Variable soil phosphorus effects on nitrogen nutrition, abundance and associated carbon costs of a savanna legume, Vachellia sieberiana grown in soils from varying altitudes

Abstract

Vachellia sieberiana (DC.) Kyal. is a leguminous indigenous tree that occurs in savannas of southern and tropical Africa. The tree is known to tolerate frost, which possibly accounts for its presence in in high-altitude areas. However, there is less abundance of this tree in high-altitude areas of KwaZulu-Natal Province, South Africa. The aim of this study was to investigate if variation in soil phosphorus affected the tree growth and nitrogen nutrition, consequently reducing its abundance in high-altitude areas. Seeds of V. sieberiana were germinated and grown in the greenhouse in soils collected from three sites in the Van Reenen Pass area of KwaZulu-Natal Province (i.e. Zandspruit, altitude 1165 m; Wyford, altitude 1326 m; and Waterfall, altitude 1697 m). These sites had different V. sieberiana tree abundance, soil phosphorus (P) and nitrogen (N) nutrient concentrations. Although the nodulating rhizobia (Mesorhizobium sp.) in V. sieberiana root nodules were similar regardless of the differences in altitude as well as P and N concentrations, the total plant biomass and N source nutrition was altered. In this regard, V. sieberiana saplings grown in glasshouse conditions and in soils collected from Zandspruit accumulated more biomass, and relied on both atmospheric derived N and soil derived N. In addition, these V. sieberiana saplings had a higher total P and N concentration. The saplings grown in the Waterfall soil which had the lowest P and N concentration, relied equally on both atmospheric- and soil- derived N, and had an increased specific N utilisation rates and carbon construction costs compared with saplings grown in soils from the other two sites. The variation in soil P and N nutrient with increasing altitude affected growth and N source preference.