A bioassay of the availability of residual 15N fertilizer eight years after application to a forest soil in interior British Columbia

Although a high proportion of fertilizer N may be immobilized in organic forms in the soil, no studies have examined the long-term availability of residual fertilizer ¹⁵N in forestry situations. We investigated this by growing lodgepole pine (Pinus contorta) seedlings in surface (0–10 cm) soil sample eight years after application of ¹⁵N-urea, ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ to lodgepole pine in interior British Columbia. After nine months of growth in the greenhouse, seedlings took up an average of 8.5% of the ¹⁵N and 4.6% of the native N per pot. Most of the mineral N in the pots without seedlings was in the form of nitrate, while pots with seedlings had very low levels of mineral N. In contrast to the greenhouse study, there was no significantuptake of ¹⁵N by trees in the field study after the first growing season, although half of the soil organic ¹⁵N was lost between one and eight years after fertilization. This indicates the need to understand the mechanisms which limit the uptake of mineral N by trees in the field, and the possible mismatch of tree demand and mineral N availability.     

Effects of calcium and aluminium on nodulation,nitrogen fixation and growth of groundnut in solution culture

Uptake of NH₄ and NO₃ by above ground parts of beech trees was studied by spraying young trees with varying concentrations of ¹⁵N labeled solutions, different N-forms, and spray regimes over four months. Following treatment, the trees were harvested and analyzed for ¹⁵N and major element content. Throughfall was collected and analyzed in addition in order to study the interaction between nitrogen uptake and cation leaching. Significant amounts of N were taken up by the above ground plant parts in all treatments as indicated by ¹⁵N analysis of the trees as well as by throughfall measurements. NH₄ uptake exceeded the uptake of NO₃ if applied in the same concentration. Uptake of N increased linearly with increasing concentration in the spray solution and with spray intensity. The uptaken N was translocated within the plant. The contribution of N from uptake by above ground parts to the total N content of tissues differed and reached a maximum level of 6% in leaves. No effect of above ground N uptake on the total N content of tissues was found. Calculating atmospheric N inputs to forest ecosystems by throughfall measurements may underestimate the actual N input.     

Water shortage affects the water and nitrogen balance in Central European beech forests

Whilst forest policy promotes cultivation and regeneration of beech dominated forest ecosystems, beech itself is a highly drought sensitive tree species likely to suffer from the climatic conditions prognosticated for the current century. Taking advantage of model ecosystems with cool-moist and warm-dry local climate, the latter assumed to be representative for future climatic conditions, the effects of climate and silvicultural treatment (different thinning regimes) on water status, nitrogen balance and growth parameters of adult beech trees and beech regeneration in the understorey were assessed. In addition, validation experiments with beech seedlings were carried out under controlled conditions, mainly in order to assess the effect of drought on the competitive abilities of beech. As measures of water availability xylem flow, shoot water potential, stomatal conductance as well as delta (13)C and delta (18)O in different tissues (leaves, phloem, wood) were analysed. For the assessment of nitrogen balance we determined the uptake of inorganic nitrogen by the roots as well as total N content and soluble N compounds in different tissues of adult and young trees. Retrospective and current analysis of delta (13)C, growth and meteorological parameters revealed that beech growing under warm-dry climatic conditions were impaired in growth and water balance during periods with low rain-fall. Thinning affected water, N balance and growth mostly of young beech, but in a different way under different local climatic conditions. Under cool, moist conditions, representative for the current climatic and edaphic conditions in beech forests of Central Europe, thinning improves nutrient and water status consistent to published literature and long-term experience of forest practitioners. However, beech regeneration was impaired as a result of thinning at higher temperatures and under reduced water availability, as expected in future climate.     

Limited transfer of nitrogen between wood decomposing and ectomycorrhizal mycelia when studied in the field

Transfer of ¹⁵N between interacting mycelia of a wood-decomposing fungus (Hypholoma fasciculare) and an ectomycorrhizal fungus (Tomentellopsis submollis) was studied in a mature beech (Fagus sylvatica) forest. The amount of ¹⁵N transferred from the wood decomposer to the ectomycorrhizal fungus was compared to the amount of ¹⁵N released from the wood-decomposing mycelia into the soil solution as ¹⁵N-NH₄. The study was performed in peat-filled plastic containers placed in forest soil in the field. The wood-decomposing mycelium was growing from an inoculated wood piece and the ectomycorrhizal mycelium from an introduced root from a mature tree. The containers were harvested after 41 weeks when physical contact between the two foraging mycelia was established. At harvest, ¹⁵N content was analyzed in the peat (total N and ¹⁵NH₄ ⁺) and in the mycorrhizal roots. A limited amount of ¹⁵N was transferred to the ectomycorrhizal fungus and this transfer could be explained by ¹⁵NH₄ ⁺ released from the wood-decomposing fungus without involving any antagonistic interactions between the two mycelia. Using our approach, it was possible to study nutritional interactions between basidiomycete mycelia under field conditions and this and earlier studies suggest that the outcomes of such interactions are highly species-specific and depend on environmental conditions such as resource availability.     

Intact amino acid uptake by northern hardwood and conifer trees

Empirical and modeling studies of the N cycle in temperate forests of eastern North America have focused on the mechanisms regulating the production of inorganic N, and assumed that only inorganic forms of N are available for plant growth. Recent isotope studies in field conditions suggest that amino acid capture is a widespread ecological phenomenon, although northern temperate forests have yet to be studied. We quantified fine root biomass and applied tracer-level quantities of U–¹³C₂–¹⁵N-glycine, ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ in two stands, one dominated by sugar maple and white ash, the other dominated by red oak, beech, and hemlock, to assess the importance of amino acids to the N nutrition of northeastern US forests. Significant enrichment of ¹³C in fine roots 2 and 5 h following tracer application indicated intact glycine uptake in both stands. Glycine accounted for up to 77% of total N uptake in the oak–beech–hemlock stand, a stand that produces recalcitrant litter, cycles N slowly and has a thick, amino acid-rich organic horizon. By contrast, glycine accounted for only 20% of total N uptake in the sugar maple and white ash stand, a stand characterized by labile litter and rapid rates of amino acid production and turnover resulting in high rates of mineralization and nitrification. This study shows that amino acid uptake is an important process occurring in two widespread, northeastern US temperate forest types with widely differing rates of N cycling.     

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine,USA

We followed the movements of ¹⁵N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha^–1 year^–1 using ¹⁵N-labelled nitric acid solutions (¹⁵N = 344 ) during four successive growing seasons (April–October). The ¹⁵N contents of foliage, bolewood, forests floor and mineral soil (0–5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.     

Nitrogen uptake from prey and substrate as affected by prey capture level and plant reproductive status in four carnivorous plant species

Uptake of nitrogen from prey and substrate and partitioning of prey-derived nitrogen were studied in the carnivorous plant species Pinguicula alpina, P. villosa, P. vulgaris and Drosera rotundifolia in a subarctic environment. Efficiency in nitrogen uptake from prey was evaluated by tracing ¹⁵N from ¹⁵N-enriched Drosophila flies fed to the plants. The in situ uptake efficiency differed somewhat between species and ranged from 29 to 41% of prey N. This efficiency was not affected by different feeding levels or plant reproductive status (flowering or non-flowering). A test of the amount of N absorbed from prey caught on flower stalks of Pinguicula villosa and P. vulgaris showed that both species took up little of what was available in prey (2.5% or less). The uptake efficiency found in greenhouse grown plants was higher than in plants in situ (40–50% vs. 30–40% respectively). This could probably best be explained by the absence of rain and a higher temperature in the greenhouse. The prey-derived ¹⁵N was traced to reproductive organs and winter buds. Non-flowering individuals allocated 58–97% of the N derived from prey to their winter buds. Flowering individuals allocated 17–43% of the N income from prey to reproduction, while 34–71% were allocated to buds. Root uptake of nitrogen was stimulated by increased prey capture. This increase in uptake of nitrogen from the substrate was larger than the potential direct uptake of nitrogen from captured prey.     

Temporal patterns of inorganic nitrogen uptake by mature sugar maple (Acer saccharum Marsh.) and red spruce (Picea rubens Sarg.) trees using two common approaches

Background: Plant uptake of nitrogen influences many ecosystem processes, yet uptake by trees in northern forests of the United States has not been quantified throughout the growing season.

Aims: To measure NH₄ ⁺ and NO₃ ^? uptake by mature sugar maple (Acer saccharum) and red spruce (Picea rubens) trees during the early, mid and late growing season.

Methods: At Hubbard Brook Experimental Forest, New Hampshire, we used two approaches to measure nitrogen uptake capacity by mature trees: an in situ depletion method using intact roots and an ex situ ¹⁵N tracer method using excised roots.

Results: NH₄ ⁺ uptake was greater than NO₃ ^? for both methods and tree species (P < 0.05). NH₄ ⁺ uptake was lowest during the early growing season, while NO₃ ^? uptake was lowest during the late growing season. Measured rates of NH₄ ⁺ uptake were 2–3 orders of magnitude greater using the in situ depletion method compared with the ex situ ¹⁵N tracer method.

Conclusions: These results demonstrate seasonal differences in nitrogen uptake by two dominant tree species in a northern forest and show that the method employed can significantly impact measured rates of uptake, which could have implications for understanding the magnitude of plant nitrogen uptake and for cross-study comparisons of this process.     

N2 fixation and nitrogen allocation to above and below ground plant parts in red clover-grasslands

Leys, used for grazing or production of forage to be conserved as silage or hay, are very important crops in northern areas. In order to measure the N₂ fixation in leys of varying ages and during different parts of the season, detailed measurements were taken of yield, N₂ fixation and the amounts of N remaining in the field after harvesting red clover (Trifolium pratense L.)-grass leys at a site in northern Sweden, where they are generally harvested twice per growing season. Entire plants, including stubble and roots, were sampled at the time of first and second harvest and, in addition, at the end of the growing season in three neighbouring fields, carrying a first, a second and a third year ley, respectively. N₂ fixation was measured by both ¹⁵N isotope dilution (ID) and ¹⁵N natural abundance (NA) methods. The proportion of clover dry matter (DM) in the stands increased from the first to the second harvest, but the grasses dominated throughout the entire season, especially below ground. The N concentrations, in both herbage and whole plants, were about twice as high in the clover as in the grasses. Seasonal variations in N concentrations were minor, and total N contents followed the same trends as DM. The clover acquired nearly all of its N from N₂ fixation: the proportion of N in clover herbage derived from N₂ fixation was often >0.8 throughout the season. The variations in the amounts of N₂ fixed during the course of the season corresponded well to the seasonal changes in clover biomass. Amounts of fixed N₂ allocated to clover herbage during the whole season were in the range 4 to 6 g N m⁻² in this unusually rainy year. Calculations of daily N allocation rates to herbage showed that N uptake rates were similar, and high, in grasses during May–June and July–August, while N₂ fixation rates in clover were about 10-fold as high in July–August as in May–June, reflecting the need for N in clover growth. The proportion of N remaining in clover stubble and roots after the first and second harvests was about 60 and 25%, respectively, while about 60% of the N in grasses remained in stubble and roots after both harvests. The considerable amounts of biomass and N that were left in field after harvesting red clover-grass leys are important for re-growth of the plants and provide substantial N fertilization for the next crop in the crop rotation.     

Rhizospheric NO interacts with the acquisition of reduced N sources by the roots of European beech (Fagus sylvatica L.)

The gas phase of the soil plays an important role in plant growth and development. We investigated the effect of rhizospheric NO as a signalling compound for N uptake of beech roots. Following exposure to NO, ammonium and glutamine uptake into roots were determined using ¹⁵N-labelling, and gene expression of selected transporters was analysed by quantitative real-time PCR. Uptake of both N sources increased significantly with elevated NO concentration. However, with one exception, this increase was not reflected in up-regulation of expression of the respective transporters.     

The vertical distribution of N and K uptake in relation to root distribution and root uptake capacity in mature Quercus robur, Fagus sylvatica and Picea abies stands

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting ¹⁵N and caesium (Cs; as an analogue to K) at 5 and 50 cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak (Quercus robur), European beech (Fagus sylvatica) and Norway spruce (Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50 cm soil depth in June 2003. After 65 days foliage and buds were harvested and the concentrations of ¹⁵N and Cs analysed. The recovery of ¹⁵N in the foliage and buds tended to be higher from 5 than 50 cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of ¹⁵N in the soil. In oak more Cs was recovered from 5 than from 50 cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50 cm soil depth compared to 5 cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50 cm soil depth compared with 5 cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depths.     

Influences of nitrogen load on the growth and photosynthetic responses of <Emphasis Type="Italic">Quercus serrata</Emphasis> seedlings to O<Subscript>3</Subscript>

The objectives of this study were to clarify the influences of nitrogen (N) load on the growth and photosynthetic responses of Quercus serrata seedlings to O₃ and to obtain basic data for evaluating the critical levels of O₃ for protecting Q. serrata forests in Japan. The effects of O₃ and/or N load on growth and photosynthetic activity of Q. serrata seedlings were investigated during the two growing seasons. Two-year-old seedlings were assigned to 12 experimental treatments, which were comprised of the combination of four gas treatments (charcoal-filtered air and three levels of O₃ at 1.0, 1.5 and 2.0 times ambient concentration) and three N treatments (0, 20 and 50 kg ha⁻¹ year⁻¹). During the second growing season, no significant interactive effects of O₃ and N load on the growth and net photosynthetic rate of the seedlings were detected. Threrfore, we concluded that N supply to the soil at ≤50 kg ha⁻¹ year⁻¹ does not significantly influence the growth and photosynthetic responses of Q. serrata seedlings to O₃. Based on the O₃ exposure-response relationships for the whole-plant growth of the seedlings, the critical level of O₃ for Q. serrata was estimated to be approximately 36 nmol mol⁻¹ as the average 15-h O₃ concentration during the one growing season.     

Biocide manipulation of N flow to investigate root/microbe competition in forest soil

Root/microbe competition was investigated as a mechanism controlling fertilizer N uptake by coniferous forest trees. Combinations of biocides both with and without ¹⁵N labelled urea, were applied to microcosms containing Sitka spruce seedlings, to selectively inhibit target microbial groups which may be competing with roots for N. After 1 growing season, concentrations of fertilizer N and total N in the trees, as well as populations of microbes and animals, were determined. Biocidal inhibition of microbial populations, particularly of fungi, was associated with significant increases in concentraions of fertilizer N and total N in Sitka spruce seedlings. Application of the fungicide benlate, for example, increased the concentration of fertilizer-derived N in spruce needles by one order of magnitude, and was associated with significant reductions in FDA-active hyphal lengths of fungi. This approach to investigating N-flow offers considerable potential for short term experiments involving competition for fertilizer/available N, where the microbial biomass represents the major sink for N in competition with roots.     

15N-ammonium and 15N-nitrate uptake of a 15-year-old Picea abies plantation

Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either ¹⁵N-ammonium or ¹⁵N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). ¹⁵N-labeling (62 mg ¹⁵N m^-2 under conditions of 1.5 g N m^-2 atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The ¹⁵N recovery within the entire stand (including soils) was 94%±6% of the applied ¹⁵N-ammonium tracer and 100%±6% of the applied ¹⁵N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the ¹⁵N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the ¹⁵N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both ¹⁵N-ammonium and ¹⁵N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees. ¹⁵N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered ¹⁵N in spruce; no enrichment could be detected in stem wood. Although ¹⁵N-ammonium and ¹⁵N-nitrate were applied in the same molar quantities (¹⁵NH ₄ ⁺ : ¹⁵NO ₃ ^- =1:1), the tracers were diluted differently in the inorganic soil N pools (¹⁵NH ₄ ⁺ /NH ₄ ⁺ : ¹⁵NO ₃ ^- /NO ₃ ^- =1:9). Therefore the measured ¹⁵N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate ¹⁵N uptake from inorganic N pools resulted in a 2–4 times higher ammonium than nitrate uptake by P. abies.     

Environmental and nutritional responses of a Pinus radiata plantation to biosolids application

Since the mid-1990s, a Pinus radiata (D. Don) plantation growing on a sandy, low fertility soil at Rabbit Island near Nelson, New Zealand received aerobically digested liquid biosolids. An experimental research trial was established on the site to investigate the effects of biosolids applications on tree growth, nutrition, soil and ground water quality. Biosolids were applied to the trial site in 1997 and 2000, at three application rates: 0 (control), 300 (standard) and 600 kg N ha⁻¹ (high). Biosolids application significantly increased tree growth. This was mainly attributed to improved N supply, demonstrated by the enhanced N concentration in the tree foliage. Soil analysis indicated that biosolids application have not caused significant changes in concentrations of most nutrients. However, biosolids treatments significantly increased the available P (Olsen P). Of the heavy metals only total Cu concentrations in the soil increased after biosolids application. Groundwater quality, which was monitored quarterly, has not been affected by biosolids application. The concentrations of nitrate and heavy metals in groundwater were well below the maximum acceptable values in drinking water standards. Biological treatment of sewage and digestion of sewage sludge resulted in the enrichment of ¹⁵N in the biosolids (δ¹⁵N values between 5.0 and 8.7‰). Such enrichment was used as a tracer to study the fate of biosolids derived N. The elevated δ¹⁵N in biosolids treated pine foliage indicated that a considerable amount N was sourced from biosolids. Analysis of δ¹⁵N in understorey plants showed that both non-legume and legume understorey plants took up N from the biosolids, and acted as a N sink, reducing N availability for leaching. Our study showed that application of biosolids to a plantation forest can significantly improve tree nutrition and site productivity without resulting in any measurable adverse effect on the receiving environment.     

Application of 15N and xylem ureide methods for assessing N2 fixation of three shrub legumes periodically pruned for forage

The apparently diminished capacity for N₂ fixation by the shrub legume Calliandra calothyrsus (Calliandra) relative to other woody perennial legumes was investigated in a field experiment in northern Queensland, Australia. In this trial, (i) the proportion of plant nitrogen (N) derived from symbiotic N₂ fixation (%Pfix) and the amounts of N₂ fixed were compared in Calliandra, Gliricidia sepium (Gliricidia) and Codariocalyx gyroides (Codariocalyx), (ii) variations in N₂ fixation due to season or tree age were determined, (iii) estimates of Pfix derived with the ¹⁵N natural abundance technique were compared with values obtained from ¹⁵N enrichment or xylem sap ureide procedures to determine whether the previous conclusions about Calliandra's ability to fix N had resulted from specific problems with the natural abundance methodology used in the earlier studies.Inoculated seedlings of each of the three shrub legume species were planted in dense stands (1.5 m rows, 0.5 m between trees) in two randomised blocks. The northern block was used solely for natural abundance measurements, while ¹⁵N-enriched KNO₃ (10 atom % ¹⁵N excess) was applied four times over a 52 week period to plots in the southern block. The non-nodulating tree legume Senna spectabilis (formally Cassia spectabilis) was used as a non-N₂-fixing reference for the ¹⁵N-based procedures, with Guinea grass (Panicum maximum) included as an additional non-fixing check. Growth by the trees above 75 cm was first cut and removed after 22 weeks and regrowth was subsequently pruned periodically for another 95 weeks. Sampling for dry matter production, N yield and estimates of Pfix were restricted to the central four of the 32 plants which constituted each replicate plot. Information generated during the 117 week study indicated that estimates of Pfix by ¹⁵N natural abundance were closely similar to values derived with ¹⁵N-enrichment or sap ureides. The data indicated that Calliandra had a reduced reliance upon N₂ fixation relative to Gliricidia and Codariocalyx for the first 65 weeks after establishment. This appeared to be due to more prolifc root growth by Calliandra than either of the other N₂-fixing species and an ability to extract a greater proportion of its N requirements from soil mineral N. However, after week 65 and for the remainder of the experiment, estimates of Pfix for Calliandra were similar to the other shrub legumes. Over 117 weeks, prunings from Calliandra and Gliricidia had removed 52–58 t dry matter ha^-1, and between 1471 and 1678 kg N ha^-1, of which 1026–1063 kg N ha^-1 was estimated to have been derived from N₂ fixation. At the time of final harvest, 65–73% of the fixed N was present in shoot regrowth of the N₂ fixing shrubs, 9–18% in the roots, 15% in the trunk, and 2–6% in fallen leaves.     

The course of 15N-ammonium nitrate in a spring barley cropping system

¹⁵N-labelled ammonium nitrate was applied to spring barley growing on a Cambisol soil in western Switzerland. Immobilization, plant uptake and disappearance of inorganic nitrogen were followed at frequent intervals. Fertilizer nitrogen disappeared shortly after its application, mainly through immobilization by soil microorganisms and absorption by the crop. Some of the added nitrogen was probably denitrified as a result of humid conditions during the first days after fertilizer application. At the end of the growing season, 31% of the added nitrogen was recovered from the aerial barley plants, and 56% was immobilized by microorganisms. Most of the fertilizer nitrogen not used by the crop was immobilized in the upper 0–30 cm soil layer. This prevented downward movement of nitrate and limited nitrogen losses. Fertilizer efficiency was mainly determined by the competition between crop uptake and microbial immobilization. Careful consideration of the time of fertilization, taking into account plant growth and weather conditions, can result in an increase in fertilizer efficiency and minimal pollution.     

Seedling ecology of two miombo woodland trees

The development of seedlings of two miombo trees, Brachystegia spiciformis Benth. and Julbernardia paniculata (Benth.) Troupin, was studied during two growing seasons (December 1989–April 1991) at a Zambian grassland site. Seed germination rates under laboratory and field conditions were not significantly different although germination in the field was delayed by 1–2 weeks due to insufficient rainfall. After one year of storage J. paniculata seed germination had declined from 67% to 17% while germination of B. spiciformis seeds remained at about 83%.Leaf production was confined to the rainy season. Leaf fall occurred during the dry season and in J. paniculata this was followed by shoot die-back during the hot dry period (August–November). Two-thirds of B. spiciformis seedlings experienced shoot die-back but shoot die-back did not necessarily result in seedling mortality. Seedling deaths occurred during the germination period (6–10 weeks after planting) and in the hot dry period (40–50 weeks after planting) during September–November. Survivorship of B. spiciformis seedlings was 74% at the end of the second growing season while this was 46% for J. paniculata.Shoot growth was negligible during the second growing season. In fact mean maximum leaf area of B. spiciformis decreased significantly from 19.7 cm² (SD=5.7) per plant at the end of the first growing season to 13.3 cm² (SD=5.8) at the end of the second growing season (t=3.31, P<0.01). However, root biomass of B. spiciformis seedlings increased 2.8 times during the second growing season.These results suggest that shoot die-back in seedlings of miombo trees is caused by drought and that the slow shoot growth is the result of allocating most of the biomass to root growth during seedling development.     

Foliage litter turnover and earthworm populations in three beech forests of contrasting soil and vegetation types

Summary Leaf litter decomposition, levels of accumulated litter as well as the abundance and biomass of earthworms were measured in three mature beech forests in southern Sweden: one mor site, one poor mull site, and one rich mull site. The disappearance rate of beech litter, measured with litter bags, increased with increasing soil fertility. On the rich mull site, the disappearance rate was much higher than in the two other forests, due to the combined effects of higher earthworm activity, more favouable soil moisture conditions, and higher litter quality. Incubating the litter in finely meshed bags (1-mm mesh) to exclude macrofauna had a great effect on litter mass loss in the rich mull site, but it had only a minor effect in the other sites. Simultaneous incubations of local and transplanted leaf litter on the three study sites showed that the substrate quality of the litter increased in the order: mor site — poor mull site — rich mull site. Lignin, N, and P concentrations of the leaf litter failed to explain the observed differences in decomposition rates, and acid/base properties are suggested to be more important. Earthworm numbers per m² were 2.5 (1 species) in the mor, 40 (6 species) in the poor mull and 220 (9 species) in the rich mull forest. Soil chemical conditions, notably pH, were suggested as the main factors determining the inter-site differences in abundance and species composition of earthworms. The role of litter decomposition and earthworm activity in the accumulation of organic matter in the forest floor in different types of beech woodlands are discussed.     

Nitrogen limitation in mycorrhizal Norway spruce (Picea abies) seedlings induced mycelial foraging for ammonium: implications for Ca and Mg uptake

Although many studies support the importance of the external mycelium for nutrient acquisition of ectomycorrhizal plants, direct evidence for a significant contribution to host nitrogen nutrition is still scarce. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus (Batsch) Fr. in a sand culture system with two compartments separated by a 45-m Nylon mesh. Hyphae, but not roots, can penetrate this net. Nutrient solutions were designed to limit seedling growth by nitrogen. Hyphal density in the hyphal compartment, host N status and shoot growth of mycorrhizal seedlings significantly increased in response to NH₄ ⁺ addition to the hyphal compartment. Labeling the compartment only accessible to hyphae with ¹⁵NH₄ ⁺ showed that the increase in N uptake in the mycorrhizal seedlings was a result of hyphal N acquisition from the hyphal compartment. These results indicate that hyphae of P. involutus may actively forage into N-rich patches and improve host N status and growth. In the mycorrhizal seedlings, which received additional NH₄ ⁺ via their external mycelium, the increase in NH₄ ⁺ supply less negatively affected Ca and Mg uptake than in nonmycorrhizal seedlings, where the additional NH₄ ⁺ was directly supplied to the roots. This was most likely due to the close link of NH₄ ⁺ uptake and H⁺ extrusion, which, in the nonmycorrhizal seedlings, lead to a strong acidification in the root compartment, and subsequently reduced Ca and Mg uptake, whereas in the mycorrhizal seedlings the site of intensive NH₄ ⁺ uptake and acidification was in the hyphal and not in the root compartment. Our data support the idea that the ectomycorrhizal mycelium connected to an N-deficient host may actively forage for N. The mycelium may also be important as a biological buffer system ameliorating negative influence of high NH₄ ⁺ supply on cation uptake.