长期施肥黄绵土有效磷含量演变及其与磷素平衡和作物产量的关系

土壤有效磷(Olsen-P)含量的变化过程及其与土壤磷素平衡和作物产量的关系是科学推荐施磷的基础.本文通过设置于黄土高原黄绵土区持续34年(1981—2015)的长期定位试验,研究了长期不同施肥处理对作物磷素携出量、土壤磷素平衡、土壤Olsen-P含量的影响及其演变过程,同时对土壤Olsen-P含量与磷素平衡和作物籽粒产量的相关关系进行了分析.试验采用裂区设计,主处理为施用有机肥(M)和不施用有机肥,副处理为不施化肥(CK)、单施氮肥(N)、氮磷肥配合施用(NP)和氮磷钾肥配合施用(NPK).结果表明: 不同施肥处理和作物类型对磷素携出量和磷素平衡都有显著影响.CK、N、NP、NPK、M、MN、MNP 和MNPK处理小麦的磷素携出量多年平均值为8.63、10.64、16.22、16.21、16.25、17.83、20.39、20.27 kg·hm^-2,而油菜为4.40、8.38、15.08、15.71、10.52、11.23、17.96、17.66 kg·hm^-2,小麦的携出量略高于油菜.土壤磷素盈亏量与磷素投入量呈显著正相关,土壤磷素盈余为零,种植小麦的最小土壤磷素投入量为10.47 kg·hm^-2,而油菜为6.97 kg·hm^-2.土壤磷素盈亏量显著影响土壤有效磷的变化过程.长期不施磷的CK和N处理,土壤有效磷含量随试验年限延长而逐渐降低,年均分别降低0.16和0.15 mg·kg^-1,而NP、NPK、M、MN、MNP和MNPK处理土壤有效磷含量随试验年限的延续而逐渐增加,年均增幅在0.02～0.33 mg·kg^-1.土壤磷素累积盈亏量与土壤有效磷含量间存在显著的正相关关系,不施用有机肥和施有机肥处理可分别用线性模型y=0.012x+9.33和y=0.009x+11.72显著拟合.不施有机肥处理小麦籽粒产量与土壤有效磷含量呈显著正相关,而施有机肥处理两者间的相关性不明显,两者的小麦籽粒产量和土壤有效磷含量可以用线性分段模型拟合.小麦土壤有效磷农学阈值为14.99 kg·hm^-2,油菜籽粒产量虽随土壤速效磷含量增加呈增加的趋势,但相关性不显著,表明在黄土高原黄绵土区,当土壤有效磷含量高于14.99 mg·kg^-1时,种植小麦应减少磷肥施用量或不施磷肥.     

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。     

Seasonal variations on water relations ofAmygdalus communis L. under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L.) of the Garrigues variety were grown in the field drip irrigated and rainfed. Leaf water potential (Ψ) and leaf conductance (g₁) were determined throughout one growing season. Pre-dawn measurement for Ψ in the irrigated treatment was consistent through the growing season, whereas in the rainfed treatment it decreased gradually. Ψ values at midday (Ψ minimum) was closely dependent on atmospheric evaporative demand, and their recovery was quicker in the wet treatment than in the dry. The g₁ values were higher in the wet than dry treatments, decreasing in both cases by leaf ageing. Maximum values for g₁ were reached when evaporative demand was highest in the day. The relationship between Ψ and g₁ revealed a decrease in the hysteresis throughout the growing season, being most marked in the dry treatment. The results highlight the close dependence of Ψ and g₁ on evaporative demand, leaf ageing and irrigtion treatment during the growing season.     

The uptake,translocation and release of phosphorus by Elodea densa

Short-term (16 h) laboratory studies of ³²P uptake by Elodea densa rooted in sediment demonstrated both foliar and root uptake, and that translocation occurred acropetally and basipetally. Root absorption is projected to provide 83–85% of total phosphorus uptake during 12–16 h photoperiod days. Measured foliar uptake and excretion rates suggest that there would be no net leakage of phosphorus into the water by undamaged actively-growing E. densa. Foliar uptake decreased and root uptake increased in the dark relative to rates under light.     

Application of the EUF procedure in sugar beet cultivation

Summary The relationship between the EUF-nutrient fractions in the soil on the one hand and the nutrient uptake of sugar beet as well as root yield and quality (polarization, α-amino N etc.) on the other is described on the basis of results obtained over several years in surveys conducted in farmers' fields (5000–6000 fields under sugar beet per year) and in field experiments (25–35 sites per year). Statistically significant close correlations with the respective parameters were found for the following EUF nutrient fractions: EUF-NO₃, EUF-P, EUF-K, EUF-Na, EUF-B and EUF-Mn. Within five years it was possible to determine the EUF-nutrient values which are required for the production of 9 t sugar/ha. These EUF values are the following: Ca: 65–70 mg/100 g at 20°C K: 11–15 mg/100 g at 20°C (depending on the clay content) Mg: 3–5 mg/100 g at 20°C Na: 2–3 mg/100 g at 20°C P: 1.4–1.6 mg/100 g at 20°C For calculation of the N fertilizer requirements of sugar beet it is suggested to use the sum of the EUF-extractable N amounts. It was found in Austria, Yugoslavia and Denmark over a period of 3 years that the EUF-N value of 1 mg/100 g soil determined between June and September was equivalent to 40 kg N/ha. If, for example, the analysed soil contains 3 mg EUF-N/100 g, 3×40=120 kg N/ha will be available to the sugar beet crop in the following year.     

Soil nitrate accumulation, leaching and crop nitrogen use as influenced by fertilization and irrigation in an intensive wheat–maize double cropping system in the North China Plain

There is a growing concern about excessive nitrogen (N) and water use in agricultural systems in North China due to the reduced resource use efficiency and increased groundwater pollution. A two-year experiment with two soil moisture by four N treatments was conducted to investigate the effects of N application rates and soil moisture on soil N dynamics, crop yield, N uptake and use efficiency in an intensive wheat–maize double cropping system (wheat–maize rotation) in the North China Plain. Under the experimental conditions, crop yield of both wheat and maize did␣not␣increase significantly at N rates above 200 kg N ha⁻¹. Nitrogen application rates affected little on ammonium-N (NH₄-N) content in the 0–100 cm soil profiles. Excess nitrate-N (NO₃-N), ranging from 221 kg N ha⁻¹ to 620 kg N ha⁻¹, accumulated in the 0–100 cm soil profile at the end of second rotation in the treatments with N rates of 200 kg N ha⁻¹ and 300 kg N ha⁻¹. In general, maize crop has higher N use efficiency than wheat crop. Higher NO₃-N leaching occurred in maize season than in wheat season due to more water leakage caused by the concentrated summer rainfall. The results of this study indicate that the optimum N rate may be much lower than that used in many areas in the North China Plain given the high level of N already in the soil, and there is great potential for reducing N inputs to increase N use efficiency and to mitigate N leaching into the groundwater. Avoiding excess water leakage through controlled irrigation and matching N application to crop N demand is the key to reduce NO₃-N leaching and maintain crop yield. Such management requires knowledge of crop water and N demand and soil N dynamics as they change with variable climate temporally and spatially. Simulation modeling can capture those interactions and is considered as a powerful tool to assist in␣the␣future optimization of N and irrigation managements. Section Editor: L. Wade     

施磷对川西北高寒草地土壤磷形态及有效性的影响

以川西北高寒草地为研究对象,采用随机区组设计,设置0、10、20、30、40、50、60 g/m²的过磷酸钙(P₂O₅,16%)施肥试验,分析土壤不同形态磷含量和有效磷(Olsen-P)含量变化特征,探究施磷对川西北高寒草地土壤磷形态及有效磷的影响。结果表明：(1)随施磷量增加,土壤总磷(TP)含量先增加后趋于平稳而Olsen-P含量减少。高水平(50、60 g/m²)施磷下氢氧化钠有机磷(NaOH-Po)及残留磷(Residual-P)是高寒草地主要的磷素累积形态,其含量显著高于不施磷处理;(2)树脂交换态磷(Resin-Pi)、碳酸氢钠无机磷(NaHCO₃-Pi)、碳酸氢钠有机磷(NaHCO₃-Po)和氢氧化钠无机磷(NaOH-Pi)含量随施磷量增加整体呈先增加后降低趋势,表层土壤30 g/m²磷肥用量下其值均为最高,分别为21.54、22.94、65.86、64.48 mg/kg。酸溶性无机磷(HCl-Pi)随施磷量增加整体呈下...     

Phosphorus speciation in Myall Lake sediment, NSW, Australia

The amount of phosphorus and its fractions in the sediment of Lake Myall, NSW, Australia, was assessed using a sequential extraction technique. Five sedimentary phosphorus reservoirs were measured, namely loosely sorbed phosphorus (NH₄Cl–P), iron associated phosphorus (BD–P), calcium bound phosphorus (HCl–P), metal oxide bound phosphorus (NaOH–P) and residual phosphorus (organic and refractory P, Res-P). Samples were taken from the deep and shallow sites of the lake. During the analysis, the average concentrations of each fraction of phosphorus was calculated. The results depicted that the total phosphorus (TP) content and chemically extractable phosphorus in both fine and coarse sediment fractions from the deep sites of the lake were significantly higher than those of the shallow sites, except for HCl–P extracted from the fine sediment fraction. Sediment TP was also strongly and positively correlated to sediment Fe. The phosphorus in the sediment mainly consisted of BD–P and Res-P, while NH₄Cl–P and HCl–P only contributed a minor part. The rank order of the different phosphorus extracts was similar for the two sites, namely Residual-P > BD–P > NaOH–P > HCl–P > NH₄Cl–P.     

Methane emissions associated with a green manure amendment to flooded rice in California

The goals of sustainable food production and mitigation of greenhouse gas emissions may be in conflict when green manures are used in flooded rice systems. A field study was initiated in early spring 1992 near Sacramento, California to quantify the potential for enhanced methane emissions following a green manure amendment to rice. Replicate flux measurements were made twice a day every 3–4 days throughout the growing season in four treatment plots: burned rice straw, spring incorporated rice straw, burned straw plus purple vetch and spring incorporated straw plus vetch. Seasonal methane emissions ranged from 66–136 g CH₄ m⁻² and were 1.5 to 1.8 times higher from the straw plus vetch treatments relative to the straw only treatments. No significant differences in emissions were found between the two straw only treatments or the straw plus vetch treatments. Methane fluxes were exponentially related to soil temperature, but no effect of redox potential or floodwater depth were observed. The potential impact of these results on the global methane budget is discussed.     

Hypereutrophication events in the Ca’Pisani lagoons associated with intensive aquaculture

Parameters of ecosystem structure and functioning were analyzed in three hypereutrophic lagoons of Ca’Pisani during the season of 2001. Lagoons are situated at wetlands of the NW Adriatic in the vicinity of the Porto Viro, Po River delta. They are associated with intensive fish culture enterprise and accept its wastewater. In June, the lagoons were found overloaded with the biomass of nitrophylic algae. At the end of July, a bloom of potentially toxic dinoflagellate Alexandrium tamarense occurred. Soon, it was supplanted by the picocyanobacterial assemblage, which arrived into the lagoons from the coastal Adriatic via the Marine channel. Wet biomass of this new picocyanobacterial bloom arrived in September attained 30–60 g m⁻³. Decrease of white disk water transparency down to 30–40 cm resulted in a gross mortality of macrophytes accompanied by spreading of floating saprobic alga Enteromorpha. Phytoplankton was dominated in June to July by small mixotrophic phytoflagellates with a wet biomass of 200–1300 mg m⁻³. Number of bacterioplankton ranged between 4 and 7 × 10⁶ ml⁻¹ and its wet biomass between 1.4 and 2.1 g m⁻¹. Its maximum of 18 × 10⁶ ml⁻¹ was observed in late August, when the mortality of macrophytes had occurred. Zooplankton and zoobenthos were found depleted in the lagoons especially during the blooms. Diel fluctuations of dissolved oxygen in the lagoons in June to July reached 150–200% of saturation. Photosynthetic oxygen production ranged between 15 and 30 g O₂ m⁻² d⁻¹. Water column deoxygenation rate was 1–1.5 mg O₂ l⁻¹ h⁻¹. Total photosynthesis production reached 3–8 g C m⁻² d⁻¹ by the share of phytoplankton 5–15%. Hyper-accumulation of total phosphorus in the water column and of toxic labile sulfides in the bottom sediments was documented. Content of inorganic phosphorus in water remained unusually high even by its intensive uptake by microplankton. The PO₄P uptake rate measured with ³²P-label ranged during the bloom of picocyanobacteria between 10 and 50 nM l⁻¹ min⁻¹, and the residence time of PO₄P between 15 and 50 min. The data were generalized via the calculation of energy balance and the deduction of the energy flow scheme in the ecosystem. Their analysis demonstrates the invalidation of ecosystems in hypereutrophic lagoons due to their overload with organic matter, with nutrients and with labile sulfides. After having depleted their animal food web, they are unable to decompose local plus external organic loading. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Estimation of long-term variation in nutrient loads from the Hii River by comparing the change in observed and calculated loads in the catchments

To study the long-term change in nutrient loads from the Hii River to Lake Shinji, water samples were taken repeatedly over a year in 1983/1984 and again in 2001/2002. Annual total nitrogen (TN) loads, estimated from observations of water quality and river flow, increased from 860 to 920 t with a corresponding increase in NO₃–N concentration during the cool season. In contrast, total phosphorus (TP) loads decreased from 96 to 62 t. Annual TN and TP loads, calculated using emission factors and annual statistics for the catchments, showed a tendency to decline from 1986 to 2002. No source could be identified which would result in the increase in TN in the catchments, therefore, the increase in observed TN loads was considered to originate in other areas. Atmospheric nitrogen deposition transported from long distances has elevated the sum of NH₄–N and NO₃–N concentration in rainwater in the cool season. Therefore, it was considered that this resulted in the increase in TN loads in the Hii River.     

Estimation of available potassium for cotton by soil analysis

Measurements of exchangeable K, percentage of exchangeable K of the total exchange capacity (EPP), K extracted in a CaCl₂ solution, and the change in free-energy of exchange of K for Ca+Mg (F), were compared as methods for estimation of available K in soil for growing irrigated cotton. A significant correlation was found between each of the methods and the response to K fertilizer. The different methods are discussed; the advantages of extraction with CaCl₂ solution are that it reflects the change in free energy and is easy to operate. For sandy soils, it is suggested that determination of exchangeable K be added.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1975 Series, No. 202-E Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1975 Series, No. 202-E     

Influence of alternate submergence and drying of rice soils prior to resubmergence on available phosphorus

Summary Four acid rice-soils (0–20 cm, air-dry, and <2 mm) were subjected to 3 cycles, each consisting of 30-day submergence followed by air-drying at room temperature or oven-drying at 105°C for 3 days. The available phosphorus was determined colorimetrically by Bray's P-2 extractant (0.03N NH₄F+0.1N HCl) using 1:20 soil extractant ratio. The beneficial effect of 30-day submergence followed by drying of soil prior to resubmergence on native phosphorus was apparent in all soils studied except Shirgaon clay soil, which was probably due to presence of relatively more active Fe in that soil.     

Benthic phosphorus regeneration in the Potomac River Estuary

The flux of dissolved reactive phosphate from Potomac riverine and estuarine sediments is controlled by processes occurring at the water-sediment interface and within surficial sediment.In situ benthic fluxes (0.1 to 2.0 mmoles m⁻² day⁻¹) are generally five to ten times higher than calculated diffusive fluxes (0.020 to 0.30 mmoles m⁻² day⁻¹). The discrepancy between the two flux estimates is greatest in the transition zone (river mile 50 to 70) and is attributd to macrofaunal irrigation. Bothin situ and diffusive fluxes of dissolved reactive phosphate from Potomac tidal river sediments are low while those from anoxic lower estuarine sediments are high. The net accumulation rate of phosphorus in benthic sediment exhibits an inverse pattern. Thus a large fraction of phosphorus is retained by Potomac tidal river sediments, which contain a surficial oxidized layer and oligochaete worms tolerant of low oxygen conditions, and a large fraction of phosphorus is released from anoxic lower estuary sediments. Tidal river sediment pore waters are in equilibrium with amorphous Fe (OH)₃ while lower estuary pore waters are significantly undersaturated with respect to this phase. Benthic regeneration of dissolved reactive phosphorus is sufficient to supply all the phosphorus requirements for net primary production in the lower tidal river and transition-zone waters of the Potomac River Estuary. Benthic regeneration supplies approximately 25% as much phosphorus as inputs from sewage treatment plants and 10% of all phosphorus inputs to the tidal Potomac River. When all available point source phosphorus data are put into a steady-state conservation of mass model and reasonable coefficients for uptake of dissolved phosphorus, remineralization of particulate phosphorus, and sedimentation of particulate phosphorus are used in the model, a reasonably accurate simulation of dissolved and particulate phosphorus in the water column is obtained for the summer of 1980.     

Phosphorus uptake and turnover by periphyton in the presence of suspended clays

We investigated the effect of suspended bentonite and kaolinite clays on phosphorus uptake and turnover by lotic periphyton in laboratory microcosms. Clays were characterized for their phosphorus affinity using laboratory batch experiments. Periphyton cultivated on glass microscope slides was subjected to a 0.02 mg L⁻¹ radiolabeled soluble reactive phosphorus solution in which a 200-mg L⁻¹ clay load was suspended. A 1-h uptake experiment was followed by a 10-day turnover experiment. Biomass normalized phosphorus uptake, and turnover rates were described by mean rate constants ranging from 0.14 to 0.17 min⁻¹ for uptake and 0.04–0.07 days⁻¹ for turnover. Mean phosphorus concentrations were compared among treatments using repeated measures analysis of variance (ANOVA). Mean phosphorus concentrations among treatments were compared using one-way ANOVA. No significant differences were found among treatments for either analysis. Under laboratory conditions, these clays appear to have little or no short-term influence upon phosphorus uptake or turnover by periphyton.     

Spatial and seasonal patterns of periphyton biomass and productivity in the northern Everglades,Florida, U.S.A.

We sampled periphyton in dominant habitats at oligotrophic and eutrophic sites in the northern Everglades during the wet and the dryseasons to determine the effects of nutrient enrichment on periphytonbiomass, taxonomic composition, productivity, and phosphorus storage. Arealbiomass was high (100–1600 g ash-free dry mass [AFDM]m⁻²) in oligotrophic sloughs and in stands of the emergentmacrophyte Eleocharis cellulosa, but was low in adjacent stands of sawgrass,Cladium jamaicense (7–52 g AFDM m⁻²). Epipelon biomasswas high throughout the year at oligotrophic sites whereas epiphyton andmetaphyton biomass varied seasonally and peaked during the wet season.Periphyton biomass was low (3–68 g AFDM m⁻²) and limitedto epiphyton and metaphyton in open-water habitats at eutrophic sites andwas undetectable in cattail stands (Typha domingensis) that covered morethan 90% of the marsh in these areas. Oligotrophic periphytonassemblages exhibited strong seasonal shifts in species composition and weredominated by cyanobacteria (e.g., Chroococcus turgidus, Scytonema hofmannii)during the wet season and diatoms (e.g. Amphora lineolata, Mastogloiasmithii) during the dry season. Eutrophic assemblages were dominated byCyanobacteria (e.g., Oscillatoria princeps) and green algae (e.g., Spirogyraspp.) and exhibited comparatively little seasonality. Biomass-specific grossprimary productivity (GPP) of periphyton assemblages in eutrophic openwaters was higher than for comparable slough assemblages, but areal GPP wassimilar in these eutrophic (0.9–9.1 g C m⁻²d⁻¹) and oligotrophic (1.75–11.49 g C m⁻²d⁻¹) habitats. On a habitat-weighted basis, areal periphytonGPP was 6- to 30-fold lower in eutrophic areas of the marsh due to extensiveTypha stands that were devoid of periphyton. Periphyton at eutrophic siteshad higher P content and uptake rates than the oligotrophic assemblage, butstored only 5% as much P because of the lower areal biomass.Eutrophication in the Everglades has resulted in a decrease in periphytonbiomass and its contribution to marsh primary productivity. These changesmay have important implications for efforts to manage this wetland in asustainable manner. This revised version was published online in July 2006 with corrections to the Cover Date.     

长期施磷对玉米-小麦轮作系统作物产量和磷素吸收及土壤磷积累的影响

本研究利用位于河北省保定市的7年田间定位试验,探讨了4个磷水平(不施磷、70%优化施磷、优化施磷、130%优化施磷)对玉米-小麦轮作系统作物产量的影响,分析了作物磷素吸收和磷肥利用效率以及土壤磷盈亏的变化。结果表明: 长期(7年)施磷能够显著提高玉米和小麦产量及磷素吸收量,且玉米和小麦产量与磷素吸收量随着施磷量的增加均呈现先上升后下降的抛物线变化趋势。各施磷水平下玉米季磷肥利用率高于小麦季,小麦季磷肥累积利用率呈现下降趋势,下降速率表现为优化施磷>70%优化施磷>130%优化施磷;玉米季磷肥累积利用率均呈现上升趋势,上升速率表现为优化施磷>70%优化施磷>130%优化施磷。长期不施磷条件下土壤表观磷盈亏量和累积磷盈亏量都表现为亏缺状态,施磷条件下土壤磷表现为盈余,且施磷量越高,累积年限越长,土壤磷盈余量越高。河北潮土在秸秆还田条件下小麦施磷量在105～150 kg·hm^-2,玉米施磷量在63～90 kg·hm^-2时,既能保证作物高产,又能使磷肥利用率保持在较高水平,减少磷素在土壤中的累积,降低环境风险。     

Partitioning of soil water among canopy trees in a seasonally dry tropical forest

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θ_v), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about –15‰ at 0.1 m to –50 to –55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of –60‰. θ_v increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θ_v dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θ_v remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season. Received: 5 October 1998 / Accepted: 23 June 1999     

Effects of fertigation scheme on N uptake and N use efficiency in cotton

While fertigation can increase fertilizer use efficiency, there is an uncertainly as to whether the fertilizer should be introduced at the beginning of the irrigation or at the end, or introduced during irrigation. Our objective was to determine the effect of different fertigation schemes on nitrogen (N) uptake and N use efficiency (NUE) in cotton plants. A pot experiment was conducted under greenhouse conditions in year 2004 and 2005. According to the application timing of nitrogen (N) fertilizer solution and water (W) involved in an irrigation cycle, four nitrogen fertigation schemes [nitrogen applied at the beginning of the irrigation cycle (N–W), nitrogen applied at the end of the irrigation cycle (W–N), nitrogen applied in the middle of the irrigation cycle (W–N–W) and nitrogen applied throughout the irrigation cycle (N&W)] were employed in a completely randomized design with four replications. Cotton was grown in plastic containers with a volume of 84 l, which were filled with a clay loam soil and fertilized with 6.4 g of N per pot as unlabeled and ¹⁵N-labeled urea for 2004 and 2005, respectively. Plant total dry matter (DM) and N content in N–W was significantly higher than in N&W in both seasons, but these were not consistent for W–N and W–N–W treatments. In year 2005, a significantly higher nitrogen derived from fertilizer (NDFF) for the whole plant was found in W–N and N–W than that in W–N–W and N&W. Fertigation scheme had a consistent effect on total NUE: N–W had the highest NUE for the whole plant, but this was not significantly different from W–N. Treatments W–N and W–N–W had similar total NUE, and N&W had the lowest total NUE. After harvesting, the total residual fertilizer N in the soil was highest in W–N, lowest in N–W, but this was not significantly different from N&W and W–N–W treatments. Total residual NO₃–N in the soil in N&W and W–N treatments was 20.7 and 21.2% higher than that in N–W, respectively. The total ¹⁵N recovery was not statistically significant between the four fertigation schemes. In this study, the fertigation scheme N–W (nitrogen applied at the beginning of an irrigation cycle) increased DM accumulation, N uptake and NUE of cotton. This study indicates that Nitrogen application at the beginning of an irrigation cycle has an advantage on N uptake and NUE of cotton. Therefore, NUE could be enhanced by optimizing fertilization schemes with drip irrigation.