Disproportional oxidation rates of ammonia and nitrite deciphers the heterogeneity of fertilizer-induced N2O emissions in agricultural soils

Nitrous oxide (N2O) is a potent greenhouse gas and ozone-depleting chemical. Although nitrogen (N) fertilizer rate is the best single predictor of N2O emissions, the spatial heterogeneity of the emission factor is striking and poorly understood. Nitrite (NO2-) dynamics have been reported to be correlated with N2O production in many soils; however, NO2- is often overlooked owing to its rare accumulation, and the underlying mechanisms of NO2- accumulation and the relationship with spatiotemporal heterogeneity of N2O emissions in response to N addition remains unclear. Here, we collected upland agricultural soils that cover the main upland agricultural production regions of China and investigated the soil-specific effects on N2O emissions, changes in microbial abundances and community structures of nitrifiers, and co-occurrence networks for ammonium (NH4+)-based fertilizer addition. Our results showed that high NO2- accumulation and high N2O emissions were generally correlated with low nxrA/AOB gene ratios (R-2 = 0.90 and 0.86, respectively, P < 0.01). Further network analyses revealed that NH4+ addition increased the network links of the nitrification functional communities by 23% and enhanced the interrelationship between AOB and nitrite oxidizers. By synthesizing our data with previous studies in the literature, we found that soil N2O emission rate and nxrA/AOB gene ratio exhibit a threshold behavior worldwide, suggesting that disproportionality of NH3 and NO2- oxidations rates are involved in the heterogeneity of fertilizer-induced N2O emissions in agricultural soils. These findings enhance our understanding of spatiotemporal heterogeneity in NO2- accumulation and N2O emissions within upland agricultural soils in view of nitrification functional communities.