雑草防除法, 耕起法および作付け様式の異なる水田における埋土種子の比較  [in Japanese] Seed Banks in Paddy Fields under Different Weed Control Methods, Tillage Systems and Cropping Patterns.  [in Japanese]

埋土種子集団は防除体系や耕起法など圃場に対する過去の人為的働きかけを反映する。そこで農生態系における雑草種の管理戦略を組み立てるために, 雑草防除法, 耕起法, 作付け様式などの管理の異なる岡山県南部の水田 8圃場の土壌を, 入水前の6月に地表から0-5, 5-10, 10-15, 15-20cmの4層に分けて採取し, 炭酸カリウム50%溶液を用いた比重分離法で埋土種子を回収した。埋土種子総数は, 手取り除草中心の圃場よりも除草剤運用の水田で少なかった。そして休耕田や粗放管理の圃場では, ノビエ類を中心に埋土種子数が除草剤連用水田の3倍以上であった。また不耕起田では, 種子の分布は表層部に偏り, 反転耕を行っている圃場では地下20cm近くまで種子の分布が多くみられた。草種別ではコナギ・アメリカコナギの種子数が, 手取り除草を続けている圃場で多くなる傾向を示した。そしてイネ単作圃場より, 麦やレンゲを栽培し秋から春にかけての耕起回数が少ない圃場で, スズメノテッポウを中心とする冬生雑草が多くなった。また主な草種において比重分離法で回収された種子と発芽法にて発生してきた本数との間には有意な回帰式を導くことができた。

The composition and density of buried seeds in an arable field reflect the past and present weed management and cultural practices in that field, as well as the types of crops grown. We observed seed banks in eight paddy fields located in the southern part of Okayarea prefecture under different weed control methods and cultural practices (Fig. 1, Table 1). The soils containing seeds were collected from four layers at 0-5, 5-10, 10-15 and 15-20cm depths from the ground surface level. Seeds in each sample were extracted by the chemical flotation method using a 50% potassium carbonate solution. Comparison of eight types of cultivation methods showed that the density of buried seeds was lowest in the case of successive herbicide treatments without tillage, followed by the same herbicide treatments to complement conventional tillage and in organic fields subjected to hand-weeding (Fig. 2). In contrast, the density of buried seeds increased, especially the seed population of Echinochloa, in fallow fields or fields with extensive cultivation. No-tillage cultivation resulted in the highest proportion of seeds in the surface layer, whereas in most of the ploughing methods seeds tended to be concentrated at a depth of 15 to 20cm (Fig. 3). The number of buried seeds of Monochoria vaginalis Presl var. Plantaginea Solms-Laub. and Heter-anthera limosa Willd. tended to increase in organic fields that were not subjected to chemical control of weeds. On the other hand, winter weeds, such as Alopecurus aequalis Sobol. var. Amurensis Ohwi were major contributors to seed banks in fields where barley or Astragalus sinicus L. were grown in winter under reduced tillage conditions, in comparison with single cropping systems of rice. The chemical flotation method using a potassium carbonate solution was effective to estimate seed banks in paddy fields, since the density of seeds in this method was correlated with the number of seeds of weed species determined by the seedling emergence method (Fig. 4).