Retention of soluble organic nutrients by a forested ecosystem

We document an example of a forested watershed at the Coweeta HydrologicLaboratory with an extraordinary tendency to retain dissolved organic matter(DOM) generated in large quantities within the ecosystem. Our objectives weretodetermine fluxes of dissolved organic C, N, and P (DOC, DON, DOP,respectively),in water draining through each stratum of the ecosystem and synthesizeinformation on the physicochemical, biological and hydrologic factors leadingtoretention of dissolved organic nutrients in this ecosystem. The ecosystemretained 99.3, 97.3, and 99.0% of water soluble organic C, N and P,respectively, produced in litterfall, throughfall, and root exudates. Exportsinstreamwater were 4.1 kg ha^–1yr^–1of DOC, 0.191 kg ha^–1 yr^–1 ofDON, and 0.011 kg ha^–1 yr^–1 ofDOP. Fluxes of DON were greater than those of inorganic N in all strata. MostDOC, DON, and DOP was removed from solution in the A and B horizons, with DOCbeing rapidly adsorbed to Fe and Al oxyhydroxides, most likely by ligandexchange. DON and DOC were released gradually from the forest floor over theyear. Water soluble organic C produced in litterfall and throughfall had adisjoint distribution of half-decay times with very labile and veryrefractory fractions so that most labile DOC was decomposed before beingleachedinto the mineral soil and refractory fractions dominated the DOC transportedthrough the ecosystem. We hypothesize that this watershed retained solubleorganic nutrients to an extraordinary degree because the soils have very highcontents of Fe and Al oxyhydroxides with high adsorption capacities and becausethe predominant hydrologic pathway is downwards as unsaturated flow through astrongly adsorbing A and B horizon. The well recognized retention mechanismsforinorganic nutrients combine with adsorption of DOM and hydrologic pathway toefficiently prevent leaching of both soluble inorganic andorganic nutrients in this watershed.     

Carbon dioxide consumption during soil development

Carbon is sequestered in soils by accumulation of recalcitrant organic matter and by bicarbonate weathering of silicate minerals. Carbon fixation by ecosystems helps drive weathering processes in soils and that in turn diverts carbon from annual photosynthesis-soil respiration cycling into the long-term geological carbon cycle. To quantify rates of carbon transfer during soil development in moist temperate grassland and desert scrubland ecosystems, we measured organic and inorganic residues derived from the interaction of soil biota and silicate mineral weathering for twenty-two soil profiles in arkosic sediments of differing ages. In moist temperate grasslands, net annual removal of carbon from the atmosphere by organic carbon accumulation and silicate weathering ranges from about 8.5 g m^–2 yr^–1 for young soils to 0.7 g M^–2 yr^–1 for old soils. In desert scrublands, net annual carbon removal is about 0.2 g m^–2 yr^–1 for young soils and 0.01 g m^–2 yr^–1 for old soils. In soils of both ecosystems, organic carbon accumulation exceeds CO₂ removal by weathering, however, as soils age, rates of CO₂ consumption by weathering accounts for greater amounts of carbon sequestration, increasing from 2% to 8% in the grassland soils and from 2% to 40% in the scrubland soils. In soils of desert scrublands, carbonate accumulation far outstrips organic carbon accumulation, but about 90% of this mass is derived from aerosolic sources that do not contribute to long-term sequestration of atmospheric carbon dioxide.     

Soil-productivity relationships and organic matter turnover in dry tropical forests of the Florida Keys

Soils and aboveground production in five types of upland forest in the Florida Keys were examined. Throughout the habitat gradient represented by these forest types, the soils were predominantly shallow and organic, forming in place directly on the limestone bedrock. However, the well-drained soils in the most productive broadleaved forests were deep enough to qualify as Histosols (Folists). Soils decreased in electrical conductivity and increased in nutrient content with increasing aboveground production. At 3–12 Mg ha^–1 yr^–1, production was within the range reported for dry tropical forests. Measured rates of decomposition were moderate or fast, and estimates of the organic C turnover of several soils based on their bomb radiocarbon signature were 100 years or less. In the face of these rapid turnover rates, we attribute the development of organic soils to the absence of mineral residues from weathering of the underlying limestone bedrock. Fast turnover of organic matter, and rapid and efficient cycling of nutrients are necessary to sustain the high production rates obtained on these shallow organic soils.     

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.     

Dissolved organic carbon and its utilization in a riverine wetland ecosystem

Variations in dissolved organic carbon (DOC) concentrations of surface waters and subsurface interstitial groundwater of riparian and wetland soils to 1.2 m depth were evaluated in a riverine wetland ecosystem over one year. DOC was monitored at seven sites within the wetland pond, two sites on the inflow stream, and one site on the outflow stream. Surface concentrations in the inflow stream ranged from 0.74 to 11.6 mg C L^–1 and those of the outflow from 2.1 to 8.0 mg C L^–1 Average DOC from stream floodplain hydrosoils (3.1 to 32.1 mg C L^–1 was greater than DOC from the sediments below the stream channel (1.6 to 6.8 mg C L^–1 Surface DOC within the wetland varied seasonally, with greatest fluctuations in concentrations through the summer and autumn (range 4.8 to 32.6 mg C L^–1 ) during intensive macrophyte growth and bacterial production. DOC was less variable during the winter months (1.7 to 3.3 mg C L^–1 Within the wetland pond, average DOC concentrations (7.1 to 48.2 mg C L^–1) in the subsurface waters were significantly greater (p < 0.05) than average surface concentrations. The microbial availability of surface and subsurface DOC to bacteria was evaluated from losses of DOC by wetland bacteria grown on the DOC. Bacterial growth efficiencies ranged from 5 to 20% and were negatively correlated to the percentage of DOC removed by bacteria (r²=0.93). Throughout the ecosystem, DOC concentrations were greatest in the subsurface waters, but at most depths this DOC was a less suitable substrate than surface DOC for utilization by bacteria.     

Organic matter chemistry and dynamics in clear-cut and unmanaged hardwood forest ecosystems

Forest harvesting alters the organic matter cycle by changing litter inputs and the decomposition regime. We hypothesized that these changes would result in differences in organic matter chemistry between clear-cut and uncut watershed ecosystems. We studied the chemistry of soil organic matter (SOM), and dissolved organic carbon (DOC) in soil solutions and stream samples in clear-cut and uncut sites at the Hubbard Brook Experimental Forest in New Hampshire using DOC fractionation techniques and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy.Alkyl-C (aliphatic compounds) and O-alkyl-C (carbohydrates) were the largest C fractions in soil and dissolved organic matter at Hubbard Brook. Alkyl-C ranged from 29–48% of soil C, 25–42% of soil solution C, and 22–42% of streamwater DOC. Carbohydrates comprised 32–49%, 36–43%, and 29–60% of C in soils, solutions, and streamwater, respectively. In both soils and soil solutions, the carbohydrate fraction decreased with increasing soil depth, while the aromaticity of organic matter increased with depth. There were no significant differences in the structural chemistry of SOM between clear-cut and uncut watersheds.The aromatic-C fractions in soil solutions at the clear-cut site ranged from 12–16%, approximately 40% greater than at the uncut site (8.5–11%). Thus, clear-cutting has resulted in the leaching of more highly decomposed organic matter, and depletion of more aliphatic compounds in the soluble organic pool. Because DOC fluxes are small compared to the SOM pool, large differences in soil solution chemistry do not substantially alter the overall composition of SOM. While the organic chemistry of stream DOC varied greatly among 3 sampling dates, there were no obvious clear-cutting effects. Thus, temporal variations in flowpaths and/or in-stream processes appear to be more important than disturbance in regulating the organic carbon chemistry of these streams.     

Longitudinal changes in δ13C of riffle macroinvertebrates from mountain to lowland sections of a gravel‐bed river

Summary 1. To understand longitudinal changes in the trophic base of benthic macroinvertebrates from mountain to lowland river sections, we investigated carbon stable isotopic compositions (δ¹³C) of macroinvertebrates and their food resources in riffles for four seasons at 14 sites along the main stem of the Toyo River, Japan. 2. At each site, δ¹³C was usually highest or nearly highest for periphyton (epilithic biofilm) and was lowest for transported leaf materials. Among macroinvertebrate groups, grazers usually had higher δ¹³C values than filterers or predators. 3. During all seasons, δ¹³C of periphyton and all macroinvertebrate groups increased downstream from mountain to upland sections, but decreased downstream from upland to lowland sections. In addition, the difference between grazer δ¹³C and filterer δ¹³C decreased from mountain to upland sections, but increased from upland to lowland sections. 4. The observed changes in δ¹³C of periphyton and macroinvertebrates from mountain to upland sections agree with previous reports: the δ¹³C of periphyton and consumers increased with stream size and productivity. The decrease in δ¹³C of periphyton and macroinvertebrates from upland to lowland sections has not been reported previously, and this may have resulted from an increased importance of terrestrial detritus relative to periphyton production in the lowland section, where riffles were infrequent and pools dominated the reach. 5. A simple mixing model of δ¹³C showed that grazers rely mostly on periphyton at all sites, whereas the importance of periphyton for filterers changed longitudinally increasing from mountain to upland sections and decreasing from upland to lowland sections. This longitudinal trend for filterers is possibly associated with the changes in the availability or quality of terrestrial detritus in transported particulate organic matter. 6. Longitudinal changes in the relative importance of autochthonous production and allochthonous detritus appear to be reflected in δ¹³C of riffle benthic communities. The longitudinal changes were not monotonic, and specific reach characteristics may be responsible for the greater importance of allochthonous detritus in mountain and lowland sections.     

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.     

Declining trend of carbon in Finnish cropland soils in 1974–2009

Soil organic matter not only affects soil properties and productivity but also has an essential role in global carbon (C) cycle. We studied changes in the topsoil C content of Finnish croplands using a dataset produced in nationwide soil monitoring. The monitoring network consisting of fields on both mineral and organic soils was established in 1974 and resampled in 1987, 1998, and 2009. Over the monitoring period from 1974 to 2009, cultivated soils showed a continuous decline in C concentration (g kg^?1). In organic soils, C concentration decreased at a mean rate of 0.2–0.3% yr^?1 relative to the existing C concentration. In mineral soils, the relative decrease was 0.4% yr^?1 corresponding to a C stock (kg m^?2) loss of 220 kg ha^?1 yr^?1. The change in management practices in last decades toward increasing cultivation of annual crops has contributed to soil C losses noted in this study. The results, however, suggest that the C losses result partly from other processes affecting cultivated soils such as climatic change or the continuing long‐term effect of forest clearance. We estimated that Finnish cropland soils store 161 Tg carbon nationwide in the topmost 15 cm of which 117 Tg is in mineral soils. C losses from mineral soils can therefore total up to 0.5 Tg yearly.     

A Quantitative Food Web Model for the Macroinvertebrate Community of a Northern German Lowland Stream

Trophic interactions and cycling of organic carbon within the macroinvertebrate community of a Northern German lowland stream were analyzed based on a compartment model. The network model describes the structure of the food web quantifying biomass, production, and consumption of their elements, of the entire system and between trophic levels. System primary production is 153.7 g C m⁻² yr⁻¹ and invertebrate production 53.3 g C m⁻² yr⁻¹. Invertebrate consumption amounts to 702.6 g C m⁻² yr⁻¹. Main flows are identified between trophic level 1 and 2 and are connected with highly productive compartments. ‘Anodonta and Pseudanodonta’ and Dreissena polymorpha show the highest consumption of all groups with 269.9 g C m⁻² yr⁻¹ and 114.1 g C m⁻² yr⁻¹, respectively. System consumption is highest on the import from the upstream lake with 532.5 g C m⁻² yr⁻¹, sediment detritus with 135.5 g C m⁻² yr⁻¹, and primary producers with 25.7 g C m⁻² yr⁻¹. The lowest predation pressure is observed for Bivalvia with an ecotrophic efficiency of <10% and highest for Chironomidae with 91%. Approximately 20% of organic matter entering the detritus pool are recycled to the living groups of the system. Transfer efficiencies between discrete trophic levels are generally low except for transfer of detrital material between level I and II.     

The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest‐wetland landscape

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost‐free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH₄) emissions. Here, we quantify the thaw‐induced increase in CH₄ emissions for a boreal forest‐wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long‐term net carbon dioxide (CO₂) exchange. Using nested wetland and landscape eddy covariance net CH₄ flux measurements in combination with flux footprint modeling, we find that landscape CH₄ emissions increase with increasing wetland‐to‐forest ratio. Landscape CH₄ emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH₄ emission of ~13 g CH₄ m^?2 is the dominating contribution to the landscape CH₄ emission of ~7 g CH₄ m^?2. In contrast, forest contributions to landscape CH₄ emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr^?1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH₄ m^?2 yr^?1 in landscape CH₄ emissions. A long‐term net CO₂ uptake of >200 g CO₂ m^?2 yr^?1 is required to offset the positive radiative forcing of increasing CH₄ emissions until the end of the 21st century as indicated by an atmospheric CH₄ and CO₂ concentration model. However, long‐term apparent carbon accumulation rates in similar boreal forest‐wetland landscapes and eddy covariance landscape net CO₂ flux measurements suggest a long‐term net CO₂ uptake between 49 and 157 g CO₂ m^?2 yr^?1. Thus, thaw‐induced CH₄ emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.     

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.     

Ground water flow paths in relation to nitrogen chemistry in the near-stream zone

Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L^–1, 1.2–1.8 mg L^–1 and 4.6–9.1 mg L^–1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L^–1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L^–1 and 0.4–2.2 mg L^–1 respectively. Ammonium — N concentrations (20–60 µg L^–1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.     

Spatial and temporal dynamics of mercury in Precambrian Shield upland runoff

Methylated and total Hg, and TOC concentrations were measured in precipitation and runoff in a first order Precambrian Shield watershed, and in precipitation, throughfall, shallow groundwater and runoff in a zero Precambrian Shield watershed. Plots dominated by open lichen-covered bedrock and another containing small patches of conifer forest and thin discontinuous surficial deposits were monitored within the zero order catchment. Methyl (3–10 fold) and non-methyl (1.4–2.8 fold) Hg concentrations changed irregularly during rainfall and snowmelt runoff events in all catchments. Temporal patterns of Hg concentration in runoff included flushing and subsequent dilution as well as peak concentrations coinciding with peak or recession flow. Mercury export was highest from lichen-covered bedrock surfaces as a result of high runoff yields and minimal opportunity for physical retention and in the case of MeHg demethylation. Forest canopy and lichen/bedrock surfaces were often net sources for Hg while forest soils were mostly sinks. However, upland soils undergoing periodic reducing conditions appear to be sites for the in situ production of MeHg.     

Dynamics of dissolved and particulate organic carbon in a saline and semiarid stream of southeast Spain (Chicamo stream)

Annual variations in the concentration of dissolved (DOC) and particulate organic carbon (CPOC = Coarse; FPOC = Fine; UPOC = Ultrafine) were studied in a 100 m-reach of the Chicamo stream, an intermittent saline stream in southeast Spain. DOC represented the most important fraction of organic carbon flowing in the Chicamo stream (>98%), with concentrations of about 1.7 mgC l^–1 during most of the year, reaching 2.5 mgC l^–1 in summer. One high flow episode during a rain storm in winter was characterized by a considerably increased concentration of DOC (9.4 mgC l^–1). CPOC was the dominant POC fraction. Positive and significant correlations were found for DOC and discharge, which support the idea of allochthonous inputs due to floods. There was no significant correlation between POC and discharge. No significant correlations were found for DOC or POC with the physico-chemical parameters measured, while a negative significant correlation was found between DOC and temperature. The export of total organic carbon from the drainage basin of the Chicamo stream was low (6.2 × 10^–4 gC m^–2 yr^–1) and typical of streams in arid and semi-arid regions. The results of a Principal Component Analysis defined three different phases. The first consisted of short periods, during which floods provide pulses of allochthonous organic carbon and nutrients, the second a dry phase (summer), defined by biotic interactions, during which the stream could acts as a `sink' of organic matter, and the third and final phase which is characterised by hydrological stability.     

Lead biogeochemistry in a central Ontario Forested watershed

To determine the sources and sinks of atmospherically deposited Pb at a forested watershed (Plastic Lake) in central Ontario, Canada, Pb pools and fluxes through upland, wetland and lake compartments were measured during 2002/2003 and compared with previous measurements taken between 1989 and 1991. In 2002/2003, annual bulk deposition of Pb was 0.49 mg m⁻² compared with 1.90–1.30 mg m⁻² in 1989–1991. Annual Pb concentrations in stream water draining the upland part of the catchment were very low (0.04 μg l⁻¹) and were approximately half those measured in 1989–1991 (0.11–0.08 μg l⁻¹). Leaching losses in stream water were small and mass balance estimates indicate almost complete retention (>95%) of atmospherically deposited Pb in upland soils. In contrast, annual Pb concentrations in stream water draining a wetland were between 0.38 and 0.77 μg l⁻¹, with the highest concentration occurring in 2002/2003 and mass balance calculations indicate that the wetland is a net source of Pb in all measured years. Lead concentrations in the lake outflow were low and the average Pb concentration measured in 2002/2003 (0.09 μg l⁻¹) was approximately half the value recorded in 1989–1991 (0.19 μg l⁻¹ both years). Annual mass balance estimates indicate that the lake retained between 2.47 mg m⁻² (1989/1990) and 1.42 mg m⁻² (2002/2003) and that in 2002/2003 68% of the Pb input to the lake is derived from the terrestrial catchment. These estimates are higher than sediment core records, which indicate around 18 mg m⁻² Pb was retained in sediment during the 1990s. Nevertheless, Pb concentrations decrease with sediment depth and ²⁰⁶Pb/²⁰⁷Pb concentrations increase with depth, a pattern also observed in mineral soils that reflects the substantial contribution of anthropogenic Pb to the watershed. Lead isotope data from soil and sediment indicate a recent anthropogenic Pb signal (²⁰⁶Pb/²⁰⁷Pb ∼ 1.185) in upper soils and sediments and an older anthropogenic signal (²⁰⁶Pb/²⁰⁷Pb ∼ 1.20) in deeper soil and sediment. Lead isotope data in sediment and vegetation indicate that practically all the Pb cycled in the forest at Plastic Lake is anthropogenic in origin.     

基于13C、15N和C/N识别南岳小流域沉积有机质的来源

农业流域有机质流失造成水体富营养化和土地退化,不仅威胁水质和粮食安全,而且会导致温室气体排放等潜在环境问题。本研究用¹³C、¹⁵N和C/N作为指纹标志物,分析了南岳小流域出口沉积有机质的来源及其在林地、稻田和菜地等典型土地利用类型土壤的空间分布特征,并结合贝叶斯稳定同位素混合模型定量估算了各土地利用类型的贡献率。结果表明: δ¹³C具有显著的空间差异,沉积物有机质(-22.6‰±0.53‰)和林地土壤(-23.13‰±1.71‰)的δ¹³C显著高于稻田土壤(-25.24‰±1.4‰)。各土地利用类型土壤的δ¹⁵N差异不显著,沉积物的均值最大,为(4.37±0.83)‰,林地最小,为(2.38±1.97)‰;林地土壤的C/N均值最大,为16.66±7.18,稻田土壤的C/N均值最小,为11.95±0.92。贝叶斯稳定同位素混合模型结果显示,林地、稻田和菜地对流域出口沉积有机质的贡献率分别为19.6%、15.7%和64.7%;稻田和菜地作为农业用地的总贡献率为80.4%。说明农业用地土壤是南岳小流域沉积有机质的主要来源,可以通过优化农田管理措施有效控制流域养分流失。     

Carbon Sequestration in Reclaimed Minesoils

Minesoils are drastically influenced by anthropogenic activities. They are characterized by low soil organic matter (SOM) content, low fertility, and poor physicochemical and biological properties, limiting their quality, capability, and functions. Reclamation of these soils has potential for resequestering some of the C lost and mitigating CO₂ emissions. Soil organic carbon (SOC) sequestration rates in minesoils are high in the first 20 to 30 years after reclamation in the top 15 cm soil depth. In general, higher rates of SOC sequestration are observed for minesoils under pasture and grassland management than under forest land use. Observed rates of SOC sequestration are 0.3 to 1.85 Mg C ha^{? 1} yr^{? 1} for pastures and rangelands, and 0.2 to 1.64 Mg C ha^{? 1} yr^{? 1} for forest land use. Proper reclamation and postreclamation management may enhance SOC sequestration and add to the economic value of the mined sites. Management practices that may enhance SOC sequestration include increasing vegetative cover by deep-rooted perennial vegetation and afforestation, improving soil fertility, and alleviation of physical, chemical and biological limitations by fertilizers and soil amendments such as biosolids, manure, coal combustion by-products, and mulches. Soil and water conservation are important to SOC sequestration. The potential of SOC sequestration in minesoils of the US is estimated to be 1.28 Tg C yr^?1, compared to the emissions from coal combustion of 506 Tg C yr^{? 1}.     

Carbon Mineralization and Labile Organic Carbon Pools in the Sandy Soils of a North Florida Watershed

The large pool of actively cycling carbon (C) held in soils is susceptible to release due to changes in landuse, management, or climate. Yet, the amount and distribution of potentially mineralizable C present in soils of various types and the method by which this soil C fraction can best be quantified, are not well established. The distribution of total organic C (TOC), extractable C pools (hot-water-extractable and acid-hydrolyzable), and in vitro mineralizable C in 138 surface soils across a north Florida watershed was found to be quite heterogeneous. Thus, these C quality parameters could not statistically distinguish the eight landuses or four major soil orders represented. Only wetland and upland forest soils, with the largest and smallest C pool size, respectively, were consistently different from the soils of other landuse types. Variations in potential C mineralization were best explained by TOC (62%) and hot-water-extractable C (59%), whereas acid-hydrolyzable C (32%) and clay content (35%) were generally not adequate indicators of C bioavailability. Within certain landuse and soil orders (Alfisol, Wetland and Rangeland, all with >3% clay content), however, C mineralization and clay content were directly linearly correlated, indicating a possible stimulatory effect of clay on microbial processing of C. Generally, the sandy nature of these surface soils imparted a lack of protection against C mineralization and likely resulted in the lack of landuse/soil order differences in the soil C pools. If a single parameter is to be chosen to quantify the potential for soil C mineralization in southeastern U.S. coastal plain soils, we recommend TOC as the most efficient soil variable to measure. Author Contributions  Conceived of or designed study: Sabine Grunwald, Nick Comerford, and James Sickman—Performed research: Mi-Youn Ahn—Analyzed data: Mi-Youn Ahn, Andrew Zimmerman, and Nick Comerford—Contributed new methods or models: Andrew Zimmerman, Nick Comerford, and James Sickman—Wrote the paper: Mi-Youn Ahn, Andrew Zimmerman, and Nick Comerford.     

Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Temperate forests of North America are thought to besignificant sinks of atmospheric CO₂. Wedeveloped a below-ground carbon (C) budget forwell-drained soils in Harvard Forest Massachusetts, anecosystem that is storing C. Measurements of carbonand radiocarbon (¹⁴C) inventory were used todetermine the turnover time and maximum rate ofCO₂ production from heterotrophic respiration ofthree fractions of soil organic matter (SOM):recognizable litter fragments (L), humified lowdensity material (H), and high density ormineral-associated organic matter (M). Turnover timesin all fractions increased with soil depth and were2–5 years for recognizable leaf litter, 5–10 years forroot litter, 40–100+ years for low density humifiedmaterial and >100 years for carbon associated withminerals. These turnover times represent the timecarbon resides in the plant + soil system, and mayunderestimate actual decomposition rates if carbonresides for several years in living root, plant orwoody material.Soil respiration was partitioned into two componentsusing ¹⁴C: recent photosynthate which ismetabolized by roots and microorganisms within a yearof initial fixation (Recent-C), and C that is respiredduring microbial decomposition of SOM that resides inthe soil for several years or longer (Reservoir-C).For the whole soil, we calculate that decomposition ofReservoir-C contributes approximately 41% of thetotal annual soil respiration. Of this 41%,recognizable leaf or root detritus accounts for 80%of the flux, and 20% is from the more humifiedfractions that dominate the soil carbon stocks.Measurements of CO₂ and ¹⁴CO₂ in thesoil atmosphere and in total soil respiration werecombined with surface CO₂ fluxes and a soil gasdiffusion model to determine the flux and isotopicsignature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cmof the soil (O + A + Ap horizons). The average residencetime of Reservoir-C in the plant + soil system is8±1 years and the average age of carbon in totalsoil respiration (Recent-C + Reservoir-C) is 4±1years.The O and A horizons have accumulated 4.4 kgC m^–2above the plow layer since abandonment by settlers inthe late-1800's. C pools contributing the most to soilrespiration have short enough turnover times that theyare likely in steady state. However, most C is storedas humified organic matter within both the O and Ahorizons and has turnover times from 40 to 100+ yearsrespectively. These reservoirs continue to accumulatecarbon at a combined rate of 10–30 gC m^{minus 2}yr^–1. This rate of accumulation is only 5–15% of the total ecosystem C sink measured in this stand using eddy covariance methods.