The effects of combined nitrogen on growth, nodulation, and nitrogen fixation of black locust seedlings.

Robinia pseudoacacia seedlings grown for 50 days in sand culture with 5 mM NH₄HO₃-supplemented nutrient solution had 74% lower N-fixation rates than seedlings grown without N. Differences in N-fixation between treatments reflected decreased nodulation in seedlings grown with NH₄NO₃. Relative growth rate was approximately 3 times greater in seedlings supplied with NH₄NO₃, indicating maximum biomass accumulation cannot be supported by N-fixation alone. Growth was positively correlated with N-fixation rate in seedlings grown without N, a correlation not found in NH₄NO₃-treated seedlings. Leaf N content was less for seedlings growtn in the nitrogen-free sand culture and observations suggested that a nitrogen deficiency in these seedlings restricted their growth.-from Authors