LONG-TERM EFFECTS OF EARTHWORMS ON MICROBIAL BIOMASS NITROGEN IN COARSE AND FINE TEXTURED SOILS

We conducted field studies of the effects of earthworms on microbial b iomass-N in sandy clay loam vs. sandy soils under no-tillage managemen t on Ultisols of the southern Appalachian Piedmont in Georgia, USA. N- 15-labeled crop residue was applied to the surface of plots with or wi thout the addition of earthworms (principally Lumbricus rubellus and A porrectodea caliginosa). Microbial biomass N and N-15 levels were meas ured at intervals over five years. Microbial N concentrations increase d in both soils but more so at the surface of the sandy soil and in de eper soil layers in the clay soil. Microbial N concentrations were con sistently lower in earthworm treatments than in controls in the clay s oil and to a lesser extent in the sandy soil. In deeper layers of the clay soil, earthworm additions increased N-15 enrichment of the microb ial pool, suggesting that earthworms increased transport of crop resid ue N into the subsoil. In the sandy soil the microbial pool was half a s large as in the clay soil, but showed a 100-300% increase in N-15 en richment during the first year, indicating substantially higher microb ial turnover. Although earthworm activity reduced standing stocks of m icrobial biomass, particularly in the fine-textured soil, it appeared to increase the turnover of the microbial-N pool as indicated by N-15 measurements. Observed changes in microbial biomass reflected rapid cy cling of labile organic matter pools in response to biological activit y, soil texture and soil management. (C) 1998 Elsevier Science B.V.