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 practices and crops on nitrous oxide (N2O) production and denitrification rates. Air-filled porosity, moisture content, respiration rate, and ammonium (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 liquid cattle manure, ammonium sulfate, sucrose, and irrigation water as well as the incorporation of alfalfa. Measurements were made on fallowed soil as well as soils under alfalfa (Medicago sativa L.) and Kentucky bluegrass (Pea pratensis L.). The temporal pattern of Nz gas production was dominated by episodic events caused by manure or sucrose application, 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 Kentucky bluegrass probably limited denitrification so that most N2O production-resulted from nitrification. Episodic events involving denitrification appeared to produce most of the N2O. Yet, the relationship between N2O production and denitrification was inconsistent, varying unpredictably with the crop, agronomic practices, and soil conditions.