Soil water availability strongly alters the community composition of soil protists

• We manipulauted soil moisture in a semi-field experiment in terrestrial model ecosystems (TME).
• Protist community composition showed a non-linear response to changing soil moisture conditions.
• Protist community composition changed already at a habitable pore space of <40 μm.
• This is an order of magnitude higher than earlier observations on individual taxa suggested.
• Changes in water availability will affect the function of the aquatic microfauna long before aboveground ‘drought’ effects become apparent.

Drought and heavy rainfall are contrasting conditions expected to result from increasingly extreme weather during climate change; and both scenarios will strongly affect the functioning of soil systems. However, little is known about the specific responses of soil microorganisms, whose functioning is intimately tied to the magnitude of the water-filled pore space in soil. Soil heterotrophic protists, being important aquatic soil organisms are considered as key-regulators of microbial nutrient turnover. We investigated the responses of distinct protist taxa to changes in soil water availability (SWA) using a modified enumeration technique that enabled quantification of protist taxa up to genus level. Our study revealed a non-linear shift of protist abundance with decreasing SWA and this became apparent at a maximum water-filled pore size of ≤40 μm. Generally, taxa containing large specimen were more severely affected by drought, but responses to either drought or rewetting of soils were not uniform among taxa. Changes in water availability may thus affect the functioning of key taxa and soil ecosystems long before aboveground “drought” effects become apparent.