Are nutrient availability and acidity‐alkalinity gradients related in Sphagnum‐dominated peatlands?

Abstract. Gradients in acidity‐alkalinity and nutrient availability were studied in 2 Sphagnum‐dominated peatlands on the southeastern Italian Alps. Decreasing concentrations of most mineral elements (Ca²⁺, Mg²⁺, Mn²⁺, Al³⁺ and Si⁴⁺) in pore water indicated a progressively lower influx of mineral‐soil water from the slightly minerotrophic conditions in the peatland margins to ombrogenous conditions in the central part of the peatlands. This was paralleled by decreasing concentrations of ash, bulk density, Ca, Fe and, partly, Mn in the peat. The nutrient gradient, as defined by pore water concentrations of N and P, was largely independent of the acidity‐ alkalinity gradient: NO^3‐ and PO₄^3‐ had similar concentrations throughout the gradient, whereas NH₄⁺ concentrations increased with increasing pore‐water pH. In contrast, the peat nutrient gradient coincided with the acidity‐alkalinity gradient, with total concentrations of N and P decreasing from the margin to the centre. Bryophytes and vascular plants had different responses along the acidity‐alkalinity gradient and the nutrient gradient. Bryophyte distribution reflected the acidity‐alkalinity gradient both in pore water and in peat. Vascular plant distribution was mainly influenced by variations in nutrient availability.     

The nature of three ancient woodland soils in southern England

Abstract. Despite a wealth of published research on the nature of woodland soils, little is known about the nature of soils on sites that have supported woodland for many hundreds of years, namely ancient woodland. The properties and variability of soils in three ancient woods; one in the New Forest, Hampshire and two in Berkshire, were compared with those under recent woods. The acidity of ancient and recent woodland soils was high and remarkably similar. Only where cultivation of soils had preceded woodland establishment was soil acidity lower. The quantity of carbon in the soils studied was inversely related to soil acidity and the ancient woods had accumulated larger quantities of carbon than their recent counterparts. The quantities of Ca²⁺, Mg²⁺ and K⁺ were larger in the ancient woods except where prior cultivation had taken place. Total and organic phosphate contents of the ancient woodland soils were also consistently larger. The nature and pattern of soil variability in ancient woodland soils was quite distinct from that found in recent woods. Overall, the variability of soil acidity, carbon content and organic phosphate was larger in the ancient woodland soils but the pattern of variability differed between the soil properties. No clear association existed between the pattern of soil acidity and individual trees. At the surface of some of the woodland soils, however, carbon distribution appeared to be associated with individual trees. At depth in the ancient woodland soils, the association with the existing vegetation cover was not so clear. It is probable that the ancient woodland soils retained relict features of previous vegetation cover. Organic phosphate distribution was very strongly associated with the present vegetation cover. The pattern of distribution of organic phosphate appeared to be stronger than that of soil acidity and carbon content.     

Enhanced exudation of malate in the rhizosphere due to AtALMT1 overexpression in blackgram (Vigna mungo L.) confers increased aluminium tolerance

• Worldwide, 50% of soil is acidic, which induces aluminium (Al) toxicity in plants, as the phyto‐availability of Al³⁺ increases in acidic soil. Plants responds to Al³⁺ toxicity by exuding organic acids into the rhizosphere. The organic acid responsible for Al³⁺ stress response varies from species to species, which in the case of blackgram (Vigna mungo L.) is citrate.
• In blackgram, an Arabidopsis malate transporter, AtALMT1, was overexpressed with the motive of inducing enhanced exudation of malate. Transgenics were generated using cotyledon node explants through Agrobacterium tumefaciens‐mediated transformation. The putative transgenics were initially screened by AtALMT1‐specific genomic DNA PCR, followed by quantitative PCR. Two independent transgenic events were identified and functionally characterized in the T₃ generation.
• The transgenic lines, Line 1 and 2, showed better root growth, relative water content and chlorophyll content under Al³⁺ stress. Both lines also accounted for less oxidative damage, due to reduced accumulation of ROS molecules. Photosynthetic efficiency, as measured in terms of F_v/F_m, NPQ and Y(II), increased when compared to the wild type (WT). Relative expression of genes (VmSTOP1, VmALS3, VmMATE) responsible for Al³⁺ stress response in blackgram showed that overexpression of a malate transporter did not have any effect on their expression. Malate exudation increased whereas citrate exudation did not show any divergence from the WT. A pot stress assay found that the transgenics showed better adaptation to acidic soil.
• This report demonstrates that the overexpression of a malate transporter in a non‐malate exuding species improves adaptation to Al³⁺ toxicity in acidic soil without effecting its stress response mechanism.

     

Impact of the TaMATE1B gene on above and below-ground growth of durum wheat grown on an acid and Al3+-toxic soil

Background and aims

Subsoil acidity with a high aluminium (Al³⁺) soil content inhibits root growth and proliferation of durum wheat (tetraploid AABB, Triticum turgidum) leading to poor nutrient and water uptake. This study evaluated the impact of Al³⁺-tolerantTaMATE1B allele on root and shoot traits of durum wheat grown in an acidic soil with a high Al³⁺concentration.

Methods

Two durum wheat lines, Jandaroi–TaMATE1B with the TaMATE1B gene introgressed from Al³⁺-tolerant bread wheat and Jandaroi–null (a sister line lacking the Al³⁺-tolerant TaMATE1B allele), were grown in rhizoboxes in a glasshouse. We mapped root growth and proliferation over time and measured shoot traits and grain yield.

Results

Introgression of the Al³⁺-tolerant TaMATE1B allele into durum wheat enabled root growth and proliferation below 0.25 m of the soil profile, where the soil pH was low (4.1, CaCl₂ extract) with high Al³⁺ content (16.5 mg kg⁻¹), and increased total root length and biomass at 42 days after sowing (DAS; Z₃₃) by 38.3 and 22%, respectively, relative to the Jandaroi–null. Differences in root growth between the two lines were apparent from tillering stage (Z₃₃) and by 50% anthesis (Z₆₄), respectively. Jandaroi–TaMATE1B had 69.2% greater root biomass, 76.2% greater root length, 5.89% greater leaf area and 18% greater shoot biomass than Jandaroi–null at 50% anthesis (Z₆₄). Time to anthesis and physiological maturity was delayed 6–7 days in Jandaroi–TaMATE1B, compared to Jandaroi–null. Jandaroi–TaMATE1B tended to have relatively greater, but not significantly different, shoot biomass, grain yield and yield components than Jandaroi–null.

Conclusions

Introgression of the Al³⁺-tolerant TaMATE1B allele into durum wheat enabled root growth and proliferation down an acidic soil profile with a high Al³⁺ concentration. We assume that in the field where plants need to acquire water at depth differences in above-ground parameters would be amplified.

     

An extracellular hydrophilic carboxy‐terminal domain regulates the activity of TaALMT1, the aluminum‐activated malate transport protein of wheat

Al³⁺‐resistant cultivars of wheat (Triticum aestivum L.) release malate through the Al³⁺‐activated anion transport protein Triticum aestivum aluminum‐activated malate transporter 1 (TaALMT1). Expression of TaALMT1 in Xenopus oocytes and tobacco suspension cells enhances the basal transport activity (inward and outward currents present in the absence of external Al³⁺), and generates the same Al³⁺‐activated currents (reflecting the Al³⁺‐dependent transport function) as observed in wheat cells. We investigated the amino acid residues involved in this Al³⁺‐dependent transport activity by generating a series of mutations to the TaALMT1 protein. We targeted the acidic residues on the hydrophilic C‐terminal domain of TaALMT1 and changed them to uncharged residues by site‐directed mutagenesis. These mutant proteins were expressed in Xenopus oocytes and their transport activity was measured before and after Al³⁺ addition. Three mutations (E274Q, D275N and E284Q) abolished the Al³⁺‐activated transport activity without affecting the basal transport activity. Truncation of the hydrophilic C‐terminal domain abolished both basal and Al³⁺‐activated transport activities. Al³⁺‐dependent transport activity was recovered by fusing the N‐terminal region of TaALMT1 with the C‐terminal region of AtALMT1, a homolog from Arabidopsis. These findings demonstrate that the extracellular C‐terminal domain is required for both basal and Al³⁺‐dependent TaALMT1 activity. Furthermore, we identified three acidic amino acids within this domain that are specifically required for the activation of transport function by external Al³⁺.     

Contrasting responses of two Sitka spruce forest plots in Ireland to reductions in sulphur emissions: results of 20 years of monitoring

Long-term trends in ion concentrations of bulk precipitation, throughfall, forest floor leachate (humus water) and shallow and deep soil water were assessed at two Sitka spruce (Picea sitchensis) stands—one on an Atlantic peat bog in the west of Ireland (Cloosh), the other on the east coast on a peaty podzol (Roundwood). Deposition at Cloosh was dominated by marine ions (sodium, [Na⁺], chloride [Cl^?], and magnesium [Mg²⁺]), whereas bulk precipitation and throughfall at Roundwood was characterized by inputs of non-marine sulphate (nmSO₄ ^2?), acidity and inorganic nitrogen (NH₄ ⁺, NO₃ ^?). Significant declines in concentrations of nmSO₄ ^2? and acidity in bulk precipitation and throughfall were observed at both sites. The decline in throughfall nmSO₄ ^2? was significantly related to reductions in European sulphur dioxide (SO₂) emissions. At Roundwood, SO₄ ^2? declined significantly in humus, shallow and deep soil water. In deep soil water this was accompanied by a long-term increase in pH and a reduction in total aluminum (Al_tot). The recovery from acidification was delayed by high concentrations of NO₃ ^?, which strongly influenced acidity and Al_tot concentrations. At Cloosh, there was a significant decline in SO₄ ^2? in humus water but long-term trends were not evident in shallow or deep soil water; SO₄ ^2? concentrations at these depths fluctuated in response to drought-events. Marine ions strongly influenced soil water chemistry at both sites; at Cloosh soil water acidity was strongly related to Na⁺ and Cl^?, while at Roundwood, Na⁺, Cl^? and Mg²⁺ influenced Al_tot concentrations. Dissolved organic carbon increased significantly in humus and soil water at Roundwood, where it was associated with declining acidity. Soil water at both sites was influenced by a combination of anthropogenic sulphur (S) and nitrogen (N) deposition, drought and sea-salt events. The study highlights the value of long-term monitoring in assessing the response of forest soils to S and N deposition against a background of climate influences on soil water through drought and sea-salt events.     

DIATOM COMMUNITY DYNAMICS IN STREAMS OF CHRONIC AND EPISODIC ACIDIFICATION: THE ROLES OF ENVIRONMENT AND TIME1

Community dynamics of epiphytic diatoms were studied for 3 years in a chronically and an episodically acidified tributary of Buck Creek, Adirondacks. Both streams experienced pulses of acidity during hydrologic events but these pulses were more pronounced in the episodically acidified stream, where pH decreased over two units (between 4.53 and 6.62) and the acid‐neutralizing capacity (ANC) became negative. In the chronically acidified stream, pH was below 4.9 and the ANC was negative 94% of the time. In this stream, high inorganic acidity following SO₄^2? enrichment from snowmelt or rainstorms alternated with high organic acidity derived from a headwaters wetland during base flow. The fluctuating water chemistry generated shifts in diatom community composition: from exclusive dominance of Eunotia bilunaris (Ehrenberg) Mills during periods of high inorganic acidity to proliferation of several subdominant species during periods of high organic acidity. In the episodically acidified stream, the pulses of acidity were associated with high NO₃^? concentrations and the corresponding high ratios of inorganic monomeric Al (Al_im) to organic monomeric Al (Al_om). Diatom communities there were dominated exclusively by E. exigua (Brébisson) Rabenhorst year round; however, this species peaked during periods of low acidity. Periods of high acidity and Al_im:Al_om ratios were marked by a decline in E. exigua and a concomitant increase in the subdominant species. Variance partitioning into terms of environmental and temporal variance, and their covariance, suggested that diatom communities in the chronically acidic stream were governed primarily by environmental factors while in the episodically acidic stream environmental and temporal factors had equal contributions.     

Comparison of Al speciation and other soil characteristics between meadow, young forest and old forest stands

The aim of this paper is to describe the influence of spruce (Picea abies) afforestation on soil chemical properties, especially on soil acidity and aluminium (Al) mobilization and speciation in soil. For our study we used a unique set of three adjacent plots, including a meadow and two spruce forest stands of different age, in otherwise comparable conditions. The plots were located in the region of Giant Mountains, north-eastern Czech Republic. In general, pH values decreased and Al concentrations increased significantly after afforestation. Speciation of KCl-extractable and water-soluble Al in soil samples was done by means of HPLC/IC method. The concentrations of Al(X)¹⁺ and Al(Y)²⁺ forms (in both extracts) are higher in humic and organically enriched (Bhs) horizons. The highest concentration of Al³⁺ in both extracts is in the B horizons of old forest.Generally, in all studied stands majority of Al in aqueous extract is in the Al(X)¹⁺ form, which indicates that a large amount of mobile Al is bound in organic complexes. It suggests that actual toxicity is rather low. On the other hand, we have proved that majority of KCl-extractable Al exists in Al³⁺ form. Thus we can conclude that disturbance of existing equilibrium may cause massive release of highly toxic Al³⁺ from soil sorption complex to the soil solution, and consequently it can endanger the whole ecosystem. Moreover, continuous soil acidification accelerated by anthropogenic factors leading to Al mobilization represents a chemical time bomb.     

Aluminum‐dependent dynamics of ion transport in Arabidopsis: specificity of low pH and aluminum responses

Low‐pH and Al³⁺ stresses are the major causes of poor plant growth in acidic soils. However, there is still a poor understanding of plant responses to low‐pH and Al³⁺ toxicity. Low‐pH or combined low‐pH and Al³⁺ stress was imposed in order to measure rhizosphere pH, ion fluxes, plasma membrane potential and intracellular H⁺ concentration in distal elongation and mature zones (MZs) along the longitudinal axis of Arabidopsis thaliana roots. Low‐pH stress facilitated H⁺ influx into root tissues and caused cytoplasmic acidification; by contrast, combined low‐pH/Al³⁺ treatment either decreased H⁺ influx in the distal elongation zone (DEZ) or induced H⁺ efflux in the MZ, leading to cytoplasmic alkalinization in both zones. Low‐pH stress induced an increase in rhizosphere pH in the DEZ, whereas combined low‐pH/Al³⁺ stress resulted in lower rhizosphere pH in both root zones compared with the low‐pH treatment alone. Low‐pH stress facilitated K⁺ efflux; the presence of Al³⁺ diminished K⁺ efflux or favored K⁺ influx into root tissues. In both zones, low‐pH treatment induced plasma membrane (PM) depolarization, which was significantly diminished (P≤ 0.05) when combined stresses (low‐pH/100 µM Al³⁺) were imposed. After 60 min of exposure, low pH caused PM depolarization, whereas low pH/100 µM Al³⁺ caused PM hyperpolarization. Thus, low pH and Al³⁺ toxicity differentially affect root tissues and, consequently, the rhizosphere, which might underpin the differential mechanisms of plant adaptation to these abiotic stresses.     

Growth of oil palm (Elaeis guineensis) seedlings on acid sulfate soils as affected by water regime and aluminium

In Malaysia, acid sulfate soils contain high amounts of aluminium and are usually utilized for oil palm cultivation. As these soils are frequently flooded during rainy periods, it is thought that this may affect the growth performance of the oil palm. A glasshouse experiment was, therefore, conducted to study the effects of water regime and aluminium on the growth of oil palm seedlings. Soils used in the experiment were Typic Sulfaquepts and Sulfic Tropaquepts from Pulau Lumut Island, Malaysia. Best growth was observed on a non-jarositic freely drained topsoil. Oil palm seedlings were found the be moderately tolerant to soil acidity. Growth was only affected if Al³⁺ and Al_sum activities in the soil solutions were above 100 and 700 M, respectively. Root length was found to be one of the better parameters to predict crop growth, while others included plant height, fron length and LAI. Soil solution attributes which could be used as indices of soil acidity for oil palm growth were pH and activities of Al³⁺, AlSO₄ ⁺ and Al_sum.     

Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO₃⁻), aluminum (Al), and calcium (Ca²⁺) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO₃⁻ concentration was about 20 μmol l⁻¹ for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l⁻¹ about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Al_im) export increased by 4−fold during the first year after the harvest. Al_im mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca²⁺ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Al_im and NO₃⁻ concentrations were strongly correlated in B-horizon soil water after the clearcut (r ² = 0.96), especially at NO₃⁻ concentrations greater than 100 μmol l⁻¹. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO₃⁻ which mixed with acidic, high Al_im soil water and decreased the concentration of Al_im in streamwater after the harvest. Five years after the harvest soil water NO₃⁻ concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO₃⁻ concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO₃⁻ concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO₃⁻ and Al_im concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.     

The spatial distribution of birds and carabid beetles in pine plantation forests: the role of landscape composition and structure

Aim To evaluate the joint and independent effects of spatial location, landscape composition and landscape structure on the distribution patterns of bird and carabid beetle assemblages in a mosaic landscape dominated by pine plantation forests. Location A continuous 3000‐ha landscape mosaic with native maritime pine Pinus pinaster plantations of different ages, deciduous woodlands and open habitats, located in the Landes de Gascogne forest of south‐western France. Methods We sampled breeding birds by 20‐min point counts and carabid beetles by pitfall trapping using a systematic grid sampling of 200 points every 400 m over the whole landscape. Explanatory variables were composed of three data sets derived from GIS habitat mapping: (1) spatial variables (polynomial terms of geographical coordinates of samples), (2) landscape composition as the percentage cover of the six main habitats, and (3) landscape structure metrics including indices of fragmentation and spatial heterogeneity. We used canonical correspondence analysis with variance partitioning to evaluate the joint and independent effects of the three sets of variables on the ordination of species assemblages. Moran's I correlograms and Mantel tests were used to assess for spatial structure in species distribution and relationships with separate landscape attributes. Results Landscape composition was the main factor explaining the distribution patterns of birds and carabids at the mesoscale of 400 × 400 m. Independent effects of spatial variables and landscape structure were still significant for bird assemblages once landscape composition was controlled for, but not for carabid assemblages. Spatial distributions of birds and carabids were primarily influenced by the amount of heathlands, young pine plantations, herbaceous firebreaks and deciduous woodlands. Deciduous woodland species had positive responses to edge density, while open habitat species were positively associated with mean patch area. Main conclusions Forest birds were favoured by an increase in deciduous woodland cover and landscape heterogeneity, but there was no evidence for a similar effect on carabid beetles. Fragmentation of open habitats negatively affected both early‐successional birds and carabids, specialist species being restricted to large heathlands and young plantations. Several birds of conservation concern were associated with mosaics of woodlands and grasslands, especially meadows and firebreaks. Conserving biodiversity in mosaic plantation landscapes could be achieved by the maintenance of a significant amount of early‐successional habitats and deciduous woodland patches within a conifer plantation matrix.     

Extreme low pH,not Al3+, is a key abiotic stressor for the extremophyte Carex angustisquama (Cyperaceae) in highly acidic solfatara fields

• Volcanic acidification creates extreme soil conditions, where rhizotoxicity from extremely low pH (2–3) and high Al³⁺ strongly inhibit plant growth. C. angustisquama is a dominant extremophyte in highly acidic solfatara fields, where no other vascular plants can survive. Here we investigated the key abiotic stressor determining survival of this extremophyte.
• Soil analyses and topographic surveys were conducted to examine the effects of low pH and Al³⁺, two major abiotic stressors in acidic soils, on the occurrence of C. angustisquama in solfatara fields. Hydroponic culture experiments were also performed to test its growth responses to these stressors.
• In field surveys, the spatial distribution of soil pH was consistent with vegetation zonation within a solfatara field. In contrast, soil exchangeable Al content was overall low due to strong eluviation. Statistical analysis also supported the significant role of soil pH in determining the distribution of C. angustisquama in a solfatara field. Furthermore, hydroponic culture experiments revealed a higher tolerance of C. angustisquama to low pH than a sister species, especially in the range pH 2–3, corresponding to the pH values of the actual habitats of C. angustisquama. Conversely, no significant interspecific difference was detected in Al³⁺ tolerance, indicating that both species had high Al³⁺ tolerance.
• This study suggests that low pH is a critical abiotic stressor leading to formation of the extremophyte in highly acidic solfatara fields. In contrast, C. angustisquama displayed high tolerance to Al³⁺ toxicity, probably acquired prior to speciation.

     

Characterization of a highly Al3+‐selective fluorescence probe based on naphthalimide‐Schiff base and its application to practical water samples

A new fluorescent Al³⁺‐probe, N‐allyl‐4‐[3,3′‐((2‐aminoethyl)azanediyl)‐bis(N´‐(2‐hydroxybenzylidene)propanehy‐drazide)]‐1,8‐naphthalimide ( L ), was designed and synthesized based on 1,8‐naphthalimide. The probe L contains 1,8‐naphthalimide moiety as the fluorophore and a Schiff base as the recognition group. The structure of L was determined by single crystal X‐ray. L emission at 526 nm increased on addition of Al³⁺ under excitation wavelength at 350 nm. L exhibited high selectivity and sensitivity fluorescence emission towards to Al³⁺ in ethanol/Tris–HCl buffer solution (1:1, v/v, pH = 7.2) as compared with other tested metal ions. A good linearity with a correlation coefficient (R²) of 0.99 was observed in the concentration range 2–10 μM. The binding constant and the detection limit of L for Al³⁺ were calculated to 2.6 × 10⁴ M^?1 and 0.34 μM, respectively. The results of experiments that including Job plot, ultraviolet–visible (UV–Vis) light titration, fluorescence titration, ESI‐MS and ¹H NMR titration, indicated a 1:1 stoichiometric complex between L and Al³⁺. L was highly effective in monitoring Al³⁺ in real‐life Yellow River and tap water samples.     

Changes in nitrogen metabolism enzyme activities ofVicia faba in response to aluminum and cadmium

Nodules of faba bean (Vicia faba L. cv. Giza 3) plants grown in pots containing clay-loam soil for 90 d have an active nitrate reductase (NR), while the leaves did not show detectable activity. Spraying the plant with increasing concentrations of Al³⁺ or Cd²⁺ (0–1000 μM) significantly inhibited the nodules NR activity, the decline being more pronounced in Cd²⁺ treatment. The specific activity of glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) were more prominent in the 60- than in 90-d-old plants; GOT was always higher than GPT. Furthermore, GOT was more sensitive to Al³⁺ and Cd²⁺ treatments and its activity was significantly decreased when the metal concentration increased. Also, Cd²⁺ proved to be more effective than Al³⁺ in suppressing the GOT activity in the nodules, with less significant effect observed in the leaves. In contrast, GPT was hardly affected by the various metal treatments, particulary in the leaves.     

Accumulation of carbon and nitrogen by old arable land reverting to woodland

The accumulation of carbon (C) and nitrogen (N) was measured on two sites on Rothamsted Farm that had been fenced off some 120 years ago and allowed to revert naturally to woodland. The sites had previously been arable for centuries. One had been chalked and was still calcareous; the other had never been chalked and the pH fell from 7.1 in 1883 to 4.4 in 1999. The acidic site (Geescroft wilderness) is now a deciduous wood, dominated by oak (Quercus robor); the calcareous site (Broadbalk wilderness) is now dominated by ash (Fraxinus excelsior), with sycamore (Acer pseudoplatanus) and hawthorn (Craetagus monogyna) as major contributors. The acidic site gained 2.00 t C ha^?1 yr^?1 over the 118‐year period (0.38 t in litter and soil to a depth of 69 cm, plus an estimated 1.62 t in trees and their roots); the corresponding gains of N were 22.2 kg N ha^?1 year^?1 (15.2 kg in the soil, plus 6.9 kg in trees and their roots). The calcareous site gained 3.39 t C ha^?1 year^?1 over the 120‐year period (0.54 t in the soil, plus an estimated 2.85 t in trees and roots); for N the gains were 49.6 kg ha^?1 yr^?1 (36.8 kg in the soil, plus 12.8 kg in trees and roots). Trees have not been allowed to grow on an adjacent part of the calcareous site. There is now a little more C and N in the soil from this part than in the corresponding soil under woodland. We argue from our results that N was the primary factor limiting plant growth and hence accumulation of C during the early stages of regeneration in these woodlands. As soil organic N accumulates and the sites move towards N saturation, other factors become limiting. Per unit area of woodland, narrow strips; that is, wide hedges with trees, are the most efficient way of sequestering C – provided that they are not short of N.     

Impact of grazing around a watering point on soil status of a semi-arid rangeland in Ethiopia

The influence of long‐term livestock grazing on the soil status of a semi‐arid rangeland was studied along a grazing gradient from a watering point in southern Afar Region of Ethiopia for two seasons. The soil samples were analysed for organic carbon (OC), total nitrogen (N), available phosphorus (P), exchangeable cations (Ca²⁺, Mg²⁺, K⁺ and Na⁺), acidity (pH), cation exchange capacity, and particle size distribution (clay, silt and sand). No significant differences (P > 0.05) were observed for particle size distribution, OC, N, P and K contents along the grazing gradient further than 1500 m from the watering point. Ca²⁺ and Mg²⁺ were found to be dominant cations particularly in the severely degraded area contributing about 74% to the exchangeable cations. The study has also recognized high concentrations of Na⁺ and more than 11% exchangeable sodium percentage. Total exchangeable bases significantly declined (P < 0.05) along the grazing gradient and were indicative of the exchange complex of the soil being saturated with Ca²⁺ and Mg²⁺ ions. Thus this study concluded that there are soil differences in the grazing gradient caused by impact of grazing, particularly in the severely degraded area.     

Tree growth and soil acidification in response to 30 years of experimental nitrogen loading on boreal forest

Relations among nitrogen load, soil acidification and forest growth have been evaluated based on short‐term (<15 years) experiments, or on surveys across gradients of N deposition that may also include variations in edaphic conditions and other pollutants, which confound the interpretation of effects of N per se. We report effects on trees and soils in a uniquely long‐term (30 years) experiment with annual N loading on an un‐polluted boreal forest. Ammonium nitrate was added to replicated (N=3) 0.09 ha plots at two doses, N1 and N2, 34 and 68 kg N ha^?1 yr^?1, respectively. A third treatment, N3, 108 kg N ha^?1 yr^?1, was terminated after 20 years, allowing assessment of recovery during 10 years. Tree growth initially responded positively to all N treatments, but the longer term response was highly rate dependent with no gain in N3, a gain of 50 m³ ha^?1 stemwood in N2 and a gain of 100 m³ ha^?1 stemwood in excess of the control (N0) in N1. High N treatments caused losses of up to 70% of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺) in the mineral soil, along with decreases in pH and increases in exchangeable Al³⁺. In contrast, the organic mor‐layer (forest floor) in the N‐treated plots had similar amounts per hectare of exchangeable base cations as in the N0 treatment. Magnesium was even higher in the mor of N‐treated plots, providing evidence of up‐lift by the trees from the mineral soil. Tree growth did not correlate with the soil Ca/Al ratio (a suggested predictor of effects of soil acidity on tree growth). A boron deficiency occurred on N‐treated plots, but was corrected at an early stage. Extractable NH₄⁺ and NO₃^?were high in mor and mineral soils of on‐going N treatments, while NH₄+ was elevated in the mor only in N3 plots. Ten years after termination of N addition in the N3 treatment, the pH had increased significantly in the mineral soil; there were also tendencies of higher soil base status and concentrations of base cations in the foliage. Our data suggest the recovery of soil chemical properties, notably pH, may be quicker after removal of the N‐load than predicted. Our long‐term experiment demonstrated the fundamental importance of the rate of N application relative to the total amount of N applied, in particular with regard to tree growth and C sequestration. Hence, experiments adding high doses of N over short periods do not mimic the long‐term effects of N deposition at lower rates.