The role of dissolved organic matter bioavailability in promoting phytoplankton blooms in Florida Bay

The clear, shallow, oligotrophic waters of Florida Bay are characterized by low phytoplankton biomass, yet periodic cyanobacteria and diatom blooms do occur. We hypothesized that allochthonous dissolved organic matter (DOM) was providing a subsidy to the system in the form of bound nutrients. Water from four bay sites was incubated under natural light and dark conditions with enrichments of either DOM ( > 1 kD, 2×DOM) or inorganic nutrients (N+P). Samples were analyzed for bacterial numbers, bacterial production, phytoplankton biomass, phytoplankton community structure, and production, nutrients, and alkaline phosphatase (AP) activity. The influence of 2×DOM enrichment on phytoplankton biomass developed slowly during the incubations and was relatively small compared to nutrient additions. Inorganic nutrient additions resulted in an ephemeral bloom characterized initially as cyanobacterial and brown algae but which changed to dinoflagellate and/or brown algae by day six. The DIN:TP ratio decreased 10-fold in the N+P treatments as the system progressed towards N limitation. This ratio did not change significantly for 2×DOM treatments. In addition, these experiments indicated that both autotrophic and heterotrophic microbial populations in Florida Bay may fluctuate in their limitation by organic and inorganic nutrient availability. Both N+P and 2×DOM enrichments revealed significant and positive response in bioavailability of dissolved organic carbon (BDOC). Potential BDOC ranged from 1.1 to 35.5%, with the most labile forms occurring in Whipray Basin. BDOC at all sites was stimulated by the 2×DOM addition. Except for Duck Key, BDOC at all sites was also stimulated by the addition of N+P. BDOC was lower in the dry season than in the wet season (5.56% vs. 16.86%). This may be explained by the distinct chemical characteristics of the DOM produced at different times of year. Thus, both the heterotrophic and autotrophic microbial communities in Florida Bay are modulated by bioavailability of DOM. This has ramifications for the fate of DOM from the Everglades inputs, implicating DOM bioavailability as a contributing factor in regulating the onset, persistence, and composition of phytoplankton blooms.     

Nutrients associated with terrestrial dissolved organic matter drive changes in zooplankton:phytoplankton biomass ratios in an alpine lake

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太湖梅梁湾水体悬浮颗粒物和CDOM的吸收特性

通过测定滤膜上悬浮颗粒物和过滤液中CDOM吸光度的方法计算得到太湖梅梁湾总颗粒物和CDOM的光谱吸收系数,并计算了各吸收组份的贡献份额以及吸收与PAR衰减的比值。总颗粒物的吸收系数从400 nm到600 nm大致呈下降趋势,到675nm附近由于叶绿素a的特征吸收会出现明显峰值,峰值高低随叶绿素a浓度的变化而变化,ap(440)在3.58~9.86 m-1间变化。非藻类颗粒物和CDOM的吸收随波长增加大致按指数规律下降,ad(440)和ag(440)的变化范围分别为2.23~7.07m-1和1.06~1.70 m-1。非藻类颗粒物在400~700 nm波段的指数函数斜率Sd的平均值为(10.91±0.62)μm-1;CDOM在280~500 nm波段指数函数斜率Sg的平均值为(15.52±0.49)μm-1。浮游藻类的光谱吸收表现为在440、675 nm附近存在两个明显的峰值,分别为(2.55±1.14)、(1.34±0.69)m-1。ap(440)与TSS、OSSI、SS均存在显著性正相关,而ad(440)则只与TSSI、SS有显著性相关,aph(440)只与OSS、Chla有显著性相关。CDOM吸收系数与DOC浓度没有显著正相关,但与Chla存在显著幂函数关系,浮游藻类降解产物是水体中CDOM的重要来源之一。水体中物质吸收主要以颗粒物为主,对总吸收的贡献率在70%以上,而颗粒物中又以非藻类颗粒物占主导,一般超过40%,总吸收对漫射衰减的贡献也在40%以上。     

Growth response of four freshwater algal species to dissolved organic nitrogen of different concentration and complexity

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Extracellular dissolved organic carbon released from phytoplankton as a source of carbon for heterotrophic bacteria in lakes of different humic content

The quantitative importance of photosynthetically produced dissolved organic carbon (PDOC) released from phytoplankton as a source of carbon for pelagic, heterotrophic bacteria was investigated in four temperate Swedish lakes, of which two had low (≈20 mg Pt 1⁻¹), and two moderately high (60–80 mg Pt 1⁻¹) humic content. The bacterial assimilation of PDOC was estimated with the ¹⁴C method, and the total production of the heterotrophic bacteria was estimated with the [3H]thymidine incorporation method. The release of PDOC from natural communities of phytoplankton was not restricted to periods of photosynthesis, but often continued during periods of darkness. Heterotrophic bacteria often assimilated the labile components of the PDOC at high rates (up to 73% of the released PDOC was assimilated during the incubation in our experiments). The contribution of PDOC to bacterial production exhibited large within-lake seasonal variations, but PDOC was at certain times, both in humic and non-humic lakes, a quantitatively very important carbon source for the heterotrophic bacteria. Under periods of comparatively low primary production, heterotrophic bacteria in humic lakes appear to utilize allochthonous, humic substances as a substrate.     

Light absorption by phytoplankton and chromophoric dissolved organic matter in the drainage basin and estuary of the Neuse River, North Carolina (U.S.A.)

1. We examined the absorption of solar radiation by phytoplankton and chromophoric dissolved organic matter (CDOM) taking into account riparian shading in the rivers, reservoirs, swamps of the Neuse River Estuary and its drainage basin. 2. In the streams, CDOM typically absorbed 55 and 64% of photons in the spectral range of 400–700 nm (photosynthetically active radiation, PAR) and 500–600 nm, respectively. The large proportion of photons absorbed by CDOM indicates high potential for abiotic photochemial reactions in the 500–600 nm region. 3. Despite the high concentration of nutrients, phytoplankton contributed little (2%) to the total absorption of PAR in the streams. Small (<30 m wide) streams typically received only 7% of incident PAR that impinged onto the more exposed reservoirs and estuary. Riparian shading and the low contribution of phytoplankton to the total absorption resulted in conditions where phytoplankton absorbed nearly two orders of magnitude less PAR in the streams than in the estuary and reservoirs. 4. The results indicated that riparian shading and non‐algal absorbing components can significantly restrict phytoplankton production in nutrient‐rich streams with a high concentration of CDOM flowing throughout forested catchments.     

Effects of different molecular weight fractions of dissolved organic matter on the growth of bacteria,algae and protozoa from a highly humic lake

Effects of different molecular size fractions (< 1000 MW, < 10 000 MW, < 100 000 MW and <0.1 μm) of dissolved organic matter (DOM) on the growth of bacteria, algae and protozoa from a highly humic lake were investigated. DOM from catchment drainage water as well as from the lake consisted mostly (59–63%) of high molecular weight (HMW) compounds (> 10 000 MW). With excess inorganic nutrients, the growth rate and yield of bacteria were almost identical in all size fractions. However, in < 1000 MW fractions and with glucose added, a longer lag phase occurred. Without added nutrients both the growth rates and biomasses of bacteria decreased towards the smaller size fractions and the percentage of dissolved organic carbon (DOC) used during the experiment and the growth efficiency of bacteria were lower than with excess nutrients. The growth efficiency of bacteria was estimated to vary between 3–66% in different MW fractions, largely depending on the nutrient concentrations, but the highest growth efficiencies were observed in HMW fractions and with glucose. The growth of algae was clearly lowest in the < 1000 MW fraction. In dim light no net growth of algae could be found. In contrast, added nutrients substantially enhanced algal growth and in deionized water with glucose, algae achieved almost the same growth rate and biomass as in higher MW fractions of DOM. The results suggested that bacteria and some algae were favoured by DOM, but protozoans seemed to benefit only indirectly, through bacterial grazing. The utilization of DOM by bacteria and algae was strongly affected by the availability of phosphorus and nitrogen.     

森林生态系统DOM的来源、特性及流动

可溶性有机物质（Dissolved Organic Matter)是森林生态系统主要的可移动碳库及重要的养分库。系统综述了森林生态系统DOM的来源,组成,性质,季节动态;DOM释放与存留机制及影响因素,森林生态系统DOM的流动及干扰对DOM动态影响等,已有研究表明DOM的森林生态系统C、N、P循环,成土作用,污染物迁移等方面起着重要作用。今后森林生态系统DOM的研究应集中于以几方面：（1）确定森林生态系统中DOM源和汇;（2）评价森林水文条件对DOM释放与存留的调节作用;（3）探讨全球气候变化对森林生态系统DOM的影响;（4）可溶性有机氮（Dissolved Organic Nitrogen),可溶性有机磷（Dissolved Organic Phosphorus)动态与可溶性有碳（Dissolved Organic Carbon）动态的差别。     

The utilization of dissolved organic compounds in aquatic environments

The uptake of dissolved organic matter by bacteria, phytoplankton and invertebrates is discussed referring to the literature. In natural waters the uptake of dissolved organic compounds appears to be primarily a bacterial process. Communication nr. 145 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Vertical transport of dissolved organic C and N under long-term N amendments in pine and hardwood forests

At the Harvard Forest, Massachusetts, a long-term effort is under way to study responses in ecosystem biogeochemistry to chronic inputs of N in atmospheric deposition in the region. Since 1988, experimental additions of NH₄NO₃ (0, 5 and 15 g N m^–2 yr^–1) have been made in two forest stands:Pinus resinosa (red pine) and mixed hardwood. In the seventh year of the study, we measured solute concentrations and estimated solute fluxes in throughfall and at two soil depths, beneath the forest floors (Oa) and beneath the B horizons.Beneath the Oa, concentrations and fluxes of dissolved organic C and N (DOC and DON) were higher in the coniferous stand than in the hardwood stand. The mineral soil exerted a strong homogenizing effect on concentrations beneath the B horizons. In reference plots (no N additions), DON composed 56% (pine) and 67% (hardwood) of the total dissolved nitrogen (TDN) transported downward from the forest floor to the mineral soil, and 98% of the TDN exported from the solums. Under N amendments, fluxes of DON from the forest floor correlated positively with rates of N addition, but fluxes of inorganic N from the Oa exceeded those of DON. Export of DON from the solums appeared unaffected by 7 years of N amendments, but as in the Oa, DON composed smaller fractions of TDN exports under N amendments. DOC fluxes were not strongly related to N amendment rates, but ratios of DOC:DON often decreased.The hardwood forest floor exhibited a much stronger sink for inorganic N than did the pine forest floor, making the inputs of dissolved N to mineral soil much greater in the pine stand. Under the high-N treatment, exports of inorganic N from the solum of the pine stand were increased >500-fold over reference (5.2 vs. 0.01 g N m^–2 yr^–1), consistent with other manifestations of nitrogen saturation. Exports of N from the solum in the pine forest decreased in the order NO₃-N> NH₄-N> DON, with exports of inorganic N 14-fold higher than exports of DON. In the hardwood forest, in contrast, increased sinks for inorganic N under N amendments resulted in exports of inorganic N that remained lower than DON exports in N-amended plots as well as the reference plot.     

Limnology of the Green Lakes Valley: phytoplankton ecology and dissolved organic matter biogeochemistry at a long-term ecological research site

ABSTRACT

Background: Surface waters are the lowest points in the landscape, and therefore serve as excellent integrators and indicators of changes taking place in the surrounding terrestrial and atmospheric environment.

Aims: Here we synthesise the findings of limnological studies conducted during the past 15 years in streams and lakes in the Green Lakes Valley, which is part of the Niwot Ridge Long Term Ecological Research (LTER) Site.

Methods: The importance of these studies is discussed in the context of aquatic ecosystems as indicators, integrators, and regulators of environmental change. Specifically, investigations into climatic, hydrologic, and nutrient controls on present-day phytoplankton, and historical diatom, community composition in the alpine lake, Green Lake 4, are reviewed. In addition, studies of spatial and temporal patterns in dissolved organic matter (DOM) biogeochemistry and reactive transport modelling that have taken place in the Green Lakes Valley are highlighted.

Results and conclusions: The findings of these studies identify specific shifts in algal community composition and DOM biogeochemistry that are indicative of changing environmental conditions and provide a framework for detecting future environmental change in the Green Lakes Valley and in other alpine watersheds. Moreover, the studies summarised here demonstrate the importance of long-term monitoring programmes such as the LTER programme.     

Dynamics of C, N, P and microbial community composition in particulate soil organic matter during residue decomposition

Decomposing residues can be an important source of nutrients for plants, especially of N and P, but the relationship between N and P release and microbial community dynamics have rarely been studied. Two pea (Pisum sativum L.) residues with contrasting chemical composition, shoots from flowering pea (Pea-Y) with 2.9 mg P and 36 mg N kg⁻¹ and from mature pea (Pea-M) with 0.3 mg P and 13 mg N kg⁻¹, were added at a rate of 20 g kg soil⁻¹ to a sandy soil low in nutrients. Particulate organic matter (POM) was isolated on days (d) 0, 5, 15, 28, 42 and 61 after residue addition and analysed for C, N, P and microbial community structure (fatty acid methyl ester analysis). The recovery of POM from residue-amended soils decreased over time to 30–40% of added amounts for both residues. Apart from d 0, the N concentration in POM was lower in residue-amended soil than in the control. Due to a rapid decrease in P concentration during the first 5 days in Pea-Y and a slow increase over the whole experiment in Pea-M, P concentrations in POM on d 61 were similar in all treatments. In Pea-Y, the dynamics of C, N and P were coupled, with amounts of C, N and P decreasing during the first 15 days and remaining stable thereafter. In Pea-M, a steady loss of C from POM was contrasted by a slight increase in P. As a result, the C/P ratio decreased from 1,330 on d 0 to 390 on d 61. The C/N ratio of Pea-M decreased only during the second phase of decomposition. The different nutrient dynamics in Pea-Y and Pea-M led to similar amounts of N and P in POM towards the end of the incubation. Microbial community composition in the POM in Pea-Y and Pea-M remained distinct from the control, even though it changed over time. POM was shown to be an important source of potentially available nutrients after addition of plant residues. In the unamended soil, stable nutrient amounts in POM suggested very low net nutrient release from native POM compared to POM after residue addition.     

Influence of dissolved organic matter source on lake bacterioplankton structure and function--implications for seasonal dynamics of community composition

It has been suggested that autochthonous (internally produced) organic carbon and allochthonous (externally produced) organic carbon are utilized by phylogenetically different bacterioplankton. We examined the relationship between the source of organic matter and the structure and function of lake bacterial communities. Differences and seasonal changes in bacterial community composition in two lakes differing in their source of organic matter were followed in relation to environmental variables. We also performed batch culture experiments with amendments of various organic substrates, namely fulvic acids, leachates from algae, and birch and maple leaves. Differences in bacterial community composition between the lakes, analysed by terminal restriction fragment length polymorphism, correlated with variables related to the relative loading of autochthonous and allochthonous carbon (water colour, dissolved organic carbon, nutrients, and pH). Seasonal changes correlated with temperature, chlorophyll and dissolved organic carbon in both lakes. The substrate amendments led to differences in both structure and function, i.e. production, respiration and growth yield, of the bacterial community. In conclusion, our results suggest that the source of organic matter influences community composition both within and among lakes and that there may be a coupling between the structure and function of the bacterial community.     

Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

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The impact of Chironomus plumosus larvae on organic matter decay and nutrient (N, P) exchange in a shallow eutrophic lake sediment following a phytoplankton sedimentation

The importance of Chironomus plumosus larvae onbenthic metabolism and nutrient exchange across thesediment–water interface was evaluated in a shalloweutrophic lake (Lake Arreskov, Denmark) following aphytoplankton sedimentation. Chironomus plumosuslarvae were added to laboratory sediment microcosms,corresponding to a density of 2825 larvae m⁻².Non-inhabited microcosms served as controls. Asedimentation pulse of organic matter was simulated byadding fresh algal material (Chlamydomonasreinhardii) to sediment cores (36 g dryweight m⁻²). The mineralization was followed bymeasuring fluxes of O₂, CO₂, dissolvedinorganic nitrogen and phosphate. A rapid clearance ofalgae from the water column in faunated microcosmssuggested that chironomids may be of major importancein controlling phytoplankton concentrations in shalloweutrophic lakes. Chironomids increased the sedimentO₂ uptake ≈ 3 times more than what wouldbe expected from their own respiration, indicating astimulation of microbial activity and decomposition oforganic matter in the sediment. Addition of algaeenhanced the release of CO₂, NH⁺ ₄ ando-P. The excess inorganic C, N and P released inamended non-inhabited sediment after 36 dayscorresponded to 65, 31 and 58% of the C, N and P inthe added algae. In sediment inhabited by Chironomus plumosus the corresponding numbers were147, 45 and 73%, indicating that mineralization oforganic matter also from the indigenous sediment poolwas stimulated by chironomids. This revised version was published online in July 2006 with corrections to the Cover Date.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Exoenzyme activities as indicators of dissolved organic matter composition in the hyporheic zone of a floodplain river

1. We measured the hyporheic microbial exoenzyme activities in a floodplain river to determine whether dissolved organic matter (DOM) bioavailability varied with overlying riparian vegetation patch structure or position along flowpaths. 2. Particulate organic matter (POM), dissolved organic carbon (DOC), dissolved oxygen (DO), electrical conductivity and temperature were sampled from wells in a riparian terrace on the Queets River, Washington, U.S.A. on 25 March, 15 May, 20 July and 09 October 1999. Dissolved nitrate, ammonium and soluble reactive phosphorus were also collected on 20 July and 09 October 1999. Wells were characterised by their associated overlying vegetation: bare cobble/young alder, mid‐aged alder (8–20 years) and old alder/old‐growth conifer (25 to >100 years). POM was analysed for the ash‐free dry mass and the activities of eight exoenzymes (α‐glucosidase, β‐glucosidase, β ‐N‐acetylglucosaminidase, xylosidase, phosphatase, leucine aminopeptidase, esterase and endopeptidase) using fluorogenic substrates. 3. Exoenzyme activities in the Queets River hyporheic zone indicated the presence of an active microbial community metabolising a diverse array of organic molecules. Individual exoenzyme activity (mean ± standard error) ranged from 0.507 ± 0.1547 to 22.8 ± 5.69 μmol MUF (g AFDM)^?1 h^?1, was highly variable among wells and varied seasonally, with the lowest rates occurring in March. Exoenzyme activities were weakly correlated with DO, DOC and inorganic nutrient concentrations. 4. Ratios of leucine aminopeptidase : β‐glucosidase were low in March, May and October and high in July, potentially indicating a switch from polysaccharides to proteins as the dominant component of microbial metabolism. 5. Principal components analysis indicated that there were patch effects and that these effects were strongest in the summer. 6. DOM degradation patterns did not change systematically along hyporheic flowpaths but varied with overlying forest patch type in the Queets River hyporheic zone, suggesting that additional carbon inputs exist. We hypothesise that the most likely input is the downward movement of DOM from overlying riparian soils. Understanding this movement of DOM from soils to subsurface water is essential for understanding both the hyporheic metabolism and the carbon budget of streams and rivers.     

喀斯特湖泊-河流系统溶解性有机质光谱特征

为探索喀斯特湖泊-河流系统溶解性有机质(DOM)光谱特征空间格局,该研究以我国贵州红枫湖、百花湖和老马河为对象,分析了表层水体DOM紫外参数(SUVA₂₅₄、SUVA₂₈₀、S_275-295、S_350-400、S_R和E2/E3),利用三维荧光-平行因子(EEM-PARAFAC)解析DOM主要成分,同时基于荧光参数(HIX、FI、BIX和β：α)揭示其来源信息。该研究运用Spearman相关性分析及主成分分析(PCA)方法,揭示DOM参数的内在联系及重要组分贡献。结果显示,红枫湖、百花湖和老马河SUVA₂₅₄和SUVA₂₈₀值较低,而E2/E3、S_275-295和S_350-400值较高,说明芳香类成分占比较少且DOM整体分子量较小。水体DOM主要成分为:红枫湖/百花湖(C1:微生物代谢类腐殖质;C2:可见光范围腐殖质;C3:色氨酸),老马河(C1:色氨酸;C2:富里酸;C3:陆源类腐殖质)。水体BIX范围在0.77-1.12之间,说明生物源和陆源输入共同贡献DOM。红枫湖与百花湖FI值小于1.4,而老马河在1.4-1.9间,说明河流内源DOM贡献率高于湖泊。水体β：α值范围在0.73-1.10之间,说明新生DOM具有显著贡献。湖泊与河流HIX值低于4,表明水体腐殖化程度低。老马河HIX与S_R和S_275-295呈正相关(P ＜ 0.05)且与FI呈负相关(P ＜ 0.05),说明DOM腐殖化程度与分子大小和来源密切联系。芳香类DOM的同质性导致SUVA₂₅₄和SUVA₂₈₀相互耦合。研究发现新生DOM多具有生物降解性,表现为BIX和β：α强相关。相对于激流系统,湖泊具有更多的水力停留时间,这可能进一步促进内源微生物的代谢,导致生物驱动的有机质快速循环,因此不同DOM生物讯号相互耦合。研究阐明了喀斯特湖泊-河流系统DOM成分与来源特征,有望为研究全球碳循环过程提供数据支撑。     

Variability and transport of suspended sediment,particulate and dissolved organic carbon in the tidal freshwater Hudson River

Measurements of suspended matter, particulate organic carbon and dissolved organic carbon were made over a three year period at stations spanning 150 km of the tidal freshwater Hudson River. Suspended matter concentrations varied from year-to-year and were not related to freshwater discharge. The increase in suspended matter with depth in vertical profiles suggests that, during medium to low flow conditions, resuspension of bottom sediments was as important a source of sediment as loadings from tributaries. Particulate organic carbon showed significant variability among stations, and both autochthonous primary production and detrital organic matter are contributing to POC standing stocks. Dissolved organic carbon represented over half of the total organic carbon in the water column and showed little variation among stations.Examining downstream changes in transport showed that there was significant production of both suspended matter and POC within the study reach during the ice-free season. Tributary loadings within the study reach do not appear to be the cause of these increases in downstream transport. Dissolved organic carbon behaved conservatively in that there was no evidence for net production or net consumption within the river.The spatial/temporal patterns and analyses of transport suggest that suspended matter and POC, but not DOC, were controlled to a significant extent by processes occurring within the river and were not simply related to loadings from outside.     

华南典型树种凋落叶的野外分解和溶解性有机质溶出动态

选用华南亚热带地区常见阔叶树种木荷和针叶树种湿地松的新近凋落叶,在野外分解0、30、60、90、150、210、240、365 d,分析溶出的溶解性有机质(DOM)浓度、组成和性质的变化,以及对土壤可溶性有机碳(DOC)的影响.结果表明: 随着分解的进行,尽管木荷叶片的DOM浓度高于松针,但是2种凋落叶DOM浓度、性质和物质组成变化规律一致;2种凋落叶的DOM浓度均呈下降趋势,芳香化程度和分子量增大,富里酸、腐殖酸类物质逐渐增多,可降解的简单芳烃蛋白(如酪氨酸)逐渐减少.在分解初期,DOM主要由亲水中性和酸性部分组成,易分解、易迁移,对表层土壤DOC影响不显著;在分解后期,DOM主要为腐殖酸和富里酸类物质,吸附性强,表层土壤DOC浓度显著下降.