How do nitrogen-fixing trees influence the extent to which forests mitigate and exacerbate climate change?

Nitrogen (N)-fixing trees can both mitigate climate change, by relieving N limitation of plant growth which promotes carbon dioxide (CO²) sequestration in plant biomass, and exacerbate climate change, by stimulating nitrification and denitrification which promotes nitrous oxide (N²O) emissions from soils. The balance between the negative radiative forcing (CO² sequestration in plant biomass) and positive radiative forcing (N²O emissions from soils) of N-fixing trees is unresolved. In this thesis I use a sequence of theoretical and empirical approaches to investigate the influence of N-fixing trees on CO² sequestration by forests and N²O emissions from forest soils, i.e., the net CO²-N²O effect of forests. The first chapter establishes a basis for the N²O effect of N-fixing trees with a meta-analysis, to accompany existing meta-analyses of the CO² effect of N-fixing trees. Chapter one demonstrates that N- fixing trees significantly increase N²O emissions from forest soils relative to non-fixing trees. The second chapter explores the controls and potential global importance of the net CO²-N²O effect of N-fixing trees using a theoretical ecosystem model. The third chapter explores the net CO²-N²O effect of N-fixing trees under manipulations of these controls with a field experiment paired with a modified version of the theoretical ecosystem model from the second chapter. Together, chapters two and three suggest that the net CO²-N²O effect of N-fixing trees is controlled by N limitation of plant growth and the extent to which N-fixing trees can regulate N fixation: N-fixing trees mitigate climate change relative to non-fixing trees under N limitation of plant growth, but N-fixing trees that cannot regulate N fixation exacerbate climate change relative to non-fixing trees under non-N limitation of plant growth. The fourth chapter represents the ecological mechanisms studied in chapters one, two and three in a land model: LM4.1-BNF is a novel representation of biological N fixation (BNF) and an updated representation of N cycling in the Geophysical Fluid Dynamics Laboratory Land Model 4.1 (LM4.1). LM4.1-BNF includes a mechanistic representation of asymbiotic BNF by soil microbes, the competitive dynamics between N-fixing and non-fixing plants, N limitation of plant growth, and N2O emissions from soils. Together these chapters elucidate the influence of N-fixing trees on the capacity of forests to mitigate and exacerbate climate change and establish a framework to analyse and project the trajectory of the net CO²-N²O effect of forests under global change.