Impact of nitrogen deposition on CO2 exchange

The methodology used to calculate the impact of increased nitrogen deposition on carbon sequestration by European forests is inspired by the approach of Holland et al. (1997) and Nadelhoffer et al. (1999). These authors assessed the additional carbon sequestration on a global scale from additional nitrogen uptake by trees and nitrogen immobilization in soils in response to nitrogen deposition, according to

AC sequestration = AN deposition x (frNuptake x C/Nstem wood + frNimmobilization x C/Nsoil) (17.5)

The basic assumption in this approach is that the additional nitrogen uptake or immobilization is reflected in carbon pool changes due to tree growth and organic matter accumulation according to the carbon/nitrogen ratios of the tree and the soil, respectively (Nadelhoffer et al., 1999; De Vries et al., 2003c). For the global scale, Holland et al. (1997) suggested that the increased carbon sequestration in forests due to increased atmospheric nitrogen deposition could take up one-third of the global CO2 emission from fossil fuel, which is equivalent to 2 Pg C/year, if most of the nitrogen deposited were taken up by trees and used to form new woody biomass. However, recent data of Nadelhoffer et al. (1999) suggest that a large part of the nitrogen deposited accumulates in the soil at low carbon/nitrogen ratio (10:40) and not in the trees at high carbon/ nitrogen ratio (200:500). Nadelhoffer et al. (1999) calculated the carbon sequestration due to increased nitrogen uptake by trees and nitrogen immobilization in soils on a global scale, assuming: (i) a constant nitrogen uptake fraction of 0.05 and a constant nitrogen immobilization fraction of 0.70, based on short-term (1-3 year) 15N-labelled tracer experiments in nine temperate forests; and (ii) an average carbon/nitrogen ratio in stem wood of 500 and in forest soils of 30. Using this approach, an additional deposition of 1 kg N/ha/year leads to a sequestration of 46 kg C/ha/year, of which 25 kg C/ha/year is retained in stem wood (0.05 x 500) and 21 kg C/ha/year in soil (0.7 x 30). These results suggest that carbon sequestration in forest trees and in forest soils in response to additional nitrogen deposition is of equal magnitude, and that the impact of nitrogen deposition on carbon sequestration in forests is much lower than the estimate made earlier by Holland et al. (1997).

De Vries et al. (2005b) adapted the approach by Nadelhoffer et al. (1999) as summarized in Table 17.4, using measured and estimated data at the 6000 level I plots. They used 1960 as the reference year for nitrogen deposition (this leads to 'reference' or background growth) and calculated the additional nitrogen deposited during 19612000 relative to the reference year 1960, to assess the contribution of increased nitrogen deposition on the increase in carbon pools in trees and soil. They included the spatial differences in nitrogen deposition for the individual plots (EMEP model estimates). The nitrogen uptake fraction ranged between 0.05 and 0.10, with high values in low deposition areas because of nitrogen deficiencies, and low values in high deposition areas. Similarly, the carbon/nitrogen ratios in trees were assumed to range from 250 to 500, with low values in high deposition areas because of the assumed luxury consumption. The nitrogen immobilization fraction was assumed to be a function of the carbon/

Table 17.4. Overview of differences between the approach used by Nadelhoffer et al. (1999) and our study to calculate the impacts of nitrogen deposition on carbon sequestration.

Nadelhoffer et al. (1999)

Our approach

Reference nitrogen deposition is negligible Constant average nitrogen deposition

Nitrogen uptake fraction is constant Nitrogen immobilization is constant

Carbon/nitrogen ratio tree is constant

Carbon/nitrogen ratio soil is constant in space and time

Reference nitrogen deposition is 1960 Spatially distributed and time-dependent nitrogen deposition

Nitrogen uptake fraction is f(nitrogen deposition) Nitrogen immobilization fraction is f(carbon/nitrogen ratio humus layer/soil, NH4/NO3 in deposition) Carbon/nitrogen ratio tree varies in space and time as f(nitrogen depositionxt)a Carbon/nitrogen ratio organic and mineral layer varies in space aBased on calculated EMEP nitrogen deposition.

bBased on the measured carbon/nitrogen ratio data from ~6000 forested plots.

nitrogen ratio of the soil organic matter layer, as described earlier, and not a constant of 70%. For the carbon/nitrogen ratio in the organic layer and mineral layer, we used the measured values at all level I plots instead of using a constant value of 30 (De Vries et al., 2003c, 2005b).

Results for the period 1960-2000 indicate an average additional carbon sequestration in stem wood of 15 million tonnes per year in response to an additional nitrogen input (above the reference nitrogen deposition of 1960) of 0.45 million tonnes per year, which translates to 33.3 kg C/ha/year per kilogram of nitrogen deposited (De Vries et al., 2005b). This is close to the 25 kg C/ha/year per kilogram of nitrogen used by Nadelhoffer et al. (1999). Assuming that the long-term carbon sequestration, corrected for CO2 emissions due to harvest and forest fires, is 33% of the carbon pool changes - an average ratio for Europe (Nabuurs and Schelhaas, 2003) - the net carbon sequestration in stem wood is 11.1 kg C/ha/year per kilogram nitrogen. For soil the sequestration is larger, namely 6.7 million tonnes per year, which translates to 14.8 kg C/ha/year per kilogram of nitrogen deposited. Overall the impact of nitrogen deposition on the total carbon sequestration by trees and soils is estimated at ~26 kg C/ha/year per kilogram of nitrogen. During 1960-2000, the average additional nitrogen deposition was 2.8 kg N/ha/year, causing an additional overall carbon sequestration of 73 kg C/ha/year. The total average carbon sequestration was estimated at 719 kg C/ha/ year (576 in trees and 143 in soil), implying a nitrogen deposition impact of ~10%. On a European scale, using a forested area of 162 million hectares, the impact of nitrogen deposition on carbon sequestration in trees and soil is 11.7 million tonnes per year on a total carbon sequestration of 117 million tonnes per year (0.117 Gt/year with 0.094 Gt/year in trees and 0.023 Gt/year in soil).

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