Long-term crop rotation and tillage effects on nitrous oxide emissions derived from crop residues, soil, and nitrogen fertilizer
Sustainable crop production, with low greenhouse gas (GHG) footprint, has become imperative to ensure food security in a world with increasing demands for agricultural products. Agriculture is an important contributor to N2O emissions – a potent GHG whose emissions are high when soil mineral-N is increased (e.g., following soil freeze-thaw cycles and N fertilization). Nitrous oxide emissions are affected by soil and crop management practices (e.g., tillage and crop rotation), are highly episodic, and subjected to spatial variations (i.e., “hot spots”). Since most of the studies from the literature assessed tillage and crop rotation effects on total N2O emissions, quantification of the contribution of specific N sources (i.e., soil, crop residues, N fertilizer) in response to contrasting tillage and crop rotation levels is needed. For this thesis, I addressed this knowledge gap with a 15N isotope study, which was conducted for two non-growing seasons - NGS (Chapter 2) and growing seasons - GS (Chapter 3). Annual emission factors for crop residues and N fertilizer were derived and the datasets were used to study spatial variation in N2O emissions (Chapter 4). For NGS, tillage induced higher N2O emissions than no-tillage, mostly derived from soil nitrogen since the contribution from crop residue was minimal (<2%). These results indicate refinements in emission inventories for cold climates are needed and that freeze-thaw nitrogen substrate release for N2O production does not come from crop residues. A similar trend, with higher emission for tilled soil, occurred for GS and total N2O accumulated over 4-yr. Also, N2O emissions over 4-yr from the diversified rotation were 1.3-times higher than the simple rotation, induced by differences in N fertilizer usage as fluxes were equivalent when scaled by N fertilizer rates. The recommended emission factor for crop residues is overestimated as measured values never surpassed 0.1% (vs. 0.6% IPCC, 2019). In both NGS and GS (annual cropping systems), spatial variation in N2O flux occurred, sometimes even within short distances (e.g., GS datasets). Ranges of uncertainties due to spatial variation were overall wider for tilled than no-tilled soil. The methodology used in this thesis advanced our knowledge on N2O emissions.