Effect of increased N use and dry periods on N2O emission from a fertilized grassland

To better understand the effects of increased N input and dry periods on soil nitrous oxide (N2O) emission, we examined a unique data-set of weather, soil microclimate, N input, and N2O emissions (using the eddy covariance method), measured at a fertilized grassland over the period 2003–2008. We found that the N2O emission (11.5 kg N ha−1 year−1), the ratio of N2O emission to N input (3.4), and the duration of elevated N2O flux (57 days) in 2003 were about two times greater than those of the following years. 2003 had the highest annual N input (343 kg N ha−1 year−1) which exceeded the agronomical requirements for Irish grasslands (up to 306 kg ha−1 year−1). In the summer of 2003, the site had a significantly higher soil temperature, lower WFPS and lowest rainfall of all years. Large N2O emission events followed rainfall after a long dry period in the summer of 2003, attributed to dominant nitrification processes. Furthermore, in the non summer periods, when temperature was lower and WFPS was higher and when there were prior N applications, lower N2O emissions occurred and were attributed to dominant denitrification processes. Throughout the study period, the N input and soil dryness related factors (duration of WFPS under 50%, summer average WFPS, and low rainfall) showed exponential relationships with N2O emission and the ratio of N2O emission to N input. Based on these findings, we infer that the observed anomalously high N2O emission in 2003 may have been caused by the combined effects of excess N input above the plant uptake rate, elevated soil temperature, and N2O flux bursts that followed the rewetting of dry soil after an unusually long dry summer period. These results suggest that high N input above plant uptake rate and extended dry periods may cause abnormal increases in N2O emissions.