This study compares vegetation biodiversity metrics, soil degradation variables, and environmental factors for two olive orchard catchments in Spain with differing environmental characteristics and vegetation management regimes. The study is innovative and multidisciplinary by evaluating biodiversity metrics relative to environmental conditions and management practices. The primary goal was to evaluate the potential for biodiversity metrics for spontaneous grass cover to serve as indicators of soil degradation. A second goal was to determine whether biodiversity differences between the two sites could be accounted for primarily by differences in site characteristics or by differences in vegetation management regimes. The study found that the site with the greater biodiversity was the site with greater soil loss and lower soil organic content. Thus, the authors concluded that biodiversity metrics have little utility as an indicator of soil degradation. The study also concludes that the differences in biodiversity metrics between the two sites were likely associated with differences in site conditions rather than with differences in tillage practices.
Unfortunately, the study was not designed to test the hypotheses. That is, there is no way of statistically testing an association between two variables (e.g., a diversity metric vs. a soil degradation metric) with only two sites. Likewise, there is no way to determine whether a difference in biodiversity between the two sites is associated with site conditions or with tillage practices because the two sites differed in both respects. Moreover, it’s not clear that the soil loss data, which came from another study, covered the same years as the biodiversity data. Thus, the author’s conclusion that biodiversity metrics “were not found to be suitable for describing soil degradation” is at best a “suggestion” this is the case. Had the authors evaluated 20 sites instead of two, the conclusion might be different. Moreover, it’s possible that biodiversity metrics are indeed related to degradation metrics, but not for systems that have already developed gullies and rills such as at the CN site. The assertion that biodiversity differences between the two sites are better explained by differences in site conditions, and not tillage practices, is further unsupported because there is no discussion of how the authors reached this conclusion. They do not describe how one would be able to differentiate between site effects versus tillage effects, and they do not discuss the possibility that tillage practices were the cause of biodiversity differences. They simply make assertions that the differences are due to site conditions.
Despite the above shortcomings, I think there is value in the data obtained in the study. Detailed studies of agroecosystems are scarce relative to the immense array of systems that exist. Moreover, such studies can be difficult to undertake such that the number of sites that can be studied may be quite limited. Thus, I think the data obtained in the present study warrant publication, but the paper would need to be recast to be more of a comparative case study and less of a hypothesis-testing study. In any event, the conclusions would need to be tempered given the small sample size.
The hypotheses and objectives (lines 86-94) are confusing and inadequately developed. What is meant by “starting hypothesis” relative to a real one? Also, “wider ecological niches” is unclear, and there is no linkage made between this term and the biodiversity metrics selected. Regarding the second hypothesis about interactions of soil and management better explaining diversity differences than environmental site conditions (lines 88-89), there is no explanation of how one would make this distinction. For clarification of objective 1, I think you mean “biodiversity indices for spontaneous grass cover”. For 2, “as a result of” should be “relative to”. Also, it is unclear what “soil management” refers to. For 3, this could be better stated as “to evaluate the relevance of biodiversity indices as indicators for ….”
The choice of diversity metrics seems reasonable, but it is not apparent why the authors restricted their analysis to site-level metrics and did not include any plot-level metrics. The data were collected at the plot level so it would have been easy to at least calculate species richness for each plot. Comparing metrics at the plot scale would provide a rigorous test of whether the metrics at this scale differed between the two sites, whereas for the metrics calculated at the site scale in the study, differences between the sites cannot be statistically tested because the sample size is only 3 (i.e., 3 years).
The terminology and metrics of interest for soil are confusing. The title addresses “soil degradation”, yet the text sometimes refers to “soil quality” and once to “soil situation.” In one place the text states the metrics of soil degradation of interest are soil loss, soil organic content, and runoff, but in other places it refers just to soil loss or soil loss and organic matter. It is also confusing when the text refers to “soil loss and soil degradation” (e.g., lines 70, 94), implying that soil loss is something different from soil degradation. When the text compares biodiversity metrics versus soil metrics, it doesn’t present all the soil degradation metrics at one time, so it is confusing to the reader whether the differences in biodiversity metrics between the sites are in the same direction as the several soil degradation metrics or not.
The Results section is full of inferences and comparisons to other studies. The Results section should just contain findings of the study. Perhaps the journal would allow a combined “Results and Discussion” section followed by a “General Discussion” section, in which case the problem could be resolved by simply renaming the section headings.
There is quite a bit of unclear text and awkward writing and grammar. I have attempted to improve some of this below.
Specific Comments by Line Number
Line 15. Specify country or institution behind Common Agricultural Policy.
Lines 15-16. Unclear what is meant by environmental quality of such covers.
Lines 16-18. Confusing. Consider “we measured biodiversity indices for spontaneous grass cover in two olive orchards with contrasting site conditions and management regimes in order to evaluate the potential for biodiversity metrics to serve as an indicator of soil degradation.”
Line 17. Unclear what “biodiversity patterns” refers to.
Line 23, and elsewhere. “Pictures” should be “photographs”.
Line 24. “frequency” of what? Unclear what “diversity” refers to. The topic of the paragraph is biodiversity indices. “Transformed” should be “modified”, as described in text.
Line 26: Consider: “Sorensen’s index showed strong differences in species composition for the the grass covers in the two olive orchard catchments.”
Line 27: What is the rationale for asserting that the differences in species composition were due to site conditions rather than management regimes? The previous paragraph stated the difference between the two catchments was in tillage practice, not site conditions.
Lines 27-29. An important point implied here is that the site with higher biodiversity indices had greater soil loss. This does not become apparent, however, until the value for soil loss is provided for the other site several lines below. The statement should be revised to make clear which site had the higher soil loss. There should be a clear statement relating biodiversity difference between the two sites with the difference in the two soil degradation metrics between the two sites in one sentence so the mismatch is clear and not spread among a lot of other text.
Lines 29-30. Regarding “were reasonably high”, text needs to make clear what this is in comparison to.
Line 32. Define “worse site conditions”.
Line 33. “biodiversity indicators” should be “biodiversity indices”. None of the indices were shown to “indicate” anything.
Line 33-34. This sentence indicates this inference is a firm conclusion of the study, whereas this was certainly not a firm conclusion. The study compared only two sites, so at best the findings “suggest” that biodiversity indices “were not found to be suitable….” (see General Comments)
Line 34. The “soil degradation” metrics included in the study need to be specified (see comments above)
Line 38. “sedimentation problems” is vague.
Line 43. Specify what program/country “Multi-annual Financial Framework” belongs to.
Line 50. “Different” should be “multiple.”
lines 54-56. Reference needed.
Lines 64-66. Unclear statement. “landscape improvement” unclear. “etc” should be deleted.
Lines 67-84. This paragraph is confusing. The topic starts with spontaneous grass covers, then indicators of soil loss and degradation, then biodiversity, then implementation of protection policies. Paragraph should be revised to make the main point first (topic sentence) and follow this with explanatory material.
Line 73. “protection” of what?
Line 81. paper is exploring use of biodiversity metrics to reflect (indicate) soil degradation, not “measure” soil degradation as written here.
Line 82. Unclear what “richer and more complex ecological niches” means. Also unclear is “efficient cover and soil protection.”
Lines 86-90. See comments above
Line 100. “were considered” by whom?
Lines 102 – 135. It is unclear what the source is for the information provided about the two study sites. Citations should be provided.
Lines 145-150. “pictures” should be “photographs.” Unclear what “to check” means.
Lines 152-153. “absolute and relative frequency of occurrence”: I don’t recall these data do not appear in the manuscript.
Lines 154. “species” refers to grasses and forbs?
Line 158. “both samples” should be “the two samples”
Line 163. H does not represent probability. Probability would be between 0 and 1 only.
LIne 165. “unclear “to a limited group…species”
Line 167. H is also increased by number of species, not just evenness of species.
Line 172. Unclear what “based on the evaluation of pictures” means.
Lines 173-175. Unclear what “suitability” refers to and what “verified” means? Also, it’s confusing that Jmod is addressed here but is not defined until below.
Line 177. Change “which would describe” to “with 1 describing”
Line 182. Revise second sentence to “Jmod is obtained by substituting Hmod for H (Equ.3).”
Line 192. Who runs gauging stations?
Line 199. Focus of study is on soil degradation, not soil quality.
LIne 199-200. Give years. It’s important to know whether these 5 years overlapped the 3 years of the present study.
LIne 215. Unclear what “checked for the weighted values” means.
LInes 216-217. Sample size should be provided here or in figures.
LInes 224-227. The basis for these inferences is not clear. An objective of the paper was to differentiate between site effects and tillage effects, yet the latter possibility is not even mentioned here. Also, “water deficit” is not represented in Table 4.
Line 232. It doesn’t seem noteworthy to claim a difference between the sites in coefficient of variation. There were only 3 variables, so the fact that two were higher in one site does not represent a pattern.
Line 233. “notably high”. What is the basis for this assertion? References should be provided.
LIne 235. State how Jmod indicates no dominant species, i.e., values were generally much closer to 1 than to 0.
Line 236. “These features”. Which features?
LInes 236-238. Statement needs a reference. Also, “wide range of colonizing species” is unclear, as is “without any clear dominant pattern.”
LIne 242. State how values show this, i.e., values were generally much closer to 0 than to 1.
LIne 243-244. Drop “It is worth noting how”. Give the mean values for winter and spring.
Lines 244-245. Unclear.
Lines 252-255. Unclear how such firm conclusions (e.g., “as a result of”) are reached here with a sample size of 2 sites.
Line 264. BD was NOT significant. Table shows p = 0.077
Lines 266-268. Much emphasis is given to the differences between sites in soil loss and runoff, yet the differences do not appear to be significant.
Line 271. Why is runoff not listed? It was one of the soil degradation metrics.
Lines 273-274: The meaning of the following terms is unclear and it’s also unclear how they were represented in the study: “wider ecological niche”, “efficient occupation of space” and “efficiency in the flow control”
Line 277-278. “impinge the spontaneous grass cover seedbank growth” is awkward.
LIne 280-284. I do not understand, possibly because I am not a soil scientist.
Line 291. Seems like “as used at CN” should be added to end of sentence after “orchards.”
Lines 301-302. But text indicated CN had the more extensive management than PG.
LIne 310. Number of species is a metric of biodiversity so it is unclear what is meant by “number of …. species and biodiversity”.
LIne 312-314. Confusing sentence. The assertion for why there were not monocotyledons at CN needs to be made. “measures” seems like incorrect word. What “management” action is referred to?
Line 320-322. Specify what “it” refers to. Provide reference.
Lines 327-329. What does “this case” refer to? “complete the information” is unclear. “allow us to confirm…..CN” is unclear.
Line 338. Specify site you are referring to.
Line 340. “high” relative to what?
Line 352. Focus of study was on soil degradation, not “soil situation” which is vague.
Table 1. Arroyo Blanco is not the site name used in the text. Provide site abbreviations (CN, PG)
Table 3. Define OM
Table 7. Specify sample size.
Figure 1. Lettering has poor resolution, and scale is unreadable. Arroyo Blanco label does not correspond to site name used in text or in figure legend.
Fig. 2. Identify which site is shown in c.
Fig. 3. Legend: “Number of species by life form …..”
Figure 4. Specify sample size for each variable. State which data came from another study and give reference. Mean +- 1.96 SE is not really appropriate given the same size was so small. It would be more meaningful to use the appropriate t value (mean +- t*SE). State which data came from another study and give reference.
I hope these comments will be useful to the authors.