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.     

Seasonal changes in the chemical quality and biodegradability of dissolved organic matter exported from soils to streams in coastal temperate rainforest watersheds

The composition and biodegradability of streamwater dissolved organic matter (DOM) varies with source material and degree of transformation. We combined PARAFAC modeling of fluorescence excitation–emission spectroscopy and biodegradable dissolved organic carbon (BDOC) incubations to investigate seasonal changes in the lability of DOM along a soil-stream continuum in three soil types: bog, forested wetland and upland forest. The percent BDOC ranged from 7 to 38% across all sites, and was significantly greater in soil compared to streamwater in the bog and forested wetland, but not in the upland forest. The percent BDOC also varied significantly over the entire sampling period in soil and streamwater for the bog and forested wetland, as BDOC peaked during the spring runoff and was lowest during the summer months. Moreover, the chemical quality of DOM in wetland soil and streamwater was similar during the spring runoff and fall wet season, as demonstrated by the similar contribution of protein-like fluorescence (sum of tyrosine and tryptophan fluorescence) in soil water and in streams. These findings suggest that the tight coupling between terrestrial and aquatic ecosystems is responsible for the delivery of labile DOM from wetland soils to streams. The contribution of protein-like fluorescence was significantly correlated with BDOC (p < 0.001) over the entire sampling period indicating DOM is an important source of C and N for heterotrophic microbes. Taken together, our findings suggest that the production of protein-rich, labile DOM and subsequent loss in stream runoff might be an important loss of labile C and N from coastal temperate watersheds.     

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.     

Dissolved organic matter (DOM) concentration and quality in a forested mid-Atlantic watershed, USA

Understanding the quantity and quality of dissolved organic matter (DOM) in potential watershed sources is critical for explaining and quantifying the exports of DOM in stream runoff. Here, we examined the concentration and quality of DOM for ten watershed sources in a 12?ha forested catchment over a two-year period. DOM composition was evaluated for: throughfall, litter leachate, soil water (zero and tension), shallow and deep groundwater, stream water, hyporheic zone, and groundwater seeps. DOM quality was measured using a suite of optical indices including UV–visible absorbance and PARAFAC modeling of fluorescence excitation-emission matrices (EEMs). DOM concentrations and quality displayed a pronounced trend across watershed sources. Surficial watershed sources had higher DOM concentrations and more humic-like DOM with higher molecular weight whereas deeper groundwater sources were rich in % protein-like fluorescence. The greater % contribution of protein-like fluorescence in groundwater suggested that a larger fraction of groundwater DOM may be bioavailable. DOM for wetland groundwater was more aromatic and humic-like than that at the well-drained riparian location. Principal component analyses (PCA) revealed that the differences in surficial watershed compartments were dictated by humic-like components while groundwater sources separated out by % protein-like fluorescence. Observations from optical indices did not provide any conclusive evidence for preferential association of dissolved organic carbon (DOC) or dissolved organic nitrogen (DON) with any particular DOM quality pools.     

Distribution of dissolved organic carbon and nitrogen in a coral reef

Dissolved organic matter (DOM) concentrations in a fringing coral reef were measured for both carbon and nitrogen with the analytical technique of high-temperature catalytic oxidation. Because of high precision of the analytical system, not only the concentrations of dissolved organic carbon and nitrogen (DOC and DON, respectively) but the C:N ratio was also determined from the distribution of DOC and DON concentrations. The observed concentrations of DOC and DON ranged 57–76 and 3.8–5.6 μmol l⁻¹, respectively. The C:N ratios of the DOM that was produced on the reef flat were very similar between seagrass- and coral-dominated areas; the C:N ratio was 10 on average. The C:N ratio of DOM was significantly higher than that of particulate organic matter (POM) that was produced on the reef flat. Production rates of DOC were measured on the reef flat during stagnant periods and accounted for 3–7% of the net primary production, depending on the sampling site. The production rate of DON was estimated to be 10–30% of the net uptake of dissolved inorganic N in the reef community. Considering that the DOM and POM concentrations were not correlated with each other, a major source of the reef-derived DOM may be the benthic community and not POM such as phytoplankton. It was concluded that a widely distributed benthic community in the coral reef released C-rich DOM to the overlying seawater, conserving N in the community.     

Temporal and spatial distributions of dissolved organic carbon and nitrogen in two small lakes on the Southwestern China Plateau

Temporal and spatial distributions of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), chlorophyll-a and inorganic nitrogen were investigated in two small mountainous lakes (Lake Hongfeng and Baihua), on the Southwestern China Plateau, based on almost 2 years’ field observation. DOC concentrations ranged from 163 μM to 248 μM in Lake Hongfeng and from 143 μM to 308 μM in Lake Baihua, respectively, during the study period. DON concentrations ranged from 7 μM to 26 μM in Lake Hongfeng and from 14 μM to 47 μM in Lake Baihua. DOC showed vertical heterogeneity with higher concentrations in the epilimnion than in the hypolimnion during the stratification period. The DON concentration profiles appeared to be more variable than the DOC profiles. Apparent DON maxima occurred in the upper layer of water. In Lake Hongfeng, DOC concentration in the surface water was highest at the end of spring and early summer. DON concentration was 2–5 μM higher in May 2003 and in June 2004 than in adjacent months. DOC and chlorophyll-a concentrations were significantly correlated (r = 0.79, P < 0.05). The period of highest concentrations of DOC in Lake Hongfeng was also the season of concentrated rainfall. Algae activity and allochthonous input might result in an increase of DOC and DON concentrations together. In Lake Baihua, the maximum concentrations of DOC and DON in the surface water occurred simultaneously in May 2003 and February 2004. DOC concentrations were significantly correlated with DON (r = 0.90, P < 0.01), indicating the common sources. Allochthonous input, biological processes, stratification and mixing were the most important factors controlling the distributions and cycling of dissolved organic matter (DOM) and inorganic nitrogen in these two lakes. Inference from the corresponding vertical distributions of DOM and inorganic nitrogen indicated that DOM played potential roles in the internal loading of nitrogen and metabolism in the water body in these small lakes. The carbon/nitrogen (C/N) ratio showed a potential significance for tracing the source and biogeochemical processes of DOM in the lakes. These results are of significance in the further understanding of biogeochemical cycling and environmental effects of DOM and nitrogen in lake ecosystems.     

Detrital Controls on Soil Solution N and Dissolved Organic Matter in Soils: A Field Experiment

We established a long-term field study in an old growth coniferous forest at the H.J. Andrews Experimental Forest, OR, USA, to address how detrital quality and quantity control soil organic matter accumulation and stabilization. The Detritus Input and Removal Treatments (DIRT) plots consist of treatments that double leaf litter, double woody debris inputs, exclude litter inputs, or remove root inputs via trenching. We measured changes in soil solution chemistry with depth, and conducted long-term incubations of bulk soils from different treatments in order to elucidate effects of detrital inputs on the relative amounts and lability of different soil C pools. In the field, the addition of woody debris increased dissolved organic carbon (DOC) concentrations in O-horizon leachate and at 30 cm, but not at 100 cm, compared to control plots, suggesting increased rates of DOC retention with added woody debris. DOC concentrations decreased through the soil profile in all plots to a greater degree than did dissolved organic nitrogen (DON), most likely due to preferential sorption of high C:N hydrophobic dissolved organic matter (DOM) in upper horizons; percent hydrophobic DOM decreased significantly with depth, and hydrophilic DOM had a much lower and less variable C:N ratio. Although laboratory extracts of different litter types showed differences in DOM chemistry, percent hydrophobic DOM did not differ among soil solutions from different detrital treatments in the field, suggesting that microbial processing of DOM leachate in the field consumed easily degradable components, thus equalizing leachate chemistry among treatments. Total dissolved N leaching from plots with intact roots was very low (0.17 g m⁻² year⁻¹), slightly less than measured deposition to this very unpolluted forest (~s 0.2 g m⁻² year⁻¹). Total dissolved N losses showed significant increases in the two treatments without roots whereas concentrations of DOC decreased. In these plots, N losses were less than half of estimated plant uptake, suggesting that other mechanisms, such as increased microbial immobilization of N, accounted for retention of N in deep soils. In long-term laboratory incubations, soils from plots that had both above- and below-ground litter inputs excluded for 5 years showed a trend towards lower DOC loss rates, but not lower respiration rates. Soils from plots with added wood had similar respiration and DOC loss rates as control soils, suggesting that the additional DOC sorption observed in the field in these soils was stabilized in the soil and not readily lost upon incubation.     

Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams

Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-like components in chromophoric DOM. In addition, excitation–emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.     

Return of Salmon-Derived Nutrients from the Riparian Zone to the Stream during a Storm in Southeastern Alaska

Spawning salmon deliver nutrients (salmon-derived nutrients, SDN) to natal watersheds that can be incorporated into terrestrial and aquatic food webs, potentially increasing ecosystem productivity. Peterson Creek, a coastal watershed in southeast Alaska that supports several species of anadromous fish, was sampled over the course of a storm during September 2006 to test the hypothesis that stormflows re-introduce stored SDN into the stream. We used stable isotopes and PARAFAC modeling of fluorescence excitation–emission spectroscopy to detect flushing of DOM from salmon carcasses in the riparian zone back into a spawning stream. During the early storm hydrograph, streamwater concentrations of NH₄–N and total dissolved phosphorus (TDP), the fluorescent protein tyrosine and the δ¹⁵N content of DOM peaked, followed by a rapid decrease during maximum stormflow. Although δ¹⁵N has previously been used to track SDN in riparian zones, the use of fluorescence spectroscopy provides an independent indicator that SDN are being returned from the riparian zone to the stream after a period of intermediate storage outside the stream channel. Our findings further demonstrate the utility of using both δ¹⁵N of streamwater DOM and fluorescence spectroscopy with PARAFAC modeling to monitor how the pool of streamwater DOM changes in spawning salmon streams.     

Effects of storm events on mobilisation and in-stream processing of dissolved organic matter (DOM) in a Welsh peatland catchment

Peatlands are important contributors of dissolved organic matter (DOM) to downstream aquatic systems. We investigated the effects of storm events on dissolved organic carbon (DOC) concentrations and DOM quality in a stream draining a Welsh peatland catchment. Intensive stream samples were collected and analysed for pH, DOC, dissolved organic nitrogen (DON), absorbance and fluorescence. Soil water samples and samples of sphagnum pore water were also collected, and a simple end-member mixing model was applied to account for changes occurring during the events. Fluorescence data were interpreted using parallel factor analysis (PARAFAC). DOC concentrations increased and pH decreased during the storm events. The soil water data and the mixing model indicated that this was due to a change of flow paths and draining of the DOC-rich acrotelm. Absorbance indices and the DOC/DON ratio suggested that the DOM released during events was less degraded. There was a striking, inversely related diurnal pattern in absorbance and fluorescence after the discharge peak. The diurnal pattern and a lack of fit with the mixing model suggested that fluorescing DOM was mainly produced in-stream. Fluorescence has been found to peak in the morning and decline during day-time due to photo-bleaching. We hypothesise that the input of additional DOM during events causes a change in the diurnal pattern, giving a peak at mid-day, when the processing of the additional DOM is highest.     

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.     

马尾松与芒萁鲜叶及凋落物水溶性有机物的溶解特征和光谱学特征

为探讨福建省三明市马尾松人工林淋溶液中水溶性有机物(DOM)的溶解特征和光谱学特征,以马尾松(Pinus massoniana)和芒萁(Dicranopteris dichotoma)鲜叶、未分解层(L层)和半分解层(F层)凋落物为研究对象进行室内淋溶,对淋溶液中的水溶性有机碳(DOC)、水溶性有机氮(DON)与水溶性有机磷(DOP)含量和紫外光谱(SUVA)、荧光光谱指标和红外光谱(FTIR)等特征进行研究。结果表明,在24 h内,随着淋溶时间的延长,DOM含量呈现有波动的上升趋势;芒萁除鲜叶的DOC含量显著高于马尾松外(P0.01),其余DOM含量均小于马尾松。两植物F层DOM的SUVA值和腐殖化指标(HIX)都显著高于鲜叶和L层(P0.05),表明F层的芳香性化合物含量和腐殖化程度越来越高。DOM的同步荧光峰值显示淋溶液中含有类蛋白和类富里酸荧光团。两植物的红外光谱显示有5个相似的吸收谱带,强度最大的吸收来自于H键键合的-OH的伸缩振动,同一植物3种样品之间红外吸收的差异证明从鲜叶-L层-F层凋落物的共轭体系逐渐增大,结构更复杂。因此,随分解进程两植被DOM的化学结构越来越复杂;马尾松较芒萁含有更多的DON和DOP,这一方面为微生物提供了更多的养分,另一方面又增加了底物分解的难度。     

Characterization of Nanming River (southwestern China) sewerage-impacted pollution using an excitation-emission matrix and PARAFAC

Nanming River, the largest urban river in Guizhou Province, southwestern China plateau, has been severely polluted for decades. This study characterizes the organic materials and their sources in the upstream and downstream waters by dissolved organic carbon (DOC), excitation emission matrix (EEM) spectroscopy, parallel factor (PARAFAC) analysis and photo-microbial experiments. DOC concentrations were low (47–120 μM C) upstream and relatively high (146–462 μM C) downstream. The PARAFAC studies on the sample EEM spectra demonstrated that the upstream dissolved organic matter (DOM) was mostly composed of one component that had a fulvic acid-like substance; downstream DOM was composed of two components with mixtures of tryptophan-like and fulvic acid-like substances. From the results of the sewerage drainage samples collected along the bank of the river, it is evident that both household detergent-like and protein-like or tryptophan-like substances are predominantly present, indicating that untreated sewerage effluents are the major sources of organic matter pollution in Nanming River. The degradation experiments conducted on river, sewerage drainage and commercial detergent samples demonstrated that the detergent-like and tryptophan-like substances are both photochemically and microbiologically more decomposable than fulvic acid-like materials under sunlight and dark incubations. These results suggest that the input of the untreated sewerage effluents along the streams is the major pollution source in Nanming River, and the fluorescent DOM was efficiently affected by both photochemical and microbial processes.     

Three-dimensional fluorescence as a tool for investigating the dynamics of dissolved organic matter in the Lake Biwa watershed

Quantitative and qualitative characterizations of dissolved organic matter (DOM) were carried out at the watershed level in central Japan by measuring dissolved organic carbon (DOC) concentration and the three-dimensional excitation–emission matrix (3-D EEM). DOC concentration was low (mean 37 ± 19 µM C) in the upstream waters, whereas, in general, it increased toward the downstream areas (mean 92 ± 47 µM C). Significant variations in DOC concentration were detected among rivers and channels. DOC concentration in the epilimnion of Lake Biwa increased during the summer period and decreased during the winter period. The lake hypolimnion has lower DOC concentration (mean 87 ± 7 µM C) compared with the epilimnion (107 ± 15 µM C). Fulvic acid (FA)-like substances in the DOM were directly characterized by 3-D EEM. The fluorescence peak for upstream DOM was found in regions with longer wavelengths (excitation/emission 386 ± 6/476 ± 5 nm) compared with downstream and lake DOM (351 ± 12/446 ± 15 nm and 341 ± 6/434 ± 6 nm, respectively). The DOC concentration is correlated with fluorescence peak intensity of FA-like substances in DOM in river waters. Such a relationship was not found in lake DOM. A blueshift of the fluorescence peak from upstream to lake DOM was observed. A decrease in fluorescence intensities was also detected during the summer period. These results may suggest that the degradation of FA-like substances in DOM occurs from natural solar irradiation. Protein-like fluorescence was significantly detected in the lake epilimnion during the summer period. A linear relationship between DOC concentration and protein-like fluorescence indicated that an autochthonous input of DOM gave rise to the increase in DOC concentration in the lake epilimnion during the summer. These results may suggest that the 3-D EEM can be used as a tool for the investigation of DOM dynamics at the watershed level with concurrent measurement of DOC concentration and the fluorescence properties of fulvic acid-like and protein-like substances.     

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.     

Leaching of dissolved organic carbon,dissolved organic nitrogen,and other solutes from coarse woody debris and litter in a mixed forest in New York State

Coarse woody debris (CWD) may play a role in nutrient cycling in temperate forests through the leaching of solutes, including dissolved organic carbon (DOC) and dissolved organic nitrogen (DON), to the underlying soil. These fluxes need to be considered in element budget calculations, and have the potential to influence microbial activity, soil development, and other processes in the underlying soil, but studies on leaching from CWD are rare. In this study, we collected throughfall, litter leachate, and CWD leachate in situ at a young mixed lowland forest in NY State, USA over one year. We measured the concentrations of DOC, DON, NH₄⁺, NO₃⁻, dissolved organic sulfur, SO₄²⁻, Cl⁻, Al, Ca, K, Mg, Na, and P, estimated the flux of these solutes in throughfall, and measured the cover of CWD to gain some insight into possible fluxes from CWD. Concentrations of DOC were much higher in CWD leachate than in throughfall or litter leachate (15 vs. 0.7 and 1.6 mM, respectively), and greater than reported values for other leachates from within forested ecosystems. Other solutes showed a similar pattern, with inorganic N being an exception. Our results suggest that microsite scale fluxes of DOC from CWD may be An high relative to throughfall and litter leaching fluxes, but since CWD covered a relatively small fraction (2%) of the forest floor in our study, ecosystem scale fluxes from CWD may be negligible for this site. Soil directly beneath CWD may be influenced by CWD leaching, in terms of soil organic matter, microbial activity, and N availability. Concentrations of some metals showed correlations to DOC concentrations, highlighting the possibility of complexation by DOM. Several solute concentrations in throughfall, including DOC, showed positive correlations to mean air temperature, and fewer showed positive correlations in litter leachate, while negative correlations were observed to precipitation, suggesting both biological and hydrologic control of solute concentrations.     

Net release of dissolved organic matter by the scleractinian coral Acropora pulchra

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm^− 2 h^− 1, respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by ¹³C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm^− 2 and 23 µmol N cm^− 2, indicating that the release of DOC and DON accounted for 0.021% h^− 1 and 0.019% h^− 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm^− 2 h^− 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.     

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。     

Bioavailability and size-fraction of dissolved organic carbon, nitrogen, and sulfur at the Arbutus Lake watershed, Adirondack Mountains, NY

The heterogeneity of DOM is closely linked with the various sources and the diversity of biogeochemical processes. We studied the spatial and temporal patterns of the quantity (bioavailable [B-] and refractory concentrations by laboratory incubations) and quality (δ¹³C, aromaticity, and size-fraction by ultrafiltration [low molecular weight (LMW) < 1 kDa and high molecular weight (HMW) > 1 kDa]) of dissolved organic C (DOC), N (DON), and S (DOS) for surface waters (two upland streams, two wetland-affected streams, and lake outlet) over a 14-month period within the Arbutus Lake watershed in the Adirondack Mountains of New York State, USA. The % BDOC and % BDON of this watershed averages ranged from 6 to 18 % and from 12 to 43 %, respectively. The DOC and DON concentrations increased as water was transported through wetland areas of the Arbutus Lake watershed. DOC and DON constituents in the surface waters passing through a wetland were composed mostly of refractory HMW components (% HMWDOC: 55 %, % HMWDON: 60 %) with a higher level of aromaticity compared to upland streams (% HMWDOC: approximately 35 %, % HMWDON: approximately 30 %). DOS was dominated by the refractory (% BDOS range 6–13 %) and LMW (% LMWDOS range 62–96 %) form and we suggest that bacterial dissimilatory sulfate reduction might play an important role in generating this distinct DOS biogeochemistry. The aromaticity was positively related to total DOC concentration, but negatively to % BDOC. Arbutus Lake DOM was dominated by bioavailable and LMW characteristics, compared to wetland-affected streams where refractory and HMW DOM fractions were more prevalent. Our study suggests the different variability of DOM characteristics among elements (C, N, S) using a “bioavailability-molecular size model” showing a diagenetic perspective due to the relative refractoriness of the LMW DOS. This study also highlights the importance of multiple approaches for understanding DOM biogeochemistry with respect to molecular size, bioavailability, aromaticity, stoichiometry, isotopic values, and elemental concentrations.     

Atmospheric N Deposition Increases Organic N Loss from Temperate Forests

Atmospheric deposition of nitrogen (N) resulting from fossil fuel combustion has increased N inputs to temperate forests worldwide with large consequences for forest productivity and water quality. Recent work has illustrated that dissolved organic N (DON) often dominates N loss from unpolluted forests and that the relative magnitude of dissolved inorganic N (DIN) loss increases with atmospheric loading. In contrast to DIN, DON loss is thought to be controlled by soil dynamics that operate independently of N supply and demand and thus should track dissolved organic carbon (DOC) following strict stoichiometric constraints. Conversely, DON loss may shift with N supply if soil (SOM) or dissolved organic matter (DOM) is stoichiometrically altered. Here, we assess these two explanations of DON loss, which we refer to as the Passive Carbon Vehicle and the Stoichiometric Enrichment hypotheses, by analyzing patterns in soil and stream C and N in forest watersheds spanning a broad gradient in atmospheric N loading (5–45 kg N ha⁻¹ y⁻¹). We show that soil N and DON losses are not static but rather increase asymptotically with N loading whereas soil C and DOC do not, resulting in enrichment of organic N expressed as decreased soil C:N and stream DOC:DON ratios. DON losses from unpolluted sites are consistent with conservative dissolution and transport of refractory SOM. As N supply increases, however, N enrichment of organic losses is greater than expected from simple dissolution of enriched soils, suggesting activation of novel pathways of DON production or direct N enrichment of DOM. We suggest that our two hypotheses represent domains of control over forest DON loss as N supply increases but also that stoichiometric enrichment of bulk soils alone cannot fully account for large DON losses in the most N-polluted forests.