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1.
A laboratory-based aerobic incubation was conducted to investigate nitrogen(N) isotopic fractionation related to nitrification in five agricultural soils after application of ammonium sulfate((NH4)2SO4). The soil samples were collected from a subtropical barren land soil derived from granite(RGB),three subtropical upland soils derived from granite(RQU),Quaternary red earth(RGU),Quaternary Xiashu loess(YQU) and a temperate upland soil generated from alluvial deposit(FAU). The five soils varied in nitrification potential,being in the order of FAU YQU RGU RQU RGB. Significant N isotopic fractionation accompanied nitrification of NH+4. δ15N values of NH+4 increased with enhanced nitrification over time in the four upland soils with NH+4 addition,while those of NO-3 decreased consistently to the minimum and thereafter increased. δ15N values of NH+4 showed a significantly negative linear relationship with NH+4-N concentration,but a positive linear relationship with NO-3-N concentration. The apparent isotopic fractionation factor calculated based on the loss of NH+4 was 1.036 for RQU,1.022 for RGU,1.016 for YQU,and 1.020 for FAU,respectively. Zero- and first-order reaction kinetics seemed to have their limitations in describing the nitrification process affected by NH+4 input in the studied soils. In contrast,N kinetic isotope fractionation was closely related to the nitrifying activity,and might serve as an alternative tool for estimating the nitrification capacity of agricultural soils.  相似文献   

2.
The effects on nitrification and acidification in three subtropical soils to which (NH4)2SO4 or urea had been added at rate of 250 mg N kg−1 was studied using laboratory-based incubations. The results indicated that NH4+ input did not stimulate nitrification in a red forest soil, nor was there any soil acidification. Unlike red forest soil, (NH4)2SO4 enhanced nitrification of an upland soil, whilst urea was more effective in stimulating nitrification, and here the soil was slightly acidified. For another upland soil, NH4+ input greatly enhanced nitrification and as a result, this soil was significantly acidified. We conclude that the effects of NH4+ addition on nitrification and acidification in cultivated soils would be quite different from in forest soils. During the incubation, N isotope fractionation was closely related to the nitrifying capacity of the soils.  相似文献   

3.
A study was conducted at two experimental tree plantations in the Pacific Northwest to assess the roles of bacteria and fungi in nitrogen (N) cycling. Soils from red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) plots in low- (H.J. Andrews) and high- (Cascade Head) productivity stands were sampled in 2005 and 2006. Fungal:bacterial ratios were determined using phospholipid fatty acid (PLFA) profiles and quantitative (Q)-PCR. Ratios from these two molecular methods were highly correlated and showed that microbial biomass varied significantly between the two experimental sites and to a lesser extent between tree types with fungal:bacterial biomass ratios lower in more N-rich plots. 15N isotope dilution experiments, with ammonium (NH4+) and nitrate (NO3?), were paired with antibiotics that blocked bacterial (bronopol) and fungal (cycloheximide) protein synthesis. This modified isotope dilution technique was used to determine the relative contribution of bacteria and fungi to net N mineralization and gross rates of ammonification and nitrification. When bacterial protein synthesis was blocked NH4+ consumption and nitrification rates decreased in all treatments except for NH4+ consumption in the Douglas-fir plots at H.J. Andrews, suggesting that prokaryotic nitrifiers are a major sink for mineral NH4+ in forest soils with higher N availability. Cycloheximide consistently increased NH4+ consumption, however the trend was not statistically significant. Both antibiotics additions also significantly increased gross ammonification, which may have been due to continued activity of extra- and intracellular enzymes involved in producing NH4+ combined with the inhibition of NH4+ assimilation into proteins. The implication of this result is that microorganisms are likely a major sink for soil dissolved organic N (DON) in soils.  相似文献   

4.
 Gross N mineralization and nitrification rates and their relationships to microbial biomass C and N and enzyme (protease, deaminase and urease) activities were determined in soils treated with dairy shed effluent (DSE) or NH4 + fertilizer (NH4Cl) at a rate equivalent to 200 kg N ha–1 at three water potentials (0, –10 and –80 kPa) at 20  °C using a closed incubation technique. After 8, 16, 30, 45, 60 and 90 days of incubation, sub-samples of soil were removed to determine gross N mineralization and nitrification rates, enzyme activities, microbial biomass C and N, and NH4 + and NO3 – concentrations. The addition of DSE to the soil resulted in significantly higher gross N mineralization rates (7.0–1.7 μg N g–1 soil day–1) than in the control (3.8–1.2 μg N g–1 soil day–1), particularly during the first 16 days of incubation. This increase in gross mineralization rate occurred because of the presence of readily mineralizable organic substrates with low C : N ratios, and stimulated soil microbial and enzymatic activities by the organic C and nutrients in the DSE. The addition of NH4Cl did not increase the gross N mineralization rate, probably because of the lack of readily available organic C and/or a possible adverse effect of the high NH4 + concentration on microbial activity. However, nitrification rates were highest in the NH4Cl-treated soil, followed by DSE-treated soil and then the control. Soil microbial biomass, protease, deaminase and urease activities were significantly increased immediately after the addition of DSE and then declined gradually with time. The increased soil microbial biomass was probably due to the increased available C substrate and nutrients stimulating soil microbial growth, and this in turn resulted in higher enzyme activities. NH4Cl had a minimal impact on the soil microbial biomass and enzyme activities, possibly because of the lack of readily available C substrates. The optimum soil water potential for gross N mineralization and nitrification rates, microbial and enzyme activities was –10 kPa compared with –80 kPa and 0 kPa. Gross N mineralization rates were positively correlated with soil microbial biomass N and protease and urease activities in the DSE-treated soil, but no such correlations were found in the NH4Cl-treated soil. The enzyme activities were also positively correlated with each other and with soil microbial biomass C and N. The forms of N and the different water potentials had a significant effect on the correlation coefficients. Stepwise regression analysis showed that protease was the variable that most frequently accounted for the variations of gross N mineralization rate when included in the equation, and has the potential to be used as one of the predictors for N mineralization. Received: 10 March 1998  相似文献   

5.
 The effects of inorganic N and organic manure, applied to a loamy arable soil, on CH4 oxidation were investigated in laboratory incubation experiments. Applications (40 mg N kg–1) of NH4Cl, (NH4)2SO4, and urea caused strong instantaneous inhibition of CH4 oxidation by 96%, 80%, and 84%, respectively. After nitrification of the added N the inhibitory effect was not fully reversible, resulting in an residual inhibition of 21%, 16%, and 25% in the NH4Cl, (NH4)2SO4, and urea treatments, respectively. With large NH4 + applications [240 mg N kg–1 as (NH4)2SO4] the residual inhibition was as high as 64%. Exogenous NO2 – (40 mg NO2 –-N kg–1) initially inhibited CH4 oxidation by 84%, decreasing to 41% after its oxidation. Therefore, applied NO2 – was a more effective inhibitor of CH4 consumption than NH4 +. Temporary accumulation of NO2 – during nitrification of added N was small (maximum: 1.9 mg NO2 –-N kg–1) and thus of minor importance with respect to the persistent inhibition after NH4 + or urea application. CH4 oxidation after NaNO3 (40 mg N kg–1) and NaCl addition did not differ to that of the untreated soil. The effect of organic manures on CH4 oxidation depended on their C/N ratio: fresh sugar beet leaves enhanced mineralization, which caused an instantaneous 20% inhibition, whereas after wheat straw application available soil N was rapidly immobilized and no effect on CH4 oxidation was found. The 28% increase in CH4 oxidation after biowaste compost application was not related to its C/N ratio and was probably the result of an inoculation with methanotrophic bacteria. Only with high NH4 + application rates (240 mg N kg–1) could the persistent inhibitory effect partly be attributed to a pH decrease during nitrification. The exact reason for the observed persistent inhibition after a single, moderate NH4 + or urea application is still unknown and merits further study. Received: 31 October 1997  相似文献   

6.
A study was carried out to investigate the potential gross nitrogen (N) transformations in natural secondary coniferous and evergreen broad-leaf forest soils in subtropical China. The simultaneously occurring gross N transformations in soil were quantified by a 15N tracing study. The results showed that N dynamics were dominated by NH4+ turnover in both soils. The total mineralization (from labile and recalcitrant organic N) in the broad-leaf forest was more than twice the rate in the coniferous forest soil. The total rate of mineral N production (NH4+ + NO3−) from the large recalcitrant organic N pool was similar in the two forest soils. However, appreciable NO3− production was only observed in the coniferous forest soil due to heterotrophic nitrification (i.e. direct oxidation of organic N to NO3−), whereas nitrification in broad-leaf forest was little (or negligible). Thus, a distinct shift occurred from predominantly NH4+ production in the broad-leaf forest soil to a balanced production of NH4+ and NO3− in the coniferous forest soil. This may be a mechanism to ensure an adequate supply of available mineral N in the coniferous forest soil and most likely reflects differences in microbial community patterns (possibly saprophytic, fungal, activities in coniferous soils). We show for the first time that the high nitrification rate in these soils may be of heterotrophic rather than autotrophic nature. Furthermore, high NO3− production was only apparent in the coniferous but not in broad-leaf forest soil. This highlights the association of vegetation type with the size and the activity of the SOM pools that ultimately determines whether only NH4+ or also a high NO3− turnover is present.  相似文献   

7.
The effects of temperature, moisture content and the addition of pig slurry on nitrification in two soils were studed. There was no accumulation of NO2?-N under the incubation conditions investigated and the accumulation of NO3?-N was linear for additions of 50–250 μg NH4+-N g? soil, either as ammonium sulphate or as pig slurry. Nitrate formation was treated as a single step, zero order process to enable a rate constant to be calculated. Nitrification rate increased with increasing moisture content up to the highest level tested, soil water potential ?8.0 kPa, corresponding to approximately 60% of water holding capacity in both soils. Measurable nitrification was found in both soils at the lowest moisture content (soil water potential ?1.5 MPa) and temperature (5° C) tested. The nitrification rate constant in soils treated with 50 μg NH4+-N g? soil was not significantly affected (P = 0.05) by the form of ammonium added. Addition of 250 μg NH4+-N as ammonium sulphate caused a marked inhibition of nitrification at all moisture contents and temperatures. Addition of 250 μg NH4+-N as pig slurry caused a marked increase in nitrification rate, the increase being greater at the higher temperatures and moisture contents.  相似文献   

8.
Gross rates of N mineralization–immobilization turnover (MIT) and gross nitrification in soil can be determined by use of a 15NH4+ pool dilution technique, under the assumption that native 14NH4+, applied 14NH4+ and microbial processes are uniformly distributed in the soil. In a laboratory investigation we compared gross N fluxes obtained from two labelling techniques applied to an arable sandy loam: (1) injection of 15NH4+ solution into intact soil cores, and (2) mixing 15NH4+ solution into disturbed soil. It was assumed that MIT obtained with the mixing technique reflected the true rates, since the assumption of uniformity was thought to be satisfied by this technique. MIT from the two techniques were not significantly different, thus non-uniform distribution of native 14NH4+, injected 15NH4+ and microbial processes in the intact-core technique did not cause a marked error in the MIT rates. In contrast the gross nitrification rates were twofold higher with the mixing technique than with the intact-core technique. Gross nitrification rates are likely to increase with the mixing technique because mixing redistributes nitrifiers and added 15NH4+, and thereby increases the contact between NH4+ and nitrifiers.  相似文献   

9.

Purpose  

The oxidation of ammonium (NH4+) to nitrate (NO3−) in the soil is an important biogeochemical process, which has major environmental implications as it can contribute to NO3− leaching and nitrous oxide (N2O) emissions. Nitrification inhibitors have been used to slow down this process to reduce NO3− leaching and N2O emissions from agricultural land. The objective of this study was to determine the effectiveness of a liquid formulation of 3,4-Dimethylpyrazole phosphate (DMPP) compared with a dicyandiamide (DCD) solution in inhibiting the growth of ammonium-oxidizing bacteria (AOB) and ammonium oxidizing archaea (AOA) and slowing down the rate of NH4+ oxidation in soil.  相似文献   

10.
An incubation study investigated the effects of nitrification inhibitors (NIs), dicyandiamide (DCD), and neem oil on the nitrification process in loamy sand soil under different temperatures and fertilizer rates. Results showed that NIs decreased soil nitrification by slowing the conversion of soil ammonium (NH4+)-nitrogen (N) and maintaining soil NH4+-N and nitrate (NO3?)-N throughout the incubation time. DCD and neem oil decreased soil nitrous oxide (N2O) emission by up to 30.9 and 18.8%, respectively. The effectiveness of DCD on reducing cumulative soil N2O emission and retaining soil NH4+-N was inconsistently greater than that of neem oil, but the NI rate was less obvious than temperature. Fertilizer rate had a stronger positive effect on soil nitrification than temperature, indicating that adding N into low-fertility soil had a greater influence on soil nitrification. DCD and neem oil would be a potential tool for slowing N fertilizer loss in a low-fertility soil under warm to hot climatic conditions.  相似文献   

11.
Temporal variations in δ15N of NH4+ and NO3− in water-saturated and unsaturated soils were examined in a laboratory incubation study. Ammonium sulfate (δ15N=−2.6‰) was added to 25 g samples of soil at concentrations of 160 mg N kg−1. Soils were then incubated under unsaturated (50% of water holding capacity at saturation, WHC) or saturated (100% of WHC) water conditions for 7 and 36 d, respectively. During 7 d incubation of unsaturated soil, the NH4+-N concentration decreased from 164.8 to 34.4 mg kg−1, and the δ15N of NH4+ increased from −0.4 to +57.2‰ through nitrification, as evidenced by corresponding increase in NO3−-N concentration and lower δ15N of NO3− (product) than that of NH4+ (substrate) at each sampling time. In saturated soil, the concentration of NH4+-N decreased gradually from 162.4 to 24.2 mg kg−1, and the δ15N values increased from +0.8 to +21.0‰ during 36 d incubation. However, increase in NO3− concentration was not observed due to loss of NO3− through concurrent denitrification in anaerobic sites. The apparent isotopic fractionation factors (αs/p) associated with decrease in NH4+ concentration were 1.04 and 1.01 in unsaturated and saturated soils, respectively. Since nitrification is likely to introduce greater isotope fractionation than microbial immobilization, the higher value for unsaturated soil probably reflected faster nitrification under aerobic conditions. The lower value for saturated soil suggests that immobilization and subsequent remineralization of NH4+ were relatively more dominant than nitrification under the anaerobic conditions.  相似文献   

12.
 Soils are a major source of atmospheric NO and N2O. Since the soil properties that regulate the production and consumption of NO and N2O are still largely unknown, we studied N trace gas turnover by nitrification and denitrification in 20 soils as a function of various soil variables. Since fertilizer treatment, temperature and moisture are already known to affect N trace gas turnover, we avoided the masking effect of these soil variables by conducting the experiments in non-fertilized soils at constant temperature and moisture. In all soils nitrification was the dominant process of NO production, and in 50% of the soils nitrification was also the dominant process of N2O production. Factor analysis extracted three factors which together explained 71% of the variance and identified three different soil groups. Group I contained acidic soils, which showed only low rates of microbial respiration and low contents of total and inorganic nitrogen. Group II mainly contained acidic forest soils, which showed relatively high respiration rates and high contents of total N and NH4 +. Group III mainly contained neutral agricultural soils with high potential rates of nitrification. The soils of group I produced the lowest amounts of NO and N2O. The results of linear multiple regression conducted separately for each soil group explained between 44–100% of the variance. The soil variables that regulated consumption of NO, total production of NO and N2O, and production of NO and N2O by either nitrification or denitrification differed among the different soil groups. The soil pH, the contents of NH4 +, NO2 – and NO3 –, the texture, and the rates of microbial respiration and nitrification were among the important variables. Received: 28 October 1999  相似文献   

13.
Incubation of soil under low partial pressures of acetylene (10 Pa) is a widely used method to specifically inhibit nitrification due to the suicide inhibition of ammonium monooxygenase (AMO), the first enzyme in NH4 + oxidation by nitrifying bacteria. Although the inhibition of AMO is irreversible, recovery of activity is possible if new enzyme is synthesized. In experiments with three different soils, NH4 + concentrations decreased and NO3 – concentrations increased soon after acetylene was removed from the atmosphere. Recovery of NO production started immediately after the removal of acetylene. The release rates of NO and N2O were higher in soil samples which were only preincubated with 10 Pa acetylene than in those which were kept in the presence of 10 Pa acetylene. In the permanent presence of 10 Pa acetylene, NH4 + and NO3 – concentrations stayed constant, and the release rates of NO and N2O were low. These low release rates were apparently due to processes other than nitrification. Our experiments showed that the blockage of nitrification by low (10 Pa) acetylene partial pressures is only reliable when the soil is kept in permanent contact with acetylene. Received: 17 July 1996  相似文献   

14.
In the last century, conversion of native North American grasslands to Juniperus virginiana forests or woodlands has dramatically altered ecosystem structure and significantly increased ecosystem carbon (C) stocks. We compared soils under recently established J. virginiana forests and adjacent native C4-dominated grassland to assess changes in potential soil nitrogen (N) transformations and plant available N. Over a 2-year period, concentrations of extractable inorganic N were measured in soils from forest and grassland sites. Potential gross N ammonification, nitrification, and consumption rates were determined using 15N isotope-dilution under laboratory conditions, controlling for soil temperature and moisture content. Potential nitrification rates (Vmax) and microbial biomass, as well as soil physical and chemical properties were also assessed. Extractable NH4+ concentrations were significantly greater in grassland soils across the study period (P ≤ 0.01), but analysis by date indicated that differences in extractable inorganic N occurred more frequently in fall and winter, when grasses were senescent but J. virginiana was still active. Laboratory-based rates of gross N mineralization (ammonification) and nitrification were greater in grassland soils (P ≤ 0.05), but only on one of four dates. Potential nitrification rates (Vmax) were an order of magnitude greater than gross nitrification rates in both ecosystems, suggesting that nitrification is highly constrained by NH4+ availability. Differences in plant uptake of N, C inputs, and soil microclimate as forests replace grasslands may influence plant available N in the field, as evidenced by seasonal differences in soil extractable NH4+, and total soil C and N accumulation. However, we found few differences in potential soil N transformations under laboratory conditions, suggesting that this grassland-to-forest conversion caused little change in mineralizable organic N pools or potential microbial activity.  相似文献   

15.

Purpose

Input of N as NH4 + is known to stimulate nitrification and to enhance the risk of N losses through NO3 ? leaching in humid subtropical soils. However, the mechanisms responsible for this stimulation effect have not been fully addressed.

Materials and methods

In this study, an acid subtropical forest soil amended with urea at rates of 0, 20, 50, 100 mg N kg?1 was pre-incubated at 25 °C and 60 % water-holding capacity (WHC) for 60 days. Gross N transformation rates were then measured using a 15N tracing methodology.

Results and discussion

Gross rates of mineralization and nitrification of NH4 +-N increased (P?<?0.05), while gross rate of NO3 ? immobilization significantly decreased with increasing N input rates (P?<?0.001). A significant relationship was established between the gross nitrification rate of NH4 + and the gross mineralization rate (R 2?=?0.991, P?<?0.01), so was between net nitrification rate of NH4 + and the net mineralization rate (R 2?=?0.973, P?<?0.05).

Conclusions

Stimulation effect of N input on the gross rate of nitrification of NH4 +-N in the acid soil, partially, resulted from stimulation effect of N input on organic N mineralization, which provides pH-favorable microsites for the nitrification of NH4 + in acid soils (De Boer et al., Soil Biol Biochem 20:845–850, 1988; Prosser, Advan Microb Physiol 30:125–181, 1989). The stimulated gross nitrification rate with the decreased gross NO3 ? immobilization rate under the elevated N inputs could lead to accumulation of NO3 ? and to enhance the risk of NO3 ? loss from humid forest soils.
  相似文献   

16.
Ammonia oxidation, the first step of nitrification, is mediated by both ammonia-oxidizing archaea (AOA) and bacteria (AOB); however, the relative contributions of AOA and AOB to soil nitrification are not well understood. In this study we used 1-octyne to discriminate between AOA- and AOB-supported nitrification determined both in soil-water slurries and in unsaturated whole soil at field moisture. Soils were collected from stands of red alder (Alnus rubra Bong.) and Douglas-fir (Pseudotsuga menziesii Mirb. Franco) at three sites (Cascade Head, the H.J. Andrews, and McDonald Forest) on acidic soils (pH 3.9–5.7) in Oregon, USA. The abundances of AOA and AOB were measured using quantitative PCR by targeting the amoA gene, which encodes subunit A of ammonia monooxygenase. Total and AOA-specific (octyne-resistant) nitrification activities in soil slurries were significantly higher at Cascade Head (the most acidic soils, pH < 5) than at either the H.J. Andrews or McDonald Forest, and greater in red alder compared with Douglas-fir soils. The fraction of octyne-resistant nitrification varied among sites (21–74%) and was highest at Cascade Head than at the other two locations. Net nitrification rates of whole soil without NH4+ amendment ranged from 0.4 to 3.3 mg N kg−1 soil d−1. Overall, net nitrification rates of whole soil were stimulated 2- to 8-fold by addition of 140 mg NH4+-N kg−1 soil; this was significant for red alder at Cascade Head and the H.J. Andrews. Red alder at Cascade Head was unique in that the majority of NH4+-stimulated nitrifying activity was octyne-resistant (73%). At all other sites, NH4+-stimulated nitrification was octyne-sensitive (68–90%). The octyne-sensitive activity—presumably AOB—was affected more by soil pH whereas the octyne-resistant (AOA) activity was more strongly related to N availability.  相似文献   

17.

Purpose

Two recent discoveries in nitrogen (N) cycling processes, i.e., archaeal ammonia oxidizers and anaerobic ammonia (ammonium) oxidation (anammox), have triggered great interest in studying microbial ammonia oxidation processes. The purpose of this review is to highlight recent progress in ammonia oxidation processes in soils and sediments and to propose future research activities in this topic.

Results and discussion

Aerobic ammonia oxidation and anammox processes are linked through the production and consumption of nitrite, respectively, thereby removing the reactive N (NH4 +, NO2 ?, NO3 ?) from soil and sediment ecosystems. Ammonia-oxidizing microorganisms are widely distributed in soils and sediments, and increasing evidence suggests that ammonia-oxidizing archaea and bacteria are functionally dominant in the ammonia oxidation of acid soils and other soils, respectively. The widespread occurrence and great variation in the abundance of anammox bacteria indicate their heterogeneous distribution and niche differentiation. Therefore, the worldwide distribution of both microbial groups in nature has stimulated researchers to investigate the physiology and metabolism of related groups, as well as appraising their contribution to N cycling.

Conclusions

We summarized the current progress and provided future perspectives in the microbiology of aerobic and anaerobic ammonia oxidation in soils and sediments. With increasing concern and interest in soil and sediment ammonia oxidation processes, studies in the microbial mechanisms underlying nitrification and anammox, as well as their interactions, are essential for understanding their contribution to the loss of N either through nitrate leaching or N-related gas emissions.  相似文献   

18.
Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH4+) and nitrate (NO3−). To compare NH4+ availability, competition between nitrifiers and heterotrophic microorganisms for NH4+, and microbial NO3− assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, 15N isotope pool dilution and 15N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH4+ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO3− and NH4+ was estimated based on the uptake of 15N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH4+ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO3− assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO3− than NH4+. Heterotrophic microbes assimilated less NH4+ than NO3− probably because NH4+ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH4+ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO3− immobilization.  相似文献   

19.
Agricultural systems that receive high amounts of inorganic nitrogen (N) fertilizer in the form of either ammonium (NH4+), nitrate (NO3−) or a combination thereof are expected to differ in soil N transformation rates and fates of NH4+ and NO3−. Using 15N tracer techniques this study examines how crop plants and soil microbes vary in their ability to take up and compete for fertilizer N on a short time scale (hours to days). Single plants of barley (Hordeum vulgare L. cv. Morex) were grown on two agricultural soils in microcosms which received either NH4+, NO3− or NH4NO3. Within each fertilizer treatment traces of 15NH4+ and 15NO3− were added separately. During 8 days of fertilization the fate of fertilizer 15N into plants, microbial biomass and inorganic soil N pools as well as changes in gross N transformation rates were investigated. One week after fertilization 45-80% of initially applied 15N was recovered in crop plants compared to only 1-10% in soil microbes, proving that plants were the strongest competitors for fertilizer N. In terms of N uptake soil microbes out-competed plants only during the first 4 h of N application independent of soil and fertilizer N form. Within one day microbial N uptake declined substantially, probably due to carbon limitation. In both soils, plants and soil microbes took up more NO3− than NH4+ independent of initially applied N form. Surprisingly, no inhibitory effect of NH4+ on the uptake and assimilation of nitrate in both, plants and microbes, was observed, probably because fast nitrification rates led to a swift depletion of the ammonium pool. Compared to plant and microbial NH4+ uptake rates, gross nitrification rates were 3-75-fold higher, indicating that nitrifiers were the strongest competitors for NH4+ in both soils. The rapid conversion of NH4+ to NO3− and preferential use of NO3− by soil microbes suggest that in agricultural systems with high inorganic N fertilizer inputs the soil microbial community could adapt to high concentrations of NO3− and shift towards enhanced reliance on NO3− for their N supply.  相似文献   

20.
 The 15N isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha–1 year–1 (F). Uniform N enrichment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 μg N g–1 day–1 and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 μg N g–1 day–1. In the F treatment, the turnover of NH4 +-N and NO3 –-N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO3 –-N pools turning over twice as fast as the NH4 +-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added 15N was recovered in the soil microbial biomass (SMB), which represented a 4–5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z<C<F), and the addition of a nitrification inhibitor (C2H2) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net rates was demonstrated and the improved methodology for 15N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions. Received: 25 May 1999  相似文献   

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