Nitrogen nutrition in nodulated field plants of the shrub tea legume Aspalathus linearis assessed using 15N natural abundance

Abstract

Provision of N, P, and Ca to field plants of A. linearis markedly (P <0.05) increased growth and N nutrition in a very acidic nutrient-poor soil. Application of P and Ca promoted a significant increase in %N derived from fixation and amounts of N fixed compared to those receiving no nutrients. N2 fixation measured under field conditions ranged from 3.8 g N plant-1 in unfertilized control to 7.1 g N plant-1 in fertilized plants. Overall, about 85% increase in N2 fixation was observed with P supply. The high N2-fixing activity in P-treated plants was confirmed by their lower (more negative) δ15N values. Age of plants also influenced growth and symbiotic activity as the δ15N values, %N derived from fixation, and N fixed were lower in 1- and 2-year-old plants compared to 3-year-old. The contribution of symbiotic fixation in unfertilized A. linearis to the N economy of the ecosystem ranged from 105 kg N ha-1 in 1-year-old plants to 128 kg N ha-1 in 3-year-old plants, clearly indicating the remarkable adaptation of this symbiosis to the very nutrient-poor, low pH conditions of Cedarberg soils.