Nitrogen versus phosphorus limitation of plant productivity over post-fire succession in Alaskan boreal forest

Background/Question/Methods

Plant productivity in boreal forest ecosystems is generally thought to be limited by nitrogen (N) availability, yet there are few experimental tests of this in natural forests. Low rates of N inputs due to the paucity of the N fixing flora and slow rates of decomposition in cold soils support the idea that N is in short supply relative to plant demand.  But disturbances characteristic of boreal forests, such as wildfire, can differentially affect the availability of N and the next most likely limiting element, phosphorus (P), raising the potential for changing nutrient limitation over secondary succession. We directly tested for N and P limitation of plant productivity in three sites that are part of a well-studied post-fire chronosequence in Interior Alaska.  The youngest site was burned in the 1999, the middle-aged site burned in 1987, and the oldest site burned in approximately 1921.  Within each site, we applied a randomized complete block design (n=4) where plots received treatments of 10 g N m^-2 yr^-1, 5 g P m^-2 yr^-1, N and P combined at the same rates, or control.  Plant composition, aboveground biomass and net primary productivity, tissue nutrient concentrations, and soil organic layer characteristics were surveyed in each plot prior to the initiation of the experiment and after five and six years of treatment.

Results/Conclusions

All three sites showed significant increases in ANPP when fertilized with N or N+P; only in the oldest site did ANPP respond to P added alone.  The size of the N fertilization effect on ANPP was largest in the young and middle-aged sites: about 200% of control levels.  In the oldest site, both N and P fertilization caused significant but small (20%) increases in ANPP.  Although foliar N concentrations of the youngest needles of the spruce trees that dominate the oldest site increased with N fertilization, there was no change in foliar P concentration in the P fertilization treatment.  There was, however, a tendency towards higher foliar N concentrations in this treatment. Soil organic layer carbon:N ratios decreased and nitrate concentrations increased relative to the control treatment, suggesting that P addition may indirectly stimulate plant productivity by accelerating decomposition, turnover and availability of N in this system.