Impact of herbivores on nitrogen cycling: contrasting effects of small and large species

Herbivores are reported to slow down as well as enhance nutrient cycling in grasslands. These conflicting results may be explained by differences in herbivore type. In this study we focus on herbivore body size as a factor that causes differences in herbivore effects on N cycling. We used an exclosure set-up in a floodplain grassland grazed by cattle, rabbits and common voles, where we subsequently excluded cattle and rabbits. Exclusion of cattle lead to an increase in vole numbers and a 1.5-fold increase in net annual N mineralization at similar herbivore densities (corrected to metabolic weight). Timing and height of the mineralization peak in spring was the same in all treatments, but mineralization in the vole-grazed treatment showed a peak in autumn, when mineralization had already declined under cattle grazing. This mineralization peak in autumn coincides with a peak in vole density and high levels of N input through vole faeces at a fine-scale distribution, whereas under cattle grazing only a few patches receive all N and most experience net nutrient removal. The other parameters that we measured, which include potential N mineralization rates measured under standardized laboratory conditions and soil parameters, plant biomass and plant nutrient content measured in the field, were the same for all three grazing treatments and could therefore not cause the observed difference. When cows were excluded, more litter accumulated in the vegetation. The formation of this litter layer may have added to the higher mineralization rates under vole grazing, through enhanced nutrient return through litter or through modification of microclimate. We conclude that different-sized herbivores have different effects on N cycling within the same habitat. Exclusion of large herbivores resulted in increased N annual mineralization under small herbivore grazing.     

Purification of adult Drosophila melanogaster lipophorin and its role in hydrocarbon transport

Lipophorin was isolated from homogenized adult Drosophila melanogaster. It is stained by Sudan Black and has a native molecular mass of 640 kD and a density of 1.12 g/ml. It consists of two glycosylated apoproteins of 240 and 75 kDa. Gas chromatography and mass spectrometry showed that lipophorins isolated separately from virgin 3-day-old male and female flies were associated with specific hydrocarbons, and that these were the same hydrocarbons found in male and female cuticles, respectively. Moreover, a pool of internal hydrocarbons was demonstrated for the first time, with chain lengths similar to those of the cuticular pool. Studies on the fate of the hydrocarbons synthesized de novo after topical applications of radiolabelled fatty acid precursors showed a decrease of the internal pool of hydrocarbons with time, concomitant with an increase of the cuticular pool. These results suggest that hydrocarbons synthesized at an internal site, possibly in oenocytes, may be transported to the cuticle of the flies by lipophorin. © 1996 Wiley-Liss, Inc.     

Beta‐diversity in temperate and tropical forests reflects dissimilar mechanisms of community assembly

Site‐to‐site variation in species composition (β‐diversity) generally increases from low‐ to high‐diversity regions. Although biogeographical differences in community assembly mechanisms may explain this pattern, random sampling effects can create this pattern through differences in regional species pools. Here, we compared assembly mechanisms between spatially extensive networks of temperate and tropical forest plots with highly divergent species pools (46 vs. 607 species). After controlling for sampling effects, β‐diversity of woody plants was similar and higher than expected by chance in both forests, reflecting strong intraspecific aggregation. However, different mechanisms appeared to explain aggregation in the two forests. In the temperate forest, aggregation reflected stronger environmental correlations, suggesting an important role for species‐sorting (e.g. environmental filtering) processes, whereas in the tropics, aggregation reflected stronger spatial correlations, more likely reflecting dispersal limitation. We suggest that biogeographical differences in the relative importance of different community assembly mechanisms contribute to these striking gradients in global biodiversity.     

Patterns in vegetation, hydrology, and nutrient availability in an undisturbed river floodplain in Poland

In the undisturbed floodplain of the Biebrza river (N.E. Poland) wecompared vegetation composition, standing crop and the nutrients in standingcrop to site factors such as flood duration and inundation depth during springfloods, summer water levels and concentrations of chemical constituents inwaterand nutrient release rates from peat. Our analysis shows a number of clearspatial patterns of biotic and abiotic variables in the ca. 1 kmwide river marginal wetland. The distribution of vegetation types follows acertain pattern: Glycerietum maximae close to the river,followed by respectively Caricetum gracilis andCaricetum elatae and finally Calamagrostietumstrictae at the margin of the river plain. Species richnessincreasesand standing crop decreases from the river towards the margin. The elevation ofthe ground surface gently rises with increasing distance from the river; floodduration and flooding depths in spring decrease in the same direction.Groundwater tables in summer are less correlated to the elevation gradient buttend to be closer to the ground surface at the valley margin. These differencesalso lead to a higher amplitude close to the river and a fairly stable watertable far away from the river. Concentrations of major ions and ammoniumincrease towards the river. Nutrient release rates are also higher closer totheriver. Absence and presence of species and the variation in species compositionof the vegetation was explained best by flood variables; variables fromgroundwater explained much less of the variance. Variations in standing cropandnutrients in standing crop corresponded better to the rates of nutrient releasefrom the organic soil than to nutrient concentrations in the soil water. Weconcluded that river hydrology and nutrient release from the soil are clearlyrelated to vegetation composition, species richness and productivity of thevegetation.     

Spatial and temporal variations in phloem sap composition of plantation-grown Eucalyptus globulus

Spontaneous bleeding of sugar-rich sap from cambial-deep incisions in the bark of trunks was demonstrated for Eucalyptus globulus and other eucalypts across a range of localities and seasonal conditions in south-west Australia. High levels of sucrose and raffinose (up to 31% w/v total sugars) were present in the exudates, and upward and downward gradients in exudate sugar concentrations were recorded between samples obtained at different heights up trunks of E. globulus. The data indicated a phloem origin for the exudates, with source:sink pressure gradients driving translocation. Concentration ratios of sugars to amino acids were consistently lower in exudate from upper (distal) than basal regions of trunks, suggesting preferential partitioning of nitrogen upwards towards the trunk apex. A comparison of phloem and xylem sap composition from one plantation over a season showed nitrate in xylem but not phloem and substantial amounts of sodium, and high concentrations of chloride and sulphate relative to phosphate in xylem and phloem. Phloem sap sampled across a range of 29 contrasting plantations of E. globulus at peak stress (autumn) showed great inter-site variability in concentrations of amino acids, sulphur, sodium and certain trace elements and in C:N and Na:K ratios of sap. Carbon isotope ratios (δ¹³C) were strongly correlated with sugar concentrations of the sap samples from these and other plantations. Use of sap compositional attributes of phloem and δ¹³C values of translocated carbon is suggested for assessing the current nutritional condition and water status of E. globulus plantings. Received: 9 April 1998 / Accepted: 20 August 1998     

Factors associated with plant species richness in a coastal tall‐grass prairie

Abstract. In this study we examine the factors associated with variations in species richness within a remnant tall‐grass prairie in order to gain insight into the relative importance of controlling variables. The study area was a small, isolated prairie surrounded by wetlands and located within the coastal prairie region, which occurs along the northwestern Gulf of Mexico coastal plain. Samples were taken along three transects that spanned the prairie. Parameters measured included micro‐elevation, soil characteristics, indications of recent disturbance, above‐ground biomass (including litter), light penetration through the plant canopy, and species richness. Species richness was found to correlate with micro‐elevation, certain soil parameters, and light penetration through the canopy, but not with above‐ground biomass. Structural equation analysis was used to assess the direct and indirect effects of micro‐elevation, soil properties, disturbance, and indicators of plant abundance on species richness. The results of this analysis showed that observed variations in species richness were primarily associated with variations in environmental effects (from soil and microtopography) and were largely unrelated to variations in measures of plant abundance (biomass and light penetration). These findings suggest that observed variations in species richness in this system primarily resulted from environmental effects on the species pool. These results fit with a growing body of information that suggests that environmental effects on species richness are of widespread importance.     

Growth chronologies of white sucker, Catostomuscommersoni, and lake trout, Salvelinusnamaycush: a comparison among lakes and between trophic levels

A growth chronology index was used to determine whether changes in ecosystem structure and function in lakes could be associated with fish growth histories. Growth chronologies were compared for white suckers, Catostomus commersoni, from Little Moose (oligotrophic), Oneida (eutrophic), and Cayuga (mesotrophic) lakes (New York) from opercular bone growth increments, and for lake trout, Salvelinus namaycush, from Little Moose Lake using otolith growth. The longevity of these species allowed the development of chronologies from 17 to 33 years in length using only contemporary collections. We used these chronologies to examine whether fish growth histories could be used as an index for ecosystem-scale changes. Specifically, we examined whether zebra mussel, Dreissena polymorpha, invasion in Oneida and Cayuga lakes in the early 1990s, and treatment of sewage effluent from dwellings around Little Moose Lake beginning during the late 1980s could be detected in white sucker and lake trout growth chronologies. White sucker growth in Oneida and Cayuga Lakes did not differ before and after zebra mussel invasions. Neither white sucker nor lake trout growth chronologies from Little Moose Lake reflect changes in growth expected with reduced productivity levels associated with improved sewage treatment. Growth chronologies of these two species did not detect the ecosystem-scale changes that occurred in the study lakes.     

Chirality differences in amino acid retention and release from the acid-extractable pool of cultured mammalian cells

In previous work, no chiral differences were found between D and L enantiomers of Leu in their ability to displace one another from the acid-extractable pool in mammalian cells. Recent evidence suggested otherwise. Our aim is to examine whether, in physiological range, D-amino acids have an equivalent ability to displace L-amino acids from the acid-extractable pool of HeLa cells, and vice versa. In the millimolar range, D-Leu and L-Leu have similar uptake and displacement properties with regard to the acid-extractable pool in HeLa cells, despite only the latter isomer being incorporated into protein. Below millimolar concentrations however, a distinct difference was found in the displacement of tritium-labelled L-Leu from the pool by unlabelled D-Leu compared with unlabelled L-Leu. Thus, unlabelled L-Leu in the external medium at 10⁻⁴ or 10⁻⁵ M displaced an equivalent amount of label from the pool as D-Leu introduced at a concentration approx. one order of magnitude higher, respectively. Reciprocal experiments, in which the acid-extractable pool was preloaded with ³H-D-Leu, confirmed this finding. The chirality difference was noted whether pool prelabelling was carried out at 37 or 0°C; but in order to avoid the complications of active transport mechanisms, the competition work reported here was done at 0°C. Similar chirality differences were observed with other hydrophobic amino acids, including His, Ile and Phe, such as, preferential displacement by the L-Leu racemer compared with the D-Leu racemer below mM levels. This was also true for the D and L forms of the non-utilisable isomer of Leu, norleucine (nLeu). We conclude that D-forms of hydrophobic amino acids have lower affinity for similar or the same intracellular binding sites involved in the acid-extractable pool than their L-forms. The significance of these chirality findings to amino acid pools in cells, and to the predominance of L-forms of amino acids in the biosphere is considered.     

Influence of urea on biological N2 fixation and N transfer from Azolla intercropped with rice

The N₂ fixed by Azolla before and after urea application during the rice cycle, the mineralisation of Azolla-N as well as its availability to rice was studied in two greenhouse experiments conducted in 1996 and 1997 and in June 1998 in Goettingen (Germany). Dry matter production of the various rice parts of experiment 1 showed a clear positive synergism between treatment with Azolla and urea with a resulting apparent N recovery by rice increasing from 40% (without Azolla) to 57% in the presence of Azolla. Part of this increase may be due to N fixed biologically by Azolla and transferred to the rice. The second experiment shed some light on the role of BNF. Using an iterative method of estimation, the daily rate of N fixation was estimated at 0.6 – 0.7 kg N ha^–1. The rate was not so much affected by the age of the Azolla crop. At this rate, the BNF would amount to up to 100 kg N ha^–1 over a 130-day season. Assuming that BNF may be inhibited for a period of 5 – 10 days following urea application due to high levels of N in the floodwater, this might reduce the BNF by between 6 and 14 kg N ha over the season. Using the mean-pool-abundance concept, it was estimated that around 75 – 80% of the Azolla-N mineralized during the growth period was actually absorbed by the rice plants. Of the N taken up by rice around 28% was derived from the biologically fixed Azolla N, the remainder was urea N cycled through the Azolla. Azolla also seems to help sustain the soil N supply by returning N to the soil in quantities roughly equal to those extracted from the soil by the rice plant.