Does an enhanced microbial diversity promote the resistance of soil multifunctionality against drought events in amended soils?

Abstract

A large fraction of the Mediterranean soils is threatened by losses of organic matter and biodiversity, which could compromise the provision of soil ecosystem services and the stability of ecosystems in the face of climate change. In this work we explore several hypotheses related to the role of C inputs and microbial diversity on soil multifunctionality and its resistance to drought in degraded Mediterranean soils. We designed a factorial experiment to test the effect of the addition of an organic amendment and of microbial diversity (using four inoculants with different abundance and diversity of soil microbiota), on the resistance of soil functionality against drought in pot mesocosms. Pots were sown with a forage mixture (Lolium rigidum and Medicago polymorpha), and plant productivity, soil chemical properties, and microbial activity and diversity were measured before and after a simulated drought event. The amendment favored soil moisture, enhancing the stability of the productivity of M. polymorpha. In contrast, the manipulation of inoculation load had a limited effect on the resistance of microbiological activity. Indeed, microbial functioning was highly resistant to reduced water inputs, probably related to the prevalence of Gram positive bacteria. Besides, the effect of microbial diversity on soil multifunctionality was limited. Structural equation modelling confirmed that the enhancement of multifunctionality after soil amendment was attributed to the direct effect of organic C on soil moisture and chemical fertility. In these degraded soils, physico-chemical limitations are the major drivers of soil multifunctionality rather than bacterial or fungal diversity.