Ibaraki Prefecture produces the largest volume of lotus root in Japan. The Teno area in Tsuchiura City (Ibaraki Prefecture) is the first to develop an infrastructure consisting of a concrete levee and to separate irrigation and drainage canals in Japanese lotus paddy fields. This was carried out over the period from 1995 through 2015. In this study, we studied the changes in runoff loads from lotus paddy fields due to the improvement of agricultural infrastructure in this area. Runoff loads of suspended solids (SS) , chemical oxygen demand (COD) and total nitrogen (TN) were the largest in March and April. This result is attributed to the manuring using basal fertilization and calcium cyanamid in this period. On the other hand, the runoff load of total phosphorus (TP) was maximum in July and August. As a cause, the elution of phosphate-phosphorus from lotus paddy soils and bottom sediments in drainage canals under lowered dissolved oxygen concentrations can be considered. Furthermore, annual balanced losses of COD, TN, and TP from lotus paddy fields were increased compared with those before the improvement of agricultural infrastructure. The separation of irrigation and drainage canals decreased soil sedimentation, which resulted in increases in losses of COD, TN, and TP from lotus paddy fields.
"},{"@language":"ja","@value":"茨城県のレンコン出荷量は全国一位であり, 全国のレンコン収穫量, 出荷量の半分近くを占めている。茨城県土浦市手野地区はレンコン専作地帯で, 1995年から2015年にかけて日本で初めてハス田群で基盤整備を実施し, コンクリート畦畔整備や用排分離等を行った。本研究では基盤整備された手野地区において流出負荷量調査を行い, 基盤整備による流出負荷量の影響を調査した。ハス田群からの排出負荷量を検討した結果, SS, COD, TNは3月から4月にかけて最大の値を示し, その要因として石灰窒素施用, 基肥施肥の影響が考えられた。TPは土壌が嫌気状態になりやすい7月から8月にかけて最大の値を示した。一年間の差引負荷量は基盤整備前と比べ, SS, COD, TN, TP全てで増加し, その要因として用排分離を整備したことにより, 田越灌漑の沈殿効果が消失した影響が考えられた。
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