Compensatory fertilization of Scots pine stands polluted by heavy metals

The results from four compensatory fertilization experiments located at dif ferent distances (0.5, 2, 4 and 8 km) along a heavy metal deposition gradie nt extending from the Harjavalta Cu-Ni smelter in SW Finland are presented. The experiments were established in middle-age Scots pine stands growing o n dryish sites of sorted glaciofluvial sediments. The soil type in all the experiments is ferric podsol. The treatments in the experiments consisted o f liming, a powdered slow-release mineral mixture and stand-specific fertil ization which comprised at least methylene urea and ammonium nitrate. Monitoring of deposition and soil solution and studies on soil chemical and microbiological properties, on the nutrient status of trees and needle lit terfall, on fine root dynamics and on the growth of the tree stands were ca rried out during a 5-year period. There was a severe shortage of exchangeable Ca and Mg in the organic layer of the most polluted stands. Although the uppermost mineral soil layer had relatively high exchangeable Ca and Mg concentrations, the trees were not a ble to utilize these nutrient reserves presumably due to the toxic effects of Cu and Ni on the plant roots and mycorrhizas. The treatments that included limestone markedly decreased the Cu and Ni con centrations in the soil solution and soil organic layer, presumably due to immobilisation through precipitation or absorption. The Ca and Mg concentra tions correspondingly increased, which certainly contributed to the partial recovery of fine root and stand growth. The powdered mineral mixture and t he combination of methylene urea and ammonium nitrate had no short-term eff ect on the microbial biomass and activity. All the fertilizer treatments in creased volume growth in the most polluted stand. The stand-specific fertil ization increased needle mass in heavily polluted stands, but the response of the needle mass to fertilizer treatments was low in the less polluted st ands. No clear evidence was found to support the role of nutrient status in tree resistance.