Study on external economy on terraced paddy fields : economic and soil scientific approach

本研究分为两个部分。一是经济分析，二是土壤科学分析。在前面的分析中，我们首先研究了高知县大藤町梯田水田的现状。根据地理和地质调查资料，确定了该滑坡带上的水田。到目前为止，水田坝的作用一直被强调为支撑水田的基础。然而，它在这个领域是不合适的。空间创造功能更为重要。此外，我们还向位于吉野河下游的大东洋町和德岛市的居民发出了调查问卷，德岛市位于吉野河下游，通过河与大东洋町相连。通过问卷调查，我们明确了居民之间的认知差异，并估计了居民支付水田维护费用的意愿。最后，探讨了该地区利用四坝草的最佳土地利用方式。土壤科学分析的主要成果是：在花田农场(Nahari-cho，Aki-un，高知)和斋藤(Saito)农场(南国，高知)进行了当地气象、土壤和水质、土壤微生物、土壤低分子量有机酸的监测。作为四坝草场利用后，由于表层形成了根垫层，地下形成了硬锅层，并在这两层之间形成了颗粒状结构，从而提高了土壤的透水性。从养分动态来看，土壤结构的这些变化有利于复垦草地生态系统的稳定性。

This study is separated into two parts. One is economic analysis and the other is soil scientific analysis. In the former analysis, we firstly studied the current situation of terraced paddy fields in Ohtoyo-cho of Kochi prefecture. With the data of geographical and geological survey, we identified that the paddy fields stand on the landslide area. Until now the function of dam of paddy fielda is stressed as a basis for supporting the fielda. It is, however, not appropriate in this area. The space-creation function is more important. Additionally we sent out questionnaires to inhabitants in Ohtoyo-cho and Tokushima-shi, which locates on the downstream of Yoshino river and is connected to Ohtoyo-cho by the river. With the questionnaires we clarified the difference of recognition between the inhabitants and estimated the willingness to pay for maintaining the paddy field. Finally we discussed the optimal land use with using siba grass in that area. The major results of soil scientific analysis are fallowed : Monitoring of the local meteology, soil and water quality, soil microorganisms, low molecular weight organic acids in soils, under reclaimed siba (Sosiba japonica) grass land was carried out at Hanada Farm (Nahari-cho, Aki-gun, Kochi) and Saito Farm (Nankoku-shi, Kochi). Upon the utilization as siba grass land, water permeability became improved, because of the formation of root mat layr at the surface and hard-pan layr in the subsurface, and eventually, development of granular structure inbetween those two layrs. These changes in soil structure gave favorable changes for the stability of the ecosystem of reclaimed grass land in terms of the nutrient dynamics.