Geologic Soil Parent Material Influence on Forest Surface Soil Chemical Characteristics in the Inland Northwest, USA

Round 1

Reviewer 1 Report

This manuscript describes soil nutrient differences due to distinct soil parent materials in the inland Northwest. I found this to a very useful manuscript to inform forest managers on potential nutrient concerns on their forests. I would like to know if there are climate differences between soil parent materials that might be affecting soil nutrition because many of these parent materials are in different regions.  I would like to see a PCA graph showing the relationship between nutrients and how the different soil parent materials are separated by nutrient types.  Where there any differences in site productivity between the soil parent materials?  I would like to see the discussion lengthened to discuss why the chemistry and composition of the parent materials are affecting soil nutrition differently. You could contrast your data with that from Perakis (Ecosystems 2006) and Hynicka (Global Change Biology 2016) about the relationship between soil N and Ca in sedimentary and basalt soils.  I believe that this manuscript should be accepted after major revisions.

Lines 16-18: I might be more visually pleasing to sort the nutrients by macro- and micronutrients and alphabetically when describing availability.

Lines 42-45: Littke et al. 2014 (Forest Science) could also be cited in this sentence.

Lines 81-83: I would like a better description of the different parent material types.  Include geologic age ranges and example rock types. Discuss any differences in climates between these parent materials as well.

Line 90: It is not necessary to say how the samples were stored.

Figure 1. Would it be possible to show the study locations on this graph or to have another figure with the study locations?

Lines 100-104: Why was NO₃ not tested?

Lines 135-144: Reconsider if all of this is necessary information for the reader.

Line 176: Ca instead of calcium

Table 2 would be easier to read if there was a space between the numbers and letters.

Table 2: I know that the SPMs are in alphabetic order, but it might make more sense to group by parent material type.  I would suggest grouping the plutonic and volcanic together and metasedimentary and sedimentary together.

I don’t know if Table 3 is necessary in this manuscript.  Consider moving it to the appendix. 

Lines 250-255: Littke et al. (Forest Science 2014) and Littke et al. (Forest Science 2016) would also be good citations for this paragraph.

Author Response

Author's Reply to the Review Report #1

Reviewer 1 Comments

This manuscript describes soil nutrient differences due to distinct soil parent materials in the inland Northwest. I found this to a very useful manuscript to inform forest managers on potential nutrient concerns on their forests.

Reviewer 1 Comments

Comment 1: I would like to know if there are climate differences between soil parent materials that might be affecting soil nutrition because many of these parent materials are in different regions.

Response 1: We have revised the text to clarify the authors found no significant relationship between broad climatic regimes and soil nutrient profiles (L87-94).

Comment 2: I would like to see a PCA graph showing the relationship between nutrients and how the different soil parent materials are separated by nutrient types. 

Response 2: We agree that a PCA analysis for data of this type would be very informative. However, for the objectives of this paper a CDF approach provides similar information to that achieved through PCA, but further provides actionable information for land resource managers. CDFs allow anyone to compare their nutrient data against these regional curves to determine their relative nutrient richness. A PCA would not provide this capability, despite being informative in other statistical ways. Given space limitations and objectives of the paper, we have decided to retain the CDFs in lieu of PCA analyses.

Comment 3: Where there any differences in site productivity between the soil parent materials?

Response 3: Broadly yes, and have been published prior (Shen et al 2001; Moore et al 2004a,b; White et al 2012; Coleman et al 2014; Kimsey et al 2019 – as referenced in this manuscript). However, the interaction between rock type, nutrition and forest productivity is very nuanced across the Inland Northwest, and greatly controlled by climatic inputs, topographic position (i.e., solar radiation), and elevation. For this paper, all broad geologic classes were spread across the broad range of climatic drivers of forest productivity, and we saw no direct relationship with differences in soil nutrient profiles. This was addressed at L85-L94. This does not imply none exist. Given more data, coupled with nonparametric analyses, these relationships could be further explored. Regardless, the findings of this paper our illustrative of forest soil nutrient profiles commonly found across this region and, coupled with foliar nutrient analyses (Moore et al 2004b) allow resource managers to identify nutrient limitations and options for amelioration.

Comment 4: I would like to see the discussion lengthened to discuss why the chemistry and composition of the parent materials are affecting soil nutrition differently. You could contrast your data with that from Perakis (Ecosystems 2006) and Hynicka (Global Change Biology 2016) about the relationship between soil N and Ca in sedimentary and basalt soils. 

Response 4: We have revised the discussion section significantly to include additional explanatory text relative to our findings and to those found elsewhere as suggested by the reviewer. We appreciate the literature suggestions provided.

Reviewer 1 Suggestions

Lines 16-18: I might be more visually pleasing to sort the nutrients by macro- and micronutrients and alphabetically when describing availability. – Revised as suggested.

Lines 42-45: Littke et al. 2014 (Forest Science) could also be cited in this sentence. – Added.

Lines 81-83: I would like a better description of the different parent material types.  Include geologic age ranges and example rock types. Discuss any differences in climates between these parent materials as well.

Revised text according to suggestion and included a new Table listing the dominant lithologies for each geological soil parent material.

Line 90: It is not necessary to say how the samples were stored. – Deleted.

Figure 1. Would it be possible to show the study locations on this graph or to have another figure with the study locations?

Locations included in Figure 1.

Lines 100-104: Why was NO3 not tested?

It was, so too NH4+. Only mineralizable N was found to have any relationship with rock class. Inorganic N forms are particularly low within our relatively warmer/drier settings within the Inland Northwest, USA.  Nearly an order of magnitude lower than that observed in warmer/wetter climes of the Pacific Northwest, USA.  Often NO3 is nearly absent, where NH4 is the dominant form, but in quantities that we find deficient for most conifer species of the region.  Thus, we often find a stronger correlation between parent materials with higher forest productivity and mineralizable N; reflecting the tight nutrient cycling link between limited atmospheric N inputs and microbial meditated conversion of organic N to inorganic N.

Lines 135-144: Reconsider if all of this is necessary information for the reader.

We wished to be transparent in our analyses and to include for the reader, particularly in the event someone wishes to perform a similar analysis. Inclusion provides the researcher with methods to replicate if desired.

Line 176: Ca instead of calcium – Revised as suggested.

Table 2 would be easier to read if there was a space between the numbers and letters. – Revised as suggested.

Table 2: I know that the SPMs are in alphabetic order, but it might make more sense to group by parent material type.  I would suggest grouping the plutonic and volcanic together and metasedimentary and sedimentary together. – Revised as suggested.

I don’t know if Table 3 is necessary in this manuscript.  Consider moving it to the appendix.

We debated this exact point internally before placing as Table 3 (now Table 4).  Every journal handles appendices differently, some are relatively easy to access, some require extra effort on the part of the reader to access. We felt this table was important for the reader to have direct access to within the body of the manuscript, thus we wish to retain. If the journal requests movement to an appendix, we will modify accordingly.

Lines 250-255: Littke et al. (Forest Science 2014) and Littke et al. (Forest Science 2016) would also be good citations for this paragraph. – Revised as suggested.

Reviewer 2 Report

This manuscript entitled “Geologic soil parent material influence on forest surface soil chemical characteristics in the Inland Northwest, USA” is well-written and it investigated the soil fertility parameters, in area under forest vegetation, and with soils formed from 5 contrasting parent materials. Although well-written, it is important to provide more details on the parent materials for the better comprehension of the results and to improve the discussions. For instance, the text mentions volcanic and plutonic rocks, but only mentioning that it does not mean these rocks differ chemically and mineralogically from each other. Hence, soils formed from them may be very similar and the differences of parameters of soil fertility may be not related to their parent material, as taken as certain by the investigation. Thus, it is needed to specify the types of rocks, instead of using only plutonic and volcanic rocks (these terms are very general). The same applies for the other terms used for PM identification (metasediment, mixed, sediment) – please specify them. Another important point is that other parameters have great influence on plant development, such as soil texture (clay, silt and sand contents), but it was not evaluated in the manuscript, which would contribute very much for the discussions. Finally, although the authors mention the importance of soils on the development of plant species, no evaluation of the vegetation was performed in this study, which makes a great gap in the paper, especially considering it is well known that different parent materials derive different soils and that plants will grow differently at each condition. Discussion section was very general and did not explain well the findings of this study. Thus, this study simply characterized the nutrient/fertility parameters in different soils, but no correlation was made with the plants (better growth, lesser mortality, greater biodiversity, etc). Some other specific suggestions are presented below.

 

L13: Please insert comma after USA

Abstract: Is it possible to define better each of the rock types mentioned here? For instance, which are the volcanic rocks that derived soils with high CEC and many nutrients? This is important to clarify because there are many divergent volcanic rocks and not all of them generate soils with high contents of nutrients. The same applies to the sedimentary rocks mentioned.

L51: Please use italic in Pinus elliottii

L60-63: Please use italic in the name of these species

Objectives: Please be more specific on the objectives of this study. As it is written, it seems soils from different PMs will be simply characterized in terms of nutrients.

L75: By data, do you mean soil samples and nutrient availability? Please clarify.

L82-85: Is it possible to provide more details on these PM types? As stated before, there are many different rocks within the same rock type, e.g., there are contrasting sedimentary rocks in terms of chemical and mineralogical composition, which will derive soils with contrasting properties as well.

L97: Please use 100 g^-1

L98: Please use cmol_c kg^-1

L104: Please use NH⁴⁺

Table 1 – Please insert the units of the soil properties presented herein

L171 – Here there is an example of the importance of specify the rock type instead of using plutonic or volcanic nomenclature. For instance, gabbro and basalt are plutonic and volcanic rocks, respectively, but they contain the same chemical and mineralogical composition, thus deriving soils very similar in terms of nutrients availability and texture. Thus, simply mentioning the rocks are volcanic and plutonic does not mean they will derive different soils, as exemplified for gabbro and basalt. Please specify your PM rock types

Table 2 – Use ^-1 superscript in meq 100g^-1

Fig. 2 – Nitrogen is not very strongly present in most parent material types, except for some sediments rich in organic materials and other less common rocks. Thus, maybe N differences among such soils are related to the vegetation above (which is a consequence of general soil conditions) instead of related to the PM type.

L237-247: These differences on vegetation growth are well reported by other studies, but no evaluation of the vegetation was performed in this study, which makes a great gap in the paper, especially considering it is well known that different parent materials derive different soils and that plants will grow differently at each condition. Thus, this study basically characterized the nutrient/fertility parameters in different soils. It would be very interesting to show that plant species grown in different soils presented different height, volume, lesser mortality, etc. Thus, great part of this paragraph seems more like a literature review.

L256-259: However, soil organic matter, plant uptake of nutrients and leaching were not evaluated in your study to state that.

Author Response

Author's Reply to the Review Report #2

Reviewer 2 Comments

Comment 1: Although well-written, it is important to provide more details on the parent materials for the better comprehension of the results and to improve the discussions. For instance, the text mentions volcanic and plutonic rocks, but only mentioning that it does not mean these rocks differ chemically and mineralogically from each other. Hence, soils formed from them may be very similar and the differences of parameters of soil fertility may be not related to their parent material, as taken as certain by the investigation. Thus, it is needed to specify the types of rocks, instead of using only plutonic and volcanic rocks (these terms are very general). The same applies for the other terms used for PM identification (metasediment, mixed, sediment) – please specify them.

Response 1: Absolutely, we completely agree. To that end we have revised the text slightly and added a new table to allow the reader to associate the dominant lithologies with these broad geological classes. These broad geological classes were chosen to 1) reflect the dominant lithological sources across this study region and 2) provide more power in detecting soil nutritional differences amongst classes. Even with 154 sampling sites, we had over 30 differing lithologies, which would yield on average only 5 sites per geological class. As anyone who has worked with soils knows, 5 samples across such a broad region are not sufficient to detect a difference at any meaningful statistical level.

Comment 2: Another important point is that other parameters have great influence on plant development, such as soil texture (clay, silt and sand contents), but it was not evaluated in the manuscript, which would contribute very much for the discussions.

Response 2: Very true. However, this is a classic case of sample size limitation vs variability across independent factors, not limited to only soil texture. Organic matter, coarse fraction, depth to soil bedrock, hydrologic relations, climatic inputs, etc can all influence these nutritional characteristics. That is well understood and acknowledged. With a larger dataset one could start to parse these dynamically linked factors on soil nutrient content/concentration. With our sample size we were able to examine the role of geologic soil parent material influence on the upper soil profiles.  Further research and samples can aid in refining these relationships as a function of other soil-site factors.

Comment 3: Although the authors mention the importance of soils on the development of plant species, no evaluation of the vegetation was performed in this study, which makes a great gap in the paper, especially considering it is well known that different parent materials derive different soils and that plants will grow differently at each condition. Discussion section was very general and did not explain well the findings of this study. Thus, this study simply characterized the nutrient/fertility parameters in different soils, but no correlation was made with the plants (better growth, lesser mortality, greater biodiversity, etc). Some other specific suggestions are presented below.

Response 3: We have enhanced the discussion by including a comparison of recent relevant research and its relationship to our findings (Littke et al, Hynicka et al, and Perakis et al).  Regarding the absence of plant-soil nutrient relationships, that data from these same research installations was published in a prior journal article (Moore, J.A.; Mika, P.G.; Shaw, T.M.; Garrison-Johnston, M.I. Foliar Nutrient Characteristics of Four Conifer Species in the Interior Northwest United States. Western Journal of Applied Forestry 2004, 19, 13–24). This is a companion paper to that publication. We inserted text to clarify this relationship.

Reviewer 2 Suggestions:

L13: Please insert comma after USA – Revised as suggested.

Abstract: Is it possible to define better each of the rock types mentioned here? For instance, which are the volcanic rocks that derived soils with high CEC and many nutrients? This is important to clarify because there are many divergent volcanic rocks and not all of them generate soils with high contents of nutrients. The same applies to the sedimentary rocks mentioned.

In the interest of space and the relative abundance of different lithologies present in these broad rock types, we kept the abstract general, but included more information within the text and added a new table listing the dominant lithologies represented in these 5 classes.

L51: Please use italic in Pinus elliottii – Revised as suggested.

L60-63: Please use italic in the name of these species – Revised as suggested.

Objectives: Please be more specific on the objectives of this study. As it is written, it seems soils from different PMs will be simply characterized in terms of nutrients.

After re-reading, we could indeed be more specific. Objectives clearly stated within the concluding paragraph of the Introduction.

L75: By data, do you mean soil samples and nutrient availability? Please clarify. – Clarified text to specifically state soil samples.

L82-85: Is it possible to provide more details on these PM types? As stated before, there are many different rocks within the same rock type, e.g., there are contrasting sedimentary rocks in terms of chemical and mineralogical composition, which will derive soils with contrasting properties as well.

Indeed. We have provided a new Table 1 to list dominant lithologies sampled across the 154 installations.

L97: Please use 100 g-1 – Revised as suggested.

L98: Please use cmolc kg-1 – Revised as suggested.

L104: Please use NH4+ – Revised as suggested.

Table 1 – Please insert the units of the soil properties presented herein – Revised as suggested.

L171 – Here there is an example of the importance of specify the rock type instead of using plutonic or volcanic nomenclature. For instance, gabbro and basalt are plutonic and volcanic rocks, respectively, but they contain the same chemical and mineralogical composition, thus deriving soils very similar in terms of nutrients availability and texture. Thus, simply mentioning the rocks are volcanic and plutonic does not mean they will derive different soils, as exemplified for gabbro and basalt. Please specify your PM rock types.

Please see new Table 1. Volcanic and plutonic sources are very likely to impact soil texture, in this example something like a gabbro, with its relatively large crystalline grain structure, will weather to a sandier soil with higher coarse fragment compared to a finer-textured soil that we would expect from the much finer-grained basalt.

Table 2 – Use -1 superscript in meq 100g-1 – Revised as suggested.

Fig. 2 – Nitrogen is not very strongly present in most parent material types, except for some sediments rich in organic materials and other less common rocks. Thus, maybe N differences among such soils are related to the vegetation above (which is a consequence of general soil conditions) instead of related to the PM type.

Inorganic N forms are particularly low within our relatively warmer/drier settings within the Inland Northwest, USA.  Nearly an order of magnitude lower than that observed in warmer/wetter climes of the Pacific Northwest, USA.  Often NO3 is nearly absent, where NH4 is the dominant form, but in quantities that we find deficient for most conifer species of the region.  Thus, we often find a stronger correlation between parent materials with higher forest productivity and mineralizable N; reflecting the tight nutrient cycling link between limited atmospheric N inputs and microbial meditated conversion of organic N to inorganic N.

L237-247: These differences on vegetation growth are well reported by other studies, but no evaluation of the vegetation was performed in this study, which makes a great gap in the paper, especially considering it is well known that different parent materials derive different soils and that plants will grow differently at each condition. Thus, this study basically characterized the nutrient/fertility parameters in different soils. It would be very interesting to show that plant species grown in different soils presented different height, volume, lesser mortality, etc. Thus, great part of this paragraph seems more like a literature review.

Revised the text to sound less literature review. Regarding the absence of plant-soil nutrient relationships, that data from these same research installations was published in a prior journal article (Moore, J.A.; Mika, P.G.; Shaw, T.M.; Garrison-Johnston, M.I. Foliar Nutrient Characteristics of Four Conifer Species in the Interior Northwest United States. Western Journal of Applied Forestry 2004, 19, 13–24). This is a companion paper to that publication. We inserted text to clarify this relationship.

L256-259: However, soil organic matter, plant uptake of nutrients and leaching were not evaluated in your study to state that.

True, but we weren’t making that statement relative to our analyses, but reflecting on statements made by James et al.  We understand how a reader could infer that we were by where we placed the citation reference.  We added a citation reference after this statement to ensure the reader understands the attribution of this observation.

Round 2

Reviewer 1 Report

The authors have done a great job in improving this manuscript.  I only have a few comments that are listed below.  This manuscript should be accepted after minor revisions.

Line 48: Himes et al. 2014 is a good citation for this section, but consider Littke et al. 2014 (Effects of Geoclimatic Factors on Soil Water, Nitrogen, and Foliar Properties of Douglas-Fir Plantations in the Pacific Northwest) for differences in soil and foliar N availability on different SPMs.

Figure 1: Change your marker for cities or sampling locations because they are the same.

Thank you for the inclusion of Table 1.  It is very helpful to see the different SPMs used in this study.

Lines 240-241: Revise this sentence to remove the double mention of Ca concentration.

Figures: It is hard to see the volcanic lines in the updated graphs.

Line 287: Citation 19 would be Himes et al. 2014, but Littke et al. 2014 would be acceptable here as well.

Lines 292-297: I believe that Perakis and Hynicka did show a difference in the relationship between N and Ca under basalt and sedimentary soils. High N basalt soils tend to have leached Ca out of the upper profile while sedimentary soils have not.

Author Response

Dear Reviewer,

Thank you for your second review.  Below are my corrections to suggested edits.

Littke citation: I'm unsure how we missed the Forest Science citation, which is the one I was looking for all along, and only found the Himes citation.  We replaced Himes et al with Littke et al. at lines where [19] is referenced.

Figure 1: Good catch - edited symbols appropriately.

L240-241: Double statement removed (revised sentence on L230-231).

Illegible volcanic lines in Figures: We are unsure why volcanic is not legible. We tried printing in B&W, greyscale and color, and it is as legible on our screens as the other lines.  If this is a problem during the typesetting stage, we will correct any issues that persist.

Regarding Perakis and Hynicka: They did find differences in Ca and other nutrients when sampling fresh sedimentary and basalt bedrock; however, those differences disappeared when looking at residual and exchangeable nutrients in the soil samples. They did mention means were different, but not statistically so.  We interpret as too much variability in the samples - either do to too few samples to overcome that variability, or the confounding of N-enrichment with Ca availability - regardless of soil bedrock source.  Since our analyses were entirely based off soil samples, not fresh bedrock, it was more appropriate to make the distinction we did.  However, to nuance our statements relative to the strong relationship with N enrichment on Ca availability, we revised L273-279 to provide a bit more context.

Again, thank you for your review and comments!

Regards, Mark

Reviewer 2 Report

The authors have improved the points raised in the first round of revisions. Please only provide the units of CEC in Table 2.

Author Response

Dear Reviewer,

Thank you for your second review.  In hindsight, units should have only been associated with Table 3, not Table 2.  We have removed units from T2 to make the table more readable and less repetitive.

Regards, Mark