Response of Soil CO2 Flux of a Crop Field to Rain Pulse.

Soil microbes can start utilizing carbon substrate immediately when environmental conditions become favorable, such as when it rains. This is so-called Birch Effect. So often in the literature we see reports which show an enhancement of soil CO₂ respiration (total CO₂ produced inside the soil profile) after rain events, especially for studies from ecosystems when the soil surface is dry. The same cannot be said for soil CO₂ flux (CO₂ transport across the soil surface). Based on a 2-year study over a crop (corn/soybean) field at Mead Nebraska with a 16-chamber automated soil CO₂ flux system, we found that the response of soil CO₂ flux to rain events depended on the amount of crop residues on the soil surface.  For a location where there was no residue on the soil surface, soil CO₂ flux could be decreased after rain events because of reducing the soil conductance for gas transport. While for a location where there was a large amount of crop residue, soil CO₂ flux was always enhanced after rain events. The coefficient of variation (CV) of soil CO₂ flux was found to be increased right after rain events. From a modeling point of view, it is necessary to treat the CO₂ respired from the microbial activities inside the soil profile differently from microbial activity on crop residue at the soil surface.