Soil chemical limitations to growth and development of Veronica officinalis L. and Carex pilulifera L.

Veronica officinalis and Carex pilulifera, widespread plants of acid soils in Europe, were grown in 50 soils of natural and seminatural ecosystems representing a wide range of soil chemical properties. The experiment was performed in a greenhouse at a soil moisture content of 55–65% WHC, ca. 60% R.H. of the air, temperature 14–16°C at night and 19–21°C by day; additional light 70 W m^-2 12 h d^-1. Properties closely related to soil acidity precluded growth of V. officinalis and limited the growth rate of C. pilulifera at soil pH-KCl < ca. 3.4. In slightly-moderately alkaline (calcareous) soils, growth was primarily limited by insufficient phosphate uptake. A low growth rate of C. pilulifera. in such soils was related to low concentrations of exchangeable soil phosphate and low tissue concentrations of phosphorus. However, in high-pH soils, secondary effects due to suboptimum trace element (probably Fe) conditions, giving rise to symptoms of chlorosis, were also indicated. The highest growth rates of both species were invariably measured in soils of intermediate acidity having very high concentrations of exchangeable phosphate. Multiple regression tests on the entire material indicated that 65–75% of the variability in several growth functions could be accounted for, when two or more soil characters were included in the equation. Besides phosphate, exchangeable Zn (in C. pilulifera) and nitrate (in V. officinalis) were of considerable importance in accounting for growth rates.     

Nitrogen Use Efficiency of Crop Plants: Physiological Constraints upon Nitrogen Absorption

Current global nitrogen fertilizer use has reached approximately one hundred billion kg per annum. In many agricultural systems, a very substantial portion of this applied nitrogen fertilizer is lost from soil to groundwaters, rivers and oceans. While soil physicochemical properties play a significant part in these losses, there are several characteristic features of plant nitrogen transporter function that facilitate N losses. Nitrate and ammonium efflux from roots result in a reduction of net nitrogen uptake. As external nitrate and ammonium concentrations, respectively, are increased, particularly into the range of concentrations that are typical of agricultural soils, elevated rates of nitrate and ammonium efflux result. The rapid down-regulation of high-affinity influx as plants become nitrogen replete further reduces the root's capacity to acquire external nitrogen; only nitrogen-starved roots absorb with both high capacity and high affinity. The results of studies using molecular biology methods demonstrate that genes encoding nitrate and ammonium transporters are rapidly down-regulated when nitrogen is resupplied to nitrogen-starved plants. Provision of ammonium to roots of plants actively absorbing nitrate imposes a block on nitrate uptake, the extent of which depends on the ammonium concentration, thus further reducing the efficient utilization of soil nitrate. During the daily variation of incoming light and during periods of low incident irradiation (i.e. heavy cloud cover) the expression levels of genes encoding nitrate and ammonium transporters, and rates of nitrate and ammonium uptake, are substantially reduced. Low temperatures reduce growth and nitrogen demand, and appear to discriminate against high-affinity nitrogen influx. In sum, these several factors conspire to limit rates of plant nitrogen uptake to values that are well below capacity. These characteristics of the plant's nitrogen uptake systems facilitate nitrogen losses from soils.     

Phosphate-solubilizing microorganisms isolated from rhizospheric and bulk soils of colonizer plants at an abandoned rock phosphate mine

The abandoned “Monte-Fresco” rock phosphate mine in Táchira, Venezuela, was sampled to study the biodiversity of phosphate-solubilizing microorganisms (PSM). Rhizosphere and bulk soils were sampled from colonizer plant species growing at a mined site where pH and soluble P were higher than the values found at a near by unmined and shrubby soil. Counting and isolating of PSM choosing strains showing high solubilization halos in a solid minimal medium with hydroxyapatite as phosphate source were evaluated using ammonia or nitrate as nitrogen sources and dextrose, sucrose, and mannitol as carbohydrate sources. A larger number of PSM were found in the rhizospheric than in the bulk soil. Six fungal strains belonging to the genus Penicillium and with high hydroxyapatite dissolution capacities were isolated from bulk soil of colonizer plants. Five of these strains had similar phenotypes to Penicillium rugulosum IR-94MF1 but they solubilized hydroxyapatite at different degrees with both nitrogen sources. From 15 strains of Gram-negative bacteria isolated from the rhizosphere of colonizer plants, 5 were identified as diazotrophic free-living encapsulated Azotobacter species able to use ammonium and/or nitrate to dissolve hydroxyapatite with glucose, sucrose and/or mannitol. Different nitrogen and carbohydrate sources are parameters to be considered to further characterize the diversity of PSM.     

Uptake capacity of amino acids by ten grasses and forbs in relation to soil acidity and nitrogen availability

Uptake capacity of organic nitrogen was studied in solution experiments on eight grasses and two forbs growing in acid soils with relatively high nitrogen mineralisation in southern Sweden. Uptake of a mixture of amino acids (alanine, glutamine, glycine), that varied between 1.6 and 6.3 μmol g(-1) dw root h(-1), could not be explained by soil data from the species' field distributions (pH, total carbon and nitrogen, potential net mineralisation of ammonium and nitrate). The ratio between organic and inorganic nitrogen (methylamine) uptake was <0.05 for the forbs, higher for the grasses with a maximum of 1.42 for Deschampsia flexuosa. The ratio was negatively correlated with measures related to soil acidity (Ellenberg's R-value, soil nitrate and total carbon) but not, as hypothesised, with the total amount of mineralised nitrogen. The total demand on nitrogen by all components of the ecosystem would probably have described the extent to which competition among and between plants and microbes induced nitrogen limitation. In a methodological study two grasses were exposed to pH 3.8, 4.5 and 6.0 and to 50, 100 and 250 μmol l(-1) of three amino acids. Uptake was also compared between intact plants and excised roots. The treatment response varied considerably between the species which stresses the importance of studying intact plants at field-relevant pH and concentrations.     

Nitrogen and Phosphorus in a Stretch of the Guadalupe River,Texas, with Five Main-Stream Impoundments by

Nitrogen and phosphorus were studied in a 168-km stretch of the Guadalupe River that had five main-stream impoundments. Flow through the study area was controlled by releases from these five reservoirs and from Canyon Reservoir, a deep-storage reservoir, located 30 km upstream. Parameters measured monthly on a diel basis at 16 stations were nitrate nitrogen, nitrite nitrogen, ammonia nitrogen, Kjeldahl nitrogen, inorganic phosphate phosphorus, organic phosphate phosphorus, and total phosphate phosphorus.Inorganic nitrogen concentrations observed in this study were as high or higher than that previously reported for other bodies of water. Nitrate nitrogen entered the study area in relatively high concentrations from Comal Springs which was a major source of water for the Guadalupe River. Water from Canyon Reservoir, the other major source of water, was relatively low in nitrate nitrogen. The concentration of nitrate nitrogen was, therefore, dependent in part upon the portion of the total river flow originating from the two sources. Increased discharge from Canyon Reservoir and utilization by plants in areas of high chlorophyll a resulted in low nitrate-nitrogen levels. Retention of water in reservoirs reduced the concentration of nitrate nitrogen due to increased utilization by plants in areas of low flow. Nitrate nitrogen, in general, reached seasonal minima in summer and maxima in winter. Nitrite nitrogen showed considerable variation with no meaningful pattern except that higher concentrations occurred in association with high chlorophyll a and high Kjeldahl nitrogen, regions and periods of low river flow, and large phytoplankton populations. There was no increase in concentration of any form of nitrogen in the vicinity of sewage outfalls and no downstream accrual.Phosphorus levels in the study area were as high or higher than those reported in studies of other bodies of water. Sewage treatment plants at New Braunfels and Seguin, Texas, were major sources of phosphorus to the Guadelupe River. Total phosphate phosphorus was determined to be the most critical phosphate parameter in assessing eutrophication. Seasonally, it ranged from a winter high to a summer low. Concentrations were highest immediately below sewage outfalls and decreased as water progressed downstream. Inorganic-phosphate-phosphorus concentrations showed no clear seasonal trend but were clearly associated with sewage outfalls. Since large standing crops of phytoplankton were observed in areas of low inorganic phosphate phosphorus, it was not considered to limit photosynthesis. Total organic phosphate phosphorus varied seasonally, with high concentrations occurring during the spring and low concentrations in the fall. Total organic phosphate phosphorus showed no correlation with sewage outfalls, but was correlated to a degree with total Kjeldahl nitrogen and chlorophyll a. No consistent pattern of diel fluctuations was evident for any phosphorus or nitrogen compounds analyzed.     

Modelling yield losses of aluminium-resistant and aluminium-sensitive wheat due to subsurface soil acidity: effects of rainfall, liming and nitrogen application

Subsurface soil acidity reduces the growth of roots, which can potentially decrease crop yields. However, the magnitude of these yield reductions is dependent on interactions between factors such as the depth and severity of subsurface soil acidity, plant resistance to acidity, and water and nutrient availability. The Agricultural Production Systems Simulator (APSIM) was used to examine effects of these factors and their interactions on wheat yields in the Mediterranean climatic regions of Western Australia. The model was linked to historical meteorological data of the region (up to 90 different seasons), and was run for three locations representing low, medium and high rainfall zones and three constant but contrasting soil acidity profiles in a deep sandy soil with two wheat cultivars differing in aluminium (Al) resistance. The simulated results showed inherently high variability between seasons in grain yield, rooting depth and nitrogen leaching. Subsurface soil acidity could decrease average grain yields by up to 60%, particularly in soil profiles with acidity in deep layers. The adverse effects of acidity on wheat yields were greater in the high than the low rainfall zone. Amelioration of acidity by 75% in the entire profile or in the top 20-cm layer improved the yield of the Al-sensitive wheat cultivar. Growing Al-resistant wheat partially eliminated the negative effects of acidity on yields in soils with severe subsurface acidity and almost fully eliminated these negative effects in soils with moderate subsurface acidity. The yield benefits arising from growing Al-resistant wheat were greater than those from ameliorating acidity in the 0–20 cm layer by liming. Increasing nitrogen input increased yields of both Al-sensitive and Al-resistant wheat grown in acid soils in all the rainfall zones, but the yield increments were much greater in the high than the low rainfall zones. Applications of nitrogen fertilisers mitigate the effect of acidity on yields of Al-sensitive wheat in soils with shallow (10–40 cm) subsurface acidity. Furthermore, the improved yield by growing Al-resistant wheat and amelioration of acidity was correlated with increased rooting depth and was associated with decreased nitrogen leaching. Possible agronomic management options to combat the subsurface acidity problem are discussed.     

Partitioning of inorganic nitrogen assimilation between the roots and shoots of cerrado and forest trees of contrasting plant communities of South East Brasil

Summary Woody plants growing in cerrado and forest communities of south-east Brasil were found to have low levels of nitrate reductase activity in their leaves suggesting that nitrate ions are not an important nitrogen source in these communities. Only in the leaves of species growing in areas of disturbance, such as gaps and forest margins, were high levels of nitrate reductase present. When pot-grown plants were supplied with nitrate, leaves and roots of almost all species responded by inducing increased levels of nitrate reductase. Pioneer or colonizing species exhibited highest levels of nitrate reductase and high shoot: root nitrate reductase activities. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase were present in leaves and roots of the species examined.¹⁵N-labelled nitrate and ammonium were used to compare the assimilatory characteristics of two species:Enterolobium contortisiliquum, with a high capacity to reduce nitrate, andCalophyllum brasiliense, of low capacity. The rate of nitrate assimilation in the former was five times that of the latter. Both species had similar rates of ammonium assimilation. Results for eight species of contrasting habitats showed that leaf nitrogen content increased in parallel with xylem sap nitrogen concentrations, suggesting that the ability of the root system to acquire, assimilate or export nitrate determines shoot nitrogen status. These results emphasise the importance of nitrogen transport and metabolism in roots as determinants of whole plant nitrogen status.     

Differences in Earthworm Densities and Nitrogen Dynamics in Soils Under Exotic and Native Plant Species

Previous studies of the invasion of two exotic plants – Berberis thunbergii and Microstegium vimineum – in hardwood forests of New Jersey have shown a significant increase of pH in soils under the invasive plants as compared with soils from under native shrubs (Vaccinium spp). We present a further investigation of soil properties under the exotic plants in question. We measured the densities of earthworms in the soil under the two exotics and the native shrubs in three parks in New Jersey. In the same populations we also measured the extractable ammonium and nitrate in the top 5 cm of the soil, as well as the respiration of the soils and the potential rates of mineralization (aerobic lab incubation). In addition, we measured the nitrate reductase activity in leaves of the two exotic plants and several native shrubs and trees. Although there were differences between parks, we observed significantly higher earthworm densities in the soil under the exotic species. The worms were all European species. Soil pH, available nitrate and net potential nitrification were significantly higher in soils under the two exotic species. In contrast, total soil C and N and net ammonification were significantly higher under native vegetation. Nitrate reductase activities were much higher in the leaves of exotic plants than in the leaves of native shrubs and trees. Changes in soil properties, especially the change in nitrogen cycling, associated with the invasion of these two plant species may permit the invasion of other weedy or exotic species. Our results also suggest that even if the two exotic species were removed, the restoration of the native flora might be inhibited by the high nitrate concentrations in the soil.     

Mineral Nutrient Requirements of Pinus silvestris and Picea abies Seedlings

The mineral nutrient requirements of Pinus silvestris L. and Picea abies Karst. were studied according to previously published methods applied to a series of various plant species. The optimum nutrient proportions are similar to those of Vaccinium, with a lower relative potassium requirement than birch and other broad-leaf species. Various ratios between ammonium and nitrate nitrogen were about equally efficient except for a minor growth reduction with pure nitrate, which gave a comparatively low nitrogen content and a high cation/nitrogen ratio. The rate of ammonium uptake was much higher than that of nitrate when both sources were supplied. The required total concentration in the nutrient solution for maximum growth is lower in pine than in spruce, but both fall within the low salt range. Both species, especially pine, are sensitive to high salt concentrations. Although pine and spruce grow on the same type of soils as Vaccinium— leached soils with low base saturation – accumulation of calcium or other cations is not as pronounced as in Vaccinium, especially not in pine. The results are compared with results from similar experiments with a series of other conifers. All the conifers have more flexible cation uptake mechanisms than Vaccinium but the results indicate tendencies to accumulation of anions, nitrate and phosphate. Recommended fertilizer compositions for forest fertilization and nurseries are discussed.     

Nitrogen use strategies of neotropical rainforest trees in threatened Atlantic Forest

The characteristics of nitrogen acquisition, transport and assimilation were investigated in species of an Atlantic Forest succession over calcareous soil in south‐eastern Brazil. Differences in behaviour were observed within the regeneration guilds. Pioneer species showed high leaf nitrogen contents, a high capacity to respond to increased soil nitrogen availability, a high capacity for leaf nitrate assimilation and were characterized by the transport of nitrate + asparagine. At the other end of the succession, late secondary species had low leaf nitrogen contents, little capacity to respond to increased soil nitrogen availability, low leaf nitrate assimilation and were active in the transport of asparagine + arginine. The characteristics of nitrogen nutrition in some early secondary species showed similarities to those of pioneer species whereas others more closely resembled late secondary species. Average leaf δ¹⁵N values increased along the successional gradient. The results indicate that the nitrogen metabolism characteristics of species may be an additional ecophysiological tool in classifying tropical forest tree species into ecological guilds, and may have implications for regeneration programmes in degraded areas.     

Soil nitrogen patterns induced by colonization of Polygonum cuspidatum on Mt. Fuji

Summary The spatial pattern of soil nitrogen was analyzed for a patchy vegetation formed by the colonization of Polygonum cuspidatum in a volcanic desert on Mt. Fuji. Soils were sampled radially from the bare ground to the center of the patch, and analyses were done for bulk density, water content, soil acidity, organic matter, organic nitrogen, and ammonium and nitrate nitrogen. The soils matured with succession from the bare ground through P. cuspidatum to Miscanthus oligostachyus and Aster ageratoides sites: bulk density decreased, and water content, organic matter, organic nitrogen, and ammonium nitrogen increased. Nitrate nitrogen showed the highest values at the P. cuspidatum site. Application of principal component analysis to the soil data discriminated two component factors which control the variation of soil characteristics: the first factor is related to soil formation and the second factor to nitrogen mineralization and nitrification. The effect of soil formation on nitrogen mineralization and nitrification was analyzed with a first-order kinetic model. The decreasing trends with soil formation in the ratios of mineral to organic nitrogen and of nitrate to ammonium nitrogen could be accounted for by the higher activity of immobilization by microorganisms and uptake by plants in the more mature ecosystem.     

Growth performance of an inland population of Plantago maritima in response to nitrogen and salinity

The inland distribution of Plantago maritima in Ireland and Britain is strongly western and generally associated with base-rich soils or with flushes in more acidic upland soils. Plants from an eastern Irish salt marsh and from an inland population growing on shallow calcareous soils in east County Clare, Ireland, were grown in culture solutions with a range of seawater dilutions (10–100%) and nitrogen (2.8–140 ppm N) concentrations added as ammonium nitrate. A low nitrogen supply (2.8 ppm N) resulted in very low dry matter production but with increased nitrogen, plant growth increased, even of inland population plants in 50% seawater. Some inland plants survived 100% seawater, but growth was much reduced. The relatively high salinity tolerance of this inland population is discussed.Nomenclature follows Flora Europaea (Tutin et al., 1964–80) for angiosperms, Watson (1968) for bryophytes and Hawksworth et al. (1980) for lichens.Thanks are due to the Central Marine Services Unit and Michael Coughlan, Microbiology Department at University College Galway for Mullaghmore soil nitrogen analyses.     

Different performances of soil phosphate tests for reflecting the effects of buffering capacity on uptake of native phosphate with time

The ability of two sodium bicarbonate (Colwell and Olsen) and two ammonium fluoride (Bray I and Bray II) soil tests to reflect the effect of phosphate buffering capacity of the soil on plant growth through time was studied on ten Argentine soils. The soils were divided into three groups (low, medium and high buffering capacity) according to a buffering index calculated from the slope of the Freundlich equation. The relation between phosphate extracted by soil tests and both relative yield and phosphate uptake of rye grass plants was affected by the phosphate buffering capacity of the soil. The effect of buffering on that relation was more marked for the sodium bicarbonate tests (specially Colwell) than for the Bray tests. This effect was consistent with time. Hence, adjustment for buffering would be more important for the sodium bicarbonate tests than for the Bray tests. Soils with high buffering capacity were able to sustain a greater rate of phosphate uptake. The effect of buffering on the relation between soil tests and both relative yield and phosphate uptake was greatest when the plants were young and decreased with time. This effect would therefore be very important for the early nutrition of annual pasture or crop species.     

Impact of low pH and aluminium on nitrogen uptake and metabolism in roots of <Emphasis Type="Italic">Lotus japonicus</Emphasis>

The effect of low pH and aluminum on nitrogen uptake and metabolism was studied in roots of Lotus japonicus grown in hydroponic cultures. The low pH slightly suppressed root elongation, and this effect was accompanied by the suppression of nitrate and ammonia uptake, as well as the nitrate reductase activity. In spite of high resistance of young Lotus plants to short-term Al application, the one-day treatment of Al strongly reduced nitrate uptake and also the activity of nitrate reductase (NRA) in the apical parts of roots. The glutamine synthetase activity was also suppressed by Al treatment, but in lower extent. On the other hand, the ammonium uptake and nitrite reductase activity stayed unchanged by Al treatment and the values were practically the same as in control plants. These results support the view that nitrate uptake and nitrate reduction might be the main processes responsible for Al induced growth retardation in Lotus plants grown in mineral acid soils.     

Water availability – a physiological constraint on nitrate utilization in plants of Australian semi-arid muiga woodlands

Nitrate was found to be the predominant form of available nitrogen in mulga soils. Nitrate reductase activities on a fresh mass basis of a range of plants from eastern (Queensland) mulga ecosystems 2 weeks after partial relief from drought were uniformly low for both herbaceous species (165 ± 25 pkat g^?1) and woody perennials (77 ± 14 pkat g^?1). Supply of nitrate for 24 h to cut transpiring shoots of woody species or application of nitrate solution to the rooting zone of herbaceous species promoted little further increase in mean shoot nitrate reductase activities. Most species exhibited high tissue nitrate concentrations during water stress and soluble organic N profiles were in many cases dominated by the osmoprotective compounds, proline or glycine betaine. Species with low levels of proline or glycine betaine showed high foliar concentrations of other compatible osmotica such as polyols or sugars. Effects of relieving water stress on nitrate reductase activity, proline, glycine betaine and nitrate levels were followed over, 3d of irrigation. Available soil nitrate rose 10-fold immediately and, following rapid restoration of leaf water status of the eight study species, a 4-fold increase occurred in mean nitrate reductase activity together with progressive decreases in mean tissue concentrations of nitrate, proline and glycine betaine over the 3 d period. Similar changes in soil nitrate, nitrate reductase activity, proline and tissue nitrate were observed in the same ecosystem following a natural rainfall event and in western (S.W. Australia) mulga following irrigation. It is concluded that, although nitrate nitrogen is present at high concentrations and is the predominant inorganic nitrogen source in soils of the mulga biogeographic region, its assimilation by perennial and ephemeral vegetation is limited primarily by water availability. A scheme is presented depicting interrelated physiological and biochemical events in typical mulga species following a rain event and subsequent drying out of the habitat.     

The effect of regeneration burning upon the nutrient status of soil in two forest types in southern Tasmania

In two forest types in southern Tasmania, eucalypt rainforest (mixed forest) and eucalypt dry sclerophyll forest, surface soils (0–10 cm) from stands that had been clear-felled and burned between 1976 and 1979 were compared with those from uncut, unburned stands. Factors compared were total organic C, N, P, K, Mg, Ca, Zn, Mn; pH; exchangeable Ca, Mg, and K; cation exchange capacity; extractable P; soil phosphate buffering capacity; and N-mineralisation rates. Sampling started in April 1979 and ended in October 1980. Within each forest type, soils from burned coupes had higher mean values for pH, exchangeable cations, percent base saturation, and nitrate-N produced during aerobic incubation, and had lower mean values for exchangeable acidity and ammonium-N produced during aerobic incubation than soils from unburned coupes. In mixed forest only, soils from burned coupes had higher mean values for extractable P and soil phosphate buffering capacity, and lower mean values for total organic C than those of unburned coupes. There were only small differences between burned and unburned soils in cation exchange capacity and ammonium-N produced during anaerobic incubation. For each burned coupe in mixed forest, with increase in time since burning there was a decrease in pH, an increase in exchangeable acidity, and a decrease in rate of production of nitrate: no changes were detected in other factors. It is concluded that, for clay soils developed on dolerite, the nutritional status of soil in both forest types is probably improved by burning. The improvement lasts for more than 4 years in mixed forest and more than two years in dry sclerophyll forest. Only minor leaching of nutrients to below 10 cm in depth is likely to occur in either type.     

Relationship of ion absorption to growth rate in taiga trees

Summary Rates of nutrient absorption were measured on excised roots of taiga tree seedlings grown in the laboratory. Phosphate and to a lesser extent ammonium (relatively immobile ions in the soil) were absorbed most rapidly by poplar and aspen, two species with rapid growth rates and most slowly by alder and/or black spruce, species with slow growth rates. In contrast, potassium (which is more mobile in soil) was absorbed most rapidly by slowly growing species. All species had low rates of nitrate and chloride absorption. Absorption rate of each ion was most temperature sensitive in those species that typically occupy the warmest soils (i.e. poplar and aspen). We suggest that in infertile soils a high capacity for uptake is an important component of root competition only in the case of mobile ions (e.g. potassium, nitrate), because only for these ions do diffusion shells of adjacent roots overlap; in contrast plants compete for immobile ions (e.g. phosphate) only by increasing absorptive surface via root growth or mycorrhizal association.     

Buckwheat (Fagopyrum esculentum Moench) has high capacity to take up phosphorus (P) from a calcium (Ca)-bound Source

Two experiments were carried out in a growth chamber to investigate the phosphorus (P)-uptake efficiency of Fagopyrum esculentum Moench (buckwheat) and Triticum aestivum (spring wheat) from a Ca-bound form. The first experiment was based on a sand-culture system with either rock phosphate (RP) or CaHPO₄ (CaHP) as the P source and nitrate or ammonium nitrate as nitrogen source. A highly calcareous soil was used in the second experiment. Buckwheat was shown to be highly efficient in taking up Ca-bound P compared to spring wheat. When plants were supplied with nitrate, the total P uptake by buckwheat from RP was nearly 10-fold higher than that of spring wheat (20.1 compared with 2.1 mg P pot^–1). Changing nitrogen source from nitrate only to ammonium nitrate increased P uptake by spring wheat substantially, but not buckwheat. High P-uptake efficiency of buckwheat was also demonstrated using the field soil, but to a lesser extent, which may be related to the difference in Zn supply between sand culture and field soil. It is suggested that buckwheat may be included in intercropping or crop rotation systems to activate P sources in calcareous soils. The principal mechanism of P uptake efficiency of buckwheat may be its ability to acidify the rhizosphere; however, further study is needed to unravel the regulation of root excretion of H⁺ and its molecular basis in order to exploit buckwheat's genetic capability to utilise sparingly soluble P from soil.     

Nitrogen Fixation and Nitrogen Assimilation in a Temperate Saline Ecosystem

As nitrogen is known to be a limiting factor for plant growth, we were interested in the relationship between soil microbial activity and the nitrogen assimilation of 5 different halophytes from 4 saline sites near the lake “Neusiedlersee”, Austria. The following were studied between May and October 1985: nitrogen fixation (¹⁵N₂ and acetylene reduction): N-mineralization; several soil characteristics and in vivo nitrate reductase activity of roots and shoots of these plants. NO^?₃, org. N- and carboxylate contents of both roots and shoots, as well as the effect of NO^?₃-fertilization on the amounts of these substances, were determined on plants growing in the field during a 3-day period in September 1985. Fertilization led to a decrease in acetylene reduction activity at most sites, and an increase in the nitrate reductase activity of the shoots of all plants. Overall, carboxylate and organic nitrogen contents of these halophytes did not change in response to fertilization. Only in the roots of Aster tripolium and Atriplex hastata was there a marked increase in the nitrate reductase activity in response to fertilization. Species growing at the same site, such as Plantago maritima and Lepidium crassifolium showed contrasting levels of assimilatory activity. Apparent low rates of ammonification and nitrification were detected in soils from the 4 sites. The results are discussed in relation to the nitrogen and carbon economies of the microorganisms and plants.     

The nature of three ancient woodland soils in southern England

Abstract. Despite a wealth of published research on the nature of woodland soils, little is known about the nature of soils on sites that have supported woodland for many hundreds of years, namely ancient woodland. The properties and variability of soils in three ancient woods; one in the New Forest, Hampshire and two in Berkshire, were compared with those under recent woods. The acidity of ancient and recent woodland soils was high and remarkably similar. Only where cultivation of soils had preceded woodland establishment was soil acidity lower. The quantity of carbon in the soils studied was inversely related to soil acidity and the ancient woods had accumulated larger quantities of carbon than their recent counterparts. The quantities of Ca²⁺, Mg²⁺ and K⁺ were larger in the ancient woods except where prior cultivation had taken place. Total and organic phosphate contents of the ancient woodland soils were also consistently larger. The nature and pattern of soil variability in ancient woodland soils was quite distinct from that found in recent woods. Overall, the variability of soil acidity, carbon content and organic phosphate was larger in the ancient woodland soils but the pattern of variability differed between the soil properties. No clear association existed between the pattern of soil acidity and individual trees. At the surface of some of the woodland soils, however, carbon distribution appeared to be associated with individual trees. At depth in the ancient woodland soils, the association with the existing vegetation cover was not so clear. It is probable that the ancient woodland soils retained relict features of previous vegetation cover. Organic phosphate distribution was very strongly associated with the present vegetation cover. The pattern of distribution of organic phosphate appeared to be stronger than that of soil acidity and carbon content.