Symbiotic Nitrogen Fixation and Nitrate Uptake by the Pea Crop

Symbiotic nitrogen fixation and nitrate uptake by pea plants (Pisum sativum L.) were studied in field and pot experiments using the ¹⁵N isotope dilution technique and spring barley as a non-fixing reference crop. Barley, although not ideal, seemed to be a suitable reference for pea in the ¹⁵N-technique. Maximum N₂ fixation activity of 10 kg N fixed per ha per day was reached around the flat pod growth stage, and the activity decreased rapidly during pod-filling. The pea crop fixed between 100 and 250 kg N ha^-1, corresponding to from 45 to 80% of
total crop N. The amount of symbiotically fixed N₂ depended on the climatic conditions in the experimental year, the level of soil mineral N and the pea cultivar. Field-grown pea took up 60 to 70% of the N-fertilizer supplied. The supply of 50 kg NO₃-N ha^-1 inhibited the N fixation approximately 15%. Small amounts of fertilizer N, supplied at sowing (starter-N), slightly stimulated the vegetative growth of pea, but the yields of seed dry matter and protein were not significantly influenced. In the present field experiments the environmental conditions, especially the distribution of rainfall during the growth season, seemed to be more important in determining the protein and dry matter yield of the dry pea crop, than the ability of pea to fix nitrogen symbiotically. However, fertilizer N supplied to pot-grown pea plants at the flat pod growth stage or as split applications significantly increased the yield of seed dry matter and protein.