Total Mineralization Rates Research Articles

Bottom-trawling signals lost in sediment: A combined biogeochemical and modeling approach to early diagenesis in a perturbed coastal area of the southern Baltic Sea

Trawl-fishing is broadly considered to be one of the most destructive anthropogenic activities toward benthic ecosystems. In this study, we examine the effects of bottom-contact fishing by otter trawls on the geochemistry and macrofauna in sandy silt sediment in an area of the Baltic Sea where clear spatial patterns in trawling activity were previously identified by acoustic mapping. We calibrated an early diagenetic model to biogeochemical data from various coring locations. Fitting measured mercury profiles allowed for the determination of the sediment mixing and burial velocity. For all sites, independent of the trawl mark density, good fits were obtained by applying the model with the same organic matter loading and parameter values, while iron fluxes scaled linearly with the burial velocity. A sensitivity analysis revealed that the fitted sulfate reduction rate, solid sulfur contents, ammonium concentration, and both the isotopic composition and concentration of dissolved inorganic carbon provided reliable constraints for the total mineralization rate, which exhibited a narrow range of variability (around ±20 % from the mean) across the sites. Also, the trawling intensity did not significantly correlate with total organic carbon contents in surficial sediment, indicating limited loss of organic matter due to trawling. The fits to the reactive iron, acid volatile sulfur, chromium(II) reducible sulfur contents, and porewater composition demonstrate that sediment burial and mixing primarily determine the redox stratification. The mixing depth did not correlate with trawling intensity and is more likely the result of bioturbation, as the analyzed macrofaunal taxonomy and density showed a high potential for sediment reworking. The extraordinarily long-lived Arctica islandica bivalve dominated the infaunal biomass, despite the expectation that trawling leads to the succession from longer-lived to shorter-lived and bigger to smaller macrofauna. Our results further suggest that a clear geochemical footprint of bottom-trawling may not develop in sediments actively reworked by tenacious macrofauna.

Levofloxacin degradation in electro‐Fenton system with Fe@Co/GF composite cathode

AbstractBACKGROUNDElements of transition metals have a distinct non‐nuclear electronic structure, and modifying electrodes with transition metals can increase electrocatalytic activity. Due to its low cost and high electrical conductivity, graphite felt (GF) is frequently used as the carbon base material for modified electrodes. GF was treated with KOH and subsequently loaded with the bimetal Fe and Co to create a cathode (Fe@Co/GF) with enhanced catalytic activity in this study.RESULTSUnder optimal experimental conditions (initial levofloxacin (LEV) concentration of 30 mg L−1, electric voltage of 8 V, GF thickness of 3 mm, an aeration rate of 3 4.5 L min−1, Na2SO4 concentration of 0.05 mol L−1, initial Fe2+ concentration of 0.1 mmol L−1, initial pH of 3, KOH activation temperature of 900 °C and Fe@Co ratio of 1:3), the LEV elimination rate was 90.38% and the mineralization rate was 88.53% within 160 min. Compared to the GF cathode, both the total LEV degradation rate and the total mineralization rate increased by approximately 20%. When Fe@Co/GF was used as the cathode in an electro‐Fenton (EF) system without an applied iron source, the Fenton reaction could still occur. In addition, Fe@Co/GF also promoted the degradation of ciprofloxacin in the EF system.CONCLUSIONAccording to morphological and other statistical data, the modified cathode has a larger specific surface area, larger pores and a greater number of active sites. Electrochemical performance measurements indicate that the modified cathode is more responsive to current. Fe@Co/GF cathode was simple to prepare and contributed significantly to the LEV removal process when used as the cathode in an EF system, thereby decreasing the LEV degradation time. © 2023 Society of Chemical Industry (SCI).

Offshore Windfarm Footprint of Sediment Organic Matter Mineralization Processes

Offshore windfarms (OWFs) offer part of the solution for the energy transition which is urgently needed to mitigate effects of climate change. Marine life has rapidly exploited the new habitat offered by windfarm structures, resulting in increased opportunities for filter- and suspension feeding organisms. In this study, we investigated the effects of organic matter (OM) deposition in the form of fecal pellets expelled by filtering epifauna in OWFs, on mineralization processes in the sediment. OM deposition fluxes produced in a 3D hydrodynamic model of the Southern Bight of the North Sea were used as input in a model of early diagenesis. Two scenarios of OWF development in the Belgian Part of the North Sea (BPNS) and its surrounding waters were calculated and compared to a no-OWF baseline simulation. The first including constructed OWFs as of 2021, the second containing additional planned OWFs by 2026. Our results show increased total mineralization rates within OWFs (27–30%) in correspondence with increased deposition of reactive organic carbon (OC) encapsulated in the OM. This leads to a buildup of OC in the upper sediment layers (increase by ∼10%) and an increase of anoxic mineralization processes. Similarly, denitrification rates within the OWFs increased, depending on the scenario, by 2–3%. Effects were not limited to the OWF itself: clear changes were noticed in sediments outside of the OWFs, which were mostly opposite to the “within-OWF” effects. This contrast generated relatively small changes when averaging values over the full modeling domain, however, certain changes, such as for example the increased storage of OC in sediments, may be of significant value for national / regional carbon management inventories. Our results add to expectations of ecosystem-wide effects of windfarms in the marine environments, which need to be researched further given the rapid rate of expansion of OWFs.

Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Abstract. Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deeper-penetrating gear (e.g. a tickler chain beam trawl) versus a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers and led to significantly lower amounts of ammonium (43 %–99 % reduction) and organic carbon in the top 10 cm of the sediment (62 %–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types and between trawling frequencies. The shallow-penetrating gear had a slightly smaller effect on benthic denitrification than the deeper-penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generate significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

CO2 Emission and Change in the Fertility Parameters of a Calcareous Soil Following Annual Applications of Deinking Paper Sludge (The Case of Tunisia)

The use of deinking paper sludge (DPS) as a fertilizer instead of sending it to landfill could play a role in reducing greenhouse gases and improving soil properties. The objectives of this study were (1) to evaluate the changes in the physical (permeability and structural stability), chemical (particularly soil pH), and biological (microbial metabolic quotient (qCO2), microbial biomass soil CO2 emissions) of a calcareous agricultural soil following two successive annual amendments with three treatments (0, 30, and 60 Mg DPS ha−1—control, DPS30, and DPS60, respectively); and (2) to determine whether the addition of N-fertilizer to these treatments (controlF, DPS30F, and DPS60F, respectively) causes changes to soil fertility. The DPS application increased soil organic matter (+0.80%: DPS60 vs. control; and +0.35%: controlF vs. DPS60F), available phosphorus (+23.14 mg kg−1: DPS60 vs. control; and +14.34 mg kg−1: DPS60F vs. controlF), potassium (+0.6 g kg−1: controlF vs. DPS30F), and calcium (+0.28 g kg−1: DPS60 vs. control). The 60 Mg DPS ha−1 rate improved permeability and structural stability, regardless of the presence or absence of N-fertilizer. On the other hand, the 60 Mg DPS ha−1 rate without N-fertilizer lead to a decrease in total mineralization rate and qCO2, thereby indicating a reduction in CO2 emissions. The rate of 60 Mg ha−1 DPS could be effectively used to enhance the permeability and stability (soil restoration) and mitigate CO2 emissions, whereas the 30 Mg ha−1 rate could be used as fertilizer to improve the fertility of calcareous soils.

Mechanisms behind the inhibition of autotrophic nitrification following rice-straw incorporation in a subtropical acid soil

The effectiveness of rice-straw incorporation to alleviate environmental deterioration and increase soil fertility is widely accepted, whereas, the effect of this management on stimulating soil nitrogen (N) transformation is not fully understood. This study was conducted to investigate the effect of rice-straw incorporation on soil N transformation. An incubation experiment was conducted with rice-straw incorporated at rates of 0 (RS0), 1.67 (RS1), 3.33 (RS2) and 6.67 g kg−1 soil (RS3). Tracing experiments with 15NH4NO3 and NH415NO3 was conducted in the first (Week 1) and tenth week (Week 10) after straw incorporation, and a numerical model was used to calculate gross rates of N transformations. Incorporation of rice-straw increased gross rates of soil organic N mineralization, ammonium (NH4+) and nitrate (NO3−) immobilization and oxidized organic-N to NO3−, by 0.2–1.7 times, 4.6–11.6 times, 20.4–74.9 times and 6.2–20.3 times, respectively. However, the stimulation of soil N transformation via rice-straw incorporation was insignificant by week 10. Over the incubation period, the stimulation of soil inorganic N production pathways (organic N mineralization and oxidized organic-N to NO3−) via rice-straw incorporation was less than on consumption pathways (NH4+ and NO3− immobilization), leading to soil inorganic N supply capacity decreasing with straw incorporation rates. Dissimilatory NO3− reduction to NH4+ was stimulated by rice-straw incorporation, as observed in both the first and tenth week. Compared with RS0, autotrophic nitrification decreased by 14%, 25% and 46% in RS1, RS2 and RS3, respectively, but this effect disappeared by week 10. However, nitrification capacity (NC, the ratio of autotrophic nitrification rate to total mineralization rate) was constrained following rice-straw incorporation both in the first and tenth weeks. Decreasing autotrophic nitrification was the most important factor contributing to decreased NO3− content with straw incorporation, followed by increasing NO3− immobilization. The gross rate of autotrophic nitrification was negatively correlated with NH4+immobilization, indicating that autotrophic nitrification inhibition may be attributed to increased NH4+ immobilization. Therefore, based on the observations of this study, rice-straw incorporation is recommended for reducing nitrification capacity and reducing risks of N losses in subtropical acid soil.

Soil resource status affects the responses of nitrogen processes to changes in temperature and moisture

The mechanisms of the different responses of soil gross nitrogen (N) transformation to increasing temperature or moisture in different types of soils are still unclear. Here, we conducted two 15N tracing experiments to investigate the effects of increasing temperature (15 °C and 25 °C) or moisture (30%, 45%, and 60% water-filled pore space (WFPS)) on soil gross N transformation rates for two soils (organic (O) and mineral (A) horizon soils) in a temperate forest. As the temperature increased from 15 to 25 °C or moisture increased from 30 to 60% WFPS, total mineralization rates increased by 4.5-fold and 2.5-fold respectively, total NH4+ immobilization rates increased by 173.2-fold and 7.6-fold respectively, and autotrophic nitrification rates increased by 0.7-fold and 0.6-fold respectively in the O horizon. Under the same treatment, the changes in autotrophic nitrification rates, NH4+ immobilization rates, and mineralization rates were much smaller in the A horizon than in the O horizon. We propose that the difference between the two horizons in the responses to changing temperature and moisture was due to the different resource status. The O horizon could provide more substrates than the A horizon, resulting in a stronger response of N processes to changing temperature and moisture. Resource status also affected the competition for substrates between NH4+ immobilization and autotrophic nitrification. The N transformation rates were higher in the O horizon than in the A horizon, consistent with higher microbial biomass N, microbial respiration rate, and amoA gene abundance in the O horizon. Our results suggest that the activity switch of microbes and the competition for resources are important biotic factors regulating potential responses of soil N cycling processes to changing abiotic factors.

Enhanced mineralization of sedimentary organic carbon induced by excess carbon from phytoplankton in a eutrophic plateau lake

Large additions of organic carbon (OC) have been introduced into the carbon cycle of lakes from algae during the process of lake eutrophication. The impact of eutrophication on OC burial and CO2 fixing has been widely studied; however, there is still a gap regarding the impact of excess OC from algae on sedimentary OC mineralization. In this study, we aim to fill this gap by analyzing in situ measurements. Three pairs of sediment cores collected from a plateau lake (Dianchi Lake) in 2006 and 2014 were used to estimate the accumulation loss rate (ALR) of OC (and thus the total mineralization rate) in the sediment. n-Alkanes, measured from the same sedimentary cores, were used to identify the source of OC. An OC mineralization experiment in a laboratory was used to confirm the enhanced effect of excess phytoplankton carbon on ALR and reveal the potential influence of microorganisms. The results indicate that the sedimentary core (core 3), with high excess OC from algae (located in an algal bloom area), possessed a higher ALR (85.66%) and a higher attenuation coefficient (0.078), indicating the low burial efficiency and short mineralization duration of OC. Sedimentary core 1, controlled by terrestrial OC, had a relatively lower ALR (64.60%) and lower attenuation coefficient (0.029), indicating a high burial efficiency and long period of OC mineralization. The mineralization of OC in core 2 was impacted by terrestrial and endogenous OC, with an ALR of 72.00% and attenuation coefficient of 0.064, which is between that of cores 1 and 3. Excess OC from algae corresponded to an increase in ALR by 32.60% when comparing core 1 to core 3. The increased ALR and attenuation coefficient could be caused by excess OC from algae. Sedimentary OC mineralization indicates that the ALR with dominantly allochthonous OC (64.60%) is much lower than that controlled by autochthonous OC (85.66%). Excess OC from phytoplankton increases the mineralization of OC: not only via increased ALR but also increased mineralization speed. The laboratory experiment on the mixture of algae and sediment suggested that excess phytoplankton OC increased the emission of CO2 by 20–70% (mineralization rate).

Seasonal Changes in Organic Matter Mineralization in a Sublittoral Sediment and Temperature-Driven Decoupling of Key Processes

Seasonal changes in the mineralization of organic compounds in sediments were investigated in temperate, sublittoral zone sediments (Tokyo Bay, Japan). The total mineralization rate and sulfate reduction rate showed large seasonal variations over the year, and although the fluctuations in both rates correlated with temperature, the latter was irregularly high in May. The concentration of organic carbon dissolved in interstitial water was specifically high in April. A culture-based experiment was also conducted under temperatures corresponding to the seasonal changes. In the culture incubated at a temperature corresponding to April (13°C), hydrolysis and fermentation proceeded, but terminal oxidation was hindered, thereby resulting in acetate accumulation. At a temperature corresponding to May (22°C), acetate oxidation coupled with sulfate reduction was observed. The temperature-related differences were also reflected in the bacterial community structure in the cultures analyzed by DGGE. In the culture incubated at the lower temperature, sulfate-reducing bacterium of incomplete oxidizer was detected, while sequence found in the culture incubated at the higher temperature was related to complete oxidizers. These results suggest that complete and incomplete-oxidizing sulfate-reducing bacteria act as distinct functional groups, responding to temperature in different ways, particularly in environments characterized by large temperature fluctuations.

Mineralization of organic matter and nitrogen transformations in the Gotland Sea deep water

Measurements of the concentrations of total CO 2 (C T), O 2, H 2S, NO 2 −, NO 3 −, and NH 4 + were performed between 2003 and 2007 during intervals of 2–3 months in the deep water of the Gotland Basin (Baltic Sea). The data were used to determine mineralization and denitrification rates in four sublayers below 150 m during a period of stagnation. To account for the vertical mixing at the boundaries of the sublayers, mixing coefficients were determined based on the temporal changes of the salinity profiles. Mass balances were established for each measured variable and yielded the sink/source rates in the individual sublayers for each time interval. The seasonality of mineralization rates was confined to water layers that were in close contact with the sediment surface, indicating that the mineralization took place mainly at the sediment surface. Moreover, it was shown that the mineralization rates did not depend on the redox conditions in the water column. The total mineralization rate in the basin was found to be 2.0 mol m –2 yr –1, thereby implying that about 80% of the particulate organic carbon input occurred by lateral transport. A redox balance indicated that manganese dioxide and possibly iron oxides contributed to the oxidation of organic matter by about 13%. Intense denitrification was observed during the transition from oxic to anoxic conditions. A value of 456 mmol–N m –2 yr –1 was found, which included a possible anammox contribution of about 5%.

Effect of Biosurfactant Addition on the Biodegradation of Phenanthrene in Soil-water System

The extent of solubility enhancement by biosurfactant was examined at various pHs prior to the biodegradation experiments. The molar solubilization ratio (MSR) was calculated from the batch solubilization experiments and the highest MSR was detected at pH 5. The effect of the biosurfactant, rhamnolipids, on the phenanthrene mineralization in soil-water system was investigated. The strain 3Y was selected for the mineralization assay and large amounts of phenanthrene were degraded at neutral pH in soil-water system without the biosurfactant. The addition of 150 mg/L rhamnolipids showed no effect on mineralization of phenanthrene in soil-water system, and total mineralization rates after 6 weeks incubation at each pH showed no differences in presence and absence of rhamnolipids. Our result indicated that the toxic effect of rhamnolipids can disappear when soil particles exist, and also the enhanced solubility of phenanthrene does not work for mineralization enhancement in this soil-water system.

Enhanced mineralization of phenol and other hydroxylated compounds in a photocatalytic process assisted with ferric ions

This study shows that the presence of low ferric ion concentrations accelerates the rate of photocatalytic oxidation and mineralization of different phenolic components. Ferric ions not only considerably affect the rates of disappearance and total mineralization of phenol, but also the concentration-time profiles of its major intermediates. At reaction conditions, three major aromatic intermediates were identified: ortho-dihydroxybenzene (o-DHB), para-dihydroxybenzene (p-DHB), and 1,4-benzoquinone (1,4-BQ). Additionally, four carboxylic acids were also detected as intermediates: fumaric acid (FuAc), maleic acid (MeAc), oxalic acid (OxAc) and formic acid (FoAc). Fe ions also increase the rate of disappearance and mineralization of the aromatic intermediates when tested separately. A new refined series-parallel kinetic network for the photocatalytic mineralization of phenol is proposed describing the reaction in higher detail than previous studies. It is concluded that the proposed mechanism adequately describes both unpromoted photocatalytic oxidation (unpromoted PC) and Fe-assisted photocatalytic oxidation (Fe-assisted PC) of phenol with the various mechanistic steps involving detectable species. Experimental data also reveal that while unpromoted PC and Fe-assisted PC reactions share a close mechanism, the latter leads to changes in the step-related kinetic constants. It is found that the overall quantum yield (OQY) increase in the Fe-assisted PC reactions for phenol ranges between 30% and 50%. For the aromatic intermediates the increment is between 25% and 38%. Two kinetic models are developed to represent the oxidation of phenol in both unpromoted PC and Fe-assisted PC systems. The first model predicts the rate of disappearance of the major aromatic components while the second one includes the carboxylic acids and allows the prediction of the total mineralization rate of phenol. Estimates of the parameters along with their statistical indicators are also reported for both systems.

Benthic carbon mineralization in the Atlantic: a synthesis based on in situ data from the last decade

Benthic oxygen uptake rates quantified by the use of microsensors and flux chambers over a period of approx. 10 yr were compiled and used to assess the organic carbon mineralization in the central and South Atlantic (35°N–50°S). Measurements were performed in situ and in the laboratory on recovered sediment cores. In contrast to the laboratory data, both the in situ diffusive (DOU) and total oxygen uptake (TOU) decreased with increasing water depth. The data demonstrated that sediment recovery alter the O 2 microdistribution and affect the measured O 2 uptake rates. The ratio between TOU and DOU, a measure of the benthic fauna-mediated oxygen uptake, decreased from 3 to 4 in shallow and productive areas to around 1 at the deeper sites. The in situ oxygen uptake rates (both diffusive and total) also correlated with the oceanic primary production. Based on the compiled in situ measurements an empirical relation between the surface water primary production (PP, g C m −2 yr −1), water depth ( z, m) and benthic mineralization deduced from the TOU and DOU was established (C-DOU=PP 0.7358 z −0.3306 (g C m −2 yr −1); C-TOU=PP 1.0466 z −0.4922 (g C m −2 yr −1)). These equations were extrapolated to the entire investigated area of the Atlantic. The mineralization mimicked the surface water primary production, with high consumption rates in the upwelling areas. For the entire area (water depth⩾1000 m) the benthic carbon mineralization was between 134 and 168×10 12 g C yr −1 (from C-DOU and C-TOU, respectively), which equals 1.7–2.1% of the surface water primary production. These rates are higher than previous estimates of benthic carbon mineralization in deep-sea sediments. Integrated for the investigated area of the Atlantic the benthic fauna-mediated carbon mineralization accounted for 35×10 12 g C yr −1 (or 21% of the total mineralization rate). Using our relations to calculate the organic carbon flux through the 1000 m depth horizon revealed that between 212 and 333×10 12 g C yr −1 sink below this depth horizon, of which 63% and 51% is remineralized in the sediments. Particulate organic carbon fluxes obtained from sediment trap data cannot support either the measured or extrapolated benthic mineralization. The areal distribution of the oxygen penetration depth (OPD) for the investigated area of the Atlantic was estimated from the relation between the in situ C-DOU and OPD measurements (OPD=114,6968 C-DOU −0.7541 (mm)).

Oxidation and origin of organic matter in surficial eastern Mediterranean hemipelagic sediments

Aerobic mineralisation of Corg in surface sediments of the deep (>2000 m water depth) eastern Mediterranean Sea has been quantified by analysis of detailed box core Corg concentration versus depth profiles and the modelling environment for early diagenetic problems MEDIA. The reactive fraction comprises 60-80% of the total Corg reaching the sediments and is largely oxidised within the surficial 10 cm. A non-reactive Corg fraction (GNR) dominates at depths >10 cm, and makes up 20-40% of the total Corg flux to the sediments. First-order rate constants for decomposition of the reactive fraction calculated from the Corg profiles range from 5.4 × 10 −3 to 8.0 × 10 −3 y −1 to 8.0 × 10 −3 y −1 . Total mineralization rates in the surface sediment are between 1.7 and 2.6 µmol Cc m −2 y −1 and thus are typical for oligotrophic, deep-sea environments. The low fluxes and rapid remineralisation of Corg are accompanied by 210 Pbexcess surface mixed layers which are only 2 cm deep, among the thinnest reported for oxygenated marine sediments. Model results indicate a mis- match between the Corg profiles and O2 microprofiles which were measured onboard ship. This can be attributed to a combination of decompression artefacts affecting onboard measurement of the O2 profiles or the leakage of oxygen into the core during handling on deck. Furthermore, the used Db values, based on 210 Pb, may not be fully appropriate; calculations with higher Db values improve the O2 fits. The surficial sediment δ 13 Corg values of ∼− 22 become less negative with increasing depth and decreasing Corg concentrations. The major δ 13 C change occurs in the top 3 to 4 cm and coincides with the interval were most of the organic carbon oxidation takes place. This indicates that the reactive fraction of organic matter, commonly assumed to be marine, has a more negative δ 13 Corg than the refractory fraction, usually held to be terrestrial. Palaeoproductivity estimates calculated from the sediment data by means of literature algorithms yield low surface pro- ductivities (12-88 gC m −2 y −1 ), which are in good agreement with field measurements of primary productivity in other studies. Such values are, however, significantly lower than those indicated by recent productivity maps of the area derived from satellite imagery (>100 gC m −2 y −1 ).

Below-ground decomposition of organic matter in forests of the mangroves Rhizophorastylosa and Avicenniamarina along the arid coast of Western Australia

Differences in biogeochemical characteristics between three Rhizophora stylosa and three Avicennia marina forests were examined in different coastal settings of arid Western Australia. Decomposition rates of sediment organic matter correlated with temperature, but did not differ significantly between Rhizophora (range: 46.5–52.9 mmol C m −2 per day) and Avicennia (range: 28.5–48.3 mmol C m −2 per day) forests. There were, however, clear differences in the dominance of specific carbon oxidation pathways between forest types. Rates of sulfate reduction were significantly greater in the Rhizophora (range: 12.9–28.2 mmol S m −2 per day) than in the Avicennia (range: 2.1–8.5 mmol S m −2 per day) forests, accounting for 54–100 and 20–55% of total mineralization rates in both forest types, respectively. Sulfate reduction rates correlated significantly with live root biomass, above-ground biomass and forest net primary production. Rates of oxic respiration were, on average, greater in the Avicennia forests (range: 12.7–37.4 mmol m −2 per day). Oxic respiration appeared to be a minor decomposition pathway in two of the three Rhizophora forests (range: 0.0–23.1 mmol m −2 per day). It was estimated that, on average, 50% ( Avicennia) to 87% ( Rhizophora) of total oxygen uptake was consumed in oxidation of reduced metabolites. Methanogenesis was not detected in any of the forests, and Mn and Fe reduction were minor carbon oxidation pathways. Rates of dissolved nitrogen and phosphorus regeneration across the sediment–water interface were slow when measurable. The percentage ratios of total sediment respiration to forest net primary production ( T COX/NPP) were low, ranging among forests between 3 and 7%. Although there may be some carbon loss via tidal export, the low T COX/NPP ratios suggest slow rates of organic matter decomposition in relation to tree productivity. A large, slowly decomposing, pool of wood and other plant detritus may be an adaptive response, as in other tropical forests, to enhance ecosystem stability and conserve essential nutrients.

Assessing organic matter mineralization, degradability and mixing rate in an ocean margin sediment (Northeast Atlantic) by diagenetic modeling

We test whether organic matter degradability, mixing activity, and total sediment mineralization can be estimated by inversion of a coupled nonlinear numerical steady-state diagenetic model. We use a single data set comprising oxygen, nitrate, ammonium and organic carbon versus depth profiles from a slope station in the Goban Spur area (1034 m, Northeast Atlantic). Based on an extensive sensitivity analysis, it appears that (1) when using all data, the total mineralization rates can be determined with reasonable precision; bioturbation and degradability are less well constrained and (2) total mineralization rates can be determined based on nitrate and oxygen profiles only; estimates of organic matter mixing rates and degradability are refined when including the solid phase organic carbon profile. The bulk mixing rates obtained for organic carbon are one order of magnitude higher than mixing rates previously estimated from Pb-210 profiles, lending validity to the hypothesis that organic particles are mixed faster than inert particles. The degradability of the organic carbon prior to its incorporation in the sediment is in the order of 10-30 yr(-1), indicating that mineralization in this slope station of the Goban Spur area is fueled mainly by freshly deposited organic matter. [KEYWORDS: Benthic oxygen-demand; deep-sea sediments; marine-sediments; hemipelagic sediments; nutrient diagenesis; equatorial pacific; water interface; carbon; bioturbation; nitrogen]