Nitrous oxide production in an eastern corn belt soil: sources and redox range
Nitrous oxide derived from soils is a main contributor to the greenhouse gas effect and a precursor to ozone-depleting substrates; however, the source processes and interacting controls are not well established. This study was conducted to estimate the magnitude and source (nitrification vs. denitrification) of N2O production as affected by the form of N fertilizer, soil water content, and redox potential (Eh). Soils from continuous corn (Zea mays L.) experimental plots with a history of eight consecutive years of either side-dressed urea–NH4NO3 (UAN) or fall liquid swine manure (FM) were collected and N2O evolution was traced in both aerobic and anaerobic incubations using 15N labeling. Partitioning results were highly variable but suggested that enhanced denitrification occurred after an extreme increase in soil water content (from 45 to 90% water-filled pore space [WFPS]) while a more coupled nitrification–denitrification process drove N2O evolution at moderate water content (55% WFPS). Manured soils at high water contents registered shorter duration peaks but with higher overall N2O production rates than those observed at moderate water content (7-d weighted average of 0.61 vs. 0.09 µg N2O kg–1 soil h–1). Under anoxic conditions, manured soils showed higher N2O production rates than UAN soils (up to 336 and 145 µg N2O kg–1 soil h–1, respectively) shortly after flooding, which coincided with a sharp drop in Eh (from 575 to 466 mV). Irrespective of the N source, a narrow, consistent Eh range for N2O production occurred under moderate reducing conditions (420–575 mV). These results indicate that soils receiving repeated manure application that are subject to intensive, recurrent soil rewetting events may be prone to higher N2O emissions.