Is the Ca:Al ratio superior to pH,Ca or Al concentrations of soils in accounting for the distribution of plants in deciduous forest?

The distributions of vascular plants in south Swedish deciduous forests were related to exchangeable (exc) and soil solution concentrations of H⁺ (pH), Ca, Al and the Ca:Al ratios within these fractions. Topsoils (0–5 cm) of 172 sites with a pH_KCl of 3.2–3.9 (corresponding to 3.7–4.4 in soil solution) were used. In the soil solution both total Al_t and quickly reacting Al_r were determined. Exchangeable concentrations were generally well related to plant distributions, the highest correlation coefficients usually being given by pH_KCl>Ca_exc>Al_exc.>(Ca:Al)_exc. The (Ca:Al)_exc ratio was clearly inferior. Out of the soil solution variables studied, Ca concentration, followed by pH, was best correlated with plant distributions, Al_t, Al_r, and the Ca:Al ratios having similar and lower coefficients. It is concluded that the use of Ca:Al ratios as a general measure of Al toxicity in controlling plant distributions is rather problematic. It seems difficult to apply evidence for Ca-Al interactions from solution culture experiments to field conditions when measured as exchangeable or soil solution concentrations of the soil.     

Effect of nitrogen form and phosphorus source on the growth,nutrient uptake and rhizosphere soil property of Camellia sinensis L.

The effects of nitrogen form and phosphorus source on the growth, nutrient uptake and rhizosphere soil property of tea (Camellia sinensis L.) were investigated in a pot experiment. The experiment was performed with a compartmental cropping device, which enables the collection of rhizosphere soil at defined distances from the root of tea plant. Nitrogen was supplied as nitrate or ammonium in combination with soluble phosphorus as Ca(H₂PO₄)₂ or insoluble P as rock phosphate. The leaf dry matter production of tea was significantly greater in the treatments with NH₄ ⁺ than NO₃ ^-, whereas dry matter production of root and stem was not significantly affected. Addition of phosphorus as either source did not influence the dry matter production. The concentrations of K in root, Mg and Ca in both the shoot and root supplied with NO₃ ^- were significantly higher than in NH₄ ⁺ and influence of P sources was minor. On the contrary, Al and Mn concentrations were significantly larger in NH₄ ^--fed plants which could be attributed to remarkably increased availability of Al and Mn caused by acidification of the rhizosphere soil (the first 1-mm soil section from the root surface) with NH₄–N nutrition. The concentration of N in shoot was also significantly higher in NH₄- than in NO₃-fed plants, indicating higher use efficiency of NH₄–N. Whatever the phosphate source, rhizosphere pH declined in ammonium compared to in nitrate treatment. The pH decrease was much larger when no P or soluble P were applied and reached 0.85–1.30 units which extended to 3–5 mm away from the root surface. Exchangeable acidity, content of exchangeable Al and Mn were also considerably higher in the rhizosphere soils of NH₄ ⁺ fed tea plants. Significant amounts of P dissolved from rock phosphate accumulated in rhizosphere of NH₄ ⁺, not NO₃ ^-, suggesting that the dissolution of rock phosphate was induced by the proton excreted by tea root fed with ammonium. With soluble P addition, shoot and root P concentrations were greater in NH₄ ⁺ than in NO₃ ^- treatment and it appeared that this difference could not be sufficiently explained by the available P content in soil which was only slightly higher in NH₄ ⁺ treatment. With rock phosphate addition, the shoot and root P concentrations were hardly affected by nitrogen form, although the available P content was much higher and accumulated in the rhizosphere soil supplied with ammonium. The reason for this was discussed with regard to the inter-relationship of Al with P uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Plant induced alteration in the rhizosphere and the utilisation of soil heterogeneity

Plant-soil interactions result in a special rhizosphere soil chemistry, differing from that of the bulk soil found only a few mm from the root. The aim of this study was to investigate adaptation mechanisms of herbs growing in acid soils through studying their rhizosphere chemistry in a greenhouse experiment and in a field study. Ten herbs were grown in acid soil (pH 4.2 in the soil solution) in the greenhouse. The concentrations of NO₃ ^-, SO₄ ^2-, phosphates, Ca²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺, NH₄ ⁺ and pH were analysed in soil solutions obtained by centrifugation. The general pattern found was a depletion of nutrients in the rhizosphere compared with their concentrations in the bulk soil. The pH increase (up to 0.7 units) in the rhizosphere soil appeared to be caused by plant uptake of NO₃ ^- (r²=0.88). The ion concentrations in the soil solution of the rhizosphere were dependent on plant species and biomass increase. Although species with a larger biomass and higher growth rates showed a higher degree of ion depletion (except for Na⁺, SO₄ ^2-) in the rhizosphere, there were also species specific responses. A field study of five herbs at five oak forest sites in Southern Sweden (Scania) was also carried out. In addition to the soil solution concentrations, the loss on ignition (LOI) and the concentrations of 0.1 M BaCl₂ extractable K⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Al ions were measured. The amount of soil solution Al was determined as free ionic (quickly reacting) Al. For all species and sites, the LOI and the concentrations of exchangeable cations were higher in the rhizosphere than in the bulk soil, apparently due to the roots preferably growing at organic-rich microsites. The concentrations of the ions as measured in the centrifuged soil solution, were either higher in the rhizosphere than in the bulk soil or were the same in both, except for NO₃ ^- and quickly reacting Al. The lower concentrations of quickly reacting Al in the rhizosphere, compared with the bulk soil could indicate the uptake of Al by the plant or the exudation of complexing substances. The pH differences were only small and mostly non-significant. Plant-soil interactions and the ability of plants to utilise heterogeneity of the soil appear to be more important for plant growth in acid soils than recognised heretofore. Rhizosphere studies provide an important means of understanding plant strategies in acid soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Response of soil chemistry to forest dieback after bark beetle infestation

We evaluated changes in the chemistry of the uppermost soil horizons in an unmanaged spruce forest (National Park Bohemian Forest, Czech Republic) for 3 years after dieback caused by a bark beetle infestation, and compared these changes with a similar undisturbed forest area. The soils below the disturbed forest received 2–6 times more elements via litter fall compared to the unaffected plot. The subsequent decomposition of litter and reduced nutrient uptake by trees resulted in a steep increase in soil concentrations of soluble N (NH₄-N, organic-bound N) and P forms in the disturbed plot. The average concentrations of NH₄-N and soluble reactive P increased from 0.8 to 4.4 mmol kg^?1 and from 0.04 to 0.9 mmol kg^?1, respectively, in the uppermost soil horizon. Decomposition of litter at the disturbed plot elevated soil concentrations of Ca²⁺, Mg²⁺ and K⁺, which replaced Al³⁺ and H⁺ ions from the soil sorption complex. Consequently, soil concentrations of exchangeable base cations increased from 120 to 200 meq kg^?1, while exchangeable Al³⁺ and H⁺ decreased 66 and 50 %, respectively, and soil base saturation increased from 40 to 70 %. The Al³⁺ liberation did not elevate concentrations of ionic Al in the soil solution, because most of the liberated Al³⁺ was rapidly complexed by dissolved organic carbon (DOC) and transformed to DOC–Al complexes. The chemical parameters investigated at the unaffected plot remained stable during the study.     

Effects of throughfall manipulation on soil nutrient status: results of 12 years of sustained wet and dry treatments

To investigate the potential effects of changing precipitation on forest ecosystems, the Throughfall Displacement Experiment (TDE) was established on Walker Branch Watershed, Tennessee, in 1993. Three different throughfall amounts were tested: ?33% (DRY); ambient (no change, AMB); and +33% (WET). Throughfall manipulations had no statistically significant effects on total C, N, exchangeable Ca²⁺, Mg²⁺, bicarbonate‐extractable P, or extractable SO₄^2? in soils after 12 years of sustained treatments. Increased K⁺ inputs in the WET treatment resulted in relative increases in exchangeable K⁺compared with the AMB and DRY treatments. Soil C, N, and extractable P declined in all treatments over the 12‐year study, and the declines in N were inexplicably large. Field observations contrasted with earlier simulations from the Nutrient Cycling Model (NuCM), which predicted greater decreases in exchangeable K⁺, Ca²⁺, Mg²⁺, and extractable P in the order WET>AMB>DRY, and no change in C, N, and extractable SO₄^2?. The failure of the NuCM model to accurately predict observed changes is attributed to the lack of mechanisms for deep rooting and the transfer of throughfall K⁺ from one plot to another in the model. Measurements of element availability using resin membranes during the final years showed higher values in wet and lower values in dry treatments compared with ambient conditions for mineral N, K, Mn, Zn, and Al, but the opposite for B, Ca, and Mg. In the cases of Ca and Mg, the patterns in resin values were similar to those at the soil exchange sites (greatest in the dry treatment) and appeared to reflect pretreatment differences. This study showed that while longer term changes in soil nutrients are likely to occur with changes in precipitation, potential changes over this 12‐year interval were buffered by ecosystem processes such as deep rooting.     

The effect of phosphate rock dissolution on soil chemical properties and wheat seedling root elongation

Soils of the Appalachian region of the United States are acidic and deficient in P. North Carolina phosphate rock (PR), a highly substituted fluoroapatite, should be quite reactive in these soils, allowing it to serve both as a source of P and a potential ameliorant of soil acidity. An experiment was conducted to evaluate the influence of PR dissolution on soil chemical properties and wheat (Triticum aestivum cv. Hart) seedling root elongation. Ten treatments including nine rates of PR (0, 12.5, 25, 50, 100, 200, 400, 800, and 1600 mg P kg^-1) and a CaCO₃ (1000 mg kg^-1) control were mixed with two acidic soils, moistened to a level corresponding to 33 kPa moisture tension and incubated for 30 days. Pregerminated wheat seedlings were grown for three days in the PR treated soils and the CaCO₃ control. Root length was significantly (P<0.05) increased both by PR treatments and CaCO₃, indicating that PR dissolution was ameliorating soil acidity. The PR treatments increased soil pH, exchangeable Ca, and soil solution Ca while lowering exchangeable Al and 0.01 M CaCl₂ extractable soil Al. Root growth in PR treatments was best described by an exponential equation (P<0.01) containing 0.01 M CaCl₂ extractable Al. The PR dissolution did not reduce total soil solution Al, but did release Al complexing anions into soil solution, which along with increased pH, shifted Al speciation from toxic to nontoxic forms. These results suggest that North Carolina PR should contribute to amelioration of soil acidity in acidic, low CEC soils of the Appalachian region.     

Effects of limestone and gypsum application to a Malaysian ultisol on soil solution composition and yields of maize and groundnut

A field experiment was conducted on an Ultisol in Malaysia to assess changes in soil solution composition and their effects on maize and groundnut yields, resulting from limestone and gypsum application. The results showed that soil solution Ca in the lime treatment remained mainly in the zone of incorporation, but in the gypsum treatment some Ca moved into 15–30 cm zone. Al³⁺ and AlSO₄ ⁺ were dominant Al species in the soil solution of nil treatment. Liming decreased Al³⁺ and AlSO₄ ⁺, but increased hydroxy-Al monomer activities. However, gypsum application resulted in an increase of AlSO₄ ⁺ activity and in a decrease of Al³⁺ activity. Relative maize and groundnut yields were negatively correlated with Al³⁺, Al(OH)²⁺ and Al_sum activities. Likewise, relative yields were negatively correlated with Al concentration and the Al concentration ratio and positively correlated with soil solution Mg concentration and Ca/Al ratio.     

Effects of CO2 and nitrogen fertilization on vegetation and soil nutrient content in juvenile ponderosa pine

This paper summarizes the data on nutrient uptake and soil responses in opentop chambers planted with ponderosa pine (Pinus ponderosa Laws.) treated with both N and CO₂. Based upon the literature, we hypothesized that 1) elevated CO₂ would cause increased growth and yield of biomass per unit uptake of N even if N is limiting, and 2) elevated CO₂ would cause increased biomass yield per unit uptake of other nutrients only by growth dilution and only if they are non-limiting. Hypothesis 1 was supported only in part: there were greater yields of biomass per unit N uptake in the first two years of growth but not in the third year. Hypothesis 2 was supported in many cases: elevated CO₂ caused growth dilution (decreased concentrations but not decreased uptake) of P, S, and Mg. Effects of elevated CO₂ on K, Ca, and B concentrations were smaller and mostly non-significant. There was no evidence that N responded in a unique manner to elevated CO₂, despite its unique role in rubisco. Simple growth dilution seemed to explain nutrient responses in almost all cases.There were significant declines in soil exchangeable K⁺, Ca²⁺, Mg²⁺ and extractable P over time which were attributed to disturbance effects associated with plowing. The only statistically significant treatment effects on soils were negative effects of elevated CO₂ on mineralizeable N and extractable P, and positive effects of both N fertilization and CO₂ on exchangeable Al³⁺. Soil exchangeable K⁺, Ca²⁺, and Mg²⁺ pools remained much higher than vegetation pools, but extractable P pools were lower than vegetation pools in the third year of growth. There were also large losses of both native soil N and fertilizer N over time. These soil N losses could account for the observed losses in exchangeable K⁺, Ca²⁺, Mg²⁺ if N was nitrified and leached as NO ₃ ^– .     

Changes in nutrient distribution in forests and soils of Walker Branch watershed,Tennessee, over an eleven-year period

Changes in vegetation, litter, and soil nutrient content were measured in selected plots on Walker Branch watershed, Tennessee, from 1972–73 to 1982. The watershed has been allowed to revert to forest since 1942, before which it consisted of small subsistence farms and woodland pastures. Changes in Ca status were of particular interest because initial nutrient cycling characterizations indicated that net Ca accumulation in vegetation could have caused large decreases in soil exchangeable Ca²⁺ within 20 years.Decreases in forest floor and subsoil (45–60 cm) N, exchangeable Ca²⁺, and Mg²⁺ content were noted in several plots from 1972 to 1982. Surface soils (0–15 cm) showed either no change or, in some cases (e.g., N and exchangeable K⁺ in certain plots), increases over the 11-year period. Reductions in forest floor and subsoil exchangeable Ca²⁺ and exchangeable Mg²⁺ on cherty, upper slope oak-hickory and chestnut oak forests were most striking. The changes in Ca²⁺ are thought to be due primarily to high rates of Ca²⁺ incorporation into woody tissues of oak and hickory species. Reductions in forest floor and subsoil exchangeable Mg²⁺ could not be accounted for by woody increment; leaching may have played a major role in causing these decreases. Changes in P and exchangeable K⁺ were variable, with both increases and decreases.There were significant increases in exchangeable Al³⁺ in both subsoils and surface soils of certain plots, but these were not accompanied by decreases in exchangeable base cations or consistent decreases in pH. Dissolution of interlayer Al from 2:1 clays may be the cause of the exchangeable Al³⁺ increases.These results suggest a general decline in fertility, especially with regard to Ca and Mg in those forests with low soil Ca and Mg supplies. Monitoring of further changes (if any) in these ecosystems will continue as the currently aggrading forests approach steady state.     

Plant availability of applied and native boron in soils with diverse properties

Laboratory and greenhouse research was conducted to study effects of soil properties on the availability of native and applied B in 14 Virginia soils. Boron absorption could be described by the Langmuir equation in 12 of the 14 soils, and maximum B adsorption (Vmax) in these 12 soils ranged from 3.3 to 26.5 mg kg⁻¹. A multiple regression equation, −19.3+3.51 pH+0.048 clay content, accounted for 89.6% of the variation in Vmax for the 12 soils. Curvilinear relationships (α=0.01) occurred between B in corn (Zea mays L.) tissue from native B and hot-water soluble B, mannitol exchangeable B, and NH₄-acetate and Mehlich III extractable B. Among these four procedures, mannitol exchangeable B correlated most closely (r=0.923) with B in corn tissue from native B. From 0.4 to 13.5% of the applied B was absorbed by corn plants and translocated to shoots. Curvilinear relationships (α=0.01) occurred between B in corn tissue from applied B and soil clay content, NH₄-oxalate extractable Al and Fe, and acidified NH₂OH·HCl extractable Mn. It is evident from these relationships that soil clay and oxyhydroxides of Al, Fe, and Mn have an affinity to adsorb B in somewhat unavailable forms.     

Influence of leaf litter type on the chemical evolution of a soil parent material (sandstone)

The influence of leaves of Quercus suber L. and Eucalyptus globulus Labill. and needles of Pinus pinaster Ait. on a sandstone substrate was assessed through lysimetric studies during a ten-year period at a site in Central Portugal. The decomposition rate of Q. suber leaf litter was similar to that of E. globulus and higher than that of P. pinaster needle litter. The proportion of nitrogen released from the Q. suber leaf litter was higher than that lost from the other organic species. Such a release was proportional to the initial nitrogen content in the substrates. The concentrations of both NH₄-N and NO₃-N were much higher in leachates collected under Q. suber leaf litter than in those collected under the other organic substrates. A similar trend was found in the leachates collected under the mineral substrate influenced by the studied organic substrates. The leachate concentrations of mineral N (especially NO₃-N) were higher from the mineral substrate under Q. suber leaf litter than from this organic substrate itself. The mineral substrate under leaf litter of E. globulus or needle litter of P. pinaster showed an increase in exchangeable base cations and pH values, and a decrease in extractable Al. Conversely, in the substrate with Q. suber leaf litter there was only a slight increase in exchangeable base cations and pH values, and a decrease in extractable Al. These results combined with those obtained in soils under E. globulus plantations indicate that changes found in these soils are due to soil and forest management practices rather than to the decomposition process of the respective of leaf litter.     

Influence of soil solution aluminum on root elongation of wheat seedlings

Wheat (Triticum aestivum L.) seedlings were grown for 4 days in an acid soil horizon treated with 10 levels each of Ca(OH)₂, CaSO₄ and CaCl₂. The treatments resulted in a wide range of Al levels and Al speciation in soil solution. Seedling root length in the Ca(OH)₂ treatments was significantly related (p<0.01) to calculated Al³⁺ activity in soil solution. The Al–SO₄ complex in soil solution had a negligible effect on the root growth of Hart wheat, thus confirming the previously reached conclusion concerning the nonphytotoxicity of Al–SO₄. The short-term seedling root growth technique used in this investigation allowed for separation of Al effects on root elongation from those on plant nutrition and should be useful for studying Al toxicity relationships in soil.     

Soil isotopically exchangeable zinc: A comparison between E and L values

The objectives of the present paper were: (i) to determine isotopically exchangeable zinc using two isotopic exchange methods (E and L values) in a series of polluted and non-polluted Swiss agricultural soils, and (ii) to evaluate the ability of chemical extraction methods to estimate plant-available soil Zn using isotopic techniques. The surface horizon (0–20 cm) of seven polluted and non-polluted soils representing a wide range in physico-chemical properties and Zn contents were sampled. An isotopic exchange kinetics (IEK) approach was used to assess, in a batch experiment, the isotopically exchangeable Zn content (E value). In order to determine the L values, a pot experiment was carried out with Lolium multiflorum (cv. Axis) in a growth chamber using a ⁶⁵Zn-isotope dilution technique. Total Zn uptake and the isotopic composition (⁶⁵Zn/^stableZn) were determined in Lolium multiflorum for five successive cuts. The amounts of zinc extracted by different chemicals were compared with L values and regression parameters were estimated. The isotopic composition in soil extracted by DTPA and EDTAAc at the end of the pot experiment was also determined. Results showed that the equation describing the increase of isotopically exchangeable Zn with time could be extrapolated to three months for polluted and non-polluted neutral and acidic soils, and that the results were not different from the amount of isotopically exchangeable Zn experimentally determined with Lolium multiflorum (L value). In alkaline soils however, results suggest that either ⁶⁵Zn sorption occurred in the batch experiment or that the concentration of Zn in the soil solution had been overestimated, leading to an overestimation of the E value compared to the L values. Furthermore, the specific activities measured in DTPA and EDTA extractions at the end of the pot experiment were significantly different compared to the specific activity of the plant, showing that both these chelating agents extract neither all the available soil Zn nor only the available soil Zn for plants. Abbreviations: C _Zn– concentration of Zn in a soil water extract (mg Zn L^?1); C _Zn?Plant– concentration of Zn in plant shoots (mg Zn kg^?1 DM); DTPA – diethylene triamine pentaacetic acid; E _1\min– amount of Zn isotopically exchangeable within one min (mg Zn kg^?1 soil); E _(t)\exp– amount of Zn isotopically exchangeable after t min derived from experimental results (mg Zn kg^?1 soil); E _(t)pred– amount of Zn isotopically exchangeable after t min predicted using kinetic parameters derived from a 100 min long isotope exchange kinetic experiment together with C _Zn, and Zn_HNO3 (mg Zn kg^?1 soil); EDTA – ethylene diamine tetraacetic acid; IC_P– isotopic composition of Zn in plant shoots; IC_DTPA– isotopic composition of Zn in the soil DTPA extract; IC_EDTA– isotopic composition of Zn in the soil EDTA extract; IC_SE– isotopic composition of Zn in the soil extracts; IEK – isotope exchange kinetics; L value – amount of plant available Zn (mg Zn kg^?1 soil); Lolium multiflorum; TEA – Triethanolamine; Zn_DTPA– Zn extractable by 0.005 M DTPA + 0.01 M CaCl2 + 0.1 M TEA; Zn_EDTA?NH4Ac– Zn extractable by 0.5 M NH₄Ac, 0.02 M EDTA; Zn_{EDTA?Ca(NO3)2}– Zn extractable by 0.005 M EDTA, 0.01 M Ca(NO₃)₂; Zn_KCl– Zn extractable by 1 M KCl; Zn_CaCl2– Zn extractable by 0.01 M CaCl₂; Zn_NaNO3– Zn extractable by 0.1 M NaNO₃; Zn_HNO3– Zn extractable by 2 M HNO₃.     

Aluminum tolerance of wheat does not induce changes in dominant bacterial community composition or abundance in an acidic soil

Aims

Aluminum-tolerant wheat plants often produce more root exudates such as malate and phosphate than aluminum-sensitive ones under aluminum (Al) stress, which provides environmental differences for microorganism growth in their rhizosphere soils. This study investigated whether soil bacterial community composition and abundance can be affected by wheat plants with different Al tolerance.

Methods

Two wheat varieties, Atlas 66 (Al-tolerant) and Scout 66 (Al-sensitive), were grown for 60 days in acidic soils amended with or without CaCO₃. Plant growth, soil pH, exchangeable Al content, bacterial community composition and abundance were investigated.

Results

Atlas 66 showed better growth and lower rhizosphere soil pH than Scout 66 irrespective of CaCO₃ amendment or not, while there was no significant difference in the exchangeable Al content of rhizosphere soil between the two wheat lines. The dominant bacterial community composition and abundance in rhizosphere soils did not differ between Atlas 66 and Scout 66, although the bacterial abundance in rhizosphere soil of both wheat lines was significantly higher than that in bulk soil. Sphingobacteriales, Clostridiales, Burkholderiales and Acidobacteriales were the dominant bacteria phylotypes.

Conclusions

The difference in wheat Al tolerance does not induce the changes in the dominant bacterial community composition or abundance in the rhizosphere soils.     

Effect of rock phosphate on soil properties and apparent phosphorus recovery in acid soil of Sumatra

The effects of rock phosphate (RP) on soil properties and apparent P recovery of corn in acid soil of Sumatra were investigated. The soil was dominated by kaolinitic minerals, very acid and low in extractable phosphorus. The experiment was conducted in two steps: incubation and greenhouse. The treatments were rock phosphate and triple superphosphate (TSP) at 0, 200, 400 and 600 µg P g^-1 for the incubation experiment, and at 0, 50, 100, 150 and 200 µg P g^-1 for greenhouse experiment In the incubation experiment, rock phosphate reduced exchangeable Al and increased pH_H2O better than TSP, and supplied Bray-1 extractable P as much as did TSP. In the greenhouse experiment, apparent P recovery values of rock phosphate were very close to those of TSP and the values tended to decrease at higher rates of P. Although corn yields of the rock phosphate treatment were lower than TSP, its relative agronomic effectiveness was quite high.     

Effects of [CO2] and nitrogen fertilization on soils planted with ponderosa pine

The effects of six years treatment with elevated [CO₂] (350, 525, and 700 μl l^-1) and nitrogen (N) (0, 10, and 20 g N m^-2 yr^-1) on soils, soil solution, and CO₂ efflux in an open-top chamber study with ponderosa pine (Pinus ponderosa Laws.) are described. The clearest [CO₂] effect was in year 6, when a pattern of lower soil N concentration and higher C/N ratio with elevated [CO₂] emerged. Statistically significant effects of elevated [CO₂] on soil total C, extractable P, exchangeable Mg²⁺, exchangeable Ca²⁺, base saturation, and soil solution HCO₃ ^- and NO₃ ^- were also found in various treatment combinations and at various times; however, these effects were inconsistent among treatments and years, and in many cases (P, Mg²⁺, Ca²⁺, base saturation) reflected pre-treatment differences. The use of homogenized buried soil bags did not improve the power to detect changes in soil C and N or help resolve the inconsistencies in soil C patterns. Nitrogen fertilization had the expected negative effects on exchangeable Ca²⁺, K⁺, and Mg²⁺ in year 6, presumably because of increased NO₃ ^- leaching, but had no consistent effect on soil C, N, or extractable P. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of soil acidification and subsequent leaching on levels of extractable nutrients in a soil

Summary The effects of soil acidification (pH values from 6.5 to 3.8), and subsequent leaching, on levels of extractable nutrients in a soil were studied in a laboratory experiment. Below pH 5.5, acidification resulted in large increases in the amounts of exchangeable Al in the soil. Simultaneously, exchangeable cations were displayed from exchange sites and Ca, Mg, K and Na in soil solution increased markedly. With increasing soil acidification, increasing amounts of cations were leached; the magnitude of leaching loss was in the same order as the cations were present in the soil: Ca²⁺>Mg²⁺>K⁺>Na⁺. Soil acidification appeared to inhibit nitrification since in the unleached soils, levels of NO ₃ ⁻ clearly declined below pH 5.5 and at the same time levels of NH ₄ ⁺ increased greatly. Significant amounts of NH ₄ ⁺ and larger amounts of NO ₃ ⁻ , were removed from the soil during leaching. Concentrations of NaHCO₃-extractable phosphate remained unchanged between pH 4.3 and 6.0 but were raised at higher and lower pH values. No leaching losses of phosphate were detected. For the unleached soils, levels of EDTA-extractable Mn and Zn increased as the soil was acidified whilst levels of extractable Fe were first decreased and then increased greatly and those for Cu were decreased slightly between pH 6.5 and 6.0 and then unaffected by further acidification. Significant leaching losses of Mn and Zn were observed at pH values below 5.5 but losses of Fe were very small and those of Cu were not detectable.     

The dynamics of protons,aluminium, and calcium in the rhizosphere of maize cultivated in tropical acid soils: experimental study and modelling

The goals of this work were to understand the dynamics of H⁺, Al and Ca in the rhizosphere of maize cultivated in tropical acid soils, and to evaluate the contribution of the dissolution kinetics of the Al-hydroxides to Al dynamics. The study of the dissolution kinetics was based on a comparison between experimental and simulated data, using a model of the chemical processes in the rhizosphere. Two Oxisols, pH 5.1 and 4.6, and one Ultisol, pH 5.2, were studied. An Al-tolerant maize variety (Zea mays L.) was grown for 14 days on a 3-mm thick soil layer. The composition of the soil and the soil solution, together with the concentration of Al in the roots, were determined throughout the experiment. The results showed that root growth (i) decreased the soil solution pH, up to one pH unit, (ii) increased Al concentration in the soil solution, (iii) increased exchangeable Al, and (iv) decreased exchangeable Ca. Soil solution pH, exchangeable Al, and exchangeable Ca were closely linked. Exchangeable Al increased 1.5 – 3.0 times, due to the dissolution of easily mobilised Al components. In addition, Al accumulation in roots depended mainly on Al in the soil solution. Modelling the interactions between H⁺, Al, and Ca proved that the main factor determining Al in the soil solution was the kinetic reactivity of the easily mobilised Al components. These components, probably poorly crystallised Al-hydroxides, are key players in the functioning of the rhizosphere in tropical acid soils.     

Application of the STICS crop model to predict nitrogen availability and nitrate transport in a tropical acid soil cropped with maize

Oxisols have a high likelihood of NO₃ ^– leaching which may strongly reduce N availability for tropical crops. The aim of this work was to evaluate the N and the water submodels of the STICS crop model for its ability to estimate N availability in N-fertilised field maize crops on two oxisols in Guadeloupe (French West Indies) with and without Al toxicity: a non-limed plot (NLI, pH_KCl 3.9, 2.1 cmol Al³⁺ kg^–1), and a limed plot (LI, pH_KCl 4.5, 0 cmol Al³⁺ kg^–1). An uncropped plot (UC, pH_KCl 4.5, 0 cmol Al³⁺ kg^–1) was used in order to fit some model parameters for soil evaporation, nitrification and NO₃ ^– transport. The model was modified in order to describe nitrification as a partially inhibited process in acid soils, and to take into account NO₃ ^– retention in oxisols. Nitrification was described as the result of the multiplicative effects of soil acidity, temperature and soil water content. Soil moisture and NO₃ ^– and NH₄ ⁺ content up to 0.8 m soil depth, above-ground biomass and N uptake by crops, and their leaf area index (LAI), were measured from sowing to the beginning of grain filling. The model described correctly the changes in soil water content during the moist and the dry periods of the experiment, and there was some evidence that capillary rise occurred in the dry period. Nitrogen mineralization, nitrification in UC, NO₃ ^– transport and plant uptake were satisfactorily simulated by the model. Because of the effect of Al toxicity on plant growth, LAI at flowering was three times higher in LI than in NLI. Some discrepancies between observed and simulated data were found for the distribution of NO₃ ^– and NH₄ ⁺ in the cropped plots. This was probably due to the change of the ionic N form absorbed by the crops as a function of soil acidity and available P in the soil. No leaching was observed below 0.8 m depth and this was associated with NO₃ ^– retention in the soil. The results showed that partial inhibition of nitrification and NO₃ ^– retention should be taken into account by crop models to obtain realistic estimates of N availability for plants in tropical acid soils.     

Depletion of Selenium in Soil Solution due to its Enhanced Sorption in the Rhizosphere of Soybean

The effect of plant roots on selenium (Se) mobility in soil was studied by a large-scale pot experiment in order to understand the environmental behavior of Se in agricultural soils under plant growth conditions. Soybean plants (Glycine max (L.) Merrill) were grown in a greenhouse for 84 d. The concentrations of Se and major elements (K, Ca, Mg, Na, and Al) in the soil solutions and in the plants were measured at different growth periods. Concentrations of Se and major cations in soil solution decreased as the soybean plants grew, while the concentrations of Al increased. It was assumed that the soybean roots released H⁺ with the uptake of cations; consequently, due to the acidification of the rhizosphere, Al³⁺ was released starting from the soil solid phase. The decreased Se concentration in the soil solution should be due to the enhancement of Se sorption onto the soil solid phase. The increase of Se sorption level in the rhizosphere was examined in a small-scale pot experiment. The soil–soil solution distribution coefficient of Se (K _d-Se) was observed as an index of Se sorption level. K _d-Se clearly increased in the rhizosphere soil after cultivation. The effects of pH and Al³⁺ in the rhizosphere on Se sorption were assessed by K _d-Se measurements at different levels of HCl and AlCl₃. In this third experiment, a decrease in pH increased K _d-Se values, but no specific effect was observed on Se sorption due to increased Al³⁺. These results show that the Se mobility in agricultural soil could be decreased by plant roots under plant growth conditions due to enhanced Se sorption in the rhizosphere.