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The acidification of allophanic Andosols by fertilizer application in relation to soil productivity was studied with special reference to the soil colloidal composition. Among the Japanese cultivated allophanic Andosols, in 95% of the samples, the exchange acidity y1 was < 6, while in 5% of the samples (30 soil samples) the exchange acidity y1 was ≧ 6. The strongly acidic allophanic Andosols (exchange acidity y1≧ 6) did not significantly differ from the weakly acidic allophanic Andosols in the contents of amorphous materials. However, the mean values of the strongly acidic allophanic Andosols for the soil pH and base saturation which were 4.5 ± 0.3 and 15 ± 9%, respectively, were relatively lower than those of weakly acidic allophanic Andosols. The content of KCl‐extractable Si in the strongly acidic allophanic Andosols dominated by allophane (exchange acidity y1≧ 6 and allophane/clay content ratio > 0.7) ranged from 51 to 141 mg kg−1, and the values increased with increasing concentrations of KCl‐extractable Al. KCl‐extractable Al in the cultivated allophanic Andosols was considered to be derived from both the allophane fraction and Al‐humus complexes. In the field studies of allophanic Andosols with successive application of fertilizers conducted at the Fujisaka Branch of Aomori Agricultural Experiment Station, the value of the exchange acidity y1 of the topsoil and subsoil exceeded 6 about 40 and 50 years after the start of the experiment, respectively. The yield of flint corn (Zea mays L. var. indurata Bailey. cv. Onoa) in the fertilizer plot decreased by the strong soil acidification. The amount of nitrogen mineralized in the soil of the fertilizer plot during a 280‐d incubation was 0.114 g kg−1 in the topsoil and 0.041 g kg−1 in the subsoil. These values were relatively low compared with those in the lime + fertilizer plot. Thus, we confirmed that the subsoil fertility of the fertilizer plot decreased by heavy application of fertilizers.

Abstract:The comprehensive relationship of backscattering coefficient (σ 0 ) values from two current X-band SAR sensors (COSMO-SkyMed and TerraSAR-X) with canopy biophysical variables were investigated using the SAR images acquired at VV polarization and shallow incidence angles. The difference and consistency of the two sensors were also examined. The chrono-sequential change of σ 0 in rice paddies during the transplanting season revealed that σ 0 reached the value of nearby water surfaces a day before transplanting, and increased significantly just after transplanting event (3 dB). Despite a clear systematic shift (6.6 dB) between the two sensors, the differences in σ 0 between target surfaces and water surfaces in each image were comparable in both sensors. Accordingly, an image-based approach using the "water-point" was proposed. It would be useful especially when absolute σ 0 values are not consistent between sensors and/or images. Among the various canopy variables, the panicle biomass was found to be best correlated with X-band σ 0 . X-band SAR would be promising for direct assessments of rice grain yields at regional scales from space, whereas it would have limited capability to assess the whole-canopy variables only during the very early growth stages. The results provide a clear insight on the potential capability of X-band SAR sensors for rice monitoring. OPEN ACCESSRemote Sens. 2014, 6 5996