Naoki Moritsuka

Effect of plant growth on the distribution and forms of soil nutrients in the rhizosphere

Abstract The effect of plant growth on the distribution and forms of soil nutrients in the rhizosphere was studied by growing plants in rhizoboxes. An experiment was carried out in a growth chamber for 17 d with three treatments, i.e. maize (Zea mays L.), kidney bean (Phaseolus vulgaris L.), and no plants. After the experiment, soil samples were collected up to about 50 mm from the central compartment (C. C.) in which roots were allowed to grow, and analyzed for the soluble, exchangeable, and total amounts of nutrients. Plant samples were also analyzed for their nutrient contents. The pattern of nutrient distribution in the rhizosphere varied among the types and forms of elements. In both maize and kidney bean, the concentrations of soluble P, K, and N decreased significantly at the C.C., up to 4 mm and up to 10 mm from the C.C., respectively. The magnitude of their depletion reflected their rate of diffusion in soil. The concentration of exchangeable K decreased significantly up to 2 mm from theC.C., whereas those of exchangeable Ca and Mg increased within 1 mm from the C.C. The total amount of nutrients, however, did not change significantly for each element. The depletion of soluble P and K mainly around the C.C. indicated the presence of their diffusion toward the root surface. Their uptake was larger than the apparent decrease of their soluble forms, suggesting that their depletion was compensated by intensive replenishment from the soil solid phase. The concentrations of N, Ca, and Mg, on the other hand, markedly decreased up to the end of the rhizobox, indicating the presence of mass flow. The depletion was proportional to the rate of transpiration by plants.

Effects of the application of heated sewage sludge on soil nutrient supply to plants

Abstract We previously reported that heating sewage sludge significantly changes the rate of N mineralization. The present study was undertaken to examine the extent to which these changes affect plant growth and nutrient supply after application to different soils. A pot experiment in which komatsuna plants (Brassica campestris L. var. rapa) were grown in a Fluvisol or an Arenosol indicated that the amount of N taken up by the plants increased significantly by heating air-dried sludge at 120°C and decreased significantly by heating at 180°C. Heat-drying of the sludge at 120°C or 180°C also increased the N uptake significantly. These plant responses could be explained by the heat-induced changes in the release of inorganic N from the sludge. In contrast to N, the sludge materials containing Fe as a coagulant immobilized soil soluble P. When the sludge was applied to an Andosol, the N-supplying effects on plant growth were offset by P adsorption onto the sludge and soil. In a successive pot experiment conducted without additional sludge application, it was further suggested that frequent sludge application is required to maintain plant growth. However, it will also lead to the accumulation of sludge components in the soil because less than 40% and 15% of the sludge N and P, respectively, were apparently recovered in two harvests of the plants. In conclusion, heated sludge materials can act as an effective organic N fertilizer provided that they are applied to a suitable soil and that the short-term effects on soil productivity are balanced with the long-term effects on environmental quality.

Effect of heating treatments on nitrogen mineralization from sewage sludge

Abstract Heating of sewage sludge has multifunctional benefits for agricultural recycling of sludge, one of which is the heat-induced changes in N mineralization from the sludge. To enhance the understanding and practical use of this phenomenon, we subjected sewage sludge to heating treatments at 120°C or 180°C for 16 h with or without air-drying as a pretreatment. During the 84-day aerobic incubation period, N mineralization from the sludge mixed with samples of an Andosol, a Fluvisol, an Arenosol and an Acrisol was significantly accelerated by the heating of air-dried sludge at 120°C and was significantly retarded by heating at 180°C, regardless of the soil types and temperatures during the incubation period. More conventional heat-drying of moist sludge at 120°C or 180°C also exerted similar but less pronounced effects. These heat-induced changes were attributed to the transformation of sludge organic N, because volatilization of N during the heating treatments was negligible. Sequential extraction of sludge N enabled detection of the heat-induced N transformations accounting for some but not all of the incubation results, indicating that mineralization of N in the heated sludge materials was determined not only by chemical extractability but also by other factors. Our results suggest that heating of sewage sludge regulates the rate of N mineralization and presents a promising method for producing various organic N fertilizers from sewage sludge.

Changes in Soil Physicochemical Properties Following Land Use Change from Paddy Fields to Greenhouse and Upland Fields in the Southeastern Basin of Dianchi Lake, Yunnan Province, China

Paddy fields in the southeastern basin of Dianchi Lake have rapidly changed to greenhouses since 1999. A total of 61 surface soil samples, including 43 greenhouse soils, 12 upland soils, and 6 paddy soils, were collected from a flat lowland area mainly used for agricultural production fields in the southeastern basin of Dianchi Lake. Analyses of the soil samples indicated that the greenhouse soils were characterized by a lower organic matter content, lower pH, and higher soluble nutrients than the paddy soils in the area. The lower organic matter content of the greenhouse soils was ascribed to environmental or management factors rather than the clay content of the soil. Accumulation of soluble nutrients, especially inorganic N, was due to over-application of fertilizers, which also caused soil acidification. The average amount of readily available N, P, and K accumulated in the greenhouse soils was estimated to be equal to or higher than the annual input of these nutrients as a fertilizer, indicating that a reduction in fertilizer application was possible and recommended. In contrast, a very low available Si content was observed in the paddy soils, suggesting the need for Si application for rice production.

Spatial relationships among different forms of soil nutrients in a paddy field

Abstract Within-field soil variability has often been reported in Japanese paddy fields. In one of such fields, we examined the relationships of spatial variations among different forms of soil nutrients and their direct influence on rice growth. Samples of surface soils collected from 100 plots (5 x 10 m) in the field were analyzed for the contents of N, P, and K as well as for the total C and clay contents. The values of the coefficient of variation (CV) for the soil properties were relatively high, ranging from 7% (total K) to 33% (inorganic N). More than 60% of the variation was spatially dependent, with ranges of less than 30 m for the contents of inorganic N and clay, 30-40 m for the contents of N (mineralizable and total), P (water-soluble, available, and total), and total C, and more than 60 m for the content of K (water-soluble, exchangeable, and nonexchangeable). These properties were grouped into two types; those significantly correlated either with the total C content (contents of mineralizable N, total N, all P forms, water-soluble K, exchangeable K, and 0.01 M HCl-extractable nonexchangeable K) or with the clay content (contents of fixed Nand NaBPh4-extractable nonexchangeable K). In a pot experiment in which rice plants were grown in soil from each plot for 50 d, the dry matter weight of shoots exhibited a large variation (CV 12%) and increased significantly with the increase of the level of the components related to the total C content, especially the content of mineralizable N. Significantly positive correlations were also found between the contents of N, P, and K in shoots and the corresponding soil nutrients occurring in a moderately labile form. These soil-plant relationships suggest that site-specific application of organic fertilizers for altering the level of the components related to the total C content in the topsoil could be effective to homogenize rice growth in the paddy field examined.

Non-destructive method for determining temporal and spatial changes of the soil solution chemistry in the rhizosphere

Abstract A non-destructive method for sampling the rhizosphere soil solution at a high spatial resolution is presented. A rhizobox, which enables to separate soils in the rhizosphere, was combined with soil solution samplers with a looped hollow fiber (LHF-sampler). In a rhizobox, LHF-samplers were installed at a distance of 0–2 mm and 8–10 mm from the root surface. A pot experiment was carried out for 17 d with two treatments, i.e. maize (Zea mays L.) and no plants. During the experiment, soil solutions were collected on day 5,9,13, and 17, and analyzed for the ionic concentrations. After the experiment, the soil samples were also collected at a distance of every 2 mm from the root surface up to 10 mm and analyzed for soluble ion concentrations in order to evaluate the degree of disturbance due to soil solution sampling. The method applied in this experiment enabled to sample the soil solution from the rhizosphere at a high spatial resolution with minimal disturbance. With plant growth, the concentrations of major ions except for SO4 2− in the soil solution collected at a distance of 0–2 mm from the root surface became significantly lower than those at 8–10 mm. This procedure led to the alteration of the ionic composition, i.e. the predominance of SO4 2− instead of NO3 −, only within a few millimeters from the root surface. These quantitative results suggested that plant roots absorb the soil solution in the rhizosphere where the level of NO3 − is relatively lower than that of the outer solution and that other anions such as SO4 2− play an important role in regulating the concentrations of cations and their supply to plants.

Dynamics of clay-fixed ammonium as a sink or source of exchangeable ammonium in a paddy soil

Clay-fixed ammonium ( ) plays an important role in the dynamics and availability of soil nitrogen (N), acting as a sink or source of exchangeable . However, quantitative information about exchange processes between exchangeable and fixed is limited. In this study, we developed a method to sequentially extract exchangeable and fixed , and evaluated whether fixed acts as a sink or source of exchangeable in a paddy soil. A paddy soil was subjected to anaerobic incubation for 70 days with and without the application of powdered rice (Oryza sativa L. cv. Hinohikari) straw as a pre-treatment, and with and without air-drying as a post-treatment. By the anaerobic incubation, the content of weakly fixed increased significantly with a significant increase in exchangeable , implying that exchangeable in the paddy soil is the main pool as a source of weakly fixed . On the other hand, a significant decrease in the exchangeable content by rice straw application before the incubation slightly lowered the content of weakly fixed . Air-drying after the incubation caused the fixation of exchangeable . Overall, the contents of exchangeable and weakly fixed were positively correlated (r = 0.58; p < 0.05), indicating that weakly fixed acted as a transitory pool between strongly fixed and exchangeable . The proposed sequential extraction method was effective to evaluate the interaction between exchangeable and fixed . Combining this method with an incubation experiment, it was demonstrated that fixed , especially weakly fixed , in the paddy soil can be released or retained in quantities sufficient to affect the dynamics of exchangeable .

Soil color analysis for statistically estimating total carbon, total nitrogen and active iron contents in Japanese agricultural soils

Abstract Soil color originates mainly from organic matter, iron mineralogy and moisture content. We aimed to find a suitable method to measure soil color sensitively and to evaluate the extent to which the color parameters can be useful for statistically estimating total carbon (C), total nitrogen (N) and active iron (Fe) contents in Japanese agricultural soils. A soil color reader (SPAD-503) was applied to two sample sets: (1) 100 surface soils collected throughout a 0.5-ha paddy field (field scale) and (2) 147 surface soils collected from agricultural fields in Japan (national scale). For analysis with this instrument, about 2 g of air-dried, finely-ground samples were packed firmly in a plastic cell, and their colors as they appeared on windows in both sides of the cell were measured. A CIE 1976 (L*, a*, b*) color space was used for color description. For the field-scale samples, the values of the coefficient of variation were around 15% for total C, total N and acid oxalate extractable iron (Feo). The L* value (lightness) was negatively correlated with the content of total C and total N (R2 = 0.18** and 0.26**, respectively), and the b* value (yellowness) was positively correlated with the Feo content (R2 = 0.59**). For the national-scale samples, the values of the coefficient of variation were around 60% for total C, total N and Feo. The L* value was negatively correlated with the content of total C and total N (R2 = 0.70** and 0.59**, respectively), but the b* value was not correlated with the content of Feo (R2 = 0.00). When the analysis was limited to 65 samples frequently used for paddy fields, the b* value was positively correlated with the Feo content (R2 = 0.52**). In conclusion, the proposed method enabled us to measure soil color sensitively with a small sample size. The L* and b* values obtained can be useful for rapid estimation of total C, total N and Feo contents in agricultural surface soils in Japan.

Evaluation of readily available nonexchangeable potassium in soil by sequential extractions with 0.01 molar hydrochloric acid

Abstract Nonexchangeable potassium (K-ne), i.e. 1 M NH4OAc-nonexchangeable K, often contributes significantly to plant nutrition. However conventional extraction methods often extract much more K-ne than plants even after intensive cropping, suggesting the difficulty in evaluating the amount of readily available soil K-ne. In this study, we used a milder extraction method (0.01 M HCl method) to examine its applicability to evaluate the amount of readily available K-ne in soil. In the first experiment, the concentration of K-ne in twenty surface soils sampled from agricultural fields in Japan and K-bearing minerals was determined by the 0.01 M HCl method, i.e. sequential extraction with 0.01 M HCl over a period of 10 d after removal of exchangeable K, and by conventional methods. The average percentage of the soil K-ne extracted by the 0.01 M HCl method amounted to 0.66% of the total K amount, and was much lower than that by a single extraction with 1 M HNO3 (2.0%) or with 0.2 M sodium tetraphenylboron for 2 d (22%). In the second experiment, the amount of K-ne removed by chemical extractions was compared with that of K-ne removed by maize plants grown for 29 d in five of the above soils. The amount of the K-ne evaluated by the 0.01 M HCl method gave the highest correlation (p < 0.05) with that of the K-ne utilized by plants among the extraction methods applied. The amount of soil K-ne extracted by the 0.01 M Hel method could therefore become a suitable index of the amount of readily available K-ne in soil. Extraction of K-ne in soils after maize planting further indicated that plants had removed K-ne more intensively than the 0.01 < HCl method probably only from the rhizosphere, although a high correlation was observed between the amount of K-ne removed by the 0.01 M Hel method and that by plants. This implies that the estimation of the amount of K-ne utilized by plants requires not only soil chemical analysis but also the evaluation of the percentage of the soil volume where the plant-induced release of K-ne actually occurs.

Distribution and forms of cadmium on the rhizosphere of Brassica juncea in Cd-contaminated soils and implications for phytoremediation

Abstract To investigate the chemical and positional availability of soil cadmium (Cd) absorbed by plants, the distribution and forms of Cd in the rhizosphere were analyzed by growing Brassica juncea L. in three artificially Cd-contaminated soils (two Fluvisols with 21.7 and 7.4 mg Cd kg−1 and an Andosol with 25.1 mg Cd kg−1 designated as FH, FL, and AH, respactively) using the rhizobox method. Mtar a 25-d growth period, plant samples were harrn vested to determine the total amount of Cd absorbed, Soil samples were also collected at a distance of every 2-mm from the root-accumulating central compartment (C.C.) for the determination of the Cd concentration in the water-soluble, exchangeable, inorganically and organically bound fractions using a sequential extraction method. The amount of Cd absorbed by the plants was 168, 31, and 12 µg pot−1 for the FH, FL, and AH soils, respactively, suggesting that the efficiency of Cd removal or phytoremediation in the Fluvisol was more than 10 times higher than that in the Andosol. In accordance with the plant uptake, a significant decrease in the content of soil Cd was observed in both inorganically and organi cally bound fractions in the Fluvisol and mostly in the organically bound fraction in the Andosol. The depletion was limited to a volume extending less than 10 mm from the C.C., and about 60% of the amount of Cd depleted was observed within 2 mm of the C.C., irrespective of the soils examined. The effective recovery rates were, therefore, 6.0, 3.2, and 0.6% for FH, FL, and AH, respectively, if only the soil within 10 mm from the C.C. was taken into account. The amounts of water-soluble and exchangeable Cd, in contrast, showed some increase, especially in the FH soil, suggesting a possible accumulation of readily available Cd in the rhizosphere in certain soil-plant combinations. These results suggest the importance of 1) selection of soil, 2) increase in the volume of the rhizosphere, and 3) minimization of the possible leaching loss of Cd, when efficient and environmentally sound phytoremediation is to be accomplished for Cd-contaminated soils.