Formation of alkali soils by neutral salt solutions

It has been reported that large tracts of land under irrigation in the western part of United States contain enough soluble salts to depress crop production and thousands of acres have been abandoned because of salinity. In soils commonly called white alkali soils, containing accumulation of neutral salts in the form of chlorides or sulphates, 0.2% of salt will reduce yields of salt sensitive plants and 1.0% will cause marked yield reduction in most crops. If the drainage is poor and water table is maintained near the surface of the soil, evaporaLion will be high and the salt will accumulate. When large amount of soluble salts accumulate at the surface, the soil is termed as saline and has a pH value below 8.5. The soils containing alkali salts such as Na.COa or NaHC03 and having a pH value above 8.5 are called alkali soils or black alkali soils. Misra (1954) has reported that a gradual replacement of calcium ions and a subsequent increase in the exchangeable Na or K in the exchange complex of the soil, due to repeated leaching of the soils with dilute salt solutions of Na or K takes place which is similar to the repeated leaching of agricultural fields by irrigation.al water containing smaller amounts of soluble salts of Na or K. He has also reported that K-soils, which are also alkaline, are formed under similar conditions of leaching of the soil with K-salLs. Greater amounts of calcium were found in the filtrate when soluble salts were used instead of water alone for leaching the soil.     

Management of forests under nutrient and water stress

Management of forest nutrition through fertilization and other forestry practices has, undoubtedly, helped to increase the productivity of forests. Relatively little attention has been given to the potential for manipulating available water, although water relations of trees have been studied extensively. It is being increasingly recognized that, for fast-growing tree species, availability of water and its interaction with nutrients have overriding influences on growth, with major implications for the development of cost effective management practices to increase yield. For example, it has been argued that leaf area development is primarily controlled by available water and that in turn, determines the response to nutrient management. Crucial questions which need to be examined inlude:At the process level, how does the interplay between water and nutrients influence nutrient dynamics in forests? How do trees and associated vegetation interact with respect to the use of water and nutrients? How does water and nutrient availability influence leaf area development, assimilation and in particular assimilate partitioning? Does improved nutrition influence water use efficiency? An urgent need for the study of nutrition of trees under water stress also arises from the many problems encountered in reforesting arid and degraded land. This paper is an overview of these issues, emphasizing the mechanisms and processes that underly the response of trees and stands to manipulations of water and nutrient availability.     

Management of forests under nutrient and water stress

Management of forest nutrition through fertilization and other forestry practices has, undoubtedly, helped to increase the productivity of forests. Relatively little attention has been given to the potential for manipulating available water, although water relations of trees have been studied extensively. It is being increasingly recognized that, for fast-growing tree species, availability of water and its interaction with nutrients have overriding influences on growth, with major implications for the development of cost effective management practices to increase yield. For example, it has been argued that leaf area development is primarily controlled by available water and that in turn, determines the response to nutrient management. Crucial questions which need to be examined inlude: At the process level, how does the interplay between water and nutrients influence nutrient dynamics in forests? How do trees and associated vegetation interact with respect to the use of water and nutrients? How does water and nutrient availability influence leaf area development, assimilation and in particular assimilate partitioning? Does improved nutrition influence water use efficiency? An urgent need for the study of nutrition of trees under water stress also arises from the many problems encountered in reforesting arid and degraded land. This paper is an overview of these issues, emphasizing the mechanisms and processes that underly the response of trees and stands to manipulations of water and nutrient availability.     

Plant water relations in lychee: Effects of solar radiation interception on leaf conductance and leaf water potential

The effect of radiation interception on leaf conductance and leaf water potential in six-years old lychee trees (Litchi chinensis Sonn. cv. Bengal) was investigated during the dry season in subtropical Queensland, Australia. A high degree of exposure of leaves to direct radiation raised leaf-air water vapour concentration gradient (Δw) and resulted in lower leaf conductance and leaf water potential. Interior leaves of the south side of trees were less sensitive to atmospheric and radiation effects and are the best indicator of drought stress in lychee. Completely random or stratified sampling is necessary to estimate a true mean value for calculation of canopy transpiration or photosynthesis.     

Estimating leaf area distribution in savanna trees from terrestrial LiDAR measurements

Vegetation structure parameters are key elements in the study of ecosystem functioning and global scale ecosystemic interactions. The detailed retrieval of many of these parameters by direct measurements is impractical due to the quantity of plant material in trees. Terrestrial LiDAR Scanners (TLSs) have been shown to hold great potential as an indirect means of estimating plant structure parameters with a high level of detail, while some studies identified a number of challenges inherent to this approach. In this study we investigate the use of a voxel-based approach to retrieve leaf area distribution of individual trees. The approach is based on the contact frequency method applied to co-registered TLS returns from two or more scanning positions. The contact frequency was computed for voxels being 10, 30, and 50 cm in size and subsequently corrected for the influence of occlusion effects, leaf inclination, the presence of non-photosynthetic material, and the laser beam size. The leaf area of voxels for which occlusion effects were too pronounced was estimated using modeled values based on the availability of light. We compared the TLS derived leaf area estimates against direct measurements, obtained by the harvesting of leaves, in a broad-leaved savanna of central Mali. The measured leaf area values of the sampled trees ranged from 30 to 530 m², and crown LAI values between 0.8 and 7.2. The leaf area estimates lay on average 14% from the reference measurements (general bias). Our method provides vertical as well as radial distributions of leaf area in individual trees, and lends itself to the estimation of savanna vegetation structural parameters with a high level of detail.     

Evaporation of intercepted rainfall from isolated evergreen oak trees: Do the crowns behave as wet bulbs?

A new approach is suggested for estimating evaporation of intercepted rainfall from single trees in sparse forests. It is shown that, theoretically, the surface temperature of a wet tree crown will depend on the available energy and windspeed. But for a fully saturated canopy under rainy conditions, surface temperature will approach the wet bulb temperature when available energy tends to zero. This was confirmed experimentally from measurements of the radiation balance, aerodynamic conductance for water vapour and surface temperature on an isolated tree crown. Net radiation over a virtual cylindrical surface, enclosing the tree crown, was monitored by a set of radiometers positioned around that surface. Aerodynamic conductance for the tree crown was derived by scaling up measurements of leaf boundary layer conductance using the heated leaf replica method. Thermocouples were used to measure the average leaf surface temperature. Results showed that a fully wet single tree crown behaves like a wet bulb, allowing evaporation of intercepted rainfall to be estimated by a simple diffusion equation for water vapour, which is not restricted by the assumptions of one-dimensional transfer models usually used at the stand scale. Using this approach, mean evaporation rate from wet, saturated tree crowns was 0.27 or 0.30 mm h^?1, when surface temperature was taken equal to the air wet bulb temperature or estimated accounting for the available energy, respectively.     

Ecophysiology of the internal cycling of nitrogen for tree growth

Internal cycling of nitrogen has been shown to be a major source of nitrogen used for the seasonal growth of both evergreen and deciduous trees providing up to 90% of N used for leaf growth of some species. The processes of internal cycling comprise seasonal nitrogen storage, followed by remobilisation during either periods of growth (e.g. in the spring) or during leaf senescence. The ecophysiology of these processes is reviewed, along with the methods used to quantify their contribution to tree growth. Nitrogen budget studies have been widely used to estimate internal cycling, particularly in relation to soil fertility. These studies have shown that as trees develop their rate of N uptake decreases, but as they grow their storage capacity increases, However, budget studies are imprecise and have not always quantified remobilisation adequately. An alternative approach has been the use of ¹⁵N to quantify N uptake and partitioning, allowing precise measurements of N storage and remobilisation to be made. The use of isotopes has allowed experiments to be run which have shown that environmental factors such as soil fertility influence the amount of N stored, but have no direct influence upon the amount of N remobilised. These methods are discussed in light of recent research on N remobilisation, which has provided an understanding of the processes of storage and remobilisation which potentially allows direct measurements to be made in field grown trees for the first time.     

The effect of nitrogen on the formation of the anisophylly on the terminal shoots of apple trees

The anisophylly on the terminal shoots of apple trees has been widely observed in various varieties (4) and it has been recognized that the normal form of this anisophylly is independent of soil moisture in natural conditions(3). This form is expressed as follows : there appears the 1st maximum leaf at about the 9th leaf numbered from the base of the shoot, the very small leaf at about the 13th leaf, and 2nd maximum leaf at about the 17th leaf, and plotting leaf-area/leaf-position curve the form makes a double-peak-curve. The authors presumed from the seasonal cycle of nutriments in the trees, that the appearance of this normal form which had the small leaf at the middle of the shoot, might have been attributed to the conversion of nutrient supply sources, viz., conversion from the so-called 1st to the 2nd-growth stage, the former controlled mainly by the reserved nutriments in the previous year, and the latter controlled by the nutriments supplied in the current year (2).     

Nutritional status of declining spruce (Picea abies (L.) Karst.): Effect of soil organic matter turnover rate

Foliar analysis was undertaken in two plots of Picea abies (L.) Karst., located in a watershed of Haute Ardenne, Belgium, in order to estimate the decline of the trees. Apart from a general Mg deficiency, the concentrations of the needles were in the same range as those determined in other European stands. Comparisons between healthy and declining trees within each plot revealed a general pattern of decline similar to that observed elsewhere in Western Europe. This was shown as lower Ca, Mg, Zn concentrations and water content and higher N and P concentrations of the needles collected from declining trees. It is concluded that this decline could be due to N over fertilization by the atmospheric deposition. The difference of decline between the two plots was attributed to the turnover rate of the soil organic layer which was less intensive in the most damaged plot.     

Nutrient relations and root mycorrhizal status of healthy and declining beech (Fagus sylvatica L.) in Southern Britain

Many studies have been carried out to investigate the wide scale problem of forest decline that is affecting vast numbers of trees throughout Europe and North American. Evidence of nutritional disturbances, both above- and below-ground, has been found for a variety of species. Furthermore, it has been suggested that differences in above-ground tree health are often reflected below ground, particularly in the root systems of declining trees. An investigation of root vitality, soil and leaf chemistry of healthy and unhealthy beech (Fagus sylvatica L.) was carried out at a number of sites in southern Britain, covering a range of different soil types. At each site, healthy and unhealthy trees growing in very close proximity were compared in order to avoid large scale environmental differences that might be expected when comparing stands of trees at different locations. Healthy trees were found to have significantly greater proportions of live mycorrhizal roots than their unhealthy neighbours at most of the sites investigated. In addition, significant differences in soil chemistry were found between trees in contrasting health, with healthy trees generally growing in soil containing higher concentrations of calcium, magnesium and potassium and lower aluminium/calcium ratios. Very few differences were found in leaf nutrient content, suggesting that soil chemistry is not yet limiting nutrient uptake. The importance of root system differences in the tolerance of unfavourable edaphic and climatic conditions is discussed with respect to tree health.     

The Impact of Simulated Acid Rain and Fertilizer Application on a Mature Sugar Maple (Acer Saccharum Marsh.) Forstt in Central Ontario Canada

The effects of two year's addition of simulated acid precipitation, with and without added fertilizer, on mycorrhizae, litter decomposition and soil and tree chemistry in a sugar maple (Acer saccharum Marsh.) dominated forest were investigated. The forest floor beneath mature sugar maple trees was irrigated at monthly intervals between May and September with local lake water acidified to pH 3, pH 4 or untreated lakewater of pH 4.9. In addition, a commercial organic slow-release fertilizer (Maple Gro) was added to the soil prior to irrigation with pH 3 spray. Trees to which no experimental spray was applied were also included as controls. Eight trees were used for each of the five treatments. Application of the acid spray alone did not acidify surface soil nor cause visible symptoms of decline in trees. The pH of the soil solution and soil leachate was increased by addition of acidified lakewater. An increase in the concentration of sulphate (SO₄ ^2-), calcium (Ca), and magnesium (Mg) in soil leachate was only recorded in plots irrigated with water acidified to pH 3 + Maple Gro. The initial rate of litter decomposition tended to be higher following application of the acid sprays, although mycorrhizal infection of sugar maple roots was reduced in the pH 3 and pH 4 treatments. Concentrations of zinc (Zn), chromium (Cr) and nickel (Ni) were highest in wood formed during the period irrigated with water acidified to pH 3. Foliar nitrogen (N) concentrations tended to be higher in all irrigated treatments, although there were no differences between treatment in any of the other nutrients measured in foliage after two years of treatment. It is concluded that the application of simulated acid rain under field conditions results in a complex interaction of events which are not reproduced in pot trials and must be fully understood before the impact of acid rain on sugar maple forests can be evaluated.     

Soil changes induced by air pollutant deposition and their implication for forests in central Europe

A survey of leaf and needle losses of European forests in 1993 revealed that 23% of the total forested area had defoliation of more than 25%. The focus of this defoliation is in Central Europe, namely in Poland, Slowakia, Czech Republic, and Germany. The annual surveys of leaf losses and discoloration indicated only small changes during the last years for the coniferous forests in Germany. However, the increasing leaf losses of oak and beech during the last years were alarming. Evaluating the potential relation between air pollutant deposition, soil changes and forest damage, we focus here on the recent changes in deposition and soil conditions, and their implication on tree root development and drought susceptability of trees. While deposition of SO₄ ^2?, H⁺ and Ca²⁺ in many Central European forests decreased in the last decade, input of NH₄ ⁺ and NO₃ ^? remained high or even increased. The H⁺ load of many forest soils today is thus still high compared to weathering rates, but the proportion of the H⁺ load resulting from turnover of deposited N has increased. Recent effects of changing depositions on acid forest soils were: depletion of soil Al-pools, release of formerly stored soil SO₄ ^2?, accumulation of N in soil organic matter, increasing N availability to trees and decreasing concentration of Ca²⁺ in the soil solution. We hypothesise that soil acidification and increased N availability will decrease the fine root biomass of trees and shift the rooting zone to upper soil layers. Increased above ground growth, observed in many areas of Europe, will furthermore decrease the root/shoot ratio. This development will finally cause increased drought susceptability of trees and is thus of destabilizing nature. The proposed chain of events might be overlapped by other effects of air pollutants on forest ecosystems, namely direct effects of gases on leaves, nutritional inbalances, and interactions with pests.     

Treated municipal wastewater for citrus irrigation 1

Treated, reclaimed municipal wastewater was evaluated on citrus trees in central Florida for over six years. The effects of irrigation with reclaimed wastewater on soil water content, soil chemical analysis, leaf mineral status, and fruit quality were compared with those of irrigation with well water. Irrigation with reclaimed water increased mineral residues in the soil profile, altered leaf mineral concentration and fruit quality, and promoted better tree performance and more weed growth relative to irrigation with well water. Higher accumulation of nitrogen (N), potassium (K), calcium (Ca), and magnesium (Mg) in soils irrigated with reclaimed water were not significantly reflected in leaf mineral status. Although leaf sodium (Na), chloride (Cl), and boron (B) concentrations were noticeably higher in reclaimed water treatments than in those of well water, they are still far below the toxicity levels. This highly treated wastewater in central Florida has been found to be a very safe and good option for increasing water supplies, but not a significant source of plant nutrients to citrus trees.     

Neutralization of Acid Droplets on Plant Leaf Surfaces

Sulfuric acid mist exposure of bush bean leaves at a low rate of precipitation suggested that acid on the leaf surface was neutralized by cations leached from leaf tissues and that Ca-S compounds were accumulated on the leaf surface (Kohno, 1994). This report summarizes visual observations of the neutralization process of acid on leaf surfaces as determined by a pH-imaging microscope. Small droplets of sulfuric acid were placed on the adaxial leaf surface and allowed to air dry under laboratory conditions. Droplets (0.1 µl) of sulfuric acid took about 7–8 minutes to dry. Leaf samples were cut at various times after the acid droplets dried. The adaxial leaf surface was placed on the pH-adjusted agar film layer on the pH-imaging sensor of the microscope. Hydrogen ions dispersed into the film layer and resulting pH distributions were visualized as pH distribution patterns. The size of the acidic area generated became smaller with time after the acid was added and allowed to dry. Results indicate that leaves could neutralize the surface acid probably by ion exchange with cations from their surface tissues and could recover from strong temporary acid stress imposed by acid rain or acid fog in a relatively short period of time. Our findings indicate that acidic precipitation at current acidity levels does not pose a direct threat to plants.     

Distribution patterns of soluble salts and gypsum in soils under large-scale irrigation agriculture in Central Asia

Abstract

Soil salinization is a serious problem in the arid and semi-arid regions of Central Asia. To address the problems, we analyzed the dynamics and distribution patterns of salts in both rice-based and cotton-based cropping fields in selected farms of southern Kazakhstan and Uzbekistan with special emphasis on the dynamics of gypsum, which had a lower solubility than Na salts, as an index of water-movement regimes in irrigated fields. Most of the rice-based plots and some of the cotton-based plots exhibited no surface accumulation of soluble salts or gypsum because of repeated washing by a huge amount of irrigation water in the former or comfortable drainage in the latter. These soils are probably free from the risk of secondary salinization under present conditions and management practices. In contrast, uncultivated plots near canals accumulate both soluble salts and gypsum in the surface soil layers, and these salts would not be leached out without a drastic change to a predominantly downward pattern of water movement. In the intermediate stages in terms of soil salinization, some soils accumulated substantial amounts of soluble salts in surface layers but relatively low amounts of gypsum. In this case, periodic irrigation could have washed out most of the gypsum and soluble salts in a downward direction and, consequently, it is possible to leach out the accumulated soluble salts by applying additional irrigation water if necessary. However, there were some cases in which soils accumulated large amounts of gypsum in surface layers as well as soluble salts, suggesting that irrigation/drainage is generally insufficient to remove gypsum with a lower solubility and that these profiles are dominated by an overall upward movement of water. For these soils, drainage facilities should be improved to ensure the efficient leaching of accumulated salts on cropping. Thus, the condition of irrigated plots in terms of the direction of water movement and resulting salt regimes can be well understood from the distribution patterns of both soluble salts (or cations) and gypsum.     

Atmospheric deposition and forest decline

This paper discusses the economic implications of alternative scientific explanations of observed forest decline. The various explanations can be grouped into two categories. One category posits direct damage to the forest canopy that is reversible. The other category posits indirect damage via soil quality changes that may be irreversible or slow to recover. The paper argues that the decision maker should take into account all of the explanations in designing a control strategy for acid deposition. It would be costly to wait until a single explanation has emerged victorious. If the irreversible decline explanation turns out to be correct, the forest damages will already have been sustained with no recovery possible. Assuming that the impact of acid deposition is to reduce forest productivity by 5% per annum then the annual losses to the commercial timber industry in Canada is estimated to be $197 million ( in 1981 dollars). To this an annual loss of $1.29 billion (in 1981 dollars) should be added for disruption to recreation and wildlife habitat values.     

Controlling dryland salinity by planting trees in the best hydrogeological setting

Revegetation by trees and shrubs is examined as the best long-term option for controlling dryland salinity. Since hydrogeological features in a catchment control and cause salinity, it is shown that management programs using trees should be developed on a hydrogeological basis through the use of hydrogeomorphic mapping. Catchment planning should ensure that trees suited to the area, and preferably of commercial value, are planted in the best hydrogeologically determined positions in sufficient numbers to reduce excess recharge generated following clearing. The programme should integrate other biological measures to increase water use, which includes improved crop and soil management and the introduction of salt-tolerant plants. Since trees will take several years to exert their full effect on recharge, engineering methods such as groundwater pumping, which lower water tables, and contour banks, which capture and redirect surface water, can best be used as short-term measures. Case studies from catchments in Western Australia are used to illustrate the application of control measures.     

赣江中游水源涵养林乔木和蕨类植物凋落物持水和失水特征

选取赣江中游典型水源涵养林中常见的3种常绿针叶树、3种落叶阔叶树、4种常绿阔叶树和5种蕨类植物的凋落叶,通过室内浸泡试验和失水试验研究了植物凋落物的持水和失水特征.结果表明:(1)凋落物的吸水速率均显示出随浸泡时间增加而降低的趋势,到8 h基本趋于平缓,24 h达到饱和状态,而失水速率随着失水时间的增加逐渐减少,到12...     

调控亏水度灌溉对成龄桃树生长和产量的影响

自1988到1990年,在北京市林业果树研究所桃园17年生的桃树上进行了调控亏水度灌溉法(RDI-Regulated Deficit Irrigation)的试验研究。试验设有3个灌溉水平,即在果实细胞分裂完成后至快速膨大前的一段时间内果实的缓慢生长阶段(RDI期间),分别按80％(对照),40％,20％美国A级皿蒸发量补给。测得的各处理桃树的叶水势情况为:在RDI期间供水量少(处在水应力状态)的桃树,其叶水势较对照低(负的最值大);当RDI结束后各处理同样充分灌水,结果表明,此间曾经受过水应力作用的桃树的叶水势较对照(80％)高(负的量值小)。这种水应力作用的结果表现在对桃树生长量的影响上为:RDI期间,枝条的生长量明显为水势的降低所抑制;而在后期桃果实快速膨大时,经受过RDI的桃树上的果实因水势较高而膨大更快,收获时单果重更大,成熟果实略多,从而提高了产量。另外经受RDI的果树用水量明显减少,从而大幅度提高了水的利用效率。