Tree seedlings suffer oxidative stress but stimulate soil enzyme activity in oil sludge-contaminated soil in a species-specific manner

The impact of plants on soil enzymatic activity and their potential for remediation of oil-contaminated soil has been widely studied but information about tree species is scarce. Here, we used seedlings of four tree species (Ailanthus altissima Mill, Fraxinus rotundifolia Mill, Melia azedarach L. and Robinia pseudoacacia L.) to investigate rhizosphere effects on key soil enzymes (dehydrogenase, urease, alkaline and acid phosphatase) and to evaluate oxidative stress in response to oil sludge-contaminated soil (0, 100, 200, 400 g kg−1). We observed a large species-specific stimulation of soil enzyme activity often far exceeding the figures in unplanted soil with peaks mostly occurring under low oil contamination (100 g kg−1). The strongest stimulatory effect on dehydrogenase and urease activity produced R. pseudoacacia at the low and intermediate and F. rotundifolia at the highest contamination level. No clear pattern emerged for acid phosphatase activity, which was equally stimulated by all species at the highest contamination level. Alkaline phosphatase stimulation was dominated by F. rotundifolia at the low and by R. pseudoacacia at the higher oil contamination levels. Foliar H2O2 rose significantly in a species-specific manner in response to oil contamination, triggering an upregulation of the antioxidant defence, which began to show signs of exhaustion at the highest pollution level and revealed that oxidative stress was highest in A. altissima. Our results imply that phytoremediation efforts can be optimized through carefully designed plant species assemblages aligning stimulatory effects on soil enzyme activity and oxidative stress tolerance with the severity of the oil pollution.