The use of N-15 natural abundance variation to examine plant and soil organic fractions in pasture under different management practises

Pasture systems lack the complexity of natural grasslands and have undistur bed soil profiles relative to arable monocultures. With controlled nitrogen (N) fertilizer inputs and measurable outputs (e.g. grazing and leaching), they can be used to investigate N-15 natural abundance variation as a tool for the study of soil-N processes. In the present study, four pastures of d ifferent sward composition and N inputs were examined. Plant shoots and a r ange of soil fractions, categorized by size, were sampled in May prior to a ny major N additions, and again in July after initial N inputs had of been made. Samples were analyzed for N-15 natural abundance (delta(15)N) and tot al N (epsilon N). In the May sample plant and soil fractions varied in both N-15 and epsilon N between treatments. The 0.5 mm and 0.2 mm soil factions were comparable within treatments, as were the silt and clay fractions. Be tween May and July changes were apparent in the delta(15)N and epsilon N Of shoots and some soil fractions within each plot these corresponded to N in puts or sward type. Changes in silt-N especially, were similar to those occ urring in the shoots. No comparable changes were seen in the larger fractio ns. Not all measured variation was explicable in this study. The inadequaci es of the approach are highlighted and suggested improvements discussed.