Differences In Nitrification Research Articles

Low-abundant but highly transcriptionally active uncharacterised Nitrosomonas drive ammonia-oxidation in the Brouage mudflat, France

Exploring differences in nitrification within adjacent sedimentary structures of ridges and runnels on the Brouage mudflat, France, we quantified Potential Nitrification Rates (PNR) alongside amoA genes and transcripts. PNR was lower in ridges (≈1.7 fold-lower) than runnels, despite higher (≈1.8 fold-higher) ammonia-oxidizing bacteria (AOB) abundance. However, AOB were more transcriptionally active in runnels (≈1.9 fold-higher). Sequencing of amoA genes and transcripts revealed starkly contrasting profiles with transcripts from ridges and runnels dominated (≈91 % in ridges and ≈98 % in runnels) by low abundant (≈4.6 % of the DNA community in runnels and ≈0.8 % in ridges) but highly active phylotypes. The higher PNR in runnels was explained by higher abundance of this group, an uncharacterised Nitrosomonas sp. cluster. This cluster is phylogenetically similar to other active ammonia-oxidizers with worldwide distribution in coastal environments indicating its potential, but previously overlooked, contribution to ammonia oxidation globally. In contrast DNA profiles were dominated by highly abundant but low-activity clusters phylogenetically distinct from known Nitrosomonas (Nm) and Nitrosospira (Ns). This cluster is also globally distributed in coastal sediments, primarily detected as DNA, and often classified as Nitrosospira or Nitrosomonas. We therefore propose to classify this cluster as Ns/Nm. Our work indicates that low abundant but highly active AOB could be responsible for the nitrification globally, while the abundant AOB Ns/Nm may not be transcriptionally active, and as such account for the lack of correlation between rate processes and gene abundances often reported in the literature. It also raises the question as to what this seemingly inactive group is doing?

Responses of nitrification and denitrification in the rhizosphere of mudflat paddy to rice genotype and nitrogen fertilization

Nitrification and denitrification are important processes in the biogeochemical cycle in agricultural soils and are influenced by plant genotype and nitrogen (N) fertilization. However, few studies have evaluated the effects of genotype on nitrification and denitrification in the rhizosphere of rice cultivated in mudflats and their response to N fertilization. In this study, we determined the net nitrification and denitrification rates, quantified the abundance of functional genes involved in nitrification and denitrification, and characterized bacterial communities in the rhizosphere of two rice varieties (japonica and hybrid rice) cultivated in mudflat soil with three N fertilization levels. Results showed that net nitrification rates were strongly influenced by genotype and was higher in the rhizosphere of hybrid rice than in that of japonica rice, which might be related to the N absorption strategy of rice, while net denitrification rates were mainly affected by N fertilization. Unlike hybrid rice, the net nitrification rate in the rhizosphere of japonica rice was strongly influenced by N fertilization and positively correlated to N fertilization levels. In contrast, the net denitrification rate of hybrid rice was strongly influenced by N fertilization and positively correlated to N fertilization levels, while N fertilization had no significant effect on the net denitrification rate of japonica rice. Ammonia-oxidizing bacteria were the main drivers of nitrification in the rhizosphere, and denitrification in the rhizosphere of hybrid rice was mainly driven by narG-, nirS-, and norB-type denitrifiers. Furthermore, differences in the assemblages of bacteria associated with the relative abundance of these functional genes provided insight into genotypic differences in nitrification and denitrification responses to N fertilization. Overall, this study provides insight into the underlying mechanisms of nitrification and denitrification influenced by rice genotypes and N fertilization in the rhizospheres of japonica and hybrid rice.

Modification of nitrifying microbial community via DC electrical field application

The performance and operational stability of nitrifying-based treatment processes are closely associated with microbial community structures and species-to-species interactions. In this work, we investigated the effect of intermittent direct current (DC) on the change of nitrifying community structure and process performance in a membrane electro-bioreactor (MEBR). Microbial populations and dynamics were characterized using a fluorescent in situ hybridization (FISH) technique. The outcomes demonstrated that the MEBR system achieved substantial nitrification rates compared to a conventional membrane bioreactor (MBR). The MEBR enhanced the removal of NH3+−N to almost 99%. FISH analysis indicated a dominance of different nitrifying bacteria species in both reactors, leading to differences in nitrification and NH3+−N removal performance. In robust MEBR system, rapid nitrifiers, such as AOB (Nitrosomonas spp.) and NOB (Nitrobacter spp.) were dominat. In contrast, slower nitrifier consortiums like AOB (Nitrosospira spp.) and NOB (Nitrospira spp.) occupied the control MBR. Taken together, the development of fast and robust nitrifying community can lead to extra savings in capital and operational costs of the WWTPs, suggesting the adoption of MEBR oxic/anoxic process. Moreover, a better understanding of biological processes taking place in the new MEBR system and microbial environmental preferences would be valuable for process control and optimization, as well as beneficial for designing bioaugmentation processes, in order to avoid potential washout of target bacteria not adapted to real conditions in bioreactors. Such bio-electrically enhanced processes presented in this work can be used in a new WWTP design or retrofitting existing treatment plants.

Seasonal and spatial characterisation of soil properties, nitrification and denitrification at the urban river-riparian interface with permeable revetments

Understanding the seasonal and spatial variation characteristics and formation mechanism of soil nitrification and denitrification on permeable revetment can provide basis for revetment design and protecting the ecological function of river-riparian interface. In this study, permeable imitation wooden pile revetments, commonly used in Shanghai, were selected. In spring, summer and autumn, soil samples were collected and tested at different distances from the river and depths in the river-riparian interface. The seasonal and spatial variation characteristics of nitrification and denitrification potentials their relationships with soil properties were analysed. The results indicated the seasonal change of soil structure indexes were not significant, but that of the material content related indexes were significant. The influence of soil depth on soil properties is greater than the distance from the river. The soil nitrification potentials during spring (4.43 ± 4.26 mg kg−1 d−1) and autumn (5.24 ± 4.54 mg kg−1 d−1) were significantly higher than that during summer (3.53 ± 2.66 mg kg−1 d−1), whereas the soil denitrification potential was significantly higher during summer (11.65 ± 9.20 mg kg−1 d−1) than during autumn (10.79 ± 8.33 mg kg−1 d−1) and spring (10.21 ± 7.11 mg kg−1 d−1). The seasonal variations of the soil nitrification and denitrification potentials in the occasionally flooded area (0–20 cm depth) were larger than in the frequently flooded area (25–45 cm depth) and the permanently flooded area (55–75 cm depth). The seasonal and spatial differences determine the state of soil submergence, resulting in the difference in soil’s properties, leading to the difference in nitrification and denitrification intensity. The results of this study can provide the guidelines for urban revetment construction for nitrogen removal in riparian zones.

Differences in nitrification and ammonium-oxidising prokaryotes in the process of wetland restoration

Ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB) are ammonium oxidising prokaryotes that can drive soil nitrification in wetlands. During the restoration of wetlands, different types of land use soils (agricultural soil [AS], restored wetland soil [RS], and natural wetland soil [NWS]) are present. However, studies on the effects of changes in the types of land use in wetlands during restoration on nitrification and the community composition of AOA and AOB are still not well understood. In this study, the differences in the potential nitrification rate (PNR) and community composition of AOA and AOB in AS, RS, and NWS were compared and discussed. The results indicated that the PNRs in the AS, RS, and NWS were on the same order of magnitude. Nitrification was mainly driven by AOB. High-throughput sequencing results showed that the genus Nitrososphaera of AOA and unclassified_o_Nitrosomonadales of AOB were only detected in the AS. Redundancy analysis (RDA) results indicated that the community composition of AOA was mostly influenced by pH, while TC was the most influential variable on the community composition of AOB. Our study provides a basis for distinguishing the roles of ammonium-oxidising prokaryotes in nitrification and further understanding the changes in nitrifying activity in wetlands during restoration.

Variable responses of nitrification and denitrification in a paddy soil to long-term biochar amendment and short-term biochar addition

Biochar-mediated change in soil N transformations has gained much attention. Biochar properties undergo changes through ageing process that may impact variably on N cycling, yet the degree to which soil N transformations may be impacted by biochar ageing process on long-time scale remains unclear. Here, we compared the effects on nitrification, denitrification and N2/N2O gas flux of short-term fresh biochar addition with long-term biochar amendment over six consecutive years in a paddy soil under identical amounts of soil organic C contents. We found that short-term treatments elicited a stronger effect on nitrification than long-term treatments, due to more positive effects on soil pH and ammonia monooxygenase gene (amoA) abundance of ammonia oxidizing bacterial (AOB). However, there were contrasting effects on soil denitrification and total N gas loss for two types of biochar addition, where the long-term treatments limited availability of C and N, and decreased net NO3− reduction and total N gas (N2O + N2) losses by up to 55.5% and 25%. Short-term biochar addition increased dissolved organic C, nutrient content and soil pH, enhanced net NO3− reduction by 88.7%, also reduced the ratio of nitrite reductase genes (nirS and nirK) and nitrous oxide reductase gene (nosZ) and N2O emissions, but increased total N gas loss by 0.2–1.2 times. Our study quantified the differences in nitrification and denitrification capacity in response to short-term biochar addition and long-term biochar amendment and bridged the knowledge-gaps of fresh and aged biochar-mediated N transformation.

Ammonia Oxidizers in a Grazing Land with a History of Poultry Litter Application.

Poultry litter (PL) is widely applied on grazing lands in Georgia. However, it is not clear how its long-term use affects soil microorganisms and their function. We examined changes in activity and community structure of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in a grazing land with a history of PL application and compared it to treatment with urea ammonium nitrate (UAN). Soil samples (0-15 cm) were collected in 2009 (after 15 yr of PL application) and in 2013 (after 2 yr of no application). The abundance and community composition of ammonia oxidizers (AO) were determined with molecular techniques that targeted Nitrification potential (NP) was used for measuring their activity. Abundance of AO was significantly higher in PL (7.41 and 7.10 log copies g soil for AOB and AOA, respectively) than in UAN plots (6.82 and 6.50 log copies g soil for AOB and AOA, respectively) in 2009. This is consistent with NP, which was higher in PL (0.78 mg NO -N kg h) than in UAN (0.50 mg NO-N kg h) plots in 2009. The abundance of AO and NP decreased in 2013. There was no treatment effect on the composition of AO. Correlation analysis suggested that AOB were functionally more important than AOA, indicating the need to target AOB for efficient management of N in PL-receiving soils. Overall, the difference in nitrification between PL and UAN was mainly caused by the change in AO abundance rather than composition, and AO were not negatively affected by an increase in PL-derived trace metal concentrations.

Differences in activity and N demand between bacteria and fungi in a microcosm incubation experiment with selective inhibition

Bacteria and fungi are important micro-organisms in the soil, but may differ in their impact on net N-mineralization. The hypothesis was tested that fungi are characterized by low microbial activity, but also low immobilization, and bacteria by high activity and high immobilization. A one-month laboratory incubation experiment with selective inhibition of fungi (cycloheximide) or bacteria (streptomycin) was conducted with samples of organic layer and mineral topsoil (0–10cm) from neutral, bacteria-dominated and acidic, fungi-dominated Luxembourg beech forests. In the control treatment, respiration was higher in neutral than in acidic soil, but net N-mineralization was lower, due to higher immobilization. In the antibiotic treatments, differences in nitrification suggest that selective inhibition indeed occurred; in all soils and horizons, nitrification was especially limited by bactericide. Besides as inhibitor of the target group, antibiotics may also serve as source of C and N for the non-target group. For both bactericide and fungicide, acidic soils showed higher net recovery of C and N from antibiotics than neutral soil, which suggests that uptake or sorption of antibiotics is higher in the latter. Clear differences between neutral and acidic soils arose when the main micro-organisms were stimulated. In bacteria-dominated neutral soil, application of fungicide led to increased microbial respiration. In fungi-dominated acidic soil, however, application of bactericide did not lead to higher respiration, but to increased net N-mineralization per unit respiration, which supports a lower immobilization. Differences between antibiotics were consistent for organic layer and mineral topsoil, with increase in activity with fungicide, and lower immobilization with bactericide. The results provide correlative and experimental evidence that reduced immobilization by fungi compensates for their lower rates of activity with respect to N-availability to the vegetation.

Controls on soil nitrification and stream nitrate export at two forested catchments

Dormant season inorganic nitrogen (N) leaching varies considerably among forested catchments with similar bedrock, forest cover and deposition history. Recent work has highlighted the importance of winter rain-on-snow (ROS) events as a source of winter nitrate (NO3-N) export, but differences among streams are likely due to differences in baseflow NO3-N concentrations, and thus soil N processes. The objective of this study was to investigate rates of N-mineralization and nitrification as well as their potential environmental controls throughout the year, but with particular focus on the winter season in south-central Ontario, Canada. Field incubations were utilized to assess differences in NO3-N and ammonium production over time and across topographic positions in two catchments with contrasting patterns of N export. Rates of nitrification were similar to rates of total mineralization, and nitrification rates were significantly higher during the summer and spring compared with the winter and fall; however, winter nitrification was substantial, and ranged from 19 to 36 % of annual rates. Seasonal differences in nitrification were largely driven by temperature, soil moisture and inorganic N concentration in soil. Rain and melting snow infiltrated the soil during ROS events, which were associated with increased NO3-N availability, particularly in well-drained soils, and ROS-induced increases in stream nitrate concentrations were largest at the catchment dominated by well-drained soil. Annual nitrification fluxes were almost two orders of magnitude greater than N deposition or NO3-N leaching fluxes at either catchment. Similar rates of NO3-N production within the two catchments suggest that consumption of NO3-N within wet soils is responsible for the 10-fold difference in NO3-N export between the two streams. Notably, these results suggest that consumption processes were important for reducing NO3-N export even during winter ROS events.

Nitrate δ 15N and δ 18O evidence for active biological transformation in the Changjiang Estuary and the adjacent East China Sea

The Changjiang Estuary has been considered as one of the most polluted estuaries in the world due to high nitrate (NO 3 − ) input, especially in spring and summer. In this study, δ 15N and δ 18O of NO 3 − , along with other chemical parameters in this area, were measured in spring to evaluate NO 3 − biogeochemical processes. A simple two end-members mixing model was used to examine the relative contribution of the Changjiang River Diluted Water and marine water to NO 3 − sources in the Changjiang Estuary and the adjacent East China Sea. The isotopic signals show that NO 3 − behaved relatively and conservatively in Transect F and Transect P where assimilation was weak possibly due to vertical mixing, while active assimilation and weak nitrification occurred in Transect D. Spatial difference in assimilation was indicated by the ∼1:1 enrichment of δ 15N and δ 18O in the three transects, while spatial difference in nitrification was reflected by deviations of δ 15N and δ 18O from assimilation line. Our results suggest that the input of the Changjiang River Diluted Water promoted NO 3 − assimilation possibly by stratifying the water column which favored the phytoplankton growth.

Initial recovery of soil carbon and nitrogen pools and dynamics following disturbance in jack pine forests: A comparison of wildfire and clearcut harvesting

Forests naturally maintained by stand-replacing wildfires are often managed with clearcut harvesting, yet we know little about how replacing wildfire with clearcutting affects soil processes and properties. We compared the initial recovery of carbon (C) and nitrogen (N) pools and dynamics following disturbance in jack pine ( Pinus banksiana) stands in northern Lower Michigan, USA, by sampling soils (Oa+A horizons) from three “treatments”: 3–6-year-old harvest-regenerated stands, 3–6-year-old wildfire-regenerated stands and 40–55-year-old intact, mature stands ( n=4 stands per treatment). We measured total C and N; microbial biomass and potentially mineralizable C and N; net nitrification; and gross rates of N mineralization and nitrification. Burned stands exhibited reduced soil N but not C, whereas clearcut and mature stands had similar quantities of soil organic matter. Both disturbance types reduced microbial biomass C compared to mature stands; however, microbial biomass N was reduced in burned stands but not in clearcut stands. The experimental C and N mineralization values were fit to a first-order rate equation to estimate potentially mineralizable pool size (C 0 and N 0) and rate parameters. Values for C 0 in burned and clearcut stands were approximately half that of the mature treatment, with no difference between disturbance types. In contrast, N 0 was lowest in the wildfire stands (170.2 μg N g −1), intermediate in the clearcuts (215.4 μg N g −1) and highest in the mature stands (244.6 μg N g −1). The most pronounced difference between disturbance types was for net nitrification. These data were fit to a sigmoidal growth equation to estimate potential NO 3 − accumulation (Nit max) and kinetic parameters. Values of Nit max in clearcut soils exceeded that of wildfire and mature soils (149.2 vs. 83.5 vs. 96.5 μg NO 3 −–N g −1, respectively). Moreover, the clearcut treatment exhibited no lag period for net NO 3 − production, whereas the burned and mature treatments exhibited an approximate 8-week lag period before producing appreciable quantities of NO 3 −. There were no differences between disturbances in gross rates of mineralization or nitrification; rather, lower NO 3 − immobilization rates in the clearcut soils, 0.20 μg NO 3 − g −1 d −1 compared to 0.65 in the burned soils, explained the difference in net nitrification. Because the mobility of NO 3 − and NH 4 + differs markedly in soil, our results suggest that differences in nitrification between wildfire and clearcutting could have important consequences for plant nutrition and leaching losses following disturbance.

Water table elevation controls on soil nitrogen cycling in riparian wetlands along a European climatic gradient

Riparian zones have long been considered as nitrate sinks in landscapes. Yet, riparian zones are also known to be very productive ecosystems with a high rate of nitrogen cycling. A key factor regulating processes in the N cycle in these zones is groundwater table fluctuation, which controls aerobic/anaerobic conditions in the soil. Nitrification and denitrification, key processes regulating plant productivity and nitrogen buffering capacities are strictly aerobic and anaerobic processes, respectively. In this study we compared the effects of these factors on the nitrogen cycling in riparian zones under different climatic conditions and N loading at the European scale. No significant differences in nitrification and denitrification rates were found either between climatic regions or between vegetation types. On the other hand, water table elevation turned out to be the prime determinant of the N dynamics and its end product. Three consistent water table thresholds were identified. In sites where the water table level is within −10 cm of the soil surface, ammonification is the main process and ammonium accumulates in the topsoils. Average water tables between −10 and −30 cm favour denitrification and therefore reduce the nitrogen availability in soils. In drier sites, that is, water table level below −30 cm, nitrate accumulates as a result of high net nitrification. At these latter sites, denitrification only occurs in fine textured soils probably triggered by rainfall events. Such a threshold could be used to provide a proxy to translate the consequences of stream flow regime change to nitrogen cycling in riparian zones and consequently, to potential changes in nitrogen mitigation.

Mechanisms of plant species impacts on ecosystem nitrogen cycling

AbstractPlant species are hypothesized to impact ecosystem nitrogen cycling in two distinctly different ways. First, differences in nitrogen use efficiency can lead to positive feedbacks on the rate of nitrogen cycling. Alternatively, plant species can also control the inputs and losses of nitrogen from ecosystems.Our current understanding of litter decomposition shows that most nitrogen present within litter is not released during decomposition but incorporated into soil organic matter. This nitrogen retention is caused by an increase in the relative nitrogen content in decomposing litter and a much lower carbon‐to‐nitrogen ratio of soil organic matter. The long time lag between plant litter formation and the actual release of nitrogen from the litter results in a bottleneck, which prevents feedbacks of plant quality differences on nitrogen cycling. Instead, rates of gross nitrogen mineralization, which are often an order of magnitude higher than net mineralization, indicate that nitrogen cycling within ecosystems is dominated by a microbial nitrogen loop. Nitrogen is released from the soil organic matter and incorporated into microbial biomass. Upon their death, the nitrogen is again incorporated into the soil organic matter. However, this microbial nitrogen loop is driven by plant‐supplied carbon and provides a strong negative feedback through nitrogen cycling on plant productivity. Evidence supporting this hypothesis is strong for temperate grassland ecosystems. For other terrestrial ecosystems, such as forests, tropical and boreal regions, the data are much more limited. Thus, current evidence does not support the view that differences in the efficiency of plant nitrogen use lead to positive feedbacks.In contrast, soil microbes are the dominant factor structuring ecosystem nitrogen cycling. Soil microbes derive nitrogen from the decomposition of soil organic matter, but this microbial activity is driven by recent plant carbon inputs. Changes in plant carbon inputs, resulting from plant species shifts, lead to a negative feedback through microbial nitrogen immobilization. In contrast, there is abundant evidence that plant species impact nitrogen inputs and losses, such as: atmospheric deposition, fire‐induced losses, nitrogen leaching, and nitrogen fixation, which is driven by carbon supply from plants to nitrogen fixers. Additionally, plants can influence the activity and composition of soil microbial communities, which has the potential to lead to differences in nitrification, denitrification and trace nitrogen gas losses. Plant species also impact herbivore behaviour and thereby have the potential to lead to animal‐facilitated movement of nitrogen between ecosystems.Thus, current evidence supports the view that plant species can have large impacts on ecosystem nitrogen cycling. However, species impacts are not caused by differences in plant quantity and quality, but by plant species impacts on nitrogen inputs and losses.

Low temperature nitrogen removal in sequencing batch reactors

A nitrogen removal study was conducted in four parallel reactors: two 8 h cycle and two 12 h cycle reactors, all utilizing 2 h anoxic “fill and mix” and 2 h “settle and decant” periods per cycle. Temperature was lowered incrementally from 15 to 7°C, 5 to 2°C. The hydraulic retention time was 24 h. Raw sewage mixed with the high-in-nitrogen pharmaceutical wastewater was used. At 15°C there was little difference in nitrification between the 8 and the 12 h cycle reactors. At and below 7°C the 12 h cycle reactors exhibited superior nitrification rates and inferior denitrification rates in comparison to the 8 h cycle reactors. Response to the decrease in temperature was discontinuous. The temperature correction factors were θ N = 1.02 (15-7°C), and θ N = 1.40 (<7-2°C). For denitrification, the effects of temperature were represented by θ DN = 1.06 (15-7°C) and θ DN = 1.30 (<7-2°C). The minimum specific nitrogen removal rates, at 2°C were K N = 8 gN(kg VSSd) −1 and K DN = 3–6 gN(kg VSSd) −1.

Aboveground Production and N and P Cycling Along a Nitrogen Mineralization Gradient on Blackhawk Island, Wisconsin

Net aboveground production (4.1—9.5 Mg°ha—1) across a series of edaphic climax forests was highly correlated with field measurements of soil N mineralization (26—84 kg°ha—1°yr—1; r2 = 0.902, P &lt; .001) and with soil silt + clay content (5—74%; r2 = 0.883, P &lt; .001). Soil N mineralization was positively correlated with litter production and N and P return in litter. Soil N mineralization was negatively correlated with litter C:N and C:P ratios and with efficiency of P use in litter production. Efficiency of N use in litter production declined with increasing N mineralization except for inefficient use of N in a hemlock stand at low N mineralization. Changes of litter quality across the mineralization—soil texture gradient were due to species replacement rather than changes in litter quality within each species. Nitrification was not correlated with aboveground production. Both mineralization and nitrification were highly correlated with humus P content. Differences in nitrification among the soils appeared to be related to PO4—P supply in the spring and early summer and to NH4—N supply in mid— to late summer.

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Effect of soil moisture levels on nitrification.

Nitrification of 100 p.p.m. added NH4-N was studied in two soils of Georgia. Moisture levels tried were equivalent to those equilibrated at soil tension of 15, 8, 4, 2, 0.3, 0.0 bars on the release curve and a flooded condition. Moisture was applied as crushed ice in a cold room at −5 °C. Samples were incubated at 25 °C with a constant stream of humidified air passing over them. Nitrification rate in Cecil sandy loam increased as the soil moisture content increased from 15 to 2 bars and decreased thereafter. Marked nitrification occurred even under flooded conditions. Unaccounted-for nitrogen amounted to 30% under flooded condition. Nitrification in Norfolk loamy sand was exceptionally high, nitrification being 73% complete within 2 weeks at 15 bars. There was practically no difference in nitrification at 15 to 0.3 bar. Unaccounted-for nitrogen amounted to over 60% in flooded soils.

The Formation of Nitrate from Ammonium Nitrogen in Soils: III Influence of Temperature and Initial Population of Nitrifying Organisms on the Maximum Rate and Delay Period

AbstractA number of soils of varying properties were incubated at several temperatures and rates of inoculation of nitrifying organisms to determine the influence of these factors on nitrification. The time‐nitrification curve generally exhibited a “delay” phase and a “maximum rate” phase followed by a phase in which the rate of nitrification declined as the substrate was exhausted and the end product concentration increased. Each phase must be measured and characterized before difference in nitrification in soils can be fully understood and described.In soils incubated at field capacity, the maximum nitrification rates increased from immeasurably low values to as great as 900 ppm. per week in some soils and the delay periods decreased from about 32 weeks to &lt; 1 day, as temperatures increased from 0 to 25° C. Increase in initial population of nitrifying organisms caused decreases in delay periods but did not appreciably affect the maximum rate above 10° C. The influence of temperature and nitrifying population on delay periods and maximum rates varied greatly among soils indicating that other factors inherent in the soils also affect nitrification.

Field nitrates in the Gezira soil. II

The yields of cotton from five three-course rotations are given; they show marked seasonal variations, the variations of the different rotations being in close accord with one another. The average yields of the rotations are related fairly closely to the average soil nitrates; in individual years this is not so, though considerable responses to nitrogenous manuring are always obtained on Gezira soil.The distribution of nitrate in Gezira soil shows an unusual accumulation at depth; though cotton roots can reach the zone of accumulation, the variability of nitrate at these depths renders it impossible to relate changes to uptake. The figures show that under growing cotton the whole of the upper 4 ft. of soil can have low nitrates (of the order of 1 p.p.m.). Unless the roots obtain nitrate from the fifth and sixth feet accumulations the plants must be taking up nitrate as fast as it is formed, and the nitrate-supplying power of the soil must be of first importance. Figures are given showing differences in nitrification in soil from the different rotations.The effect of heavy nitrogenous manuring on soil nitrate is shown to be largely confined to the year of application, and no accumulations could be demonstrated. The average recovery of added nitrogen in this experiment is only of the order of 10%. The yields of the heavily manured treatments show the same violent fluctuations as do the unmanured treatments.In the three-course rotation experiment surface nitrates in August and September in the wider rotations are strongly negatively correlated with the rainfall of the period 1 July to 15 August. Over the period July to September the distribution of nitrates in the upper 2 ft. of the profile changes; there are differences between the rotations in this respect.