Effects of Different Nitrogen Substitution Practices on Nitrogen Utilization, Surplus, and Footprint in the Sweet Maize Cropping System in South China
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsSummary. This study evaluates the impact of nitrogen substitution (NS) practices in sweet maize cropping systems in South China. The authors assess four organic materials—maize straw, biochar, cow manure, and biogas residue—as partial replacements for synthetic nitrogen fertilizers. The study investigates their effects on nitrogen use efficiency (NUE), nitrogen balance (NB), nitrogen footprint (NF), and nitrogen losses using field experiments over three cropping seasons. The findings indicate that NS practices enhance nitrogen absorption, reduce nitrogen surplus, and lower nitrogen losses, with biochar substitution demonstrating the most effective performance.
General Comments. The study is highly relevant, addressing the urgent issue of nitrogen overuse and its environmental consequences in agriculture. It contributes to sustainable nitrogen management strategies in subtropical maize cropping systems.
The randomized block design and multi-season approach enhance the study's reliability. The manuscript presents detailed evaluations of nitrogen uptake, efficiency, surplus, and footprint, providing a well-rounded assessment. The manuscript is well-structured, logically progressing from problem identification to results and discussion.
The study presents multiple statistical comparisons but lacks clarity on the power of the statistical tests. Were assumptions of normality and homogeneity of variance tested before conducting ANOVA?
The effect size should be reported along with p-values to provide a more quantitative measure of the impact of NS practices.
The selection of a 20% substitution rate is not well justified. Was this choice based on previous literature, pilot studies, or practical constraints? Would a higher or lower substitution rate yield different results?
The nitrogen footprint (NF) calculations consider both direct and indirect emissions, but there is limited discussion on uncertainties associated with emission factors used in the calculations.
The study assumes that organic materials have a neutral pre-existing footprint since they are considered waste products. However, some organic materials, such as biogas residues, may have varying emissions based on processing methods. This limitation should be acknowledged.
While the study spans three cropping seasons, it does not address potential long-term soil health effects of NS practices. For example, how do these practices influence soil microbial activity, organic matter content, or nutrient cycling beyond nitrogen?
The methods are detailed, but additional information on the cost-effectiveness and feasibility of each NS practice for large-scale application would strengthen the practical relevance of the findings.
Specific Comments
The abstract effectively summarizes the study but could be more concise. Consider reducing redundancy in reporting numerical improvements across different indicators. (Lines 11–32)
The introduction provides a solid background on nitrogen management challenges but could benefit from a clearer articulation of the knowledge gap. The statement, "How to reduce nitrogen fertilizer application while ensuring crop yield and environmental benefits has become a hot issue of global concern," could be supported by more recent references. (Lines 37–102)
Were the four organic materials applied in their raw form, or were any pre-processing steps (e.g., composting, drying) performed? The application method (e.g., surface application, incorporation into the soil) should be specified for clarity. (Lines 126–135)
The C/N ratios of the organic materials vary significantly. Were adjustments made to mitigate potential nitrogen immobilization effects in high-C/N materials like biochar? (Table 2)
The trends in nitrogen use efficiency across seasons (NUE trends) are informative but could be presented more clearly with standard error bars and post-hoc test results. (Figure 2)
The CB treatment shows a significantly lower nitrogen footprint than the control, but the biological explanation for this difference is not discussed in detail. Was biochar’s ability to enhance microbial nitrogen retention measured? (Table 6)
The discussion is well-structured but could benefit from a more explicit comparison with previous studies. Were similar reductions in nitrogen surplus observed in other maize cropping studies? The statement "Biochar’s highly porous structure effectively adsorbs nitrogen in the soil" is crucial but should be supported with quantitative evidence from literature or experimental results. (Line 444)
The conclusion summarizes key findings well but could highlight recommendations for future research, such as exploring higher substitution rates or investigating microbial responses to NS practices. (Lines 502–513)
Author Response
Please see the attachment
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsTitle of the article needs refinement, as it does not fully align with the research's objectives and tasks.
Clarification is needed in line 117: The study was conducted starting in 2020, but results were obtained only in 2022? An explanation must be provided.
In Figure 1, the authors did not specify the period for which the data are presented: Is this for one of the studied years, or is it averaged data across multiple years? Additionally, if data are presented, they should be described in detail, but the authors only provided a chart.
In the research methodology, the authors stated that synthetic fertilizers were replaced with organic ones. However, the decomposition rate of different organic fertilizers and their nutrient availability in the first and subsequent years does not amount to 100%. Fertilizer efficacy depends on the type of fertilizer, composition, application technology, and numerous other factors. Thus, replacing mineral fertilizers with organic ones at a rate of 300 kg/ha would not be equivalent across all organic fertilizers studied, and the total amount of available nitrogen would vary. Furthermore, adding mineral nitrogen to plant residues accelerates the decomposition of organic matter, thereby reducing its quantity.
In Section 2.2, the authors did not specify the type of nitrogen mineral fertilizer used.
A more detailed description of the raw material used for biogas production is required.
Section 2.2 lacks an explanation of soil tillage practices, which significantly influence organic matter decomposition processes and future root system development.
Clarification is needed: Why was corn harvested three times a year? (lines 157–158). Additionally, the rationale for evaluating yield based on grain (for sweet corn) requires explanation. Sweet corn is typically used fresh (for direct consumption or canning) or for producing dry grain.
The active root system of sweet corn penetrates the soil up to 0.5–0.7 m, but the authors analyzed soil samples only at a 0.2 m depth. An explanation for selecting this specific depth is necessary. Moreover, the soil tillage depth is not mentioned in preceding sections.
In the description of Table 4, the moisture content at which sweet corn grain was analyzed is unclear.
The Conclusion section requires improvement. This section should present specific, concrete results obtained in the study.
Author Response
Please see the attachment
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors made a consistent effort to improve the quality of the article and I do not have any further comments to add.
