Going Beyond Soil Conservation with the Use of Cover Crops in Mediterranean Sloping Olive Orchards

Round 1

Reviewer 1 Report

Please see the comments attached. 

Comments for author File: Comments.pdf

Author Response

General Comments

The article is interesting but a bit disjointed. So many acronyms are used in the paper, making it difficult to understand. More importantly, they were not described well even when used for the first time. Some of the approaches used in the analysis are questionable. For example, lumping all the cover crops treatment (8 years) for soil erosion and rainfall after not seeing significant differences in the 7th and 8th years may not reflect the actual treatment difference. Soil erosion is driven by precipitation. The monthly and annual rainfall may differ from month to month and year to year. (see specific comments for details). Further, which cover crops treatment would you recommend for soil erosion control if you lumped the treatments together? Many statements in the paper need more explanation. On multiple occasions, authors discuss literature findings in the discussion section first before presenting their research findings. I recommend discussing your result first and relate your findings with the literature. Some sentences throughout were not well connected to your results and discussion. Please see specific comments for more details.

R: We would like to thank Reviewer #1 for his/her thorough review in order to improve quality and clearness of the manuscript. In the next lines we have addressed the General and the Specific Comments made by Reviewer #1. We really appreciate all of them since have help us to improve the manuscript a lot.

Specific comments

Abstract

Line 19: Explain the acronyms C-NAT, CC-GRA, and CC-MIX. Must explain when you are using the first time, either in abstract or other areas.

R: A brief description of the managements has been included in L19-20 in the abstract of the revised version of the manuscript (RM).

Line 20: Is not the microbial distribution comes under biological soil properties of soil health.

R: We agree with the Reviewer’s opinion and microbial communities have been grouped within biological properties. Corrected in L21-22 of RM.

Line 25: Provide the significance level in parenthesis.

R: The words ‘significant’ and ‘significantly’ were not meaning ‘significant statistical differences’ in this paragraph and therefore, it has been modified. L25-29 of RM. We prefer not to talk to specific significances in the abstract and the general overview of results was provided instead.

1. Introduction

Line 44: Is there any bare soil management ever. I recommend rewording and rewrite the sentence.

R: This sentence has been reworded and rewritten for clarity in L44-53 of RM.

“Soil managements based on tillage or use of herbicides to control adventitious vegetation, has been proved unsustainable in terms of soil conservation, since can reach up to soil losses systematically above 20 t·ha^-1·year^-1 on Mediterranean sloping areas at the short [4] and long term [5].”.

Lines 47-48: In your abstract (line 17) authors mentioned that the effect of cover crops on olive orchard is unknown. However, here the stating cover crops have been proven as an alternative to conventional tillage….. It's confusing now. So, please clarify which statement is true.

R: In L17 of the original abstract we stated that “In that sense, the influence of different plant species on soil quality indicators in olive orchards remains unknown yet.”. There are many studies indicating positive effects of cover crops on different soil properties and soil erosion control in woody crops under Mediterranean conditions. However, which is still unclear is how the composition of the cover crop (both homogeneous or composed by several plant species) can influence to these soil properties, specially to those related to biological parameters. Therefore, these two statements in L17 and L47-48 are not contradictory.

Line 48: Provide a reference.

R: Two new references have been included in the RM, and all the references have been numbered again:

6. de Graaff, J.; Eppink, L. A. A. J. Olive oil production and soil conservation in southern Spain, in relation to EU subsidy policies. Land Use Policy 1999, 16(4), 259-267. https://doi.org/10.1016/S0264-8377(99)00022-8
7. De Graaff, J.; Kessler, A.; Duarte, F. Financial consequences of cross-compliance and flat-rate-per-ha subsidies: The case of olive farmers on sloping land. Land Use Policy 2011, 28(2), 388-394. https://doi.org/10.1016/j.landusepol.2010.08.001

Line 64: Can you provide an example of some biological indicators in parenthesis.

R: Done. Some examples have been provided in the RM in order to complement the ones already mentioned in the text. The paragraph has been rewritten:

L70-77: “In the last decade, the use of biological indicators has gained importance for assessing soil health (e.g., micro-meso fauna, enzymatic activities, microbial biomass, etc.). As a contrast to the relatively high number of studies describing the benefits of using cover crops on physical and chemical soil quality indicators in Mediterranean olive orchards, those assessing specifically the effects of ground cover management (also considering different plant species composition) on soil biological properties, and more specifically on the microbial community structure and its activity are less numerous [e.g., 20-29].”.

Line 65: High number of relatively high number f studies describing the benefits of using cover crops on physical and chemical soil quality…..Again this contradicts your statement in the abstract (line 17). Please make sure your statements are accurate and are not conflicting.

R: Thanks for the comment. The effect of cover crops in physico-chemical properties has been broadly studied; on the contrary the effect on biological properties have not been evaluated as extensively in similar agroecosystems. However, multi-specific cover crops’ (conformed by different plant species) effect on soil health is still unclear, being needed more research. We have rewritten this sentence (see the previous answer).

2. Materials and Methods

Can you add a figure to depict the field experiment? It will help readers to understand the study.

If you do not have a picture of your experimental design, a well-drawn map/figure shown olive trees and your treatments will help.

R: Figure S1 has been included describing the experimental plots.

Lines 118- 134: Please explain your acronyms so that they will help readers. There are many and are confusing as well.

R: The acronyms we have included in these lines, are fully described throughout the text as they appear (see Section 2.2). These acronyms correspond to the different soil managements (TILL, CC-GRA, CC-MIX and CC-NAT).

Results and Discussions

Lines 312-317: You might not have observed a significant difference in rainfall and soil loss in the 7th and 8th years between CC-GRA and CC-MIX. However, it does not necessarily mean you would not see the treatment difference for another six years. Therefore, grouping the CC-GRA and CC-MIX for eight years is incorrect. Please analyze them separately. In addition, provide the monthly and annual rainfall pattern in the location for all eight years to help readers understand it better.

R: Thanks for the comment. We think there is a misunderstanding as we have not grouped CC-MIX and CC-GRA for the whole period (eight years). Both cover crop managements were grouped the last two years (when CC-MIX was established) and just because no statistically significant differences in runoff and soil loss were found. Nevertheless, we have reviewed the whole dataset for all the managements and years, and have present results of CC-GRA and CC-MIX managements independently, we have re-done some calculations and run new statistical analysis in order to re-write Sections 3.1 and 4.1 of RM. Finally, we have made a new plot for Figure S2 of the RM.

The aim of this study is not the seasonal characterisation of the temporal evolution of ground cover or soil properties within a year. For this reason, we consider that rainfall patterns (monthly or annually) for the eight years, do not offer relevant information in this study. In that way, the data provided in this manuscript regarding soil erosion, were annual and cumulative ones. However, annual rainfall is presented.

In any case, we assume that Reviewer#1 is talking about runoff instead of rainfall, as no statistical analysis of rainfall distribution was included in this study.

Line 324-325: Now, you (authors) say the annual rainfall and soil loss were statistically different between management. So, this supports the comment in lines 312-317. Please do not just lump sum the treatments. The rainfall and sediment loss are directly related, and sediment loss the function of each rainfall event. Thus, please rerun your stats again, separating the erosion measurement treatments and updating your findings in this section.

R: Findings updated. See the previous answer.

Lines 323-326: The sentence structure is confusing. Please rewrite and simplify.

R: We have rewritten the paragrah and simplify it in L397-403 of RM.

“For each year, runoff coefficients were moderately lower in the temporary cover crops (CC) treatments compared to TILL, ranging from 0.8-20.9% (6^th and 8^th years) and 1.4-24.6% (2^nd and 1^st years), respectively. Regarding soil loss, this trend was similar varying from 0.1 to 14.5 t·ha^-1 (6^th and 8^th years) in CC and from 2.0 to 65.8 t·ha^-1 (2^nd and 5^th years) in TILL. Only statistically significant differences among managements (P<0.05) were found for annual runoff (6^th year) and soil loss (4^thand 5^th years) (Figure S1B).”.

Lines 332-333: Correlation to what? Please explain it better.

R: The sentence has been slightly modified. The correlations are between two variables, measuring the strength of association between them and the direction of the relationship. This can be found in Table S1.

Rewritten in L409-411 of RM:

“In general terms, the analysed physicochemical soil properties showed correlations between them (i.e., strong correlations were observed between pH and organic C, AL and CaCO₃ content or texture parameters and organic C) (Table S1).”.

Lines 341-342: Please rewrite this sentence.

R: Done. Rewritten in L418-422 of RM:

“An intermediate situation was found for CC-GRA and CC-MIX managements. Similar results were observed in most cases concerning these properties (Figure 1B).”.

Lines 343-345: If the organic N is not significant, no need to focus on it. Please discuss only the significant results.

R: Thanks for the comment. However, we consider that not statistically significant results are also relevant to understand or to explain soil health. We decided to include a short comment about Nitrogen results as we mentioned it in the description of the managements in Section 2.2.:

L165-167: “For the cover crop treatments, except CC-NAT, the inter-rows were fertilized at the time of seeding with Nitrogen (50 kg·ha^-1) during the fall by direct application, and additional fertilization spaced 1-3 years depending on cover crop status.”

Lines 346-361: Did you measure the herbaceous mass of cover crops? Without measuring the herbaceous mass of those cover crops, how can some estimate the amount of organic matter added into the systems?

R: Thanks for the comment. No, we did not measure herbaceous mass of cover crops in this study. This manuscript is not presenting a soil carbon balance and therefore no above/below ground biomass was measured in order to estimate organic C at the top 10 cm of soil. The data shown in Figure 1 came from laboratory measurements (see Section 2.5).

In figure 1 B, there was a significant difference in organic carbon in between till and CC-mix. I am really surprised to see the difference in Organic carbon within two years of using CC-MIX (Lines 127-128 states, the CC-mix was used for two years). Please explain more about how and what contributed such a significant difference in soil organic carbon within two years of using cover crops in a Mediterranean climate.

R: Thanks for the comment. To understand and interpret CC-MIX soil properties, the history of the management must be taken into account as it is not only based on two years of a sown mixture of plant species. This cover crop mixture is preceded by two years of tillage and four years of a gramineous cover crop, as described in Section 2.2. Therefore, high(er) values of soil organic carbon could be expected due among other factors, ground cover biomass for 4 years in total.

Another intriguing aspect in your Figures 1, A and B was why you have a higher percentage of clay but a lower percentage of sand in tilled plots? Does the soil texture change in such a manner within 8-10 years? If yes, please explain what has caused this in detail.

R: Thanks for the comment. Comparing TILL and CC-NAT sampled areas, we are comparing a relatively high disturbed soil top profile (first 10-15 cm) since 2003 to a non-disturbed area since 1985. Therefore, variations in soil texture might be most likely to soil mechanical disturbances.

Lines 381-406: Figure 2. If the results are not significant, I will delete the picture. I would also recommend focusing your discussion on significant results only. That will help to keep your paper short and more interesting.

R: This figure has been moved to supplementary material as Figure S5 in the RM.

Lines 443-446: Why the CC-GRA had the same number of bacterial genera as in CC-NAT? Does that mean you do not need to maintain your ground cover as in CC-NAT for a long period to benefit soil health? Or you can achieve the same or similar microbial diversity with CC-GRA within few years, so there is no need to maintain CC-NAT?

R: Having same number of bacterial taxa does not mean that the diversity is similar as was demonstrated by beta-diversity results which showed that both treatments have a completely different bacterial structure.

Lines 497-506: Authors have mentioned the CC-GRA showing intermediate value multiple times. What does that intermediate mean? Discuss in terms of significance or not? If it is not significant, it is statistically the same. So, there is no need to mention the intermediate terminology.

R: Thanks for the comment. With intermediate values we mean that they are within the most extreme data measured, collected or recorded in our study. In other words, that CC-GRA shows, in many aspects, a behaviour that is in between TILL and CC-MIX or CC-NAT. We believe that these results are also important.

Despite differences are not statistically significant in all the cases, we consider that providing mean data or intermediate status is also relevant to understand soil managements effect.

Lines 630-634: Wow! Was there such a huge difference n soil loss between till and CC? 33 tons per hectare compared to 3.2 tons. That is why it's important not to lump your CC treatments together. It will be interesting to see how CC-MIX differs from CC-GRA etc. Further, is that difference is only due to cover crops? I doubt. There should be an important role of rainfall as well. So, providing rainfall averages for months and years would help to understand the ground reality.

R: Thanks for the comment. As suggested, we have reviewed the whole dataset for all the managements and years, split CC-GRA and CC-MIX managements, re-make some calculations and run new statistical analysis in order to re-write Sections 3.1 and 4.1 of RM. Figure S2 of the RM was plotted again.

The aim of this study was the assessment of annual/cumulated rainfall, runoff and soil loss. In that sense, no monthly precipitation was shown. Annual precipitation had already been included in the original version of the manuscript but we have also maintained it in Figure S2 of RM.

Line 342: Please rewrite the sentence.

R: Done. Please, see L914-917 of the RM.

“Our higher erosion records in the CC treatments could be explained mainly due to a combination of rainy years and a concentration of high intensity events in the period after the sowing in fall, in which the soil presented a very low ground cover (data not shown).”.

Lines 653-656: I recommend discussing your results first, followed by the literature like in other sections.

R: Done. Please, see Section 4.1. of the RM.

Lines 663-665: Do you think the high consumption of the cover crops impacts overall nitrogen fertilization and costs of olive orchards?

R: The cover crops’ demands regarding water or nutrients consumption is out of the scope of this study so we cannot state if these demands are high or low. Cover crops managements were fertilised mainly to assure a proper establishment of the ground cover, specially during the first years. However, any additional agricultural operation (tillage included), implies additional (economic) costs.

It is also worthy to mention, that the use of cover crops is promoted as a sustainable management practice by different agricultural policies, being subsidised in different situations as a tool to improve the environmental costs.

Line 721: Reference(s)

R: Two new references have been added:

81. Bonanomi, G.; De Filippis, F.; Cesarano, G.; La Storia, A.; Ercolini, D.; Scala, F. Organic farming induces changes in soil microbiota that affect agro-ecosystem functions. Soil. Biol. Biochem. 2016, 103, 327-336. https://doi.org/10.1016/j.soilbio.2016.09.005
82. Wolińska, A.; Górniak, D.; Zielenkiewicz, U.; Goryluk-Salmonowicz, A.; Kuźniar, A.; Stępniewska, Z.; Błaszczyk, M. Microbial biodiversity in arable soils is affected by agricultural practices. Int. Agrophys. 2017, 31(2), 259-271. https://doi.org/10.1515/intag-2016-0040

Lines 724-726: As stated earlier, including cover crops' herbage mass would help to understand this dynamic better.

R: Thanks for the comment. We agree that this information would be very useful for many purposes such as carbon cycling. Unfortunately, the evolution and quantification of ground cover biomass was out of the scope of this study. And we cannot provide data that we did not measured since it was not the scope of the study.

Lines 748-752: Discuss your results first and relate your findings with the literature.

R: Section 4.3. Discussion section has been reorganised.

Lines 758-763: This sentence is so long, vague, and unclear. It isn't easy to understand. Please break it into simple sentences.

R: This sentence has been rewritten in L914-917 of the RM:

“Our higher erosion records in the CC treatments could be explained mainly due to a combination of rainy years and a concentration of high intensity events in the period after the sowing in fall, in which the soil presented a very low ground cover (data not shown).”.

Line 790: Discuss your significant difference results, not the trends.

R: Thanks for the comment. As it has been explained in previous comments, we consider that exposing and discussing trends is as relevant as presenting statistically significant results. This information cannot be unnoticed as in many cases, it helps to discuss the current status of the managements and leads to discuss about which are the main drivers of statistically significant results.

Conclusions

Which cover crop types would you recommend for the Mediterranean olive production system.

R: It would depend on the aim of the cover crop (soil erosion control, enhancement of biodiversity, etc.) and the particular conditions of each farm such as soil characteristics or existing seed benchmark. More research is necessary to find (flowering) plant species of fast-growing short life-cycle which emerge early in fall to get a proper ground cover and have a high self-seeding capacity.

Reviewer 2 Report

Dear Authors,

The presented manuscript fully corresponds to the subject of the journal. The data on the influence of the types of tillage in sloping olive orchards on different soil properties were analyzed. The article can be accepted for publication after minor revision.

Comments

Please check the text again for correction of some small typos (like L323 2nd, L334 Al -> AL)

LL 239-238 Please indicate the activity of which enzymes were studied

When discussing the results, could you give a general scheme of the influence of the type of cover vegetation on the change in soil properties?

Yours faithfully,

Reviewer

Author Response

The presented manuscript fully corresponds to the subject of the journal. The data on the influence of the types of tillage in sloping olive orchards on different soil properties were analyzed. The article can be accepted for publication after minor revision.

R: Thanks for the positive feedback. Changes have been included in the revised version of the manuscript (RM).

Comments

Please check the text again for correction of some small typos (like L323 2nd, L334 Al -> AL)

R: Done. The text has been carefully revised and all typos have been corrected.

LL 239-238 Please indicate the activity of which enzymes were studied.

R: These are listed in Figure 4 of the RM: Alkaline phosphatase, Esterase (C4), Esterase Lipase (C8), Lipase (C14), Leucine arylamidase, Valine arylamidase, Cystine arylamidase, Trypsyn, α-chymotrypsin, Acid phosphatase, Naphthol-AS-BI-phosohohydrolase, -acetyl β-galactosidase, β-glucuronidase, α-glucosidase, N-acetyl-β-glucosaminidase, α-mannosidase and α-fucosidase.

We did not list them in the text due to the long number but we have referred to Figure 4 instead. See L 243 of RM.

When discussing the results, could you give a general scheme of the influence of the type of cover vegetation on the change in soil properties?

R: An introduction on the Discussion section has been added in which we have provided a general overview of main influence of using a CC; L 872-884 of RM. Also, some remarks can be found in L687-689, L692-696, L701-704 or L711-715 in Section 4.1. of the RM.

Round 2

Reviewer 1 Report

The authors have addressed all the concerns that I have raised. Thank you.