Soil humic acids were extracted from an Entisol, an Inceptisol and an Andisol in this study. We examined the effects of different concentrations of humic acids on growth and nutrient uptake of rice (Oryzasativa) seedlings grown hydroponically. Rice growth was inhibited and the symptoms of chlorosis appeared at the addition of 75 to 100 mg L^<-1> of humic acids. The concentration of K in rice seedlings was almost constant, but the concentrations of Mg and Ca increased with increasing concentration of humic acids. In culture solution, Ca^<2+> and Mg^<2+>, which were concentrated near the negatively charged surface of the humic acids, would move to the root surface by means of ion exchange with H^+ released from the root surface and then absorbed by the roots, resulting in the increase of Ca and Mg concentrations in the seedlings. Fe^<3+> and Cu^<2+> did not undergo ion exchange with H^+ because they were coordinated with the carboxyl groups and formed the complexes with humic acids. Fe and Cu concentrations in the seedlings decreased, leading to chlorosis, because Fe^<3+> and Cu^<2+> were not absorbed as rapidly by the roots. The addition of large amount of Fe^<3+> for 20 days mitigated the chlorosis of rice seedlings hydroponically cultivated with 100 mg L^<-1> of humic acids. The addition of 1.2 mg L^<-1> Fe^<3+> solution prevented the chlorosis of rice seedlings. The results suggest that the addition of Fe equivalent to more than 5% of the carboxyl groups of humic acids can stimulate the growth of rice plant by preventing chlorosis development.

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To elucidate the pattern of nitrogen N mineralization in paddy soils, we used moist and air-dried soil samples with various fertility levels collected from 100 fields throughout Japan. We incubated moist soils for 4, 7, or and 10 weeks and air-dried soils for 1, 2, 4, or and 10 weeks at 30℃ under submerged conditions and measured N -mineralization during incubation. In addition, we investigated the effect of initial soil moisture content before submergence on the N -mineralization rate, as well as and the relationship of rice yield to the amount of nitrogen N supplied by soil N -mineralization and nitrogen N fertilizer. (1) The pattern of N -mineralization during incubation of moist soils varied by the geographic location and soil group of the collected soil samples. Soils collected from the Sea of Japan sea side of the Tohoku region and Niigata areas exhibited rapid N mineralization after 7 weeks of incubation. Different patterns of N -mineralization increase were observed in Soils amended with organic matter (OM) in the form of compost or rice straw showed different patterns of N mineralization increase. (2) The pattern of N -mineralization during incubation of air-dried soil was did not found to vary by geographic location or OM amendment. Substantial N -mineralization occurred by 2 to 4 weeks of incubation, resulting in clear differences in N -fertility among soil samples. In addition, after 4 weeks of incubation of air-dried soil, we observed a positive correlation between the changes in total nitrogen N and total carbon C resulting from OM amendment and showed a positive correlation with the change in N -mineralization. (3) In soil samples with different As the initial moisture content decreased, the amount of N -mineralization at 4 weeks of incubation was found to increased with decreasing moisture content. The moisture content of soils collected from fields cultivated underused for rice-soybean rotation was lower than that of soils collected from nearby fields used for in the same geographical area undergoing continuous rice production, and the increase in N -mineralization associated with the decrease in lower moisture content was smaller. A strong positive correlation was observed between The difference in the moisture content of OM-amended or wetland rice-soybean rotation soils and that of the control soil and showed a strong positive correlation with the difference in N -mineralization during the incubation of air-dried soil. (4) We observed a strong quadratic correlation between The yield of staple wetland rice cultivars showed a strong quadratic correlation and with the combined nitrogen N supply from soil N -mineralization (estimated at rice maturity) and nitrogen N fertilizer.

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We evaluated a simple and rapid method for measuring nitrogen N mineralization (available nitrogenN) during wet incubation of air-dried paddy soils. Using 116 soil samples collected from around the Japan, we investigated the relationship between total organic carbon C (TOC) or total organic nitrogenN, recovered by static hot-water (80 ℃) extraction for 16 h or cold-water (25 ℃) extraction for 1 h accompanied by with shaking, and available nitrogenN: 1) Although we observed a positive correlation (R^2=0.59) between TOC assessed by hot-water (80 ℃) extraction of air dried soils over a 16-h period and showed a positive correlation (R^2=0.59) with available nitrogenN, the root-mean-square error (RMSE) was also large. In addition, the coefficient of determination between extracted organic nitrogen N + additional ammonia-N generated by heating and available nitrogen N was low (R^2=0.22). 2) TOC assessed by cold-water extraction of soils with shaking and available nitrogen N increased with decreasing initial soil moisture content. The amount of TOC extracted varied dependeding on the temperature at which the soil samples were dried. 3) We observed a strong correlation (R^2=0.80) between TOC assessed by cold-water extraction of air-dried soil samples with 1 h of shaking and showed a strong correlation (R^2=0.80) with available nitrogenN. To eliminate the influence of drying temperature, we again performed cold-water (25 ℃) extractedion with 1 h of shaking, using soil samples that were oven-dried at 105 ℃ for 24 h. The result was a strong, positive correlation between TOC and available nitrogen N (R^2=0.86) and a reduction in the RMSE. The relationship between TOC extracted using the above method assessed by cold-water extraction and available nitrogen N varied slightly by with sample source (geographical location) and soil type. As such Thus, it may be possible to increase the accuracy of estimates of available nitrogen N estimates by establishing soil groupings.

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