Root-derived nitrous oxide emissions from an Upper Midwest agricultural ecosystem

Agroecosystems are the dominant source of anthropogenic nitrous oxide (N2O) emissions globally, yet the partitioning of nitrogen sources supporting N2O emissions is not well understood. Fertilizer-derived N2O emissions receive significant attention, while N2O emissions from organic nitrogen sources, particularly belowground sources, are rarely studied. Here, in situ corn roots (Zea mays L.) were isotopically-labeled with nitrogen (N) and carbon (C) to examine effects of different long-term management systems on root-derived N2O emissions measured during the following soybean crop in southwest Minnesota, USA. Systems differed in management intensity (tillage and fertilization), crop rotation diversity (two or four crops), and fertilizer type (inorganic or organic). The average contribution of root-derived nitrogen to cumulative N2O-N emitted over the growing season was 8%, and was higher in 2-year (11%) than 4-year rotations (6%). The fractional loss of root-derived N as N2O, which is an estimate of the annual emission factor for root-derived N2O, was small (0.07-0.52%). Management intensity effects on root-derived N2O emissions and on the root-derived fraction of N2O emitted differed between two growing seasons as did the effects of fertilizer type on root-derived N cycling rates. Overall, rotation diversity (2 vs. 4-year rotations) exhibited the strongest management effect on root-derived N2O emissions, suggesting that root-derived N2O emissions could be mitigated by greater crop rotation diversity.