Chemical Characteristics and Acid Buffering Capacity of Surface Soils in Japanese Forests

To determine the geological distribution of acid buffering capacity and exchangeable Al of forest soils in Japan, surface soils under forest vegetation were collected nationwide from a total of 1,034 sites. Generally, surface soils in Japanese forests are mostly acidic and low in both exchangeable cation content and exchangeable Al. The median of soil pH(H₂O), total exchangeable cations, and exchangeable Al are 5.1, 76 mmol₍₊₎Kg^?1, and 19.6 mmol₍₊₎kg^?1, respectively. Acid buffering capacities of selected soils were determined using a soil column, which was comparable to the capacity that resulted from cation exchanges with protons. Soils with high buffering capacity and low exchangeable Al are widely distributed in Japan, and overlap with the areas of Andisol distribution. Volcanogenic materials seem to mask soil characteristics derived from underlying geology even though they are not Andisols. However, central to western Honshu Island, Shikoku Island, and northern Kyushu Island showed weak acid buffering capacities with high exchangeable Al potential in surface soils.     

Fate of heavy metals in a strongly acidic shooting‐range soil: small‐scale metal distribution and its relation to preferential water flow

To assess the mobility of Pb and associated metals in a highly contaminated shooting range soil (Losone, Ticino, Switzerland), we investigated the spatial distribution of the metals and their relation to preferential water flow paths. A 2.2 m² plot located 40 m behind the stop butt was irrigated with a solution containing bromide and Brilliant Blue, a slightly sorbing dye. A soil profile 50 cm in width was sampled down to 80 cm with a spatial resolution of 2.5 cm, resulting in 626 samples. Concentrations of elements (12 ≤ Z ≤ 92) were determined by energy‐dispersive Xray fluorescence spectrometry, and Brilliant Blue concentrations were determined with a chromameter. In the acidic (pH 3), organic matter‐rich, well drained Dystric Cambisol, maximum concentrations of 80.9 g kg^‐1 Pb, 4.0 g kg^‐1 Sb, and 0.55 g kg^‐1 Cu were measured in the topsoil. Within 40 cm soil depth, however, Pb, Sb, and Cu approached background concentrations of 23 mg kg^‐1, 0.4 mg kg^‐1, and 9.4 mg kg^‐1, respectively. The even horizontal distribution and the steep gradient along soil depth indicate tight metal binding in the topsoil, and a fairly homogeneous transport front. In contrast, water flow through the profile was highly heterogeneous. In the uppermost 20 cm, preferential flow was initiated by heterogeneous infiltration at the soil surface, but had no influence on metal distribution. Below 20 cm, however, preferential flow originated from larger tree roots, and metal concentrations were significantly elevated along these macropores. Spatial distributions of Pb, Sb, and Cu were similar, suggesting that all three metals are strongly retained in the topsoil and transported along preferential water flow paths in the subsoil.     

Lead and soil properties distributions in a roadside soil: Effect of preferential flow paths

Preferential flow is the non-uniform movement of water and solutes through soils that bypasses a portion of the soil matrix. It limits the effective sorbing capacity of the soil and it has been related to a reduced solute residence time accounting for an increased risk of groundwater contamination. A dye tracer experiment and a subsequent soil sampling procedure in three adjacent soil profiles were performed in a moderately Pb contaminated roadside soil aiming to make visible different flow domains, to test whether preferential flow paths influence the metal content depth distribution, to study differences in selective soil properties resulting from flow domains and, lastly, to investigate whether preferential flow induces changes in the solid phase distribution of lead. Identified preferential flow domains showed greater lead concentration along with lower pH and greater amounts of both inorganic and organic C contents than unstained soil matrix domains. The ratio humic acids (HAs) to fulvic acids (FAs) contents was significantly lower in the preferential paths than in the matrix domains which might be related with sizable differences in the humification process. Lastly, based on the three-step BCR sequential extraction procedure, it has been shown that preferential flow phenomena induce differences in metal distribution between both identified flow domains. While lead content in the soluble and exchangeable (FI) as well as bound to organic matter fractions (FIII) were greater in the matrix than in the preferential flow domains, the amount of metal bound to Fe, Al and Mn (hydr)oxides (FII) was significantly greater in the preferential flow domain than in the matrix one. This indicates that more stable lead sorption mechanisms, thus leading to a lower metal mobility, might partially be responsible for the larger amount of total metal content found associated to the preferential flow paths. Overall, our results suggest that preferential flow domains may behave as metal storage compartments in the soils due to differences in soil properties promoted by means of the heterogeneous water flow distribution.     

Phosphorus availability to corn from wood ash-amended soils

Wood ash is a residual material produced during biomass burning. In the northeastern United States up to 80 % of the ash is spread on agricultural lands as a liming amendment with the remainder being disposed of in landfills. As well as raising soil pH, wood ash also adds plant nutrients to soil. This study is an examination of the plant availability of the P in 8 different soils amended with one wood ash. Plant availability was assessed by measuring the biomass and P concentration of corn (Zea mays) L.) plants grown in the greenhouse for 28 d in soil amended with either CaCO₃ (control), wood ash to supply 200 mg kg^?1 total P, or monocalcium phosphate (MCP) to supply 200 mg kg^?1 total P and CaCO₃. Both corn growth and P uptake were highest in the MCP treatments, intermediate in the wood ash treatments, and lowest in the controls for all soil types. The soil property which seemed to have the greatest influence on P availability was pH buffer capacity. The soils with the greatest capacity to buffer OH additions also tended to exhibit the greatest absolute P uptake from wood ash-amended soils and the greatest P uptake relative to that from MCP-amended soils. The ability of soil test extractants to predict uptake of P in the three soil treatments was examined. A buffered ammonium acetate extradant overestimated P availability in the ash-amended soils relative to the MCP-amended soils. An unbuffered, acid, fluoride-containing extract provided a measure of P levels that was consistent with P uptake from all soil treatments. In this study the predictive relationship was as follows: P uptake = 0.017× (Bray P, mg kg^?1) + 1.19; r = 0.81.     

Cadmium sorption by two acid soils as affected by clearing and cultivation

Abstract

The objective of this study was to determine the effect of clearing and cultivation on the sorption of cadmium (Cd) by two acid soils from Zimbabwe with differing cultivation stories. In their original state, not cleared‐not cultivated (virgin soils), the two soils exhibited noticeable and similar capacities to sorb Cd. The Mazowe soil contains the highest level of organic matter (40 g kg^‐1) and a effective cation exchange capacity (ECEC) of 144 mmol_c kg^‐1. Yet, Bulawayo soil (23.5 g kg^‐1 organic matter and ECEC of 146 mmol_c kg^‐1) has higher pH and Mn and Fe oxide content and these characteristics seemed to counteract the effect of lower organic matter. After 50 years of cultivation, The Mazowe soil has lost 60% of its organic matter and ECEC, and consequently the ability of its soil matrix to bind Cd has proportionally decreased. In Bulawayo (cleared in 1983 and first ploughed in 1984), on the contrary, the organic matter and ECEC of the cultivated soil remains over 95% of the values on its virgin counterpart. In this soil, the retaining ability for Cd has not still been affected. In the two soils Cd sorption was highly pH‐dependent. The extent of sorption was minimal under acidic conditions and increased sharply as the pH was raised. The immediate reversibility of the sorption process proved to be very low. When sorption and desorption data were compared it was clear that soil characteristics like high organic matter and oxide content which showed to enhanced Cd sorption, contributed at the same time to slow down the backward reaction.     

Cation exchange capacities of organo-mineral particle-size fractions in soils from long-term experiments

Soils with and without organic manuring from 10 long-term manurial experiments in East Germany were fractionated into organo-mineral particle-size separates by ultrasonic disaggregation and sedimentation/decantation. The cation exchange capacities (CECs) buffered at pH 8.1 were determined for the size fractions fine+medium clay, coarse clay, fine, medium and coarse silt, sand, and for the total soil samples. In the samples from nine field experiments the CECs decreased with increased equivalent diameters (fine+medium clay: 489–8 13 mmol_c kg^?1, coarse clay: 367–749 mmol_c kg^?1, fine silt: 202–587 mmol_c kg^?1. medium silt: 63–345 mmol_c kg^?1, coarse silt: 12–128 mmol_c kg^?1 and sand: 10–156 mmol_c kg^?1. The CECs varied with genetic soil type, mineralogical composition of the <6.3-μm particles, and the C and N contents of the size fractions. In a pot experiment examining the role of various organic materials in the early stages of soil formation, the clay-size fractions had the largest CECs (85–392 mmol_c kg^?1), followed by the medium-silt (1 9-222 mmol_c kg^?1) and fine-silt fractions (23–192 mmol_c kg^?1). The effect of organic amendments on CEC was in general: compost>fresh farmyard manure = straw + mineral fertilizer = mineral fertilizer.     

Sorption and Fractionation of Copper in Soil at a Sewage Irrigation Farm in Australia

Abstract

Copper (Cu) is an important heavy metal to be considered in soil contamination, because high concentrations of copper in soil produce toxic effects and may accumulate in plant tissues. In Australia's oldest sewage irrigation farm, located in Werribee, Victoria, soil in the land filtration area is contaminated by Cu. However, Cu content in herbage tissues is in the normal range and has been trending downward since 1979. Therefore, studies on the sorption capacity and sequential extraction of Cu in soil at the Werribee Farm is of significance, not only for better understanding the mechanism of transport, chemical processes, and plant uptake of Cu, but also in providing information for the practical management of sewage farm soils. Methods of combining sorption isotherms with sequential extraction procedures were adopted, and the results showed that the soil in the land filtration area at Werribee Farm has a high sorption capacity for Cu, and distribution coefficients, K_f of Cu, were 629 L kg^?1 in surface soils (0–20 cm) and 335 L kg^?1 in subsurface soils (20–40 cm). The sequential extraction fractions demonstrate that exchangeable and carbonate fractions are very low, only comprising 3.49 to 5.49% of total copper. The other fractions are also discussed. This characteristic of Cu in soil is related to the low concentration of Cu in plant tissues.     

Liming and sulfur amendments improve growth and yields of maize in Rubona Ultisol and Nyamifumba Oxisol

Aluminum toxicity is a major limitation to crop production on highly weathered and leached soils in Rwanda. Moreover, sulfur though widely deficient in Rwanda acidic soils has received little attention by soil fertility researchers. A field experiment on maize response and soil nutrients status to liming materials of travertines at 3.4 t ha^?1, ash wood 1.2 t ha^?1 of CaO equivalent and sulfur at 10 kg ha^?1 combined with NPK at 80, 60, and 45 kg ha^?1 respectively was conducted in Rubona Ultisol and Nyamifumba Oxisol. Results revealed that travertine and wood ash increased the soil pH from 4.7 to 5.8 or higher and decreased exchangeable Al³⁺ and H⁺ to near 0 cmolc kg^?1. Soil nutrients generally increased to high or medium ranges for crop production. Leaf dry biomass, plant height and maize grain yields were significantly higher in Rubona Ultisol than in Nyamifumba Oxisol. Plots that received wood ash, with NPKS or with NPK, generally had higher maize yields, followed by those which received travertines and NPKS or NPK which had maize growth response as compared to the control plots which received NPK only. Thereby, a combination of wood ash with NPKS or NPK, travertines with NPKS was found to neutralize soil aluminum toxicity, increase soil nutrients status to required levels for plant growth and increase maize yields significantly.     

Dissolved organic matter sorption on sub soils and minerals studied by 13C-NMR and DRIFT spectroscopy

Sorption on the mineral matrix is an important process restricting the movement of dissolved organic matter (DOM) in soils. In this study, we aimed to identify the chemical structures responsible for the retention of DOM by sorption experiments with total DOM and acidic humic substances (AHS), containing humic and fulvic acids, on soil samples and minerals (goethite, ferrihydrite, and amorphous Al(OH)₃). The AHS remaining in solution after sorption were studied by ¹³C nuclear magnetic resonance (NMR) analysis, and total DOM and AHS for bed on the surfaces of minerals by diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy. The soil samples were taken from strongly sorbing Bw horizons of two Inceptisols rich in pedogenetic Fe (29 and 35 g kg ^?1) and containing little C (7 and 22 g kg^?1). The ¹³C-NMR spectra showed that sorption causes a preferential removal of aromatic and carboxyl C from the solution, whereas alkyl-C accumulates in the solution. No change was observed for O-alkyl C. The DRIFT spectra of sorbed total DOM and AHS showed a relative increase of the band intensity of carboxyl groups compared to DOM in the initial solution, confirming the importance of those groups for the sorption to mineral surfaces. The spectra also indicated reactions of carboxyl groups with metals at the mineral surfaces. The extent to which the carboxyl groups are bound depended on the surface coverage with DOM and the type of mineral.     

Modelling the impact of within-storm variability of rainfall on the loading of solutes to preferential flow pathways

Rainfall variability within a storm can have a significant impact on the amount of chemical transported by surface runoff and preferential flow. Previously, studies have evaluated only a few slowly varying rainfall patterns and related sorption capacities. We use a bounded random cascade approach to generate 50 000 realizations of realistic rainfall intensity patterns within a single storm event (96 minutes duration, mean intensity of 18.75 mm hour^?1) to explore the effects on the partitioning of rainfall and linearly sorbing solutes between fast preferential flow (loading) and slow flow in the soil matrix for a silt loam and a sandy clay. Loading and infiltration are modelled by a near‐surface mixing model and Green–Ampt infiltration. The statistical properties of loading were evaluated from these simulations. For this storm the mean total of resident solute mixing from the soil to preferential flow reached a maximum for a retardation factor R ～ 5. Much smaller loadings occurred for more weakly sorbing and more strongly sorbing solutes. The variability of loading tended to decrease with increasing R. Ensemble averaged rainfall patterns were derived which related to the magnitude of loading. The patterns of rainfall generating large preferential flows did not necessarily lead to large solute loading. Early peaking, mid‐storm peaking and late peaking rainfall contributed to large solute loadings, depending upon soil and chemical properties. These patterns result from a balance between the amount of preferential flow generated and the amount of solute available when preferential flow is triggered. The results suggest that the use of R as a measure of the mobility of resident solutes depends on the flow pathway considered. In addition, characterization of flux distributions in soil with weakly sorbing, resident tracers, may underestimate the potential for rapid transport of strongly sorbing solutes subject to natural variations in rainfall.     

Influence of Lime and Gypsum on Yield and Yield Components of Soybean and Changes in Soil Chemical Properties

Soybean is one of the most important legume crops in the world. Two greenhouse experiments were conducted to determine the influence of liming and gypsum application on yield and yield components of soybean and changes in soil chemical properties of an Oxisol. Lime rates used were 0, 0.71, 1.42, 2.14, 2.85, and 4.28 g kg^?1 soil. Gypsum rates applied were 0, 0.28, 0.57, 1.14, 1.71, and 2.28 g kg^?1 soil. Lime as well as gypsum significantly increased grain yield in a quadratic fashion. Maximum grain yield was achieved with the application of 1.57 g lime per kg soil, whereas the gypsum requirement for maximum grain yield was 1.43 g per kg of soil. Lime significantly improved soil pH, exchangeable soil calcium (Ca) and magnesium (Mg) contents, base saturation, and effective cation exchange capacity (ECEC). However, lime application significantly decreased total acidity [hydrogen (H) + aluminum (Al)], zinc (Zn), and iron (Fe) contents of the soil. The decrease in these soil properties was associated with increase in soil pH. Gypsum application significantly increased exchangeable soil Ca, base saturation, and ECEC. However, gypsum did not change pH and total acidity (H + Al) significantly. Adequate soil acidity indices established for maximum grain yield with the application of lime were pH 5.5, Ca 1.8 cmol_c kg^?1, Mg 0.66 cmol_c kg^?1, base saturation 53%, Ca saturation 35%, and Mg saturation 13%. Soybean plants tolerated acidity (H + Al) up to 2.26 cmol_c kg^?1 soil. In the case of gypsum, maximum grain yield was obtained at exchangeable Ca content of 2.12 cmol_c kg^?1, base saturation of 56%, and Ca saturation of 41%.     

Changes in soil solution composition and pH in urine-affected areas of pasture

Changes in soil solution composition and concentrations of exchangeable cations and mineral N in undisturbed cores of pasture soil were investigated in two experiments following applications of sheep urine to the cores. The major cations applied in the urine were K⁺ and Na⁺, and the major anions were HCO₃^? and Cl^?. Addition of urine increased concentrations of exchangeable K⁺, Na⁺ and NH₄⁺ and measured ionic strength of the soil solution throughout the surface 15 cm of soil, demonstrating that the urine moved through the core by macropore flow immediately following addition. Immediately following urine application the ionic strength in soil solution in the surface 2.5 cm of soil increased from 4–6 MM to 24–41 mM. Hydrolysis of urine-urea was extremely rapid, and in less than 1 d high concentrations of NH₄⁺-N (i.e. 270–370 mg N kg^?1) had accumulated in the surface 0–2.5 cm of the urine patch, and soil pH had risen by over one unit. Nitrification then proceeded and, after approximately 15 d, NO₃^? became the dominant form of mineral N present. During nitrification, soil pH declined and the ionic strength of the soil solution increased substantially with NO₃^? becoming the dominant anion present in solution. There were concomitant increases in the concentrations of Ca²⁺ and, to a lesser extent, Mg²⁺ in the soil solution as NO₃^? concentrations increased. After approximately 30 d, concentrations of exchangeable NO₃^? had risen to 250–330 mg N kg^?1, soil solution NO₃^? concentrations had increased to about 80 mmol, dm^?3, and ionic strength in the soil solution had increased to 130–140 mM. These results demonstrate the dominating effect of N transformations in causing large fluctuations in the pH, ionic composition and ionic strength of the soil solution in the urine patch. It was concluded that nutrient availability in the patch was affected directly by nutrient addition in urine, and also probably indirectly through the fluctuations in soil solution pH and ionic strength that occur.     

Release of trace elements from wood ash by nitric acid

Currently wood ash is being used as a soil amendment. Its use is regulated based on trace element content. However, no published information exists on solubilities of trace elements in wood ash. We investigated the release of environmentally-significant trace elements (Cd, Cr, Cu, Pb and Zn) from wood ash as a function of pH and of particle size. Wood ash was sampled from three sources in Maine and sieved into <0.5 mm, 0.5–1 mm, and 1–2 mm fractions. Elemental compositions were determined using a HNO₃/H₂O₂ digestion. Sub-samples (1 g) from each of the nine samples (three sources and three size fractions) were reacted with 50 mLs of standardized HNO₃ for a week using a range of acid concentrations (0.01–0.25 M) to achieve a range in final pH values. The resulting solutions were filtered and analyzed. The compositions of the three wood ashes varied widely. The dominant elements were Si (9.7–34%), Ca (5.8–21%), K (0.8–5.7%), Al (0.8–4.9%), and Mg (0.5–3.0%). Trace elements were present in the following concentrations ranges: Cd (1.9–12 mg kg^?1), Cr (24–92 mg kg^?1), Cu (33–75 mg kg^?1), and Zn (130–1400 mg kg^?1). Both Cd and Zn were released readily from the ashes at final pH values of approximately 6.5 and below. In the final pH range of 3–4, 80–100% of the total Cd and 70–90% of the total Zn was released by the ashes. All three wood ashes showed somewhat different patterns of Cr release. Level of Cr(VI) in a water extract of the ash fractions was found to be a much better predictor of relative Cr solubility than total Cr. Solubility of Cu was low, and Pb was very insoluble. There was little influence of particle size on release of trace elements. The relatively high Cd concentration of wood ash compared with soil, and its relative solubility in wood ash, should be considered in evaluating the potential environmental impact of spreading wood ash on land.     

Effect of wood ash on soil chemistry of a pine stand in Northern Germany

Addition of wood ash to acid soils will affect the soil chemistry of forests in a number of ways which were assessed for a pine stand in northern Germany. A field experiment was carried out in a fifty‐year old pine stand on a sandy Podzol at Fuhrberg (Lüneburger Heide, Lower Saxony/Germany) which involved depositing wood ash (2.4 t ha^—1) on the surface. Soil solution chemistry was investigated monthly at different depths for 24 months. Prior to and 19 months after the ash addition, exchangeable cations and amounts of heavy metals were determined at different depths. Two to four months after addition of wood ash, maximum mean concentrations in the soil solution of Ca were 240 μmol l^—1 at 0 cm (surface of mineral soil) and 100 μmol l^—1 at 100 cm and of K 980 μmol l^—1 and 140 μmol l^—1, respectively. The pH values in soil solutions dropped temporarily by 0.3 units at 0 and 10 cm depth. Nitrate concentrations increased at all depths and maximum mean concentration was 230 μmol l^—1 at 100 cm. Concentrations of Pb and Cr in soil solution did not change significantly (p < 0.05) after ash addition. Concentrations of Cd and Zn increased significantly at some depths but stayed well below the legal limit for drinking water and below the limits given by the German recommendation for soil conservation. Nineteen months after ash addition, the cation exchange capacity (corrected for the release of cations from the ash) of the upper 6 cm of the organic layer was almost doubled and amounts of exchangeable Ca and Mg increased significantly in the upper 8 cm of the organic layer. Amounts of Zn were increased in the entire organic layer, but changes were significant only in the upper 4 cm. The results of this study suggest that ash from untreated wood (using modest additions) may be recommended for amelioration of forest soils.     

Inorganic and organic ligands induced changes in Pb sorption of arid ‎clay and calcareous loamy sand soils

The objective of this study was to investigate sorption, desorption, and immobilization of Pb in the clay and calcareous loamy sand soils treated with inorganic ligands (NO₃^?, Cl^? and H₂PO₄^?). Pb sorption was also determined in the presence of oxalate and citrate. The maximum Pb sorption capacities (q) ranged from 42.2 to 47.1 mmol kg^?1 for the clay soil, and from 45.2 to 47.0 mmol kg^?1 for loamy sand soil. It was observed that the binding energy constant (k) for Pb sorbed onto loamy sand soil (528–1061) is higher than that for clay soil (24.38–55.29). The loamy sand soil-sorbed greater quantities of Pb compared to the clay soil when initial pH was ≥ 3. However, it had lower sorption capacity at the lowest initial pH of 2. Additionally, the greatest Pb sorption and immobilization occurred in the soil treated with H₂PO₄. In the clay soil, the sorption of Pb was depressed at 0.1 mol kg^?1 of Cl^?, as compared with other ligands. Concerning organic acids, citrate ligand showed the highest decrease in Pb sorption. It could be concluded that the nature of Pb sorption can depend on the type and quantity of ligands present, as well as the soil type.     

Impact of Soil Sorption Characteristics and Bedrock Composition on Phosphorus Concentrations in two Bohemian Forest Lakes

Phosphate (PO₄-P) sorption characteristics of soils and bedrock composition were determined in catchments of two mountain lakes, Ple?né Lake (PL) and ?ertovo Lake (CT), situated in the Bohemian Forest (Czech Republic). The aim was to explain higher terrestrial P export to mesotrophic PL compared to oligotrophic CT. Concentrations of Al and Fe oxides were the dominant parameters affecting soil ability to adsorb PO₄-P. Depending on concentrations of Al and Fe oxides, P sorption maxima varied from 9.7 to 70.5 mmol kg^?1 and from 7.4 to 121 mmol kg^?1 in organic and mineral soil horizons, respectively. The catchment weighted mean PO₄-P sorption capacity was 3.4 mol m^?2 and 11.9 mol m^?2 in the PL and CT soils, respectively. The higher PO₄-P sorption capacity in the CT catchment was predominantly associated with higher pools of soil and Fe oxides. The CT bedrock (mica schist) released one order of magnitude less P than the PL bedrock (granite) within a pH range of catchment soils (pH_CaCl2 of 2.5–4.5). The higher ability of PL bedrock to release P and the lower ability of PL soils to adsorb PO₄-P thus contributed to the higher terrestrial P loading of this lake.     

Application of wood ash accelerates soil respiration and tree growth on drained peatland

Forested peatlands contain large pools of terrestrial carbon. As well as drainage, forest management such as fertilizer application can affect these pools. We studied the effect of wood ash (application rates 0, 5 and 15 t ha^?1) on the heterotrophic soil respiration (CO₂ efflux), cellulose decomposition, soil nutrients, biomass production and amount of C accumulated in a tree stand on a pine‐dominated drained mire in central Finland. The ash was spread 13 years before the respiration measurements. The annual CO₂ efflux was statistically modelled using soil temperature as the driving variable. Wood ash application increased the amounts of mineral nutrients of peat substantially and increased soil pH in the uppermost 10 cm layer by 1.5–2 pH units. In the surface peat, the decomposition rate of cellulose in the ash plots was roughly double that in control plots. Annual CO₂ efflux was least on the unfertilized site, 238 g CO₂‐C m^?2 year^?1. The use of wood ash nearly doubled CO₂ efflux to 420–475 g CO₂‐Cm^?2 year^?1 on plots fertilized with 5–15 t ha^?1 of ash, respectively. Furthermore, ash treatments resulted also in increased stand growth, and during the measurement year, the growing stand on ash plots accumulated carbon 11–12 times faster than the control plot. The difference between peat C emission and amount of C sequestered by trees on the ash plots was 43–58 g C m^?2, while on the control plot it was 204 g C m^?2. Our conclusion is that adding wood ash as a fertilizer increases more C sequestration in the tree stand than C efflux from the peat.     

Phosphorus accumulation and leaching risk of greenhouse vegetable soils in Southeast China

Over-fertilization has caused significant phosphorus(P) accumulation in Chinese greenhouse vegetable production(GVP) soils. This study, for the first time, quantified profile P accumulation directly from soil P measurements, as well as subsoil P immobilization, in three alkaline coarse-textured GVP soil profiles with 5(S5), 15(S15), and 30(S30) years of cultivation in Tongshan, Southeast China. For each profile, soil samples were collected at depths of 0–10(topsoil), 10–20, 20–40, 40–60, 60–80, and 80–100 cm. Phosphorus accumulation was estimated from the difference in P contents between topsoil and parent material(60–100 cm subsoil). Phosphorus mobility was assessed from measurements of water-soluble P concentration(PSol). Finally, P sorption isotherms were produced using a batch sorption experiment and fitted using a modified Langmuir model. High total P contents of 1 980(S5), 3 190(S15), and 2 330(S30) mg kg~(-1) were measured in the topsoils versus lower total P content of approximately 600 mg kg~(-1) in the 80–100 cm subsoils. Likewise, topsoil PSol values were very high, varying from 6.4 to 17.0 mg L~(-1). The estimated annual P accumulations in the topsoils were 397(S5), 212(S15), and 78(S30) kg ha~(-1) year~(-1). Sorption isotherms demonstrated the dominance of P desorption in highly P-saturated topsoils, whereas the amount of adsorbed P increased in the 80–100 cm subsoils with slightly larger P adsorption capacity. The total P adsorption capacity of the 80–100 cm subsoils at a solution P concentration of0.5 mg L~(-1) was 15.7(S5), 8.7(S15), and 6.5(S30) kg ha~(-1), demonstrating that subsoils were unable to secure P concentrations in leaching water below 0.5 mg L~(-1) because of their insufficient P-binding capacity.     

Phosphate-induced zinc retention in a tropical semi-arid soil

Zinc (Zn) deficiency symptoms and sporadic responses to applied Zn are being observed frequently in the Nigerian savanna, and one cause is thought to be the growing use of phosphorus (P) fertilizers. This study was designed to test the hypothesis of P-induced Zn retention in the soils. Soil mixed eith P was incubated at field capacity for 3 weeks at 30 ± 2°C. P levels added to the soil were 0, 500, 1000 and 2000 mg per kg soil. After 3 weeks of incubation, water-soluble Zn in soil decreased by 92% and exchangeable Zn by 78% with 2000 mg kg^?1 of applied P. Zn levels ranging from 0 to 200 mg kg^?1 were added to the P-incubated soil to determine the Zn sorption isotherm and retention capacity. The P-treated soil retained 93 ± 2% of added Zn compared with 52 ± 2% of the control soil. P treatment changed the Zn sorption isotherm from an L-curve isotherm to an H-curve isotherm, indicating strong affinity of P-treated soil for Zn, probably as a result of the formation of Zn-phosphate complexes on the soil surface and precipitation at sufficiently large concentrations of P and Zn. At 2000 mg P kg^?1, up to 90% of Zn retained by the soil was bound in solid form as ZnHPO₄. Varying the soil pH from 3.5 to 9.0, Zn retention by the soil was related to Zn hydrolysis with maximum adsorption occurring at pH 7.3 ± 0.2. The dependence of sorbed Zn on Zn(OH)₂° at pH 3.5–7.4 of P-treated soil indicated that significant van der Waals forces might be involved in Zn retention. The implication of the results of this study for the region is that fertilizer-P placement around a growing crop plant, commonly practised to maximize fertilizer-P efficiency, can potentially limit Zn solubility and availability.     

Effects of organic and inorganic calcium compounds on soil-solution pH and aluminium concentration

The reactions of two organic (citrate and fulvate) and two inorganic (chloride and phosphogypsum) calcium compounds were studied during leaching of columns of unsaturated acidic soil. The potential of these compounds to decrease the aluminium concentration in the soil solution and remove exchangeable aluminium, and their effects on soil acidity are described. The calcium citrate solution increased the soil solution pH from 5 to a maximum value of 7 in the upper portion of the column. In contrast, the fulvate, calcium chloride and phosphogypsum solutions had little effect on soil-solution pH. Treatment with calcium citrate, or fulvate solution that contained 51 mm Na, removed most of the exchangeable aluminium from the column. The cation exchange sites in the upper portion of the column were saturated with calcium, and the cation exchange capacity of the soil was increased from 35 to c. 80 mmol_c kg^?1 in the calcium citrate treatment. Leachate from this treatment contained low (< 2 mm ) calcium concentrations and high aluminium concentrations. In contrast, the above changes were not shown by the calcium chloride and phosphogypsum treatments. In these treatments the calcium concentration in the leachate was equal to that in the inflowing solution, which indicated that calcium was transported through the entire column. These results suggest that calcium alone was ineffective in displacing aluminium from the cation exchange sites and a strong complexing agent such as citrate or fulvate is needed to mobilize the exchangeable aluminium.