Nitrogen Availability and Nitrogen Use Efficiency in Loblolly Pine Stands

The effects of nitrogen availability on N cycling and N use efficiency (NUE) were examined in natural and fertilized loblolly pine stands on the upper coastal plain of South Carolina. Indices of N availability, based on potential rates of N mineralization, ranged from 1.6 to 11 kg°ha — 1 (8 wk) — 1 in the stands, and concentrations of N in foliage, wood, fine roots, and needlefall increased with greater N availability. Litterfall dry masses, net aboveground production, and litterfall nitrogen were all positively correlated with N availability, while indices of NUE decreased with increased N availability. Mechanisms that could explain increased NUE in low—N sites were examined in the field and in a phytotron study. First, nitrogen retranslocation on a per needle basis did not change significantly across the N—availability gradient, and thus could not account for the change in NUE efficiency. For the stands as a whole, however, substantially more N was retranslocated at the highest levels of N availability. Second, an increase in N uptake efficiency could not account for an increase in NUE with low N availability, since phytotron—grown seedlings fertilized with N had significantly higher rates of N uptake per unite root mass and lower root: shoot ratios than N—limited seedlings. Third, net production per unit N within pine seedlings was significantly higher in the N—limited plants, suggesting that an increase in the amount of carbon fixed per unit of tissue N could account for the observed increased in NUE in loblolly pine stands at low levels of N availability.