Increased temperature sensitivity of net DOC production from ombrotrophic peat due to water table draw-down

The production and release of dissolved organic carbon (DOC) from peat soils is thought to be sensitive to changes in climate, specifically changes in temperature and rainfall. However, little is known about the actual rates of net DOC production in response to temperature and water table draw‐down, particularly in comparison to carbon dioxide (CO₂) fluxes. To explore these relationships, we carried out a laboratory experiment on intact peat soil cores under controlled temperature and water table conditions to determine the impact and interaction of each of these climatic factors on net DOC production. We found a significant interaction (P < 0.001) between temperature, water table draw‐down and net DOC production across the whole soil core (0 to −55 cm depth). This corresponded to an increase in the Q₁₀ (i.e. rise in the rate of net DOC production over a 10 °C range) from 1.84 under high water tables and anaerobic conditions to 3.53 under water table draw‐down and aerobic conditions between −10 and − 40 cm depth. However, increases in net DOC production were only seen after water tables recovered to the surface as secondary changes in soil water chemistry driven by sulphur redox reactions decreased DOC solubility, and therefore DOC concentrations, during periods of water table draw‐down. Furthermore, net microbial consumption of DOC was also apparent at − 1 cm depth and was an additional cause of declining DOC concentrations during dry periods. Therefore, although increased temperature and decreased rainfall could have a significant effect on net DOC release from peatlands, these climatic effects could be masked by other factors controlling the biological consumption of DOC in addition to soil water chemistry and DOC solubility. These findings highlight both the sensitivity of DOC release from ombrotrophic peat to episodic changes in water table draw‐down, and the need to disentangle complex and interacting controls on DOC dynamics to fully understand the impact of environmental change on this system.     

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.     

Photochemical mineralization of synthetic lignin in lake water indicates enhanced turnover of aromatic organic matter under solar radiation

The degradation of ¹⁴C-[ring]-labelled syntheticlignin (¹⁴C-DHP) and dissolved organic carbon(DOC) from lake water were studied simultaneously.¹⁴C-DHP was incubated in humic lake water (colour173 mg Pt l^-1) for 7 d in the dark or under solarradiation. In the dark <0.4% of the introduced¹⁴C-DHP label and 4% of the indigenous DOC weremineralized, indicating that the ¹⁴C-labelledaromatic rings of DHP and the humic DOC weremicrobiologically recalcitrant. Under solar radiation(116 MJ m^-2), 17–21% of the ¹⁴C-labelledcarbons in DHP and 18–23% of the indigenous DOC weremineralized in 7 d. Simultaneously the watersolubility of ¹⁴C-DHP increased. Solar radiationconverted the aromatic cores of synthetic lignin toCO₂ and soluble organic photoproducts. Theresults suggest that solar radiation plays a key rolein the decomposition of natural polyaromatic matter.     

Fate and stability of dissolved organic carbon in topsoils and subsoils under beech forests

Dissolved organic carbon (DOC) from Oa horizons has been proposed to be an important contributor for subsoil organic carbon stocks. We investigated the fate of DOC by directly injecting a DOC solution from ¹³C labelled litter into three soil depths at beech forest sites. Fate of injected DOC was quantified with deep drilling soil cores down to 2 m depth, 3 and 17 months after the injection. 27 ± 26% of the injected DOC was retained after 3 months and 17 ± 22% after 17 months. Retained DOC was to 70% found in the first 10 cm below the injection depth and on average higher in the topsoil than in the subsoil. After 17 months DOC in the topsoil was largely lost (− 19%) while DOC in the subsoil did not change much (− 4.4%). Data indicated a high stabilisation of injected DOC in the subsoils with no differences between the sites. Potential mineralisation as revealed by incubation experiments however, was not different between DOC injected in topsoil or subsoils underlining the importance of environmental factors in the subsoil for DOC stabilisation compared to topsoil. We conclude that stability of DOC in subsoil is primary driven by its spatial inaccessibility for microorganisms after matrix flow while site specific properties did not significantly affect stabilisation. Instead, a more fine-textured site promotes the vertical transport of DOC due to a higher abundance of preferential flow paths.

     

Controls on fluxes and export of dissolved organic carbon in grasslands with contrasting soil types

Export of dissolved organic carbon (DOC) from grassland ecosystems can be an important C flux which directly affects ecosystem C balance since DOC is leached from the soil to the groundwater. DOC fluxes and their controlling factors were investigated on two grassland sites with similar climatic conditions but different soil types (Vertisol vs. Arenosol) for a 2.5-year period. Parts of both grasslands were disturbed by deep ploughing during afforestation. Contrary to what was expected, ploughing did not increase DOC export but surface soil DOC concentrations decreased by 28% (Vertisol) and 14% (Arenosol). DOC flux from the soil profile was negatively influences by the clay content of the soil with seven times larger DOC export in the clay-poor Arenosol (55 kg C ha^?1 a^?1) than in the clay-rich Vertisol (8 kg C ha^?1 a^?1). At the Arenosol site, highest DOC concentrations were measured in late summer, whereas in the Vertisol there was a time lag of several months between surface and subsoil DOC with highest subsoil DOC concentrations during winter season. DOC export was not correlated with soil organic carbon stocks. Large differences in ¹⁴C concentrations of 22–40 pMC between soil organic carbon and DOC in the subsoil indicated that both C pools are largely decoupled. We conclude that DOC export at both sites is not controlled by the vegetation but by physicochemical parameters such as the adsorption capacity of soil minerals and the water balance of the ecosystem. Only in the acidic sandy Arenosol DOC export was a significant C flux of about 8% of net ecosystem production.     

Leaffall Phenology in a Subtropical Wet Forest in Puerto Rico: From Species to Community Patterns

Leaffall phenology is an important periodical event in forests, contributing to mobilization of organic matter from primary producers to soil. For seasonal forests, leaffall periodicity has been related to rainfall regime and dry season length. In weakly seasonal forests, where there is no marked dry season, other climatic factors could trigger leaf shed. In this study, we aimed to determine if other climatic variables (wind speed, solar radiation, photosynthetic photon flux density [PPFD], day length, temperature, and relative humidity) could be better correlated with patterns of litter and leaffall in a weakly seasonal subtropical wet forest in Puerto Rico. Leaffall patterns were correlated mainly with solar radiation, PPFD, day length, and temperature; and secondarily with rainfall. Two main peaks of leaffall were observed: April–June and August–September, coinciding with the periods of major solar radiation at this latitude. Community leaffall patterns were the result of overlapping peaks of individual species. Of the 32 species analyzed, 21 showed phenological patterns, either unimodal (16 species), bimodal (three species), or multimodal (two species). Lianas also presented leaffall seasonality, suggesting that they are subject to the same constraints and triggering factors affecting trees. In addition to solar radiation as a main determinant of leaffall timing in tropical forests, our findings highlight the importance of interannual variation and asynchrony, suggesting that leaffall is the result of a complex interaction between environmental and physiological factors.     

Impacts of elevated atmospheric ozone on peatland below-ground DOC characteristics

Rising concentrations of tropospheric ozone are having detrimental impacts on the growth of crop and forest species and some studies have reported inhibition of the allocation of carbon below ground. The effects of ozone on peatland ecosystems have received relatively little attention, despite their importance within the global carbon cycle. During this study, cores from a Welsh minerotrophic fen and ombrotrophic bog were exposed to four ambient/elevated ozone concentration regimes representing current and predicted 2050 profiles. A large and significant reduction in the concentration of porewater dissolved organic carbon (DOC) was recorded in the fen cores exposed to the elevated ozone concentrations (up to ?55%), with a concurrent shift to a higher molecular weight of the remaining soil carbon. No effects of ozone on DOC concentrations or characteristics were recorded for the bog cores. The data suggest higher ozone sensitivity of plants growing in the fen-type peatland, that the impacts on the vegetation may affect soil carbon characteristics through a reduction in root exudates and that there may have been a shift in the source of substrate DOC for microbial consumption from vegetation exudates to native soil carbon. There may also have been a direct effect of ozone molecules reacting with soil organic matter after being transported into the soil through the aerenchyma tissue of the overlying vegetation. These qualitative changes in the soil carbon in response to elevated ozone may have important implications for carbon cycling in peatland ecosystems, and therefore climate change.     

Modelling the effects of climate change on an acidic upland stream

Most modelling studies of soil and surface water recovery from acidification assume a constant influence of climate over the simulation period. Given the likelihood of future climate change, and recent identification of links between climatic fluctuations and surface water trends on decadal time scales, an attempt is made to simulate a number of climate-related effects on the recovery of an acidified stream in mid-Wales. Empirical relationships were derived to model past and future variations in (i) runoff, as a function of rainfall and temperature; (ii) stream chloride concentration and sea-salt deposition, as a function of the North Atlantic Oscillation Index (NAOI); and (iii) stream dissolved organic carbon (DOC) concentration, as a function of summer temperatures and sulphur deposition. Incorporating these relationships in the MAGIC dynamic model reproduced a significant part of the observed variation in streamwater chemistry. Current scenarios of rising UK temperature and NAOI, and falling rainfall and acid deposition, were used to forecast effects of climate change on soil and water recovery from acidification. Results suggest that increasing DOC would have a significant negative impact on soil recovery, with elevated organic acidity lowering soil water pH and severely depleting soil base saturation. The resulting increase in soil base cation export would, however, have a positive impact on streamwater recovery. For increased sea-salt deposition, effects are essentially the opposite; increased base cation inputs enhance the recovery of soil base saturation, but displace acidity to the stream. However, simulated effects of raising DOC considerably outweighed those of increased sea-salt deposition. It is argued that greater consideration of these, and other, climatic effects on biogeochemical cycles needs to be taken when predicting the recovery of soils and surface waters from acidification.     

Soil Iron Content as a Predictor of Carbon and Nutrient Mobilization in Rewetted Fens

Rewetted, previously drained fens often remain sources rather than sinks for carbon and nutrients. To date, it is poorly understood which soil characteristics stimulate carbon and nutrient mobilization upon rewetting. Here, we assess the hypothesis that a large pool of iron in the soil negatively affects fen restoration success, as flooding-induced iron reduction (Fe³⁺ to Fe²⁺) causes a disproportionate breakdown of organic matter that is coupled with a release of inorganic compounds. We collected intact soil cores in two iron-poor and two iron-rich drained fens, half of which were subjected to a rewetting treatment while the other half was kept drained. Prolonged drainage led to the mobilization of nitrate (NO₃^-, > 1 mmol L^-1) in all cores, regardless of soil iron content. In the rewetted iron-rich cores, a sharp increase in pore water iron (Fe) concentrations correlated with concentrations of inorganic carbon (TIC, > 13 mmol L^-1) and dissolved organic carbon (DOC, > 16 mmol L^-1). Additionally, ammonium (NH₄⁺) accumulated up to phytotoxic concentrations of 1 mmol L^-1 in the pore water of the rewetted iron-rich cores. Disproportionate mobilization of Fe, TIC, DOC and NH₄⁺ was absent in the rewetted iron-poor cores, indicating a strong interaction between waterlogging and iron-mediated breakdown of organic matter. Concentrations of dissolved phosphorus (P) rose slightly in all cores upon rewetting, but remained low throughout the experiment. Our results suggest that large pools of iron in the top soil of drained fens can hamper the restoration of the fen’s sink-service for ammonium and carbon upon rewetting. We argue that negative effects of iron should be most apparent in fens with fluctuating water levels, as temporary oxygenation allows frequent regeneration of Fe³⁺. We conclude that rewetting of iron-poor fens may be more feasible for restoration.     

Vegetation and climate controls on potential CO2, DOC and DON production in northern latitude soils

Climatic change may influence decomposition dynamics in arctic and boreal ecosystems, affecting both atmospheric CO₂ levels, and the flux of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) to aquatic systems. In this study, we investigated landscape‐scale controls on potential production of these compounds using a one‐year laboratory incubation at two temperatures (10° and 30 °C). We measured the release of CO₂, DOC and DON from tundra soils collected from a variety of vegetation types and climatic regimes: tussock tundra at four sites along a latitudinal gradient from the interior to the north slope of Alaska, and soils from additional vegetation types at two of those sites (upland spruce at Fairbanks, and wet sedge and shrub tundra at Toolik Lake in northern Alaska). Vegetation type strongly influenced carbon fluxes. The highest CO₂ and DOC release at the high incubation temperature occurred in the soils of shrub tundra communities. Tussock tundra soils exhibited the next highest DOC fluxes followed by spruce and wet sedge tundra soils, respectively. Of the fluxes, CO₂ showed the greatest sensitivity to incubation temperatures and vegetation type, followed by DOC. DON fluxes were less variable. Total CO₂ and total DOC release were positively correlated, with DOC fluxes approximately 10% of total CO₂ fluxes. The ratio of CO₂ production to DOC release varied significantly across vegetation types with Tussock soils producing an average of four times as much CO₂ per unit DOC released compared to Spruce soils from the Fairbanks site. Sites in this study released 80–370 mg CO₂‐C g soil C^?1 and 5–46 mg DOC g soil C^?1 at high temperatures. The magnitude of these fluxes indicates that arctic carbon pools contain a large proportion of labile carbon that could be easily decomposed given optimal conditions. The size of this labile pool ranged between 9 and 41% of soil carbon on a g soil C basis, with most variation related to vegetation type rather than climate.     

Processes controlling DOC in pore water during simulated drought cycles in six different UK peats

The effect of episodic drought on dissolved organic carbon (DOC) dynamics in peatlands has been the subject of considerable debate, as decomposition and DOC production is thought to increase under aerobic conditions, yet decreased DOC concentrations have been observed during drought periods. Decreased DOC solubility due to drought-induced acidification driven by sulphur (S) redox reactions has been proposed as a causal mechanism; however evidence is based on a limited number of studies carried out at a few sites. To test this hypothesis on a range of different peats, we carried out controlled drought simulation experiments on peat cores collected from six sites across Great Britain. Our data show a concurrent increase in sulphate (SO₄) and a decrease in DOC across all sites during simulated water table draw-down, although the magnitude of the relationship between SO₄ and DOC differed between sites. Instead, we found a consistent relationship across all sites between DOC decrease and acidification measured by the pore water acid neutralising capacity (ANC). ANC provided a more consistent measure of drought-induced acidification than SO₄ alone because it accounts for differences in base cation and acid anions concentrations between sites. Rewetting resulted in rapid DOC increases without a concurrent increase in soil respiration, suggesting DOC changes were primarily controlled by soil acidity not soil biota. These results highlight the need for an integrated analysis of hydrologically driven chemical and biological processes in peatlands to improve our understanding and ability to predict the interaction between atmospheric pollution and changing climatic conditions from plot to regional and global scales.     

The effects of sheep faeces on soil solution composition

To elucidate changes occurring in soil solution under sheep faeces we collected faeces from sheep grazing on an improved podzol in Scotland in summer and re-applied it (150 g of fresh weight) to patches, each with an area of 0.018 m². Faeces and/or intact soil cores (0–50 mm deep) from underneath the patches were sampled at intervals up to 224 days and nutrients measured in the soil solution isolated by centrifugation. The treatment with faeces increased the soil solution concentrations of P, N, dissolved organic carbon (DOC), Ca, Mg and K. The greatest increases occurred at day 56 for P, Ca, Mg and K, day 84 for N, and day 112 for DOC. Most of the P in the faeces applied to the soil was in inorganic form, but organic P made a substantial proportion (nearly 50%) of the P in soil solution at the time of maximum effect. The faecal residue became indistinguishable from the soil between 84 and 112 days following periods of persistent rainfall. The data provides a new insight into the effects of sheep faeces on soil solution composition unperturbed by chemical extraction and a contrast with the effects of sheep urine on soil solution composition.Section Editor: N.J. Barrow     

Linking dynamics of dissolved organic carbon in a forested lake with environmental factors

Dynamics of dissolved organic carbon concentration (DOC) and capacity toabsorb light (color) are determined by in-lake and external properties andprocesses. In this study, the influence of external factors such as rainfallandsolar radiation on DOC and color dynamics was assessed for a small forestedlake. DOC and absorption coefficients at 440 nm (a₄₄₀)ranged 4-fold from 0.46 to 1.62 mM and from 3.4 to 14.8m^–1, respectively. DOC and a₄₄₀ variedsynchronously, but an important percentage of the variability (26%) ina₄₄₀ was not explained by DOC. The resulting twofold variation inthemolar absorption coefficient of DOC suggested significant seasonal changes inchromophoric content. Both DOC and a₄₄₀ were positive andsignificantly related to cumulative rainfall. Solar radiation, however, onlyappeared to influence a₄₄₀ dynamics. This influence was mediated byphotobleaching. Photobleaching coefficients (k_b) were higher in falland spring relative to the summer. This seasonal variability in k_bvalues was related to monthly rainfall. The influence of photobleaching ona₄₄₀ dynamics was evaluated by comparing the half life ofa₄₄₀ in the water column with water residence time (WRT). For thestudy lake, photobleaching contributed notably to a₄₄₀ dynamicsduring the dry periods when WRT was longer than the a₄₄₀ half life .DOC dynamics, however, were not related to solar radiation becausephotomineralization was considerably slower than photobleaching.     

Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Water draining from the Everglades marshes of southern Florida containshigh concentrations of dissolved organic C (DOC), N (DON), and in somelocations, P (DOP). These dissolved organic nutrients carry over 90% of the Nand organic C, and about 25% of the P exported downstream in the Everglades.Ourobjectives were to describe the most important aspects of the origin and fateofdissolved organic matter (DOM) in the Everglades, and to describe the processescontrolling its concentration and export. Concentrations of dissolved organicnutrients are influenced by local plant production, decomposition, and sorptionequilibrium with peat. The drained peat soils of the Everglades AgriculturalArea and the more productive marshes of the northern Everglades produce some ofthe highest concentrations of DOC and DON in the Everglades watershed. Inportions of the marshes of the northern Everglades, P enrichment was correlatedwith higher local DOC and DON concentrations and greater production of solubleplant matter. Microbial degradation of Everglades DOM was very slow; less than10% of the DOC was lost after 6 months of incubation in the laboratory andsupplements of inorganic nutrients failed to speed the decomposition. Exposureto solar radiation increased the subsequent decay rate of the remaining DOC(25%in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7%of the DON, and degraded about 50% of the humic substances over 21 days insterile porewater samples and thus degraded DOM faster than microbialdegradation. The humic substances appeared to inhibit biodegradation of theother fractions of the DOC since hydrophilic organic acids decomposed fasterwhen isolated from the humic substances.The fate of DOC and DON is closely linked as indicated by a generally narrowrange of C/N ratios. In contrast, high concentrations of DOP were associatedwith P enrichment (at least in pore water). The DOC was composed of about 50%humic substances, 33% hydrophilic acids, and 15% hydrophilic neutralsubstances,typical of DOC from other environments, despite the fact that it originatesfroma neutral to slightly alkaline peatland. Despite high exports of DON (3.9g m^–2 y^–1 from one area), themarshes of the northern Everglades are a sink for DON on a landscape scale. Theagricultural fields of the Everglades Agricultural Area, however, exported netquantities of DON. High concentrations of DOC desorbed from the agriculturalsoils when water with no DOC was added. Sorption experiments indicated thathighconcentrations of dissolved organic matter flowing into the marshes from theEverglades Agricultural Area could suppress the further desorption ofadditionalsoluble organic matter through physicochemical mechanisms. While biologicalfactors, plant production and microbial decomposition are important inproducingpotentially soluble organic nutrients, physicochemical sorption equilibria,hydrology, and degradation by solar radiation are also likely to control theexport of this material on the landscape scale.     

Influence of drought-induced acidification on the mobility of dissolved organic carbon in peat soils

A strong relationship between dissolved organic carbon (DOC) and sulphate (SO₄^2?) dynamics under drought conditions has been revealed from analysis of a 10‐year time series (1993–2002). Soil solution from a blanket peat at 10 cm depth and stream water were collected at biweekly and weekly intervals, respectively, by the Environmental Change Network at Moor House‐Upper Teesdale National Nature Reserve in the North Pennine uplands of Britain. DOC concentrations in soil solution and stream water were closely coupled, displaying a strong seasonal cycle with lowest concentrations in early spring and highest in late summer/early autumn. Soil solution DOC correlated strongly with seasonal variations in soil temperature at the same depth 4‐weeks prior to sampling. Deviation from this relationship was seen, however, in years with significant water table drawdown (>?25 cm), such that DOC concentrations were up to 60% lower than expected. Periods of drought also resulted in the release of SO₄^2?, because of the oxidation of inorganic/organic sulphur stored in the peat, which was accompanied by a decrease in pH and increase in ionic strength. As both pH and ionic strength are known to control the solubility of DOC, inclusion of a function to account for DOC suppression because of drought‐induced acidification accounted for more of the variability of DOC in soil solution (R²=0.81) than temperature alone (R²=0.58). This statistical model of peat soil solution DOC at 10 cm depth was extended to reproduce 74% of the variation in stream DOC over this period. Analysis of annual budgets showed that the soil was the main source of SO₄^2? during droughts, while atmospheric deposition was the main source in other years. Mass balance calculations also showed that most of the DOC originated from the peat. The DOC flux was also lower in the drought years of 1994 and 1995, reflecting low DOC concentrations in soil and stream water. The analysis presented in this paper suggests that lower concentrations of DOC in both soil and stream waters during drought years can be explained in terms of drought‐induced acidification. As future climate change scenarios suggest an increase in the magnitude and frequency of drought events, these results imply potential for a related increase in DOC suppression by episodic acidification.     

Environmental heterogeneity and biotic interactions mediate climate impacts on tropical forest regeneration

Predicting the fate of tropical forests under a changing climate requires understanding species responses to climatic variability and extremes. Seedlings may be particularly vulnerable to climatic stress given low stored resources and undeveloped roots; they also portend the potential effects of climate change on future forest composition. Here we use data for ca. 50,000 tropical seedlings representing 25 woody species to assess (i) the effects of interannual variation in rainfall and solar radiation between 2007 and 2016 on seedling survival over 9 years in a subtropical forest; and (ii) how spatial heterogeneity in three environmental factors—soil moisture, understory light, and conspecific neighborhood density—modulate these responses. Community‐wide seedling survival was not sensitive to interannual rainfall variability but interspecific variation in these responses was large, overwhelming the average community response. In contrast, community‐wide responses to solar radiation were predominantly positive. Spatial heterogeneity in soil moisture and conspecific density were the predominant and most consistent drivers of seedling survival, with the majority of species exhibiting greater survival at low conspecific densities and positive or nonlinear responses to soil moisture. This environmental heterogeneity modulated impacts of rainfall and solar radiation. Negative conspecific effects were amplified during rainy years and at dry sites, whereas the positive effects of radiation on survival were more pronounced for seedlings existing at high understory light levels. These results demonstrate that environmental heterogeneity is not only the main driver of seedling survival in this forest but also plays a central role in buffering or exacerbating impacts of climate fluctuations on forest regeneration. Since seedlings represent a key bottleneck in the demographic cycle of trees, efforts to predict the long‐term effects of a changing climate on tropical forests must take into account this environmental heterogeneity and how its effects on regeneration dynamics play out in long‐term stand dynamics.     

Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from 13C and 14C

Export of dissolved organic carbon (DOC) from forested catchmentsis governed by competing processes of production, decomposition, sorptionand flushing. To examine the sources of DOC, carbon isotopes (¹⁴Cand ¹³C) were analyzed in DOC from surface waters, groundwatersand soils in a small forested catchment on the Canadian Shield in centralOntario. A significant fraction (greater than 50%) of DOCin major inflows to the lake is composed of carbon incorporated into organicmatter, solubilized and flushed into the stream within the last 40 years. Incontrast, ¹⁴C in groundwater DOC was old indicating extensiverecycling of forest floor derived organic carbon in the soil column beforeelution to groundwater in the lower B and C soil horizons. A small uplandbasin had a wide range in ¹⁴C from old groundwater values atbaseflow under dry basin conditions to relatively modern values during highflow or wetter antecedent conditions. Wetlands export mainly recently fixedcarbon with little seasonal range. DOC in streams entering the small lakemay be composed of two pools; an older recalcitrant pool delivered bygroundwater and a young labile pool derived from recent organic matter.The relative proportion of these two pools changes seasonally due thechanges in the water flowpaths and organic carbon dynamics. Althoughchanges in local climate (temperature and/or precipitation) may alterthe relative proportions of the old and young pools, the older pool islikely to be more refractory to sedimentation and decomposition in thelake setting. Delivery of older pool DOC from the catchment andsusceptibility of this older pool to photochemical decomposition mayconsequently be important in governing the minimum DOC concentrationlimit in lakes.     

Site‐to‐site variability and temporal trends of DOC concentrations and fluxes in temperate forest soils

Here, we report site‐to‐site variability and 12–14 year trends of dissolved organic carbon (DOC) from organic layers and mineral soils of 22 forests in Bavaria, Germany. DOC concentrations in the organic layer were negatively correlated with mean annual precipitation and elevation whereas air temperature had a positive effect on DOC concentrations. DOC fluxes in subsoils increased by 3 kg ha^?1 yr^?1 per 100 mm precipitation or per 100 m elevation. The highest DOC concentrations were found under pine stands with mor humus. Average DOC concentrations in organic layer leachates followed the order: pine>oak>spruce>beech. However, the order was different for mean DOC fluxes (spruce>pine>oak>beech) because of varying precipitation regimes among the forest types. In 12 of 22 sites, DOC concentrations of organic layer leachates significantly increased by 0.5 to 3.1 mg C L^?1 yr^?1 during the sampling period. The increase in DOC concentration coincided with decreasing sulfate concentration, indicating that sulfate concentration is an important driver of DOC solubility in the organic layer of these forest sites. In contrast to the organic layer, DOC concentrations below 60 cm mineral soil depth decreased by <0.1–0.4 mg C L^?1 yr^?1 at eight sites. The negative DOC trends were attributed to (i) increasing adsorption of DOC by mineral surfaces resulting from desorption of sulfate and (ii) increasing decay of DOC resulting from decreasing stabilization of DOC by organo‐Al complexes. Trends of DOC fluxes from organic layers were consistent with those of DOC concentrations although trends were only significant at seven sites. DOC fluxes in the subsoil were with few exceptions small and trends were generally not significant. Our results suggest that enhanced mobilization of DOC in forest floors contributed to the increase of DOC in surface waters while mineral horizons did not contribute to increasing DOC export of forest soils.     

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.     

TRANSFORMATION AND ALLELOPATHY OF NATURAL DISSOLVED ORGANIC CARBON AND TANNIC ACID ARE AFFECTED BY SOLAR RADIATION AND BACTERIA1

The aim of this study was to test whether abiotic and biotic factors may affect allelopathic properties. Therefore, we investigated how solar radiation and bacteria influence allelopathic effects of the plant‐derived, polyphenolic tannic acid (TA) on microalgae. Using a block design, lake water samples with and without TA were exposed to solar radiation or kept in darkness with or without bacteria for 3 weeks. Dissolved organic carbon (DOC), specific size fractions of DOC analyzed by chromatography–organic carbon detection (LC‐OCD), and concentrations of total phenolic compounds (TPC) were measured to follow the fate of TA in lake water with natural DOC exposed to photolytic and microbial degradation. DOC and TPC decreased in dark‐incubated lake water with TA and bacteria indicating microbial degradation. In contrast, exposure to solar radiation of lake water with TA and bacteria did not decrease DOC. Chromatographic analyses documented an accumulation of DOC mean size fraction designated as humic substances (HS) in sunlit water samples with TA. The recalcitrance of the humic fraction indicates that photolytic degradation may contribute to a DOC less available for bacterial degradation. Subsequent growth tests with Desmodesmus armatus (Chodat) E. Hegewald showed low but reproducible difference in algal growth with lower algal growth rate cultured in photolytically and microbially degraded TA in lake water than cultured in respective dark treatments. This finding highlights the importance of photolytic processes and microbial degradation influencing allelopathic effects and may explain the high potential of allelochemicals for structuring the phytoplankton community composition in naturally illuminated surface waters.