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.

Growth and yield of two chickpea (Cicer arietinumL.) varieties in Canterbury, New Zealand

“…Although some authors have reported that chickpeas are capable of fixing all the N they need (Mahler et al 1988), others have reported that the crop does not fix adequate N for maximum growth even when vigourous nodules are present (Beck et al 1991 ). This work indicates that in Canterbury, whether the crop is inoculated with suitable Rhizobium or not, N fertiliser is required for maximum growth and yield.…”

“…Chickpeas are not considered strong N fixers even when adequate nodules are present. Beck et al ( 1991 ) reported that chickpeas obtained from 8-81 % of their plant N requirements from fixed atmospheric N. The amount depended on both the site and time of sowing.…”

Enhancing legume N2 fixation through plant and soil management

“…For instance, values for N2 fixation were high and there was a small net contribution of fixed N to soil following seed harvest of winter sown chickpea in Syria (Table 8), whereas both crop N and Pfix were low in spring sowings where drought limited growth as early as anthesis. In contrast, in France, differences between N2 fixed by winter-sown and spring-sown chickpea were still observed, but were not so marked because of extended moisture availability throughout the later stages of growth (Beck et al, 1991; Table 8). …”

Feedback regulation of symbiotic N2 fixation under drought stress

“…For cool-season food legumes, Beck et al [2] concluded that even if the drought stress effects on N 2 fixation do not always directly affect grain yield, drought may result in a significant decrease in the total N balance.…”