Microbial nitrogen transformations in earthworm burrows

Earthworms play an active role in soil nitrogen cycling. Past research has shown that earthworm castings are enriched in NO3 ÿ and NH4 + and show a high potential for microbial nitri®cation and denitri®cation. Little information is available on microbial populations and N transformations in the 1±2 mm thick soil lining of earthworm burrows (the drilosphere). We measured nitrifying bacterial populations, denitrifying bacterial populations, nitri®cation rates and denitri®cation rates of drilosphere and nondrilosphere soils. These measurements, in addition to measurements of NO3 ÿ concentration, NH4 + concentration, soluble organic-C, pH and water content, were performed on drilosphere material from laboratory microcosms inoculated with Lumbricus terrestris L. and on drilosphere material collected from earthworm burrows in long term no-till plots. The drilosphere soil was enriched in NO3 ÿ, NH4 + and soluble organic C and these soils had elevated populations of nitrifying and denitrifying bacteria relative to nondrilosphere soil. Drilosphere soil also had higher nitri®cation and denitri®cation rates. We postulate that earthworm-derived C and N deposited in the drilosphere facilitates the enrichment of N-transforming bacterial populations and that the elevated N-transformation rates results in an enrichment of NO3 ÿ in the earthworm burrow. This phenomenon has the potential for increased downward NO3 ÿ transport; however, the extent to which this potential is realized is not known. # 1999 Elsevier Science Ltd. All rights reserved.