Reviewer #2: ‘Comment on soil-2021-17’, Lukas Kohl, 30 Mar 2021

Lu and co-authors report on a study that investigates the impact of transition that conventional rice paddy/oilseed rape to vegetable cultivation has on soil properties. They report that increased over-fertilization and decreased organic matter inputs lead to nutrient accumulation and organic matter depletion in subtropical agricultural soils.

This is an important and timely study that fits well into the SOIL journal. The authors do not provide sufficient details on the methods to provide a full judgement on the validity of their results, but my overall impression is that their conclusions are well supported by the data. The manuscript is written in technically correct English, but I think that some sections need to be substantially improved before publication (see below). I therefore think that the authors should be given the chance to substantially improve the manuscript before publication (major revision or rejection with invitation to resubmit).

Response: Thank you very much for your kind comments and valuable suggestions for further improving the quality of current manuscript. We have revised the manuscript carefully, and all changes in the revised manuscript are made using Track Changes to make reviewing easy.

Specifically, I think that the quality of the different section of the manuscript varies a lot. In my opinion, the results and discussion sections are well developed and can be published with little modification, whereas introduction and methods sections need substantial improvement. The introduction is very short and does not provide a full overview about what is known about crop-to-vegetable transition impacts on soils. The text also misses ‘flow’ from one paragraph to the next. I would recommend to broaden the introduction, and look at crop-to-vegetable-transitions studies conducted throughout various climate systems. In addition, the introduction could explain how the different parameters measured are affected by this transition, and what results are therefore expected. In the methods section, a lot more detail needs to be provided with regards to data sources and analytical methods.

Response: Many thanks, we accept all your comments and suggestions, and have revised and added more information to the manuscript, particularly in the Introduction and M&M sections, e.g. “On average, the sown area of non-grain crops has increased by >12% since 1980 in China, with a corresponding decrease in grain crop cultivation (Chen et al., 2016)” (please see Line 35-36), “Numerous studies have reported on the serious soil degradation in greenhouse vegetable fields which have been converted from maize-wheat rotation in the North China Plain, as a result of high total fertilizer inputs and increased frequency of soil cultivation, resulting in a significant decrease of soil C/N ratio and pH, enrichment of P and K, and salinization (Ju et al., 2007; Yan et al., 2012). However…” (please see Line 46-49), “However, there is no information available about the effects of land use changes from cereals to vegetable production on the concentration of SOC and TN in southwest China.” (please see Line 58-59). In addition, in the M&M section, we have added a database of soil properties and provided further analytical parameters in the Supplementary Information. We now believe that the revised manuscript is much improved and is suitable for publication.

I have one major scientific concern that should be addressed by the authors. The manuscript states that soil properties of fields converted to vegetable farming were similar to those retained in conventional crop rotation. I think this authors need to provide more evidence for this point as it is an essential condition for the validity of their findings. In particular, I am wondering if the authors have any information whether the fields that were transitioned from conventional to vegetable farming were chosen randomly, of if farmers took soil properties into consideration when deciding if a given field was converted or not. If the latter were the case, the initial conditions of these field would be different from those retained in traditional crop rotation, which would limit the authors ability to link difference in soil properties to farming practices. This also applies to effects of time-since-conversion, since the farmer’s considerations could change over time.

Response: The comparisons of paddy-rape fields with similar field following conversion to vegetable production are based on matched soil types, i.e. soils that have developed from the same parent material and under the same conditions of regional climatic conditions and agricultural production practices. We do not have any information on whether the fields that farmers transitioned from conventional to vegetable farming were chosen randomly. According to the farmer survey, the most important factor influencing the choice of land-use conversion is for economic benefit, because the output-input ratio of vegetables is significantly greater than that of cereal crops and oil crops. Additionally, factors such as limitation of agricultural irrigation conditions for paddy production, topography, and farm size may have influenced farmer choice of which fields to target for transition. Whilst we understand this reviewer’s comment, the biggest differences between vegetable production and paddy-rape rotation are the nutrient inputs (including organic and inorganic, please see Table 1) and frequency of cultivation. So, we believe these factors would far outweigh any small variations in e.g. soil texture.

Minor comments:

L37: per unit nutrient inputs: Specify which unit you refer to (area?)

Response: Yes, it should be “per unit area of nutrient inputs”. We have revised text in the new version of the manuscript.

L64-70: the data presented here should be supported by references.

Response: Thanks. We have added an appropriate reference in Line 80.

L81: farmers’ survey methodology: I assume this refers to an established survey or similar undertaking, but this is likely lost in the translation, and comes across quite confusingly. I recommend clarifying this.

Response: Thanks, we have added a sentence to clarify this. “A survey of management practices was conducted by face-to-face interviews with farmers in Jinping County in Guizhou Province. Based on farmers’ survey methodology (Jia et al., 2013), two of the most important vegetable production townships in Jinping County, Xinhua and Dunzhai, were selected in this area. In each township, four villages were randomly selected, and 15-20 farmers from each village who had managed paddy-rape rotation before switching to vegetable cropping system were randomly surveyed. Survey questions were related to crop varieties and the management of the two cropping systems, including sowing rate, fertilizer (chemical or organic) application rate, tillage frequency, crop-residue management, crop growth cycle, cropping duration, and yield.” Please see Line 97-103.

Section 2.3: A lot more detail is needed here. Ideally, a reference should be provided for each method, along with enough details such that the reader can reproduce the measurements.

Response: Accepted and done. Please see Line 118-129.

Section 2.4: Here as well more detail needs to be provided. What were the nutrient concentration in each fertilizer, and how were they obtained (source?).

Response: Accepted and done. We have added more details information in the Section 2.4, and have shown as listed in Table S1 and Table S2.

Section 2.5: The use of a two-segment fitted line in Fig 2b is inappropriate because there are only 3 independent datapoints (underlying replicates are not independent of each other). Essentially, this just connects two points with a line in between. I also don’t think this contributes to the findings of the paper, so I would recommend removing it. Also, provide an overview on the amount if each fertilizer used in each treatment group.

Response: Thanks for your thoughtful analysis and suggestion. Actually, according to nonlinear regression analysis, the linear-with-plateau model produced the best fit for the plots of SOC concentration of 0-20 cm soil layer in the vegetable fields (VF) and number of tillage operations since conversion to VF based on 48 groups datapoints, which were hidden in the manuscript instead of these 3 independent datapoints (the mean values for the corresponding grouping parameters). In addition, we would prefer to keep the corresponding Fig. 2b, as considering tillage frequency is the key factor affecting the decomposition of SOC in agricultural soils (Pires et al., 2016; Six et al., 2000), and quantifying the correlation between the change of SOC and tillage frequency is very important.

L152: I assume the unit here should be Mg ha-1 instead of Mg ha-1 yr-1

Response: Done.

L196: explain ‘extension service’

Response: Accepted and corrected as requested. “The annual application rates of fertilizer N, P and K in the vegetable production (Table 1) were much higher than those recommended (N, 600-765, P, 79-144, K, 398- 498 kg ha^-1) based on crop requirements and soil properties, recommended by more than 30 agricultural research institutions nationwide (i.e. the extension service, Zhang et al., 2009).” Please see Line 230-233.

L207-8 weighted mean: weighted by what?

Response: We have clarified this sentence in Line 243-244.

286-288 check repeated statement.

Response: Done. We have deleted the repeated sentence.

Table 1 and 2: standard deviation or some other measure variance should state along with each value

Response: Accepted and done. We have added the standard deviation in the Table 1 and Table 2, respectively.

Fig 2. See comment on segmented fit above. Also, I guess with tillage frequency you mean number of tillage’s since conversion? It would be simpler to just state number or years since conversion here. You could also add the PRF datapoint to the plot (time since conversion = 0)

Response: Yes, you are right. We have corrected the statement of abscissa for Fig. 2B, and added the paddy-rape rotation datapoint to the plot.

Fig 3: Wider bars would make the figure easier to read.

Response: Done.

Fig 4: Did you actually calculate the cumulative P surplus for each site, or did you just multiply the average surplus with the number of years? If the latter, it would be better to just state years since conversion.

Response: Yes, we calculated the cumulative P surplus for each site.

Data availability: Raw data should be placed in a publicly available repository to meet the Copernicus/EGU data policy.

Response: Done.

Thank you very much for your consideration.

Kind regards,

(Ming Lu and Dunyi Liu)

Reviewer #3: 'Comment on soil-2021-17', Anonymous Referee #3, 13 Apr 2021

The present study intends to assess the effects over time of conversion from paddy rice-oilseed rape rotation (PRF) to vegetables (VF) cropping on soil organic carbon, total nitrogen, pH and nutrients considered as soil quality parameters. The subject fall within the general scope of the journal and it is a new and original contribution. In fact, this work provides useful data on soil organic matter within vegetables the cropping systems and confirms that is a sensitive indicator in environment degradation studies relative to land use and is worthy of publication in Soil. However, the manuscript requires major revision to improve the text and clarify some points before being acceptable for publication:

Response: Thanks for your kind comments and generally positive evaluation. We have revised the manuscript as per your suggestions. Please see below for details.

-Introduction section should include a work hypothesis.

Response: You are right, and we have added the corresponding hypothesis content to the Introduction section. “However, the impacts of land conversion from cereal cropping to vegetable production may be more adverse in tropical and subtropical regions compared to temperate regions due to higher temperatures and rainfall combined, low organic inputs and high frequency of tillage practices (NAPCC, 2016; Powlson et al., 2016; Sarker et al., 2018). Based on the background information, here we hypothesise that the conversion of cereals to vegetable production would results in significant degradation in agricultural soils in southwest China.” Please see Line 49-53.

- In my opinion, the first objective is encompassed in the second objective because this previous survey is necessary to select the different VF farms according to the time elapsed since the conversion to vegetables as well as to discuss the differences found between both types of crops. In any case, this survey generates a database with little data and regional or local relevance.

Response: Yes, we agree with the reviewer, and have revised the objectives accordingly. Please see Line 68-73, “The objectives of this study were to assess the impacts of land use conversion from paddy-rape rotation to vegetable production on soil pH, SOC, TN, soil C/N ratio, available P (AP), CaCl2-soluble P (CaCl2-P), and available base cations (K, Ca, and Mg). In the present study, paddy rice-oilseed rape rotation fields were used as a reference, because all vegetable fields in this region were converted from paddy soil. This will allow the comparison between the conditions on areas that changed to open-field vegetable production with surrounding areas that have remained under paddy rice-oilseed rape rotations.”

- The experimental design is poorly explained, which combined with a poor presentation of statistical results, makes it very difficult to be certain what the researchers actually did. Were used plots randomly set up? For each time since conversion from PRF to VF; were all 12 plots located in the same farm? What is plot size? Where were 12 samples of PRF collected? Adjacent to what? Please add the distance between plots. Thus, information about plots distribution would help.

Response: We have revised Section 2.2 text carefully as per your suggestions: The field size of paddy-rape rotation and vegetable cropping were 0.03-0.13 and 0.01-0.08 ha, respectively.” (please see Line 104-105). “First, we randomly collected 36 soil samples from a total of 133 commercial vegetable fields that were converted from paddy-rape rotation 1-3 years (n=12), 5-10 years (n=12), and ≥15 years (n=12) ago. Second, twelve paddy-rape rotation fields and the adjacent (< 100 m) vegetable fields were also randomly sampled for comparison. The soil type of the selected sites and the Local/regional climatic conditions and agricultural production practices were similar” (please see Line 108-112).

- Line 92: Why were the soil samples collected in September?

Response: To ensure the consistency of soil sampling, because both the paddy rice cropping and the first of three consecutive (non-heading) Chinese cabbage seasons are generally harvested in September according to the Fig. 1B.

- The effects of type of crop and soil depth on measured variables could be tested by a two-way analysis of variance. This statistical analysis could support the discussion about the differences along soil depth detected between PRF and VF crops.

Response: We agree. As per your suggestion, we have added the two-way ANOVA results on the differences of soil properties along soil depth detected between PRF and VF crops. Please see Table 2 and Figure 3, 5 and 6.

- Please, provide information about methodology followed for analysing soil parameters.

Response: Accepted and done. Please see Section 2.3.

- Line 88: “To investigate the effects of long-term fertilization….” This is not correct because the soil samples were collected also from VF crops that were converted from PRF 1-3 years and 5-10 years.

Response: Thanks, we have revised the sentence as “To investigate the effect of conventional fertilization on nutrient surplus, enrichment/depletion, and leaching them into the soil profile of vegetable field, a multistage sampling technique was used to select representative fields for sampling soils from each cropping system.” Please see Line 106-108.

- For the measurement of SOC following the dichromate digestion method soil samples are sieved to 2 mm and ground to a power-like consistency. However, the authors used soil samples passed through a 0.15 mm sieve for SOC and TN analyses, which prevents the comparison with other studies.

Response: Thanks for your meticulous mention. There many studies, which were mainly cited in the manuscript to make comparisons with the current study, have reported that soil samples could sieved to 0.15 to 0.25 mm (Yan et al., 2012; Wang et al., 2014; Gómez et al., 2020). Therefore, we think methodology and data generated in this manuscript are comparable.

- Lines 154-156: Could you please add some more information in Mat & Meth. about tillage operations (e.g type of machinery, tillage depth, tillage timing)? a better description may help to support the suggestions made by the authors about the effects of tillage practices on soil organic carbon.

Response: Yes, thanks for your suggestions. We have carefully revised and added more details about tillage operations in M&M section. Please see Line 90-91.

- Line 117: the authors state that the accumulation of P surplus was calculated as the annual P surplus multiplicated by planting duration. I guess that they meant that annual P surplus was multiplicated by years since conversion to vegetable cultivation. This should be clarified.

Response: Yes, this has been clarified. Please see Line 144-146.

- Expressions like 1.92 or 0.80 mg P kg-1 (see abstract and through text) must be arranged by keeping the homogeneity of significant figures that your equipment generates. What is the precision of your method? Probably significant figures are "out" of the precision of your method!

Response: Here, we used a UV-Visible spectrophotometer method to determine the soil P (including AP and CaCl₂-P) concentrations, please see Line 124-127. This method is accurate enough to allow us to keep two decimal places.

- Some confusing sentences. What do the author's mean by “farmers´ survey methodology” or “local extension service”?

Response: We apologize for causing you any confusion, and have added some sentences to clarify this “farmers’ survey methodology” and “local extension service”. Please see Line 96-104 and Line 230-233.

- Lines 150-151: Where are the data of bulk density in the manuscript?

Response: We have added the bulk density data to the Table 2.

- Lines 212-221: This topic is not addressed in this study. Then, I suggest deleting it.

Response: Thanks for your suggestion. The discussion section in this paragraph is to further discuss the feasible strategies of improving the nutrient utilization efficiency and the soil quality of vegetable field in vegetable cropping systems. In our opinion, this is highly linked with the theme of this study.

- Lines 286-288: These two sentences are repetitive. One sentence should be deleted.

Response: Done. We have deleted the repeated sentence.

- The variance of data presented in Tables 1 and 2 should be expressed using standard deviation or standard error. 

Response: Done.

- What data are shown in Table 1? Are average values of 36 surveys related to VF management?

Response: Table 1 is the investigated inputs and output for inventory analysis in paddy rice-oilseed rape rotation and vegetables production based on farmer surveys.

- Lines 165-166: The authors state that for every 100 kg ha-1 of P surplus in the VF, AP concentration in the 0-20 cm soil layer increased by 1.92 mg kg-1. However, according to the equation obtained by the authors for the data of the Figure 4 (y = 19.234 X + 43.257), for 100 kg ha-1 of P surplus in the VF, AP concentration in the 0-20 cm would increase by 45 mg kg-1.

Response: We apologize for this mistake, and have revised the sentence: “For every 100 kg ha^-1 of P surplus in the VF, AP concentration in the 0-20 cm soil layer increased 1.92 mg kg^-1”. Please see Line 197-198.

- Lines 235-246: This topic is not addressed in this study. Then, I suggest deleting it.

Response: Accepted and corrected as requested.

- Lines 255-258: In relation to inorganic N losses, the ammonium fixing capacity of the soils used (Ultisol) should be mentioned.

Response: We have added the details of the ammonium fixing capacity of the soils used (Ultisol) into the Section 4.2: “The slightly lower pH in subsoil in the vegetable field compared to that in the paddy-rape rotation might have slowed nitrification such that part of the inorganic N remained in the ammonium form sufficiently long enough for some immobilization to occur (Figure 6; Marschner, 2012). However, Nieder et al. (2011) indicated that the ammonium fixing capacity of the Ultisol, the type of soil used in this study, is extremely low. This may increase the risk of ammonium losses in this region, a potential environmental issue that warrants more attention in the future.” Please see Line 285-290.

- Line 266 (P enrichment): It has been omitted that when the soil pH reaches values less than 5.3 as occurred by conversion to VF, the presence of Al and Fe in clay minerals of Ultisols can contribute to fixing process of P.

Response: We agree and have added this sentence to the Line 304-306.

- The English of the manuscript needs to be revised. They erroneously interchanged the term P surplus by surplus P throughout the text. For example, the authors write “the accumulation of surplus P” in the caption of Figure 4, whereas the title of x they write “accumulation of P surplus”.

Response: We apologize for this unintentional omission and have carefully revised the scientific terminology as P surplus. Thanks.

Thank you very much for your consideration.

Kind regards,

(Ming Lu and Dunyi Liu)

Cover Letter

Jun. 1^st, 2021

Dear Prof. García-Orenes,

Thank you for processing our manuscript entitled “Significant soil degradation is associated with intensive vegetable cropping in subtropical area: A case study in southwest China” (soil-2021-17) quickly. We appreciate the comments/suggestions from you and the reviewer on our paper.

We have revised the manuscript carefully, addressing all your and reviewer’s comments and suggestions. Details of revision are given below. We also made a few minor edits in the manuscript to improve the language.

All changes made in the revised manuscript are done in Track Changes to make reviewing easy. We believe that the revised manuscript is much improved and is suitable for publication.

Thank you again and look forward to hearing from you soon.

Yours sincerely,

Dunyi Liu

College of Resources and Environment, Southwest University

Tiansheng Road, Chongqing 400716, China

Tel: 023-68251082; E-mail: liudy1989@swu.edu.cn