Evidence that fungi can oxidize NH4+ to NO3− in a grassland soil

The contribution of bacteria and fungi to NH₄⁺ and organic N (N_org) oxidation was determined in a grassland soil (pH 6.3) by using the general bacterial inhibitor streptomycin or the fungal inhibitor cycloheximide in a laboratory incubation study at 20°C. Each inhibitor was applied at a rate of 3 mg g^?1 oven‐dry soil. The size and enrichment of the mineral N pools from differentially (NH₄¹⁵NO₃ and ¹⁵NH₄NO₃) and doubly labelled (¹⁵NH₄¹⁵NO₃) NH₄NO₃ were measured at 3, 6, 12, 24, 48, 72, 96 and 120 hours after N addition. Labelled N was applied to each treatment, to supply NH₄⁺‐N and NO₃^?‐N at 3.15 μmol N g^?1 oven‐dry soil. The N treatments were enriched to 60 atom % excess in ¹⁵N and acetate was added at 100 μmol C g^?1 oven‐dry soil, to provide a readily available carbon source. The oxidation rates of NH₄⁺ and N_org were analysed separately for each inhibitor treatment with a ¹⁵N tracing model. In the absence of inhibitors, the rates of NH₄⁺ oxidation and organic N oxidation were 0.0045 μmol N g^?1 hour^?1 and 0.0023 μmol N g^?1 hour^?1, respectively. Streptomycin had no effect on nitrification but cycloheximide inhibited the oxidation of NH₄⁺ by 89% and the oxidation of organic N by more than 30%. The current study provides evidence to suggest that nitrification in grassland soil is carried out by fungi and that they can simultaneously oxidize NH₄⁺ and organic N.     

Simulating the dynamics of glucose and NH4+ in alkaline saline soils of the former Lake Texcoco with the Detran model

The turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with ¹⁴C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of ¹⁴C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg ¹⁴C‐labelled glucose kg^?1 and 200 mg NH₄⁺‐N kg^?1, and incubated aerobically. Production of ¹⁴CO₂ and CO₂, dynamics of NH₄⁺, NO₂^– and NO₃^–, and microbial biomass ¹⁴C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because > 90 mg ¹⁴C kg^?1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH₄⁺ and NO₃^– in the NH₄⁺‐amended soil could not be simulated unless an immobilization of NH₄⁺ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH₄⁺ was added to alkaline saline soils.     

TRANSFORMATION OF 15N-LABELLED AMMONIUM IN TWO SOILS DIFFERING IN NH+4-FIXING CAPACITY

Two soils differing in ammonium fixation capacity were incubated for 127 days with ¹⁵N-ammonium sulphate. In a gley soil with high NH⁺₄-fixing capacity caused by smectites with a charge up to 0.8 per formula unit, the major part of the added ammonium was first fixed by minerals and then released slowly during incubation. The proportion of labelled N in the nitrate fraction increased during the first weeks and then decreased permanently. In contrast, in a histosol with low NH⁺₄-fixing capacity, the exchangeable fraction contained most of the labelled NH⁺₄, this being highly available to microorganisms and therefore subject to nitrification. About 50% of the added ¹⁵NH₄ was lost from the histosol in 127 days, but only about 20 per cent was lost from the gley soil.     

Measuring gross nitrogen mineralization, and nitrification by 15 N isotopic pool dilution in intact soil cores

The isotope dilution method for measuring gross rates of N mineralization, immobilization, and nitrification was applied to intact soil cores so that the effects of soil mixing were avoided. Soil cores were injected with solutions of either ¹⁵NH₄⁺ or ¹⁵NO₄^?; gross mineralization rates were calculated from the decline in “N enrichment of the NH: pool during a 24-h incubation; gross nitrification rates were calculated from the decline in ¹⁵N enrichment of the NO^?₃ pool; gross rates of NH₄⁺ and NO₃^? consumption were calculated from disappearance of the ¹⁵N label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added “N with ambient pools was not a necessary assumption but that bias in distribution of added label, coincident with a non-random distribution of microbial processes, would cause significant errors in rate estimates. Rate estimates were also sensitive to errors in initial ¹⁵N and ¹⁴N pool size estimates, In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added ¹⁵NH₄⁺ within minutes of adding the label to sterilized soil. Extracting a subset of soil cores at the beginning of an incubation is recommended for obtaining initial pool size estimates. Gross immobilization is probably stimulated by addition of inorganic ¹⁵N substrate and, therefore, is overestimated by the isotope dilution method. As an alternative method, a non-linear equation is given for calculating the gross immobilization rate from the appearance of ¹⁵N in chloroform-labile microbial biomass; but incomplete extraction of biomass N may result in low estimates. Details of the isotope dilution methodology (injection rates, concentrations, experimental artefacts, etc.) are described and discussed. When care is taken to understand the underlying assumptions and sources of error, the isotope dilution method provides a powerful tool for measuring gross rates of microbial transformations of soil nitrogen in intact soil cores.     

Effect of elevated CO2 on soil N dynamics in a temperate grassland soil

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.     

Simultaneous nitrification and diffusion in soil. I. The effects of the addition of a low level of ammonium chloride

The nitrification of NH₄⁺ and the simultaneous diffusion of NH₄⁺, NO₃^? and H⁺ following the addition of ammonium chloride to a fine sandy loam soil was analysed experimentally and theoretically. Experimentally, the concentration profiles of mineral N and pH were analysed 140h and 284 h after the homogeneous addition of 11 μmoles NH₄Cl_cm^?3 of soil to one part of a composite soil column. The mathematical model presented includes a kinetic model of nitrifier growth and activity, the adsorption equilibria of NH₄⁺ and soil acidity with the soil solid phase and the influence of other ions on the diffusion characteristics of each diffusing ion. The predictions of the model were generated using parameters derived from independent experiments so that the predictions did not depend on data derived from the experimental concentration profiles. Good agreement was found between experimental and predicted profiles. The use of the model for predicting the penetration of NH₄⁺ and NO₃^? into the soil is demonstrated.     

Microbial populations immobilizing NH4-N and NO3-N differ in their sensitivity to sodium chloride salinity in soil

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.     

Effect of reforestation on turnover of 15N-labelled nitrate and ammonium in relation to changes in soil microflora

The effects of incubation time, vegetation type (represented by a pine plantation, a protected and a periodically burnt eucalypt forest), lime and finely ground pine needles on the transformation of (¹⁵NH₄)₂SO₄ and K¹⁵NO₃ were studied in incubation experiments with a sandy lateritic podzolic soil from south-east Queensland. Microorganisms were counted so as to relate N transformations to particular groups of microorganisms.The heterotrophic miroflora utilized NH⁺₄ as a source of N in preference to NO^?₃, and autotrophic nitrifiers seemed to be weak competitors for NH⁺₄. Lime caused a slight loss of NO^?₃ and this was accompanied by an increase in the population of denitrifying bacteria.Lime promoted immobilization of NH⁺₄ by heterotrophic bacteria and subsequent mineralization by nitrifying bacteria, but when pine needles were also added the nitrifiers were suppressed and immobilization by heterotrophic bacteria dominated. Pine needles alone stimulated fungi to immobilize NH⁺₄.While reforestation with exotic pines caused a loss of total-N there was evidence of increased turnover, i.e. more rapid immobilization and nitrification, in pine plantation soils. Prescribed burning also promoted nitrification while reducing total-N.     

Effect of ammonium-, nitrite- and nitrate nitrogen on anaerobic nitrogenase activity in soil

Sandy loam soil, with added glucose, was incubated anaerobically under N₂ and subjected to repeated 1-h C₂H₂ reduction assays. In the presence of 1% glucose the addition of 50 μg NH₄⁺ ?N/g or of 20 μg NO^?₃ N/g (untreated soil contained 1.2 μg NH⁺₄?N and 7.10 μg NO^?₃-N/g) caused at least some suppression of nitrogenase activity. Activity developed when the KCl-extractable soil inorganic nitrogen concentration dropped below 35 μg/g. In the presence of 0.1 or 0.05% glucose the addition of 5 μg NH⁺₄?N/g caused some suppression of nitrogenase activity. However, activity developed when the soil NH₄⁺-N concentration dropped below about 4 μg/g. With 0.1% glucose and 5 μg added NO^?₂ N/g, activity did not develop until the soil NO^?₂ -N concentration dropped to zero. Added NO^?₃ N was rapidly reduced and denitrified to NO^?₂- N, N₂O-N and NH⁺₄ N and furthermore caused some inhibition of CO₂ evolution. The data from NH₄^?-addition experiments are consistent with a nitrogenase repression/ derepression threshold of 4 and 35μg NH⁺₄-N/g at 0.05 and 1% glucose concentrations, respectively. The data from NO^?₂- and NO^?₃-addition experiments suggest a combination of repression and toxicity effects in the presence of added NO^?₃ N.     

铵硝营养对水稻氮效率和矿质养分吸收的影响

NH_4~+和NO_3~–是对植物有效的两种主要无机氮源。水稻一般被认为是偏好NH_4~+的植物,但是在NO_3~–条件下,水稻也能良好地生长。大多数关于水稻铵硝营养的报道是在pH 6.0左右的水培条件下开展的,但是对于酸性条件下水稻铵硝营养研究很少。随着土壤酸化的加重及一些边际酸性土壤被用作水稻种植,研究酸性条件下水稻的铵硝营养具有重要意义。本文采用水培试验,在pH 5.0的条件下,通过添加和不添加pH缓冲剂MES(2-(N-吗啡啉)乙磺酸),研究了NH_4~+和NO_3~–对水稻生长、氮效率和矿质养分(N、P、K、Ca、Mg、Fe、Zn、Cu、Mn)吸收的影响。结果表明,在不添加MES的条件下,水稻地上部生长(株高、叶绿素含量、干重)在NH_4~+和NO_3~–之间没有显著差异,而添加MES后,NH_4~+处理的水稻地上部生长优于NO_3~–。不管是否添加MES,NO_3~–处理的水稻地下部生长(根长、根表面积和根物质量)优于NH_4~+。水稻含氮量和氮利用效率在不同NH_4~+和NO_3~–处理之间没有显著差异,但是NH_4~+处理的水稻氮吸收效率高于NO_3~–。与NO_3~–相比,NH_4~+增加了水稻地上部P和Fe含量,而降低了水稻地上部Ca、Mg、Zn、Cu和Mn含量,对K含量影响较小。上述结果表明,NH_4~+有利于改善水稻地上部生长,提高氮吸收效率、地上部P和Fe含量,而NO_3~–则有利于水稻发根,提高地上部Ca、Mg、Zn、Cu和Mn含量。     

Comparison of NH4 pool dilution techniques to measure gross N fluxes in a coarse textured soil

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.     

The influence of increased NH4+ deposition rates on aluminium chemistry in the percolate of acid soils

The influence of different NH₄⁺ loads on aluminium speciation in percolating water was studied on acid luvisols with similar soil-pH values but with different soil adsorption surface characteristics. All of the applied NH₄⁺ was nitrified and therefore led to proton production. Thus, Al displacement in the percolate was increased. The extent of this enhanced translocation was found to be chiefly a function of the amount of exchangeable base cations which buffered the protons produced by nitrification. The proportion of the different Al species in the percolate was altered at large NH₄⁺ loads. Very large A1³⁺ concentrations eliminated Al species bound to dissolved organic matter and particles, presumably because of flocculation and precipitation of organic ligands. Furthermore, increased NO₃^? and A1³⁺ concentrations enhanced the desorption of F^? giving rise to an additional increase in Al displacement by the formation of Al fluoro complexes in the aqueous phase.     

Simultaneous nitrification and diffusion in soil. III. The effects of the addition of ammonium sulphate

The simultaneous nitrification and diffusion of NH₄⁺, applied as ammonium sulphate to laboratory columns, was followed experimentally and with a simulation model. Ammonium was applied as a fertilizer band at levels equivalent to 69 kg N ha^?1 to a 1 cm depth. The concentration profiles of NH₄⁺, NO₃^?, SO^2?₄ and pH were measured in two columns for incubation times of 214 and 286 h. The simulation model provided for the precipitation and ion pair formation of CaSO₄, the adsorption equilibria of NH₄⁺ and soil acid with the soil solid phase, and nitrifier growth and activity. In general, good agreement was found between the experimental and simulated concentration profiles. The effect of CaSO₄ precipitation on the diffusion of N was investigated using model simulations of the diffusion of NH₄⁺ in the absence of nitrification. The simulations suggested that the reactions of SO^2?4 in the soil could markedly affect the spread of NH₄⁺ from a band of (NH₄)₂SO₄.     

Acetylene and N-serve effects upon N2O emissions from NH4+ and NO3− treated soils under aerobic and anaerobic conditions

N₂O emissions from soils treated with NH₄⁺-N under aerobic conditions in the laboratory were 3- to 4-fold higher than those from controls (no extra N added) or when NO₃^?-N was added. Although the emission of N₂O-N in these field and laboratory experiments represented only 0.1–0.8% of the applied fertilizer NH₄⁺-N and are therefore not significant from an agronomic standpoint, these studies have conclusively demonstrated that the oxidation of applied ammoniacal fertilizers (nitrification) could contribute significantly to the stratospheric N₂O pool.Like N-serve, acetylene was shown to be a potent inhibitor of nitrification as it stopped the oxidation of NH₄⁺-N to (NO₃⁺-N + NO₂^?)-N and hence reduced the evolution of N₂O from nitrification within 60 min after its addition.Although high amounts of NO₃^?-N were present, the rate of denitrification was very low from soils with moisture up to 60% saturation. The further increase in the degree of saturation resulted in several-fold increase of denitrification which eventually became the predominant mechanism of gaseous N losses under anaerobic conditions.     

定期淹水强碱性盐渍土中14C标记葡萄糖和NH4+的动态

Flooding an extremely alkaline（pH 10.6） saline soil of the former Lake Texcoco to reduce salinity will affect the soil carbon（C）and nitrogen（N） dynamics.A laboratory incubation experiment was done to investigate how decreasing soil salt content affected dynamics of C and N in an extremely alkaline saline soil.Sieved soil with electrical conductivity（EC） of 59.2 dS m^-1 was packed in columns,and then flooded with tap water,drained freely and conditioned aerobically at 50%water holding capacity for a month.This process of flooding-drainage-conditioning was repeated eight times.The original soil and the soil that had undergone one,two,four and eight flooding-drainage-conditioning cycles were amended with 1000 mg glucose-¹⁴C kg^-1 soil and 200 mg NH₄⁺-N kg^-1soil,and then incubated for 28 d.The CO₂ emissions,soil microbial biomass,and soil ammonium（NE₄⁺）,nitrite（NO₂^-） and nitrate（NO₃^-） were monitored in the aerobic incubation of 28 d.The soil EC decreased from 59.2 to 1.0 dS m_¹ after eight floodings,and soil pH decreased from 10.6 to 9.6.Of the added ¹⁴C-labelled glucose,only 8%was mineralized in the original soil,while 24%in the soil flooded eight times during the 28-d incubation.The priming effect was on average 278 mg C kg^-1 soil after the 28-d incubation.Soil microbial biomass C(mean 66 mg C kg^-1 soil) did not change with flooding times in the unamended soil,and increased 1.4 times in the glucose-NH₄⁺-amended soil.Ammonium immobilization and NO₂^- concentration in the aerobically incubated soil decreased with increasing flooding times,while NO₃^- concentration increased.It was found that flooding the Texcoco soil decreased the EC sharply,increased mineralization of glucose,stimulated nitrification,and reduced immobilization of inorganic N,but did not affect soil microbial biomass C.     

Pathways and dynamics of NO3 and NH4 applied in a mountain Picea abies forest and in a nearby meadow in central Switzerland

To evaluate the pathways and dynamics of inorganic nitrogen (N) deposition in previously N-limited ecosystems, field additions of ¹⁵N tracers were conducted in two mountain ecosystems, a forest dominated by Norway spruce (Picea abies) and a nearby meadow, at the Alptal research site in central Switzerland. This site is moderately impacted by N from agricultural and combustion sources, with a bulk atmospheric deposition of 12 kg N ha⁻¹ y⁻¹ equally divided between NH₄⁺ and NO₃⁻. Pulses of ¹⁵NH₄⁺ and ¹⁵NO₃⁻ were applied separately as tracers on plots of 2.25 m². Several ecosystem pools were sampled at short to longer-term intervals (from a few hours to 1 year), above and belowground biomass (excluding trees), litter layer, soil LF horizon (approx. 5-0 cm), A horizon (approx. 0-5 cm) and gleyic B horizon (5-20 cm). Furthermore, extractable inorganic N, and microbial N pools were analysed in the LF and A horizons. Tracer recovery patterns were quite similar in both ecosystems, with most of the tracer retained in the soil pool. At the short-term (up to 1 week), up to 16% of both tracers remained extractable or entered the microbial biomass. However, up to 30% of the added ¹⁵NO₃⁻ was immobilised just after 1 h, and probably chemically bound to soil organic matter. 16% of the NH₄⁺ tracer was also immobilised within hours, but it is not clear how much was bound to soil organic matter or fixed between layers of illite-type clay. While the extractable and microbial pools lost ¹⁵N over time, a long-term increase in ¹⁵N was measured in the roots. Otherwise, differences in recovery a few hours after labelling and 1 year later were surprisingly small. Overall, more NO₃⁻ tracer than NH₄⁺ tracer was recovered in the soil. This was due to a strong aboveground uptake of the deposited NH₄⁺ by the ground vegetation, especially by mosses.     

Does the potential ammonium fixation of soils have an impact on the optimum nitrogen fertilizer rate?

Field experiments were conducted to determine the effect of nitrogen (N) fertilizer forms and doses on wheat (Triticum aestivum L.) on three soils differing in their ammonium (NH₄) fixation capacity [high = 161 mg fixed NH₄-N kg^?1 soil, medium = 31.5 mg fixed NH₄-N kg^?1 soil and no = nearly no fixed NH₄-N kg^?1 soil]. On high NH₄⁺ fixing soil, 80 kg N ha^?1 Urea+ ammonium nitrate [NH₄NO₃] or 240 kg N ha^?1 ammonium sulfate [(NH₄)₂SO₄]+(NH₄)₂SO₄, was required to obtain the maximum yield. Urea + NH₄NO₃ generally showed the highest significance in respect to the agronomic efficiency of N fertilizers. In the non NH₄⁺ fixing soil, 80 kg N ha^?1 urea+NH₄NO₃ was enough to obtain high grain yield. The agronomic efficiency of N fertilizers was generally higher in the non NH₄⁺ fixing soil than in the others. Grain protein was highly affected by NH₄⁺ fixation capacities and N doses. Harvest index was affected by the NH₄⁺ fixation capacity at the 1% significance level.     

C and N availability affects the 15N natural abundance of the soil microbial biomass across a cattle manure gradient

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.     

Effect of sodium chloride on CO2 evolution,ammonification, and nitrification in a Sassafras sandy loam

Sodium chloride, at rates up to 100 mg g^?1, was added to a Sassafras sandy loam amended with finely-ground alfalfa to determine the effect of NaCl on CO₂ evolution, ammonification, and nitrification in a 14-week study. A NaCl concentration of 0.25 mg g^?1 significantly reduced CO₂ evolution by 16% in unamended soil and 5% in alfalfa-amended soil. Increasing NaCl progressively reduced CO₂ evolution, with no CO₂ evolved from the soil receiving 100 mg NaCl g^?1. A 0.50 mg NaCl g^?1 rate was required before a significant reduction in decomposition of the alfalfa occurred. The NO^?₂-N + NO^?₃-N content of the soil was significantly reduced from 40 to 37 μg g^?1 at 0 and 0.25 mg NaCl g^?1, respectively in the unamended soil. In the alfalfa amended soil, nitrification was significantly reduced at 5 mg NaCl g^?1. At 10 mg NaCl g^?1, nitrification was completely inhibited, there being only 6 and 2 μg NO^?₂-N + NO^?₃-N g^?1 in the alfalfa amended and unamended soil, respectively. In the alfalfa amended soil NH⁺₄-N accumulated from 6 μg g^?1 at the 0 NaCl rate to a maximum of 54 μg g^?1 with 25 mg NaCl g^?1. These higher NH⁺₄-N values resulted in a 0.5 unit increase in the pHw over that of the 0 NaCl rate in the alfalfa amended soil. At NaCl concentrations above 25 mg g^?1 there was a reduction in NH⁺₄-N. The addition of alfalfa to the soil helped to alleviate the adverse affects of NaCl on CO₂ evolution and nitrification.     

坡缕石包膜对尿素氮行为的影响

采用静态吸收和土柱淋溶试验方法,分析对比了3种不同用量坡缕石包膜尿素与普通尿素施入土壤后对尿素氮行为的影响,结果表明:在土壤中施用坡缕石包膜尿素较普通尿素减少10.38%～26.24%的氨挥发损失,减少5.88%～27.74%的氮素(NO3--N+NH4+-N)淋溶损失,20%的坡缕石包膜尿素能显著提高土柱土壤NH4+-N含量,3种坡缕石包膜尿素都能极显著提高土柱土壤NO3--N含量.坡缕石包膜后能减少尿素氨的挥发,降低NH4+-N和NO3--N的淋失,提高土壤NH4+-N和NO3--N含量,以20%的坡缕石包膜尿素的综合生态效应最好.