Availability Of Organic Matter Research Articles

High connectivity among Vesicomyid bivalves from cold seeps and deep-sea fans of Congo

Chemosynthetic ecosystems are scattered in the deep ocean, harbouring highly specialized communities, among which the degree of connectivity and dispersal is scarcely studied. This gap is largely due to limited range distribution, either real or due to highly partial exploration, preventing the availability of a sufficient number of samples for population genetic analysis. For the few species that escaped this gap, large-scale panmixia was often reported, raising wonders as to the evolutionary mechanisms involved in the first steps of speciation. Vesicomyid bivalves are one of the most abundant groups of chemosynthetic fauna, for which depth was proposed as an essential driver of differentiation. Early stages of speciation are thus expected to involve genetic differentiation along depth gradients. The vesicomyid bivalve Christineconcha regab was studied across widely separated localities along the Western African margin, from the cold-seeps of Regab pockmarks located at 3150 m depth on the Congo margin to the turbiditic lobes of the Congo deep-sea fan located at 5000 m depth, using mitochondrial (COI) sequences and eight microsatellite loci. Despite rather high density in relation to high organic matter availability, results obtained showed rather low levels of genetic diversity at both mtDNA and microsatellites. The main consistent pattern of differentiation was observed across depths from Regab pockmark (South-Western part) and lobe areas (Lobe B and C). This is likely due to the largest sample sizes characterizing Regab_SW and Lobe C allowing the detection of faint genetic differentiation, and possibly to a stronger signature in the demographically declining (thus acknowledgedly under sampled) Lobe B. Other significant results were not congruent among markers, suggesting low statistical power due to limited sample size or the occurrence of chaotic genetic patchiness. Altogether, the results suggest the occurrence of effective gene flow at regional scale, and departure from equilibrium in the recently discovered lobes of the Congo River, possibly resulting from unstable environmental conditions and recurrent events of extinction recolonization.

Effects of multi-resistant ScALDH21 transgenic cotton on soil microbial communities

Transgenic crops are increasingly prevalent worldwide, and evaluating their impact on soil microbial communities is a critical aspect of upholding environmental safety. Our previous research demonstrated that overexpression of ScALDH21 from desiccant-tolerant moss, Syntrichia caninervis, in cotton revealed multi-resistance to drought, salt, and biotic stresses. We conducted metabarcoding using high-throughput sequencing to evaluate the effect of ScALDH21 transgenic cotton on soil microbial communities. We further conducted soil tests to analyze the chemical properties of transgenic and non-transgenic cotton, including the total content and availability of chemical elements (K, P, and N), organic matter, and pH value. Both transgenic and non-transgenic cotton fields exhibited soil pH values higher than 8. The presence of transgenic cotton significantly enhanced the availability of available K and the total content of total P in the soil. Alpha and beta diversity indices of soil microbiota showed no difference between two transgenic and non-transgenic cotton groups. Dominant clades of fungal and bacterial genera were equivalent at the phylum and genus levels in all three groups. The correlation analysis of microbial communities and soil environmental factors revealed the absence of significant differences between transgenic and non-transgenic cotton genotypes. Functional predictions of soil microbial communities indicated that microbial community function did not show significant differences between transgenic and non-transgenic cotton samples. These findings are essential for evaluating the environmental effects of transgenic crops and supporting the secure implementation of transgenic cotton.

Hybridization of anaerobic digestion with solar energy: A solution for isolated livestock farms

Intensive farming causes an important amount of greenhouse gas emissions. This scenario can be significantly reduced by the implementation of renewable technologies and transforming farms from energy consumers to energy providers. In the particular case of livestock production, biogas and solar energy reduce greenhouse gas emissions and the energy demand of the installations. However, the implementation of these technologies requires solutions adapted to local scenarios, such as connectivity to the energy grids. In this work, a biogas/biomethane production system, energetically covered with hybrid solar panels is proposed as a solution for isolated areas where biodegradable substrates (manure) are abundant. Thus, the electrical and thermal requirements of the digester are supplied by solar panels, reducing the biogas self-consumption and the energy inputs from the electrical grid. Hybrid solar panels also provide sufficient energy for operation of an upgrading system to obtain biomethane of fuel vehicle quality, increasing the energy self-sufficiency of the agricultural activities. This solution has been simulated in five different climatic regions corresponding to areas of intense pig farming activity. The results demonstrate the sustainable bioenergy production in isolated farms with limited connection to the energy grid and organic matter availability. Furthermore, the economic study showed that the proposed technology is competitive compared to other technologies in the energy sector.

Redox potential as a soil health indicator – how does it compare to microbial activity and soil structure?

Background and aimsRedox potential is a promising soil health indicator, which integrates the combined effect of chemical oxidation–reduction reactions into a single measurement. However, this method has been tested only on a few soils. The aim of this study was to test redox potential as a soil health indicator, to see if it responds to management practices and to compare it with existing soil health metrics for microbial activity (“CO2burst”) and soil structure.MethodsWe collected 35 soil samples in agricultural fields from a carbon farming trial, where contrasting management practices have been applied to increase carbon stock. The soil samples were dried, rewetted and analyzed for redox and microbial respiration during rewetting. In addition, soil structure, texture and organic matter content was measured. The data was analyzed for correlations between the indicators and for the differences between management and control fields.ResultsRedox was well correlated with microbial activity, structure, and texture. A low redox state was connected to high microbial activity, indicating bioavailable organic matter availability. Soils with good structure had an oxidized redox status, possibly reflecting high gas-transport. The carbon farming practices resulted in lower oxidation, possibly due to build-up of plant residues.ConclusionsThe findings supported the use of redox as a soil health indicator, but highlighted further research needs for identifying the shared mechanisms linking structure, redox and microbial activity. As such, redox can be a low-cost additional measurement to map changes in soil health, but it cannot replace existing structure or microbial activity measurements.

Ectomycorrhizal fungi integrate nitrogen mobilisation and mineral weathering in boreal forest soil.

Tree growth in boreal forests is driven by ectomycorrhizal fungal mobilisation of organic nitrogen and mineral nutrients in soils with discrete organic and mineral horizons. However, there are no studies of how ectomycorrhizal mineral weathering and organic nitrogen mobilisation processes are integrated across the soil profile. We studied effects of organic matter (OM) availability on ectomycorrhizal functioning by altering the proportions of natural organic and mineral soil in reconstructed podzol profiles containing Pinus sylvestris plants, using 13 CO2 pulse labelling, patterns of naturally occurring stable isotopes (26 Mg and 15 N) and high-throughput DNA sequencing of fungal amplicons. Reduction in OM resulted in nitrogen limitation of plant growth and decreased allocation of photosynthetically derived carbon and mycelial growth in mineral horizons. Fractionation patterns of 26 Mg indicated that magnesium mobilisation and uptake occurred primarily in the deeper mineral horizon and was driven by carbon allocation to ectomycorrhizal mycelium. In this horizon, relative abundance of ectomycorrhizal fungi, carbon allocation and base cation mobilisation all increased with increased OM availability. Allocation of carbon through ectomycorrhizal fungi integrates organic nitrogen mobilisation and mineral weathering across soil horizons, improving the efficiency of plant nutrient acquisition. Our findings have fundamental implications for sustainable forest management and belowground carbon sequestration.

Effect of different feed nitrogen on nutrient flow, performance, and productivity in the minapadi system

Cultivation with the minapadi method can increase the availability of nutrients and soil organic matter from the decomposition of feed residues and weeds. This system implements environmentally friendly cultivation and creates a sustainable ecosystem because rice plants use fish waste as a source of nutrients, and fish use weeds and pests in rice as a food source. This study aims to find the best feed formula in the minapadi system based on differences in nitrogen content and to analyze the effect of nitrogen from the artificial feed given on nitrogen flow, performance, and productivity in the minapadi system. Using a randomized block design method with four treatments of different feed doses repeated three times to obtain twelve experimental plot units, feed production using local raw materials that are cheap and easy to obtain, soil quality, water, and growth performance are the parameters observed. The results showed that the soil and water quality at the study site was quite good, so the growth performance of rice plants and fish was also good. The treatment of the feed doses given showed that the feed with the highest nitrogen content, namely 9%, gave the best growth performance for rice plants, while 7% nitrogen content gave the best growth for tilapia. The excessive levels of protein are not suitable for fish. However, they are handy for rice plants in the Minapadi system because nutrients that dissolve in water, nutrients from fish feces and urine, and nutrients that are sedimentation on the soil surface, are utilized by rice plants as a nutrient source.

Zooplankton derived organic matter as a food source for benthic coral

Zooplankton derived organic matter, such as fecal pellets and carcasses, may be a vital source of nutrients for benthic corals. In shallow tropical reefs, settling particles (organic matter) often reach the benthos, in contrast with deeper systems where the particles are recycled/transformed prior to reaching the benthos. Furthermore, zooplankton fecal pellets aggregate together and entrain additional organic matter, aiding in the transference of organic matter to reefs. These settling particles can be ingested by coral as a supplement to the nutrients obtained from their symbionts (zooxanthellae). Settling particles can provide essential lipids, which are produced by phytoplankton and repackaged by zooplankton, to coral reefs. Settling particles collected in the Upper Florida Keys Reef Tract were used to evaluate the role of zooplankton in the transportation of organic matter. The general chemical composition remained constant in the sediment particles, and total sediment flux generally decreased as distance from shore increased. Through lipid and stable isotope analyses, zooplankton sources were determined to contribute approximately 50% of the total lipid content in the particles with smaller contributions coming from phytoplankton, bacteria, vascular plants, and coral. The repackaging of phytoplankton derived organic matter by zooplankton demonstrates zooplankton as a primary driver of settling particles in coral reefs. The shallow water column in the reef habitat results in less recycling and remineralization of sediment particles compared to the open ocean, leading to the direct transport of organic matter from the surface to the benthos. Therefore, zooplankton abundance and composition play a vital role in determining the availability of organic matter and settling particles on coral reefs. Without zooplankton transporting organic matter throughout the water column reefs would likely decline, meaning it is imperative that zooplankton derived organic matter is considered when studying coral reefs.

Macaronichnus ‘co-occurrence’ in offshore transition settings: Discussing the role of tidal versus fluid muds influence

Macaronichnus is a key trace fossil in palaeoceanographic, palaeoclimatic, and petroleum exploration research. Small ichnosubspecies such as Macaronichnus segregatis segregatis, M. s. lineiformins, M. s. maeandriformis and M. s. spiriformis, typically occur in wave-dominated foreshore sands where large M. s. degiberti was never found. The latter shows a wide environmental distribution, occurring in sandy deposits of tidal channels, tidal bar sand sandridges, tidal-flat sand sheets, shorefaces, bioturbated sandy shelf, shelf storm-sheets, shelf sand ridges, and upper slopes. Small M. segregatis and large M. s. degiberti have not been observed to date due the ecological segregation of the tracemakers. An abundant record of large M. s. degiberti in a Tortonian (Late Miocene) mixed carbonate-siliciclastic unit from the Betic Cordillera (southern Spain) has been studied. Occurrence of M. s. degiberti is the result of the interaction of tidal and waves, storm influenced environment determining high-energy conditions and associated palaeoenvironmental parameters as shifting substrates, organic matter availability, and oxygenated pore and bottom-waters. Locally, associated to M. s. degiberti appear small, sinuous traces infilled by light material that were originally assigned to M. s. maeandriformis, and very rare M. s. spiriformis. However, the absence of the typical rim of Macaronichnus avoid a conclusive assignment. The coexistence of both small traces (?M. s. maeandriformis) and large M. s. degiberti is identified in the deposits underlying mudstone layers, revealing the importance of mud deposition during tidal slack water intervals or linked to fluid mud events favouring the co-occurrence of the trace makers of both Macaronichnus ichnosubespecies. This fact would have significant palaeobiological and palaeoecological implications, and could be the first record of both ichnosubspecies in the same intervals.

Fish Productivity Response to Water Quality Variations: A Case Study of Nyumba ya Mungu Dam, in Pangani Water Basin, Tanzania

Fish habitat degradation, such as water pollution due to the rapid expansion of incompatible land uses, is one of the noteworthy challenges to freshwater ecosystems. Elevated contents of nutrients and contaminants in water are some of the imperative challenges. This research was conducted to investigate the link between water qualities and fish productivity in the Nyumba ya Mungu Dam, whose fishery potential had decreased by 95% between 1972 and 2018. Physicοchemical characteristics, namely, temperature, pH, dissolved oxygen (DO), turbidity, total nitrogen, total phosphorus, chlorophyll a, and the water depth of fishing net location, were assessed following standard procedure. Fish catches per unit effort representing fish biomass/productivity were monitored for twelve months from January to December 2019. Pearson’s correlation analysis indicated that fish catch per unit effort was positively and significantly correlated with turbidity (r = 0.461, P < 0.01) and TP (r = 0.405, P < 0.01). Stepwise multiple regression model results indicated that turbidity, dissolved oxygen, and the depth of the fishing net location are the statistically significant predictors of fish catch per unit effort which represented fish biomass. Findings further indicated that the model combining turbidity and dissolved oxygen predicted a 24% change in fish catch per unit effort, whereas turbidity, dissolved oxygen, and the depth of fishing net location contributed a 28.9% change in fish catch per unit effort. In conclusion, fish productivity in the NMD is limited by organic matter availability and nutrient levels that cause algae bloom occurrence. Turbidity is an indicator of organic matter availability and the effect of algae bloom on fish productivity. Inflow of nutrients to the dam nourishes the algae biomass, thus creating vicious cycles on fish productivity as fish species in the dam failed to take advantage of high primary production by algae.

Niche differentiation of microbial community shapes vertical distribution of recalcitrant dissolved organic matter in deep-sea sediments

Sedimentary organic matter provides carbon substrates and energy sources for microorganisms, which drive benthic biogeochemical processes and in turn modify the quantity and quality of dissolved organic matter (DOM). However, the molecular composition and distribution of DOM and its interactions with microbes in deep-sea sediments remain poorly understood. Here, molecular composition of DOM and its relationship with microbes were analyzed in samples collected from two sediment cores (∼40 cm below the sea floor), at depths of 1157 and 2253 m from the South China Sea. Results show that niche differentiation was observed on a fine scale in different sediment layers, with Proteobacteria and Nitrososphaeria dominating the shallow sediments (0–6 cm) and Chloroflexi and Bathyarchaeia prevailing in deeper sediments (6–40 cm), indicating correspondence of microbial community composition with both geographical isolation and the availability of organic matter. An intimate link between the DOM composition and microbial community further indicates that, microbial mineralization of fresh organic matter in the shallow layer potentially resulted in the accumulation of recalcitrant DOM (RDOM), while relatively low abundance of RDOM was linked to anaerobic microbial utilization in deeper sediment layers. In addition, higher RDOM abundance in the overlying water, as compared to that in the surface sediment, suggests that sediment might be a source of deep-sea RDOM. These results emphasize the close relation between the distribution of sediment DOM and different microbial community, laying a foundation for understanding the complex dynamics of RDOM in deep-sea sediment and water column.

Benthic fluxes and mineralization processes at the scale of a coastal lagoon: Permeable versus fine-grained sediment contribution

Benthic fluxes of biogenic compounds play a major role in the biogeochemistry of shallow aquatic environments. Quantifying these fluxes at the scale of a lagoon is a challenge, especially when sediments are heterogeneous and fluxes at the sediment-water interface combine diffusive and advective transport processes. Diagenetic processes and associated benthic fluxes were quantified across different seasons in a lagoon of the French Mediterranean coast (La Palme lagoon) from vertical profiles of pore water and sedimentary solid fraction carried out at many representative stations of the lagoon. The northwestern part of the lagoon is covered with fine-grained sediment with low permeability and the rest of the lagoon contains permeable sandy sediment. We obtained vertical profiles with centimetre-scale resolution of water content, salinity, and major particulate and dissolved biogenic compounds of C, N, P, Si, S, Fe and Mn. This study allowed us to refine the sedimentary mapping of the lagoon, to specify the spatio-temporal evolution of biogeochemical processes, and to determine more precisely the part of the diffusive fluxes of nutrient compared to advective fluxes. Comparison of the vertical profiles with a molecular diffusion transport model shows that molecular diffusion is the dominant process in fine-grained sediments, while sandy sediments are dominated by advection due to circulation of lagoon water in shallow sediments. Benthic respiration renders fine-grained and sandy sediments anoxic from the first few mm below the sediment-water interface, particularly due to the availability of labile organic matter. Benthic dissolved inorganic nitrogen fluxes are of the same order of magnitude in both sediment types, despite the different flux mechanisms. This suggests that the intensity of organic matter mineralization processes is the same in fine and sandy sediments. Benthic phosphate fluxes are greater in sandy than in fine sediments because phosphorus is more efficiently retained in the solid fraction of fine sediments. Thus, sandy sediments play a dominant role in the pelagic-benthic coupling of the lagoon.

Organic matter sources and flows in tundra wetland food webs.

Arctic lowland tundra is often dominated by wetlands. As numbers and types of these wetlands change with climate warming, their invertebrate biomass and assemblages may also be affected. Increased influx of nutrients and dissolved organic matter (DOM) from thawing peat may alter the relative availability of organic matter (OM) sources, differentially affecting taxa with disparate dependence on those sources. In five shallow wetland types (<40 to 110 cm deep) and in littoral zones of deeper lakes (>150 cm), we used stable isotopes (δ13C, δ15N) to compare contributions of four OM sources (periphytic microalgae, cyanobacteria, macrophytes, peat) to the diets of nine macroinvertebrate taxa. Living macrophytes were not distinguishable isotopically from peat that likely contributed most DOM. Within invertebrate taxa, relative OM contributions were similar among all wetland types except deeper lakes. Physidae snails consumed substantial amounts of OM from cyanobacteria. However, for all other taxa examined, microalgae were the dominant or a major OM source (39-82%, mean 59%) in all wetland types except deeper lakes (20‒62%, mean 31%). Macrophytes and macrophyte-derived peat, likely consumed mostly indirectly as DOM-supported bacteria, ranged from 18‒61% (mean 41%) of ultimate OM sources in all wetland types except deeper lakes (38-80%, mean 69%). Invertebrate consumption of microalgal C may often have involved bacterial intermediates, or a mix of algae with bacteria consuming peat-derived OM. High production of periphyton with very low δ13C values were favored by continuous daylight illuminating shallow depths, high N and P levels, and high CO2 concentrations from bacterial respiration of peat-derived DOM. Although relative OM sources were similar across wetland types except deeper lakes, total invertebrate biomass was much higher in shallow wetlands with emergent vegetation. Impacts of warming on the availability of invertebrate prey to waterbirds will likely depend not on shifts in OM sources, but more on changes in overall number or area of shallow emergent wetlands.

A molluscan class struggle: exploring the surprisingly uneven distribution of chemosymbiosis among two major mollusk groups

Many bivalves and gastropods from marine reducing environments such as deep ocean hydrothermal vents and seeps host chemosynthetic bacteria in a nutritional symbiosis. Despite their functional similarities, the distribution of chemosymbiosis in these two mollusk classes is surprisingly uneven: the number of bivalve species known to host chemosynthetic symbionts is more than twenty times that of gastropods, and chemosymbiotic bivalves are reported from a far greater diversity of marine habitats. Here we explore the potential drivers for this trend, including but not limited to physiological differences, habitat characteristics, and sampling bias. Sampling bias likely contributes to the magnitude of the observed discrepancy, but we posit that the phenomenon itself reveals how intrinsic (e.g. morphology) and extrinsic (e.g. organic matter availability) factors might have shaped the distribution of extant gastropod and bivalve associations. These observations also serve as an impetus for increasing investigation into gastropods and other mollusks from chemically reducing environments to better understand the evolution and ecology of chemosymbiosis among molluscan hosts.

Decreasing Richness and Biomass During a Flood Pulse Observed in a Southeastern US Coastal Floodplain Following a Multi-Year, Supra-Seasonal Drought

Floodplains of the southeastern United States exhibit high biological abundance and diversity, maintained by periodic inundation from seasonal flooding. To examine the relationship between invertebrate community composition and seasonal flooding following periods of drought, we quantified aquatic macroinvertebrate communities monthly in an inundated floodplain during the annual flood pulse (December-April) in two years: one following a multi-year drought and one during a larger than average flood. We predicted that floodplain communities would differ in richness, biomass, and community composition between years and that differences would be driven by the annual flood pulse and organic matter availability. We collected macroinvertebrates from a floodplain of the Altamaha River, a free-flowing 6th order river in the Coastal Plain region of the southeastern US. With invertebrates identified to genus, we estimated richness, abundance, biomass, community composition, and proportions of functional feeding group abundance. Richness was generally higher in the drought year but decreased throughout the flood pulse, while during the flood year richness was lower and increased over time. Biomass decreased throughout the flood pulse following the drought year and increased during the flood year. There was high overlap in invertebrate community composition based on abundance data during both years of the study with collector gatherers being the most abundant functional feeding group. Our study provides insight to the response of aquatic communities to an extended drought and subsequent return to normal flow in a lower river wetland.

Nitrite pathway in A2/O WWTPs: Modelling organic matter reduction, operational cost and N2O emissions

Deciding whether the implementation of the nitrite pathway is useful for the treatment of a given wastewater depends on several criteria such as organic matter availability, N and P removal, N2O emissions and operational costs. This work is a simulation-based study with a conventional Anaerobic/Anoxic/Oxic (A2/O) WWTP where the nitrite pathway can be implemented. The outcomes are general correlations to calculate the minimum COD requirements for a certain influent and a practical decision tree on the opportunities of nitrite pathway in A2/O WWTPs as a function of the influent wastewater composition (for P and N concentrations in the ranges 3–11 mgPO4−3-P·L−1 and 20–60 mg NH4+-N·L−1). This study shows that the implementation of the nitrite pathway reduces the COD requirements (depending on the influent, between 9 and 68%). It also can lead to a reduction in aeration costs. For an equivalent COD in the influent, the implementation of the nitrite pathway compared to the nitrate pathway results in a reduction of 41-47% in aeration costs and similar sludge production, leading to a reduction in the operational cost index of 10-16%. However, it is essential to note that this strategy can also lead to increased N2O emissions, with an emission factor for the nitrite pathway in the range of 2.5–17 times that of the nitrate pathway.

Temporal evolution of shallow marine diagenetic environments: Insights from carbonate concretions

Early diagenesis of marine organic matter dramatically impacts Earth’s surface chemistry by changing the burial potential of carbon and promoting the formation of authigenic mineral phases including carbonate concretions. Marine sediment-hosted carbonate concretions tend to form as a result of microbial anaerobic diagenetic reactions that degrade organic matter and methane, some of which require an external oxidant. Thus, temporal changes in the oxidation state of Earth’s oceans may impart a first-order control on concretion authigenesis mechanisms through time. Statistically significant variability in concretion carbonate carbon isotope compositions indicates changes in shallow marine sediment diagenesis associated with Earth’s evolving redox landscape. This variability manifests itself as an expansion in carbon isotope composition range broadly characterized by an increase in maximum and decrease in minimum isotope values through time. Reaction transport modelling helps to constrain the potential impacts of shifting redox chemistry and highlights the importance of organic carbon delivery to the seafloor, marine sulfate concentrations, methane production and external methane influx. The first appearance of conclusively anaerobic oxidation of methane-derived concretions occurs in the Carboniferous and coincides with a Paleozoic rise in marine sulfate. The muted variability recognized in older concretions (and in particular for Precambrian concretions) likely reflects impacts of a smaller marine sulfate reservoir and perhaps elevated marine dissolved inorganic carbon concentrations. Causes of the increase in carbon isotope maximum values through time are more confounding, but may be related to isotopic equilibration of dissolved inorganic carbon with externally derived methane. Ultimately the concretion isotope record in part reflects changes in organic matter availability and marine oxidation state, highlighting connections with the subsurface biosphere and diagenesis throughout geologic time.

Influence of abiotic and biotic factors on benthic marine community composition, structure and stability: a multidisciplinary approach to molluscan assemblages from the Miocene of northern Germany

AbstractThe Miocene mica‐clay deposits of Groß Pampau (northern Germany) are well known for their diverse assemblages of marine mammals. Despite numerous systematic and biostratigraphic studies, an in‐depth palaeoecological analysis of its molluscan assemblages and a comprehensive palaeoenvironmental reconstruction are lacking. Here, we integrate new faunal, sedimentological and geochemical data to reconstruct the marine palaeoecosystem of the Upper Miocene sedimentary succession of Groß Pampau, and to identify the drivers controlling the composition, ecological structure and temporal dynamics of its macrobenthic molluscan assemblages. Fossil evidence, coupled with analyses of clay mineral composition, grain size distribution and geochemical data (total organic carbon, total nitrogen, δ13C, δ18O, δ15N of sediment and shells), suggests a warm–temperate, mesotrophic, low‐energy, offshore marine setting mostly below storm wave base and a pronounced surface‐to‐bottom water temperature gradient. Low variability in sedimentological and geochemical signals indicates generally stable physicochemical conditions, whereas the occurrence of the opportunistic species Varicorbula gibba suggests occasionally unfavourable bottom conditions, possibly related to transient hypoxia. Canonical correspondence analysis indicates that the distribution of molluscan assemblages correlates with total organic carbon and nitrogen content, suggesting organic matter availability at the sea floor as a controlling factor. A pattern of repetitive punctuated stasis of molluscan assemblages is defined by the temporal persistence in taxonomic and ecological composition, occasionally interrupted by shifts to a different faunal configuration. We suggest that both stable environmental conditions and biotic interactions (i.e. the top‐down control exerted by carnivorous gastropods and environmental modification by ubiquitous burrowing deposit feeders) probably contributed to the observed temporal stability.

Dissolved organic nitrogen removal and its mechanisms during simulated soil aquifer treatment

Soil aquifer treatment systems are known to further remove contaminants in wastewater effluent when applied through infiltration into the ground. Dissolved organic nitrogen (DON) in the effluent, a precursor for nitrogenous disinfection by-products (DBPs) such as N-nitrosodimethylamine (NDMA), is of great concern upon subsequent use of the groundwater infiltered into the aquifer. In this study, the vadose zone of the soil aquifer treatment system was simulated using 1 m laboratory soil columns under unsaturated conditions representing the vadose zone. The final effluent of a water reclamation facility (WRF) was applied to these columns to investigate the removal of N species with a focus on DON, as well as NDMA precursors. DON removal achieved was up to 99 % with an average of 68 % and was accompanied by a 52 % nitrate increase suggesting the occurrence of ammonification and nitrification through the soil columns. Around 62 % of total DON removal was seen at <10 cm travel distance, which was in accordance with higher adenosine triphosphate (ATP) concentrations at the top of the column due to more oxygen and organic matter availability. Total Dissolved N removal was drastically lowered to 4.5 % in the same column without microbial growth, which highlights the importance of biodegradation. The columns were capable of removing 56 % of the fluorescent dissolved organic matter (FDOM). Soil columns could remove NDMA precursors up to 92 % through the column with the initial concentration of 89.5 ng/L, possibly due to the removal of DON fractions. The results demonstrate the capability of the vadose zone in further treatment of DON and other organic matter before reaching the groundwater through infiltration or indirect discharge to surface water. Differences in applied water quality and the site-specific oxic conditions in SAT systems could lead to variable removal efficiencies.

Dual indicators link geochemistry to microbiota in blue carbon soils

In blue carbon ecosystems, biomarkers and indicators have been developed to assess soil biogeochemical processes and history. In this study, we investigate blue carbon soils to determine if geochemical characteristics can predict soil microbial characteristics, and secondly to investigate if these two indicators can reveal novel information about processes about soil formation and alteration. First, phospholipid fatty acids (PLFAs) identified soil microbial functional groups and abundances. Microbes varied strongly among the three blue carbon ecosystems – seagrass, mangrove and saltmarsh – and to a lesser extent with soil age/depth. Then, x-ray fluorescence using a core scanner (ITRAX) was used to identify geochemical traits of the soil. Lastly, statistical procedures were carried out to test the ability of ITRAX-derived geochemical characteristics to explain the variation in PLFA composition. The soil geochemical indicators for organic matter (OM) source and turnover explained most of the variability in PLFAs, followed by indicators for redox and grain size. For both indicators the saltmarsh soils were associated with relatively higher OM availability and reactivity and microbial abundance. High OM samples also supported a range of microbial functional groups, including fungi and the co-existence of prokaryotic aerobic and anaerobic metabolisms. In contrast, increased depths representing older soil ages showed decreases in OM reactivity and shifts in OM processing through reduced active microbial abundance. We suggest that x-ray fluorescence data used as an indicator in conjunction with other biomarkers can be used to assess multiple aspects of blue carbon soils (biology, provenance, physicochemistry), while also possessing opportunity for future development as rapid field technique.

Stream respiration exceeds CO2 evasion in a low‐energy, oligotrophic tropical stream

AbstractCarbon dioxide (CO2) can be either imported to streams through groundwater and subsurface inputs of soil‐respired CO2 or produced internally through stream metabolism. The contribution of each source to the CO2 evasion flux from streams is not well quantified, especially in the tropics, an underrepresented region in carbon (C) cycling studies. We used high‐frequency measurements of dissolved O2 and CO2 concentrations to estimate the potential contribution of stream metabolism to the CO2 evasion flux in a tropical lowland headwater stream. We found that the stream was heterotrophic all year round, with net ecosystem productivity (NEP) values ranging from 0.84 to 4.06 g C m−2 d−1 (median 1.29 g C m−2 d−1; here we expressed gross primary productivity (GPP) as a negative flux and ecosystem respiration (ER) as a positive flux). Positive NEP values were the result of a relatively low and stable GPP through the seasons, compared to a higher and more variable ER favored by the high temperatures and organic matter availability, particularly during the wet season. The CO2 evasion flux was relatively low due to low turbulence (median: 1.09 g C m−2 d−1). As a result, daily NEP rates exceeded the CO2 evasion flux with a potential contribution of 129% (median; 120–175% interquartile range), despite the strong seasonal changes in flow regime and landscape connectivity. The CO2 excess was likely transported downstream, where it was ultimately emitted to the atmosphere. Our results highlight the overwhelming importance of ER to the C cycle of low‐energy, oligotrophic tropical streams.