The Effects of Soil Drying Out and Rewetting on Nitrogen and Carbon Leaching—Results of a Long-Term Lysimeter Experiment

Round 1

Reviewer 1 Report

The main objective of the paper was to establish the working hypothesis on the fact that the water and solute balance of agriculturally used soils is adversely affected by drought events (amount of seepage and solute concentration).

Accordingly, a 29 years’ time series of lysimeter measurements was evaluated with following objectives) to identify experimental years with extremely dry weather (dry years), ii) to characterize the influence of extreme drying on seepage formation, and iii) to reveal the associated effects on the N and DOC leaching and their hazard potentials regarding water quality.

Indeed, I believe the significance of this study is valuable, however it should be improved before it is published.

In particular:

- in the introduction:

• the sentence that goes from line 47 to line 49 must be justified by numerous bibliographic references or in any case by very important references.
• From line 46 to line 70 some indices used to measure drought or evapotranspiration are described, but this part, however, does not bind very well to the rest of the introduction which remains quite detached. It is advisable to develop this part more (from line 71 to line 113) and to better tie the previous part (from line 46 to line 70).
• It is suggested to introduce and develop the concept of soil ecosystem services- it is advisable to add a map of the study area.

- In the section on materials and methods it is suggested not to go into too technical details but in the case of moving these parts to special appendices.

- For the results and discussion section it is suggested to map the results, when possible, as in Figure 3.

- As far as the conclusions are concerned, they should be rewritten because they are almost entirely a synthesis of the results, instead the part from line 437 to 443 must be developed, in which the sums of the entire work are summed up and related to what is contained in the introduction to grasp the meaning and general importance of the paper, also going to highlight any limitations and possible advancements in the research.

Author Response

First, we would like to thank the reviewer for the thorough and constructive review as well as for their encouragement. We have revised the manuscript according to the helpful suggestions and comments.  In the following, we respond to the reviewer´s comments point by point.

The main objective of the paper was to establish the working hypothesis on the fact that the water and solute balance of agriculturally used soils is adversely affected by drought events (amount of seepage and solute concentration).

Accordingly, a 29 years’ time series of lysimeter measurements was evaluated with following objectives) to identify experimental years with extremely dry weather (dry years), ii) to characterize the influence of extreme drying on seepage formation, and iii) to reveal the associated effects on the N and DOC leaching and their hazard potentials regarding water quality.

Indeed, I believe the significance of this study is valuable, however it should be improved before it is published.

Thank you for the valuable advice, which we have largely implemented.

In particular:

- in the introduction:

• the sentence that goes from line 47 to line 49 must be justified by numerous bibliographic references or in any case by very important references.

We have rephrased this sentence to improve the clarity of the statement. Thank you for pointing this out (lines 47-49).

• From line 46 to line 70 some indices used to measure drought or evapotranspiration are described, but this part, however, does not bind very well to the rest of the introduction which remains quite detached. It is advisable to develop this part more (from line 71 to line 113) and to better tie the previous part (from line 46 to line 70).
  
  

To achieve the requested better connection of the two main content areas of the introductory chapter, a corresponding adjustment wording was added (lines 72-76).

• It is suggested to introduce and develop the concept of soil ecosystem services- it is advisable to add a map of the study area.

We would also like to thank the unknown reviewer for this information. We have tried to take up the concept of ecosystem services in the adaptation formulation already mentioned to make clear the multi-causality of the processes presented (lines 72-76). Furthermore, we have added an overview map of Europe and Germany to Figure 3 to give the international readership an idea of the location of the study area.

In the section on materials and methods it is suggested not to go into too technical details but in the case of moving these parts to special appendices.

We have made cuts in the Material and Methods chapter in line with the reviewer's proposal so as not to present too many technical details at this point (lines 134-136, 143-144, 184-186, 191-193). 

- For the results and discussion section it is suggested to map the results, when possible, as in Figure 3.

We would also like to thank the reviewer for this advice. Unfortunately, a graphical presentation of the results comparable to Figure 3 was not possible. However, we have now shown the climatic water balance calculated in the individual lysimeter years in Figure 4 by inserting an additional y-axis and thus hope to have improved the clarity of the presentation of results.

- As far as the conclusions are concerned, they should be rewritten because they are almost entirely a synthesis of the results, instead the part from line 437 to 443 must be developed, in which the sums of the entire work are summed up and related to what is contained in the introduction to grasp the meaning and general importance of the paper, also going to highlight any limitations and possible advancements in the research.

The Conclusions chapter has been reformulated. We hope that our statements and the associated indications for research and agricultural practice have now been formulated more clearly.

Reviewer 2 Report

What does "lysimeter year" mean? Please explain this and standardize the records of years to one form, e.g. 1991/92 (occurs: 1991 or 1991/1992).

Author Response

What does "lysimeter year" mean? Please explain this and standardize the records of years to one form, e.g. 1991/92 (occurs: 1991 or 1991/1992).

We would like to take this opportunity to thank the reviewer for reviewing the manuscript and for his commentary. We have now clearly defined the term "lysimeter year" used by us and the associated evaluation period of lysimeter data in chapter 2.1 and then used this uniformly for the presentation of the lysimeter results (lines 159-161).

Reviewer 3 Report

The paper studies the effect of droughts on leaching of nitrate and organic carbon by means of lysimeters in northeastern Germany. Perhaps the most interesting is the use of long-term data. The paper is interesting and well written. The topic is of interest for Water. I have some just a few comments, that need to be addressed.

1. Line 125. You use gravity-flow (free drainage) lysimeters. These lysimeters alter the water flux at the bottom, because it only permits outflow when the soil at the bottom is saturated and never permits upward fluxes such as capillary rise. This may be particularly important during droughts. Do you think that this can be a problem? It least you may want to discuss it.

2. Figure 2. I think the proper name of the vertical axis is "CWB", not "range". The horizontal axis seems to represent the pdf (probability density function) not the "CWB". Does the horizontal position of the "Data" points have some meaning?

3. Line 159-160 and 243-245. How is the CWB calculated? According to 159-160, it is CWB = P - EP. Is this the same as the Standardized Precipitation Evapotranspiration Index (line 243-245)?

4. Line 278-279: What do you mean with "at the 0.001 level"? Can you be more specific?

5. Figure 4. I think it would be useful to add a graph of the mean CWB versus time for comparison with the seepage. Also, you mention a lot the CWB in particular years throughout the text. So, it would be nice to have some graphical reference.

6. Line 307-309. Preferential flow, mentioned before, can shorten the travel time of two years. So, we can expect leachate earlier than one or two years after a drought.

7. Line 396-398. I do not follow the logic of this sentence. The effect of increased respiration or oxidation of organic matter would contribute to low, not high, DOC concentrations.

Author Response

The paper studies the effect of droughts on leaching of nitrate and organic carbon by means of lysimeters in northeastern Germany. Perhaps the most interesting is the use of long-term data. The paper is interesting and well written. The topic is of interest for Water. I have some just a few comments, that need to be addressed.

We would like to take this opportunity to thank the unknown reviewer for reviewing the manuscript and for his valuable comments.

1. Line 125. You use gravity-flow (free drainage) lysimeters. These lysimeters alter the water flux at the bottom, because it only permits outflow when the soil at the bottom is saturated and never permits upward fluxes such as capillary rise. This may be particularly important during droughts. Do you think that this can be a problem? It least you may want to discuss it.

The text was extended by an adaptation formulation pointing out the problems of the lysimeter type used (lines 144-148). The design deficiencies of the gravity lysimeters used, which the reviewer mentions, are well known and are controversially discussed in the literature. We were able to show in previous studies that the lysimeters we used to reflect the soil water balance of sites with comparable (sandy) substrates with high accuracy. We, therefore, assume that the statements made here, which include a measurement period of 29 years and 21 lysimeters, are reliable.

2. Figure 2. I think the proper name of the vertical axis is "CWB", not "range". The horizontal axis seems to represent the pdf (probability density function) not the "CWB". Does the horizontal position of the "Data" points have some meaning?

Thank you very much for this important hint. Figure 2 has been revised according to the advice given.  The horizontal position of the data points represents the result of a class formation as it is performed independently by the used statistics program (Origin).

3. Line 159-160 and 243-245. How is the CWB calculated? According to 159-160, it is CWB = P - EP. Is this the same as the Standardized Precipitation Evapotranspiration Index (line 243-245)?

The CWB was calculated by us as presented by the reviewer. The CWB is not directly comparable with the Standardized Precipitation Evapotranspiration Index (SPEI), since the latter takes into account many more parameters in comparison. The procedure to calculate the index is detailed and involves a climatic water balance, the accumulation of deficit/surplus at different time scales, and adjustment to a log-logistic probability distribution. Mathematically, the SPEI is similar to the standardized precipitation index (SPI), but it includes the role of temperature. However, the SPEI approach is also essentially based on the calculation of the climatic water balance.

4. Line 278-279: What do you mean with "at the 0.001 level"? Can you be more specific?

Thank you very much for this comment. We have made a rewording in the manuscript with the reference to the significance level used (line 289).

5. Figure 4. I think it would be useful to add a graph of the mean CWB versus time for comparison with the seepage. Also, you mention a lot the CWB in particular years throughout the text. So, it would be nice to have some graphical reference.

This hint was also very valuable. We have now presented the climatic water balance calculated in the individual lysimeter years in Figure 4 by inserting an additional y-axis and hope to have achieved better clarity in the graphical presentation of results.

6. Line 307-309. Preferential flow, mentioned before, can shorten the travel time of two years. So, we can expect leachate earlier than one or two years after a drought.

We agree with the reviewer that soil solution flux rates can vary greatly due to preferential flow paths. The displacement velocities estimated during preliminary investigations with conservative tracers were confirmed by the lysimeter investigations. However, there remains uncertainty in quantifying the displacement velocity in soil. The formulation questioned by the reviewer has been adjusted accordingly (lines 318-321).

7. Line 396-398. I do not follow the logic of this sentence. The effect of increased respiration or oxidation of organic matter would contribute to low, not high, DOC concentrations.

The main external conditions that interact with the soil organic matter decomposition are temperature and humidity. Studies on soil warming showed that the activity of soil microorganisms, as well as organic matter decomposition, is strongly affected by soil properties threatened by climate changes. These are the core statements that should be covered by our wording. The sentence in question has been removed and replaced by a sentence indicating the importance of soil temperature and soil moisture-dependent processes for the N and DOC concentration in the soil solution (lines 411-413).