Tree species effect on soil organic matter and soil microorganisms in trace element contaminated soils

Abstract

In forest stands different tree species might have contrasted signals on soil organic matter due to their patterns of leaf litter and rood exudates, thus leading to species-specific interactions with the microbial communities in the soils underneath. In trace element-contaminated soils, organic matter is also associated with the dynamics of these elements in the soil-plant system. In a trace element contaminated area in SW Spain, different native tree species were planted in order to reduce the mobility of these contaminants. After 15 years of tree growth, we evaluated whether different tree species had specific influences on: 1) trace element stabilization in soil, 2) the quality of soil organic matter and 3) the functioning of the soil microbial community. Two areas with different soil bedrock and contamination level (North and South) were selected in the Guadiamar Green Corridor. Soils and leaf litter were collected underneath three native tree species: white poplar, stone pine (Pinus pinea) and wild olive, as well as in a treeless soil covered by herbaceous plants. Main soil chemical properties were analysed, as well as soil microbial biomass and enzyme activities. The signal of different chemical groups litter and in the soil particulate organic matter (POM) was analysed by Fourier-transform infrared spectroscopy (FT-IR). Soil trace element availability was significantly higher in the North area due to the acidic soil pH and lower soil organic matter. Soils underneath pine trees in the North area presented higher acidification, which was linked to a more intense signal of those peaks characteristic of acid functional groups (pectin, resins and waxes) in the litter and the POM underneath this species. The comparison of FT-IR spectra of POM in both areas suggested a higher degree of trasnformation of the POM in the Southern area. Microbial biomass was however similar underneath the trees in both areas, but lower in treeless soil. The effects of trees on enzyme activities depended on the enzyme, however in general enzyme activities were highly driven by soil acidification and contamination. In conclusion, we found that trace elements availability increased underneath pine trees which are not beneficial due to their acidification effect through leaf litter deposition.