水库磷收支及其调控措施研究进展

磷是主要的富营养化限制性因子,其收支途径和调控措施的研究是水库生态安全的重要保障.为推动水库磷的系统研究,进一步完善磷调控体系,本文归纳了水库磷的收支途径,分析了不同因素对磷收支的影响,并对常规调控措施进行了梳理.水库磷输入的主要来源包括上游河流、地表径流、生活生产污染、养殖投入、大气沉降和底泥释放.其中上游河流的输入是水库磷最大的来源.磷主要的输出途径为泄洪或放水,并受不同排水方式的影响.另外生物收获也会输出部分磷.水库磷收支平衡受水库功能、水文条件和水体理化性质等因素的影响,不同季节和地域的收支情况差异很大.在磷的调控治理上,人工湿地、前置库、底泥疏浚和生物操纵法是目前采取的主要措施.为有效去除磷,需根据水库不同生态特性和水质管理目标,选择或结合多种不同的调控方式.因此,未来的研究应突出水库生态特性,进一步对水库磷收支及其影响因素进行系统分析,并开展水库磷收支与其他营养物质之间影响关系的研究,在此基础上构建科学完善的管理体系,是有效削减水库磷的根本保障.     

红壤小流域坡地不同利用方式对土壤磷素流失的影响

以浙江德清县排溪冲小流域生态系统为实例,采用定位土芯Ｅｕ示踪和无界径流小区法研究了流域内坡地不同利用方式对土壤磷素流失的影响,结果表明：（１）坡地不同利用方式磷素流失差异明显,流失最严重提是竹园,其次是旱地作物和新建果园,再次是幼龄茶园,林地和未开发利用的荒草地磷素的流失较轻;（２）泥沙结合态磷（Ｐａｒｔｉｃｕｌａｔｅ ｐｈｏｓｐｈｏｒｕｓ,ＰＰ）随泥沙迁移流失是红壤坡地磷素流的的主要形式,６种利     

Factors influencing fractional phosphorus composition in sediments of Spanish reservoirs

Total phosphorus in sediment (P_sed) and its fractional composition (reactive phosphate extracted with NaOH, NaOH-RP, reactive phosphate extracted with HCl, HCl-RP, and residual phosphate, residual-P) have been determined in superficial sediments of 43 Spanish reservoirs located in different limnological regions and with different trophic states. Data were evaluated by statistical analysis to examine the influence of regional distribution and trophic status. Relations with calcium, manganese, iron and aluminium contents have also been studied.In the western part of Spain, reservoirs presented the highest values on average of P_sed, NaOH-RP and residual-P (1296, 328 and 877 µg g^–1 dw., respectively) and the lowest values of HCl-RP (91.0 µg g^–1 dw.). The main phosphorus fractions were residual-P (> 50%) and NaOH-RP (>10%). In the eastern area, P_sed NaOH-RP and residual-P attained the lowest values on average (502, 4 and 330 µg g^–1 dw., respectively), whereas HCl-RP presented the highest values (167 µg g^–1 dw.). The main fractions were residual-P (> 50%) and HCl-RP (> 25%).Trophic status seemed to be a secondary factor controlling P_sed. The highest contents of P_sed were found in eutroohic reservoirs, but only when those of the same region were compared, and the statistical significance (ANOVA F test) of the observed differences was very small (p < 0.057).     

长兴县合溪水库集雨区苗木地不同施肥管理模式对径流磷素流失的影响

为保护主要饮用水源合溪水库水质,长兴县将合溪水库集雨区农业种植结构向苗木种植方式调整,但缺乏优化施肥管理模式。针对该问题,选取集雨区典型苗木种植地进行径流小区定位试验,设置了撒施(对照)、地膜覆盖、秸秆覆盖、区外设置植草缓冲带、条施、穴施6个处理,研究不同施肥管理模式下苗木地径流磷素流失特征,筛选最优施肥管理模式,实现最大化的削减农田磷素流失。结果表明:不同施肥管理模式下,苗木地地表径流年总磷素径流流失通量大小顺序为:撒施(对照)地膜覆盖条施穴施区外植草缓冲带秸秆覆盖,撒施(对照)、地膜覆盖、条施、穴施、区外设置植草缓冲带和秸秆覆盖六种施肥管理模式下的年总磷素径流流失通量分别为:9.60、9.14、5.49、4.44、2.48、1.37 kg hm-2a-1,说明穴施、区外设置植草缓冲带和秸秆覆盖是较佳的施肥管理模式,若将其优化组合将显著消减合溪水库集雨区苗木地径流磷素流失及其对合溪水库水质的潜在影响。此外,不同施肥管理模式下,苗木地径流水样磷素流失的首要形态均为颗粒态磷,其次为溶解态磷,其中,撒施(对照)、地膜覆盖、秸秆覆盖、区外植草缓冲带、条施和穴施小区径流年均流失的颗粒态磷占流失总磷的比例分别是66.09%、70.69%、67.97%、71.63%、68.11%和67.87%。     

Effects of phosphate-solubilizing bacteria application on soil phosphorus availability in coal mining subsidence area in Shanxi

The effect of phosphate-solubilizing bacteria (PSB) application on phosphorus (P) availability in reclaimed soil in coal mining subsidence region was investigated. Seven treatments were carried out including control, chicken manure (CM), PSB, PSB + tricalcium phosphate (TCP), CM?+?TCP, PSB?+?ground phosphate rock (GPR) and CM?+?GPR. The results showed soil Olsen-P concentration and phosphatase level as well as the yield of pakchoi (Brassica chinensis L.) were significantly higher in PSB application treatments compared to the corresponding CM application treatments. Soil phosphatase, invertase and urease contents were increased most significantly in PSB treatment, 1.18-, 1.31- and 2.32-fold higher than those in the control, respectively. Soil Ca₂-P, Ca₈-P, Fe-P and Al-P concentrations exhibited the greatest increases in PSB?+?TCP treatment, while occluded-P showed minor changes in different treatments. Application of PSB fertilizer reduced the transformation of Olsen-P to Ca₁₀-P, thus increasing P availability in reclaimed soil of coal mining subsidence area.     

施磷对间套作玉米叶面积指数、干物质积累分配及磷肥利用效率的影响

通过2011、2012年连续两年田间试验, 研究了“小麦/玉米/大豆”间套作体系中不同施磷处理（小麦0、45、90、135、180 kg P₂O₅·hm^-2, 记为WP₀、WP₁、WP₂、WP₃、WP₄; 玉米0、37.5、75、112.5、150 kg P₂O₅·hm^-2, 记为MP₀、MP₁、MP₂、MP₃、MP₄）对玉米叶面积指数、干物质积累动态和磷肥利用效率的影响.结果表明: 在玉米与小麦共生期, 施磷明显增加了玉米叶面积指数（LAI）和群体光合势（LAD）, 促进了茎、叶干物质的积累（DMA）; 玉米拔节以后LAI、LAD、生长率（CGR）和DMA均随磷肥施用量的增加呈先增加后减少的趋势, 最大值都出现在MP₂或MP₃处理; 玉米生殖生长期营养器官的干物质输出随着磷肥施用量的增加而增加.玉米籽粒产量和体系总产量均随磷肥施用量的增加先增大后减少,都以P₃处理为最高,分别为6588和11955 kg·hm^-2.玉米磷肥表观利用率（PARE）以MP₂处理最高（26.3%）,分别比MP₁（14.4%）、MP₃（19.0%）、MP₄（10.4%）处理高82.6%、38.4%和152.9%.综上,在麦/玉/豆间套作体系中,适量施用磷肥可促进玉米的生长、减轻小麦对玉米的影响,同时可提高玉米当季磷肥利用率,玉米的磷肥施用量在75～112.5 kg P₂O₅·hm^-2为宜.     

Examination through a principal component analysis of the phosphorus exchanges between sediments and water in trophically different reservoirs

Phosphorus exchange at the sediment-water interface coupled with several parameters were assessed in several reservoirs with geologically different catchment basins and different trophic status in Morocco and France.The results showed that these exchanges were regulated by a combination of factors: physical chemical variability of the environment, the geological composition of catchment basins and the trophic status of the lake.In the hypereutrophic Villerest, iron-bound phosphorus is the major form of phosphorus trapped by the sediment whereas, in Moroccan reservoirs, calcium-bound phosphorus prevailed.We suggest that a drastic control of phosphorus inputs into the waters must be done through a large program of dephosphatization of tributaries to avoid Microcystis aeruginosa bloom formation in Villerest (Aleya et al., 1993) and calcium-bound phosphorus dissociation in Moroccan reservoirs with upward release of bioavailable phosphorus.

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Résumé Les échanges de phosphore au niveau de l'interface eau-sédiment couplés á la distribution temporelle de divers éléments chimiques et biologiques ont été étudiés dans divers réservoirs de niveaux trophiques différents, au Maroc et en France.Nos résultats mettent clairement en évidence une influence directe de l'environnement physico-chimique, de la nature géologique des bassins versants et de l'état trophique du lac sur la dynamique du phosphore au sein de cette interface.De plus, il apparait que dans le lac hypereutrophe de Villerest (Roanne, France), le phosphore est majoritairement complexé au fer alors que dans les retenues marocaines, ce sont les complexes phosphore-calcium qui prédominent.Nous préconisons un contrôle drastique des apports en phosphore á travers l'installation et la multiplication d'unités de déphosphatation afin d'éviter d'une part, la prolifération massive de la Cyanobactérie Microcystis aeruginosa á Villetest (Aleya et al., 1994) et d'autre part la dissociation des complexes phhosphore-calcium au sein des retenues marocaines avec libération de phosphore biodisponible.

     

喀斯特不同土地利用类型裂隙土壤有机碳及磷素赋存特征

以喀斯特石漠化区不同土地利用类型裂隙土壤为研究对象,运用野外调查和实验室内分析相结合方法,探索其有机碳和磷素含量变化及其赋存特征,以期为喀斯特地区开展石漠化治理和植被恢复提供理论依据。结果表明：4种土地利用类型裂隙土层土壤有机碳赋存含量变化范围为16.067-39.436 g/kg,总体呈现出随土层深度增加而降低的变化趋势;土壤全磷、有效磷赋存含量变化范围分别为0.093-0.274 g/kg、3.836-8.025 mg/kg,整体上在裂隙表层显著高于其他土层,具有上层高下层低的特点;同时,土壤有机碳和磷素总体上属于中度变异。乔木林地和灌丛地的C/P总体上表现出随土层的加深而减小的趋势,而草地和撂荒地先减小后增加,土壤C/P在各土地利用类型裂隙土层变化范围为86.499-268.343,磷的有效性较低;随土层深度的增加,各土地利用类型裂隙土壤有机碳、全磷和有效磷含量逐渐在减少,有机碳对土壤碳磷比、有效磷含量变化有一定影响。     

Drainage history and land use pattern of a Swedish river system — their importance for understanding nitrogen and phosphorus load

During the 19th and the first half of the 20th century, approximately 300 km² of lakes and wetlands, representing 29% of the River Kavlingean catchment in Southern Sweden, were drained to make land available for agriculture. Published accounts of nutrient loads from the catchment indicated that until the mid 20th century, factories and urban point sources were the major contributors of both nitrogen and phosphorus. By the middle of the 20th century, the construction of sewage treatment plants had effectively reduced phosphorus pollution. Concurrently, the land drained in the previous century underwent a more intense cultivation, with productivity being maintained by commercial fertilizers. Subsequently, net nutrient loads from agriculture continued to increase, reaching an annual load of 2652 tons total-nitrogen and 70 tons total-phosphorus for the River Kävlingeån. Whilst high nutrient leakage from agricultural watersheds may be a problem which is only recently recognized, it had its origins in nearly a hundred years of commonly accepted agricultural policy.To assess the importance of agriculture as the major source of nutrients to the River Kävlingeån system, three tributary catchment areas, differing in terms of their land use patterns (high, medium and low intensity of agricultural use), were studied and compared with literature figures. Results indicated that agricultural nutrient loss areal coefficients were substantially higher than the literature figures, demonstrating the role of agriculture as source of nutrients to the River Kävlingeån system. The agricultural land use policies of the last fifty years were revealed to be most important with regard to this role. Of such land use policies, the cultivation of the last 10–15% of land employed for agricultural use (primarily riparian ecotones) may be of most significance. The literature indicates that intense agricultural use of this final 10–15% may account for a ca. 50% increase in nitrogen loss. This suggests that one solution to the problem of agricultural diffuse pollution may lie in the restoration and sustainable management of riparian ecotones of agricultural streams.     

乌江流域不同营养水平的梯级水库沉积物中磷形态特征

分析了乌江干流及支流不同营养水平的梯级水库(东风-贫营养、红岩-中营养、乌江渡-富营养)沉积物粒度的垂向分布,并以改进的连续提取法(SEDEX)调查了不同磷形态垂向分布特征。结果表明:3水库沉积物粒径均随深度的增加而减小,东风水库粒径最大,红岩水库其次,处于下游的乌江渡水库粒径最小;乌江渡、红岩和东风水库沉积物总磷(TP)的平均含量分别为2.07、1.22和1.11mg.g-1,总无机磷分别占到总磷53.8%、44.5%和81.9%,有机磷占TP的41.9%、55.5%和18.1%;3水库各形态磷所占比例不同,其中,乌江渡水库交换态磷所占比例相对较大,与其富营养化状态一致;乌江渡水库沉积物表层10cm,各形态磷含量随深度的增加呈下降趋势,红岩水库除交换态磷,其他各形态磷变化趋势不明显。东风水库各形态磷变化趋势不明显;河流梯级开发使下游水库颗粒物粒径变小,从而使下游水体中的磷更易吸附和释放,下游水库沉积物与水体交换也更强烈,除总磷外,交换态磷、自生磷和有机磷也可较好地反映水库富营养程度及演变。     

基于改进TOPSIS方法的三峡库区生态敏感区土地利用系统健康评价

土地利用系统健康评价研究能够有效引导土地合理利用,协调城市发展与自然生态保护之间的矛盾。构建基于PSR模型的土地利用系统健康评价指标体系,并采用改进TOPSIS方法对三峡库区生态敏感区的典型区域—重庆市进行实证分析。结果表明:1)研究区土地利用系统健康综合分值整体呈现T型带状分布格局,可分为四个健康等级,即健康、临界健康、不健康、病态。2)渝东北、渝东南和重庆市西南片区部分地区因其土地生态系统脆弱敏感,土地利用风险性大和生态系统稳定性差,土地生态系统呈现病态和不健康状态,属于高风险-高压力区域;重庆市主城区环线区域因其属于城市核心拓展区和人类活动频繁区域,人口压力指数和土地利用压力指数较大,土地利用风险性较小,健康度较为良好,是低风险-中度压力区域。3)PSR模型能够较好地改变现有研究主要关注自然资源环境的状况,更准确地反映土地利用系统健康的各要素之间的关系和影响土地生态系统健康的关键因素,为三峡库区生态敏感区土地利用系统健康状态起到一定的预警作用。4)以改进TOPsis方法计算土地利用系统健康指数,消除了不同指标量纲的影响,并能充分利用原始数据的信息,能充分反映各方案之间的差距,客观真实的反映实际情况。5)为保障三峡库区生态敏感区土地利用系统的健康发展,应加强土地利用规划与调整,控制人类过度开发,维持生态系统正常功能。     

供磷水平及方式对杉木幼苗根系生长和磷利用效率的影响

为研究杉木幼苗根系生长、形态学指标及养分利用效率对土壤磷素异质分布的响应规律,选择杉木种子园单株采种培育的半同胞家系实生幼苗为研究对象,采用室内沙培控磷盆栽试验,设计低浓度供磷(8 mg/kg KH₂PO₄)、正常供磷(16 mg/kg KH₂PO₄)和高浓度供磷(32 mg/kg KH₂PO₄)3个供磷水平,每个供磷水平分别采用2种供磷方式(局部供磷和均匀供磷)进行根部施磷。结果表明:(1)从供磷水平来看,低浓度供磷下的杉木根长、根系生物量、根冠比、根系及全株的磷素利用效率均显著大于正常供磷和高浓度供磷,而根平均直径相反;随着供磷水平的提高,杉木苗高和地上部生物量无显著差异,而比根长表现出逐渐降低的趋势。(2)从供磷方式来看,局部供磷处理的杉木苗高、根长、根系表面积、比根长、地上部生物量、根系及全株的磷素利用效率均显著大于均匀供磷处理,而根平均直径和根冠比则相反。总体上,低浓度局部供磷处理下杉木可明显增强其根系的形态可塑性,从而优化根系在养分异质土壤里的...     

排水对小兴安岭森林沼泽湿地溶解性有机碳和有效氮磷的影响

选取不同排水年限的兴安落叶松人工林湿地(1974年排水、1985年排水、1992年排水、2003年排水)和天然森林沼泽湿地(兴安落叶松沼泽湿地)为研究对象,探讨排水对小兴安岭森林沼泽湿地土壤溶解性有机碳(DOC)和有效氮磷的影响。结果表明,天然沼泽排水后,在土壤垂直剖面上,不同排水年限的森林湿地与天然沼泽湿地的土壤溶解性有机碳含量均呈递减变化。与天然森林沼泽湿地相比,排水湿地各土层DOC含量均显著低于天然沼泽湿地(P0.05)。天然森林沼泽,表层(0—10 cm)的土壤SOC含量、DOC/SOC、土壤有效氮含量均大于排水森林沼泽,但是有效磷含量却低于排水森林沼泽(P0.05)。在土壤表层(0—10 cm),排水年限与DOC、SOC、DOC/SOC、土壤有效氮呈显著性负相关,与有效磷呈显著性正相关(P0.05)。天然沼泽排水后,表层(0—10 cm)土壤的DOC含量与有效氮(铵态氮、硝态氮)含量成正比,与有效磷含量成反比(P0.05)。     

Iron:phosphorus ratio in surface sediment as an indicator of phosphate release from aerobic sediments in shallow lakes

Analysis of Danish lakes showed that both mean winter and mean summer concentrations of lake water total phosphorus in the trophogenic zone correlated negatively with the total iron to total phosphorus ratio (Fe:P) in surface sediments. No correlation was found between the water total phosphorus concentration and either the sediment phosphorus concentration alone or with sediment calcium concentration. The increase in total phosphorus from winter to summer, which is partly a function of net internal P-loading, was lowest in lakes with high Fe:P ratios in the surface sediment.A study of aerobic sediments from fifteen lakes, selected as representative of Danish lakes with respect to the sediment Fe and phosphorus content, showed that the release of soluble reactive phosphorus was negatively correlated with the surface sediment Fe:P ratio. Analysis of phosphate adsorption properties of surface sediment from 12 lakes revealed that the capability of aerobic sediments to buffer phosphate concentration correlated with the Fe:P ratio while the maximum adsorption capacity correlated with total iron. Thus, the Fe:P ratio may provide a measure of free sorption sites for orthophosphate ions on iron hydroxyoxide surfaces.The results indicate that provided the Fe:P ratio is above 15 (by weight) it may be possible to control internal P-loading by keeping the surface sediment oxidized. Since the Fe:P ratio is easy to measure, it may be a useful tool in the management of shallow lakes.     

兴安落叶松林火烧迹地土壤有效磷与土壤微生物生物量磷时空演变特征

野火是大兴安岭活跃的生态干扰因子,显著影响火烧迹地土壤有效磷(AP, Available Phosphorus)和土壤微生物生物量磷(MBP, Microbial Biomass Phosphorus),本文旨在了解兴安落叶松林火烧迹地AP、MBP的时空演变特征,并在此基础上探究两者间的偶联机制。采用“以空间换时间”的研究方法,于大兴安岭塔河地区兴安落叶松林火烧迹地选取实验样地,于未过火兴安落叶松林选取对照样地,踏查每个样地的海拔、坡度、坡向、坡位信息,测定火烧迹地土壤AP、MBP含量,分析兴安落叶松林火烧迹地AP与MBP的时空演变特征。火干扰后,火烧迹地土壤AP、MBP含量均随恢复时间表现出先减少后增加的趋势,恢复初期火烧迹地MBP含量显著低于未过火样地,AP含量显著高于未过火样地(P<0.05);不同海拔火烧迹地AP、MBP含量差异显著(P<0.05),不同海拔未过火样地AP、MBP含量均无显著差异(P>0.05)。火烧迹地土壤MBP、AP的随机森林回归模型的模型总解释度约为84%,而未过火样地的模型总解释度约为60%,两个模型均达到了极显著水平(P<0.0...     

The influence of land use on the sedimentation of the river delta in the Kyrönjoki drainage basin

Sediment surface samples (0–2 cm) from 66 sites, and longer cores (up to 540 cm) from 9 sites in the estuary of the river Kyrönjoki, Western Finland, were analysed for organic content, P, Fe, Mn, Pb, Cd, Cu and Zn. One core was dated on the basis of annual laminations.Chemical analyses of the cores showed that organic matter and heavy metal content have increased in recent decades. The heavy metal content was clearly lower than in areas polluted by industry. The sedimentation rate in the delta increased between the 1930s and 1950. It decreased in the 1960s, and has been below 1930s levels since 1970. The sedimentation rates of organic matter were fairly stable all through the period measured, even though the organic content increased.The increase in organic matter and the heavy metal content of the sediment in recent decades is evidently due to the increased intensity of agriculture, forestry and peat harvesting in the drainage basin. Drainage of peatlands in particular has increased erosion and the organic sediment load of the river. Reservoir building after 1970 has decreased the sedimentation rates in the delta.     

Effects of phosphorus and nitrogen enrichment on the phytoplankton in a tropical reservoir (Lobo Reservoir,Brazil)

The effects of enrichment with phosphate (0–500 µg. 1^–1) and forms of nitrogen (nitrate, nitrite, ammonia an and urea) (0–3500 µgg. ^–1) on the phytoplankton growth of Lobo Reservoir (Brazil) were studied in July, 1979. Suspended matter, chlorophyll a, cell concentrations and the carotenoid:cchlorophyll ratio were estimated following 14 days of in situ incubation. Phosphate alone caused no significant effects, but enrichment with nitrogen caused a substantial increase on the growth of phytoplankton. Comparison between the different forms of nitrogen showed insignificant effects after their additions with 350 µg. ^–1 and in combination with phosphate. However, when nitrogen was added in large quantities (3 500 µg. ^–1), significant differences between the nitrogeneous forms were found, with urea causing the strongest effect. In July, nitrogen is mhe main limiting nutrient to phytoplankton growth of Lobo Reservoir.Supported by CNPq and FAPESP.     

中亚热带森林更新方式对土壤磷素的影响

为了深入了解磷(P)在中亚热带森林生态系统内的有效性,在三明市梅列区陈大采育场黄坑工区,选择天然林采伐后采取不同更新方式的多种森林,以米槠天然林为对照,研究森林不同更新方式对中亚热带森林土壤全磷、有效磷及可溶性有机磷的影响。结果显示:在0—100 cm土层,(1)土壤全磷平均含量大小顺序依次为米槠天然林(NF)(0.49±0.09)g/kg,米槠轻度干扰人促更新林(LAR)(0.35±0.04)g/kg,米槠强度干扰人促更新林(HAR)(0.34±0.03)g/kg,马尾松人工林(PIM)(0.32±0.02)g/kg,杉木人工林(CUL)(0.3±0.03)g/kg,人促更新林比人工林高,NF显著高于其它的林分(P0.05);(2)土壤有效磷(Na HCO3提取)平均含量大小顺序依次为NF(0.41±0.39)mg/kg,LAR(0.26±0.2)mg/kg,HAR(0.23±0.16)mg/kg,PIM(0.17±0.05)mg/kg,CUL(0.13±0.06)mg/kg,NF显著高于其它林分,LAR显著比人工林高(P0.05)。(3)在0—10 cm土层,各林分可溶性有机磷含量在夏季最高,冬季最低,温度和降水量的季节变化是影响其重要因子之一;同一季节,人促更新林比人工林高,NF显著高于人工林(P0.05)。结果表明,全磷、有效磷和可溶性有机磷含量随人为干扰强度的增强呈降低趋势,其与年凋落物量和土壤有机碳储量呈显著正相关,与土壤容重呈显著负相关,全磷和有效磷在土壤剖面呈表聚性特征。相比于人工林经营,采取人促天然更新的方式,更有利于中亚热带森林养分的贮存和转化,有利于森林的长期经营和管理。     

Effects of fertility management strategies on phosphorus bioavailability in four West African soils

Low phosphorus (P) in acid sandy soils of the West African Sudano-Sahelian zone is a major limitation to crop growth. To compare treatment effects on total dry matter (TDM) of crops and plant available P (P-Bray and isotopically exchangeable P), field experiments were carried out for 2 years at four sites where annual rainfall ranged from 560 to 850 mm and topsoil pH varied between 4.2 and 5.6. Main treatments were: (i) crop residue (CR) mulch at 500 and 2000 kg ha^–1, (ii) eight different rates and sources of P and (iii) cereal/legume rotations including millet (Pennisetum glaucum L.), sorghum [Sorghum bicolor (L.) Moench], cowpea (Vigna unguiculata Walp.) and groundnut (Arachis hypogaea L.). For the two Sahelian sites with large CR-induced differences in TDM, mulching did not modify significantly the soils' buffering capacity for phosphate ions but led to large increases in the intensity factor (C_P) and quantity of directly available soil P (E _1min). In the wetter Sudanian zone lacking effects of CR mulching on TDM mirrored a decline of E _1min with CR. Broadcast application of soluble single superphosphate (SSP) at 13 kg P ha^–1 led to large increases in C _P and quantity of E _1min at all sites which translated in respective TDM increases. The high agronomic efficiency of SSP placement (4 kg P ha^–1) across sites could be explained by consistent increases in the quantity factor which confirms the power of the isotopic exchange method in explaining management effects on crop growth across the region.     

Contribution of dissolved calcium and magnesium to phytoplanktonic particulate phosphorus concentration at the heads of two river reservoirs

At the heads of two river reservoirs, the Ishitegawa Dam and the Nomura Dam Reservoirs in Japan, the concentrations of phytoplanktonic particulate phosphorus (PP) were compared with those of dissolved calcium (Ca) and magnesium (Mg), using multiple regression analyses on the data taken from samples which registered more than 6.0 µg l^–1 in chlorophyll a concentration. Of the 27 monthly samples, 16 from the Ishitegawa Reservoir and 17 from the Nomura Reservoir were used. A significant regression line, logPP = k ₁F + k ₂, was obtained, where k ₁ (> 0) and k ₂ were constants and F (named the Ca-Mg index) consisted of log(Ca/Mg) – 0.5 log(ca + Mg) in mol concentration in Ca and Mg, in common with the two reservoirs (r ² = 0.730 & 0.913).