Submitted to: Journal of Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/19/1999
Publication Date: N/A
Interpretive Summary: Earthworms play an important role in soil. Earthworms move and degrade crop residues, releasing nutrients back into the soil. Through their burrowing activities, earthworms can improve soil aeration and water infiltration. Past research has shown that earthworm castings are enriched in the plant-required nutrients, nitrate and ammonium. Earthworm casts also have been observed to have elevated populations of bacteria involved in nitrogen cycling reactions. This study was done to see how nitrogen cycling bacteria in the lining of the earthworm burrows would affect nitrate concentrations. We found high concentrations of nitrogen cycling bacteria in the soil lining the earthworm burrows. The activities of these bacteria caused the burrow lining to be high in nitrate. The significance of this work lies in the fact that although it may be suspected that the nitrate in the burrow lining is susceptible to leaching, past work indicates that worm burrows transmit a relatively small percentage of the total areal rainfall. Thus, we conclude that most of the inorganic nitrogen formed in the earthworm burrow will not be lost from the rooting zone, but rather it will be available for crop use. These results highlight the importance of earthworms to soil health and should encourage farmers to adopt practices that facilitate the development of earthworm populations.
Technical Abstract: 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 soil lining earthworm burrows (drillosphere soil). This study evaluates microbial N cycling in soil associated with earthworm burrows. Nitrifying bacterial populations, denitrifying bacterial populations, nitrification rates, and denitrification rates were performed on drillosphere and non-drillosphere soils. These measurements, in addition to measurements of NO3-/ concentration, NH4+/ concentration, soluble organic-C, pH, and water content, were performed on drillosphere material from laboratory microcosms inoculated with Lumbricus terrestris, and on drillosphere material collected earthworm burrows in long-term no-till plots. It was generally observed that drillosphere soil was enriched in NO3-/, NH4+/, and soluble organic C, and that these soils had elevated populations of nitrifying and denitrifying bacteria relative to non-drillosphere soil. Drillosphere soil also had higher nitrification and denitrification rates. We conclude that earthworm-derived C and N deposited in the drillosphere facilitates the enrichment of N-transforming bacterial populations, and that the elevated N-transformation rates result 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 currently not known.