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.     

太湖地区主要土壤中的固定态铵及其有效性

测定了太湖地区主要土壤中固定态铵的储量和表土的固铵能力,并通过盆栽试验研究了它们的有效性。土壤的固定态铵含量和表土的固铵能力因母质而异,长江冲积物发育的土壤最高,次为黄土状物质的,第四纪红色粘土的最低。各土壤0—20厘米土层中的固定态铵总量平均占全氮总量的18.5%,0—100厘米土体中占34%。各土壤“固有的”固定态铵的有效性差异较大,视土层等的不同,在0—13%间。“新固定”的来自肥料或土壤有机质矿化释出的铵则很高,一般均能被当季作物完全吸收利用。渍水条件并不能提高固定态铵的有效性。讨论了铵的固定作用在土壤氮素肥力中的意义;指出,由于铵的固定作用和不同土壤的固铵能力各异,常用的淹水培育法所得到的土壤氮素矿化量值不但一般偏低,而且难于进行相互比较。     

Nitrogen in particle size fractions of soils incubated for five years with 15N-ammonium and 14C-hemicellulose

Four soils with a range of clay and silt contents were incubated for 5 a with ¹⁵N-labelled (NH₄)SO₄ and ¹⁴C-labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions. Between 29% and 48% of the added N was found in organic form. The ¹⁵N atom percentage excess decreased in the order: clay > whole soil > silt > sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand <0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively. All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small. The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated.     

Diffusion of NH+4 and NO−3 mineralized from organic N in soil

The mineralization of native soil organic matter and the simultaneous diffusion of zero NH⁺₄ and NO^?₃ to a solution sink of zero N concentration was analysed experimentally and theoretically for a fine sandy loam soil. Experimentally, the NH₄ and NO₃ ions produced in an incubated unsaturated soil column were allowed to diffuse through a sintered glass plate into a stirred solution sink. The distribution of NH⁺₄ and NO^?₃ in the soil column was measured after various incubation times. The rate of ammonification was measured directly during incubation and the rate of nitrification modelled from nitrifier growth kinetics. A Freundlich equation was used to describe the equilibrium between soluble and exchangeable NH⁺₄ in the soil. Terms for the microbial transformation of N and the adsorption-desorption of NH⁺₄ were combined with diffusion equations which were solved numerically using finite difference methods. The model constructed was used to predict the NH⁺₄ and NO^?₃ con-centration distributions in the soil column, and good agreement was obtained between the experimental and predicted concentration profiles. The use of the model for predicting the diffusive flux of mineral N to the outer surfaces of soil peds, where it is vulnerable to leaching, was demonstrated.     

Verhalten von Dünger-NH4 in Böden unterschiedlicher tonmineralischer Zusammensetzung im Inkubationsversuch

Fate of fertilizer ammonium in soils with different composition of clay minerals in an incubation experiment In an incubation experiment with three different soils (gray brown podsolic soil from loess, alluvial gley, and brown earth, derived from basalt) the specific adsorption (fixation) and release of fertilizer NH₄⁺ was investigated. In one treatment 120 mg NH₄–N/kg soil was added, while the other treatment (control) received no nitrogen. Soils samples were taken every ten days and analyzed for nonexchangeable and exchangeable NH₄⁺ and NO₃^?. The experimental results are showing that the specific adsorption of applied NH₄⁺ was related to the type of clay minerals. While the loess soil, rich in illite, and the alluvial soil, rich in expansible clay minerals, bound about 40% of the added NH₄⁺ specifically, the soil derived from basalt with mainly kaolinite bound only about 10 %. From the recently “fixed” fertilizer NH₄⁺ about a half was nitrified during the incubation period of about 9 weeks. In the control there was no significant release of specifically bound NH₄⁺. Obviously this NH₄⁺ is located more deeply in the interlayers of the clay minerals and not available to microorganisms.     

Freisetzung von spezifisch an Tonminerale gebundenem Ammoniumstickstoff durch Bepflanzung mit Welschem Weidelgras (Lolium multiflorum)

Utilization of day fixed ammonium nitrogen by ryegrass (Lolium multiflorum) In a pot experiment with ryegrass (Lolium multiflorum) the availability of ammonium fixed by clay minerals of a soil treated with ¹⁵NH₄ has been studied. The following results were obtained: After the treatment of the soil with labelled NH₄⁺ it contained 62.7 mg fixed NH₄-N/kg soil including 4.5 mg ¹⁵NH₄-N/kg soil. Nitrogen uptake of ryegrass and N turnover reactions in the soil reduced this content of ¹⁵NH-N to about 0.9 mg/kg soil. This shows, that about 80 % of the fixed ¹⁵NH₄⁺ had been released through out a growing period comprising 3 cuts, a part of which was taken up by the plants. From the native fixed ammonium 3.3 to 4.8 % were released.     

盆栽和田间条件下土壤15N标记肥料氮的转化

利用¹⁵N在盆栽条件下研究了铵的矿物固定作用对肥料氮在三种土壤中转化的影响.结果表明,红壤性水稻土不固定肥料铵,但在白土和夹沙土中,56-77%的肥料氮被土壤矿物所固定,这些“新固定”的固定态铵的有效性很高,其中90%以上在30-50天内即被水稻所吸收,或者为微生物所利用转变为生物固定态氮.生物固定态氮对当季作物的有效性远较“新固定”的固定态铵的低.田间微区试验的结果还表明,甚至第二、三季作物吸收的残留肥料氮中,20-86%的氮也系来自固定态铵.作者认为,对具有较强固铵能力的土壤来说,只有了解铵的矿物固定作用,才能正确了解肥料氮的其它转化过程.     

Following the decomposition of ryegrass labelled with 13C and 15N in soil by solid-state nuclear magnetic resonance spectroscopy

Investigating the biogeochemistry of plant material decomposition in soil has been restricted by difficulties extracting and identifying organic compounds. In this study the decomposition of ¹³C- and ¹⁵N-labelled Lolium perenne leaves mixed with mineral soil has been investigated over 224 days of incubation under laboratory conditions. Decomposition was followed using short-term rates of CO₂ evolution, the amounts of ¹³C and ¹⁵N remaining were determined by mass spectrometry, and ¹³C and ¹⁵N solid-state nuclear magnetic resonance (NMR) spectroscopy was used to characterize chemically the plant material as it decomposed. After 224 days 48% of the added ¹³C had been lost with a rapid period of C0₂ evolution over the first 56 days. The fraction of cross-polarization magic angle spinning (CP MAS) ¹³C NMR spectra represented by O-alkyl-C signal probably in carbohydrates (chemical shift, 60–90 p.p.m.) declined from 60 to 20% of the spectrum (chemical shift, 0–200 p.p.m.) over 224 days. The rate of decline of the total ¹³C exceeded that of the 60–90 p.p.m. signal during the first 56 days and was similar thereafter. The fraction of the CP MAS ¹³C NMR spectra represented by the alkyl- and methyl-C (chemical shift, 10–45 p.p.m.) signal increased from 5 to 14% over the first 14 days and was 19% after 224 days. CP MAS ¹³C NMR of ¹³C- and ¹⁵N-L. perenne contained in 100-μm aperture mesh bags incubated in the soil for 56 days indicated that the remaining material was mainly carbohydrate but there was an increase in the alkyl- and methyl-C associated with the bag's contents. After 224 days incubation of the labelled ¹³C- and ¹⁵N-L. perenne mixed with the soil, 40% of the added N had been lost. Throughout the incubation there was only one signal centred around 100 p.p.m. detectable in the CP MAS ¹⁵N NMR spectra. This signal corresponded to amide ¹⁵N in peptides and may have been of plant or microbial origin or both. Although there had been substantial interaction between the added ¹⁵N and the soil microorganisms, the associated redistribution of ¹⁵N from plant to microbial tissues occurred within the amide region. The feasibility of following some of the component processes of plant material decomposition in soil using NMR has been demonstrated in this study and evidence that microbial synthesis contributes to the increase in alkyl- and methyl-C content of soil during decomposition has been represented.     

Influence of NH4-application on gross N turnover rates in soil

Determination of gross N mineralization rate in soil, by use of the isotopic pool dilution approach implies that ¹⁵NH₄⁺ is applied homogeneously to soil. Since the labeling is applied to the product of the mineralization, the application of ¹⁵NH₄⁺ should in theory not alter the mineralization rate. However, recent studies have indicated inverse relation between the amounts of ¹⁵NH₄⁺ applied and the determined gross N mineralization rates, due to overestimated rates when ‘low’ amounts of ¹⁵NH₄⁺ were added, as a result of preferential ¹⁵NH₄⁺ consumption. We present here results from a similar study. We observed no effect from the amount of applied NH₄⁺ on the measured gross N mineralization rates. Our results indicate, that the inverse relation as described earlier, probably was due to underestimated rates when ‘high’ amounts of ¹⁵NH₄⁺ were added, as a result of preferential ¹⁴NH₄⁺ consumption, when the applied ¹⁵NH₄⁺ was incomplete distribution in the soil.     

MEASUREMENT OF LOSSES FROM FERTILIZER NITROGEN DURING INCUBATION IN ACID SANDY SOILS AND DURING SUBSEQUENT GROWTH OF RYEGRASS, USING 15N-LABELLED FERTILIZERS

Ammonium sulphate and calcium nitrate both containing excess ¹⁵N were applied to four acid sandy soils; two were from old arable fields and two from grassland, selected so that one of each pair was about pH 5 and the other about pH 6 (in water). The soils were incubated for 6 weeks at 21°C in large glazed earthenware pots, one set with the nitrification inhibitor 2-chloro-6-(trichloromethyl)-pyridine added and another without inhibitor. Ammonium and nitrate N were determined at intervals, and the total-N at the start and after 6 weeks. The atom per cent ¹⁵N in the mineral-N extracted from soils treated with ammonium sulphate was determined after 0, 3, and 6 weeks, and in the total-N of all the soils given N-fertilizer at 0 and 6 weeks. Much added N was immobilized at first, but some was re-mineralized during the second half of the incubation. Mineral-N extracted from soils treated with ammonium sulphate contained less ¹⁵N than the fertilizer added, showing that part of the apparent re-mineralization during the second half was from unlabelled soil organic matter. After incubating for 6 weeks less than 5 per cent of the N added as nitrate was lost but about 5 per cent of the labelled-N added as ammonium sulphate was lost from the two grassland soils. Adding the inhibitor prevented this loss. After incubating, the soil remaining in each jar was halved to provide duplicate pots and sown with ryegrass. A similar series of pots with the same treatments (but with unlabelled fertilizer) was also prepared from the soils that had been stored slightly moist and at 21°C; these were sown with ryegrass. All pots were harvested after 42 days and again after 70 days. More than 93 per cent of the labelled-N was recovered in plants and soil, except from the two grassland soils to which calcium nitrate was added. It is concluded that while a little nitrogen may be lost during nitrification in some of these soils, more nitrogen may be lost during the growth of grass, when nitrate is present in relatively large amounts. The nitrification inhibitor decreased yields of grass at the first cutting on grassland soils treated with ammonium, but increased them on soil treated with nitrate, suggesting that changing the proportions of nitrate to ammonium by adding the inhibitor alters the growth rate and yield of grass.     

陕西省几种代表性土壤NH4+吸附、解吸动力学特征研究

采用连续液流法测定了五种土壤吸附、解吸ＮＨ＾＋４的动力学性质。研究表明：（１）ＮＨ＾＋４吸附、解吸平衡时间及反应速率，平衡时的吸附、解吸量及吸附平衡常数均随土壤粘粒和ＣＥＣ不同而变化；（２）不同动力学模型及同一模型对不同土壤的拟合性不同。     

不同铵态氮水平对水稻根际固氮活性的影响

本试验观察了在杭州生态条件下土壤中不同NH₄⁺-N水平对水稻根际固氮活性的影响。试验的结果表明:1.NH₄⁺-N肥在一定时间内对水稻根际固氮活性具有明显的抑制效应,施用量越大,其抑制作用越严重。土壤速效氮浓度与水稻根际固氮活性之间呈高度(或中度)负相关,不同生育期两者的相关系数r值在-0.4288—0.9945之间。2.土壤速效氮对水稻土柱固氮活性抑制的起始浓度为20ppm。3.NH₄⁺-N对水稻根际固氮活性的抑制时间随施用量而不同,低氮区在20天左右,中氮区和高氮区在25—30天左右。此后施氮区对水稻根际固氮活性具有促进作用。     

不同热解温度茶渣生物质炭对茶园土壤吸附解吸NH4-N的影响

生物质炭对铵根的吸附解吸影响着土壤的固氮效果,为探讨茶渣生物质炭对茶园土吸附—解吸NH_4~+—N性能的影响,减少土壤中氮素的淋失,提高氮素利用效率,通过模拟培养试验,采用平衡吸附法及HCL解吸法,研究了不同热解温度下制备的茶渣生物质炭在不同添加比例(0.35%,0.70%,1.40%,2.80%)下,茶园土对NH_4~+—N吸附解吸的特性。结果表明:施用生物质炭能有效增强茶园土对NH_4~+—N的吸附,并随生物质炭添加量的增加而增强。同一生物质炭添加量下,4种生物质炭处理下茶园土对NH_4~+—N的吸附量大小表现为BC400BC300BC500BC600。生物质炭的CEC含量是影响土壤吸附NH_4~+—N能力的主要因素。土壤对NH_4~+—N的吸附过程均以Langmuir方程拟合达到显著水平(0.953 7R~20.995 5),以单层吸附为主。施用生物质炭后,土壤产生了解吸滞后,有效降低了茶园土对NH_4~+—N的解吸率,BC400的解吸率最低。茶渣生物质炭能够增强土壤对NH_4~+—N的吸附,降低对NH_4~+—N的解吸,有利于提高土壤对氮素的吸持能力,其中BC400,2.80%处理下效果最佳。     

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.     

Blocking the release of potassium from clay interlayers by small concentrations of NH4+ and Cs+

To understand the process and the kinetics of potassium release from the clay interlayer in natural and arable soils in more detail, I tested the hypotheses that large, monovalent cations, especially NH₄⁺ and Cs⁺, can reduce the release rates of K⁺ which is exchanged by Ca²⁺, even if these monovalent cations are present in concentrations of only a few μm . Percolation experiments were carried out with different illitic soil materials, some containing vermiculite, with 5 m m CaCl₂ at pH 5.8 and 20°C, in some cases for over 7000 h. NH₄⁺ and Cs⁺ both caused a large decrease in the rate at which K⁺ was released, Cs⁺ especially. Suppression began at 5 μm NH₄⁺ Blocking by 20 μm NH₄⁺ was easily reversible: the release rates readily increased when NH₄⁺ was omitted from the exchange solution. Blocking by 2 μm Cs⁺ was equal to approximately 90% of that at 10 μm Cs^+. Larger concentrations of Cs⁺ than 10 μm did not further reduce release but rather caused a slight increase, probably because of enhanced exchange of K⁺ by Cs⁺ without exfoliation of the interlayer space. Blocking by Cs⁺ was not reversible within > 7000 h of percolation by 5 m m CaCl₂. The blocking effect was reproduced in several different soil materials using 10 μm Cs⁺ but was most pronounced in vermiculite-rich samples. As NH₄⁺ is present in most arable soils, at least in concentrations of a few μm , I conclude that the observed effects are of significance in the K dynamics processes in soils, for example near the roots of plants. Further, very small concentrations of Cs⁺ in exchange solutions containing a large background of Ca²⁺ appear to be useful for suppressing K⁺ release from the interlayer in laboratory studies, probably without significantly altering the exchange at outer mineral surfaces.     

THERMODYNAMICS AND THERMOCHEMISTRY OF THE EXCHANGE REACTION BETWEEN NH4+ AND Mn2+ IN A MONTMORILLONITE CLAY

The exchange reaction between NH₄⁺ and Mn³⁺ was studied on a montmorillonite clay at several temperatures and different ionic strengths. Manganese was preferred to ammonium; this preference increased with the temperature and dilution of the dialysate. Comparison with published data concerning exchanges involving NH₄⁺ and the alkaline-earths showed that in the sequence of increasing selectivity: Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺, Mn²+ lies between Mg²⁺ and Ca²⁺. The enthalpy change was measured calorimetrically and calculated by application of the van't Hoff law to the temperature coefficient of the equilibrium constants. Both values were in good agreement. The excellent recoveries of Mn²⁺ at the end of the exchange reaction and the constancy of the cation exchange capacity over the whole range of surface composition ruled out the possibility of significant adsorption in the MnOH⁺ form. The behaviour of manganese was very similar to that of the alkaline-earth cations.     

Slow-release 15N fertilizer formulations to measure N2-fixation by isotope dilution

Pot experiments were carried out to examine the effects of slow-release fertilizer formulations on estimates of N₂-fixation determined by the isotope dilution method. Soybeans were used as the N₂-fixing plants, with non-nodulated soybeans and maize as the non-fixing controls. The ¹⁵N-fertilizer formulations used were (¹⁵NH₄)SO₄, K¹⁵NO₃, gypsum-pelleted K¹⁵NO₃, (¹⁵NH₄)₂SO₄ + glucose, ground plant material enriched with ¹⁵N or ¹⁵N-oxamide. The estimate of the amount of N₂ fixed by the nodulated soybean plants depended upon both the control plant and the fertilizer formulation used. Maize took up N later than non-nodulated soybean and estimates of soil N-pool (soil “A” value + fertilizer N added) calculated from the enrichment of this control were about twice as large as those calculated from the enrichment of non-nodulated soybean receiving the same fertilizer treatment. As a consequence, estimates of N₂-fixation relative to this control were lower than those relative to non-nodulating soybean (mean 140 mg N per pot compared with 292 mg N per pot). With unstablilized ¹⁵N salts errors were sufficient to produce negative estimates of fixation relative to maize. Even with a “well-matched” control (non-nodulated soybean) estimates of fixation varied with fertilizer formulation.     

不同铵钾比对高铵下拟南芥地上部和根系生长的影响

钾在缓解植物铵毒害的过程中起着重要的作用。本文研究了高铵(30 mmol/L)条件下,不同铵钾比(7.5︰1和150︰1)对拟南芥(Col-0)主根、侧根以及地上部生长的影响。结果表明:30 mmol/L NH4+条件下,高铵钾比(150)处理显著加重了拟南芥铵毒害现象,地上部和根系生长所受的抑制作用更为明显并导致更严重的氧化胁迫。相比低铵钾比水平,在高铵处理下,高铵钾比使得拟南芥主根伸长量降低57.4%,侧根数量减少33.3%,而地上部鲜重减轻69.9%。DAB(3,3￠-二氨基联苯胺,3,3￠-diaminobenzidine)叶片染色结果表明,不加铵处理下,外源不同钾水平(0.2和4.0 mmol/L)对拟南芥叶片的氧化胁迫作用没有显著差异;而高铵处理下,相比低铵钾比处理,高铵钾比显著增加了叶片中过氧化氢的含量,加重了其氧化胁迫。伊文思蓝(Evans blue,EB)染色结果表明,不加铵处理下,外源不同钾水平对拟南芥地上部和根部的膜透性没有显著差异,而高铵处理下,高铵钾比显著增强了拟南芥地上部和根部的膜透性,表明其对细胞的伤害程度加重。可见,高铵抑制拟南芥根系和地上部生长,高铵钾比则会加重这种抑制,其原因除了高浓度钾能减少植物对铵的吸收外,可能与高铵钾比条件加剧了植物的氧化胁迫有关。因此,适宜的铵钾比在植物应对铵毒害的过程中发挥重要作用。     

湖南几种耕地土壤固定添加铵的动力学研究

通过野外调查取样、室内培养试验和分析测定，研究了湖南省几种成土母质发育的旱地土壤和稻田土壤固定添加铵的动力学特性。结果表明，供试土壤对添加铵的固定速度很快，尤其在反应的前8～12h内速度更快，12h后速度逐渐变慢，24h以后，土壤对外源铵的固定已基本达到平衡。数学拟合表明，一级动力学方程和Elovich方程两种动力学模型能较好地拟合供试土壤固定添加铵的动力学特性，经统计均达极显著水平，抛物扩散方程也有较好的拟合效果，零级方程较差。由一级动力学方程求得的不同土壤固铵动力学参数：理论最大固铵量（A）、反应速率常数（b）以及反应半时值明显不同。耕型石灰性紫色土、耕型酸性紫色土、耕型棕色石灰土和耕型石灰岩红壤的理论最大固铵量和反应半时值分别为212．3mg kg^-1、179．0mg kg^-1、142．9mg kg^-1.13．7mg kg^-1和29．75h、25．96h、27．18h、23．49h；紫泥田、河沙泥、灰泥田和红黄泥的理论最大固铵量和反应半时值分别为86．2mg kg^-1、68．7mg kg^-1,31．8mg kg^-1、19．1mg kg^-1和14．50h、15．10h、15．51h、18．43h。耕型石灰性紫色土、耕型酸性紫色土、耕型棕色石灰土和耕型石灰岩红壤的反应速率常数分别为0．0233 h^-1、0．0267h^-1、0．0255h^-1、0．0295h^-1；紫泥田、河沙泥、灰泥田和红黄泥的反应速率常数分别为0．0478h^-1、0．0459h^-1、0．0447h^-1、0．0376h^-1.除耕型石灰岩红壤以外，旱地土壤的理论最大固铵量和反应半时值均明显大于水田土壤，而反应速率常数明显小于水田土壤。