Nitrogen Uptake Kinetics of Kentucky Bluegrass As a Determinant of Nitrogen Use Efficiency.

A nutrient solution screening procedure was used to identify Kentucky bluegrass (Poa pratensis L.) cultivars having high vs. low NO₃-N uptake capacity. Subsequent column lysimeter studies were carried out to examine if advantageous nitrogen uptake ability would translate into increased competitiveness and reduced nitrate leaching. Tillers of 80 Kentucky bluegrass cultivars were rooted and transferred to a flow-through solution culture system. After establishment, plants were treated to develop N deficiency.  ¹⁵N-labeled KNO₃ was introduced at high (1 mM) and low (0.05 mM) concentrations to screen for differences in the kinetic uptake parameters V_max and K_m, respectively.  After a brief uptake period, plants were harvested, dried, and analyzed for ¹⁵N content to determine N uptake rate. There were significant and substantial differences among the cultivars for uptake at both high and low N concentrations. Accordingly, ‘Julia’ and ‘Midnight’ were selected as representative cultivars exhibiting superior and inferior uptake ability, respectively.  To examine the role of uptake capacity on competitiveness, the two cultivars were planted alternately in a grid pattern using column lysimeters filled with medium-coarse sand.  After establishment, ¹⁵N-labeled potassium nitrate solution (1mM and 0.05mM) was applied to each column. After an uptake/flush period, individual plants of each cultivar were harvested and pooled for ¹⁵N analysis.  Cultivar ‘Julia’ exhibited only slightly higher (6.6%, p = 0.067) nitrate uptake than ‘Midnight’ at 1mM N, and no difference at 0.05mM N.