Source of inorganic N affects the cost of growth in a legume tree species (Virgilia divaricata) from the Mediterrean-type Fynbos ecosystem

Abstract

Aims In Mediterranean-type ecosystem, the Cape Fynbos, legumes may be able to switch between soil N and atmospheric N2 sources during growth to adjust the carbon costs of N acquisition. This study investigated the utilization of different inorganic N sources by Virgilia divaricata, a native legume from the Mediterranean-type ecosystem of the Cape Floristic Region. Methods Plants were cultivated in sterile quartz sand, supplied with 25% strength Long Ashton nutrient solution, modified to contain 500 μM Phosphate. At the phosphate level (500 μM), plants were treated with 500 μM NH4NO3 (treatment named N), or grown in N-free nutrient solution and inoculated with effective Burkholderia sp. (Bact.) or treated with combined N sources (500 μM NH4NO3) and inoculated with effective Burkholderia sp. (N+Bact.). Important Findings The application of NH4NO3 to the legumes resulted in a greater increase in plant dry matter. Carbon construction costs were higher in plants that were supplied with mineral and symbiotic N sources. Maximum photosynthetic rates per leaf area was maintained, irrespective of the N sources. Although the plant roots were nodulated, the plant dependence on N2 fixation decreased with addition of N. Roots and nodules of the plants solely reliant on N2 fixation showed an increase in glutamine content. These results show that V. divaricata is highly adapted for growth at the forest margin. Fynbos and possibly anthropic soils by utilizing both atmospheric and soil N sources.