Short-term nitrogen transformations in bulk and root-associated soils under ryegrass

The balance between gross rates of N mineralization and N consumption in so ils is influenced both by the presence or absence of plants, and by physica l soil disturbance. Studies are needed which evaluate the impact of disturb ances caused either by plant harvest (grazing or clipping) or by the activi ties of soil fauna on soil N transformations. Using N-15 pool dilution tech niques, we compared gross N transformations in bulk and root-associated soi ls under ryegrass grown in greenhouse conditions. Over a period of 98 d, ry egrass plants were clipped periodically, or left unclipped in the presence or absence of earthworms. After 98 d of ryegrass growth, (NH4)-N-15-N (33 a tom% N-15) or (NO3)-N-15-N (10 atom% N-15) solutions were injected througho ut the soil. Virtually all of the N-15 added was recovered in ryegrass and soil after 48 h of incubation, but up to 30% of the (NH4)-N-15-N and 44% of the (NO3)-N-15-N were unaccounted for after 30 d. Gross N mineralization a nd NH4-N consumption rates were higher in the root-associated than in bulk soils of all treatments. The rate of gross N mineralization was highest in the pots with clipped plants, followed by the unclipped plus earthworm and the unclipped minus. earthworm treatments. Our results indicate that rates of gross N transformations differ between bulk and ryegrass root-associated soils, and that clipping of plants and the presence of earthworms exert ma rked effects on short-term N transformations. (C) 2001 Elsevier Science Ltd . All rights reserved.