Carbon sequestration by a jack pine stand following urea application

Aboveground C accumulation in a productive, 45-year-old, fire-origin jack pine ( Pinus banksiana Lamb.) forest in northeastern Ontario was examined for 30 years after fertilization. Nitrogen was applied as urea at five levels (0, 56, 112, 224, and 448 kg N ha −1) in a Latin square design with five replicates in May 1970. Gross and net C accumulation rates were calculated at 5-year intervals until 1985 and, again, in 1999 from local tree biomass equations and tree diameters, heights and stocking. Incremental response to fertilization was assessed for the 0, 224 and 448 treatments using cores obtained in 1999. Comparisons of post-fertilization to pre-fertilization growth revealed that a N addition of 448 kg ha −1 produced a statistically significant increase in 10-year ( p=0.05) dbh increment ratio over the control in co-dominant trees. Dominant and intermediate trees did not respond significantly to N additions. Nitrogen addition did not affect gross and net stand C increment beyond 10 years or the maximum amount of C in the stand over 30 years. The unfertilized pine stand was able to capture sufficient inherent site N to expand its crown mass and N content to maturity and thereby maintain exceptional growth rates. It appears that C accumulation by this over-stocked, productive semi-mature jack pine stand was only weakly constrained by N supply.