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1.
The importance of heterotrophic nitrification was studied in soil from a mixed-conifer forest. Three sites in the forest were sampled: a clear cut area, a young stand and a mature stand. In the mature stand, the mineral soil (0–10 cm) and the organic layer were sampled separately. Gross rates of N mineralization and nitrification were measured by 15NH
4
+
and 15NO
3
–
isotopic pool dilution, respectively. The rates of autotrophic and heterotrophic nitrification were distinguished by use of acetylene as a specific inhibitor of autotrophic nitrification. In samples supplemented with 15NH
4
+
and treated with acetylene, no 15NO
3
–
was detectable showing that the acetylene treatment effectively blocked the autotrophic nitrification, and that NH
4
+
was not a substrate for heterotrophic nitrification. In the clear cut area, autotrophic nitrification was the most important NO
3
–
generating process with total nitrification (45 ug N kg –1h –1) accounting for about one-third of gross N mineralization (140 ug N kg –1 h –1). In the young and mature forested sites, gross nitrification rates were largely unaffected by acetylene treatment indicating that heterotrophic nitrification dominated the NO
3
–
generating process in these areas. In the mature forest mineral and organic soil, nitrification (heterotrophic) was equal to only about 5% of gross mineralization (gross mineralization rates of 90 ug N kg –1 h –1 mineral; 550 ug N kg –1 h –1 organic). The gross nitrification rate decreased from the clear cut area to the young forest area to the mineral soil of the mature forest (45; 17; 4.5 ug kg –1 h –1 respectively). The 15N isotope pool dilution method, combined with acetylene as an inhibitor of autotrophic nitrification provided an effective technique for assessing the importance of heterotrophic nitrification in the N-cycle of this mixed-conifer ecosystem. 相似文献
2.
Aerobic N 2O production was studied in nitrifying humus from urea-fertilized pine forest soil. Acetylene and nitrapyrin inhibited both NH 4+ oxidation and N 2O production, indicating that N 2O production was closely associated with autotrophic NH 4+ oxidation. N 2O production was enhanced by low soil pH; it was negligible above pH 4.7. When soil pH decreased from 4.7 to 4.1, the relative amount of N 2O-N produced from NH 4+-N oxidized increased exponentially to 20%. There was also some evidence that N 2O formation was stimulated by salts (potassium sulfate and sodium phosphates). The maximum rate of N 2O-N production was 0.17 μg of N 2O-N per g of soil per h. When humus was treated with NO 2−, N 2O evolved immediately, indicating chemical formation, but no N 2O was formed on the addition of NO 3−. The amount of N 2O-N evolved was 0.6 to 4.6% of NO 2−-N added. A high concentration of NO 2− and low soil pH enhanced chemical production of N 2O. There was no accumulation of NO 2− during nitrification. The calculations indicated that chemical formation of N 2O was not the main source of N 2O during NH 4+ oxidation. After the addition of inhibitors of NH 4+ oxidation the soils contained NO 3−, but no N 2O was produced. The results suggest that enhanced autotrophic NH 4+ oxidation is a potential source of N 2O in fertilized acid forest soil. 相似文献
3.
Seasonal variation of dissolved organic C (DOC) and its effects on microbial activity and N dynamics were studied during two
consecutive years in soils with different organic C concentrations (hilltop and hillslope) in a tropical deciduous forest
of Mexico. We found that DOC concentrations were higher at the hilltop than at the hillslope soils, and in both soils generally
decreased from the dry to the rainy season during the two study years. Microbial biomass and potential C mineralization rates,
as well as dissolved organic N (DON) and NH 4+ concentrations and net N immobilization were higher in soils with higher DOC than in soils with lower DOC. In contrast, net
N immobilization and NH 4+ concentration were depleted in the soil with lowest DOC, whereas NO 3− concentrations and net nitrification increased. Negative correlations between net nitrification and DOC concentration suggested
that NH 4+ was transformed to NO 3− by nitrifiers when the C availability was depleted. Taken together, our results suggest that available C appears to control
soil microbial activity and N dynamics, and that microbial N immobilization is facilitated by active heterotrophic microorganisms
stimulated by high C availability. Soil autotrophic nitrification is magnified by decreases in C availability for heterotrophic
microbial activity. This study provides an experimental data set that supports the conceptual model to show and highlight
that microbial dynamics and N transformations could be functionally coupled with DOC availability in the tropical deciduous
forest soils.
Responsible Editor: Chris Neill 相似文献
4.
Soil emission of gaseous N oxides during nitrification of ammonium represents loss of an available plant nutrient and has an important impact on the chemistry of the atmosphere. We used selective inhibitors and a glucose amendment in a factorial design to determine the relative contributions of autotrophic ammonium oxidizers, autotrophic nitrite oxidizers, and heterotrophic nitrifiers to nitric oxide (NO) and nitrous oxide (N 2O) emissions from aerobically incubated soil following the addition of 160 mg of N as ammonium sulfate kg −1. Without added C, peak NO emissions of 4 μg of N kg −1 h −1 were increased to 15 μg of N kg −1 h −1 by the addition of sodium chlorate, a nitrite oxidation inhibitor, but were reduced to 0.01 μg of N kg −1 h −1 in the presence of nitrapyrin [2-chloro-6-(trichloromethyl)-pyridine], an inhibitor of autotrophic ammonium oxidation. Carbon-amended soils had somewhat higher NO emission rates from these three treatments (6, 18, and 0.1 μg of N kg −1 h −1 after treatment with glucose, sodium chlorate, or nitrapyrin, respectively) until the glucose was exhausted but lower rates during the remainder of the incubation. Nitrous oxide emission levels exhibited trends similar to those observed for NO but were about 20 times lower. Periodic soil chemical analyses showed no increase in the nitrate concentration of soil treated with sodium chlorate until after the period of peak NO and N 2O emissions; the nitrate concentration of soil treated with nitrapyrin remained unchanged throughout the incubation. These results suggest that chemoautotrophic ammonium-oxidizing bacteria are the predominant source of NO and N 2O produced during nitrification in soil. 相似文献
5.
The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N 2O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N 2O production from the intensive vegetable fields and to identify the affecting factors, a 15N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with 15NH 4NO 3 or NH 4 15 NO 3. The average N 2O emission rate was 24.2 ng N?kg ?1 h ?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg ?1 h ?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N 2O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N 2O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N 2O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N 2O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N 2O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO 3 - or NH 4 + ) could also significantly affect the sources and rates of N 2O emission. 相似文献
6.
The capacity for dissimilatory reduction of NO 3− to N 2 (N 2O) and NH 4+ was measured in 15NO 3−-amended marine sediment. Incubation with acetylene (7 × 10 −3 atmospheres [normal]) caused accumulation of N 2O in the sediment. The rate of N 2O production equaled the rate of N 2 production in samples without acetylene. Complete inhibition of the reduction of N 2O to N 2 suggests that the “acetylene blockage technique” is applicable to assays for denitrification in marine sediments. The capacity for reduction of NO 3− by denitrification decreased rapidly with depth in the sediment, whereas the capacity for reduction of NO 3− to NH 4+ was significant also in deeper layers. The data suggested that the latter process may be equally as significant as denitrification in the turnover of NO 3− in marine sediments. 相似文献
7.
Dissimilatory reduction of NO 2− to N 2O and NH 4+ by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N 2O production by this mechanism. In batch cultures with defined media, NO 2− reduction to NH 4+ was favored by high glucose and low NO 3− concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO 3− concentrations. With succinate as the energy source, little or no NO 2− was reduced to NH 4+ but N 2O was produced. Resting cell suspensions reduced NO 2− simultaneously to N 2O and free extracellular NH 4+. Chloramphenicol prevented the induction of N 2O-producing activity. The Km for NO 2− reduction to N 2O was estimated to be 0.9 mM NO 2−, yet the apparent Km for overall NO 2− reduction was considerably lower, no greater than 0.04 mM NO 2−. Activities for N 2O and NH 4+ production increased markedly after depletion of NO 3− from the media. Amendment with NO 3− inhibited N 2O and NH 4+ production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH 4+ but not of N 2O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N 2O at rates equal to the wild type. These observations suggest that N 2O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO 2− to NH 4+. 相似文献
8.
We used the buried bag incubation method to study temporal patterns of net N mineralization and net nitrification in soils at Ta-Ta-Chia forest in central Taiwan. The site included a grassland zone, (dominant vegetation consists of Yushania niitakayamensis and Miscanthus transmorrisonensis Hayata) and a forest zone ( Tsuga chinensis var. formosana and Yushania niitakamensis). In the grassland, soil concentration NH 4
+ in the organic horizon (0.1–0.2 m) ranged from 1.0 to 12.4 mg N kg –1 soil and that of NO 3
– varied from 0.2 to 2.1 mg N kg –1 soil. In the forest zone, NH 4
+ concentration was between 2.8 and 25.0 mg N kg –1 soil and NO 3
–varied from 0.2 to 1.3 mg N kg –1 soil. There were lower soil NH 4
+ concentrations during the summer than other seasons. Net N mineralization was higher during the summer while net nitrification rates did not show a distinct seasonal pattern. In the grassland, net N mineralization and net nitrification rates were between –0.1 and 0.24 and from –0.04 to 0.04 mg N kg –1 soil day –1, respectively. In the forest zone, net N mineralization rates were between –0.03 and 0.45 mg N kg –1 soil day –1 and net nitrification rates were between –0.01 and 0.03 mg N kg –1 soil day –1. These differences likely result from differing vegetation communities (C 3 versus C 4 plant type) and soil characteristics. 相似文献
9.
The oxidation of NH 4+ by Nitrosomonas europaea was insensitive to 10 mM NaClO 3 (sodium chlorate) but was strongly inhibited by NaClO 2 (sodium chlorite; Ki, 2 μM). The oxidation of NO 2− by Nitrobacter winogradskyi was inhibited by both ClO 3− and ClO 2− ( Ki for ClO 2−, 100 μM). N. winogradskyi reduced ClO 3− to ClO 2− under both aerobic and anaerobic conditions, and as much as 0.25 mM ClO 2− was detected in the culture filtrate. In mixed N. europaea-N. winogradskyi cell suspensions, the oxidation of both NH 4+ and NO 2− was inhibited in the presence of 10 mM ClO 3− after a 2-h lag period, despite the fact that, under these conditions, ClO 2− was not detected in the filtrate. The data are consistent with the hypothesis that, in mixed culture, NH 4+ oxidation is inhibited by ClO 2− produced by reduction of ClO 3− by the NO 2− oxidizer. The use of ClO 3− inhibition of NO 2− oxidation in assays of nitrification by mixed populations necessitates cautious interpretation unless it can be shown that the oxidation of NH 4+ is not affected. 相似文献
10.
High-resolution NO 3− profiles in freshwater sediment covered with benthic diatoms were obtained with a new microscale NO 3− biosensor characterized by absence of interference from chemical species other than NO 2− and N 2O. Analysis of the microprofiles obtained indicated no nitrification during darkness, high rates of nitrification and a tight coupling between nitrification and denitrification during illumination, and substantial rates of NO 3− assimilation during illumination. Nitrification during darkness could be induced by purging the bulk water with O 2 gas, indicating that the stimulatory effect on nitrification by illumination was caused by algal production of O 2. NH 4+ addition did not stimulate nitrification during darkness when O 2 was restricted to the upper 1-mm layer, and there was thus a low nitrification potential in the permanently oxic top 1 mm of the sediment. 相似文献
11.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
12.
Ammonia (NH 3)-oxidizing bacteria (AOB) and thaumarchaea (AOA) co-occupy most soils, yet no short-term growth-independent method exists to determine their relative contributions to nitrification in situ. Microbial monooxygenases differ in their vulnerability to inactivation by aliphatic n-alkynes, and we found that NH 3 oxidation by the marine thaumarchaeon Nitrosopumilus maritimus was unaffected during a 24-h exposure to ≤20 μM concentrations of 1-alkynes C 8 and C 9. In contrast, NH 3 oxidation by two AOB ( Nitrosomonas europaea and Nitrosospira multiformis) was quickly and irreversibly inactivated by 1 μM C 8 (octyne). Evidence that nitrification carried out by soilborne AOA was also insensitive to octyne was obtained. In incubations (21 or 28 days) of two different whole soils, both acetylene and octyne effectively prevented NH 4+-stimulated increases in AOB population densities, but octyne did not prevent increases in AOA population densities that were prevented by acetylene. Furthermore, octyne-resistant, NH 4+-stimulated net nitrification rates of 2 and 7 μg N/g soil/day persisted throughout the incubation of the two soils. Other evidence that octyne-resistant nitrification was due to AOA included (i) a positive correlation of octyne-resistant nitrification in soil slurries of cropped and noncropped soils with allylthiourea-resistant activity (100 μM) and (ii) the finding that the fraction of octyne-resistant nitrification in soil slurries correlated with the fraction of nitrification that recovered from irreversible acetylene inactivation in the presence of bacterial protein synthesis inhibitors and with the octyne-resistant fraction of NH 4+-saturated net nitrification measured in whole soils. Octyne can be useful in short-term assays to discriminate AOA and AOB contributions to soil nitrification. 相似文献
13.
The effects of select monoterpenes on nitrogen (N) mineralization and nitrification potentials were determined in four separate laboratory bioassays. The effect of increasing monoterpene addition was an initial reduction in NO 3
–-N production (nitrification inhibition), followed by a reduction in the sum of NH 4
+-N and NO 3
–-N (inhibition of net N mineralization and net immobilization at high monoterpene additions. Monoterpenes could produce this pattern by inhibiting nitrification, reducing net N mineralization, enhancing immobilization of NO 3
–-N relative to NH 4
+-N, and/or stimulating overall net immobilization of N by carbon-rich material.Initial monoterpene concentrations in the assay soils were about 5% of the added amount and were below detection after incubation in most samples.Potential N mineralization-immobilization, nitrification, and soil monoterpene concentrations were determined by soil horizon for four collections from a ponderosa pine ( Pinus ponderosa) stand in New Mexico. Concentrations of monoterpenes declined exponentially with soil depth and varied greatly within a horizon. Monoterpene content of the forest floor was not correlated with forest floor biomass. Net N mineralization was inversely correlated with total monoterpene content of all sampled horizons. Nitrification was greatest in the mineral soil, intermediate in the F-H horizon, and never occurred in the L horizon. Nitrification in the mineral soil was inversely correlated with the amount of monoterpenes in the L horizon that contain terminal unsaturated carbon-carbon bonds ( r
2 = 0.37, P 0.01). This pattern in the field corresponded to the pattern shown in the laboratory assays with increasing monoterpene additions. 相似文献
14.
Although they drain remarkably similar forest types, streams of the Hubbard Brook Experimental Forest (HBEF) vary widely in
their NO 3
− concentrations during the growing season. This variation may be caused by differences in the terrestrial systems they drain
(for example, varying forest age or composition, hydrology, soil organic matter content, and so on) and/or by differences
between the streams themselves (for example, contrasting geomorphology, biotic nitrogen [N] demand, rates of instream nitrogen
transformations). We examined interstream variation in N processing by measuring NH 4
+ and NO 3
− uptake and estimating nitrification rates for 13 stream reaches in the HBEF during the summers of 1998 and 1999. We modeled
nitrification rates using a best-fit model of the downstream change in NO 3
− concentrations following short-term NH 4
+ enrichments. Among the surveyed streams, the fraction of NH 4
+ uptake that was subsequently nitrified varied, and this variation was positively correlated with ambient streamwater NO 3
− concentrations. We examined whether this variation in instream nitrification rates contributed significantly to the observed
variation in NO 3
− concentrations across streams. In some cases, instream nitrification provided a substantial portion of instream NO 3
− demand. However, because there was also substantial instream NO 3
− uptake, the net effect of instream processing was to reduce rather than supplement the total amount of NO 3
− exported from a watershed. Thus, instream rates of nitrification in conjunction with instream NO 3
− uptake were too low to account for the wide range of streamwater NO 3
−. The relationship between streamwater NO 3
− concentration and rates of instream nitrification may instead be due to a shift in the competitive balance between heterotrophic
N uptake and nitrification when external inputs of NO 3
− are relatively high.
Received 11 October 2000; accepted 14 December 2001. 相似文献
15.
The representation of NO 3
– dynamics within forest growth simulation models could improve forest management. An extensive literature review revealed an 88% probability of measuring a higher relative nitrification index (i.e. RNI = [NO 3
–] ÷ [NO 3
– + NH 4
+]) in mineral soil horizons than in forest floors, across a wide range of conifer and hardwood ecosystems. We then hypothesised that humus form and fine root density could be used as two crude variables to predict changes in in situ, potential and relative nitrification rates. Twenty-seven trench plots were established in 1999, across nine contrasting hardwood and coniferous stands in the Eastern Townships of Québec. Forest floor and mineral soil samples were collected from each plot, and from a 1 m radius surrounding each plot, on three dates during summer 2000. In situRNI values increased significantly in trench plots as the season progressed. Potential nitrification rates (i.e. NO 3
– concentrations following incubation) were two orders of magnitude higher in forest floor than in mineral soil samples. RNI was significantly higher in mineral soil than in forest floor samples after incubations, but the relative increase in RNI due to trenching was higher in forest floor samples. Indices of available C were significantly higher in forest floor than mineral soil samples, and decreased only in forest floor samples during incubations. Likewise, trenching significantly reduced available C in forest floor samples only. Seasonal changes in soil temperature and fine root growth were the most plausible explanations for seasonal changes in NO 3
– dynamics, whereas other factors such soil acidity and moisture appeared less important in determining NO 3
– dynamics in this study. We conclude that crude assessments of humus form and fine root density have the potential to be used as calibration parameters for the simulation of NO 3
– dynamics in forest growth and yield models. 相似文献
16.
Background and aims Continuous vegetable cultivation in greenhouses can easily induce soil degradation, which considerably affects the development of sustainable vegetable production. Recently, the reductive soil disinfestation (RSD) is widely used as an alternative to chemical soil disinfestations to improve degraded greenhouse vegetable soils. Considering the importance of nitrogen (N) for plant growth and environment effect, the internal N transformation processes and rates should be well investigated in degraded vegetable soils treated by RSD, but few works have been undertaken. Methods Three RSD-treated and three untreated degraded vegetable soils were chosen and a 15?N tracing incubation experiment differentially labeled with 15NH 4NO 3 or NH 4 15NO 3 was conducted at 25 °C under 50 % water holding capacity (WHC) for 96 h. Soil gross N transformation rates were calculated using a 15?N tracing model combined with Markov Chain Monte Carlo Metropolis algorithm (Müller et al. 2007), while the emissions of N 2O and NO were also measured. Results RSD could significantly enhance the soil microbial NH 4 + immobilization rate, the heterotrophic and autotrophic nitrification rates, and the NO 3 ? turnover time. The ratio of heterotrophic nitrification to total inorganic N supply rate (mineralization + heterotrophic nitrification) increased greatly from 5.4 % in untreated vegetable soil to 56.1 % in treated vegetable soil. In addition, low release potential of NO and N 2O was observed in RSD-treated vegetable soil, due to the decrease in the NO and N 2O product ratios from heterotrophic and autotrophic nitrifications. These significant differences in gross N transformation rates, the supply processes and capacity of inorganic N, and the NO and N 2O emissions between untreated and treated vegetable soils could be explained by the elimination of accumulated NO 3 ?, increased pH, and decreased electrical conductivity (EC) caused by RSD. Noticeably, the NO 3 ? consumption rates were still significantly lower than the NO 3 ? production rates in RSD-treated vegetable soil. Conclusions Except for improving soil chemical properties, RSD could significantly alter the supply processes of inorganic N and reduce the release potential of N 2O and NO in RSD-treated degraded vegetable soil. In order to retard the re-occurrence of NO 3 ? accumulation, acidification and salinization and to promote the long-term productivity of greenhouse vegetable fields, the rational use of N fertilizer should be paid great attention to farmers in vegetable cultivation. 相似文献
17.
Spatial variability of soil total nitrogen (N), available N (KCl extractable NH 4+ and NO 3−), and spatial patterns of N mineralization and nitrification at a stand scale were characterized with geostatistical and
univariate analysis. Two extensive soil spatial samplings were conducted in an evergreen broadleaf forest in Sichuan province,
southwestern China in June and August 2000. In a study area of 90 × 105 m 2, three soil samples were collected from each 5 × 5 m 2 plot ( n = 378) in June and August, and were analyzed for total N and available N contents. Net N mineralization and nitrification
were measured by in situ core incubation and the rates were estimated based on the difference of NH 4+ and NO 3− contents between the two sampling dates. Total N, NH 4+, and NO 3− were all spatially structured with different semivariogram ranges (from high to low: NH 4+, NO 3−, and total N). The semivariograms of mineralization and nitrification were not as spatially structured as available N. NH 4+ was the dominant soil inorganic N form in the system. Both NH 4+ and NO 3− affected spatial patterns of soil available N, but their relative importance switched in August, probably due to high nitrification
as indicated by greatly increased soil NO 3− content. High spatial auto-correlations (>0.7) were found between available N and NH4 +, available N and NO 3− on both sampling dates, as well as total N measurements between both sampling dates. Although significant, the spatial auto-correlation
between NH 4+ and NO 3− were generally low. Topography had significant but low correlations with mineralization ( r = −0.16) and nitrification ( r = −0.14), while soil moisture did not. The large nugget values of the calculated semivariograms and high-semivariance values,
particularly for mineralization and nitrification, indicate that some fine scale (<5 m) variability may lie below the threshold
for detection in this study. 相似文献
18.
Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH 4 +) and nitrate (NO 3 ?). Previous long-term studies indicate efficient turnover of atmogenic NH 4 + to NO 3 ? in weathered, acidic soils of the subtropics, leading to excessive NO 3 ? leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ 15N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. 15NH 4NO 3, NH 4 15 NO 3 and 15N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (N 2O) were monitored for nine days. Incorporation of 15NO 3 ? into soil organic N was negligible and 80% of the applied label was lost from the top soil (0–15 cm) primarily by leaching within 9 days. In contrast, 15NH 4 + was largely retained in soil organic N. However, instant production of 15NO 3 ? in the 15NH 4 + treatment suggested active nitrification. In both the 15NH 4 + and 15N-glutamic acid treatments, the 15N enrichment in the NO 3 ? pool exceeded that in the NH 4 + pool one day after 15N application, suggesting preferential nitrification of added 15NH 4 + with subsequent dilution of the NH 4 + pool and/or immobilization of 15NH 4 + followed by heterotrophic nitrification. The cumulative recovery of 15N in N 2O after 9 days ranged from 2.5 to 6.0% in the 15NO 3 ? treatment, confirming the previously reported significant response of N 2O emission to N deposition. Source partitioning of 15N 2O demonstrated a measurable contribution of nitrification to N 2O emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH 4 + in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO 3 ? and NH 4 +) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO 3 ? in the soil and (ii) rapid immobilization-mineralization of NH 4 + followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and N 2O emission. 相似文献
19.
The effect of exogenous NH 4+ on NO 3− uptake and in vivo NO 3− reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO 3− uptake. Pretreatment with NH 4Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO 3− uptake. When NH4+ was given at the start of NO3− nutrition, the time pattern of NO3− uptake was the same as in plants receiving no NH4+. After 6 hours, however, the NO3− uptake rate (NUR) and root NRA were inhibited by NH4+ to a maximum of 45% and 60%, respectively. The response of the NUR of NO3−-induced plants depended on the NH4Cl concentration. Below 1 millimolar NH4+, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH4+ at 2 millimolar and above caused a rapid and transient stimulation of NO3− uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO3−-induced plants, root NRA responded less and more slowly to NH4+ than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH4+ inhibition of the NUR of NO3−-induced plants. We conclude that repression of the NUR by NH4+ depends on NH4+ assimilation. The repression by NH4+ was least at the lowest and highest NH4+ levels tested (0.04 and 25 millimolar). 相似文献
20.
Net uptakes of K + and NO 3− were monitored simultaneously and continuously for two barley ( Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K + and NO 3− uptake in the absence of NH 4+ or Cl −. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH 4+, K +, Cl −, or NO 3− affected the cultivars very differently: (a) the presence of NH 4+ as NH 4Cl stimulated net NO 3− uptake in Prato barley but inhibited net NO 3− uptake in Olli barley; (b) Cl − inhibited net NO 3− uptake in Prato but had little effect in Olli; and (c) NH 4+ as (NH 4) 2SO 4 inhibited net K + uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl − initially inhibited net K + uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH 4+ and Cl − uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption. 相似文献
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