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

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 (R[2] = 0.18** and 0.26**, respectively), and the b[*] value (yellowness) was positively correlated with the Feo 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 Feo. 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 Feo (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 Feo 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 Feo contents in agricultural surface soils in Japan

The extractability of potassium and radiocaesium in soils developed from granite and sedimentary rock in Fukushima, Japan

Potassium (K) and radiocaesium (RCs) were chemically extracted from soils derived from granite (G soils) and sedimentary rock (S soils) in Fukushima, Japan. The extractants employed were 1 M HNO3, concentrated HNO3, and HF + HClO4. As S soils contain a lower amount of trioctahedral 2:1 phyllosilicates than G soils, the RCs/K ratio was higher in S soils than in G soils with 1 M HNO3 extraction, indicating that the potential risk of soil-to-plant transfer of RCs is higher in S soils than in G soils. In conclusion, information about surface geology is important in predicting the spatial pattern of soil characteristics related to transferability of RCs

Soluble selenium content of agricultural soils in Japan and its determining factors with reference to soil type, land use and region

<div><p></p><p>To evaluate labile selenium (Se) content in agricultural soils in Japan and to investigate its determining factors, 178 soil samples were collected from the surface layer of paddy or upland fields in Japan and their soluble Se contents were determined. Two grams of soil was extracted with 20 mL of 0.1 mol L⁻¹ sodium sulfate (Na₂SO₄) solution for 30 min in boiling water, and the released Se was reduced to Se (IV) after organic matter decomposition. The concentration of Se (IV) was then determined by high performance liquid chromatography (HPLC) with a fluorescence detector after treatment with 2,3-diaminonaphthalene (DAN) and extraction with cyclohexane. Soluble Se content ranged from 2.5 to 44.5 μg kg⁻¹ with geometric and arithmetic means of 11.4 and 12.8 μg kg⁻¹, respectively, and corresponded to 3.2% of the total Se on average. The overall data showed log-normal distribution. In terms of soil type, Non-allophanic Andosols and Volcanogenous Regosols had relatively high soluble Se content, and Wet Andosols and Lowland Paddy soils had relatively low soluble Se content. In terms of land use, upland soils had significantly higher soluble Se content than paddy soils (p < 0.01). The soluble Se content had significant positive correlation with total organic carbon (TOC) content of the extract, soil pH and total Se content (p < 0.01). In conclusion, total Se content in combination with soil pH was the main determining factor of the soluble Se content of agricultural soils in Japan.</p></div

Impact of radiocesium contamination in flood sediment deposited after the 2019 typhoon on decontaminated fields of Fukushima Prefecture, Japan

International audienceSediment that has deposited after the flood generated by the 2019 typhoon (referred to as Flood Sediment, FS) was collected along two rivers in eastern Fukushima Prefecture, Japan to determine the total and exchangeable radiocesium (137 Cs), and acid-extractable potassium (K) contents. Then, these FS parameters were compared with those of decontaminated soils (DS) in nearby agricultural fields to discuss the potential transfer risk of 137 Cs from rivers to nearby remediated soils. While FS had about four times higher total 137 Cs content, it showed a three-times lower exchangeable 137 Cs content than DS. Furthermore, acid-extractable K referred to as non-exchangeable K (Nex-K) content was high enough to restrict 137 Cs transfer from soil to crops for both FS and DS. From these comparisons, we concluded that deposition of FS onto DS may not increase the transfer of 137 Cs from soil to crops

Significance of Plant-induced Solubilization of Soil Nitrogen: A Case of Komatsuna Plants Grown in Fertilized Soils

Plant-induced solubilization of soil nitrogen (N) is a key process for plants to utilize the recalcitrant form of N. Toevaluate its contribution to plant uptake, the factors affecting the contribution and the forms of N solubilized by plants, we analyzed the results of a pot experiment in which komatsuna (Brassica rapa L. var. peruviridis) was grown in 3 different soils applied with 5 types of sewage sludge as a source of N for plants. The amount of N solubilized by plants, which was defined experimentally as the difference in the amount of solubilized N between the planted and unplanted treatments, varied with the soil types more than the types of sewage sludge. This accounted for 30% (Arenosol), 15% (Fluvisol) and 1.6% (Andosol) of the amount of N uptake on average. These percentages were high when the level of soil soluble N after the experiment was below approximately 30 mg kg-1. Sequential analyses of insoluble N in soil after the experiment indicated the occurrence of plant-induced solubilization of both bio-soluble and acid-soluble N in many of the Arenosol and Fluvisol treatments and that of acid-soluble N in the Andosol treatments. The plant-induced solubilization in the Andosol resulted in the accumulation of more labile bio-soluble N rather than enhanced plant uptake. For komatsuna grown in fertilized soil, the depletion of soluble N in the root zone seems to be important for the increase in the contribution of the plant-induced solubilization to uptake but not for the occurrence of the solubilization

Spatial variability of nitrous oxide emissions and their soil-related determining factors in an agricultural field.

To evaluate spatial variability of nitrous oxide (N2O) emissions and to elucidate their determining factors on a field-scale basis, N2O fluxes and various soil properties were evaluated in a 100- x 100-m onion (Allium cepa L.) field. Nitrous oxide fluxes were determined by a closed chamber method from the one-hundred 10- x 10-m plots. Physical (e.g., bulk density and water content), chemical (e.g., total N and pH), and biological (e.g., microbial biomass C and N) properties were determined from surface soil samples (0–0.1 m) of each plot. Geostatistical analysis was performed to examine spatial variability of both N2O fluxes and soil properties. Multivariate analysis was also conducted to elucidate relationships between soil properties and observed fluxes. Nitrous oxide fluxes were highly variable (average 331 µg N m-2 h-1, CV 217%) and were log–normally distributed. Log-transformed N2O fluxes had moderate spatial dependence with a range of >75 m. High N2O fluxes were observed at sites with relatively low elevation. Multivariate analysis indicated that an organic matter factor and a pH factor of the principal component analysis were the main soil-related determining factors of log-transformed N2O fluxes. By combining multivariate analysis with geostatistics, a map of predicted N2O fluxes closely matched the spatial pattern of measured fluxes. The regression equation based on the soil properties explained 56% of the spatially structured variation of the log-transformed N2O fluxes. Site-specific management to regulate organic matter content and water status of a soil could be a promising means of reducing N2O emissions from agricultural fields