Predicting the efficacy of the nitrification inhibitor dicyandiamide in pastoral soils

Identification of soil, environmental, or microbial properties linked with efficacy of the nitrification inhibitor dicyandiamide (DCD) in high and low-input pastoral farming system soils. Soils were collected from under 25 pastures. Potential nitrification rate (PRN) was quantified in the presence and absence of DCD, and percentage efficacy of DCD in reducing PNR calculated. PNR and %DCD efficacy were statistically tested (REML analysis) for relationships to a suite of edaphic (33), environmental (5), and microbiological (8) variables. Microbiological properties included measurement of bacterial and archaeal ammonia monooxygenase genes (amoA qPCR) and soil DNA content. DCD reduced PRN by an average of 36 %. The percent DCD efficacy was not related to system intensity, soil type, nor PNR (all P > 0.05). However the numbers of bacterial amoA genes (r = 0.46; P < 0.05), and ratios of bacterial:archaeal amoA (r = -0.53; P < 0.05), were strongly correlated to %DCD efficacy. In both high and low input systems, models best explaining variance in %DCD efficacy fitted AOA: AOB g soil(-1) as the first varaible (P < 0.05). Characterisation of soils based on ammonia oxidising communities may increase the ability to predict the % efficacy of DCD between sites and provide for more targeted application of this nitrification inhibitor.