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1.
High nitrification rates which convert ammonium (NH4+) to the mobile ions NO2− and NO3− are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH4+) or organic N. Selective nitrification inhibitors and 15N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH4+-N and organic-N by labelling the NH4+ or NO3− pools separately or both together with 15N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a 15N tracing model was developed. This model consists of the three N-pools NH4+, NO3− and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH4+ to NO3− and not the conversion of organic N to NO3−. There was evidence that slurry-enhanced oxidation of NH4+ to NO3− was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH4+ by approximately a factor of three.  相似文献   

2.
The response of terrestrial ecosystems to elevated atmospheric CO2 is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO2 Enrichment (FACE) for six years. A 15N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO2 on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a 15N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH4+ and from organic-N to NO3− and analysed the rate of dissimilatory NO3− reduction to NH4+ (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO2. At the same time the gross rate of NH4+ immobilization increased by 20%, while NH4+ oxidation to NO3− was reduced by 25% under elevated CO2. The NO3− dynamics under elevated CO2 were characterized by a 52% increase in NO3− immobilization and a 141% increase in the DNRA rate, while NO3− production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH4+ pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH4+ under elevated CO2. The advantage of such a shift is that NH4+ is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO2.  相似文献   

3.
The effects of repeated synthetic fertilizer or cattle slurry applications at annual rates of 50, 100 or 200 m3 ha−1 yr−1 over a 38 year period were investigated with respect to herbage yield, N uptake and gross soil N dynamics at a permanent grassland site. While synthetic fertilizer had a sustained and constant effect on herbage yield and N uptake, increasing cattle slurry application rates increased the herbage yield and N uptake linearly over the entire observation period. Cattle slurry applications, two and four times the recommended rate (50 m3 ha−1 yr−1, 170 kg N ha−1), increased N uptake by 46 and 78%, respectively after 38 years. To explain the long-term effect, a 15N tracing study was carried out to identify the potential change in N dynamics under the various treatments. The analysis model evaluated process-specific rates, such as mineralization, from two organic-N pools, as well as nitrification from NH4+ and organic-N oxidation. Total mineralization was similar in all treatments. However, while in an unfertilized control treatment more than 90% of NH4+ production was related to mineralization of recalcitrant organic-N, a shift occurred toward a predominance of mineralization from labile organic-N in the cattle slurry treatments and this proportion increased with the increase in slurry application rate. Furthermore, the oxidation of recalcitrant organic-N shifted from a predominant NH4+ production in the control treatment, toward a predominant NO3− production (heterotrophic nitrification) in the cattle slurry treatments. The concomitant increase in heterotrophic nitrification and NH4+ oxidation with increasing cattle slurry application rate was mainly responsible for the increase in net NO3− production rate. Thus the increase in N uptake and herbage yield on the cattle slurry treatments could be related to NO3− rather than NH4+ production. The 15N tracing study was successful in revealing process-specific changes in the N cycle in relationship to long-term repeated amendments.  相似文献   

4.
Legumes increase the plant-available N pool in soil, but might also increase NO3− leaching to groundwater. To minimize NO3− leaching, N-release processes and the contribution of legumes to NO3− concentrations in soil must be known. Our objectives were (1) to quantify NO3−-N export to >0.3 m soil depth from three legume monocultures (Medicago x varia Martyn, Onobrychis viciifolia Scop., Lathyrus pratensis L.) and from three bare ground plots. Furthermore, we (2) tested if it is possible to apply a mixing model for NO3− in soil solution based on its dual isotope signals, and (3) estimated the contribution of legume mineralization to NO3− concentrations in soil solution under field conditions. We collected rainfall and soil solution at 0.3 m soil depth during 1 year, and determined NO3− concentrations and δ15N and δ18O of NO3− for >11.5 mg NO3−-N l−1. We incubated soil samples to assess potential N release by mineralization and determined δ15N and δ18O signals of NO3− derived from mineralization of non-leguminous and leguminous organic matter.Mean annual N export to >0.3 m soil depth was highest in bare ground plots (9.7 g NO3−-N m−2; the SD reflects the spatial variation) followed by Medicago x varia monoculture (6.0 g NO3−-N m−2). The O. viciifolia and L. pratensis monocultures had a much lower mean annual N export (0.5 and 0.3 g NO3−-N m−2). The averaged NO3−-N leaching during 70 days was not significantly different between field estimates and incubation for the Medicago x varia Martyn monoculture.The δ15N and δ18O values in NO3− of rainfall (δ15N: 3.3±0.8‰; δ18O: 30.8±4.7‰), mineralization of non-leguminous SOM (9.3±0.9‰; 6.7±0.8‰), and mineralization of leguminous SOM (1.5±0.6‰; 5.1±0.9‰) were markedly different. Applying a linear mixing model based on these three sources to δ15N and δ18O values in NO3− of soil solution during winter 2003, we calculated 18-41% to originate from rainfall, 38-57% from mineralization of non-leguminous SOM, and 18-40% from mineralization of leguminous SOM.Our results demonstrate that (1) even under legumes NO3−-N leaching was reduced compared to bare ground, (2) the application of a three-end-member mixing model for NO3− based on its dual isotope signals produced plausible results and suggests that under particular circumstances such models can be used to estimate the contributions of different NO3− sources in soil solution, and (3) in the 2nd year after establishment of legumes, they contributed approximately one-fourth to NO3−-N loss.  相似文献   

5.
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   

6.
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   

7.
An incubation experiment was conducted to study the response to sodium chloride (NaCl) salinity of microbial population immobilizing NH4+- and NO3−-N using glucose as an easily oxidizable C source. Immobilization of NH4+-N was faster than that of NO3−-N and was complete within 12 h of -incubation. Presence of NaCl retarded the process of N immobilization; that of NO3−-N being more affected. Remineralization of immobilized N started within 48 h in case of both NH4+- and NO3−-N and was faster for the latter. Both remineralization and nitrification were significantly delayed in the presence of NaCl; inhibition being more at 4000 mg NaCl kg−1 soil. The inhibitory effect of NaCl on remineralization of N was relatively more for NH4+-treated soil. The results of the study suggested a higher sensitivity to NaCl of microorganisms assimilating NO3−. However, remineralization of N from NO3−-assimilating microbial population was less affected by NaCl salinity compared to NH4+-assimilating population.  相似文献   

8.
Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO2 concentrations and global warming through soil respiration. We conducted a comparative study to determine NH4+ and NO3− concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO3−-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH4+-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg−1 d−1) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH4+ and, NO3− concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.  相似文献   

9.
The assumption in using the chloroform fumigation technique for microbial biomass determination is that microbes are killed or at least inactivated by the treatment. Problems associated with transformations of the N released on or during fumigation have so far only been associated with the fumigation-incubation method. A laboratory and a field study were carried out to investigate the possible N transformations during biomass determination by the fumigation-extraction method. Labelled NH4NO3 (either the NO3−, NH4+ or both pools were 15N enriched) was applied to the soil and biomass determinations made at intervals subsequently. The size and enrichment of the ammonium (NH4+), and nitrate (NO3−) pools were determined before and after chloroform fumigation. The 15N enrichment of the NH4+ pool after fumigation could only be explained if immobilisation of ammonium occurred at some time during the 24 h fumigation period. The extent of this immobilisation was calculated. In addition, there was evidence that nitrification occurred during the fumigation procedure at the start of the laboratory study and throughout the field study. The laboratory and field study differed mainly in the dynamics related to NO3− uptake and release. There was evidence for uptake of NO3− by the microbial biomass with and without utilization. We conclude that the 15N enrichment in the microbial biomass cannot be accurately determined when N transformations and release of non-utilized N occurs during fumigation. The possible immobilisation of mineral N during fumigation will affect the magnitude of the factor used to convert measured microbial biomass N to actual microbial biomass N in soil.  相似文献   

10.
A 15N tracing study was carried out to identify microbial and abiotic nitrogen (N) transformations in a south Chilean Nothofagus betuloides forest soil which is characterized by low N inputs and absence of human disturbance. Gross N transformation rates were quantified with a 15N tracing model in combination with a Markov chain Monte Carlo sampling algorithm for parameter estimation. The 15N tracing model included five different N pools (ammonium (NH4+), nitrate (NO3−), labile (Nlab) and recalcitrant (Nrec) soil organic matter and adsorbed NH4+), and ten gross N transformation rates. The N dynamics in the N. betuloides ecosystem are characterized by low net but high gross mineralization rates. Mineralization in this soil was dominated by turnover of Nlab, while immobilization of NH4+ predominantly entered the Nrec pool. A fast exchange between the NH4+ and the adsorbed NH4+ pool was observed, possibly via physical adsorption on and release from clay lattices, providing an effective buffer for NH4+. Moreover, high NH4+ immobilization rates into the Nrec pool ensure a sustained ecosystem productivity. Nitrate, the most mobile form of N in the system, is characterized by a slow turnover and was produced in roughly equal amounts from NH4+ oxidation and organic N oxidation. More than 86% of the NO3− produced was immediately consumed again. This study showed for the first time that dissimilatory nitrate reduction to ammonium (DNRA) was almost exclusively (>99%) responsible for NO3− consumption. DNRA rather than NO3− immobilization ensures that NO3− is transformed into another available N form. DNRA may therefore be a widespread N retention mechanism in ecosystems that are N-limited and receive high rainfalls.  相似文献   

11.
A new 15N tracing model was developed to analyse nitrogen (N) transformations in old grassland soil. There was a need to develop a new model because existing models such as FLUAZ were not able to simulate the observed N dynamics. The new features of the model are: (a) simulation of heterotrophic nitrification, (b) simulation of dissimilatory nitrate (NO3−) reduction to ammonium (NH4+) (DNRA), (c) release of adsorbed or stored fertiliser N into the available mineral N pools and (d) immobilisation of NH4+ and NO3− into two separate organic N pools with different re-mineralisation characteristics. The tracing model contains six N pools and nine simultaneous N transformations either at zero- or first-order kinetics. The model is set up in the modelling software ModelMaker which contains non-linear optimisation routines based on the Marquardt-Levenberg algorithm. The model is able to simulate data obtained from triple labelling studies where either the NH4+, the NO3− or both pools were labelled with 15N. The flexible modelling environment allows the user to develop the model further.  相似文献   

12.
Elevated CO2 may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO2-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO2. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to 13C labeling at ambient (392 μmol mol−1) and elevated (792 μmol mol−1) CO2 concentrations, in soil containing 15N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-13C was enhanced by 50% under elevated compared to ambient CO2. Concurrently, plant 15N uptake increased (+7%) under elevated CO2 while 15N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO2. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO2 can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO2.  相似文献   

13.
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14.
The availability of C and N to the soil microbial biomass is an important determinant of the rates of soil N transformations. Here, we present evidence that changes in C and N availability affect the 15N natural abundance of the microbial biomass relative to other soil N pools. We analysed the 15N natural abundance signature of the chloroform‐labile, extractable, NO3–, NH4+ and soil total N pools across a cattle manure gradient associated with a water reservoir in semiarid, high‐desert grassland. High levels of C and N in soil total, extractable, NO3–, NH4+ and chloroform‐labile fractions were found close to the reservoir. The δ15N value of chloroform‐labile N was similar to that of extractable (organic + inorganic) N and NO3– at greater C availability close to the reservoir, but was 15N‐enriched relative to these N‐pools at lesser C availability farther away. Possible mechanisms for this variable 15N‐enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more 15N‐enriched substrate with decreasing C availability.  相似文献   

15.
Experimentation with dynamics of soil carbon pools as affected by elevated CO2 can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis (δ13C) were used to investigate labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa free air CO2 enrichment (FACE) experiment, in which elevated CO2 (FACE: 560 μmol mol−1) and ambient CO2 (Control: 360 μmol mol−1) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO2 transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Similarly, old C in the recalcitrant pool under elevated CO2 was higher than that under ambient CO2, indicating that elevated CO2 reduces the decay of the old C in recalcitrant pool. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 12 and 13 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO2 enhanced the shift of the carbon from labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO2 and mitigating harmful CO2 under future atmospheric conditions.  相似文献   

16.
Stable 15N isotope dilution and tracer techniques were used in cultivated (C) and uncultivated (U) ephemeral wetlands in central Saskatchewan, Canada to: (1) quantify gross mineralization and nitrification rates and (2) estimate the relative proportion of N2O emissions from these wetlands that could be attributed to denitrification versus nitrification-related processes. In-field incubation experiments were repeated in early May, mid-June and late July. Mean gross mineralization and nitrification rates (10.3 and 3.1 mg kg−1 d−1, respectively) did not differ between C and U wetlands on any given date. Despite these similarities, the mean NH4+ pool size in the U wetlands (17.2 mg kg−1) was two to three times that of the C wetlands (6.7 mg kg−1) whereas the mean NO3− pool size in U wetlands (2.2 mg kg−1) was less than half that of C wetlands (5.8 mg kg−1). Mean N2O emissions from the C wetlands decreased from 112.8 to 17.0 ng N2O m2 s−1 from May to July, whereas mean U-wetland N2O emissions ranged only from 31.8 to 51.1 ng N2O m2 s−1 over the same period. This trend is correlated to water-filled pore space in C wetlands, demonstrating a soil moisture influence on emissions. Denitrification is generally considered the dominant emitter of N2O under anaerobic conditions, but in the C wetlands, only 49% of the May emissions could be directly attributed to denitrification, decreasing to 29% in July. In contrast, more than 75% of the N2O emissions from the U wetlands arose from denitrification of the soil NO3− pool throughout the season. These land use differences in emission sources and rates should be taken into consideration when planning management strategies for greenhouse gas mitigation.  相似文献   

17.
Emissions of N2O and CH4 and CH4 oxidation rates were measured from Lolium perenne swards in a short-term study under ambient (36 Pa) and elevated (60 Pa) atmospheric CO2 at the Free Air Carbon dioxide Enrichment experiment, Eschikon, Switzerland. Elevated pCO2 increased (P<0.05) N2O emissions from high N fertilised (11.2 g N m−2) swards by 69%, but had no significant effect on net emissions of CH4. Application of 13C-CH4 (11 μl l−1; 11 at.% excess 13C) to closed chamber headspaces in microplots enabled determination of rates of 13C-CH4 oxidation even when net CH4 fluxes from main plots were positive. We found a significant interaction between fertiliser application rate and atmospheric pCO2 on 13C-CH4 oxidation rates that was attributed to differences in gross nitrification rates and C and N availability. CH4 oxidation was slower and thought to be temporarily inhibited in the high N ambient pCO2 sward. The most rapid CH4 oxidation of 14.6 μg 13C-CH4 m−2 h−1 was measured in the high fertilised elevated pCO2 sward, and we concluded that either elevated pCO2 had a stimulatory effect on CH4 oxidation or inhibition of oxidation following fertiliser application was lowered under elevated pCO2. Application of 14NH415NO3 and 15NH415NO3 (10 at.% excess 15N) to different replicates enabled determination of the respective contributions of nitrification and denitrification to N2O emissions. Inhibition of CH4 oxidation in the high fertilised ambient pCO2 sward, due to competition between NH3 and CH4 for methane monooxygenase enzymes or toxic effects of NH2OH or NO2− produced during nitrification, was hypothesised to increase gross nitrification (12.0 mg N kg dry soil−1) and N2O emissions during nitrification (327 mg 15N-N2O m−2 over 11 d). Our results indicate that increasing atmospheric concentrations of CO2 may increase emissions of N2O by denitrification, lower nitrification rates and either increase or decrease the ability of soil to act as a sink for atmospheric CH4 depending on fertiliser management.  相似文献   

18.
Alternative silvicultural systems were introduced in Coastal Western Hemlock forests of British Columbia, Canada, to reduce disturbance incurred by conventional clear-cutting and to maintain the forest influence on soil nutrient cycling. As we hypothesized, in situ pools and net mineralization of NH4 + were lower under no and low disturbance (old-growth forest and shelterwood) compared to clear-cuts (high disturbance); in situ pools and net production of NO3 − were very low across all treatments. Gross transformation rates of NH4 + increased while those of NO3 − decreased with increasing disturbance, suggesting that these processes were uncoupled. We conclude that shelterwood harvesting reduces the impact on forest floor NH4 + cycling compared to clear-cutting, and that neither low nor high disturbance intensity results in substantial NO3 − accumulation, as what occasionally occurs in other ecosystems. We hypothesize that the uncoupling of NH4 + and NO3 − dynamics may be due to the predominance of heterotrophic nitrification by lignin-degrading fungi that oxidize organic N rather than NH4 +–N, and whose activities are suppressed at high NH4 + concentrations.  相似文献   

19.
We compared gross N fluxes by 15N pool dilution in a coarse-textured agricultural soil when 15N was applied to the soil NH4+ pool by either: (i) mixing a 15NH4NO3 solution into disturbed soil or (ii) injection of 15NH3 gas into intact soil cores. The two techniques produced similar results for gross N mineralization rates indicating that NH4+ production in soil was not altered by soil disturbance, method of application (gas vs. solution), or amount of N applied. This was not the case for immobilization rates, which were twofold higher when 15N label was applied to the soil NH4+ pool with the mixing technique compared to the injection technique. This was attributed to the fact that more NH4+ was applied with the mixing technique. Estimates of gross nitrification were accompanied by large error terms meaning differences between 15N labeling methods could not be accurately assessed for this process rate.  相似文献   

20.
A laboratory investigation was performed to compare the fluxes of dinitrogen (N2), N2O and carbon dioxide (CO2) from no-till (NT) and conventional till (CT) soils under the same water, mineral nitrogen and temperature status. Intact soil cores (0-10 cm) were incubated for 2 weeks at 25 °C at either 75% or 60% water-filled pore space (WFPS) with 15N-labeled fertilizers (100 mg N kg−1 soil). Gas and soil samples were collected at 1-4 day intervals during the incubation period. The N2O and CO2 fluxes were measured by a gas chromatography (GC) system while total N2 and N2O losses and their 15N mole fractions in the soil mineral N pool were determined by a mass spectrometer. The daily accumulative fluxes of N2 and N2O were significantly affected by tillage, N source and soil moisture. We observed higher (P<0.05) fluxes of N2+N2O, N2O and CO2 from the NT soils than from the CT soils. Compared with the addition of nitrate (NO3−), the addition of ammonium (NH4+) enhanced the emissions of these N and C gases in the CT and NT soils, but the effect of NH4+ on the N2 and/or N2O fluxes was evident only at 60% WFPS, indicating that nitrification and subsequent denitrification contributed largely to the gaseous N losses and N2O emission under the lower moisture condition. Total and fertilizer-induced emissions of N2 and/or N2O were higher (P<0.05) at 75% WFPS than with 60% WFPS, while CO2 fluxes were not influenced by the two moisture levels. These laboratory results indicate that there is greater potential for N2O loss from NT soils than CT soils. Avoiding wet soil conditions (>60% WFPS) and applying a NO3− form of N fertilizer would reduce potential N2O emissions from arable soils.  相似文献   

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