Phosphorus pools and other soil properties in the rhizosphere of wheat and legumes growing in three soils in monoculture or as a mixture of wheat and legume

Background and aims

Phosphorus and nitrogen availability and forms are affected by soil properties as well as by plant species and further modulated by soil microbes. Additionally, close contact of the roots of two plant species may affect concentrations and forms of N and P. The aim of this study was to assess properties related to N and P cycling in the rhizosphere of wheat and legumes grown in monoculture or in wheat/legume mixtures in three soils differing in pH.

Methods

Faba bean, white lupin and wheat were grown in three soils differing in pH (4.8, 7.5 and 8.8) in monoculture or in mixed culture of wheat and legumes. Rhizosphere soil was collected at flowering and analyzed for P pools by sequential fractionation, available N as well as community structure of bacteria, fungi, ammonia oxidizers, N₂-fixers and P mobilizers by polymerase chain reaction (PCR)—denaturing gradient gel electrophoresis (DGGE).

Results

Soil type was the major factor determining plant growth, rhizosphere nutrient dynamics and microbial community structure. Among the crop species, only faba bean had a significant effect on nitrification potential activity (PNA) in all three soils with lower activity compared to the unplanted soil. Soil type and plant spieces affected the community composition of ammonia-oxidizing archaea (AOB), ammonia-oxidizing archaea (AOA), N₂-fixers (nifH), P mobilizers (ALP gene) and fungi, but not that of bacteria. Among the microbial groups, the AOA and nifH community composition were most strongly affected by crop species, cropping system and soil type, suggesting that these groups are quite sensitive to environmental conditions. All plants depleted some labile as well as non-labile P pools whereas the less labile organic P pools (NaOH extractable P pools, acid extractable P pools) accumulated in the rhizosphere of legumes. The pattern of depletion and accumulation of some P pools differed between monoculture and mixed culture as well as among soils.

Conclusions

Plant growth and rhizosphere properties were mainly affected by soil type, but also by crop species whereas cropping system had the least effect. Wheat and the legumes depleted less labile inorganic P pools in some soils whereas less labile organic P pools (NaOH extractable P, acid extractable P) accumulated in the rhizosphere of legumes.     

Cadmium availability at different soil pH to transgenic tobacco overexpressing ferritin

Knowledge on physiological mechanisms and plant metabolism can be used to enhance metal uptake. The capacity to uptake metals of transgenic tobaccos overexpressing ferritin in plastids (P6) or in cytoplasm (C5) and a control tobacco (A) is assessed in three polluted soils from the same soil series, with a similar Cd content, but displaying pH from 5.8 to 7 (8b2, 8b3, S11). Differences in dry leave weight were not significant between the three tobaccos growing on each soil. Iron concentration in ferritin overexpression either in P6 or in C5 tobaccos increased only on the S11 soil, which had a soil pH 7, in comparison to A tobacco. In both 8b2 and 8b3 soils at pH lower than 7, the leaf Fe content was not different between the three tobaccos. Only the P6 tobacco growing in the S11 soil accumulated more Cd, Zn and Mn compared with the A and C5 tobaccos. This increase represents, compared to A tobacco, 88% for Fe, 120% for Mn, 30% for Cd, 80% for Zn. In soils with a pH higher than 7, the ferritin synthesis is over-expressed in the P6 tobacco cells inducing a iron deficiency. These metal accumulations were increased, owing to a rhizosphere modification. The A and P6 tobaccos uptake Cd in the same soil pool. The Cd totality in the 8b2 soil is labile and in the S11 soil, the labile Cd represents 86–88% of the total Cd.     

Effect of soil chloride level on cadmium concentration in sunflower kernels

Understanding soil factors related to cadmium (Cd) uptake and accumulation in plants is important for development of agronomic technologies, and breeding strategy to produce low Cd crops. The objective of the study was to examine the effect of soluble salts (chloride and sulfate) and other soil factors on the Cd concentration in sunflower (Helianthus annuus L.) kernels. Commercial nonoilseed hybrid kernels and soils were sampled from 22 farmer's production fields in North Dakota and Minnesota. The sites sampled included saline and nonsaline variants from 7 soil series. Soils were sampled at four depths. Relationships between kernel Cd level and soil physical and chemical characteristics were examined. The soil pH covered a narrow range (7.3–8.1) at these sampled sites. Regression analysis showed that there was no correlation between kernel Cd and soil pH at any depth. The kernel Cd level was highly correlated with DTPA-extractable Cd in all 4 depths, and with clay content in sub-soils. Soil chloride and sulfate concentrations varied among soil series and within soil series. The absence of a statistically significant effect of soil sulfate level on kernel Cd concentration, indicated that soil sulfate levels did not affect Cd uptake by sunflower plants. However, soil chloride levels in sub-soil were correlated with kernel Cd. The most important soil factor was DTPA-extractable Cd. When chloride was included in the multiple regression equations, R square (R²) values improved significantly. These results demonstrate that soil chloride concentration is another important factor related to Cd uptake in sunflower plants.     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

Efficiency of zinc fertilization for flooded rice

Summary Several Zn-deficient soils from the major rice growing areas in the USA were characterized with respect to extractable and labile Zn and other parameters including available P, pH, organic matter, and cation exchange capacity. A greenhouse experiment was conducted to determine whether the above factors influence the response of several rice cultivars to ZnSO₄ or ZnEDTA applied as a mixed preplant treatment or to the floodwater at panicle initiation.Response of the rice to Zn varied widely among soils with labile Zn (L value) being a poor indicator of Zn availability and soil pH, and DTPA or 2 N MgCl₂ extraction of Zn being the most reliable indices. Soil incorporation prior to flooding was more effective than floodwater application for the initial crop, but both methods of placement were comparable for a subsequent ratoon crop. In either situation, ZnSO₄ was superior to ZnEDTA. Recovery of Zn from ZnSO₄ was generally less than 5 percent where Zn response was obtained and up to 14 percent from nonresponsive soils.     

Phytoavailability of Cd and Zn in soil estimated by stable isotope exchange and chemical extraction

The distribution of labile Cd and Zn in two contrasting soils was investigated using isotopic exchange techniques and chemical extraction procedures. A sewage sludge amended soil from Great Billings (Northampton, UK) and an unamended soil of the Countesswells Association obtained locally (Aberdeen, UK) were used. ¹¹⁴Cd and ⁶⁷Zn isotopes were added to a water suspension of each soil and the labile metal pool (E-value) determined from the isotope dilution. Samples were obtained at 13 time points from 1h to 50 days. For the sewage sludge amended soil, 29 g Cd g^–1 (86% of total) and 806 g Zn g^–1 (65% of total) were labile and for the Countesswells soil the value was 8.6 g Zn g^–1 (13% of total); limits of detection prevented a Cd E-value from being measured in this soil. The size of the labile metal pool was also measured by growing plants for 90 days and determining the isotopic content of the plant tissue (L-value). Thlaspi caerulescensJ. & C. Presl (alpine penny cress), a hyperaccumulator of Zn and Cd, Taraxacum officinale Weber (dandelion) and Hordeum vulgare L. (spring barley) were used. L-values were similar across species and lower than the E-values. On average the L-values were 23±0.8 g Cd g^–1 and 725±14 g Zn g^–1 for the Great Billings soil and 0.29±0.16 g Cd g^–1 and 7.3±0.3 g Zn g^–1 for the Countesswells soil. The extractable metal content of the soils was also quantified by extraction using 0.1 M NaNO₃, 0.01 M CaCl₂, 0.5 M NaOH, 0.43 M CH₃COOH and 0.05 M EDTA at pH 7.0. Between 1.3 and 68% of the total Cd and between 1 and 50% of the total Zn in the Great Billings soil was extracted by these chemicals. For the Countesswells soil, between 6 and 83% of the total Cd and between 0.1 and 7% of the total Zn was extracted. 0.05 M EDTA and 0.43 M CH₃COOH yielded the greatest concentrations for both soils but these were less than the isotopic estimates. On the whole, E-values were numerically closer to the L-values than the chemical extraction values. The use of isotopic exchange provides an alternative estimate of the labile metal pool within soils compared to existing chemical extraction procedures. No evidence was obtained that T. caerulescens is able to access metal within the soil not freely available to the other plants species. This has implications for long term remediation strategies using hyperaccumulating plant species, which are unlikely to have any impact on non-labile Cd and Zn in contaminated soil.     

Cadmium uptake and bioaccumulation in selected cultivars of durum wheat and flax as affected by soil type

Accumulation of cadmium (Cd) in crop plants is of great concern due to the potential for food chain contamination through the soil-root interface. Although Cd uptake varies considerably with plant species, the processes which determine the accumulation of Cd in plant tissues are affected by soil factors. The influence of soil type on Cd uptake by durum wheat (Triticum turgidum var. durum L.) and flax (Linum usitatissimum L.) was studied in a pot experiment under environmentally controlled growth chamber conditions. Four cultivars/lines of durum wheat (Kyle, Sceptre, DT 627, and DT 637) and three cultivars/lines of flax (Flanders, AC Emerson, and YSED 2) were grown in two Saskatchewan soils: an Orthic Gray Luvisol (low background Cd concentration; total/ABDTPA extractable Cd: 0.12/0.03 mg kg^-1, respectively) and a Dark Brown Chernozem (relatively high background Cd concentration; total/ABDTPA Cd: 0.34/0.17 mg kg^-1 respectively). Plant roots, stems, newly developed heads, and grain/seeds were analyzed for Cd concentration at three stages of plant growth: two and seven weeks after germination, and at plant maturity. The results showed that Cd bioaccumulation and distribution within the plants were strongly affected by both soil type and plant cultivar/line. The Cd concentration in roots leaves and stems varied at different stages of plant growth. However, all cultivars of both plant species grown in the Chernozemic soil accumulated more Cd in grain/seeds than plants grown in the Orthic Gray Luvisol soil. The different Cd accumulation pattern also corresponded to the levels of ABDTPA extractable and metal-organic complex bound soil Cd found in both soils. Large differences were found in grain Cd among the durum wheat cultivars grown in the same soil type, suggesting the importance of rhizosphere processes in Cd bioaccumulation and/or Cd transport processes within the plant. Distribution of Cd in parts of mature plants showed that durum grain contained up to 21 and 36% of the total amount of Cd taken up by the plants for the Orthic Gray Luvisol and Chernozemic soils, respectively. These results indicate the importance of studying Cd speciation, bioaccumulation and cycling in the environment for the management of agricultural soils and crops.     

Metal uptake by iron-efficient and inefficient oats

Metal uptake by oats depending on plant responses to Fe-deficiency stress was investigated. Coker 227 oats classified as Fe-efficient and TAM 0–312 oats as Fe-inefficient cultivars (Hopkins et al., 1992) were grown either alone or in combination in three sandy soils using a pot experiment. These soils were from a field trial with sludge-borne metals applications leading to an increased metal content. Plant shoots were harvested one month after growth. Because soil pH increased from 5.4 to 6.8, shoot Fe level decreased in the Fe-inefficient TAM 0–312 oats compared to Coker 227 oats when plants were grown alone. In combination, TAM 0–312 oats had a negative impact on the availability of Fe in the Fe-efficient Coker 227 oats. Especially, Coker 227 and TAM 0–312 shoots showed chlorosis in mixed culture with high Zn and Mn content in the soil (soil B). However, Fe content in TAM 0–312 shoots in mixed culture did not increase compared to monoculture in all soils. In metal-contaminated soils, TAM 0–312 oats grown alone obtained less Zn and Cd than Coker 227 oats. Additionally at soil pH 6.8, shoot Ni and Mn levels were also lower in TAM 0–312 oats than in Coker 227 oats. Shoot Zn, Cd, and Ni levels decreased in Coker 227 oats from mixed cultures, and were not different compared to those in TAM 0–312 oats. Cu uptake was similar in all treatments except for the mixed culture in soil B. Coker 227 oats have been found to release a phytosiderophore whereas TAM 0–312 did not (Brown et al., 1991). Results indicated that phytosiderophores may lead to a higher Zn, Cd and Ni supply in the rhizosphere of Coker 227 oats and to higher metal contents in their shoots than in TAM 0–312 oats which did not activate such mechanisms.     

Immobilization and phytoavailability of cadmium in variable charge soils. I. Effect of phosphate addition

The effect of phosphate on the surface charge and cadmium (Cd) adsorption was examined in seven soils that varied in their variable-charge components. The effect of phosphate on immobilization and phytoavailability of Cd from one of the soils, treated with various levels of Cd (0–10 mg Cd kg^–1 soil), was further evaluated using mustard (Brassica juncea L.) plants. Cadmium immobilization in soil was evaluated by a chemical fractionation scheme. Addition of phosphate, as KH₂PO₄, increased the pH, negative charge and Cd adsorption by the soils. Of the seven soils examined, the three allophanic soils (i.e., Egmont, Patua and Ramiha) exhibited greater increases in phosphate-induced pH, negative charge and Cd²⁺ adsorption over the other four non-allophanic soils (i.e., Ballantrae, Foxton, Manawatu ad Tokomaru). Increasing addition of Cd enhanced Cd concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Addition of phosphate effectively reduced the phytotoxicity of Cd. There was a significant inverse relationship between dry matter yield and Cd concentration in soil solution. Addition of phosphate decreased the concentration of the soluble + exchangeable Cd fraction but increased the concentration of inorganic-bound Cd fraction in soil. The phosphate-induced alleviation of Cd phytotoxicity can be attributed primarily to Cd immobilization due to increases in pH and surface charge.     

The bioavailability of cadmium to lettuce and cabbage in soils previously treated with sewage sludges

The application of sewage sludges to soils may lead to increased soil-Cd levels. The bioavailability of Cd is determined by the interaction of a number of soil physico-chemical and plant variables, of which pH is the most important. Duplicate samples of sludge-treated soils were transferred to tubs in the field, one of each pair being limed to pH 7±0.5. Lettuce and cabbage were grown to maturity and analysed for Cd. Liming always reduced Cd uptake by the plants. Three soil extractants, 1 M NH₄NO₃, 0.05 M EDTA-(Na)₂ and 0.05 M CaCl₂ were used as indices of Cd bioavailability. CaCl₂ proved to be the most effective for both lettuce and cabbage. Multiple linear regression equations were derived to describe the uptake and accumulation of Cd by both crops. The relative influence of soil variables differed between the two species. Unlike those of a number of pot experiments conducted in glasshouses, the data from this experiment are comparable with those of crop samples taken from the field.     

Microbial biomass C,N and P in two arctic soils and responses to addition of NPK fertilizer and sugar: implications for plant nutrient uptake

The soil microbial carbon (C), nitrogen (N) and phosphorus (P) pools were quantified in the organic horizon of soils from an arctic/alpine low-altitude heath and a high-altitude fellfield by the fumigation-extraction method before and after factorial addition of sugar, NPK fertilizer and benomyl, a fungicide. In unamended soil, microbial C, N and P made up 3.3–3.6%, 6.1–7.3% and 34.7% of the total soil C, N and P content, respectively. The inorganic extractable N pool was below 0.1% and the inorganic extractable P content slightly less than 1% of the total soil pool sizes. Benomyl addition in spring and summer did not affect microbial C or nutrient content analysed in the autumn. Sugar amendments increased microbial C by 15 and 37% in the two soils, respectively, but did not affect the microbial nutrient content, whereas inorganic N and P either declined significantly or tended to decline. The increased microbial C indicates that the microbial biomass also increased but without a proportional enhancement of N and P uptake. NPK addition did not affect the amount of microbial C but almost doubled the microbial N pool and more than doubled the P pool. A separate study has shown that CO₂ evolution increased by more than 50% after sugar amendment and by about 30% after NPK and NK additions to one of the soils. Hence, the microbial biomass did not increase in response to NPK addition, but the microbes immobilized large amounts of the added nutrients and, judging by the increased CO₂ evolution, their activity increased. We conclude: (1) that microbial biomass production in these soils is stimulated by labile carbon and that the microbial activity is stimulated by both labile C and by nutrients (N); (2) that the microbial biomass is a strong sink for nutrients and that the microbial community probably can withdraw substantial amounts of nutrients from the inorganic, plant-available pool, at least periodically; (3) that temporary declines in microbial populations are likely to release a flush of inorganic nutrients to the soil, particularly P of which the microbial biomass contained more than one third of the total soil pool; and (4) that the mobilization-immobilization cycles of nutrients coupled to the population dynamics of soil organisms can be a significant regulating factor for the nutrient supply to the primary producers, which are usually strongly nutrient-limited in arctic ecosystems.     

Long-term effects of metals in sewage sludge on soils,microorganisms and plants

Summary This paper reviews the evidence for impacts of metals on the growth of selected plants and on the effects of metals on soil microbial activity and soil fertility in the long-term. Less is known about adverse long-term effects of metals on soil microorganisms than on crop yields and metal uptake. This is not surprising, since the effects of metals added to soils in sewage sludge are difficult to assess, and few long-term experiments exist. Controlled field experiments with sewage sludges exist in the UK, Sweden, Germany and the USA and the data presented here are from these long-term field experiments only. Microbial activity and populations of cyanobacteria,Rhizobium leguminosarum bv.trifolii, mycorrhizae and the total microbial biomass have been adversely affected by metal concentrations which, in some cases, are below the European Community's maximum allowable concentration limits for metals in sludge-treated soils. For example, N₂-fixation by free living heterotrophic bacteria was found to be inhibited at soil metal concentrations of (mg kg^–1): 127 Zn, 37 Cu, 21 Ni, 3.4 Cd, 52 Cr and 71 Pb. N₂-fixation by free-living cyanobacteria was reduced by 50% at metal concentrations of (mg kg^–1): 114 Zn, 33 Cu, 17 Ni, 2.9 Cd, 80 Cr and 40 Pb.Rhizobium leguminosarum bv.trifolii numbers decreased by several orders of magnitude at soil metal concentrations of (mg kg^–1): 130–200 Zn, 27–48 Cu, 11–15 Ni, and 0.8–1.0 Cd. Soil texture and pH were found to influence the concentrations at which toxicity occurred to both microorganisms and plants. Higher pH, and increased contents of clay and organic carbon reduced metal toxicity considerably. The evidence suggests that adverse effects on soil microbial parameters were generally found at surpringly modest concentrations of metals in soils. It is concluded that prevention of adverse effects on soil microbial processes and ultimately soil fertility, should be a factor which influences soil protection legislation.     

Phytoextraction of cadmium with Thlaspi caerulescens

The in situ phytoextraction of cadmium from soils can only be achieved using plants that are both tolerant to high Cd concentrations and able to extract sufficient amounts of the metal. However, very few plant species are capable of remediating Cd polluted soils in a reasonable time frame. This paper aims to show that the population of the hyperaccumulator Thlaspi caerulescens J. & C. Presl. from Viviez (south of France), which has a high Cd-accumulating capability, is an efficient tool to remove Cd from contaminated soils. Roots of T. caerulescensViviez proliferate in hot spots of metals in soils which is particularly advantageous because of heterogeneity of the distribution of metal in polluted soils. Isotopic techniques showed that plants from this population acquire Cd from the same pools as non-accumulating species, but that it was much more efficient than non-hyperaccumulators at removing the metal from the soil labile pool. This is due: to (i) a specific rooting strategy, and (ii) a high uptake rate resulting from the existence in this population of Cd-specific transport channels or carriers in the root membrane. Growth and overall extraction can be improved with appropriate N fertilisation, supplied either as mineral fertilisers or uncontaminated sewage sludge. Selecting bigger plants is possible from within a suitable Cd-accumulating population to improve the phytoextraction process. Growing the Cd-accumulating populations results in a reduction in the availability of Cd and Zn as shown with field and lysimeter experiments conducted for several years. As a result, on a practical aspect, Cd hyperaccumulating populations of T. caerulescens may be used as a tool to efficiently reduce the availability of Cd in soils, providing appropriate populations are used.     

Plant uptake of sulphur as related to changes in the HI-reducible and total sulphur fractions in soil

Although considerable progress has been made in relating extractable soil S to plant S availability, most of these studies determined the extractable soil S at the beginning of the experiment to use as an index of soil S status. This bears little or no relationship to the S taken up by plants during the entire growing season. The present study investigates the changes in extractable soil S with time and relates these to changes in plant S uptake. Six soils with different long-term fertiliser histories (0, 21, 42 kg of S as superphosphate ha^–1 applied since 1952) and animal camping treatments (camp and non-camp) were used in two pot systems (with and without plants). Carrier-free ³⁵SO₄–S was added to the soils, to provide the information on the transformations of recently added S between the different extractable S forms in soils and whether these transformations could predict plant-available S. The soils were pre-conditioned and then transferred to the glasshouse, where one set of pots were planted with perennial ryegrass (Lolium perenne L.) while the other set was left uncropped. Periodic plant harvests and soil samplings at four weekly intervals were conducted over a period of 20 weeks to determine plant S uptake and amounts of extractable soil S and ³⁵S forms using five extractants. Same extractions of soil S and ³⁵S were conducted for the initial soils. Results showed that HI-reducible and total soil S extracted by CaCl₂, KH₂PO₄ and by KCl at 40°C were utilised significantly by plants but not those extracted by NaHCO₃ and NaOH extractants. However, after the 8th week, plants continued to take up S even though levels of S extracted from the soil by CaCl₂, KH₂PO₄ and by KCl at 40°C remained low and unchanged. These results suggest that soil S taken up by plants after the 8th week period originated directly from the mineralisation of soil organic S from S pools other than those present in the extractable soil S forms. Similar results were shown by ³⁵S data, thereby confirming the complexity of determining plant S availability based on soil S extraction methods.     

Biogeochemical implications of labile phosphorus in forest soils determined by the Hedley fractionation procedure

Forest ecologists and biogeochemists have used a variety of extraction techniques to assess labile vs. non-labile soil P pools in chronosequences, the balance between biological vs. geochemical control of P transformations across a wide range of soil orders, the role of plants with either N-fixing or mycorrhizal symbionts in controlling soil P fractions, and to make inferences about plant-available P. Currently, variants of the sequential extraction procedure developed by M. J. Hedley and co-workers afford the greatest discrimination among labile and non-labile organic and inorganic P pools. Results of recent studies that used this technique to evaluate P fractions in forest soils indicate the following: (1) in intact, highly weathered forest soils of the humid tropics, Hedley-labile P values are several times larger than extractable P values resulting from mildly acidic extracting solutions which were commonly used in the past 2 decades; (2) pools of Hedley-labile P are several times larger than the annual forest P requirement and P required from the soil annually in both temperate and tropical forests; (3) long-term trends in non-labile P pools during pedogenesis are adequately represented by the Walker and Syers' model of changes in P fractionation during soil development. However, to better represent trends in pools that can supply plant-available P across forest soils of different age and weathering status, the paradigm should be modified; and (4) across a wide range of tropical and temperate forest soils, organic matter content is an important determinant of Hedley-labile P.     

Phytoavailability of Cadmium (Cd) to Pak Choi (Brassica chinensis L.) Grown in Chinese Soils: A Model to Evaluate the Impact of Soil Cd Pollution on Potential Dietary Toxicity

Food chain contamination by soil cadmium (Cd) through vegetable consumption poses a threat to human health. Therefore, an understanding is needed on the relationship between the phytoavailability of Cd in soils and its uptake in edible tissues of vegetables. The purpose of this study was to establish soil Cd thresholds of representative Chinese soils based on dietary toxicity to humans and develop a model to evaluate the phytoavailability of Cd to Pak choi (Brassica chinensis L.) based on soil properties. Mehlich-3 extractable Cd thresholds were more suitable for Stagnic Anthrosols, Calcareous, Ustic Cambosols, Typic Haplustalfs, Udic Ferrisols and Periudic Argosols with values of 0.30, 0.25, 0.18, 0.16, 0.15 and 0.03 mg kg⁻¹, respectively, while total Cd is adequate threshold for Mollisols with a value of 0.86 mg kg⁻¹. A stepwise regression model indicated that Cd phytoavailability to Pak choi was significantly influenced by soil pH, organic matter, total Zinc and Cd concentrations in soil. Therefore, since Cd accumulation in Pak choi varied with soil characteristics, they should be considered while assessing the environmental quality of soils to ensure the hygienically safe food production.     

Phosphorus availability indices in calcareous lebanese soils

Summary Three indices of available P were evaluated with 20 mainly calcareous Lebanese soils using wheat (Triticum aestivum L.) and tomato (Lycopersicum esculentum L.) in the greenhouse. Both the NaHCO₃-and anion exchange resin-extractable P were significantly correlated with P uptake. The relationship for NaOH–Na₂C₂O₄ was not significant. Langmuir isotherm and actual sorption parameters were also poorly related to uptake. In general, soil properties were not significantly related to extractable P or crop uptake. Sequential inclusion of selected soil properties with test values only slightly improved the multiple correlation coefficients.Contribution from the Department of Soils, Irrigation and Mechanization, American University of Beirut, Beirut, Lebanon. Journal No. 523B. Sci. Paper Series No.297.     

Prediction of long-term fertilizer nitrogen requirements of maize in the tropics using a nitrogen balance model

A simple N balance model was used to calculate fertilizer requirement for a target N uptake by maize. Nitrogen uptake from soil sources and target uptake of N with fertilizer N additions were obtained from fertilizer trials in Africa and Latin America. Most experiments had data for only one cropping period, although some from Latin America had data for four to six crops. The transfer coefficient of fertilizer N to the crop was adjusted to realize maximum recovery of fertilizer N under best methods of fertilizer application. The time constants of transfer of soil N to the crop were allowed to vary and were affected mainly by soil texture. Where 4 to 6 cropping periods were available good agreement between actual and predicted fertilizer N requirements was obtained. With this approach long-term fertilizer N requirements for 14 sites were predicted using first cropping period N uptake. This study showed that pools of organic N in more coarse-textured soils were usually smaller and declined more rapidly than in fine-textured soils. Labile organic N pools declined with time under all simulations, but approached equilibrium within 10 croppings seasons. Equilibrium N uptake from the soil organic N pool was predicted to be 31 kg ha^–1 for the more coarse-textured soils and 36 kg ha^–1 for the fine-textured soils. Long-term projections of fertilizer requirements using input data of the field experiments were reasonable, and effects of legume green manures and other amendments could be clearly evaluated.     

Soil phosphorus movement and budget after 13 years of fertilized cultivation in the Amazon basin

Agronomic studies on soil phosphorus dynamics have primarily focused on the plant-available inorganic P pool. However organic P and less labile inorganic pools can contribute significantly to plant P uptake. The objectives of this study were to determine the changes in inorganic and organic P pools of varying lability in and below the plowlayer after 13 years of continuous cultivation and fertilization on a Typic Paleudult in Yurimaguas, Peru. The field experiment was established after slash and burn of a secondary forest and included non-fertilized and fertilized treatments. The yearly cropping pattern consisted of an upland rice (Oryza sativa),-corn (Zea mays),-soybean (Glycine max) rotation. A modified version of the Hedley et al. procedure was used to sequentially fractionate soil P into increasingly recalcitrant organic and inorganic pools. Plowlayer accumulation of the fertilizer P occurred in all P pools. The greatest increase was in the NaOH extractable inorganic P pool. In the non-fertilized plots, the organic P decreased by 42%. Phosphorus fertilization resulted in significant movement of P below plowlayer. The accumulation occurred mostly in inorganic and organic P pools that are not quantified by traditional soil-P test methods. In fertilized plots sub-plowlayer total P increased by 90 g g^–1 (87%) while resin extractable P increased only 4 g g^–1. Phosphorus content of the organic P pools below the plowlayer increased by 24 g g^–1 (50%) in fertilized plots. The inclusion of less labile P pools in studies of P movement and the evaluation of P fertilizer residual values could lead to a better understanding of P dynamics and hence better management of P fertilization.     

Potential of Salix as phytoextractor for Cd on moderately contaminated soils

Biomass production and Cd uptake in Salix (willow) is generally high. The aim of this study was to quantify the amounts of Cd in different plant and soil compartments in commercial Salix stands grown on different soil types and to use these data for estimation of the long-term effects of Cd removal by Salix at stem harvest on the soil Cd content by calculating Cd balances. Ten commercial short rotation willow coppice stands, planted with clone 78183 of Salix viminalis (L.) were investigated in order to include three different soil types with respect to pH. Stands with high stem biomass production generally had lower Cd concentrations in the plant and grew on more alkaline soils with low Cd availability when compared to stands with low stem biomass production. The highest output of Cd by Salix is therefore not necessarily achieved by stands with the highest yields or the highest Cd concentrations, as both these parameters have to be considered. The net removal of Cd from the plough layer by the Salix crop under different conditions varied between 2.6 and 16.5 g Cd ha^–1 year^–1. The long-term effects of Salix cultivation on soil Cd content, i.e. after 6–7 cutting cycles (25 years), were calculated and we reached the conclusion that there is a high potential in Salix to remove significant amounts of Cd at stem harvest. Salix can therefore presumably be used to counteract Cd pollution of agricultural soils in Sweden provided that the Cd in furnace ashes is taken care of.