A recent field study published in the journal Biochar by authors Sihang Deng, Qun Gao, Ling Han, Xin Tong, Wenrui Shen, Anqi Liu, Hongkwan Lee, Zhencheng Ye, Suo Liu, Ke Sun, Xinghui Xia, and Yunfeng Yang reveals that the baseline fertility of urban green spaces determines how soil responds to organic additions. Urban green spaces face significant threats from organic matter depletion and fertility decline caused by rapid urbanization. While land managers widely implement protective measures such as biochar and compost, the effectiveness of these treatments has historically remained poorly understood across different urban landscapes. By testing single and combined applications of kitchen waste biochar and garden waste compost across three distinct city parks in Beijing with varying baseline nutrient levels, the researchers uncovered that the initial background status of the soil dictates the final ecological outcome.

The investigation demonstrates that applying organic amendments yields the most significant improvements in soils that start out starved of vital nutrients. The researchers measured changes in soil organic carbon, total carbon, and total nitrogen following the field treatments. They discovered that the positive effects of the interventions were up to 14.4-fold stronger in nutrient-poor park soils compared to nutrient-rich soils. In contrast, the nutrient-rich sites responded poorly, showing the weakest overall changes. Unexpectedly, the combined application of biochar and compost even caused a decline in total carbon and total nitrogen at the wealthiest soil site, meaning that over-applying amendments to already fertile urban grounds can counteract conservation goals.

The underlying biological reason for this stark difference comes down to a structural shift in how underground microbial communities behave after receiving amendments. In the nutrient-poor environments, the additions triggered a strong increase in fungal diversity, boosting the overall ratio of fungi to bacteria. The fungal networks in these poorer soils became significantly more connected and stable over time, allowing them to efficiently lock away carbon and nitrogen into the ground. On the flip side, the nutrient-rich soils reacted to the surplus of organic material by shifting toward rapid bacterial growth. This explosion of bacteria decreased the presence of helpful fungi, and the bacteria quickly consumed the available carbon, accelerating carbon emissions and destabilizing the existing soil carbon storage pools.

Furthermore, the study identified specific groups of microorganisms that act as direct indicators of these opposing environmental outcomes. Certain groups of fast-growing bacteria were linked to higher carbon dioxide emissions and lower nitrogen retention. Conversely, specific types of fungi, including certain plant pathogens, actually helped accumulate and keep nutrients in the soil, likely by slowing down the rate at which above-ground plants drew nutrients out of the earth. Fungal communities ultimately showed a far greater number of varieties directly linked to positive soil storage outcomes than bacterial communities did. This confirms that fungi serve as the essential biological engines driving successful soil restoration and long-term fertility in city environments.

These discoveries provide city planners and land managers with a practical conceptual framework for managing urban soils more effectively. Rather than applying a uniform strategy across all city parks, soil management must become precise and tailored to local conditions. Because the organic amendments showed minimal changes across different seasons, the improvements made to soil health and carbon capture remain durable over the long term. Ultimately, the research proves that governments and urban foresters must actively prioritize highly depleted, nutrient-poor municipal green spaces for biochar and compost interventions, ensuring that limited resources are directed where they will deliver the maximum possible ecological payoff.

Source: Deng, S., Gao, Q., Han, L., Tong, X., Shen, W., Liu, A., Lee, H., Ye, Z., Liu, S., Sun, K., Xia, X., & Yang, Y. (2026). Fungi enhance biochar and compost effects on carbon accrual in nutrient-deficient urban greenspace soils. Biochar, 8(85).