Calcium and iron as key drivers of brown moss composition through differential effects on phosphorus availability

Brown moss-dominated rich fens are characterized by minerotrophic conditions, in which calcium (Ca) and iron (Fe) concentrations show large variations. We examined the relative importance of Ca and Fe in relation to the occurrence of three typical brown moss species: Scorpidium scorpioides, Scorpidium cossonii, and Hamatocaulis vernicosus. Peat chemistry was examined in 24 stands of brown moss-dominated rich fens: 12 in the Netherlands and 12 in central Sweden. Ca and Fe turned out to be important drivers of brown moss composition. Fens dominated by Scorpidium scorpioides or Scorpidium cossonii were characterized by high pore water Ca-concentrations and total soil Ca-contents, but low P-availability. In these Ca-rich, but Fe-poor fens, foliar N:P ratios of vascular vegetation exceeded 20?g?g^?1, indicating phosphorus (P)-limitation due to Ca-P precipitation or low P-sorption capacity due to low Fe-levels. In contrast, fens dominated by Hamatocaulis vernicosus were characterized by high pore water Fe-concentrations and total soil Fe-contents, but also relatively high P-availability. N:P ratios in these fens were below 13.5?g?g^?1, indicating potential nitrogen (N)-limitation. We conclude that the relative roles of Ca and Fe, as related to the geohydrological conditions present, strongly determine the brown moss composition in rich fens through their differential effects on plant P-availability.     

Phosphorus sorption characteristics of the Bronx River bed sediments

Abstract

Phosphorus (P) is a major nutrient for plant growth, and it is often the primary limiting nutrient in freshwater ecosystems controlling algal blooms. The Bronx River of New York City, New York, USA includes freshwater and coastal water systems. The water quality of both fresh and saline water is lower than the standard levels designated by New York State, and classified as Class B and Class I waters, respectively. Algal blooms and oxygen depletion within the river have degraded the water quality, endangered fishing, and limited recreational use. The internal loading of P, an important bioavailability indicator in the Bronx River, is determined by the sorption processes, i.e., cycling of P between solid and liquid phases. The objectives of this study were to understand how P sorption characteristics affect the internal loading of P and the conditions that might give rise to a flux of P from sediment to the water column, and to estimate the effects of physicochemical properties of the sediments on P sorption parameters. Bed sediments were collected from 15 sites along the Bronx River, from the origin in Westchester Davis Brook, Kensico Dam through the Bronx to the Sound View Park estuary. Phosphorus sorption maximum (S_max) were significantly correlated with oxalate–extractable iron (Ox–Fe) and aluminum (Ox–Al), acid-extractable calcium (HCl–Ca) and magnesium (HCl–Mg), and total organic matter (OM), suggesting that not only metal ions affected P sorption characteristics, but OM also influenced the P sorption processes. This study also showed that originally sorbed P (S₀) was significantly correlated with Ox–Fe, Ox–Al, HCl–Mg, and OM. The extremely high values of the percentage of sorbed P retained in sediments (>98% for all sites except the two estuary sites: site 13 of 88% and site 14 of 92%) suggest that a large flux of P to the water column from the sediments could potentially occur under changing hydro-climatic conditions, such as the changes in pH, ionic strength and redox conditions, which may, in turn, exacerbate eutrophic conditions and subsequent algal blooms.     

Calcicole behaviour of Callisthene fasciculata Mart., an Al‐accumulating species from the Brazilian Cerrado

• Most aluminium (Al)‐accumulating species are found on soils with high Al saturation and low Ca availability (Ca poor). Callisthene fasciculata Mart. (Vochysiaceae), however, is an Al‐accumulating tree restricted to Ca‐rich soils with low Al saturation in the Brazilian Cerrado savanna. Here we tested its calcicole behaviour, and the possible role of organic acids in detoxification of Al during the early stages of plant development.
• We assessed growth, dry mass, nutrients, Al and organic acids in seedlings grown for 50 days on two contrasting Cerrado soils; one with high Ca concentrations and low Al saturation and the other with low Ca availability and high Al saturation.
• Relative to plants on Ca‐rich soil, plants on Ca‐poor soil had necrotic spots and bronzing of leaves. Roots and shoots contained reduced concentrations of P and Cu, but higher concentrations of Fe, Al and citrate. Despite lower concentrations in the soil, Ca and Mg increased in shoots. Shoot concentrations of oxalate were also higher.
• We confirmed C. fasciculata as an Al‐accumulating species with calcicole behaviour. The increased concentrations of organic acids in plants with higher Al accumulation suggest that high availability of soluble Al does not prevent occurrence of this species on soils with high Al saturation. Instead, the absence of C. fasciculata from Ca‐poor soils is probably due to imbalances in tissue Fe, Cu and Zn imposed by this soil type.

     

Environmental Control of Root Exudation of Low-Molecular Weight Organic Acids in Tropical Rainforests

The availability of phosphorus (P) can limit net primary production (NPP) in tropical rainforests growing on highly weathered soils. Although it is well known that plant roots release organic acids to acquire P from P-deficient soils, the importance of organic acid exudation in P-limited tropical rainforests has rarely been verified. Study sites were located in two tropical montane rainforests (a P-deficient older soil and a P-rich younger soil) and a tropical lowland rainforest on Mt. Kinabalu, Borneo to analyze environmental control of organic acid exudation with respect to soil P availability, tree genus, and NPP. We quantified root exudation of oxalic, citric, and malic acids using in situ methods in which live fine roots were placed in syringes containing nutrient solution. Exudation rates of organic acids were greatest in the P-deficient soil in the tropical montane rainforest. The carbon (C) fluxes of organic acid exudation in the P-deficient soil (0.7?mol?C?m^?2?month^?1) represented 16.6% of the aboveground NPP, which was greater than those in the P-rich soil (3.1%) and in the lowland rainforest (4.7%), which exhibited higher NPP. The exudation rates of organic acids increased with increasing root surface area and tip number. A shift in vegetation composition toward dominance by tree species exhibiting a larger root surface area might contribute to the higher organic acid exudation observed in P-deficient soil. Our results quantitatively showed that tree roots can release greater quantities of organic acids in response to P deficiency in tropical rainforests.     

Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil

Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (P_i) availability in the rhizosphere and are frequently found to occur simultaneously. In non-calcareous soils, a major proportion of P_i is strongly sorbed to metal oxi(hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with P_i for the same sorption sites (ligand exchange) or solubilize P_i via ligand-promoted mineral dissolution. Root-induced co-acidification may also further promote P_i release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main P_i solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased P_i solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20–50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble P_i, presumably due to ionic-strength-driven changes in the electric surface potential favoring a higher P_i retention. Across a wider soil pH range (pH 3–8), P_i solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing P_i was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe- and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing P_i.     

Nutrient dynamics in small mesotrophic fens surrounded by cultivated land

Summary Release of inorganic N and P in the organic soils of three small quaking fens in The Netherlands was studied by means of in situ incubation of the peat soil in plastic bottles. One of the fens had higher biomass production and lower species richness than the other two. The former fen is located in an area with downward groundwater percolation, whereas the latter fens are situated in an area of upward seepage of groundwater rich in calcium and bicarbonate.Mineralization of N proved to be slow in the low-productive fens, and 6 times faster in the highly productive fen. In the latter fen the amount of N mineralized during a certain period exceeded by far the amount accumulated in that period in the above-ground parts of the vegetation.The release of inorganic P was extremely slow in the two low-productive fens and rapid in the highly productive fen. The release rates were not related to the total P content of the soils. The slow P release in the low-productive fens may be due to the continuous inflow of groundwater rich in Ca, Al and Fe compounds.It can be concluded that the soil is a potentially large source of inorganic N and P in quaking fens. The importance of the soil versus other N and P sources is greater in nutrient-rich fens.     

Can tissue element concentration patterns at the individual species level indicate the factors underlying vegetation gradients in wetlands?

Question: Do tissue element concentrations at the individual species level vary along major vegetation gradients in wetlands, and can they indicate environmental conditions? Location: West Carpathians. Methods: Total plant species composition was recorded in plots distributed along a poor to rich gradient within spring fens and along the gradient from fens to wet meadows. Eriophorum angustifolium (Cyperaceae) and three broadleaf dicotyledonous herb species were collected from the vegetation plots. Tissue N, P, K, Ca and Fe concentrations, N:P and N:K ratios of the species were determined. Each variable was correlated with the sample scores along the first two axes of the DCA ordination, which represented the two main vegetation gradients. Results: K and Ca concentrations in a particular species correlated well with the vegetation gradients, thus indicating changes in the element availability to the species. The trends were sometimes contradictory to known patterns at the community level, but the differences could be ecologically interpreted. Contrary to Ca and K, patterns in N, P and Fe concentration appeared to be more species‐specific. E. angustifolium had a lower K and Ca concentration than the broadleaf herbs. Conclusions: Compared to community‐level measurements, element concentrations in individual species correlated less with observed vegetation gradients. Trends found at the species level may indicate changes in ecological conditions affecting the species, although they need not correspond with trends found at the community level. We conclude that the species‐level approach cannot substitute, but can advance, the community‐level approach in searching for mechanisms underlying vegetation gradients within wetlands.     

On the tropical soils; The influence of organic matter (OM) on phosphate bioavailability

Application of organic manure (OM) and crop residues in agricultural soils can potentially influence positively or negatively the availability of soil phosphorus (P) through soil mineralization, sorption, or desorption of soil-bound P. Traditionally, the addition of OM can reduce the capacity of the soil colloids to adsorb P, thus increasing the release of P in soil solution, but also added OM can increase the adsorption site and increase the fixation or sorption of P to soil colloids, thus reducing the availability of P in soil solution and loss to the environment. The highly weathered tropical soils (HWTS) are susceptible to P insufficiency because HWTS have high P adsorption and fixation; this is mainly due to high concentration of P adsorbent. The main P adsorbents in HWTS include Al, Fe, Ca, and clay minerals, which are principally the same binding or adsorbent for OM compounds, but in excess, are toxic (Al and Fe) to crops. Thus, the presence of OM in HWTS can compromise the adsorption and availability of P in agricultural soils following phosphatic fertilizer applications. In this study, the influence of OM on P adsorption and availability was characterized to have a clear understanding of how OM influences P availability in agricultural soils, especially in highly weathered tropical soil. It is clearly outlined that the application of OM and crop residues can positively or negatively influence the availability of P in agricultural soils for plant uptake and dictate the P that is available for loss to the environment. Thus, the addition of organic matter as a strategy to increase P bioavailability for plant uptake must be treated with care because their contribution is not strait forward to be positive in many agricultural soils.     

Phosphorus retention and movement across an ombrotrophic-minerotrophic peatland gradient

An understanding of the mechanisms controlling nutrient availability and retention in and across ecosystems allows for a greater understanding of the role of nutrients in maintaining ecosystem structure and function. To examine the underlying mechanisms of phosphorus (P) cycling in northern peatlands, we compared the retention and movement of P across a natural hydrologic/pH gradient in nine peatlands by applying as a light rain an in situ tracer amount of ³²PO₄ ^–3 to track changes in P pools (vegetation, soil, microbial) over 30 days. The ³¹P concentrations of available P, microbial P, and the root P at 10–20 cm did not differ across the gradient, although total soil P and aboveground vegetation P content (g m^–2) increased from bog to rich fen. Total retention of ³²P in the first 24 hours of application was greatest in the bogs and intermediate fens (90–100%) and was very low (20–50%) in the rich fens. Retention of P in the different pools was dependent on the type of peatland and changed with time. In the first 24 hours in the bogs and intermediate fens, the microbial pool contained the largest amount of ³²P, but by the seventh day, the aboveground vegetation contained the largest amount. In the rich fen, the recovered ³²P was almost equally divided between the aboveground vegetation and the litter layer with very little in other pools. Therefore, although bogs and intermediate fens have a small total P pool, they have similar P availability to rich fens because of rapid cycling and efficient retention of P.     

Leaf litter inputs decrease phosphate sorption in a strongly weathered tropical soil over two time scales

In strongly weathered soils, leaf litter not only returns phosphorus (P) to the soil environment, it may also modify soil properties and soil solution chemistry, with the potential to decrease phosphate sorption and increase plant available P. Using a radioactive phosphate tracer (³²P) and 1 h laboratory incubations we investigated the effect of litter inputs on phosphate sorption over two time scales: (1) long-term field litter manipulations (litter addition, control and litter removal) and (2) pulses of litter leachate (i.e. water extracts of leaf litter) from five species. Leachate pulse effects were compared to a simulated throughfall, which served as a control solution. Soil receiving long-term doubling of leaf litter maintained five-fold more phosphate in solution than the litter removal soil. In addition to the quantity of phosphate sorbed, the field litter addition treatment decreased the strength of phosphate sorption, as evaluated through extraction of sorbed ³²P using a weakly acidic ammonium fluoride solution (Bray 1). In litter removal soil, leachate pulses significantly reduced phosphate sorption in comparison to the throughfall control for all five species evaluated. However, the ability of leachate pulses to reduce phosphate sorption decreased when soil had received field litter inputs. Across soils the effect of leachate pulses on phosphate sorption increased with net sorption of dissolved organic C, with the exception of leachate from one species that had a higher index of aromatic C concentration. These results demonstrate that litter inputs, as both long-term inputs and short-term leachate pulses, can decrease the quantity and strength of phosphate sorption, which may increase the biological availability of this key nutrient.     

Biological and Geochemical Sinks for Phosphorus in Soil from a Wet Tropical Forest

In many tropical and volcanic soils, phosphorus (P) availability is strongly influenced by geochemical sorption, which binds P to soil minerals. The aim of this study was to determine whether biological demand or soil sorption strength was the primary control over phosphate availability and retention in a wet tropical soil with high sorption capacity and low P availability. We added ³²PO₄ to soil from the upper two horizons and assessed the ability of soil microbes to immobilize the added phosphate in the presence of strong sorption. We added phosphate at two concentrations, one representing background turnover that adds low concentrations of P to the soil solution, and the other representing nutrient pulses that can add fairly high fluxes of P to the soil solution. Sorption and microbial immobilization were rapid for both concentrations, consuming most added P within 30 min. Thus, little P remained in the soil solution or extractable pools, which are considered more available to plants. Although soil sorption strength was almost identical for the two horizons, immobilization of tracer P was approximately three times greater in the upper horizon, where most microbial activity was located. This result suggests that microbial demand controlled how P was partitioned into biological versus geochemical sinks. Further evidence for microbial control is suggested by the movement of tracer P from the sorbed pool into the microbial pool when demand was stimulated by the addition of carbon (C). We also explored how increased nitrogen (N) and P availability changed P dynamics in this nutrient poor soil. In contrast to the unfertilized soil, long-term N and P fertilization substantially reduced biological control over inorganic P. P fertilization saturated the soils, overwhelming biological P demand, whereas N fertilization appeared to increase available P through reduced P sorption. Where biological demand for P is high and P becomes available in the soil solution, microbes may play an important role in controlling P partitioning into biological versus geochemical sinks even in soils that have high sorption capacity.     

Dynamics of nitrogen and phosphorus retention during wetland ecosystem succession

We compared the mechanisms of nitrogen (N) and phosphorus (P) removal in four young (<15 years old) constructed estuarine marshes with paired mature natural marshes to determine how nutrient retention changes during wetland ecosystem succession. In constructed wetlands, N retention begins as soon as emergent vegetation becomes established and soil organic matter starts to accumulate, which is usually within the first 1–3 years. Accumulation of organic carbon in the soil sets the stage for denitrification which, after 5–10 years, removes approximately the same amount of N as accumulating organic matter, 5–10 g/m²/yr each, under conditions of low N loadings. Under high N loadings, the amount of N stored in accumulating organic matter doubles while N removal from denitrification may increase by an order of magnitude or more. Both organic N accumulation and denitrification provide for long-term reliable N removal regardless of N loading rates. Phosphorus removal, on the other hand, is greatest during the first 1–3 years of succession when sediment deposition and sorption/precipitation of P are greatest. During this time, constructed marshes may retain from 3 g P/m²/yr under low P loadings to as much as 30 g P/m²/yr under high loadings. However, as sedimentation decreases and sorption sites become saturated, P retention decreases to levels supported by organic P accumulation (1–2 g P/m²/yr) and sorption/precipitation with incoming aqueous and particulate Fe, Al and Ca. Phosphorus cycling in wetlands differs from forest and other terrestrial ecosystems in that conservation of P is greatest during the early years of succession, not during the middle or late stages. Conservation of P by wetlands is largely regulated by geochemical processes (sorption, precipitation) which operate independently of succession. In contrast, the conservation of N is controlled by biological processes (organic matter accumulation, denitrification) that change as succession proceeds.     

L'epoca Della Differenziazione Delle Gemme Fiorali Nell'olivo

Abstract

Calcium, as in general for all plants, is very important for fruit trees and a tight correlation between leaf content and cropping efficiency has been found.

The annual removal of calcium oxide of an orchard has been estimated in kg per hectare: pear 200, apple 180, peach 150, grape 60–130, olive 35–70, kiwi 55–60.

The rootstock affects the calcium uptake from the soil and content of the scion; frequently a higher calcium content is found in trees grafted an dwarfing rootstocks as pear on quince, apple on M9 and M26, peach on Damas.

By the horticultural point of view, calcium is responsible of two main problems: chlorosis due to high active Ca content in the soil and bitter pit, on the apposite, due to a low Ca level in the fruits.

From soil and leaf studies it seems clear that lime-induced Fe chlorosis results from two conditions: a) slow availability of Fe in the soil, and b) immobilization of Fe in the tree in forms that are not available for chlorophyll formation.

Breeding tolerant rootstocks has been the practical solution of chlorosis for most of the susceptible species.

Bitter pit is a physiological disorder of apple fruits, sometime already evident before picking, more frequently after harvesting, during the storage.

The prevention of the disorder is, essentially, a good horticultural practice (pruning, fertilization, irrigation, fruit thinning). Very effective are also Ca sprays as chloride or nitrate, or citrate, or phosphate, starting after setting, 4–5 times every 10–12 days.     

Nest and foraging‐site selection in Yellowhammers Emberiza citrinella: implications for chick provisioning

Capsule Vegetation structure and invertebrate abundance interact to influence both foraging sites and nestling provisioning rate; when invertebrate availability is low, adults may take greater risks to provide food for their young.

Aims To investigate nesting and foraging ecology in a declining farmland bird whose fledging success is influenced by the availability of invertebrate prey suitable for feeding to offspring, and where perceived predation risk during foraging can be mediated by vegetation structure.

Methods Provisioning rates of adult Yellowhammers feeding nestlings were measured at nests on arable farmland. Foraging sites were compared with control sites of both the same and different microhabitats; provisioning rate was related to habitat features of foraging‐sites.

Results Foraging sites had low vegetation density, probably enhancing detection of predators, or high invertebrate abundance at high vegetation density. Parental provisioning rate decreased with increasing vegetation cover at foraging sites with high invertebrate abundance; conversely, where invertebrate abundance was low, provisioning rate increased with increasing vegetation cover.

Conclusions Vegetation structure at foraging sites suggests that a trade‐off between predator detection and prey availability influences foraging site selection in Yellowhammers. Associations between parental provisioning rate and vegetation variables suggest that where invertebrate abundance is high birds increase time spent scanning for predators at higher vegetation densities; however, when prey are scarce, adults may take more risks to provide food for their young.     

Effects of pH and bicarbonate on the nutrient status and growth of three Lupinus species

Aims

High pH, and high bicarbonate (HCO₃⁻) and calcium (Ca) availability characterise calcareous soils. High [Ca] only partially explains why some Lupinus species are calcifuge, so we explored high [HCO₃⁻] and high pH.

Methods

We grew six Lupinus genotypes in hydroponics with pH 5, 6.5 and 8^a (adjusted by KOH), and 8^b (adjusted by KHCO₃). Leaf symptoms and areas, root appearance and biomass were recorded; whole leaf and root nutrient concentrations, and leaf cellular phosphorus (P), Ca and potassium (K) concentrations were determined using elemental X-ray microanalysis.

Results

Chlorosis was observed in young leaves at high pH for L. angustifolius and L. cosentinii, and P deficiency at high pH for all genotypes. High pH decreased iron (Fe) and zinc (Zn) uptake in all genotypes. It also decreased lateral root growth, the uptake of P, K, Ca, and manganese (Mn) by all sensitive species; and translocation of P, Fe, Zn, Mn, and Ca to leaves in most sensitive species. However, leaf [Ca], leaf [K], [K] within each measured cell type, and translocation of K and Ca to leaves of L. pilosus and L. cosentinii at pH 8 were greater than at pH 5 and 6.5. Compared with pH 8^a, all L. angustifolius genotypes translocated more P, Fe, Zn, Mn and K from roots to leaves at pH 8^b. High pH did not affect the leaf cell types that accumulated P and Ca, but decreased the leaf cellular [P].

Conclusions

Lupinus angustifolius and L. cosentinii were sensitive to high [HCO₃⁻] and/or high pH; L. pilosus was relatively tolerant. High pH decreased lateral root growth and nutrient uptake, inhibiting growth of sensitive species. High [HCO₃⁻] diminished the negative effect of pH 8 on nutrient translocation to leaves in most L. angustifolius genotypes. This knowledge provides critical insights into the habits of Lupinus species to guide breeding of calcicole plants.

     

Phosphorus transformations along a soil/vegetation series of fire-prone, dolomitic, semi-arid shrublands of southern Spain Soil P and Mediterranean shrubland dynamic

Models of P transformations duringpedogenesis and with succession have developed fromstudies in temperate humid regions with neutral toacidic soils. Little is known about P biogeochemistryand P availability in semi-arid Mediterranean-typeshrublands with alkaline soils. We studied Ptransformations in a series of semi-arid, dolomiticshrublands in southeastern Spain, ranging from afrequently-burned, open gorse-scrubland on erodedTypic Xerorthents to a long-unburned, mature garrigueon Entic Haploxerolls. In contrast to the commonpattern of decreasing total P concentrations in thesoil profile with soil development due to leaching,total P increased markedly in this system. This is dueto concentration increases of relatively insolubleelements (P, Al, Fe, Ti) as karstification of parentmaterial (dolomitic marbles with up to 94% Ca-Mgcarbonate) during pedogenesis released bicarbonateand, subsequently, Ca and Mg leached from the profileat a higher rate. The total element to Ti ratiosindicated that the relative ion weathering losssequence, from easily weathered to resistant ions, wasCa>Mg > > > Fe>=Al>Ti, showing that P islost from these shrublands at an intermediate rate. Inone extreme of the series (the open gorse-scrubland),most soil P was Ca bound and organic P concentrations,organic matter content and phosphatase activity werevery low, as predicted by the model of Walker andSyers for the initial stages of soil development.However, this site showed the highest inorganic soilsolution P concentration, low soil P fixation capacityand the lowest foliar N:P ratios. Soils from theintermediate stages of the series showed the highestlabile inorganic and labile organic P concentrations.At the other extreme of the series (the maturegarrigue), a high proportion of soil P was in occludedinorganic and organic forms as predicted by the Walkerand Syers'; model. However, Ca bound P still accountedfor the largest single P fraction. Soils showed veryhigh sorption capacity (and high extractable Fe and Alconcentrations) and released very little P tosolution. Increasing values for NaOH extractable Po,organic matter and phosphatase activity indicate thatcycling of P through organic matter is increasinglyimportant with ecosystem development through theseries.     

Soil phosphorus supply controls P nutrition strategies of beech forest ecosystems in Central Europe

Phosphorus availability may shape plant–microorganism–soil interactions in forest ecosystems. Our aim was to quantify the interactions between soil P availability and P nutrition strategies of European beech (Fagus sylvatica) forests. We assumed that plants and microorganisms of P-rich forests carry over mineral-bound P into the biogeochemical P cycle (acquiring strategy). In contrast, P-poor ecosystems establish tight P cycles to sustain their P demand (recycling strategy). We tested if this conceptual model on supply-controlled P nutrition strategies was consistent with data from five European beech forest ecosystems with different parent materials (geosequence), covering a wide range of total soil P stocks (160–900 g P m^?2; <1 m depth). We analyzed numerous soil chemical and biological properties. Especially P-rich beech ecosystems accumulated P in topsoil horizons in moderately labile forms. Forest floor turnover rates decreased with decreasing total P stocks (from 1/5 to 1/40 per year) while ratios between organic carbon and organic phosphorus (C:P_org) increased from 110 to 984 (A horizons). High proportions of fine-root biomass in forest floors seemed to favor tight P recycling. Phosphorus in fine-root biomass increased relative to microbial P with decreasing P stocks. Concomitantly, phosphodiesterase activity decreased, which might explain increasing proportions of diester-P remaining in the soil organic matter. With decreasing P supply indicator values for P acquisition decreased and those for recycling increased, implying adjustment of plant–microorganism–soil feedbacks to soil P availability. Intense recycling improves the P use efficiency of beech forests.     

Fate and bioavailability of phosphorus loaded to iron oxyhydroxide nanoparticles added to weathered soils

Aims

The low phosphorus (P) fertilizer use efficiency in weathered, P deficient soils calls for better fertilizer formulations. We previously formulated nanoparticles containing P (NP-P) that were a successful fertilizer in nutrient solution. This study was set up to test the fate and the bioavailability of nanofertilizer-P and of that of native (colloid) P naturally present in soil.

Methods

The NP-P consisted of nano-ferrihydrite (~ 10 nm) loaded with phosphate (P-nFh) and stabilized with either natural organic matter (NOM) or hexametaphosphate (HMP). Natural colloid concentrations were increased with KOH addition, as deflocculating agent, to soil; all tests used samples from P deficient, highly weathered soils.

Results

Pot trials with rice seedlings did not reveal larger P uptake in the NP-P amended soils compared to equal doses of soluble PO₄ or soluble HMP. Total Fe concentrations in soil solutions were unaffected by NP-P addition, whereas natural colloidal Fe and P markedly increased by KOH addition. The bioavailability of native colloidal P, mobilized by KOH addition, could not be assessed due to lack of growth, likely related to collapse of the soil structure.

Conclusions

This study showed that P-loaded iron oxyhydroxide NPs insufficiently enhanced soluble P in soil to offer benefits over soluble fertilizers, likely because of a combined effect of lower diffusivity of NPs compared to P_i and lower bioavailability of NP-P than P_i. Smaller particles or small labile organic colloids might offer an improvement in both aspects.

     

Reversibility of phosphorus sorption by ferruginous nodules

Ferruginous nodules sorb significant amounts of available soil and fertiliser phosphate. The effect of this sorption on phosphorus availability of an agricultural soil was tested by sequential extraction and by exhaustive cropping with millet (Pennisetum typhoides) in a greenhouse trial following fertilisation of the original soil containing 70% nodules and of prepared samples containing various mixes of separated soil fines and nodules. Phosphorus sorption maxima by the soil fines and nodules were 190 mg kg⁻¹ and 380 mg kg⁻¹ respectively. Samples of fines and nodules which had sorbed 110 and 194 mg kg⁻¹ were submitted to 8 successive extractions with 0.01 M KCl, after which P desorption amounted to 117 mg kg⁻¹ and 103 mg kg⁻¹ respectively. Hysteresis between sorption and desorption was negligible for the soil fines and increased with increasing nodule content of the samples. In the greenhouse experiment, P uptake at the first cropping was highest in the soil fines at all levels of phosphorus applied. Subsequent croppings, however, showed higher P uptake in the concretionary soils. These results indicate a higher initial P release from the soil fines with cropping followed by an earlier exhaustion of phosphorus. At the end of the greenhouse experiment, yields were low in spite of the large quantities of P still remaining in the soils. Phosphorus fractionation showed that, of the P left in the soil after cropping 20% was in labile, 29% in Fe or Al-associated, and 51% in low-availability forms.     

Ecological studies on the Lime Hill serpentine,Scotland

Summary

Lime Hill is an example of a serpentine outcrop which shows only to a limited extent the features of vegetation often associated with ultrabasic rock. Plant and soil analyses from this site are presented and compared with those from other Scottish serpentines. Substantial quantities of heavy metals are present at Lime Hill. It is suggested that the cause of the absence of serpentine-characteristic plants lies in a relatively low soil Mg : Ca ratio and/or high P and K levels.