Associate Professor University of Illinois at Urbana-Champaign, Illinois, United States
Most soil-emitted nitrous oxide (N2O) is generated during “hot moments” of denitrification, periods of heightened microbial metabolism, which contribute disproportionately to annual N2O budgets. It is well understood that soil moisture, nitrate (NO3-), and organic carbon (OC) are important drivers of denitrification, but despite knowing these drivers, our ability to predict hot moments of N2O at the field scale remains poor. We tested the hypothesis that spatial variation in soil OC (SOC) availability controls where N2O hot moments can occur. We conducted a laboratory experiment using soil samples collected from 20 locations over four ha of one maize field in Champaign County, Illinois. We predicted that soils with more particulate organic matter (POM) would have more microbially accessible OC and exhibit higher denitrification potential than soils with more mineral-associated organic matter (MAOM), due to mineral protection of OC in MAOM. We measured denitrification potential using denitrification enzyme assays without carbon amendment, allowing us to detect the effect of native OC availability on denitrification potential. We also incubated all soil samples at 33% gravimetric water content for 72 hr and measured a suite of soil chemical properties pre- and post-incubation to gain insight into controls on denitrification potential and soil OC availability.
The results from the incubation experiment supported our hypothesis that variation in SOC availability controls denitrification potential but contradicted our prediction that POM-C would represent more microbially accessible C. A multiple linear regression analysis revealed that MAOM-C concentration, DOC concentration, and C-mineralization rate were all strongly positively correlated with denitrification potential (p < 0.05 in all cases), whereas the POM-C bulk soil fraction was strongly negatively correlated with denitrification potential (p = 0.02). This indicates that microbes utilized MAOM-C and DOC for denitrification, but not POM-C. Additionally, simple linear regression showed that MAOM-C was positively correlated with the DOC concentration post-incubation (R2 = 0.22, p < 0.05), suggesting that DOC may be released from the MAOM-C pool to fuel denitrification. Together, these findings suggest that MAOM-C may be a better OC source for denitrification than POM-C, highlighting the importance of SOC accessibility as a crucial valve for denitrification potential. Quantifying spatial variation in MAOM-C could enable improvements in predicting hot moments of denitrification at the field scale.