A gas-flow soil core method to measure field denitrification rates

A method was developed for rapid measurement of soil denitrification under conditions where natural soil structure and aeration status is maintained. Air was continuously recirculated by means of a membrane pump through a soil core and a sample loop of a gas chromatograph equipped with an electron capture detector. Addition of acetylene to the recirculating air permitted measurement of denitrification in the soil core. Because of the rapid distribution of C₂H₂ and removal of N₂O provided by the gas flow, denitrification rates could usually be determined in less than 2 h. By means of external 6-way and 8-way valves, four soil cores could be simultaneously analyzed on one gas Chromatograph equipped with dual detectors. Soil cores could also be stored at 4°C for later analysis without affecting the denitrification rate. The detection limit for denitrification rate measurements was 0.5 ngN g^?1 soil day^?1 or approximately 2.6 g N ha^?1 day^?1. Coefficients of variation for repeated measurements on the same soil core were usually less than 15%, but coefficients of variation for repacked or natural cores of the same soil were much higher (70–90%) Disruption of the natural soil structure by sieving increased the denitrification rate in an aggregated clay loam soil, but decreased the rate in a non-aggregated sandy soil. These results illustrate the importance of maintaining natural soil structure during denitrification measurements. The effect of pumping gas through soil was evaluated by comparing denitrification rates in soil cores where C₂H₂ was allowed to distribute into the soil by passive diffusion with rates obtained by pumping. Lower denitrification rates were observed in the static incubation presumably due to limited diffusion of C₂H₂ into or N₂O out of the denitrifying sites in the soil. This diffusion limitation could be overcome in the static incubations if C₂H₂ was initially distributed through the soil by pumping. This gas flow method is well suited to the study of soil denitrification rates under nearly natural conditions because the indigenous substrates and anaerobic microsites are preserved, the rapidity in which denitrification rates can be measured, and the high sensitivity and relatively low analytical variability of the method.