Evaluation of the CO2 flush as a soil health indicator

The CO2 flush is recommended as an indicator of soil biological health. We conducted field and greenhouse experiments to evaluate the effect of several factors likely to influence C mineralization (presence/absence of plants, plant species, time of season, N fertility) on the CO2 flush. The field experiment was conducted with barley grown in 2017 (two sampling times) and 2018 (three sampling times). In greenhouse experiments, barley was grown for 4, 6, or 8 weeks; barley, corn, crimson clover, soybean, and ryegrass were grown for 4 weeks; and corn and barley were grown for 5 weeks at 4 levels of N. All had unplanted controls. Root biomass, microbial biomass carbon (MBC), dissolved organic carbon (DOC), and the amount of CO2-C released during 24 h after rewetting dried soil were measured. MBC was determined by the microwave method, and DOC was extracted by water with C quantified using a Shimadzu TOC-VCPH. A LI-COR infrared gas analyzer was used to quantify CO2. We found that planted soil had a greater CO2 flush than bare or unplanted soil, but the difference was not large, ranging between 15 and 40%. Root biomass did not consistently correlate with the CO2 flush. In unfertilized soils, the CO2 flush was not influenced by plant species. In fertilized soils, the CO2 flush was significantly higher in soils planted to corn than soils planted to barley at the two highest nitrogen levels. We found strong correlations between DOC and the CO2 flush, and inconsistent correlations between MBC and the CO2 flush. Because the CO2 flush was not strongly influenced by collection time, plant species, or N fertility, the CO2 flush may be a robust soil health indicator among different crops and sampling times.