Patterns Of Nitrification Research Articles

Ancient landscapes and the relationship with microbial nitrification.

Ammonia oxidizing archaea (AOA) and bacteria (AOB) drive nitrification and their population dynamics impact directly on the global nitrogen cycle. AOA predominate in the majority of soils but an increasing number of studies have found that nitrification is largely attributed to AOB. The reasons for this remain poorly understood. Here, amoA gene abundance was used to study the distribution of AOA and AOB in agricultural soils on different parent materials and in contrasting geologic landscapes across Australia (n = 135 sites). AOA and AOB abundances separated according to the geologic age of the parent rock with AOB higher in the more weathered, semi-arid soils of Western Australia. AOA dominated the younger, higher pH soils of Eastern Australia, independent of any effect of land management and fertilization. This differentiation reflects the age of the underlying parent material and has implications for our understanding of global patterns of nitrification and soil microbial diversity. Western Australian soils are derived from weathered archaean laterite and are acidic and copper deficient. Copper is a co-factor in the oxidation of ammonia by AOA but not AOB. Thus, copper deficiency could explain the unexpectedly low populations of AOA in Western Australian soils.

The effects of rice-straw biochar addition on nitrification activity and nitrous oxide emissions in two Oxisols

Nitrification rates in Oxisols vary with soil pH and substrate availability. Biochar can be used to improve acid soils. The aim of this study was therefore to investigate the interactive impacts of 1% and 5% (w/w) rice-straw biochar application on nitrification, ammonia oxidizer populations and nitrous oxide (N2O) emissions over short periods of microcosm incubation in two agricultural Oxisols derived from granite (RGU) and tertiary red sandstone (RTU), respectively. We measured soil nitrate (NO3−) and ammonium (NH4+) concentrations during the incubation and used nitrification kinetic model to assess the response of nitrification to biochar addition. We also performed real-time quantitative polymerase chain reaction (qPCR) to quantify the copies of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) genes, and collected N2O gas at various intervals during the 56-day incubation. The addition of ammonium sulfate ((NH4)2SO4-N) stimulated nitrification in both soils. In RGU, biochar treatments altered soil nitrification patterns to a first-order reaction model; this stimulation was more pronounced with the increase of biochar application rates. In RTU, 1% biochar treatment increased nitrification rate constants, and 5% biochar treatment altered nitrification patterns from a zero-order to a first-order reaction model. Treating the two soils with 5% biochar rates significantly increased AOB gene copy numbers up to 7.88- and 14-fold compared with the no biochar controls in RGU and RTU, respectively, while the treatments had little or reduced effect on AOA gene copy numbers. Biochar addition significantly reduced cumulative N2O emissions up to 37.6% in RGU and 46.4% in RTU, respectively. These results underscore the potential of biochar in the restoration of nitrification and the reduction of greenhouse gas N2O emission in Oxisols.

Distribution of nitrifiers and nitrification associated with different sizes of aggregates along a 2000 year chronosequence of rice cultivation

Periodic short-term redox cycles induced by paddy management over long time periods have strong effects on soil biogeochemical processes, including nitrification. Heterogeneous distribution of microbes has been well documented and the spatial structural patterns can be related to soil function. Soil aggregation, distribution patterns of ammonia-oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) associated with different sizes of aggregates, and nitrification were studied for sites under a 300 and 2000year duration of rice cultivation (P300 and P2000). Long-term rice cultivation improved large macroaggregate formation from silt+clay fractions. AOB and AOA were mainly distributed within small macroaggregates (2.0–0.25mm), and their distribution patterns were similar between farming durations. Nitrification patterns associated with various sizes of aggregates coincided with whole soil, but nitrification rates were significantly different for various aggregate sizes. The highest net nitrification occurred for macroaggregates and the least for large macroaggregates, which coincided with nitrifier populations. For P300, net nitrification associated with small macroaggregates was 10.6% and 26.6% higher than that with whole soil and large macroaggregates, respectively. For P2000, net nitrification associated with small macroaggregates was 37.6% and 49.5% higher than that with whole soil and large macroaggregates, respectively. A long duration of intensive rice cultivation history depressed nitrifier populations and nitrification rates at both whole soil scale and aggregate scale.

Activity, Abundance, and Diversity of Nitrifying Archaea and Denitrifying Bacteria in Sediments of a Subtropical Estuary: Bahía del Tóbari, Mexico

Fixed nitrogen (N) removal from estuaries via coupled nitrification–denitrification plays a significant role in the global N cycle and the biogeochemistry of individual estuaries. Much of our understanding of these processes is drawn from temperate estuaries, yet tropical and subtropical estuaries may respond differently to N inputs. I tested the hypothesis that nitrification is limited within subtropical estuaries by comparing nitrification and denitrification potentials, and the abundance of archaeal ammonia monooxygenase (amoA) and bacterial nitrite reductase (nirS) genes, across five sites in Bahia del Tobari, Mexico. Sampling was conducted when agricultural runoff supplied substantial quantities of N (ca. 20–80 μM ammonium), yet nitrification was detected at a single site. Denitrification was measured at four sites, and three displayed nitrate uptake rather than net nitrification—indicating a N sink within these sediments. Bacterial nirS genes uniformly outnumbered archaeal amoA genes (3- to 49-fold) and were more abundant in the northern part of the estuary. Patterns of community similarity among different sites were also different for nirS and archaeal amoA: similarities between sites based on nirS were often greater than for amoA, and sites were more rarely statistically different from each other. While amoA abundance was inversely related to temperature, neither amoA nor nirS was correlated with nitrification or denitrification potentials. My results are broadly consistent with known and proposed patterns of nitrification and denitrification in subtropical estuarine sediments, including the idea that nitrification is limited within subtropical estuarine sediments.

Diversity and abundance of ammonia oxidizing archaea in tropical compost systems

Composting is widely used to transform waste materials into valuable agricultural products. In the tropics, large quantities of agricultural wastes could be potentially useful in agriculture after composting. However, while microbiological processes of composts in general are well established, relatively little is known about microbial communities that may be unique to these in tropical systems, particularly nitrifiers. The recent discovery of ammonia oxidizing archaea (AOA) has changed the paradigm of nitrification being initiated solely by ammonia oxidizing bacteria. In the present study, AOA abundance and diversity was examined in composts produced from combinations of plant waste materials common in tropical agriculture (rice straw, sugar cane bagasse, and coffee hulls), which were mixed with either cow- or sheep-manure. The objective was to determine how AOA abundance and diversity varied as a function of compost system and time, the latter being a contrast between the start of the compost process (mesophilic phase) and the finished product (mature phase). The results showed that AOA were relatively abundant in composts of tropical agricultural wastes, and significantly more so than were the ammonia-oxidizing bacteria. Furthermore, while the AOA communities in the composts were predominatly group I.1b, the communities were diverse and exhibited structures that diverged between compost types and phases. These patterns could be taken as indicators of the ecophysiological diversity in the soil AOA (group I.1b), in that significantly different AOA communties developed when exposed to varying physico-chemical environments. Nitrification patterns and levels differed in the composts which, for the mature material, could have significant effects on its performance as a plant growth medium. Thus, it will also be important to determine the association of AOA (and diversity in their communities) with nitrification in these systems.

Diurnal patterns of denitrification, oxygen consumption and nitrous oxide production in rivers measured at the whole‐reach scale

Summary1. Denitrification, net oxygen consumption and net nitrous oxide flux to the atmosphere were measured in three small rivers (discharge approximately 2–27 m3 s−1) at the whole reach scale during Spring and Summer, 2002. Two of these rivers (Iroquois River and Sugar Creek in north‐west Indiana – north‐east Illinois, U.S.A.) drained agricultural catchments and the other (Millstone River in central New Jersey, U.S.A.) drained a mixed suburban–agricultural catchment.2. Denitrification, oxygen consumption and N2O flux were measured based on net changes in dissolved gas concentrations (N2, O2, and N2O) during riverine transport, correcting for atmospheric exchange. On each date, measurements were made during both light and dark periods.3. Denitrification rates in these rivers ranged from 0.31 to 15.91 mmol N m−2 h−1, and rates within each river reach were consistently higher during the day than during the night. This diurnal pattern could be related to cyclic patterns of nitrification driven by diurnal variations in water column pH and temperature.4. Oxygen consumption ranged from 2.56 to 241 mmol O2 m−2 h−1. In contrast to denitrification, net oxygen consumption was generally higher during the night than during the day.5. River water was consistently supersaturated with N2O, ranging from 102 to 209% saturated. Net flux of N2O to the atmosphere ranged from 0.4 to 60 μmol N m−2 h−1. Net flux of N2O was generally higher at night than during the day. The high flux of N2O from these rivers strengthens the argument that rivers are an important contributor to anthropogenic emissions of this greenhouse gas.

Sources of spatial and temporal variability of inorganic nitrogen in pure and mixed deciduous temperate forests

We investigated the influence of tree canopy composition and structure on the spatial and temporal variability of (i) concentrations of inorganic N (NH 4 + and NO 3 −) and (ii) net N-mineralization and net nitrification, within the temperate forest floor. We compared a pure European beech stand (PS) with a mixed beech–hornbeam one (MS). Three sampling areas were chosen in each stand. Within the PS, the tree locations represented a decreasing gradient of light intensity reaching the forest floor. Within the MS they represented a gradient in the amount of hornbeam leaves present in the litter. In the field NH 4 + and NO 3 − concentrations were measured in the upper mineral soil (UMS) and the overlying organic layers (OL and OF+OH). Field exposures using buried bags were carried out on UMS over 1 year to measure in situ net N-mineralization and net nitrification. Potential net N-mineralization and net nitrification were investigated in summer with UMS, OL and OF+OH incubated at 28 °C for 28 days in the laboratory. We hypothesize that with the presence of a mull-forming species (hornbeam) within a stand dominated by a moder-forming one (European beech), (i) the spatial and (ii) temporal patterns of soil inorganic N concentrations, net N-mineralization and net nitrification would be different in the two stands. Our main results show that tree species composition has an influence on both spatial and temporal patterns of nitrification. The PS exhibited its highest peaks of UMS NO 3 − concentration and net nitrification in spring and early summer while they were highest in the MS in winter. Furthermore, PS exhibited a higher rate of net nitrification than MS. We discuss this unexpected result and suggest that dissolved organic C may be the controlling factor for net nitrification in the MS.

Analytical methods for defining stand–clearcut edge effects demonstrated for N mineralization

Edge effects are becoming an important forest management consideration, but information regarding the influence of edges on N cycling variables has not been well documented. In addition, the quantification of edge effects can benefit from the application of complementary spatial analysis methods. Forest floor N mineralization and environmental variables were intensively measured 5 years after harvest along transects crossing the north and south edges of a 1-ha clearcut, in a high-elevation Engelmann spruce – subalpine fir forest. Wavelet analysis and depth-of-edge influence (DEI) methods were used to locate and measure the spatial extent of edge effects on N mineralization. Then variance partitioning (partial redundancy analysis) was used to examine the influence of edges on N mineralization relative to the influence of other environmental factors. Initial NO3-N content and net nitrification markedly increased in the opening within 2–6 m of each edge. Net ammonification did not exhibit obvious edge-related spatial patterns. Spatial patterns of nitrification appeared to be more closely related to spatial changes in substrate quality than to soil temperature and moisture. Results of the wavelet and DEI analyses provided quantification of locations and functional extents of edge effects.

Nitrification in the Upper Mississippi River: patterns, controls, and contribution to the NO3−budget

We measured nitrification rates in sediment samples collected from a variety of aquatic habitats in Navigation Pool 8 of the Upper Mississippi River (UMR) 7 times between May 2000 and October 2001. We also conducted nutrient-enrichment experiments and analyzed vertical profiles of sediment to determine factors regulating nitrification. Nitrification rates were relatively high compared to other ecosystems (ranging from 0–8.25 μg N cm−2 h−1) and exhibited significant temporal and spatial patterns. Nitrification rates were greatest during the summer and spring compared to autumn and winter (ANOVA, p < 0.05) and were greater in contiguous backwater and impounded habitats compared to main and side-channel habitats (p < 0.05). Regression analysis indicated that nitrification rates were weakly (r2 = 0.18, p < 0.0001) related to temperature and exchangeable NH4+ of the sediment. However, nutrient-enrichment experiments showed that NH4+ availability did not limit nitrification in 3 sediment types with variable organic matter. Vertical profiles of sediment cores demonstrated that oxygen concentration and nitrification had similar patterns suggesting that nitrification may be limited by oxygen penetration into sediments. We conclude that temperature and sediment NH4+ can be useful for predicting broad-scale temporal and spatial nitrification patterns, respectively, but oxygen penetration into the sediments likely regulates nitrification rates in much of the UMR. Overall, we estimated that nitrification produces 6982 mt N/y of NO3− or 7% of the total annual NO3− budget.

Mineralization and nitrification patterns at eight northeastern USA forested research sites

Nitrogen transformation rates in eight northeastern US research sites were measured in soil samples taken in the early season of 2000 and the late season of 2001. Net mineralization and nitrification rates were determined on Oa or A horizon samples by two different sampling methods—intact cores and repeated measurements on composite samples taken from around the cores. Net rates in the composite samples ( n=30) showed three different temporal patterns: high net nitrification with minimal NH 4 + accumulation, high net nitrification and high NH 4 + accumulation, and minimal net nitrification and moderate NH 4 + accumulation. The 4-week net rates in intact cores were about half that of the rates from the composite samples but were well related ( R 2>0.70). Composite samples from sites that exhibited high net nitrification were incubated with acetylene and net nitrification was completely stopped, suggesting an autotrophic pathway. Gross mineralization and nitrification (2000 only) rates were estimated using the isotope dilution technique. Gross rates of nitrification and consumption in intact cores were relatively low. Gross rates of mineralization and net rates of nitrification were both related to the soil C/N ratio, with higher rates generally occurring in sites containing Acer saccharum as a dominant or co-dominant species. The comparison of methods suggests that all provide a similar hierarchy of potential rates but that the degree of net nitrification is strongly influenced by the degree of sample disturbance. Differences between sites appear to be related to an interaction of soil (C/N) and vegetation ( A. saccharum contribution) characteristics.

Impact of seasonal variations and nutrient inputs on nitrogen cycling and degradation of hexadecane by replicated river biofilms.

Biofilm communities cultivated in rotating annular bioreactors using water from the South Saskatchewan River were assessed for the effects of seasonal variations and nutrient (C, N, and P) additions. Confocal laser microscopy revealed that while control biofilms were consistently dominated by bacterial biomass, the addition of nutrients shifted biofilms of summer and fall water samples to phototrophic-dominated communities. In nutrient-amended biofilms, similar patterns of nitrification, denitrification, and hexadecane mineralization rates were observed for winter and spring biofilms; fall biofilms had the highest rates of nitrification and hexadecane mineralization, and summer biofilms had the highest rates of denitrification. Very low rates of all measured activities were detected in control biofilms (without nutrient addition) regardless of season. Nutrient addition caused large increases in hexadecane mineralization and denitrification rates but only modest increases, if any, in nitrification rates, depending upon the season. Generally, both alkB and nirK were more readily PCR amplified from nutrient-amended biofilms. Both genes were amplified from all samples except for nirK from the fall control biofilm. It appears that bacterial production in the South Saskatchewan River water is limited by the availability of nutrients and that biofilm activities and composition vary with nutrient availability and time of year.

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24-year-old, treated with (NH4)2SO4 since 1989 at the rate of 35 kg N·ha–1·year–1). Net nitrification was 141, 114, and 115 kg N·ha–1·year–1, for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO3– of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO3– in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems.

Influence of Initial C/N Ratio on Chemical and Microbial Composition during Long Term Composting of Straw.

Shredded straw of Miscanthus was composted in 800-L boxes with different amounts of pig slurry added as nitrogen source. The impact of the different initial C/N ratios (11, 35, 47, 50, and 54) on the composting process and the end product was evaluated by examining chemical and microbiological parameters during 12 months of composting. Low initial C/N ratios caused a fast degradation of fibers during the first three months of composting (hemicellulose: 50-80%, cellulose: 40-60%), while high initial C/N ratios resulted in 10-20% degradation of both hemicellulose and cellulose. These differences were reflected in the microbial biomass and respiration, which initially were higher in low C/N treatments than in high C/N treatments. After 12 months of composting, this situation was reversed. Composts with high initial C/N ratios had high microbial biomass (15-20 mg ATP g-1 OM) and respiration rates (200 mg CO2 h-1 g-1 OM) compared to treatments with low initial C/N ratios (less than 10 mg ATP g-1 OM and 25 mg CO2 h-1 g-1 OM). This could be explained by the microorganisms being nitrogen limited in the high C/N ratio treatments. In the low C/N ratio treatments, without nitrogen limitation, the high activity in the beginning decreased with time because of exhaustion of easily available carbon. Different nitrogen availability was also seen in the nitrification patterns, since nitrate was only measured in significant amounts in the treatments with initial C/N ratios of 11 and 35. The microbial community structure (measured as phospholipid fatty acid, PLFA, profile) was also affected by the initial C/N ratios, with lower fungal/bacterial ratios in the low compared to the high C/N treatments after 12 months of composting. However, in the low C/N treatments higher levels of PLFAs indicative of thermophilic gram-positive bacteria were found compared to the high C/N treatments. This was caused by the initial heating phase being longer in the low than in the high C/N treatments. The different fungal/bacterial ratios could also be explained by the initial heating phase, since a significant correlation between this ratio and heat generated during the initial composting phase was found.

SOIL MICROBIAL CONTROL OF NITROGEN LOSS FOLLOWING CLEAR-CUT HARVEST IN NORTHERN HARDWOOD ECOSYSTEMS

Establishing the relationship among the spatial distribution of forest ecosystems, N cycling processes, and N loss following harvesting could enable land managers to anticipate and predict the potential for N loss at the scale of local and regional landscapes. In the Great Lakes region, northern hardwood forests with distinct floristic, edaphic, and physiographic characteristics vary predictably across the landscape in N cycling processes, especially in rates of nitrification. Although their landscape distribution and patterns of N cycling are well established, it is uncertain whether this type of information could be used to predict landscape-level patterns of N loss following overstory harvest or other types of disturbance. We studied the microbial processes in soil that control the retention and loss of N following clear-cutting in two northern hardwood ecosystems that are widely distributed in the upper Great Lakes states and differ floristically, edaphically, and in N cycling processes. The overstory of one hardwood ecosystem is dominated by Acer saccharum and Quercus rubra, and the other is dominated by A. saccharum and Tilia americana. These ecosystems differ in annual rates of net nitrification (5 vs. 15 g NO3−-N·m−2·yr−1), and we studied paired intact and clear-cut plots within them to understand whether spatial patterns of nitrification could be used to predict N loss following harvest. We measured microbial biomass, NO3− leaching, denitrification, and microbial N transformations for one year following a clear-cut harvest. Clear-cut harvest led to significant loss of N through NO3− leaching in both ecosystems, averaging 4.9 g N·m−2·yr−1 in clear-cut plots and 0.2 g N·m−2·yr−1 in intact plots. Denitrification was low in both ecosystems (0.08–0.42 g N·m−2·yr−1) and did not increase significantly following clear-cutting (0.16–0.29 g N·m−2·yr−1). Averaged across ecosystems, annual net N mineralization increased by a factor of 2 in clear-cut plots (14.2 g N·m−2·yr−1) relative to intact plots (7.3 g N·m−2·yr−1); net nitrification similarly increased after harvest (11.4 g N·m−2·yr−1 clear-cut vs. 5.5 g N·m−2·yr−1 intact). Gross rates of N mineralization and nitrification displayed a response similar to that of net rates. Although gross rates of N immobilization increased following clear-cutting, microbial biomass did not change. Thus, increased turnover of N through microbial biomass ([biomass N]/[gross N immobilization]) resulted in the observed increase in net N mineralization rates. Our results indicate that microbial immobilization of N was not an important process of N retention following clear-cut harvest in sugar maple-dominated northern hardwood forests. Rather, increases in net N mineralization following harvest increased substrate availability to nitrifying bacteria, which eventually resulted in substantial losses of N through leaching. Regardless of initial differences in net nitrification, harvesting led to similar rates of rates NO3− leaching from both northern hardwood ecosystems. We conclude that N loss following clear-cutting in these forests cannot be predicted by rates of nitrification prior to harvest. Rather, N loss depends on high initial rates of net N mineralization and on the effects of changes in microclimatic conditions and heterotrophic activity on NH4+ availability to nitrifying bacteria following overstory removal.

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

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

Nitrification and denitrification in Wadden Sea sediments (Königshafen, Island of Sylt, Germany) as measured by nitrogen isotope pairing and isotope dilution

Seasonal variation of sediment denitrif~cation and nitrification in darkness was measured by the nitrogen isotope pairing and the nitrate isotope dilution techniques in 3 different sediment types: coarse sand, fine sand and muddy sand (S, A and M, respectively) in intertidal Konigshafen, Island of Sylt, Germany. The temporal pattern of denitrification based on nitrate from the overlying water could be explained by variations in the nitrate concentration and oxygen penetration depth in all 3 sediment types. Highest rates (8 to 48 pm01 m-2 h-') were observed in April 1994 when nitrate concentration in the overlying water was 65 to 70 pM and the lowest (0.2 to 1.1 pm01 m-2 h-') in summer and fall when nitrate was depleted from the water phase. The annual mean dark denitrification, of which 69 to 15% was due to overlying water nitrate, increased in the sequence A, S and M in accord with increasing sediment oxygen uptake. AU 3 sediments types exhibited low nitrification in early winter. At S and M, maximum nitrification was measured in spring and late summer (8 to 17 and 27 to 28 pm01 m--', respectively), whereas nitrification was repressed in mid summer. Maximum dark nitrification here generally occurred at the time of the highest activity of the benthic microalgae (measured as oxygenbased dally gross primary production). At Stn A, maximum nitrification was measured in mid summer (26 pm01 m-2 h-'), when daily gross primary production was low. The 2 different seasonal patterns of nitrification can be explained by differences in oxygen and ammonium availability. In all sediment types, a low degree of coupling between nitrification and denitrification (in relation to total nitrification) was evident, especially during summer or autumn. Annual mean dark nitrification, of which 11 to 27 % was coupled to denitrification, was higher in the muddy sediment than in the 2 sandy sediments.

Seasonal Patterns of Nitrification and Denitrification in a Natural and a Restored Salt Marsh

Seasonal patterns of microbially-mediated nitrogen cycling via the nitrification-denitrification pathway were compared between a natural and a restored salt marsh. Sedimentary denitrification rates, measured with a modification of the acetylene block technique, were approximately 44 times greater in the natural marsh relative to an adjacent transplanted marsh. Nitrification rates were similar at both sites. The difference in denitrification rates was attributed to oxygen inhibition at low tide and tidal flushing of porewater nutrients at high tide in the coarse sediments of the restored marsh. Denitrification was positively correlated with nitrification throughout the year in the natural marsh with a seasonal fall peak in denitrification corresponding to a maximum in porewater ammonia concentration. A weak correlation existed between the two processes in the restored marsh, where nitrification rates exceeded denitrification rates by a factor of 20. Transplanted marsh denitrification rates exhibited a spring peak, corresponding to elevated porewater ammonia concentrations. Our findings demonstrate functional differences in microbial nitrogen dynamics of a young (0–3 yr) restored marsh relative to a mature (>50 yr) salt-marsh system. *** DIRECT SUPPORT *** A01BY070 00008

Nitrification activity in submerged soils and its relation to denitrification loss

Nitrification activity (formation of NO2−+ NO3−per unit soil weight) was measured in the surface layer of 15 presubmerged soils incubated in petri dishes under flooded but aerobic conditions. soils with pH above 5 nitrified quickly, whereas soils with pH below this level did not nitrify or nitrified slowly. The pH values between 7 and 8.5 were optimal for nitrification. Organic-matter levels in the 15 soils of our study did not influence their nitrification activities. In a follow-up greenhouse pot study, after a period of 3 weeks, 15N-balance measurements showed that the loss of N through apparent denitrification did not follow the nitrification patterns of the soils observed in the petri dishes. Apparent denitrification accounted for 16.8% and 18.9% loss of 15N from a soil with insignificant nitrification activity and a soil with high nitrification activity, respectively. These results, thus, indicate a lack of correspondence between the nitrification activities of soil and the denitrification loss of N when the former was measured in the dark and the latter was estimated in the light. Soils that nitrified in the darkness of the incubator did not nitrify in the daylight in the greenhouse.

Nitrification activity in New Zealand grassland soils

Abstract Initial Nitrification Activity (INA) was measured in 68 New Zealand grassland soil sampies (0–75 mm) using a 17-h laboratory perfusion technique to provide a measure of the nitrifying activity. Considerable variation (< 0.02–5.70 μg N oxidised/g soil/h)was found between soils, yellow-brown loams having higher INA values than other soil groups. Variation of pH, % total N, % organic C, and C:N ratio explained only 12–44% of the variation of INA. Four characteristic patterns of nitrification were found when soils were perfused for 300 h. Significant populations of ammonium oxidisers(1.6 × 104–2.2 × 106/g soil) and nitrite oxiders (5.4 × 104-1.9 × 107/g soil) were found in the top 75 mm of 13 soils, even those oflow INA. Field measurements of nitrification activity in 2 soils of low and medium INA values supported the predicted difference in INA. The mean rate of nitrification under field conditions was 0.68 × 10-2 μg N oxidised/g soil/h in the soil of low INA (Wharekohe silt loam) and 5.3 × 10-2 μg N oxidised/g soil/h the soil of medium INA (Kiripaka silt loam). This difference resulted in an average NH 4 - N: NO 3 - N ratio of 16:1 in the former and 1:1 in the latter soil.

Humus nitrification patterns in the broadleaved evergreen Mediterranian Zone

Humus nitrification patterns in the broadleaved evergreen Mediterranian Zone