Effects of forest clear-cutting on the sulphur, phosphorus and base cations fluxes through podzolic soil horizons

Clear-cutting considerably alters the flow of nutrients through the forest ecosystem. These changes are reflected in soil solution concentrations and fluxes. The effects of clear-cutting (stems only) on the fluxes of water soluble phosphorus (P), sulphur (S) and base cations (Ca, Mg and K) through a podzolic soil were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall (throughfall + stemflow) and soil percolate from below the organic (O), eluvial (E) and illuvial (B) horizons were collected for 4 years before and for 3 years after cutting. Annual deposition loads (kg ha^–1) to the forest floor were less after clear-cutting, averaging 1.7 S, 0.84 Ca, 0.14 Mg, 0.64 K and 0.10 P. Before cutting, the loads were 4.6 S, 2.7 Ca, 0.70 Mg, 6.2 K and 0.20 P. Annual fluxes of total S and sulphate (SO ₄ ^2– ) from below the O-horizon were also lower (33%) after clear-cutting, total S averaging 2.0 kg ha^–1, the flux from below the B-horizon also diminished after clear-cutting. The flux of total P (mainly inorganic) from below the O-horizon increased threefold (6.9 kg ha^–1; sum over the 3-year period) compared to period before cutting. The fluxes of base cations from below the O-horizon increased twofold. The flux of K⁺ from below the O- and E-horizons was most strongly correlated with that of phosphate (PO ₄ ^3– ) and those of Ca²⁺ and Mg²⁺ with the DOC flux. Increased fluxes of P and base cations to the mineral soil generated only slightly increased fluxes from below the B-horizon. The retention of base cations and P in the mineral soil indicates there was little change in leaching to ground and surface waters after clear-cutting.     

韶山针阔叶混交林凋落物层的淋溶及缓冲作用

在韶山森林设立4个10m×10m的标准样地,分别收集凋落物、凋落物层淋滤液和冠层穿透水,研究了韶山森林凋落量季节动态,凋落物淋滤液和冠层穿透水特征以及凋落物层对酸沉降的缓冲作用.结果表明:(1)韶山森林凋落高峰出现在秋季,凋落量随着海拔增加而增加;(2)凋落物淋滤液中盐基阳离子浓度除夏季K ＞Ca2 外,其余季节均为:Ca2 ＞K ＞NH 4 ＞Mg2 ＞Na ,与冠层穿透水中阳离子浓度分布基本是一致的;(3)除Ca2 外,凋落物淋滤液和穿冠水中各阳离子浓度相关性显著,K 达到了极显著相关水平,证实了韶山森林通过凋落物养分归还的K 主要来自森林冠层的滤出;(4)除个别点外,凋落物淋滤液中各阳离子总浓度较冠层穿透水均有不同程度的增加,秋季增幅最大,冬季次之,这与凋落物量变化一致;(5)韶山森林冠层穿透水pH值变动范围为4.58～7.13,最低值出现在冬季,最高值出现在夏季,凋落物淋滤液pH值变动范围为5.02～6.69,均高于韶山表层土壤pH平均值5.0,且最低值出现在春季,最高值出现在秋季;(6)冬季凋落物淋滤液pH值较冠层穿透水均增加,增幅范围在0.06～1.35之间,而其他季节(除样地C的秋季外)pH值均有所下降,这表明韶山森林凋落物层冬季具有较强的酸缓冲作用,而其他季节由于盐基阳离子的滤出不足以抵消凋落物本身分解产生的有机酸类物质的酸化作用,而使凋落物淋滤液pH值降低.     

Cation selectivity of and cation binding to the cGMP-dependent channel in bovine rod outer segment membranes

The properties of the cGMP-dependent channel present in membrane vesicles prepared from intact isolated bovine rod outer segments (ROS) were investigated with the optical probe neutral red. The binding of neutral red is sensitive to transport of cations across vesicular membranes by the effect of the translocated cations on the surface potential at the intravesicular membrane/water interface (Schnetkamp, P. P. M. J. Membr. Biol. 88: 249-262). Only 20-25% of ROS membrane vesicles exhibited cGMP-dependent cation fluxes. The cGMP-dependent channel in bovine ROS carried currents of alkali and earth alkali cations, but not of organic cations such as choline and tetramethylammonium; little discrimination among alkali cations (K greater than Na = Li greater than Cs) or among earth alkali cations (Ca greater than Mn greater than Sr greater than Ba = Mg) was observed. The cation dependence of cGMP-induced cation fluxes could be reasonably well described by a Michaelis-Menten equation with a dissociation constant for alkali cations of about 100 mM, and a dissociation constant for Ca2+ of 2 mM. cGMP-induced Na+ fluxes were blocked by Mg2+, but not by Ca2+, when the cations were applied to the cytoplasmic side of the channel. cGMP-dependent cation fluxes showed a sigmoidal dependence on the cGMP concentration with a Hill coefficient of 2.1 and a dissociation constant for cGMP of 92 microM. cGMP-induced cation fluxes showed two pharmacologically distinct components; one component was blocked by both tetracaine and L-cis diltiazem, whereas the other component was only blocked by tetracaine.     

Gross precipitation and throughfall chemistry in legume species planted in Northeastern México

Plant cover modifies throughfall chemistry, and the solute concentration is dependent on the plant species at any given site. The chemistry of gross rainfall and throughfall of four endemic species planted in northeastern Mexico was evaluated from March 1996 to March 1997. Chemical solutes measured included Ca, K, Mg, Na, Fe, Mn, Cu, and Zn. Dry deposition and canopy leaching fluxes were estimated following the canopy budget model. Variance analyses tested the statistical dependence of the total and net fluxes on the species and seasons. Regression analysis tested the dependence of chemical concentrations on rainfall depth and lag time between rains. A total of 52 rainfall events were recorded during the study period summing 523 mm. Significant differences were noted on the total and net fluxes between the plant species. For total flux, average throughfall (37.8 kg ha^?1 year^?1) almost doubled the flux of solutes compared to rainfall (24.1 kg ha^?1 year^?1). Pithecellobium ebano (Berland.) C.H. Mull. (43.3 kg ha^?1 year^?1), Acacia berlandieri Benth. (38.7 kg ha^?1 year^?1), and Pithecellobium pallens (Bent.) Standl. (38.4 kg ha^?1 year^?1) recorded the highest total flux of solutes, and Acacia rigidula Benth. (30.9 kg ha^?1 year^?1) the smallest. Chemical solutes showed significant differences for total and net fluxes. Ca was the dominant cation with 48% and 52% of the total constituent flux for rainfall and throughfall, respectively. However, K, Mg and Cu approximately doubled in throughfall in contrast to gross rainfall. Species with the largest aboveground biomass had lower throughfall volumes (i.e., higher interception rates) but higher chemical solute inputs to the forest floor. Rainfall depth and lag time between rains explained part of the variation for most species, stressing the partial dependence of the washing effect and the amount of dry deposition on canopies. This research discusses the importance and the sources of incoming solutes on the studied plant species.     

The effects of acid deposition on the biogeochemical cycles of major nutrients in miniature red spruce ecosystems

As part of an experimental study of air pollution effects on tree growth and health, we combined process studies with an ecosystem approach to evaluate the effects of acidic deposition on soil acidification, nutrient cycling and proton fluxes in miniature red spruce ecosystems. Ninety red spruce saplings were transplanted into 1-m diameter pots containing reconstructed soil profiles and exposed to simulated acid rain treatments of pH 3.1, 4.1 and 5.1 for four consecutive growing seasons. All the principal fluxes of the major elements were measured. During the first year of treatments, the disturbance associated with the transplanting of the experimental trees masked any treatment effects by stimulating N mineralization rates and consequent high N0₃ ⁻ cation, and H⁺ flux through the soil profile. In subsequent years, leaching of base cations and labile Al was accelerated in the most intensive acid treatment and corresponding declines in soil pH and exchangeable pools of Ca and Mg and increases in exchangeable Al concentrations were observed in the organic horizon. Leaching of Ca²⁺ and Mg²⁺ also was significantly higher in the pH 4.1 than in the pH 5.1 treatment. Flux of Ca from foliage and soil was increased in response to strong acid loading and root uptake increased to compensate for foliar Ca losses. In contrast, K cycling was dominated by root uptake and internal cycling and was relatively insensitive to strong acid inputs. Cation leaching induced by acidic deposition was responsible for the majority of H⁺ flux in the pH 3.1 treatment in the organic soil horizon whereas root uptake accounted for most of the H⁺ flux in the pH 4.1 and 5.1 treatments. Although no measurable effects on tree nutrition or health were observed, base cation leaching was significantly accelerated by acidic deposition, even at levels below that observed in the eastern U.S., warranting continued concern about acid deposition effects on the soil base status of forested ecosystems.     

Ion flux from precipitation to peat soil in spruce forest–<Emphasis Type="Italic">Sphagnum</Emphasis> bog communities in the Ochiishi district,eastern Hokkaido,Japan

To evaluate the contribution of proton flux from precipitation on peat acidification in mire ecosystems, we estimated ion fluxes to peat soils from bulk deposition in Sphagnum-dominated bogs and from throughfall plus stem flow in spruce forests in three cool-temperate ombrogenous mires in the Ochiishi district, northeastern Japan. We tested the hypothesis that proton fluxes from the atmosphere to peat soils are affected by vegetation types, leading to the consequent difference in soil acidity. The proton flux in bulk deposition was higher than that in throughfall plus stem flow, but the concentration of H⁺ in the peat surface water in Sphagnum bogs was lower than that in spruce forests. The inverse relationship between proton flux and soil water acidity means that the soil water acidity could not be explained quantitatively by proton flux from the atmosphere to the peat surface. The ion fluxes of sea-salt components were dependent on the distance from the coast to the mires. This means that the sea-salt accumulation in the peat surface soil can be directly attributed to the high flux of sea-salt from precipitation. The flux of sea-salts deposited on the mires positively correlated with the H⁺ concentration of the peat surface water in each community, implying that the acidity of peat surface water depends on the cation fluxes from precipitation.     

Taxonomic effect on plant base concentrations and stoichiometry at the tips of the phylogeny prevails over environmental effect along a large scale gradient

Despite the well‐known importance of all elements to plant growth and nutrient fluxes in ecosystems, most studies to date have been restricted to the roles of foliar nitrogen (N) and phosphorus (P). Much less is known about cycling and pools of base cations in ecosystems and the drivers of variation in cation concentrations among plant species, even though these cations are paramount for plant and ecosystem function. In particular, little is known about the contributions of taxonomic position and environmental variation on base cation concentrations. The extent to which concentrations of elements in plants are determined by phenotypic response to their availability in current environments versus by inherent species‐specific uptake and processing adaptations, should be most directly evident at the tips of the phylogeny, where inherent variation among species should reflect relatively recent adaptation to environmental variation since their common ancestry. To test this hypothesis, we explored the geographic pattern and the effects of taxonomy, climate and soil on concentrations and stoichiometry of the base cations potassium (K), sodium (Na), calcium (Ca) and magnesium (Mg) across a lineage of Artemisia species and their close relatives across northern China. We found that species identity explained the largest proportion of the total variance for all four base cations (38.3–53.8%) and their stoichiometry (35.2–59.6%). K, Na and Ca concentrations increased significantly with climate seasonality, while Ca concentration decreased with annual temperature and precipitation. Plant K concentration, K:Ca and K:Mg were negatively correlated with soil organic carbon concentrations, but positively with soil pH. Our results suggest that taxonomy still needs to be fully considered for interpreting variation in vegetation nutrition and stoichiometry along broad geographical gradients even for species at the tips of the phylogeny.     

Divalent cation effects on acetylcholine-activated channels at the frog neuromuscular junction

The effects of the divalent cations Ca and Mg on the properties of ACh-activated channels at the frog neuromuscular junction were studied using a two-microelectrode voltage clamp. The divalent cation concentration was varied from 2 to 40 mM in solutions containing 50% normal Na. The reversal potential was determined by interpolation of the acetylcholine (ACh)-induced current versus voltage relationship. The single-channel conductance and the mean channel lifetime were calculated from fluctuation analysis of the ACh-induced end-plate current. Extracellular Na and/or divalent cations affected the reversal potential of endplate channels in a way that cannot be described by the Goldman-Hodgkin-Katz equation or by a simple two-barrier, one-binding site model of the channel if the assumption was made that permeability ratios were constant and not a function of ion concentrations. Increasing the divalent cation concentration decreased the single-channel conductance to approximately 10 pS in solutions with 50% Na and 40 mM divalent cation concentrations. The effect of the divalent cations Ca and Mg on the mean channel lifetime was complex and dependent on whether the divalent cation was Ca or Mg. The mean channel lifetime was not significantly changed in most solutions with increased Ca concentration, while it was slightly prolonged by increased Mg concentration.     

Nutrient cycling in Pinus sylvestris stands in eastern Finland

Nutrient cycling within three Pinus sylvestris stands was studied in eastern Finland. The aim of the study was to determine annual fluxes and distribution of N, P, K, Ca, Mg, Zn, Fe, B, and Al in the research stands. Special emphasis was put on determining the importance of different fluxes, especially the internal cycle within the trees in satisfying the tree nutrient requirements for biomass production. The following nutrient fluxes were included, input; free precipitation and throughfall, output; percolation through soil profile, biological cycle; nutrient uptake from soil, retranslocation within trees, return to soil in litterfall, release by litter decomposition. The distribution of nutrients was determined in above- and belowground tree compartments, in ground and field vegetation, and in soil.The nitrogen use efficiencies were 181, 211 and 191 g of tree aboveground dry matter produced per g of N supplied by uptake and retranslocation in the sapling, pole stage and mature stands, respectively. Field vegetation was more efficient in nitrogen use than trees. Stand belowground/aboveground and fine root/coarse root biomass ratios decreased with tree age. With only slightly higher fine root biomass, almost three times more nitrogen had to be taken-up from soil for biomass production in the mature stand than in the sapling stand.The annual input-output balances of most nutrients were positive; throughfall contained more nutrients than was lost in mineral soil leachate. The sulphate flux contributed to the leaching of cations, especially magnesium, from soil in the mature stand.Retranslocation supplied 17–42% of the annual N, P and K requirements for tree aboveground biomass production. Precipitation and throughfall were important in transferring K and Mg, and also N in the sapling stand. Litterfall was an important pathway for N, Ca, Mg and micro nutrients, especially in the oldest stands.     

Effect of divalent cations on ion fluxes and leaf photochemistry in salinized barley leaves

Photosynthetic characteristics, leaf ionic content, and net fluxes of Na(+), K(+), and Cl(-) were studied in barley (Hordeum vulgare L) plants grown hydroponically at various Na/Ca ratios. Five weeks of moderate (50 mM) or high (100 mM) NaCl stress caused a significant decline in chlorophyll content, chlorophyll fluorescence characteristics, and stomatal conductance (g(s)) in plant leaves grown at low calcium level. Supplemental Ca(2+) enabled normal photochemical efficiency of PSII (F(v)/F(m) around 0.83), restored chlorophyll content to 80-90% of control, but had a much smaller (50% of control) effect on g(s). In experiments on excised leaves, not only Ca(2+), but also other divalent cations (in particular, Ba(2+) and Mg(2+)), significantly ameliorated the otherwise toxic effect of NaCl on leaf photochemistry, thus attributing potential targets for such amelioration to leaf tissues. To study the underlying ionic mechanisms of this process, the MIFE technique was used to measure the kinetics of net Na(+), K(+), and Cl(-) fluxes from salinized barley leaf mesophyll in response to physiological concentrations of Ca(2+), Ba(2+), Mg(2+), and Zn(2+). Addition of 20 mM Na(+) as NaCl or Na(2)SO(4) to the bath caused significant uptake of Na(+) and efflux of K(+). These effects were reversed by adding 1 mM divalent cations to the bath solution, with the relative efficiency Ba(2+)>Zn(2+)=Ca(2+)>Mg(2+). Effect of divalent cations on Na(+) efflux was transient, while their application caused a prolonged shift towards K(+) uptake. This suggests that, in addition to their known ability to block non-selective cation channels (NSCC) responsible for Na(+) entry, divalent cations also control the activity or gating properties of K(+) transporters at the mesophyll cell plasma membrane, thereby assisting in maintaining the high K/Na ratio required for optimal leaf photosynthesis.     

Changes in the composition of rainwater upon passage through the canopies of trees and of ground vegetation in a Dutch oak-birch forest

The composition of canopy throughfall water in an oak-birch woodland, heavily affected by atmospheric deposition of N and S, changed markedly upon contact with the above-ground parts of the ground vegetation, which consisted mainly of bracken. The fluxes of nitrate and H⁺ decreased, simultaneously with an increase in the flux of bicarbonate, indicative of above-ground uptake of nitrate by the ground vegetation. This above-ground assimilation takes place in spite of abundant availability of inorganic nitrogen in the root zone of the ground vegetation. Fluxes of phosphate were somewhat lower, and those of ammonium somewhat higher in throughfall of the ground vegetation than in that of the tree layer. Although those differences were not statistically significant, they do suggest assimilation of some P and extra net dry deposition of atmospheric ammonia below the tree canopies.     

Tests of an electrostatic screening hypothesis of the inhibition of neurotransmitter release by cations at the frog neuromuscular junction.

We have investigated an electrostatic screening hypothesis of cationic inhibition of quantal release at the neuromuscular junction of the frog (Rana pipiens). According to this hypothesis, increasing the extracellular concentration of an inhibitory cation reduces the quantal content (m) of the end-plate potential by reducing the ability of negative surface charge to attract Ca2+ to the external surface of the presynaptic membrane. The inhibitory power of various cations should depend only on their net ionic charge and should increase strongly with increasing charge. We have demonstrated, in Ringer's solutions containing modified concentrations of Na+, Ca+, and Mg2+, that at fixed concentrations of Ca2+ and Na+ (a) the dependence of m on [Mg2+]0 is satisfactorily accounted for by electrostatic theory and (b) the dependence of m on the univalent cation concentration of the modified Ringer's solution is satisfactorily predicted from the Mg2+ inhibition of m. (Glucosamine or arginine was used to replace a fraction of the Na+ content of Ringer's solution in the latter experiments.) These results are consistent with electrostatic screening actions of Mg2+ and univalent cations in the inhibition of m. We have also re-examined the inhibition of m caused by the addition to Ringer's solution of two trace concentration divalent cations, Mn2+ and Sr2+. Our data suggest that the inhibition of m by Sr2+ at high quantal contents may also be due to surface charge screening, while the potent inhibitory actions of Mn2+ may be due to its ability to bind negative surface charge.     

Calcium-sensitive univalent cation channel formed by lysotriphosphoinositide in bilayer lipid membranes.

A calcium sensitive univalent cation channel could be formed by lysotriphosphoinositide on an artificial bilayer membrane made of oxidized cholesterol. The modified membrane was selectively permeable to univalent cations, but was only very sparingly permeable to anions or divalent cations. Selectivity sequence among group IA cations was Rb+ greater than Cs+ greater than Na+ greater than K+ greater than Li+. The conductance of the membrane was increased up to a value of about 10-2 ohm-1/cm2 with an increase in the concentration of univalent cation, and was drastically depressed by a relatively small increase in the concentration of calcium ion or other divalent cations. The sequence of depressing efficiency among divalent cations was Zn+ greater than Cd2+ greater than Ca2+ greater than Sr2+ greater than Mg2+.     

Reductions in external divalent cations evoke novel voltage-gated currents in sensory neurons

It has long been recognized that divalent cations modulate cell excitability. Sensory nerve excitability is of critical importance to peripheral diseases associated with pain, sensory dysfunction and evoked reflexes. Thus we have studied the role these cations play on dissociated sensory nerve activity. Withdrawal of both Mg(2+) and Ca(2+) from external solutions activates over 90% of dissociated mouse sensory neurons. Imaging studies demonstrate a Na(+) influx that then causes depolarization-mediated activation of voltage-gated Ca(2+) channels (Ca(V)), which allows Ca(2+) influx upon divalent re-introduction. Inhibition of Ca(V) (ω-conotoxin, nifedipine) or Na(V) (tetrodotoxin, lidocaine) fails to reduce the Na(+) influx. The Ca(2+) influx is inhibited by Ca(V) inhibitors but not by TRPM7 inhibition (spermine) or store-operated channel inhibition (SKF96365). Withdrawal of either Mg(2+) or Ca(2+) alone fails to evoke cation influxes in vagal sensory neurons. In electrophysiological studies of dissociated mouse vagal sensory neurons, withdrawal of both Mg(2+) and Ca(2+) from external solutions evokes a large slowly-inactivating voltage-gated current (I(DF)) that cannot be accounted for by an increased negative surface potential. Withdrawal of Ca(2+) alone fails to evoke I(DF). Evidence suggests I(DF) is a non-selective cation current. The I(DF) is not reduced by inhibition of Na(V) (lidocaine, riluzole), Ca(V) (cilnidipine, nifedipine), K(V) (tetraethylammonium, 4-aminopyridine) or TRPM7 channels (spermine). In summary, sensory neurons express a novel voltage-gated cation channel that is inhibited by external Ca(2+) (IC(50)～0.5 μM) or Mg(2+) (IC(50)～3 μM). Activation of this putative channel evokes substantial cation fluxes in sensory neurons.     

Tree species shape the elemental composition in the lichen Hypogymnia physodes transplanted to pairs of spruce and beech trunks

The lichen Hypogymnia physodes was sampled from spruce trunks and we used to assess natural elements in the throughfall from pairs of neighbouring beech (Fagus sylvatica) and spruce (Picea abies) canopies in an unmanaged forest reserve. The beech bark (pH = 4.36 ± 0.13) was less acidic than spruce bark (3.71 ± 0.06). After a 1 yr transplantation onto trunks, lichens on beech had significantly higher concentrations of Ca, K, Mg and P than on spruce, and lower Mn, Zn and C, but had similar Al, B, Fe, N, Na, S and Si concentrations. Base cations (Ca, Mg, K) in lichens highly significantly increased with bark pH, with no overlap between tree species neither for base cations, nor for pH. The results are consistent with the view that trees modify the elemental composition of lichens in their dripzone, and that trees at least to some extent can modify the elemental chemistry of their local surroundings and thus influence ecosystem processes. We discuss lichen transplantation as a method to estimate long-term effects of tree species on local chemical environments.     

Litterflow chemistry and nutrient uptake from the forest floor in northwest Amazonian forest ecosystems

Samples of the fraction of net rainfall passing through the forest floor collected at monthly intervals in four pristine forests in Colombian Amazonia, during the period between 1995–1997 were analysed for solute concentrations to estimate the element fluxes from the forest floor into the mineral soil and root nutrient uptake from these forest floors. Results were compared with inputs by throughfall, stemflow, litterfall and fine root decay. Element concentrations were tested for their relationship with litterflow amounts, rainfall intensity and length of the antecedent dry period and differences in element fluxes between ecosystems were assessed. Concentrations of elements in litterflow followed a similar pattern as those in throughfall, which indicates that element outputs from the forest floor are strongly related to those inputs in throughfall. In the forests studied, the average concentrations of elements as K, Mg, orthoP and the pH of the litterflow decreased relative to that in throughfall in most events, while the concentration of elements such as dissolved organic carbon, H, SO₄ and Si increased in litterflow from these forests. Element concentrations in litterflow showed a poor correlation with variables such as litterflow amounts, rainfall intensity and antecedent dry period, except for K which showed a significant correlation (p>0.95) with analysed variables in all forests. Outputs were significantly different between forests (p>0.95); these fluxes, which particularly concerned cations, being the largest in the flood plain, while for anions outputs increased from the flood plain to the sedimentary plain. After adding the nutrient contributed by litter decomposition and fine root decay, the net outputs of main elements from the forest floors were still smaller than inputs by net precipitation (throughfall+stemflow) indicating that the litter layers clearly acted as a sink for most nutrients. Accordingly, the element balances confirm that the forest floors acted as a sink for nutrients coming in by throughfall, stemflow, litterfall and fine root decomposition. P, Mg and N appeared to be the most limiting nutrients and the forests studied efficiently recycled these nutrients.     

Cationic modulation of follicle-stimulating hormone binding to granulosa cell receptor

Magnesium (Mg2+) increases binding of follicle-stimulating hormone (FSH) to membrane-bound receptors and increases adenylyl cyclase activity. We examined the effects of divalent and monovalent cations on FSH binding to receptors in granulosa cells from immature porcine follicles. Divalent and monovalent cations increased binding of [125I]iodo-porcine FSH (125I-pFSH). The divalent cations Mg2+, calcium (Ca2+) and manganese, (Mn2+) increased specific binding a maximum of 4- to 5-fold at added concentrations of 10 mM. Mg2+ caused a half-maximal enhancement of binding at 0.6 mM, whereas Ca2+ and Mn2+ had half-maximal effects at 0.7 mM and 0.8 mM, respectively. The monovalent cation potassium (K+) increased binding a maximum of 1.5-fold at an added concentration of 50 mM, whereas the monovalent cation (Na+) did not increase binding at any concentration tested. The difference between K+ and Na+ suggested that either enhancement of binding was not a simple ionic effect or Na+ has a negative effect that suppresses its positive effect. Ethylenediamine tetraacetic acid, a chelator of Mg2+, prevented binding of 125I-pFSH only in the presence of Mg2+, whereas pregnant mare's serum gonadotropin, a competitor with FSH for the receptor, prevented binding in both the absence and the presence of Mg2+. Guanyl-5-ylimidodiphosphate (Gpp[NH]p) inhibited binding of 125I-pFSH in the absence or presence of Mg2+, but only at Gpp(NH)p concentrations greater than 1 mM. We used Mg2+ to determine if divalent cations enhanced FSH binding by increasing receptor affinity or by increasing the apparent number of binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)     

Physicochemical selectivity for cations and CEC of grass roots

Summary A study of root selective adsorption of Ca, Mg, K and Na was made with rootlets of 20 species of Gramineae (from natural vegetation) exhibiting low cation exchange capacities (CEC) scattered over a narrow range 7.6–16.7 meq 100 g^-1 dry matter). For each sample of rootlets successive measurements were made of its CEC (amount of Ca adsorbed from 0.05 N calcium nitrate solution) and the amounts of Ca, Mg, K and Na adsorbed from a tetracationic nitrate solution having equal concentrations (0.05 N) of each cation. Rootlet selectivity for bivalent cations increased with CEC and was greater for Ca than for Mg. In contrast, the selectivity for monovalent cations increased with decreasing CEC and was greater for K than for Na. A root's selective adsorption may therefore be governed by its CEC, in conformation with the exchange laws, as is the case with clays and resins. Paper read at the XII International Botanical Congress (mineral nutrition session) held in Leningrad, July, 1975.     

Rainfall partitioning into throughfall and stemflow and associated nutrient fluxes: land use impacts in a lower montane tropical region of Panama

Land-use change alters catchment hydrology by influencing the quality and quantity of partitioned rainfall. We compared rainfall partitioning (throughfall, stemflow and interception) and nutrient concentrations in rainfall, throughfall and stemflow in three land-use types [primary forest (PF), secondary forest (SF) and agriculture (A)] in Panama. Measurements of throughfall were highly variable which may have masked seasonal and land use differences but it was clear that throughfall at agricultural sites made up a larger proportion of gross precipitation than at forest sites. Of incident precipitation, 94% became throughfall in agriculture sites while 83 and 81% of gross precipitation became throughfall in PF and SF, respectively. The size of the precipitation event was the main driver of variation in throughfall and stemflow. Consistent patterns in nutrient cycling were also difficult to identify. Vegetation has a vital role in delivering nutrients as throughfall deposition of K was often larger than precipitation deposition. A canopy budget model indicated that canopy exchange was often more dominant than dry deposition. Throughfall was generally enriched with nutrients, especially K and Mg, with enrichment factors of up to 17 and 5 for K and Mg, respectively, in PF. In contrast, Ca was sometimes taken up by the canopy. Values of nutrient deposition were high (with up to 15, 3, 30 and 15 kg ha^?1 month^?1 in stand deposition of Ca, Mg, K and Na, respectively in PF), possibly due to the slash-and-burn agricultural practices in the area or marine inputs. Throughfall and stemflow are vital sources of nutrients in these ecosystems.     

Base-cation Cycling by Individual Tree Species in Old-growth Forests of Upper Michigan,USA

The influence of individual tree species on base-cation (Ca, Mg, K, Na) distribution and cycling was examined in sugar maple (Acer saccharum Marsh.), basswood (Tilia americana L.), and hemlock (Tsuga canadensis L.) in old-growth northern hardwood – hemlock forests on a sandy, mixed, frigid, Typic Haplorthod over two growing seasons in northwestern Michigan. Base cations in biomass, forest floor, and mineral soil (0–15 cm and 15–40 cm) pools were estimated for five replicated trees of each species; measured fluxes included bulk precipitation, throughfall, stemflow, litterfall, forest-floor leachate, mineralization + weathering, shallow-soil leachate, and deep-soil leachate. The three species differed in where base cations had accumulated within the single-tree ecosystems. Within these three single-tree ecosystems, the greatest quantity of base cations in woody biomass was found in sugar maple, whereas hemlock and basswood displayed the greatest amount in the upper 40 cm of mineral soil. Base-cation pools were ranked: sugar maple > basswood, hemlock in woody biomass; sugar maple, basswood > hemlock in foliage; hemlock > sugar maple, basswood in the forest floor, and basswood > sugar maple, hemlock in the mineral soil. Base-cation fluxes in throughfall, stemflow, the forest-floor leachate, and the deep-soil leachate (2000 only) were ranked: basswood > sugar maple > hemlock. Our measurements suggest that species-related differences in nutrient cycling are sufficient to produce significant differences in base-cation contents of the soil over short time intervals (<65 years). Moreover, these species-mediated differences may be important controls over the spatial pattern and edaphic processes of northern hardwood-hemlock ecosystems in the upper Great Lakes region.