Impact of changing precipitation patterns on the plant-microbial response to rewetting

Abstract : Water availability governs terrestrial nutrient cycles by impacting the functioning of both plants and of soil microorganisms. The predicted changes in precipitation patterns (i.e. the magnitude and frequency of precipitation events) associated with climate change, will thus likely have important consequences on ecosystem functioning. Dry and seasonally dry ecosystems are particularly vulnerable to changes in precipitation patterns, as they are already constrained to a large extent by water availability. However, more mesic systems may also experience dry periods that may impact plant-soil functions. In this thesis, experiments in soil-only systems and plant-soil systems were used to gain insight into how the legacy effects of several weeks of exposure to contrasted precipitation patterns set the scene for the rewetting response of the system. First, in an experiment using soil-only mesocosms, we evaluated the effects of contrasting precipitation regimes on the actively growing as well as the inactive bacterial and fungal communities 2 and 5 days after rewetting, using an 18O-SIP (stable isotope probing) approach by applying H218O followed by metagenomics targeting soil bacteria and fungi. Second, we performed two separate and complementary experiments using plant-soil mesocosms with wheat plant cover. The first plant-soil experiment focused on soil depth. It determined the effects of contrasting precipitation patterns on the flux of C from plants to microbes and the microbial response to rewetting at different soil depths, using a heavy isotope tracer approach (13C-CO2) and 18O-SIP with metagenomics respectively. The second plant-soil experiment evaluated the effects of a history of contrasting precipitation patterns on the dynamics of the rewetting response of the plant-soil system over time (over 29 hours post-rewetting). In addition, two levels of N inputs allowed to determine how N availability modulated plant-soil responses. The response of the potentially active soil bacterial and fungal communities to rewetting was assessed using targeted metagenomics. The responses of biogeochemical cycles were evaluated using heavy isotope tracers (13C-CO2 and 15N-NO3-) to quantify C flux from plants to soil microorganisms and plant-microbial competition for N over time post-rewetting.We found that precipitation patterns shaped plant morphology and physiology, microbial community composition as well as soil N cycling in our systems, which set contrasting scenes for the rewetting responses in our systems. In particular, infrequent precipitation patterns (cycles of longer dry periods followed by larger magnitude rain events) resulted in increased microbial N transformation potentials and smaller inorganic N pools. The rewetting responses were determined by evaluating C dynamics (plant-microbial coupling and soil CO2 efflux rate), N dynamics (plant-microbial competition for N and soil N2O efflux rate) and microbial dynamics (composition of active and potentially active bacterial and fungal communities after rewetting). First, we found that plant-microbial coupling (i.e the microbial assimilation of C from fresh photosynthate) may be reduced under more infrequent precipitation patterns, especially near the soil surface, and under conditions of low N availability. Our findings also suggest that whilst in soil-only systems, dead microbial cells appear to be a major source fuelling soil CO2 efflux pulse upon rewetting, in plant-soil systems root respiration plays an important role in the magnitude of the CO2 efflux upon rewetting. Second, concerning soil N dynamics, we found, in concurrence with previous studies, that soil microorganisms were the stronger competitor for N over short time scales, likely due to their overall fast response rates and high affinity for substrate, whilst plants outcompeted soil microbes for soil N assimilation, over longer time scales likely taking advantage of the fast microbial turnover (...).
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Ilonka Engelhardt. Impact of changing precipitation patterns on the plant-microbial response to rewetting. Agricultural sciences. Université Bourgogne Franche-Comté, 2018. English. ⟨NNT : 2018UBFCK017⟩. ⟨tel-02144321v2⟩

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