Changes in soil N pools in response to earthworm population manipulations in agroecosystems with different N sources

Responses of soil N pools to field manipulation of earthworm populations (reduced, unaltered or increased each spring and autumn) were evaluated within each of three agroecosystems based on different N sources: NH4NO3 fertilizer, cow manure or a legume-rye winter cover crop. Our objectives were to determine the effects of earthworms on soil N dynamics in agroecosystems based on different organic or inorganic sources of N, and to examine potential interactive effects of agroecosystem treatments and field-scale earthworm manipulations on soil N pools and potential N losses. Earthworm manipulations began in spring 1991, and were repeated each spring and fall. Soil microbial biomass N was determined by fumigation-extraction on six dates in 1992 and four dates in 1993. Extractable inorganic soil N (0–15 cm) was measured in January and approximately every 2 weeks during the growing seasons of 1992 and 1993. Additionally, the post-growing season vertical distribution (0–15, 15–30, and 30–45 cm) of extractable soil NO3N was evaluated in November of 1992 and 1993. Earthworm manipulations affected microbial biomass N and extractable inorganic N pools in bulk soil samples. Microbial biomass N was significantly higher in the earthworm reduction treatments. There were significant earthworm × agroecosystem interactions affecting soil NO3. In the inorganically fertilized system, earthworm additions resulted in elevated amounts of extractable NO3 during the growing season of both years. Extractable NH4 concentrations were increased by earthworm additions in 1993, but only in the inorganically fertilized system. Earthworm additions also increased the concentration of soil NO3 at lower depths after the growing season, especially in the inorganically fertilized system. These results suggest that earthworms can alter N cycling processes in agroecosystems, and that these changes are sufficient to be detected by bulk soil sampling. Our results also indicate that the net effects of earthworm activity can vary with agroecosystem management practices. Earthworms may increase N availability by reducing microbial immobilization and enhancing mineralization. However, increased amounts of soil NO3 at the end of the growing season, and increased concentrations in lower soil horizons, could lead to increased leaching losses from inorganically fertilized systems. The implications of these changes for ecosystem-level nutrient fluxes will require further investigation.