NUMERICAL CALCULATION OF PRESSURE FIELDS IN SONOCHEMICAL REACTORS - LINEAR EFFECTS IN HOMOGENEOUS PHASE

During the last 50 years sonochemistry was under active investigation. Nevertheless, there is still a lack of a sound theoretical basis for the design of sonochemical reactors. Furthermore, sonochemical reactions are not understood in detail on a molecular level. In order to calculate the yield of chemical reactions in reactors of different shapes one needs to know the number of cavitation bubbles in the reactor. These bubbles are generated by oscillating pressures in the liquid. Therefore, as a first step of the design of sonochemical reactors pressure fields in homogeneous media in reactors of different geometric shapes are calculated .

Balance of assimilative and dissimilative nitrogen processes in a diatom-rich tidal flat sediment

Tidal flat sediments are subject to repetitive mixing and resuspension events. In a short-term (24 h) <sup>15</sup>N-labelling experiment, we investigated reactive nitrogen cycling in a tidal flat sediment following an experimentally induced resuspension event. We focused on (a) the relative importance of assimilatory versus dissimilatory processes and (b) the role of benthic microalgae therein. <sup>15</sup>N-labelled substrate was added to homogenized sediment, and <sup>15</sup>N was subsequently traced into sediment and dissolved inorganic nitrogen (DIN) pools. Integration of results in a N-cycle model allowed us to quantify the proportion of major assimilatory and dissimilatory processes in the sediment. <br><br> Upon sediment disturbance, rates of dissimilatory processes like nitrification and denitrification were very high, but declined rapidly towards a steady state. Once this was reached, the balance between assimilation and dissimilation in this tidal mudflat was mainly dependent on the nitrogen source: nitrate was utilized almost exclusively dissimilatory via denitrification, whereas ammonium was rapidly assimilated, with about a quarter of this assimilation due to benthic microalgae (BMA). Benthic microalgae significantly affected the nitrogen recycling balance in sediments, because in the absence of BMA activity the recovering sediment turned from a net ammonium sink to a net source. <br><br> The driving mechanisms for assimilation or dissimilation accordingly appear to be ruled to a large extent by external physical forcing, with the entire system being capable of rapid shifts following environmental changes. Assimilatory pathways gain importance under stable conditions, with a substantial contribution of BMA to total assimilation

Oxidation kinetics and inverse isotope effect of marine nitrite-oxidizing isolates

Nitrification, the step-wise oxidation of ammonium to nitrite and nitrate, is important in the marine environment because it produces nitrate, the most abundant marine dissolved inorganic nitrogen (DIN) component and N-source for phytoplankton and microbes. This study focused on the second step of nitrification, which is carried out by a distinct group of organisms, nitrite-oxidizing bacteria (NOB). The growth of NOB is characterized by nitrite oxidation kinetics, which we investigated for 4 pure cultures of marine NOB (Nitrospina watsonii 347, Nitrospira sp. Ecomares 2.1, Nitrococcus mobilis 231, and Nitrobacter sp. 311). We further compared the kinetics to those of non-marine species because substrate concentrations in marine environments are comparatively low, which likely influences kinetics and highlights the importance of this study. We also determined the isotope effect during nitrite oxidation of a pure culture of Nitrospina (Nitrospina watsonii 347) belonging to one of the most abundant marine NOB genera, and for a Nitrospira strain (Nitrospira sp. Ecomares 2.1). The enzyme kinetics of nitrite oxidation, described by Michaelis-Menten kinetics, of 4 marine genera are rather narrow and fall in the low end of half-saturation constant (Km) values reported so far, which span over 3 orders of magnitude between 9 and >1000 µM NO2-. Nitrospina has the lowest Km (19 µM NO2-), followed by Nitrobacter (28 µM NO2-), Nitrospira (54 µM NO2-), and Nitrococcus (120 µM NO2-). The isotope effects during nitrite oxidation by Nitrospina watsonii 347 and Nitrospira sp. Ecomares 2.1 were 9.7 ± 0.8 and 10.2 ± 0.9‰, respectively. This confirms the inverse isotope effect of NOB described in other studies; however, it is at the lower end of reported isotope effects. We speculate that differences in isotope effects reflect distinct nitrite oxidoreductase (NXR) enzyme orientations

High Resolution Measurements of Nitrous Oxide (N2O) in the Elbe Estuary

Nitrous oxide (N2O) is one of the most important greenhouse gases and a major sink for stratospheric ozone. Estuaries are sites of intense biological production and N2O emissions. We aimed to identify hot spots of N2O production and potential pathways contributing to N2O concentrations in the surface water of the tidal Elbe estuary. During two research cruises in April and June 2015, surface water N2O concentrations were measured along the salinity gradient of the Elbe estuary by using a laser-based on-line analyzer coupled to an equilibrator. Based on these high-resolution N2O profiles, N2O saturations, and fluxes across the surface water/atmosphere interface were calculated. Additional measurements of DIN concentrations, oxygen concentration, and salinity were performed. Highest N2O concentrations were determined in the Hamburg port region reaching maximum values of 32.3 nM in April 2015 and 52.2 nM in June 2015. These results identify the Hamburg port region as a significant hot spot of N2O production, where linear correlations of AOU-N2Oxs indicate nitrification as an important contributor to N2O production in the freshwater part. However, in the region with lowest oxygen saturation, sediment denitrification obviously affected water column N2O saturation. The average N2O saturation over the entire estuary was 201% (SD: ±94%), with an average estuarine N2O flux density of 48 μmol m−2 d−1 and an overall emission of 0.18 Gg N2O y−1. In comparison to previous studies, our data indicate that N2O production pathways over the whole estuarine freshwater part have changed from predominant denitrification in the 1980s toward significant production from nitrification in the present estuary. Despite a significant reduction in N2O saturation compared to the 1980s, N2O concentrations nowadays remain on a high level, comparable to the mid-90s, although a steady decrease of DIN inputs occurred over the last decades. Hence, the Elbe estuary still remains an important source of N2O to the atmosphere

Nitrogen isotopic inventory of the Lena River Delta

Permafrost-affected soils around the Arctic Ocean contain a large reservoir of organic matter including nitrogen, which partly reach the river after thawing, degradation and erosion of permafrost. After mobilization, reactive remineralised nitrogen is either used for primary production, microbial processing or is simply transported to coastal waters. We have analyzed soil, suspended matter and dissolved inorganic and organic nitrogen for their contents and 15N stable isotope composition to create a baseline for a nitrogen inventory of the Lena River Delta in 2019/2020. We used samples from two transect cruises through the delta in March and August 2019, a monitoring program at Samoylov Island in the central delta (2019/2020), and different soil type samples from Samoylov Island. Our data shows that the nitrogen transported from the delta to the Laptev Sea were dominated by dissolved organic nitrogen (DON) and nitrate, which occur in similar amounts of approx. 10 μmol/L. DON was available during the whole year. Nitrate showed a clear seasonal pattern: increase from late summer until the spring flood, during summer the nitrate concentration are close to zero. During the spring flood the nitrogen concentration are higher with up to 100 μmol/L. The nitrogen stable isotope values of the different nitrogen components ranges mainly between 0.5 and 4.5‰, and were subsequently enriched from the soils via suspended particulate matter (SPM)/sediment and DON to nitrate. During the spring flood, the stable isotope signature of nitrate suggested a strong source of atmospheric deposition. The 15N values are depleted with appox. -8‰ and the 18O values are enriched up to 60‰. Our data provides a baseline for isoscape analysis and can be used as an endmember signal for modeling approaches

Metabolic alkalinity release from large port facilities (Hamburg, Germany) and impact on coastal carbon storage

Metabolic activities in estuaries, especially these of large rivers, profoundly affect the downstream coastal biogeochemistry. Here, we unravel the impacts of large industrial port facilities, showing that elevated metabolic activity in the Hamburg port (Germany) increases total alkalinity (TA) and dissolved inorganic carbon (DIC) runoff to the North Sea. The imports of particulate inorganic carbon, particulate organic carbon, and particulate organic nitrogen (PIC, POC, and PON) from the upstream Elbe River can fuel up to 90 % of the TA generated in the entire estuary via calcium carbonate (CaCO3) dissolution. The remaining at least 10 % of TA generation can be attributed to anaerobic metabolic processes such as denitrification of remineralized PON or other pathways. The Elbe Estuary as a whole adds approximately 15 % to the overall DIC and TA runoff. Both the magnitude and partitioning among these processes appear to be sensitive to climatic and anthropogenic changes. Thus, with increased TA loads, the coastal ocean (in particular) would act as a stronger CO2 sink, resulting in changes to the overall coastal system's capacity to store CO2.</p

Permafrost land-ocean interactions: fluxes, transport processes and degradation pathways

Permafrost-affected soils around the Arctic Ocean contain a large reservoir of organic matter including nitrogen, which partly reaches the riverine system after thawing, degradation and erosion of permafrost. After mobilization, reactive nitrogen in form of dissolved organic nitrogen (DON) ordissolved inorganic nitrogen (DIN: ammonium and nitrate) is either used for primary production, microbial turnover and/or is transported to coastal waters where it serves as a key source of nutrition for the marine food web. In this study, we have followed the nitrogen released from permafrost soil via the Lena River into the Laptev Sea and used the natural abundance of 15N stable isotopes to identify sources, sinks and processes. Therefore, we have investigated different soil. We present a comprehensive data set from two transect cruises (03/08 2019) through the delta, and the outcome of a monitoring program (2018 - 2021) at Samoylov Island in the central delta. High-frequency monitoring and cruise data shows that the nitrogen transported from the river to the Laptev Sea was dominated by DON and nitrate, which occurred in similar amounts of approx. 10 μmol L–1 in the river water. The nitrate concentration decreased during the early summer and increased from late summer throughout the winter until the spring flood. During the spring flood, the nitrogen concentration was up to ten times higher. Thus, spring floods transport approx. 20 % of the annual load of reactive nitrogen into the Laptev Sea just at the onset of the growing season. The nitrogen stable isotope values of the different nitrogen components ranged mainly between 0.5 and 4.5‰, and were subsequently enriched from the permafrost soils via suspended particulate matter/sediment and DON to nitrate, which indicate an oligotrophic ecosystem. Using a Bayesian mixing model, the stable isotope signature of nitrate suggested a strong source of atmospheric deposition during the spring flood. During the rest of the year, soils are the main source of the reactive nitrogen, which is transported to the marine realm