The organic carbon dynamics of a moorland catchment in N. W. England

The carbon cycle was quantified in the catchment of Doe House Gill, which drains high-relief moorland, with thin organic-rich soils (leptosols and podzols) 10–25 cm deep, in northern England. The soil C pool of 8,300 g m^-2 is due mainly to humic acid and older humin. If steady state is assumed, and a single soil C pool, the average ¹⁴C content of the whole soil (93% modern) yields a mean carbon residence time of 800 years, although this varied from 300 to 1,600 years in the four samples studied. Stream water fluxes of dissolved and particulate organic carbon (DOC, POC) were 2.5 and 0.4 g m⁻² a⁻¹ respectively in 2002–2003, lower than values for some other upland streams in the UK. The C pool, flux, and isotope data were used, with the assumption of steady state, to calibrate DyDOC, a model that simulates the soil carbon cycle, including the generation and transport of DOC. According to DyDOC, the litter pool (ca. 100 gC m⁻²) turns over quickly, and most (>90%) of the litter carbon is rapidly mineralised. The soil is calculated to gain only 16 gC m⁻² a⁻¹, and to lose the same amount, about 80% as CO₂ and 20% as DOC. From the DO¹⁴C content of 107.5% modern (due to “bomb carbon”) the model could be calibrated by assuming all DOC to come directly from litter, but DOC is more likely a mixture, derived from more than one soil C pool. The seasonal variability exhibited by stream water DOC concentration (maximum in September, minimum in January) is attributed mainly to variations in rainfall and evapotranspiration, rather than in the metabolic production rate of “potential DOC”. The model predicts that, for a Q ₁₀ of 2, the total soil organic C pool would decrease by about 5% if subjected to warming over 200 years. DyDOC predicts higher DOC fluxes in response to increased litter inputs or warming, and can simulate changes in DOC flux due to variations in sorption to soil solids, that might occur due to acidification and its reversal.     

Production of Dissolved Organic Carbon in Canadian Forest Soils

To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC concentration in the leachate and carbon dioxide (CO₂) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils (<2 to >50 mg l⁻¹). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g⁻¹ soil C day⁻¹, falling to averages of 0.01 mg g⁻¹ soil C day⁻¹; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from less than 0.01 to 0.12 mg g⁻¹ soil C day⁻¹ (0.5–47.6 mg g⁻¹ soil C), with an average of 0.021 mg g⁻¹ soil C day⁻¹ (8.2 mg g⁻¹ soil C). DOC production rate was weakly related to temperature, equivalent to Q₁₀ values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO₂–C and DOC showed a quotient (CO₂–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO₂–C:DOC production (log₁₀ transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion of CO₂ is produced. The CO₂–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO₂–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO₂–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples.     

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.     

Identifying DOC gains and losses during a 20-year record in the Trout Beck catchment,Moor House,UK

The turnover of organic carbon in rivers could represent a large source of greenhouse gases to the atmosphere and studies have suggested that of the order of 70% of the dissolved organic carbon exported from soils could be lost in rivers before it flows to continental seas. The Environmental Change Network (ECN) monitoring of the dominantly peat-covered Trout Beck catchment within the Moor House site enabled the amount of dissolved organic carbon (DOC) lost within a stream over a 20-year period to be estimated. The study compared DOC concentrations of precipitation, shallow and deep soil waters with those at the catchment outlet. The mass balance between source and outlet was reconstructed by two methods: a single conservative tracer; and based upon a principal component analysis (PCA) using multiple tracers. The study showed the two methods had different outcomes, with the PCA showing a DOC gain and the single tracer showing a DOC loss. The DOC gain was attributed to an unmeasured groundwater contribution that dominates when the river discharge is lower. The DOC loss was related to the in-stream residence time, the soil temperature and month of the year, with longer in-stream residence times, warmer soils and summer months having larger DOC losses. The single tracer study suggested a 10 year average loss of 8.77 g C m⁻² year⁻¹, which is 33.1 g CO_2eq m⁻² year⁻¹, or 29% of the DOC flux from the source over a mean in-stream residence time of 4.33 h.     

Dissolved organic carbon affects soil microbial activity and nitrogen dynamics in a Mexican tropical deciduous forest

Seasonal variation of dissolved organic C (DOC) and its effects on microbial activity and N dynamics were studied during two consecutive years in soils with different organic C concentrations (hilltop and hillslope) in a tropical deciduous forest of Mexico. We found that DOC concentrations were higher at the hilltop than at the hillslope soils, and in both soils generally decreased from the dry to the rainy season during the two study years. Microbial biomass and potential C mineralization rates, as well as dissolved organic N (DON) and NH₄⁺ concentrations and net N immobilization were higher in soils with higher DOC than in soils with lower DOC. In contrast, net N immobilization and NH₄⁺ concentration were depleted in the soil with lowest DOC, whereas NO₃⁻ concentrations and net nitrification increased. Negative correlations between net nitrification and DOC concentration suggested that NH₄⁺ was transformed to NO₃⁻ by nitrifiers when the C availability was depleted. Taken together, our results suggest that available C appears to control soil microbial activity and N dynamics, and that microbial N immobilization is facilitated by active heterotrophic microorganisms stimulated by high C availability. Soil autotrophic nitrification is magnified by decreases in C availability for heterotrophic microbial activity. This study provides an experimental data set that supports the conceptual model to show and highlight that microbial dynamics and N transformations could be functionally coupled with DOC availability in the tropical deciduous forest soils. Responsible Editor: Chris Neill     

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.     

137Cs and 40K isotopes in forest and wasteland soils in a selected region of eastern Poland 20 years after the Chernobyl accident

The vertical ¹³⁷Cs profile of forest and wasteland soils was analyzed in the south of the Podlasie Lowland area (Eastern Poland) about 20 years after the Chernobyl accident. In addition, the concentration of ⁴⁰K in soils of the investigated area was measured. Below the litter layer (mean thickness 3 cm), the soil samples were collected up to a depth of 12 cm and then divided into three layers: 0–3, 3–7, 7–12 cm. The behavior of ¹³⁷Cs and ⁴⁰K isotopes in soils was analyzed depending on the depth from which the soil samples were collected, as well as on the content of organic carbon, pH of soil and its granulometric composition. It was established that the density of ¹³⁷Cs in the litter layer equals 2.17 kBq m⁻²; it is the highest in layer 0–3 cm where it equals 3.44 kBq m⁻², and it decreases with the depth to the value of 0.76 kBq m⁻² in layer 7–12 cm. No similar pattern was observed in wasteland soils. The concentrations of ⁴⁰K in forest and wasteland soils did not change significantly with depth.     

Soil organic carbon budget and fertility variation of black soils in Northeast China

Black soils in Northeast China are characteristic of high soil organic carbon (SOC) density and were strongly influenced by human activities. Therefore, any change in SOC pool of these soils would not only impact the regional and global carbon cycle, but also affect the release and immobilization of nutrients. In this study, we reviewed the research progress on SOC storage, budget, variation, and fertility under different scenarios. The results showed that the organic carbon storage of black soils was 646.2 TgC and the most potential sequestration was 2887.8 g m⁻². According to the SOC budget, the net carbon emission of black soils was 1.3 TgC year⁻¹ under present soil management system. The simulation of CENTURY model showed that future climate change and elevated CO₂ concentration, especially the increase of precipitation, would increase SOC content. Furthermore, fertilization and cropping sequence obviously influenced SOC content, composition, and allocation among different soil particles. Long-term input of organic materials such as manure and straw renewed original SOC, improved soil structure and increased SOC accumulation. Besides, soil erosion preferred to transport soil particles with low density and fine size, decreased recalcitrant SOC fractions at erosion sites and increased activities of soil microorganism at deposition sites. After natural grasslands were converted into croplands, obvious variation of soil chemical nutrients, physical structure, and microbial activities had taken place in surface and subsurface soils, and represented a degrading trend to a certain degree. Our studies suggested that adopting optimal management such as conservation tillage in black soil region is an important approach to sequester atmospheric CO₂ and to slow greenhouse effects.     

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.     

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.     

Dynamics of dissolved organic <Superscript>14</Superscript>C in throughfall and soil solution of a Norway spruce forest

Dissolved organic carbon (DOC) is an important component of the C cycle in forest ecosystems, but dynamics and origin of DOC in throughfall and soil solution are yet poorly understood. In a 2-year study, we analyzed the radiocarbon signature of DOC in throughfall and soil solution beneath the Oa horizon and at 90 cm depth in a Norway spruce forest on a Podzol soil. A two-pool mixing model revealed that throughfall DOC comprised mainly biogenic C, i.e. recently fixed C, from canopy leaching and possibly other sources. The contribution of fossil DOC from atmospheric deposition to throughfall DOC was on average 6% with maxima of 8–11% during the dormant season. In soil solution from the Oa horizon, DO¹⁴C signature was highly dynamic (range from −8‰ to +103‰), but not correlated with DOC concentration. Radiocarbon signatures suggest that DOC beneath the Oa horizon originated mainly from occluded and mineral associated organic matter fractions of the Oa horizon rather than from the Oi or Oe horizon. Relatively old C was released in the rewetting phase following a drought period in the late summer of 2006. In contrast, the DO¹⁴C signature indicated the release of younger C throughout the humid year 2007. In soil solutions from 90 cm depth, DO¹⁴C signatures were also highly dynamic (−127‰ to +3‰) despite constantly low DOC concentrations. Similar to the Oa horizon, the lowest DO¹⁴C signature at 90 cm depth was found after the rewetting phase in the late summer of 2006. Because of the variation in the DO¹⁴C signatures at this depth, we conclude that DOC was not equilibrated with the surrounding soil, but also originated from overlaying soil horizons. The dynamics of DO¹⁴C in throughfall and soil solution suggest that the sources of DOC are highly variable in time. Extended drought periods likely have a strong influence on release and translocation of DOC from relatively old and possibly stabilized soil organic matter fractions. Temporal variations as well as the input of fossil DOC needs to be considered when calibrating DOC models based on DO¹⁴C signatures.     

Microbial activity and organic matter dynamics during 4 years of irrigation with treated wastewater

The global changes in rainfall frequency and quantity have subjected arid and semi-arid regions to long periods of drought. As this phenomenon corresponds to increasing trend of water shortage, the use of treated wastewater (TWW) has been suggested as an alternative for irrigation of agricultural crops in these areas. The aim of the study was to investigate the short- and middle-term effects of TWW irrigation on the soil microbial activities and organic carbon content. The microbial community activity was measured every 1–3 months for 4 years in a persimmon (Diospyros kaki) orchard. These activities were used here as an indicator for the soil health. The hydrolysis activity (detected by fluorescein diacetate hydrolysis (FDA) assay) increased during the irrigation season and was significantly higher in soils irrigated with TWW compared to those irrigated with freshwater (FW). This activity was also negatively correlated with dissolved organic carbon (DOC) concentrations during the irrigation season, suggesting that the community degraded the DOC in the soils regardless of its origin. The irrigation season was also characterized by an increase in nitrification potential in both TWW- and FW-irrigated soils, which coincided with high concentrations of nitrate (50 mg kg⁻¹ soil). Overall, there was an increase in all measured activities during the irrigation season, and they were higher in the TWW soils. However, it appears that after each irrigation season, the potential activity of the community returned to levels similar to or even slightly lower than those of FW-irrigated soil during the wet season, suggesting that the periodic irrigation did not significantly change the soil microbial activity.     

Holocene Carbon Stocks and Carbon Accumulation Rates Altered in Soils Undergoing Permafrost Thaw

Permafrost soils are a significant global store of carbon (C) with the potential to become a large C source to the atmosphere. Climate change is causing permafrost to thaw, which can affect primary production and decomposition, therefore affecting ecosystem C balance. To understand future responses of permafrost soils to climate change, we inventoried current soil C stocks, investigated ∆¹⁴C, C:N, δ¹³C, and δ¹⁵N depth profiles, modeled soil C accumulation rates, and calculated decadal net ecosystem production (NEP) in subarctic tundra soils undergoing minimal, moderate, and extensive permafrost thaw near Eight Mile Lake (EML) in Healy, Alaska. We modeled decadal and millennial soil C inputs, decomposition constants, and C accumulation rates by plotting cumulative C inventories against C ages based on radiocarbon dating of surface and deep soils, respectively. Soil C stocks at EML were substantial, over 50 kg C m⁻² in the top meter, and did not differ much among sites. Carbon to nitrogen ratio, δ¹³C, and δ¹⁵N depth profiles indicated most of the decomposition occurred within the organic soil horizon and practically ceased in deeper, frozen horizons. The average C accumulation rate for EML surface soils was 25.8 g C m⁻² y⁻¹ and the rate for the deep soil accumulation was 2.3 g C m⁻² y⁻¹, indicating these systems have been C sinks throughout the Holocene. Decadal net ecosystem production averaged 14.4 g C m⁻² y⁻¹. However, the shape of decadal C accumulation curves, combined with recent annual NEP measurements, indicates soil C accumulation has halted and the ecosystem may be becoming a C source. Thus, the net impact of climate warming on tundra ecosystem C balance includes not only becoming a C source but also the loss of C uptake capacity these systems have provided over the past ten thousand years.     

Sources and sinks of dissolved organic carbon in a forested swamp catchment

Concentrations of dissolved organic carbon (DOC) were measured in precipitation, throughfall, stemflow, and soil, peat and stream water in a 50 ha catchment with a central 5 ha swamp at Mont St. Hilaire, Quebec. DOC concentrations in precipitation were low (2.0 mg L^–1), but increased in passage through the tree canopies as throughfall (9.1–14.6 mg L^–1) and stemflow (23.1–30.1 mg L^–1). For the period July 1–November 15, 1987, 0.5 g DOC m^–2 was imported as precipitation, and forest canopies contributed a further 1.4–1.7 g m^–2 2 to the soil surface. DOC concentrations were higher (46.0 and 67.6 mg L^–1) in upland soil organic horizons, but decreased with depth because subsoil mineral horizons acted as a major sink of DOC. A laboratory experiment using leaf leachate revealed that subsoil horizons were able to adsorb DOC, with equilibrium DOC concentrations ranging from 3 to 19 mg L^–1. Soil organic carbon appeared to be an important determinant of equilibrium DOC concentrations. The swamp was a major source of DOC, with an overall average DOC concentration of 58.6 mg L^–1 and showed strong spatial and temporal variations related to hydrologic and thermal regimes. During base flow periods, stream DOC concentrations were small (< 3 mg L^–1), dominated by water fed from springs draining upland soils. During high flows, stream DOC concentrations increased through the contribution of DOC-rich water originating in the swamp. Sources, sinks and transport of DOC are thus a function of a complex set of inter-related biotic and abiotic process.     

Sedimentation in Boreal Lakes—The Role of Flocculation of Allochthonous Dissolved Organic Matter in the Water Column

We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (<0.48 km²), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l⁻¹. Stable isotope analysis suggests that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m² lake area was correlated to DOC concentration in the water (R ² = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R ² = 0.58). The net efflux of C as CO₂ from the water to the atmosphere was likewise correlated to DOC concentration (R ² = 0.52). The losses of organic carbon from the water column via mineralization to CO₂ and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.     

Spatial variability of UVR attenuation and bio-optical factors in shallow coral-reef waters of Malaysia

Biologically diverse coral-reef ecosystems are both directly and indirectly susceptible to changes in the spectral ultraviolet radiation (UVR) distribution. The purpose of this study was to (1) measure the variability of UVR and photosynthetically active radiation (PAR) penetration in the water above coral reefs around the Malaysian peninsula, (2) measure the variability and distribution of UVR-specific biogeochemical factors, and (3) determine the impact of biogeochemical variability as it affects the UVR:PAR ratio. Downwelling UVR and PAR irradiance and bio-optically derived biogeochemical factors were measured at 14 coral survey stations around the Malaysian peninsula from August 10–29, 2007. The West Coast was characterized by relatively shallow mean 10% UV-B (320 nm) penetration (1.68 ± 1.12 m), high chlorophyll (3.00 ± 4.72 μg l⁻¹), high chromophoric dissolved organic matter (CDOM; 6.61 ± 3.31 ppb), high particulate organic carbon (POC; 190.65 ± 97.99 mg m⁻³), and low dissolved organic carbon (DOC; 1.34 ± 0.65 mg m⁻³). By contrast, the East Coast was characterized by relatively deep mean 10% UV-B penetration (5.03 ± 2.19 m), low chlorophyll (0.34 ± 0.22 μg l⁻¹), low CDOM (1.45 ± 0.44 ppb), low POC (103.21 ± 37.93 mg m⁻³), and relatively high DOC (1.91 ± 1.03 mg m⁻³). The UVR:PAR ratio was relatively higher on the East Coast relative to the West Coast, suggesting variable concentrations of UVR-specific absorbing components. At all sites, UVR attenuation coefficients showed significant correlations with CDOM, but were spatially dependent with regard to chlorophyll a, POC, and DOC. The results suggest that bio-optically significant CDOM and DOC factors are uncoupled in coral-reef communities of Malaysia. Furthermore, the results support prior studies that show chromophorically active concentrations of DOM and POC are significantly altering the amount of UVR penetration above coral reefs and may be notable factors in regulating intricate biogeochemical cycles around benthic coral communities in Malaysia.     

Quantifying the effects of switchgrass (Panicum virgatum) on deep organic C stocks using natural abundance 14C in three marginal soils

Perennial bioenergy crops have been shown to increase soil organic carbon (SOC) stocks, potentially offsetting anthropogenic C emissions. The effects of perennial bioenergy crops on SOC are typically assessed at shallow depths (<30 cm), but the deep root systems of these crops may also have substantial effects on SOC stocks at greater depths. We hypothesized that deep (>30 cm) SOC stocks would be greater under bioenergy crops relative to stocks under shallow‐rooted conventional crop cover. To test this, we sampled soils to between 1‐ and 3‐m depth at three sites in Oklahoma with 10‐ to 20‐year‐old switchgrass (Panicum virgatum) stands, and collected paired samples from nearby fields cultivated with shallow rooted annual crops. We measured root biomass, total organic C, ¹⁴C, ¹³C, and other soil properties in three replicate soil cores in each field and used a mixing model to estimate the proportion of recently fixed C under switchgrass based on ¹⁴C. The subsoil C stock under switchgrass (defined over 500–1500 kg/m² equivalent soil mass, approximately 30–100 cm depth) exceeded the subsoil stock in neighboring fields by 1.5 kg C/m² at a sandy loam site, 0.6 kg C/m² at a site with loam soils, and showed no significant difference at a third site with clay soils. Using the mixing model, we estimated that additional SOC introduced after switchgrass cultivation comprised 31% of the subsoil C stock at the sandy loam site, 22% at the loam site, and 0% at the clay site. These results suggest that switchgrass can contribute significantly to subsoil organic C—but also indicated that this effect varies across sites. Our analysis shows that agricultural strategies that emphasize deep‐rooted grass cultivars can increase soil C relative to conventional crops while expanding energy biomass production on marginal lands.     

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.     

A comparison of soil carbon pools and profiles in wetlands in Costa Rica and Ohio

Wetlands are large carbon pools and play important roles in global carbon cycles as natural carbon sinks. This study analyzes the variation of total soil carbon with depth in two temperate (Ohio) and three tropical (humid and dry) wetlands in Costa Rica and compares their total soil C pool to determine C accumulation in wetland soils. The temperate wetlands had significantly greater (P < 0.01) C pools (17.6 kg C m⁻²) than did the wetlands located in tropical climates (9.7 kg C m⁻²) in the top 24 cm of soil. Carbon profiles showed a rapid decrease of concentrations with soil depth in the tropical sites, whereas in the temperate wetlands they tended to increase with depth, up to a maximum at 18–24 cm, after which they started decreasing. The two wetlands in Ohio had about ten times the mean total C concentration of adjacent upland soils (e.g., 161 g C kg⁻¹ were measured in a central Ohio isolated forested wetland, and 17 g C kg⁻¹ in an adjacent upland site), and their soil C pools were significantly higher (P < 0.01). Among the five wetland study sites, three main wetland types were identified – isolated forested, riverine flow-through, and slow-flow slough. In the top 24 cm of soil, isolated forested wetlands had the greatest pool (10.8 kg C m⁻²), significantly higher (P < 0.05) than the other two types (7.9 kg C m⁻² in the riverine flow-though wetlands and 8.0 kg C m⁻² in a slowly flowing slough), indicating that the type of organic matter entering into the system and the type of wetland may be key factors in defining its soil C pool. A riverine flow-through wetland in Ohio showed a significantly higher C pool (P < 0.05) in the permanently flooded location (18.5 kg C m⁻²) than in the edge location with fluctuating hydrology, where the soil is intermittently flooded (14.6 kg C m⁻²).     

A comparative study of dissolved organic carbon transport and stabilization in California forest and grassland soils

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year⁻¹ for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models.