Dissolved Nitrogen, Phosphorus, and Sulfur forms in the Ecosystem Fluxes of a Montane Forest in Ecuador

The N, P, and S cycles in pristine forests are assumed to differ from those of anthropogenically impacted areas, but there are only a few studies to support this. Our objective was therefore to assess the controls of N, P, and S release, immobilization, and transport in a remote tropical montane forest. The study forest is located on steep slopes of the northern Andes in Ecuador. We determined the concentrations of NO₃-N, NH₄-N, dissolved organic N (DON), PO₄-P, dissolved organic P (DOP), SO₄-S, dissolved organic S (DOS), and dissolved organic C (DOC) in rainfall, throughfall, stemflow, lateral flow (in the organic layer), litter leachate, mineral soil solution, and stream water of three 8–13 ha catchments (1900–2200 m a.s.l.). The organic forms of N, P, and S contributed, on average, 55, 66, and 63% to the total N, P, and S concentrations in all ecosystem fluxes, respectively. The organic layer was the largest source of all N, P, and S species except for inorganic P and S. Most PO₄ was released in the canopy by leaching and most SO₄ in the mineral soil by weathering. The mineral soil was a sink for all studied compounds except for SO₄. Consequently, concentrations of dissolved inorganic and organic N and P were as low in stream water (TDN: 0.34–0.39 mg N l⁻¹, P not detectable) as in rainfall (TDN: 0.39–0.48 mg N l⁻¹, P not detectable), whereas total S concentrations were elevated (stream water: 0.04–0.15, rainfall: 0.01–0.07 mg S l⁻¹). Dissolved N, P, and S forms were positively correlated with pH at the scale of soil peda except inorganic S. Soil drying and rewetting promoted the release of dissolved inorganic N. High discharge levels following heavy rainstorms were associated with increased DOC, DON, NO₃-N and partly also NH₄-N concentrations in stream water. Nitrate-N concentrations in the stream water were positively correlated with stream discharge during the wetter period of the year. Our results demonstrate that the sources and sinks of N, P, and S were element-specific. More than half of the cycling N, P, and S was organic. Soil pH and moisture were important controls of N, P, and S solubility at the scale of individual soil peda whereas the flow regime influenced the export with stream water.     

Nitrogen Cycling and Mass Balance for a Forested Catchment in the Italian Alps. Assessment of Nitrogen Status

During 1999–2001 the chemical composition and fluxes were measured in rainfall, throughfall, soil solution and stream water in a remote forested site in the Italian Alps. The analysis of temporal patterns revealed the differential behaviour of nitrogen and sulphur and suggested that different mechanisms controlled their flux. No important changes in sulphate concentration and fluxes emerged as the solution passed through the various components of the forest ecosystem, and temporal variations of SO₄ in the soil solution and stream were likely driven by the physical process of dilution. The availability of nitrate and ammonia, by contrast, was drastically reduced as throughfall water entered the soil and passed through the mineral layers, irrespective of season. The calculated hydrochemical budget based on throughfall and soil solution N fluxes revealed that ~80% N retention in the forest soil, corresponding to 12 kg ha⁻¹ yr⁻¹, despite a relatively high N deposition loading (15 kg ha⁻¹ yr⁻¹). Most of the leached nitrogen (90%) was in the organic form. Indicators of the N status of this ecosystem, such as C/N ratio in solid and solution phase of the soil and N foliage content as well as land use history were examined. Despite the strong N retention in the forested part of the catchment, the stream water N–NO₃ levels were consistently above 10 μg l⁻¹ suggesting that the Val Masino catchment as a whole was less efficient in processing atmospheric N inputs. This contrasting N behaviour illustrates the role of landscape features, such as the soil cover and vegetation type, that is characteristic of an alpine catchment.     

Response of dissolved organic matter in the forest floor to long-term manipulation of litter and throughfall inputs

Dissolved organic matter (DOM) contributes to organic carbon either stored in mineral soil horizons or exported to the hydrosphere. However, the main controls of DOM dynamics are still under debate. We studied fresh leaf litter and more decomposed organic material as the main sources of DOM exported from the forest floor of a mixed beech/oak forest in Germany. In the field we doubled and excluded aboveground litter input and doubled the input of throughfall. From 1999 to 2005 we measured concentrations and fluxes of dissolved organic C and N (DOC, DON) beneath the Oi and Oe/Oa horizon. DOM composition was traced by UV and fluorescence spectroscopy. In selected DOM samples we analyzed the concentrations of phenols, pentoses and hexoses, and lignin-derived phenols by CuO oxidation. DOC and DON concentrations and fluxes almost doubled instantaneously in both horizons of the forest floor by doubling the litter input and DOC concentrations averaged 82 mg C l⁻¹ in the Oe/Oa horizon. Properties of DOM did not suggest a change of the main DOM source towards fresh litter. In turn, increasing ratios of hexoses to pentoses and a larger content of lignin-derived phenols in the Oe/Oa horizon of the Double litter plots in comparison to the Control plots indicated a priming effect: Addition of fresh litter stimulated microbial activity resulting in increased microbial production of DOM from organic material already stored in Oe/Oa horizons. Exclusion of litter input resulted in an immediate decrease in DOC concentrations and fluxes in the thin Oi horizon. In the Oe/Oa horizon DOC concentrations started to decline in the third year and were significantly smaller than those in the Control after 5 years. Properties of DOM indicated an increased proportion of microbially and throughfall derived compounds after exclusion of litter inputs. Dissolved organic N did not decrease upon litter exclusion. We assume a microbial transformation of mineral N from throughfall and N mineralization to DON. Increased amounts of throughfall resulted in almost equivalently increased DOC fluxes in the Oe/Oa horizon. However, long-term additional throughfall inputs resulted in significantly declining DOC concentrations over time. We conclude that DOM leaving the forest floor derives mainly from decomposed organic material stored in Oe/Oa horizons. Leaching of organic matter from fresh litter is of less importance. Observed effects of litter manipulations strongly depend on time and the stocks of organic matter in forest floor horizons. Long-term experiments are particularly necessary in soils/horizons with large stocks of organic matter and in studies focusing on effects of declined substrate availability. The expected increased primary production upon climate change with subsequently enhanced litter input may result in an increased production of DOM from organic soil horizons.     

Concentrations and fluxes of dissolved organic carbon in an age-sequence of white pine forests in Southern Ontario,Canada

We determined concentrations and fluxes of dissolved organic carbon (DOC) in precipitation, throughfall, forest floor and mineral soil leachates from June 2004 to May 2006 across an age-sequence (2-, 15-, 30-, and 65-year-old) of white pine (Pinus strobus L.) forests in southern Ontario, Canada. Mean DOC concentration in precipitation, throughfall, leachates of forest floor, Ah-horizon, and of mineral soil at 1 m depth ranged from ∼2 to 7, 9 to 18, 32 to 88, 20 to 66, and 2 to 3 mg DOC L⁻¹, respectively, for all four stands from April (after snowmelt) through December. DOC concentration in forest floor leachates was highest in early summer and positively correlated to stand age, aboveground biomass and forest floor carbon pools. DOC fluxes via precipitation, throughfall, and leaching through forest floor and Ah-horizon between were in the range of ∼1 to 2, 2 to 4, 0.5 to 3.5, and 0.1 to 2 g DOC m⁻², respectively. DOC export from the forest ecosystem during that period through infiltration and groundwater discharge was estimated as ∼7, 4, 3, and 2 g DOC m⁻² for the 2-, 15-, 30-, and 65-year-old sites, respectively, indicating a decrease with increasing stand age. Laboratory DOC sorption studies showed that the null-point DOC concentration fell from values of 15 to 60 mg DOC L⁻¹ at 0 to 5 cm to <15 mg DOC L⁻¹ at 50 cm. Specific ultraviolet light absorption at 254 nm (SUVA₂₅₄) increased from precipitation and throughfall to a maximum in forest floor and decreased with mineral soil depth. No age-related pattern was observed for SUVA₂₅₄ values. DOC concentration in forest floor soil solutions showed a positive exponential relationship with soil temperature, and a negative exponential relationship with soil moisture at all four sites. Understanding the changes and controls of DOC concentrations, chemistry, and fluxes at various stages of forest stand development is necessary to estimate and predict DOC dynamics on a regional landscape level and to evaluate the effect of land-use change.     

Concentration and Fluxes of Dissolved Organic Carbon (DOC) in Three Norway Spruce Stands along a Climatic Gradient in Sweden

Leaching of dissolved organic carbon (DOC) from the forest floor and transport in soil solution into the mineral soil are important for carbon cycling in boreal forest ecosystems. We examined DOC concentrations in bulk deposition, throughfall and in soil solutions collected under the O and B horizons in three Norway spruce stands along a climatic gradient in Sweden. Mean annual temperature for the three sites was 5.5, 3.4 and 1.2 °C. At each site we also examined the effect of soil moisture on DOC dynamics along a moisture gradient (dry, mesic and moist plots). To obtain information about the fate of DOC leached from the O horizon into the mineral soil, ¹⁴C measurements were made on bulk organic matter and DOC. The concentration and fluxes of DOC in O horizon leachates were highest at the southern site and lowest at the northern. Average DOC concentrations at the southern, central and northern sites were 49, 39 and 30 mg l⁻¹, respectively. We suggest that DOC leaching rates from O horizons were related to the net primary production of the ecosystem. Soil temperature probably governed the within-year variation in DOC concentration in O horizon leachates, but the peak in DOC was delayed relative to that of temperature, probably due to sorption processes. Neither soil moisture regime (dry, mesic or moist plots) nor seasonal variation in soil moisture seemed to be of any significance for the concentration of DOC leached from the O horizon. The ¹⁴C measurements showed that DOC in soil solution collected below the B horizon was derived mainly from the B horizon itself, rather than from the O horizon, indicating a substantial exchange (sorption–desorption reactions) between incoming DOC and soil organic carbon in the mineral soil.     

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

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

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

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

Environmental chemistry of rivers and lakes, part v: a comparative study of the chemical and physicochemical characteristics of organic carbon dissolved in river and lake waters

To determine the chemical and physicochemical characteristics of dissolved organic carbon in the Ado River and the Yasu River, the main rivers flowing into Lake Biwa, the adsorption behavior onto hydrous iron oxide (HIO) and the reactivity to KMnO₄ oxidant were investigated in parallel with measurement of the distribution profiles of dissolved organic carbon (DOC) along the rivers. In one year of observation at the mouths of the two rivers, DOC concentrations were found to vary in the Ado over the range 0.28–1.21 mg C l⁻¹ and in the Yasu over the range 1.01–2.68 mg C l⁻¹. Act-DOC, one of the fractions separated from the total DOC by its adsorption-active character onto HIO at pH 4, was thought primarily to control the variation of total DOC, as in Lake Biwa. The int-DOC, another fraction separated by its adsorption-inert or -inactive character onto HIO, remained at almost a steady value around 0.18 ± 0.07 mg C l⁻¹ in the Ado, which was lower than that (0.35 ± 0.05 mg C l⁻¹) in Lake Biwa. The act-DOC in river waters was reactive to KMnO₄ oxidant, showing a linear relation with the amount of permanganate consumed for the reaction (chemical oxygen demand: COD). In river waters, the relation can be approximated by a straight line expressed as COD (mg O₂ l⁻¹) = 0.64 × act-DOC (mg C l⁻¹) − 0.02. In contrast, in the lake water the relation was COD (mg O₂ l⁻¹) = 0.97 × act-DOC (mg C l⁻¹) − 0.50. Received: March 3, 1999 / Accepted: December 2, 1999     

Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m⁻² year⁻¹). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m⁻² year⁻¹ in the north to +9 g C m⁻² year⁻¹ in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m⁻² year⁻¹. An erratum to this article can be found at     

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.     

Abiotic reaction of nitrite with dissolved organic carbon? Testing the Ferrous Wheel Hypothesis

The Ferrous Wheel Hypothesis (Davidson et al. 2003) postulates the abiotic formation of dissolved organic N (DON) in forest floors, by the fast reaction of NO₂ ⁻ with dissolved organic C (DOC). We investigated the abiotic reaction of NO₂ ⁻ with dissolved organic matter extracted from six different forest floors under oxic conditions. Solutions differed in DOC concentrations (15–60 mg L⁻¹), NO₂ ⁻ concentrations (0, 2, 20 mg NO₂ ⁻-N L⁻¹) and DOC/DON ratio (13.4–25.4). Concentrations of added NO₂ ⁻ never decreased within 60 min, therefore, no DON formation from added NO₂ ⁻ took place in any of the samples. Our results suggest that the reaction of NO₂ ⁻ with natural DOC in forest floors is rather unlikely.     

The effects of dissolved organic carbon,acidity and seasonality on metal geochemistry within a forested catchment on the Precambrian Shield,central Ontario,Canada

Metal pollution, in combination with other environmental stressors such as acid deposition and climate change, may disturb metal biogeochemical cycles. To investigate the influence of dissolved organic carbon, acidity and seasonality on metal geochemistry, this study has described concentrations of 19 metals as they pass through an acidified forested catchment on the Precambrian Shield in south-central Ontario, Canada. Metal, dissolved organic carbon (DOC) and sulphate (SO₄ ²⁻) concentrations fluctuate throughout the catchment compartments as the water passes through and interacts with vegetation, soils and bedrock. Relationships among metals, DOC and SO₄ ²⁻ are most pronounced in compartments where DOC and SO₄ ²⁻ exhibit high variability, namely in the throughfall, organic horizon soil water, and wetland-draining stream. Metal, DOC and SO₄ ²⁻ concentrations varied seasonally in the streams, and temporal coherence occurred among metal, DOC and SO₄ ²⁻ concentrations in the organic horizon soil water and the wetland-draining stream (PC1). In the wetland-draining stream, the highest DOC, Cr, Cu, Fe, Pb, and V concentrations occur in the summer, whereas concentrations of SO₄ ²⁻ and most other metals peak in the fall after a period of drought. Despite the rural location, provincial water quality objectives for surface water were exceeded for many metals when the peak fall values occurred.     

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.     

Dissolved organic carbon concentrations in four Norway spruce stands of different ages

Boreal forests are increasing in age partly due to reduced logging and efficient wildfire control. As a result, they also stock more carbon. Whether increased forest C stock causes greater production of dissolved organic carbon (DOC) is uncertain. DOC in bulk precipitation, throughfall and soil water was studied in 10-, 30-, 60- and 120-year-old stands of Norway spruce (Picea abies (L.) Karst.) DOC concentrations in throughfall and O horizon soil water followed the order 10 < 30 < 60 = 120 and 10 = 30 < 120 < 60, respectively. DOC fluxes followed the order 10 = 30 < 60 = 120 in throughfall, while no significant difference between stands was found for O horizon soil water. Above-ground tree litter varied according to 10 < 30 < 60 = 120, a pattern identical to that for DOC concentrations in throughfall and resembling but not identical to that for DOC concentrations in O horizon soil water. This indicates additional sources for DOC in soil water. Seasonality in DOC concentrations was observed at the base of the O horizon, and seasonality in DOC fluxes in both throughfall and O horizon soil water. Our results suggest differences in the polarity of DOC between the 10-year stand and the others, which we interpret as reflecting the lack of grown trees and possibly the different vegetation on the 10-year stand.     

The influence of wetlands, decaying organic matter, and stirring by wildlife on the dissolved oxygen concentration in eutrophicated water holes in the Seronera River, Serengeti National Park, Tanzania

The dissolved oxygen concentration (DO) was sampled during a diurnal cycle in three water holes heavily used by wildlife and with distinctive biological features along the Seronera River. The DO fluctuated widely (by up to 11.5 mg l⁻¹) as a function of time, mechanical stirring and aeration by animals, and the presence of fringing wetlands. The DO cycle was successfully modeled (within 0.3 mg l⁻¹) by assuming that the four dominant processes were photosynthesis and respiration by algae near the surface, trapping by wetlands, decomposition of dead organic matter on the bottom, and stirring/aeration by hippos. The rate of DO decline from the decay of dead organic matter was equal to the rate of DO removal by algal respiration at night.     

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

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

Gaps and Soil C Dynamics in Old Growth Northern Hardwood–Hemlock Forests

Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these forests are not well understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great Lakes region, USA, were assessed from measurements of litter and soil C stocks, surface C efflux, and soil microbial indices over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha⁻¹) compared to gap-edges (39.5 Mg C ha⁻¹) and the closed forest (38.0 Mg C ha⁻¹). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha⁻¹) compared to forest plots (15.3 Mg C ha⁻¹). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha⁻¹ y⁻¹) compared to edges and the closed forest (9.2 and 8.9 Mg C ha⁻¹ y⁻¹). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha⁻¹) than in the contiguous forest (0.09 Mg N ha⁻¹). The metabolic quotient (qCO₂) was significantly greater in the forest (0.0031 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹) relative to gaps or edges (0.0014 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest, gaps have significantly less labile C, significantly greater in situ surface C efflux, and significantly lower decreased qCO₂ values.     

Leaf litter decomposition in a tropical peat swamp forest in Peninsular Malaysia

It has long been assumed that the peat underlying tropical peat swamp forests accumulates because the extreme conditions (water logged, nutrient poor, anaerobic and acidic—pH 2.9–3.5) impede microbial activity. Litterbag studies in a tropical Malaysian peat swamp (North Selangor peat swamp forest) showed that although the sclerophyllous, toxic leaves of endemic peat forest plants (Macaranga pruinosa, Campnosperma coriaceum, Pandanus atrocarpus, Stenochlaena palustris) were barely decomposed by bacteria and fungi (decay rates of only 0.0006–0.0016 k day⁻¹), leaves of M. tanarius, a secondary forest species were almost completely decomposed (decay rates of 0.0047–0.005 k day⁻¹) after 1 year. Thus it is intrinsic properties of the leaves (that are adaptations to deter herbivory in the nutrient poor environment) that impede microbial breakdown. The water of the peat swamp was very high in dissolved organic carbon (70–84 mg l⁻¹ DOC). Laboratory studies revealed initial rapid leaching of DOC from leaves (up to 1,720 mg l⁻¹ from 4 g of leaves in 7 days), but the DOC levels then fell rapidly. The leaching of DOC resulted in weight loss but the physical structure of the leaves remained intact. It is suggested that the DOC is used as a substrate for microbial growth hence lowering the concentration of DOC in the water and transferring energy from the leaves to other trophic levels. This would explain how nutrient poor tropical peatswamps support diverse, abundant flora and fauna despite low nutrient levels and lack of rapid litter cycling such as occurs in other types of tropical rainforests.     

Production of Total Potentially Soluble Organic C, N, and P Across an Ecosystem Chronosequence: Root versus Leaf Litter

Dissolved organic matter (DOM) plays several important roles in forest ecosystem development, undergoing chemical, physical and/or biological reactions that affect ecosystem nutrient retention. Very few studies have focused on gross rates of DOM production, and we know of no study that has directly measured DOM production from root litter. Our objectives were to quantify major sources of total potentially water-soluble organic matter (DOM_tps) production, with an emphasis on production from root litter, to quantify and compare total potentially soluble organic C, N, and P (DOC_tps, DON_tps, and DOP_tps) production, and to quantify changes in their production during forest primary succession and ecosystem development at the Mt. Shasta Mudflows ecosystem chronosequence. To do so, we exhaustively extracted freshly senesced root and leaf and other aboveground litter for DOC_tps, DON_tps, and DOP_tps by vegetation category, and we calculated DOM_tps production (g m⁻² y⁻¹) at the ecosystem level using data for annual production of fine root and aboveground litter. DOM production from throughfall was calculated by measuring throughfall volume and concentration over 2 years. Results showed that DOM_tps production from root litter was a very important source of DOM_tps in the Mount Shasta mudflow ecosystems, in some cases comparable to production from leaf litter for DON_tps and larger than production from leaf litter for DOP_tps. Total DOC_tps and DON_tps production from all sources increased early in succession from the 77- to the 255-year-old ecosystem. However, total DOP_tps production across the ecosystem chronosequence showed a unique pattern. Generally, the relative importance of root litter for total fine detrital DOC_tps and DON_tps production increased significantly during ecosystem development. Furthermore, DOC_tps and DON_tps production were predominantly driven by changes in biomass production during ecosystem development, whereas changes in litter solubility due to changes in species composition had a smaller effect. We suggest that DOM_tps production from root litter may be an important source of organic matter for the accumulation of SOM during forest ecosystem development. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Shauna M. Uselman, Robert G. Qualls, and Juliane Lilienfein conceived of or designed the study and performed research. SMU analyzed data and wrote the article. SMU and RGQ contributed new methods or models.     

Chemical Characteristics and Origin of Dissolved Organic Matter in the Yukon River

Monthly (or bi-weekly) water samples were collected from the Yukon River, one of the largest rivers in North America, at a station near the US Geological Survey Stevens Village hydrological station, Alaska from May to September 2002, to examine the quantity and quality of dissolved organic matter (DOM) and its seasonal variations. DOM was further size fractionated into high molecular weight (HMW or colloidal, 1 kDa–0.45 μm) and low molecular weight (LMW, <1 kDa) fractions. Dissolved organic carbon (DOC), colored dissolved organic matter (C-DOM) and total dissolved carbohydrate (TCHO) species were measured in the size fractionated DOM samples. Concentrations of DOC were as high as 2830 μmol-C l⁻¹ during the spring breakup in May and decreased significantly to 508–558 μmol-C l⁻¹ during open-water season (June–September). Within the DOC pool, up to 85% was in the colloidal fraction (1 kDa–0.45 μm) in early May. As DOC concentration decreased, this colloidal portion remained high (70–85% of the bulk DOC) throughout the sampling season. Concentrations of TCHO, including monosaccharides (MCHO) and polysaccharides (PCHO), varied from 722 μmol-C l⁻¹ in May to 129 μmol-C l⁻¹ in September, which comprised a fairly constant portion of bulk DOC (24±2%). Within the TCHO pool, the MCHO/TCHO ratio consistently increased from May to September. The C-DOM/DOM ratio and the size fractionated DOM increased from May to September, indicating that DOM draining into the Yukon River contained increased amounts of humified materials, likely related to a greater soil leaching efficiency in summer. The average composition of DOM was 76% pedogenic humic matter and 24% aquagenic CHO. Characteristics of soil-derived humic substances and low chlorophyll-a concentrations support a dominance of terrestrial DOM in Yukon River waters.