There are large uncertainties in the GHG emission estimates and in the estimated effects of nitrogen deposition on those emissions. The range in values presented in Table 17.6 may in reality be even larger due to other aspects not included in the quantification, such as the occurrence of forest disturbances, the neglect of off-site carbon sequestration in hardwood products, the neglect of indirect N2O emissions and the occurrence of lag times between changes in nitrogen deposition and GHG emissions.
The neglect of off-site carbon sequestration causes an underestimation of the real nitrogen deposition impact, whereas the other aspects cause an overestimation.
Information on effects of disturbances on the productivity of forest ecosystems, either natural as a consequence of fire, wind, pest or disease, or managed such as forest logging, has been achieved by the use of chrono-sequences of forest stands at different ages. Results show that disturbances decrease photosynthetic capacity, completely in the case of logging, and typically convert a forest from a carbon sink into a carbon source (see also Hymus and Valentini, Chapter 2, this volume). The time required for a stand to become carbon-neutral and ultimately sequester carbon after the initial disturbance depends on the type and intensity of the disturbance and on post-disturbance management for evergreen forests (Thornton et al., 2002). Periods generally vary between 10 and 12 years but periods of ~25 years have also been found (Hymus and Valentini, Chapter 2, this volume). In this context, it is also important to mention that N2O emissions can substantially increase in the period following clear-cutting. In an overview article, Bowden (1986) estimated an increase of up to 0.5 kg N2O-N/ha/year, but this might be much higher in nitrogen-saturated forests. First results for such forests indicate huge emissions of even more than 3 kg N2O-N/ha/ year in the first 4-7 years after clear-cutting (Butterbach-Bahl, 2006, personal communication). Including these estimates over the rotation time of a forest could increase the average N2O emission by ~0.2-0.5 kg N2O-N/ha/year, which is very substantial compared to the estimates presented earlier (see also Table 17.6). Similarly, nitrogen leaching is substantially increased in this period. This was already shown in an overview of numerous studies from the 1960s and 1970s in the USA (Vitousek and Melillo, 1979), and was recently substantiated by an additional overview of numerous studies since the 1980s in Europe and partly in the USA (Gundersen et al., 2005). The results show that, in general, the nitrate concentration in soil and stream waters increases directly after clear-cutting with peak concentrations within 2-3 years. Gundersen et al. (2005) found that the highest responses (the difference in nitrate concentration between cut and reference stands) were observed in central Europe (5 mg N/l in stream or seepage water as a mean over the region). Assuming an average precipitation excess of 100-300 mm/year would imply an increase in the nitrogen leaching rate of 5-15 kg/ha/ year. Using the standard fraction of 2.5% for indirect N2O emission for each kilogram of nitrogen leached (Mosier et al., 1998) would cause an estimated increase in indirect N2O emissions of ~0.1-0.3 kg N2O-N/ ha/year. The nitrate concentration, however, often returns to pre-cutting levels within 5 years.
In this study we assume that wood which is harvested and removed from a site is ultimately released as CO2 into the atmosphere. Thus we only account for the carbon sequestered in standing biomass. However, harvested wood can often reside in solid wood products, recycled products or landfills for centuries. Sometimes a large fraction of harvested wood is also used for energy production. This type of full accounting is often used for carbon sequestration, and results show that increases in off-site carbon can be sizeable, perhaps matching increases in on-site carbon (e.g. Pacala et al., 2001). If nitrogen deposition accelerates forest growth, the potential for the off-site carbon sequestration (storage in products or in landfills, bioenergy offsets of fossil fuel emissions) is thus increased.
The impact of additional nitrogen deposition on indirect N2O emissions from surface water, induced by additional nitrogen leaching or runoff, has not been included in the calculation because of its extreme uncertainties. In general, nitrogen leaching is negligible below an atmospheric input of 10 kg N/ha/year (Dise et al., 1998; Gundersen et al., 1998; De Vries et al., 2003a). At higher nitrogen inputs, leaching is clearly higher in 'nitrogen-enriched' sites (carbon/nitrogen ratio in the organic layer below 23) than in 'carbon-enriched' sites (carbon/nitrogen ratio in the organic layer above 23). In the first case, a linear relationship has been derived according to nitrogen leaching = -4.3 + 0.67 x nitrogen deposition in kg N/ha/year (final report CNTER project 2005; Gundersen, 2006, personal communication). Using an average nitrogen deposition of 12.3 kg N/ha/year for forests in Europe (De Vries et al., 2005b) would imply an average nitrogen leaching rate of 5.3 kg N/ha/year, which is close to 40% of the nitrogen deposition. However, in most cases the carbon/ nitrogen ratio of the organic layer is above 23 and nitrogen leaching is generally lower. On average, the nitrogen leaching rate at 121 intensively monitored plots was only ~1 kg N/ ha/year, which is ~7% of the nitrogen input (~13 kg N/ha/year; De Vries et al., 2001). In the IPCC approach, nitrogen leaching is estimated at 30% of the nitrogen input (Mosier et al., 1998). Using the standard fraction of 2.5% for indirect N2O emissions for each kilogram of nitrogen leached (Mosier et al., 1998) and a leaching fraction varying between 0.1 and 0.4 implies a net N2O emission ranging from ~0.25% to 1.0% for each additional kilogram of nitrogen deposited. Compared to the average estimated value of 1.8-4.0% for direct N2O emissions, this implies an average increase of ~10-20%, but the uncertainty in this value is high and the contribution may also be high at certain plots.
Considering the various factors affecting N2O emissions, it is in general not correct to assume that N2O emissions will decrease proportionally with reduced nitrogen deposition. Because the soil is a large reservoir of nitrogen, especially in nitrogen-saturated forest soils, decreases or increases in atmospheric nitrogen deposition may not cause direct changes in N2O emissions. This is illustrated by a key experiment, published by Borken et al. (2002), describing the application of normal 'polluted' versus cleaned (to produce natural, unpolluted precipitation) throughfall to soil under roofed plots of a 70-year-old Norway spruce plantation in Germany. Although the average N2O emission at the end of the experiment was slightly lower in the cleaned sites, no significant differences in N2O emission rates were found after 7 years of treatment (Borken et al., 2002). The results suggest that soil acidification and nitrogen eutrophication had a negligible effect on N2O emissions of this temperate spruce forest. It should be noted that the N2O emissions from the spruce forest were low (~0.3 kg N2O-N/ha/ year) and thus the mitigation potential was limited. Clearly the results cannot be extrapolated either to deciduous forests or 'high N2O out' forest systems or to forests that are largely nitrogen-limited like many boreal forests. Nitrogen-input manipulations, i.e. N2O response studies in various systems, would be an important addition to quantify the mitigation potential of reduced nitrogen inputs. Borken et al. (2002) also monitored the CH4 oxidation rate and, like for N2O emissions, no changes in CH4 oxidation were observed upon a 7-year reduction of nitrogen deposition. The mechanism behind these observations remains obscured. It may be the result of a lag phase in the response of a forest that has received high nitrogen inputs over a long period, or because the original nitrogen inputs were below a level where strong effects are seen. Clearly further study is necessary to elucidate the potential to reverse N2O emission levels and CH4 oxidation capacities of forests exposed to elevated nitrogen deposition levels.
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