Earthworm effects on N dynamics and soil respiration in microcosms receiving organic and inorganic nutrients

We designed a microcosm experiment to investigate the effects of earthworms on N cycling processes and microbial activity, in soil receiving organic or inorganic nutrient amendments. Cylindrical microcosms contained 16l. of field-collected soil that received 1 of 3 nutrient amendments, added to the upper 5 cm of soil at a rate of 150 kg N ha−1; (1) granular NH4NO3 fertilizer; (2) straw-packed dairy cow manure; and (3) air-dried hairy vetch (Vicia villosa) legume residue. There were 4 replicates of each nutrient treatment without earthworms, and 4 replicates with a total of 21 earthworms added per microcosm (a mixed community of Lumbricus terrestris and Aporrectodea tuberculata). The microcosms were incubated for 112 days. Soil respiration was measured continuously and extractable NO3− and NH4+ and microbial biomass-N were measured, periodically, at 0–5 and 5–15 cm. Earthworms had significant effects on amounts of extractable NO3−, which increased with time, and were greatest in soil that received NH4NO3 fertilizer and least in soil that received legume residues. On the final sample date (112 days), earthworms increased amounts of extractable NO3− at the 0–5 cm soil depth 1.83-, 1.88- and 1.26-fold in microcosms supplied with NH4NO3, manure and legume trestments, respectively. Earthworms increased the amounts of extractable NH4+, after 112 days, at the 0–5 cm soil depth, by 1.60-, 4.00- and 1.30-fold, in microcosms that had received the NH4NO3, manure and legume treatments, respectively. Earthworms significantly reduced the amounts of microbial biomass-N at the 5–15 cm soil depth, and this effect was greatest in microcosms that had received the NH4NO3 and manure treatments. Earthworms increased soil respiration rates during the first 15 days by from 1.24- to 2.42-fold and increased cumulative soil respiration (112 days) by 1.84-, 1.37- and 1.24-fold, respectively, in microcosms treated with the fertilizer, manure and legume treatment. These results indicate earthworms increased the amounts of extractable N by feeding on the microbial biomass, and increasing the turnover and mineralization of microbial tissues.