EFFECTS OF ELEVATED ATMOSPHERIC CO2 ON SOIL ORGANIC CARBON STOCKS AND STABILITY IN THE MOJAVE DESERT

Abstract

Elevated atmospheric CO2 (eCO2) is perturbing the global carbon (C) cycle. There is high uncertainty in how this disturbance will impact soil organic carbon (SOC) stocks. Quantifying SOC stocks and susceptibility to disturbance in deserts is critical because of the widespread geographic distribution of arid ecosystems. This thesis investigates the long-term effects of eCO2 on SOC stocks at the Nevada Desert Free-Air Carbon Dioxide Enrichment Facility (NDFF). Chapter one compares two methods of soil inorganic carbon removal on SOC stock measurements and how these methods differ by CO2 treatment. We find that 30% of SOC can be lost using acid washing relative to fumigation and that this effect is greatest in soils from control plots at NDFF. These results indicate that SOC stabilization may change after long-term exposure to eCO2. Chapter two directly applies the finding from chapter one to understand variation in SOC stock measurements from NDFF across multiple studies. The result of a direct comparison demonstrates how statistical choices, acid pretreatment methods, and heterogeneity of soils can impact our understanding of eCO2 effects at NDFF. Chapter three quantifies changes in SOC stability using a two-pool density fractionation. We find that eCO2 decreases fast-cycling particulate organic matter and does not affect slower-cycling mineral associated organic matter. Chapter four quantifies eCO2 effects on microbial mineralization in NDFF soils. We measure soil C priming with a 13C-labelled glutamic acid substrate using a short-term incubation and find eCO2 effects are strongest in water amended soils. Priming is not affected by eCO2 in water or C amended soils these conditions but is negatively correlated with SOC concentration. Together, these chapters indicate NDFF soils are primarily C limited and the increase in organic C availability from eCO2 treatment drives loss of SOC stocks. Addition of C in the form of acidic root exudates may destabilize organic matter and promote mineralization via increased microbial access, particularly from the C-rich particulate organic matter pool. These results emphasize the need to study multiple drivers of global change in arid ecosystem to fully understand impacts on SOC stocks and the global C cycle.