Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Effects of climate extremes on N2O fluxes in a long-term fenced semiarid grassland

Auteur : LI, Linfeng

Université de soutenance : Griffith University,

Grade : Doctor of Philosophy (PhD) 2021

Résumé partiel
Nitrous oxide (N2O) flux is the third most important anthropogenic greenhouse gas and soil is the single largest source of N2O. However, the responses of N2O to climate extremes are largely unknown, although which is increasing in magnitude and frequency. This thesis examined the impacts of multiple climate extremes, including drought, heat wave, hot drought (drought in combination of heat wave), heavy rainfall, as well as precipitation variability, on N2O fluxes and underlying abiotic and microbial mechanisms as well as how the seasonal timing regulated such responses based on a manipulative experiment in a semiarid grassland from 2014 to 2016. N2O fluxes were measured over the whole grass growing season from May to September. Meanwhile, soil carbon (C) and nitrogen (N) availability indicators (dissolved organic C (DOC) and soil inorganic N (SIN)) as well as the abundances of soil functional genes (archaeal amoA, bacterial amoA, nosZ, narG, nirK, and nirS) involved in nitrification and denitrification were measured. In Chapter 2, based on forty-six published studies of N2O fluxes and relevant soil functional genes (SFG), we found increased temperature increased N2O emissions by 33% at the global scale. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N2O emissions (most effectively induced by open-top chambers). Whole-day or whole-year warming treatment significantly enhanced N2O emissions, but day-time, night-time or short-season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N2O emission by an average of 55%, while decreased precipitation suppressed N2O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U-shape relationship with soil moisture ; further insight into biotic mechanisms underlying N2O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. The results indicate that climate change substantially affects N2O emission and highlight the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change. In Chapter 3, N2O emission was suppressed during the droughts. Meanwhile, drought reduced soil water content (SWC), microbial biomass carbon (MBC), SIN, and DOC contents, and the abundance of archaeal amoA, nirK, and narG. After the drought events and once the soil was rewetted, the SIN, DOC, soil functional genes, and resultant N2O emission were completely recovered to the magnitude of the ambient control. In contrast, these variables overall remained steady in response to heat waves over the growing season.

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