Biological nitrogen fixation, accumulation of soil nitrogen and nitrogen balance for white clover (Trifolium repens L.) and field pea (Pisum sativum L.) grown for seed

The N accumulation, biological N-2 fixation and simple N balance of white clover and field pea grown for seed were determined in field experiments. Both crops accumulated similar amounts of dry matter but pea accumulated more N (427 kg N ha(-1)) than white clover (387 kg N ha(-1)). However, white clover fixed 327 kg N ha(-1), which was significantly higher than that (286 kg N ha(-1)) fixed by pea. At final harvest, the per cent N-2 derived from the air (%Ndfa) was also significantly higher for white clover (90%) than pea (69%). Amounts of N-2 fixed were strongly correlated with dry matter yield in both crops. There was no significant difference in amount of N-2 fixed or %Ndfa when N-2 fixation was estimated based on N-15 enrichment of individual plant component or the whole plant. The N harvest index for both crops (4-28%) was considerable lower than %Ndfa (69-90%), and hence the overall simplified N balances for these crops were positive. The net N gain due to N-2 fixation was high(171-313 kg N ha(-1)) and may play an important role in the N benefits to subsequent crops and/or contribute to increased soil N fertility. (C) 2000 Elsevier Science B.V. All rights reserved.