Increased atmospheric deposition of nitrogen increases [NH+] in the soil and usually decreases CH4 uptake by well-drained soils although reverse effects can also be observed in nitrogen-deficient forests as discussed in Section 17.2. For the Hoglwald (spruce) in Germany, Butterbach-Bahl et al. (1998, 2002a) found the following relationship:
Reworking this on an annual basis leads to
CH4 uptake (kg CH4/ha/year)
Equation (17.10b) indicates that the uptake rate of atmospheric CH4 is decreased by 4.1% for each kilogram of NH4-N entering the system. As with N2O emissions, a rather extreme impact of nitrogen deposition on CH4 uptake was found for pine forests in north-east Germany (Butterbach-Bahl et al, 2002a), but this result cannot be considered a representative. Assuming the Hoglwald forest as a representative for Europe and considering an additional average NH4 deposition of 1.2 kg N/ha/year during 1960-2000, there would have been an average decrease in atmospheric CH4 uptake of 0.05 kg CH4/ha/year at this site. This amount equals 1.6% of the total uptake of 3.04 kg CH4/ ha/year at zero NH4-N input (see Eq. 17.10b). Clearly, the effect of the increased atmospheric nitrogen deposition on CH4 uptake by forest soils is small. Furthermore, this result is rather uncertain, given the small number of studies and the complexities involved. Therefore, no further attempt has been made to quantify the effects of atmospheric deposition on net CH4 uptake by European forest soils.
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