Miller et al. 2008

Miller MN, Zebarth BJ, Dandie CE, Burton DL, Goyer C, Trevors JT. 2008. Crop residue influence on denitrification, N2O emissions and denitrifier community abundance in soil. Soil Biology & Biochemistry 40: 2553-2562.

These authors conducted a factorial experiment using packed soil cores to examine the influence of varying levels of available carbon and nitrogen on the process of denitrification. The treatments consisted of addition of glucose at three levels and KNO3 at four levels in experiment 1, and additions of either red clover or barley straw crop residues with or without additional KNO3. They measured soil chemistry, including extractable organic carbon and NO3- concentration based on K2SO4 extractions, as well as N2O production, the molar ratio of N2O (N2O : (N2O + N2)), and a handful of bacterial genes by qPCR.

The experimental setup was very similar to what we used in SLSC 802 (Special Topics) in the fall of 2010; cylindrical soil cores with gas-exchange holes in the sides were filled with soil at 1 g cm-3 bulk density and a water content of 70% and placed in 1 L canning jars with lids fitted with a perforable septum for gas sampling. One important difference between this experiment and what we have done is that in this experiment, gas measurements were of total cumulative gas production, whereas we flushed each jar with ambient air after each sampling event. Presumably this difference will have important effects on the formation of anaerobic conditions and microbial consumption of N2O previously produced under less anoxic conditions.

Not surprisingly, minimal denitrification activity was found in treatments without added NO3-. Starting NO3- concentrations were 3 mg NO3--N kg-1 soil, and fell in all treatments without added NO3- to less than 1 mg NO3--N kg-1. Once this supply of readily available nitrate was used, it appears the bacteria ceased denitrification activity, or at least it was reduced.
The red clover had a much lower C:N ratio than the barley straw, 13:1 and 45:1, respectively, and more labile carbon. This difference appears to have driven the observed difference in denitrification activity, in a manner that reflects the results of the simple-C-source experiment 1. In general, more labile C and more available N leads to stronger denitrification activity and greater production of N2O; in sealed jars such as these, strong respiration under these conditions leads to anaerobic conditions and a fall in the molar ratio of N2O as nosZ-equipped microbes consume N2O as a terminal electron acceptor.

Extractable organic carbon (EOC) was a relatively poor predictor of denitrification, compared to respiration as measured by CO2 production. EOC is a measure of the instantaneous size of the pool of labile C, while respiration represents carbon that has already passed through a microbe’s metabolism. The distinction here may be between two different pools of carbon, as well as between an instantaneous snapshot measure and a series of measurements readily convertible to an estimate of the rate of a process.

In conclusion, these authors reiterate their finding that available C and available N (especially as NO3-) are strong predictors of denitrifying activity, across a range of C and N sources. I read this paper for the class SLSC 802 in the fall of 2010, but the portions describing denitrification physiology and especially the qPCR information will be generally useful to my other projects.