| 13 May 2026
Impact of perennial forage plants on microbial carbon use efficiency in weathered Ethiopian soils
Abstract. Aims: Ethiopian soils are highly weathered and in urgent need of remediation. Increasing the soil carbon (C) pool by perennials potentially remediates soil degradation while sequestering atmospheric CO2. The soil microbial loop is thought to play an important role in soil C sequestration as it contributes significantly to the stabilization of plant litter C. The amount of stabilized litter C has been hypothesized to depend on microbial carbon use efficiency (mCUE), which is sensitive to soil nutrients and plant inputs. Here, we investigate the effect of two perennial forage crops on mCUE in four weathered Ethiopian soils.
Methods: We cultivated the grass Urochloa cv. Cayman as single stand and in mixture with the legume Desmodium intortum in a fully factorial greenhouse experiment with four distinct farmers’ soils from the Ethiopian Rift Valley (Amhara and Sidama). Microbial CUE was assessed after 104 days of plant growth using 18O-H2O incorporation.
Results: Microbial CUE varied between the four soils, with Amhara soils showing lower mCUE than Sidama soils. Overall, mCUE was little affected by plant growth but increased significantly with plant dry weight (DW) in one of the two Sidama soils. Mass specific growth rates remained small in Amhara soils, indicating that factors other than plant C input inhibited microbial growth. Plant composition had no effect in Amhara soils while the Urochloa × Desmodium mixture tended to have lower mCUE in Sidama soils.
Conclusions: Perennials may improve mCUE in weathered soils, but this stimulation strongly depends on soil nutrient availability and suggests that there are critical nitrogen (N) and phosphorous (P) thresholds below under which mCUE does not respond to growing plants. This implies that highly degraded soils must be ameliorated before using perennials to stimulate nutrient cycling and restore soil health.
Principal criteria
Excellent (1)
Good (2)
Fair (3)
Poor (4)
Scientific significance:
Does the manuscript represent a substantial contribution to scientific progress within the scope of SOIL (novel concepts, theories, methods, or data; interdisciplinary)?
2
Scientific quality:
Are the scientific approach and applied methods valid and reproducible? Are the results discussed in a thorough and balanced way (consideration of related and relevant work, including appropriate references)?
2
Presentation quality:
Are the scientific results and conclusions presented in a clear, concise, and well-structured way (appropriate number and quality of figures/tables, appropriate use of English)?
1
Summary and recommendation:
This paper quantifies the effect of two types of perennial forage crops on microbial carbon use efficiency (CUE) in four highly weathered soils. Soils were collected from four different Farming Training Centers in Ethiopia, which allowed the authors to study the effects of perennial crops on microbial CUE across different soil physicochemical features.
This paper integrated a 104 day fully factorial greenhouse experiment to examine plant dry weight, microbial CUE, mass-specific growth rate, respiration, and microbial biomass carbon across soils planted with Urochloa cv. Cayman as single stand and in mixture with the legume Desmodium intortum. The paper uses linear regression and two-way ANOVA to examine the effect of the predictor variables and their interactions on the response variables. The results are overall robust and contribute to knowledge of microbial CUE under different soils under plant treatments. However, not all assumptions and conclusions were clear or supported by the data. Notably, the effects of soil and plant type could not be isolated, nor could the effects of soil pH and soil phosphorus, and critical nitrogen thresholds were not explicitly explained but assumptions were made in the discussion, conclusion, and abstract. After these minor revisions, and further suggestions detailed below, I recommend this article to be accepted for publication.
Yes, the paper addresses the important topic of soil remediation in highly weathered soils using plant remediation approaches to improve microbial CUE as microbial CUE may control the stabilization of soil litter C. The authors measured the effect of perennial plant treatment, soil, and their interactions on microbial carbon use efficiency. Additional microbial metrics were measured including mass-specific growth, respiration, and microbial biomass carbon.
2. Does the paper present novel concepts, ideas, tools, or data?
Yes, this article contributes to existing work that seeks to address how plant remediation approaches can influence microbial CUE, which is hypothesized to regulate soil litter C stabilization. The research is conducted on four highly weathered agricultural Ethiopian soils that are amended with fertilizer and used for maize production. This work contributed to advancing knowledge on modern agricultural practices by utilizing four different soils from Farmers’ Training Centers in Ethiopia to explore plant remediation effects on microbial CUE.
3. Does the paper address soils within a multidisciplinary context?
Yes, soils here are addressed through the lens of soil fertility, agricultural sciences, climate change science, and microbiology through examining the effects of plant remediation using two perennial crops on microbial CUE.
4. Is the paper of broad international interest?
This paper would be of broad interest to farmers, microbiologist, ecologist, climate change scientist, soil scientist, and politicians as it relates to soil sustainability and crop production, and general audience as it relates to improving soil health through plant remediation approaches.
5. Are clear objectives and/or hypotheses put forward?
Yes, the author's motivation for the work and aims were clearly laid out. The hypotheses are well laid out at the end of the introduction, which were 1) microbial CUE and response to plant inputs depend on soil properties, with nutrient poor soils having a limited response , 2) legume inclusion would increase microbial CUE by alleviating nitrogen limitation, 3) plant functional diversity exerts a greater influence on microbial CUE than plant biomass.
6. Are the scientific methods valid and clear outlined to be reproduced?
Yes, each methodology has a clear subheader, followed by description, formulas, and citations.
7. Is the soil type/classification adequately described?
Soil pH, soil textural class, CEC, and certain nutrients are well described, however there is no overarching soil classification data. Soil texture class is used interchangeably here with soil type, but these are two different things. If possible, I think it would be helpful to list the soil classification as soil type, or make it clear the authors refer to a soils textural class as the soil type. Including soil classification data may yield more insight into plant and microbial responses to plant remediation strategies since soils in the different regions actually had overlapping soil texture classes.
8. Are analyses and assumptions valid?
Yes, the analysis is generally valid. However, the assumptions and conclusions could use additional clarification. Since microbial CUE was not able to be measured in the unplanted soil controls, the authors could not isolate the effect of the soil from the effect of the potted plants. The soils from each site were sampled in five locations (corner and center) at depths between 0 - 25 cm, which could be mixing different soil layers with unique physicochemical properties. Additionally, the authors did not mention why the data was appropriate for linear regression and two-way ANOVA.
9. Are the presented results sufficient to support the interpretations and associated discussion?
It is not clear how the analysis supports the conclusion that highly degraded Sub-Saharan soils must be amended with phosphorus fertilizer before plant remediation approaches occur. The soils that had higher microbial CUE did have more P, but they also had higher pH which the authors noted in the discussion. Since the effects of soil pH from soil phosphorus could not be isolated, the statement that P amendment must occur before plant remediation approaches warrants further investigation.
It is also unclear why the authors stated that there are critical nitrogen (N) thresholds below which microbial CUE does not respond to growing plants in the abstract. Can the authors state this threshold? Total nitrogen was highest in soils with the lowest microbial CUE so this statement could use revision to make it more clear.
10. Is the discussion relevant and backed up?
The discussion is well written but could benefit from clearer sub sections. The authors note that the significant correlation between microbial CUE and plant biomass could suggest that increased biomass is a driving force for increasing microbial CUE if P is available. Yet, microbial CUE in Baricha soils that had high P was not significantly correlated with plant biomass. The authors state that the perennial mixture significantly lowered mCUE in one soil, yet there was no change in microbial respiration.
Line 298 - 299 - soils from the Amhara and the Sidama region had overlapping soil textural classes (silt loam) so the statement that different soil textures may contribute to different microbial CUE should be revised to clarify that the same soil textural classes had different responses based on clay content.
11. Are accurate conclusions reached based on the presented results and discussion?
Please see section 6 and section 7.
12. Do the authors give proper credit to related and relevant work and clearly indicate their own original contribution?
Yes
13. Does the title clearly reflect the contents of the paper and is it informative?
Yes
14. Does the abstract provide a concise and complete summary, including quantitative results?
Yes, however the importance of legumes for microbial CUE is not established in the introductory section of the abstract but it is an important component of the approach and results. The abstract conclusion sentences about nitrogen thresholds and fertilizer amendment strategy should be revised as the results and discussion did not clearly state the necessary nitrogen threshold for plant remediation to be successful. Without isolating the effects of pH and phosphorous content, it's unclear if soils should be amended to increase pH or increase phosphorus.
15. Is the overall presentation well structured?
Yes.
16. Is the paper written concisely and to the point?
Yes, the paper is well written and clear, with clearly laid out importance statements, aims, hypothesis, and approach.
17. Is the language fluent, precise, and grammatically correct?
Yes
18. Are the figures and tables useful and all necessary?
Yes
19. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used according to the author guidelines?
Yes
20. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
It's not clear if the authors measured respiration in the unplanted controls (methods section:). It is clear they could not measure microbial CUE or mass-specific growth, but can it be stated whether respiration was measured in unplanted controls?
The caption for Figure 3 could be more descriptive by explaining that regression equations are shown based on p-value significance and linear regression was done on a per pot basis. The light color lines and small text are somewhat difficult to see in Figure 3, especially in panel C that is crowded with the legend and panel D where the equation is in small light pink colored text. Can the color legend be shifted outside of the four panels?
The legend for figure 4 states that different letters indicate significance, but no letters are shown.
21. Are the number and quality of references appropriate?
Yes
22. Is the amount and quality of supplementary material appropriate and of added value?
There is no supplementary table. A table of all statistical analysis used to test the effect of the predictor variable and interaction of predictor variables on the response variable would help summarize the data.