Interacting effects of pH,aluminium and base cations on growth and mineral composition of the woodland grasses Bromus benekenii and Hordelymus europaeus

The influence of base cation concentrations on pH and aluminium sensitivity of the woodland grasses Bromus benekenii and Hordelymus europaeus was studied in flowing solution culture experiments. Plants were exposed to low pH (3.9, experiment 1) and Al concentrations of 19 and 37 M (experiment 2) at two base cation (Ca+Mg+K) levels, all within the ranges measured in natural forest soil solutions. Elevated base cation concentrations ameliorated both H and Al toxicity, as indicated by increased root and shoot growth. In the third experiment, interactions between pH (4.3 and 4.0) and Al (0 and 19 M) were investigated. It was shown that the combined toxicity effects of H and Al were not greater than the separate H or Al effects. Tissue concentrations of base cations and Al increased with increasing concentrations in the solution, but were also influenced by the base cation : Al ratio. Relating the experimental evidence with the composition of forest soil solutions suggests an important role of soil pH and Al in controlling the distribution of the two species. Growth conditions also differ at various soil depths. Concentrations of free cationic Al were higher and base cation concentrations lower at 5–10 cm than at 0–5 cm soil depth. Increasing base cation concentrations may protect roots from both H and Al injury during periods of drought when concentrations of most elements increase in the soil solution, whereas molar ratios between base cations, H and Al remain unchanged.     

The effect of soil phosphorus on the external mycelium growth of arbuscular mycorrhizal fungi during the early stages of mycorrhiza formation

The effects of soil P amendments and time of application on the formation of external mycelium by different arbuscular mycorrhizal (AM) fungi were studied. In the first experiment the external mycelium produced in the soil by the AM fungus Glomus etunicatum Beck. and Gerd., during the early stages of root colonization (7 and 14 days after inoculation), was quantified by the soil-agar film technique. A Brazilian Oxisol was used with three different phosphate levels, varying from deficient to supra-optimal for the plant. Significant differences were observed in the phosphate and inoculation treatments for plant dry weight, P content in the tissue, root length and root colonization, at fourteen days after planting. At 7 days, mycelium growth, root colonization and their relationship were reduced at supra-optimal P concentrations. Applications of P one week after planting reduced mycelium growth and root colonization more than when applied to the soil before planting. In a second experiment the arbuscular mycorrhizal (AM) fungi, Scutellospora heterogama (Nicol. and Gerd.) Walker and Sanders and E3 were tested and compared with Glomus etunicatum. For the species studied, the length of external hyphae per unit of colonized root length was affected by small P additions but no further significant differences were observed at high P levels. The three AM endophytes showed marked differences in their response to P in the soil: Scutellospora heterogama, although producing external mycelium more profusely than the Glomus spp., showed a higher sensitivity to soil P supply.     

A simple technique for producing 13C or 14C-labelled fronds of Lemna gibba and their use in soil incubation investigations

The duckweed Lemna gibba required light and a suitable energy source such as sucrose, glucose or fructose, for maximum growth in culture. The requirement for light was relatively unimportant and the plants grew well in a photon flux density of only 52 μmol m^-2s^-1 PAR. The uptake and incorporation of uniformly labelled ¹⁴C-glucose into fronds was related only to the concentration of the sugar. When incubated with soil, labelled L. gibba behaved in a manner similar to that of labelled ryegrass roots which had been produced by a more elaborate technique using a ¹⁴CO₂ labelled atmosphere. During incubation with soil for 224 days the L. gibba material (specific activity 6133 Bq mg^-1 d. wt) lost 64% of its radioactivity as ¹⁴CO₂ and ryegrass (specific activity 6634 Bq mg^-1 d. wt) lost 49%. Alkaline extracted humic and fulvic acids from soil had specific activities for the L. gibba incubation of 3409 and 407 Bq mg^-1 solid and for ryegrass roots of 4609 and 546 Bq mg^-1 solid respectively. The production of ¹³C or ¹⁴C-labelled L. gibba can be undertaken using only simple equipment producing material the specific radioactivity of which can be controlled by adjusting the activity of the sugar energy source.     

Nutrient distribution in a Swedish tree species experiment

The influence of four tree species on the distribution of nutrients between different compartments of the ecosystem was examined. In a randomized block (n=3) experiment in south-western Sweden, Ca, Mg and K were determined as exchangeable amounts in the mineral soil and as total amounts in the O+A₁ horizons (topsoil) and in the aboveground tree biomass. N contents were determined in all compartments as well as P contents of the aboveground tree biomass and the topsoil. The four tree species planted were: silver fir [Abies alba Mill.] (AA), grand fir [Abies grandis Lindl.] (AG), Norway spruce [Picea abies L. Karst.] (PA) and Japanese larch [Larix leptolepis (Sieb. och Zucc.) Endl.] (LL). At the age of 35–36 years, the total stemwood production of the most productive species, AG, was estimated at 471 m³ ha⁻¹. In relation to AG, LL had produced 80%, PA 73% and AA 37%. The system totals [aboveground tree biomass total + topsoil total + exchangeable (Ca, Mg, K) or total (N) in the mineral soil] of Ca, K and N did not differ significantly at the 5% level between the investigated species. For Mg, the system total in LL was significantly higher than for the other species. There was an indication that LL and AA contained higher amounts of Ca, Mg, K and N in the topsoil but less in the biomass than did AG and PA (partly significant). In the mineral soil, there were no significant differences in the exchangeable pools of Ca and K, nor in the total amounts of N. The biomass nutrient concentrations generally decreased in the order: AA > PA > AG > LL. At stem or whole-tree harvest, the Ca export per biomass unit would more than double in the case of PA compared to LL. LL also contained less N in the biomass than the other species. However, the N content in the biomass did not differ between the most (AG) and the least (AA) productive species, although the production of dry weight biomass (standing + harvested) of AG had been twice that of AA. It is concluded that the nutrient budget of a managed forest may vary considerably depending on the choice of tree species.     

Phosphorus depletion in the rhizosphere as influenced by soil moisture

To study the influence of soil moisture on phosphorus (P) depletion in the rhizosphere, maize (Zea mays cv. Trak) was pre-grown in vermiculite filled-PVC tubes for 9 days and then the plants with the tubes were transplanted into soil columns maintained at two soil moisture levels () of 0.14 and 0.20 cm³ cm^–3 for 10 days. The soil columns were separated at 1 cm depth by a nylon screen of 53 m inner mesh size, into 1 cm soil layer above and 3 cm soil column below screen. A root mat developed over the screen, but root hairs only could penetrate it. Regardless of the soil moisture level in the columns, and adequate and equal water and nutrients supply was maintained via wicks from an external nutrient solution to the plant roots in vermiculite. After 10 days, the soil columns were separated from the root mats, quickly frozen in liquid nitrogen and sliced into thin layers (0.2mm) using a refrigerated microtome to give soil samples at defined distances from the root mats for analyses. Lower soil moisture (=0.14) resulted in narrower and steeper depletion profile of 0.5 M NaHCO₃ extractable P (NaHCO₃-P_i) as compared to higher soil moisture (=0.20). Depletion of P in soil solution in the immediate vicinity of root mats did not differ much but the extension of the depletion zones was 0.10 cm at =0.14 and 0.20 cm at =0.20. The depletion up to 0.05cm with =0.14 and up to 0.07 cm with =0.20 was uniform, and may be attributed to the depletion in the root hair zone. Beyond the root hair zones, the theory of diffusion and mass flow was able to explain the observed differences in shape and extent of the P depletion profiles at the two soil moisture levels.     

Effect of snow removal on leaf water potential,soil moisture,leaf and soil nutrient status and leaf peroxidase activity of sugar maple

Effect of removal of snow cover in winter was investigated in an 80-year-old sugar maple (Acer saccharum Marsh.) stand in southern Quebec. We hypothesized that winter soil frost would induce some of the decline symptoms observed in sugar maple stands in southern Quebec in the early 1980's. Snow was continuously removed from around trees for a one week (partial removal) or for a four-month period (complete removal) during the 1990–1991 winter. Foliage and soils were sampled periodically during the summer of 1991. The complete snow removal treated trees showed decreased leaf water potential and increased peroxidase activity over most of the growing season. Foliar Ca was reduced in both snow removal treatments early in the growing season while foliar N was reduced in the complete snow removal trees late in the growing season. Soil NO ₃ ^– and K⁺ were elevated in both snow removal treatments at various times throughout the growing season. Prolonged soil frost in a sugar maple stand can induce lower leaf water potential, higher leaf peroxidase activity and early leaf senescence during the following growing season. Soil frost may have reduced nutrient uptake without affecting significantly the leaf nutrient status.     

Direct assessment of symbiotically fixed nitrogen in the rhizosphere of alfalfa

Rhizodeposition has been proposed as one mechanism for the accumulation of significant amounts of N in soil during legume growth. The objective of this experiment was to directly quantify losses of symbiotically fixed N from living alfalfa (Medicago sativa L.) roots to the rhizosphere. We used ¹⁵N-labeled N₂ gas to tag recently fixed N in three alfalfa lines [cv. Saranac, Ineffective Saranac (an ineffectively nodulated line), and an unnamed line in early stages of selection for apparent N excretion] growing in 1-m long polyvinylchloride drainage lysimeters in loamy sand soil in a greenhouse. Plants were in the late vegetative to flowering growth stage during the 2-day labelling period. We determined the fate of this fixed N in various plant organs and soil after a short equilibration period (2 to 4 days) and after one regrowth period (35 to 37 days). Extrapolated N₂ fixation rates (46 to 77g plant^–1 h^–1) were similar to rates others have measured in the field. Although there was significant accretion of total N in rhizosphere compared to bulk soil, less than 1% was derived from newly fixed N and there were no differences between the excreting line and Saranac. Loss of N in percolate water was small. These results provide the first direct evidence that little net loss of symbiotically-fixed N occurs from living alfalfa roots into surrounding soil. In addition, these results confirm our earlier findings, which depended on indirect ¹⁵N labelling techniques. Net N accumulation in soil during alfalfa growth is likely due to other processes, such as decomposition of roots, nodules, and above ground litter, rather than to N excretion from living roots and nodules.     

Iron toxicity and other chemical soil constraints to rice in highland swamps of Burundi

Iron toxicity is suspected to be a major nutritional disorder in rice cropping systems established on flooded organic soils that contain reductible iron. A pot trial was carried out to assess Fe toxicity to rice in flooded Burundi highland swamp soils with a wide range of organic carbon contents. Soil and leaf analyses were performed and total grain weight was determined. Clear Fe toxicity was diagnosed, based on leaf Fe content at panicle differentiation. Leaf Fe contents higher than 250 g g^–1 dry matter induced lower Mg (and probably Mn) uptake, and a 50% total grain weight reduction. These features were associated with exchangeable Fe equivalent fractions higher than 86%. Besides, several non-Fe toxic soils exhibited an Mg-Mn imbalance.     

Plant growth and nutrient uptake characteristics of Fe-deficiency chlorosis susceptible and resistant subclovers

The objective of this study was to evaluate the growth and nutrient-uptake characteristics of Fe-deficiency resistant and susceptible subclover (Trifolium subterraneum L., T. yanninicum Katzn. and Morley, T. brachcalycinum Katzn. and Morley) cultivars on a calcareous soil. Ten subclover cultivars showing varying susceptibilities to Fe-deficiency chlorosis (Karridale, Nangeela, Geraldton, Mt. Barker, Woogenellup, Larisa, Trikkala, Rosedale, Koala and Clare) were grown on a low-Fe, calcareous soil (Petrocalcic Paleustoll) under moist (18% water content, 85% of water holding capacity) and water-saturated conditions using a Cone-tainer^® culture system. Chlorosis and its correlation with growth traits and mineral nutrition of the 10 cultivars were examined. The Fe-deficiency susceptibilities of the 10 cultivars decreased in the above order under the moist condition, but in slightly different order under the saturated condition. Shoot and root dry weights, total dry weight, and root-to-shoot ratio were each negatively correlated with chlorosis under both soil-moisture conditions, as was total shoot content of P, Ca, Fe, Mn and Zn. Shoot P and Fe concentrations were each positively correlated with chlorosis under the moist soil condition. Iron and Cu utilization efficiencies (biomass per unit weight of nutrient) in the shoot were each negatively correlated with chlorosis under the moist soil condition. These results suggest that there may be several characteristics of Fe-deficiency chlorosis resistance in subclovers, such as a more effective soil-Fe mobilizing mechanism(s), more balanced nutrition, lower required Fe concentration in the shoot, higher shoot-Fe utilization efficiency, and higher root/shoot ratio under Fe-deficiency stress conditions.