Decline of soil fertility during forest conversion of secondary forest to Chinese fir plantations in subtropical China

The effects of forest conversion on soil fertility are still not well understood in subtropical zones. This issue was addressed by comparing chemical properties of soil in a secondary forest and a Chinese fir (Cunninghamia lanceolata Hooker) plantation at the Huitong Experimental Station of Forest Ecology. Total N, available P, ‐N, cation exchange capacity (CEC) and exchangeable Al³⁺ and H⁺ of soil were significantly lower in the pure Chinese fir plantation (PCP) than in the secondary forest while soil organic carbon (SOC), total K and exchangeable Na⁺ had a tendency to decrease in the PCP. In contrast, soil pH and percentage base saturation (PBS) significantly increased due to forest conversion, and available K, ‐N and exchangeable Ca²⁺, Mg²⁺ and K⁺ tended to increase in the PCP. Some underlying processes responsible for the differences in soil fertility between the secondary forest and the Chinese fir plantation were low litterfall and root input to soil and site preparation in coniferous plantations. There was no significant difference in the effect of slope position on chemical properties of soil in the PCP and the secondary forest. Results indicated that the conversion of secondary forests to coniferous plantations leads to a decline in soil fertility. Copyright © 2010 John Wiley & Sons, Ltd.     

A model for the impact of soil solution Ca: Al ratio,soil moisture and temperature on tree base cation uptake

A model for tree base cation uptake has been developed, dependent on the soil solution concentration of Al³⁺, divalent base cations such as Ca²⁺, Mg²⁺ and H⁺ ions, modelled with a Mikaelis-Menten type of expression based on the molar BC∶Al ratio, where BC is the sum of the divalent non-toxic base cations Ca²⁺ and Mg²⁺. The expression has the form of counteracting adsorption isoterms for BC and Al to the tree root. The effect of toxic levels of Hg and Pb is incorporated into the model, using root adsorption as the mechanism, and parameterization against experimental data. The expression is modified with an expression accounting for the effect of heavy metal toxicity and soil water content. The dependence of the uptake rate on soil moisture content can be shown to have the form of a Freundlich adsorption isotherm for water. The available data indicate an activation energy of 47 kJ^?1 mol for base cation uptake to trees. Data from the literature was used to estimate the rate coefficients and ion selectivity coefficients for typical coniferous and decideous trees in Sweden and Germany. The model indicates that Ca²⁺ and Mg²⁺ is effective in mitigating Al³⁺ toxicity to tree roots, and that increasing the Ca²⁺+Mg²⁺ soil solution concentration has a promotive effect on base cation uptake. Above a certain limit base cation uptake becomes independent of the solution base cation concentration. This is consistent with field observations, and may be developed to become a tool for assessing the impact of soil chemistry changes on forest growth rate and health status. Field data from the Swedish Forest Survey indicate that uptake depend on the square root of the soil solution base cation availability originating from weathering and deposition input, which is consistent with the BC∶Al expression of the model.     

Identification and Characterization of High Acid Tolerant and Aluminum Resistant Yeasts Isolated from Tea Soils

From acidic tea soils of Kagoshima Prefecture in Japan, some soil properties were determined and 38 strains of acid tolerant microorganisms were isolated. Different Al³⁺ concentrations were applied to YG media to estimate Al resistance. Selected microbial strains could grow strongly in the liquid media in the presence of 100 mM Al³⁺ and survive even in 300 mM Al³⁺ at pH 3.0. Their base sequences of 28S rDNA-D1/D2 were determined and sequence data were searched using the Basic Local Alignment Search Tool (BLAST) system. The results of sequencing revealed that the isolates belong to two different species, Cryptococcus sp. and Candida palmioleophila. When cultivated with various Al³⁺ concentrations, the yeast growth was inhibited at a concentration of 200 mM. Pre-cultivation of these strains with 0–30 mM Al³⁺ did not promote the growth response caused by Al³⁺. Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS) was used to assess the elimination of Al. The amount of Al remaining in culture media was decreased considerably after cultivation. Due to a capacity for resistance to significant Al concentrations as well as high Al elimination, these acid tolerant and Al resistant yeasts may have potential applications in the bio- and phyto-remediation of Al and acid-contaminated soils.     

Effects of aluminium on growth and nutrition in birch seedlings under magnesium- or calcium-limiting growth conditions

Growth and nutrition of birch seedlings (Betula pendula Roth) with and without 0.5 mM Al³⁺ were studied. The seedlings were cultivated at growth-limiting Mg- or Ca-conditions by adding one of these elements at relative addition rates (R_Mg, R_Ca) of 0.15 day^?1. The concentration of Ca²⁺ in the Mg-limited treatments was 0.01.0.1 or 1 mM and Mg²⁺ was given in equal molarities in the Ca-limited treatments. The relative growth rates, R_G, of the plants and plant parts attained values close to R_Mg or R_Ca and were not affected by the concentration of Ca²⁺, Mg²⁺ or Al³⁺ in the culture solution. Uptake of Ca, at Ca-limitation, increased significantly in the roots after addition of Al, and decreased in all plant parts when the Mg concentration of the culture solution was raised. Less clear effects were found on Mg uptake at Mg-limitation. The inhibitory effect of Al on Mg uptake decreased with time as a result of the growth technique. The root mass fraction was significantly larger in plants limited in Ca (24%) than in plants limited in Mg (20%). The usefulness in using the balance between Ca and Al or ratios between Ca, Mg, K and Al as diagnostic tools for assessing the nutritional status of trees is discussed.     

Soybean root growth in relation to ionic composition in magnesium-amended acid subsoils: Implications on root citrate ameliorating aluminum constraints

Abstract

Hydroponic studies with soybean (Glycine max [L.] Merr.) have shown that µmol L^?1 additions of Mg²⁺ were as effective in ameliorating Al rhizotoxicity as additions of Ca²⁺in the mmol L^?1 concentration range. The objectives of this study were to assess the ameliorative effects of Mg on soybean root growth in acidic subsoils and to relate the soil solution ionic compositions to soybean root growth. Roots of soybean cultivar Plant Introduction 416937 extending from a limed surface soil compartment grew for 28 days into a subsurface compartment containing acid subsoils from the Cecil (oxidic and kaolinitic), Creedmoor (montmorillonitic) and Norfolk (kaolinitic) series. The three Mg treatments consisted of native equilibrium soil solution concentrations in each soil (50 or 100 µmol L^?1) and MgCl₂ additions to achieve 150 and 300 µmol L^?1 Mg (Mg150 and Mg300, respectively) in the soil solutions. Root elongations into Mg-treated subsoils were compared with a CaCO₃ treatment limed to achieve a soil pH value of 6. Subsoil root growth responses to the Mg treatments were less than for the lime treatments. Root length relative to the limed treatments for all subsoils (RRL) was poorly related to the activity of the soil solution Al species (Al³⁺ and Al-hydroxyl species) and Mg²⁺. However, the RRL values were more closely related to the parameters associated with soil solution Ca activity, including (Ca²⁺), (Al³⁺)/(Ca²⁺) and (Al³⁺)/([Ca²⁺] + [Mg²⁺]), suggesting that Ca could be a primary factor ameliorating Al and H⁺ rhizotoxicity in these subsoils. Increased tolerance to Al rhizotoxicity of soybean by micromolar Mg additions to hydroponic solutions, inducing citrate secretion from roots to externally complex toxic Al, may be less important in acid subsoils with low native Ca levels.     

Aluminium Toxicity Risk for Pinus pinaster in Acid Soils (Galicia,NW Spain)

The influence of bedrock on aluminium toxicity and aluminium speciation in the soil solution was studied in four Pinus pinaster plots. Growth and biomass parameters in the acidic soils were also evaluated in relation to different Al toxicity indices. The plots were developed over slate, biotitic schist, mica schist and granite. Samples of rhizospheric and non‐rhizospheric soil, 1‐year‐old needles and roots were collected in each study plot. Total Al, reactive Al, acid‐soluble Al, non‐labile and labile Al and Al species (Al³⁺, Al‐OH, Al‐F and Al‐SO₄) were determined in soil solution. Reactive Al dominated over the acid‐soluble Al, and the non‐labile Al predominated over the labile Al in all soils, but particularly over mica schist. In the biotitic schist soil, the Al forms and total Al were lower, whereas concentrations were always higher over mica schist. The Al forms considered most toxic were Al³⁺ and Al‐OH, and Al concentrations were highest over slate and mica schist. Al toxicity indices in soil, needle and roots showed a risk of toxicity in mica schist, slate and granite. The stand site indices over slate and mica schist were lower, consistent with the high labile Al and Al³⁺ + Al‐OH in soil solution. Despite the high stand site index over granite, the growth efficiency was low, in accordance with very low ratios of Ca/Al in needles or fine roots. This confirmed the adaptation of maritime pine to granitic substrates. Copyright © 2016 John Wiley & Sons, Ltd.     

Toxicity factors in acidic forest soils: attempts to evaluate separately the toxic effects of excessive Al3+ and H+ and insufficient Ca2+ and Mg2+ upon root elongation

The acidic soils of temperate forests and some pastures on former forest land characteristically contain large amounts of aluminium and hydrogen ions and small amounts of calcium and magnesium ions. The relative importance of these potential toxicity factors are assessed from published data from soils collected in the United States and Europe. Activities of ions in the soil solutions and at the surfaces of root‐cell plasma membranes were computed with electrostatic models. Activities of Al³⁺ in soil solutions ({Al³⁺}) peaked at pH 4.1, and Al³⁺ activities at the surface of the plasma membrane ({Al³⁺}₀) achieved a broad maximum between pH 4.1 and 4.8; thus, Al³⁺ intoxication is likely to be more severe in soils at pH 4.1 than in more acidic ones. Intoxication (assessed by root elongation) correlated somewhat ambiguously with ion activities, but Al³⁺‐ and H⁺‐induced depletion of Ca²⁺ and Mg²⁺, or both, from the cell surface appears to play a role in toxicity. By contrast, experiments in solution culture, where intercorrelation among {Al³⁺}, {H⁺}, and {Ca²⁺} could be avoided, clearly demonstrated the following extrinsic and intrinsic effects. 1 The ions Al³⁺ and H⁺ are intrinsic toxicants. 2 They are also extrinsic toxicants because of the electrostatic displacement of Ca²⁺ from the surface of the plasma membrane. 3 They are extrinsic ameliorants because each electrostatically displaces the other from the surface of the plasma membrane. 4 The ion Ca²⁺ is an extrinsic ameliorant because of the electrostatic displacement of Al³⁺ and H⁺ from the surface of the plasma membrane. 5 It is an intrinsic ameliorant of intrinsic H⁺ toxicity, but not intrinsic Al³⁺ toxicity. 6 It meets an intrinsic requirement. 7 The ion Mg²⁺ resembles Ca²⁺ in item 4 but not items 5 and 6 in short‐term cultures. In acidic soils, Al³⁺ may prevent H⁺ from becoming an intrinsic toxicant (item 3) and may induce an insufficiency of Ca²⁺ and Mg²⁺ (item 2). These findings have implications for the mechanisms by which woodland plants tolerate very acidic soils.     

CALCIUM: ALUMINIUM EXCHANGE EQUILIBRIA IN CLAY MINERALS AND ACID SOILS1

Exchange reactions between 0.0in AlCl₃ solutions of different pH and Ca-saturated montmorillonite, vermiculite, illite, and soils from the Park Grass Experiment at Rothamsted and the Deerpark Experiment, Wexford, Ireland, showed that Al³⁺ and Al(OH)₂⁺ were adsorbed from solutions of pH > 4.0 and Al³⁺ and H⁺ from solutions of pH < 3.0. When Al was adsorbed, the cation exchange capacity of Ca-saturated soils and clays increased. Conventional Ca: Al exchange isotherms showed that Al³⁺ was strongly preferred to Ca²⁺ on all soils and clays. The equilibrium constant for Ca: Al exchange, K, was identical for soils before and after oxidizing their organic matter and did not vary, for any exchanger, with Al-saturation or the initial pH of the AlCl₃ solution. This proved the validity of the procedure used for calculating exchangeable Al³⁺. K values for Ca:Al exchange favoured Al³⁺ in the order: vermiculite > Park Grass soil > Deerpark soil > illite > montmorillonite. The influence of surface-charge densities of the clay minerals on this order is discussed and a method proposed and tested for calculating the K value of a soil from its mineralogical composition.     

Field application of industrial by-products as Al toxicity amendments: chemical and mineralogical implications

Lime, gypsum and various gypsum‐like by‐products have long been applied to soil surfaces as ameliorants of soil acidity and aluminium and manganese toxicity. We examined changes in chemical and mineralogical properties at two different depths in two acid soils one year after the application of gypsum, phosphogypsum + dolomitic residue, red gypsum + dolomitic residue, sugar foam, and sugar foam + mined gypsum. All treatments were found to increase the proportion of Ca²⁺ and decrease those of Al³⁺ and Mn²⁺ in the exchange complex of the surface and subsurface horizons, thus reducing its effective Al and Mn saturation. However, the mined gypsum treatment resulted in losses of Mg²⁺ from the Ap horizon of the soils, and the sugar foam treatment was not so effective with the AB horizons as the other treatments. The combined application of both gypsum‐like by‐products and the dolomitic residue proved the most effective choice with a view to reducing the effective Al and Mn saturation of the exchange complex in the Ap and AB horizons. In addition, both treatments reduced Mg²⁺ losses at both depths. Finally, all treatments resulted in the formation and retention on mineral and organic surfaces of a large fraction of the Al³⁺ released by exchange with Ca²⁺ as Al polymers. This is quite consistent with the observed changes in the CuCl₂‐, oxalate‐ and DTPA‐extractable Al contents as well as by SEM and EDS analyses. Based on these results, the use of the appropriate mixtures of these by‐products is an effective alternative to that of mined gypsum and lime to alleviate soil acidity and reduce toxic concentrations of Al³⁺ and Mn²⁺ in agricultural acid soils.     

Phosphorus Alleviates Aluminum Toxicity in Al-Sensitive Wheat Seedlings

The role of phosphorus (P) in the amelioration of aluminum (Al) toxicity to plants is still unclear. The aim of this study was to examine the amelioration of Al toxicity by P supply. The study involved growing Al-sensitive wheat seedlings for 13 days in an acidic soil [pH 4.5 in calcium chloride (CaCl₂)] with increasing added rates of P (0, 20, 40, and 80 mg P kg^?1 soil) and Al [0, 50, and 150 mg aluminum chloride (AlCl₃) kg^?1 soil]. The results indicated that the effects of Al toxicity in this soil could be fully alleviated by the application of P at 50 mg AlCl₃ kg^?1. The 150 mg kg^?1 AlCl₃ treatment significantly reduced root growth, but this was partially overcome by the 80 mg kg^?1 P treatment. High P significantly reduced the concentration of Al in the apoplast, root, and shoot. It is possible that an insoluble Al-P complex forms in the soil and this decreases Al bound in apoplast as well as uptake into the roots. High P decreased the translocation of Al from root to shoot. This study also concluded that detoxification of Al³⁺ by P mainly occurs in soil but not within the plant tissue.     

Increases in pH and soluble salts influence the effect that additions of organic residues have on concentrations of exchangeable and soil solution aluminium

It has been suggested that additions of organic residues to acid soils can ameliorate Al toxicity. For this reason the effects of additions of four organic residues to an acid soil on pH and exchangeable and soil solution Al were investigated. The residues were grass, household compost, filter cake (a waste product from sugar mills) and poultry manure, and they were added at rates equivalent to 10 and 20 t ha^?1. Additions of residues increased soil pH measured in KCl (pH_(KCl)) and decreased exchangeable Al³⁺ in the order poultry manure > filter cake > household compost > grass. The mechanism responsible for the increase in pH differed for the different residues. Poultry manure treatment resulted in lower soil pH measured in water (pH_(water)) and larger concentrations of total (Al_T) and monomeric (Al_mono) Al in soil solution than did filter cake. This was attributed to a soluble salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al³⁺ and H⁺ back into soil solution. The considerably larger concentrations of soluble C in soil solution originating from the poultry manure may also have maintained greater concentrations of Al in soluble complexed form. There was a significant negative correlation (r = ?0.94) between pH_(KCl) and exchangeable Al. Concentrations of Al_T and Al_mono in soil solution were not closely related with pH or exchangeable Al. The results suggest that although additions of organic residues can increase soil pH and decrease Al solubility, increases in soluble salt and soluble C concentrations in soil solution can substantially modify these effects.     

Influence of Different Forms of Iron and Aluminum on the Nature of Soil Acidity of Some Inceptisols,Alfisols, and Ultisols

Abstract

Fifteen acid soils of Mizoram representing Ultisols and Inceptisols, and Madhya Pradesh, representing Alfisols, were studied to characterize the nature of acidity in relation to different forms of iron (Fe) and aluminum (Al). The mean contents of Fe and Al were extracted by various extracting reagents and were found to be in descending order as followed: dithionite>oxalate>pyrophosphate>ammonium acetate>KCl. The electrostatically bonded EB‐H⁺ and EB‐Al³⁺ acidity comprised 28.3 and 71.7% of exchangeable acidity whereas EB‐H⁺, EB‐Al³⁺, exchangeable, and pH‐dependent acidities comprised 9.8, 30.7, 40.5, and 59.5% of total potential acidity. All forms, of acidity showed significant correlation with pH_k and organic carbon. Among the different forms, Fe and Al caused most of the variations in different forms of soil acidity but the effect of different forms of Al are more active and directly participate in the formation of EB‐H⁺, EB‐Al³⁺, and exchangeable acidity.     

Apparent cation-exchange equilibria in podzolic forest soils

The Gaines–Thomas selectivity coefficient, K, was used to express the relation between the cations in solution and the cations in exchange sites in podzolic forest soils. Soil solution was obtained by centrifuging a fresh bulked soil sample. Exchangeable cations HX, AlX, CaX, MgX and KX and effective cation-exchange capacity, CEC_e, were determined with 0.1 m BaCl₂. Apparent values of K indicated a preference of Ca²⁺ over Mg²⁺ and over Al³⁺ in O, A and B horizons (log K_Al–Ca < 0 and log K_Mg–Ca < 0), whereas log K_K–Ca and log K_H–Ca exceeded zero. The horizons were similar with respect to log K_H–Ca, and the differences in log K_Mg–Ca were small. Log K_K–Ca and log K_Al–Ca increased in the horizons in the order O < A < B. Log K_Al–Ca was not significantly correlated with the fraction AlX/CEC_e. Log K_Mg–Ca was positively correlated with the fractions HX/CEC_e and AlX/CEC_e, and negatively correlated with log (CaX/MgX). The selectivity coefficient of binary cation exchange seemed to be applicable to in situ soil solutions. However, the fraction of each cation on exchange sites should be based on the CEC_e rather than on the sum of the two cations. The latter, also, seemed to be acceptable in cases of exchangeable cations with a large relative content in soil, e.g. in Al³⁺–Ca²⁺ exchange in A and B horizons, and in H⁺–Ca²⁺ exchange in O and A horizons.     

杉木人工林皆伐对林地径流水化学特征的影响

杉木人工林皆伐前后径流水化学特征研究结果表明，杉木人工林皆伐后引起林地土壤养分大量流失，皆伐后2年内土壤N素、P素、盐基离子（K^ ,Na^ ,Ca^2 ,Mg^2 ）的可溶性盐分的流失增量分别为20.54kg/hm^2、0.817kg/hm^2、42.67kg/hm^2和774.44kg/hm^2，表晨杉木人工林具有一定的调节、富集和保持土壤养分的能力。     

潮土条件下长期施肥对作物产量、氮吸收与土壤剖面中硝酸盐含量分布的影响

Soil properties were investigated in sites where three succeeding generations of Chinese fir (Cunninghamia lanceolata (Lambert) Hooker) in Nanping, Fujian, China, were cultivated in order to show the impact of a repeated monoculture on site productivity. Compared with the first generation (FG) stand the soil structure deteriorated in the second generation (SG) and the third generation (TG) stands. For instance, the destruction rate of the peds increased by 55%-115% in the SG and the TG stands compared to the FG stand. Soil nutrient storage and nutrient availability also decreased in the SG and the TG stands. For surface soils of 0-20 cm, the organic matter content, total N and P, and available N and P decreased by 3%-20% relative to those in the FG stand. For many soil parameters, the differences between the FG stand and the SG and the TG stands were statistically significant (LSD test, P < 0.05). Furthermore, with each succeeding generation of Chinese fir, the total number of soil microbes declined, the soil enzyme activity weakened, and the soil biological activity decreased. In order to maintain sustainable site productivity, new silvicultural practices need to be developed for management of Chinese fir plantations.     

Nifedipine influence on calcium and aluminum absorption in roots of three sorghum cultivars

A 1,4‐dihydroxypyridine type of ion channel blocker, nifedipine [1,4‐dihydro‐2,6‐dimethyl‐4‐(2‐nitrophenyl)‐3,5‐pyridinedicarboxylate dimethyl ester], was tested on the root absorption of Al³⁺ and Ca²⁺ by sorghum [Sorghum bicolor (L.) Moench] cultivars with varying acid stress tolerance. In an acid stress sensitive cultivar, Funk G522DR, nifedipine (1 μM) influenced Ca²⁺ but not Al³⁺ absorption. In one acid stress tolerant cultivar, SC574, nifedipine (1 μM) influenced both Ca²⁺ and Al³⁺ absorption. In a second acid stress tolerant cultivar, SC283, nifedipine (1 μM) did not influence Ca²⁺ but did influence Al³⁺ absorption. Considerable genetic diversity is present in Ca²⁺ and Al³⁺ absorption between sorghum cultivars.     

pH and solubility of aluminium in acidic forest soils: a consequence of reactions between organic acidity and aluminium alkalinity

Data from two Podzol O and E horizons, sampled in 1-cm layers at 13 points within 2 m × 2 m plots, were used to test the hypothesis that the composition of hydrogen ions (H) and aluminium (Al) adsorbed to the solid-phase soil organic matter (SOM) determines pH and Al solubility in organic-rich acidic forest soils. Organically adsorbed Al was extracted sequentially with 0.5 m CuCl₂ and organically adsorbed H was determined as the difference between total acidity titrated to pH 8.2 and Al extracted in 0.5 m CuCl₂. The quotient between fractions of SOM sites binding Al and H (N_Al/N_H) is shown to determine the variation in pH and Al solubility. It is furthermore shown that models in which pH and Al solubility are linked via a pH-dependent solubility of an Al hydroxide and in which cation exchange between Al³⁺ and Ca²⁺, rather than cation exchange between Al³⁺ and H⁺, is the main pH-buffering process cannot be used to simulate pH or Al solubility in O and E horizons. The fraction of SOM sites adsorbing Al increased by depth in the lower O and throughout the E horizon at the same magnitude as sites adsorbing H decreased. The fraction of sites binding the cations Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ remained constant. It is suggested that a net reaction between Al silicates (proton acceptors) and protonated functional groups in SOM (proton donors) is the long-term chemical process determining the composition of organically adsorbed H and Al in the lower part of the O and in the E horizon of Podzols. Thus, in the long term, pH and Al solubility are determined by the interaction between organic acidity and Al alkalinity.     

Effects of Alum-treated Waste Water Sludge on Barley Growth

Alum sludge derived from a municipal wastewater plant was used as a soil amendment in a greenhouse study with barley (Hordeum vulgare) as the test crop. Treatment variables included the soil pH (4.5, 5.1 and 6.5), the amount of Al in the sludge (control = 30 mg Al_T/g; alum sludges = 38 and 52 mg Al_T/g), and the sludge application rate (100 and 270 kg N_T/ha). Soil amendment with the two alum sludges reduced soil pH, increased Al³⁺ activity in the soil solution, and reduced barley growth over the 6-week experiment. Barley growth decreased as the Al³⁺ activity in the sludged soil solution increased, but for a given Al³⁺ the phytotoxicity of Al was markedly pH dependent. For example, at a pH of 5.0 ± 0.1 an Al³⁺ activity of 0.5 μM was sufficient to inhibit plant growth by about 50% this IC₅₀ value increased five-fold to about 2.5 μM when the soil pH was 4.5 ± 0.1. This decrease in the toxicity of Al50 with acidification was explained in terms of a competitive interaction between the H⁺-ion and Al³⁺ at the root surface. Stepwise multiple regression allowed the prediction of aerial leaf biomass from soil pH and sludge application rate.     

华蓥市山区典型林分水源涵养功能评价

为研究南方丘陵山区典型人工林的生长现状和涵养水源能力,选取华蓥市6种林分作为研究对象,通过测定林下枯落物层和土壤层特征,分析比较不同林地枯落物和土壤的持水能力,对林地水源涵养能力进行评价。结果表明:杉木纯林(近自然经营)枯落物现存蓄积量最大,枯落物持水能力排序为杉木纯林(近自然经营)杉木纯林马尾松—檵木混交林(近自然经营)杉木—响叶杨混交林柏木纯林马尾松—杉木混交林;杉木纯林的土壤层总孔隙度、毛管孔隙度、有效持水量均为最优,分别达到67.67%,60.39%,138.11 t/hm~2;6种林地土壤有效持水量大小顺序为杉木纯林杉木纯林(近自然经营)马尾松—杉木混交林杉木—响叶杨混交林柏木纯林马尾松—檵木混交林(近自然经营);综合枯落物层和土壤层持水能力可知,杉木纯林(近自然经营)的水源涵养能力最强,为309.77 t/hm~2,马尾松—杉木混交林最弱。     

Determination of free Al3+ in soil solutions by capillary electrophoresis

Colorimetric and ion exchange methods are commonly used to distinguish and measure Al species in natural waters. Unfortunately they also include weakly complexed Al species in their ‘reactive' or ‘labile' Al fractions and thus are of limited value for the estimation of free Al³⁺. Capillary electrophoresis (CE) has the potential for direct measurement of Al³⁺, and its performance has been verified experimentally. The method also detected the stable and positively charged AlOx⁺ complex formed with oxalic acid. It was compared with a colorimetric and an ion exchange method by analysing artificial solutions containing low molecular weight organic acids as well as soil extracts and seepage waters and was found to be the only method closely matching the theoretically calculated values of free Al³⁺. In samples from the upper soil horizons of an acid forest soil less than 14% of total Al was present as free Al³⁺, whereas the colorimetric method found more than 65%, and the ion exchange method more than 80% of total Al in a ‘reactive' or ‘labile' form. The latter methods thus would seriously overestimate Al toxicity, whereas using CE Al toxicity is likely to be only slightly underestimated.