Dinitrogen Fixation and Nitrogen Balance in Cool-Season Food Legumes

Abstract

Understanding the magnitude of N₂ fixation and export of plant N, particularly in the harvested grain and straw, is necessary to assess the potential for grain legumes to contribute to long-term agricultural production stability. The proportions of total plant N derived from N₂ fixation in four grain legume crops, faba bean (Vicia faba L.), chickpea (Cicer arietinum L.), lentil (Lens culinaris L), and pea (Pisum sativum L.), were estimated in three field experiments conducted for two seasons in Syria (Vertic Chromoxeralf soil) and one season in France (Calcaric Cambisol). Since cultural practices and cultivars affect grain yield, the impact on N₂ fixation of sowing date in chickpea and Sitona insect control in lentil, and of cultivar selection of pea and faba bean, was evaluated. By calculating the proportion of total plant N derived from N₂ fixation (%Ndfa) using ¹⁵N isotope dilution with barley (Hordeum vulgare L.) and non-nodulating chickpea as reference crops, the effects of removal of N in grain and straw, relative to N fixed and plant uptake of soil mineral N, were estimated. Pea and lentil had similar %Ndfa values across locations, seasons, and cultural practices, with an average 70%Ndfa. In chickpea, winter sowing increased %Ndfa to 72 from 26% in the spring-sown crop. Dinitrogen fixation in the spring-sown crop was higher in France (44%Ndfa), while fixation in the winter crop was higher in Syria (80%). Faba bean obtained 9O%Ndfa in France, but only 69% in Syria. The large-seeded type fixed higher N (74%Ndfa) than the small-seeded type (64%) in Syria but not in France. The calculated N balance where only grain was removed ranged from 44 kg N ha-' net gain to 44 kg N ha⁻¹ loss. Where both seed and straw were removed, nearly all calculations were negative, with loss of up to 70 kg N ha⁻¹ from soil.