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 populatio ns (reduced, unaltered or increased each spring and autumn) were evalu ated 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 dynam ics in agroecosystems based on different organic or inorganic sources of N, and to examine potential interactive effects of agroecosystem tr eatments and field-scale earthworm manipulations on soil N pools and p otential N losses. Earthworm manipulations began in spring 1991, and w ere repeated each spring and fall. Soil microbial biomass N was determ ined by fumigation-extraction on six dates in 1992 and four dales in 1 993. Extractable inorganic soil N (0-15 cm) was measured in January an d approximately every 2 weeks during the growing seasons of 1992 and 1 993. Additionally, the post-growing season vertical distribution (0-15 , 15-30, and 30-45 cm) of extractable soil NO3-N was evaluated in Nove mber of 1992 and 1993. Earthworm manipulations affected microbial biom ass N and extractable inorganic N pools in bulk soil samples. Microbia l biomass N was significantly higher in the earthworm reduction treatm ents. There were significant earthworm x agroecosystem interactions af fecting soil NO3. In the inorganically fertilized system, earthworm ad ditions resulted in elevated amounts of extractable NO3 during the gro wing season of both years. Extractable NH4 concentrations were increas ed by earthworm additions in 1993, but only in the inorganically ferti lized system. Earthworm additions also increased the concentration of soil NO3 at lower depths after the growing season, especially in the i norganically fertilized system. These results suggest that earthworms can alter N cycling processes in agroecosystems, and that these change s are sufficient to be detected by bulk soil sampling. Our results als o indicate that the net effects of earthworm activity can vary with ag roecosystem management practices. Earthworms may increase N availabili ty by reducing microbial immobilization and enhancing mineralization. However, increased amounts of soil NO3 at the end of the growing seaso n, and increased concentrations in lower soil horizons, could lead to increased leaching losses from inorganically fertilized systems: The i mplications of these changes for ecosystem-level nutrient fluxes will require further investigation. (C) 1997 Elsevier Science Ltd.