Does drainage history mediate the response of greenhouse gas emissions to precipitation in an agricultural field?
Precipitation is expected to increase in intensity and frequency in the Midwestern United States due to climate change. This may result in increased flooding of poorly drained upland soils, which can feed back on climate change by altering greenhouse gas (GHG) emissions. In the short term, GHG emissions may respond to changes in oxygen, water, and nutrient availability caused by flooding. Over a longer timescale, more frequent or longer duration flooding may alter the soil microbial community. Here we determined how soil drainage history affects the response of soil GHG emissions to rain events. We measured nitrous oxide (N2O) and carbon dioxide (CO2) fluxes across a ponding gradient (low, intermediate, and severe-ponding) in an agricultural soybean field in Urbana, Illinois (n = 6 per ponding class) on 11 sampling dates over a three week time period that included nine small rain events (< 15 mm) and only two large rain events (> 15 mm) that led to flooded conditions in the severe-ponding class. Due to auto-correlation in fluxes among sampling dates, we only performed statistical tests on fluxes measured on June 23 and 24, which occurred before and after a 35 mm rain event on the afternoon of June 23.
N2O emissions from the low-ponding class increased from 3.05 ± 0.12 to 25.8 ± 12.3 ng-N cm-2 h-1 after the June 23 rain event (p = 0.002) whereas N2O emissions from the severe-ponding class decreased from 14.7 ± 3.30 to 4.58 ± 1.99 ng-N cm-2 h-1 (p = 0.001). Before the June 23 rain event, CO2 emissions were similar among ponding classes, but afterwards, CO2 emissions dropped in both the severe and intermediate ponding classes (p < 0.001). Post-rain emissions followed this pattern across the gradient: severe-ponding (2.33 ± 0.56 ug-C cm-2 h-1) < intermediate (5.24 ± 1.10 ugC cm-2 h-1) < low-ponding (20.7 ± 4.70 ug-C cm-2 h-1) (p < 0.001). These responses occurred after both >15 mm rain events but not after the smaller rain events, suggesting diffusion limitation of soil-atmosphere gas exchange under flooded conditions. The opposing patterns in N2O emissions in the severe-ponding versus low-ponding classes suggest an additional effect of drainage history on the processes leading to soil N2O production and consumption. Thus, the regional climate change feedback effect from precipitation-induced changes in soil GHG emissions in the Midwest will depend on the relative area that has historically flooded after high rainfall events versus not.