NITRIFICATION, DENITRIFICATION, AND N-LIBERATION ASSOCIATED WITH 2 TYPES OF ORGANIC HOT-SPOTS IN SOIL

Nitrification, denitrification and nitrogen liberation were studied wh ile liquid cattle manure and duck litter decomposed in aerobic soil co ntained 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 ti mes of the nitrifying bacterial population as low as 1 d. Nitrificatio n rates of 400 nmol N cm(-2) h(-1) were reached after 7 d of incubatio n, with most of the nitrifying activity situated < 4 mm from the organ ic matter-soil interface. This location of the nitrification zone caus ed a tight coupling between nitrification and denitrification, and 40% of the liberated N was converted to N-2 during the 3-week experiment. A decreasing N liberation with time caused a reduction in both nitrif ication 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) thro ugh a 1 cm thick soil phase. The high rate of NH4+ liberation seemed t o inhibit nitrification in the aerobic soil. Ten days without measurab le nitrification activity was followed by a slowly increasing nitrifyi ng activity situated > 5 mm from the aerobic-anaerobic interface. This suggests that a very high NH4+ outflux from anaerobic organic materia l to a surrounding aerobic soil may increase the spatial separation be tween the nitrification and denitrification zones, thereby significant ly reducing the process of coupled nitrification-denitrification. (C) 1998 Elsevier Science Ltd. All rights reserved.