Nitrification, denitrification, and N-liberation associated with two types of organic hot-spots in soil

Nitrification, denitrification and nitrogen liberation were studied while liquid cattle manure and duck litter decomposed in aerobic soil contained in a diffusion chamber. The high NH4+ outflux from the liquid manure initially saturated the indigenous nitrification potential and caused an exponential increase in nitrifying activity with doubling times of the nitrifying bacterial population as low as 1 d. Nitrification rates of 400 nmol N cm−2 h−1 were reached after 7 d of incubation, with most of the nitrifying activity situated <4 mm from the organic matter-soil interface. This location of the nitrification zone caused a tight coupling between nitrification and denitrification, and 40% of the liberated N was converted to N2 during the 3-week experiment. A decreasing N liberation with time caused a reduction in both nitrification and coupled nitrification–denitrification, and only 20% of the initial activity remained at the end of the experiment. Incubation of duck litter led to NH4+ outflux rates of 800 nmol N cm−2 h−1 through a 1 cm thick soil phase. The high rate of NH4+ liberation seemed to inhibit nitrification in the aerobic soil. Ten days without measurable nitrification activity was followed by a slowly increasing nitrifying activity situated >5 mm from the aerobic–anaerobic interface. This suggests that a very high NH4+ outflux from anaerobic organic material to a surrounding aerobic soil may increase the spatial separation between the nitrification and denitrification zones, thereby significantly reducing the process of coupled nitrification–denitrification.