Carbon and Greenhouse Gas Dynamics in Annual Grasslands: Effects of Management and Potential for Climate Change Mitigation

Ecosystem management practices that sequester carbon (C) may play an important role in mitigating climate change. Grasslands managed for livestock (e.g., rangelands) constitute the largest land-use area globally. Critical components of the long-term sustainability of rangelands are the maintenance of net primary production (NPP) and soil organic carbon (C) pools. However, overgrazing, plant invasions, and climate change have led to significant C losses from many rangeland ecosystems. Thus, management practices may have considerable potential to restore or increase grassland C storage and help mitigate climate change. Practices that promote C sequestration may have valuable co-benefits, including increased forage production and improved soil water holding capacity. Despite the potential for C sequestration through management interventions, the question remains largely unexplored in grassland ecosystems. I used a combination of laboratory experiments, field manipulations, and modeling simulations to examine the effects of rangeland management practices on C sequestration and greenhouse gas emissions. The specific goals of this research were to 1) assess the immediate and carry-over effects of management practices on the net C balance and greenhouse gas emissions in grasslands amended with compost, 2) measure changes to soil C and N stocks following amendment, 3) investigate the long-term fate of compost C and net climate change mitigation potential, and 4) explore the extent of tradeoffs between C sequestration strategies and vegetation characteristics. In the first chapter, I conducted a three-year field manipulation replicated within and across valley and coastal grassland sites to determine the effects of a single application of composted organic matter amendment on net ecosystem C balance. Amendments increased C losses through soil respiration, and estimates of net C storage were sensitive to models of respiration partitioning of autotrophic and heterotrophic components. Over the three-year study, amendments increased C inputs by stimulating net primary production by 2.1 ± 0.8 at the coastal grassland and 4.7 ± 0.7 Mg C ha-1 at the valley grassland. Carbon gains through above- and belowground NPP significantly outweighed C losses, with the exception of a sandy textured soil at the coastal grasslands. Treatment effects persisted over the course of the study. Net ecosystem C storage increased by 25 to 70 % over three years, not including direct C inputs from the amendment.The purpose of chapter two was to further investigate changes to rangeland soil C and N stocks three years after a one-time application of composted organic material. Increases in bulk soil C, though often difficult to detect over short timeframes, were significant at the valley grassland study site. Physical fractionation of soil revealed greater amounts of C and N in the free and occluded light fractions by 3.31 ± 1.64 and 3.11 ± 1.08 Mg C/ha in the valley and coastal grassland, respectively. Analysis of the chemical composition of soil fractions by diffuse reflectance infrared Fourier transform (DRIFT) showed chemical protection and inclusion of compost C into the light fractions. The combination of physical and chemical analyses suggests that the newly incorporated C was physically protected and less available for decomposition.In the third chapter, I employed the ecosystem biogeochemical model, DAYCENT, to investigate the short (10 yr), medium (30 yr), and long-term (100 yr) climate change mitigation potential of compost amendments to grasslands. Climate change mitigation potential was estimated as the balance of total ecosystem C sequestration minus soil greenhouse gas emissions and indirect emissions of N2O via nitrate leaching. The model was parameterized using site-specific characteristics and validated with data from the three-year field manipulation. Model simulations included variations in the applications rate and C:N ratio of the composted material. Above- and belowground NPP and soil C pools increased under all amendment scenarios. The greatest increase of soil C occurred in the slow pool. Ecosystem C sequestration rates were highest under low C:N scenarios, but these scenarios also resulted in greater N2O fluxes. Single or short-term applications of compost resulted in positive climate change mitigation potential over 10 and 30-year time frames, despite slight offsets from increased greenhouse gas emissions. Finally, chapter four examined important tradeoffs between rangeland C sequestration activities and vegetation characteristics. I measured aboveground biomass, plant N content, vegetation communities, and the abundance of noxious weed species for four years following single management events of compost amendment, keyling plowing, and a combination of amendment and plowing. During the first year, plant N content and aboveground biomass was significantly higher in the amended plots and lower in the plowed plots. In the amended plots, forage quantity and quality increases were sustained over the four-year study. During spring grazing events, cows consumed more forage from amended plots without adversely increasing grazing impacts on residual biomass. Plant communities at both grasslands were relatively resistant to management events, however there were short-term declines in the abundance of a noxious annual grass at the valley grassland and increases in a noxious forb at the coastal grassland. Grassland management practices, such as the application of composted organic matter, have considerable potential to mitigate climate change while improving plant production, soil fertility, and diverting organic wastes from landfills. This research illustrates the potential for grassland management to sequester while explicitly considering impacts on greenhouse gas emissions, plant production, and vegetation communities over multiple time frames. Overall, my dissertation contributes toward a better understanding of the role of ecosystem management interventions in climate change mitigation