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Abstract Microbial oxidation of ammonia controls the rate of nitrification in the majority of soils. Both nitrification rate and the composition of communities of ammonia oxidising archaea (AOA) and ammonia oxidising bacteria (AOB) are influenced by drought, with evidence that AOA are more sensitive to periods of drought than AOB. This has been explained by greater sensitivity of AOA to ammonia concentration, which will increase in soil solution during drought, but an alternative, previously unexplored explanation, is greater sensitivity of AOA to matric and/or osmotic stress. A soil microcosm experiment was designed to distinguish these different explanations in which AOA and AOB abundances (amoA abundance) and nitrification rate were measured over 28 days in nine treatments corresponding to all combinations of three soil matric potentials and three initial ammonia concentrations. Comparison of amoA abundance dynamics suggested that AOA were more susceptible to reduced matric potential than AOB, irrespective of soil ammonia concentration. The greater sensitivity of soil AOA to osmotic stress was also tested in 10-day cultures of representative strains of AOA and AOB in liquid medium containing different concentrations of NaCl and sorbitol as osmo-inducer. AOA were significantly more sensitive to osmotic stress than AOB. These results provide evidence for greater sensitivity of AOA than AOB to both components of water stress, matric and osmotic potential, representing an additional niche differentiation between these two essential groups of ammonia oxidisers. Highlights • NH 3 -oxidising archaea (AOA) more sensitive to drought than NH 3 -oxidising bacteria (AOB). • Water stress more important than NH 3 conc. in explaining effects of drought. • Pure culture studies supported greater sensitivity of AOA to water stress. • Evidence that desiccation tolerance is niche differentiation factor for AOA and AOB. • Findings challenge proposals that NH 3 conc. explains AO responses to drought. [ABSTRACT FROM AUTHOR]

The nitrous oxide and molecular N emissions from 5-cm length subsamples taken from 20-cm length sample corers containing eutric Cambisol soil fertilised either with urea, ammonium or nitrate for 1 year have been examined using gas chromatography. At the beginning of the incubation, the same N rate (260 kg N/ha) was added to the soil and kept constant during the experiment. The total abundance of the soil Bacteria and Archaea and that of nitrifiers and denitrifiers was estimated by quantitative PCR of the corresponding biotic variables 16S rRNA, amoA and napA, narG, nirK, nirS, norB, nosZI and nosZII genes. The abiotic variables dissolved oxygen, pH, exchangeable NH4+-N and NO3−-N contents and total C and total N were also analysed. None of the three fertilisers affected the total abundance of Bacteria and Archaea and nitrification was the main driver of nitrous oxide production in the 0- to 5-cm and 5- to 10-cm soil layers while denitrification was in the 10- to 15-cm and 15- to 20-cm soil horizons. Parallel to the reduction in the content of dissolved oxygen along the soil profile, there was a decrease in the total and relative abundance of the bacterial and archaeal amoA gene and an increase in the abundances of the denitrification genes, mainly in the 10- to 15-cm and 15- to 20-cm soil layers. A non-metric multidimensional scaling plot comparing the biotic and abiotic variables examined in each of the four 5-cm soil subsamples and the whole 20-cm sample showed a disparate effect of N fertilisation on N gas emissions and abundance of nitrifiers and denitrifiers bacterial and archaeal communities. [ABSTRACT FROM AUTHOR]

Ammonia oxidation, the primary and rate-limiting step of nitrification, is mediated by both ammonia-oxidizing archaea (AOA) and bacteria (AOB). However, the dominant environmental driver of the activity, abundance, diversity and composition of ammonia-oxidizers has not been well understood enough, and the relative contribution of AOA and AOB to nitrification is still under debate. Soils treated with different fertilization regimes in an over 35-year field experiment were collected to explore the variation in function and structure of ammonia-oxidizer communities and corresponding driver. The highest nitrification activity (44.49 mg N kg −1 soil d −1 ) was found in only organic fertilizer (O) treated soil, whereas the lowest activity (2.81 mg N kg −1 soil d −1 ) was observed in only mineral fertilizer (NPK) treated soil. Moreover, as 1-octyne was employed to discriminate AOA- and AOB-supported nitrification, AOA dominated (93.53%) the nitrification in the Control soil, while AOB contributed dominantly (84.73–89.10%) in all of the organic amended soils, and NPK-treated soil showed an almost equal contribution to AOA (45.79%) and AOB (54.21%). Compared with the Control soil, AOA abundance increased in soils with organic and low chemical fertilizer but decreased in only chemically treated soil, whereas the AOB abundance in all fertilized soils was greatly enhanced. The AOA activity was linearly dependent on AOA abundance, whereas the AOB activity was exponentially correlated with AOB abundance. The sequences of AOA and AOB in the Control soil were mostly affiliated with group I.1b thaumarchaeota and genus Nitrosospira clusters 3a.1. Soil treated with NPK increased the abundance of AOA that belonged to group I.1a-associated lineage, whereas more abundant AOB was related to Nitrosospira clusters 3a.2 and 8b. In contrast, the O-treated soil showed more abundant AOB that belonged to Nitrosospira clusters 3b and 8b. As revealed by aggregated boosted tree analysis, the soil ammonium (NH 4 + ) content was identified as the dominant driver of activity and diversity of AOA, and soil pH was considered to be the major influencing factor in abundance and composition of AOA; the AOB composition was mainly affected by soil NH 4 + content, the relative activity and diversity by soil pH, and the relative abundance by soil electrical conductivity (EC). Collectively, different fertilization regimes will result in variations in activity, abundance, diversity and composition of ammonia-oxidizers with distinct drivers. Our research could be helpful to identify better strategies for the mitigation of nitrate production in agricultural soils. [ABSTRACT FROM AUTHOR]

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH ) and nitrate (NO ). Previous long-term studies indicate efficient turnover of atmogenic NH to NO in weathered, acidic soils of the subtropics, leading to excessive NO leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. NHNO, NHNO and N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (NO) were monitored for nine days. Incorporation of NO into soil organic N was negligible and 80% of the applied label was lost from the top soil (0-15 cm) primarily by leaching within 9 days. In contrast, NH was largely retained in soil organic N. However, instant production of NO in the NH treatment suggested active nitrification. In both the NH and N-glutamic acid treatments, the N enrichment in the NO pool exceeded that in the NH pool one day after N application, suggesting preferential nitrification of added NH with subsequent dilution of the NH pool and/or immobilization of NH followed by heterotrophic nitrification. The cumulative recovery of N in NO after 9 days ranged from 2.5 to 6.0% in the NO treatment, confirming the previously reported significant response of NO emission to N deposition. Source partitioning of NO demonstrated a measurable contribution of nitrification to NO emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO and NH ) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO in the soil and (ii) rapid immobilization-mineralization of NH followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and NO emission. [ABSTRACT FROM AUTHOR]

High concentrations of ammonium (NH 4 + ) in soil have been shown to inhibit nitrification, and fertilizer injection as conducted during CULTAN (controlled uptake long-term ammonium nutrition) management might thus have the potential to reduce N 2 O emission from arable soil. We conducted an incubation experiment with different NH 4 + concentrations in soil that resembled concentrations as expected at and around injection spots (5000, 2250, 1000, 450, 0 mg NH 4 + -N kg −1 soil) directly after fertilization and after dilution due to plant uptake or precipitation. N 2 O emission was measured in dynamic soil mesocosms over a period of 21 days. Acetylene inhibition and 15 N tracer approaches were used to calculate the relative contribution of nitrification and denitrification to N 2 O emission. An isotopomer approach was applied to gain further insight into N 2 O producing processes. We expected lower contribution of nitrification-derived N 2 O to total N 2 O emission and a higher N 2 O/NO 3 − ratio from nitrification with increasing NH 4 + levels. Nitrification indeed declined with increasing NH 4 + level, and no nitrification occurred in the 5000 mg NH 4 + -N kg −1 soil treatment. A pool dilution approach showed that gross nitrification in 450 mg NH 4 + -N kg −1 soil (nitrification rate: 4.96 mg NO 3 − -N kg soil d −1 ) was by a factor of 2.6 and 6 higher than in 1000 and 2250 mg NH 4 + -N kg −1 soil treatments. In the 5000 mg NH 4 + -N kg −1 soil treatment, gross nitrification occurred at very small rates (0.1 mg NO 3 − -N kg soil d −1 ). Similarly, N 2 O emission declined with increasing NH 4 + level. The N 2 O yield of nitrification was between 0.07 and 0.15% of NO 3 − production, but was not affected by increasing NH 4 + level. Nitrification was the dominant source of N 2 O throughout the incubation at all NH 4 + levels, and there was no significant change in the relative contribution of nitrification and denitrification with NH 4 + level or time. This finding indicates that denitrification derived N 2 O emissions were similarly reduced at high NH 4 + levels. Applying the non-equilibrium technique to our 15 N tracer data revealed heterogeneous distribution of denitrification in soil, with at least two distinct NO x − (NO 3 − + NO 2 − ) pools and spatial separation of NO x − formation and consumption. The isotopomer approach provided reasonable results in comparison with the acetylene inhibition and 15 N tracer approaches and indicated substantial contribution of nitrifier denitrification and/or coupled nitrification-denitrification (10–40%) to total N 2 O production. [ABSTRACT FROM AUTHOR]

In our study at Mt. Kilimanjaro, East Africa, we quantified gross rates of ammonification, nitrification, nitrogen immobilization, and dissimilatory nitrate reduction to ammonium in soils across different land uses, climate zones (savanna, montane forest ecosystems, extensive agroforest homegarden, and intensively managed coffee plantation), and seasons (dry, wet, and transition from dry to wet season) to identify if and to what extent conversion of natural ecosystems to cultivated land has affected key soil microbial nitrogen turnover processes. Overall variation of gross soil nitrogen turnover rates across different ecosystems was more pronounced than seasonal variations, with the highest turnover rates occurring at the transition between dry and wet seasons. Nitrogen production and immobilization rates positively correlated with soil organic carbon and total nitrogen concentrations as well as substrate availability of dissolved organic carbon and nitrogen r > 0.67, P < 0.05), but did not correlate with soil ammonium and nitrate concentrations. Soil nitrogen turnover rates were highest in the montane Ocotea forest (ammonification 29.84, nitrification 12.67, NH immobilization 38.92, NO immobilization 10.74, and DNRA 1.54 µg N g SDW d) and progressively decreased with decreasing annual rainfall and increasing land-use intensity. Using indicators of N retention and characteristics of soil nutrient status, we observed a grouping of faster, but tighter N cycling in the (semi-) natural savanna and Ocotea forest. This contrasted with a more open N cycle in managed systems (the homegarden and coffee plantation) where N was more prone to leaching or gaseous losses due to high nitrate production rates. The partly disturbed (selected logging) lower montane forest ranged between these two groups. [ABSTRACT FROM AUTHOR]

The impact of temperature on bacterial processes is well known; however temperature related data on nitrification rates in aquaculture systems are fragmented and compiled from different studies. We sought to determine ammonium and nitrite removal kinetics over a temperature range from 6 to 36 °C by using moving bed bio-elements from a freshwater RAS in steady state operated at 18 °C. The impact of temperature on ammonium and nitrite oxidation rates was evaluated by transferring the colonized bio elements to six liter batch reactors (triplicated setup). Each reactor was acclimatized for 24 h at each of the six temperatures (6, 12, 18, 24, 30 and 36 °C) and then spiked with ammonium chloride or sodium nitrite under identical conditions. The average surface specific TAN removal (STR) increased a six-fold from 6 to 30 °C (0.04–0.25 g TAN m −2 day −1 ) and dropped significantly at 36 °C–0.14 g TAN m −2 day −1 . The surface specific nitrite removal (SNR) increased linearly from 0.04 g N m −2 d −1 at 6 °C–0.14 g N m −2 day −1 at 30 °C, decreasing to 0.12 g N m −2 day −1 at 36 °C. Throughout the temperature range tested, STR remained significantly larger than SNR. The temperature coefficient, Ɵ (6–30 °C) for ammonium oxidation was 1.079; for nitrite oxidation the temperature coefficient was found to be 1.054. The data provided by this study can be applied dimensioning future RAS that utilizes temperature ranges below 10 and above 30 °C. [ABSTRACT FROM AUTHOR]

Inhibition of nitrification of ammoniacal nitrogen pig slurry after its application to the soil can mitigate nitrogen (N) losses by nitrate (NO3-) denitrification and leaching, with economical and environmental benefits. However, the use of this strategy is incipient in Brazil and, therefore, requires further assessment. The aim of this study was to evaluate the efficiency of dicyandiamide (DCD) nitrification inhibitor in slowing the nitrification of ammoniacal N applied to the soil with pig slurry (PS). For this, incubation was performed in laboratory, where nitrification was assessed by NO3- accumulation in the soil. Rates of 2.8, 5.7 and 11.3 kg DCD ha-1 were compared, being applied to the soil during PS addition. Nitrification was inhibited by DCD, and inhibition magnitude and duration depended on DCD applied rate. At a dose of 11.3kg ha-1 DCD, nitrification was completely inhibited in the first 12 days. During the first month after PS application, each 2.8 kg of DCD increase applied per hectare promoted NO3--N reduction in the soil of 13.3 kg ha-1, allowing longer ammoniacal N maintenance in the soil. [ABSTRACT FROM AUTHOR]

Oxygen concentration is considered to be the most important factor influencing nitrification and mineralization rates in agricultural soils. However, the sensitivities of nitrification and N mineralization in paddy soils to oxygen concentrations are not well known. We examined nitrification activities and N mineralization rates of six paddy soils with pH ranging from 5.23 to 7.83 and incubated at 25°C and 60% water-holding capacity in laboratory after ammonium was added at concentrations of 10, 30 and 50 mg N kg−1of soil and the headspace gases were replaced with stock gases whose oxygen concentrations were 20%, 10% and 2%, respectively. The tested paddy soils had a very wide range of nitrification activities so that the nitrate ratio in inorganic N varied from > 95% after 1 day incubation to < 25% after 7 days of incubation. The nitrate ratio correlated with the soil pH. Nitrate content and its ratio, and mineralization rate were not suppressed when the oxygen concentration in the headspace decreased from 20% to 2%. Our results suggested that nitrifiers and microbial communities involved in N mineralization might have adapted to the environment with low oxygen concentration in paddy soils in which oxygen concentration is deficient during the flooding period. However, oxygen concentration would be even lower than 2% in paddy soils under the flooding conditions. Thus, the minimum oxygen concentration at which nitrification activities in paddy soils are suppressed needs to be explored further. [ABSTRACT FROM AUTHOR]

Little is known about the effects of nitrogen (N) fertilization rates on ammonia-oxidizing bacteria (AOB) and archaea (AOA) and their differential contribution to ammonia oxidation, particularly in greenhouse-based high N input vegetable soils. Potential ammonia oxidation (PAO) of these vegetable soils was evaluated under five levels of N fertilization (with urea) in the presence or absence of the bacterial protein synthesis inhibitors kanamycin and gentamicin. Abundance and community composition of AOB and AOA were evaluated by quantitative polymerase chain reaction (PCR) and clone libraries. The five annual N fertilization rates studied were 100% (N870: 300, 270 and 300 kg N ha −1 for tomato, cucumber and celery, respectively), 80% (N696), 60% (N522), 40% (N348) and 0% (N0) of the conventional N rate. The PAO decreased significantly with increasing N fertilization rates irrespective of the presence of kanamycin plus gentamicin. PAO was significantly lower in the presence than in the absence of kanamycin plus gentamicin, and was decreased by 71.9% under N0, 77.2% under N348, 54.9% under N522, 49.9% under N696, and 51.6% under N870. The abundance of bacterial and archaeal amoA genes was significantly decreased at the highest N fertilization rate. The clone sequences of AOB and AOA were mostly affiliated with the genus Nitrosospira and group 1.1b thaumarchaeota. Changes in community composition were more pronounced in AOB than in AOA after long-term (6-year) N fertilization. Our results suggest that AOB may play a more important role than AOA in NH 3 oxidation in such high N input vegetable soils. [ABSTRACT FROM AUTHOR]

Future climate change is predicted to influence soil moisture regime, a key factor regulating soil nitrogen (N) cycling. To elucidate how soil moisture affects gross N transformation in a cultivated black soil, a 15 N tracing study was conducted at 30%, 50% and 70% water-filled pore space (WFPS). While gross mineralization rate of recalcitrant organic N (N rec ) increased from 0.56 to 2.47 mg N kg −1 d −1 , the rate of labile organic N mineralization declined from 4.23 to 2.41 mg N kg −1 d −1 with a WFPS increase from 30% to 70%. Similar to total mineralization, no distinct moisture effect was found on total immobilization of ammonium, which primarily entered the N rec pool. Nitrate (NO 3 − ) was mainly produced via autotrophic nitrification, which was significantly stimulated by increasing WFPS. Unexpectedly, heterotrophic nitrification was observed, with the highest rate of 1.06 mg N kg −1 d −1 at 30% WFPS, contributing 31.8% to total NO 3 − production, and decreased with WFPS. Dissimilatory nitrate reduction to ammonium (DNRA) increased from near zero (30% WFPS) to 0.26 mg N kg −1 d −1 (70% WFPS), amounting to 16.7–92.9% of NO 3 − consumption. A literature synthetic analysis from global multiple ecosystems showed that the rates of heterotrophic nitrification and DNRA in test soil were comparative to the forest and grassland ecosystems, and that heterotrophic nitrification was positively correlated with precipitation, soil organic carbon (SOC) and C/N, but negatively with pH and bulk density, while DNRA showed positive relationships with precipitation, clay, SOC, C/NO 3 − and WFPS. We suggested that low pH and bulk density and high SOC and C/N in test soil might favor heterotrophic nitrification, and that C and NO 3 − availability together with anaerobic condition were crucial for DNRA. Overall, our study highlights the role of moisture in regulating gross N turnover and the importance of heterotrophic nitrification for NO 3 − production under low moisture and DNRA for NO 3 − retention under high moisture in cropland. [ABSTRACT FROM AUTHOR]

Invasive plants often benefit from changes that they impose on soil microbes via positive plant–soil feedback, but the mechanisms that underlie these changes, and the legacy of their effects, remain poorly quantified. We investigated the impacts of an invasive annual grass, Microstegium vimineum , on the structure and functioning of soil microbial communities in a multi-year, field-based common garden experiment. Given previous reports that M . vimineum can both elevate nitrification rates in soil and benefit from enhanced nitrate availability, we sought to answer the following questions: 1) Does M . vimineum alter the abundance or composition of soil nitrifying microbial communities (ammonia oxidizing archaea and bacteria, AOA and AOB, respectively)? 2) Are such effects reversible or do soil legacy effects persist after M . vimineum is no longer present? After three years, invaded plots had greater AOA abundances than uninvaded native dominated plots, as well as different AOA community structure. However, after seven years, and following a period of M . vimineum replacement by native plants in the invaded plots, AOA abundances and nitrification rates declined toward levels found in uninvaded plots. Collectively, our results suggest that while the impacts of M . vimineum invasions on nitrogen cycling likely relate to their association with AOA, these effects may not persist if M . vimineum declines over time and native plants and their associated microbes are able to re-establish. [ABSTRACT FROM AUTHOR]

Adding biochar into soils has potential to manipulate soil nitrification process due to its impacts on nitrogen (N) cycling, however, the exact mechanisms underlying the alteration of nitrification process in soils are still not clear. Nitrification in an acidic orchard soil amended with peanut shell biochar (PBC) produced at 400 °C was investigated. Nitrification was weakened by PBC addition due to the decreased NH 4 + -N content and reduced ammonia-oxidizing bacteria (AOB) abundance in PBC-amended soils. Adding phenolic compounds (PHCs) free biochar (PBC-P) increased the AOB abundance and the DGGE band number, indicating that PHCs remaining in the PBC likely reduced AOB abundance and diversity. However, PBC addition stimulated rape growth and increased N bioavailability. Overall, adding PBC could suppress the nitrification process and improve N bioavailability in the agricultural soils, and thus possibly mitigate the environmental negative impacts and improving N use efficiency in the acidic soils added with N fertilizer. [ABSTRACT FROM AUTHOR]

A field experiment was initiated whereby a series of replicated plots received annual applications of ivermectin, monensin, and zinc bacitracin, either singly or in a mixture. Pharmaceuticals were added at concentrations of 0.1 mg/kg soil or 10 mg/kg soil. The authors collected soil samples in 2013, before and after the fourth annual application of pharmaceuticals. In addition, a 30-d laboratory experiment was undertaken with the same soil and same pharmaceuticals, but at concentrations of 100 mg/kg soil. The impact of the pharmaceuticals on nitrification rates, on the abundance of ammonia-oxidizing bacteria (AOB), and on the abundance of ammonia-oxidizing archaea (AOA) was assessed. None of the pharmaceuticals at 0.1 mg/kg had any effect on nitrification. Referenced to control soil, nitrification was accelerated in soil exposed to 100 mg/kg zinc bacitracin or 10 mg/kg of the pharmaceutical mixture, but none of the treatments inhibited nitrification. Neither AOB abundance nor AOA abundance was affected by the pharmaceuticals at 0.1 mg/kg. At 10 mg/kg, monensin, zinc bacitracin, and a mixture of all 3 pharmaceuticals suppressed the abundance of AOB, and zinc bacitracin and the mixture increased AOA abundance. The decrease in AOB abundance and increase in AOA abundance when exposed to 10 mg/kg soil suggests that AOB are more sensitive to these chemicals and that AOA populations can expand to occupy the partially vacated niche. Environ Toxicol Chem 2015;34:618-625. © 2014 Canadian Crown in right of Canada. Published 2014 SETAC [ABSTRACT FROM AUTHOR]

Field studies demonstrate increased ammonia (NH3) abatement efficiencies (up to 90%) of subsurface application and incorporation of animal manures compared to conventional surface application methods. However, a portion of the nitrogen (N) conserved may come at the expense of increased nitrous oxide (N2O) emissions produced during denitrification and nitrification processes in the soil. Research of N2O emissions subsequent to subsurface manure application is limited (particularly in the U.S.) and results are variable. Cumulative N2O emissions typically range from 0.1% to 3% of the total N applied following manure injection; however, only a few studies indicate a statistical difference in N2O emissions when compared to surface application methods. Circumstances attributed to increased emissions vary among studies and include the concentration of readily metabolizable water-soluble carbon (C) in manure slurry compared to background levels in the soil, soil moisture conditions pre- and post-subsurface application that drive nitrification and denitrification processes, localized N form and oxygen (O2) concentration at the injection site, and injection depth, which can dictate the length of the diffusion path of N2O to the atmosphere. Research also draws correlations of N2O emission rates with subsurface application method, atmospheric conditions during and subsequent to application, and solids separation and dilution of manure slurry. Results thus far suggest that increased N2O emission is not an inevitable consequence of NH3 abatement strategies and should not dissuade subsurface manure application practices. Furthermore, potential reductions in indirect N2O emissions may be realized with subsurface application methods when compared to high NH3 volatilization and N deposition associated with surface application methods. Considering these tradeoffs and the ongoing debate on the role of anthropogenic greenhouse gases such as N2O and their role in climate change warrants more investigation of the factors that contribute to N2O emissions. This review synthesizes the available research on N2O emissions following subsurface application of animal manure to both grassland and arable land, compares results from different application techniques, and examines the conditions and circumstances that lead to N2O emissions. [ABSTRACT FROM AUTHOR]

This work is dedicated to the biological treatment of wastewater with increased salinity using a combination of partial nitritation and Anammox processes. Two one-stage deammonification moving bed biofilm reactors were operated with the increase in NaCl concentration every two weeks by 5 and 2.5 g/L. The strategy with a step of 5 g/L of salinity increase led to complete inhibition of the process at the salinity level of 15 g/L. The strategy with a step of 2.5 g/L gave possibility to adapt bacteria to the elevated salinity. After reaching the salinity level of 10 g NaCl/L, the reactor was operated during 92 days with a nitrogen removal rate of 0.39 ± 0.19 g N/(m2·day) (0.078 ± 0.038 kg N/m3·day) and an average nitrogen removal efficiency of 59%. It was shown that conductivity cannot be used for monitoring the process when a reactor is treating wastewater with increased salinity, whereas pH can be correlated to effluent ammonium concentration regardless of wastewater salinity. [ABSTRACT FROM PUBLISHER]

In the present study we investigated long-term climate imprints (160 year period) on tree-ring δ15N signatures of sessile oak (Quercus petraea) at sites with contrasting water availability (i.e. Cambisols vs. Regosols in Luxembourg, Central Europe, with 175 mm and 42 mm available water capacity, respectively). We hypothesized that tree-ring δ15N signatures constitute a sensitive indicator to long-term shifts in climatic conditions. Our findings revealed a close positive correlation between winter and spring temperatures and tree-ring δ15N signatures. These relationships were stronger for Cambisol than for Regosol sites. If entire chronologies were considered, peaks in annual mean temperatures closely corresponded with peaks in tree-ring δ15N signatures, with both annual mean temperatures and δ15N signatures reaching their maxima within the last two decades. In addition, we found a weak but significant impact of February precipitation on δ15N signatures, but only for Cambisols. We hypothesize that these findings are attributable to climate- (particularly temperature-) mediated nitrification rates in forest soils. As nitrification is a strongly fractionating process that produces 15N-depleted nitrate and higher isotopic ratios for ammonium in the topsoil, increased nitrification leads to 15N-enriched pools of Ninorg in the upper soil horizons and therefore higher δ15N signatures in plant tissues. Weaker correlations at Regosol sites were likely related to dryer and more acidic site conditions, both of which may reduce nitrification rates. Comparisons of oak and beech trees in the study area point to species-specific trajectories of wood nitrogen isotopes, likely related to the partitioning of ammonium and nitrate among species. In conclusion, tree-ring δ15N signatures may serve as an integrator of terrestrial N cycling and as such constitute a valuable tool in the identification of spatial and temporal patterns of N cycling in relation to environmental changes. Due to the mediating effects of the isotopic composition of the respective N sources, analyses of the isotopic composition of airborne N loads would support the interpretation of wood δ15N patterns, particularly in areas that are subject to high N pollution. [ABSTRACT FROM AUTHOR]

Heavy applications of ammonium-based fertilizers greatly accelerated the acidification of agricultural soils in southern China. The effects of dicyandiamide on soil pH and contents of ammonium nitrogen (NH4+-N) and [nitrate (NO3−) + nitrite (NO2−)-N] were investigated with incubation experiments to evaluate the inhibiting effect of dicyandiamide on soil acidification induced by ammonium-based fertilizers. Results showed application of urea or ammonium bicarbonate increased the pH of an acidic soil at the beginning of incubation, whereas nitrification led to a decrease in soil pH in the later stages. Dicyandiamide elevated soil pH through inhibition of nitrification. At the end of an 85-day incubation, soil pH was 1.30 and 2.20 units greater than the unfertilized sample, and 0.66 and 1.56 units greater if dicyandiamide was added with either 14.29 or 28.57 mmol N kg−1of urea. Similar results occurred for ammonium bicarbonate. Therefore, application of urea or ammonium bicarbonate with dicyandiamide reduced the soil acidification caused by ammonium-based fertilizers. [ABSTRACT FROM AUTHOR]

Appropriate remediation targets or universal guidelines for polar regions do not currently exist, and a comprehensive understanding of the effects of diesel fuel on the natural microbial populations in polar and subpolar soils is lacking. Our aim was to investigate the response of the bacterial community to diesel fuel and to evaluate if these responses have the potential to be used as indicators of soil toxicity thresholds. We set up short- and long-exposure tests across a soil organic carbon gradient. Utilizing broad and targeted community indices, as well as functional genes involved in the nitrogen cycle, we investigated the bacterial community structure and its potential functioning in response to special Antarctic blend (SAB) diesel fuel. We found the primary effect of diesel fuel toxicity was a reduction in species richness, evenness, and phylogenetic diversity, with the resulting community heavily dominated by a few species, principally Pseudomonas. The decline in richness and phylogenetic diversity was linked to disruption of the nitrogen cycle, with species and functional genes involved in nitrification significantly reduced. Of the 11 targets we evaluated, we found the bacterial amoA gene indicative of potential ammonium oxidation, the most suitable indicator of toxicity. Dose-response modeling for this target generated an average effective concentration responsible for 20% change (EC20) of 155 mg kg-1, which is consistent with previous Macquarie Island ecotoxicology assays. Unlike traditional single-species tolerance testing, bacterial targets allowed us to simultaneously evaluate more than 1,700 species from 39 phyla, inclusive of rare, sensitive, and functionally relevant portions of the community. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a greenhouse gas that contributes to the destruction of stratospheric ozone, and agricultural soil is an important source of N2O. Aerobic soils are sinks for atmospheric methane (CH4), a greenhouse gas. Ammonia monooxygenase (AMO) can oxidize CH4, but CH4is mostly oxidized by methane monooxygenase (MMO), and CH4oxidation by AMO is generally negligible in the soil. We monitored the N2O and CH4fluxes after urea application in fields containing different soils using an automated sampling system to determine the effects of environmental and microbial factors on the N2O and CH4fluxes. The soil types were Low-humic Andosol (Gleyic Haplic Andosol), yellow soil (Gleyic Haplic Alisol) and gray lowland soil (Entric Fluvisol). Cumulative N2O emissions from the yellow soil were higher than those from other soil types, although the difference was not significant. The CH4uptake level by Andosol was one order of magnitude higher than that by other soils. There were significant relationships between the ammonia oxidation potential, AOB and AOAamoAcopy numbers, and the CH4uptake. In contrast, the gene copy numbers of methane-oxidizing bacteria (MOB)pmoAwere below the detection limit. Our results suggested that the AMOs of AOB and AOA may have more important roles than those previously considered during CH4oxidation in agricultural soils treated with N fertilizers. [ABSTRACT FROM PUBLISHER]

Limited research has been conducted on how atmospheric carbon dioxide (CO2) affects water and soil nitrogen (N) transformation in wetland ecosystems. A stable isotope technique is suitable for conducting a detailed investigation of mechanistic nutrient transformations. Nutrient ammonium sulfate (NH4)2SO4input in culture water under elevated CO2(700μL L−1) and ambient CO2(380μL L−1) was studied to analyze N transformations with N blanks for both water and soil. It was measured by15N pool dilution using analytical equations in a riparian wetland during a 3-month period. Soil gross ammonium (NH4+) mineralization and consumption rates increased significantly by 22% and 404%, Whereas those of water decreased greatly by −57% and −57% respectively in enriched CO2. In contrast, gross nitrate (NO3−) consumption and nitrification rates of soil decreased by −11% and −14% and those of water increased by 29% and 27% respectively in enrichment CO2. These may be due to the extremely high soil microbial biomass nitrogen (MBN), which increased by 94% in elevated soil. The results can show when CO2concentrations are going to rise in the future. Consequently soil microbial activity initiates the decreased N concentration in sediment and increased N concentration in overlying water in riparian wetland ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (δN). Methods: Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for δN. Results: All foliar δN values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 ‰). δN of soils and roots were also more negative in mor ridge forests by about 3 ‰. Foliar δN values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar δN in the two forest types suggesting a changing uptake ratio of different N forms between forest types. Conclusions: These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root δN values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar δN between the two forest types. [ABSTRACT FROM AUTHOR]

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We investigated the impact of nitrogen (N), phosphorus (P), potassium (K) (NPK) and NPK plus glucose-balanced fertilization compared with N-only fertilization on the soil pH, NH4 +, NO3 −, ammonia-oxidizing bacterial community, bacterial community and function during microcosm incubation. The NPK and NPK plus glucose treatments resulted in significantly reducing soil acidification and NO3 − accumulation compared with the N-only fertilization. The terminal restriction fragment size measuring 283 (Nitrosospira) and 54 bp (unidentified) were predominant in the soil ammonia-oxidizing bacterial composition for all treatments. The N-only fertilization did not change the ammonia-oxidizing bacterial community, the bacterial community composition based on terminal restriction fragment length polymorphism analysis, and the bacterial functional diversity based on Biolog EcoPlateTM incubation. The NPK and NPK plus glucose treatments resulted in a shift in the soil ammonia-oxidizing bacterial community and bacterial community composition, and significantly increased the bacterial functional diversity (average well colour development, Richness and Shannon index). Nitrosomonas species were detected in the soil upon NPK and NPK plus glucose treatment on incubation day 9 but not on days 1 and 31. The effect of NPK treatment on the bacterial community composition was transient; a new 116 bp fragment was present on incubation day 9, but the data returned to their original values by day 31. In contrast, treatment with NPK plus glucose resulted in the appearance of a new 116 bp fragment that remained until incubation day 31. These results demonstrated that the balanced fertilization of N, P, K and glucose, plays an important role in regulating ammonia-oxidizing bacterial community quickly, and promoting nitrification functions. The results also showed the importance of balanced fertilization in reducing acidification, improving bacterial community structure and function in latosolic red soil. Therefore, optimizing the ammonia oxidation process by balanced fertilization may be helpful to reduce the loss of soil nitrogen. [ABSTRACT FROM AUTHOR]

Zeolite minerals may improve nitrogen availability to plants in soil and reduce losses to the environment. A study was conducted to determine the influence of clinoptilolite (CL) on nitrogen (N) mineralization from solid dairy manure (224 kg N ha−1) in a sandy soil. Clinoptilolite was added to soil at six rates (0 to 44.8 Mg CL ha−1), each sampled during 11 sampling dates over a year. Over time, nitrate (NO3)-N increased, ammonium (NH4)-N decreased, but total inorganic N increased. Clinoptilolite did not influence the nitrification rates of initial manure NH4-N or mineralization of organic N (ON) over time. It is possible that adsorption of manure-derived potassium (K) outcompeted the NH4-N for CL exchange sites. The ON concentration was constant up to 84 days and then decreased by approximately 18% over the remaining time of the study across all treatments. Clinoptilolite use in this sandy soil did not alter mineralization of N from dairy manure. [ABSTRACT FROM AUTHOR]

Abstract: Soil nitrification rates in northeastern USA forested ecosystems appear to be regulated by a number of factors and are likely influenced by continuing N deposition. Among other factors, rates across small watersheds have been found to relate to the soil C/N ratio and tree species composition. We measured potential net nitrification rates in the Ranch Brook watershed, a relatively large (9.6km2) forested basin in north central Vermont, to determine if relationships found in smaller catchments were evident at a larger scale. The stream network was divided into eight reaches to determine the variability within the watershed. Sampling points (6–15 along each reach, total of 74) were established along transects that paralleled the major watershed tributaries. At each point, we measured net rates of nitrification and ammonification in the uppermost humified soil horizon (Oa or A), using a one-day lab incubation. The basal area and density of all tree species were measured in a 10-m radius plot, along with a number of topographic metrics such as slope, aspect and elevation. In a stepwise regression, 39% of the variability in net nitrification rates was explained by the density of Picea rubens, elevation and the thickness of the forest floor. When net nitrification rates were normalized to soil C concentration, 60% of the variability was explained by soil N concentration, C/N ratio, elevation and the density of P. rubens. The significant negative influence of P. rubens density, and not basal area, was consistent with a previous cross-site study of 10 smaller northeastern USA watersheds. No influence of sugar maple basal area or density was found. Other relationships, similar to those found in smaller watersheds, were net nitrification rates predicted by the fraction of inorganic N as , net nitrification predicted by the C/N ratio and the C/N ratio predicted by tree species. The consistent influence of tree species on potential net nitrification rates demonstrates a role for future forest management in influencing ecosystem processes. [Copyright &y& Elsevier]

Abstract: Changes in the soil water regime, predicted as a consequence of global climate change, might influence the N cycle in temperate forest soils. We investigated the effect of decreasing soil water potentials on gross ammonification and nitrification in different soil horizons of a Norway spruce forest and tested the hypotheses that i) gross rates are more sensitive to desiccation in the Oa and EA horizon as compared to the uppermost Oi/Oe horizon and ii) that gross nitrification is more sensitive than gross ammonification. Soil samples were adjusted by air drying to water potentials from about field capacity to around −1.0 MPa, a range that is often observed under field conditions at our site. Gross rates were measured using the 15N pool dilution technique. To ensure that the addition of solute label to dry soils and the local rewetting does not affect the results by re-mineralization or preferential consumption of 15N, we compared different extraction and incubation times. T0 times ranging from 10 to 300 min and incubation times of 48 h and 72 h did not influence the rates of gross ammonification and nitrification. Even small changes of water potential decreased gross ammonification and nitrification in the O horizon. In the EA horizon, gross nitrification was below detection limit and the response of the generally low rates of gross ammonification to decreasing water potentials was minor. In the Oi/Oe horizon gross ammonification and nitrification decreased from 37.5 to 18.3 mg N kg−1 soil d−1 and from 15.4 to 5.6 mg N kg−1 soil d−1 when the water potential decreased from field capacity to −0.8 MPa. In the Oa horizon gross ammonification decreased from 7.4 to 4.0 mg N kg−1 soil d−1 when the water potential reached −0.6 MPa. At such water potential nitrification almost ceased, while in the Oi/Oe horizon nitrification continued at a rather high level. Hence, only in the Oa horizon nitrification was more sensitive to desiccation than ammonification. Extended drought periods that might result from climate change will cause a reduction in gross N turnover rates in forest soils even at moderate levels of soil desiccation. [Copyright &y& Elsevier]

Abstract: Despite their widespread use in household activities and various industries, information on the toxicity of quaternary ammonium compounds (QACs) to microbial activities in soil is scant. This study investigated the effect of three commonly used QACs namely hexadecyltrimethyl ammonium bromide (HDTMA), octadecyltrimethyl ammonium bromide (ODTMA) and Arquad on dehydrogenase and potential nitrification activities in three different soils. The toxicity of QACs on the dehydrogenase activity and potential nitrification in these soils followed the order: HDTMA>ODTMA>Arquad and Arquad>HDTMA>ODTMA, respectively. HDTMA, ODTMA and Arquad exhibited toxicity to dehydrogenase activity at concentration of 50, 100 and 750mgkg−1 soil, respectively, whereas potential nitrification was inhibited by HDTMA and ODTMA even at 50mgkg−1 soil. Arquad exhibited toxicity to potential nitrification at comparatively higher concentration of 250mgkg−1 soil, with the severity of toxicity very intense at higher concentrations. The nature of QACs and soil properties influenced the toxicity. The toxic effect of QACs on soil microbial activities was more influenced by the relative release of sorbed QACs in soils. This study provides valuable information on the toxicological properties of some widely used QACs on important soil microbial activity parameters. To our knowledge, this is the first report. [ABSTRACT FROM AUTHOR]

Abstract: The majority of soil urease activity measurements have been based on the rate of ammonium production under optimal conditions. However, such procedures do not exclude ammonium consumption by the nitrification process. The purpose of this study was to determine the percentage of soil urease activity that is underestimated due to soil nitrification. Six soils with diverse properties were incubated using a standard procedure for measuring soil urease activity. The dynamics of nitrite and nitrate were observed during the incubation. Our results showed that the percentage of underestimation ranged from 7.38% to 15.97%, depending on soil types and whether or not a buffer was used. We recommend that nitrification be taken into account when soil urease activity is assayed by the ammonium production rate method. [ABSTRACT FROM AUTHOR]

Abstract: The incorporation of two legume materials increased soil pH, while the nitrification of the ammonium–nitrogen (N) from the ammonification of organic N weakened the liming potential of these materials at the later stages of incubation experiments. The addition of dicyandiamide (DCD) enhanced the liming potential of the two legume materials through its inhibition of nitrification of ammonium-N. At the end of the 60-d incubation, soil pH of the treatment using Chinese milk vetch shoots with DCD was 1.48 units higher than that of control and 1.16 units higher than of the treatment using only Chinese milk vetch. The corresponding data for the pea straw were 1.24 and 1.01 units higher, respectively. [Copyright &y& Elsevier]

Abstract: Ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) co-exist in soil, but their relative distribution may vary depending on the environmental conditions. Effects of changes in soil organic matter and nutrient content on the AOB and AOA are poorly understood. Our aim was to compare effects of long-term soil organic matter depletion and amendments with labile (straw) and more recalcitrant (peat) organic matter, with and without easily plant-available nitrogen, on the activities, abundances and community structures of AOB and AOA. Soil was sampled from a long-term field site in Sweden that was established in 1956. The potential ammonia oxidation rates, the AOB and AOA amoA gene abundances and the community structures of both groups based on T-RFLP of amoA genes were determined. Straw amendment during 50 years had not altered any of the measured soil parameters, while the addition of peat resulted in a significant increase of soil organic carbon as well as a decrease in pH. Nitrogen fertilization alone resulted in a small decrease in soil pH, organic carbon and total nitrogen, but an increase in primary production. Type and amount of organic matter had an impact on the AOB and AOA community structures and the AOA abundance. Our findings confirmed that AOA are abundant in soil, but showed that under certain conditions the AOB dominate, suggesting niche differentiation between the two groups at the field site. The large differences in potential rates between treatments correlated to the AOA community size, indicating that they were functionally more important in the nitrification process than the AOB. The AOA abundance was positively related to addition of labile organic carbon, which supports the idea that AOA could have alternative growth strategies using organic carbon. The AOB community size varied little in contrast to that of the AOA. This indicates that the bacterial ammonia oxidizers as a group have a greater ecophysiological diversity and potentially cover a broader range of habitats. [Copyright &y& Elsevier]

Abstract: The effects of bacterial-feeding nematodes on nitrification and the ammonia-oxidizing bacteria (AOB) community composition were studied in soil microcosms. Sterilized soils were inoculated with mixed soil bacteria (obtained by filtering) or with bacteria and bacterial-feeding nematodes, after which the dynamic inorganic nitrogen concentration was measured weekly. After 28 days of incubation, denaturing gradient gel electrophoresis (DGGE) based on PCR amplification of the amoA gene was used to analyze the AOB community composition. In addition, a clone library from the amoA gene fragments was established using clones randomly selected and sequenced from the two treatments. The results showed that the presence of bacterial-feeding nematodes led to significantly greater NH4 + and NO3 − contents over the entire incubation period, indicating that bacterial-feeding nematodes promoted both N mineralization and nitrification. The results of DGGE showed that the AOB community composition was significantly changed in the presence of bacterial-feeding nematodes. Furthermore, the sequencing results suggested that Nitrosospira sp. was the dominant species in the treatment without nematodes, while Nitrosomonas sp. and Nitrosospira sp. were the dominant species in the treatment with nematodes. Such changes in the AOB community may be one of explanation of the important role that nematodes play in promoting nitrification. [Copyright &y& Elsevier]

The aim of this work was to couple a nitrogen (N) sub-model to already existent hydrological lumped (LU4- N) and semi-distributed (LU4-R-N and SD4-R-N) conceptual models, to improve our understanding of the factors and processes controlling nitrogen cycling and losses in Mediterranean catchments. The N model adopted provides a simplified conceptualization of the soil nitrogen cycle considering mineralization, nitrification, immobilization, denitrification, plant uptake, and ammonium adsorption/desorption. It also includes nitrification and denitrification in the shallow perched aquifer. We included a soil moisture threshold for all the considered soil biological processes. The results suggested that all the nitrogen processes were highly influenced by the rain episodes and that soil microbial processes occurred in pulses stimulated by soil moisture increasing after rain. Our simulation highlighted the riparian zone as a possible source of nitrate, especially after the summer drought period, but it can also act as an important sink of nitrate due to denitrification, in particular during the wettest period of the year. The riparian zone was a key element to simulate the catchment nitrate behaviour. The lumped LU4-N model (which does not include the riparian zone) could not be validated, while both the semi-distributed LU4-R-N and SD4-RN model (which include the riparian zone) gave satisfactory results for the calibration process and acceptable results for the temporal validation process. [ABSTRACT FROM AUTHOR]

Abstract: Humic acids (HAs) play an important role in the global nitrogen cycle by influencing the distribution, bioavailability, and ultimate fate of organic nitrogen. Ammonium oxidation by autotrophic ammonia-oxidizing bacteria (AOB) is a key process in ecosystems and is limited, in part, by the availability of NH4 +. We evaluated the impact of HAs on soil AOB in microcosms by applying urea (1.0%, equal to 10 mg urea/g soil) with 0.1% bHA (biodegraded lignite humic acids, equal to 1 mg/g soil), 0.1% cHA (crude lignite humic acids) or no amendment. AOB population size, ammonium and nitrate concentrations were monitored for 12 weeks after urea and HA application. AOB densities (quantified by real-time PCR targeting the amoA) in the Urea treatments increased about ten-fold (the final abundance: 5.02 × 107 copies (g of dry soil)−1) after one week of incubation and decreased to the initial density after 12 weeks incubation; the population size of total bacteria (quantified by real-time PCR with a universal bacterial probe) decreased from 1.12 × 1010 to 2.59 × 109 copies (g of dry soil)−1 at week one and fluctuated back to the initial copy number at week 12. In the Urea + bHA and Urea + cHA treatments, the AOB densities were 4 and 6 times higher, respectively, than the initial density of approximately 5.07 × 106 copies (g of dry soil)−1 at week 1 and did not change much up to week 4; the total bacteria density changed little over time. The AOB and total bacteria density of the controls changed little during the 12 weeks of incubation. The microbial community composition of the Urea treatment, based on T-RFLP using CCA (canonical correspondence analysis) and pCCA (partial CCA) analysis, was clearly different from those of other treatments, and suggested that lignite HAs buffered the change in diversity and quantity of total bacteria caused by the application of urea to the soil. We hypothesize that HAs can inhibit the change in microbial community composition and numbers, as well as AOB population size by reducing the hydrolysis rate from urea to ammonium in soils amended with urea. [Copyright &y& Elsevier]

Abstract: Recent research has proven soil nitrite to be a key element in understanding N-gas production (NO, N2O, N2) in soils. NO is widely accepted to be an obligatory intermediate of N2O formation in the denitrification pathway. However, studies with native soils could not confirm NO as a N2O precursor, and field experiments mainly revealed ammonium nitrification as the source of NO. The hypothesis was constructed, that the limited diffusion of NO in soil is the reason for this contradiction. To test this diffusion limitation hypothesis and to verify nitrite and NO as free intermediates in native soils we conducted through-flow (He/O2 atmosphere) 15N tracer experiments using black earth soil in an experimental set up free of diffusion limitation. All of the three relevant inorganic N soil pools (ammonium, nitrite, nitrate) were 15N labelled in separate incubation experiments lasting 81h based on the kinetic isotope method. During the experiments the partial pressure of O2 was decreased in four steps from 20% to about 0%. The net NO emission increased up to 3.7μgNkg−1 h−1 with decreasing O2 partial pressure. Due to the special experimental set up with little to no obstructions of gas diffusion, only very low N2O emission could be observed. As expected the content of the substrates ammonium, nitrate and nitrite remained almost constant over the incubation time. The 15N abundance of nitrite revealed high turnover rates. The contribution of nitrification of ammonium to the total nitrite production was approx. 88% under strong aerobic soil conditions but quickly decreased to zero with declining O2 partial pressure. It is remarkable that already under the high partial pressure of 20% O2 12 % of nitrite is generated by nitrate denitrification, and under strict anaerobic conditions it increases to 100%. Nitrite is present in two separate endogenous pools at least, each one fed by the nitrification of ammonium or the denitrification of nitrate. The experiments clearly revealed that nitrite is almost 100% the direct precursor of NO formation under anaerobic as well as aerobic conditions. Emitted N2O only originated to about 100% from NO under strict anaerobic conditions (0–0.2% O2), providing evidence that NO is a free intermediate of N2O formation by denitrification. To the best of our knowledge this is the first time that NO has been detected in a native soil as a free intermediate product of N2O formation at denitrification. These results clearly verify the “diffusion limitation” hypothesis. [Copyright &y& Elsevier]

Al stress and ammonium–nitrogen nutrition often coexist in acidic soils due to their low pH and weak nitrification ability. Rice is the most Al-resistant species among small grain cereal crops and prefers NH4 + as its major inorganic nitrogen source. This study investigates the effects of NH4 + and NO3 − on Al toxicity and Al accumulation in rice, and thereby associates rice Al resistance with its NH4 + preference. Two rice subspecies, indica cv. Yangdao6 and japonica cv. Wuyunjing7, were used in this study. After treatment with or without Al under conditions of varying NH4 + and NO3 − supply, rice seedlings were harvested for the determination of root elongation, callose content, biomass, Al concentration and medium pH. The results indicated that Wuyunjing7 was more Al-resistant and NH4 +-preferring than Yangdao6. NH4 + alleviated Al toxicity in two cultivars compared with NO3 −. Both NH4 +-Al supply and pretreatment with NH4 + reduced Al accumulation in roots and root tips compared with NO3 −. NH4 + decreased but NO3 − increased the medium pH, and root tips accumulated more Al with a pH increase from 3.5 to 5.5. Increasing the NO3 − concentration enhanced Al accumulation in root tips but increasing the NH4 + concentration had the opposite effect. These results show NH4 + alleviates Al toxicity for rice and reduces Al accumulation in roots compared with NO3 −, possibly through medium pH changes and ionic competitive effects. Making use of the protective effect of NH4 +, in which the Al resistance increases, is advised for acidic soils, and the hypothesis that rice Al resistance is associated with the preferred utilization of NH4 + is suggested. [ABSTRACT FROM AUTHOR]

In N-polluted, minimally managed soils, recent research has shown that ammonium-N, as well as nitrate-N, may move down through soil profiles. There is a need, therefore, to reassess what controls N species transformations below the rooting depth in such soils. The changes with depth down to 1 m in extractable ammonium-N and nitrate-N concentrations have been studied for two heavily N-impacted acid grassland soils near York, UK, for fresh soils and for soils incubated after either deionised water (d.w.) or ammonium-N spiking. Although in close proximity, the profiles showed marked differences in their relationships of ammonium- and nitrate-N to soil pH, C%, N% and C:N ratio. One profile was slightly more acidic at the surface, which seems to have changed the distribution of organic matter throughout the soil profile. Trends in ammonium production with depth were clearer when ammonium-N concentrations were recalculated on the basis of ammonium-N per unit mass of soil organic C. This then allowed a consistent trend with soil pH to be discerned for both profiles. Ammonium-N spiking showed that ammonium substrate availability was limiting potential net nitrification rate at 20–60 cm depth for both profiles. Potential mineral-N production was considerable at depth, which would facilitate transport of N to surface waters and/or groundwaters. [ABSTRACT FROM AUTHOR]

The effects of forest management (thinning) on gross and net N conversion, the balance of inorganic N production and consumption, inorganic N concentrations and on soil microbial biomass in the Ah layer were studied in situ during eight intensive field measuring campaigns in the years 2002–2004 at three beech ( Fagus sylvatica L.) forest sites. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Since the sites are characterized either by cool-moist microclimate (NE site and NW site) or by warm-dry microclimate (SW site) and thinning took place in the year 1999 at the NE and SW sites and in the year 2003 at the NW site the experimental design allowed to evaluate (1) short-term effects (years 1–2) of thinning at the NW site and (2) medium-term effects (years 4–6) of thinning under different microclimate at the SW and NE site. Microbial biomass N was consistently higher at the thinning plots of all sites during most of the field campaigns and was overall significantly higher at the SWT and NWT plots as compared to the corresponding untreated control plots. The size of the microbial biomass N pool was found to correlate positively with both gross ammonification and gross nitrification as well as with extractable soil NO concentrations. At the SW site neither gross ammonification, gross nitrification, gross ammonium (NH) immobilization and gross nitrate (NO) immobilization nor net ammonification, net nitrification and extractable NH and NO contents were significantly different between control and thinning plot. At the NET plot lower gross ammonification and gross NH immobilization in conjunction with constant nitrification rates coincided with higher net nitrification and significantly higher extractable NO concentrations. Thus, the medium-term effects of thinning varied with different microclimate. The most striking thinning effects were found at the newly thinned NW site, where gross ammonification and gross NH immobilization were dramatically higher immediately after thinning. However, they subsequently tended to decrease in favor of gross nitrification, which was significantly higher at the NWT plot as compared to␣the␣NWC plot during all field campaigns after␣thinning except for April 2004. This increase␣in␣gross nitrification at the NWT plot (1.73 mg N kg−1 sdw day−1 versus 0.48 mg N kg−1 sdw day−1 at the NWC plot) coincided with significantly higher extractable NO concentrations (4.59 mg N kg−1 sdw at the NWT plot versus 0.96 mg N kg−1 sdw at the NWC plot). Pronounced differences in relative N retention (the ratio of gross NH immobilization + gross NO immobilization to gross ammonification + gross nitrification) were found across the six research plots investigated and could be positively correlated to the soil C/N ratio ( R = 0.94; p = 0.005). In sum, the results obtained in this study show that (1) thinning can lead to a shift in the balance of microbial inorganic N production and consumption causing a clear decrease in the N retention capacity in the monitored forest soils especially in the first two years after thinning, (2)␣the resistance of the investigated forest ecosystems to disturbances of N cycling by thinning may vary with different soil C contents and C/N ratios, e. g. caused by differences in microclimate, (3) thinning effects tend to decline with the growth of understorey vegetation in the years 4–6 after thinning. [ABSTRACT FROM AUTHOR]

Ammonia-oxidizing bacteria (AOB) perform the rate-limiting step of nitrification, a key ecosystem process that in part determines the fate of nitrogen in ecosystems. However, little is known about the factors that determine soil AOB diversity or composition, especially in tropical systems. Using a set of study systems in Costa Rica, we examined whether plant diversity or land-use influenced AOB diversity or composition and whether AOB diversity or composition were associated with nitrification rates. We characterized the molecular diversity and composition of AOB via polymerase chain reaction amplification, cloning, and sequencing of 16S rDNA. We found that AOB diversity or composition did not change significantly across plant diversity treatments. In contrast, AOB differed among land-use types in some measures of diversity and in composition, and differences in AOB composition among land-use types were correlated with potential rates of nitrification. Our results suggest that anthropogenic changes of ecosystems can alter microbial communities in ways that may affect the processes they mediate. [ABSTRACT FROM AUTHOR]

Temporal variations in δ15N of NH4+ and NO3− in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ15N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg−1. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH4+-N concentration decreased from 164.8 to 34.4 mg kg−1, and the δ15N of NH4+ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO3−-N concentration and lower δ15N of NO3− (product) than that of NH4+ (substrate) at each sampling time. In saturated soil, the concentration of NH4+-N decreased gradually from 162.4 to 24.2 mg kg−1, and the δ15N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO3− concentration was not observed due to loss of NO3− through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (αs/p) associated with decrease in NH4+ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH4+ were relatively more dominant than nitrification under the anaerobic conditions. [Copyright &y& Elsevier]

A model of nitrogen release from ammoniacal fertilizers into soil (e.g. from a controlled release nitrogen (CRN) source) and subsequent nitrification has been developed to assess possibilities for reducing losses of N and environment pollution. Emphasis was placed on the effects of changes in ammonium concentration and pH on nitrification rates, and the interactions of these effects with the rate of release of N from fertilizers. The model considers ammonium release and transport from a spherical source, such as a coated granule of CRN or a nest containing either granules of CRN or an easily dissolved fertilizer (EDF). It is shown that the appearance of nitrate in the soil decreases as the size of the N source increases. This is due to both a decrease in the rate of nitrification, caused by accumulation of ammonium near the source, and a slower relative release (i.e. smaller ratio of N released per unit time to the total mass initially in the source, which is inversely proportional to the radius of a granule or nest). Placement of many CRN granules in a nest is shown to be an effective way to reduce nitrification, particularly in soils having low microbial activities. The less the mobility of ammonium in the soil, the slower will be the nitrification because ammonium accumulates close to the source. A CRN nest leads to more effective reduction of nitrification than an EDF nest and N in it is released more slowly into the soil. [ABSTRACT FROM AUTHOR]

A 15N tracing experiment was carried out to investigate gross N dynamics in coastal wetland soils. The results showed that the cultivation of coastal wetland has resulted in a significant increase in the gross mineralization rate, gross nitrification rate and gross microbial immobilization of ammonium rate (p<0.01). However, the effect of cultivation on the gross microbial immobilization of nitrate rate was not significant. The gross nitrification rate was much higher than nitrate immobilization (p<0.01), indicating that cultivation of coastal wetland could increase the risk of losses of N from the soil through leaching. [ABSTRACT FROM AUTHOR]