Interactive comment on “ Direct contribution of nitrogen deposition to nitrous oxide emissions in a temperate beech and spruce forest – a 15 N tracer study ” by N .

Abstract. The impact of atmospheric nitrogen (N) deposition on nitrous oxide (N2O) emissions in forest ecosystems is still unclear. Our study assessed the direct contribution of N deposition to N2O emissions in temperate forests exposed to chronic high N depositions using a 15N labelling technique. In a Norway spruce stand (Picea abies) and in a beech stand (Fagus sylvatica) at the Solling, Germany, we used a low concentrated 15N-labelled ammonium-nitrate solution to simulate N deposition. Nitrous oxide fluxes and 15N isotope abundances in N2O were measured using the closed chamber method combined with 15N isotope analyses. Emissions of N2O were higher in the beech stand (2.6 ± 0.6 kg N ha−1 yr−1) than in the spruce stand (0.3 ± 0.1 kg N ha−1 yr−1). We observed a direct effect of N input on 15N-N2O emissions, which lasted for less than three weeks and was mainly caused by denitrification. No further increase in 15N enrichment of N2O occurred during a one-year experiment, which was probably due to immobilisation of deposited N. The annual emission factor for N2O from deposited N was 0.1% for the spruce stand and 0.6% for the beech stand. Standard methods used in the literature applied to the same stands grossly overestimated emission factors with values of up to 25%. Only 6–13% of the total N2O emissions were derived from direct N depositions. Whether the remaining emissions resulted from accumulated anthropogenic N depositions or native soil N, could not be distinguished with the applied methods. The 15N tracer technique is a useful tool, which may improve estimates of the current contribution of N deposition to N2O emissions.


General comments
This paper presents experimental results of a 15N tracer study, comprising a short-term (3 weeks) and a long-term (1 year) experiment, on the direct contribution of deposited N to nitrous oxide soil emissions in two adjacent Norway spruce and beech forest stands in the Solling, Central Germany. It provides evidence that the direct contribution of throughfall-deposited N to N2O emissions is much smaller than previously thought, using other methods to determine the ratio of N2O from N deposition to total N2O emissions of the soil. The topic of this paper is well in the scope of Biogeosciences. The study appears to be well conducted, except for the fact that N2O chamber measure-C4271 BGD 7, C4271-C4274, 2010 Interactive Comment Full Screen / Esc

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Interactive Discussion Discussion Paper ments on the control plots were conducted with significantly less chambers than in the labeling treatments (two in the case of the short-term experiment compared to four for each treatment; and three control chambers in the long-term experiment compared to seven chambers for each treatment plot). The own results are put nicely into the context of a large range of other results, pinpointing the fact that most other approaches overestimate the direct contribution of N deposition to N2O soil emissions. If the editors of Biogeosciences do not see a general problem in the lower number of control chamber replicates, I recommend publication of the paper after consideration of some points which are specified below. The paper would also benefit from language editing by a native speaker.

Specific comments
p. 8346, l. 11: Here and throughout the manuscript: the formula 15N2O is misleading, or incorrect sensu stricto. I am quite sure that you have analyzed 15N14NO or 14N15NO, i.e. m/z 45, not m/z 46 (as it would be the case for "true" 15N2O). Therefore, I recommend replacing 15N2O with 15N-N2O, as you have used on p. 8357, l. 4, throughout the manuscript to be chemically correct.
p. 8350, l. 6: "Two chambers served as control." This is not really the standard of good scientific practice, especially as the data of the control chambers form the basis for all your calculations of increased emissions after N addition and of 15N-excess of N2O.
p. 8350, l. 7: What do you mean with "irrigation"? "Irrigation event"? p. 8350, l. 24+25: "bi-weekly" or "biweekly" is ambiguous, it could mean "twice per week" or "every other week". p. 8350, l. 26-28: This sentence is not clear to me. Do you mean: "During one irrigation event in the one-year experiment, amounts of added N and water were equal to the amounts added with one irrigation event in the short-term experiment"? p. 8356, l. 8-11: I don't understand this paragraph. In the previous paragraph you C4272 BGD 7, C4271-C4274, 2010 Interactive Comment Full Screen / Esc

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Interactive Discussion Discussion Paper describe broadly the differences between the labeling treatments, and here you state that there were no differences in 15N-N2O fluxes between labeling treatments. I can also see differences in the nitrate labeling treatments of a factor of 10 or more with respect to 15N-N2O fluxes. p. 8356, l. 19: Here and in the following, do you mean 15NH415NO3? p. 8357, l. 1-2: "In the beech stand, we observed no differences in 15N2O flux for both treatments. . .": I can see clear differences. The question is, whether the differences were significant.
p. 8357, l. 2-3: ". . .and they followed the same seasonal trend as the total N2O flux in both treatments. . .": Again, I see differences. N2O flux in beech reached its maximum in June, whereas 15N-N2O fluxes reached their maximum in July. Please describe the results more carefully.
p. 8360, l. 14-18: For N2O fluxes, soil moisture is at least as important as soil temperature, if not much more (except for freeze-thaw events). Thus, you should also consider soil moisture when deriving EFR from a regression analysis.
p. 8361, l. 5-8: This sentence needs to be rephrased. The meaning is not clear.
p. 8362, l. 22: Write "to the emission" instead of "on the emission".