Soluble Organic Nitrogen Pools in Forest soils of Subtropical Australia

Soil soluble organic N (SON) plays an important role in N biogeochemical cycling. In this study, 22 surface forest soils (0–10 cm) were collected from southeast Queensland, Australia, to investigate the size of SON pools extracted by water and salt solutions. Approximately 5–45 mg SON kg⁻¹, 2–42 mg SON kg⁻¹ and 1–24 SON mg kg⁻¹ were extracted by 2 M KCl, 0.5 M K₂SO₄ and water, on average, corresponding to about 21.1, 13.5 and 7.0 kg SON ha⁻¹ at the 0–10 cm forest soils, respectively. These SON pools, on average, accounted for 39% (KCl extracts), 42% (K₂SO₄ extracts) and 43% (water extracts) of total soluble N (TSN), and 2.3% (KCl extracts), 1.3% (K₂SO₄ extracts) and 0.7% (water extracts) of soil total N, respectively. Large variation in SON pools observed across the sites in the present study may be attributed to a combination of factors including soil types, tree species, management practices and environmental conditions. Significant relationships were observed among the SON pools extracted by water, KCl and K₂SO₄ and microbial biomass N (MBN). In general, KCl and K₂SO₄ extracted more SON than water from the forest soils, while KCl extracted more SON than K₂SO₄. The SON and soluble organic C (SOC) in KCl, K₂SO₄ and water extracts were all positively related to soil organic C, total N and clay contents. This indicates that clay and soil organic matter play a key role in the retention of SON in soil.     

Inducing condensed tannin production in Colophospermum mopane: Absence of response to soil N and P fertility and physical damage

The effects of available soil N and P and the effect of simulated browsing (leave removal) on foliar condensed tannin (CT) concentration were tested on young Colophospermum mopane (J. Kirk ex Benth) J. Leonard (Mopane) plants. Although clear differences in growth occurred between different levels of soil N, no differences in foliar CT concentration were found. Changes in available soil P and physical damage did not affect the plant growth or chemical composition. The complete absence of response of CT concentration to physical damage and soil nutrients may be related to the age of the trees in this study. It is proposed that existing theories on the interaction between soil properties and carbon based defences in trees are expanded, to include the potential responses of young trees to different soil nutrient levels.     

Responses of tropical cladocerans to a gradient of resource quality

1. The response of three tropical cladocerans to a gradient of resource quality was compared in a series of growth bioassays using seston collected from five lakes of different depth and trophic structure in Michigan, U.S.A. To assess the food quality in terms of digestibility, assimilation experiments were performed with ³²P‐labelled seston from the same lakes. Animals were also analysed for P‐content in their tissues at the end of these assays. 2. In general, assimilation efficiency was higher when animals fed on seston from shallow compared to deep lakes, and was significantly correlated with growth rates, suggesting that shallow lakes have the best food resources in terms of digestibility and P availability. 3. Results also showed that all cladoceran species responded similarly to the resource gradient, with lower growth rates in deep lakes and higher growth rates in shallow lakes, although the strength of response (sensitivity) was different among the species tested. 4. The cladoceran Moina micrura was the most sensitive species, and also displayed the highest P‐content and maximal growth rate, a pattern consistent with the growth rate hypothesis. 5. However, seston C : P ratio and growth rates in the different resources did not correlate with the animals’ P‐contents, showing an uncoupling between RNA‐phosphorus demands for growth and seston food quality. 6. In conclusion, our results support the idea that digestion resistance in algae is a major constraint to cladocerans in natural plankton communities.     

Do phytoplankton communities correctly track trophic changes? An assessment using directly measured and palaeolimnological data

1. Measurements of total phosphorus (TP) concentrations since 1975 and a 50‐year time series of phytoplankton biovolume and species composition from Lake Mondsee (Austria) were combined with palaeolimnological information on diatom composition and reconstructed TP‐levels to describe the response of phytoplankton communities to changing nutrient conditions. 2. Four phases were identified in the long‐term record. Phase I was the pre‐eutrophication period characterised by TP‐levels of about 6 μg L^?1 and diatom dominance. Phase II began in 1966 with an increase in TP concentration followed by the invasion of Planktothrix rubescens in 1968, characterising mesotrophic conditions. Phase III, from 1976 to 1979, had the highest annual mean TP concentrations (up to 36 μg L^?1) and phytoplankton biovolumes (3.57 mm³ L^?1), although reductions in external nutrient loading started in 1974. Phases II and III saw an expansion of species characteristic of higher nutrient levels as reflected in the diatom stratigraphy. Oligotrophication (phase IV) began in 1980 when annual average TP concentration, Secchi depth and algal biovolume began to decline, accompanied by increasing concentrations of soluble reactive silica. 3. The period from 1981 to 1986 was characterised by asynchronous trends. Annual mean and maximum total phytoplankton biovolume initially continued to increase after TP concentration began to decline. Reductions in phytoplankton biovolume were delayed by about 5 years. Several phytoplankton species differed in the timing of their responses to changing nutrient conditions. For example, while P. rubescens declined concomitantly with the decline in TP concentration, other species indicative of higher phosphorus concentrations, such as Tabellaria flocculosa var. asterionelloides, tended to increase further. 4. These data therefore do not support the hypotheses that a reduction in TP concentration is accompanied by (i) an immediate decline in total phytoplankton biovolume and (ii) persistence of the species composition characterising the phytoplankton community before nutrient reduction.     

Influence of Arbuscular Mycorrhizal (AM) Symbiosis on Phosphorus Leaching through Soil Cores

Two experiments with soil cores were carried out to investigate the effects of arbuscular mycorrhizal (AM) fungal colonization on mobility of phosphorus (P) during leaching of repacked columns of a soil with a loamy sand texture. Trifolium subterraneum plants inoculated with an AM fungus or not inoculated were grown in cores with low or high P concentrations for 8 or 10 weeks in the glasshouse. Cores were then irrigated with 2500 mL water and the leachate collected. Plant growth and the amounts of P removed by plants, remaining in soil as available P and removed dissolved in leachate were measured. Mycorrhizal fungal colonization and development of external hyphae were also determined. Inoculation and/or P application significantly increased plant growth and plant P removal and decreased P leaching. In low P soils AM fungal colonization significantly increased plant P uptake and decreased soil available P and total dissolved P in leachates. Lower P leaching from cores with AM plants under low P conditions was related to enhancement of plant growth and to scavenging and removal of P from the soil by roots and/or external hyphae. When P was applied AM effects were not observed and available P remaining in the soil after leaching was much higher, regardless of AM fungal colonization.     

Cluster Roots: A Curiosity in Context

Cluster roots are an adaptation for nutrient acquisition from nutrient-poor soils. They develop on root systems of a range of species belonging to a number of different families (e.g., Proteaceae, Casuarinaceae, Fabaceae and Myricaceae) and are also found on root systems of some crop species (e.g., albus, Macadamia integrifoliaandCucurbita pepo). Their morphology is variable but typically, large numbers of determinate branch roots develop over very short distances of main root axes. Root clusters are ephemeral, and continually replaced by extension of the main root axes. Carboxylates are released from cluster roots at very fast rates for only a few days during a brief developmental window termed an ‘exudative burst’. Most of the studies of cluster-root metabolism have been carried out using the crop plant L. albus, but results on native plants have provided important additional information on carbon metabolism and exudate composition. Cluster-root forming species are generally non-mycorrhizal, and rely upon their specialised roots for the acquisition of phosphorus and other scarcely available nutrients. Phosphorus is a key plant nutrient for altering cluster-root formation, but their formation is also influenced by N and Fe. The initiation and growth of cluster roots is enhanced when plants are grown at a very low phosphate supply (viz. ≤1 μM P), and cluster-root suppression occurs at relatively higher P supplies. An important feature of some Proteaceae is storage of phosphorus in stem tissues which is associated with the seasonality of cluster-root development and P uptake (winter) and shoot growth (summer), and also maintains low leaf [P]. Some species of Proteaceae develop symptoms of P toxicity at relatively low external P supply. Our findings with Hakea prostrata (Proteaceae) indicate that P-toxicity symptoms result after the capacity of tissues to store P is exceeded. P accumulation in H. prostrata is due to its strongly decreased capacity to down-regulate P uptake when the external P supply is supra-optimal. The present review investigates cluster-root functioning in (1) L.albus (white lupin), the model crop plant for cluster-root studies, and (2) native Proteaceae that have evolved in phosphate-impoverished environments.     

Growth,nutrient acquisition and ectomycorrhizae of <Emphasis Type="Italic">Eucalyptus regnans</Emphasis> F. Muell. seedlings in fertilized or diluted air-dried and undried forest soil

Seedlings of Eucalyptus regnans (mountain ash) grow poorly in undried forest soil, where they develop purple coloration in the foliage, but their growth is markedly improved when forest soil has been air dried. Whether this growth promotion is purely due to improved nutrient status of the soil, as a result of air drying, was investigated. In several pot experiments, E. regnans seedlings were grown (i) in air-dried and undried forest soil with addition of different levels of complete fertiliser, (ii) in air-dried or undried soil diluted to different extents with sand, or (iii) in undried soil mixed with different amounts of air-dried soil. Seedling dry weight, P content and incidence of ectomycorrhizal root tips were determined.In all experiments, the dry weights of seedlings were 3–6 times greater in 100% air-dried soil than in 100% undried soil. Fertiliser application resulted in a significant increase in dry weight of seedlings in both air-dried and undried soil, but the dry weights in air-dried soil were always significantly greater than those in undried soil at the same level of fertiliser application. Even at the highest level of fertiliser application, the growth difference between seedlings in air-dried and undried soil remained. When air-dried soil was diluted with sand, there was a significant reduction in seedling dry weight only when soil was diluted to 20% or less (air-dried soil:total mix). Conversly, when air-dried soil was mixed with undried soil, there was a proportional decrease in seedling dry weight with increasing amounts of undried soil. In all experiments, the dominant ectomycorrhizal morphotypes in 100% air-dried soil were different from those in undried soil. Fertilisation and dilution of air-dried and undried soil did not result in a reduction in the overall incidence of ectomycorrhizal root tips, although the frequency of occurrence of different ectomycorrhizal morphotypes was affected.It is concluded that the growth difference between seedlings in air-dried and undried forest soils is not due solely to differences in the direct availability of nutrients in the soils, and different ectomycorrhizae may indirectly affect nutrient availability to the plant.     

Rapid whole-plant bioassay for phosphorus phytoavailability in soils

Chemical P extraction from soils is an indirect and frequently questionable index for P availability. To monitor the dynamics of P availability in soils more directly following the application of P fertilizer, manure or sludge, a rapid, whole-plant bioassay was developed using tomato (Lycopersicon esculentum Mill.), Chinese cabbage (Brassica rapa L. var.pekinensis) and wheat (Triticum aestivum L.). Plant P extracted in 0.1 M H₂SO₄ (P_i) and total P (P_t) concentration or content in stem, leaves or whole shoots were highly correlated (P < 0.01) with P fertilizer rates or water-soluble (WSP) or Olsen P in various soils, over wide ranges of soil P status. The whole-plant P_i content was found to be as informative as the more complicated indices of P_t or P_iconcentration. The assay was used to compare availability of fertilizer-P and sewage-sludge-P after incorporation into alluvial soil during 1–100 days of incubation. While both soil and plant indices had shown that fertilizer-P was more highly available than sewage-sludge-P in each period, the bioassay was much more sensitive than the Olsen-P or WSP soil indices in showing P fixation and decrease of availability during incubation time. The bioassay is sufficiently rapid (5–12 days) to allow a study of short-term changes in soil-P availability following incorporation of various P additives, and it is applicable to a very wide range of P availability values (6–535 mg Olsen-P kg^–1), extending from lower than desired for crop production to higher than permitted from an environmental standpoint.     

Reaction equilibrium for lipase-catalyzed condensation in organic solvent systems

Lipase-catalyzed condensation in an organic solvent is useful for the syntheses of esters. To reasonably design and optimize the reaction conditions, knowledge of the reaction equilibrium is required. The interaction of water with other reactants and the quantitative predictions for adsorption of water by a desiccant are discussed. The solvent effects on the reaction equilibrium are also elucidated in mixtures of nitrile and tert-alcohol.     

Effects of Nutrient Enrichment on Decomposition and Fungal Colonization of Sweet Chestnut Leaves in an Iberian Stream (Central Portugal)

This study assessed the effect of nutrient enrichment on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation of sweet chestnut (Castanea sativa Miller) leaves in a 3rd order stream (Central Portugal), with medium to high background values of nutrients. Coarse and fine mesh leaf bags were attached to nutrient diffusing substrata containing NaNO₃, KH₂PO₄, both nutrients, or no additions. Leaf breakdown rates were similar in the four treatments and in the two mesh sizes (k=−0.0155 to −0.0219 day⁻¹). Phosphorus content of P or N + P enriched leaves was higher than in the other treatments after 28 days, but there were no differences in N concentrations. Ergosterol concentrations associated with decomposing leaves were similar among treatments. The peak sporulation rates of aquatic hyphomycetes were stimulated by the addition of N + P and N but not by P alone. Results from the experiment provide evidence that leaf breakdown in the study stream, as a model for streams with naturally medium to high level of nutrients, was not nutrient-limited, and that fungal reproductive activity was limited by dissolved N but not by dissolved P in stream water.