Denitrifying enzyme activity and carbon availability for denitrification following manure application

Abstract

A field experiment was established which permitted the comparison of several methods to estimate carbon (C) availability for denitrifiers. Treatments consisted of a control (CONT), NH₄NO₃ (FERT), liquid dairy cattle manure (LCM), and solid (bedded) beef cattle manure (SBM) applied in early May and incorporated 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 application. Gas production was related to denitrifying enzyme activity (DEA) and available C content. Available C content was estimated by several methods: 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 correlated with dN; Cresp, Cext, Cphen, and DEA were more strongly correlated 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 treatments. These variables were relatively high early in the 49‐d period with the LCM treatment but remained at relatively high levels for the SBM treatment throughout the period. These observations indicated a large quantity of readily available C applied in LCM, but a more sustained release of available C during the decomposition of bedding straw in SBM. Indices of available C were more useful than DEA in predicting the short‐term temporal pattern of dN rate. At this scale DEA was largely determined by available C supply. None of the methods used was superior for estimating C availability to denitrifiers. It is recommended that the easiest method be used for this purpose.