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.     

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.     

Acclimation effect on fatty acids of the coral Montipora digitata and its symbiotic algae

Lipids play a key role in thermal and photo-acclimation processes, yet they are often neglected in stress studies. We investigated the influence of different light intensities and an increase of temperature on the fatty acid composition of the coral Montipora digitata and its symbiotic algae (i.e., zooxanthellae). Coral branches were subjected to 3 different light intensities (7, 30 and 95% sea surface photosynthetic active radiation) in filtered seawater for 35 days. Fatty acids as methyl esters were determined using gas chromatography (GC) and verified by GC-mass spectrometry. Different light intensities, but only in combination with increased temperature, significantly affected the fatty acid composition of the coral host and zooxanthellae. Temperature and light intensity increases caused reductions in the proportion of polyunsaturated fatty acids in both the host and symbionts. Most changes occurred in the host coral, which suggests that the host is more susceptible to environmental change than the symbiont, or that the host shields the symbionts from environmental change.     

Effects of coral reef benthic primary producers on dissolved organic carbon and microbial activity

Benthic primary producers in marine ecosystems may significantly alter biogeochemical cycling and microbial processes in their surrounding environment. To examine these interactions, we studied dissolved organic matter release by dominant benthic taxa and subsequent microbial remineralization in the lagoonal reefs of Moorea, French Polynesia. Rates of photosynthesis, respiration, and dissolved organic carbon (DOC) release were assessed for several common benthic reef organisms from the backreef habitat. We assessed microbial community response to dissolved exudates of each benthic producer by measuring bacterioplankton growth, respiration, and DOC drawdown in two-day dark dilution culture incubations. Experiments were conducted for six benthic producers: three species of macroalgae (each representing a different algal phylum: Turbinaria ornata – Ochrophyta; Amansia rhodantha – Rhodophyta; Halimeda opuntia – Chlorophyta), a mixed assemblage of turf algae, a species of crustose coralline algae (Hydrolithon reinboldii) and a dominant hermatypic coral (Porites lobata). Our results show that all five types of algae, but not the coral, exuded significant amounts of labile DOC into their surrounding environment. In general, primary producers with the highest rates of photosynthesis released the most DOC and yielded the greatest bacterioplankton growth; turf algae produced nearly twice as much DOC per unit surface area than the other benthic producers (14.0±2.8 µmol h⁻¹ dm⁻²), stimulating rapid bacterioplankton growth (0.044±0.002 log10 cells h⁻¹) and concomitant oxygen drawdown (0.16±0.05 µmol L⁻¹ h⁻¹ dm⁻²). Our results demonstrate that benthic reef algae can release a significant fraction of their photosynthetically-fixed carbon as DOC, these release rates vary by species, and this DOC is available to and consumed by reef associated microbes. These data provide compelling evidence that benthic primary producers differentially influence reef microbial dynamics and biogeochemical parameters (i.e., DOC and oxygen availability, bacterial abundance and metabolism) in coral reef communities.     

Estrone and estradiol-17 beta concentration in tissue of the scleractinian coral, Montipora verrucosa

Spawnings of scleractinian corals are affected by light, temperature, and other environmental cues, but no studies elucidate physiological mechanisms that regulate coral gametogenesis. We hypothesized that estrogens may act as bioregulators of coral reproduction. Estrone (E1) and estradiol-17 beta (E2) concentrations were measured in homogenates of tissue and skeleton from M. verrucosa. Tissue samples were collected monthly throughout the year, and more frequently in July and August around spawning. Steroids were extracted with diethyl ether, purified via celite chromatography and assayed with radioimmunoassay. Non-specific binding in coral tissue varied with sample weight and was elevated relative to standards. Monthly mean E1 ranged from 20-70 ng E1 g ash-free dry weight (AFDW)-1, with highest values in April. Smaller asynchronous peaks occurred in early July, prior to spawning. Monthly mean E2 ranged from 8-25 ng E2 g AFDW-1, with highest values in February and March. Peaks in E2 preceded peaks in E1, indicating metabolism of a pool of estrogen. E1 was positively correlated with protein concentration, which is consistent with a bioregulatory role of estrogens. Estrogen peaks in spring and prior to the July spawn corroborate our hypothesis that estrogens regulate coral gametogenesis and spawning.     

Effects of herbivore exclusion and nutrient enrichment on coral reef macroalgae and cyanobacteria

Although phase shifts on coral reefs from coral-dominated to algal-dominated communities have been attributed to the effects of increased nutrient availability due to eutrophication and reduced herbivore abundance due to overfishing and disease, these factors have rarely been manipulated simultaneously. In addition, few studies have considered the effects of these factors on benthic, filamentous cyanobacteria (blue-green algae) as well as macroalgae. We used a combination of herbivore-exclusion cages and nutrient enrichment to manipulate herbivore abundance and nutrient availability, and measured the impacts of these treatments on macroalgal and cyanobacterial community structure. In the absence of cages, surface cover of the cyanobacterium Tolypothrix sp. decreased, while surface cover of the cyanobacteria Oscillatoria spp. increased. Cyanobacterial cover decreased in partial cages, and Tolypothrix sp. cover decreased further in full cages. Lower cyanobacterial cover and biomass were correlated with higher macroalgal cover and biomass. Dictyota bartayresiana dominated the partial cages, while Padina tenuis and Tolypiocladia glomerulata recruited into the full cages. Palatability assays demonstrated that herbivore-exclusion shifted macroalgal species composition from relatively unpalatable to relatively palatable species. Nutrient enrichment interacted with herbivore exclusion to increase the change in cover of D. bartayresiana in the uncaged and fully caged plots, but did not affect the final biomass of D. bartayresiana among treatments. Nutrient enrichment did not significantly affect the cover or biomass of any other taxa. These results stress the critical role of herbivory in determining coral reef community structure and suggest that the relative palatabilities of dominant algae, as well as algal growth responses to nutrient enrichment, will determine the potential for phase shifts to algal-dominated communities.     

Effects of desiccation and rewetting on the release and decomposition of dissolved organic carbon from benthic macroalgae

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转Bt水稻对土壤可溶性有机碳氮及微生物学性质的影响

研究了3种转Bt水稻［克螟稻（KMD）、华恢1号（HH1）和Bt汕优63（BtSY63）］及对应亲本在2年大田条件下对土壤可溶性有机碳（DOC）和可溶性有机氮（DON）及微生物学性质的影响.结果表明：测定指标均随采样时间发生显著变化.与对应亲本相比,转Bt水稻对土壤DOC、DON和微生物生物量氮（MBN）的影响不显著,而对土壤微生物生物量碳（MBC）、基础呼吸（BR）和微生物代谢熵（qCO₂）的影响在大田种植第1年某些时段达到显著水平（P<0.05）,但这种影响没有持续到第2年;3种亲本水稻土壤DOC、DON及微生物学性质差异均不显著,但相应转Bt水稻土壤MBC、BR、qCO₂差异显著,BtSY63土壤MBC和BR显著高于KMD及HH1,而qCO₂显著低于KMD及HH1.转Bt水稻2年的大田种植对土壤DOC和DON及微生物学性质的影响较小,但3种转Bt水稻之间微生物学性质的差异比亲本之间的差异大,表明长期监测可能有助于发现转Bt水稻对土壤生态系统结构和功能的影响.     

菌渣化肥配施对稻田土壤微生物量碳氮和可溶性碳氮的影响

菌渣作为一种养分丰富的有机物料还田,可减少化肥施用,同时保持土壤肥力;而土壤微生物量碳、氮和可溶性碳、氮是土壤活性碳氮库的重要组成部分,其含量和比例变化对土壤肥力均具有重要作用。因此,探讨不同比例菌渣化肥配施对土壤微生物量碳、氮及可溶性碳、氮的影响,评价菌渣在优化土壤肥力方面的生态作用具有重要意义。本研究在水稻田间定位试验条件下,设置3个化肥水平(C) 0%、50%、100%,菌渣相对用量(F) 0%、50%、100%,共9个处理,分析了各处理土壤微生物量碳(MBC)、氮(MBN)和可溶性碳(DOC)、氮(DON)的变化特征,及其占土壤有机碳(SOC)和全氮(TN)的比例与相关关系。结果表明:菌渣化肥配施后,微生物量碳和可溶性碳、氮均在C100F50最高,微生物量氮在C50F100最高,与不施肥处理相比,分别显著增加了49.40%、43.65%、83.52%、207.19%;MBC/SOC和DOC/SOC均随着菌渣化肥配施量的增加而减少,MBN/TN和DON/TN均在C100F50最高。相关分析表明,MBC、DOC与SOC,MBN与TN均呈极显著正相关,DON和TN呈显著正相关。总体来讲,菌渣化肥配施能够显著提高土壤微生物量碳、氮和可溶性碳、氮含量,但不是随着用量的增加一直呈增加趋势,高量菌渣或者化肥下会有降低趋势;菌渣化肥配施降低了土壤微生物量和可溶性碳氮比,因此适宜的菌渣化肥配施是提高土壤有机碳周转速度、微生物活性及其氮素供应能力和有效性的最佳选择。     

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.     

Effect of water currents on organic matter release by two scleractinian corals

Organic matter release by scleractinian corals fulfils an important ecological role as energy carrier and particle trap in reef ecosystems, but the hypothetically stimulating impact of water currents, an essential and ubiquitous environmental factor in coral reefs, on this process has not been investigated yet. This study therefore quantifies organic matter release by two species of scleractinian corals subjected to ambient water current velocities ranging from 4 to 16?cm?s^?1 using closed-system flow-through chambers. Findings revealed that particulate organic matter (POM) concentration was significantly increased in the flow-through chambers in all investigated coral species compared to still water conditions, while no effect on dissolved organic carbon (DOC) concentration could be observed. These results suggest that POM release by corals may be controlled by hydro-mechanical impacts, while DOC fluxes are rather influenced by the physiological condition of the corals. Hence, this study indicates that previous POM release quantification results are conservative estimates and may have underestimated in situ POM release through corals in reef environments. The contribution of coral-derived POM to biogeochemical cycles in reef ecosystems, therefore, may be more pronounced than already assumed.     

Controls on the release of dissolved organic carbon and nitrogen from a deciduous forest floor investigated by manipulations of aboveground litter inputs and water flux

Although dissolved organic matter (DOM) released from the forest floor plays a crucial role in transporting carbon and major nutrients through the soil profile, its formation and responses to changing litter inputs are only partially understood. To gain insights into the controlling mechanisms of DOM release from the forest floor, we investigated responses of the concentrations and fluxes of dissolved organic carbon (DOC) and nitrogen (DON) in forest floor leachates to manipulations of throughfall (TF) flow and aboveground litter inputs (litter removal, litter addition, and glucose addition) at a hardwood stand in Bavaria, Germany. Over the two-year study period, litter manipulations resulted in significant changes in C and N stocks of the uppermost organic horizon (Oi). DOC and DON losses via forest floor leaching represented 8 and 11% of annual litterfall C and N inputs at the control, respectively. The exclusion of aboveground litter inputs caused a slight decrease in DOC release from the Oi horizon but no change in the overall leaching losses of DOC and DON in forest floor leachates. In contrast, the addition of litter or glucose increased the release of DOC and DON either from the Oi or from the lower horizons (Oe + Oa). Net releases of DOC from the Oe + Oa horizons over the entire manipulation period were not related to changes in microbial activity (measured as rates of basal and substrate-induced respiration) but to the original forest floor depths prior to manipulation, pointing to the flux control by the size of source pools rather than a straightforward relationship between microbial activity and DOM production. In response to doubled TF fluxes, net increases in DOM fluxes occurred in the lower forest floor, indicating the presence of substantial pools of potentially soluble organic matter in the Oe + Oa horizons. In contrast to the general assumption of DOM as a leaching product from recent litter, our results suggest that DOM in forest floor leachates is derived from both newly added litter and older organic horizons through complex interactions between microbial production and consumption and hydrologic transport.     

Effects of herbivores, nutrient enrichment, and their interactions on macroalgal proliferation and coral growth

We conducted a 20-week manipulative field experiment on shallow forereefs of the Florida Keys to assess the separate and interactive effects of herbivory and nutrient enrichment on the development of macroalgal communities and the fitness of the corals Porites porites and Siderastrea siderea. Excluding large herbivorous fishes produced macrophyte blooms both with and without nutrient enrichment. In contrast, there were no direct effects of nutrient enrichment. There were, however, small, but significant, interactive effects of herbivory and enrichment on macroalgal cover. Following nutrient enrichment, total macroalgae and the common seaweeds Dictyota spp. were suppressed in the presence, but not in the absence, of large herbivorous fishes—suggesting that fishes were selectively feeding on nutrient-enriched macrophytes. Access by large herbivores prevented algal overgrowth of corals, but these large fishes also directly grazed both corals. Excluding fishes did not alter survivorship of either coral species, but did decrease parrotfish grazing scars on both corals and increased the net growth of P. porites. Nutrient additions had no direct effects on the survivorship of corals, but there was a trend (P = 0.097) for nutrients to stimulate the growth of P. porites. The preponderance of experiments available to date indicates that loss of key herbivores is a major factor driving macroalgal blooms on coral reefs; anthropogenic nutrient pollution generally plays a more minor role.     

The effect of land use on dissolved organic carbon and nitrogen uptake in streams

1. Agricultural and urban land use may increase dissolved inorganic nitrogen (DIN) concentrations in streams and saturate biotic nutrient demand, but less is known about their impacts on the cycling of organic nutrients. To assess these impacts we compared the uptake of DIN (as ammonium, NH₄⁺), dissolved organic carbon (DOC, as acetate), and dissolved organic nitrogen (DON, as glycine) in 18 low‐gradient headwater streams in southwest Michigan draining forested, agricultural, or urban land‐use types. Over 3 years, we quantified uptake in two streams in each of the three land‐use types during three seasons (spring, summer and autumn). 2. We found significantly higher NH₄⁺ demand (expressed as uptake velocity, V_f) in urban compared to forested streams and NH₄⁺V_f was greater in spring compared to summer and autumn. Acetate V_f was significantly higher than NH₄⁺ and glycine V_f, but neither acetate nor glycine V_f were influenced by land‐use type or season. 3. We examined the interaction between NH₄⁺ and acetate demand by comparing simultaneous short‐term releases of both solutes to releases of each solute individually. Acetate V_f did not change during the simultaneous release with NH₄⁺, but NH₄⁺V_f was significantly higher with increased acetate. Thus, labile DOC V_f was not limited by the availability of NH₄⁺, but NH₄⁺V_f was limited by the availability of labile DOC. In contrast, neither glycine nor NH₄⁺V_f changed when released simultaneously indicating either that overall N‐uptake was saturated or that glycine and NH₄⁺ uptake were controlled by different factors. 4. Our results suggest that labile DOC and DON uptake can be equivalent to, or even higher than NH₄⁺ uptake, a solute known to be highly bioreactive, but unlike NH₄⁺ uptake, may not differ among land‐use types and seasons. Moreover, downstream export of nitrogen may be exacerbated by limitation of NH₄⁺ uptake by the availability of labile DOC in headwater streams from the agricultural Midwestern United States. Further research is needed to identify the factors that influence cycling of DOC and DON in streams.     

氮素添加和CO2浓度升高对白羊草根际和非根际土壤水溶性有机碳、氮的影响

采用盆栽控制试验对黄土丘陵区白羊草在不同CO₂浓度(400和800 μmol·mol^-1)和施氮水平(0、2.5、5.0 g N·m^-2·a^-1)条件下根际和非根际土壤水溶性有机碳(DOC)和水溶性有机氮(DON)的变化特征进行研究.结果表明: CO₂浓度升高对白羊草根际和非根际土壤DOC、水溶性总氮(DTN)、DON、水溶性铵态氮(NH₄⁺-N)、水溶性硝态氮(NO₃^--N)含量均无显著影响.施氮显著提高了根际和非根际土壤DTN、NO₃^--N含量和根际土壤DON含量,显著降低了根际土壤DOC/DON.在各处理条件下,根际土壤DTN、NO₃^--N和DON含量均显著低于非根际土壤,根际土壤DOC/DON显著高于非根际土壤.短期CO₂浓度升高对黄土丘陵区土壤水溶性有机碳、氮含量无显著影响,而氮沉降的增加在一定程度上改善了土壤中水溶性氮素缺乏的状况,但并不足以满足植被对水溶性氮素的需求.     

Response of a scleractinian coral, Stylophora pistillata, to iron and nitrate enrichment

The purpose of this study was to determine whether the addition of iron alone or in combination with nitrate affects growth and photosynthesis of the scleractinian coral, Stylophora pistillata, and its symbiotic dinoflagellates. For this purpose, we used three series of two tanks for a 3-week enrichment with iron (Fe), nitrate (N) and nitrate+iron (NFe). Two other tanks were kept as a control (C). Stock solutions of FeCl(3) and NaNO(3) were diluted to final concentrations of 6 nM Fe and 2 &mgr;M N and continuously pumped from batch tanks into the experimental tanks with a peristaltic pump. Results obtained showed that iron addition induced a significant increase in the areal density of zooxanthellae (ANOVA, p=0.0013; change from 6.3+/-0.7x10(5) in the control to 8.5+/-0.6x10(5) with iron). Maximal gross photosynthetic rates normalized per surface area also significantly increased following iron enrichment (ANOVA, p=0.02; change from 1.23+/-0.08 for the control colonies to 1.81+/-0.24 &mgr;mol O(2) cm(-2) h(-1) for the iron-enriched colonies). There was, however, no significant difference in the photosynthesis normalized on a per cell basis. Nitrate enrichment alone (2 &mgr;M) did not significantly change the zooxanthellae density or the rates of photosynthesis. Nutrient addition (both iron and nitrogen) increased the cell-specific density of the algae (CSD) compared to the control (G-test, p=0.3x10(-9)), with an increase in the number of doublets and triplets. CSD was equal to 1.70+/-0.04 in the Fe-enriched colonies, 1.54+/-0.12 in the N- and NFe-enriched colonies and 1.37+/-0.02 in the control. Growth rates measured after 3 weeks in colonies enriched with Fe, N and NFe were 23%, 34% and 40% lower than those obtained in control colonies (ANOVA, p=0.011).     

Effects of dissolved organic carbon and salinity on bioavailability of mercury.

Hypotheses that dissolved organic carbon (DOC) and electrochemical charge affect the rate of methylmercury [CH3Hg(I)] synthesis by modulating the availability of ionic mercury [Hg(II)] to bacteria were tested by using a mer-lux bioindicator (O. Selifonova, R. Burlage, and T. Barkay, Appl. Environ. Microbiol. 59:3083-3090, 1993). A decline in Hg(II)-dependent light production was observed in the presence of increasing concentrations of DOC, and this decline was more pronounced at pH 7 than at pH 5, suggesting that DOC is a factor controlling the bioavailability of Hg(II). A thermodynamic model (MINTEQA2) was used to select assay conditions that clearly distinguished among various Hg(II) species. By using this approach, it was shown that negatively charged forms of mercuric chloride (HgCl3-/HgCl(4)2-) induced less light production than the electrochemically neutral form (HgCl2), and no difference was observed between the two neutral forms, HgCl2 and Hg(OH)2. These results suggest that the negative charge of Hg(II) species reduces their availability to bacteria and may be one reason why accumulation of CH3Hg(I) is more often reported to occur in freshwater than in estuarine and marine biota.     

Controls on the dynamics of dissolved organic carbon and nitrogen in a Central European deciduous forest

Despite growing attention concerning therole of dissolved organic matter (DOM) inelement cycling of forest ecosystems, thecontrols of concentrations and fluxes of bothdissolved organic carbon (DOC) and nitrogen(DON) under field conditions in forest soilsremain only poorly understood. The goal ofthis project is to measure the concentrations and fluxes of DON, NH₄ ⁺, NO₃ ^–and DOC in bulkprecipitation, throughfall, forest floorleachates and soil solutions of a deciduousstand in the Steigerwald region (northernBavaria, Germany). The DOC and DONconcentrations and fluxes were highest inleachates originating from the Oa layer of theforest floor (73 mg C L^–1, 2.3 mg NL^–1 and about 200–350 kg C, 8–10 kg Nha^–1 yr^–1). They were observed to behighly variable over time and decreased in themineral topsoil (17 mg C L^–1, 0.6 mg NL^–1 and about 50–90 kg C, 2.0 to 2.4 kg Nha^–1 yr^–1). The annual variability ofDOC and DON concentrations and subsequentialDOC/DON ratios was substantial in allsolutions. The DOC and DON concentrations inthroughfall were positively correlated withtemperature. The DOC and DON concentrationsdid not show seasonality in the forest floorand mineral soil. Concentrations were notrelated to litterfall dynamics but didcorrespond in part to the input of DOC and DONfrom throughfall. The throughfall contributionto the overall element fluxes was higher forDON than for DOC. Concentrations and fluxes ofDON were significantly correlated to DOC inthroughfall and the Oi layer. However, thecorrelation was weak in Oa leachates. Inaddition, seasonal and annual variation ofDOC/DON ratios indicated different mechanismsand release rates from the forest floor forboth components. The concentrations of DOC andDON in forest floor leachates were in mostcases dependent neither on the pH value orionic strength of the solution, nor on thewater flux or temperature changes. As aconsequence, the DOC and DON fluxes from theforest floor into the mineral soil werelargely dependent on the water flux if annualand biweekly time scales are considered.     

Degradability of dissolved soil organic carbon and nitrogen in relation to tree species

The degradability and chemical characteristics of water-extractable dissolved organic carbon (DOC) and nitrogen (DON) from the humus layer of silver birch (Betula pendula Roth), Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) stands were compared in short-term incubation of soil solutions. For all extracts the degradation of DOC and DON was low (12-17% loss) and increased in the order: birch, spruce and pine. In the humus layer under pine a relatively larger pool of rapidly degrading dissolved soil organic matter (DOM) was indicated by the [3H]thymidine incorporation technique, which measures the availability of DOM to bacteria. The degradation of DOC was explained by a decrease in the hydrophilic fraction. For DON, however, both the hydrophilic and hydrophobic fractions tended to decrease during incubation. No major differences in concentrations of hydrophilic and hydrophobic fractions were detected between tree species. Molecular size distribution of DOC and DON, however, revealed slight initial differences between birch and conifers as well as a change in birch extract during incubation. The depletion of very rapidly degrading fractions (e.g., root exudates and compounds from the litter) may explain the low degradability of DOM in the humus layer under birch.