Impact of dicyandiamide on the internal nitrogen cycle of a volcanic, silt loam soil receiving effluent

Abstract

There are concerns that land application of dairy shed effluent onto grazed pasture compromises groundwater quality. One approach to minimizing the environmental impact of spreading effluent aims at maximizing the retention of effluent-N in the receiving soil by using nitrification inhibitors. The effectiveness and persistence of such an inhibitor, dicyandiamide (DCD), was tested on a freely draining silt loam soil. Trials were performed in a greenhouse. Soil received 10 mm of dairy shed effluent twice monthly. The first application of effluent was amended with DCD-N at a rate equivalent to 170 μg g − receiving soil (dry weight basis). On alternate weeks, soil received 15 mm rainfall events. After 99 days, there had been a significant ( P < 0.05) reduction in total N leached from the 150-mm profile depth of DCD-amended soil, compared with unamended soil. The increment in total N conserved in soil as a result of DCD-amendment equated to 22 kg N ha −1. Ammonium-N concentrations in DCD-amended soil increased significantly ( P < 0.05) 8 days following DCD-application and remained higher than in unamended soil for up to 99 days. After 22 days, nitrate-N concentration in unamended soil was twice that of amended soil. Levels of microbial biomass C were higher in amended soil than in controls from Days 8 to 22, but were similar after 99 days. Microbial biomass N was significantly higher in DCD-amended soil than in control soil throughout the trial period of 99 days. Degradation rates of DCD were estimated for a range of soil temperatures (6–22°C). The half-life of DCD at 22°C (summer ambient) was 39 days. The threshold at which DCD underwent rapid decomposition, largely due to an undetectable lag phase, lay between 13 and 16°C. DCD reduced nitrate leaching and resulted in a net increase in microbial N-immobilization following application of effluent onto freely draining soil despite high soil temperatures and simulated rainfall events.