Selection of reference plants for ¹⁵N natural abundance assessment of N₂ fixation by crop and pasture legumes in south-west Australia

Abstract

The 15N natural abundance (S15N) of the shoot total N of a range of non-N2 fixing potential reference species was compared with that of nodulated field pea (Pisum sativum L.), narrow leafed lupin (Lupinus angustijolius L.) or subterranean clover (Trijolium subterraneum L.) across a range of field sites, to which N fertilizers had not been applied in the season of study. Shoot S15N values of reference species lay mostly within the range from +3 to +5%o and there was some evidence of lower S15N values in gramineaceous than dicotyledonous non-legume species. Continuous sampling within crops of each legume showed S15N values to differ consistently between and within potential reference species through the season. The S15N values of seedlings of four non legume species in a lupin crop were closer to that of soil N03-N (S15N = 4.2%o) than soil NH4-N (S15N = 7.9%0). Shoot S15N values of non-nodulated pea, lupin and subterranean clover, and a range of possible reference species all sand-cultured on a defined nitrate source (S15N = 7.5%), suggested little or no discrimination during utilization of nitrate. However, when four candidate reference species were sand cultured with nodulated actively fixing subterranean clover on the same nitrate source, the S15N of the ryegrass was lowered significantly below that of the NO3. Field plot comparisons of nine potential reference species with nodulated field pea showed certain species to resemble field pea more closely than others in terms of the S15N value of their shoots. Reference plants sampled within or well outside the rooting zone of an actively fixing legume (subterranean clover, field pea or lupin) showed significantly lower shoot S15N of mixed grass components when harvested in root contact with, as opposed to well distant from, subterranean clover. A similar effect was observed for barley within v. outside the rooting zone of pea, but no such effects were observed between capeweed and subterranean clover, pea and radish, or for any of five reference plants matched with lupin. The data are utilized to select appropriate reference plants for field assessments of N2 fixation under south-west Australian conditions.