Development of nitrification hot-spots around degrading red clover (Trifolium pratense) leaves in soil

Litterbags with clover leaves in soil were buried in the field to study the influence of degrading clover leaves on the development of elevated nitrif ication activity (hot-spots) in space and time. Potential NH3 oxidation act ivity indicating the population size of ammonia-oxidizing bacteria was meas ured in soil samples taken in a zone a few millimetres thick around the lea ves and in bulk soil. On 3 sampling days during leaf degradation, the poten tial NH4+ oxidation was significantly higher in leaf-associated soil than i n bulk soil, the largest difference (factor of 2) occurring 24 days after b urial. At all sampling occasions, NH4+ oxidation rates followed a normal di stribution, except for a lognormal distribution in the leaf-associated soil sampled 24 days after burial when some very active samples (hot-spots) app eared. In a similar laboratory experiment we examined the effects of soil w ater on the development of nitrification hot-spots. We observed that the de velopment of hot-spots did not take place in soil incubated at 60% of water -holding capacity (WHC), whereas they developed faster when the soil was in cubated at approximately 100% WHC. It was concluded that soil water in comb ination with easily degradable organic N were essential parameters for the development of nitrification hot-spots in this soil.