Urban greenspaces provide critical ecosystem services, including carbon storage, nutrient cycling, and public recreational space. However, rapid urbanization continuously threatens these habitats by depleting soil organic matter, disrupting nutrient cycles, and degrading overall soil fertility. To counter these impacts, land managers frequently apply protective soil amendments such as biochar and municipal green waste compost. While these treatments are widely recognized for improving agricultural soils, their efficacy across heterogeneous urban landscapes remains poorly understood. Biochar offers long-term carbon stabilization but can suffer from high production costs and high carbon-to-nitrogen ratios that induce temporary plant nitrogen limitations. Conversely, compost supplies essential nutrients but can trigger elevated nitrous oxide emissions or nutrient leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More if applied improperly. Understanding how initial soil conditions dictate amendment success is necessary for optimizing urban soil management.

To uncover the biological mechanisms controlling these soil responses, the research team conducted a field experiment across three contrasting urban greenspaces in Beijing, China. The selected sites included a nutrient-poor campus soil at Peking University, an intermediate-nutrient park soil at Haidian Park, and a nutrient-rich residential soil at Changchun Garden. Plots at each site received single or combined additions of kitchen-waste biochar and garden-waste compost at a rate of one kilogram per square meter. Over the course of the experiment, the researchers monitored soil organic carbon, total carbon, total nitrogen, available nutrients, and soil carbon dioxide emissions. Additionally, high-throughput DNA sequencing of bacterial and fungal communities was performed to examine changes in microbial richness, life-history traits, network stability, and community assembly mechanisms across the different soil fertility gradients.

The findings revealed a striking contrast in how baseline soil fertility modulates amendment outcomes. Biochar and compost additions significantly enhanced soil organic carbon, total carbon, and total nitrogen contents, but the magnitude of these gains was up to 14.4-fold greater in nutrient-poor soils compared to nutrient-rich soils. In depleted soils, amendments elevated fungal richness, increased the fungal-to-bacterial richness ratio, and strengthened fungal network connectivity and stability. Fungal lineages, particularly saprotrophic and plant-interactive groups, facilitated carbon build-up and nutrient retention. Conversely, applying amendments to the nutrient-rich soil yielded minimal or even negative impacts on carbon and nitrogen storage. In fertile soils, the nutrient surplus favored fast-growing bacteria with high ribosomal RNA operon copy numbers. This surge in bacterial activity accelerated native carbon mineralization and carbon dioxide flux, while fungal diversity, network connectivity, and network robustness dropped significantly.

Partial least squares path modeling confirmed that fungal community traits and network structures were the primary drivers mediating carbon accrual and soil fertility enhancements. While bacterial communities responded primarily through stochastic assembly processes, fungal communities were governed by deterministic environmental selection. In nutrient-deficient urban soils, boosting fungal abundance and network stability allows microbes to process organic inputs efficiently without spurring excessive respiratory carbon loss. In contrast, adding excess organic matter to already fertile soils disrupts fungal networks, triggering bacterial-driven carbon consumption that destabilizes existing soil carbon pools. Rather than applying amendments uniformly across all urban parks and gardens, municipal restoration strategies must prioritize nutrient-depleted sites where ecological and carbon sequestration returns are highest.

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, Article 85.