Effect of sewage sludge in Sitka spruce plantations on a reclaimed site
Severely nitrogen and phosphorus deficient restored opencast coal mine sites planting with Sitka spruce ( Picea sitchensis , Bong. (Carr.)) were treated with both sewage sludge and inorganic fertilizer. Cake sludge was applied before planting at the rate of 100 and 200 tds/ha and liquid sludge at the rte of 227 m3/ha at the age of 3 in selected plots. The highest treatments (S200L) supplied approximately 2051 kg N/ha and 983 kg P/ha which is sufficient to establish an internal nutrient cycle in such degraded sites. Sewage sludge significantly increased the tree growth and foliage N and P concentrations both over the control and inorganic fertilizer treatments. Foliage N concentrations up to 1.95&'37 increased the maximum height growth above which the height growth declined due to excess N concentrations. Both the mean needle weight and height growth increased in response to the increased foliar P concentrations and the response is still not complete. Whole tree sampling showed that there was significantly more tree biomass in sludge treatments in comparison to the control treatments. Regression equations based on independent variables of stem diameter at 5 cm from the ground were developed to estimate total tree dry biomass. Ground vegetation dry matter, nutrient content and species diversity significantly increased in sludge treatments. Soil pH increased, bulk density decreased and organic C, total N, available P, K and Mg increased following sludge application in comparison to the control treatments. Nitrate leaching losses increased immediately after the liquid sludge applications in highest sludge treatments but with time fell down and remained within the standard limit for potable water. Drain water nutrient leaching losses remained well below the soil leachate except for magnesium. Nitrogen mineralization potential increased with increased application rate indicating increased cycling of N within the ecosystem and hence long-term growth response potential to the applied treatments.