Potential effects of leaf litter on water quality in urban watersheds

Leaf litter plays a critical role in regulating ecological functions in headwater forest streams, whereas the effects of leaves on water quality in urbanized streams are not fully understood. This study examined the potential importance of leaf litter for the release and transformations of organic carbon and nutrients in urban streams, and compared the effects with other types of natural organic substrates (periphyton and stream sediment). Nutrients and organic carbon were leached from senescent leaves of 6 tree species in the laboratory with deionized water, and maximal releases, leaching rate constants, composition and bioavailability of the leached dissolved organic carbon (DOC) were determined. Stream substrates (leaf debris, rocks with periphyton, and sediment) were seasonally collected from urban and forest reference streams of the NSF Baltimore Long-term Ecological Research Site and incubated with overlying stream water to estimate areal fluxes of DOC and nitrogen. Leaf litter leaching showed large ranges in maximal releases of DOC (7.0–131 mg g^?1), dissolved organic nitrogen (DON; 0.07–1.39 mg g^?1) and total dissolved phosphorus (TDP; 0.14–0.70 mg g^?1) among tree species. DOC leaching rate constants, carbon to nitrogen ratios, and DOC bioavailability were all correlated with organic matter quality indicated by fluorescence spectroscopy. Results from substrate incubation experiments showed far higher DOC and DON release and nitrate retention with leaf debris than with sediment, or rocks with periphyton. DOC release from leaf debris was positively correlated with stream nitrate retention at residential and urban sites, with the highest values observed during the fall and lowest during the summer. This study suggests the potential importance of leaf litter quantity and quality on fostering DOC and nutrient release and transformations in urban streams. It also suggests that species-specific impacts of leaves should be considered in riparian buffer and stream restoration strategies.     

Effect of dissolved organic carbon quality on microbial decomposition and nitrification rates in stream sediments

1. Microbial decomposition of dissolved organic carbon (DOC) contributes to overall stream metabolism and can influence many processes in the nitrogen cycle, including nitrification. Little is known, however, about the relative decomposition rates of different DOC sources and their subsequent effect on nitrification. 2. In this study, labile fraction and overall microbial decomposition of DOC were measured for leaf leachates from 18 temperate forest tree species. Between 61 and 82% (mean, 75%) of the DOC was metabolized in 24 days. Significant differences among leachates were found for labile fraction rates (P < 0.0001) but not for overall rates (P=0.088). 3. Nitrification rates in stream sediments were determined after addition of 10 mg C L^–1 of each leachate. Nitrification rates ranged from below detection to 0.49 μg N mL sediment^–1 day^–1 and were significantly correlated with two independent measures of leachate DOC quality, overall microbial decomposition rate (r=–0.594, P=0.0093) and specific ultraviolet absorbance (r=0.469, P=0.0497). Both correlations suggest that nitrification rates were lower in the presence of higher quality carbon. 4. Nitrification rates in sediments also were measured after additions of four leachates and glucose at three carbon concentrations (10, 30, and 50 mg C L^–1). For all carbon sources, nitrification rates decreased as carbon concentration increased. Glucose and white pine leachate most strongly depressed nitrification. Glucose likely increased the metabolism of heterotrophic bacteria, which then out‐competed nitrifying bacteria for NH₄⁺. White pine leachate probably increased heterotrophic metabolism and directly inhibited nitrification by allelopathy.     

Leaf Litter Fuels Methanogenesis Throughout Decomposition in a Forested Peatland

Decomposing leaf litter is a large supply of energy and nutrients for soil microorganisms. How long decaying leaves continue to fuel anaerobic microbial activity in wetland ecosystems is poorly understood. Here, we compare leaf litter from 15 tree species with different growth forms (angiosperms and gymnosperms, deciduous, and longer life span), using litterbags positioned for up to 4 years in a forested peatland in New York State. Periodically, we incubated partially decayed residue per species with fresh soil to assess its ability to fuel microbial methane (CH₄) production and concomitant anaerobic carbon dioxide (CO₂) production. Decay rates varied by leaf type: deciduous angiosperm > evergreen gymnosperm > deciduous gymnosperm. Decay rates were slower in leaf litter with a large concentration of lignin. Soil with residue of leaves decomposed for 338 days had greater rates of CH₄ production (5.8 µmol g^?1 dry mass d^?1) than less decomposed (<0.42 µmol g^?1 dry mass d^?1) or more decomposed (2.1 µmol g^?1 dry mass d^?1) leaf residue. Species-driven differences in their ability to fuel CH₄ production were evident throughout the study, whereas concomitant rates of CO₂ production were more similar among species and declined with degree of decomposition. Methane production rates exhibited a positive correlation with pectin and the rate of pectin decomposition. This link between leaf litter decay rates, biochemical components in leaves, and microorganisms producing greenhouse gases should improve predictions of CH₄ production in wetlands.     

Decomposition of Fire Exposed Eucalyptus Leaves in a Portuguese Lowland Stream

We compared fire exposed with normal abscised eucalyptus leaves incubated in a stream running through eucalyptus plantations in central Portugal, in terms of breakdown rates, microbial activity, diversity and macroinvertebrate abundance. Although leaves exposed to fire had lower nutritional value, mass loss was similar for both leaf types (k = 0.0089–0.0095 d^–1 for fire and k = 0.0084–0.00103 d^–1 for normal leaves). Fungal biomass was similar among treatments, whereas sporulation and microbial respiration were lower in fire exposed leaves. Both leaf types had similar aquatic hyphomycetes communities. Physical fragmentation was important in fire exposed leaves breakdown. Invertebrates colonized leaves in low numbers in both treatments. Alteration of leaf litter quality determined by fires in streams does not seem to determine changes in ecosystem functioning in a short term. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microbial activity under the ice cover of the shallow Neusiedler See (Austria, Central Europe)

In an attempt to assess bacterioplankton production and growth yieldunder low temperature conditions and to compare bacterioplankton withphytoplankton production in the ice-covered water column of the shallowNeusiedler See, outdoor measurements under near in situ conditions wereperformed during the winter of 1995/96. During the investigation period,mean chlorophyll (Chl) a concentration was 21.03 ± 14.95 µg Chla l^-1. Phytoplankton primary production integrated over thewater column ranged from 1.35 to production integrated over the water columnranged from 1.35 to 4.23 mg C m^-2 d^-1 (mean± SD = 2.46 ± 1.06 mg C m^-2d^-1). Bacterial abundance varied from 20 to 40×10⁵ ml^-1 for most of the investigationperiod and increased by the end of March concomitantly with the increase intemperature from 1.3 to 6.3 °C within 5 days. Mean bacterial productionwas 15.3 ± 12.8 µg C l^-1 d^-1(range: 3.0 to 41.7 µg C l^-1 d^-1) and meanbacterial growth rate 0.23 ± 0.16 d^-1 following closelythe pattern in bacterial production. DOC concentration declined linearlyfrom 20.7 mg C l^-1 to 16.45 mg C l^-1 over the 4months period of ice cover. The contribution of humic substances to thetotal DOC pool declined from 43.6% at the end of November to37.3% at the end of March. Calculated on an area basis, phytoplanktonproduction amounted to only 16% of bacterial production which makesit unlikely that phytoplankton supply substrate for bacterioplankton growthin significant quantities when the lake is ice covered. From the observeddecline in DOC over the investigation period and assuming only negligibleinput of DOC from other sources we calculated an average DOC uptake by thebacterioplankton community of 47.5 µg C l^-1d^-1 resulting in a bacterial growth efficiency of 15.9%for the ice covered conditions. Based on the growth efficiency we estimatethat pelagic primary production amounts to 2.8% of the bacterialcarbon demand. This might indicate that the bacterioplankton in NeusiedlerSee sustain their high growth rates at low temperatures (<2°C formost of the investigation period) by using probably the DOC originating fromthe previous season. This DOM stems most likely from the decay of the reedPhragmites australis and its epiphytes and, probably of minor importance,from phytoplankton leachates.     

杭州西湖北里湖荷叶枯落物分解及其对水环境的影响

湖泊水生植物枯落物的分解过程影响着枯落物的淤积以及营养元素向水体和底泥的释放,进而影响湖泊水环境。用分解袋法研究了杭州西湖北里湖荷叶枯落物的分解速率和营养动态。荷叶枯落物的分解速率表现快慢交替的阶段性特点,分解速率常数峰值出现在6月,其余时段呈现波动状态。枯落物氮、磷含量变化趋势基本一致,呈现下降-逐渐上升-渐趋稳定的变化特点。氮、磷积累指数(NAI)呈现释放-积累-释放,整体以释放为主的变化特征。在此基础上推算了北里湖荷叶枯落物一个分解周期残留量的变化及氮、磷释放情况,探讨枯落物分解对水环境的影响。荷叶枯落物入湖量以立枯体总量的20%估算,残留量从11月至翌年1月急剧增加,在1月底达到峰值,随后持续下降,至10月底尚残留1.675 t干重,合30.45 kg/m²干重(按荷塘面积计算),与残留量峰值相比减少了74.39%。枯落物分解导致的氮、磷释放在11月至翌年1月持续增高,2、3月份释放量趋于下降,4、5月份出现净积累,6-10月保持净释放。整个分解周期氮、磷总释放分别为92.247 kg和6.421 kg,相当于北里湖水中氮、磷含量分别增加0.143 mg/L和0.010 mg/L。由于挺水植物生长过程吸收的主要是沉积物中的氮、磷营养盐,因此,荷花的生长和枯落分解过程具有促进氮、磷从沉积物迁移到水体,增加水中氮、磷含量的作用。     

Strong seasonal patterns of DOC release by a temperate seaweed community: Implications for the coastal ocean carbon cycle

Release of dissolved organic carbon (DOC) by seaweed underpins the microbial food web and is crucial for the coastal ocean carbon cycle. However, we know relatively little of seasonal DOC release patterns in temperate regions of the southern hemisphere. Strong seasonal changes in inorganic nitrogen availability, irradiance, and temperature regulate the growth of seaweeds on temperate reefs and influence DOC release. We seasonally surveyed and sampled seaweed at Coal Point, Tasmania, over 1 year. Dominant species with or without carbon dioxide (CO₂) concentrating mechanisms (CCMs) were collected for laboratory experiments to determine seasonal rates of DOC release. During spring and summer, substantial DOC release (10.06–33.54 μmol C · g DW⁻¹ · h⁻¹) was observed for all species, between 3 and 27 times greater than during autumn and winter. Our results suggest that inorganic carbon (C_i) uptake strategy does not regulate DOC release. Seasonal patterns of DOC release were likely a result of photosynthetic overflow during periods of high gross photosynthesis indicated by variations in tissue C:N ratios. For each season, we calculated a reef-scale net DOC release for seaweed at Coal Point of 7.84–12.9 g C · m⁻² · d⁻¹ in spring and summer, which was ~16 times greater than in autumn and winter (0.2–1.0 g C · m⁻² · d⁻¹). Phyllospora comosa, which dominated the biomass, contributed the most DOC to the coastal ocean, up to ~14 times more than Ecklonia radiata and the understory assemblage combined. Reef-scale DOC release was driven by seasonal changes in seaweed physiology rather than seaweed biomass.     

针阔凋落叶混合分解过程中可溶性有机碳释放的动态特征

为了调整低效马尾松(Pinus massoniana, P)人工纯林的林分结构,探明其与乡土阔叶树种凋落叶混合分解过程中的可溶性有机碳(DOC)释放规律,该研究以马尾松、香樟(Cinnamomum camphora, C)和香椿(Toona sinensis, T)的凋落叶为研究对象,将其按照不同树种和质量比例组合为15个处理(3个单一树种处理 + 12个混合处理)后进行野外凋落叶分解实验,并探讨DOC释放最佳的凋落叶树种组合以及混合比例。结果表明:(1)马尾松和大部分混合处理凋落叶(除了PT64)在分解初期(0～6个月)的DOC含量均显著升高,出现富集现象,之后随着分解时间的延长而降低,在分解中后期(12～18个月)或分解末期(18～24个月)再次出现小幅度的碳富集现象。阔叶所占比例越高,其后期DOC含量越低。(2)分解前期(0～6个月)凋落叶DOC释放的拮抗效应较强(58.33%),仅有8.33%(1/12)的混合处理表现出协同效应。之后(6～18个月)其协同效应逐渐增强(91.67%),分解末期(18～24个月)凋落叶的协同效应有所减弱(66.67%)。在所有混合处理中,PT64在整个分解期间均出现协同效应,其次为PT73、PCT622和PCT613在大部分分解时期(3/4)出现协同效应。(3)偏最小二乘法(PLS)回归分析表明,凋落叶初始质量因子中N含量、P含量、木质素含量、缩合单宁含量、C/N、C/P、木质素/N以及木质素/P是影响该研究区域中凋落物DOC释放的重要因素。总体而言,马尾松与阔叶凋落叶混合后的DOC释放受到树种、混合比例及分解时间的共同影响。相对于其他混合处理,阔叶占比大于等于30.00%且含有香椿(T)的凋落叶混合处理(PT64、PT73、PCT622和PCT613)更能促进DOC的释放。     

Nutrient limitation to the decomposition of water hyacinth ( Eichhornia crassipes)

The responses of decomposition to N and P supply were investigated in three leaf types of water hyacinth (Eichhornia crassipes (Mart.) Solms): dead green leaves collected from Donghu Lake; green, and brown leaves collected from outdoor tanks. The ratios of C:N, C:P, lignin:N and lignin:P were lowest in the green leaves collected from Donghu Lake, and highest in the brown leaves collected from outdoor tanks. Decomposition constant (k) of water hyacinth varied greatly, ranged from 0.006 to 0.099 d^–1. Leaf litters decayed most quickly within the initial two weeks during the experimental period, but decomposition rate decreased significantly in the following days. Decomposition and nutrient (N and P) release were fastest in the green leaves collected from Donghu Lake, intermediate in the green leaves collected from outdoor tanks, slowest in the brown leaves collected from outdoor tanks. Statistical analyses revealed that the effects of P-availability on decomposition rate and N, P release rate of the three litter types were significant, whereas the impacts of N-availability was insignificant (p > 0.05) except for the brown leaves collected from outdoor tanks. These results suggest that decomposition rate and nutrient content dynamics of water hyacinth differ with their growth habitats, and could partly be regulated by nutrient availability, especially by P-availability, in the environments.     

Correlation of foliage and litter chemistry of sugar maple, Acer saccharum, as affected by elevated CO2 and varying N availability, and effects on decomposition

Rising atmospheric carbon dioxide has the potential to alter leaf litter chemistry, potentially affecting decomposition and rates of carbon and nitrogen cycling in forest ecosystems. This study was conducted to determine whether growth under elevated atmospheric CO₂ altered the quality and microbial decomposition of leaf litter of a widely distributed northern hardwood species at sites of low and high soil nitrogen availability. In addition, we assessed whether the carbon–nutrient balance (CNB) and growth differentiation balance (GDB) hypotheses could be extended to predict changes in litter quality in response to resource availability. Sugar maple (Acer saccharum) was grown in the field in open‐top chambers at 36 and 55 Pa partial pressure CO₂, and initial soil mineralization rates of 45 and 348 μg N g^?1 d^?1. Naturally senesced leaf litter was assessed for chemical composition and incubated in the laboratory for 111 d. Microbial respiration and the production of dissolved organic carbon (DOC) were quantified as estimates of decomposition. Elevated CO₂ and low soil nitrogen resulted in higher litter concentrations of nonstructural carbohydrates and condensed tannins, higher C/N ratios and lower N concentrations. Soil N availability appears to have had a greater effect on litter quality than did atmospheric CO₂, although the treatments were additive, with highest concentrations of nonstructural carbohydrates and condensed tannins occurring under elevated CO₂–low soil N. Rates of microbial respiration and the production of DOC were insensitive to differences in litter quality. In general, concentrations of litter constituents, except for starch, were highly correlated to those in live foliage, and the CNB/GDB hypotheses proved useful in predicting changes in litter quality. We conclude the chemical composition of sugar maple litter will change in the future in response to rising atmospheric CO₂, and that soil N availability will exert a major control. It appears that microbial metabolism will not be directly affected by changes in litter quality, although conclusions regarding decomposition as a whole must consider the entire soil food web.     

Relationship between fresh leaf traits and leaf litter decomposition of 20 plant species in Kerqin sandy land, China

20 plant species (10 monocots and 10 dicots) grown in Kerqin sandy grassland were incubated under indoor conditions to monitor the amount and rate of CO₂ release from the leaf litter. 11 traits of mature fresh leaves including caloric value, contents of Mg, P, N, K, C, C/N, N/P, specific leaf area, dry matter content and leaf surface area were measured to determine the relationship between CO₂ release and leaf characteristics. All those traits have great variation among the 20 species with over 3 fold differences between the maximum and minimum values, and a few traits such as leaf Mg content reached as high as 9 folds. After 28 d's incubation, the average CO₂ release amount from all the species was (4121 ± 1713) μg kg^?1 dry soil. The highest level from Chenopodium acuminatum was (8767 ± 177) μg kg^?1 dry soil, which was 5 folds higher than the lowest level ((1669 ± 47)μg kg^?1 dry soil) from Digitaria sanguinalis. However, CO₂ release rate showed the same trend in all the 20 species, i.e., the leaf litter decomposed faster initially (0–4 d), and gradually slowed down during extended cultural periods. Comparison between monocots and dicots showed that these two taxonomic groups had significant differences in terms of the amount and rate of CO₂ released from leaf litter, and N and C contents, leaf C/N, and dry matter content of mature leaves. Contents of N, C and dry matter, and C/N of mature leaves are significantly correlated with CO₂ release from leaf litter decomposition, which has been revealed by the Pearson correlation test. It can be concluded that these three traits of mature leaves can be used indirectly to predict decomposition rate of the leaf litter.     

Decomposition pathways of 13C-depleted leaf litter in forest soils of the Swiss Jura

Decomposition of leaf litter and its incorporation into the mineral soil are key components of the C cycle in forest soils. In a ¹³C tracer experiment, we quantified the pathways of C from decomposing leaf litter in calcareous soils of a mixed beech forest in the Swiss Jura. Moreover, we assessed how important the cold season is for the decomposition of freshly fallen leaves. The annual C loss from the litter layer of 69–77% resulted mainly from the C mineralization (29–34% of the initial litter C) and from the transfer of litter material to the deeper mineral soil (>4 cm) by soil fauna (30%). Although only 4–5% of the initial litter C was leached as dissolved organic carbon (DOC), this pathway could be important for the C sequestration in soils in the long term: The DOC leached from the litter layer was mostly retained (95%) in the first 5 cm of the mineral soil by both physico-chemical sorption and biodegradation and, thus, it might have contributed significantly to the litter-derived C recovered in the heavy fraction (>1.6 g cm^?3) at 0–4 cm depth (4% of the initial litter C). About 80% of the annual DOC leaching from the litter layer occurred during the cold season (Nov–April) due to an initial DOC flush of water-soluble substances. In contrast, the litter mineralization in winter accounted for only 25% of the annual C losses through CO₂ release from the labelled litter. Nevertheless, the highest contributions (45–60%) of litter decay to the heterotrophic soil respiration were observed on warm winter days when the mineral soil was still cold and the labile litter pool only partly mineralized. Our ¹³C tracing also revealed that: (1) the fresh litter C only marginally primed the mineralization of older SOM (>1 year); and (2) non-litter C, such as throughfall DOC, contributed significantly to the C fluxes from the litter layer since the microbial biomass and the DOC leached from the litter layer contained 20–30% and up to 60% of unlabelled C, respectively. In summary, our study shows that significant amounts of recent leaf litter C (<1 year) are incorporated into mineral soils and that the cold season is clearly less important for the litter turnover than the warm season in this beech forest ecosystem.     

Decomposition of leaf litter of Dombeya goetzenii in the Njoro River, Kenya

Decomposition of the leaves of Dombeya goetzenii (K. Schum) in the Njoro River is described and analysed. The loss of the ash-free dry mass was rapid during the first 14 d of exposure in the wet and humid zones. The leaves in the litter bags in the humid and wet zones were processed at a rate (±SD) of 0.005±0.001 d^-1 and 0.021±0.001 d^-1, respectively. The processing rates of the leaves in the wet zone differed significantly from those observed in the humid zone (t-value, p<0.05). The interchanged litter bags (i.e. from wet to humid zones vice versa) showed that the processing rates of the leaves in the litter bags interchanged from the wet zone to the humid zone was about 200 times lower than that of the leaves retained in the wet zone throughout; fourfold higher in the leaves in the litter bags which were transferred to the wet zone than in those leaves of the litter bags which were retained in the humid zone throughout the experiment. It took approximately 38 months for 90% of the leaf dry mass to be processed in the humid zone whilst it took 4 months for a similar percentage to be processed in the wet zone. It is concluded that the immersion and emersion of leaf litter, which may occur in the wet and humid zones, respectively, are important aspects of the decomposition process which may influence the quantity of nutrients in stream ecosystems.     

Relative rates of delivery of xylem solute to shoot tissues: Possible relationship to sequential leaf senescence

The rates of delivery of regulatory solutes such as cytokinins and mineral ions from the roots to competing shoot tissues can influence rates of metabolism and development. A 15 min pulse of a synthetic xylem mobile and phloem-immobile solute, acid fuchsin, was used to quantify relative rates of solute delivery to competing organs on excised transpiring bean shoots (Phaseolus vulgaris L. cv. Contender) at different stages of development. Stem, flower and fruit tissues received comparatively low rates of solute delivery. The relative rate of solute delivery to newly opened leaves was initially low, but increased during rapid leaf expansion and then declined progressively as the leaves exceeded 70% of their final area. The relative rate of solute delivery to tissues of the basal primary leaves declined progressively from 2 weeks onwards. This decline appeared to be caused by progressive internally regulated increases in both stomatal resistances and hydraulic resistances to xylem flow up to and into the leaf blade. Thus combined abaxial and adaxial stomatal resistance values in the primary leaves (Rs) increased from 3 to ≥ 7 s cm^?1 between 2 and 5 weeks. Similarly, mean values for the connection resistances (Rc) to hydraulic flow into the primary leaves rose from 7 to 13 TPa · s · m^?1 between 2 and 4 weeks. In the same period pathway resistance from stem to primary leaf petioles (Rp), as determined by direct pressure flow assay, increased from 7 to 15 TPa · s · m^?1. Senescence-associated declines in protein and chlorophyll levels in the primary leaves were initiated in parallel with, or after, declines in relative rates of solute delivery. The provision of extra illumination at the basal leaf level between 2 and 5 weeks did not prevent declines in chlorophyll and soluble protein or increases in stomatal resistance. We suggest that internally programmed changes in the hydraulic architecture of the plant progressively divert xylem-transported root supplies of nutrients and cytokinins from basal to more apical leaves and thus regulate the progressive senescence of leaves from the base upwards.     

Litterfall dynamics in a mixed conifer-angiosperm forest in northern New Zealand

Litterfall in a mixed conifer-angiosperm temperate forest in northern New Zealand was traced for 5 years to determine the patterns of litter production and turnover for conifer and angiosperm components of the forest. Basal area and above-ground biomass was shared approximately equally between conifer (mostly Agathis australis; New Zealand kauri) and angiosperm species (plus tree ferns). The five-year mean annual litterfall, excluding macro-litter, was 7.76± 0.39(SEM) t ha^?1 and ranged from 6.77±0.70 t ha^?1 in 1983–4 to 8.79±1.00 t ha^?1 in 1987–8. Mean monthly litterfall showed a strong seasonal pattern with low rates in winter and early spring, increasing to a peak in early autumn. There were major differences in the nature and timing of litterfall between the conifer and angiosperm fractions. Angiosperm leaf litter reached a maximum in early summer, while conifer litterfall showed highest rates for leaves, twigs and cone scales in late summer-autumn. Conifer reproductive structures (strobili and cone scales) contributed from 13 to 21% of total litterfall, a value high relative to other temperate forests. However, conifer leaf turnover was low relative to that for the angiosperms. Size of the microlitter store was 16.16±1.97 t ha^?1 prior to conifer cone fall, and 18.70±2.02 t ha^?1 following it, and conifer litter made up 76–78% of the total litter store. The estimated mean annual decomposition constant, k, was 0.39 overall, 0.33 for conifer leaf litter and 0.71 for angiosperm leaf litter, values which agree well with previously published rates for decomposition in this forest stand. Differences in the costs of biomass production and rates of turnover, as measured by litterfall and decomposition, may help to explain the functional coexistence of conifers and angiosperms in mixed forests.     

不同水分条件下杨树-玉米复合系统凋落物分解特性

为探讨水分变化对农林复合生态系统凋落物分解特性的影响,以河西走廊杨树（Populus）-玉米（Zea mays）凋落物为研究对象,设置正常水分（9200 m³/hm²,对照）,轻度干旱胁迫（减少15%,7800 m³/hm²）,中度干旱胁迫（减少30%,6400 m³/hm²）3种不同水分处理条件,采用分解袋法研究了不同水分条件下杨树叶和玉米秸秆的质量残留率、分解速率和养分含量变化特征。结果表明：（1）随着干旱胁迫的加剧,两种凋落物的质量残留率均增加,而分解速率降低。经过164 d的分解后,杨树叶和玉米秸秆的质量残留率分别为70.43%-77.49%、63.55%-68.29%。分析表明：水分和时间对各类型凋落物的质量残留率均有极显著的影响（P<0.001）,但二者的交互作用不显著（P>0.05）;干旱胁迫显著降低了玉米秸秆的分解速率,但杨树叶的分解速率却只是在中度干旱胁迫下显著降低（P<0.05）。对于不同类型凋落物而言,分解速率表现为玉米秸秆>杨树叶。（2）两种类型凋落物的氮（N）残留率在分解过程中表现为降低的趋势,但随着干旱程度的加大,N的残留率增加,表明水分抑制了N的释放过程。分解164d后,同一类型凋落物不同水分条件下的N残留率均存在显著差异。对于同一水分条件下不同凋落物而言,玉米秸秆的N残留率最低,而杨树叶最高。总的来说,水分降低对干旱区农林复合系统内凋落物的分解和氮元素含量具有显著的抑制作用。     

Instream release of dissolved organic matter from coarse and fine particulate organic matter of different origins

Dissolved organic matter (DOM), produced through leaching from particulate organic matter (POM), is an essential component of the carbon cycle in streams. The present study investigated the instream DOM release from POM, varying in size and chemical quality. We produced large and medium sized fine particulate organic matter (L-FPOM, 250–500 μm; M-FPOM, 100–250 μm) of defined quality by feeding five types of coarse particulate organic matter (CPOM) to shredding amphipods (Gammarus spp.). Microscopic observations showed that L-FPOM and M-FPOM mainly consisted of the fecal pellets of amphipods, and incompletely eaten plant fragments, respectively. DOM release experiments were conducted by exposing CPOM and M- and L-FPOM fractions in natural stream water over a two week period. For CPOM, the release of dissolved organic carbon (DOC) by leaching was highest during the first 6 h (3.64–23.9 mg C g C^?1 h^?1) and decreased rapidly afterwards. For M- and L-FPOM, the DOC release remained low during the entire study period (range: 0.008–0.15 mg C g C^?1 h^?1). Two-way ANOVA revealed that the DOC release rate significantly differed with POM source and size fraction, both at day 1 and after a week of exposure. Multiple regression analyses revealed a significant correlation of elemental contents and lignin content to DOC release rate after a week of exposure. Overall, the results indicated that DOC release rate of FPOM, on a carbon basis, is comparable to that of CPOM after leaching, while size and source of POM significantly affect DOC release rate.     

Bacterial Colonization and Ectoenzymatic Activity in Phytoplankton-Derived Model Particles. Part II. Cleavage and Uptake of Carbohydrates

Abstract The bacterial colonization and development of the ectoenzymatic glucosidase activity and glucose uptake were followed together with bacterial growth (measured as thymidine incorporation) in laboratory experiments, using phytoplankton-derived particles incubated in rolling tanks. Bacterial colonization of the particles was rapid. In the particles, bacterial turnover rates (production/biomass) were low (0.02 to 0.14 d⁻¹). In the ambient water, turnover rates increased from 0.1 d⁻¹ to 23.3 d⁻¹, until the end of the experiment. In the control, lacking any particles, turnover of bacteria ranged from 0.3 to 7.6 d⁻¹. Similarly, glucose uptake rates, per bacterium, were 1 to 2 orders of magnitude lower for particle-attached bacteria than for their free-living counterparts. Generally, K_m values for glucosidase activity declined, over the incubation period, in particles and free-living bacteria until 168 h, and slightly increased, thereafter, to values of approximately 0.1 μM. Particle-attached bacteria exhibited significantly lower uptake rates of both thymidine and glucose, per bacterium, throughout the incubation. The per-cell ectoenzymatic activity was similar in particle-associated and free-living bacteria during the initial phase of the experiment, but was significantly higher after ≈200 h. Dissolved total (TCHO), as well as monomeric carbohydrates (MCHO), declined continuously in both particles and ambient water; they remained constant in the control; TCHO comprised about 50% of the dissolved organic carbon (DOC) in the particles. In ambient water TCHO contribution to DOC varied, with only one exception, between 25 and 45%; and in the control, between 20 and 50%. The shift detectable in the relation between ectoenzymatic activity and uptake of glucose between free-living and attached bacteria over the incubation period may reflect changes in the physiological status of the bacteria. Received: 3 February 1997; Accepted: 6 November 1997     

SEASONAL PATTERNS OF LEAF PHOTOSYNTHETIC CAPACITY IN SUCCESSIONAL NORTHERN HARDWOOD TREE SPECIES

Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO₂ Exchange Rate (CER) (10–15 μmol m ² s ¹) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4–5 μmol m ² s ¹). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 μmol/mol for species in the understory to 51 and 56 μmol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2–5 mm/s) than leaves in the understory (1–2 mm/s). All species maintained leaf intercellular CO, mole fractions (c,) near 200 uML/L until autumn, when c, increased during leaf senescence.     

QUANTITATIVE PATTERNS OF LEAF EXPANSION: COMPARISON OF NORMAL AND MALFORMED LEAF GROWTH IN VITIS VINIFERA CV. RUBY RED

Developmental patterns produced during normal expansion of the leaf of Vitis vinifera cv. Ruby Red are quantitatively characterized from the distribution of relative growth rates and growth velocity vectors and are compared to patterns produced during the development of an abnormally shaped leaf. The bilateral symmetry of the V. vinifera leaf, which is present in the normal leafand absent in the malformed leaf, is shown during growth by the patterns of velocity isolines. Ellipses formed by the isolines around the midrib during normal development are distorted during development of the malformed leaf. During normal growth, tissue elements are displaced in rather straight lines, resulting in streamlines which radiate outward from the petiole. Element motion in the abnormally developing leaf causes curving of streamlines. Relative growth in area of elements located in an area in the normal leaf are higher than those in the malformed leaf. The most frequently observed category of relative area growth in the normal leaf is 40%-59% d~¹, while 20%–39% d^−-1 predominates in the abnormal leaf. A spatial gradient in growth appears during normal development with lowest relative growth (20%–39% d^−-1) present in the tip region, intermediate values (40%–59% d^−-1) in the midsection, and maximum growth (> 60% d^−-1) appearing in the basal region. During development of the abnormally shaped leaf, the gradient along the midrib is disrupted with low magnitudes of growth (<20% d^−-1) appearing in the midsection of the leaf where intermediate values are expected. The theoretical and numerical distinctions between two common expressions for relative growth (relative elemental growth and exponential growth rate) are discussed. Relative elemental growth is shown to become increasingly larger than the exponential growth rate as the magnitude of growth increases relative to initial size of the tissue element. Numerical methods for evaluating relative growth based on finite element areas are compared to methods based on displacement and velocity gradients and are shown to produce similar results.