Factors controlling long-term changes in soil pools of exchangeable basic cations and stream acid neutralizing capacity in a northern hardwood forest ecosystem

Understanding the factors regulating the concentrations of basic cations in soils and surface waters is critical if rates of recovery are to be predicted in response to decreases in acidic deposition. Using a dynamic simulation model (PnET-BGC), we evaluated the extent to which atmospheric deposition of strong acids and associated leaching by strong anions, atmospheric deposition of basic cations through changes in emissions of particulate matter, and historical forest cutting have influenced soil pools of exchangeable basic cations and the acid-base status of stream water at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Historical deposition of basic cations was reconstructed from regression relationships with particulate matter emissions. Simulation results indicate that the combination of these factors has resulted in changes in the percent soil base saturation, and stream pH and acid neutralizing capacity (ANC) from pre-industrial estimates of 20%, 6.3 and 45 eq L^–1, respectively, to current values of 10%, 5.0 and –5 eq L^–1, respectively. These current values fall within the critical thresholds at which forest vegetation and aquatic biotic are at risk from soil and surface water acidification due to acidic deposition. While the deposition of strong acid anions had the largest impact on the acid-base status of soil and stream water, the reduction in deposition of basic cations associated with reductions in particulate emissions was estimated to have contributed about 27% of the depletion in soil Ca²⁺ exchange pool and 15% of the decreases in stream water concentrations of basic cations. Decline in stream water concentrations of basic cation occurred under both increasing and decreasing exchangeable pools, depending on the process controlling the acid base status of the ecosystem. Model calculations suggest that historical forest cutting has resulted in only slight decreases in soil pools of exchangeable basic cations, and has had a limited effect on stream ANC over the long-term.     

Release of carboxylic anions and protons by tomato roots in response to ammonium nitrate ratio and pH in nutrient solution

The exudation of certain organic anions and protons by roots which may affect solubility of metals and P and uptake by plants, is affected by nitrogen form and pH. The objective of this work was to study exudation of carboxylates and H⁺/OH^– by tomato plants in response to NH₄/NO₃ ratio and pH in nutrient solution. Four NH₄/(NH₄+NO₃) ratios (R= 0, 0.33, 0.67 and 1) and constant vs. variable solution pH treatments were investigated. The sum of the exudation rates of all carboxylates tended to decline with increasing R, particularly tri- and dicarboxylates. The molar fraction of the exuded tri- and dicarboxylates, averaged over all treatments and plant ages, increased in the order tartarate 2%), malate (6%), succinate (15%), citrate (26%) and fumarate (46%). At R=1 the solution pH dropped from 5.2 to 3 and at R=0 increased to 8. The R corresponding to the pH stat of tomato plant was 0.3. For the constant solution pH treatment, the effect of solution pH on carboxylate exudation rate was small as compared to the effect of R. The exudation of citrate and H⁺ efflux which were initiated when NO₃ and NH₄ uptake rates per plant exceeded certain threshold values, increased with plant age.     

Effects of drainage and temperature on carbon balance of tussock tundra micrososms

We examined the importance of temperature (7°C or 15°C) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soil and vegetation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO₂ exchanges, methane (CH₄) emissions, and dissolved C losses on intact blocks of tussock (Eriophorum vaginatum) and intertussock (moss-dominated). We hypothesized that under increased temperature and/or enhanced drainage, C losses from ecosystem respiration (CO₂ respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net source of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO₂ storage (NEP) in tussock tundra micrososms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross photosynthesis. Elevated temperature alone had no effect on NEP because CO₂ losses from increased ecosystem respiration at elevated temperature were compensated by increased CO₂ uptake (gross photosynthesis). Although CO₂ losses from ecosystem respiration were primarily limited by drainage, CH₄ emissions, in contrast, were dependent on temperature. Furthermore, substantial dissolved C losses, especially organic C, and important microhabitat differences must be considered in estimating C balance for the tussock tundra system. As much as 20% of total C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored 2x more C and emitted 5x more methane than intertussocks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra.     

Temporal changes of mangrove vegetation boundaries in Amazônia: Application of GIS and remote sensing techniques

The present work analyses a 25-year timeseries of radar and satellite images, andidentifies areas with losses and gains ofvegetation coverage along 166 km of Para'scoastline (North Brazil) and in the innerparts of the Bragança peninsula. Thegeomorphology of this region hassignificantly changed in recent years. Aresult of these changes has been a retreatof the mangrove vegetation along thecoastline, mainly due to landward sandmigration, which covers the mudflat andasphyxiates the vegetation. Image analysissuggests that the loss of vegetationcoverage has been the dominating process inthe last 25 years, occurring on 42% ofthe Bragança coastline and adjacentareas. Vegetation has remained stable along 39% of the coastline, while mudsedimentation has allowed mangrovedevelopment along 19% of it. On the otherhand, during this period of time, mangroveshave invaded 3.4 km² ( 38%) of theelevated herbaceous flats in the highestsectors of the Bragança peninsula.Despite other possible causes for mangrovedeath along the coastline, such as littoraldrift currents or cyclical changes incoastal sediment dynamics, the invasion ofmangrove into herbaceous elevated flats inthe central peninsula cannot be attributedto these factors. The current dynamics ofvegetation coverage change seem to becompatible with a long-term trend relatedto the predicted rates of sea-level rise.     

Impacts of the C4 sedge Cyperus papyrus L. on carbon and water fluxes in an African wetland

Fluxes of CO₂ and H₂O vapour were measured by eddy covariance from a stand of the C₄ emergent sedge Cyperus papyrus (papyrus), which formed a fringing swamp on the north-west shore of Lake Naivasha, Kenya. The fluxes of CO₂ and H₂O vapour between the papyrus swamp and the atmosphere were large but variable, depending on the hydrology of the wetland system and the condition of the vegetation. These measurements, combined with simulation modelling of annual fluxes of CO₂, show that papyrus swamps have the potential to sequester large amounts of the carbon (1.6 kg C m^–2 y^–1) when detritus accumulates under water in anaerobic conditions, but they are a net source of carbon release to the atmosphere (1.0 kg C m^–2 y^–1) when water levels fall to expose detritus and rhizomes to aerobic conditions. Evapotranspiration from papyrus swamps (E) was frequently lower than evaporation from open water surfaces (E _o) and plant factors have a strong influence on the flux of water to the atmosphere. For the period of measurement E/E_o was 0.36.     

Iron deficiency in chickpea in the Mediterranean region and its control through resistant genotypes and nutrient application

Iron-deficiency chlorosis is commonly observed in some genotypes of chickpea (Cicer arietinum L.) in the Mediterranean region of West Asia and North Africa when grown on calcareous soils. An evaluation of 3267 germplasm lines of kabuli-type chickpea for iron-deficiency chlorosis on the calcareous soil (calcium carbonate content 20%, pH8.5) of the principal research station of the International Centre for Agricultural Research in the Dry Areas (ICARDA) at Tel Hadya, northern Syria, revealed that most of the lines were tolerant while only 25 lines showed susceptibility. Foliar spray of 0.5% FeSO₄ at the onset of chlorosis was effective in correcting the symptoms but did not result in significant increase in crop yield.Studies on the inheritance of resistance to iron-deficiency chlorosis revealed that the resistance was dominant and is governed by a single gene.To ensure elimination from the breeding material of chickpea genotypes inefficient in Fe-use on the calcareous soils of the Mediterranean region, a negative selection for Fe-deficiency chlorosis in the segregating populations in the field has proved effective. The method for field screening of large number of breeding lines and germplasm is described.     

Altitudinal changes in composition and structure of mountain-temperate vegetation: a case study from the Western Carpathians

Changes in composition and structure of plant communities in relation tothe soil and snow cover variation were analyzed along an altitudinal transect(1150–1750 m) from the mountain-temperate forests to a woodyshrub community and alpine meadows on Mt Velký Gápel', Slovakia.The soils below the treeline (1510 m) had a more developedorganic layer above the mineral substratum. Generally, soil depth decreased asthe altitude increased, although the maximum values were recognized at a middlealtitude in a beech stand. Snow was redistributed by westerly winds from theridgeline down to the upper forest margin. Mean snow depth decreased withaltitude up to almost snow-free sites around the summit. In the 48 plots at 16sites we recorded 118 taxa including 6 tree, 7 shrub, 18 grass, 42 herb, 5fern,25 moss and 15 lichen species. The species diversity showed no distinctrelationship to altitude but declined with canopy consolidation. The TWINSPANfloristic classification distinguished five groups of community typescharacterised by different dominants, and a further three clusters of samplesfrom transition zones. Horizontal compositional heterogeneity increased inareaswhere trees were aggregated and tree basal area was smaller. Vegetationcomposition became more patchy at open-canopy Acerpseudoplatanus–Abies alba mixed forest at 1150 m,in Picea abies forest limit 1470 m, andin Pinus mugo krummholz at 1590 m. Speciesturnover of the entire transect was 6.1 half-changes as estimated by DCA.Despite this heterogeneity, none of the 15 elevational bands had significantaggregation of species' limits. Vegetation varied continuously, with individualspecies overlapping in transition zones delimited by dominant taxa. Thecoincident aggregation of up-slope and down-slope boundaries was found at abelt1430–1510 m. This discrete ecotone corresponds to a shiftfrom the closed coniferous forest to P. mugo krummholz.Thesecond inherent up-slope boundary aggregation indicated the P.mugo krummholz – alpine meadow vegetation transition at1700 m. Spatial analysis (K-function) of eight forest plots(0.12 ha each) showed that at lower elevation, adult trees of thebroad-leaf forest were closer to a random arrangement while at higherelevation,trees of evergreen coniferous stands became aggregated toward the forest limitwith the highest intensity from 2 to 4 m. Altitudinal gradient andrelated factors explained 35% of the variance in vegetation data.Canonical correspondence analysis also showed that main vegetation changesabovethe treeline area were associated with the topographic pattern of pine shrubsand snow cover.     

Carbon-13 variation with depth in soils of Brazil and climate change during the Quaternary

Paleoecological and geomorphological studies indicate that, during the middle Holocene, there was a predominance of drier conditions with grassy savannahs replacing forests across the South American continent. Modern savannahs are composed mainly of C4 plants and soils developed under this type of vegetation show enrichment in ¹³C compared to soils under C3 vegetation cover. If soils contain stabilized organic matter formed in the middle Holocene, we hypothesize that former C4 vegetation would be evidenced by a large enrichment of ¹³C in soil organic matter (SOM). We investigate this possibility examining the depth variation of carbon isotopic composition in 21 soil profiles collected by different researchers at 14 different sites in Brazil. Of these, profiles from only three sites showed a marked increase of ¹³C with depth (9–10 enrichment in ¹³C difference between the surface soil and deepest depth); two sites showed intermediate enrichment (4–5), and nine sites showed a small enrichment of approximatelly 2.5. The majority of sites showing all-C3 derived SOM were in the Amazon region. Possible causes for the absence of a large ¹³C enrichment with depth are: (1) dominance of C3 rather than C4 grasses in mid-Holocene savannahas, (2) soil profiles did not preserve organic matter derived from mid-Holocene plants, (3) the retreat of forest areas did not occur on a regional scale, but was a much more localized phenomenon.     

Patterns and mechanisms of soil acidification in the conversion of grasslands to forests

Grassland to forest conversions currently affect some of the world's most productive regions and have the potential to modify many soil properties. We used afforestation of native temperate humid grassland in the Pampas with eucalypts as an experimental system to 1) isolate forest and grassland imprints on soil acidity and base cation cycling and 2) evaluate the mechanisms of soil acidification. We characterized soil changes with afforestation using ten paired stands of native grasslands and Eucalyptus plantations (10–100 years of age). Compared to grasslands, afforested stands had lower soil pH (4.6 vs.5.6, p < 0.0001) and 40% lower exchangeable Ca (p < 0.001) in the top 20 cm of the soil. At three afforested stands where we further characterized soil changes to one meter depth, soil became increasingly acidic from 5 to 35 cm depth but more alkaline below 60 cm compared to adjacent grasslands, with few differences observed between 35 and 60 cm. These changes corresponded with gains of exchangeable acidity and Na in intermediate and deeper soil layers. Inferred ecosystem cation balances (biomass + forest floor + first meter of mineral soil) revealed substantial vertical redistributions of Ca and Mn and a tripling of Na pools within the mineral soil after afforestation. Soil exchangeable acidity increased 0.5–1.2 kmol_c.Ha^–1.yr^–1 across afforested stands, although no aboveground acidic inputs were detected in wet + dry deposition, throughfall and forest floor leachates. Our results suggest that cation cycling and redistribution by trees, rather than cation leaching by organic acids or enhanced carbonic acid production in the soil, is the dominant mechanism of acidification in this system. The magnitude of soil changes that we observed within half a century of tree establishment in the Pampas emphasizes the rapid influence of vegetation on soil formation and suggests that massive afforestation of grasslands for carbon sequestration could have important consequences for soil fertility and base cation cycles.     

Correlated photosynthetic responses and habitat factors of two successional tree species

Summary Ulmus alata and Diospyros virginiana are components of the shrubearly tree communities of old-field succession in several areas in the deciduous forests of eastern North America. In these habitats, the plants experience high insolation, high temperatures, and low soil moisture during the summer. They exhibit pronounced daily changes in water potential and usually develop more negative water potentials as the season progresses. The species light saturate at 1,150 E m^-2 sec^-1 with photosynthetic rates of 15 mg CO₂ dm^-2 h^-1 for U. alata and 17 mg CO₂ dm^-2 h^-1 for D. virginiana. The optimum temperatures for photosynthesis are 25°C. Ulmus alata maintains maximum photosynthesis to water potentials of-14 bars and recovers from-20 bars to 60% of maximum photosynthesis within 10 hrs after watering. When they are deprived of water, twigs of D. virginiana exhibit faster decline in photosynthesis and leaf conductance than twigs of U. alata. The two species have somewhat different response to the environmental of high insolation and low water supply. Unlike Ulmus, Diospyros virginiana has some adaptations which may explain the persistence of a few individuals in mature forests.     

Spatial variation in pyritization of trace metals in salt-marsh soils

In the present study, the pyritization of trace metals was studied in 12soils from 3 salt marshes in the Ría of Ortigueira (NW Spain). Theconcentrations of trace metals in the pyrite fraction were related tophysicochemical conditions, physiographical position in the salt marsh, and thepresence or absence and type of vegetation. Redox conditions in soils from thelow-salt marsh and from the creek bottom were strongly reducing throughout theprofile, and there were higher concentrations of Fe and some trace metals (Cuand Mn) in the pyrite fraction (soluble in HNO₃) than in thereactivefraction (soluble in 1N HCl). In contrast, the trace-metal content in pyritefraction in the surface layers of the high-salt marsh was low. In some of thesoils, there was a significant increase in the pyrite content below 25cm, and levels of Fe, Mn and Cu incorporated into this fractionwere similar to those in the reactive fraction. The degree of pyritizationvaried greatly among metals in the order: CuFe-1NHCl>dithionite-Fe>NiMn>>Zn>Cr, although when we consideredonly the amorphous forms (ascorbate-Fe) as reactive-Fe, the order was:ascorbate-Fe>>Cu>NiMn>>Zn>Cr. These differences appearedto be a consequence of the different geochemical behaviour of each metal(mainlyin terms of the thermodynamic stability of sulfides and reaction kinetics),except for Zn. The low concentrations of Zn obtained may have been due to thesolubility of ZnS in 1N HCl, which meant that it was extracted with thereactivefraction. Finally, we observed a direct relationship between DOP and DTMP,whichwas independent of the geochemical behaviour of each metal and of itsconcentration in the soil. Thus, the strong correlation between pyrite-Fe andthe metals associated with this fraction appears to indicate that these metalscoprecipitate with pyrite rather than form metal sulfides.     

The traction effect of lateral roots of Pinus yunnanensis on soil reinforcement: a direct in situ test

A traction effect by lateral roots is one way in which roots can contribute to lateral in-plane reinforcement of a shallow soil mass. ln contrast to the effect of vertically-extending roots, whereby soil is reinforced by an increase in its shear strength, the traction effect reinforces the soil by enhancing the tensile strength of the rooted soil zone. To verify whether or not a traction effect exists in the root system of a pine forest (Pinus yunnanensis French) in the Hutiaoxia Gorge, SW China, and to investigate the magnitude of this effect, a direct in situ test was conducted at two sites in the gorge. The results from the two sites showed that, in the topmost soil (020cm), the lateral roots are able to provide a tractive force of up to 421.19561.05 N over a vertical cross-section area of 20×50 cm², or an increase in the pulling resistance of the rooted soil by 33.237.58%.This is equivalent to a tensile strength increase of the shallow rooted soil by 4.25.6 KPa. The test results imply that, together with the pine's vertical roots, which anchor the shallow rooted soil zone to the deep and more stable soil mass, the lateral roots of the pine trees, through their traction effect, are able to mitigate against shallow instability in the forested slopes, such as shallow slide and creep, to a certain degree.     

A nonconjugative mobilizable broad host range plasmid of Acinetobacter sp. that determines HgCl2 resistance

Summary HgCl₂-resistant strains of Acinetobacter sp. obtained from the soil at the Khaidarkan mercury mine (Kirghiz SSR) were found to contain, apart from large plasmids (60 kb), a small plasmid (7.5 kb) designated pKL1. It was established by conjugative crosses and transformation that pKL1 is a broad host range mobilizable plasmid and that it carries the Hg^r determinant. The restriction map of pKL1 was constructed; the site of the Hg^r determinant and the regions essential to replication were localized. A comparison of these results with earlier data suggests that microorganisms belonging to one microbiocenosis may carry Hg^r determinants on plasmids with highly different structures and properties.Deceased on July 16, 1985     

Stable soil nitrogen accumulation and flexible organic matter stoichiometry during primary floodplain succession

Large increases in nitrogen (N) inputs to terrestrial ecosystems typically have small effects on immediate N outputs because most N is sequestered in soil organic matter. We hypothesized that soil organic N storage and the asynchrony between N inputs and outputs result from rapid accumulation of N in stable soil organic pools. We used a successional sequence on floodplains of the Tanana River near Fairbanks, Alaska to assess rates of stable N accumulation in soils ranging from 1 to 500+ years old. One-year laboratory incubations with repeated leaching separated total soil N into labile (defined as inorganic N leached) and stable (defined as total minus labile N) pools. Stable N pools increased faster (2 g N m^–2 yr^–1) than labile N (0.4 g N m^–2 yr^–1) pools during the first 50 years of primary succession; labile N then plateaued while stable and total N continued to increase. Soil C pools showed similar trends, and stable N was correlated with stable C (r² = 0.95). From 84 to 95 % of soil N was stable during our incubations. Over successional time, the labile N pool declined as a proportion of total N, but remained large on an aerial basis (up to 38 g N m^–2). The stoichiometry of stable soil N changed over successional time; C:N ratios increased from 10 to 22 over 275 years (r² = 0.69). A laboratory ¹⁵N addition experiment showed that soils had the capacity to retain much more N than accumulated naturally during succession. Our results suggest that most soil N is retained in a stable organic pool that can accumulate rapidly but is not readily accessible to microbial mineralization. Because stable soil organic matter and total ecosystem organic matter have flexible stoichiometry, net ecosystem production may be a poor predictor of N retention on annual time scales.     

HPLC methods for detection of uniconazole-P in soils and plant tissues

High-performance liquid chromatographic (HPLC) methods have been developed for the detection of uniconazole-P [(E)-1-(4-chlorophenyl)-4,4,-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol; XE-1019; the active ingredient in Prunit and Sumagic] in soil and plant tissue samples. Methanolic extracts of soil and plant samples were dried to the aqueous phase, the pH adjusted to 11, and partitioned against methylene chloride. The methylene chloride phases were washed with pH 11 water and then passed through C-18 solid phase extraction (SPE) columns. The soil extracts were then dried and the residues taken up in 1 ml acetonitrile of which 20 l were injected directly onto a C-18 reverse phase analytical column for HPLC analysis. Plant tissue extracts were purified by partitioning and passing through a sequence of Florisil/C-18/Florisil SPE columns before HPLC analysis. Recovery of uniconazole-P was 70% from soils and 40% from plant tissues. Quantitative detection of 10 parts per billion (ppb) uniconazole-P in plant tissues and soil samples was feasible following these procedures. The soil cleanup procedures were also used to detect uniconazole-P in leachates collected from container-grown plants.     

Features and conservation of the Brazilian Pantanal wetland

The Pantanal is a 140,000 km² sedimentary floodplain in western Brazil and one of the largest wetlands in the world. The main landscapes and phytophysiognomies, according to flood origin, are briefly described and some of the characteristic plant species are mentioned: (a) river flood (1–5 m) on clayey eutrophic soils with gallery forests, pioneer forests and scrub, Tabebuia and Copernicia parks, seasonal swamps, grasslands and oxbow lakes; and (b) rain flood (10–80 cm) mainly on dystrophic sandy soils (72% of the total area) with savanna (cerrado) grasslands and woodlands, with or without ponds. Regulating factors of the vegetation such as wet-and-dry cycle and management are considered. Dynamics of the vegetation, in particular the aquatic types, are shortly depicted. The role of grazing for conservation is discussed, and we suggest that 200 years of cattle ranching apparently did not cause major changes in the vegetation, except turning tall grass into short swards, as the domestic herd found a nearly empty niche. However, severe threats to the flora and fauna of the Pantanal originate outside the floodplain. Siltation of the Taquari river is pointed out as the worst environmental problem, changing the hydrology (wet-and-dry to wet), fauna and flora, e.g. eliminating riparian forest.This revised version was published online in March 2005 with corrections to the issue cover date.     

Pepsin fragmentation of botulinum type E neurotoxin: Isolation and characterization of 112, 48, 46, and 16 kD fragments

Controlled digestion of 150 kD single chain botulinum type E neurotoxin with pepsin atpH 6.0 produced 112, 48, 46, and 16 kD fragments. These were chromatographically purified; their locations in the 1300 amino acid residue long neurotoxin were determined by identifying the amino terminal 10 residues of 112 and 48 kD fragments, 50 residues of 46 kD fragment, and 59 residues of 16 kD fragment. The 48 and 112 kD fragments contain the N-terminal segment of the neurotoxin (i.e., residue no. 1 to 425 and 1 to 990, respectively), the 46 kD fragment corresponds to 407 residues of the C-terminal region, and the 16 kD fragment contains the 140 residues from a segment nearer to the C-terminus. The 48 kD fragment is similar to the 50 kD N-terminal light chain of the 150 kD dichain neurotoxin, which is generated by tryptic cleavage of the 150 kD single chain neurotoxin, and is separated from the 100 kD C-terminal heavy chain by dithiothreitol (DTT) reduction of an intrachain disulfide bond in the presence of 2 M urea (Sathyamoorthy and DasGupta,J. Biol. Chem. 260, 10461, 1985). The pepsin-generated 48 kD fragment, unlike the light chain, was isolated without exposure to DTT and urea. The single chain 112 kD fragment following trypsin digestion yielded 48 and 60 kD fragments that were separable after DTT reduction of the intrachain disulfide which links them. The N-terminal residues of the smaller fragment were identical to that of the single chain 150 kD neurotoxin; the single chain 112 kD fragment is therefore the neurotoxin minus the 50 kD C-terminal half of the heavy chain. The biological activities of the 48 and 112 kD fragments can be demonstrated in permeabilized PC12 cells (Lomnethet al., J. Neurochem. 57, 1413, 1991); they inhibit norepinephrine release.     

Absolute measurement of cell expansion in plant cell suspension cultures

A method is described for the measurement of intracellular volume (V_i) in cell cultures. In principle, any stable compound that neither penetrates the plasma membrane nor binds to the cells can be used to trace the total extracellular (apoplastic) volume and hence to estimate the intracellular volume. No suitable coloured or UV-absorbing compound could be found among those tested; the main problems were binding to the cell surface and/or instability in the medium. However, [¹⁴C]mannitol was an acceptable apoplastic marker, by use of which we showed that 21–47% of total packed cell volume (PCV) was intracellular, and 14–33% of total settled cell volume (SCV) was intracellular. Therefore, measurements of PCV and SCV misrepresent cell expansion to a variable extent. Cultures of Acer, Rosa, Spinacia and Zea achieved final symplastic volumes of only 9, 14, 6 and 6%, respectively, of the total suspension culture volume.     

Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest

Overwinter and snowmelt processes are thought to be critical to controllersof nitrogen (N) cycling and retention in northern forests. However, therehave been few measurements of basic N cycle processes (e.g.mineralization, nitrification, denitrification) during winter and littleanalysis of the influence of winter climate on growing season N dynamics.In this study, we manipulated snow cover to assess the effects of soilfreezing on in situ rates of N mineralization, nitrification and soilrespiration, denitrification (intact core, C₂H₂ – based method),microbial biomass C and N content and potential net N mineralization andnitrification in two sugar maple and two yellow birch stands with referenceand snow manipulation treatment plots over a two year period at theHubbard Brook Experimental Forest, New Hampshire, U.S.A. The snowmanipulation treatment, which simulated the late development of snowpackas may occur in a warmer climate, induced mild (temperatures >–5 °C) soil freezing that lasted until snowmelt. The treatmentcaused significant increases in soil nitrate (NO₃ ^–)concentrations in sugar maple stands, but did not affect mineralization,nitrification, denitrification or microbial biomass, and had no significanteffects in yellow birch stands. Annual N mineralization and nitrificationrates varied significantly from year to year. Net mineralization increasedfrom 12.0 g N m^–2 y^–1 in 1998 to 22 g N m^–2 y^–1 in 1999 and nitrification increased from 8 g N m^–2 y^–1 in 1998 to 13 g N m^–2 y^–1 in 1999.Denitrification rates ranged from 0 to 0.65 g N m^–2 y^–1. Ourresults suggest that mild soil freezing must increase soil NO₃ ^– levels by physical disruption of the soil ecosystem and not by direct stimulation of mineralization and nitrification. Physical disruption canincrease fine root mortality, reduce plant N uptake and reduce competitionfor inorganic N, allowing soil NO₃ ^– levels to increase evenwith no increase in net mineralization or nitrification.     

Substrate, climate, and land use controls over soil N dynamics and N-oxide emissions in Borneo

Nitrogen (N) enrichment of tropical ecosystems is likely to increase with rapid industrial and agricultural development, but the ecological consequences of N additions in these systems are not well understood. We measured soil N- oxide emissions and N transformations in primary rain forest ecosystems at four elevations and across two substrate types on Mt. Kinabalu, Borneo, before and after short-term experimental N additions. We also measured N pools and fluxes across a land use gradient of primary forest, burned secondary forest, and fertilized agriculture. Background soil N₂O and NO emissions in primary forest decreased with elevation, and soils derived from sedimentary substrates had larger pools of inorganic N, rates of nitrification, and N-oxide fluxes than ultrabasic soils when there were significant differences between substrate types. N-oxide emissions after N additions and background rates of nitrification were low in all soils derived from ultrabasic substrates compared to sedimentary substrates, even at lowland sites supporting, diverse Dipterocarp forests growing on morphologically similar Oxisols. Rates of potential nitrification were good predictors of N-oxide emissions after N additions. N₂O and NO fluxes were largest at low elevations and on sedimentary-derived soils compared to ultrabasic-derived soils, even at the smallest addition of N, 15kgNha^–1. Because current methods of soil classification do not explicitly characterize a number of soil chemical properties important to nutrient cycling, the use of soil maps to extrapolate biogeochemical processes to the region or globe may be limited in its accuracy and usefulness. In agricultural systems, management practices were more important than substrate type in controlling N-oxide emissions and soil N cycling. N-oxide fluxes from agricultural fields were more than an order of magnitude greater than from primary forests on the same substrate type and at the same elevation. As primary forests are cleared for intensive agriculture, soil N₂O and NO emissions are likely to far exceed those from the most N-saturated tropical forest ecosystems. This study highlights the inter-dependence of climate, substrate age, N deposition, and land-use practices determining N cycling and N-oxide emissions in humid tropical regions.