Net nitrogen mineralization and nitrification rates in forest soil in northeastern Taiwan

We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg⁻¹ day⁻¹, and net nitrification rates ranged from 2.2 to 11.6 mg N kg⁻¹ day⁻¹. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg⁻¹ day⁻¹ and net nitrification rates ranged from 0.02 to 2.8 mg N kg⁻¹ day⁻¹. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha⁻¹. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.     

Long-term manuring and fertilization effects on soil organic carbon pools in a Typic Haplustept of semi-arid sub-tropical India

Soil is a potential C sink and could offset rising atmospheric CO₂. The capacity of soils to store and sequester C will depend on the rate of C inputs from plant productivity relative to C exports controlled by microbial decomposition. Management practices, such as no-tillage and high intensity cropping sequences, have the potential to enhance C and N sequestration in agricultural soils. An investigation was carried out to study the influence of long-term applications of fertilizers and manures on different organic C fractions in a Typic Haplustept under intensive sequence of cropping with maize–wheat–cowpea in a semi-arid sub-tropic of India. In 0–15 cm, the bulk density was lowest (1.52 Mg m⁻³) in plots treated with 100% NPK + FYM, while the control treatment showed the highest value (1.67 Mg m⁻³). Balanced application of NPK (100% NPK) showed significantly lower bulk density (1.56 Mg m⁻³) over either 100% N (1.67 Mg m⁻³) or 100% NP (1.61 Mg m⁻³) in surface soils. The application of super-optimal dose of NPK (150% NPK) showed higher total organic C (TOC) (12.9 g C kg⁻¹) over either 50% NPK (9.3 g C kg⁻¹) or 100% NPK (10.0 g C kg⁻¹) in 0–15 cm soil layer. There was an improvement in TOC in 100% NPK or 100% NP (9.3 g C kg⁻¹) over 100% N (8.7 g C kg⁻¹) in the same depth. The application of FYM with 100% NPK showed 15.2, 9.9 and 5.2 g C kg⁻¹ in 0–15, 15–30 and 30–45 cm, respectively. Application of graded doses of NPK from 50 to 150% of recommendation NPK significantly enhanced other organic C fractions like, microbial biomass C (MBC), particulate organic C (POC) and KMnO₄ oxidizable C (KMnO₄–C) in all the three soil depths. The TOC in 0–45 cm soil depth in 150% NPK (63.5 Mg C ha⁻¹) was increased by 39% over that in 50% NPK treatment (51.5 Mg C ha⁻¹) and 29% over that in 100% NPK treatment (54.1 Mg C ha⁻¹). Integrated use of farmyard manure with 100% NPK (100% NPK + FYM) emerged as the most efficient management system in accumulating largest amount of organic C (72.1 Mg C ha⁻¹) in soil. Nevertheless, this treatment also sequestered highest amount of organic C (731 kg C ha⁻¹ year⁻¹). Particulate organic carbon, a physically protected carbon pool in soil, could well be protected in sub-surface soil layers than in surface soil layer as a means of carbon aggradations. Microbial metabolic quotient (qCO₂) was significantly lower in 100% NPK + FYM over other treatments to indicate this to be the most efficient manuring practice to preserve organic carbon in soil where it facilitates aggradations of more recalcitrant organic C in soil. As compared to POC, total TOC proved to be a better predictor of MBC as it strongly correlated with total carbon mineralized from soil.     

Total selenium content of agricultural soils in Japan

To evaluate the selenium (Se) level in agricultural soils in Japan and to investigate its determining factors, 180 soil samples were collected from the surface layer of paddy or upland fields in Japan and their total Se contents were determined. Finely ground soil (50 mg) was wet-digested with HNO₃ and HClO₄ solution and the released Se was reduced to Se(IV). The concentration of Se(IV) was then determined by high-performance liquid chromatography with a fluorescence detector after treatment with 2,3-diaminonaphthalene and extraction with cyclohexane. The total Se content ranged from 0.05 to 2.80 mg kg⁻¹ with geometric and arithmetic means of 0.43 and 0.51 mg kg⁻¹, respectively. The overall data showed a log-normal distribution. In terms of soil type, volcanic soils and peat soils had relatively high Se content and regosols and gray lowland soils had relatively low Se content. In terms of land use, upland soils had significantly higher Se content than paddy soils. Among regions, soils in the Kanto, Tohoku, Hokkaido and Kyushu regions had relatively high content. The total Se content had a significant positive correlation with the organic carbon content ( P  < 0.01) and the equation for the estimation of total Se content with organic carbon suggested that on average approximately 48% (0.24 mg kg⁻¹) of the total Se was in inorganic forms and approximately 52% (0.25 mg kg⁻¹) was in organic forms. Soil pH, on the contrary, did not show a significant relationship with the total Se content. In conclusion, the organic matter content, in combination with volcanic materials, was the main determining factor of the total Se content of agricultural soils in Japan.     

Aluminum toxicity of synthetic aluminum–humus complexes derived from non-allophanic and allophanic Andosols and its amelioration with allophanic materials

Natural non-allophanic Andosols often show aluminum (Al) toxicity to Al-sensitive plant roots. The significance of Al–humus complexes to Al toxicity has been emphasized. Allophanic Andosols also possess Al–humus complexes, but they rarely show any toxicity. In the present study, using model substances, we tested the toxicity of Al–humus complexes and its amelioration with allophanic materials. We extracted humic substances from the A horizons of a non-allophanic Andosol and an allophanic Andosol using a NaOH solution, and reacted the humic substances and partially neutralized AlCl₃ solution at pH 4. Allophanic material was purified from commercial Kanuma pumice. Plant growth tests were conducted using a medium containing the Al–humus complexes (50 g kg⁻¹), the allophanic material (0, 90, 180 and 360 g kg⁻¹) and perlite. The root growth of barley ( Hordeum vulgare L.) and burdock ( Arctium lappa ) was reduced in the media containing the Al–humus complexes derived from both the non-allophanic and allophanic Andosols when the allophanic material was not added. With the addition of the allophanic materials, particularly in the 360 g kg⁻¹ treatment, the growth of the barley roots was improved markedly. Although the root growth of the burdock tended to improve with allophanic materials, the effect was weaker than that for barley. Monomeric Al in a solution of the medium was not detected (< 0.05 mg L⁻¹) following the addition of 360 g kg⁻¹ of allophanic materials, whereas 0.8–1.7 mg L⁻¹ Al was recorded without the allophanic material.     

Zinc fractions and solubility in a tropical semi-arid soil under long-term cultivation

There is an increasing incidence of Zn deficiency in savanna soil under intensive cultivation. The sizes of Zn pools and Zn²⁺ solubility were studied in a savanna soil under long-term cultivation and varying management practices which included fertilization with NPK, farmyard manure (FYM), NPK + FYM and a control which received neither NPK nor FYM for 50 years. An uncultivated natural site was sampled as a reference for assessing the effect of management history on Zn fractions and solubility. Total Zn ranged from 38 to 63 mg kg^-1 in the natural site, and from 28 to 57 mg kg^-1 in the cultivated fields. The FYM-fertilized field maintained total and extractable Zn on par with the natural site. Cultivation and management history did not affect the concentration of diethylenetriaminepentaacetic acid- (DTPA) extractable Zn, water-soluble Zn (SOL-Zn), exchangeable Zn (EXCH-Zn) and organically complexed Zn (OM-Zn). For the natural site, residual Zn (RES-Zn) accounted for 48% of total Zn, whereas Mn-oxide-bound Zn (MnOX-Zn) accounted for between 40% and 61% of total Zn in the cultivated fields. Cultivation caused the depletion of RES-Zn especially in FYM- and FYM + NPK-fertilized soils. Solubility studies indicated that some mechanism involving Zn and Fe, but independent of pH, appeared to control Zn²⁺ concentration in soil solution consistent with the reasonably constant values of pZn²⁺ + 2pFe³⁺ in soil solution.     

Indicator of potential residual carbon in soils after exogenous organic matter application

An indicator to evaluate the proportion of exogenous organic matter (EOM) remaining in soils over the long-term after application has been developed. A database was constructed with analytical data corresponding to 83 EOMs, including sludges, composts, animal wastes, mulches, plant materials and fertilizers. The data included results of proximal analysis (soluble, SOL, hemicellulose-, HEM, cellulose-, CEL, and lignin-like, LIC, fractions, in g kg⁻¹ total organic matter) and of carbon (C) mineralization during long-term incubations under laboratory conditions (in g kg⁻¹ exogenous organic C, EOC). The potential residual organic C after EOM application to soil was assessed from the extrapolation of the incubation results. Then, partial least square regression was used to relate EOM characteristics to the proportion of potentially residual organic C previously determined from the incubations. The biochemical fractions of EOM were not predictive enough to develop the indicator. The proportion of organic C mineralized during 3 days of incubation (C_3d) was cumulated and appeared to be the most predictive variable of residual organic C. The proposed indicator of residual organic carbon in soils (expressed as g EOC kg⁻¹) was I_ROC = 445 + 0.5 SOL – 0.2 CEL + 0.7 LIC – 2.3 C_3d. The indicator was calculated for the main types of EOM applied to soils. When compared with the few field data of residual C measured in long-term field experiments, the values provided by the indicator seemed to be over-estimated (i.e. EOC degradation could be faster under field conditions than during laboratory incubations).     

Long-term farmyard manure application effects on properties of a silty clay loam soil under irrigated wheat–soybean rotation

Increasing importance has been placed on the use of agricultural soils for the mitigation of atmospheric CO₂ through sequestration of soil C. Although crop productivity is sustained mainly through the application of organic manure in the Indian Himalayas, little information is available on C sequestration, C content in different aggregate size fractions and soil water transmission properties (infiltration and saturated hydraulic conductivity) as affected by long-term manure addition. We analyzed results of an 8-year experiment, initiated in 1995–1996 on a silty clay loam soil, to determine the influence of fertilizer and fertilizer + farmyard manure (FYM) application on those important soil properties. The overall increase in soil organic C (SOC) content in the 0–45 cm soil depth in NPK + FYM treatment as compared to NPK and control treatments was 11.0 and 13.9 Mg C ha⁻¹ at the end of 8 years, respectively. Application of FYM significantly reduced soil bulk density and increased mean weight diameter (MWD) and SOC contents in different aggregate size fractions. Soil organic C content in macroaggregates was greater than in microaggregates. The response of SOC content to FYM application was dependent upon inorganic fertilization and more upon balanced application of NPK than N only. Steady state infiltration rate under NPK + FYM (1.98 cm h⁻¹) was higher than under unfertilized (0.72 cm h⁻¹) and NPK (1.2 cm h⁻¹). Soil water sorptivity (calculated from Philip's equation) under NPK + FYM (1.06 cm min^−0.5) was higher than under NPK (0.61 cm min^−0.5). We conclude that hill farmers in northern India should be encouraged to use FYM along with chemical fertilizers to increase SOC content and improve soil physical properties.     

Methane and nitrous oxide fluxes from a farmed Swedish Histosol

Fluxes of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) from histosolic soils (which account for approximately 10% of Swedish agricultural soils) supporting grassley and barley production in Sweden were measured over 3 years using static chambers. Emissions varied both over area and time. Methane was both produced and oxidized in the soil: fluxes were small, with an average emission of 0.12 g CH₄ m⁻² year⁻¹ at the grassley site and net uptake of −0.01 g CH₄ m⁻² year⁻¹ at the barley field. Methane emission was related to soil water, with more emission when wet. Nitrous oxide emissions varied, with peaks of emission after soil cultivation, ploughing and harrowing. On average, the grassley and barley field had emissions of 0.20 and 1.51 g N₂O m⁻² year⁻¹, respectively. We found no correlation between N₂O and soil factors, but the greatest N₂O emission was associated with the driest areas, with < 60% average water-filled pore space. We suggest that the best management option to mitigate emissions is to keep the soil moderately wet with permanent grass production, which restricts N₂O emissions whilst minimizing those of CH₄.     

Desalination of sea water by akadama soil and akadama soil-inorganic adsorbents mixtures

(pp. 25–32)
The effectiveness of drip fertigation, which is known to control fertilizer application, for reducing nitrate in spinach and for improving the other qualities of spinach was investigated. Fertilizer application can be controlled effectively by drip fertigation. In 2002 and 2003, two spinach cultivars were grown in a plastic greenhouse with 4, 8 or 12 g N m⁻² of fertilizer application by drip fertigation, and with 8, 12 or 16 g N m⁻² of fertilizer application as a basal application. Nitrate concentration of petiole sap extracted by a garlic squeezer was significantly correlated with the water-extractable nitrate concentration. Nitrate concentrations of petiole sap extracted from plants treated with 12, 8 and 4 g N m⁻² of fertilizer by drip fertigation were constant, gradually decreased and significantly decreased, respectively, during the last 2 weeks. When nitrate concentration decreased during the last week, nitrate concentration in spinach at harvest was less than 3,000 mg kg⁻¹ FW. Thus it was thought that the pattern of the time course of nitrate in petiole xylem sap is a good indicator for getting spinach with low nitrate. The sugar concentration was negatively correlated with applied nitrogen quantity and the nitrate concentration. The total oxalic acid concentration in spinach treated by drip fertigation was significantly lower than that in spinach treated by basal application, independent of the amount of applied nitrogen. Thus drip fertigation is advantageous for improving crop quality.     

Geologic Nitrogen as a Source of Soil Acidity

The origin of highly acidic (pH<4.5) barren soils in the Klamath Mountains of northern California was examined. Soil parent material was mica schist that contained an average of 2,700 mg N kg⁻¹, which corresponds to 7.1 Mg N ha⁻¹ contained in a 10-cm thickness of bedrock. In situ soil solutions were dominated by H⁺, labile-monomeric Al³⁺ and NO₃⁻, indicating that the barren area soils were nitrogen saturated—more mineral nitrogen available than required by biota. Leaching of excess NO₃⁻ has resulted in removal of nutrient cations and soil acidification. Nitrogen release rates from organic matter free soil ranged from 0.0163 to 0.0321 mg N kg⁻¹ d⁻¹. Nitrogen release rate from fresh ground rock was 0.0465 mg N kg⁻¹ d⁻¹. This study demonstrates that geologic nitrogen may represent a large and reactive nitrogen pool that can contribute significantly to soil acidification.     

Depth distribution of soil organic C and N after long-term soybean cropping in Texas

Crop management practices have potential to enhance subsoil C and N sequestration in the southern U.S., but effects may vary with tillage regime and cropping sequence. The objective of this study was to determine the impacts of tillage and soybean cropping sequence on the depth distribution of soil organic C (SOC), dissolved organic C (DOC), and total N after 20 years of treatment imposition for a silty clay loam soil in central Texas. A continuous soybean monoculture, a wheat–soybean doublecrop, and a sorghum–wheat–soybean rotation were established under both conventional (CT) and no tillage (NT). Soil was sampled after soybean harvest and sectioned into 0–5, 5–15, 15–30, 30–55, 55–80, and 80–105 cm depth intervals. Both tillage and cropping intensity influenced C and N dynamics in surface and subsurface soils. No tillage increased SOC, DOC, and total N compared to CT to a 30 cm depth for continuous soybean, but to 55 cm depths for the more intensive sorghum–wheat–soybean rotation and wheat–soybean doublecrop. Averaged from 0 to 105 cm, NT increased SOC, DOC, and total N by 32, 22, and 34%, respectively, compared to CT. Intensive cropping increased SOC and total N at depths to 55 cm compared to continuous soybean, regardless of tillage regime. Continuous soybean had significantly lower SOC (5.3 g kg⁻¹) than sorghum–wheat–soybean (6.4 g kg⁻¹) and wheat–soybean (6.1 g kg⁻¹), and 19% lower total N than other cropping sequences. Dissolved organic C was also significantly higher for sorghum–wheat–soybean (139 mg C kg⁻¹) than wheat–soybean (92 mg C kg⁻¹) and continuous soybean (100 mg C kg⁻¹). The depth distribution of SOC, DOC, and total N indicated treatment effects below the maximum tillage depth (25 cm), suggesting that roots, or translocation of dissolved organic matter from surface soils, contributed to higher soil organic matter levels under NT than CT in subsurface soils. High-intensity cropping sequences, coupled with NT, resulted in the highest soil organic matter levels, demonstrating potential for C and N sequestration for subsurface soils in the southern U.S.     

Stock, turnover time and accumulation of organic matter in bulk and density fractions of a Podzol soil

Temperate forest soils store large amounts of organic matter and are considered as net sinks for atmospheric carbon dioxide. Information about the sink strength and the turnover time of soil organic carbon (SOC) is required to assess the potential response of soils to climate change. Here we report on stocks, turnover times (TT) and accumulation of SOC in bulk soil and density fractions from genetic horizons of a Podzol in the Fichtelgebirge, Germany. Stocks of SOC, total nitrogen and exchangeable cations determined in nine quantitative soil pits strongly varied with stone content and thickness of horizons in both the organic layer and the mineral soil. On the basis of radiocarbon signatures, mean turnover times of 4, 9 and 133 years, respectively, were calculated for Oi, Oe and Oa horizons from three soil pits, using a non-steady-state model. The Oa horizons accumulated 4–8 g C m⁻² year⁻¹ whereas the Oi and Oe horizons were close to steady-state during the past decade. Free particulate organic matter (FPOM) was the most abundant fraction in the Oa and EA horizons with TT of 70–480 years. In the B horizons, mineral associated organic matter (MAOM) dominated with over 40% of total SOC and had TT of 390–2170 years. In contrast to other horizons, MAOM in the Bsh and Bs horizon had generally faster TT than occluded particulate organic matter (OPOM), possibly because of sorption of dissolved organic carbon by iron and aluminium oxides/hydroxides. Our results suggest that organic horizons with relatively short turnover times could be particularly vulnerable to changes in climate or other disturbances.     

Retention of copper by a calcareous soil and its textural fractions: influence of amendment with two agroindustrial residues

Metal availability in soils is strongly related with sorption processes and the possible association of the metal ions with a particular particle-size fraction. Therefore, studies of metal retention by a soil will be aided if retention by different size fractions is also studied. Sorption of copper on a calcareous soil and its textural fractions was studied in batch assays. The soil was amended over 3 years with two agroindustrial residues, a composted olive mill sludge and vinasse. Sorption of Cu on the calcareous soil was very large (110 mmol kg⁻¹) and was enhanced by both amendments. Metal retention by the clay fraction of the unamended soil was less than that of the whole soil, but increased dramatically after amendment with olive mill sludge. This was caused by the larger calcite content in this fraction as well as the increase in organic matter content. The amount of Cu sorbed was very large in the silt fraction, again because of the carbonate content of this fraction (300–460 g kg⁻¹). Copper sorption decreased dramatically after removal of carbonate. Copper retention tended to be enhanced by organic amendments. This was particularly evident in the silt fraction, as a consequence of the organic matter accumulation in this fraction.
Copper sorption on the calcareous soil and its silt fractions (unamended and amended) was irreversible. By contrast, desorption was measurable from all the carbonate-free samples (both whole soil and textural fractions), although in all cases a large hysteresis was observed. We conclude that carbonate was the main component responsible for the lack of reversibility.     

Effects of tillage and nitrogen management on soil chemical and physical properties after 23 years of continuous sorghum

Long-term tillage and nitrogen (N) management practices can have a profound impact on soil properties and nutrient availability. A great deal of research evaluating tillage and N applications on soil chemical properties has been conducted with continuous corn (Zea Mays L.) throughout the Midwest, but not on continuous grain sorghum (Sorghum bicolor (L.) Moench). The objective of this experiment was to examine the long-term effects of tillage and nitrogen applications on soil physical and chemical properties at different depths after 23 years of continuous sorghum under no-till (NT) and conventional till (CT) (fall chisel-field cultivation prior to planting) systems. Ammonium nitrate (AN), urea, and a slow release form of urea were surface broadcast at rates of 34, 67, and 135 kg N ha⁻¹. Soil samples were taken to a depth of 15 cm and separated into 2.5 cm increments. As a result of lime applied to the soil surface, soil pH in the NT and CT plots decreased with depth, ranging from 6.9 to 5.7 in the NT plots and from 6.5 to 5.9 in the CT plots. Bray-1 extractable P and NH₄OAc extractable K was 20 and 49 mg kg⁻¹ higher, respectively, in the surface 2.5 cm of NT compared to CT. Extractable Ca was not greatly influenced by tillage but extractable Mg was higher for CT compared to NT below 2.5 cm. Organic carbon (OC) under NT was significantly higher in the surface 7.5 cm of soil compared to CT. Averaged across N rates, NT had 2.7 Mg ha⁻¹ more C than CT in the surface 7.5 cm of soil. Bulk density (Δ_b) of the CT was lower at 1.07 g cm⁻³ while Δ_b of NT plots was 1.13 g cm⁻³. This study demonstrated the effect tillage has on the distribution and concentration of certain chemical soil properties.     

The status of atmospheric concentrations of ammonia in an intensive dairy farming area in central Japan

(pp. 41–46)
Silicon availability in 36 commercial nursery bed soils was evaluated by four methods the phosphate buffer (pH 6.2, 40 mmol L⁻¹), incubation, supernatant and acetate buffer (pH 4.0, 1 mol L⁻¹) Methods. The influence of silicon availability in the nursery bed soils on the silicon uptake of rice Oryza sativa L. cv. Hitomebore seedlings and the effect of silicon fertilizer application were examined in a glass house in 2002.
The results revealed that the best correlation between silicon content in rice seedlings and available silicon in soils was obtained with the phosphate buffer-solution method ( r  = 0.86). More precise evaluation of available silicon was achieved by grouping soils based on these phosphate absorption coefficients (PAC). The correlation coefficients between silicon content in rice seedlings and available silicon in soils were 0.92 and 0.72 for volcanic soils (PAC > 1500) and non volcanic soils (PAC < 1500), respectively.
We concluded that the phosphate buffer method is the most easily adjusted method for estimation of silicon availability in nursery bed soils, and silicon fertilizers should be applied when silicon availability in non-volcanic nursery bed soils goes below 200 mg kg⁻¹, whereas the level is less than 350 mg kg⁻¹ in volcanic soils.     

Inhibition of nitrate accumulation in tropical grassland soils: effect of nitrogen fertilization and soil disturbance

In acid soils in the Eastern Plains of Colombia, forage grasses planted on land prepared before the previous dry season produced 40–50% more dry matter than when land was prepared immediately before planting. Virtually no NO₃⁻ accumulated in surface (0–10 cm) soil from three native undisturbed savanna sites. Where land was ploughed before the dry season, NO₃⁻ levels increased gradually after a 2–3 month lag, and dropped at the beginning of the rains. In samples incubated for 4 weeks, more NO₃⁻ accumulated in the wet than the dry season. A similar 2–3-month lag occurred when land was ploughed after the dry season. NH₄⁺ levels were higher in ploughed than savanna soils, and rose in all soils at the beginning of the rains. More NO₃⁻ and NH₄⁺ accumulated on incubation in pots than in soil cores. Forage grasses inhibited NO₃⁻ accumulation in the soil, relative to plant-free plots, and legumes stimulated it. N fertilization overcame this inhibition except in the case of Brachiaria humidicola .     

Loss of availability of phosphate in New Zealand soils

Phosphate sorption was measured by the method of Barrow (1980) using a laboratory incubation procedure for up to 60 d on four soils which had different mineralogies but medium to high phosphate retention. All the soils had slow reactions where phosphate sorption continued, but at a decreasing rate, with time. The rate of decrease in the slow reactions was similar on all the soils. Phosphate became less available to plants during the slow reactions, and results of a pot trial with white clover showed that, on all the soils, phosphate incubated with the soils for 218 d was about 65% as effective as phosphate incubated for 10d.
When 700 mg P kg⁻¹ was added to allophanic soils (Andisols), about 100 mg kg⁻¹ was strongly adsorbed, about 200 mg kg⁻¹ became unavailable in about 200 days and the remainder was weakly adsorbed. A similar result was obtained on Waiarikiki soil (Inceptisol), which contained ferrihydrite and Al-humus as the predominant reactive species. On the Kerikeri soil (Oxisol) about 150 mg P kg⁻¹ became unavailable with time as a result of reactions with geothite, hematite and Al-humus.
The phosphate uptake by the microbial biomass was similar to the uptake by the clover, and immobilization of phosphate in the biomass can contribute to the loss of availability of phosphate in soils.     

Practical phytoextraction in cadmium-polluted paddy fields using a high cadmium accumulating rice plant cultured by early drainage of irrigation water

Some indica rice varieties are potential phytoextractors for paddy fields polluted with Cd because of their high biomass and because they can accumulate Cd to moderate levels in their shoots. To establish a practical phytoextraction system, phytoextraction using two indica rice cultivars (MORETSU and IR-8) was carried out in a paddy field polluted with moderate Cd levels (2.91 and 2.52 mg kg⁻¹, respectively). The Cd concentration and Cd uptake of MORETSU increased when irrigation water was drained at the maximum tillering stage, and the paddy soil was under oxidative conditions until harvesting. The Cd uptake of MORETSU and IR-8 increased and reached 516 and 657 g ha⁻¹, respectively, at the beginning of October. After phytoextraction using these high Cd accumulating rice varieties for 2 years, the Cd concentration in the paddy field decreased by 18% compared with the initial Cd concentration. The Cd concentration in the rice grains of a japonica ordinary rice variety (HINOHIKARI) subsequently grown on the field after the phytoextraction was lower than the concentration in rice grown on a non-phytoextracted field. These results suggest that phytoextraction using high Cd accumulating rice varieties with early drainage of irrigation water is a practical remediation system for moderate Cd polluted paddy fields in southwest Japan.     

Distribution coefficients of Cd, Co, Ni, and Zn in soils

Batch adsorption experiments were conducted with a mixture of solutes at low equilibrium concentrations of Cd (0.7-12.6 μg1⁻¹), Co (18-118μg1⁻¹, Ni (22-330 μg 1⁻¹), and Zn (40-1480 μg1⁻¹) in 38 different soils. Statistical correlations indicated that metal sorption onto the soils was influenced by the presence of clays and hydrous oxides of Fe and Mn. Based on calculated distribution coefficients for these metals, Co will generally exhibit the highest mobility in soils, but the mobility of Zn will increase faster with decreasing pH. Two types of empirical relationships are developed from these data to estimate values for the distribution coefficients.     

Effect of liming and phosphate additions on sulphate leaching in soils

The effect of lime (CaCO₃) and phosphate additions on surface charge characteristics and their effect on the leaching of sulphate were examined for two soils (Patua loam and Tokomaru silt loam) which differed in their adsorption capacities for sulphate.
Incubation of soils with either CaCO₃ (0–600 mmol kg⁻¹) or phosphate (0-208 mmol kg⁻¹) resulted in a two- to five-fold increase in the net negative charge and a similar decrease in the adsorption of sulphate. The effect of either lime or phosphate addition on both the surface charge and sulphate adsorption was more pronounced for the allophanic Patua soil than for the Tokomaru soil containing mainly vermiculite.
In a column experiment, liming induced the leaching of sulphur either by the desorp-tion of adsorbed sulphate or by the mineralization of organic sulphur. During a miscible displacement study, addition of either CaCO₃ or phosphate resulted in an early breakthrough of sulphate in the leachate. In a pulse experiment, in which soils were incubated with sulphate (3.12 mmol kg⁻¹) for 1 week and subsequently leached with water, more added sulphate was lost in the leachate of the soils previously incubated with either CaCO₃ or phosphate.