Soil phosphorus supply affects nodulation and N:P ratio in 11 perennial legume seedlings

Developing new perennial pasture legumes for low-P soils is a priority for Australian Mediterranean agro-ecosystems, where soil P availability is naturally low. As legumes tend to require higher P inputs than non-legumes, the ability of these plants to fix N-2 under varying soil P levels must be determined. Therefore, the objective of this study was to investigate the influence of soil P supply on plant N status and nodule formation in 11 perennial legumes, including some novel pasture species. We investigated the effect of applying soil P, ranging from 0 to 384 mu g P/g dry soil, on plant N status and nodulation in a glasshouse. Without exogenous P supply, shoot N concentration and N: P ratio were higher than at 6 mu g P/g soil. Shoot N concentration and N: P ratio then changed little with further increase in P supply. There was a close positive correlation between the number of nodules and shoot P concentration in 7 of the 11 species. Total nodule dry weight and the percentage of plant dry weight that consisted of nodules increased when P supply increased from 6 to 48 mu g P/g. Without exogenous P addition, N: P ratios partitioned into a two-group distribution, with species having a N: P ratio of either >70 or