The different effects of sewage sludge and manure compost on the biomass of both species in Pb/Zn tailings could be explained by the fact that the total and water-soluble N content in sewage sludge was 43 870 and 1452 mg/kg respectively, compared to 19 052 and 187 mg/kg respectively in manure compost. Hence, the total N and water-soluble N were 2.3 and 7.8 times greater than these in manure compost respectively. Sewage sludge allowed better plant growth because N is an essential element for making proteins and chlorophyll. In addition, soluble P must be below 20–30 mg/kg in soils, otherwise it will cause phytotoxicity (Charman and Murphy, 1992). While application of manure compost to Pb/Zn tailings increased water-soluble P in the substrata with increased application rates (>50 mg/kg in 88.10 t/ha manure compost) (Table 4). These are possible reasons for the lack of growth of vetiver and common reed during manure compost amendment, when compared with the growth in pure Pb/Zn tailings (PZ). Nonetheless, the biomass of common reed when treated with 88.10 t/ha sewage sludge (PZS20) in Pb/Zn tailings was seriously prohibited, and the plant died after 2 weeks; the presence of heavy metals (high DTPA-extractable Zn: >700 mg/kg) probably limited the growth and neutralized the effect of extra nutrients. Furthermore, the excessive soluble salts (EC = 5.62 mS/cm in 88.10 t/ha sewage sludge) in Pb/Zn tailings also contributed to the decline in growth. Anderson and Ingram (1989) suggested that an EC of more than 5 mS/cm in soils will inhibit the growth of plants. The enhancement effect on the growth of both species with NPK fertilizer was due to the additional nutrient that was available for plant growth and the improvement of physical properties, e.g. soil aeration, in both tailings by organic wastes. Furthermore, the results also implied that further increases in sewage sludge dosages above 10% would not increase biomass production of the plant species in Pb/Zn tailings. In general, the application of sewage sludge to mine tailings could ease its disposability, in addition to improving the properties of the toxic tailings for plant growth.
The different effects of sewage sludge and manure compost on the biomass of both species in Pb/Zn tailings could be explained by the fact that the total and water-soluble N content in sewage sludge was 43 870 and 1452 mg/kg respectively, compared to 19 052 and 187 mg/kg respectively in manure compost. Hence, the total N and water-soluble N were 2.3 and 7.8 times greater than these in manure compost respectively. Sewage sludge allowed better plant growth because N is an essential element for making proteins and chlorophyll. In addition, soluble P must be below 20–30 mg/kg in soils, otherwise it will cause phytotoxicity (Charman and Murphy, 1992). While application of manure compost to Pb/Zn tailings increased water-soluble P in the substrata with increased application rates (>50 mg/kg in 88.10 t/ha manure compost) (Table 4). These are possible reasons for the lack of growth of vetiver and common reed during manure compost amendment, when compared with the growth in pure Pb/Zn tailings (PZ). Nonetheless, the biomass of common reed when treated with 88.10 t/ha sewage sludge (PZS20) in Pb/Zn tailings was seriously prohibited, and the plant died after 2 weeks; the presence of heavy metals (high DTPA-extractable Zn: >700 mg/kg) probably limited the growth and neutralized the effect of extra nutrients. Furthermore, the excessive soluble salts (EC = 5.62 mS/cm in 88.10 t/ha sewage sludge) in Pb/Zn tailings also contributed to the decline in growth. Anderson and Ingram (1989) suggested that an EC of more than 5 mS/cm in soils will inhibit the growth of plants. The enhancement effect on the growth of both species with NPK fertilizer was due to the additional nutrient that was available for plant growth and the improvement of physical properties, e.g. soil aeration, in both tailings by organic wastes. Furthermore, the results also implied that further increases in sewage sludge dosages above 10% would not increase biomass production of the plant species in Pb/Zn tailings. In general, the application of sewage sludge to mine tailings could ease its disposability, in addition to improving the properties of the toxic tailings for plant growth.
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