Carbon and nitrogen limitations on soybean seedling development.

Carbon and nitrogen limitations on symbiotically grown soybean seedlings (Glycine max [L.] Merr.) were assessed by providing 0.0, 1.0, or 8.0 millimolar NH(4)NO(3) and 320 or 1,000 microliters CO(2)/liter for 22 days after planting. Maximum development of the Rhizobium-soybean symbiosis, as determined by acetylene reduction, was measured in the presence of 1.0 millimolar NH(4)NO(3) under both levels of CO(2). Raising NH(4)NO(3) from 0.0 to 8.0 millimolar under 320 microliters CO(2)/liter increased plant dry weight by 251% and Kjeldahl N content by 287% at 22 days after planting. Increasing NH(4)NO(3) from 1.0 to 8.0 millimolar under 320 microliters CO(2)/liter increased total dry weight and Kjeldahl N by 100 and 168%, respectively, on day 22. Raising CO(2) from 320 to 1,000 microliters CO(2)/liter during the same period had no significant effect on Kjeldahl N content of plants grown with 0.0 or 1.0 millimolar NH(4)NO(3). The maximum CO(2) treatment effects were observed in plants supplied with 8.0 millimolar NH(4)NO(3), where dry weight and Kjeldahl N content were increased 64% and 20%, respectively. An increase in shoot CO(2)-exchange rate associated with the CO(2)-enrichment treatment was reflected in a significant increase in leaf dry weight and starch content for plants grown with 1,000 microliters CO(2)/liter under all combined N treatments. These data show directly that seedling growth in symbiotically grown soybeans was limited primarily by N availability. The failure of the CO(2)-enrichment treatment to increase total plant N significantly in Rhizobium-dependent plants indicates that root nodule development and functioning in such plants was not limited by photosynthate production.