We evaluated weed competition effects on the N economy of field pea (Pisum sativum L.) and the subsequent crop to address the paucity of such information. Plots were seeded to pea, canola (Brassica napus L.) and barley (Hordeum vulgare L.) in 1997 and 1998. Weeds, augmented by cross-seeding experimental plots with oat (Avena sativa L.), were removed with herbicides one and four weeks after crop emergence (WAE). The subsequent barley crop received 0 or 6 g N m(-2). Mean percentage of N derived from the atmosphere (%Ndfa) for the 2 yr, estimated by N-15 isotopic dilution, was 81% for the 4-WAE treatment and 51% for the 1-WAE treatment, indicating that a pea plant subjected to greater weed competition derived more of its N from symbiotic fixation. Total N fixed by pea was not affected by the time of weed removal, however, and total N uptake and seed yield were greater with early weed removal due to less competition for soil N. Early weed removal resulted in net N export in pea seeds (because of higher production) while later weed removal resulted in gains of 1.1 to 1.3 g N m(-2). However, time of weed removal during pea cultivation had no effect on the yield or N uptake of the subsequent barley crop. Higher barley yield and N uptake following pea than following barley were mostly the result of greater N availability. Nitrogen fertilization benefited the subsequent barley regardless of preceding crop type.
