DENITRIFICATION FOLLOWING HERBICIDE APPLICATION TO A GRASS SWARD

The application of the herbicide Roundup (glyphosate), and subsequent death of a predominately bromegrass (Bromus inermis Leyss.) and blue g rass (Pea pratensis L.) sward, resulted in a 20- to 30-fold increase i n denitrification rate 14 and 49 d after application compared to herbi cide-untreated and fallowed soil treatments. The regulation of denitri fication by O-2, carbon and NO3- availabilities was assessed by measur ement of various sail variables. The regulation of denitrification by C and NO3- availabilities was further studied in a laboratory experime nt in which denitrification was measured following NO3- and glucose-C addition to soil from the field treatments. Elevated denitrification i n the herbicide-treated soil was attributed to increased soil moisture and NO3- contents resulting from the death of vegetation. The death o f the grass sward did not increase available C to denitrifiers, wherea s the absence of vegetation in the fallowed soil 1 yr following herbic ide application reduced available C. This study indicates that herbici de application to a grass sward increases denitrification and hence ma y contribute to greater nitrous oxide emission and N loss from soil.