in 21 different forests across Canada.The original annual FLDM was formulated with three main litter pools (fast, slow, and very slow decomposing litter) to address the fact that forest litter is naturally composed of a mixture of organic compounds that decompose at different rates. The annual FLDM was shown to provide better simulations than more complex models like CENTURY and SOMM.The revised monthly model retains the original structure of the annual FLDM, but separates litter decomposition from nitrogen (N) mineralization. In the model, monthly soil temperature, soil moisture, and mean January soil temperature are shown to be the most important controlling variables of within-year variation in decomposition. Use of the three variables in a process-based definition of litter decomposition is a significant departure from the empirical definition in the annual model. The revised model is shown to give similar calculations of residual mass and N concentration as the annual model (r² = 0.91, 0.78), despite producing very different timeseries of decomposition over six years. It is shown from a modelling perspective that (i) forest litter decomposition is independent of N mineralization, whereas N mineralization is dependent on litter decomposition, and (ii) mean January soil temperature defines litter decomposition in the summer because of winter-temperatures’ role in modifying forest-floor microorganism community composition and functioning in the following summer.     

鼎湖山森林演替序列水文过程中总有机碳(TOC)变化规律及其对土壤碳平衡的贡献

森林水文过程中的总有机碳转运对土壤有机碳平衡起着重要的作用,但我们对于水文过程对碳平衡的贡献机理所知甚少.本研究针对鼎湖山季风常绿阔叶林演替序列不同森林生态系统(马尾松林、针阔混交林和季风常绿阔叶林(简称季风林))的大气降水、穿透水、树干流、凋落物淋洗水以及地表径流中的总有机碳(TOC)进行了三年(2002年4月-2005年5月)观测,以此来分析水文学过程中TOC的变化规律和水文学过程对不同成熟度森林生态系统土壤有机碳积累的贡献.每场雨后进行水样的采集,采集的水样装入棕色玻璃瓶中,加硫酸至pH值小于2,放置于实验室冰箱冷藏待测.TOC用日本岛津公司生产的5000A型TOC-V分析仪测定.研究结果及推论如下:鼎湖山森林水文学过程中TOC浓度和总量变化呈现规律性的变化.大气降水中的TOC浓度和总量分别为(3.65±0.59)mg·L~(-1)和51.8104 kg·hm~(-2)·a~(-1),大气降水是鼎湖山森林生态系统水文循环过程中TOC的主要来源.穿透水(DTF)中TOC浓度和总量均为:松林>混交林>季风林,其中季风林TOC浓度显著低于其他两种林型.松林树干流的TOC浓度显著高于混交林和季风林.凋落物淋洗水TOC浓度和总量大小依次均为:松林>混交林>季风林,且三林型间存在显著差异(p<0.05).径流中TOC浓度和总量均较小,且无明显差异.在湿季5月份,穿透水、树干流、凋落物淋洗水的TOC浓度呈现下降趋势.干季(10月)开始以后,穿透水、树干流、凋落物淋洗水中的TOC浓度又逐步回升.地表径流中TOC浓度干湿季变化趋势不明显.干季中各水文学分量TOC浓度大于湿季,但TOC总量呈现相反趋势.在森林水文学过程中,凋落物淋洗水所携带的有机碳量是土壤有机碳输入的最大项,季风林、混交林、松林中TOC总量分别为246.983 kg·hm~(-2)·a~(-1),255.187kg·hm~(-2)·a~(-1)和261.876kg·hm~(-2)·a~(-1);其次是直接到达土壤表面的穿透水,季风林、混交林、松林中TOC总量分别为28.152kg·hm~(-2)·a~(-1),37.410kg·hm~(-2)·a~(-1)和43.176kg·hm~(-2)·a~(-1);树干流中有机碳浓度虽高,但总量很微小,季风林、混交林、松林中TOC总量分别为4.663kg·hm~(-2)·a~(-1),5.910kg·hm~(-2)·a~(-1)和4.566kg·hm~(-2)·a~(-1),所以对土壤有机碳收入贡献不大.径流所携带的TOC总量很小,季风林、混交林、松林中分别为8.707kg·hm~(-2)·a~(-1),9.318kg·hm~(-2)·a~(-1),7.220kg·hm~(-2)·a~(-1).由此可知,水文过程输入土壤的TOC总量远大于径流所带走的TOC总量,导致了水文过程中的TOC存留在土壤中,对土壤有机碳(SOC)的积累起着重要作用.季风林、混交林和马尾松林土壤每年通过水文学过程净输入的有机碳量分别为(27.1+1.65)g·m~(-2),(28.9±2.79)g·m~(-2)和(30.2±2.65)g·m~(-2).水文学过程中的这部分有机碳由于占总有机碳比例较小往往被忽视,但是正是由于水分在土壤中的下渗使得有机碳的分布趋于均匀,这将更加利于SOC的积累和保存.     

Modelling solute uptake and incorporation into vegetation and litter

The concepts and algorithms of the Baldwin, Nye and Tinker model describing solute movement from bulk soil solution to roots (DIFMAS) and a model of solute dynamics and accumulation in plant tissues and litter (DRYADS) are presented. Foliar uptake of solutes and gases are included in the DRYADS code. These models form components in a coupled system of models having hourly resolution of carbon, water, and solute dynamics in terrestrial ecosystems. Applications showing successive hourly, monthly, and annual results illustrate the utility of the models. The DRYADS model sensitivity to both leaf solute conductivity and root solute conductivity parameters suggest the importance of careful experimental determination of these plant properties. The tissues of solute entry (leaves, roots) initially accumulate solutes in a fixed form in preference to the more remote tissues (stems, fruits). Model application results show that root sapwood is the first major site of trace contaminant accumulation from soil-borne pollutants. The algorithms describing solute movement along a concentration gradient in phloem and as mass flow in the xylem transpiration stream result in high mobility of solutes in vegetation. The simulated diurnal pattern of root solute uptake showed that more than 85% of solutes were taken up during the daylight hours. The simulations further showed that contaminants had the greatest effect on the litter system. Toxic effects of contaminants on decomposition resulted in lower mineralization losses and accumulation of contaminant in litter with continuing deposition.     

Long-term forest-floor litter dynamics in Canada's boreal forest: Comparison of two model formulations

Litter decomposition is a key component in ecosystem material cycling that determines (i) forest soil carbon (C) and nutrient content, (ii) release of carbon dioxide to the atmosphere, and (iii) generation and mass transfer of dissolved organic carbon from terrestrial to aquatic ecosystems. In this study, we provide simulations of long term forest-floor litter dynamics generated with both (i) an existing forest nutrient cycling and biomass growth model (ForNBM) with a single-pool formulation of forest-floor litter decomposition (Zhu et al., 2003. Ecol. Model. 169, 347-360), and (ii) a revised version of the model produced by substituting the single-pool formulation with a three-pool version of the formulation tested against data from litterbag experiments (FLDM; Zhang et al., 2010. Ecol. Model. 221, 1944-1953). This is done to determine the importance of subdividing the litter mass into categories of rates of decay (i.e., fast, slow, and very slow) on model accuracy. Forest-forest litter dynamics simulated with the two models are subsequently compared against field measurements collected at several northern jack pine (Pinus banksiana) stands along a southwest-northeast oriented transect (climate gradient) associated with the Boreal Forest Transect Case Study in northwest Canada. Initial comparison shows that the single-pool formulation underpredicts residual litter mass when forests are <65 years old, largely due to the improper treatment of the very slow decomposing litter component. This underprediction is resolved when the three-pool formulation is used. From a ecosystems-response point of view, the revised ForNBM (with the three-pool formulation) demonstrates that (i) forest-floor litter initially increases with forest growth and reaches a plateau once the forest matures; (ii) the forest floor stores more litter and C at the southern and warmer sites than at the northern sites; and (iii) in a similar climate regime, the forest floor stores more litter and C at productive than at nutrient-poor sites.     

CERES — A model of forest stand biomass dynamics for predicting trace contaminant,nutrient, and water effects. II. Model application

In a previous paper (Dixon et al., 1978) the ecosystem model, CERES, was described. In this paper an application of CERES is presented to show the versatility of the model and its capability to simulate solute transport. Simulation results for 10 days in May illustrate the hourly patterns of photosynthesis, leaf sugar levels and translocation. A 1-year simulation shows seasonal dynamics of biomass fluxes in plants and litter.The third simulation shows the effects of a lead mine-smelter complex on an oak-hickory forest in southeastern Missouri. Results from a 6-year simulation with CERES coupled with other models in the Unified Transport Model illustrate toxic metal effects on litter decomposition and slight reduction of root growth with the parameters chosen. Heavy metal pollutants from the lead mine and smelter complex were introduced to the ecosystem as wetfall and dryfall. The DRYADS and DIFMAS models calculated the heavy metal uptake by the vegetation and accumulation in litter. During the 6-year period, the steady annual input of plant litter and the reduced decomposition rate resulted in an increase in litter mass of nearly 50%.     

Sources and concentrations of vascular plant material in sediments of Buzzards Bay,Massachusetts, USA

Samples of surface sediment from Buzzards Bay and creek sediment from Great Sippewissett Marsh were analyzed for lignin and stable carbon isotope composition in 1984. The lack of change in composition of lignins in detritus of Spartina alterniflora over two years of decomposition and similar aldehyde/acid ratios of lignin oxidation products of plant and sediment samples indicated minimal diagenesis of lignins in sediments. Remains of non-woody angiosperm tissues made up the bulk of the vascular plant debris in Great Sippewissett Marsh and Buzzards Bay sediments. These plant remains were evenly distributed over the sampling area in Buzzards Bay. Based on model calculations, salt marshes potentially contributed a significant fraction of the total amount of vascular plant debris in coastal marine sediments. The bulk of the organic matter in Buzzards Bay sediments, however, was derived from phytoplankton; vascular plant remains made up only 5 to 7% of the total amount of organic carbon in these sediments.     

Modelling carbon and nitrogen dynamics in forest ecosystems of Central Russia under different climate change scenarios and forest management regimes

The individual-based stand-level model EFIMOD was used for large-scale simulations using standard data on forest inventories as model inputs. The model was verified for the case-study of field observations, and possible sources of uncertainties were analysed. The approach developed kept the ability for fine-tuning to account for spatial discontinuity in the simulated area. Several forest management regimes were simulated as well as forest wildfires and climate changes. The greatest carbon and nitrogen accumulations were observed for the regime without cuttings. It was shown that cuttings and wildfires strongly influence the processes of carbon and nitrogen accumulations in both soil and forest vegetation. Modelling also showed that the increase in annual average temperatures resulted in the partial relocation of carbon and nitrogen stocks from soil to plant biomass. However, forest management, particularly harvesting, has a greater effect on the dynamics of forest ecosystems than the prescribed climate change.     

The trait contribution to wood decomposition rates of 15 Neotropical tree species

The decomposition of dead wood is a critical uncertainty in models of the global carbon cycle. Despite this, relatively few studies have focused on dead wood decomposition, with a strong bias to higher latitudes. Especially the effect of interspecific variation in species traits on differences in wood decomposition rates remains unknown. In order to fill these gaps, we applied a novel method to study long-term wood decomposition of 15 tree species in a Bolivian semi-evergreen tropical moist forest. We hypothesized that interspecific differences in species traits are important drivers of variation in wood decomposition rates. Wood decomposition rates (fractional mass loss) varied between 0.01 and 0.31 yr(-1). We measured 10 different chemical, anatomical, and morphological traits for all species. The species' average traits were useful predictors of wood decomposition rates, particularly the average diameter (dbh) of the tree species (R2 = 0.41). Lignin concentration further increased the proportion of explained inter-specific variation in wood decomposition (both negative relations, cumulative R2 = 0.55), although it did not significantly explain variation in wood decomposition rates if considered alone. When dbh values of the actual dead trees sampled for decomposition rate determination were used as a predictor variable, the final model (including dead tree dbh and lignin concentration) explained even more variation in wood decomposition rates (R2 = 0.71), underlining the importance of dbh in wood decomposition. Other traits, including wood density, wood anatomical traits, macronutrient concentrations, and the amount of phenolic extractives could not significantly explain the variation in wood decomposition rates. The surprising results of this multi-species study, in which for the first time a large set of traits is explicitly linked to wood decomposition rates, merits further testing in other forest ecosystems.     

Incubation time, functional litter diversity, and habitat characteristics predict litter-mixing effects on decomposition

Plant diversity influences many fundamental ecosystem functions, including carbon and nutrient dynamics, during litter breakdown. Mixing different litter species causes litter mixtures to lose mass at different rates than expected from component species incubated in isolation. Such nonadditive litter-mixing effects on breakdown processes often occur idiosyncratically because their direction and magnitude change with incubation time, litter species composition, and ecosystem characteristics. Taking advantage of results from 18 litter mixture experiments in streams, we examined whether the direction and magnitude of nonadditive mixing effects are randomly determined. Across 171 tested litter mixtures and 510 incubation time-by-mixture combinations, nonadditive effects on breakdown were common and on average resulted in slightly faster decomposition than expected. In addition, we found that the magnitude of nonadditive effects and the relative balance of positive and negative responses in mixtures change predictably over time, and both were related to an index of functional litter diversity and selected environmental characteristics. Based on these, it should be expected that nonadditive effects are stronger for litter mixtures made of functionally dissimilar species especially in smaller streams. Our findings demonstrate that effects of litter diversity on plant mixture breakdown are more predictable than generally thought. We further argue that the consequences of current worldwide homogenization in the composition of plant traits on carbon and nutrient dynamics could be better inferred from long-duration experiments that manipulate both functional litter diversity and ecosystem characteristics in "hotspots of biodiversity effects," such as small streams.     

Chemical composition of rivers and streams in the forest and savanna zones of Nigeria

The aim of this study is to investigate the differences in the chemical conditions of lotic waterbodies in the two major ecosystems in Nigeria, the forest and savanna zones. The forest waters were slightly acidic (mean±SD pH = 6.72±0.58) while the savanna waters were slightly alkaline (pH = 7.11±0.33). The cationic order of dominance in the forest waters was Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ in contrast to Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ in savanna waters. The forest waters were chlorided (typical of coastal and/or marine waters) whereas the savanna waters were carbonated in nature, typical of the worldwide freshwater. Organic carbon was significantly higher in forest waters than in the savanna waters (p < 0.05) while nutrient compounds were significantly higher in savanna waters than in forest waters. The seasonal variation of the chemical parameters was generally more evident in savanna than in forest waters. The differences in water quality between the two major vegetation zones reflect the differences in the biogeochemical processes and nutrient cycling that characterise forest and savanna ecosystems.     

A simulation model to evaluate the impacts of invasive earthworms on soil carbon dynamics

Recent studies have reported that earthworm invasions alter native communities and impact nutrient cycling in terrestrial ecosystems. We developed a simulation model to evaluate the potential impacts of earthworm invasions on carbon dynamics, taking into consideration earthworm feeding strategies and priming effects on the microorganisms through their casting activities. Responses of carbon stocks (forest litter, soil organic matter, microbial biomass and earthworm populations) and carbon fluxes (litter decomposition, earthworm consumption, and microbial respiration) were used to evaluate an earthworm invasion of a forest ecosystem. Data from a northern temperate forest (Arnot Forest, New York) were adapted for model calibration and evaluation. Simulation results suggest that the impact and outcome of earthworm invasions are affected by pre-invasion resource availability (litter and soil organic matter), invasive earthworm assemblages (particularly feeding strategy), and invasion history (associated with earthworm population dynamics). The abovementioned factors may also determine invasion progress of earthworm species. The accuracy of the model could be improved by the addition of environmental modules (e.g., soil water regimes), precise parameters accounting for individual species attributes under different environmental conditions (e.g. utilization ability of different types of food resources), as well as earthworm population dynamics (size and structure) and interactions with predators and other invasive/indigenous species during the invasion progress. Such an earthworm invasion model could provide valuable evaluation of the complicated responses of carbon dynamics to earthworm invasions in a range of forest ecosystems, particularly under global change scenarios.     

Riparian forest composition affects stream litter decomposition despite similar microbial and invertebrate communities

Cross-boundary flows of energy and nutrients link biodiversity and functioning in adjacent ecosystems. The composition of forest tree species can affect the structure and functioning of stream ecosystems due to physical and chemical attributes, as well as changes in terrestrial resource subsidies. We examined how variation in riparian canopy composition (coniferous, deciduous, mixed) affects adjacent trophic levels (invertebrate and microbial consumers) and decomposition of organic matter in small, coastal rainforest streams in southwestern British Columbia. Breakdown rates of higher-quality red alder (Alnus rubra) litter were faster in streams with a greater percentage of deciduous than coniferous riparian canopy, whereas breakdown rates of lower-quality western hemlock (Tsuga heterophylla) litter were independent of riparian forest composition. When invertebrates were excluded using fine mesh, breakdown rates of both litter species were an order of magnitude less and were not significantly affected by riparian forest composition. Stream invertebrate and microbial communities were similar among riparian forest composition, with most variation attributed to leaf litter species. Invertebrate taxa richness and shredder biomass were higher in A. rubra litter; however, taxa evenness was greatest for T. heterophylla litter and both litter species in coniferous streams. Microbial community diversity (determined from terminal restriction fragment length polymorphisms) was unaffected by riparian forest or litter species. Fungal allele richness was higher than bacterial allele richness, and microbial communities associated with lower-quality T. heterophylla litter had higher diversity (allele uniqueness and richness) than those associated with higher-quality A. rubra litter. Percent variation in breakdown rates was mostly attributed to riparian forest composition in the presence of invertebrates and microbes; however, stream consumer biodiversity at adjacent trophic levels did not explain these patterns. Riparian and stream ecosystems and their biotic communities are linked through exchange and decomposition of detrital resources, and we provide evidence that riparian forest composition affects stream ecosystem catabolism despite similarities in microbial and invertebrate communities.     

Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.     

亚热带6种针叶和阔叶树种凋落叶分解比较

针叶和阔叶树种分别代表了不同的生活型,其凋落叶片具有不同的分解速率.应用分解网袋法,在中国亚热带地区,选取了具有代表性的3种针叶树种(马尾松Pinus massoniana、水杉Metasequoia glyptostroboides和杉木Cunninghamia lanceolata)和3种阔叶树种(木荷Schima superb、乐昌含笑Michelis chapensis和青冈Cyclobalanopsisgtauca)的凋落叶,放置于样地杭州千岛湖的林地中,经过1 a的分解实验,分析不同类型树种凋落叶的分解特征.6种树种凋落叶质量损失过程基本符合Olson指数模型,其中,3种针叶树种(马尾松、水杉和杉木)凋落叶的分解系数k值(分别为0.51、0.30和0.44),明显小于3种阔叶树种(木荷、乐昌含笑和青冈)凋落叶的分解系数k值(分别为0.55、1.12和0.66);同时,针叶树种(马尾松、水杉和杉木)凋落叶分解50%和95%所需时间(分别为1.36、2.31、1.78 a和5.87、9.99、7.68 a),大于阔叶树种(木荷、乐昌含笑和青冈)凋落叶的分解时间(分别为1.26、0.62、1.05 a和5.45、2.68、4.54a).多元回归分析表明,凋落物分解系数与初始钾元素含量显著相关(P<0.05).一元线性回归分析表明,凋落物的分解系数与初始钾元素和初始木质素含量均具有显著性差异(P<0.05).亚热带地区针、阔叶树种凋落叶分解的差异与自身质量密切相关,其中初始木质素与钾元素含量是控制凋落物分解的主要因素.图2表2参23