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

Abstract

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.\nIn 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.\nIn 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. Additionally, there were no clear interactions of drought and heat wave on these variables. The results suggest that N2O fluxes had low resistance but high resilience to droughts while N2O fluxes were resistant to heat wave alone or in combination with drought, and that soil water content-induced changes in C and N substrates and the community size of soil functional microorganisms jointly influenced N2O responses to the climate extremes.\nIn Chapter 4, early-season drought affected N2O fluxes with high interannual variability, due to corresponding changes in DOC and the abundances of soil functional genes (bacterial amoA, nirK, nirS, and nosZ). In contrast, mid-season drought had consistently negative effects on SIN, DOC and the abundance of archaeal amoA, nirK, and narG and thereby N2O emissions during the treatment across the three years (but these variables were completely recovered at the end of the growing season). Similarly, late-season drought reduced DOC and SIN, changed the abundance of nirK, nirS, and nosZ, but had little effects on N2O flux over the three years. Our results highlight that N2O fluxes in response to drought events depends on seasonal timing of the drought and that soil functional genes and C/N substrates co-regulated N2O fluxes during the early- and mid-season drought but not the late-season drought owing to low-temperature limitation.\nIn Chapter 5, mid-season heavy rainfall significantly promoted soil N2O emissions, attributable to increases in the denitrifying nirK and nirS abundances at the higher soil water contents. However, archaeal and bacterial amoA and narG genes did not change significantly due to counteracting effects of increased soil water content (positive) and soil pH (negative). Enhanced accumulative gaseous emissions and probably leaching under mid-season heavy rainfall led to reduction in soil total N. In contrast, late-season heavy rainfall did not change N2O emissions and soil total N contents even though soil water content, soil pH and nirK and nirS abundance were significantly increased, perhaps due to limitation by low temperature. The results highlight that heavy rainfall timing during the plant growing season strongly regulates N2O emission responses and that increase in heavy rainfalls in the middle part of the growing season may potentially cause a positive feedback to global warming and exacerbate N limitation in terrestrial ecosystems.\nIn Chapter 6, although changes in precipitation variability altered the seasonal mean SWC and SIN. However, MBC and abundances of the soil functional genes involved in nitrification and denitrification remained largely unaffected. Furthermore, seasonal mean N2O fluxes and soil total N content were not affected by the precipitation variability. The results suggested that soil biochemical properties related to N2O production in this semiarid grassland were insensitive to the precipitation variability within the growing season.\nIn summary, the findings in the thesis enhanced our understanding of the impacts of climate extremes on N2O fluxes, timing effects of the extreme events and the underlying ecological mechanisms, which is of significance in assessing ecosystem N cycling and predicting global warming feedback effects under changing climate.