Effect of cultivation on the mineralization of nitrogen in soil

Summary The effect of cultivation (ploughing followed by rotavation) on the mineralization of soil nitrogen was measured at 2 sites on a silt loam soil. Both sites had a predominantly arable cropping history but one had been under grass for the previous 2 years and the other had carried wheat. Mineralization of N was slightly faster in cultivated soil but the difference was only significant at the site previously under grass. At this site cultivated soil contained 7 kg ha^–1 more mineral N than uncultivated soil 2 weeks after treatment, and 9 kg ha^–1 after 6 weeks. The corresponding figures for the site that had grown wheat were 4 and 6 kg N ha^–1.     

Growth and development of seminal and crown root systems in N-limited barley,and their contributions to nitrate acquisition during vegetative and generative growth

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under nitrogen limitation, controlled by the relative rate of nitrate-N addition (RA), in solution culture. The seminal and crown root systems were kept apart, but in contact with the same nutrient solution throughout culturing. Growth, nitrate uptake, and in vitro nitrate reductase (NR) activity in the different root parts were studied at plant ages from 40 (late vegetative stage) to 110 (mid grain-filling) days. The RA was during this time interval stepwise decreased from 0.08 day^–1 to 0.005 day^–1. The ratio between seminal root dry weight and total plant dry weight decreased drastically during post-anthesis growth, whereas the contribution by crown roots remained unchanged. Tissue nitrogen concentrations in seminal roots did not change with time, but decreased in crown roots after day 80. The NR activity decreased with age in both seminal and crown roots. The V_max for net nitrate uptake decreased throughout the experiment in the seminal root system, but not in the crown root system. The kinetic properties (V_max and K_M) were used to calculate the nitrate concentration required to maintain a relative rate of nitrate-N uptake that equals the relative addition rate. These concentrations (2 to 5 mmol m^–3) were found to closely match actually measured nitrate concentrations in the nutrient solution (1 to 6 mmol m^–3). From uptake kinetics, it was deduced that the contribution by seminal roots to total nitrate uptake at these concentrations decreased from more than 50% in vegetative plants, to about 20% just after main shoot anthesis, and to less than 5% during grain-filling. ei]Section editor: H Lambers     

Effect of phosphate fertiliser and plant density on phosphate inflow into ryegrass roots in soil

Summary Ryegrass plants were grown in pots of Horotiu sandy loam, either singly or as a dense sward, with a range of P fertiliser rates and regular harvests. Plants were non-mycorrhizal. P inflows into roots increased with P fertiliser rate. Sward plants absorbed up to 14.3% of the P fertiliser added, and single plants up to 7.6%. Sward plants absorbed most of the fertiliser P available to them within two weeks of germinating. After that, root growth ceased except at the highest P fertiliser rate used, and P inflow into roots decreased from 10^–15 to 10^–16 mole/cm/sec. Single plants, however, had continuous root growth and P uptake throughout the two month experiment, except for those at the two lowest fertiliser rates. Singly grown plants absorbed much more P than each plant in the dense swards. In single plants, root length/root weight ratio increased with increasing P fertiliser. Plant growth was dependant on continued P uptake by the roots, which only occurred if new roots were continally produced and new volumes of soil tapped for P. Non-growing root systems absorbed very little P. re]19760201     

Effects of gibberellic acid spray on nitrogen yield efficiency of mustard grown with different nitrogen levels

Mustard is cultivated throughout the world for oil in its seeds. Itrequires high nitrogen input for improved productivity but the nitrogen appliedto the soil is not fully utilised by the crops due to various constraints. Theobjective of the reported research was to determine if foliar- appliedgibberellic acid (GA₃) could enhance crop growth and increasenitrogen-use efficiency. A field experiment was conducted during 1997–98in which GA₃ (10^–5 M) was applied tofoliage at 40d after sowing (pre-flowering) to mustard grown with 0, 40(sub-optimal), 80 (optimal) and 120 (supra-optimal) kgN ha^–1. Foliar spray of GA₃ was effectiveonly when plants received sufficient N (80 kgN ha^–1). GA₃ sprays significantly enhancedplant dry mass, leaf area, carbon dioxide exchange rate, plant growth rate,cropgrowth rate and relative growth rate. GA₃ -treated plants showedenhanced nitrogen-use efficiency through redistribution of N to seeds.     

Silicon accumulation and water uptake by wheat

Silicon (Si) content in cereal plants and soil-Si solubility may be used to estimate transpiration, assuming passive Si uptake. The hypothesis for passive-Si uptake by the transpiration stream was tested in wheat (Triticum aestivum cv. Stephens) grown on the irrigated Portneuf silt loam soil (Durixerollic calciorthid) near Twin Falls, Idaho. Treatments consisted of 5 levels of plant-available soil water ranging from 244 to 776 mm provided primarily by a line-source sprinkler irrigation system. Evapotranspiration was determined by the water-balance method and water uptake was calculated from evapotranspiration, shading, and duration of wet-surface soil. Water extraction occurred from the 0 to 150-cm zone in which equilibrium Si solubility (20°C) was 15 mg Si L^–1 in the A_p and B_k (0–58 cm depth) and 23 mg Si L^–1 in the B_kq (58–165 cm depth).At plant maturity, total Si uptake ranged from 10 to 32 g m^–2, above-ground dry matter from 1200 to 2100 g m^–2 and transpiration from 227 to 546 kg m^–2. Silicon uptake was correlated with transpiration (Si_up=–07+06T, r²=0.85) and dry matter yield with evapotranspiration (Y=119+303ET, r²=0.96). Actual Si uptake was 2.4 to 4.7 times that accounted for by passive uptake, supporting designation of wheat as a Si accumulator. The ratio of Si uptake to water uptake increased with soil moisture. The confirmation of active Si uptake precludes using Si uptake to estimate water use by wheat.     

Growth of benthic freshwater algae on dairy manures

A potential alternative to land application of livestock manures for cropproduction is the production of algae to recover the nitrogen andphosphorus present in the manure. Compared to terrestrial plants,filamentous algae have exceedingly high growth and nutrient uptake rates. Moreover, they are capable of year-round growth in temperate climates,can be harvested on adapted farm-scale equipment, and yield a biomassthat should be valuable as an animal feed supplement. The objective of thisresearch was to evaluate algal turf scrubber (ATS) technology to removenitrogen, phosphorus and chemical oxygen demand from raw andanaerobically digested dairy manure. Laboratory-scale ATS units wereoperated by continuously recycling wastewater and adding manure effluentsdaily. ATS units were seeded with algal consortia from a nearby streamand grown using dairy manures from two different dairy farms. Algalbiomass was harvested weekly and dried prior to analysis for total Kjeldahlnitrogen, total phosphorus, and inorganic constituents. Wastewater sampleswere analyzed for total Kjeldahl nitrogen, ammonium, nitrate,orthophosphate, conductivity and chemical oxygen demand. Using atypical manure input containing 0.6–0.96 g total nitrogen day^-1,the dried algal yield was approximately 5 g m^-2 day^-1. Thedried algae contained approximately 1.5–2% phosphorus and 5–7%nitrogen. Algal nitrogen and phosphorus accounted for 42–100% ofinput ammonium-nitrogen (33–42% of total nitrogen) and 58–100%of input total phosphorus, respectively.     

Effect of salinity on mycorrhizal onion and tomato in soil with and without additional phosphate

Summary Vesicular-arbuscular mycorrhizal fungi (VAM) are known to increase plant growth in saline soils. Previous studies, however, have not distinguished whether this growth response is due to enhanced P uptake or a direct mechanism of increased plant salt tolerance by VAM. In a glasshouse experiment onions (Allium cepa L.) were grown in sterilized, low-P sandy loam soil amended with 0, 0.8, 1.6 mmol P kg^–1 soil with and without mycorrhizal inoculum. Pots were irrigated with saline waters having conductivities of 1.0, 2.8, 4.3, and 5.9 dS m^–1. Onion colonized withGlomus deserticola (Trappe, Bloss, and Menge) increased growth from 394% to 100% over non-inoculated control plants when soil P was low ( 0.2 mmol kg^–1 NaHCO₃-extractable P) at soil saturation extract salinities from 1.1 dS m^–1 to 8.8 dS m^–1. When 0.8 and 1.6 mM P was added no dry weight differences due to VAM were observed, however, K and P concentrations were higher in VAM plants in saline treatments.Glomus fasciculatum (Gerdeman and Trappe) andGlomus mosseae (Nicol. and Gerd.) isolates increased growth of VAM tomato 44% to 193% in non-sterilized, saline soil (10 dS m^–1 saturation extract) despite having little effect on growth in less saline conditions when soil P was low. Higher tomato water potentials, along with improved K nutrition by VAM in onion, indicate mechanisms other than increased P nutrition may be important for VAM plants growing under saline stress. These effects appear to be secondary to the effects of VAM on P uptake.     

Cultivation and carrageenan yield and quality ofKappaphycus alvarezii in the waters of Vietnam

The carrageenan-producing red algaKappaphycus alvarezii (Doty) Doty was brought to Vietnam from Japan in 1993. Branch fragments of this species were cultivated in a pond, lagoon, inlet and offshore in Vietnam for the first time. The best daily growth rate (DGR) of plants grown in the lagoon area attained 9–11 % day^–1 in May to June (cold season). The water temperature and salinity in this area ranged from 27.2–32.4 °C and 31.4–33.7 °C, respectively. DGR of plants grown in the inlet ranged from 7 to 9% day^–1 in June. Grazing by fish has been observed to occur in this area. The DGR of plants grown in the pond ranged from 5–6% in January–July, but decreased to less than 4% day^–1 in August (hot season). K. alvarezii in Vietnam showed a carrageenan yield of 18.8–24.6% and gel strength of 1566–1712 g cm^–2. These values are similar ones obtained fromK. alvarezii cultivated in the Philippines and Indonesia.     

Osmotic adjustment inSorghum bicolor (L. Moench) grown under moisture stress in soil and osmotically modified solution cultures

Sorghum (Sorghum bicolor L. Moench) plants were grown in solution culture and stressed at three rates of decreasing leaf water potential (−0.123, −0.068 and −0.029 MPa day⁻¹) achieved by the incremental addition of an osmoticum, polyethylene glycol (PEG) 6000 to the solutions. Plants were also grown in soil and given different amounts of water which resulted in rates of decreasing leaf water potentials of −0.130 and −0.073 MPa day⁻¹. The rate of stress and the culture system influenced the accumulation of solutes in the cell, but not cell volume. A rapid stress (−0.123 and −0.130 MPa day⁻¹) to approximately −1.6 MPa leaf water potential resulted in 0.75 and 0.16 MPa of osmotic adjustment in the PEG and soil culture respectively. At moderate stress (−0.068 and −0.073 MPa day⁻¹) respective values were 1.68 and 0.58 MPa. There were some visual symptoms in the solution grown plants characteristic of uptake of high molecular weight PEG. However the relative growth rates of these plants were equal to or greater than those of the soil grown plants. In view of the differences in plant water status of soil and PEG solution cultured plants it was concluded that the use of the latter system would not be entirely suitable for some studies of drought resistance in sorghum, as related to crop performance in the field.     

Growth and nutrition of small Betula pendula plants at different relative addition rates of iron

Summary Small birch plants (Betula pendula Roth.) were cultivated in a hydroponic spray solution where the relative addition rate of iron (R_Fe; g g^–1 day^–1), was the growth-controlling variable. All other elements were added in free access. An additional treatment was performed where all nutrients, including iron, were in free access (FA). The plants showed deficiency symptoms at steady-state growth and severe limitation of iron, R_Fe 0.05 and 0.10 day^–1. There were few symptoms at R_Fe of 0.15 or above. Plant relative growth rate (R_G; g g^–1 day^–1), equalled the relative rate of increase in iron supply, R_Fe. Internal iron concentration of the plants ranged from 40 to 70 g g^–1 dry weight (DW) over the range for which iron supply was limiting growth. At FA, the internal concentration was approximately 200 g g^–1 DW without further increase in R_G, demonstrating that iron may be taken up in excess without affecting growth. Internal concentrations of macronutrients were stable at the different R_Fe, except for Ca and Mg in shoots which were higher at low iron supply. Uptake rates of iron, calculated per root growth rate (mol g^–1 root DW), were approximately twice as high at R_Fe 0.20 as at 0.05 day^–1. The effect of iron limitation on dry matter allocation to leaves was small, with increases in the root fraction being largely at the expense of the stem. Leaf area ratio was constant regardless of R_Fe and the specific leaf area tended to increase with increasing iron limitation. Net assimilation rate decreased by a factor of 6 from free access to severe iron limitation, largely accounting for the differences in plant R_G.     

The formation,morphology and anatomy of cluster root of Lupinus albus L. as dependent on soil type and phosphorus supply

The effect of phosphorus supply on the formation, morphology and anatomy of cluster roots of Lupinus albus L. cv Ultra grown in a loam and two sandy soils was examined relative to its effect on total root length, shoot weight and the phosphorus concentration of the shoots. The loam soil was most conducive to the formation of cluster roots. Cluster roots growing in the sandy soils developed to a lesser extent on plants of an equivalent phosphorus status, suggesting that some biotic or abiotic factors independent of phosphorus supply were also operating. The presence of mature cluster rootlets on a length of lateral root increased the root surface area by 14–22 times of an equal length of lateral roots not bearing cluster rootlets. The application of phosphorus decreased cluster-root length, whereas total root length showed a steady increase. There was an inverse relationship between cluster-root production and phosphorus concentration in shoots ranging from 2 to 8.5 mg g^–1 with the critical phosphorus level for maximum shoot growth being around 2.5 mg g^–1. Cluster roots formed in solution culture were not well developed in comparison with those grown in the loam soil or nutrient solution with added loam soil. The organisation of the cluster rootlet was similar to that of the lateral roots. Mature rootlets lacked an apical meristem and a vascular cambium with a reduced root cap and cortical tissue.     

Brief pre- and post-irrigation sprinkling with fresh water reduces foliar salt uptake in maize and barley sprinkler irrigated with saline water

Brief pre- and post-irrigation sprinkling treatments using freshwater were tested to determine if these practices could reduce the uptake of salts through leaves when saline water is used to sprinkler irrigate crops. Maize and barley were sprinkler irrigated 2 to 3 times per week for 30 min with saline water (4.2 dS m^–1, 30 mmol L^–1 NaCl and 2.8 mmoles L^–1 CaCl₂ for maize and 9.6 dS m^–1, 47 mmoles L^–1 NaCl and 23.5 mmoles L^–1 CaCl₂ for barley) in separate experiments with plants grown in pots outdoors. The soil surface of all pots was covered to prevent salinization of the soil by the sprinkling water. One half of the sprinkled plants was grown in nonsaline soil to study the effects of pre-wetting and post-washing when ion uptake was primarily through leaves. The other half of the sprinkled plants was grown in soil salinized by drip irrigation, in order to evaluate the effects of pre-wetting and post-washing when Na⁺ and Cl^- uptake was through both leaves and roots.Post-washing with freshwater (5 min) reduced the leaf sap concentrations of Cl^- in saline-sprinkled plants from 56 to 43 mmol L^–1 in maize and from 358 to 225 mmol L^–1 in barley (averages for plants grown in nonsaline and saline soil). Na⁺ concentrations in leaf sap were reduced from 93 to 65 mmoles L^–1 (maize) and from 177 to 97 mmoles L^–1 (barley) by the post-washing. Pre-wetting had a small effect on ion uptake through leaves, the only significant reduction in seasonal means being in leaf Na⁺ concentrations for plants grown in nonsaline soil. Pre-wetting and post-washing, when combined, reduced leaf Cl^- concentrations to levels similar to those of nonsprinkled plants grown in saline soil; however, Na⁺ concentrations in leaves remained 3.5 times (maize) and 1.5 times (barley) higher than those of nonsprinkled plants. When pre-wetting and post-washing were not applied, sprinkled barley plants grown in saline soil had grain yields which were 58% lower than nonsprinkled plants grown in saline soil, but the reduction in grain yield was only 17% when the freshwater treatments were given. We conclude that a brief period of post-washing with freshwater is essential when saline water is employed in sprinkler irrigation. By comparison, the benefits from pre-wetting were small in these experiments. ei]T J Flowers     

Phosphorus effects on root growth and development in two maize genotypes

Soil phosphorus (P) availability is critical for the early growth and development of maize (Zea mays L.). Soil P also affects root morphological and physiological characteristics that are important for P uptake. The objective of this study was to evaluate the effects of P on seedling root growth and development of two maize genotypes differing in root system plasticity. Two maize genotypes, CM37 (high plasticity) and W153R (low plasticity), were selected based on a preliminary study. Maize plants were evaluated at six vegetative stages of development for three soil P treatments (0, 45, and 300 mg kg^-1). Seedlings were grown in a controlled environment using a soil with low native P, Maddock sandy loam (sandy, mixed Udorthentic Haploborolls). The addition of P decreased the time to reach a given growth stage and increased the relative growth rate of roots to a greater degree in CM37 than in W153R. The effects of P on shoot dry weight and root surface area during the V4–V6 growth period appeared to be related to the effects of P on development and relative growth rates during the V1–V3 growth period. Evaluation of the time course of phenotypic change is an important consideration when developing adapted genotypes for specific environments.     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.     

A15N tracer study to compare nitrogen supply by Azolla and ammonium sulphate to IR8 rice plants grown under flooded conditions

Summary A pot experiment was carried out using a Bangladesh sandy loam paddy soil of pH 6.9 to compare the rates at which nitrogen from Azolla and ammonium sulphate was available to a high yielding rice variety, IR8, grown for 60 days in pots with 4 cm standing flood water.¹⁵N tracer studies confirm that nitrogen from ammonium sulphate was more available to the rice plants than from Azolla. An application of 6, 9 and 18 mg N of Azolla pot^–1 (each pot contained 250 g soil) increased shoot dry matter yields by 13, 29 and 49% for an uptake of 19, 36 and 85% more nitrogen; the corresponding increases on using ammonium sulphate were 33, 54 and 114% for an increased uptake of 57, 90 and 177% more nitrogen, respectively. About 34% of applied¹⁵N of Azolla was taken up by the rice plants in 60 days but 61% of¹⁵N of the ammonium sulphate was absorbed during this period. About 45% of the Azolla-N was released in 60 days, 55% remained in the soils as undecomposed material and 11% was lost as gas. The gaseous loss of¹⁵N from ammonium sulphate was 14%; 25% remained in the soils.     

Effects of vesicular-arbuscular mycorrhizae on growth and phosphorus and zinc nutrition of peanut (Arachis hypogaea L.) in an Oxisol from subtropical Australia

Peanut plants (cv. Shulamit) were grown in an Oxisol soil in pots in the glasshouse to assess effects of soil sterilization and inoculation with spores of vesicular-arbuscular mycorrhizal fungi (VAMF) on the response to five rates of phosphorus (0 to 240 kg P ha^–1) and two rates of zinc (0 and 10 kg Zn ha^–1) fertilizers.Both P and Zn nutrition were affected by VAMF activity but the dominant role of VAMF in this soil type was in uptake of P. In the absence of VAMF there was a clear threshold level of P application (60 kg P ha^–1) below which plants grew poorly, which resulted in a sigmoidal response of dry matter to applied P. The maximum response was not fully defined because dry matter production continued to increase up to 240 kg P ha^–1. Tissue P concentration of non-mycorrhizal plants increased linearly with P rate and was always significantly less than that in mycorrhizal plants.Mycorrhizal plants responded without threshold to increasing P rate, attaining maximum dry matter at 120 kg P ha^–1 in inoculated sterilized soil and at 30 kg P ha^–1 in nonsterile soil. These differences in maximal P rates and in the greater dry matter produced in sterile soil at high P rates were attributed to the negative effects of the root-knot nematodeMeloidogyne hapla in nonsterile soil.Plant weight did not respond to zinc fertilizer but tissue Zn concentration increased with applied Zn. Tissue Zn concentration and uptake were increased by VAMF.     

Interaction of nitrate uptake and nitrogen fixation in faba beans

An experiment was carried out to determine the relationship between nitrate uptake and nitrogen fixation of faba beans. Therefore inoculated and uninoculated faba beans were grown in nutrient solution with different nitrate concentrations. Nitrate uptake was measured every two days during the growing period. At the end of the experiment the nitrate uptake kinetics were determined with a short time depletion technique and nitrogen fixation was measured with the acetylene reduction method. A limitation of nitrate uptake due to nitrogen fixation was relatively small. Nitrate concentrations of approximately 1 mol m^–3 and 5 mol m^–3 decreased nitrogen fixation to values of 16% and 1% of the control plants which received no nitrate nitrogen. A reduction of nitrogen fixation was mainly due to a decrease of specific nitrogen fixation per unit nodule weight and to a lesser extent due to a reduction of nodule growth. Only the maximum nitrate influx (I_max) seemed to be influenced by nitrogen fixation. Michaelis-Menten constants (K_m) and minimum NO _inf3 ^– -concentrations (C_min) were not significantly influenced by nitrogen fixation.     

Removing Constraints on the Biomass Production of Freshwater Macroalgae by Manipulating Water Exchange to Manage Nutrient Flux

Freshwater macroalgae represent a largely overlooked group of phototrophic organisms that could play an important role within an industrial ecology context in both utilising waste nutrients and water and supplying biomass for animal feeds and renewable chemicals and fuels. This study used water from the intensive aquaculture of freshwater fish (Barramundi) to examine how the biomass production rate and protein content of the freshwater macroalga Oedogonium responds to increasing the flux of nutrients and carbon, by either increasing water exchange rates or through the addition of supplementary nitrogen and CO₂. Biomass production rates were highest at low flow rates (0.1–1 vol.day⁻¹) using raw pond water. The addition of CO₂ to cultures increased biomass production rates by between 2 and 25% with this effect strongest at low water exchange rates. Paradoxically, the addition of nitrogen to cultures decreased productivity, especially at low water exchange rates. The optimal culture of Oedogonium occurred at flow rates of between 0.5–1 vol.day⁻¹, where uptake rates peaked at 1.09 g.m⁻².day⁻¹ for nitrogen and 0.13 g.m⁻².day⁻¹ for phosphorous. At these flow rates Oedogonium biomass had uptake efficiencies of 75.2% for nitrogen and 22.1% for phosphorous. In this study a nitrogen flux of 1.45 g.m⁻².day⁻¹ and a phosphorous flux of 0.6 g.m⁻².day⁻¹ was the minimum required to maintain the growth of Oedogonium at 16–17 g DW.m⁻².day⁻¹ and a crude protein content of 25%. A simple model of minimum inputs shows that for every gram of dry weight biomass production (g DW.m⁻².day⁻¹), Oedogonium requires 0.09 g.m⁻².day⁻¹ of nitrogen and 0.04 g.m⁻².day⁻¹ of phosphorous to maintain growth without nutrient limitation whilst simultaneously maintaining a high-nutrient uptake rate and efficiency. As such the integrated culture of freshwater macroalgae with aquaculture for the purposes of nutrient recovery is a feasible solution for the bioremediation of wastewater and the supply of a protein resource.     

Effects of aluminium on growth and nutrient uptake of small Picea abies and Pinus sylvestris plants

Summary The effects of aluminium concentrations between 0.2 and 30 mM at pH 3.8 ±0.2 on small plants of Norway spruce [(Picea abies (L.) Karst], Scots pine (Pinus sylvestris L.), and Scots pine infected with the ectomycorrhizal fungus Suillus bovinus (L. ex Fr.) O. Kuntze were investigated. The plants were grown at maximum relative growth rate (R_G % day^–1) with free access but very low external concentrations of nutrients. Steady-state conditions with respect to relative growth rate (R_G) and internal nutrient concentrations were achieved before addition of aluminium, which was added as AlCl₃ and/or Al(NO₃)₃. There were reductions in rg at aluminium concentrations of 0.3 mM in spruce, 6 mM in pine and 10 mM in ectomycorrhizal pine, i. e. at aluminium concentrations considerably higher than those normally occurring in the top layer of the mineral soil where most fine roots are found. Nutrient uptake rate per unit root growth rate was calculated for different nutrient elements. The uptake rate of calcium and magnesium was reduced at aluminium concentrations of 0.2 mM (spruce), 1 mM (pine) and 3 mM (ectomycorrhizal pine), without influencing R_g. The results question the validity of the hypothesis of aluminium toxicity to forest tree species at low external concentrations.     

Ethylene and carbon dioxide concentrations of soils as influenced by rhizosphere of crops under field and pot conditions

A method for collecting low volumes of soil gas from a small region, and a technique for determining small concentrations of ethylene using an enrichment process are described. Using these methods, it was found that ethylene and carbon dioxide (CO₂) concentrations of soils varied considerably depending on the presence or absence of a rhizosphere. Ethylene was much higher (31–375 nL L^–1; mean: 207) in non-cropped areas (i.e., soils without rhizosphere) than in the rhizosphere region (8–136 nL L^–1; mean: 38) of a field in which maize or soybean were grown. On the other hand, CO₂ concentrations were higher in rhizosphere than in non-rhizosphere soil, especially in pot experiments. The rate of ethylene decomposition was, however, much greater in rhizosphere soil (55 nL g^–1 day^–1) than in non-rhizosphere soil (34 nL g^–1 day^–1). Higher microbial activity was presumed to result in the decrease of ethylene concentration and the increase in CO₂ in rhizosphere regions. The implications of these results in relation to the influence of ethylene in rhizosphere on plant growth, and the role of soil microbes on decomposition of ethylene is discussed.