Effect of calcium levels on redistribution of Sr85 in bush bean plants

Summary When bush bean plants previously exposed to Sr⁸⁵ were transferred to solutions containing different levels of calcium without further addition of Sr⁸⁵, and calcium had relatively little, but some, effect on the redistribution of the Sr⁸⁵ in the plants. Calcium did tend to restrict movement of strontium. Sr⁸⁵ tended to concentrate in stems or old leaves, usually more so than calcium. New leaves particularly had lower Sr/Ca ratios than other plant parts indicating some discrimination in favor of calcium.     

Pilot Study of Bioaccumulation and Distribution of Cesium,Potassium, Sodium and Calcium in King Oyster Mushroom (Pleurotus Eryngii) Grown Under Controlled Conditions

This pilot study presents preliminary results on interrelations between alkali and alkaline earth elements during their transfer to mycelium and fruitbodies of saprophytic fungi. The accumulation and distribution of four elements (cesium, potassium, sodium, and calcium) was evaluated in king oyster mushroom (Pleurotus eryngii) cultivated under controlled conditions. Elemental composition of caps, stipes, and the substrate was analyzed by atomic absorption/emission spectroscopy to evaluate discrimination, concentration, and transfer factors. The transfer factors determined for all the investigated elements were different and can be put in the following order: Cs > K > Na > Ca. There has been a higher accumulation of cesium in caps than in stipes. Distribution of cesium in fruitbodies depended on the presence of other ions in the substrate. The addition of Ca²⁺ limited the transport of cesium and potassium from stipes to caps. Sodium and calcium were mainly accumulated in the stipes. In a control experiment, without supplementation with K⁺, Na⁺, and Ca²⁺, ～ 62% of the cesium present in the substrate was extracted by mycelium and transported to the fruitbodies. Possible applications of fruiting saprophytic fungi in bioremediation are discussed.     

Identifying calcium sources at an acid deposition-impacted spruce forest: a strontium isotope,alkaline earth element multi-tracer approach

Depletion of calcium from forest soils has important implications for forest productivity and health. Ca is available to fine feeder roots from a number of soil organic and mineral sources, but identifying the primary source or changes of sources in response to environmental change is problematic. We used strontium isotope and alkaline earth element concentration ratios of trees and soils to discern the record of Ca sources for red spruce at a base-poor, acid deposition-impacted watershed. We measured ⁸⁷Sr/⁸⁶Sr and chemical compositions of cross-sectional stemwood cores of red spruce, other spruce tissues and sequential extracts of co-located soil samples. ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios together provide a tracer of alkaline earth element sources that distinguishes the plant-available fraction of the shallow organic soils from those of deeper organic and mineral soils. Ca/Sr ratios proved less diagnostic, due to within-tree processes that fractionate these elements from each other. Over the growth period from 1870 to 1960, ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios of stemwood samples became progressively more variable and on average trended toward values that considered together are characteristic of the uppermost forest floor. In detail the stemwood chemistry revealed an episode of simultaneous enhanced uptake of all alkaline earth elements during the growth period from 1930 to 1960, coincident with reported local and regional increases in atmospheric inputs of inorganic acidity. We attribute the temporal trends in stemwood chemistry to progressive shallowing of the effective depth of alkaline earth element uptake by fine roots over this growth period, due to preferential concentration of fine roots in the upper forest floor coupled with reduced nutrient uptake by roots in the lower organic and upper mineral soils in response to acid-induced aluminum toxicity. Although both increased atmospheric deposition and selective weathering of Ca-rich minerals such as apatite provide possible alternative explanations of aspects of the observed trends, the chemical buffering capacity of the forest floor-biomass pool limits their effectiveness as causal mechanisms.     

Changes in Sr/Ca, Ba/Ca and 87Sr/86Sr ratios between trophic levels in two forest ecosystems in the northeastern U.S.A.

The variability and biologicalfractionation of Sr/Ca, Ba/Ca and ⁸⁷Sr/⁸⁶Srratios were studied in a soil–plant–invertebrate–bird food chain in two forested ecosystems withcontrasting calcium availability in the northeasternU.S.A. Chemical measurements were made of the soilexchange pool, leaves, caterpillars, snails, and boththe femurs and eggshells of breeding insectivorousmigratory songbirds. ⁸⁷Sr/⁸⁶Sr values weretransferred up the food chain from the soil exchangepool to leaves, caterpillars, snails and eggshellswithout modification. Adult birds were the oneexception; their ⁸⁷Sr/⁸⁶Sr values generallyreflected those of lower trophic levels at each site,but were lower and more variable, probably becausetheir strontium was derived in part from foods intropical winter habitats where lower⁸⁷Sr/⁸⁶Sr ratios are likely to predominate. Sr/Ca and Ba/Ca ratios decreased at each successive trophiclevel, supporting previous suggestions that Sr/Ca andBa/Ca ratios can be used to identify the trophic levelat which an organism is primarily feeding. The changesin Sr/Ca and Ba/Ca ratios we measured for vegetationand insects were comparable to similar measurementsmade previously (but based on single samples of eachorganism) in an alpine ecosystem. Changes in Sr/Ca andBa/Ca ratios between birds and their food have notpreviously been measured, but the values we obtainedwere similar to those for herbivorous and carnivorousmammals at similar trophic levels. Our results provideevidence that supports the use of Sr/Ca ratios in thedetermination of human paleodiets and suggests thatSr/Ca ratios may also provide a useful tool in studiesof modern food webs. Furthermore, our findings suggestthat ⁹⁰Sr from nuclear fallout will notbioaccumulate in forests and that changes in Sr/Caratios between trophic levels will need to beconsidered in some cases when using⁸⁷Sr/⁸⁶Sr as a tracer of calciumbiogeochemistry.     

The effect of calcium on the growth ofChlorella andScenedesmus

The paper deals with the problem of the significance of Ca²⁺ for the growth of algae, its participation in the process of nitrate and ammonium nitrogen utilization, and with the possibility of substituting Sr²⁺ for it. It was revealed that calcium represents a microbiogenous element for the algae tested, which takes part in the utilization of nitrate nitrogen. It cannot be replaced by strontium especially at a high growth rate of mixotrophically cultivated algae (ina glucose medium).     

The Role of Encrusting Coralline Algae in the Diets of Selected Intertidal Herbivores

Kalk Bay, South Africa, has a typical south coast zonation pattern with a band of seaweed dominating the mid-eulittoral and between two molluscan-herbivore dominated upper and lower eulittoral zones. Encrusting coralline algae were very obvious features of these zones. The most abundant herbivores in the upper eulittoral were the limpet, Cymbula oculus (10.4 ± 1.6 individuals m⁻²; 201.65 ± 32.68 g.m⁻²) and the false limpet, Siphonaria capensis (97.07± 19.92 individuals m⁻²; 77.93 16.02 g.m⁻²). The territorial gardening limpet, Scutellastra cochlear, dominated the lower eulittoral zone, achieving very high densities (545.27 ± 84.35 m⁻²) and biomass (4630.17 ± 556.13 g.m⁻²), and excluded all other herbivores and most seaweeds, except for its garden alga and the encrusting coralline alga, Spongites yendoi (35.93 ± 2.26% cover). In the upper eulittoral zone, encrusting coralline algae were only present in the guts of the chiton Acanthochiton garnoti (30.5 ± 1.33%) and the limpet C. oculus (2.9 ± 0.34%). The lower eulittoral zone limpet, Scutellastra cochlear also had a large percentage of encrusting coralline algae in its gut with limpets lacking gardens having higher (45.1 ± 1.68%) proportions of coralline algae in their guts than those with gardens (25.6 ± 0.8%). Encrusting coralline algae had high organic contents, similar to those of other encrusting and turf-forming algae, but higher organic contents than foliose algae. Radula structure, grazing frequencies as a percentage of the area grazed (upper eulittoral 73.25 ± 3.60% d⁻¹; lower eulittoral 46.0 ± 3.29% d⁻¹), and algal organic content provided evidence to support the dietary habits of the above herbivores. The data show that many intertidal molluscs are actively consuming encrusting coralline algae and that these seaweeds should be seen as an important food source.     

Accumulation and subcellular distribution of cations in relation to the growth of potassium-deficient barley

Abstract Growth of barley (Hordeum vulgare L., cv. Georgie) was insensitive to soil K content above about 150 mg kg^?1, but at lower levels it declined. The reduction in yield was greater in soils containing approximately 10 mg Na kg^?1 than in soils with about 90 mg kg^?1 of Na. Growth was unaffected by changes in shoot K concentration above 75 mol m^?3, but declined at lower concentrations, and the decrease was less in plants grown in soils with high Na. Growth responses were not simply related to tissue K concentrations because plants grown in soils with extra Na had higher yields but lower K concentrations. When soil Na was low, plants accumulated Ca as tissue K declined, but when Na was provided this ion was accumulated. Plant Mg concentrations were generally low but increased as K decreased. The Ca and Mg were osmotically active. There were highly significant inverse linear relationships between yield and either the Ca or Mg concentrations in the shoots. X-ray microanalysis was used to examine the compartmentation of cations in leaves from barley plants (cv. Clipper) grown in nutrient solutions with high and low K concentrations. In plants grown with 2.5 mol m^?3 K, this was the major cation in both the cytoplasm and vacuole of mesophyll cells. However, in plants grown with 0.02 mol m^?3 K it declined to undetectable levels in the vacuole, although it was still detectable in the cytoplasm. In all plants, Ca was mainly located in epidermal cells. The implication of the results for explaining responses to K. in terms of compartmentation of solutes is discussed.     

SALINITY AND NUTRIENT LIMITATIONS ON GROWTH OF BENTHIC ALGAE FROM TWO ALKALINE SALT LAKES OF THE WESTERN GREAT BASIN (USA)1

Enrichment cultures of littoral benthic algae from Mono Lake, California, and Abert Lake, Oregon, were grown under conditions of varied salinity and nutrient content. Field-collected inocula were composed mainly of diatoms and filamentous blue-green and green algae. The yield of long-term cultures (30 days) showed tolerance over a broad salinity range (50–150 g·L^?1) for Mono Lake-derived algae. Algae from Abert Lake had a lower range of tolerance (25–100 g·L^?1) Organic content and pigment concentrations of algae from both lakes were also reduced above the tolerated salinity level. Within the range of salinity tolerance for Mono Lake algae, initial growth rates and organic content were reduced by increased salinity. The effects of macro- and micronutrient enrichment on algal growth in Mono Lake water were also tested. Only nitrogen enrichment (either as ammonium or nitrate) stimulated algal growth. Although the benthic algae cultured here had wide optima for salinity tolerance, the rates of growth and storage were limited by increased salinity within the optimum range. Although the lakes compared had similar species composition, the range and limits of tolerance of the algae were related to salinity of the lake of origin.     

ALGAL EXCRETION AND BACTERIAL ASSIMILATION IN HOT SPRING ALGAL MATS1

Benthic algal-bacterial mats are present in the effluents of alkaline hot springs at temperatures between 50 and 73 C. The thin surface layer is composed of the unicellular blue-green alga Synechococcus lividus. Also present in the surface layer and forming thick, orange mats beneath it, are filamentous, phototrophic, gliding bacteria of the genus Chloroflexis, also capable of heterotrophic growth. The very low species diversity and the constancy of the hot spring environment, make these mats a good ecosystem for studying the transfer of nutrients from the algae to the bacteria. To determine whether the alga might supply organic materials to the bacterium, excretion by natural populations of S. lividus was studied in the field by means of short-term radioisotope experiments. Under optimal conditions for photosynthesis, between 3 and 12% of the total ¹⁴C fixed was excreted as ¹⁴C-labeled organic compounds. Variations in cell density at concentrations of S. lividus approximating those found in the mat had no effect on the percentage excretion. However, at cell densities below a threshold, level, the percentage excretion increased with diminishing cell density. Except at very low light intensities the percentage of fixed carbon excreted, was very similar for all light intensities tested. Excretion at temperatures approaching the upper limit for growth was not significantly different from the percentage excretion values observed at lower temperatures. ¹⁴C-labeled organic compounds excreted during algal photosynthesis could be subsequently assimilated by natural populations of the bacteria present in the mat.     

FINE STRUCTURE OF A TERRESTRIAL CYANOBACTERIAL MAT FROM ANTARCTICA1

The fine structure of a cyanobacterial mat collected from the fellfield ecosystems of Signy Island, Antarctica, was examined using some novel light and scanning electron microscope techniques. The mat was up to 5 mm thick and was distinctly layered. The surface of the mat consisted of nonliving material above a zone of Phormidium autumnale (Ag.) Gom. filaments. We suggest that the surface layer protects the cyanobacterium from the effects of desiccation or high irradiance. Lower layers were less structured than the upper layer and included other taxa of cynobacteria and eukaryotic algae, although still dominated by Phormidium. The lowest layers consisted of dead organic material. The mat bound large amounts of inorganic material within and between the subsurface layers.     

Biogeochemical consequences of the transformation of native Cerrado into Pinus caribaea plantations in Brazil

Under the same climatic and edaphic conditions, native savanna vegetation in Brazil, the Cerrado, shows a lower stature and canopy cover than planted Pinus caribaea Morelet forests. To assess the differences in biogeochemical element cycling we compared the nutrient economy of Cerrado and Pinus on three replicate plots of each forest type. The mean nutrient storage in the soil organic layer under Pinus (N: 2630; P: 141; K: 103; Ca: 131; Mg: 20 kg ha^–1) was substantially higher than under Cerrado (N: 23; P: 1.2; K: 0.83; Ca: 5.8; Mg: 1.0 kg ha^–1) probably because the Pinus roots explored a larger soil volume. The Pinus trees had a higher nutrient-use efficiency as indicated by higher mean litter mass per unit nutrient in litter (N: 108; P: 2290; K: 729; Ca: 1360; Mg: 5420; S: 1190; Fe: 2960; Mn: 9990, Zn: 145000) than the Cerrado trees (N: 94; P: 1810; K: 619; Ca: 302; Mg: 938, S: 746; Fe: 1800; Mn: 7880; Zn: 63700). Mean annual small litterfall collected in 0.25-m² samplers between May 1997 and April 1999 was 2.1 Mg ha^–1 in Cerrado and 7.8 in Pinus. The litterfall rates of the 1–3 week collection intervals correlated negatively with the soil matric potential indicating that litterfall was partly related to water stress. The fluxes of N (73 kg ha^–1 year^–1), P (3.7), K (11), S (7.0), and Mn (0.83) to the soil with litterfall under Pinus were greater than the litterfall+turnover of the grass/herbs layer under Cerrado (N: 39, P: 2.8, K: 8.6, S: 5.4, Mn: 0.79 kg ha^–1 year^–1), those of Zn (0.06–0.07) were similar, and those of Ca (Pinus: 5.9/Cerrado: 10), Mg (1.5/4.4), and Fe (2.9/4.0) were smaller. Mean residence times of the organic matter and of all elements were longer in the soil organic layer under Pinus (3.7–26 years in the Oi horizon, 8.1–907 years in the whole organic layer) than under Cerrado (0.22–3.6 years in the Oi horizon, the only organic horizon under Cerrado). Our results demonstrate that the main differences in biogeochemical element cycling between the Pinus forest and the Cerrado consisted of a larger nutrient storage in the organic layer, a higher nutrient-use efficiency, and slower nutrient release rates from the organic layer in the Pinus forest than in the Cerrado. Nutrient cycling as assessed by the nutrient fluxes with litterfall was only partly faster in the Pinus forest than in the Cerrado.     

Relationship of strontium and calcium concentrations with the parameters of cell structure in Siberian spruce and fir tree-rings

This study addressed distribution of calcium and strontium in Siberian spruce (Picea obovata Ledeb.) and Siberian fir (Abies sibirica Ledeb.) tree-rings and its dependence on these woody species cell structure. Calcium concentration was found to decrease gradually from earlywood to latewood, whereas strontium showed an opposite trend. However, their trends at the scale of several rings are co-directed in the samples analyzed. A strong linear relationship was identified between the distribution of Sr/Ca concentration ratio and tree-ring density profile for both woody species. Radiographic density of Siberian spruce tree-ring cell walls and Ca and Sr concentrations in them were determined to have negative correlation with cell wall thickness. In earlywood of annual rings of a spruce the radiographic density of cell wall reaches 2.0 g/cm³ and decreases to 1.2 g/cm³ in latewood. The hypothesis put forward in this study to explain these strontium and calcium distributions in the tree-rings is that the concentrations of the element ions change with development of different cell wall layers. The high value of radiographic density of a cellular wall in earlywood and its relationship with cell wall thickness can be explained by the presence of ions of calcium in a cellular wall. Ions of calcium absorb X-ray radiation more strongly in comparison with light chemical elements. It can become the reason of observable relationship between radiographic density of cell wall and cell wall thickness.     

Sclerophylly in Qualea Parviflora (Vochysiaceae): Influence of Herbivory, Mineral Nutrients, and Water Status

Qualea parviflora Mart. (Vochysiaceae) is a deciduous tree, commonly observed in campo sujo, cerrado sensu stricto and cerradão vegetation types in Brazilian cerrado (savannas). In this study we investigated herbivory, nutritional, and water status effects on leaf sclerophylly of Q. parviflora. Twenty fully expanded leaves were taken from 10 plants in each vegetation type four times a year. Mean leaf concentration of N, P, K, Ca, C, Al, Si, and percentage of total phenols, herbivory and tannins were measured on a plant basis. Leaf specific mass (LSM) (g m^?2), a sclerophylly index, and pre-dawn leaf water potential (MPa) were also recorded. Soil samples below each tree were collected to quantify N–NO₃, N–NH₄, P, K, Mn, soil moisture, organic matter, Si, and Al. Qualea parviflora showed a LSM from 69 to 202 g m^?2 and leaves were younger and less sclerophyllous in November (beginning of rainy season). Q. parviflora inhabiting the cerradão had leaves with higher concentration of nutrients and lower sclerophylly while trees in campo sujo and cerrado sensu stricto did not show significant differences in leaf sclerophylly. The concentrations of N, P, K and tannins had an inverse relationship with leaf age. Concentration of phenols, Al, C, Ca, Si, C/N and Ca/K increased with leaf age. The concentrations of P and Ca/K ratio in leaves explained 60% of variation observed in leaf sclerophylly. We did not find any significant relationship between the level of sclerophylly and water potential or herbivory. Our results corroborate the hypothesis that predicts lower concentrations of essential macronutrients would be the main factors influencing higher sclerophylly in leaves of Q. parviflora plants in Cerrado.     

EFFECT OF CYSTEINE AND METHIONINE ON SULFATE UPTAKE AND PRIMARY PRODUCTIVITY BY AXENIC ALGAL CULTURES AND LAKE MICROPLANKTON COMMUNITIES1

The presence of up to 500 μg sulfur·l^?1 of an equimolar mixture of cysteine and methionine had virtually no effect on the SO₄^2- uptake rate of Navicula pelliculosa, (Bréb.) Hilse whereas the rate of Ankistrodesmus falcatus (Corda) Ralfs was decreased by the presence of 500 μg S· l^?1 and Anabaena flos-aquae (Lyngbye) Bréb. by 50 μg S·l^?1. Primary productivity in these axenic cultures was affected (decreased) only in A. falcatus. The C:S uptake ratio was lowest in N. pelliculosa and highest in A. falcatus. Considering these species as representative of groups of naturally occurring algae, patterns of SO₄^2- uptake and primary productivity in a eutrophic and a moderately oligotrophic lake reflected the results of the algal culturing experiments: SO₄^2- uptake rates, relative to primary productivity, were higher in the presence of diatoms and bluegreen algae and lower when green algae were present; the addition of the cysteine I methionine mixture to the lake waters decreased the rate of microplankton SO₄^2- uptake in correlation with the makeup of the algal community; primary productivity decreased upon the addition of cysteine I methionine when green algae were relatively abundant. It is concluded that, in most fresh water systems, the effects of organic sulfur pollution on algal SO₄^2- uptake and primary productivity are insignificant as compared to other ecological changes that occur due to that pollution.     

Effect of long-term fertilization and cropping on the potassium supplying capacity of soils

Soil samples from a long term field experiment, to study the K depletion pattern, were analysed for K. The values of supply parameters pK?1/2p (Ca+Mg) and free energy δG _r ⁰ were calculated using the data on water soluble and exchangeable cations. Correlation coefficient values between pK?1/2p (Ca+Mg) vs. total K removed, K removal by pearl millet in 1980 and soil available potassium were positive and statistically significant. This indicated that these soils have reached the stage where they would start responding to potassium application. Also, negative but significant correlation of δG _r ⁰ with pK?1/2p (Ca+Mg) and available K, were found. δG _r ⁰ values were positive indicating the affinity of such soils for K and the values, for K fertilized plots were lower in comparison to plots where no K was applied. pK?1/2p (Ca+Mg) parameter was found to be a better measure of K availability over δG _r ⁰ due to either fixation of K or greater contribution of nonexchangeable K fraction towards plant utilized K.     

Evidence for aqueous soluble vitamin D-like substances in the calcinogenic plant, Tristetum flavescens

A crude aqueous extract of the leaves of $\text{T. flavescens}$ when administered orally to vitamin D-deficient chicks produced significant increases in plasma phosphate but had little effect on plasma calcium. When chicks, fed a high strontium diet to inhibit endogenous 1,25(OH)₂ vitamin D₃ production and intestinal calcium transport, were dosed with the extract or synthetic 1,25(OH)₂D₃⁴⁵Ca absorption from the duodenum $\text{in vivo}$ was stimulated, whereas vitamin D₃ was ineffective. Partial purification of the crude extract on a Sephadex GH25 column yielded two factors, one of which mimicked 1,25 (OH)₂D₃ activity in chicks fed the high strontium diet while the other produced a significant increase in plasma phosphate. The presence of these substances, together with previously demonstrated organic solvent soluble vitamin D-type activity, may account for the calcinogenic nature of the plant.     

Studies on the Inhibition of Intestinal Absorption of Radioactive Strontium: V. The Effect of Administration of Calcium Alginate

A method of selective suppression of absorption of radioactive strontium from ingested food material is described which allows calcium to be available to the body. In the present study the effects of administering calcium alginate were determined. Studies on the relative binding of Ca and Sr by calcium alginate in vivo are also important to the investigation of calcium-strontium exchange.Samples of calcium alginate were obtained commercially or prepared from sodium salts and the binding properties with Ca⁴⁵ and Sr⁸⁹ were tested in vivo. There was a free exchange of radioactive calcium as well as of strontium with bound inactive calcium. The amount of Ca⁴⁵ retained in the gut by calcium alginate is proportionally less than Sr⁸⁹, so that the ratio Sr⁸⁹/Ca⁴⁵ in the bone is consistently and significantly lower in treated rats. Calcium alginate acts differently from other calcium salts, although in a manner similar to sodium alginate.     

Nonliving biomass of marine macrophytes as arsenic(V) biosorbents

The present study was aimed at assessing the performance of different nonliving macrophytes sampled in the Adriatic coast in arsenic(V) sorption. Full factorial experiments were carried out where the main factors were the macrophyte species (brown algae: Cystoseira, Dictyopteris, and Eisenia; green algae: Caulerpa and Ulva; red algae: Ceramium, Gracilaria, and Porphyra; and seagrass: Zostera), biosorbent washing pre-treatment (deionized water, acid pH?2 and basic pH?10), equilibrium pH (in the range 1 to 8), under relatively high (10?mg?L^?1) and relatively low (100?μg?L^?1) arsenic concentration. All species exhibited significant adsorption. Indeed, they showed a good performance, with the highest observed value of about 1.3?±?0.1?mg?g^?1 for the red alga Ceramium and the seagrass Zostera, comparable with those of activated carbon and other low-cost adsorbents reported in the literature under similar experimental conditions. Moreover, red algae known in the literature to be bad cationic metal sorbents showed very good arsenic sorption performance. This work shows that the performance of arsenic biosorption depends on many factors: the different composition and structure of outer layer of the macrophytes, arsenic speciation and functional group availability under different pH, and eventual counter-ion interactions with arseniate.     

Soil Ca alters processes contributing to C and N retention in the Oa/A horizon of a northern hardwood forest

Recent studies on the effects of calcium (Ca) additions on soil carbon (C) cycling in organic soil horizons present conflicting results, with some studies showing an increase in soil C storage and others a decrease. We tested the legacy effects of soil Ca additions on C and nitrogen (N) retention in a long-term incubation of soils from a plot-scale field experiment at the Hubbard Brook Experimental Forest, NH, USA. Two levels of Ca (850 and 4250 kg Ca/ha) were surface applied to field plots as the mineral wollastonite (CaSiO₃) in summer of 2006. Two years after field Ca additions, Oa/A horizon soils were collected from field plots and incubated in the laboratory for 343 days to test Ca effects on C mineralization, dissolved organic carbon (DOC) export, and net N transformations. To distinguish mineralization of soil organic C (SOC) from that of more recent C inputs to soil, we incubated soils with and without added ¹³C-labeled sugar maple leaf litter. High Ca additions increased exchangeable Ca and pH compared to the control. While low Ca additions had little effect on mineralization of SOC or added litter C, high Ca additions reduced mineralization of SOC and enhanced mineralization of litter C. In litter-free incubations, δ¹³C of respired C was enriched in the high Ca treatment compared to the control, indicating that Ca suppressed mineralization of ¹³C-depleted SOC sources. Leaching of DOC and NH₄ ⁺ were reduced by Ca additions in litter-free and litter-amended soils. Our results suggest that Ca availability in these organic soils influences mineralization of SOC and N primarily by stabilization processes and only secondarily through pH effects on organic matter solubility, and that SOC binding processes become important only with relatively large alterations of Ca status.     

Chemical composition of forest floor and consequences for nutrient availability after wildfire and harvesting in the boreal forest

In boreal forests of eastern Canada, wildfire has gradually been replaced by clearcut harvesting as the most extensive form of disturbance. Such a shift in disturbance may influence the chemical properties of the forest floor and its capacity to cycle and supply nutrients, with possible implications for forest productivity. We compared the effects of stem-only harvesting (SOH), whole-tree harvesting (WTH) and wildfire on the chemical composition of forest floor organic matter and nutrient availability for plants, 15–20 years after disturbance in boreal coniferous stands in Quebec (Canada). The forest floor on plots of wildfire origin was significantly enriched in aromatic forms of C with low solubility, whereas the forest floor from SOH and WTH plots was enriched with more soluble and labile C compounds. The forest floor of wildfire plots was also characterized by higher N concentration, but its high C:N and high concentration of ¹⁵N suggest that its N content could be recalcitrant and have a slow turnover rate. Total and exchangeable K were associated with easily degradable organic structures, whereas total and exchangeable Ca and Mg were positively correlated with the more recalcitrant forms of C. We suggest that the bulk of Ca and Mg cycling in the soil–plant system is inherited from the influx of exchangeable cations in the forest floor following disturbance. The buildup of Ca and Mg exchangeable reserves should be greater with wildfire than with harvesting, due to the sudden pulse of cation-rich ash and to the deposition of charred materials with high exchange capacity. This raises uncertainties about the long-term availability of Ca and Mg for plant uptake on harvested sites. In contrast, K availability should not be compromised by either harvesting or wildfire since it could be recycled rapidly through vegetation, litter and labile organic compounds.