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.     

Watershed Glacier Coverage Influences Dissolved Organic Matter Biogeochemistry in Coastal Watersheds of Southeast Alaska

The Coast Mountains of southeast Alaska are currently experiencing some of the highest rates of glacier volume loss on Earth, with unknown implications for proglacial stream biogeochemistry. We analyzed streamwater for δ¹⁸O and dissolved organic matter (DOM) biogeochemistry (concentration, δ¹³C-dissolved organic carbon (DOC), and fluorescence characterization) during the 2012 glacial runoff season from three coastal watersheds in southeast Alaska that ranged in glacier coverage from 0 to 49% and a glacier outflow stream. Our goal was to assess how DOM biogeochemistry may change as receding glaciers are replaced by forests and glaciers contribute less meltwater to streamflow. Discharge and streamwater δ¹⁸O varied seasonally reflecting varying contributions of rainfall and snow/icemelt to streamflow over the runoff season. Mean DOC concentrations were lowest in the glacial outflow and highest in the non-glacial stream reflecting an increasing contribution of vascular plant-derived carbon with decreasing watershed glaciation. Fluorescence and δ¹³C-DOC signatures indicated that DOM shifted from vascular plant-derived, humic-like material in the non-glacial stream toward more δ¹³C-DOC enriched, glacier-derived DOM in the glacial outflow. Streamwater δ¹⁸O was significantly correlated to DOC concentration, δ¹³C-DOC, and protein-like fluorescence of streamwater DOM (all P < 0.05), demonstrating that changes in the source of streamwater across the glacial watershed continuum have important implications for the amount and quality of stream DOM export. Overall, our findings show that continued glacial recession and subsequent changes in glacial runoff could substantially influence the biogeochemistry of coastal temperature watersheds by altering the timing, magnitude, and chemical signature of DOM delivered to streams.     

黄土高原不同植被覆盖对流域水文的影响

以山西省吉县蔡家川流域为对象,研究了植被覆盖类型对流域水文的影响.结果表明:不同植被覆盖的流域年径流系数分别为:林地流域1.6%～2.3%,以农、牧为主的流域3.1%～3.9%;各流域基流系数差异显著,人工林流域为零,次生林为主的流域1.0%～1.5%,以农、牧为主的流域2.5%～2.8%;在雨季人工林流域的径流总量是次生林流域的3.37倍、农地流域的1.9倍,而农地流域的基流量是次生林流域的2.2倍;短历时高强度降雨条件下,人工林流域、次生林流域地表径流量分别是农地流域的10.8倍和2.2倍;在历时较长的暴雨条件下,人工林流域单位面积上的洪峰流量是农地流域的3.4倍,次生林流域的6.9倍;在长历时、大雨量条件下,农地流域的径流量是次生林流域的1.8倍.水平梯田的水源涵养功能与次生林植被相当,次生林植被的水源涵养功能远好于人工植被,在水资源短缺的黄土高原应提倡植被的自然恢复.     

Amazon deforestation alters small stream structure, nitrogen biogeochemistry and connectivity to larger rivers

Human activities that modify land cover can alter the structure and biogeochemistry of small streams but these effects are poorly known over large regions of the humid tropics where rates of forest clearing are high. We examined how conversion of Amazon lowland tropical forest to cattle pasture influenced the physical and chemical structure, organic matter stocks and N cycling of small streams. We combined a regional ground survey of small streams with an intensive study of nutrient cycling using ¹⁵N additions in three representative streams: a second-order forest stream, a second-order pasture stream and a third-order pasture stream. These three streams were within several km of each other and on similar soils. Replacement of forest with pasture decreased stream habitat complexity by changing streams from run and pool channels with forest leaf detritus (50% cover) to grass-filled (63% cover) channel with runs of slow-moving water. In the survey, pasture streams consistently had lower concentrations of dissolved oxygen and nitrate (NO₃ ^?) compared with similar-sized forest streams. Stable isotope additions revealed that second-order pasture stream had a shorter NH₄ ⁺ uptake length, higher uptake rates into organic matter components and a shorter ¹⁵NH₄ ⁺ residence time than the second-order forest stream or the third-order pasture stream. Nitrification was significant in the forest stream (19% of the added ¹⁵NH₄ ⁺) but not in the second-order pasture (0%) or third-order (6%) pasture stream. The forest stream retained 7% of added ¹⁵N in organic matter compartments and exported 53% (¹⁵NH₄ ⁺?=?34%; ¹⁵NO₃ ^??=?19%). In contrast, the second-order pasture stream retained 75% of added ¹⁵N, predominantly in grasses (69%) and exported only 4% as ¹⁵NH₄ ⁺. The fate of tracer ¹⁵N in the third-order pasture stream more closely resembled that in the forest stream, with 5% of added N retained and 26% exported (¹⁵NH₄ ⁺?=?9%; ¹⁵NO₃ ^??=?6%). These findings indicate that the widespread infilling by grass in small streams in areas deforested for pasture greatly increases the retention of inorganic N in the first- and second-order streams, which make up roughly three-fourths of total stream channel length in Amazon basin watersheds. The importance of this phenomenon and its effect on N transport to larger rivers across the larger areas of the Amazon Basin will depend on better evaluation of both the extent and the scale at which stream infilling by grass occurs, but our analysis suggests the phenomenon is widespread.     

Stream chemistry and hydrologic pathways during snowmelt in a small watershed adjacent Lake Superior

In regions with airborne contaminants and large snowpacks, there is concern over the impact that snowmelt chemical pulses — periods of sharp increase in meltwater solute concentration — could have on aquatic resources during spring runoff. A major variable in determining such an effect is the flow path of snowmelt solutes to the stream or lake. From December 1988, to late April 1989, the quality and quantity of precipitation, snowmelt, soil solution and streamwater were measured in a 176-ha gauged watershed on the south shore of Lake Superior. The main objectives were to (1) examine the change in flow path meltwaters take to the stream during distinct winter and spring hydrologic periods, (2) quantify ecosystem-level ion budgets prior to, during, and following snowmelt, and (3) examine if streamwater chemistry might be a sensitive indicator of change in ecosystem flow paths. Prior to peak snowmelt, groundwater made up 80% of stream discharge. During peak snowmelt, the groundwater level rose to the soil surface resulting in lateral water movement through near-surface macropores and as overland flow. Near the end of snowmelt, melt-waters exerted a piston action on deeper soil solution again increasing its relative contribution to streamwater discharge. Net groundwater drawdown during the study resulted in streamwater discharge about equal to precipitation inputs. Unfrozen soils and brief mid-winter thaws resulted in steady snowmelt throughout early and mid-winter. The snowpack lost > 50% of most ions prior to the period of major snowmelt and high stream discharge in late March and early April. Snowmelt and streamwater NO₃ ^– and NH₄ pulses occurred before the period of overland flow and peak streamwater discharge (April 4–24). During overland flow, stream discharge of total N, P, DOC, and AI peaked. Nutrient budgets computed for distinct hydrologic periods were much more helpful in explaining ecosystem pathways and processes than were changes in solute concentration. For the study period, watershed base cation (C_B) discharge was 23 times input and SO₄ ^2– discharge exceeded input by 42%. H⁺ was the most strongly conserved ion with output < 0.2% of input. Also conserved were NH₄ ⁺ with only 1.4% of input leaving the ecosystem and NO₃ ^– with output equal to 9.4% of input.     

Effect of Native Vegetation Loss on Stream Ecosystem Processes: Dissolved Organic Matter Composition and Export in Agricultural Landscapes

Stream and river ecosystems are dependent on energetic inputs from their watersheds and thus shifts in land use from forest cover to agriculture will affect stream community composition and function. The disruption of forest-aquatic linkages alters the organic matter resources in agricultural streams. Dissolved organic matter (DOM) is the dominant form of organic matter in aquatic ecosystems, and a microbial energy source that is important for stream respiration. The concentrations and characteristics of DOM are regulated by both terrestrial (for example, terrestrial organic matter supply) and in-stream processes (for example, microbial respiration and periphyton production) that are influenced by land management. The effects of watershed land use and topographic, soil and climatic variables on DOM quantity (dissolved organic carbon concentration and load), source (terrestrial or in-stream) and quality (composition and lability) were measured in 14 streams across an agricultural land-use gradient. DOC concentration was positively correlated with watershed pasture cover and negatively correlated with watershed relief. No watershed variables were important correlates of DOC load. Stream DOM was primarily of terrestrial origin, but DOM in agricultural streams had a greater proportion of sources from in-stream sources. This may be due to reduced connection with riparian vegetation and increased in-stream primary production. We suggest that maintaining watershed tree cover greater than 52% and ensuring less than 10% of the length of riparian corridor is cleared for pasture could minimize changes to DOM composition. This is important to avoid flow-on effects for stream ecosystem processes that are mediated by DOM. Long-term DOM monitoring will be valuable for assessing the functional impacts of land-use change.     

DOC and DIC in Flowpaths of Amazonian Headwater Catchments with Hydrologically Contrasting Soils

Organic and inorganic carbon (C) fluxes transported by water were evaluated for dominant hydrologic flowpaths on two adjacent headwater catchments in the Brazilian Amazon with distinct soils and hydrologic responses from September 2003 through April 2005. The Ultisol-dominated catchment produced 30% greater volume of storm-related quickflow (overland flow and shallow subsurface flow) compared to the Oxisol-dominated catchment. Quickflow fluxes were equivalent to 3.2 ± 0.2% of event precipitation for the Ultisol catchment, compared to 2.5 ± 0.3% for the Oxisol-dominated watershed (mean response ±1 SE, n = 27 storms for each watershed). Hydrologic responses were also faster on the Ultisol watershed, with time to peak flow occurring 10 min earlier on average as compared to the runoff response on the Oxisol watershed. These different hydrologic responses are attributed primarily to large differences in saturated hydraulic conductivity (K _s). Overland flow was found to be an important feature on both watersheds. This was evidenced by the response rates of overland flow detectors (OFDs) during the rainy season, with overland flow intercepted by 54 ± 0.5% and 65 ± 0.5% of OFDs for the Oxisol and Ultisol watersheds respectively during biweekly periods. Small volumes of quickflow correspond to large fluxes of dissolved organic C (DOC); DOC concentrations of the hydrologic flowpaths that comprise quickflow are an order of magnitude higher than groundwater flowpaths fueling base flow (19.6 ± 1.7 mg l⁻¹ DOC for overland flow and 8.8 ± 0.7 mg l⁻¹ DOC for shallow subsurface flow versus 0.50 ± 0.04,mg l⁻¹ DOC in emergent groundwater). Concentrations of dissolved inorganic C (DIC, as dissolved CO₂–C plus HCO₃⁻–C) in groundwater were found to be an order of magnitude greater than quickflow DIC concentrations (21.5 mg l⁻¹ DIC in emergent groundwater versus 1.1 mg l⁻¹ DIC in overland flow). The importance of deeper flowpaths in the transport of inorganic C to streams is indicated by the 40:1 ratio of DIC:DOC for emergent groundwater. Dissolved CO₂–C represented 92% of DIC in emergent groundwater. Results from this study illustrate a highly dynamic and tightly coupled linkage between the C cycle and the hydrologic cycle for both Ultisol and Oxisol landscapes: organic C fluxes strongly tied to flowpaths associated with quickflow, and inorganic C (particularly dissolved CO₂) transported via deeper flowpaths.     

Forest sources and pathways of organic matter transport to a blackwater stream: a hydrologic approach

Quantitative information regarding landscape sources and pathways of organic matter transport to streams is important for assessing impacts of terrestrial processes on aquatic ecosystems. We quantified organic C, a measure of organic matter, flowing from a blackwater stream draining a 12.6 km² watershed on the upper Atlantic Coastal Plain in South Carolina, and utilized a hydrologic approach to partition this outflow between its various pathways from upland and wetland forest sources. Results of this study indicate that 28.9 tonnes C yr^–1 were exported in stream flow, which was estimated to be 0.5% of the annual C input from forest detritus to the watershed. Upland forest, which covers 94% of the watershed area, contributed only 2.0 tonnes C yr^–1 to stream flow, which amounted to 0.04% of detritus annually produced by the upland forest. Organic matter was transported from uplands to the stream almost entirely through groundwater. Apparently, upland soils are too sandy to support overland flow, and the sloping topography insufficiently extensive or steep enough to drive important quantities of interflow. Riparian wetland forest, which covers only 6% of the watershed area, contributed 26.9 tonnes C yr^–1 to stream flow, amounting to about 10.2% of detritus annually produced by the wetland forest. Dissolved organic C leached from wetland soil accounted for 63% of all organic C entering the stream, and was transported chiefly in baseflow. These results indicate that upland detritus sources are effectively decoupled from the stream despite the sandy soils and quantitatively confirm that even small riparian wetland areas can have a dominant effect on the overall organic matter budget of a blackwater stream. In view of the recognized importance of dissolved organic matter in facilitating transport of other substances (e.g., cation nutrients, metals, and insoluble organic compounds), our results suggest that the potential for movement of these substances through wetland soils to streams in this region is high.     

Changes in the character of DOC in streams during storms in two Midwestern watersheds with contrasting land uses

Dissolved organic carbon (DOC) dynamics in streams is important, yet few studies focus on DOC dynamics in Midwestern streams during storms. In this study, stream DOC dynamics during storms in two Midwestern watersheds with contrasting land uses, the change in character of stream DOC during storms, and the usability of DOC as a hydrologic tracer in artificially drained landscapes of the Midwest are investigated. Major cation/DOC concentrations, and DOC specific UV absorbance (SUVA) and fluorescence index (FI) were monitored at 2–4 h intervals during three spring storms. Although DOC is less aromatic in the mixed land use watershed than in the agricultural watershed, land use has little impact on stream DOC concentration during storms. For both watersheds, DOC concentration follows discharge, and SUVA and FI values indicate an increase in stream DOC aromaticity and lignin content during storms. The comparison of DOC/major cation flushing dynamics indicates that DOC is mainly exported via overland flow/macropore flow. In both watersheds, the increase in DOC concentration in the streams during storms corresponds to a shift in the source of DOC from DOC originating from mineral soil layers of the soil profile at baseflow, to DOC originating from surficial soil layers richer in aromatic substances and lignin during storms. Results also suggest that DOC, SUVA and FI could be used as hydrologic tracers in artificially drained landscapes of the Midwest. These results underscore the importance of sampling streams for DOC during high flow periods in order to understand the fate of DOC in streams.     

Deforestation of watersheds of Panama: nutrient retention and export to streams

A series of eight watersheds on the Pacific coast of Panama where conversion of mature lowland wet forest to pastures by artisanal burning provided watershed-scale experimental units with a wide range of forest cover (23, 29, 47, 56, 66, 73, 73, 91, and 92 %). We used these watersheds as a landscape-scale experiment to assess effects of degree of deforestation on within-watershed retention and hydrological export of atmospheric inputs of nutrients. Retention was estimated by comparing rainfall nutrient concentrations (volume-weighted to allow for evapotranspiration) to concentrations in freshwater reaches of receiving streams. Retention of rain-derived nutrients in these Panama watersheds averaged 77, 85, 80, and 62 % for nitrate, ammonium, dissolved organic N, and phosphate, respectively. Retention of rain-derived inorganic nitrogen, however, depended on watershed cover: retention of nitrate and ammonium in pasture-dominated watersheds was 95 and 98 %, while fully forested watersheds retained 65 and 80 % of atmospheric nitrate and ammonium inputs. Watershed forest cover did not affect retention of dissolved organic nitrogen and phosphate. Exports from more forested watersheds yielded DIN/P near 16, while pasture-dominated watersheds exported N/P near 2. The differences in magnitude of exports and ratios suggest that deforestation in these Panamanian forests results in exports that affect growth of plants and algae in the receiving stream and estuarine ecosystems. Watershed retention of dissolved inorganic nitrogen calculated from wet plus dry atmospheric deposition varied from 90 % in pasture- to 65 % in forest-dominated watersheds, respectively. Discharges of DIN to receiving waters from the watersheds therefore rose from 10 % of atmospheric inputs for pasture-dominated watersheds, to about 35 % of atmospheric inputs for fully forested watersheds. These results from watersheds with no agriculture or urbanization, but different conversion of forest to pasture by burning, show significant, deforestation-dependent retention within tropical watersheds, but also ecologically significant, and deforestation-dependent, exports that are biologically significant because of the paucity of nutrients in receiving tropical stream and coastal waters.     

Conversion to soy on the Amazonian agricultural frontier increases streamflow without affecting stormflow dynamics

Large‐scale soy agriculture in the southern Brazilian Amazon now rivals deforestation for pasture as the region's predominant form of land use change. Such landscape‐level change can have substantial consequences for local and regional hydrology, but these effects remain relatively unstudied in this ecologically and economically important region. We examined how the conversion to soy agriculture influences water balances and stormflows using stream discharge (water yields) and the timing of discharge (stream hydrographs) in small (2.5–13.5 km²) forested and soy headwater watersheds in the Upper Xingu Watershed in the state of Mato Grosso, Brazil. We monitored water yield for 1 year in three forested and four soy watersheds. Mean daily water yields were approximately four times higher in soy than forested watersheds, and soy watersheds showed greater seasonal variability in discharge. The contribution of stormflows to annual streamflow in all streams was low (<13% of annual streamflow), and the contribution of stormflow to streamflow did not differ between land uses. If the increases in water yield observed in this study are typical, landscape‐scale conversion to soy substantially alters water‐balance, potentially altering the regional hydrology over large areas of the southern Amazon.     

Use of ESI-FTICR-MS to Characterize Dissolved Organic Matter in Headwater Streams Draining Forest-Dominated and Pasture-Dominated Watersheds

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) has proven to be a powerful technique revealing complexity and diversity of natural DOM molecules, but its application to DOM analysis in grazing-impacted agricultural systems remains scarce. In the present study, we presented a case study of using ESI-FTICR-MS in analyzing DOM from four headwater streams draining forest- or pasture-dominated watersheds in Virginia, USA. In all samples, most formulas were CHO compounds (71.8–87.9%), with other molecular series (CHOS, CHON, CHONS, and CHOP (N, S)) accounting for only minor fractions. All samples were dominated by molecules falling in the lignin-like region (H/C = 0.7–1.5, O/C = 0.1–0.67), suggesting the predominance of allochthonous, terrestrial plant-derived DOM. Relative to the two pasture streams, DOM formulas in the two forest streams were more similar, based on Jaccard similarity coefficients and nonmetric multidimensional scaling calculated from Bray-Curtis distance. Formulas from the pasture streams were characterized by lower proportions of aromatic formulas and lower unsaturation, suggesting that the allochthonous versus autochthonous contributions of organic matter to streams were modified by pasture land use. The number of condensed aromatic structures (CAS) was higher for the forest streams, which is possibly due to the controlled burning in the forest-dominated watersheds and suggests that black carbon was mobilized from soils to streams. During 15-day biodegradation experiments, DOM from the two pasture streams was altered to a greater extent than DOM from the forest streams, with formulas with H/C and O/C ranges similar to protein (H/C = 1.5–2.2, O/C = 0.3–0.67), lipid (H/C = 1.5–2.0, O/C = 0–0.3), and unsaturated hydrocarbon (H/C = 0.7–1.5, O/C = 0–0.1) being the most bioreactive groups. Aromatic compound formulas including CAS were preferentially removed during combined light+bacterial incubations, supporting the contention that black carbon is labile to light alterations. Collectively, our data demonstrate that headwater DOM composition contains integrative information on watershed sources and processes, and the application of ESI-FTICR-MS technique offers additional insights into compound composition and reactivity unrevealed by fluorescence and stable carbon isotopic measurements.     

Influences of land use and stream size on particulate and dissolved materials in a small Amazonian stream network

We investigated the influences of forest or pasture land use and stream size on particulate and dissolved material concentrations in a network of second to third order streams in Rondônia, in the Brazilian Amazon. During the dry season, a second order stream originating in pasture had lower concentrations of dissolved oxygen and nitrate, higher concentrations of chlorophyll, total suspended solids, particulate organic carbon, particular organic nitrogen, ammonium, and phosphate than a second order stream originating in forest. Where the second order forest stream exited forest and entered pasture, concentrations of dissolved oxygen dropped from 6 mg/L to almost 0 mg/L and nitrate concentrations dropped from 12 M to 2 M over a reach of 2 km. These changes indicated a strong influence of land use. During the rainy season, differences among reaches of all particulate and dissolved materials were diminished. Concentrations of oxygen, chlorophyll, total suspended solids, particulate organic carbon and nitrogen, nitrate, ammonium, and phosphate in the third order pasture stream more closely resembled the second order forest stream than the second order pasture stream, suggesting that conditions in the channels of larger pasture streams more strongly control concentrations of these materials. If this pattern is widespread in stream networks of regions that consist of a mosaic of forest and pasture lands, it may have important consequences for understanding the effects of deforestation on larger rivers of the Amazon Basin. This would indicate that the effects of forest clearing on the concentrations of many suspended and dissolved materials will be most easily detected in very small streams but potentially difficult to detect in larger streams and rivers.     

Forest harvest legacies control dissolved organic carbon export in small watersheds,western Oregon

This study examined physical and biological controls on dissolved organic carbon (DOC) fluxes from conifer-forest watersheds in the H.J. Andrews Experimental Forest of Oregon. We tested how DOC export was related to streamflow and legacies of wood on the forest floor three to five decades after harvest of old-growth forest in seven watersheds spanning the rain to snow elevation gradient. Three watersheds had old-growth forest and four had 30 to 50-year-old forest established after clearcutting of old-growth forest. Mean annual DOC flux in the watersheds was related to the biomass of forest floor wood, which was two or three times higher in watersheds with old-growth forest compared to young forest, and was inversely related to elevation, a measure of snowpack depth and duration. In contrast, fluxes of inorganic elements such as Si and Ca did not vary with harvest history or forest floor characteristics. Annual fluxes of DOC, Si, and Ca were linearly related to annual runoff, and annual volume-weighted concentrations of these ions declined by?<?0.6% with several-fold increases in annual runoff. Across all years, DOC concentrations peaked before the peak of the hydrograph in all watersheds, which we interpret as representing movement, likely via preferential and surficial flow, of organic materials mineralized and solubilized during the long dry summers in this ecosystem. DOC concentrations relative to stream flow exhibited clockwise hysteresis loops in each water year, also suggesting that soluble DOC produced in the dry summer is exported in the fall. DOC concentration differences between reference and harvested watersheds also peaked in late summer or early fall, suggesting that the source of the additional DOC from reference watersheds came from coarse woody debris that remains moist during the dry summers and that was significantly greater in watersheds with elevated DOC. Taken together, our results suggest that forest floor wood is a previously unappreciated control on the supply of DOC that can be exported, and runoff is a secondary control on total DOC flux to streams. The legacy of forest harvest on DOC flux can be observed for decades, as total ecosystem carbon stocks, especially coarse woody debris, may require centuries to develop after old-growth forest harvest.     

Riparian land use and the relationship between the benthos and litter decomposition in tropical montane streams

1. Although stream–catchment interactions have been analysed in some detail in temperate environments, little is known about the effects of land‐use changes in the tropics. Here, we analyse differences in benthic communities (macroinvertebrates and fungi) under two contrasting land uses (mature secondary forest and pasture) in montane streams in north‐western Ecuador and their influence on the rates of litter processing. 2. Between 2005 and 2006, we used a combination of coarse and fine mesh bags to study the relative contribution of macroinvertebrates and fungi to processing of two types of litter, Alnus acuminata and Inga spectabilis, in three‐first‐order streams running through mature secondary montane forests and adjacent downstream reaches running through pastures. At the same time, we characterised the assemblages of shreddering macroinvertebrates and fungi communities and the litter processing rates in stream reaches under both vegetation types. 3. Litter processing rates attributable to invertebrate feeding (coarse mesh bags) were significantly slower in streams running through pastures. Nevertheless, shredder diversity and richness were similar between pasture and forest sections, while shredder abundance was significantly higher in forest streams (mainly Phylloicus sp. :Trichoptera). Fungal reproductive activity and litter processing rates were low (fine mesh bags) and did not differ significantly between pasture and forest stream reaches. 4. Phylloicus sp. abundance was the best predictor of the percentage of litter remaining in coarse mesh bags across pasture and forest sites. Neither shredder diversity nor their species richness was a significant predictor of mass loss, as most of the decomposition was performed by a single keystone species. Although litter decomposition by microbial decomposers was low, fungal biomass (but not diversity) was the best variable explaining the percentage of litter remaining in fine mesh bags. 5. Our data suggest that, in these Neotropical montane streams, land use can have a significant impact on the rates of critical ecosystem processes, such as litter decomposition. In this study, this effect was not mediated by a major shift in the structure of the benthos, but by a decrease in the abundance and relative representation of a single species whose life history makes it critical to litter processing. 6. This study highlights the significant role that macroinvertebrate fauna can have in the processing of litter in Neotropical streams and the predominant role that single species can have in terms of controlling stream ecosystem‐level processes. Understanding the extent to which these patterns affect the long‐term and large‐scale functioning of stream ecosystems still needs further research and will become increasingly important in terms of managing lotic ecosystems in the context of rapid land‐use change.     

Modeling changes in soil organic matter in Amazon forest to pasture conversion with the Century model

Land use and land cover changes in the Brazilian Amazon region have major implications for regional and even global carbon cycling. We analyzed the effects of the predominant land use change, conversion of tropical forest to pasture, on total soil C and N, using the Century ecosystem model and data collected from the Nova Vida ranch, Western Brazilian Amazon. We estimated equilibrium organic matter levels, plant productivity and residue carbon inputs under native forest conditions, then simulated deforestation following the slash and burn procedure. Soil organic matter dynamics were simulated for pastures established in 1989, 1987, 1983, 1979, 1972, 1951, and 1911. Using input data from the Nova Vida ranch, the Century model predicted that forest clearance and conversion to pasture would cause an initial decline in soil C and N stocks, followed by a slow rise to levels exceeding those under native forest. Simulated soil total C and N levels (2500 g C m^?2 and 245 g N m^?2 in the 0–20 cm layer) prior to conversion to pasture were close to those measured in the native forest. Simulated above‐ and below‐ground biomass for the forest and pasture were comparable with literature values from this region. The model predicted the long‐term changes in soil C and N under pasture inferred from the pasture chronosequence, but there was considerable variation in soil C stocks for pastures <20 years in age. Differences in soil texture between pastures were relatively small and could not account for much of the variability between different pastures of similar ages, in either the measured or simulated data. It is likely that much of the variability in C stocks between pastures of similar ages is related to initial C stocks immediately following deforestation and that this was the largest source of variability in the chronosequence. Internal C cycling processes in Century were evaluated using measurements of microbial biomass and soil δ¹³C. The relative magnitude and long‐term trend in microbial biomass simulated by the model were consistent with measurements. The close fit of simulated to measured values of δ¹³C over time suggests that the relative loss of forest‐derived C and its replacement by pasture‐derived C was accurately predicted by the model. After 80 years, almost 90% of the organic matter in the top 20 cm was pasture derived. While our analysis represents a single ‘case study’ of pasture conversion, our results suggest that modeling studies in these pasture systems can help to evaluate the magnitude of impacts on C and N cycling, and determine the effect of management strategies on pasture sustainability.     

Spatial relationships of aluminum chemistry in the streams of the Hubbard Brook Experimental Forest,New Hampshire

Aluminum chemistry was evaluated in two headwater streams in the White Mountains of New Hampshire. Observed elevational trends in stream aluminum chemistry may be related to spatial variations of vegetation type and mineral soil depth within the watersheds. At the highest elevations maximum densities of spruce and fir vegetation occur and aluminum appears to be mobilized predominantly by transformations involving dissolved organic matter. At the mid-elevations hardwood vegetation predominates and the mechanism of aluminum mobilization shifts to dissolution by strong acids within the mineral soil. At the lowest elevations, relatively thick mineral soil seems to limit aluminum mobility, resulting in low concentrations in streamwater. Comparison of these results with an earlier study of an adjacent watershed, indicates that subtle differences in watershed characteristics such as tree species distribution and topography may cause significant variations in stream aluminum chemistry. Control of aluminum mobility by imogolite minerals was not indicated by the stream chemistry of these watersheds. To determine the relationship between acidic deposition and aluminum mobility, natural variations which occur in the aluminum cycle must be addressed.     

河流流量对流域下垫面特性的响应

以美国切斯比克湾地区为例,对该区域150个小流域的下垫面特性(包括土地利用类型、地面不透水系数和土壤物理属性)进行了提取,根据1984—2004年间逐日流量观测数据计算出了33个水文指标,运用逐步回归方法在不同地理区分析了9种下垫面特性对其中17个重要水文指标的影响。结果表明:随着草地和林地比例的增加,流量趋于减小、流量变化趋于稳定,随着建设用地和不透水层的增加,流量增加、流量变化剧烈,随着土壤水文组等级的升高,流量减少;在整个切斯比克湾流域,对流域下垫面特性响应最为显著的水文指标是高脉冲个数及历时,在阿巴拉契亚高地地区响应最为显著的水文指标是年极值流量、高脉冲个数及历时,在皮德蒙特山地响应最为显著的水文指标是高脉冲个数及历时,在沿海平原地区响应最为显著的水文指标是高脉冲个数及历时、流量变化的速率与频率。     

美国大陆五个山区集水区的河溪结构分析

河溪结构及其关联的空间特征,是研究集水区中能量和物质移动规律所不可缺少的内容,利用美国地质调查局的水文数据库和地理信息系统（ＡＲＣ／ＩＮＦＯ）,对美国大陆５个主要山地中典型集水区河溪的数据量、密度,分布结构以及河溪岸边带的组成,进行了分析和比较。５个山地分别是：西北太平洋沿岸,西部卡斯卡特山,中部落矶山,东部阿巴拉契亚山和南部欧扎克山,发现这５个集水区中的河溪网络结构非常相似,一级河溪大约占河溪总     

Characterization and qualitative changes in DOM chemical characteristics related to hydrologic conditions in a Pampean stream

The aim of our study was; (i) to characterize the composition of DOM in stream water and their potential sources (groundwater, overland flow, subsurface flow and rain water) and (ii) to analyze changes in DOM concentration and composition under different hydrological conditions (baseflow and high flow) in a third-order Pampean stream (Argentina). Pampean streams are mainly fed by the shallow aquifer under baseflow conditions and they lack of riparian forest. In addition, water velocity is low due to the gentle slope of the region and nutrient levels are high, favoring the development of rich macrophyte communities. DOM optical properties in the stream and end members were determined by combining absorbance-fluorescence spectroscopy techniques. Our results indicated that DOM chemical characteristics in the stream were mainly modulated by a differential contribution of end members to stream water depending on hydrological conditions. We observed that DOM in groundwater showed a microbial origin while DOM in runoff was terrestrially-derived. DOC concentration and inputs of humic substances from the riparian zone increased with discharge at high flow conditions. Due to the strong link between DOC properties and the riparian environment, structural alterations in the stream channel and changes in riparian vegetation (forestation) may result in changes in DOM composition and dynamics.