| 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.
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.