Predicting nitrogen supply from dairy effluent applied to contrasting soil types

A closed incubation assay was conducted to identify attributes that can be used to predict nitrogen (N) supply from dairy cattle effluents applied to contrasting soil types. The experimental design included six slurry (DM 7.9–13%) and six solid (DM 15.9–42.5%) effluents applied to a Horotiu sandy loam or a Templeton silt loam with treatments incubated at 70% of field capacity at 20°C for 2, 5, 14, 21, 35, 42, 63, 96, 119 and 175 days. After 175 days, inorganic N supply (INS) ranged from −38.6 to 82.8% of total effluent N applied while net N mineralisation (NNM) ranged from −107.4 to 65.1% of organic effluent N applied. The best predictor of INS at day 175 was the log ratio of total C to water extractable N (P < 0.001, r = −0.85) and the log ratio of DM to water extractable N (P < 0.001, r = −0.85) (water extractable N = water soluble N + hot water extractable N). These measures were inversely related to INS and effective at explaining both net mineralisation and immobilisation effects. Importantly, correlations between INS and C:N ratio measures were improved considerably when total N was substituted for a labile N measure, in this case water extractable N. We were unable to identify an effluent measure to consistently predict NNM.