Influence of the quality of organic amendments on Mediterranean agricultural soils following a drought episode

Soil organic matter loss is a major concern in agricultural Mediterranean soils due to its influence on soil water retention and hence increased susceptibility to drought conditions. An experiment was conducted to study the influence of organic inputs' quality on the resistance of a C-poor soil to drought. Four organic materials were added to the soil in a mesocosm experiment: 1) leonardite; 2) earthworm humus; 3) biosolid compost; and 4) dry biomass of Vicia faba, simulating the incorporation of a legume cover crop. Pots were sown with a forage mixture (Lolium perenne and Medicago polymorpha), and 45 days following C-addition a drought episode (30 % reduction in water inputs) was simulated in half of the replicates. Soil resistance was evaluated by measuring soil chemical properties, biological activity (enzyme activities and respiration rate), biomass and the fungi:bacteria ratio in the microbial community, and plant productivity. With the exception of earthworm humus, organic-C addition had positive effects on soil chemical properties and water retention. V. faba biomass was the most effective material for enhancing water retention (16.5 % higher than in the non-amended soil), microbial activity and abundance (2.5 and 8.4 times greater bacterial and fungal abundances than in the non-amended soil, respectively), and soil N and P availability. Soils enriched with V. faba biomass also had the highest fungi:bacteria ratio by the end of the experiment. Drought reduced plant productivity, but all soil function indices showed a high resistance to drought, regardless of the organic matter quality or the fungi:bacteria ratio.