Source: pellet ammonium-nitrate (50# bags from local agricultural supply vendor)
Rate: 11.2 g N m^-2 y^-1
Application Method: broadcast by hand

Fertilizer applications:

3/15/2005	5.6 g N m-2 applied
4/21/2005	5.6 g N m-2 applied
3/14/2006	11.2 g N m-2 applied
3/7/2007	11.2 g N m-2 applied
3/25/2008	11.2 g N m-2 applied

Background Information:

Based on the observations at the Duke FACE experiment, and the need to provide parameters for diagnostic and prognostic modeling of carbon sequestration in forests at regional scales, three adjustments were made to the experimental design of the Duke FACE experiment before the 2005 growing season: (1) Each plot was split into two halves with one-half of each plot fertilized at a rate of 11.2 g N m^-2 yr^-1 (Oren et al. 2001); (2) The prototype FACE plot was incorporated as a part of the "replicated" FACE experiment thereby increasing the statistical power for assessing the effects of elevated [CO₂] on NEP (n=4). We also established four non-instrumented control plots (each of the same size and split for N fertilization) in which forest NPP and nutrient cycling are measured. These additional plots will reduce the uncertainty in estimates of variables under ambient [CO₂]; and (3) without neglecting aboveground research, research will be focused on identifying, quantifying and modeling belowground processes that control C transfer and storage under current and future atmospheric CO₂ concentration.

Loblolly pine has a capacity for high recovery rate following disturbance, and shows a rapid response rate to N addition. We allowed two growing seasons for the stand to recover from the damage inflicted by the December 2002 ice storm. At the end of the second growing season we quantified the spatial variability of growth and litter production within each plot. This facilitated the partition of each plot along the N-S or E-W axis such that the two halves have similar growth and litter production rates. We analyzed information on trees >=8 cm in diameter for all species, and the pine component alone, collected in each plot quarter. The information included tree density, basal area, biomass, 2004 woody biomass increment, and 2004 litterfall. As can be expected there are broad correlations between basal area, biomass, biomass increment and litterfall, but the correlations are not overwhelmingly strong. We therefore emphasized in the analyses and subsequent plot-split decision the biomass increment and litterfall rather than standing biomass (the latter can be used as a covariate). The analyses of increment and litterfall produced similar results so both fluxes were combined to a rough measure of production. Because the pine dominates biomass increment and litterfall, the results of analyses for all species and pine only were nearly the same. The two possible splits in each plot (N-S versus E-W) were assessed, and the split with the most similar production in the two halves was selected.

Next, within each CO₂ treatment, the halves were assigned into two sets such that they have similar average production (biomass increment + litterfall). A paired t-test (pairing the two halves of each plot; N=3) showed no significant difference (P>0.85), indicating that the split generated similar initial production in both the ambient and elevated CO₂ sets. Any differences that might appear after the application of fertilizers would indicate nitrogen limitation. Following, each set within each CO₂ treatment was assigned, as a whole, to the fertilization treatment. To assign the sets to be fertilized or serve as control, each set within a treatment was compared with both sets in the other treatment. The P values were between 0.10 and 0.16 in all comparisons. The difference between the P value in the analyses above (0.85) and the lower P values in these analyses reflects the CO₂ effect on woody biomass production + litterfall in 2004.

The similarity in the test results allowed us to select the "control" (i.e., unfertilized) sets that preserve the most recent statistics when comparisons were made based on the whole-plot data. In 2004, the control halves under elevated CO₂ had higher "productivity" than those under ambient CO₂, with a P<0.1 - similar to previous whole-plot P values. For the group of 4 new plots (plots 9-12), splits were based on the same approach but using standing biomass only because we have no growth or litterfall data. Note that plots 7 (the prototype) and plot 8 (its reference) were split in 1998.

The outcome (numbers are the plot quadrants) is:

plot	C	F
1	1/4	2/3
2	3/4	1/2
3	1/4	2/3
4	1/2	3/4
5	3/4	1/2
6	3/4	1/2
7	3/4	1/2
8	2/3	1/4
9	1/2	3/4
10	1/2	3/4
11	1/4	2/3
12	3/4	1/2

The partition was done as in the prototype; a narrow trench was dug to a depth of 70 cm (more than twice the depth of the fine roots at the site), an impermeable sheet was inserted in the trench, and soil and stones collected on a tarp used as backfill.

Nitrogen fertilization, using Ammonium Nitrate pellets, was applied in two half-doses in March and April of 2005, and is being applied in a single full-dose annually in March since then. The rate (11 g N m^-2 y^-1) is similar to the rate used in the prototype, an order of magnitude higher than the local addition of N in deposition (~0.8 g m^-2 y^-1). The rate represents a compromise between fertilization operation in forestry, and high deposition rates found in certain areas in northeast U.S. and in Europe.

References:

Oren R, Ellsworth D, Johnsen K, Phillips N, Ewers B, Maier C, Schäfer K, McCarthy H, Hendrey G, McNulty S, and G Katul. 2001. Soil fertility limits carbon sequestration by forest ecosystems in a CO₂-enriched atmosphere. Nature 411:469-472. [PDF]