UC Riverside

Drylands compose approximately 40% of the earth’s land surface; a hallmark of these systems is low average soil moisture and infrequent precipitation (or irrigation) which limit biogeochemical cycling rates. However, following a re-wetting event, soil metabolism can recover within minutes, leading to discrete pulses of high rates of carbon dioxide and nitrogen oxide trace gas flux that can exceed those of more mesic systems. Although evidence suggests that moisture, temperature, and substrate availability are predominant abiotic predictors of soil metabolism, these mechanisms have not been well-described interactively and in the context of re-wetting pulses. In this dissertation, I utilized a unique network of soil chambers and trace gas analyzers to quantify patterns of carbon dioxide (CO2), nitrous oxide (N2O), and nitric oxide (NO) pulses driven by temperature-moisture-substrate interactions. In an urban shrubland context (Chapter 1), I investigated pulse consequences of exotic grass litter accumulation during shrubland-grassland ecosystem type conversion. I showed that rewetting pulses of CO2, N2O, and NO differ across seasonal wetting history, and that invasive grass litter may provide a labile C source that stimulates CO2 and N2O, but not NO, emissions from soils in this system. In a high-temperature agricultural context (Chapter 2), I explored the potential for subsurface drip irrigation to reduce soil trace gas emissions compared to traditional furrow irrigation management. I found substantial evidence that drip irrigation reduced water use, irrigation-induced pulses, and per-yield emissions of CO2, N2O, and NO from alfalfa and sudangrass fields. Particularly, the benefits of drip irrigation were strongest for fertilized sudangrass and in hot summer months, suggesting that irrigation is a strong control over emissions in optimal temperature and substrate conditions. In a desert context (Chapter 3), I tested interactive mechanisms limiting CO2 pulses by explicitly manipulating temperature and coupled carbon-nitrogen availability. I found evidence that C and N stoichiometry, not strictly limitation, determined CO2 pulse responses to soil re-wetting, and pulse responses to temperature were complicated by interactions with soil moisture immediately following rewetting. My dissertation research identifies a theme that temperature-moisture-substrate interactions drive pulse responses to dryland soil rewetting.