Reviews and syntheses: Constraining the Nitrous Oxide (N₂O) Budget Across Scales: A Critical Review of Sources, Sinks, Estimation Methods, and Knowledge Gaps

Abstract. Nitrous oxide (N₂O) budget assessments integrate evidence from microbial processes, surface flux measurements, emission inventories, and atmospheric observations, but these approaches do not necessarily constrain the same quantities. We systematically reviewed 80 peer-reviewed studies published between 2002 and 2024, using a PRISMA-based screening procedure to examine research themes, environmental systems, and estimation approaches. We additionally audited the evidence endpoints and accounting boundaries of 23 selected studies. Of the 80 studies, 53 were classified as source-focused, 22 as budget-imbalance-focused, and five as sink-focused. Although 12 studies were originally classified as top-down, six provided observation-constrained spatial emission inversions; two inferred sources through other atmospheric approaches, and four primarily examined atmospheric lifetime, transport variability, or observing-system capability. The audit also showed that microbial N₂O reduction, locally negative surface exchange, and atmospheric chemical removal are distinct quantities: evidence for one cannot, by itself, establish the magnitude of another. Some integrated assessments incorporate shared model inputs or source products, warranting caution when interpreting agreement as independent confirmation. These findings distinguish the distribution of research methods from the comparability and independence of the budget constraints they provide. Greater transparency in accounting boundaries, evidence provenance, and observational evaluation is needed to improve N₂O budget interpretation.