Memory Effect of Past N Fertilization Increases N2O Emissions from Unfertilized Controls.

Nitrous oxide (N₂O) is a potent greenhouse gas that accounts for about half of agricultural greenhouse gas emissions.  The IPCC default method for estimating N₂O emissions, by using a fixed percentage of applied N (1%), approximates a global average emissions factor fairly accurately, but ignores much of the inherent variability in N₂O emissions due to variation in environmental and agronomic factors.  Recent efforts have attempted to account for factors such as N fertilizer type, soil type, and climate, but few studies have examined the role of historical management practices, which are likely to have a significant influence on N₂O emissions, and thus N₂O emissions factors (EF).  In this study, we specifically sought to understand the role of historical N management on N₂O emissions.  We measured N₂O emissions during a ten-day incubation from soils collected from a long-term (20-yr) agricultural experiment site that differed only in historical N fertilization (84 kg N ha^-1 per growing season vs. a 0 N control).  Historically fertilized soils in the wheat phase of the two-year rotation had 4.4 times greater cumulative N₂O emissions during a ten-day incubation than the corresponding historically unfertilized soils. The historically fertilized soils entering the fallow phase of the same rotation had 18 times greater cumulative N₂O emissions than historically unfertilized soils.  Higher N₂O emissions were positively correlated with extractable N followed by potentially mineralizable N.  Results show that historical N fertilization has the potential to significantly increase N₂O emissions independent of N fertilization during the growing season.  This finding builds upon similar conclusions from other long-term experiments and suggests that prior N fertilization history should be accounted for when obtaining background emissions of N₂O in agricultural systems.