Aluminum status of synthetic Al–humic substance complexes and their influence on plant root growth

Abstract

Aluminum (Al)–humus complexes are abundant in the A horizons of non-allophanic Andosols and contribute to the unique properties of volcanic ash soils, such as high reactivity with phosphate ions and a low bulk density. Natural non-allophanic Andosols commonly show Al toxicity to plant roots. There have been very few studies examining the contribution of Al–humus complexes to the Al toxicity of plant roots, although the complexes are the probable source of the toxic Al. We extracted humic substances from the A horizon of a non-allophanic Andosol using NaOH solution and reacted the humic substances and partially neutralized the AlCl₃ solution at three pH conditions (pH 4.0, 4.5 and 5.5) to prepare pure Al–humic substance complexes. The Al solubility study (equilibrium study in 10^?2 mol L^?1 CaCl₂) and the Al release study (a stirred-flow method using 10^?3 mol L^?1 acetate buffer solution adjusted to pH 3.5) indicated that all the synthetic complexes easily and rapidly release monomeric Al into the liquid phase with slight changes in pH and ion strength, although the Al contents and their extent of polymerization are considerably different among the complexes. A plant growth test was conducted using a medium containing the Al–humic substance complexes and perlite mixture. Root growth in burdock (Arctium lappa) and barley (Hordeum vulgare L.) was reduced equally by all three complex media, and the roots showed the typical injury symptoms of Al toxicity. These results indicate that in soils dominated by Al–humus complexes the Al released from the Al–humus complexes, as well as the exchangeable Al adsorbed by soil minerals, is definitely toxic to plant roots.     

Aluminum solubility of strongly acidified allophanic Andosols from Kagoshima Prefecture,southern Japan

Abstract

The aluminum solubility of acidified soils both from furrows and under tree canopies of a tea garden was studied using equilibrium experiments in 0.01 mol L^?1 CaCl₂ solution systems. The soils were originally classified as allophanic Andosols. The furrow soils were more severely acidified because of the heavy application of nitrogen fertilizer, especially in the upper soil horizons (pH[H₂O] of 3.6–3.8 in the A1 and 2A2 horizons). These acidified soils were characterized by the dissolution of allophanic materials (allophane, imogolite and allophane-like materials) and by an increase in Al–humus complexes. Ion activity product (IAP) values of the strongly acidified soil horizons were largely undersaturated with respect to imogolite (allophanic clay) or gibbsite. Plots of p(Al³⁺) as a function of pH strongly indicated that Al solubility of the soils was largely controlled by Al–humus complexes, especially in the A1 horizon. In the canopy soils, which were more weakly acidified (pH[H₂O] 4.9–5.0), Al solubility was close to that of gibbsite and allophanic materials, indicating that the solubility is partly controlled by these minerals.     

The impact of long-term cultivation and management history on the status and dynamics of cobalt in a savanna Alfisol in Nigeria

The status of cobalt (Co) in savanna soils of Nigeria is largely unknown, and a long-term experiment including inorganic fertilizer (NPK) and farmyard manure (FYM) and uncultivated land provided information on the way management affected the dynamics of Co in the soil. Total Co increased with increasing depth, whereas readily extractable Co decreased. The mean concentration of Co (5.6–7.9 mg kg⁻¹) was close to the mean value of 8 mg kg⁻¹ reported for soils worldwide, whereas the concentration of extractable Co was less than that reported in most soils. Regression analysis indicated that total Fe predicted up to 78% of the soil Co. The potentially available Co correlated strongly with pedogenic or reducible Mn oxides extracted with dithionite–citrate–bicarbonate. Mass balance calculations showed that fertilization with either NPK or FYM caused losses of between 0.8 and 1.1 g Co m⁻² after 50 years of cultivation against the uncultivated site as a reference. However, Co increased by 1.8 g m⁻² in the soil receiving FYM + NPK, suggesting that the Co of the soil was best maintained under this management probably because of incidental additions of Co in the manures. Furthermore, the positive Co balance in the FYM + NPK plot was partly enhanced by its larger contents of clay, Fe and pedogenic Mn oxides than in either the FYM or NPK plots. Clay, Fe and pedogenic oxides served as Co sinks in this particular savanna soil.     

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.     

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.     

Sequential extraction methods for soil organic nitrogen

(pp. 825–831)
This study was carried out to clarify the effects of soil nitrate before cultivation and amounts of basal-dressed nitrogen on additional N application rate and yields of semi-forced tomato for three years from 1998 to 2000. The amounts and timing of additional N dressing were determined based on diagnosis of petiole sap nitrate. The top-dressing was carried out with a liquid fertilizer when the nitrate concentration of a leaflet's petiole sap of leaf beneath fruit which is 2–4 cm declined below 2000 mg L⁻¹.
For standard yield by the method of fertilizer application based on this condition, no basal-dressed nitrogen was required when soil nitrate before cultivation was 150 mg kg⁻¹ dry soil or higher in the 0–30 cm layer; 38 kg ha⁻¹ of basal-dressed nitrogen, which corresponds to 25% of the standard rate of fertilizer application of Chiba Prefecture, was optimum when soil nitrate before cultivation was 100150 mg kg⁻¹ dry soil; 75 kg ha⁻¹ of basal-dressed nitrogen, which corresponds to 50% of the standard, was optimum when soil nitrate before cultivation was under 100 mg kg⁻¹ dry soil. A standard yield was secured and the rate of nitrogen fertilizer application decreased by 49–76% of the standard by keeping the nitrate concentration of tomato petiole sap between 1000–2000 mg L⁻¹ from early harvest time to topping time under these conditions.     

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).     

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.     

Effect of supplied phytosiderophore on 59Fe absorption and translocation in Fe-deficient barley grown hydroponically in low phosphorus media

Hydroponically grown barley plants ( Hordeum vulgare L. cv. Minorimugi) under iron-deficient (–Fe) and high phosphorus (P) conditions (500 µmol L⁻¹) showed Fe chlorosis and lower growth compared with plants grown in –Fe and low P conditions (50, 5 and 0.5 µmol L⁻¹). To understand the physiological role of P in regulating the growth of plants in –Fe medium, we carried out an Fe feeding experiment using four P levels (500, 50, 5 and 0.5 µmol L⁻¹) and phytosiderophores (PS), mugineic acid. Our results suggest that plants grown in a high P medium had higher absorption activity of ⁵⁹Fe compared with plants grown in low P media, irrespective of the presence or absence of added PS. Translocation of ⁵⁹Fe from roots to shoots was not affected by the P level. The relative translocation rate of ⁵⁹Fe increased with decreasing levels of P in the medium. In general, the addition of PS enhanced the absorption of ⁵⁹Fe and its translocation. Taken together these results suggest that the lower relative translocation rate of Fe in high P plants may be induced by the physiological inactivation of Fe in the roots, and the higher absorption activity of Fe in high P conditions possibly results from the response of barley plants to Fe deficiency.     

Organic carbon stocks and soil erodibility in Canary Islands Andosols

Soil organic carbon (SOC) plays a key role in the structural stability of soils and in their resistance against erosion. However, and as far as andic soils are concerned, these mechanisms and processes, as well as the influence of the different types of SOC on aggregate stability, are not fully understood. The targets of this paper are: (i) to determine the content and forms of SOC in Andosols under evergreen forest vegetation [laurel (Laurus) and heather (Erica) forest] and (ii) to find out the role of soil organic matter (SOM) in the aggregate stability and in the resistance of Andosols to water erosion. Soil samples have been collected in 80 sites in a 40 km² area under udic soil moisture regime. In them, fulvic and humic acids, Walkley–Black SOC, pyrophosphate-extractable SOC, Fe and Al, potassium sulphate extractable SOC, dissolved SOC, acid oxalate-extractable Fe, Al and Si, USLE K-factor and aggregate stability have been determined. The Andosols over volcanic ash are Aluandic Andosols (non-allophanic Andosols), whereas over basaltic lava flows are Silandic Andosols (allophanic Andosols). The surface (0–30 cm) samples analyzed contain 9.5–30 kg C m^− 2 being significantly higher in allophanic Andosols (p < 0.5). Organic carbon adsorbed onto the mineral fraction (extractable pyrophosphate, C_p) accounts for 35–55% of the total SOC. All samples show a high stability to slaking and raindrop impact, being the first one highly correlated (r = 0.6) with pyrophosphate extractable C (C_p), Fe (Fe_p), and Al (Al_p) in allophanic Andosols, unlike non-allophanic ones. The stability to raindrop impact correlates with pyrophosphate extractable C (C_p) and Fe (Fe_p) in both types of soils (r = 0.3–0.6, p < 0.05). These findings suggest that the high stability to both slaking and water-drop impact is due to the occurrence of allophane–Fe–OC complexes, rather than to the total OC, and the active Fe and Al forms, generated by the weathering of volcanic materials, constitute an essential constituent responsible for C sequestration and resistance to degradation in these soils.     

Horizontal distribution of main hydroxyanthraquinones in soil

Six hydroxyanthraquinones (chrysophanol, chrysotalunin, microcarpin, physcion, 7,7′-biphyscion, and hinakurin) present in the samples of 26 surface soils were quantitatively analyzed, and the contents of HAQs in soil types were compared. The soil samples had been collected from 19 Umbric Andosols and seven Distric Cambisols, and the Andosols were further subdivided into those with allophanic soil materials (exchange acidity (y ₁<5 mL 100 g^-l) and nonallophanic soil materials (y _l≧5 mL 100 g^-l). The following results were obtained. (1) It was determined quantitatively for the first time that chrysotalunin was the major hydroxyanthraquinone in many soils. (2) The amounts of major dime ric hydroxyanthraquinones (chrysotalunin, 7,7′-biphyscion, and microcarpin) in non-allophanic soil materials were significantly larger than those in allophanic soil materials. As the contents of chrysotalunin in Andosols were positively correlated with y _l, which was reported to be positively correlated with aluminum toxicity and exchangeable aluminum in soil, it is suggested that toxic aluminum may be involved in the production of soil hydroxyanthraquinones.     

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.     

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.     

Metal-humus complexes in A horizons of Thai and Korean red and yellow soils

Carbon, Al and Fe (C_pyr, Al_pyr and Fe_pyr) were extracted with 0.1 m Na₄P₂O₇ from 26 A horizon samples of tropical Thai and temperate Korean soils (Ultisols, Alfisols, Oxisols and Inceptisols). The soils, except for one Thai Inceptisol, had similar total C (0.35–3.29%) and C_pyr/total C ratios (0.20–0.41). There were approximately linear relationships between total C or C_pyr and clay content; two groups of soils gave different linear relationships. A curvilinear relationship between C_pyr and (Al + Fe)_pyr (milli-atom kg⁻¹) that can be approximated by an equation: C_pyr= 53 (Al_pyr+ Fe_pyr)^1/2– 24 was also found for most Thai and Korean soils. The above relationships indicated that total C and C_pyr would be close to zero at zero clay or zero (Al + Fe)_pyr. It was inferred that clay-humus interaction has a primary importance in the determination of humus content in red and yellow soils in tropical and temperate regions and that the main role of clay is to supply Al and Fe that complex and stabilize humus against microbial degradation.     

Measurement of ammonia volatilization loss using a dynamic chamber technique: A case study of surface-incorporated manure and ammonium sulfate in an upland field of light-colored Andosol

The present study aimed to elucidate ammonia (NH₃) volatilization loss following surface incorporation (0–15 cm mixing depth) of nitrogen (N) fertilizer in an upland field of light-colored Andosol in central Japan. A dynamic chamber technique was used to measure the NH₃ effluxes. Poultry manure, pelleted poultry manure, cattle manure, pelleted cattle manure and ammonium sulfate were used as N fertilizers for basal fertilization to a bare soil with surface incorporation. All three experiments in summer and autumn 2007 and in summer 2008 showed negligible NH₃ volatilization losses following the application of all N fertilizers with the same application rate of 120 kg N ha⁻¹ as total N; these negligible losses were primarily ascribed to chemical properties of the soil, that is, its high cation exchange capacity (283 mmol_c kg⁻¹ dry soil) and relatively low pH(H₂O) (5.9). In addition, the surface incorporation, the very small ratio of ammoniacal N to total N for the manure, and the decrease in soil pH to ≤5.5 following applications of ammonium sulfate were also advantageous to the inhibition of NH₃ volatilization loss from the field-applied N fertilizers.     

Relationships between the High Aluminum Concentration and Other Components in Soil Solution of Acidic Soil in Kumagaya, Central Japan

A plot with a high aluminum (Al) concentration in soil solution was found in Kumagaya, central Japan; the maximum was 4.0 mg L⁻¹ as total dissolved Al (TD-Al) at a depth of 10 cm in August 2000. The soil type was Dystric Andosols with three horizons, A/Bw1/Bw2, which contained a considerable quantity of Al extracted by dithionite-citrate and acid-oxalate extractants. The upper two horizons were acidic with soil pH (H₂O) of 4.4 and 4.6. The fundamental cause of the high TD-Al concentration was the low pH with a very low base saturation of less than 2%. The seasonal change in TD-Al concentrations in soil solution at a depth of 10 cm was significantly and positively correlated with the nitrate concentrations but weakly and rather negatively correlated with the sulfate concentrations. Nitrification functioned as the direct acid source to cause the TD-Al concentrations to fluctuate strongly, whereas sulfate adsorption onto the soil at the research plot functioned as a sink of proton.     

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.     

Increased predicted losses of phosphorus to surface waters from soils with high Olsen-P concentrations

Abstract. Diffuse soluble reactive P (SRP) & total P (TP) loads from over 50 major river catchments in Northern Ireland were predicted using an export coefficient modelling approach. Phosphorus export coefficients for each CORINE land cover class, derived from satellite imagery, allowed the prediction of P loads from a breakdown of the CORINE land cover classes by catchment using a GIS. This approach was validated using observed P loads calculated from flow and concentration data. Mean measured Olsen-P concentrations in the soil A-horizon were also determined on a catchment basis. Plots of P loads to the watercourse versus Olsen-P concentrations in the soil showed a breakpoint around 22 mg Olsen- P l⁻¹ for both SRP & TP data. Below Olsen-P concentrations of 22 mg l⁻¹, SRP & TP losses were essentially independent of Olsen-P at 0.28 and 0.63 kg P ha⁻¹ yr⁻¹, respectively. Above Olsen-P concentrations of 22 mg l⁻¹, there was considerable spread in the P loss data. Nevertheless, significant upward trends in SRP and TP losses to watercourses were detected with increasing Olsen-P at a rate of approximately 0.5 and 1.0 kg P ha⁻¹ yr⁻¹, for SRP and TP respectively, for each 10 mg l⁻¹ increase in Olsen-P.     

Phosphate absorption of intact komatsuna plants as influenced by phosphite

Phosphite (     ; Pi) uptake in cell suspension culture, information on how Phi affects the Pi uptake of intact plants remains to be determined. The present study was conducted to investigate the effect of Phi on Pi absorption of intact komatsuna plants ( Brassica rapa var. peruviridis cv. Ajisai) in hydroponic culture. Phosphite markedly decreased Pi absorption of the intact komatsuna plants under both low (0.05 mmol L⁻¹ ) and high (0.5 mmol L⁻¹) Pi supply, although the growth (both shoots and roots) and water uptake of the high Pi-supplied plants was not affected by Phi. The inhibiting effect of Phi was small at 0.2 mmol L⁻¹, but became large at 2 mmol L⁻¹. Using relatively large seedlings (28 days old) to better assess the influence of Phi on Pi absorption early in the treatment, the results indicated that there was an immediate decrease in Pi absorption within the first 2-day period of Phi treatment when the water absorption of the plants was not affected. Taken together, the results suggested that there was a strong inhibiting effect of Phi on Pi uptake of intact komatsuna plants and this effect is exerted most likely by competition between Phi and Pi at uptake level. We speculate that the application of Phi to plant roots in an environment that is unfavorable for Phi-to-Pi conversion (e.g. hydroponic culture) may need to increase the amount of required Pi fertilization of plants to compensate for the reduction in Pi uptake by Phi. Further research is needed to confirm our results.     

Increasing nitrate removal at low temperatures by incorporating organic matter into paddy fields

Denitrification of paddy fields is a key process for improving water quality in fields where nitrate concentrations are high. The objective of the present study was to understand the effects of incorporating organic carbon (C) into soil on the denitrification rate of paddy fields in winter. On 11 December 2007, separate paddy field plots were prepared by incorporating 5 Mg ha⁻¹ of rice straw (RS), 11 Mg ha⁻¹ of rice straw compost (RSC) or a control. A field with a high concentration of nitrate in the water (averaging 18 mg N L⁻¹) was irrigated until 29 March. During the experiment, the daily average soil temperature at a depth of 0.05 m ranged between 3 and 15°C. The nitrate concentration in the surface water in the RS plot, where the residence time was 2 days, decreased more than the concentration in the control or RSC plots. The total estimated nitrate removal from each plot in relation to the other plots was RS > RSC = control. Measurements of the soil from each plot on 29 February 2008 showed that incorporation of RS significantly increased the denitrification potential, even at low temperatures (5–10°C). Furthermore, the RS plot contained more dissolved organic C than the control or RSC plots. This result indicates that supplying RS effectively increases denitrification under low-temperature conditions.