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 (Fe o ). The L* value (lightness) was negatively correlated with the content of total C and total N (R 2 = 0.18** and 0.26**, respectively), and the b* value (yellowness) was positively correlated with the Fe o content (R 2 = 0.59**). For the national-scale samples, the values of the coefficient of variation were around 60% for total C, total N and Fe o . The L* value was negatively correlated with the content of total C and total N (R 2 = 0.70** and 0.59**, respectively), but the b* value was not correlated with the content of Fe o (R 2 = 0.00). When the analysis was limited to 65 samples frequently used for paddy fields, the b* value was positively correlated with the Fe o content (R 2 = 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 Fe o contents in agricultural surface soils in Japan.

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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.