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.     

Riparian nitrogen dynamics in two geomorphologically distinct tropical rain forest watersheds: subsurface solute patterns

Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian zones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN; NH₄ ⁺- and NO₃ ⁻-N) were low in stream water (< 110 ug/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO₃ ⁻-N (550 ug/L) and oxygen concentrations were high (5.2 mg/L). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH₄ ⁺-N (200–700 ug/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 ug/L) than at Icacos (600 ug/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH₄ ⁺ and NO₃ ⁻ were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appear to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes through the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zone. However, geomorphology appears to strongly modify the importance of specific biotic components.     

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH₄-N + NO₃-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN– DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kgha^{minus 1} yr^{minus 1}) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO₃-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO₃-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.     

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.     

Fate and Transport of Organic Nitrogen in Minimally Disturbed Montane Streams of Colorado,USA

In two montane watersheds that receive minimal deposition of atmospheric nitrogen, 15–71% of dissolved organic nitrogen (DON) was bioavailable in stream water over a 2-year period. Discharge-weighted concentrations of bulk DON were between 102 and 135 μg/l, and the C:N ratio differed substantially between humic and non-humic fractions of DON. Approximately 70% of DON export occurred during snowmelt, and 40% of that DON was biologically available to microbes in stream sediments. Concentrations of bioavailable DON in stream water were 2–16 times greater than dissolved inorganic nitrogen (DIN) during the growing season, and bioavailable DON was depleted within 2–14 days during experimental incubations. Uptake of DON was influenced by the concentration of inorganic N in stream water, the concentration of non-humic DON in stream water, and the C:N ratio of the non-humic fraction of dissolved organic matter (DOM). Uptake of DON declined logarithmically as the concentration of inorganic N in stream water increased. Experimental additions of inorganic N also caused a decline in uptake of DON and net production of DON when the C:N ratio of non-humic DOM was high. This study indicates that the relative and absolute amount of bioavailable DON can vary greatly within and across years due to interactions between the availability of inorganic nutrients and composition of DOM. DOM has the potential to be used biotically at a high rate in nitrogen-poor streams, and it may be generated by heterotrophic microbes when DIN and labile DOM with low relative nitrogen content become abundant.     

Impacts of Labile Organic Carbon Concentration on Organic and Inorganic Nitrogen Utilization by a Stream Biofilm Bacterial Community

In aquatic ecosystems, carbon (C) availability strongly influences nitrogen (N) dynamics. One manifestation of this linkage is the importance in the dissolved organic matter (DOM) pool of dissolved organic nitrogen (DON), which can serve as both a C and an N source, yet our knowledge of how specific properties of DOM influence N dynamics are limited. To empirically examine the impact of labile DOM on the responses of bacteria to DON and dissolved inorganic nitrogen (DIN), bacterial abundance and community composition were examined in controlled laboratory microcosms subjected to various combinations of dissolved organic carbon (DOC), DON, and DIN treatments. Bacterial communities that had colonized glass beads incubated in a stream were treated with various glucose concentrations and combinations of inorganic and organic N (derived from algal exudate, bacterial protein, and humic matter). The results revealed a strong influence of C availability on bacterial utilization of DON and DIN, with preferential uptake of DON under low C concentrations. Bacterial DON uptake was affected by the concentration and by its chemical nature (labile versus recalcitrant). Labile organic N sources (algal exudate and bacterial protein) were utilized equally well as DIN as an N source, but this was not the case for the recalcitrant humic matter DON treatment. Clear differences in bacterial community composition among treatments were observed based on terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes. C, DIN, and DON treatments likely drove changes in bacterial community composition that in turn affected the rates of DON and DIN utilization under various C concentrations.     

Dissolved organic carbon and nitrogen in urban and rural watersheds of south-central Texas: land use and land management influences

Dissolved organic carbon (DOC) and nitrogen (DON) concentrations were quantified in urban and rural watersheds located in central Texas, USA between 2007 and 2008. The proportion of urban land use ranged from 6 to 100% in our 12 study watersheds which included nine watersheds without waste water treatment plants (WWTP) and three watersheds sampled downstream of a WWTP. Annual mean DOC concentrations ranged 20.4–52.5 mg L^?1. Annual mean DON concentrations ranged 0.6–1.9 mg L^?1. Only the rural watersheds without a WWTP had significantly lower DOC concentrations compared to those watersheds with a WWTP but all the streams except two had significantly reduced DON compared to those with a WWTP. Analysis of the nine watersheds without a WWTP indicated that 68% of the variability in mean annual DOC concentration was explained by urban open areas such as golf courses, sports fields and neighborhood parks under turf grass. There was no relationship between annual mean DON concentration and any land use. Urban open area also explained a significant amount of the variance in stream sodium and stream sodium adsorption ratio (SAR). Ninety-four percent of the variance in annual mean DOC concentration was explained by SAR. Irrigation of urban turf grass with domestic tap water high in sodium (>181 mg Na⁺ L^?1) may be inducing sodic soil conditions in watershed soils in this region resulting in elevated mean annual DOC concentrations in our streams.     

Bioavailability and composition of dissolved organic carbon and nitrogen in a near coastal catchment of south-western Australia

The bioavailability and composition of dissolved organic carbon (DOC) and nitrogen (DON) were examined in 10 major sub-catchments of the Swan-Canning estuary, which bisects the city of Perth, in south-western Australia. Catchments contain a mix of forest, agriculture, and urban-dominated land-use, with the degree of development increasing near the city center. We incubated water samples from the 10 sub-catchments for 14 days at 25°C, and measured changes in DOC and DON and dissolved inorganic nitrogen (DIN). A greater proportion of DON (4–44%) was decomposed compared to DOC (1–17%). Both agricultural and urban catchments had high proportions of bioavailable DOC and DON, but overall DOC and DON losses were greatest in urban catchments. Using resin isolation techniques, we found that DOC was concentrated in the hydrophobic (humic) fraction, whereas DON had both hydrophobic and hydrophilic (non-humic) fractions. Hydrophobic DOC content was positively related to DOC decomposition. In contrast, DON decomposition was highly correlated with hydrophilic DON content and inversely related to the hydrophilic DOC/DON ratio, indicating a labile fraction of DON from non-humic sources. Taken together, these relationships suggest that bioavailable DOC may be supplied in part from terrestrial plant material, but bioavailable DON is likely to be from highly labile sources, possibly autochthonous or anthropogenic. Overall, labile DON was greater than initial DIN concentration at seven of ten sites and was even dominant in highly developed catchments. This study highlights the importance of organic N in urbanizing coastal catchments that, in addition to DIN, may serve as a readily available source of N for in-stream and estuarine production.     

Salt impacts on organic carbon and nitrogen leaching from senesced vegetation

Senesced vegetation is exposed to a wide range of salt concentrations in surface waters resulting from human activities which include deicing salts and irrigation water chemistry. Both dissolved organic carbon (DOC) and salt concentrations are rising in northern hemisphere watersheds, yet there has been little investigation of sodium as a potential mechanism for DOC increases. The objective of this study was to investigate the impact of solution sodicity and salinity on DOC and dissolved organic nitrogen (DON) leaching from five types of senesced and cut vegetation. Vegetation was soaked for 24 h in a series of sodium chloride (NaCl)–calcium chloride (CaCl₂) solutions with sodium adsorption ratios (SAR) of 2, 10, or 30 and electrical conductivities of 0.1 dS m^?1 through 3.0 dS m^?1. Vegetation was also soaked in a sodium bicarbonate (NaHCO₃) solution at SAR = 30 and stream water from local watersheds with a range of sodicity and salinity. The mass of both DOC and DON released increased as SAR increased in the NaCl solutions, but the total salinity had inconsistent effects on DOC and DON release. NaHCO₃ leached similar amounts of DOC and DON as NaCl. The SAR of the stream water solutions was able to explain 88 % of the variability in DOC leached from vegetation (p < 0.05). The results indicated that sodicity, quantified by SAR, had a significant impact on DOC and DON leaching from senesced vegetation and could be a potential mechanism to explain the observed increases in surface water DOC.     

Hydrological and biological processes modulate carbon,nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec,Canada)

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO₃) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.     

An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils

Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH₄ ⁺–N?+?NO₃ ^?–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25?cm depth. Fluxes of DIN were generally dominated by NO₃ ^?–N; but NH₄ ⁺–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH₄ ⁺–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH₄ ⁺–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p?<?0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p?<?0.01) and surface litter (p?<?0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively.     

Biogeochemical processes in the groundwater discharge zone of urban streams

The influence of biogeochemical processes on nitrogen and organic matter transformation and transport was investigated for two urban streams receiving groundwater discharge during the dry summer baseflow period. A multiple lines of evidence approach involving catchment-, and stream reach-scale investigations were undertaken to describe the factors that influence pore water biogeochemical processes. At the catchment-scale gaining stream reaches were identified from water table mapping and groundwater discharge estimated to be between 0.1 and 0.8 m³ m^?2 d^?1 from baseflow analysis. Sediment temperature profiles also suggested that the high groundwater discharge limited stream water infiltration into the sediments. At the stream reach-scale, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations were higher in stream water than in groundwater. However, DOC and DON concentrations were greatest in sediment pore water. This suggests that biodegradation of sediment organic matter contributes dissolved organic matter (DOM) to the streams along with that delivered with groundwater flow. Pore water ammonium (NH₄ ⁺) was closely associated with areas of high pore water DOM concentrations and evidence of sulfate (SO₄ ^2?) reduction (low concentration and SO₄:Cl ratio). This indicates that anoxic DOM mineralization was occurring associated with SO₄ ^2? reduction. However the distribution of anoxic mineralization was limited to the center of the streambed, and was not constrained by the distribution of sediment organic matter which was higher along the banks. Lower sediment temperatures measured along the banks compared to the center suggests, at least qualitatively, that groundwater discharge is higher along the banks. Based on this evidence anoxic mineralization is influenced by groundwater residence time, and is only measurable along the center of the stream where groundwater flux rates are lower. This study therefore shows that the distribution of biogeochemical processes in stream sediments, such as anoxic mineralization, is strongly influenced by both the biogeochemical conditions and pore water residence time.     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

亚热带典型林分对表层和深层土壤可溶性有机碳、氮的影响

林分类型是影响土壤可溶性有机碳、氮库大小的重要因素,但目前对其研究主要集中在表层土壤(0～10 cm).本研究以亚热带地区天然林、毛竹林、格式栲人工林和杉木人工林为对象,用3种不同的浸提方式(冷水、热水和KCl溶液)提取表层(0～10 cm)和深层(40～60 cm)土壤中可溶性有机碳(DOC)和有机氮(DON),研究林分类型对表层和深层土壤可溶性有机碳、氮库的影响.结果表明: 林分类型对表层土壤DOC及其占土壤总有机碳(TOC)的比重有显著影响,深层土壤受林分类型的影响不显著;不同林分土壤DON含量仅在表层土壤存在显著差异,在深层土壤差异不显著.林分间土壤微生物生物量碳的差异仅在表层土壤达到显著水平.DON占土壤总氮(TN)的比重在各林分表层和深层土壤间差异均不显著.3种浸提方法得到的DOC和DON库大小顺序为热水>KCl>冷水,不同浸提方法得到的DOC库及DON库的相关性均达到显著水平,表明冷水、热水和KCl溶液浸提得到的有机碳、氮库含有相似组分.冷水和热水浸提方法得到的表层土壤DOC和DON含量及DOC占TOC比重在天然林和毛竹林均显著大于格式栲和杉木人工林,表明天然林和毛竹林土壤可溶性有机碳、氮含量高于格式栲和杉木人工林,更有利于土壤肥力的恢复.     

Nitrogen retention in the riparian zone of catchments underlain by discontinuous permafrost

1. Riparian zones function as important ecotones that reduce nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. 2. In this study, riparian zone nitrogen retention was examined in a high permafrost catchment (approximately 53% of land area underlain by permafrost) and a low permafrost catchment (approximately 3%). To estimate the contribution of the riparian zone to catchment nitrogen retention, we analysed groundwater chemistry using an end‐member mixing model. 3. Stream nitrate concentration was over twofold greater in the low permafrost catchment than the high permafrost catchment. Riparian groundwater was not significantly different between catchments, averaging 13 μm overall. Nitrogen retention, measured using the end‐member mixing model, averaged 0.75 and 0.22 mmol N m^?2 day^?1 in low and high permafrost catchments, respectively, over the summer. The retention rate of nitrogen in the riparian zone was 10–15% of the export in stream flow. 4. Our results indicate that the riparian zone functions as an important sink for groundwater nitrate and dissolved organic carbon (DOC). However, differences in stream nitrate and DOC concentrations between catchments cannot be explained by solute inputs from riparian groundwater to the stream and differences between streams are probably attributable to deeper groundwater inputs or flows from springs that bypass the riparian zone.     

Effect of High Flow Events on In-Stream Dissolved Organic Nitrogen Concentration

Dissolved organic nitrogen (DON) can comprise a large and biologically important fraction of total dissolved N in surface water. Biotic and abiotic processes result in heterogeneous DON concentrations and bioavailability in soils, and as hydrologic connectivity expands and flow paths change in watersheds, novel sources of DON can be mobilized and transported to surface water. Although the relationship between in-stream DOC concentration and stream discharge has previously been examined in the literature, up to now there has not been a synthesis examining how DON concentrations, loads, and composition change during transitions from base flow to pulse flow conditions. We present a meta-analysis examining the effect of high flow on DON concentration ([DON]). The ratio of mean pulse flow [DON] to mean base flow [DON] (P:B) was calculated for individual events and averaged (geometric) within and then across sites to generate an overall effect size. For 47 sites (78 events), mean P:B was 1.58, which was significantly different from unity. This moderate increase in DON concentration contributed to over a more than 10-fold average increase in the rate of DON yield from base flow to high flow. The response of [DON] to high flow was significant in catchments where individual storm events or snowmelt runoff events were responsible for elevated flows, whereas the response was not significant in catchments where high discharge resulted from a mixture of upstream snowmelt and rain events. Additionally, an examination of DOC:DON ratios during high flow indicates that multiple sources of DON may be mobilized during high flow. Finally, current models of annual DON export may be improved by including a positive relationship between discharge and DON.     

Effect of Reduced Winter Precipitation and Increased Temperature on Watershed Solute Flux, 1988–2002, Northern Michigan

Since 1987 we have studied weekly change in winter (December–April) precipitation, snowpack, snowmelt, soil water, and stream water solute flux in a small (176-ha) Northern Michigan watershed vegetated by 65–85 year-old northern hardwoods. Our primary study objective was to quantify the effect of change in winter temperature and precipitation on watershed hydrology and solute flux. During the study winter runoff was correlated with precipitation, and forest soils beneath the snowpack remained unfrozen. Winter air temperature and soil temperature beneath the snowpack increased while precipitation and snowmelt declined. Atmospheric inputs declined for H⁺, NO₃⁻, NH₄⁺, dissolved inorganic nitrogen (DIN), and SO₄²⁻. Replicated plot-level results, which could not be directly extrapolated to the watershed scale, showed 90% of atmospheric DIN input was retained in surface shallow (<15 cm deep) soils while SO₄²⁻ flux increased 70% and dissolved organic carbon (DOC) 30-fold. Most stream water base cation (C_B), HCO₃⁻, and Cl⁻ concentrations declined with increased stream water discharge, K⁺, NO₃⁻, and SO₄²⁻ remained unchanged, and DOC and dissolved organic nitrogen (DON) increased. Winter stream water solute outputs declined or were unchanged with time except for NO₃⁻ and DOC which increased. DOC and DIN outputs were correlated with the percentage of winter runoff and stream discharge that occurred when subsurface flow at the plot-level was shallow (<25 cm beneath Oi). Study results suggest that the percentage of annual runoff occurring as shallow lateral subsurface flow may be a major factor regulating solute outputs and concentrations in snowmelt-dominated ecosystems.     

Potential effects of leaf litter on water quality in urban watersheds

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

Degrading seagrass (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) ecosystems: a source of dissolved matter in the Mediterranean

Diurnal variation of dissolved oxygen (DO), organic and inorganic carbon (DOC, DIC), nitrogen (DON, DIN), and phosphorus (DOP, DIP) flux across the sediment–water interface was assessed in fish farm impacted and pristine seagrass (Posidonia oceanica) meadows in the Aegean Sea (Greece). DIC consumption decreased by 52% and DO production decreased by 60% in the light, suggesting reduced photosynthetic performance of the plant community under the fish cages probably due to organic matter loading. In light there was 4 and 15 times higher release of dissolved inorganic and organic matter, respectively, compared to dark incubations under the cages, indicating that fish farming impact is more intense during daytime. DO was taken up, while DIC was released in the dark in both stations, representing a direct measure of mineralization. Dissolved inorganic matter flux (as the sum of DIN and DIP fluxes) was positively related to DIC flux, rendering mineralization as the main driver of nutrient flux under the cages. On average, the impacted meadow released DIN and DIP both in light and dark, while efflux of dissolved organic matter (as the sum of DOC, DON, and DOP fluxes) increased by 132% in the light and by 21% in the dark, implying that the degrading seagrass meadow is a source of dissolved matter to the surrounding water. Shoot density and leaf production were negatively correlated with both diel DIN and DIP fluxes, showing that meadow regression is accompanied by DIN and DIP release from the sediment. Hence, nutrient efflux can adequately illustrate meadow deterioration and, therefore, can be used as indicator of P. oceanica community health.     

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.