Net NH4 Research Articles

Variation in NH 4+ mineralization and microbial communities with stand age in lodgepole pine ( Pinus contorta) forests, Yellowstone National Park (USA)

Soils and vegetation were analyzed in 20 lodgepole pine ( Pinus contorta) forest stands, varying in age from 50 to 350 years, that had initiated following stand-replacing fire. Our goal was to determine how nitrogen availability (NH 4 +–N) and microbial community composition varied with stand age-class and to determine whether differences could be explained by canopy, soil, or understory characteristics. Gross NH 4 + mineralization was measured using laboratory isotopic pool dilution, and microbial community composition was evaluated using microbial membrane lipids. The microbial community composition of stands in the 300–350 age class was distinct from stands in younger age classes. Microbial community composition among sites varied with pH, % organic matter, and phosphorus. Gross NH 4 + mineralization rates averaged 1.45±0.07 mg NH 4 + kg soil −1 d −1 while consumption averaged 1.37±0.20 mg NH 4 + kg soil −1 d −1, resulting in low net NH 4 + mineralization rates (0.08±0.18 mg NH 4 + kg soil −1 d −1), but rates were not significantly different with stand age-class at p<0.05. At p<0.10, net NH 4 + mineralization was significantly higher in the 300–350 age class compared to the 125–175 age class. None of the measured variables significantly explained NH 4 + consumption and net mineralization patterns. However, gross NH 4 + mineralization rates were best explained by information on microbial community structure (i.e. lipids). Variation among stands within a given age-classes was high, indicating that patterns of N cycling across landscapes reflect substantial heterogeneity among mature stands.

Ammonium cycling under a strong oxygen gradient associated with the Oxygen Minimum Zone off northern Chile (~23°S)

Ammonium (NH4 + ) cycling rates under different dissolved oxygen (DO) conditions in the water column, at a coastal upwelling area off northern Chile (~23° S), were estimated. Net NH4 + regeneration (eukaryotes and prokaryotes) and NH4 + oxidation (nitrifying bacteria) rates were exam- ined by means of selective inhibitor assays (cycloheximide and allylthiourea) under dark conditions. Whole water samples for incubations were taken in the oxycline, low-DO zone (30 m; 69 µM DO), and in the upper boundary of the Oxygen Minimum Zone (OMZ), suboxic zone (50 m; 1.2 × 10 6 , >0.9 × 10 5 and >37 cells ml -1 , respectively), indicating a potential large impact on NH4 + cycling.

Nitrogen mineralization in a pecan (Carya illinoensis K. Koch)?cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Information on temporal and spatial patterns of N mineralization is critical in designing tree-crop mixed systems that could maximize N uptake while minimizing N loss. We quantified N mineralization rates in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in northwestern Florida with (non-barrier) and without tree-crop belowground interactions (barrier separating the root systems of pecan and cotton). Monthly rates of mineralization were estimated using buried bag incubations over a 15-month period. In addition, seasonal mineralization rates and cotton lint yield on soils supplied with two sources of N—inorganic fertilizer and organic poultry litter—were assessed. Results indicated that temporal variations in net NH4 and NO3 accumulation and mineralization rates were driven primarily by environmental factors and to a lesser degree by initial soil NH4 and NO3 levels. Mineralization varied by belowground interaction treatment during the initial growing season, when the non-barrier treatment exhibited a higher mineralization rate than the barrier treatment, likely due to reduced nutrient uptake by cotton in the non-barrier or a higher degree of immobilization in the barrier treatment. Mineralization during the second growing season was similar for both treatments. Source of N had no effects on N transformation in the soil. Lint yield reductions were observed in the non-barrier treatment during both years compared to the barrier treatment, likely due to interspecific competition for water. Yield differences between treatments in the second growing season were likely compounded by a diminishing pre-study fallow effect. Source of N was found to have a significant effect on cotton yield, with inorganic fertilizer resulting in 39% higher lint compared to poultry litter in the barrier treatment.

Temporal variability of nitrogen cycling in continental-shelf sediments of the upwelling ecosystem off central Chile

Abstract The continental shelf region off central Chile (∼36 °S), one of the widest and most productive areas of the eastern South Pacific, is an important site of coastal upwelling. In order to understand how seasonal and inter-annual variability in bottom-water physical and chemical conditions affect benthic nutrient regeneration and sediment characteristics in this area, ammonium (NH 4 + ) and nitrate (NO 3 − ) fluxes at the water–sediment interface were experimentally quantified (March 1998–April 2001), along with net NH 4 + production, potential nitrification and denitrification rates (November 1998–August 2000). NH 4 + fluxes to the overlying water up to 10.4 mmol m −2 d −1 , occurred during the upwelling season (i.e. austral spring and summer), while NH 4 + removal from the water column up to −5.7 mmol m −2 d −1 during non-favorable upwelling conditions was observed (i.e., austral winter and the 1997–1998 El Nino condition). The fate of the benthic N regenerated as NH 4 + appears to be controlled by the amount of labile organic carbon (here indexed as chlorophyll- a ) in the surface sediment and, indirectly, by the bottom-water oxygen concentration. The balance between net NH 4 + production and potential nitrification (4.4–34.3 and 0.3–2.9 mmol m −2 d −1 , respectively) does not support the observed NH 4 + fluxes, suggesting the occurrence of other NH 4 + dissamilative (by dissolved metal or anammox) or assimilative consuming processes. Throughout the entire study period, the sediments acted as a large sink for NO 3 − (−3.4±1.4 mmol m −2 d −1 ) and as an important denitrification site (0.6–2.9 mmol m −2 d −1 ) coupled with NO 3 − produced by nitrification (58–97%). Other processes such as NO 3 − ammonification or active NO 3 − uptake by Thioploca mats could account for NO 3 − uptake from the water column.

Characteristics of renal Rhbg as an NH4+ transporter

Rhbg is one of two recently cloned nonerythroid glycoproteins belonging to the Rh antigen family. Rhbg is expressed in basolateral membranes of intercalated cells of the kidney cortical collecting duct and some other cell types of the distal nephron and may function as NH(4)(+) transporters. The aim of this study was to characterize the role of Rhbg in transporting NH(4)(+). To do so, we expressed Rhbg in Xenopus laevis oocytes. Two-electrode voltage-clamp and H(+)-selective microlectrodes were used to measure NH(4)(+) currents, current-voltage plots, and intracellular pH (pH(i)). In oocytes expressing Rhbg, 5 mM NH(4)(+) induced an inward current of 93 +/- 7.7 nA (n = 20) that was significantly larger than that in control oocytes of -29 +/- 7.1 nA (P < 0.005). Whole cell conductance, at all tested potentials (-60 to +60 mV), was significantly more in oocytes expressing Rhbg compared with H(2)O-injected oocytes. In Rhbg oocytes, 5 mM NH(4)(+) depolarized the oocyte by 28 +/- 3.6 mV and decreased pH(i) by 0.30 +/- 0.04 at a rate of -20 +/- 2.5 x 10(-4) pH/s. In control oocytes, 5 mM NH(4)(+) depolarized V(m) by only 20 +/- 5.8 mV and pH(i) decreased by 0.07 +/- 0.01 at a rate of -2.7 +/- 0.6 x 10(-4) pH/s. Raising bath [NH(4)(+)] in increments from 1 to 20 mM elicited a proportionally larger decrease in pH(i) (DeltapH(i)), larger depolarization (DeltaV(m)), and a faster rate of pH(i) decrease. Bathing Rhbg oocytes in 20 mM NH(4)(+) induced an inward current of 140 +/- 7 nA that was not significantly different from 178 +/- 23 nA induced in H(2)O-injected (control) oocytes. The rate of pH(i) decrease induced by increasing external [NH(4)(+)] was significantly faster in Rhbg than in H(2)O-injected oocytes at all external NH(4)(+) concentrations. In oocytes expressing Rhbg, net NH(4)(+) influx (estimated from NH(4)(+)-induced H(+) influx) as a function of external [NH(4)(+)] saturated at higher [NH(4)(+)] with a V(max) of approximately 30.8 and an apparent K(m) of 2.3 mM (R(2) = 0.99). These data strongly suggest that Rhbg is a specific electrogenic transporter of NH(4)(+).

Carbon and Nitrogen Dynamics During Incubation of Manured Soil

Denitrification N losses during manure net mineralizable N assays may lead to miscalculation of the manure's N‐supplying capacity. In this study we measured denitrification, manure properties, gas fluxes, nutrient pools, and mineralizable N during laboratory incubation of manured soil. Different dairy manures (n = 107) were added to soil at a rate of 0.1 mg N g−1 Manured and control soils were incubated and sampled weekly for soil mineral N, CO2 flux, and N2O flux. The denitrification enzyme activity (DEA) was measured at the end of the experiment. Weekly N2O and CO2 production increased in the manured soils during the first 3 wk of incubation. There was a positive correlation between added manure C and cumulative CO2 production. Nitrate content increased in all soils throughout the 6‐wk period, but the increase was more marked in the manured soils. In most manured soils, ammonium concentration was initially high then declined rapidly during the first 2 wk. This high net NH4+ decline in the manured soils suggests that N was immobilized during the incubation. Microbial biomass N should be determined during manure mineralization assays to account for all potential manure N sinks. No correlation existed between DEA and N pools or gas fluxes in the manured soils. Manures with negative N mineralization had an average C/N of 19.0, while manures with positive N mineralization had an average C/N of 16.0. On average, denitrification accounted for approximately 5% of the added manure N. Higher proportions of denitrified N were observed in some manures, supporting our hypothesis that N losses through denitrification may be significant in manure mineralizable N assays.

Cation transport by the neuronal K(+)-Cl(-) cotransporter KCC2: thermodynamics and kinetics of alternate transport modes.

Both Cs(+) and NH(4)(+) alter neuronal Cl(-) homeostasis, yet the mechanisms have not been clearly elucidated. We hypothesized that these two cations altered the operation of the neuronal K(+)-Cl(-) cotransporter (KCC2). Using exogenously expressed KCC2 protein, we first examined the interaction of cations at the transport site of KCC2 by monitoring furosemide-sensitive (86)Rb(+) influx as a function of external Rb(+) concentration at different fixed external cation concentrations (Na(+), Li(+), K(+), Cs(+), and NH(4)(+)). Neither Na(+) nor Li(+) affected furosemide-sensitive (86)Rb(+) influx, indicating their inability to interact at the cation translocation site of KCC2. As expected for an enzyme that accepts Rb(+) and K(+) as alternate substrates, K(+) was a competitive inhibitor of Rb(+) transport by KCC2. Like K(+), both Cs(+) and NH(4)(+) behaved as competitive inhibitors of Rb(+) transport by KCC2, indicating their potential as transport substrates. Using ion chromatography to measure unidirectional Rb(+) and Cs(+) influxes, we determined that although KCC2 was capable of transporting Cs(+), it did so with a lower apparent affinity and maximal velocity compared with Rb(+). To assess NH(4)(+) transport by KCC2, we monitored intracellular pH (pH(i)) with a pH-sensitive fluorescent dye after an NH(4)(+)-induced alkaline load. Cells expressing KCC2 protein recovered pH(i) much more rapidly than untransfected cells, indicating that KCC2 can mediate net NH(4)(+) uptake. Consistent with KCC2-mediated NH(4)(+) transport, pH(i) recovery in KCC2-expressing cells could be inhibited by furosemide (200 microM) or removal of external [Cl(-)]. Thermodynamic and kinetic considerations of KCC2 operating in alternate transport modes can explain altered neuronal Cl(-) homeostasis in the presence of Cs(+) and NH(4)(+).

Ammonia Volatilization and Selected Soil Characteristics Following Application of Anaerobically Digested Pig Slurry

Ammonia volatilization occurs shortly following land application of pig slurry. Several slurry and soil characteristics modulate the intensity of this process, and their net effect on volatilization is still hard to predict. Our aim was to compare volatilization following application of anaerobically stored (ASPS) and anaerobically digested (ADPS) pig slurry to a bare loamy soil (loamy, mixed, frigid, Aeric Haplaquept). Ammonia volatilization was measured using wind tunnels. Soil pH and water, NH+4, NO−3, and volatile fatty acid (VFA) contents were monitored in the 0‐ to 0.5‐, 0.5‐ to 2‐, 2‐ to 5‐, and 5‐ to 10‐cm soil layers to explain volatilization rates. Following slurry application, pH increased by 1 to 3 units in the top 2 cm of soil, resulting in high volatilization rates in the first 6 h of experiment. Thereafter, pH decreased more slowly in ASPS than ADPS plots, possibly due to the degradation of VFAs present in ASPS. After 2 d, 35% of slurry‐added NH+4–N was lost as NH3–N for both slurries, corresponding well to the net decrease found in soil NH+4–N content. After 9 d, net soil NH+4–N disappearance accounted for about 60% of slurry‐added NH+4–N for both slurries, whereas NH3–N losses represented only 40%. Therefore, for the first 2 d of the experiment NH3 volatilization explained most of the decline in soil NH+4 Afterwards, biological processes, such as immobilization and nitrification, were assumed to play a significant role in slurry NH+4 disappearance. Despite marked changes in slurry properties, anaerobic digestion did not significantly modify the proportion of slurry N that was lost as NH3 Ammonia volatilization was related mostly to soil pH and NH+4 content in the top 2 cm of soil. Below 5‐cm depth, slurry application had little effect on soil pH, water, VFA, or mineral N content. This finding stresses the importance of stratified soil sampling when studying the short‐term effects of pig slurry on NH3 volatilization and associated soil properties.

Autotrophic ammonia oxidation in a deep-sea hydrothermal plume

Direct evidence for autotrophic ammonia oxidation is documented for the first time in a deep-sea hydrothermal plume. Elevated NH 4 + concentrations of up to 341±136 nM were recorded in the plume core at Main Endeavour Field, Juan de Fuca Ridge. This fueled autotrophic ammonia oxidation rates as high as 91 nM day −1, or 92% of the total net NH 4 + removal. High abundance of ammonia-oxidizing bacteria was detected using fluorescence in situ hybridization. Ammonia-oxidizing bacteria within the plume core (1.0–1.4×10 4 cells ml −1) accounted for 7.0–7.5% of the total microbial community, and were at least as abundant as methanotrophs. Ammonia-oxidizing bacteria were a substantial component of the particle-associated communities (up to 51%), with a predominance of the r-strategist Nitrosomonas-like cells. In situ chemolithoautotrophic organic carbon production via ammonia oxidation may yield 3.9–18 mg C m −2 day −1 within the plume directly over Main Endeavour Field. This rate was comparable to that determined for methane oxidation in a previous study, or at least four-fold greater than the flux of photosynthetic carbon reaching plume depths measured in another study. Hence, autotrophic ammonia oxidation in the neutrally buoyant hydrothermal plume is significant to both carbon and nitrogen cycling in the deep-sea water column at Endeavour, and represents another important link between seafloor hydrothermal systems and deep-sea biogeochemistry.

Ammonia-oxidizing activity under extremely oligotrophic conditions in strongly acid tea soils

The number and the activity of chemolithotrophic ammonia-oxidizing bacteria (CAB) in acid soils were determined and compared with those in neutral soils. Among the soils tested, CAB were predominant in acid tea soils but were not conspicuous in an acid grassland soil. In general, the activity decreased along with the net concentration of NH3 , suggesting that the major factor for CAB activity in acid soils was the substrate availability, regardless of the soil type and soil pH. In one of the acid tea soils tested, NO2 − was actively produced in a hundred-lower range of the net NH3 concentration, compared with other acid soils, and the activity increased sharply with a moderate increase in the NH3 concentration, presumably due to the high cell activity under oligotrophic conditions and not because of the abundance of CAB. A semi-quantitative analysis was carried out based on PCR and probing of the ammonia monooxygenase gene. It was suggested that CAB correlating to Nitrosomonas eutropha and Nitrosomonas sp. TK794 were predominant in the acid tea soils.

Inorganic nutrient and oxygen fluxes across the sediment–water interface in the inshore macrophyte areas of a shallow estuary (Lake Illawarra, Australia)

Rates of inorganic nutrient and oxygen fluxes, and gross community primary productivity were investigated using incubated cores in July, August and September 2001, in a seagrass meadow of Lake Illawarra, a barrier estuary in New South Wales, Australia. The results indicated that rates of gross primary productivity were high, varying from C = 0.62 to 1.89 g m−2 d−1; low P/R ratios of 0.28–0.48 define the system as heterotrophic and indicate that more carbon is respired than is produced. In order to determine the effect of macroalgae on O2 and nutrient fluxes, measurements were also conducted on cores from which the macroalgae had been removed. The results showed that the O2 fluxes during light incubations were significantly lower in the cores without macroalgae (P 80%). NO2−+ NO3− and o-P fluxes were always very low during the sampling period. The increasing tendency of net nutrient effluxes, especially NH4+ from July to September, is consistent with the increase of the water temperature and seagrass biomasses. However, in September, significantly lower light, dark and net NH4+ effluxes were found in the cores with macroalgae (SA-sediments) compared with the cores without macroalgae (S-sediments). These results support the hypothesis that actively-growing dense macroalgal mats (i.e., algal blooms in September) may act as a filter reducing the flux of nutrients to the water column.

Coupling soil–plant–atmosphere exchange of ammonia with ecosystem functioning in grasslands

The exchange of ammonia (NH 3) between the atmosphere and the land surface is controlled by both atmospheric and land surface processes and can thus be bi-directional. Whether emission or deposition occurs depends on the nitrogen (N) status of the ecosystem. Resistance models have been developed to represent this bi-directional pattern of NH 3 exchange. Major pathways include exchange with leaf cuticles, plant tissues (via stomata) and with the soil surface. However, the parameters that quantify the emission potential of the foliage and ground surface, the NH 3 stomatal compensation point, χ s, and the soil surface NH 3 concentration, χ soil, respectively, are entirely empirical in these models and do not consider the influence of the plant and soil N status. On the other side, grassland ecosystem models simulate the ecosystem N dynamics, but until now NH 3 biosphere–atmosphere exchange was only treated in a very simple manner. A two-layer resistance model for NH 3 exchange has thus been combined with the grassland ecosystem model PaSim in order to link NH 3 exchange with the ecosystem N dynamics. For this purpose, the plant substrate N pool in previous versions of PaSim has been divided between apoplastic and symplastic compartments, and the apoplastic substrate N pool has been linked to the stomatal NH 3 exchange. In addition, soil ammoniacal N (NH x ) has been partitioned between the soil surface and several soil layers, and the soil surface NH 3 exchange has been linked to the soil surface ammonium (NH 4 +). The new combined model has been parameterised and applied to an intensively managed grassland site in Southern Scotland. The comparison with micrometeorological measurements of NH 3 fluxes has shown that the model can qualitatively reproduce the effects of cutting and fertilisation on the net NH 3 exchange above the canopy. In particular, the model reproduces the expected strong coupling of the NH 3 exchange with the dynamics of the apoplastic substrate N pool. However, peak NH 3 emissions are underestimated, and it is postulated that this could be related to the representation of leaf litter emissions from the soil surface in the model, and to the simulated soil NH x dynamics. Nevertheless, this new version of PaSim is a valuable tool for investigating the influence of different management scenarios or of climate change on NH 3 exchange.

15N MEASUREMENTS OF AMMONIUM AND NITRATE UPTAKE BY ULVA FENESTRATA (CHLOROPHYTA) AND GRACILARIA PACIFICA (RHODOPHYTA): COMPARISON OF NET NUTRIENT DISAPPEARANCE, RELEASE OF AMMONIUM AND NITRATE, AND 15N ACCUMULATION IN ALGAL TISSUE1

Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by 15N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen&gt; 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH4+ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw−1·h−1 in U. fenestrata and 32 μmol N·g dw−1·h−1 in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH4+ release rate of 11 μmol N·g dw−1·h−1. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH4+ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH4+ disappearance and 15N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH4+ concentrations, provided that isotope dilution is taken into account. On average, 96% of added 15NH4+ was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO3− was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO3− pools) or physiological/metabolic changes required before the uptake of NO3−. During the subsequent days, NO3− uptake rates were similar for the two macroalgae but much lower than NH4+ uptake rates (1.97–3.19 μmol N·g dw−1·h−1). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added 15NO3− was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH4+ uptake experiments conducted with N‐rich macroalgae and that determination of 15N atom % enrichment of the dissolved NH4+ is recommended to avoid poor isotope recovery and underestimation of uptake rates.

Leaf–atmosphere NH3 exchange of white clover (Trifolium repens L.) in relation to mineral N nutrition and symbiotic N2 fixation

Plant-atmosphere NH(3) exchange was studied in white clover (Trifolium repens L. cv. Seminole) growing in nutrient solution containing 0 (N(2) based), 0.5 (low N) or 4.5 (high N) mM NO(3)(-). The aim was to show whether the NH(3) exchange potential is influenced by the proportion of N(2) fixation relative to NO(3)(-) supply. During the treatment, inhibition of N(2) fixation by NO(3)(-) was followed by in situ determination of total nitrogenase activity (TNA), and stomatal NH(3) compensation points (chi(NH(3))) were calculated on the basis of apoplastic NH4(+) concentration ([NH4(+)]) and pH. Whole-plant NH(3) exchange, transpiration and net CO(2) exchange were continuously recorded with a controlled cuvette system. Although shoot total N concentration increased with the level of mineral N application, tissue and apoplastic [NH4(+)] as well as chi(NH(3)) were equal in the three treatments. In NH(3)-free air, net NH(3) emission rates of <1 nmol m(-2) s(-1) were observed in both high-N and N(2)-based plants. When plants were supplied with air containing 40 nmol mol(-1) NH(3), the resulting net NH(3) uptake was higher in plants which acquired N exclusively from symbiotic N(2) fixation, compared to NO(3)(-) grown plants. The results indicate that symbiotic N(2) fixation and mineral N acquisition in white clover are balanced with respect to the NH4(+) pool leading to equal chi(NH(3)) in plants growing with or without NO(3)(-). At atmospheric NH(3) concentrations exceeding chi(NH(3)), the NH(3) uptake rate is controlled by the N demand of the plants.

Microbial activity and nitrogen mineralization in forest mineral soils following heating: evaluation of post-fire effects

Heat generated during fire induces chemical oxidation of soil organic matter thereby altering carbon (C) and nitrogen (N) transformations. Prior soil fire history and soil moisture content at the time of heating can be confounding factors in the interpretation of the influence of heat on soil processes. In this study we evaluated how soil heating (160 and 380°C) under three moisture regimes (−0.03, −1.0, and −1.5 MPa) influences microbial activity and N mineralization in two soils: (1) not exposed to fire for the past 80 years, (2) recently exposed to wildfire. Initially, the fire exposed soil had lower basal respiration rates and lower concentrations of microbial biomass C, potentially mineralizable nitrogen (PMN), soluble hexose sugars, and NH 4 +–N, but higher NO 3 −–N concentrations than the soil not exposed to fire. Both soils responded similarly to elevated temperatures. Higher temperatures resulted in greater microbial mortality and a greater release of soluble sugars and NH 4 +–N. PMN concentrations increased at 160°C, but decreased at 380°C in both soils. The highest NH 4 +–N concentrations were observed in soils not previously exposed to fire that were incubated at −0.03 MPa after heating. Soils previously exposed to fire had low NH 4 +–N concentrations and high NO 3 −–N concentrations. Heating at low soil water potentials resulted in elevated concentrations of microbial biomass C and soluble sugars, and lower NH 4 +–N and NO 3 −–N concentrations. Initial C availability appeared to be an important factor in the recovery of microbial biomass during 14-d post-heating incubation, which was greatest after heating at 380°C and −1.5 MPa. Both soils demonstrated slow rates of recovery of nitrifying organisms despite high rates of net NH 4 +–N accumulation. It appears that low soil water potential at the time of heat exposure reduces losses of mineralizable N.

Laboratory methods of estimating potentially mineralizable nitrogen in organic potting mixes. I. Development of incubation and chemical methods

Organic potting mixes supply plant nutrients through the mineralization of organic matter. However, laboratory methods of estimating mineralization in soilless potting mixes have not been developed. Using 100 potting mix formulations, this study investigated whether an anaerobic incubation method or two chemical methods (alkaline KMnO4 and HCl) could be used to predict potentially mineralizable N (PMN) in organic potting mixes, using N uptake by ryegrass herbage as the benchmark method of estimation of PMN. The anaerobic incubation method failed to produce NH4 + from many mixes, and was considered unsuitable as a PMN test. The alkaline KMnO4 and HCl methods produced NH4 + in all cases, and correlation between net NH4 + production and ryegrass herbage N was good (r=0.66, P<0.001 and r=0.80, P<0.001, respectively). However, when initially available N was included with the NH4 + produced by the tests, correlations with ryegrass herbage N were improved (r=0.95, P<0.001 and r=0.92, P<0.001 respectively). It was concluded that the two chemical methods, modified to include initially available N, have potential as laboratory methods of estimating PMN in organic potting mixes.

High Immobilization of NH4+ in Danish Heath Soil Related to Succession, Soil and Nutrients: Implications for Critical Loads of N

During recent decades heathlands havechanged into grasslands in regions with high atmosphericnitrogen deposition. In regions with intermediatedeposition level (e.g., Denmark) changes have been lesspronounced which may be due to delay or decrease inresponse of the ecosystem. The mor layer (O horizon) mayplay an important role for this delay due to high sinkstrength for N. In this study, the capacity for netNH4+ immobilization and mineralization wasstudied during short- and long-term incubations (2–36 days)of mor samples from Danish dry inland heaths. High short-term capacity for net NH4+ immobilization wasfound to be a general characteristic of Danish heath morlayers both under heather (Calluna vulgaris) andcrowberry (Empetrum nigrum ssp nigrum), the latterdominating late stages in heathland succession. The netNH4+ immobilization was higher under youngcompared to old or dead vegetation, and higher on lessnutrient poor soils than on extremely nutrient poor soils.The addition of N, P and C stimulated CO2 productionand net NH4+ immobilization, but not net Nmineralization. The immobilization of 15NH4+caused release of dissolved organic N, increased N anddecreased C/N ratio in the microbial biomass, and indicatedgrowth of microorganisms with other metabolic abilitiesthan the indigenous population. No evidence was obtained ofstabilization of immobilized 15NH4+ intosoil organic matter during the experiment. On background ofthe results and current knowledge it was concluded that therecognition of the high capacity for net NH4+immobilization in mor layers does not allow for a raiseof critical loads for N for northern dry inland heaths.

Stomatal compensation points for ammonia in oilseed rape plants under field conditions

Compensation points for gaseous exchange of ammonia (NH 3) between stomata and the atmosphere were determined in an oilseed rape ( Brassica napus) canopy by analysing the concentrations of NH 4 + and H + in leaf apoplastic solution. This bioassay approach was applied for the first time in the field, allowing the first intercomparison with compensation points derived from micrometeorological measurements. Apoplastic NH 4 + and H + concentrations differed between leaf heights but values were relatively stable over time, both diurnally and during a 2-week period. Stomatal NH 3 compensation points calculated on the basis of apoplastic NH 4 + and H + concentrations and corrected for ambient leaf temperatures were found to correlate positively with the net NH 3 emission from the canopy estimated by micrometeorological measurements. As there was little diurnal variability in apoplastic concentrations, this correlation was largely due to the effect of temperature on NH 3 solubility and NH 4 + dissociation in the apoplast, together with similar effects of temperature on the net NH 3 flux. Very high NH 4 + concentrations were also found in extracts of fallen litter and resulted in NH 3 partial pressures significantly exceeding NH 3 levels in the atmosphere close to the ground. By comparison of vertical atmospheric NH 3 concentration profiles in the plant canopy with the stomatal NH 3 compensation points determined here at three different plant heights, as well as NH 3 partial pressures in the litter, it is shown that plant residues on the soil surface would have been the primary NH 3 source while attached leaves acted as an NH 3 sink. Although it was not possible to measure apoplastic concentrations of siliques (seed cases), bulk tissue NH 4 +/H + concentrations and vertical atmospheric NH 3 concentration profiles indicate that these may have acted as an NH 3 source.

Carbon and nitrogen mineralization in sediments of the Bangrong mangrove area, Phuket, Thailand

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 22:199-213 (2000) - doi:10.3354/ame022199 Carbon and nitrogen mineralization in sediments of the Bangrong mangrove area, Phuket, Thailand Erik Kristensen1,*, Frede Ø. Andersen1, Nikolaj Holmboe1, Marianne Holmer1, Nalinee Thongtham2 1Institute of Biology, Odense University, SDU, 5230 Odense M, Denmark 2Phuket Marine Biological Center, PO Box 60, Phuket 83000, Thailand *E-mail: ebk@biology.ou.dk ABSTRACT: Carbon and nitrogen mineralization were determined along a transect from a mangrove forest to a seagrass meadow in the Bangrong area, Phuket Island, Thailand. Vertical sediment profiles of carbon oxidation were measured as sulfate reduction rates (SRR) using the 35S technique and by monitoring net TCO2 and DOC production and Fe(III) reduction using anaerobic sediment incubations (Œjar¹ technique). Nitrogen transformations were measured simultaneously as net NH4+ and DON production. In addition, total benthic metabolism and net nitrogen exchange were determined as fluxes of O2, TCO2, DOC, and DIN (NO3- and NH4+) across the sediment-water interface. Rates of carbon and nitrogen transformations in this vascular-plant (high C:N)-dominated area were low compared with areas fuelled by detritus of marine origin (low C:N). It appears that the high content of structural biopolymers (e.g. lignocelluloses) hampers microbial activity. Suboxic respiration with Fe(III) as electron acceptor accounted for 70 to 80% of the total carbon oxidation in the rooted mangrove forest sediment, whereas SRR and aerobic respiration were responsible for about 20 and <6%, respectively. The role of SRR decreased to about 10% and aerobic respiration increased to 45-65% in an adjacent bioturbated mudflat, while Fe(III) respiration decreased to 30-40%. At the sand flat and seagrass meadow outside the mangrove forest, Fe(III) respiration only accounted for 15 and ~0%, respectively, whereas SRR was responsible for 20 to 45% of the total carbon oxidation. However, the most important electron acceptor in the area outside was oxygen (55 to 75%). The shift in dominance of electron acceptors along the transect is primarily related to the presence of roots and infauna, but the sediment composition (e.g. grain size, organic content and iron content) is believed to be an important co-factor. The net production of ammonium in the sediment was not balanced by fluxes of DIN across the sediment-water interface. The missing nitrogen was assigned to a rapid and efficient bacterial ammonium assimilation at the sediment surface as indicated by ammonium turnover times of about 1 d. KEY WORDS: Mangrove forest · Seagrass · Carbon · Nitrogen · Mineralization · Sulfate reduction · Fe(III) reduction · Benthic metabolism Full text in pdf format PreviousExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 22, No. 2. Online publication date: September 08, 2000 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2000 Inter-Research.

Efficacy of laidlomycin propionate in low-protein diets fed to growing beef steers: effects on steer performance and ruminal nitrogen metabolism.

We conducted two experiments to evaluate the effect of the ionophore laidlomycin propionate (LP) on steer performance and ruminal N metabolism. Experiment 1 was a 91-d growth study evaluating the growth and ruminal characteristics of steer calves consuming supplemental LP. Steers (n = 96; 255 +/- 3 kg; four steers/pen; six pens/treatment) were used in a randomized complete block design with a 2 x 2 factorial arrangement of treatments consisting of two levels of dietary CP (formulated to be 10.5 and 12.5% of DM) with and without LP (11 mg/kg diet DM). Ruminal fluid was collected via stomach tube on d 91 from one steer randomly selected from each pen. No CP x LP interactions were observed with performance data (P > .64). Final weight and total gain were greater (P < .07) for 12.5% CP and LP compared with 10.5% CP and control steers, respectively. Also, DMI was increased (P = .08) with 12.5% CP but not with LP supplementation (P = .36). In addition, ADG and gain:feed ratio were greater (P < .03) for both 12.5% CP and supplemental LP. Ruminal NH3 N concentration was greater (P < .09) with 12.5% CP and LP. Total VFA concentration and molar proportion of acetate were not affected by treatment (P > .11). However, propionate concentration was increased (P < .09) with 12.5% CP and LP, and acetate:propionate was lower (P = .02) with LP supplementation. In Exp. 2, six steers were used in a replicated 3 x 3 Latin square design to compare ruminal fermentation and protein degradation in steers without ionophore feeding or adapted to LP or monensin. In vitro deamination of amino acids by adapted ruminal microbes was also assessed. Ionophore supplementation decreased (P = .07) ruminal NH3 N concentration compared with control steers, and LP increased (P = .02) ruminal NH3 N compared with monensin. Molar proportion of acetate was decreased (P = .02) and propionate increased (P = .01) with ionophore treatment. Consequently, ionophore supplementation depressed the acetate:propionate ratio (P = .01). In situ degradation rate of soybean meal (SBM) CP was greater (P = .09) with ionophore treatment, but estimates of SBM undegradable intake protein were not altered by treatment (P > .25). Microbial specific activity of net NH3 N release and alpha-amino N degradation were decreased (P < .04) with ionophores. Based on this study, LP and monensin did not affect the extent of ruminal degradation of SBM CP but decreased amino acid deamination.