Severe drought conditions modify the microbial community structure, size and activity in amended and unamended soils

Biological activity could be affected severely by the impact on soil quality of drought, which can be very severe in Southern areas of Europe. The objective of this work was to assess, under controlled laboratory conditions, whether a long period of severe drought (six months) can affect the structure, size and activity of the microbial community of a semiarid soil, as well as the influence of organic amendments on these effects. The soil was incubated for 180 days under controlled conditions (25 °C and 60/80% day/night relative humidity), with two treatments: unamended (US) and amended (AS) with manure compost (100 t ha−1). Two levels of irrigation were imposed: (1) well-watered (MUS and MAS), the soil being maintained at 60% of its water-holding capacity (WHC), and (2) dry soils, without irrigation (DUS and DAS). The drought conditions caused a significant inhibition of C and N mineralisation, and affected negatively the size and activity of the soil microbial biomass. Thus, after 180 days under drought conditions, the non-watered soils showed higher organic carbon content than the well-watered soils. Likewise, the stressed soils showed significantly lower values of water-soluble N, ATP content, microbial biomass C, alkaline phosphomonoesterase activity and total functional diversity than the well-watered soils. There was a significant decrease in the total amount of each fatty acid in DUS and DAS with respect to MUS and MAS after 180 days under drought. The physiology of the microbial community was affected more strongly by water stress than was the microbial community structure, changes in the structure caused by drought being less pronounced in amended than in unamended soils. Furthermore, the organic amendments increased the soil organic matter content, hence improving the size and activity of the soil microbial biomass and helping soil to retain moisture.