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Nitrogen Form and Availability Measured with Ion Exchange Resin in a Loblolly Pine Stand on the Coastal Plain of North Carolina

Duarte, N.. 2002. M Sc North Carolina State University


Understanding how management affects long-term site productivity will help sustain forest productivity. In 1991, the USDA Forest Service installed the Long-Term Soil Productivity study in the Croatan National Forest in Craven County, NC. The study objectives were to compare indices of plant-available inorganic N from ion-exchange resin (IER) in bead and membrane form and aerobic incubations in a 8-week study, and to analyze the effects of harvest intensity and site preparation on N availability and tree growth ten years after planting. Site treatments included three levels of organic matter removal and two levels of compaction arranged in a strip-plot design replicated in three blocks. Block 1 consisted primarily of Goldsboro soil, while Blocks 2 and 3 consisted of Lynchburg. The two-month study showed no correlation of the N availability among the three techniques. The short duration of the study and the low soil fertility at the site may have contributed to the lack of correlation, indicating that the two forms of IER may not be equally effective at low nutrient concentrations. Through the 2001 growing season, IER membranes were used to measure soil NO3and NH4 biweekly. Non-linear analysis indicate that total inorganic N recovered from the IER membranes was significantly influenced by interactions between block, organic matter removal treatment and compaction level (p=0.005). Total basal area of loblolly pines was significantly impacted by block (p=0.015) and organic matter removal treatment (p=0.032). At year ten, treatment effects can still be seen on both soil processes and tree productivity.