the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Mar 2026
Field application of rice straw–sewage sludge compost in Mediterranean citrus orchards: effects on soil properties, nutrient status and fruit quality
Abstract. Intensive agricultural practices have degraded soil fertility and polluted natural resources in Spanish citrus orchards, highlighting the need for more sustainable management strategies. Composting rice straw (RS) and sewage sludge (SS)—two residues that are difficult to manage in Mediterranean regions—offers an environmentally sound alternative for residue valorisation and soil fertility restoration. This study assessed the agronomic performance of compost produced from RS and SS (RS/SS) at an industrial scale, in comparison with compost derived from pruning residues and sewage sludge (PR/SS), which is commonly produced in Mediterranean composting facilities. The effects of compost application at two rates over two consecutive years were evaluated through analyses of soil physical, chemical, and biological properties, as well as foliar nutrient concentrations, yield, and fruit quality. RS/SS compost contained higher nutrient levels, particularly P, suggesting its potential as a P-rich organic fertiliser. Compost application reduced soil pH and slightly increased electrical conductivity without exceeding critical thresholds. Active lime decreased, while organic matter, N, P, K, and Zn contents increased, accompanied by improvements in soil biological indicators. Effects on foliar nutrient status, yield, and fruit quality were limited. These results indicate that industrial-scale RS/SS compost application represents a sustainable strategy for residue valorisation and soil fertility improvement in Mediterranean citrus systems, enhancing soil properties and reducing dependence on mineral fertilisers.
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Status: open (until 02 Jul 2026)
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CC1: 'Comment on egusphere-2026-1497', Engracia Madejón Rodríguez, 14 May 2026
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AC1: 'Reply on CC1', Isabel Rodríguez Carretero, 16 May 2026
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Dear Dr. Madejón,
Thank you very much for your detailed and constructive comments on our manuscript. We highly appreciate the time and effort devoted to the review process.
Your suggestions regarding the focus of the introduction and discussion, as well as the interpretation of the differences between the two experimental seasons, are very valuable and will help us to improve the manuscript substantially.
We will carefully consider all your comments and suggestions in the revised version of the manuscript.
Kind regards,
Isabel Rodríguez-Carretero
Citation: https://doi.org/10.5194/egusphere-2026-1497-AC1
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AC1: 'Reply on CC1', Isabel Rodríguez Carretero, 16 May 2026
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RC1: 'Comment on egusphere-2026-1497', Anonymous Referee #1, 03 Jun 2026
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The paper ‘Field application of rice straw–sewage sludge compost in Mediterranean citrus orchards: effects on soil properties, nutrient status and fruit quality ‘ by Rodríguez-Carretero et al. deals with the use of compost obtained from rice straw and sewage sludge on citrus trees. Although the Materials and Methods section states that the experiment was conducted over two growing seasons, the manuscript actually reports two independent experiments performed in two consecutive years. This distinction has important implications for the interpretation of the results. In particular, COMP1 and COMP2 were not directly compared under the same experimental conditions, as they were applied in different growing seasons. Therefore, conclusions regarding differences between the two composts are not fully supported. Furthermore, the duration of each experiment (one growing season) is insufficient to draw robust conclusions regarding changes in soil properties, especially those related to soil organic matter and soil physicochemical characteristics, which generally require long-term investigation.
Additional comments are provided below:
- Line 107, spell out ANP
- Line 137: actually here 2 experiments are reported, 1 year-long each
- The analysis of pruning residues, rice straw and sewage sludge would help to understand the fertilization potential of composts
- Line 96, point 1 cannot be achieved, there is no comparison between COMP1 and COMP2,
- The difference between the two composts clearly depends on the proportion of sewage sludge (line 107)
- Line 98, point 2 is quite ambiguous since non comparison between the 2 compost were carried out
- Were COMP1 and COMP2 applied to the same soils or on different plots
- Line 200, The compost pH appears relatively low. Was the sewage sludge treated with gypsum? This hypothesis may be supported by the increase in soil Ca reported in line 269. Please clarify the source of Ca.
- Line 204 spell out OOC
- In table, when ANOVA is not significant, no mean separation is required
- Line 247: What is meant by “optimal” soil organic matter content? Please provide a reference range and clarify whether this statement refers to agronomic recommendations.
- Line 250: OM decreased in the 2 seasons, how do you explain it?
- Table 6 different font
- No comparison between COMP1 and COMP2 can be made (line 326-328)
- Comparison of chemical analysis between COMP1 and COMP2 (line 334) are adventurous since only 4 replications were used
- Line 347, 2 years of compost application are unlikely to be sufficient to produce soil physicochemical changes, Such effects generally require long-term study
- Line 385-386: I suggest that the Authors estimate the amount of C applied through compost relative to the total soil C pool. Without such information, the reported increase appears difficult to reconcile with the amount of compost applied and may reflect analytical variability rather than a true treatment effect.
- Since compost was not tilled into the soil, this reduces the effect on soil physical characteristics
Citation: https://doi.org/10.5194/egusphere-2026-1497-RC1 -
RC2: 'Comment on egusphere-2026-1497', Susana Ferreira, 05 Jun 2026
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The manuscript is generally well organised and presents relevant results regarding soil fertility and nutrient dynamics after compost application. However, some aspects related to experimental design, statistical interpretation, and discussion of the results should be clarified or interpreted more cautiously before the manuscript can be considered for publication.
Specific comments
- Lines 92–99: The hypotheses and objectives should be reformulated more carefully. Since COMP1 and COMP2 were applied in different growing seasons, direct comparisons between both composts should be interpreted cautiously.
- Section 2.2.1 (Lines 130–143): Although the manuscript refers to “two consecutive seasons”, the experiments differed between seasons regarding compost type and fertilisation management. This limitation should be acknowledged more explicitly.
- Lines 139–142: The reduction in P fertilisation during S2 introduces an additional management factor that may influence comparisons between seasons.
- Section 2.2.5: The statistical approach should be described in greater detail. It would be useful to clarify more explicitly whether the seasons were analysed independently.
- Lines 247–250: The decrease in soil organic matter between seasons deserves additional discussion. Climatic conditions, mineralisation rates, irrigation management, or sampling variability could have contributed to this behaviour.
- Lines 265–278 and Figure 1: Several soil parameters showed statistically significant differences; however, the agronomic relevance of some changes should be discussed more carefully.
- Lines 288–290 and Figure 2: The microbial and dehydrogenase activity results are interesting, but these parameters are naturally variable and strongly affected by environmental conditions. This should be acknowledged more explicitly.
- Lines 318–321: Since compost application did not significantly affect yield or most fruit quality parameters, the conclusions should better reflect that the main observed effects were related to soil properties.
- Lines 323–341: The discussion comparing COMP1 and COMP2 should be moderated because both composts were not evaluated simultaneously under identical field conditions.
- The discussion would benefit from a more explicit consideration of the limitations associated with extrapolating these results to drip-irrigated Mediterranean citrus orchards, since the study was conducted under flood irrigation conditions.
Technical comments
- Line 53: Spell out “ANP” at first mention.
- Lines 116 and 165: Spell out “OOC” at first mention.
- Table 6 formatting should be revised for consistency.
- Some sentences in the Discussion section could be simplified to improve readability.
Citation: https://doi.org/10.5194/egusphere-2026-1497-RC2
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- 1
This article presents a relevant study on the use of rice straw as an alternative bulking agent for sewage sludge composting and evaluates its agronomic effects under real field conditions in a commercial citrus orchard. A major strength of the work is its practical approach, as it combines industrial-scale compost production with field application over two growing seasons, providing valuable evidence of the potential of RS–SS compost as a sustainable strategy for organic waste valorisation and soil fertility improvement. However, the manuscript also presents some weaknesses and aspects that should be improved before it can be considered suitable for publication in a journal such as SOIL. In particular, the introduction and discussion would benefit from a clearer focus, reduced repetition, stronger synthesis of the results, and a more explicit interpretation of the differences observed between the two experimental seasons.
Introduction
A major strength of this work is the identification of rice straw as an alternative bulking agent for sewage sludge composting. Finding sustainable substitutes for pruning residues as bulking materials is currently an important concern within the sector, and the authors should place greater emphasis on this aspect of the study to:
Reducing the legislative background and focusing only on the most relevant policies. Shortening repetitive explanations about compost benefits that are already well established.
Improving the flow between paragraphs so the narrative moves more directly from the problem (rice straw management) to the proposed solution (RS–SS compost).
Clarifying the novelty earlier, especially the industrial-scale composting and its application in citrus orchards under flood irrigation.
Reducing citation density where possible.
Materials and Methods:
This experimental description is clear, detailed, and generally well structured. It provides essential information about the field conditions, treatments, experimental design, and management practices, which supports reproducibility. The use of a commercial citrus orchard and two growing seasons is also a strong point because it increases the practical relevance of the study.
However, the section could be improved by making it more concise and easier to follow. Some sentences are overly long and contain too many details at once. I would also recommend:
Explaining more explicitly why mineral fertilisation was maintained together with compost treatments, since this is important for interpreting the results.
Specify the total number of experimental plots used. As I understand the design, there are two composts plus the control, two application rates, and three replicates per treatment, resulting in a total of 15 experimental plots. Is that interpretation, correct?
Results
In the Results section, little emphasis is placed on the differences observed in soil determinations between the two seasons. There is a decrease in almost all parameters except for microbial biomass C. Could a statistical analysis be performed to evaluate the differences between the two seasons?
Discussion:
The discussion is scientifically solid and very well referenced, but it is excessively long, repetitive, and sometimes reads more like an extended literature review than a focused interpretation of the results. Many ideas are repeated several times (especially regarding OM, salinity, nutrient availability, and previous studies), which weakens the overall impact of the discussion.
A major weakness is that the discussion does not sufficiently emphasize the differences between S1 and S2, despite the two-season approach being one of the strengths of the experiment. The seasonal responses are mentioned, but they are not deeply interpreted. For example:
Why were microbial effects significant in S1 but not in S2?
Why did some soil parameters respond only in S2?
Did the second compost application produce cumulative effects?
Were climatic or management differences between seasons relevant?
These aspects should become central to the discussion because they provide the most interesting biological and agronomic interpretation of the study.
I would improve the discussion by:
Reducing repetition and shortening literature comparisons.
Focusing more on interpretation rather than describing every result again.
Structuring the discussion around key themes (soil fertility, microbial activity, plant nutrition, yield).
Explicitly comparing S1 vs S2 responses throughout the text.
Highlighting the novelty of the RS–SS compost and the industrial-scale validation more clearly.
Shortening considerably. secondary explanations and excessive citations
Overall, the discussion contains valuable information, but it needs stronger synthesis and clearer emphasis on the temporal dynamics between the two campaigns.