Denitrifying enzyme activity and carbon availability for denitrification following manure application

A field experiment was established which permitted the comparison of severa l methods to estimate carbon (C) availability for denitrifiers. Treatments consisted of a control (CONT), NH,NO, (FERT), liquid dairy cattle manure (L CM), and solid (bedded) beef cattle manure (SBM) applied in early May and i ncorporated at rates of 255 kg N ha(-1) as total N (FERT) or ammoniacal N ( LCM, SBM). Denitrification (dN) and N2O production rates were measured for soil cores taken from the 2.5-7.5 cm layer during a 49-d period following a pplication. Gas production was related to denitrifying enzyme activity (DEA ) and available C content. Available C content was estimated by several met hods: water-soluble C (Csol), 0.5 M K2SO4-extractable C (Cext), aerobically respired C (Cresp), water-soluble phenolic acids (Cphen), and potentially available C (Cpot). All five indices of available C and DEA were strongly c orrelated with dN; Cresp, Cert, Cphen, and DEA were more strongly correlate d with N2O production then were Cpot and Csol. The temporal patterns of dN and N2O production as well as C availability differed with the two manure t reatments. These variables were relatively high early in the 49d period wit h the LCM treatment but remained at relatively high levels for the SBM trea tment throughout the period. These observations indicated a large quantity of readily available C applied in LCM, but a more sustained release of avai lable C during the decomposition of bedding straw in SBM. Indices of availa ble C were more useful than DEA in predicting the shortterm temporal patter n of dN rate. At this scale DEA was largely determined by available C suppl y. None of the methods used was superior for estimating C availability to d enitrifiers. It is recommended that the easiest method be used for this pur pose.