How do root exudates prime the decomposition of soil organic matter following drought?

Abstract

When dry soil is rewetted, contrasting microbial responses can be induced, ranging from a more resilient type of response with faster growth recovery to a more sensitive type of response with a slower growth recovery. These microbial responses will also affect soil carbon (C) cycling in the rhizosphere. However, whether the microbially-controlled priming of soil organic matter (SOM) mineralization is differently susceptible to rhizodeposits after rewetting remains elusive. We here compared two soils with microbial communities that were either resilient or sensitive to drying-rewetting (a drought cycle), where root exudation was simulated with semi-continuous additions of ¹³C-labelled glucose under stable moisture, or following a drought cycle. Under stable moisture, simulated rhizodeposits increased soil C mineralization by ca. 20% in the soil with drought-sensitive microbes and lower nitrogen (N) availability, triggering a stronger priming effect than in the soil with drought-resilient microbes. This suggested that the microbial N-demand explained the priming responses under stable moisture. A cycle of drought strengthened the priming of soil C mineralization in the soil with drought-resilient microbes, leading to 100% increases in soil C mineralization, while it was weakened to undetectable levels in the soil with drought-sensitive microbes. The weakened response to rhizodeposits was linked to high levels of resource per unit viable microbes, likely reducing the microbial N-demand and thus SOM-mining. In contrast, in the soil with drought-resilient microbes, a lesser disruption of the microbial community coincided with a smaller liberation of resources, likely strengthening microbial demands for resources and inducing bacterial mining of SOM for nutrients. Our study suggests that the trait distributions that define microbial drought resilience will determine if rhizodeposits will induce sequestration or release of soil C after a drought cycle, where drought resilient communities will result in soil C loss, while drought sensitive communities will result in soil C accrual.