A Meta-Analysis of the Nonlinearity of Direct Annual N2O Emissions in Response to Nitrogen Fertilization.

Nitrous oxide (N₂O) is a major greenhouse gas. Total input of synthetic and organic nitrogen (N) fertilizer is the best single predictor of total N₂O emissions. Knowledge of more precise N₂O emission response is essential for improving global emission budgets of N₂O and understanding efficient mitigation strategies for emission reduction is an important task. For present meta-analysis we selected studies from published literature with at least 3 N fertilizer rates under identical field conditions. We calculated N₂O emission factors (EF) as a percentage of applied N converted to N₂O emissions additional to emissions from non-fertilized field. We investigated the rate of change in N₂O EF with N application rate. We found that N₂O response grew significantly faster than linearly (positive rate of EF change), a model that is currently being applied to describe N₂O emissions in fertilized systems. Faster-than-linear mean N₂O response was present for synthetic fertilizers, majority of crops, soils with high organic content, low mean annual temperatures, and low pH. N-fixing crops have higher rate of EF change than non-fixing crops. Higher rate of EF change is also present in soils with organic carbon content > 1.5%, acidic soils, in experiments with single fertilization event, and experiments with chamber area < 0.2 m2 compared to the respective complementary groups. Our results agree with previous estimates present in the primary literature, but have higher degree of certainty in the resulting emissions model. We have generated a global default variable EF (Tier 1) and a set of variable EFs for particular crops and soil types (Tier 2: for specific regions – where the dataset is sufficiently abundant) that we propose can be used as a more biologically appropriate value for estimating direct N₂O emissions from agricultural cropland. We suggest that the use of one or more of these EFs will improve the accuracy of national and regional inventories for direct N₂O emissions from agricultural land.