Soil nitrogen availability and nitrification in Mediterranean shrublands of varying fire history and successional stage

The short-term effect of a single fire, and the long-term effect of recent fire history and successional stage on total and mineral N concentration, net nitrogen mineralization, and nitrification were evaluated in soils from a steep semi-arid shrubland chronosequence in southeast Spain. A single fire significantly increased soil mineral N availability and net nitrification. Increasing fire frequency in the last few decades was. associated with a sharp decrease in surface soil organic matter and total N concentrations and pools, and with changes in the long-term N dynamic patterns. The surface-soil extractable NH₄ ⁺:NO₃ ^– ratio increased throughout the chronosequence. All net mineralized N in laboratory incubations from all sites was converted to nitrate, suggesting that allelochemic inhibition of net nitrification is probably not important in this system. Net nitrification in samples during incubation increased through the sere. The maximum rate of net nitrification (k_max) increased through the first three stages of the sere. A linear relationship was found between total soil N and N mineralization, and both k_max and net nitrification for the first three stages of the sere, suggesting that total N and ammonification are likely to be the control mechanisms of nitrification within the sere. The oldest site exhibited the lowest specific k_max and the highest, potential soil respiration rate suggesting that a lower N quality and increasing competition for ammonium might also limit nitrification at least in the long-unburned garrigue site.     

Nitrification and nitrous oxide production potentials in aerobic soil samples from the soil profile of a Finnish coniferous site receiving high ammonium deposition

Abstract Using aerobic soil slurry technique nitrification and nitrous oxide production were studied in samples from a pine site in Western Finland. The site received atmospheric ammonium deposition of 7–33 kg N ha⁻¹ a⁻¹ from a mink farm. The experiments with soil slurries showed that the nitrification potential in the litter layer was higher at pH 6 than at pH 4. However, the nitrification potentials in the samples from the organic and mineral horizons at pH 6 and 4 were almost equal. Also N₂O was produced at a higher rate at pH 6 than at pH 4 in slurries of the litter layer samples. The reverse was true for samples from the organic and mineral horizons. The highest N₂O production and nitrification rates were measured in the suspensions of litter layer samples. Nitrification activity in field-moist soil samples was lower than the activity in the slurries indicating that the availability of ammonium limited nitrification in these soils. Acetylene (2.5 kPa) retarded nitrification activity (70-–100%) and N₂O production (40 – 90%) in soil slurries. Acetylene inhibited the N₂O production by 40–60% during the first 3 days after its addition to field-moist samples incubated in aerobic atmosphere. After 3 days the inhibition became much lower (4–5%). The results indicate that, in soil profiles of boreal coniferous forests receiving ammonium deposition, chemolithotrophic nitrification may have importance in the N₂O production, and that changes in soil pH affect differently nitrification as well as N₂O production in litter and deeper soil layers.     

Mineralization of soil nitrogen in three forest communities from the New England region of New South Wales

Nitrogen (N) mineralization rates and the temperature response patterns of mineral N production in surface (0–7.6 cm) soils were compared in laboratory incubation studies based on disturbed, composite samples. Seasonal variation in the field levels of mineral N, and mineralization potential of intact (7.6 × 5.6 cm diameter) soil cores, were also investigated. Ammonification proceeded rapidly in each soil. Nitrification did not occur in grassy forest (GF) soil but was active in both layered forest (LF) and mossy forest (MF) soils, especially the former. Total mineral N production was greatest in MF and least in LF. Ammonification in disturbed samples was maximal at 50°C in all three soils with a secondary peak at 10°C in LF soil. Nitrification in LF and MF soils was most rapid at 25°C. Several species of ammonifying bacteria with different temperature optima were isolated, indicating that the process of ammonification is a composite of the activities of a variety of decomposer microbes. Mean field levels of mineral N and NH₄–N throughout the year were greatest in MF and least in LF. Seasonal fluctuations in NH₄–N were evident, concentrations being universally low in mid-winter (about 1.5 μgg^-1), increasing to a maximum in late summer (about 5 μg g^-1 in LF: 16–18 μg g^-1 in GF and MF). Field levels of NO₃–N were more constant and never more than 5 μg g^-1 in any community. Both total mineralization and ammonification in intact cores were greatest in MF and least in LF while nitrification was greatest in LF and almost negligible in GF, thus confirming the results obtained with disturbed samples. The potential for mineralization was large in mid-winter when the amount of mineral N was very low, and small in late summer when field levels were higher: this is interpreted as indicating that seasonal climatic factors regulate the availability of substrates for decomposers. Spatial variability in field levels of mineral N and mineral N production in the laboratory was evidenced by significant ‘sampling site’ effects in each community: however, at the sampling intensity used, the presence of bark mounds around Eucalyptus saligna trees could not be shown to affect these attributes. The inability of GF soil to nitrify when incubated in the laboratory could not be ascribed to a high C/N ratio, low pH, lack of substrate ammonium, or a low population of autotrophic nitrifying bacteria. No attempt was made to investigate the presence of allelopathic nitrification inhibitors. No evidence was obtained to support the view that nitrification is atypical of climax communities in situ. The most productive forest (LF) had the greatest capacity to nitrify and the least productive community (GF) the smallest capacity to do so.     

根系分泌物C∶N对刺槐林地土壤理化特征和土壤呼吸的影响

为探究根系分泌物C∶N对土壤养分循环及微生物活性的影响,本研究以黄土高原人工刺槐林为对象,在生境条件基本一致的15、25、35、45 a刺槐林地取原位土壤,通过模拟不同C∶N的根系分泌物(只添加N、C∶N=10、C∶N=50、C∶N=100和只添加C)添加至土壤,以去离子水作为对照,分析根系分泌物C∶N对土壤碳、氮、磷、pH值等理化特征和土壤呼吸的影响。结果表明: 1)有机碳含量与根系分泌物C∶N呈正相关,根系分泌物C∶N=10时土壤有机碳(SOC)分解较快,高根系分泌物C∶N(C∶N=100)能延缓SOC分解,而只添加C处理对SOC无显著影响。2)不同C∶N根系分泌物处理对全氮的影响不明显,碳添加能促进微生物对铵态氮的吸收,氮添加能促进铵态氮的硝化,随着根系分泌物C∶N增加,土壤中铵态氮含量下降。3)氮添加会导致土壤pH值下降,增加土壤全磷含量。4)刺槐林地土壤呼吸值与根系分泌物C∶N呈正相关,随着C∶N增加,根系分泌物对25和35 a人工刺槐林土壤呼吸的促进作用更显著。综上,根系分泌物C∶N值越高,对人工刺槐林土壤呼吸的促进作用越显著。研究结果进一步加深了对森林根系-土壤-微生物互作过程的认识。     

新异养硝化菌Colloides sp.JZ1-1的鉴定及其硝化性能

从驯化后的活性污泥中筛分、诱变出一株性能较好的异养硝化菌JZ1-1.经形态及生理生化特性分析,鉴定菌株JZ1-1为胶样菌属(Colloides sp.).分别考察了碳源、C/N、pH、溶解氧、温度和铵态氮初始浓度对JZ1-1硝化性能的影响.结果表明:菌株对柠檬酸钠的利用较好;C/N为10 ～14、30℃、pH 6-9和转速150 r·min-1以上有利于铵态氮的降解;菌株对中高浓度铵态氮废水(100 mg·L-1≤铵态氮浓度≤500 mg·L-1)的降解效果显著.经5次继代培养,菌株的稳定性较好.     

Heterotrophic nitrification by Arthrobacter sp. (strain 9006) as influenced by different cultural conditions,growth state and acetate metabolism

An Arthrobacter sp. (strain 9006), isolated from lake water, accumulated nitrite up to about 15 mg N/l, but no nitrate. In a mineral medium supplemented with tryptone, yeast extract, acetate and ammonium, the cells released nitrite into the medium parallel to growth or when growth had virtually ceased. The nitrite formed was proportional to the initial acetate concentration, indicating an involvement of acetate metabolism with nitrification. The organism grew with a wide variety of organic carbon sources, but washed cells formed nitrite from ammonium only in the presence of citrate, malate, acetate or ethanol. Magnesium ions were required for nitrification of ammonium and could not be replaced by other divalent metal ions. Analysis of the glyoxylate cycle key enzymes in washed suspensions incubated in a minimal medium revealed that isocitrate lyase and malate synthase were most active during the nitrification phase. Nitrite accumulation but not growth was inhibited by glucose, tryptone and yeast extract. A possible explanation for the different nitrification patterns during growth is based on the regulatory properties of glyoxylate cycle enzymes.Abbreviations IL Isocitrate lyase [threo-D_s-isocitrate glyoxylate-lase, E.C. 4.1.3.1.] - MS malate synthase [l-malate glyoxylate-lyase (CoA-acetylating), E.C. 4.1.3.2.]     

Effect of influent COD/N ratio on biological nitrogen removal (BNR) from high-strength ammonium industrial wastewater

The effect of influent COD/N ratio on biological nitrogen removal (BNR) from high-strength ammonium industrial wastewater was investigated. Experiments were conducted in a modified Ludzack–Ettinger pilot-plant configuration for 365 days. Total nitrification of an influent concentration of 1200 mg NH₄⁺–N l⁻¹ was obtained in this period. Influent COD/N ratios between 0.71 and 3.4 g COD g N⁻¹ were tested by varying the nitrogen loading rate (NLR) supplied to the pilot plant. An exponential decrease of nitrification rate was observed when the influent COD/N ratio increased.

The experimental COD/N ratio for denitrification was 7.1±0.8 g COD g N⁻¹ while the stoichiometric ratio was 4.2 g COD g N⁻¹. This difference is attributable to the oxidation of organic matter in the anoxic reactor with the oxygen of the internal recycle. The influence of influent COD/N ratio on the treatment of high-strength ammonium industrial wastewater can be quantified with these results. The influence of COD/N ratio should be one of the main parameters in the design of biological nitrogen removal processes in industrial wastewater treatment.     

Stable-Isotope Analysis of a Combined Nitrification-Denitrification Sustained by Thermophilic Methanotrophs under Low-Oxygen Conditions

To simulate growth conditions experienced by microbiota at O(inf2)-limited interfaces of organic matter in compost, an experimental system capable of maintaining dual limitations of oxygen and carbon for extended periods, i.e., a pO(inf2)-auxostat, has been used. (sup15)N tracer studies on thermophilic (53(deg)C) decomposition processes occurring in manure-straw aggregates showed the emission of dinitrogen gas from the reactor as a result of simultaneous nitrification and denitrification at low pO(inf2) values (0.1 to 2.0%, vol/vol). The N loss was confirmed by nitrogen budget studies of the system. Depending on the imposed pO(inf2), 0.6 to 1.4 mmol of N/day (i.e., 20 to 40% of input N) was emitted as N(inf2). When the pO(inf2) was raised, the rates of both nitrification and denitrification increased instantaneously, indicating that ammonia oxidation was limited by oxygen. In auxostats permanently running at pO(inf2) >= 2% (vol/vol), the free ammonium pool was almost completely oxidized and was converted to nitrite plus nitrate and N(inf2) gas. Labelling of the auxostat with [(sup13)C]carbonate was conducted to reveal whether nitrification was of autotrophic or heterotrophic origin. Incorporation of (sup13)CO(inf2) into population-specific cellular compounds was evaluated by profiling the saponifiable phospholipid fatty acids (FAs) by using capillary gas chromatography and subsequently analyzing the (sup13)C/(sup12)C ratios of the individual FAs, after their combustion to CO(inf2), by isotope ratio mass spectrometry. Apart from the observed label incorporation into FAs originating from a microflora belonging to the genus Methylococcus (type X group), supporting nitrification of a methylotrophic nature, this analysis also corroborated the absence of truly autotrophic nitrifying populations. Nevertheless, the extent to which ammonia oxidation continued to exist in this thermophilic community suggested that a major energy gain could be associated with it.     

Factors influencing spatial variability in nitrogen processing in nitrogen-saturated soils

Nitrogen (N) saturation is an environmental concern for forests in the eastern U.S. Although several watersheds of the Fernow Experimental Forest (FEF), West Virginia exhibit symptoms of N saturation, many watersheds display a high degree of spatial variability in soil N processing. This study examined the effects of temperature on net N mineralization and nitrification in N-saturated soils from FEF, and how these effects varied between high N-processing vs. low N-processing soils collected from two watersheds, WS3 (fertilized with [NH4]2SO4) and WS4 (untreated control). Samples of forest floor material (O1 horizon) and mineral soil (to a 5-cm depth) were taken from three subplots within each of four plots that represented the extremes of highest and lowest rates of net N mineralization and nitrification (hereafter, high N and low N, respectively) of untreated WS4 and N-treated WS3: control/low N, control/high N, N-treated/low N, N-treated/high N. Forest floor material was analyzed for carbon (C), lignin, and N. Subsamples of mineral soil were extracted immediately with 1 N KCl and analyzed for NH4+ and NO3- to determine preincubation levels. Extracts were also analyzed for Mg, Ca, Al, and pH. To test the hypothesis that the lack of net nitrification observed in field incubations on the untreated/low N plot was the result of absence of nitrifier populations, we characterized the bacterial community involved in N cycling by amplification of amoA genes. Remaining soil was incubated for 28 d at three temperatures (10, 20, and 30 degrees C), followed by 1 N KCl extraction and analysis for NH4+ and NO3-. Net nitrification was essentially 100% of net N mineralization for all samples combined. Nitrification rates from lab incubations at all temperatures supported earlier observations based on field incubations. At 30 degrees C, rates from N-treated/high N were three times those of N-treated/low N. Highest rates were found for untreated/high N (two times greater than those of N-treated/high N), whereas untreated/low N exhibited no net nitrification. However, soils exhibiting no net nitrification tested positive for presence of nitrifying bacteria, causing us to reject our initial hypothesis. We hypothesize that nitrifier populations in such soil are being inhibited by a combination of low Ca to Al ratios in mineral soil and allelopathic interactions with mycorrhizae of ericaceous species in the herbaceous layer.     

不同硝化抑制剂对红壤氮素硝化作用及玉米产量和氮素利用率的影响

本研究分析添加不同种硝化抑制剂及其组合的高效稳定性氯化铵氮肥对红壤硝化作用、玉米产量和氮肥利用率的影响,旨在筛选出适合酸性红壤的高效稳定性氯化铵态氮肥。在氯化铵中分别添加硝化抑制剂2-氯-6-三甲基吡啶(CP)、3,4-二甲基吡唑磷酸盐(DMPP)和双氰胺(DCD)及其组合,制成6种高效稳定性氯化铵态氮肥,以不施氮肥(CK)和施氯化铵(N)为对照,进行等氮量玉米盆栽试验。结果表明: 与N处理相比,CP+DMPP和DMPP+DCD处理红壤中铵态氮含量提高56%～62%,显著高于CP、DMPP和DCD处理;土壤表观硝化率显著降低33%～34%。添加硝化抑制剂及其组合的6个处理均显著提高了玉米生物量和氮肥吸收利用率。与N处理相比,单独添加硝化抑制剂处理生物量均显著高于硝化抑制剂组合处理,平均提高1.3倍;添加DCD处理效果最显著,玉米籽粒产量、吸氮量和氮肥吸收利用率分别显著提高4.1、6.3和4.4倍。为了达到既能低成本又能提高产量和氮肥利用率的效果,在红壤上添加硝化抑制剂DCD是最佳选择。     

The release,diffusion and nitrification of nitrogen in soils surrounding sulfur-coated urea granules

Summary We have compared sulfur-coated urea granules (SCU) with ammonium sulfate granules (AS) in regard to nitrogen (N) release, diffusion, nitrification and the effect of irrigation. In the experiments plastic containers were filled with six layers of soil, separated from each other by fine nylon cloths. The fertilizer granules were placed between the two central layers, and irrigation was simulated by application of tap water to the uppermost layer. Nitrogen release from the SCU was slow, and after three months, 29.5% of the applied N remained in the granules. At the end of the experiment there was a deficit of 37.1% N in the case of the AS granules, while there was virtually none with the SCU. Throughout the experiment, N from SCU remained at a relatively even level, while 95% of the N applied as AS had disappeared after irrigation. Nitrification was rapid in both cases.     

Nitrogen transformation rates are affected by cover soil type but not coarse woody debris application in reclaimed oil sands soils

Forest floor mineral soil mix (FMM) and peat mineral soil mix (PMM) are cover soils commonly used for reclamation of open‐pit oil sands mining disturbed land in northern Alberta, Canada; coarse woody debris (CWD) is another source of organic matter for land reclamation. We investigated net nitrogen (N) transformation rates in FMM and PMM cover soils near and away from CWD 4–6 years after oil sands reclamation. Monthly net nitrification and N mineralization rates varied over time; however, mean rates across the incubation periods and microbial biomass were greater (p < 0.05) in FMM than in PMM. Net N mineralization rates were positively related to soil temperature (p < 0.001) and microbial biomass carbon (p = 0.045). Net N transformation rates and inorganic N concentrations were not affected by CWD; however, the greater ¹⁵N isotope ratio of ammonium near CWD than away from CWD indicates that CWD application increased both gross N mineralization/nitrification (causing N isotope fractionation) and gross N immobilization (no isotopic fractionation). Microbial biomass was greater near CWD than away from CWD, indicating the greater potential for N immobilization near CWD. We conclude that (1) CWD application affected soil microbial properties and would create spatial variability and diverse microsites and (2) cover soil type and CWD application had differential effects on net N transformation rates. Applying FMM with CWD for oil sands reclamation is recommended to increase N availability and microsites.     

Dicyandiamide and 3,4-dimethyl pyrazole phosphate decrease N2O emissions from grassland but dicyandiamide produces deleterious effects in clover

The application of nitrogen fertilisers leads to different ecological problems such as nitrate leaching and the release of nitrogenous gases. N₂O is a gas involved in global warming, therefore, agricultural soils can be regarded as a source of global warming. Soil N₂O production comes from both the nitrification and denitrification processes. From an ecological viewpoint, using nitrification inhibitors with ammonium based fertilisers may be a potential management strategy to lower the fluxes of N₂O, thus decreasing its undesirable effect. In this study, the nitrification inhibitors (NIs) dicyandiamide (DCD) and 3,4-dimethyl pyrazole phosphate (DMPP) have been evaluated as management tools to mitigate N₂O emissions from mineral fertilisation and slurry application in grassland systems (experiments 1 and 2), and to assess the phytotoxic effect of these inhibitors per se on clover (experiment 3). Both nitrification inhibitors acted in maintaining soil nitrogen (N) in ammonium form, decreasing cumulative N₂O emissions. DCD, but not DMPP, produced phytotoxic effects and yield reduction in white clover. A nutrient imbalance, which led to a senescence process visually observed as chlorosis and necrosis at the border of the leaves, was noted.     

Impact of snowpack decrease on net nitrogen mineralization and nitrification in forest soil of northern Japan

Winter climate change is an important environmental driver that alters the biogeochemical processes of forest soils. The decrease in snowpack amplifies soil freeze–thaw cycles and decreases the snowmelt water supply to soil. This study examined how snow decrease affects nitrogen (N) mineralization and nitrification in forest soil in northern Japan by conducting an in situ experimental snowpack manipulation experiment and a laboratory incubation of soil with different moisture, temperature and freeze–thaw magnitudes. For the incubation studies, surface mineral soil (0–10 cm) was collected from a cool-temperate natural mixed forest and incubated using the resin core method during the winter. In the field, there were two treatments: 50 and 100 % snow removal and control plots. The increase in the soil freeze–thaw cycle increased net N mineralization and marginally decreased the net nitrification in soil. The dissolved organic carbon (DOC) and DOC/DON ratio in soil increased with the decrease in snowpack especially during the snow melt period. These results suggested that the change in substrate quality by the increase in freeze–thaw cycles caused the significant enhancement of microbial ammonium production in soil. The lower soil moisture and higher gross immobilization of inorganic N by soil microbes may be maintaining the slow net nitrification and low nitrate leaching in freeze–thaw cycles with less snowpack. The results indicate that winter climate change would strongly impact N biogeochemistry through the increase in ammonium availability in soil for plants and microbes, whereas it would be unlikely that nitrate loss from surface soil would be enhanced.     

The effects of nitrate: ammonium ratios and dicyandiamide on the nitrogen response of Zea mays L. in a high rainfall area on an acid soil

Hydroponic studies under controlled environmental conditions indicated that maize plants respond better to combinations of nitrate and ammonium nutrition than to either form supplied separately but that this response depended upon the total N concentration. An attempt was made to maintain different nitrate: ammonium ratios and concentrations in the soil by the addition of a nitrification inhibitor. Five nitrate: ammonium ratios at three N application rates were tested with and without dicyandiamide (H₂NC(NH)NHCN) on a low-pH, sandy soil for two years. Treatments were applied to field-grown maize in two applications, one at planting and the other at 21 to 30 days after planting. Under favourable climatic conditions for crop growth the optimum nitrate: ammonium ratio for grain yield was between 3:1 and 1:1 over all N rates. Under unfavourable climatic conditions, ratios of 3:1 and 1:1 showed in contrast to all other ratios no grain yield depressions at high N rates. Dicyandiamide did not interact with N rates or ratios, but did increase grain yield over all N treatments under favourable conditions. N ratio interactions with N rates and dicyandiamide were also shown for N concentrations of the leaves at anthesis, for the grain at harvest and for mineral N in different soil layers at anthesis. These results imply that nitrate: ammonium ratios between 3:1 and 1:1 should be recommended at the optimum N rate on a low-pH sandy soil in a high rainfall area for maize production.     

稳定性铵态氮肥在黑土和褐土中的氮素转化特征

以稳定性氯化铵为氮源,采用室内培养的方法,研究0.20、0.50、1.00 g N·kg^-1干土3种浓度的稳定性铵态氮肥在黑土、褐土中的氮素转化特征.结果表明: 在褐土中,随着氯化铵添加量的增加,土壤中发生硝化作用的时间逐渐推迟,添加0.20、0.50 g N·kg^-1干土处理开始发生明显硝化反应的时间分别为第3、7天,在高浓度氮量(1.00 g N·kg^-1干土)添加下硝化作用受到明显抑制;在黑土中,各浓度氮量添加处理开始发生硝化反应的时间相同,均为第3天,且随着添加量的增加,硝化作用潜势逐渐减弱.只加铵态氮肥的处理中,添加0.20 g N·kg^-1干土的氯化铵氮肥在褐土和黑土中的硝化反应时间分别可维持3周和2周左右;添加0.50 g N·kg^-1干土的氯化铵氮肥在褐土和黑土中的硝化反应时间分别可维持4周和3周左右.与单施氯化铵相比,黑土和褐土在0.20、0.50 g N·kg^-1干土添加浓度下,按纯氮量的1.0%添加3,4-二甲基吡唑磷酸盐(DMPP)、4.0%添加二氰二胺(DCD)均能显著抑制硝化作用,降低硝态氮的含量,抑制硝化作用潜势.综上,在褐土中,随着氯化铵添加浓度增加,土壤硝化作用受到抑制效果大于黑土.在0.20、0.50 g N·kg^-1干土外源铵态氮时,添加抑制剂可以显著抑制铵态氮的硝化作用.因此室内硝化抑制剂培养试验时,建议铵态氮添加量不超过1.00 g N·kg^-1干土,以0.50 g N·kg^-1干土效果最好.     

Manure composition affects net transformation of nitrogen from dairy manures

The plant available nitrogen (PAN) content of dairy manure is commonly calculated using concentration and availability coefficients for organic nitrogen (N) and ammonium N (NH₄), but the carbon (C) fraction of the manure also influences the availability of N over time. We evaluated the interactive effect of manure C and N from nine dairy manures during a 176 days aerobic incubation. All of the manures had appreciable NH₄ content, and varied widely in fibrous C. The incubation was conducted using sandy loam (coarse-loamy, mixed, frigid, Typic Haplorthod) and silt loam (fine, illitic, non-acid, frigid, Aeric Epiaquepts) soils at 25°C and 60% water-filled pore space. There were clear differences in nitrate (NO₃) accumulation over time, including manures that resulted in net nitrification and net immobilization. For both soils, the rate of nitrification at 7 and 56 days after application, and the amount of NO₃ accumulated at the end of the incubation (176 days) were strongly correlated (r = –0.88) with C: NH₄ and also to the ratio of neutral detergent fiber (NDF):NH₄ (r = –0.90). The addition of manure C also resulted in significant net immobilization, compared to addition of mineral N fertilizer alone. These studies demonstrate that increased understanding of manure C and N interactions may lead to improved prediction of manure PAN.     

Nitrogen (N) Dynamics in the Mineral Soil of a Central Appalachian Hardwood Forest During a Quarter Century of Whole-Watershed N Additions

The structure and function of terrestrial ecosystems are maintained by processes that vary with temporal and spatial scale. This study examined temporal and spatial patterns of net nitrogen (N) mineralization and nitrification in mineral soil of three watersheds at the Fernow Experimental Forest, WV: 2 untreated watersheds and 1 watershed receiving aerial applications of N over a 25-year period. Soil was sampled to 5 cm from each of seven plots per watershed and placed in two polyethylene bags—one bag brought to the laboratory for extraction/analysis, and the other bag incubated in situ at a 5 cm depth monthly during growing seasons of 1993–1995, 2002, 2005, 2007–2014. Spatial patterns of net N mineralization and nitrification changed in all watersheds, but were especially evident in the treated watershed, with spatial variability changing non-monotonically, increasing then decreasing markedly. These results support a prediction of the N homogeneity hypothesis that increasing N loads will increase spatial homogeneity in N processing. Temporal patterns for net N mineralization and nitrification were similar for all watersheds, with rates increasing about 25–30% from 1993 to 1995, decreasing by more than 50% by 2005, and then increasing significantly to 2014. The best predictor of these synchronous temporal patterns across all watersheds was number of degree days below 19°C, a value similar to published temperature maxima for net rates of N mineralization and nitrification for these soils. The lack of persistent, detectable differences in net nitrification between watersheds is surprising because fertilization has maintained higher stream-water nitrate concentrations than in the reference watersheds. Lack of differences in net nitrification among watersheds suggests that N-enhanced stream-water nitrate following N fertilization may be the result of a reduced biotic demand for nitrate following fertilization with ammonium sulfate.     

Nitrite effect on ammonium and nitrite oxidizing processes in a nitrifying sludge

Nitrite accumulation can be undesirable in nitrifying reactors used for the biological elimination of nitrogen from wastewaters because the ammonium oxidation process was seen to be inhibited. There is a need to better understand the effects of nitrite on both ammonium and nitrite oxidizing processes. In this paper, the effect of nitrite on the nitrifying activity of a sludge produced in steady-state nitrification was evaluated in batch cultures. At 25 mg N/l of added nitrite, nitrification was successfully carried out. Addition of higher nitrite concentrations to nitrifying cultures (100 and 200 mg N/l) provoked inhibitory effects on the nitrification respiratory process. Nitrite at 100 and 200 mg N/l induced a significant decrease in the values for nitrate yield (−20% and −34%, respectively) and specific rate of nitrate formation (−26% and −67%, respectively), while the ammonium consumption efficiency kept high and the specific rate of ammonium oxidation did not significantly change. This showed that the nitrite oxidizing process was more sensitive to the presence of nitrite than the ammonium oxidizing process. These results showed that as a consequence of nitrite accumulation in nitrification systems, the activity of the nitrite oxidizing bacteria could be more inhibited than that of the ammonium oxidizing bacteria, provoking a higher accumulation of nitrite in the medium.     

Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest

The effects of the understory shrub Ledum palustre on soil N cycling were studied in a hardwood forest of Interior Alaska. This species releases high concentrations of phenolic compounds from green leaves and decomposing litter by rainfall. Organic and mineral soils sampled underneath L. palustre and at nearby non-Ledum sites were amended with L. palustre litter leachates and incubated at controlled conditions. We aimed to know (i) whether L. palustre presence and litter leachate addition changed net N cycling rates in organic and mineral soils, and (ii) what N cycling processes, including gross N mineralization, N immobilization and gross N nitrification, were affected in association with L. palustre. Our results indicate that N transformation rates in the surface organic horizon were not affected by L. palustre presence or leachate addition. However, mineral soils underneath L. palustre as well as soils amended with leachates had significantly higher C/N ratios and microbial respiration rates, and lower net N mineralization and N-to-C mineralization compared to no Ledum and no leachates soils. No nitrification was detected. Plant presence and leachate addition also tended to increase both gross N mineralization and immobilization. These results suggest that soluble C compounds present in L. palustre increased N immobilization in mineral soils when soil biota used them as a C source. Increases in gross N mineralization may have been caused by an enhanced microbial biomass due to C addition. Since both plant presence and leachate addition decreased soil C/N ratio and had similar effects on N transformation rates, our results suggest that litter leachates could be partially responsible for plant presence effects. The lower N availability under L. palustre canopy could exert negative interactions on the establishment and growth of other plant species.