Nitrogen Concentrations In Pore Water Research Articles

Effects of Saltwater Pulses on Soil Microbial Enzymes and Organic Matter Breakdown in Freshwater and Brackish Coastal Wetlands

Coastal freshwater and brackish wetlands are exposed to pulses of saltwater during times of reduced freshwater flows (i.e., dry seasons, droughts), periodic storm surges, and increased tidal extent associated with rising seas. The effects of saltwater pulses on belowground processing rates of detrital organic matter as mediated by microbial activities are uncertain. Our objectives were to quantify whether and how pulses of saltwater (i) change soil porewater physicochemistry, (ii) change soil microbial extracellular enzyme activities, and (iii) change root litter breakdown over time in freshwater and brackish marshes. From 2014 to 2016, we simulated saltwater intrusion as monthly in situ pulsed additions of artificial seawater in experimental dosing chambers (1.4 m diameter) within freshwater and brackish marshes of Everglades National Park. At monthly intervals, we collected soil porewater chemistry, and measured microbial extracellular enzymes, elemental stoichiometric ratios, and breakdown rates (k) of incubated (0–30 cm depth) root litter and compared these responses over time. Saltwater pulses increased sulfate and nitrogen concentrations in porewater at the freshwater site. However, saltwater pulses generally decreased porewater constituents (e.g., dissolved organic carbon, dissolved nitrogen and phosphorus species) at the brackish site. One saltwater pulse increased root litter k by 1.25-fold in the brackish marsh. However, long-term (740 days) k in brackish wetlands, and both short- and long-term k in freshwater wetlands, were not affected by 24 monthly pulses of saltwater. Enzyme activities fluctuated with time and did not respond to multiple saltwater pulses. Our results suggest that detrital organic matter stocks and associated soil microbial activities are relatively resistant to single and multiple (n = 24) pulses of saltwater.

The influence of hyporheic upwelling fluxes on inorganic nitrogen concentrations in the pore water of the Weihe River

Hyporheic zone is an important region of nitrogen removal in river systems. Hyporheic exchange generally leads to heterogeneous redox environments, which are conducive to nitrogen transformation. This study seeks to determine the influence of hyporheic upwelling fluxes on inorganic nitrogen (NH4+, NO3−, and NO2−) concentrations in the sediment pore water of the Weihe River, China. The patterns and magnitudes of hyporheic water exchange on 12 August 2016 were derived by a one-dimensional heat transport model, and inorganic nitrogen concentrations in the pore water, surface water, and groundwater were obtained. The results indicated that hyporheic water exchange was characterized by upwelling at each point during the test period. Moreover, NH4+ dominated the hyporheic zone from 0 to 45 cm, likely due to organic nitrogen mineralization. Additionally, a non-linear relationship was observed between NH4+ concentrations and upwelling fluxes. This relationship was derived by analyzing the effect of upwelling on biogeochemical activity and nitrogen transformation. Notably, increasing upwelling fluxes less than 400 mm/d resulted in high NH4+ concentrations, whereas fluxes exceeding 400 mm/d led to low NH4+ concentrations. Overall, the variations in inorganic nitrogen associated with hyporheic water exchange are of great importance for controlling nitrogen pollution and maintaining sustainable health in river systems.

Potential Drivers of the Level and Distribution of Nitrogen in the Hyporheic Zone of Lake Taihu, China

The hyporheic zone is the connection between surface water and groundwater that often plays an important function in nutrient transport and transformation, and acts as an active source of or sink for nutrients to the surface water, depending on its potential water flow patterns. Bottom surface water and sediments in the shallow hyporheic zone (approximately 100 cm depth) were sampled at 12 sites near the shoreline and two sites at the center of Lake Taihu (China) during spring and winter of 2016. Concentrations of total nitrogen, ammonium, nitrate, and nitrite in the bottom surface water and porewater (obtained from sediments using a frozen centrifugation method) were analyzed in a laboratory to establish the nitrogen distribution and potential drivers. The results show that, in general, the quality of bottom water and porewater near the shoreline was poor compared to that at the center, and it gradually improved from the northwestern to the southeastern zones of Lake Taihu. No significant relationship in nitrogen concentration was found between the bottom water and porewater in surface sediments. Nitrogen concentrations in porewater differed between sampling sites and sediment depths in Lake Taihu. Vertical profiles of nitrogen in porewater and differences in nitrogen between the winter and spring seasons indicated that potential upwelling water flow occurred in the hyporheic zone in the south, west, north, and center zones of Lake Taihu, but potentially weak water flow in variable directions likely occurred in the east zone. A strong reducing environment dominated the deep parts of the hyporheic zone (i.e., below 40 cm depth), while a weak oxidizing environment dominated the shallow parts. Furthermore, the decreasing total nitrogen and ammonium nitrogen from the deep to shallow depths in the hyporheic zones in the south, west, north, and center zones indicated that potential anammox and/or denitrification processes occurred. In the east zone, potential weak nitrification processes occurred in the hyporheic zone, and plant fixation and sedimentation of nitrogen also contributed to the surface sediments. In conclusion, the hyporheic zone near the shoreline in the south, west, and north sites of Lake Taihu acts as an active source of nitrogen for the lake water due to potential upwelling water flows, whereas the east site acts as an active source or sink due to seasonally variable directions in water flow. Water flow and biogeochemistry in the hyporheic zone jointly influence nutrient distribution in the hyporheic zone and even switch or alternate the source/sink function of sediment in surface water.

Effects of organic perturbation on marine sediment betaproteobacterial ammonia oxidizers and on benthic nitrogen biogeochemistry

The failure of denitrification to remove nitrogen build-up from aquatic systems is often attributed to sediment chemical conditions inhibiting nitrification and therefore the supply of suitable substrates to be denitrified. We investigated the effects of organic fish farm pollution on nitrogen- cycle dynamics and betaproteobacterial ammonia-oxidizing bacteria (β-AOB) community structure to elucidate the potential role of the nitrifier community on nitrogen biogeochemistry in marine sedi- ments. Porewater nitrogen concentrations, denitrification rates, β-AOB 16S rDNA gene quantifica- tion, denaturing gradient gel electrophoresis (DGGE) community fingerprints and infaunal counts were determined in samples collected from beneath fish cages and at adjacent, non-impacted control sites. The study was conducted over 2 full, 1 yr production cycles. Although nitrogen cycling was sig- nificantly altered beneath cages, changes appeared to result from a reduction in the proportion of ammonia nitrified rather than from inhibition of nitrification per se. DGGE revealed β-AOB commu- nities shifted rapidly and remained diverse at both cage and reference sites. Quantitative PCR (qPCR) showed β-AOB numbers did not decline in absolute terms but did decline as a proportion of the total bacterial community at cage sites and at the end of the stocking periods. Sediment infaunal commu- nity analysis showed significant effects of organic loading and indicated more bioirrigation at impacted sites. Despite the induction of conditions thought to be detrimental to nitrification and to β- AOB (low oxygen, reduced sediments, low pH, and high sulphide concentrations), these communities remained diverse and apparently viable, perhaps a result of heavy sediment bioirrigation. However, despite the increase in denitrification, nitrogen left the sediment predominantly as ammonia, thus producing potential point sources of eutrophication.