| 03 Jul 2026
Decoupling phosphorus availability and fixation in tropical soils: roles of iron and aluminum oxides across agroecological gradients in Nigeria
Abstract. Phosphorus (P) deficiency and fixation are major constraints to crop productivity in tropical soils, yet their large-scale interactions remain poorly understood. This study provides a national-scale assessment of how short-range-ordered iron (Feₒₓ) and aluminum (Alₒₓ) oxides, soil properties, and climate jointly regulate P availability across Nigeria. A total of 1,382 topsoil samples (0–20 cm), collected using a hierarchical Soils4Africa framework, were analyzed for extractable P, Feₒₓ, Alₒₓ, soil organic carbon (SOC), texture, and pH, and integrated with precipitation data. We developed a phosphorus sorption index (PSI) to quantify fixation potential as the molar ratio of oxide abundance to available P. Soil properties varied strongly along agroecological gradients, with Feₒₓ, Alₒₓ, SOC, precipitation, and PSI increasing from semi-arid savannas to humid forest systems, while pH declined. Contrary to the expected inverse relationship, extractable P was positively associated with Feₒₓ, Alₒₓ, SOC, and precipitation, but negatively related to PSI. Multivariate analyses clarified this apparent paradox: SOC exerted the strongest positive effect on P availability, whereas Alₒₓ had a strong negative effect consistent with sorption, and precipitation influenced P both directly and indirectly through oxide accumulation. These results demonstrate that P availability and fixation are not simply inversely coupled but are co-regulated by climatic and biological processes. The PSI effectively captured spatial variation in fixation potential, with the highest values in humid regions characterized by intensive weathering. These findings highlight the dominant role of agroecological gradients in structuring P dynamics and reveal contrasting nutrient constraints across regions: oxide-rich humid soils are limited by fixation, whereas northern savannas are constrained by low inherent P. This study provides a mechanistic basis for site-specific phosphorus management and supports the development of sustainable nutrient strategies to improve P-use efficiency in tropical agroecosystems.
This manuscript addresses an important topic in tropical soil fertility by examining the relationship between phosphorus (P) availability, phosphorus fixation, and oxalate-extractable Fe and Al oxides across Nigeria using a large national-scale dataset (n = 1,382). The study is novel in its geographical scope and potentially valuable for nutrient management in tropical agroecosystems. The manuscript is generally well structured and presents interesting findings, especially the concept that P availability and fixation can be partially decoupled along agroecological gradients. However, several methodological, analytical, and interpretational issues should be addressed before publication.
Recommendation: Major Revision
Major Comments
The manuscript indicates that only 20% of samples underwent wet chemistry analysis and that MIR spectroscopy models were used to predict soil properties for the remaining samples. However, model validation statistics are not reported.
Critical metrics are missing:
Without these metrics, it is impossible to assess the reliability of the predicted soil properties that support all subsequent analyses.
Recommendation: Include a comprehensive calibration and validation table and provide prediction performance figures in supplementary materials.
The manuscript introduces:
as a phosphorus sorption index.
Several concerns arise:
The negative correlation between PSI and available P may be partly inherent to index construction rather than an independent ecological phenomenon.
Recommendation: Provide theoretical justification and sensitivity analysis. Discuss limitations of using PSI as a proxy rather than a direct measurement of sorption capacity.
Because available phosphorus is included directly in the PSI equation, conclusions such as:
"PSI was negatively correlated with P"
are mathematically expected to some extent.
The manuscript should clarify:
The kriging approach is inadequately described. Important information is missing:
Although a PSI map is presented, no geostatistical diagnostics are shown.
Recommendation: Report variogram parameters and kriging validation results.
The SEM is one of the manuscript's key analytical tools, yet important information is absent:
Without goodness-of-fit measures, it is difficult to evaluate the robustness of the SEM conclusions.
The manuscript argues that positive correlations between Feox and available P reflect broader pedogenic conditions and biological cycling.
While plausible, this explanation is not directly tested.
Alternative explanations include:
The discussion should acknowledge these alternatives and avoid overinterpreting observational correlations as causal mechanisms.
The study spans a large geographic area, yet key drivers of phosphorus availability are absent:
These variables can strongly affect available P and may confound observed relationships.
The limitations should be explicitly discussed.
Minor Comments
Section title:
"3.1 Geological Influence and interaction with agroecology"
appears after another section also numbered "3.1".
Renumber appropriately (likely 3.2).
Several equations are poorly rendered:
These should be reformatted using proper mathematical notation.
Examples include:
Professional copyediting is recommended.
Several duplicated citations exist:
Tables 1–3 provide means but not pairwise significance groupings (letters or post-hoc results).
The regression table is useful but should additionally report: