Natural 15N abundance of soil N pools and N2O reflect the nitrogen dynamics of forest soils

Natural N-15 abundance measurements of ecosystem nitrogen (N) pools and N-15 pool dilution assays of gross N transformation rates were applied to investigate the potential of delta N-15 signatures of soil N pools to reflect the dynamics in the forest soil N cycle. Intact soil cores were collected from pure spruce (Picea abies (L.) Karst.) and mixed spruce-beech (Fagus sylvatica L.) stands on stagnic gleysol in Austria. Soil delta N-15 values of both forest sites increased with depth to 50 cm, but then decreased below this zone. delta N-15 values of microbial biomass (mixed stand: 4.7 +/- 0.8 parts per thousand, spruce stand: 5.9 +/- 0.9 parts per thousand) and of dissolved organic N (DON; mixed stand: 5.3 +/- 1.7 parts per thousand, spruce stand: 2.6 +/- 3.3 parts per thousand) were not significantly different; these pools were most enriched in N-15 of all soil N pools. Denitrification represented the main N2O-producing process in the mixed forest stand as we detected a significant N-15 enrichment of its substrate NO3- (3.6 +/- 4.5 parts per thousand) compared to NH4+ (-4.6 +/- 2.6 parts per thousand) and its product N2O (-11.8 +/- 3.2 parts per thousand). In a N-15-labelling experiment in the spruce stand, nitrification contributed more to N2O production than denitrification. Moreover, in natural abundance measurements the NH4- pool was slightly N-15-enriched (-0.4 +/- 2.0 parts per thousand) compared to NO3- (-3.0 +/- 0.6 parts per thousand) and N2O (-2.1 +/- 1.1 parts per thousand) in the spruce stand, indicating nitrification and denitrification operated in parallel to produce N2O. The more positive delta N-15 values of N2O in the spruce stand than in the mixed stand point to extensive microbial N2O reduction in the spruce stand. Combining natural N-15 abundance and N-15 tracer experiments provided a more complete picture of soil N dynamics than possible with either measurement done separately.