Sorption of dissolved organic matter by mineral soils of the Siberian forest tundra

Because of low net production in arctic and subarctic surface water, dissolved organic matter (DOM) discharged from terrestrial settings plays an important role for carbon and nitrogen dynamics in arctic aquatic systems. Sorption, typically controlling the export of DOM from soil, may be influenced by the permafrost regime. To confirm the potential sorptive control on the release of DOM from permafrost soils in central northern Siberia, we examined the sorption of DOM by mineral soils of Gelisols and Inceptisols with varying depth of the active layer. Water‐soluble organic matter in the O horizons of the Gelisols was less (338 and 407 mg C kg^?1) and comprised more dissolved organic carbon (DOC) in the hydrophobic fraction (HoDOC) (63% and 70%) than in the O horizons of the Inceptisols (686 and 706 mg C kg^?1, 45% and 48% HoDOC). All A and B horizons from Gelisols sorbed DOC strongly, with a preference for HoDOC. Almost all horizons of the Inceptisols showed a weaker sorption of DOC than those of the Gelisols. The C horizons of the Inceptisols, having a weak overall DOC sorption, sorbed C in the hydrophilic fraction (HiDOC) stronger than HoDOC. The reason for the poor overall sorption and also the preferential sorption of HiDOC is likely the high pH (pH>7.0) of the C horizons and the smaller concentrations of iron oxides. For all soils, the sorption of HoDOC related positively to oxalate‐ and dithionite–citrate‐extractable iron. The A horizons released large amounts of DOC with 46–80% of HiDOC. The released DOC was significantly (r=0.78, P<0.05) correlated with the contents of soil organic carbon. From these results, we assume that large concentrations of DOM comprising large shares of HiDOC can pass mineral soils where the active layer is thin (i.e. in Gelisols), and enter streams. Soils with deep active layer (i.e. Inceptisols), may release little DOM because of more frequent infiltration of DOM into their thick mineral horizons despite their smaller contents of reactive, poorly crystalline minerals. The results obtained for the Inceptisols are in agreement with the situation observed for streams connecting to Yenisei at lower latitudes than 65°50′ with continuous to discontinuous permafrost. The smaller sorption of DOM by the Gelisols is in agreement with the larger DOM concentrations in more northern catchments. However, the Gelisols preferentially retained the HoDOC which dominates the DOC in streams towards north. This discrepancy can be explained by additional seepage water from the organic horizons that is discharged into streams without intensive contact with the mineral soil.     

Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long-term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy-nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by-products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.     

Stream dissolved organic matter bioavailability and composition in watersheds underlain with discontinuous permafrost

We examined the impact of permafrost on dissolved organic matter (DOM) composition in Caribou-Poker Creeks Research Watershed (CPCRW), a watershed underlain with discontinuous permafrost, in interior Alaska. We analyzed long term data from watersheds underlain with varying degrees of permafrost, sampled springs and thermokarsts, used fluorescence spectroscopy, and measured the bioavailabity of dissolved organic carbon (DOC). Permafrost driven patterns in hydrology and vegetation influenced DOM patterns in streams, with the stream draining the high permafrost watershed having higher DOC and dissolved organic nitrogen (DON) concentrations, higher DOC:DON and greater specific ultraviolet absorbance (SUVA) than the streams draining the low and medium permafrost watersheds. Streams, springs and thermokarsts exhibited a wide range of DOC and DON concentrations (1.5–37.5 mgC/L and 0.14–1.26 mgN/L, respectively), DOC:DON (7.1–42.8) and SUVA (1.5–4.7 L mgC⁻¹ m⁻¹). All sites had a high proportion of humic components, a low proportion of protein components, and a low fluorescence index value (1.3–1.4), generally consistent with terrestrially derived DOM. Principal component analysis revealed distinct groups in our fluorescence data determined by diagenetic processing and DOM source. The proportion of bioavailable DOC ranged from 2 to 35%, with the proportion of tyrosine- and tryptophan-like fluorophores in the DOM being a major predictor of DOC loss (p < 0.05, R ² = 0.99). Our results indicate that the degradation of permafrost in CPCRW will result in a decrease in DOC and DON concentrations, a decline in DOC:DON, and a reduction in SUVA, possibly accompanied by a change in the proportion of bioavailable DOC.     

中亚热带森林转换对土壤可溶性有机质数量与光谱学特征的影响

选取中亚热带福建三明格氏栲天然林及其转换而成的木荷、锥栗及福建柏等3种人工林表层土壤(0—10 cm)可溶性有机质(DOM)为对象,对其数量和光谱学特征进行了研究,以探讨森林转换对土壤DOM的影响。结果表明,天然林转换成上述3种人工林后,0—5 cm土壤可溶性有机碳(DOC)浓度显著降低(P0.05),降低程度分别为66.1%,69.9%及29.4%,可溶性有机氮(DON)浓度也有所下降;除福建柏外,其余两种人工林5—10 cm土壤DOC及DON浓度均低于天然林。各林分0—5 cm土壤DOC及DON浓度均高于5—10 cm土层。两个土层中,天然林土壤DOM的芳香化及腐殖化程度均显著高于人工林(P0.05),但荧光效率值低于人工林;荧光光谱图显示,天然林土壤DOM在芳香性脂肪族及木质素类复杂结构荧光基团处的吸收大于人工林;各林分土壤DOM傅里叶红外光谱出现吸收谱带的位置相似,其中吸收强度最大的为形成氢键的—OH的伸缩振动,此外还有芳香性CC伸缩振动、有机羧酸盐COO-反对称伸缩振动、碳水化合物中烷氧基C—O的振动等,人工林土壤DOM中碳水化合物的比例增加是其结构简单的主要原因。土壤DOM中结构复杂、分子量大的组分不易向下迁移;天然林与人工林间土壤DOM数量及光谱学特征的差异主要与凋落物输入及营林措施的干扰有关;本研究所涉及的3种人工林中,福建柏更有利于土壤养分的累积。     

植被恢复对侵蚀红壤可溶性有机质含量及光谱学特征的影响

为了解植被恢复对侵蚀红壤可溶性有机质含量及结构特征的影响, 以福建省长汀县河田镇植被恢复后的侵蚀红壤及对照裸地为研究对象, 对两试验地0-60 cm深土壤中可溶性有机质的含量及光谱学特征进行了比较研究。结果表明: 侵蚀红壤植被恢复后, 土壤可溶性有机碳含量显著提高, 在土表到60 cm深度的6个10 cm土层中, 植被恢复土壤可溶性有机碳含量分别提高为对照裸地相应土层的5.6、4.7、4.6、3.1、2.4及2.2倍。可溶性有机氮含量在两试验地之间的差异在各土层中不一致。植被恢复各土层侵蚀红壤可溶性有机质的芳香化指数显著高于对照裸地, 荧光发射光谱腐殖化指数略高于对照裸地, 植被恢复后的侵蚀红壤与对照裸地间荧光同步光谱腐殖化指数无明显差异。荧光同步光谱图中, 两试验地侵蚀红壤可溶性有机质的吸收主要为类蛋白质及芳香性脂肪族荧光基团的吸收。傅里叶红外光谱结果显示, 与对照裸地相比, 植被恢复后的侵蚀红壤土壤可溶性有机质中官能团种类更多, 且含有更多芳香碳及羧基碳。两试验地土壤可溶性有机质均表现为芳香化及腐殖化程度随土层的加深而降低。相关性分析显示, 土壤可溶性有机质的芳香化及腐殖化指数与土壤碳氮总量有极显著正相关关系。总之, 侵蚀红壤经植被恢复后, 土壤可溶性有机碳含量及可溶性有机质的芳香化指数显著提高, 可溶性有机质的腐殖化指数略有增大, 可溶性有机质结构更复杂, 更不易被分解, 因此有利于土壤肥力的恢复。     

杉木人工林土壤可溶性有机质及其与土壤养分的关系

通过在福建省来舟林场对不同栽植代数杉木人工林土壤可溶性有机碳(DOC)和氮(DON)及土壤养分的研究,其结果表明,随着杉木栽植代数的增加林地土壤DOC和DON含量逐渐下降,在0～10cm土层内第3代杉木林土壤DOC和DON含量分别是第1代杉木林的83.9%和87.1%、第2代杉木林的90.6%和96.9%,在10～20cm土层内第3代杉木林土壤DOC和DON含量分别是第1代杉木林的80.2%和81.5%、第2代杉木林的81.8%和90.0%。在不同林地和土层内土壤DOC含量之间的差异性达到了显著或极显著水平,而DON含量之间的差异性不显著。不同栽植代数杉木林土壤养分的变化趋势与DOM一致,随着杉木连栽,土壤养分含量呈下降趋势。在0～10cm土层内第3代杉木林土壤全氮、全钾、铵态氮和速效钾含量分别是第1代杉木林的83.1%、60.4%、68.1%和44.3%,是第2代杉木林的84.6%、68.5%、74.4%和58.7%;在10～20cm土层内第3代杉木林土壤全氮、全钾、铵态氮和速效钾含量分别是第1代杉木林的74.0%、53.4%、57.6%和54.6%,是第2代杉木林的94.8%、59.5%、74.3%和65.5%。经相关性分析,在各土层内土壤DOC和DON含量与土壤全氮、全钾、铵态氮和速效钾等土壤养分含量存在着不同程度的相关性。     

Fluorescence characteristics and biodegradability of dissolved organic matter in forest and wetland soils from coastal temperate watersheds in southeast Alaska

Understanding how the concentration and chemical quality of dissolved organic matter (DOM) varies in soils is critical because DOM influences an array of biological, chemical, and physical processes. We used PARAFAC modeling of excitation–emission fluorescence spectroscopy, specific UV absorbance (SUVA₂₅₄) and biodegradable dissolved organic carbon (BDOC) incubations to investigate the chemical quality of DOM in soil water collected from 25 cm piezometers in four different wetland and forest soils: bog, forested wetland, fen and upland forest. There were significant differences in soil solution concentrations of dissolved organic C, N, and P, DOC:DON ratios, SUVA₂₅₄ and BDOC among the four soil types. Throughout the sampling period, average DOC concentrations in the four soil types ranged from 9–32 mg C l⁻¹ and between 23–42% of the DOC was biodegradable. Seasonal patterns in dissolved nutrient concentrations and BDOC were observed in the three wetland types suggesting strong biotic controls over DOM concentrations in wetland soils. PARAFAC modeling of excitation–emission fluorescence spectroscopy showed that protein-like fluorescence was positively correlated (r ² = 0.82; P < 0.001) with BDOC for all soil types taken together. This finding indicates that PARAFAC modeling may substantially improve the ability to predict BDOC in natural environments. Coincident measurements of DOM concentrations, BDOC and PARAFAC modeling confirmed that the four soil types contain DOM with distinct chemical properties and have unique fluorescent fingerprints. DOM inputs to streams from the four soil types therefore have the potential to alter stream biogeochemical processes differently by influencing temporal patterns in stream heterotrophic productivity.     

Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Water draining from the Everglades marshes of southern Florida containshigh concentrations of dissolved organic C (DOC), N (DON), and in somelocations, P (DOP). These dissolved organic nutrients carry over 90% of the Nand organic C, and about 25% of the P exported downstream in the Everglades.Ourobjectives were to describe the most important aspects of the origin and fateofdissolved organic matter (DOM) in the Everglades, and to describe the processescontrolling its concentration and export. Concentrations of dissolved organicnutrients are influenced by local plant production, decomposition, and sorptionequilibrium with peat. The drained peat soils of the Everglades AgriculturalArea and the more productive marshes of the northern Everglades produce some ofthe highest concentrations of DOC and DON in the Everglades watershed. Inportions of the marshes of the northern Everglades, P enrichment was correlatedwith higher local DOC and DON concentrations and greater production of solubleplant matter. Microbial degradation of Everglades DOM was very slow; less than10% of the DOC was lost after 6 months of incubation in the laboratory andsupplements of inorganic nutrients failed to speed the decomposition. Exposureto solar radiation increased the subsequent decay rate of the remaining DOC(25%in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7%of the DON, and degraded about 50% of the humic substances over 21 days insterile porewater samples and thus degraded DOM faster than microbialdegradation. The humic substances appeared to inhibit biodegradation of theother fractions of the DOC since hydrophilic organic acids decomposed fasterwhen isolated from the humic substances.The fate of DOC and DON is closely linked as indicated by a generally narrowrange of C/N ratios. In contrast, high concentrations of DOP were associatedwith P enrichment (at least in pore water). The DOC was composed of about 50%humic substances, 33% hydrophilic acids, and 15% hydrophilic neutralsubstances,typical of DOC from other environments, despite the fact that it originatesfroma neutral to slightly alkaline peatland. Despite high exports of DON (3.9g m^–2 y^–1 from one area), themarshes of the northern Everglades are a sink for DON on a landscape scale. Theagricultural fields of the Everglades Agricultural Area, however, exported netquantities of DON. High concentrations of DOC desorbed from the agriculturalsoils when water with no DOC was added. Sorption experiments indicated thathighconcentrations of dissolved organic matter flowing into the marshes from theEverglades Agricultural Area could suppress the further desorption ofadditionalsoluble organic matter through physicochemical mechanisms. While biologicalfactors, plant production and microbial decomposition are important inproducingpotentially soluble organic nutrients, physicochemical sorption equilibria,hydrology, and degradation by solar radiation are also likely to control theexport of this material on the landscape scale.     

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

The flux and composition of carbon (C) from land to rivers represents a critical component of the global C cycle as well as a powerful integrator of landscape‐level processes. In the Congo Basin, an expansive network of streams and rivers transport and cycle terrigenous C sourced from the largest swathe of pristine tropical forest on Earth. Increasing rates of deforestation and conversion to agriculture in the Basin are altering the current regime of terrestrial‐to‐aquatic biogeochemical cycling of C. To investigate the role of deforestation on dissolved organic and inorganic C (DOC and DIC, respectively) biogeochemistry in the Congo Basin, six lowland streams that drain catchments of varying forest proportion (12%–77%) were sampled monthly for 1 year. Annual mean concentrations of DOC exhibited an asymptotic response to forest loss, while DIC concentrations increased continuously with forest loss. The isotopic signature of DIC became significantly more enriched with deforestation, indicating a shift in source and processes controlling DIC production. The composition of dissolved organic matter (DOM), as revealed by ultra‐high‐resolution mass spectrometry, indicated that deforested catchments export relatively more aliphatic and heteroatomic DOM sourced from microbial biomass in soils. The DOM compositional results imply that DOM from the deforested sites is more biolabile than DOM from the forest, consistent with the corresponding elevated stream CO₂ concentrations. In short, forest loss results in significant and comprehensive shifts in the C biogeochemistry of the associated streams. It is apparent that land‐use conversion has the potential to dramatically affect the C cycle in the Congo Basin by reducing the downstream flux of stable, vascular‐plant derived DOC while increasing the transfer of biolabile soil C to the atmosphere.     

Chronic catchment nitrogen enrichment and stoichiometric constraints on the bioavailability of dissolved organic matter from leaf leachate

1. Chronic nitrogen (N) deposition may alter the bioavailability of dissolved organic matter (DOM) in streams by multiple pathways. Elevated N deposition may alter the nutrient stoichiometry of DOM as well as nutrient availability in stream water. 2. We evaluated the influence of a decadal‐scale experimental N enrichment on the relative importance of DOM nutrient content and inorganic nutrient availability on the bioavailability of DOM. We measured the consumption of dissolved organic carbon (DOC) and changes in nutrient concentration, DOM components and enzyme activity in a bottle incubation assay with different DOM and nutrient treatments. To evaluate the effect of DOM stoichiometry, we used leaf leachates of different carbon/N/phosphorus (C : N :P) ratio, made from leaf litter sourced in the reference and N‐enriched catchments at the Bear Brook Watershed in Maine (BBWM). We also manipulated the concentration of inorganic N and P to compare the effect of nutrient enrichment with DOM stoichiometry. 3. DOC from the N‐enriched catchment was consumed 14% faster than that from the reference catchment. However, mean DOC consumption for both leachates was more than doubled by the simultaneous addition of N and P, compared to controls, while the addition of N or P alone increased consumption by 42 and 23%, respectively. The effect of N and/or P enrichment consistently had a greater effect than DOM source for all response variables considered. 4. We subsequently conducted DOC uptake measurements using leaf leachate addition under ambient and elevated N and P in the streams draining the reference and N‐enriched catchments at BBWM. In both streams, DOC uptake lengths were shorter when N and P were elevated. 5. Although both DOM stoichiometry and inorganic nutrient availability affect DOM bioavailability, N and P co‐limitation appears to be the dominant driver of reach‐scale processing of DOM.     

耕作扰动对喀斯特土壤可溶性有机质及其组分迁移淋失的影响

土壤可溶性有机质(DOM)及其组分淋失特征研究对深入理解干扰作用下土壤碳氮养分损失机制具有重要意义,本研究基于翻耕模拟试验,分析喀斯特石灰土可溶性有机碳(DOC)、可溶性有机氮(DON)、及DOM官能团组分的淋失动态特征及其对不同耕作频率的响应,并探讨其影响因子。结果表明:(1)土壤DOC与DON的淋失量均随翻耕频率的增加而增加,但4个官能团特征参数对翻耕频率响应均不显著;DOC/DON淋失比随翻耕频率的增大而减少,DON占淋溶水可溶性总氮(TDN)比例随翻耕频率增加而增加。(2)DOC和DON月淋失量同时受翻耕处理与季节变化及其交互作用的影响,4个官能团特征参数仅受季节变化的影响;翻耕处理实施后,DOC月淋失量表现为初期大、后期小,各处理间差异性逐渐降低;但DON月淋失量初期小、后期大,各处理间差异性逐渐增大。(3)DOC淋失量与4个官能团特征参数呈显著负相关(P0.05),与Ca~(2+)、NH_4~+-N的淋失量呈显著正相关;DON淋失量与4个官能团参数无显著相关关系,与Mg~(2+)淋失量呈显著正相关关系。以上结果表明耕作扰动会加剧土壤DOM淋失,但淋失组分中稳定性组分没有变化,意味着耕作干扰将导致土壤有机质的持续损失,且由于其淋失组分碳氮比(DOC/DON)随扰动频率增加而降低,DON/TDN比随扰动频率增加而增加,持续的耕作干扰将大大增加水体氮素污染风险。     

Composition,Dynamics, and Fate of Leached Dissolved Organic Matter in Terrestrial Ecosystems: Results from a Decomposition Experiment

Fluxes of dissolved organic matter (DOM) are an important vector for the movement of carbon (C) and nutrients both within and between ecosystems. However, although DOM fluxes from throughfall and through litterfall can be large, little is known about the fate of DOM leached from plant canopies, or from the litter layer into the soil horizon. In this study, our objectives were to determine the importance of plant-litter leachate as a vehicle for DOM movement, and to track DOM decomposition [including dissolve organic carbon (DOC) and dissolved organic nitrogen (DON) fractions], as well as DOM chemical and isotopic dynamics, during a long-term laboratory incubation experiment using fresh leaves and litter from several ecosystem types. The water-extractable fraction of organic C was high for all five plant species, as was the biodegradable fraction; in most cases, more than 70% of the initial DOM was decomposed in the first 10 days of the experiment. The chemical composition of the DOM changed as decomposition proceeded, with humic (hydrophobic) fractions becoming relatively more abundant than nonhumic (hydrophilic) fractions over time. However, in spite of proportional changes in humic and nonhumic fractions over time, our data suggest that both fractions are readily decomposed in the absence of physicochemical reactions with soil surfaces. Our data also showed no changes in the ¹³C signature of DOM during decomposition, suggesting that isotopic fractionation during DOM uptake is not a significant process. These results suggest that soil microorganisms preferentially decompose more labile organic molecules in the DOM pool, which also tend to be isotopically heavier than more recalcitrant DOM fractions. We believe that the interaction between DOM decomposition dynamics and soil sorption processes contribute to the ¹³C enrichment of soil organic matter commonly observed with depth in soil profiles. published online 2004     

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

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氮沉降对杉木人工幼林土壤溶液可溶性有机物质浓度及光谱学特征的影响

为探究氮沉降对亚热带杉木人工幼林土壤溶液可溶性有机物质(DOM)浓度及光谱学特征的影响,采用负压法,对0～15和15～30 cm土层土壤溶液DOM进行了2年的动态监测及光谱学特征研究.结果表明:氮沉降显著减少了各土层土壤溶液可溶性有机碳(DOC)浓度,增加了芳香化指数(AI)及腐殖化指数(HIX),但对可溶性有机氮(DON)无显著影响.土壤溶液DOM浓度存在明显的季节变动,夏秋季显著高于春冬季.傅里叶红外光谱结果表明,森林土壤溶液DOM在6个区域的相似位置存在吸收峰,其中1145～1149 cm^-1的吸收峰最强.三维荧光光谱表明,DOM主要以类蛋白质物质(Ex/Em=230 nm/300 nm)和微生物降解产物(Ex/Em=275 nm/300 nm)为主,施氮使0～15 cm土层类蛋白质物质减少.氮沉降可能主要是通过降低土壤pH、抑制土壤碳矿化和刺激植物生长等途径显著抑制土壤溶液DOC浓度,而表层被抑制的DOC成分以类蛋白质物质和羧酸盐物质为主.氮沉降短期可能有利于土壤肥力的储存,但随着氮沉降量的积累,土壤中营养物质将难以得到有效利用.     

Effects of salinity and light on organic carbon and nitrogen uptake in a hypersaline microbial mat

Utilization of dissolved organic matter (DOM) is thought to be the purview of heterotrophic microorganisms, but photoautotrophs can take up dissolved organic nitrogen (DON) and dissolved organic carbon (DOC). This study investigated DOC and DON uptake in a laminated cyanobacterial mat community from hypersaline Salt Pond (San Salvador, Bahamas). The total community uptake of (3)H-labeled substrates was measured in the light and in the dark and under conditions of high and low salinity. Salinity was the primary control of DOM uptake, with increased uptake occurring under low-salinity, 'freshened' conditions. DOC uptake was also enhanced in the light as compared with the dark and in samples incubated with the photosystem II inhibitor 3(3,4-dichlorophenyl)-1, 1-dimethylurea, suggesting a positive association between photosynthetic activity and DOC uptake. Microautoradiography revealed that some DOM uptake was attributed to cyanobacteria. Cyanobacteria DOM uptake was negatively correlated with that of smaller filamentous microorganisms, and DOM uptake by individual coccoid cells was negatively correlated with uptake by colonial coccoids. These patterns of activity suggest that Salt Pond microorganisms are engaged in resource partitioning, and DOM utilization may provide a metabolic boost to both heterotrophs and photoautrophs during periods of lowered salinity.     

亚热带米槠林不同更新方式对土壤可溶性有机质降解性的影响

可溶性有机质(DOM)的生物降解性影响着土壤有机质的存留和释放,对深入认识森林土壤养分循环意义重大。为探究森林更新对土壤DOM降解特征的影响,选取亚热带地区米槠天然林(NF)、米槠次生林(SF)和米槠人工促进天然更新林(AR)土壤DOM溶液为研究对象,进行室内降解(42 d)试验。结果表明: 1)3种林分土壤可溶性有机碳(DOC)的降解率和易降解DOC的比例均为SF>AR>NF;可溶性有机氮(DON)和微生物生物量碳(MBC)是显著影响易降解DOC比例的因子;2)难降解组分占3种林分土壤DOC的大部分(72.3%～94.6%),其周转时间长,有利于稳定土壤有机质(SOC)的形成;3)土壤DOM最初的腐殖化指数(HIX_em)会影响易降解DOC的周转时间。DOM光谱结构随降解过程呈现动态变化,说明DOM中易降解组分被消耗完后,微生物会转而降解芳香类和疏水性物质以获取碳源。综上,米槠天然林更新为次生林和人促林后增加了易降解DOC的比例,提高了土壤DOM生物可降解性,不利于SOC的积累。     

Chemical and seasonal controls on the dynamics of dissolved organic matter in a coniferous old-growth stand in the Pacific Northwest,USA

Soil organic matter (SOM) is the largest terrestrial C pool, and retention and release of dissolved organic matter (DOM) cause formation and loss of SOM. However, we lack information on how different sources of DOM affect its chemical composition, and how DOM chemical composition affects retention. We studied seasonal controls on DOM production and chemical controls on retention in soils of a temperate coniferous forest. The O horizon was not usually the dominant source for dissolved organic C (DOC) or N (DON) as has been reported for other sites. Rather, net production of both DOC and DON was often greater in the shallow mineral soil (0–10 cm) than in the O horizon. DOM production in the shallow mineral soil may be from root exudation as well as turnover of fine roots and microflora in the rhizosphere. In the field, the two acid fractions (hydrophobic and hydrophilic acids) dominated the soil solution at all depths. A major portion of net production and removal of total DOC within the soil column was explained by increases and decreases in these fractions, although a shift in chemical composition of DOM between the O and mineral soil horizons suggested different origins of DOM in these layers. A larger loss of the free amino fraction to deep soil water at this study site than at other sites suggested lower retention of labile DON. Field DOM removal measurements suggest that field-measured parameters may provide a good estimate for total DOM retained in mineral soil.     

隔离降雨对亚热带米槠天然林土壤可溶性有机质数量及光谱学特征的影响

全球气候变化背景下区域降雨格局变化可能深刻影响土壤可溶性有机质(DOM)的数量和质量.为了解亚热带森林土壤DOM对降雨减少的响应,通过6年不同强度(对照、-30%、-60%)的隔离降雨模拟试验,采用光谱技术,研究了降雨减少对亚热带米槠天然林不同深度土壤DOM数量和结构的影响.结果表明: 与对照相比,隔离降雨使0～10 cm土层中可溶性有机碳(DOC)和可溶性有机氮(DON)含量显著降低,其中-30%处理DOC下降幅度小于DON, 而-60%处理DOC下降幅度大于DON,0～10 cm土层中DOC和DON含量都显著高于10～20 cm土层.-30%处理土壤DOM中源于微生物代谢的芳香类腐殖质和烷烃比例上升;-60%处理土壤DOM中微生物代谢产物的相对贡献率减少.除了隔离降雨后水分变化等直接影响外,微生物活性也是本试验区影响DOM数量和结构的重要因素.     

A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream

Patterns of dissolved organic carbon (DOC) and nitrogen (DON) delivery were compared between times of stormflow and baseflow in Paine Run, an Appalachian stream draining a 12.4 km² forested catchment in the Shenandoah National Park (SNP), Virginia. The potential in-stream ecological impact of altered concentrations and/or chemical composition of DOM during storms also was examined, using standardized bacterial bioassays. DOC and DON concentrations in Paine Run were consistently low during baseflow and did not show a seasonal pattern. During storms however, mean DOC and DON concentrations approximately doubled, with maximum concentrations occurring on the rising limb of storm hydrographs. The rapid response of DOM concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. Stormflow (4% of the time, 36% of the annual discharge) contributed >50% of DOC, DON and NO₃ ^– flux in Paine Run during 1997. In laboratory bacterial bioassays, growth rate constants were higher on Paine Run stormflow water than on baseflow water, but the fraction of total DOM which was bioavailable was not significantly different. The fraction of the total stream DOC pool taken up by water column bacteria was estimated to increase from 0.03 ± 0.02% h^–1 during baseflow, to 0.15 ± 0.04% h^–1 during storms. This uptake rate would have a minimal effect on bulk DOM concentrations in Paine Run, but storms may still have considerable impact on the bacterial stream communities by mobilizing them into the water column and by supplying a pulse of DOM.     

落叶松林土壤可溶性碳、氮和官能团特征的时空变化及与土壤理化性质的关系

土壤可溶性有机碳(DOC)、可溶性有机氮(DON)及其官能团特征在土壤碳、氮循环中作用非常重要。对25个不同年龄落叶松林样地、4个深度(0-20、20-40、40-60和60-80 cm)土壤DOC、DON、有机物官能团(芳香性、分子量和疏水性)特征指标(254、260、272 nm和280 nm的单位吸光度值SUVA:吸光度值/DOC含量)和土壤理化指标(土壤全碳SOC、全氮SON、pH值、电导率、容重)进行测定,旨在探究它们的时、空变化特征及与土壤理化指标相关关系。在空间尺度上,与SOC、SON一致,表层土壤DOC、DON多显著高于深层(P<0.05),但是4个单位吸光度值SUVA₂₅₄、SUVA₂₆₀、SUVA₂₇₂和 SUVA₂₈₀均不存在差异(P>0.05);在时间尺度上,仅表层土壤DOC、SOC 和SON随落叶松年龄显著线性增长(P<0.05),而深层DOC、SOC、SON、不同层土壤DON及各官能团指标均没有显著变化(P>0.05)。可见,土壤可溶性有机物内碳的累积(7 mg kg^-1 a^-1)是SOC累积的一部分(762 mg kg^-1 a^-1),但其DON及可溶性有机物芳香性比例、分子量大小及疏水性容量等官能团特征并未受落叶松生长时间以及土壤深度的显著影响。进一步回归分析表明这些官能团指标随土壤DOC含量增加而指数下降,深层土壤同时受DON显著影响。表层土壤DOC、DON与土壤SOC、SON、土壤电导率显著正相关(P<0.05),深层相关不显著(P>0.05),而官能团指标与土壤理化性质的相关性在各个土层均不显著,显示出表层土壤可溶性有机物的量,而不是官能团组成对土壤理化性质影响显著,而深层土壤可溶性有机物量对土壤理化性质不构成显著影响。对于从可溶性组分、官能团角度,分析落叶松人工林成长过程中土壤碳、氮时空变化具有科学意义。