Round 1

Reviewer 1 Report

The manuscript “Long- and short-term inorganic Nitrogen runoff from a forested karst catchment in Austria” uses a 27-year dataset of water chemistry, soil chemistry, throughfall N deposition, tree nutrient content, and other metrics to describe the dynamics of N cycling and leaching within a watershed in Austria. I think the manuscript does a great job not only summarizing many different analyses that have been completed in the watershed, but providing a synthesis of potential long-term changes due to past measurements.

Generally this is a well-written manuscript and clearly communicates the questions, methods, and results. I highlighted a few of sentences where the meaning was unclear below, and there are a few formatting issues that I didn’t check, so the manuscript should be read closely before final submission.

I did not find any major issues with the manuscript. I highlighted a few areas where the figures could be slightly modified for clearer communication, and a couple potential additions.

Line 14: Put “increased gaseous N emissions” at the beginning of the list since it causes the others.

Line 21: Make sure -1s are superscript throughout.

Line 26: Is “weakly decreasing N deposition” supposed to be “weakly decrease leaching due to N deposition”?

Line 27: Add “the” before future in “predicted in future”

Line 34: Again, put “increased gaseous N emissions” at the beginning of the list since and mention that it leads to the other elements.

Line 36: Throughout you use 1980ties (and other decades), where I am more familiar with the use of 1980s. If this is an intentional choice, the “mid 1980ies” here is missing a t.

Line 83-84: It might be worth adding something about increased forest fire with increasing temperatures that would lead to stand replacement.

Figure 1: If possible, it would be useful to label IP I, IP II, and IP III

Line 259: My preference would be to use the units kg/ha/yr throughout the document. In each instance where you use kg/ha you directly mention it as an annual number, but just in comparing the data in the text to the figures, it would align better.

Figure 3: I would be interested in seeing a rolling 3- or 5-year average of temperature or precipitation on this figure. You mention in the text that temp increased 1.5 degrees from 1991 to 2105, but it would be interesting to see how this trends on smaller increments.

Figure 4: It’s unclear what the two bars in the figure A are.

Line 276: Reference error

Line 390: Remove “In” from “In a modelling study”

Line 392: “save” to “safe”

Author Response

Please see the attachment

Reviewer 2 Report

Long- and short-term inorganic Nitrogen runoff from a forested karst catchment in Austria

Dirnböck et al.

Article aims

The aims with this article are somewhat difficult to assess, since there is no specific section at the end of the introduction describing this.

However, embedded in the abstract and Intro I find some sentences that can be interpreted as aims and hypotheses:

1. Evaluate the drivers of dissolved inorganic N (DIN) concentrations and fluxes from a karst catchment
2. Explore vegetation, soil, and other catchment sinks in order to explain the likely effects of N deposition on nitrate leaching and to provide an outlook to the future.
3. Down-regulation of tree growth can be explained by increasing deficiency of P and K
4. We hypothesized increased soil N storage and decreasing C:N ratios in the soils,
5. With climate scenario data, the application of a hydrological model, and the knowledge we have gained through analyzing the long-term data from Zöbelboden, we discuss the potential impact of the manifold drivers on nitrate discharge in the future.

Main conclusions

1. In the the Zöbelboden catchment, soil and vegetation still immobilize N but the rate of N input exceeds their sink capacity, hence N retention and leaching loss occurs in the same time (i.e. partial N retension).
2. Elevated but stable DIN discharge and pulses of nitrate runoff during years with stand disturbances.
3. Identified tree growth as the main sink for inorganic N

Overall technical state of the manuscript

The technical state of the manuscript is not good with a very large number of technical and other mistakes. Obviously, the authors have not performed an internal review before submitting the paper. Consequently, in the review I will not correct any mistakes, the authors have to do this themselves.

Methodology

LTER Zöbelboden was monitored since 1992, so it is a long monitoring period.

Historic N deposition for the sites was monitored as bulk precipitation.

The total deposition of N was estimated as the sum of throughfall and canopy exchange. The latter was based on a canopy exchange model with sodium as the tracer ion. In order to get long-term deposition of N they scaled reconstructed deposition from 1880 to 2000 to the measurements.

I can not find any description of the monitoring of throughfall. Furthermore, the specific application of the canopy exchange model is not described. There are in this model many assumption that need to be validated for the conditions at this particular site.

In order to get long-term deposition of N the authors scaled reconstructed deposition from 1880 to 2000 to the measurements. They refer to one reference, Schöpp et al., but there is no information about how this was applied. This is not acceptable.

There was an extensive monitoring of foliage nutrient concentrations as well as soil chemistry.

The most important monitoring for the focus of this study was catchment hydrology and N measurements in the runoff with, N concentration data between 2000 and 2018, a relatively long time period. Analyses for NO3--N, NH4+-N and total N were available, hence also organic N could be estimated.

Hydrochemical modelling was made with the “VarKarst model”. I do not know this model, and I do not have the time to look up the references. However, the model is described in some detail.

The authors provide discharge projections, based on RCP 8.5 scenarios.

Presentation of results

Authors present the total annual inorganic N deposition between 2000 and 2018 as mean values +/- s.d. (I presume, it is not stated). They do not provide any information about statistical analysis of trends over time.

They state that the share of inorganic N dry deposition was higher for in the mixed deciduous forests than the spruce forests. To me this is strange, it is usually the opposite due to the higher LAI of the spruce forest. But again, it is unclear how the canopy exchange model was applied to estimate the dry deposition.

The authors state that total inorganic N deposition before 1900 was below 10 kg N ha-1 y-1, but there is no way to verify this number in this study, see above.

Engardt et al estimated that the mean total deposition of inorg-N across Europe was in the order of 3 kg N/ha/yr in the beginning of 1900-hundred. However, there was no information about the geographical distribution.

Magnuz Engardt , David Simpson, Margit Schwikowski & Lennart Granat (2017) Deposition of sulphur and nitrogen in Europe 1900–2050. Model calculations and comparison to historical observations, Tellus B: Chemical and Physical Meteorology, 69:1, 1328945, DOI: 10.1080/16000889.2017.1328945.

Figure 4: Can Exc, is that supposed to reflect dry deposition? Is it calender year or hydrological year? Why is there no NH4+ (=bulk deposition) to the spruce forest in 1994?

The changes in DIN discharge are shown but I can not find any information about the DON discharge. In many cases DON discharge is larger that DIN discharge, and this is very important when analyzing catchment budgets of N.

Future projections in this study seem only to include discharge water amounts. There is a large text section and a large figure 7 devoted to this issue. The conclusion is that in the coming 40 years, the change of mean monthly discharge is not substantial. It seems to me that there must have been earlier studies dealing with this issue?

Discussion

The authors state “In spite of these high inputs, during the last two decades still 70-83% of the added DIN was retained in the catchment’s vegetation, soils, the epikarst, the vadose or saturated zone.”

I think that we can never expect that the N retention will be zero in this type of catchments. The concentrations of DIN that are measured in the run-off at this site are very high to me.

The section “4.1. Catchment N retention” is very long and it is unclear to me to what extent the discussion is based on results obtain in this study, and what is based on results from the literature.

Again, I do not understand why discharge of DON is not included in the discussions, e.g. in this sentence “the difference between the DIN, which was deposited into the forests and soils, and the measured and modelled [38] DIN discharge, we inferred that approximately ~70-83% was retained in the catchment during the last two decades”

Overall assessment

Long-term data on monitoring of air pollution input and output from forest ecosystems are always valuable. However, in my mind, the authors of this paper do not manage to present something new from the interpretation of the results of this monitoring. That there is only partial retention of N is this type of catchment is not new.

I suggest that the authors consider again the interpretation of the results and then re-submit a new version in a much better state. Furthermore, the authors have to take discharge of DON into account.

Author Response

Please see attachment