Competition for Ammonium between Nitrifying and Heterotrophic Bacteria in Continuously Percolated Soil Columns

Although the absence of nitrate formation in grassland soils rich in organic matter has often been reported, low numbers of nitrifying bacteria are still found in these soils. To obtain more insight into these observations, we studied the competition for limiting amounts of ammonium between the chemolithotrophic ammonium-oxidizing species Nitrosomonas europaea and the heterotrophic species Arthrobacter globiformis in the presence of Nitrobacter winogradskyi with soil columns containing calcareous sandy soil. The soil columns were percolated continuously at a dilution rate of 0.007 h^-1, based on liquid volumes, with medium containing 5 mM ammonium and different amounts of glucose ranging from 0 to 12 mM.A. globiformis was the most competitive organism for limiting amounts of ammonium. The numbers of N. europaea and N. winogradskyi cells were lower at higher glucose concentrations, and the potential ammonium-oxidizing activities in the uppermost 3 cm of the soil columns were nonexistent when at least 10 mM glucose was present in the reservoir, although 10⁷ nitrifying cells per g of dry soil were still present. This result demonstrated that there was no correlation between the numbers of nitrifying bacteria and their activities. The numbers and activities of N. winogradskyi cells decreased less than those of N. europaea cells in all layers of the soil columns, probably because of heterotrophic growth of the nitrite-oxidizing bacteria on organic substrates excreted by the heterotrophic bacteria or because of nitrate reduction at reduced oxygen concentrations by the nitrite-oxidizing bacteria. Our conclusion was that the nitrifying bacteria were less competitive than the heterotrophic bacteria for ammonium in soil columns but that they survived as viable inactive cells. Inactive nitrifying bacteria may also be found in the rhizosphere of grassland plants, which is rich in organic carbon. They are possibly reactivated during periods of net mineralization.     

Inhibition of nitrification by waste tea (‘Tea Fluff’)

Summary Ammonium fertilizer applied to tea soils is readily converted to nitrate by the nitrifying bacteria in soil. Excess nitrate in soil could undergo rapid leaching losses under high rainfall conditions. Data is presented in this paper to show that waste tea could be effectively used to retard and delay nitrate production and thereby prevent loss of nitrogen as nitrate by leaching. Evidence is also presented to show that waste tea readily liberates ammonium nitrogen in soil.     

Effects of Grazing by Flagellates on Competition for Ammonium between Nitrifying and Heterotrophic Bacteria in Chemostats

The enhanced mineralization of organic nitrogen by bacteriophagous protozoa is thought to favor the nitrification process in soils, in which nitrifying bacteria have to compete with heterotrophic bacteria for the available ammonium. To obtain more insight into this process, the influence of grazing by the bacteriovorous flagellate Adriamonas peritocrescens on the competition for limiting amounts of ammonium between the ammonium-oxidizing species Nitrosomonas europaea and the heterotrophic species Arthrobacter globiformis was studied in the presence of Nitrobacter winogradskyi in continuous cultures at dilution rates of 0.004 and 0.01 h^-1. The ammonium concentration in the reservoir was maintained at 2 mM, whereas the glucose concentration was increased stepwise from 0 to 7 mM. A. globiformis won the competition for limiting amounts of ammonium when the glucose concentration in the reservoirs increased, in agreement with previously described experiments in which the flagellates were not included. The numbers of nitrifying bacteria decreased as the numbers of heterotrophic bacteria rose with increasing glucose concentrations. Critical C/N ratios, i.e., ratios between glucose and ammonium in the reservoirs at which no nitrate was found in the culture vessels, of 12.5 and 10.5 were determined at dilution rates of 0.004 and 0.01 h^-1, respectively. Below these critical values, coexistence of the competing species was found. The numbers of nitrifying bacteria decreased more in the presence of flagellates than in their absence, presumably by selective predation on the nitrifying bacteria, either in the liquid culture or on the glass wall of the culture vessels. Despite this, the rate of nitrate production did not decrease more in the presence of flagellates than in their absence. This demonstrates that no correlation has to be expected between numbers of nitrifying bacteria and their activity and that a constant nitrification rate per cell cannot be assumed for nitrifying bacteria. Above the critical C/N ratios, low numbers of nitrifying bacteria were still found in the culture vessels, probably because of attachment of the nitrifying bacteria to the glass wall of the culture vessels. Like the numbers of heterotrophic bacteria, the numbers of flagellates increased when the glucose concentrations in the reservoirs increased. Numbers of 2 × 10⁵ and 12 × 10⁵ flagellates ml^-1 were found at 7 mM glucose at dilution rates of 0.004 and 0.01 h^-1, respectively. It was concluded that the critical C/N ratios were practically unaffected by the presence of protozoa. Although nitrate production rates were equal in the presence and absence of flagellates, the numbers of nitrifying bacteria decreased more strongly in their presence. This indicates a higher activity per nitrifying cell in the presence of flagellates.     

Nitrate production in the rhizosphere of Plantago species

Summary The production of nitrate in an old established dune grassland soil and its uptake by plants was studied by comparing amounts of mineral nitrogen and numbers of nitrifying bacteria in the rhizosphere on the one hand, and on the other accumulated nitrate and levels of nitrate reductase (NaR) of individual plants of three Plantago species,i. e., P. major, P. lanceolata andP. coronopus. For these three Plantago species andP. media basal levels of NaR in the absence of nitrate were determined in plants grown in culture solutions. The basal NaR levels ofP. major andP. media (species occurring on nutrient-rich soils) were significantly higher than those ofP. lanceolata andP. coronopus (species found on nutrient-poor soils). NaR activity increased in the presence of nitrate and was suppressed by ammonium.From the numbers of nitrifying bacteria in the rhizosphere and NaR activity in the leaves it was concluded that nitrate was produced in the root environments of the three Plantago species and that the compound was taken up by the plants. NaR activities and numbers of nitrifying bacteria were higher for individuals ofP. major than for those ofP. lanceolata andP. coronopus. No correlation was found between the ammonium levels and the numbers of nitrifying bacteria in the soil, and no indications of inhibition of nitrifying bacteria in the rhizosphere were obtained. For individuals ofP. lanceolata a correlation was found between the numbers of nitrifying bacteria in the soil and NaR activity in the leaves. The results are discussed in relation to the ecological habitats of the three species.Grassland Species Research Group Publication No.38.     

黄土丘陵区不同植被土壤氮素转化微生物生理群特征及差异

采用最大或然计数法(most probable number, MPN)对黄土高原洞子沟流域不同植被恢复阶段土壤氮素微生物生理群(氨化细菌、亚硝化细菌、反硝化细菌)数量分布特征进行了测定,结果表明:1)土壤氨化细菌、亚硝化细菌和反硝化细菌数量随植被恢复而增加,三者最大值分别为最小值的74、4和31倍,其中氨化细菌和反硝化细菌的数量在铁杆蒿群落最低,辽东栎群落最高,亚硝化细菌数量在丁香群落最低,辽东栎群落最高;2)植被恢复对各氮素生理群影响不同,对氨化细菌影响最大,其次分别为反硝化细菌和亚硝化细菌;3)各氮素生理群数量差异较大,氨化细菌>反硝化细菌>亚硝化细菌。研究区氨化细菌占总数的75%-80%,反硝化细菌占20%-25%时,生态系统最为稳定;4)土壤理化性质与各功能菌关系紧密,其中,土壤容重和硝态氮含量与微生物数量相关性最大,全钾、矿化氮和微生物量氮也表现出很大的相关性。     

Seasonal denitrification in flooded and exposed sediments from the Amazon floodplain at Lago Camaleão

Denitrification processes were measured by the acetylene-blockage technique under changing flood conditions along the aquatic/terrestrial transition zone on the Amazon floodplain at Lago Camaleão, near Manaus, Brazil. In flooded sediments, denitrification was recorded after the amendment with NO ₃ ^– (100 mol liter^–1) throughout the whole study period from August 1992 to February 1993. It ranged from 192.3 to 640.7 mol N m^–2 h^–1 in the 0- to 5-cm sediment layer. Without substrate amendment, denitrification was detected only during low water in November and December 1992, when it occurred at a rate of up to 12.2 mol N m^–2 h^–1 Higher rates of denitrification at an average rate of 73.3 mol N m^–2 h^–1 were measured in sediments from the shallow lake basin that were exposed to air at low water. N₂O evolution was never detected in flooded sediments, but in exposed sediments, it was detected at an average rate of 28.3 mol N m^–2 h^–1 during the low-water period. The results indicate that under natural conditions there is denitrification and hence a loss in nitrogen from the Amazon floodplain to the atmosphere. Rates of denitrification in flooded sediments were one to two orders of magnitude smaller than in temperate regions. However, the nitrogen removal of exposed sediments exceeded that of undisturbed wetland soils of temperate regions, indicating a considerable impact of the flood pulse on the gaseous turnover of nitrogen in the Amazon floodplain.     

Seed bank of seasonally flooded grassland: experimental simulation of flood and post-flood

Wetland seed banks comprise the propagules of plant species that have species-specific germination requirements for germination in either flooded or dry conditions. At the community level, wetland structure and succession during and after a seasonal flooding event depends upon the early life-history requirements of species, including germination under flooded and dry conditions. We examined the effects of simulated flood and post-flood scenarios on seedling emergence from a seed bank of seasonally flooded grassland in the Pantanal, Brazil. Field samplings were conducted in both wet and dry seasons, both of which were subject to flood and post-flood conditions. A total of 70 species emerged from the seed bank, dominated by Poaceae and Cyperaceae. Sixteen species were exclusive to the wet and one exclusive to the dry season. The richness of perennial species was higher under flood conditions, while the richness of annuals was greater post-flood. In general, the aquatic and amphibious species exhibited a significant germination response to flooding. Terrestrial species only germinated in post-flood conditions, with higher richness in the dry season. Four species had high seedling abundance in both treatments. The capacity of regeneration by seeds is high in these grasslands and can be increased by seasonal flooding and drawdown. In these seasonally flooded grasslands, we observed three main germination strategies: under flooded conditions, aquatic and amphibious species; post-flood conditions, an explosion of annual amphibious and terrestrial species; and in moist soil, perennial terrestrial species. The differential responses to flooding versus post-flood conditions help to maintain the structure and species richness in the community over time.     

Molecular analysis of the diversity of nitrifying bacteria in the soils of the forest and steppe zones of European Russia

Nitrifying bacteria play a key role in the global nitrogen cycle due to their ability to convert reduced nitrogen compounds (ammonium) to oxidized ones (nitrite and nitrate). Recent investigations based on the methods of molecular ecology revealed that bacteria are responsible for nitrification in natural ecosystems. At the same time, data on the species composition of the nitrifiers in soil microbial communities are scarce. Soil samples collected in the forest and steppe areas of European Russia and the enrichment cultures of nitrifying bacteria isolated from these samples were used for molecular studies of the diversity of the amoA gene encoding the synthesis of the key enzyme of autotrophic ammonium oxidation. The nitrifying bacteria of the genera Nitrosospira and Nitrosovibrio were found in all the studied soils from natural biocenoses and agrocenoses.     

Nitrification in soils of secondary vegetational successions from Eucalyptus forest and grassland to cool temperate rainforest in Tasmania

Rates of nitrification in well drained granitic soils from forest stands and grassland of differing successional status and from beneath isolated individuals of several tree species were compared in a series of laboratory experiments. Fresh samples were perfused with distilled water or nutrient solution for 10 to 14 weeks at 20°C. The following treatments were applied to the soils singly and in combination: 200 and 400 g N g^–1 as (NH₄)₂SO₄; 100 g P g^–1 as KH₂PO₄; 4000 g CaCO₃ g^–1; inoculation of non-nitrifying soil with nitrifying soil; perfusion of nitrifying soil with leachate from non-nitrifying soil.Nitrification was absent or occurred at only a low rate in many soils; it generally increased as succession proceeded from nature grassland or eucalypt forest towards climax temperate rainforest, but decreased in mature climax forests. However, the influence of individual tree species was often paramount. Nitrification was stimulated by disturbance of a stand by disease. A possible inhibitor of nitrification in a rainforest soil could not be removed by leaching with water, nor transferred via the leachate to a nitrifying soil. Addition of P was without effect on either total amount of nitrate produced or on net mineralisation of soil N, but sometimes increased the rate of nitrification of added ammonium. Non-nitrifying rainforest soil of pH 4.3 was induced to nitrify only after addition of (NH₄)₂SO₄, inoculation with a nitrifying soil, and addition of CaCO₃ to raise pH by 3 units. However, once nitrification had commenced it could continue with little change in rate while pH decreased to a value of 3.4.It was concluded that rate of nitrification is dependent upon the presence of particular tree species in a stand, upon its history of disturbance, and hence in part upon the stand's successional status. It is not limited by pHper se within the range found in these soils, although an increase in pH may be necessary to initiate nitrification. In some soils the rate of nitrification may be limited by the level of ammonium substrate, and nitrifiers are virtually absent from others. Overall microbial activity is limited by lack of utilisable carbon substrate.     

Biogeochemical Contributions of Tree Islands to Everglades Wetland Landscape Nitrogen Cycling During Seasonal Inundation

In the Florida Everglades, tree islands are conspicuous heterogeneous elements in the herbaceous wetland landscape. We characterized the biogeochemical role of a seasonally flooded tree island during wet season inundation, specifically examining hydrologically mediated flows of nitrogen (N) and N retention by the tree island. We estimated ecosystem N standing stocks and fluxes, soil and litter N transformation rates, and hydrologic fluxes of N to quantify the net ecosystem N mass flux. Results showed that hydrologic sources of N were dominated by surface water loads of nitrate (NO₃) and ammonium (NH₄). Nitrate immobilization by soils and surficial leaf litter was an important sink for surface water dissolved inorganic N (DIN). We estimated that the net annual DIN retention by a seasonally flooded tree island was 20.5 ± 5.0 g m⁻² during wet season inundation. Based on the estimated tree island surface water DIN loading rate, a seasonally flooded tree island retained 76% of imported DIN. As such, seasonally flooded tree islands have the potential to retain 55% of DIN entering the marsh landscape via upstream canal overland flow in the wet season. By increasing reactive surface area and DOC availability, we suggest that tree islands promote convergence of elements that enhance DIN retention. Tree islands of this region are thus important components of landscape-scale restoration efforts that seek to reduce sources of anthropogenic DIN to downstream estuaries.     

The importance of autotrophic versus heterotrophic oxidation of atmospheric ammonium in forest ecosystems with acid soil

Abstract The role of autotrophic and heterotrophic nitrifying microorganisms in the oxidation of atmospheric ammonium in two acid and one calcareous location of a Dutch woodland area was investigated. In soil slurries nitrate formation was completely inhibited by acetylene, a specific inhibitor of autotrophic ammonium-oxidizing bacteria. A survey of nitrifiers in the forest soils showed that both autotrophic ammonium- and nitrite-oxidizing bacteria were present in high numbers and evidence was obtained that autotrophic bacteria are able to nitrify below pH 4. These results show that autotrophic nitrifying bacteria may account for most of the nitrification in the examined soils. To assess the contribution of heterotrophic nitrifiers, about 200 strains of heterotrophic bacteria and 21 morphologically distinct fungal strains were isolated from the acid soil locations and tested for their ability to nitrify. Only one Penicillium strain produced nitrate in test media, but its nitrate formation when added to acid soils was poor. These findings indicate that in the investigated soil heterotrophs are of minor importance in the oxidation of atmospheric ammonium.     

Soil nutrient supply in natural and managed forests

The effect of a laboratory addition of 10, 100 and 500 mg Cd kg_{dry soil} ^-1 on ammonification and nitrification was studied using soil samples of two unpolluted grassland soils. Calcareous and non-calcareous soil were selected for this purpose. Various parameters of nitrifying activity were investigated simulataneously: activity during long-term laboratory incubations in the presence and absence of a substrate, mineralization potentials, and potential activity of both ammonium and nitrite oxidizers during short-term incubations in soil slurries. Cadmium was added as aqueous CdCl₂.Additions of both 100 and 500 mg Cd kg_{dry soil} ^-1 doses significantly lowered the ability of both soils to nitrify 100 g added NH₄ ⁺-N g_{dry soil} ^-1 as a substrate, which was reflected in a decreased rate of nitrate formation (maximum inhibition reached 60% in the calcareous soil and 45% in the non-calcareous soil). Furthermore, these two concentrations of Cd caused an abnormal accumulation of nitrite immediately after incorporation, particularly in the calcareous soil. The addition of 10 mg Cd kg_{dry soil} ^-1 intensified N-mineralization in both soils, probably as a consequence of a higher concentration of readily metabolized substrate originating from killed bacteria or fungi. An excess of nitrate was then formed as a final step. The harmful effect of cadmium was more pronounced in calcareous soil, probably due to the higher sensitivity of nitrite-oxidizers in these soil samples.     

Nitrogen conservation in a tropical rain forest

Summary A series of plots in an Amazonian Rain Forest were trenched and treated with calcium to determine the effects of perturbation on numbers and activity of nitrifying bacteria. Although treatment resulted in between 2 and 26% of the nitrogen being lost from the humus layer, virtually all of it was in the NH ₄ ⁺ form. Numbers of nitrifying bacteria in the plots were relatively low. The low numbers and low activity of nitrifying bacteria is attributed to the low pH and high concentration of tannins in the root mat. It is hypothesized that the suppression of nitrifying bacteria results in nitrogen conservation in the rain forest.     

Nitrification in intertidal sponge <Emphasis Type="Italic">Cinachyrella cavernosa</Emphasis>

The nitrification process in different sections of the sponges remains unresolved, despite several studies on the nitrogen cycling pathways in the tissues of temperate and Arctic sponges. In this study, the abundance, diversity and activity of the associated nitrifying organisms in intracellular, intercellular, extracellular and cortex of a tropical intertidal sponge, Cinachyrella cavernosa, were investigated using most probable number, next-generation sequencing and incubation method, respectively. The nitrification rate and the abundance of nitrifying bacteria showed significant difference among different sections. The nitrification rate in C. cavernosa was 2–12× higher than the reported values in other sponge species from temperate and Arctic regions. Nitrification rate in sponge cortex was 2× higher than in intercellular and extracellular sections. Ammonium and nitrite oxidisers ranged from 10³ to 10⁴ CFU g^?1 in the sponge with a high number of ammonium and nitrite oxidisers in the cortex. Nitrifiers belonging to Nitrosomonas, Nitrospira, Nitrospina, Nitrobacter and Nitrosopumilus were present in different sections of the sponge, with nitrifying archaea dominating the intracellular section and nitrifying bacteria dominating other sections. This study reports for the first time the nitrification inside the sponge cells. The study also suggests that the intertidal sponge, C. cavernosa, harbours metabolically active nitrifiers in different sections of the sponge body with different rates of nitrification. Thus, nitrifiers play an important role in ammonia detoxification within the sponge and also contribute to the nitrogen budget of the coastal ecosystem.     

Microbial populations in trifluralin-treated soil

Summary This study examined the effects of trifluralin (,,-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), a soil incorporated herbicide, on soil microflora both in the general soil environment and in the rhizosphere of trifluralin damaged wheat roots. Two Dark Brown Chernozemic soils were treated with various trifluralin rates in the growth chamber and wheat [Triticum aestivum L. Neepawa] was seeded. Trifluralin generally had no effect on fungi, bacteria, or actinomycete populations in either the general soil or in the rhizosphere. CO₂ evolution was unchanged when trifluralin was added to the soil. In wheat plots, at two field locations, there were no significant effects of trifluralin (1.0 kg ha^–1) on soil fungi, bacteria, actinomycete, denitrifying bacteria, and nitrifying Nitrobacter propulations. A pure culture study with 42 soil microorganisms showed that many isolates were inhibited at 400 to 100,000 g g^–1 but not at concentrations <16 g g^–1. Similar data were obtained from tests on four different soils. These studies indicate that trifluralin is unlikely to cause changes in the numbers of soil microorganisms when used at recommended levels.     

Production and export of dissolved C in arctic tundra mesocosms: the roles of vegetation and water flow

To better understand carbon (C) cycling in arctic tundra we measureddissolved C production and export rates in mesocosms of three tundra vegetationtypes: tussock, inter-tussock and wet sedge. Three flushing frequencies wereused to simulate storm events and determine potential mass export of dissolved Cunder increased soil water flow scenarios. Dissolved C production and exportrates differed between vegetation types (inter-tussock < tussock < wetsedge). In the absence of flushing, dissolved organic C (DOC) dominatedproduction in tussock and inter-tussock soils but was consumed in wet sedgesoils (8.3, 32.7, and –0.4 g C g soil^–1day^–1). Soil water dissolved C concentrations declined over time when flushedat high and medium frequencies but were variable at low flushing frequency.Total yield of dissolved C and DOC increased with increased flushing frequency.The ratio of DOC to dissolved inorganic C exported dropped with increasedflushing under tussock but not inter-tussock or wet sedge vegetation. Massexport per liter of water added declined as flushing frequency increased intussock and inter-tussock mesocosms. Export and production of dissolved C werestrongly correlated with above ground biomass, but not with photosynthetic ratesor below ground biomass. DOC quality was examined by measuring production ofToolik Lake bacteria fed mesocosm soil water. When normalized for DOCconcentration, wet sedge soil water supported significantly higher bacterialproduction. Our results indicate that arctic tundra soils have high potentialsfor dissolved C export, that water flow and vegetation type mainly controldissolved C export, and that responses of aquatic microbes to terrestrial inputsdepend on the vegetation type in the watershed.     

Plant Trait Diversity Buffers Variability in Denitrification Potential over Changes in Season and Soil Conditions

Background

Denitrification is an important ecosystem service that removes nitrogen (N) from N-polluted watersheds, buffering soil, stream, and river water quality from excess N by returning N to the atmosphere before it reaches lakes or oceans and leads to eutrophication. The denitrification enzyme activity (DEA) assay is widely used for measuring denitrification potential. Because DEA is a function of enzyme levels in soils, most ecologists studying denitrification have assumed that DEA is less sensitive to ambient levels of nitrate (NO₃ ⁻) and soil carbon and thus, less variable over time than field measurements. In addition, plant diversity has been shown to have strong effects on microbial communities and belowground processes and could potentially alter the functional capacity of denitrifiers. Here, we examined three questions: (1) Does DEA vary through the growing season? (2) If so, can we predict DEA variability with environmental variables? (3) Does plant functional diversity affect DEA variability?

Methodology/Principal Findings

The study site is a restored wetland in North Carolina, US with native wetland herbs planted in monocultures or mixes of four or eight species. We found that denitrification potentials for soils collected in July 2006 were significantly greater than for soils collected in May and late August 2006 (p<0.0001). Similarly, microbial biomass standardized DEA rates were significantly greater in July than May and August (p<0.0001). Of the soil variables measured—soil moisture, organic matter, total inorganic nitrogen, and microbial biomass—none consistently explained the pattern observed in DEA through time. There was no significant relationship between DEA and plant species richness or functional diversity. However, the seasonal variance in microbial biomass standardized DEA rates was significantly inversely related to plant species functional diversity (p<0.01).

Conclusions/Significance

These findings suggest that higher plant functional diversity may support a more constant level of DEA through time, buffering the ecosystem from changes in season and soil conditions.     

The microbial nitrogen cycle

This special issue highlights several recent discoveries in the microbial nitrogen cycle including the diversity of nitrogen-fixing bacteria in special habitats, distribution and contribution of aerobic ammonium oxidation by bacteria and crenarchaea in various aquatic and terrestrial ecosystems, regulation of metabolism in nitrifying bacteria, the molecular diversity of denitrifying microorganisms and their enzymes, the functional diversity of freshwater and marine anammox bacteria, the physiology of nitrite-dependent anaerobic methane oxidation and the degradation of recalcitrant organic nitrogen compounds. Simultaneously the articles in this issue show that many questions still need to be addressed, and that the microbes involved in catalyzing the nitrogen conversions still harbour many secrets that need to be disclosed to fully understand the biogeochemical nitrogen cycle, and make future predictions and global modelling possible.     

Characteristics of ammonia oxidation potentials and ammonia oxidizers in mineral soil under <Emphasis Type="Italic">Salix polaris</Emphasis>–moss vegetation in Ny-Ålesund,Svalbard

Although nitrification is a unique and important process in the nitrogen cycle with respect to ammonium consumption and nitrate production, limited information on this process is available for high-Arctic soils. We elucidated the ammonia oxidation potentials (AOPs) and characteristics of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in mineral soils under climax vegetation, i.e., Salix polaris (polar willow)–moss vegetation, on a coastal hill in Ny-Ålesund, Svalbard. AOPs at 10 °C were determined by incubation with sufficient substrate (2 mM ammonium). The ammonia monooxygenase subunit A (amoA) genes of AOB and AOA were analyzed by using quantitative polymerase chain reaction and pyrosequencing. AOPs ranged from 1.1 to 14.1 ng N g^?1 dry soil h^?1—relatively low but of a similar order to the gross nitrification rates reported in another Svalbard study. AOP was positively correlated with thickness of the moss layer (P < 0.01), soil water content, and ammonium nitrogen content (P < 0.05). The population sizes of both AOB and AOA were not significantly related to AOP or edaphic factors. For AOB-amoA, six major operational taxonomic units (OTUs) were identified, all of which were classified into the Nitrosospira Mount Everest cluster. For AOA-amoA, six major OTUs were also identified, five of which were grouped with sequences from cold environments within clade A of the Nitrososphaera cluster, i.e., species known to have low, or no, AOP. It is, therefore, possible that the AOPs measured at the study site were driven mainly by psychrotolerant AOB.     

Soil nitrogen patterns induced by colonization of Polygonum cuspidatum on Mt. Fuji

Summary The spatial pattern of soil nitrogen was analyzed for a patchy vegetation formed by the colonization of Polygonum cuspidatum in a volcanic desert on Mt. Fuji. Soils were sampled radially from the bare ground to the center of the patch, and analyses were done for bulk density, water content, soil acidity, organic matter, organic nitrogen, and ammonium and nitrate nitrogen. The soils matured with succession from the bare ground through P. cuspidatum to Miscanthus oligostachyus and Aster ageratoides sites: bulk density decreased, and water content, organic matter, organic nitrogen, and ammonium nitrogen increased. Nitrate nitrogen showed the highest values at the P. cuspidatum site. Application of principal component analysis to the soil data discriminated two component factors which control the variation of soil characteristics: the first factor is related to soil formation and the second factor to nitrogen mineralization and nitrification. The effect of soil formation on nitrogen mineralization and nitrification was analyzed with a first-order kinetic model. The decreasing trends with soil formation in the ratios of mineral to organic nitrogen and of nitrate to ammonium nitrogen could be accounted for by the higher activity of immobilization by microorganisms and uptake by plants in the more mature ecosystem.