Copper and zinc fractions affecting microorganisms in long-term sludge-amended soils

The influences of Zn and Cu on soil enzyme activities (acid phosphatase. al kaline phosphatase, arylsulfatase, cellulase. dehydrogenase, protease (z-FL ase), urease, beta -D-glucosidase and beta -D-fructofuranosidase (invertase )) and microbial biomass carbon were investigated in agricultural soils ame nded with municipal sewage sludge or compost since 1978. The trace metals i n the soils were fractionated using a sequential extraction method. Long-te rm application of the sewage sludge and composts caused accumulations of Cu and Zn in the soils, ranging from 140 to 144 and from 216 to 292 mg kg(-1) , respectively. The percentage of Cu was highest in the NaOH- and HNO3-extr actable fractions (44-51% and 38-46%. respectively), while the percentage o f Zn was highest in the HNO3- and EDTA-extractable fractions (65-83% and 11 -32%, respectively). Although the percentage of the bioavailable fractions (sum of KNO3 + H2O-, NaOH-, and EDTA-extractable amounts) of Cu (53-64%) wa s higher than that of Zn (15-37%), the percentage of the most labile fracti ons (KNO3 + H2O) of Zn (2.1-5.9%) was larger than that of Cu (1.1-2.4%). Th e size of the microbial biomass carbon increased with the application of se wage sludge or compost. For some enzymes, however, the ratio of the enzyme activity to microbial biomass was lower in the soils amended with sewage sl udge or compost than that in the control soil. The soil enzyme activities w ere more adversely affected by Zn than by Cu. From a multiple regression an alysis, it was found that dehydrogenase, urease, and beta -D-glucosidase ac tivities were reduced by the KNO3 + H2O-extractable fraction of Zn in the s oils. These microbial activities seem to be sensitive to Zn stress, indicat ing the possibility that they might be useful bioindicators for evaluation of the toxic effects of Zn on microorganisms in the soils. (C) 2001 Elsevie r Science Ltd, All rights reserved.