The use of 15N 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 undisturbed 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 different sward composition and N inputs were examined. Plant shoots and a range of soil fractions, categorized by size, were sampled in May prior to any 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 total 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. Between 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 inputs or sward type. Changes in silt-N especially, were similar to those occurring in the shoots. No comparable changes were seen in the larger fractions. Not all measured variation was explicable in this study. The inadequacies of the approach are highlighted and suggested improvements discussed.