Influence of Constant and Fluctuating Water Contents on Nitrous Oxide Emissions from Soils under Varying Crop Rotations

Soil water contents and cropping history influence soil organic matter decomposition, N cycling, and NO 3 − losses. However, soil water content fluctuates within and between seasons, which adds complexity to these biochemical processes. The impacts of constant soil water contents (30, 45, 60, 75, and 90% water‐filled pore space, WFPS), wet–dry (WD) cycles (90–30% WFPS), and crop rotation (monoculture corn [ Zea mays L.], a 2‐yr corn–soybean [ Glycine max (L.) Merr.] rotation, or a 3‐yr corn–soybean–winter wheat [ Triticum aestivum L.] rotation) on N 2 O emissions, inorganic N, and denitrification enzyme activity (DEA) were investigated using repacked soil cores incubated for 50 d. The 30% WFPS treatment resulted in a net accumulation of inorganic N after 50 d. In the constant‐WFPS treatments, increasing water content increased DEA, reduced soil inorganic N, and enhanced N 2 O emissions. When water contents fluctuated in five WD cycles, there was a considerable decrease in NO 3 − concentrations, high DEA activity, but low N 2 O emissions, which were similar to the emissions for the 30% WFPS treatment. Hence, fluctuating soil water content provided a soil environment that may have stimulated N 2 O reduction to N 2 or other microbial processes such as NO 3 − immobilization. At the wettest WFPS (90% WFPS), N 2 O emissions were greater under monoculture corn than the 3‐yr rotation, which is consistent with the larger decreases in inorganic N and higher DEA levels. Nitrous oxide emissions accounted for between 7.5 and 27.4% of NO 3 − loss when soils were incubated at 90% WFPS but only 3.86 to 5.12% with the WD cycles. Nitrous oxide emissions were found to be influenced by soil water content to a greater extent than crop rotation.