Nitrification potential and urease activity in a mineral subsoil

Preferential flow phenomena are common in many soils and may transport various N-compounds others than nitrate beneath the root-zone. In a field study, urea was transported to a depth of at least 2.3 m during snow melt. Field measurements strongly indicated presence of both urease activity and a nitrification potential in the mineral subsoil. We investigated this in more detail by laboratory incubations of soil sampled at five depths (40 to 230 cm) at a site where urea had previously infiltrated and from a nearby virgin site. All soil samples were incubated for about 45 d with or without urea added (3.8 mM). Production of N2O (from ammonia oxidation) and CO2 (from hydrolysis) was measured as head-space accumulation in gas tight serum bottles. NH4-N and NO3-N produced was extracted and quantified frequently during the incubation. Viable numbers (MPN) of ammonia-oxidizing bacteria were estimated by serial dilution on microtiter plates. Initial numbers of ammonia oxidizers were also estimated by non-linear regression of the amount of nitrate vs time. A nitrification potential existed in all samples, but was considerably higher in the soil from the urea-treated site compared to the virgin site. This demonstrated that urea had enhanced growth of nitrifying bacteria in this mineral subsoil. Nitrous oxide production started at about the same time in all samples (within 200 h), even in those samples where the NO3-N production was very delayed, or even absent. This apparent lag in NO3-production during which a “burst” of N2O occurred, was attributed to resuscitation of a severely-starved community of ammonia-oxidizing bacteria. Our work indicates that N2O production might be a good indicator of ammonia-oxidizing activity because it seems to be produced prior to detectable amounts of nitrite. All soil samples had urease activity, but samples from the virgin soil were unable to completely hydrolyse the added urea. From our data it seems reasonable that ammonia oxidizers contributed to a complete hydrolysis of urea.