Changes in phosphorus concentrations and pH in the rhizosphere of some agroforestry and crop species

The aim of this work was to assess whether agroforestry species have the ability to acquire P from pools unavailable to maize. Tithonia diversifolia(Hemsley) A. Gray, Tephrosia vogelii Hook f., Zea mays and Lupinus albusL. were grown in rhizopots and pH change and depletion of inorganic and organic P pools measured in the rhizosphere. Plants were harvested at the same growth stage, after 56 days for maize and white lupin and 70 days for tithonia and tephrosia, and the rhizosphere sampled. The rhizosphere was acidified by tithonia (pH change –0.3 units to pH 4.8) and lupins (–0.2 units to 4.9), alkalinised by tephrosia (+0.4 units to pH 5.4), and remained unchanged with maize growth. Concurrent with acidification in the rhizosphere of tithonia there was a decline in resin-P (0.8 g P g^–1). However, there was also a decline in NaOH extractable inorganic P (NaOH-P_i) (5.6 g P g^–1 at the root surface) and organic P pools (NaOH-P_o) (15.4 g P g^–1 at 1.5 mm from the root), which would not be expected without specific P acquisition mechanisms. Alkalinisation of tephrosia rhizosphere was accompanied by changes in all measured pools, although the large depletion of organic P (21.6 g P g^–1 at 5 mm from the root) suggests that mineralisation, as well as desorption of organic P, was stimulated. The size of changes of both pH and P pools varied with distance away from the rhizoplane. Decline of more recalcitrant P pools with the growth of the agroforestry species contrasted with the effect of maize growth, which was negligible on resin-P and NaOH-P_i, but led to an accumulation of P as NaOH-P_o (14.2 g P g^–1 at 5 mm from the root). Overall the depletion of recalcitrant P pools, particularly P_o, suggests that the growth of tithonia and tephrosia enhance desorption and dissolution of P, while also enhancing organic P mineralisation. Both species appear to have potential for agroforestry technologies designed to enhance the availability of P to crops, at least in the short term.     

Roots,soil water and crop yield: tree crop interactions in a semi-arid agroforestry system in Kenya

Variations in soil water, crop yield and fine roots of 3–4 year-old Grevillea robusta Cunn. and Gliricidia sepium (Jacq.) Walp. growing in association with maize (Zea mays L.) were examined in semiarid Kenya during the long rains of 1996 and 1997. Even although tree roots penetrated more deeply than maize roots, maximum root length densities for both tree species and maize occurred in the top 200 mm of the soil profile where soil moisture was frequently recharged by rains. Populations of roots in plots containing trees were dominated by tree roots at the beginning of the growing season but because tree roots died and maize root length increased during the cropping season, amounts of tree and maize roots were similar at the end of the season. Thus, there was evidence of temporal separation of root activity between species, but there was no spatial separation of the rooting zones of the trees and crops within that part of the soil profile occupied by crop roots. Tree root length density declined with increasing distances from rows of trees and with depth in the soil profile. Although Grevillea trees were largest, plots containing G. sepium trees always contained more tree roots than plots containing G. robusta trees and Gliricidia was more competitive with maize than Grevillea. Overall, Gliricidia reduced crop yield by about 50% and Grevillea by about 40% relative to crop yield in control plots lacking trees and reductions of crop yield were greatest close to trees. There was less soil moisture in plots containing trees than in control plots. Such difference between control plots and plots containing trees were maximal at the end of the dry season and there was always less soil moisture close to trees than elsewhere in the plots. Plots containing Gliricidia trees contained less soil water than plots containing Grevillea trees.     

Effect of five tree crops and a cover crop in multi-strata agroforestry at two fertilization levels on soil fertility and soil solution chemistry in central Amazonia

The spatio-temporal patterns of soil fertility and soil solution chemistry in a multi-strata agroforestry system with perennial crops were analysed as indicators for the effects of crop species and management measures on soil conditions under permanent agriculture in central Amazonia. The study was carried out in a plantation with locally important tree crop species and a leguminous cover crop at two fertilization levels on a xanthic Ferralsol. Soil fertility to 2 m soil depth was evaluated 3.5 years after the establishment of the plantation, and soil solution chemistry at 10, 60 and 200 cm soil depth was monitored over 2 years. Several soil fertility characteristics exhibited spatial patterns within the multi-strata plots which reflected the differing properties of the plant species and their management, including the fertilizer input. Significant differences between species could be detected to 150 cm depth, and between fertilization treatments to 200 cm depth. Favourable effects on nutrient availability in the soil were found for annatto (Bixa orellana) (P, K) and cupuaçu (Theobroma grandiflorum) (Ca, Mg) in comparison with peach palm (Bactris gasipaes) and Brazil nut (Bertholletia excelsa). Nutrient concentrations of the soil solution showed pronounced fluctuations in the topsoil, corresponding to fertilizer applications. Large nutrient concentrations in the soil solution were accompanied by increased concentrations of aluminium and low pH values, caused by exchange reactions between fertilizer and sorbed acidity and reinforced by the acidifying effect of nitrification. The soil solution under the leguminous cover crop Pueraria phaseoloides had relatively large N concentrations during periods when those under the tree crops were small, and this could partly explain why no yield responses to N fertilization were observed at this site.     

外来植物肿柄菊对入侵生境的生态影响及其防控对策

入侵杂草肿柄菊对我国热带和亚热带地区的生物多样性和生态系统功能产生严重影响,且其入侵呈现快速扩张态势。为提高对肿柄菊入侵危害的认识水平,加强对肿柄菊的监测预警及防控,本文通过搜集、整理和分析相关文献资料,总结肿柄菊在世界各国不同地区的入侵情况及其影响,分析肿柄菊在我国入侵的历史、分布现状及其入侵对遗传多样性、物种多样性和生态系统多样性的影响。结合肿柄菊生态入侵防控研究与应用实践,指出肿柄菊入侵防控存在的主要问题和面临的挑战,并针对肿柄菊监测和高效防控技术体系提出高度重视肿柄菊入侵危害影响的分类风险评价、多元化开展肿柄菊入侵扩张的基础科学研究、建立早监测早预防的预警机制和构建针对肿柄菊入侵防控的综合技术体系等防控对策。     

Seasonal dynamics in the concentrations of macronutrients and organic constituents in green and senesced leaves of three agroforestry species in southern Ethiopia

Foliar inputs from indigenous agroforestry trees and shrubs could provide sufficient nutrients and organic matter to sustain crop growth. However, concentrations of foliar nutrients and organic constituents show considerable seasonal, inter- and/or intra-species variations. To determine this variability, green and senesced leaves were sampled during dry and wet seasons from Cordia africana, Albizia gummifera and Milletia ferruginea trees at Wondo Genet, southern Ethiopia. Cordia is a deciduous, non-leguminous tree, while Albizia and Milletia are semi-deciduous and leguminous trees. Leaves were analyzed for concentrations of ash, N, P, K, cellulose, lignin, soluble polyphenols, and condensed tannins. Results from statistical analyses showed significant seasonal variations (P < 0.001) in concentrations of all leaf constituents, except for P and cellulose. Foliar concentrations of ash, N, soluble polyphenols, and condensed tannins were higher during the wet season while those of K and lignin were higher during the dry season. Green leaves had significantly higher (p < 0.001) N and P concentrations than senesced leaves, while senesced leaves had higher concentrations of K, cellulose, soluble polyphenols, and condensed tannins. The ‘ Relative Percentage Changes’ in concentration of N and P in senesced leaves, i.e., their enrichment or depletion with such nutrients relative to those in green leaves, were significantly higher (P < 0.001) for Cordia than Albizia and Milletia. On the other hand, there was no consistent pattern in the enrichment or depletion of senesced leaves with organic constituents, but these leaves were in most cases more enriched with organic constituents than green leaves. Over all, the percentage depletion or enrichment ranged from about 8% to 38% for N; 24% to 63% for P; −141% to 48% for K; −44% to 15% for cellulose; −44% to 51% for lignin; −203% to −61% for soluble polyphenols; and −290% to 11% for condensed tannins. It was concluded that variations in species and life-form (legume versus non-legume), season, and developmental stage of leaves could affect the quality of organic material from agroforestry species, which has important implications for management of organic residues in tropical agricultural systems.     

外来入侵植物肿柄菊在云南的扩散风险研究

预测外来入侵植物的潜在入侵范围以及对不同环境的适应性,能够为外来植物入侵提供预警和预防.该文通过实地调查肿柄菊(Tithonia diversifolia)在云南的入侵范围,以及收集世界其他地区分布点,结合12个关键环境变量,利用MaxEnt模型对肿柄菊适生区范围和环境限制因子进行分析.结果表明:在世界范围适生区分布格...     

Short term effects of radiata pine and selected pasture species on soil organic phosphorus mineralisation

Silvopastoral systems comprise part of the continued expansion of conifer plantings on grassland in New Zealand. Greater understanding of the short term dynamics of soil organic P in such systems will further our knowledge about soil carbon and phosphorus relationships which will enable improved nutrient management in the field. A glasshouse experiment was carried out to examine the short-term effects (36 weeks) of combinations of radiata pine (Pinus radiata), lucerne (Medicago sativa L.) and perennial ryegrass (Lolium perenne L.) grown in the same soil type with a range of carbon (C) and phosphorus (P) levels on plant P uptake and the specific mineralisation rate (SMR). The SMR is defined as net mineralisation rate (i.e. gross mineralisation less microbial and geochemical uptake) and calculated from organic P decline as a percentage of organic P in the original soil before planting. This included an investigation of the effect of tree ectomycorrhizal (EM) hyphae on soil organic P. Plant P uptake was positively correlated with water soluble organic carbon (WSOC) and SMR, which in turn was closely related to soil C levels. The soils with high WSOC and C levels (which also contained high levels of labile inorganic and organic P) enabled high P uptake. Although P uptake was the greatest under radiata pine, the trees tended to deplete inorganic P to a lesser extent than the forages. When tree and forage species were combined, P uptake by forages was similar to when the forages were grown alone. The various soil and plant treatments significantly affected SMR. The two low C soils, showed the greatest organic P mineralisation while a high C soil, which contained significant levels of bicarbonate extracted inorganic P at planting and was under a long established undisturbed pasture, showed the least mineralisation. Trees grown alone showed the greatest SMR, EM hyphae and trees with lucerne were slightly lower than trees alone, while the forages showed the lowest SMR. The findings of this study showed that changes in organic P are strongly influenced by interactions between plant species (radiata pine, lucerne, ryegrass) and soil properties as determined by land use and management.     

三种苦苣苔对石灰土和红壤的适应性分析

为探讨苦苣苔科植物对其岩溶生境的适应性,该研究选取黄花牛耳朵(Primulina lutea)、紫花报春苣苔(Pri.purpurea)和桂林蛛毛苣苔(Paraboea guilinensis)三种苦苣苔科植物,将其栽种在石灰土及红壤两种不同类型的土壤中,观测记录其生长性状并对其叶片元素含量进行测定和比较。植株采集过程中,同时采集自然生境中三种苦苣苔科植物叶片及取样植物基部土壤,并对叶片及土壤元素的含量进行测定,作为今后苗圃试验的参照。结果表明:三种苦苣苔科植物在两种土壤上的生长状况及适应性具有差异,其在石灰土上生长良好,在红壤上生长较差;在两种不同土壤中,除N外,桂林蛛毛苣苔的叶片其他元素(P、K、Mn、Mg、Ca、Zn、Cu)差异极显著(P0.01);除P外,紫花报春苣苔的叶片其他元素(N、K、Mn、Mg、Ca、Zn、Cu)差异极显著(P0.01);除N、Cu、Ca外,黄花牛耳朵的叶片元素(P、K、Mn、Mg、Zn)差异极显著(P0.01);三种植物的叶片元素比值,除少数值没有差异外,大部分指标差异都极显著;对叶片元素与栽培土壤元素的相关性分析,发现植物叶片Mn元素与土壤中N、Ca、Mg、Zn、Mn、有机质含量等呈正相关,土壤P元素与叶片中N、P元素呈正相关,而与叶片中Zn元素呈负相关关系。在其他栽培条件一致的条件下,土壤因素及物种差别是造成黄花牛耳朵、紫花报春苣苔和桂林蛛毛苣苔适应性产生差异的主要原因。     

陕西省3种主要树种叶片、凋落物和土壤N、P化学计量特征

以陕西省29个县(市)39个样点的刺槐、辽东栎和油松林为研究对象,分析比较不同树种乔木叶片、凋落物与土壤N、P化学计量特征及其与经纬度、海拔、年均温度和年降水等环境因子间关系的异同以及三者之间可能存在的关系,以期为认识陕西省主要森林树种养分限制状况、制定合理的植被管理和恢复措施提供理论依据。结果表明:3树种叶片N、P含量及比值均为刺槐辽东栎油松,与叶片相比,凋落物中N、P含量变化幅度较小,为刺槐辽东栎油松,N∶P比值为油松辽东栎刺槐。10—20 cm与0—10 cm土层相比,3树种中除辽东栎中P含量差异不显著外,其它指标N、P含量及N∶P比值均显著下降(P0.05)。刺槐、辽东栎和油松叶片N、P含量与土壤N、P含量均没有显著相关性,以刺槐、辽东栎和油松3种植物叶片为总体来说,P含量与土壤P含量显著正相关(P0.05)。叶片N、P含量均大致表现出随着年均温度和年降水的增加而增加,随着经纬度的增加而降低的趋势,这一点在刺槐叶中最为明显。凋落物N含量随着年均温度和年降水的增加而增加,随着纬度和经度的增加而降低;P含量随着年降水和经度的增加而降低;N∶P比值随着年均温度和年降水的增加而增加,随着纬度的增加而降低。研究区内,土壤N、P含量随着纬度、海拔的增加和年均温度、年降水、经度的降低而增加,N∶P比值则呈相反的趋势。3树种土壤N、P含量及N∶P比值中,P含量比N含量受环境影响更大,且0—10 cm和10—20 cm土层N、P含量及N∶P比值与各环境因子的关系基本一致。     

The association between the phytoplankton, Rhopalosolen species (Chlorophyta; Chlorophyceae), and Anopheles gambiae sensu lato (Diptera: Culicidae) larval abundance in western Kenya

Algae are important food resources of the larvae of the African malaria vectors, Anopheles gambiae Giles and Anopheles arabiensis Patton (Anopheles gambiae sensu lato), and other zooplankton, but empirical evidence remains meager about the agal flora in ephemeral water bodies. The animals present in natural aquatic habitats in western Kenya were sampled from July to November 2002 to study abiotic and biotic environmental factors determining A. gambiae sl larval abundance. The five highest concentrations of third and fourth instars and pupae (hereafter referred to as old-stage larvae) were sampled in conjunction with the unicellular epizoic green algae, Rhopalosolen species (Chlorophyta; Chlorophyceae). Canonical correspondence analysis revealed that the presence of Rhopalosolen species was the most important determinant of the animal assemblage. The density of old-stage A. gambiae sl larvae was positively correlated with the presence of Rhopalosolen species, but the density of first and second instars of A. gambiae sl was not. The water bodies with Rhopalosolen sp. yielded larger mosquitoes in spite of the higher density of larvae. We demonstrated that the productivity of water bodies in terms of the larvae of malaria vectors can differ in magnitude depending on the agal flora. We discuss phytoplankton as a regulator of mosquito larval populations.     

Selenium uptake by plants as a function of soil type,organic matter content and pH

In pots containing sandy soils at two levels (pH 5 and 7) to which 0.5 mg Se L^-1 soil had been added, an increase in the proportion of clay soil or peat soil led to a decrease in the uptake of Se by spring wheat grain (Triticum aestivum L., var. Drabant) and winter rape plants (Brassica napus L., var. Emil). The effect was most pronounced for the smallest additions of clay and peat soils. Differences in Se uptake between the two pH levels were greatest in treatments where the additions of clay and peat soils were small. At the high pH, an increase in clay content from 7% to 39% resulted in a decrease in Se uptake of 79% for wheat and 70% for rape. At the low pH, the uptake decreased by 72% and 77%, respectively. At the higher pH, an increase in the content of organic matter from 1.4% to 39% resulted in decreases in Se uptake of 88% for wheat grain and 69% for rape. At the low pH, Se uptake decreased by 63% and 48%, respectively. Adding peat soil to clay soil had little effect on Se uptake. Among the limed, unmixed clay, sand and peat soils to which Se had not been added, uptake was highest from the sandy soil, i.e. 8.3 ng Se/g wheat grain and 42 ng Se/g rape. The lowest uptake rates were obtained in the clay soil, i.e. 3.0 ng Se/g for wheat grain and 9.0 ng Se/g for rape.     

Utilization of organic phosphorus in calcium chloride extracts of soil by barley plants and hydrolysis by acid and alkaline phosphatases

Organic phosphorus is often a major part of total phosphorus in soil solution. The role of this fraction as a P source for plants and the mechanism involved in its transfer from soil to plant is still unclear. We studied the utilization of organic phospharus in 0.01 M calcium chloride extracts by barley and its hydrolysis by isolated acid and alkaline phosphatases. Calcium chloride extracts were used as a nutrient solution in 24 hrs assays. Concentration of organic and inorganic P in equilibrium calcium chloride extracts was 7.8 and 1.8 µmol P L^-1, respectively, which was similar to the soil solution P concentration. When soil microbial biomass was destroyed by autoclaving, organic P concentration increased to 64.8 µmol P L^-1 whereas the inorganic P was hardly changed. Inoculation of the autoclaved soil with non-sterile soil and incubation for 5 days decreased the organic P concentration to 27.9 µmol P L^-1 but did not change inorganic P. In this study barley plants utilized organic P from all extracts. The greatest reduction of organic P concentration occurred in fresh extracts of the autoclaved soil. Inorganic P was depleted to traces in all extracts. Organic P was hydrolyzed by isolated acid and alkaline phosphatases. We conclude that organic P in soil solution is a heterogeneous pool of organic P compounds originating from microbial biomass. Its initial availability to plants was nigh but its susceptibility to phosphatase hydrolysis was quickly reduced but not completely lost.     

The effects of tree rhizodeposition on soil exoenzyme activity, dissolved organic carbon, and nutrient availability in a subalpine forest ecosystem

Previous studies have found that root carbon inputs to the soil can stimulate the mineralization of existing soil carbon (C) pools. It is still uncertain, however, whether this “primed” C is derived from elevated rates of soil organic matter (SOM) decomposition, greater C release from microbial pools, or both. The goal of this research was to determine how the activities of the microbial exoenzymes that control SOM decomposition are affected by root C inputs. This was done by manipulating rhizodeposition with tree girdling in a coniferous subalpine forest in the Rocky Mountains of Colorado, USA, and following changes in the activities of nine exoenzymes involved in decomposition, as well as soil dissolved organic C, dissolved organic and inorganic nitrogen (N), and microbial biomass C and N. We found that rhizodeposition is high in the spring, when the soils are still snow-covered, and that there are large ephemeral populations of microorganisms dependent upon this C. Microbial N acquisition from peptide degradation increased with increases in microbial biomass when rhizodeposition was highest. However, our data indicate that the breakdown of cellulose, lignin, chitin, and organic phosphorus are not affected by springtime increases in soil microbial biomass associated with increases in rhizodeposition. We conclude that the priming of soil C mineralization by rhizodeposition is due to growth of the microbial biomass and an increase in the breakdown of N-rich proteins, but not due to increases in the degradation of plant litter constituents such as cellulose and lignin.     

氮磷比对有机磷环境中微藻砷代谢的生态风险效应北大核心

【目的】探究了以单酯磷D-葡萄糖-6-磷酸二钠(D-glucose-6-disodium phosphate,GP)为唯一磷源时含砷(As5+)水体中不同氮磷质量比对铜绿微囊藻(Microcystis aeruginosa)生长及砷代谢和微囊藻毒素(microcystins,MCs)释出的影响。【方法】将氮磷饥饿状态的藻细胞于含砷水体中不同氮磷比条件下进行实验,通过测定藻细胞密度(OD680)、叶绿素a(chlorophyll a,Chla)、实际光合产率(Yield)、超氧化物歧化酶(superoxide dismutase,SOD)、砷的存在形态以及微囊藻毒素含量,分析该藻在砷胁迫下的生理响应以及砷代谢机制。【结果】氮磷饥饿状态藻细胞对较高GP水平(0.1 mg/L)下的低氮磷比有更好的适应性,较低GP水平(0.02 mg/L)下的高氮磷比能显著促进培养初期藻细胞的OD680、Chla和Yield;SOD在培养初期与末期受氮磷比影响显著。GP环境下铜绿微囊藻经8 d培养后氮磷比为10:0.1介质中的砷表现为以亚砷酸盐(As^(3+))为主,占水体总砷(total arsenic,TAs)含量的78.8%,其余氮磷比环境中仍以As^(5+)为主,藻体砷形态则均以As^(5+)为主,氮磷比为1:0.1时有机砷占藻体TAs比例最高。藻细胞砷代谢受GP水平影响显著,较高GP环境(0.1 mg/L)下砷的代谢总量也更高,氮磷比为10:0.1时砷代谢以As^(5+)的还原和As^(3+)释出为主,低GP环境下(0.02 mg/L)砷代谢的甲基化水平提高。介质中MCs的含量与GP水平有关,较高GP的低氮磷比水体中MCs含量最低。【结论】研究结果对全面了解有机磷源含砷水体中藻华暴发及砷生态风险的科学管控具有重要意义。     

Hydrolysis of organic phosphates by corn and soybean roots

Because of the importance of organic phosphates as sources of P for plants, this work was performed to study the hydrolysis of nine organic phosphates by sterile, intact corn (Zea mays L.) and soybean (Glycine max L.) roots. Results showed that the rates of hydrolysis ofp-nitrophenyl phosphate (PNP) in buffered solutions by roots of three varieties of corn and three varieties of soybean ranged from 13 to 22 μmol PO₄−P g⁻¹ root h⁻¹ and from 2.1 to 2.2 μmol PO₄−P 0.1 g⁻¹ root h⁻¹, respectively. The average rate of hydrolysis of PNP in nonbuffered solutions was 2- to 3-fold lower for corn roots and 6- to 10-fold lower for soybean roots as compared with those obtained with buffered solutions. The orthophosphate released from hydrolysis of organic P compounds in buffered solutions during a 48-h incubation of corn roots showed that the maximum rate of hydrolysis of PNP was 4 to 6 times greater than the commonly used substrates: α- and β-glycerophosphates, phenolphthalein diphosphate, and glucose-6-phosphate. The rates of hydrolysis of glucose-6-phosphate and glucose-1-phosphate were similar and about 6- to 12-fold lower than that of PNP. Phosphoethanolamine and phosphocholine were hydrolyzed slightly, ando-carboxyphenyl phosphate was not hydrolyzed. The rates of hydrolysis of organic P compounds in nonbuffered solutions by corn and soybean roots were 1 to 3 and 1 to 10 times lower than those in buffered solutions, respectively. The trends in rates of hydrolysis by soybean roots of buffered organic P substrates were similar to those observed with corn roots, with the exception of glucose-1-phosphate and phosphoethanolamine.     

Changes in nodule and root biomass of Sesbania sesban and Leucaena leucocephala following coppicing

A pot study was conducted to measure the extent of and determine the factors controlling fine root and nodule shedding following coppicing of Sesbania sesban and Leucaena leucocephala. Fine (<2 mm) root biomass decreased below pre-cutting values, but the decreases were not statistically significant in either species. Living (white) nodule biomass decreased and dead (brown) nodule biomass increased significantly two weeks after cutting in both species. These changes were relatively greater in Sesbania than in Leucaena. In the uncut treatments of both species, fine root and nodule biomass were correlated with leaf biomass, and in the cut treatments, root and nodule biomass returned to near this apparent equilibrium by two weeks after cutting. Stem growth rate per unit leaf area was not different between cut and uncut treatments, nor was it correlated with root:leaf ratios in either species. Leucaena allocated a greater fraction of its total biomass below-ground, and a greater fraction of its below-ground biomass to coarse (>2 mm) roots than Sesbania. These results are consistent with the hypothesis that relatively lower allocation to below-ground storage tissue is the cause for Sesbania's relatively greater sensitivity to cutting.     

Growth,mineral nutrition,organic constituents and rate of photosynthesis inSesbania grandiflora L. grown under saline conditions

Summary Sesbania showed a luxuriant growth in soil with an electrical conductivity of up to 10 m Scm⁻¹. Under saline conditions Na and Cl accumulated at different rates in the plants. Accumulation of these ions in the leaf rachis compared with leaflets appears to be an adaptive feature of this legume. Maintenance of an optimum K level and accumulation of Ca are also indicative of a salt-tolerance mechanism. Accumulation of Fe in the roots of salt-stressed plants is noteworthy. Organic acids and soluble sugars which accumulated in plants under stress condition may play a role in osmotic adjustment. The level of proline, however, remained unaltered. Though the chlorophyll content of the leaves decreased, the photosynthetic rate was found to be enhanced by saline conditions. The probable relationships between these changes and the salt tolerance mechanism in the plant have been discussed.     

土壤拟杆菌与梭菌分解多糖类有机物质的研究进展与展望

土壤有机质的分解需要一系列环境微生物的参与,包括上游微生物进行有机质的初步分解、中间微生物的再分解利用和下游微生物的最终代谢过程.由微生物分泌的胞外水解酶将复杂有机质转变为简单有机质是整个分解过程的关键.近年来,针对肠道复杂有机质的微生物分解研究取得了显著进展,本文首先比较了有机质分解过程在人体肠道、反刍动物瘤胃和土壤...     

The dynamics and ecology of exotic tropical species in floating plant mats: Lake Naivasha,Kenya

The floating water fern Salvinia molesta has occurred in Lake Naivasha since the early 1960s and during this period has obstructed fishing activities and navigation. In recent years the extent of Salvinia has declined markedly. Since 1988, a second floating weed, Eichhornia crassipes (water hyacinth), has colonised the lake. Salvinia formed large mats around the edges, bays and lagoons and had very few plant or invertebrate species associated with it. Mats of E. crassipes, however, support a greater abundance and variety of animal and plant species, which shows a relationship with mat size. This seems to have facilitated plant succession but without a zonation typical of the classic hydroseral sequence for the lake.     

Competition in tree row agroforestry systems. 2. Distribution, dynamics and uptake of soil inorganic N

The effect of tree row species on the distribution of soil inorganic N and the biomass growth and N uptake of trees and crops was investigated beneath a Grevillea robustaA. Cunn. ex R. Br. (grevillea) tree row and Senna spectabilisDC. (senna) hedgerow grown with Zea mays L. (maize) and a sole maize crop, during one cropping season. The hypothesis was that a tree with a large nutrient uptake would have a greater competitive effect upon coexisting plants than a tree that takes up less and internally cycles nutrients. The field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya. Soil nitrate and ammonium were measured to 300 cm depth and 525 cm distance from the tree rows, before and after maize cropping. Ammonium concentrations were small and did not change significantly during the cropping season. There was > 8 mg nitrate kg^–1 in the upper 60 cm and at 90–180 cm depth at the start of the season, except within 300 cm of the senna hedgerow where concentrations were smaller. During the season, nitrate in the grevillea-maize system only decreased in the upper 60 cm, whereas nitrate decreased at almost every depth and distance from the senna hedgerow. Inorganic N (nitrate plus ammonium) decreased by 94 kg ha^–1 in the senna-maize system and 33 kg ha^–1 in the grevillea-maize system.The aboveground N content of the trees increased by 23 kg ha^–1 for grevillea and 39 kg ha^–1 for senna. Nitrogen uptake by maize was 85 kg ha^–1 when grown with grevillea and 65 kg ha^–1 with senna. Assuming a mineralisation input of 50 kg N ha^–1season^–1, the decrease in inorganic soil N approximately equalled plant N uptake in the grevillea-maize system, but exceeded that in the senna-maize system. Pruning and litter fall removed about 14 kg N ha^–1 a^–1 from grevillea, and > 75 kg N ha^–1 a^–1 from senna. The removal of pruned material from an agroforestry system may lead to nutrient mining and a decline in productivity.