Low-phosphorus conditions affect the nitrogen nutrition and associated carbon costs of two legume tree species from a Mediterranean-type ecosystem

Abstract

The role of phosphorus nutrition in two-legume tree species from the Mediterranean-type ecosystem of the Cape Floristic Region (CFR) in South Africa was investigated. There is very little information about the functional adaptations of nitrogen (N) and phosphorus (P) nutrition in these legume trees growing in nutrient-poor soils. Nodulated Virgilia divaricata and V. oroboides tree saplings were grown in sterilised sand and supplied with Long Ashton nutrient solution, which was modified to contain either sufficient-phosphate (500 µM) or low-phosphate (5 µM) nutrient solution for 90 days. During low-P conditions, the growth of V. divaricata was not affected, whereas V. oroboides showed a decrease in growth. The decrease in V. oroboides under low-P conditions was related to the lower P uptake, which resulted in an alteration in belowground biomass allocation, which consequently affected on the N nutrition and carbon (C) cost of growth. In this regard, V. oroboides plants allocated less biomass to roots and nodules, as a proportion of whole plant growth. The impact of this was a decline in N nutrition, growth respiration and photosynthetic costs in V. oroboides. In contrast, V. divaricata maintained its P concentrations, photosynthetic costs and increased its nodule allocation under low-P conditions, to the benefit of N nutrition. The two CFR tree legumes appear to have different adaptations to low-P conditions, which may influence their N and P acquisition in their naturally low-P environment.