Nitrification in soil as a function of time

The dependence of nitrification on time may be expressed as ΣN=C/(m+1) ·t ^m+1 since its logarithmic form log ΣN=K logt+q suggests the possibility of a linear relationship between log ΣN and logt such as was found in more than 50 cases of nitrification in different soils. It was further shown that the equations for the integration curve and for the rate curve are of the same form, differing only in the constants.     

Suction force variation in double-hybrid maize as a function of soil moisture

In experiments with double-hybrid maize grown in the field and in a greenhouse, we analysed the value of the suction force as related to the regime of irrigation. The results showed that the level of water supply to the plants is in good connection with the value of the suction force and the coefficient H (the relative degree of water saturation of the cells) analysed at the same time. In the case of the 2 hybrids under investigation the highest value of coefficient H was found to be between 35–48. The data indicate that the use of these two indices should take part in a physiological method for the settlement and the application of water rules in irrigation.     

土壤含水量、pH及有机质对DMPP硝化抑制效果的影响

采用室内模拟培养试验方法,研究了土壤含水量(40％、60％和80％田间最大持水量)、pH(4、7、10)及有机质(原土和去除有机质)对3,4-二甲基吡唑磷酸盐(DMPP)在潮棕壤中硝化抑制效果的影响.结果表明:随土壤含水量的降低,DMPP的降解趋于缓慢,硝化抑制效应增强.不同土壤pH处理下,以pH为10的处理硝化抑制效果最好,DMPP的存留量最高,pH为7和4处理硝化抑制效果较低,DMPP存留量较少.去除有机质可延长DMPP的存留时间,显著降低土壤表观硝化率.     

Effect of soil microstructure on the relationship between the intensity of nitrification and soil moisture

The relationship between the logarithm of nitrification intensity and moisture (expressed as pF) did not have an ideal linear course in structural soils in which pores of a certain size predominated. When increasing the incubation moisture it could be observed that the intensity of nitrification decreases at pF 3.5-4.4 and increases again on further increasing the incubation moisture. The detected anomaly was observed when applying moistures corresponding to peaks of derivation pF-curves. It can be assumed that the relationship between the intensity of nitrification and moisture is modified by soil microstructure; therefore, this relationship was studied in more detail in the present communication.     

Net mineralization and nitrification rates in a clay soil measured and predicted in permanent grassland from soil temperature and moisture content

Summary Net mineralization of N and net nitrification in field-moist clay soils (Evesham-Kingston series) from arable and grassland sites were measured in laboratory incubation experiments at 4, 10 and 20°C. Three depth fractions to 30 cm were used. Nitrate accumulated at all temperatures except when the soil was very dry (=0.13 cm³ cm^–3). Exchangeable NH₄-ions declined during the first 24 h and thereafter remained low. Net mineralization and net nitrification approximated to zero-order reactions after 24 h, with Q₁₀ values generally <1.6. The effect of temperature on both processes was linear although some results conformed to an Arrhenius-type relationship. The dependence of net mineralization and net nitrification in the field soil on soil temperature (10 cm depth) and moisture (0–15, 15–25, 25–35 cm depths) was modelled using the laboratory incubation data. An annual net mineralization of 350 kg N ha^–1 and net nitrification of 346 kg N ha^–1 were predicted between September 1980 and August 1981. The model probably overstressed the effect of soil moisture relative to soil temperature.     

Stomata constellation in the leaves of cotton, maize and wheat plants as a function of soil moisture and environment

In two locations, one subtropical and the other semidesert, the following three agricultural plant species were studied: cotton, maize and wheat. In each zone species were examined in irrigated and nonirrigated experimental plols for four leaf parameters: density of stomata and epidermis cells and length of stomala and leaf area. The following conclusions follow from the results: 1) The water deficit in the soil accompanied by extreme climatic conditions (August-September) led to a significant increment in the density of stomata and epidermis cells; in some of the varieties a decrease in length of stomata was observed. The decrease in leaf area in the nonirrigated plant was accompanied by an increase in ash content. Identical xeromorphic changes were stated in selected leaves the area of which was equal to those of the irrigated plants. In spite of the maximal number of stomata per unit area on both sides of Ihe blade a full turgor was maintained in the cells of the nonirrigated plants during the hottest and driest months and they survived until rains started (Novemher), although no significant changes were then noted in soil moisture within the sphere of the root dispersion.     

Dry matter response of ryegrass to ammonium and nitrate sources of nitrogen as a function of soil texture and moisture

Summary InLolium perenne cv. S23 grown in pots of soil to which nitrogen was applied at rates of 0, 60, 120 and 180 mg N/kg, ammonium significantly increased root mass compared with nitrate, whereas both nitrogen sources produced similar yields of herbage. Total dry matter production in the herbage and roots increased within the textural sequence: loamy sand, coarse sandy loam and loam. The increases in the latter two soils over the loamy sand were 35% and 50%, respectively. Depletions of available water of 50% and 80% reduced yields at the first harvest by 19% and 50% compared with the field capacity. The corresponding reductions in the second harvest were only 16% and 10%, because residual effects were then limiting. The amount of dry matter produced per unit of nitrogen absorbed indicated that textural class was less restrictive than moisture supply to efficient use of nitrogen within the plant. Furthermore, its efficient physiological use was limited at the 50% depletion, although its absorption from the soil was not affected. The results illustrated the inefficiency of the substitution of nitrogen for soil moisture: even at low yield levels three times more nitrogen was absorbed per unit increase in dry matter at 80% depletion of available water compared with the field capacity. The significant first-order interactions that affected yield were moisture by soils, and moisture by nitrogen rate.     

Transpiration as a function of soil temperature and soil water stress

An apparatus was developed for the measurement of transpiration rates of Trifolium repens. The transpiration rates were measured under controlled conditions of soil water stress and soil temperature. Other environmental parameters such as air temperature, relative humidity, light intensity and air speed were held constant. Diffusive resistances were calculated and stomatal aperture changes were recorded for all treatment combinations. A significant interaction between soil water stress and soil temperature was observed for stomatal closures. Stomatal closure was observed even in the so-called wet range of soil water stress. An increase in mesophyll resistance or incipient drying was observed for several treatment combinations. The mesophyll resistance was shown to increase as soil water stress increased.     

Chemical forms of plant and soil iron as influenced by soil moisture

Summary Experiments were conducted to determine whether soil moisture content has an effect on the chemical forms of plant and soil iron. Soybean plants, variety Lee, were grown on Adelanto loam soil under greenhouse conditions. Two different moisture levels, 75 per cent and 120 per cent of the moisture equivalent, were maintained in soil samples placed in individual containers. The same moisture treatments were used for separate soil samples on which the test plants were grown.Soil iron forms were determined in the moisture-treated soil by using different extracting agents. A significant decrease in soil iron extracted with 10^–4 M EDTA from soil at the high moisture level was attributed to a relative increase in the free calcium ion.Soybean plants grown under the high moisture level were chlorotic while those under the low moisture level were green in appearance. Plant samples were taken at two stages of growth for subsequent analysis.The chemical analysis of the leaf tissues have shown the presence of equal amounts of total iron and less amounts of water-soluble and active iron in chlorotic tissues as compared to non-chlorotic tissues. The difference found between chlorotic and non-chlorotic plants in the amount of iron in the water extract was in the trichloroacetic acid-soluble fraction. The water- and salt-soluble protein nitrogen was approximately the same in chlorotic and non-chlorotic leaves.     

土壤硝化作用的抑制剂调控及其机理

既要保证氮肥在优质高效农业生产中的促进作用,又要保证农业和环境的可持续发展,如何提高氮肥利用率、减轻氮肥污染成了解决问题的关键.从氮素在土壤中的生物化学转化过程入手,通过抑制剂的施用来调控氮素的转化,减缓硝化过程的进行,是解决该问题的一种有效措施.本文从硝化抑制剂的定义、筛选硝化抑制剂的条件、国内外较受关注的硝化抑制剂品种以及硝化抑制机理等几个方面对目前的研究进展进行综述,并提出目前研究中存在的问题及今后的研究方向.     

The influence of the soil structure and moisture content on the number of bacteria and the degree of nitrification

ИзгчалоAЬ одноврeмeнноe дeйствиe сBрCкСC@ы поGвKi и Aодeрeа влаBи на количe бактeрий и интeнсивноAтЬ нитрифика. ции в гумуA-карбонаBной почвe. Путeм просeиваниHя образцоCпочвы чeрeз сиBа были в044B;цeлeны слeдующиe фракции: I — агрeгaаты мeнЬшe 1 мм; II — агрeгаты размe@ами в 1—2; мм; III — аг@eгаты размeрами в 2—3 мм; IV — агтeгаBы размeрами в 3—4 мм; V — агрeгаBы размeрами в 4—5 мм. Образцы брали три @аза а год. Для опрeДeлeния количeства бактeрий полЬзовалисЬ прямым мeтодeм флуорeсцeнтной м0438;H043A;H0440;H043E;скопии по Strugger в модификации по Seifert. Пeрeд опрeдeлeниeм биогeнности образцы насыщли влагой до заранee установлeнного уровня и инкCбировали 48 час. при тeмпeратгрe 20° С Нитрификацию опрeдeлOли послe 14-днeвной ?нкгбации образFов, насKщeнных влвгой до опрeдeлeнного уровня и хранившихся в тeрмостатe пр? 20° А. Бы;ло установлeно, что количeство бактeрий зависит от содeржания влаги и 043F;очти нe зависит оB размeров агрeгатов (рис. 1). ИнтeнсивностЬ нитрификации зависит как от содeржаниO влаги, так ? оB размeров агр;eгатов. НаиболЬHая интeнсивностЬ наблюдаласЬ в слу чаe агрeгатов пe@вой фракции, т. e. при диамeтрах мeнЬHe 1 мм. С гвeличeниeм диамeтров агрeгатов интeнсивностЬ нитрификации постeпeнно поннжаласЬ (@ис. 2). Понижeниe интeнсивности нитрификации отличалосЬ опрeдeлCнной правилЬностЬю, инаосновании ee анализа мы пришли к выводу, что инBeнсивностЬ нитрификаFии прямо пропорFионадЬна вeдиGинe спeцифичeAкой повeрхносBи агрeгатов. ДоказатdдЬство зтого привeдeно на рис. З. Для протeрки того, как на интeнсивносBЬ нитрификации дeйствуeт радмeлЬчeниe агрeгатов, мы поставнди опыт, при котором апрeгаBы V фракции были размeлЬчeны до размeров I фракции. Послe измeлЬчeниO интeнсивностЬ ниBририкаFии знаGиBeлЬно повысиласЬ (рис. 4).     

Influence of added manganese on nitrification in a low-manganese soil

Summary Nitrification was studied in a Leefield sand, sufficiently low in Mn to produce severe Mn deficiency in soybeans. The soil was treated with 100 g/g NH₄ ⁺-N, 0, 10, 100 and 1000 g/g Mn, and incubated at field capacity for six weeks at 30°C. Weekly determinations of (NO₃ ^-+NO₂ ^-)-N and NH₄ ⁺-N revealed that nitrification was not increased above the control level by any of the Mn rates. In the control, the decrease in soil pH associated with nitrification caused an increase in extractable Mn from 0.12 to 1.56 g/g during the experimental period. The 1000 g/g rate of Mn completely inhibited nitrification for the first five weeks of the experiment, but nitrification began to occur during the sixth (last) week. It is postulated that Mn was being converted to unavailable forms such that by the sixth week the level of active Mn had been reduced sufficiently to permit nitrification to begin. re]19760512     

Saccadic target selection as a function of time

Recent evidence indicates that stimulus-driven and goal-directed control of visual selection operate independently and in different time windows (van Zoest et al., 2004). The present study further investigates how eye movements are affected by stimulus-driven and goal-directed control. Observers were presented with search displays consisting of one target, multiple non-targets and one distractor element. The task of observers was to make a fast eye movement to a target immediately following the offset of a central fixation point, an event that either co-occurred with or soon followed the presentation of the search display. Distractor saliency and target-distractor similarity were independently manipulated. The results demonstrated that the effect of distractor saliency was transient and only present for the fastest eye movements, whereas the effect of target-distractor similarity was sustained and present in all but the fastest eye movements. The results support an independent timing account of visual selection.     

Acceleration by montmorillonite of nitrification in soil

A soil naturally containing montmorillonite (M) was amended with 10% M and sequentially perfused with glycine, with fresh glycine being added every 16–17 d after nitrification of the previously added glycine-nitrogen had reached a plateau. In some systems, the old perfusates were replaced each time with a fresh glycine solution; in others, the initial perfusate was not replaced but only adjusted each time to the original 200 ml volume and a comparable glycine concentration (140 μg NH₂-N/ml). The incorporation of M enhanced the rates of heterotrophic degradation of glycine and subsequent autotrophic nitrification, but these stimulatory effects decreased with each successive perfusion. The reasons for these decreases are not known, but they did not appear to be related to inorganic nutrition, as perfusion with a mixed cation solution after five perfusion cycles did not significantly enhance nitrification in either the check or M-amended soils during three subsequent perfusions with glycine. The enhancement of nitrification by M appeared to be a result, in part, of the greater buffering capacity of the M-amended soil, as indicated by lesser reductions in the pH of perfusates from the M-amended soil, by titration curves of the soils, and by the greater and longer stimulation of nitrification in the cheek soil amended with 1% CaCO₃, which had a greater buffering capacity than did M. The stimulation by CaCO₃ may also have been partially the result of providing CO₂ for the autotrophic nitrifyers. Significant concentrations of nitrite accumulated only in perfusates from soil amended with CaCO₃. Air-drying and remoistening the soils enhanced nitrification of subsequently added glycine, especially in the check soil. The importance of pH-mediation, of the production of inhibitors, and/or of feed-back inhibition was indicated by the lower rate and extent of nitrification in systems wherein the perfusates were not replaced between successive additions of glycine. Although the results of these studies confirmed previous observations that M enhances the rate of nitrification in soil, the mechanisms responsible for this stimulation are still not known.     

Phosphorus depletion in the rhizosphere as influenced by soil moisture

To study the influence of soil moisture on phosphorus (P) depletion in the rhizosphere, maize (Zea mays cv. Trak) was pre-grown in vermiculite filled-PVC tubes for 9 days and then the plants with the tubes were transplanted into soil columns maintained at two soil moisture levels () of 0.14 and 0.20 cm³ cm^–3 for 10 days. The soil columns were separated at 1 cm depth by a nylon screen of 53 m inner mesh size, into 1 cm soil layer above and 3 cm soil column below screen. A root mat developed over the screen, but root hairs only could penetrate it. Regardless of the soil moisture level in the columns, and adequate and equal water and nutrients supply was maintained via wicks from an external nutrient solution to the plant roots in vermiculite. After 10 days, the soil columns were separated from the root mats, quickly frozen in liquid nitrogen and sliced into thin layers (0.2mm) using a refrigerated microtome to give soil samples at defined distances from the root mats for analyses. Lower soil moisture (=0.14) resulted in narrower and steeper depletion profile of 0.5 M NaHCO₃ extractable P (NaHCO₃-P_i) as compared to higher soil moisture (=0.20). Depletion of P in soil solution in the immediate vicinity of root mats did not differ much but the extension of the depletion zones was 0.10 cm at =0.14 and 0.20 cm at =0.20. The depletion up to 0.05cm with =0.14 and up to 0.07 cm with =0.20 was uniform, and may be attributed to the depletion in the root hair zone. Beyond the root hair zones, the theory of diffusion and mass flow was able to explain the observed differences in shape and extent of the P depletion profiles at the two soil moisture levels.     

Available soil moisture as a basis for land capability assessment in semi arid regions

Rangelands are extensive areas in arid or semi arid regions. They have many uses (e.g. grazing, dry land farming, wildlife habitat, recreation and mining). Generally vegetation is sparse and the principal plant species are adapted to erratic rainfall events. Rainfall use efficiency (RUE) is the quotient of annual primary production divided by annual rainfall i.e. the number of kilograms aerial dry matter produced by 1 ha in 1 year per millimetre of rain. It decreases with increasing aridity. Reasonably well managed arid and semi arid grazing lands are usually in the 3.0–6.0 value range. Available soil moisture is the principal determinant of productivity. The role and significance of the major parameters (rainfall, soil depth, slope and salinity, texture, cover, erosion etc) are considered. Available moisture can be predicted if the appropriate relative productivity indices (RPI) are used in a parametric way. The predictions can be used to classify land according to its capability to support plant growth. Equations have been derived which enable land to be classified on the basis of its potential productivity. Links between land capability and vegetation cover are given.