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− and NH4+ and show a high potential for microbial nitrification and denitrification. 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, nitrification rates and denitrification rates of drilosphere and nondrilosphere soils. These measurements, in addition to measurements of NO3− 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−, 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 nitrification and denitrification 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− in the earthworm burrow. This phenomenon has the potential for increased downward NO3− transport; however, the extent to which this potential is realized is not known.