土壌の富栄養化が農村地域の河川周辺雑草群落の動態に及ぼす影響  [in Japanese] Effect of Soil Eutrophication on Weedy Riparian Communities around Agricultural Areas  [in Japanese]

畜産や農業に伴う土壌の富栄養化が耕地水系に沿って分布する河川周辺雑草群落に及ぼす影響を明らかにすることを目的として, 茨城県新治郡の帆崎川において雑草群落の構造と土壌環境の調査を行なった。スギ植林地に近い上流部では土壌中の窒素および炭素含量は少なく, 植生はメヒシバやイヌタデなどの雑草が優占するものの, ツリフネソウなどの野草的な種も多く含んでおり多様性が高いことが特徴であった。これに対して水田地帯を流れる中流部では土壌中の窒素および炭素舎量は上流部の5倍以上あり, 好窒素的な一年草のミゾソバやカナムグラが寡占して多様性の低い群落が形成されていた。また下流部では護岸工事がなされており, 攪乱の強い中洲ではクサヨシが1種優占群落を形成していた。中流部付近にはシロザが純群落を作る豚糞堆積場があり, この土壌は中流部のさらに2倍程度の窒素と炭素を含み, C/N比が低かった。これらのことから豚糞堆積場からの有機物や水田に施用された化学肥料が, 降雨時に流出して土壌が富栄養化し中流部の群落の多様性を低下させたものと考えられた。従来から良く知られている都市河川と同じように, 農村地域集水域の河川植生の動態も農業や畜産の集約化に伴う人為的な影響を強く受けていることが示唆された。

Structure and dynamics of weedy riparian communities around agricultural areas and the effect of soil eutrophication were studied along the Hozaki river, Ibaraki Prefecture, central Japan. Communities with a high species diversity, consisting of a mixture of weedy species, e.g., Digitaria ciliaris and Persicaria longiseta, with ruderal species, e.g., Impatiens textori, were established in the upper stream area near a Cryptomeria japonica plantation, and nitrogen and carbon contents of the soil were significantly low. On the contrary, in the midstream area around paddy fields, nitrogen and carbon contents of the soil were five times higher than in the upper stream area, and communities with a low species diversity, consisting mainly of Persicaria thunbergii and Humulus japonicus, were established. In the holm of the lower stream, where the embankment was covered by concrete, Phalaris arundinacea dominated alone due to severe disturbances associated with the construction work. Near the study sites of the midstream area, a pure community of Chenopodium album was established near a pile of pig dung. The nitrogen and carbon contents of the soil at the site were nearly twice as high as those of the site in the midstream area. Soil eutrophication in the midstream area due to organic matter from pig dung or chemical fertilizer from paddy fields, resulted in the predominance of nitrophilous weeds with a low species diversity. It is concluded that human impact affected the riparian vegetation not only around urban areas due to the presence of factories or home effluents, but also around agricultural areas through modern and intensive farming including rice cultivation or live-stock breeding.