DENITRIFICATION AND NITROUS-OXIDE PRODUCTION IN SOIL FALLOWED OR UNDER ALFALFA OR GRASS

A field study was carried out to assess the effects of agronomic pract ices and crops on nitrous oxide (N2O) production and denitrification r ates. Air-filled porosity, moisture content, respiration rate, and amm onium (NH4) and nitrate (NO3) concentrations were also measured along with nitrogen (N2) gas production using the static soil core procedure with acetylene (C2H2). Agronomic practices included applications of l iquid cattle manure, ammonium sulfate, sucrose, and irrigation water a s well as the incorporation of alfalfa. Measurements were made on fall owed soil as well as soils under alfalfa (Medicago sativa L.) and Kent ucky bluegrass (Pea pratensis L.). The temporal pattern of Nz gas prod uction was dominated by episodic events caused by manure or sucrose ap plication, or alfalfa incorporation in conjunction with irrigation or rainfall events. Rainfall or irrigation alone did not increase N2 gas production unless following upon a dry period. In wetted soil, the N2O production rate was lower than the denitrification rate, except under Kentucky bluegrass. The very low NO3 concentration in soil under Kent ucky bluegrass probably limited denitrification so that most N2O produ ction-resulted from nitrification. Episodic events involving denitrifi cation appeared to produce most of the N2O. Yet, the relationship betw een N2O production and denitrification was inconsistent, varying unpre dictably with the crop, agronomic practices, and soil conditions.