QUANTIFYING CARBON FLUXES AND ISOTOPIC SIGNATURE CHANGES ACROSS GLOBAL TERRESTRIAL ECOSYSTEMS
This thesis is a collection of three research articles to quantify carbon fluxes and isotopic signature changes across global terrestrial ecosystems. Chapter 2, the first article of this thesis, focuses on the importance of an under-estimated methane soil sink for contemporary and future methane budgets in the pan-Arctic region. Methane emissions from organic-rich soils in the Arctic have been extensively studied due to their potential to increase the atmospheric methane burden as permafrost thaws. However, this methane source might have been overestimated without considering high affinity methanotrophs (HAM, methane oxidizing bacteria) recently identified in Arctic mineral soils. From this study, we find that HAM dynamics double the upland methane sink (~5.5 TgCH4yr-1) north of 50°N in simulations from 2000 to 2016 by integrating the dynamics of HAM and methanogens into a biogeochemistry model that includes permafrost soil organic carbon (SOC) dynamics. The increase is equivalent to at least half of the difference in net methane emissions estimated between process-based models and observation-based inversions, and the revised estimates better match site-level and regional observations. The new model projects double wetland methane emissions between 2017-2100 due to more accessible permafrost carbon. However, most of the increase in wetland emissions is offset by a concordant increase in the upland sink, leading to only an 18% increase in net methane emission (from 29 to 35 TgCH4yr-1). The projected net methane emissions may decrease further due to different physiological responses between HAM and methanogens in response to increasing temperature. This article was published in Nature Climate Change in March 2020.
In Chapter 3, the second article of this thesis, I develop and validate the first biogeochemistry model to simulate carbon isotopic signatures (δ13C) of methane emitted from global wetlands, and examined the importance of the wetland carbon isotope map for studying the global methane cycle. I incorporated a carbon isotope-enabled module into an extant biogeochemistry model to mechanistically simulate the spatial and temporal variability of global wetland δ13C-CH4. The new model explicitly considers isotopic fractionation during methane production, oxidation, and transport processes. I estimate a mean global wetland δ13C-CH4 of -60.78‰ with its seasonal and inter-annual variability. I find that the new model matches field chamber observations 35% better in terms of root mean square estimates compared to an empirical static wetland δ13C-CH4 map. The model also reasonably reproduces the regional heterogeneity of wetland δ13C-CH4 in Alaska, consistent with vertical profiles of δ13C-CH4 from NOAA aircraft measurements. Furthermore, I show that the latitudinal gradient of atmospheric δ13C-CH4 simulated by a chemical transport model using the new wetland δ13C-CH4 map reproduces the observed latitudinal gradient based on NOAA/INSTAAR global flask-air measurements. I believe this study is the first process-based biogeochemistry model to map the global distribution of wetland δ13C-CH4, which will significantly help atmospheric chemistry transport models partition global methane emissions. This article is in preparation for submission to Nature Geoscience.
Chapter 4 of this thesis, the third article, investigates the importance of leaf carbon allocation for seasonal leaf carbon isotopic signature changes and water use efficiency in temperate forests. Temperate deciduous trees remobilize stored carbon early in the growing season to produce new leaves and xylem vessels. The use of remobilized carbon for building leaf tissue dampens the link between environmental stomatal response and inferred intrinsic water use efficiency (iWUE) using leaf carbon isotopic signatures (δ13C). So far, few studies consider carbon allocation processes in interpreting leaf δ13C signals. To understand effects of carbon allocation on δ13C and iWUE estimates, we analyzed and modeled the seasonal leaf δ13C of four temperate deciduous species (Acer saccharum, Liriodendron tulipifera, Sassafras albidum, and Quercus alba) and compared the iWUE estimates from different methods, species, and drought conditions. At the start of the growing season, leaf δ13C values were more enriched, due to remobilized carbon during leaf-out. The bias towards enriched leaf δ13C values explains the higher iWUE from leaf isotopic methods compared with iWUE from leaf gas exchange measurements. I further showed that the discrepancy of iWUE estimates between methods may be species-specific and drought sensitive. The use of δ13C of plant tissues as a proxy for stomatal response to environmental processes, through iWUE, is complicated due to carbon allocation and care must be taken when interpreting estimates to avoid proxy bias. This article is in review for publication in New Phytologist.