Soil legacy and organic amendment role in promoting the resistance of contaminated soils to drought

The application of organic amendments is a common practice in the restoration of contaminated and degraded soils. We designed an experiment in pot mesocosms to study the effect of organic amendment (comparing a short-term addition of biosolid compost with soils amended 17 years ago) and soil legacy (comparing soils exposed or not to extensive grazing over a remediation process) on the resistance of a degraded Mediterranean soil against a simulated drought event. Pots were sown with a forage mixture (Lolium rigidum and Medicago polymorpha), and soil resistance was evaluated by measuring soil chemical properties, biological activity (soil enzyme activities and respiration rate), soil microbial community composition and plant production. Our results showed a positive effect of the organic amendment and the exposure to extensive grazing on soil properties, increasing soil water retention and the stability of plant production under drought conditions. The long-term amendment addition avoided the negative consequences of drought on L. rigidum production (the species with the lowest tolerance to water stress), while M. polymorpha biomass in soils exposed to grazing was 12 times the biomass in the non-amended and non-exposed soil. However, soil biological activity (enzyme activities and respiration rate) as well as microbial diversity were not limited by the simulated drought conditions under any type of soil management legacy, demonstrating the great adaptation of the microbial communities to water stress conditions in semiarid Mediterranean soils. Soils exposed to organic matter inputs in the long-term, through amendment addition or exposure to grazing, showed high similarities in terms of bacterial and fungal composition.