Seasonal Patterns of Growth and Nitrogen Fixation in Field-Grown Pea

The seasonal patterns of growth and symbiotic N2 fixation under field conditions were studied by growth analysis and use of15N-labelled fertilizer in a determinate pea cultivar (Pisum sativum L.) grown for harvest at the dry seed stage. The patterns of fertilizer N-uptake were almost identical in pea and barley (the non-fixing reference crop), but more fertilizer-N was recovered in barley than in pea.

The estimated rate of N2 fixation in pea gradually increased during the pre-flowering and flowering growth stages and reached a maximum of 10 kg N fixed per ha per day nine to ten weeks after seedling emergence. This was the time of early pod-development (flat pod growth stage) and also the time for maximum crop growth rate and maximum green leaf area index.

A steep drop in N2 fixation rate occurred during the following week. This drop was simultaneous with lodging of the crop, pod-filling (round pod growth stage) and the initiation of mobilization of nitrogen from vegetative organs. The application of fertilizer-N inhibited the rate of N2 fixation only during that period of growth, when the main part of fertilizer-N was taken up and shortly after.

Total accumulation of fixed nitrogen was estimated to be 244, 238 and 213 kg N ha−1 in pea supplied with nil, 25 or 50 kg NO3−−N ha−1, respectively. About one-fourth of total N2 fixation was carried out during preflowering, one fourth during the two weeks of flowering and the remainder during post-flowering.

About 55% of the amount of N present in pods at maturity was estimated to be derived from mobilization of N from vegetative organs. “Starter” N (25 or 50 kg NO3−−N ha−1) did not significantly influence either dry matter and nitrogen accumulation or the development of leaf area. Neither root length and root biomass determined 8 weeks after seedling emergence nor the yield of seed dry matter and nitrogen at maturity were influenced by fertilizer application.