Abstract
Suspended solids (SS) often run off from paddy fields during puddling for rice transplanting. Because SS adsorb and transfer hazardous chemicals such as persistent organic pollutants, they are expected to reduce SS in runoff from paddy fields. In late April-early May 2002, at eleven plots around Tsukuba City in Ibaraki Prefecture, we evaluated the ecosystem function of aquatic vegetation around paddy fields to retain SS during puddling. The plots were classified into four vegetation types, Arundinella hirta-Isachne globosa community, Equisetum palustre community, Leersia japonica community and Oenanthe javanica community, based on an ordination diagram of detrended correspondence analysis (DCA) and dominant species of the plots. The aquatic vegetation removed 46-98% of total SS (TSS), with an average of 72%, and TSS concentrations measured at six plots in effluent water from the aquatic vegetation were below 0.1g/L, thus satisfying the effluent standards for agricultural lands discharging into rivers. Removal rates of TSS increased with increasing vegetation biomass index (vegetation cover (%)×vegetation height (m)) up to 80, and were consistently about 95% at higher values of the index. The removal rate was positively correlated with the plant cover of Solidago altissima but not correlated with scores for the first and second axes of DCA. The particle size of SS peaked at 10-30μm in effluent water from paddy fields, and aquatic vegetation efficiently removed SS less than 30μm in diameter. These results suggest that aquatic vegetation with a large biomass has a high potential to reduce SS runoff from paddy fields.
