Effects of heavy metal accumulation associated with pesticide application on the decomposition of cellulose and orchard grass in soils

The effects of heavy metal accumulation associated with the application of Bordeaux mixtures and lead arsenate on the decomposition of cellulose powder and orchard grass shoots in soils mere investigated by laboratory incubation experiments. The largest amounts of total Cu, Pb, and As found in the soils used were 1,108, 1,271, and 205 mg kg(-1) soil, respectively, and the amounts of Cu and Pb extracted with 0.1 RI CaCl2 were each negatively correlated with the soil pH. The rates of decomposition and C mineralization of the cellulose powder added to the soils decreased exponentially with the amount of 0.1 sl CaCl2-extractable Cu and increased exponentially with the soil pH over the 35-d period of incubation, though they did not show a close relation to the total amounts of heavy metals. When the pH was modified in acid soils by the addition of Ca(OH)(2), the increase in the soil pH enhanced the decomposition and C mineralization of added cellulose powder regardless of the accumulation of heavy metals. These results suggested that the decomposition of cellulose powder in soil was inhibited not only by the increase in the 0.1 nr CaCl2-extractable Cu, but also by the decrease in soil pH. No close relationship was detected between the total amount of heavy metals and the mineralization rate of the added orchard grass-C in the soils throughout the 70-d period of incubation. However, only during the early period of incubation (up to 7 d), the mineralization rate was negatively correlated with the amounts of 0.1 M CaCl2-extractable Cu and Pb, and positively with the soil pH. The addition of Ca(OH)(2) decreased the mineralization rate of orchard grass in an acid soil with heavy metal accumulation, whereas it did not affect appreciably the mineralization rate in an acid soil without heavy metal accumulation. It was suggested that the increases in the amounts of 0.1 M CaCl2-extractable Cu and Pb induced by the decline in soil pH may be responsible for the inhibition of the decomposition of orchard grass observed during the early period of incubation.