Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy

This paper reviews the current knowledge of microbial processes affecting C sequestration in agroecosystems. The microbial contribution to soil C storage is directly related to microbial community dynamics and the balance between formation and degradation of microbial byproducts. Soil microbes also indirectly influence C cycling by improving soil aggregation, which physically protects soil organic matter (SOM). Consequently, the microbial contribution to C sequestration is governed by the interactions between the amount of microbial biomass, microbial community structure, microbial byproducts, and soil properties such as texture, clay mineralogy, pore-size distribution, and aggregate dynamics. The capacity of a soil to protect microbial biomass and microbially derived organic matter (MOM) is directly and/or indirectly (i.e., through physical protection by aggregates) related to the reactive properties of clays. However, the stabilization of MOM in the soil is also related to the efficiency with which microorganisms utilize substrate C and the chemical nature of the byproducts they produce. Crop rotations, reduced or no-tillage practices, organic farming, and cover crops increase total microbial biomass and shift the community structure toward a more fungal-dominated community, thereby enhancing the accumulation of MOM. A quantitative and qualitative improvement of SOM is generally observed in agroecosystems favoring a fungal-dominated community, but the mechanisms leading to this improvement are not completely understood. Gaps within our knowledge on MOM-C dynamics and how they are related to soil properties and agricultural practices are identified.

/pdf/bacterial-and-fungal-contributions-to-carbon-sequestration-3i9a4yeaey.pdf

Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

Rivers may be efficient environments for metabolizing terrestrial organic carbon that was previously thought to be recalcitrant, owing to pockets that provide geophysical opportunities by retaining material for longer, and to the adaptation of microbial communities, which has enabled them to exploit the energy that escapes upstream ecosystems. Metabolism of terrestrial organic carbon in freshwater ecosystems is responsible for a large amount of carbon dioxide outgassing to the atmosphere, in contradiction to the conventional wisdom that terrestrial organic carbon is recalcitrant and contributes little to the support of aquatic metabolism. Here, we combine recent findings from geophysics, microbial ecology and organic geochemistry to show geophysical opportunity and microbial capacity to enhance the net heterotrophy in streams, rivers and estuaries. We identify hydrological storage and retention zones that extend the residence time of organic carbon during downstream transport as geophysical opportunities for microorganisms to develop as attached biofilms or suspended aggregates, and to metabolize organic carbon for energy and growth. We consider fluvial networks as meta-ecosystems to include the acclimation of microbial communities in downstream ecosystems that enable them to exploit energy that escapes from upstream ecosystems, thereby increasing the overall energy utilization at the network level.

Biophysical controls on organic carbon fluxes in fluvial networks

Properties of fluorescent dissolved organic matter in the Gironde Estuary

Protocols for the Joint Global Ocean Flux Study (JGOFS) Core Measurements

The biogeochemistry of Lena River: organic carbon and nutrients distribution

Since the damming of the Nile, the Rhone River is the main freshwater and sediment supplier to the Mediterranean Sea. We estimated for the period 1987–1996, the dissolved and particulate organic carbon (DOC and POC), dissolved inorganic carbon (DIC), and total suspended matter (TSM) fluxes of the Rhone River to the Mediterranean Sea to be 1.1 ± 0.2, 1.6 ± 0.5, 16.2 ± 0.3 × 1010 moles C yr−1, and 9.9 ± 6.4 × 106 t yr−1, respectively. PIC flux was estimated to be 8.2 ± 5.4 × 109 moles C yr−1. On the basis of literature data, we estimated that nearshore bacterial respiration of Rhone derived labile‐POC and ‐DOC (LPOC and LDOC) might produce in a few days ∼0.21 and 0.12 × 1010 moles CO2 yr−1, respectively. Extended to the whole Mediterranean, this study suggests that bacterial respiration of labile organic carbon derived from Mediterranean rivers might rapidly (days) produce 2.6‐11 × 1010 moles CO2 yr−1. On the continental shelf, up to 4.7 × 1010 moles of organic carbon introduced by primary production and Rhone export would escape each year to sedimentation and bacterial mineralization and would be exported off the shelf. Moreover, as total carbon fixed by phytoplankton exceeds (+ 5.2 × 1010 moles C yr−1) the CO2 produced by bacterial respiration (on average), the biological system on the shelf, could be considered as an autotrophic system and then a sink for atmospheric CO2. However, these numbers need further examination because of the the large uncertainties associated currently to the bacterial growth efficiency values (± 100%)

/pdf/carbon-inputs-of-the-rhone-river-to-the-mediterranean-sea-28fy24dbjq.pdf

Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications

Colloidal organic C and trace metals from the waters of a highly productive coastal environment (the Venice Lagoon, Italy) have been separated by a cross-flow ultrafiltration device. On average, 18% of organic C, 34% of Cd, 46% of Cu, 87% of Fe, 18% of Ni, 58% of Pb, and 54% of Mn which previously would have been considered in the dissolved phase are actually associated with colloidal material. Thus, past studies overestimate the dissolved trace-metal concentration in the nearshore environment. Compared to total concentration, the proportion of the colloidal fraction represents on average 15% of organic C, 18% of Cd, 28% of Cu, 11% of Fe, 11% of Ni, 29% of Pb, and 12% of Mn. This fraction acts differently from the truly dissolved and macroparticulate phases. The behavior of organic C and trace elements during mixing between freshwater and seawater is more complicated than expected when a colloidal fraction is involved. The flocculation of colloids, encountered normally during estuarine mixing, is not very significant on the time scale of mixing in the lagoon. Conversely, the interaction between colloidal and truly dissolved phases seems important. The partitioning of trace metals between different fractions of organic C appears variable, Fe and Mn are preferentially tied to macroparticulate organic matter, and Cu and Cd are preferentially tied to colloidal organic matter in seawater. Truly dissolved organic C appears to be important for Ni. Pb is mainly associated with macroparticulate organic matter at most stations except in the highly productive region where Pb prefers colloidal organic C.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1995.40.1.0119

Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)

This chapter presents a discussion on dissolved organic matter (DOM) in the coastal zone. The chapter emphasizes the processes that occur in rivers and estuaries and are important to marine DOM. The river inputs to the coastal zone together with the intense physical and biological activity in coastal waters and hence, it makes that part of the marine realm one of the most dynamic in terms of DOM. Globally, riverine inputs of DOM represent about 0.25 Gt C per year. Despite its refractory character—recognized by many authors—a relatively large fraction of that DOM is degraded after mixing with seawater, with turnover times ranging from days to years. DOM behavior is also modified by the physical–chemical processes occurring in the mixing zone—particularly flocculation and photo oxidation. These processes affect the structure and the biodegradability of organic matter, thereby influencing the fate of DOM in the coastal zone. In addition to riverine inputs, seasonal autochthonous production contributes to a large amount of DOM present in the coastal zone. The chapter concludes with a discussion on the mechanisms of DOM accumulation and export from coastal realms.

Gustave Cauwet

Papers

The biogeochemistry of Lena River: organic carbon and nutrients distribution

Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications

Significance of colloids in the biogeochemical cycling of organic carbon and trace metals in the Venice Lagoon (Italy)

Chapter 12 – DOM in the Coastal Zone

HTCO method for dissolved organic carbon analysis in seawater: influence of catalyst on blank estimation