Brief Summary of the manuscript:

Barnes and co-authors investigate the effects of wildfire burn severity on phosphorus (P) mobilization in Douglas-fir forests and sagebrush shrublands. Through laboratory leaching experiments, they examine how different burn severities influence particulate and aqueous P release. The authors find that higher burn severity increases particulate P mobilization while decreasing aqueous P availability, with particulate P controlled by total char P and aqueous P driven by solubility changes. Using nuclear magnetic resonance and X-ray absorption spectroscopy, they show that organic P compounds are thermally mineralized into inorganic calcium- and magnesium-bound forms. The study highlights that fire severity and vegetation type drive post-fire shifts in P cycling, with implications for nutrient transport and ecosystem recovery.

The paper is well-written and presents compelling results on phosphorus (P) transformations following fire. The findings provide valuable insights into how burn severity and vegetation type influence P retention and mobilization. However, several issues need to be addressed before the manuscript is suitable for publication.

I recommend the paper for publication after moderate revision, focusing on clarifying key mechanisms, improving data presentation, and addressing inconsistencies in comparisons.

Major Issues Requiring Revision:

1. Unjustified Burn Severity Comparisons:

The authors compare moderate-severity burns in sagebrush shrublands to high-severity burns in Douglas-fir forests without justification. This prevents direct comparisons and raises concerns about bias in data interpretation. Either compare the same burn severities across vegetation types or provide a clear justification for the chosen comparison.

1. Unclear Mechanisms of P Transformation and Mobilization:

The study claims burn severity influences P transformations, yet the chemical mechanisms behind these changes remain vague. For example, the authors state that aqueous P mobilization is “composition-controlled” by Ca-Pi, but later indicate that Ca-Pi concentrations are similar across vegetation types, suggesting other factors must be involved.

1. Fire Temperatures in Experimental Burns Are Lower Than Real Wildfires:

The highest recorded burn temperature for sagebrush (530°C) and Douglas-fir (704°C) is significantly lower than real wildfire conditions, which can exceed 1,000°C. Since P volatilization occurs above ~700°C, the study may underestimate P losses in real wildfire conditions. Discuss how P retention might differ if sagebrush shrubland was burned at higher temperatures (e.g., 800–1,200°C).

1. Post-Fire Ecosystem Recovery:

The manuscript discusses P mobilization and transformation but does not address how these changes affect ecosystem recovery after fire. It is unclear whether particulate P will eventually become bioavailable or remain locked in ash.

1. Missing Data or Discussion on Ash Color:

The conclusion states that ash color (black charring and white ash) increases with burn severity, yet this was not discussed in the results or presented in any figure or table. If ash color was recorded, include data or observations in the results and discuss its significance. If it was not recorded, remove the statement from the conclusion.

Minor Revisions:

Abstract:

Lines 33–34: The magnitude of P mobilization (e.g., “Burning increased particulate P mobilization (6.9-fold and 29-fold) but decreased aqueous P release (3.8-fold and 30.5-fold)”) varies significantly between Douglas-fir and sagebrush. Why? Briefly mention the mechanism driving these differences.

Line 29–31: The sentence “However, it is unclear if post-fire responses are primarily driven by changes to the molecular composition of the charred material or from the transport of P-containing compounds.” is difficult to follow. Consider rewording.

Line 39: “Thermally mineralized to inorganic P moieties”—clarify how this affects P availability in soils.

Introduction:

Lines 60–63 and 67-68: Long sentences—consider breaking them up for clarity.

Lines 111–119: Burn severity should be introduced earlier when discussing fire intensity and nutrient cycling.

Methods:

What was the collection timeframe? Seasonal variations can influence plant moisture content, affecting fire behavior and P release.

The geographic description (“Pacific Northwest, USA”) is too vague. Include specific sites or coordinates for clarity.

Lines 138–140: The statement that “Douglas-fir burns at higher intensities due to fuel loading, while sagebrush burns at lower intensities” is too general. Explain why these fuel differences affect fire behavior.

Lines 146–147: What ratio of woody to canopy material was used? The relative proportion of wood vs. foliage affects combustion properties and nutrient release.

No mention of initial sample preparation—were plant materials cleaned, dried, or processed before burning?

Results and Discussion:

Lines 282–284: The study may underestimate P volatilization since wildfires can exceed 1,000°C, causing greater P losses. Acknowledge this limitation.

Lines 292–302: The comparison between moderate-severity sagebrush and high-severity Douglas-fir needs clear justification.

Lines 308–313: Which forms of P are retained vs. combusted? Clarify whether organic or inorganic P compounds are responsible.

Lines 328–331: The phrase “selective protection” explain what protects P from mineralization.

Lines 373–395: The manuscript claims pyrophosphate forms from orthophosphate, but why did sagebrush chars produce less pyrophosphate than Douglas-fir chars, despite having more phytate initially?

Lines 460–463: The mechanism by which pH affects P solubility is not clearly explained. Report actual pH values measured.

Lines 478–479: “This has important implications for P compounds are transported…” grammatical error—revise for clarity.

Conclusions:

Lines 483–485: The first sentence should summarize the main findings upfront before interpretation.

Lines 486–487: The conceptual model (Fig. 6) is mentioned but not explained. Briefly describe its significance.

Lines 500–503: The phrase “more burned” is vague—does this mean greater P retention, more mass loss, or another factor?

Lines 506–509: The statement on shifting fire severity lacks context—explain why this matters for P retention and post-fire recovery.

Lines 510–517: Organic soil horizons are introduced but were not a focus of the study. Instead, discuss how P transformations affect bioavailability over time.

Dear authors,

This paper is an important contribution to understanding the biogeochemical impacts of fire on phosphorus fate and mobility. The study employs a robust methodological approach, including NMR and XANES analyses, to investigate how burn severity and vegetation type influence phosphorus composition and mobilization. The findings are relevant for understanding post-fire nutrient cycling and have implications for both terrestrial and aquatic ecosystems.

Major Comments:

1. Providing information on the geological and soil characteristics of the study site, including soil type and pH, would help contextualize phosphorus dynamics. Additionally, details on climate conditions such as rainfall, temperature, and seasonality would improve the interpretation of post-fire phosphorus mobilization and retention. If the bedrock is limestone, it could increase soil calcium concentrations and pH, reducing bioavailable phosphorus and promoting the formation of Ca-bound phosphorus (Ca-P). This may influence the phosphorus speciation in the original soil, affecting the composition of the phosphorus pool from which the organic samples were taken. If available, correlating soil properties with phosphorus fractions could further strengthen the study.
2. Soil Phosphorus Pools and Pre-Fire Conditions:
   
   It would be valuable to include data on the total phosphorus concentration in the soils from which the organic matter was sourced. Since the initial phosphorus pools in the soil can influence post-fire phosphorus mobilization and retention, this information could provide important context for interpreting the results. If this data is available, correlating soil phosphorus status with the observed trends would further strengthen the study.
3. Use of the Term “Total P Concentration” (Line 99 and elsewhere):
   
   The term "total P concentration" is unclear. It would be helpful to clarify whether this refers to the sum of all phosphorus fractions or a specific measurement of P concentration. Consistently defining this term throughout the text would improve clarity.

Specific Comments:

• Line 65: Expand on the impacts of climate change on fire regimes. The statement that fires are expected to increase in intensity and severity could be strengthened by elaborating on the mechanisms driving this trend, as climate change is expected to exacerbate fire risk.
• Line 285 (Figure 1): The distinction between moderate and high temperatures for Douglas fir is unclear. The figure appears to show overlapping temperature ranges - maybe you can think of a different phrasing? Additionally, the lines on the figure should be explained—do they represent the 25th and 75th percentiles? Are they median or mean values?
• Line 334 (Figure 2): The color scheme should be adjusted to better differentiate between organic P and inorganic P. Additionally, the use of "moderate" and "high" temperatures for Douglas-fir is unclear, as it appears inconsistent with figure 1.
• Line 453 (Figure 3): Similar to Figure 1, further clarification is needed. What do the dots above the boxes represent? How many samples were measured? The figure suggests that Douglas-fir moderate fire has significantly higher leachable P than low-temperature burns, and that high-temperature burns are significantly higher than moderate burns. For sagebrush, in the lower right panel, the "a" and "a" labels above the boxes indicate significant differences, yet the values appear different. Please review and clarify.
• Line 455: The impact of pH on phosphorus solubility is mentioned, but actual pH values are not provided. I suggest including these values in the results.
• Line 470: The proportion of Na-P is relatively high in Douglas-fir. A brief discussion of the potential role of Na-P in post-fire phosphorus dynamics would be useful.