The influence of macrofaunal burrow spacing and diffusive scaling on sedimentary nitrification and denitrification: An experimental simulation and model approach

The influence of burrow spacing on nitrification and denitrification was simulated experimentally using sediment plugs of different thicknesses immersed in aerated seawater reservoirs. Different plug thicknesses mimic different distances between oxygenated burrow centers and produce similar changes in aerobic–anaerobic reaction balances as a function of diffusive transport scaling. The thicknesses used were roughly equivalent to transport scales (interburrow spacing) that could be produced by burrow abundances of ~400 to 50,000 m-2, depending on burrow lumen radii (e.g., 0.05–1 cm). Following the exposure of anoxic sediment plugs to aerated water, an efficient aerobic nitrification zone was established within the first ~2–3 millimeters of sediment. At pseudo-steady state, the thinnest plug (2 mm) simulating highest burrow density, was entirely oxic and the denitrification rate nil. Denitrification was stimulated in anoxic regions of the thicker plugs (5, 10, and 20 mm) compared to the initial value in experimental sediment. Maximum nitrification rates and the highest denitrification/nitrification ratio between oxic nitrification and adjacent denitrification zones occurred for the intermediate plug thickness of 5 mm. Of the oxic/anoxic composites, the thickest plug showed the least efficient coupling between nitrification/denitrification zones (lowest denitrification/nitrification ratio). Both the thickness of the oxic layer and the total net remineralization of dissolved inorganic N varied inversely with plug thickness. A set of diffusion–reaction models was formulated assuming a range of possible nitrification kinetic functions. All model forms predicted optimal nitrification–denitrification and ammonification–denitrification coupling with relative oxic–anoxic zonation scales comparable to intermediate plug thicknesses (5–6 mm). However, none of the commonly assumed kinetic forms for nitrification could produce the observed NO-3 profiles in detail, implying that natural sediment populations of nitrifiers may be less sensitive to O2 than laboratory strains. Our experimental and model results clearly show that rates of N remineralization and the balance between stimulation/inhibition of denitrification are highly dependent on sedimentary biogenic structure and the particular geometries of irrigated burrow distributions

Nitrification-denitrification in WSP: a mechanism for permanent nitrogen removal in maturation ponds

A pilot-scale primary maturation pond was spiked with 15N-labelled ammonia (15NH4Cl) and 15N labelled nitrite (Na15NO2), in order to improve current understanding of the dynamics of inorganic nitrogen transformations and removal in WSP systems. Stable isotope analysis of δ15N showed that nitrification could be considered as an intermediate step in WSP, which is masked by simultaneous denitrification, under conditions of low algal activity. Molecular microbiology analysis showed that denitrification can be considered a feasible mechanism for permanent nitrogen removal in WSP, which may be supported either by ammonia-oxidising bacteria (AOB) or by methanotrophs, in addition to nitrite-oxidising bacteria (NOB). However, the relative supremacy of the denitrification process over other nitrogen removal mechanisms (e.g., biological uptake) depends upon phytoplanktonic activity

Anoxic nitrification in marine sediments

Nitrate peaks are found in pore-water profiles in marine sediments at depths considerably below the conventional zone of oxic nitrification. These have been interpreted to represent nonsteady- state effects produced by the activity of nitrifying bacteria, and suggest that nitrification occurs throughout the anoxic sediment region. In this study, ΣNO3 peaks and molecular analysis of DNA and RNA extracted from anoxic sediments of Loch Duich, an organic-rich marine fjord, are consistent with nitrification occurring in the anoxic zone. Analysis of ammonia oxidiser 16S rRNA gene fragments amplified from sediment DNA indicated the abundance of autotrophic ammonia-oxidising bacteria throughout the sediment depth sampled (40 cm), while RT-PCR analysis indicated their potential activity throughout this region. A large non-steady-state pore-water ΣNO3 peak at ~21 cm correlated with discontinuities in this ammonia-oxidiser community. In addition, a subsurface nitrate peak at ~8 cm below the oxygen penetration depth, correlated with the depth of a peak in nitrification rate, assessed by transformation of 15N-labelled ammonia. The source of the oxidant required to support nitrification within the anoxic region is uncertain. It is suggested that rapid recycling of N is occurring, based on a coupled reaction involving Mn oxides (or possibly highly labile Fe oxides) buried during small-scale slumping events. However, to fully investigate this coupling, advances in the capability of high-resolution pore-water techniques are required

Meta-analysis reveals ammonia-oxidizing bacteria respond more strongly to nitrogen addition than ammonia-oxidizing archaea

Shifts in microbial communities driven by anthropogenic nitrogen (N) addition have broad-scale ecological consequences. However, responses of microbial groups to exogenous N supply vary considerably across studies, hindering efforts to predict community changes. We used meta-analytical techniques to explore how amoA gene abundances of ammonia-oxidizing archaea (AOA) and bacteria (AOB) respond to N addition, and found that N addition increased AOA and AOB abundances by an average of 27% and 326%, respectively. Responses of AOB varied by study type, ecosystem, fertilizer type, and soil pH, and were strongest in unmanaged wildland soils and soils fertilized with inorganic N sources. Increases in nitrification potential with N addition significantly correlated with only AOB. Our analyses suggest that elevated N supply enhances soil nitrification potential by increasing AOB populations, and that this effect may be most pronounced in unmanaged wildland soils

Effect of dissolved oxygen and chemical oxygen demand to nitrogen ratios on the partial nitrification/denitrification process in moving bed biofilm reactors

Partial nitrification was reported to be technically feasible and economically favorable, especially for wastewater with high ammonium concentration or low C/N ratio. In this study, the effect of dissolved oxygen (DO) and influent ratio of chemical oxygen demand to nitrogen (COD/N) ratio on biological nitrogen removal from synthetic wastewater was investigated. Experiments were conducted in moving bed biofilm reactors (MBBRs) on partial nitrification process in pilot-plant configuration for 300 days. DO levels were changed from 0.04 to 0.12 and 0.42 to 3.4 mg/l in the anoxic (R1) and aerobic (R2) reactors, respectively. The optimum DO for partial nitrification was between 1-1.5 mg/l in the aerobic reactor (R2). Influent COD/N ratios between 20 and 2 g COD/g-N were tested by changing the nitrogen loading rate (NLR) supplied to the pilot plant. During operational conditions when the DO concentration in aerobic reactor was above 1 mg/l, near complete organic carbon removal occurred in the total MBBRs system. The effluent total nitrogen concentration in the operational conditions (1.7-2.1 mg O2/l and NH+ 4-N=35.7 mg N/l) was obtained in the range of 0.85-2 mg/l. The highest nitrite accumulation (50%- 52%) took place at the DO concentration of 1-1.5 mg/l and increased with decreasing COD/N ratio in aerobic reactor (R2). This study showed that the average nitrification rate at various COD/N ratios is about 0.96 gN/m2 per day while the maximum nitrification rate is about 2 gN/m2 per day at COD/N ratios lower than 6. The experimental COD/N ratio for denitrification was close to complete sum of NO2 - and NO3 - (NOx) removal efficiency (about 99%) at COD/N ratio equal 14 in the operational conditions in the anoxic reactor (R1)

Effects of Silvopasture Establishment on Aqueous and Gaseous Soil N Losses at the University of New Hampshire Organic Dairy Research Farm

The expansion of local agriculture in the New England region is putting increased pressure on farmers to expand their arable land base. While clear-cutting is a traditional method of converting forested land to agriculture, it is known for having adverse ecological impacts. To minimize these impacts, farmers can create a silvopasture which incorporates a portion of the original forest canopy into pastures or crop fields. This study evaluates the impact of land-use changes for agriculture on soil nitrogen (N) retention. In particular, this study investigates the differences in soil N turnover, gaseous loss, and aqueous loss among an established forest, established pasture, clear-cut converted pasture, and converted silvopasture systems over a 30day incubation period. We found significant differences in N mineralization, immobilization, and denitrification among treatments, with evidence that a forest-to-silvopasture conversion can successfully support soil N retention within the first two years of implementation. This may have been due to the presence of coarse woody debris inputs from forest cutting and its effect on the soil carbon (C) to N ratio. Nitrogen retention in silvopastures may also result from partial preservation of the forest canopy. Our results suggest that farmers looking to expand their agricultural land base through forest clearing may be able to use silvopastures for as a way of retaining soil nutrients while at the same time putting land into production

Coupled anoxic nitrification/manganese reduction in marine sediments

Pore water and solid phase distributions of oxygen, manganese, and nitrogen from hemipelagic and shelf sediments sometimes indicate a close coupling between the manganese and nitrogen redox cycles. Reaction coupling must be sustained in part by biological reworking of Mn-oxide-rich surface sediments into underlying anoxic zones. Surface sediment from Long Island Sound (USA) was used in laboratory experiments to simulate such intermittent natural mixing processes and subsequent reaction evolution. Mixed sediment was incubated anoxically under either diffusively open (plugs) or closed conditions (jars). In closed anoxic incubations, pore water NO3 2 increased regularly to a maximum (up to 17 mM) after one to several days, and was subsequently depleted. Mn21 was produced simultaneously with NO3 2. NO2 2 was also clearly produced and subsequently reduced, with a formation-depletion pattern consistent with coupled nitrificationdenitrification in the anoxic sediment. Manipulative additions of Mn-oxides (5–10 mmol g21 net) demonstrated that net anoxic NO3 2 production correlated directly with initial Mn-oxide content. During initial net NO3 2 production there was no evidence for SO4 22 reduction. A direct correlation was also observed between anoxic nitrification rates and estimated sulfate reduction rates; the larger nitrification rates, the larger the eventual net sulfate reduction rates. Diffusively-open incubations using sediment plugs of four different thicknesses (2, 5, 10 and 20 mm) exposed to anoxic overlying water, also showed net production of pore water NO3 2 (;15–20 mM) despite the absence of NO3 2 in the overlying water for at least five days. In general, higher nitrate concentrations were maintained in the open relative to the closed incubations, due most likely to lower concentrations of dissolved reductants for NO3 2 in the open system. These experiments imply simultaneous coupling between the benthic nitrogen, manganese, and sulfur redox cycles, involving anoxic nitrification and sulfide oxidation to SO4 22. Anoxic nitrate production during Mn reduction indicates that nitrification and denitrification can occur simultaneously in subsurface sediments, without vertical stratification. The existence of anoxic nitrification implies new reaction pathways capable of increasing coupled sedimentary nitrificationdenitrification, particularly in bioturbated or physically mixed deposits

NITROGEN CYCLING IN A FOREST STREAM DETERMINED BY A 15N TRACER ADDITION

Nitrogen uptake and cycling was examined using a six‐week tracer addition of 15N‐labeled ammonium in early spring in Walker Branch, a first‐order deciduous forest stream in eastern Tennessee. Prior to the 15N addition, standing stocks of N were determined for the major biomass compartments. During and after the addition, 15N was measured in water and in dominant biomass compartments upstream and at several locations downstream. Residence time of ammonium in stream water (5–6 min) and ammonium uptake lengths (23–27 m) were short and relatively constant during the addition. Uptake rates of NH4 were more variable, ranging from 22 to 37 μg N·m−2·min−1 and varying directly with changes in streamwater ammonium concentration (2.7–6.7 μg/L). The highest rates of ammonium uptake per unit area were by the liverwort Porella pinnata, decomposing leaves, and fine benthic organic matter (FBOM), although epilithon had the highest N uptake per unit biomass N. Nitrification rates and nitrate uptake lengths and rates were determined by fitting a nitrification/nitrate uptake model to the longitudinal profiles of 15N‐NO3 flux. Nitrification was an important sink for ammonium in stream water, accounting for 19% of the total ammonium uptake rate. Nitrate production via coupled regeneration/nitrification of organic N was about one‐half as large as nitrification of streamwater ammonium. Nitrate uptake lengths were longer and more variable than those for ammonium, ranging from 101 m to infinity. Nitrate uptake rate varied from 0 to 29 μg·m−2·min−1 and was ∼1.6 times greater than assimilatory ammonium uptake rate early in the tracer addition. A sixfold decline in instream gross primary production rate resulting from a sharp decline in light level with leaf emergence had little effect on ammonium uptake rate but reduced nitrate uptake rate by nearly 70%. At the end of the addition, 64–79% of added 15N was accounted for, either in biomass within the 125‐m stream reach (33–48%) or as export of 15N‐NH4 (4%), 15N‐NO3 (23%), and fine particulate organic matter (4%) from the reach. Much of the 15N not accounted for was probably lost downstream as transport of particulate organic N during a storm midway through the experiment or as dissolved organic N produced within the reach. Turnover rates of a large portion of the 15N taken up by biomass compartments were high (0.04–0.08 per day), although a substantial portion of the 15N in Porella (34%), FBOM (21%), and decomposing wood (17%) at the end of the addition was retained 75 d later, indicating relatively long‐term retention of some N taken up from water. In total, our results showed that ammonium retention and nitrification rates were high in Walker Branch, and that the downstream loss of N was primarily as nitrate and was controlled largely by nitrification, assimilatory demand for N, and availability of ammonium to meet that demand. Our results are consistent with recent 15N tracer experiments in N‐deficient forest soils that showed high rates of nitrification and the importance of nitrate uptake in regulating losses of N. Together these studies demonstrate the importance of 15N tracer experiments for improving our understanding of the complex processes controlling N cycling and loss in ecosystems

Increased salinity improves the thermotolerance of mesophilic nitrification

Nitrification is a well-studied and established process to treat ammonia in wastewater. Although thermophilic nitrification could avoid cooling costs for the treatment of warm wastewaters, applications above 40 A degrees C remain a significant challenge. This study tested the effect of salinity on the thermotolerance of mesophilic nitrifying sludge (34 A degrees C). In batch tests, 5 g NaCl L-1 increased the activity of aerobic ammonia-oxidizing bacteria (AerAOB) by 20-21 % at 40 and 45 A degrees C. For nitrite-oxidizing bacteria (NOB), the activity remained unaltered at 40 A degrees C, yet decreased by 83 % at 45 A degrees C. In a subsequent long-term continuous reactor test, temperature was increased from 34 to 40, 42.5, 45, 47.5 and 50 A degrees C. The AerAOB activity showed 65 and 37 % higher immediate resilience in the salt reactor (7.5 g NaCl L-1) for the first two temperature transitions and lost activity from 45 A degrees C onwards. NOB activity, in contrast to the batch tests, was 37 and 21 % more resilient in the salt reactor for the first two transitions, while no difference was observed for the third temperature transition. The control reactor lost NOB activity at 47.5 A degrees C, while the salt reactor only lost activity at 50 A degrees C. Overall, this study demonstrates salt amendment as a tool for a more efficient temperature transition for mesophilic sludge (34 A degrees C) and eventually higher nitrification temperatures

Influence of shellfish farming activities on nitrification, nitrate reduction to ammonium and denitrification at the water-sediment interface of the Thau lagoon, France

The seasonal patterns of nitrification, denitrification and dissimilatory ammonium production (DAP) rates were studied in the sediment of 2 stations in the Thau lagoon (south of France). The station ZA was located within the shellfish farming zone and thestation B was the reference site. A marked effect of shellfish farming on bacterial activities was observed. Spatial differences were associated with discrepancies in the organic content and the reduction state of sediments, i.e. highest reductive processes (denitrification and DAP) were noted in shellfish farming area, whereas the oxidative process (nitrification) was predominant outside the farming zone. At both stations, the DAP activity increased in September (autumn) concomitant with an increase of the C/N ratio in the sediment due to the sedimentation of the summer phytoplanktonic production. Nitrification and denitrification rates exhibited maxima in November (winter) corresponding to dissolved inorganic nitrogen inputs from the surrounding land. In the shellfish farming site, 98% of nitrate was reduced to NH4+ and 2% to N2O, showing that the most of the NO3- was reduced to ammonium and remained available for the ecosystem