Contribution of crop residue, soil, and fertilizer nitrogen to nitrous oxide emissions varies with long-term crop rotation and tillage.

ABSTRACT

Agriculture is an important contributor to N2O emissions - a potent greenhouse gas - with high peaks occurring when soil mineral nitrogen (N) is high (e.g., after mineralization of organic N and N fertilizer application). Nitrogen dynamics in soil and consequently N2O emissions are affected by crop and soil management practices (e.g., crop rotation and tillage), an effect mostly assessed in the literature through comparisons of total N2O emission. Hence, information is scarce on the effect of these management practices on specific N sources affecting N2O emissions (i.e., N fertilizer, soil, above and belowground crop residues) - a knowledge gap explored in this study with the use of 15N tracers. The isotope approach enabled refinement on global N2O budget by directly determining the emission factors (EF) of above and belowground crop residues that vary in chemical composition and comparison with default EF values (e.g., IPCC EFs). Our experiment was conducted over the full-cycle of long-term crop rotations to (i) compare N2O totals and intensity, under no-tillage and conventional tillage, simple and diverse rotation; (ii) partition total N2O emissions into soil, N fertilizer, above and belowground crop residue N sources; (iii) compare the 12-month EF of crop residue against the default values proposed by IPCC (2019). For the tillage effect, annual N2O emissions were from 1.2- to 2.0-times higher on CT than NT soil due to 40% increased soil N derived N2O emission in CT. The diversified crop rotation emitted 1.3-times higher N2O than the simple rotation over the full-cycle of the rotations, but the effect was due to differences in N fertilizer rate between the rotations since emissions were equivalent when scaled by N rate. Finally, our results suggested that default IPCC EF are overestimated for crop residues under CT and NT, simple and diverse rotations as measured EFs never surpassed 0.1%.