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RESULTS

Biochar Application Mitigates GHG Emissions in Urban Forest Soils

We find that with an increasing gradient of biochar application (0 t/ha, 20 t/ha, 40 t/ha), there is a significant enhancement in the absorption of greenhouse gases and a reduction in their emissions. In Figure 8, the CO₂ flux analysis from July to October shows that higher biochar treatments correlate with increased CO₂ uptake, potentially due to improved soil quality and enhanced photosynthetic activity among treated plants. The peak in CO₂ flux during August across all treatment levels could be reflective of maximal biological activity during this period, further amplified by biochar application.

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The methane flux also responds positively to increased levels of biochar application, with the 20 t/ha and 40 t/ha treatments showing a more pronounced reduction in methane emissions compared to the control (Figure 8). This suggests that biochar may modify soil properties in a way that promotes methane consumption by microbial communities or alters soil moisture conditions favorably. The trend towards lower emissions or higher uptake in the later months, particularly under the 40 t/ha treatment, aligns with the seasonal decline in temperatures, indicating a sustained impact of biochar on methane dynamics under varying environmental conditions. This evidence underscores the potential of biochar as a strategy for enhancing greenhouse gas sequestration in forest ecosystems.

Figure 8. Temporal variation of CH4 and CO2 fluxes in response to biochar treatment levels in urban forestry

Limited Influence of Biochar on Short-term Tree Growth Metrics Across Multiple Application Rates

The analysis of tree growth metrics over three months (July, August, and September) across different biochar application levels (0 t/ha, 20 t/ha, 40 t/ha) revealed that biochar application does not significantly affect tree growth in terms of height, diameter at breast height (DBH), crown diameter, and base to crown measurements. The height and DBH of trees showed minimal variation, maintaining consistent values across all treatment levels throughout the study period. Similarly, measurements from the base to the crown displayed no substantial changes, indicating that vertical growth and overall tree stature were not significantly influenced by the application of biochar (Figure 9).

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However, there was a slight trend towards increased crown diameter in the treatments with 20 t/ha and 40 t/ha of biochar, particularly by September. This suggests a potential for biochar to facilitate radial growth, possibly due to improved soil conditions such as increased nutrient availability or enhanced soil structure. Despite these observations, the changes were not statistically significant, and the overall influence of biochar on the growth attributes appears to be subtle within the timeframe of this study.

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Given the lack of significant findings, the results highlight the need for further research to elucidate the longer-term and more comprehensive impacts of biochar on forest growth dynamics. Future studies should explore the effects of biochar on different soil types and under various climate conditions to better understand its role in forestry and urban greening initiatives. This study underscores the importance of cautious interpretation of biochar benefits and the necessity for extended observation periods to capture its potential effects on tree physiology and growth.

Figure 9. Impact of biochar treatment on tree performances over time in urban forestry

Biochar Application Absorbs Soil Ammonium Nitrogen in Urban Forestry

In Figure 10A, the results for NH4+ concentrations show a clear upward trend with increased levels of biochar, especially in the higher application rates of 20 t/ha and 40 t/ha, which are statistically significant with p-values less than 0.01 in August and September. This suggests that biochar application at these levels can significantly enhance the soil's ammonium content, which could be indicative of biochar’s influence on nitrogen retention or ammonification processes in the soil.

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Figure 10B, detailing NO3- concentrations, indicates a different pattern where there are no significant differences across the treatments over the months, as denoted by the p-values greater than 0.05. The levels of nitrate do not show a consistent increase or decrease that correlates with the biochar application rates. This could imply that while biochar impacts ammonium levels, its effect on nitrates is less pronounced or that the nitrification process (conversion of ammonium to nitrate) might be inhibited or unaffected by biochar.

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These findings suggest that biochar has a differential impact on the forms of nitrogen in the soil, enhancing ammonium levels significantly while not altering nitrate levels in a notable way. This might be beneficial for agricultural practices that require enhanced nitrogen retention in the form of ammonium. However, the lack of impact on nitrate levels also points to the need for further study to understand the complex interactions of biochar with soil nitrogen cycles and its implications for soil fertility and plant nutrition over longer periods and in different soil types.

Figure 10. Ammonium and nitrate nitrogen concentration in different months under varying biochar application rates

The Figure 11A shows a noticeable increase from June to September across all treatments, with the highest observed increase in the 40 t/ha treatment. However, statistical analysis indicates that the differences between treatments are not significant (P>0.05), suggesting that while there is an apparent increase in microbial biomass carbon with higher biochar applications, these changes are not statistically robust at this stage of the research.

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Figure 11B represents the MBN levels, which appear relatively stable across both months and treatments. Similar to the MBC results, there is no significant difference between the biochar treatments in terms of microbial biomass nitrogen (P>0.05). The data indicate that biochar application does not substantially affect the MBN levels within the two months observed.

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The observations suggest a potential for biochar to impact soil microbial biomass, particularly carbon content, although the changes are not statistically significant with the current dataset. This may indicate a delayed or gradual effect of biochar on microbial communities, which could become more apparent with longer-term studies. Further research is required to determine the full impact of biochar on soil microbial biomass carbon and nitrogen, ideally through extended monitoring to capture seasonal variations and long-term trends. This would help clarify whether the initial trends observed here are sustained over time and how they might influence soil health and fertility in the context of biochar application rates.

Figure 11. Microbial Biomass Carbon (MBC) and Microbial Biomass Nitrogen (MBN) in different months under varying biochar application rates

CONCLUSION

In our comprehensive investigation into the effects of biochar derived from urban forestry waste, we observed nuanced outcomes across various metrics including soil properties, tree growth, and greenhouse gas dynamics. While there were subtle enhancements in nutrient availability and microbial biomass, these changes were not consistently statistically significant, highlighting a complex interaction between biochar and soil dynamics that does not translate uniformly into dramatic improvements.

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Our findings regarding urban tree growth indicated slight, non-significant increases in growth metrics such as diameter at breast height and crown diameter with higher rates of biochar application. These results suggest that while biochar may contribute to some aspects of radial growth, its impact on overall tree height and structural development is limited within the short duration of our study.

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Notably, our research demonstrated a significant influence of biochar on reducing greenhouse gas emissions, particularly with a marked increase in CO₂ absorption and a reduction in methane emissions as biochar application increased. These results were among the more pronounced effects observed, suggesting that biochar can enhance greenhouse gas sequestration. However, the overall magnitude of these reductions was relatively small, and the implications of these results for broader urban forestry practices remain uncertain.

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Given the mixed outcomes observed in this study, further research is essential to assess the long-term and scaled-up impacts of biochar in urban forestry settings. Although biochar shows potential for improving certain environmental metrics and contributing to greenhouse gas mitigation, the modest scale of these benefits and the variability in response across different parameters underscore the need for additional investigation. This is crucial to determine whether the strategic incorporation of biochar is justified and economically viable as a widespread practice in urban forestry and environmental management. Thus, while biochar presents certain advantages, its role as a cornerstone in urban ecological sustainability is yet to be firmly established, warranting a cautious and well-researched approach to its application.

NEXT STEPS

  • Extended Monitoring and Data Collection:

    • Duration and Scope: Conduct a longitudinal study over multiple growing seasons to capture seasonal variations and long-term effects of biochar on greenhouse gas dynamics, soil health, and tree growth.

    • Metrics: Focus on measuring soil microbial biomass, nutrient availability, pH levels, organic carbon content, tree growth parameters (height, diameter), and greenhouse gas fluxes (CO2 and CH4).

  • Scale of Biochar Application:

    • Experimental Design: Implement trials with varied biochar application rates (e.g., 0 t/ha, 20 t/ha, 40 t/ha) across multiple urban forestry sites to assess the impact at different scales and in different urban forest ecosystems.

    • Controlled vs. Natural Settings: Compare results from controlled (experimental plots) and natural (real-world urban forests) settings to evaluate practical applicability and ecological impacts.

  • Net Carbon Emissions Calculation:

    • Material Source Analysis: Investigate the carbon footprint of producing biochar from urban forestry waste, including collection, processing, and transportation.

    • Lifecycle Assessment: Perform a comprehensive lifecycle assessment of biochar to quantify net carbon savings, considering both the reduction in greenhouse gases and the emissions involved in biochar production and application.

  • Integration with Urban Waste Management:

    • Waste Stream Analysis: Collaborate with municipal waste management services to accurately quantify urban forestry waste streams and potential biochar production capacity.

    • Policy Impact Study: Analyze how biochar use can be integrated into existing urban waste management policies and the potential for policy-driven incentives.

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