山田 佳裕 1) ,井桁 明丈 2) ,中島 沙知 3) ,三戸 勇吾 3) ,小笠原貴子 4) , 和田 彩香 4) ,大野 智彦 5) ,上田 篤史 2),8) ,兵藤不二夫 2) ,今田 美穂 2) , 谷内 茂雄 2) ,陀安 一郎 6) ,福原 昭一 7) ,田中 拓弥 2) ,和田英太郎 2),9)The runoff of suspended substances,nitrogen,and phosphorus by enforced draining during the ploughing season
Experiments in paddy fieldsThe runoff of suspended substances (SS),nitrogen,and phosphorus by enforced draining during the ploughing season was calculated from experiments in two paddy fields.The area of a paddy field was 3000m 2 .The total fluxes were 33 kg (SS), 85 g (total-N), and 46 g (total-P) in Field A, with a small decrease in water levels (8mm), 110kg (SS),589 g (total-N),and146 g (total-P) in Field B with a major decrease in water levels (72mm). Furthermore, the nitrogen/phosphorus ratios (mol ratios) of the waste water by the enforced draining were 4.1 (Field A) and 8.8 (Field B).In comparison with N/P of the phytoplankton (16),it was found that the phosphorus flowed out efficiently with the enforced draining.Increasing the supply of the phosphorus to Lake Biwa is not desirable in view of the current problems of eutrophication.The careless water management of paddy fields was a crucial factor in any plan to improve the water quality of Lake Biwa.
Nitrogenous compounds with high delta(15)N values were recently found in human-dominated small rivers in the Lake Biwa area. A detailed survey was performed to determine the distribution and variation of delta(15)N values in nitrogenous compounds in a representative small river (Hebisuna River) that flows into Lake Nishinoko, an inner bay of Lake Biwa. A high delta(15)N value was detected in the lower reaches of the river and the inner bay, most likely due to denitrification. These results strongly suggest that denitrification in small river systems such as the Hebisuna watershed has contributed to (15)N enrichment in the Lake Biwa ecosystem during the past 40 years. We also observed a clear, stepwise, positive correlation between population density and delta(15)N values for particulate organic matter or sediments. These results demonstrate that delta(15)N (POM) and delta(15)N (sediment) are helpful indicators for assessing nitrogen loading from domestic sewage. Moreover, they will aid in the development of new concepts in the environmental capacity of river ecosystems and its relationship to redox conditions. Finally, our data suggest that a population density of 100-200 persons per km(2) is the upper limit for a watershed in which only simple conventional sewage treatment is in effect.
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