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

The balance between gross rates of N mineralization and N consumption in soils is influenced both by the presence or absence of plants, and by physical soil disturbance. Studies are needed which evaluate the impact of disturbances caused either by plant harvest (grazing or clipping) or by the activities of soil fauna on soil N transformations. Using 15N pool dilution techniques, we compared gross N transformations in bulk and root-associated soils under ryegrass grown in greenhouse conditions. Over a period of 98 d, ryegrass plants were clipped periodically, or left unclipped in the presence or absence of earthworms. After 98 d of ryegrass growth, 15NH4–N (33 atom% 15N) or 15NO3–N (10 atom% 15N) solutions were injected throughout the soil. Virtually all of the 15N added was recovered in ryegrass and soil after 48 h of incubation, but up to 30% of the 15NH4–N and 44% of the 15NO3–N were unaccounted for after 30 d. Gross N mineralization and 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 marked effects on short-term N transformations.