Long-term effects of trace elements contamination on soil microbial biomass and enzyme activities

Abstract

Phytostabilisation technology in post‐closure metal‐mine industry may be a feasible reclamation solution to avoid the transfer of trace elements to adjacent systems and to provide long‐term benefits increasing the functionality of the reclaimed ecosystem. In a contaminated and remediated area in SW Spain, we determined the long‐term effects of different tree species on soil functionality. After the Aznalcóllar mine‐spill, a largescale phytostabilisation plan was launched including sludge and soil removal, amendment addition and native tree species afforestation. We selected two areas in the Guadiamar Green Corridor, the North and South fields, due to their different soil properties and contamination levels. Both fields were affected by the spill and were afforested by a mixed plantation pattern with specimens of the same age. Three native tree species were selected for this study: white poplar (Populus alba), stone pine (Pinus pinea) and wild olive (Olea europaea), and compared with adjacent treeless areas. Nineteen years after the phytostabilisation implementation, we measured soil physical‐chemical parameters and available and total trace element concentrations. For functional measurements, we analysed the microbial biomass C and N as well as a varied enzyme activities to measure the hydrolysis of assimilated products of C, N and P. A long‐term marked contamination effect was still significant along the Guadiamar Green Corridor. The North field, closer to the mine tailings, presented contamination concentrations above the Lower Guideline Values for total As, Cu, Pb and Zn. Tree afforestation was found to be positive for the increase of soil fertility and microbial biomass on trace element contaminated soils. The studied tree species were found to affect differently the soil chemistry and microbial communities. White poplar and stone pine were the tree species with the greatest effects on soil. Soils under white poplar presented less acidification and more N content, while soils under stone pine were acidified and presented the highest C:N ratio. Regarding microbial biomass, stone pine was found to reduce the biomass growth. Regarding enzyme activities, tree species had different effects depending on the specific enzyme; however soil acidification and contamination were the main factors affecting them. In conclusion, our study demonstrates the importance of properly planning the post‐closure reclamation according to the nature of the mining activity and the local conditions and properties of the mine location. When phytostabilisation technology is selected, the effects of tree species should be taken into account to improve reclamation success and develop a self‐sustaining ecosystem.