Biochar application can decrease urban trees’ defect rate and mortality after transplanting by optimizing soil water condition

Reducing tree defects after transplanting is essential for sustainable urban greening. As urban areas face increasing challenges from droughts and flooding, optimizing soil water conditions to support the successful establishment of newly planted trees has become increasingly important. While biochar is widely recognized as an effective strategy for enhancing soil water conditions, its long-term effects on soil pore structure, aggregate modification, and their fundamental impact on defect rate and mortality in transplanted trees remain poorly explored. To address this knowledge gap, we hypothesized that applying biochar to the subsoil with newly planted trees would decrease tree defect rate and mortality by improving the micro-aggregate formation and pore size distribution. To investigate how biochar decreases plant defect rate and mortality, we conducted a four-year field experiment by planting 30 six-year-old trees of representative urban roadside species each for the control and biochar treatment, respectively. For biochar treatment, the wood chip biochar of 4% by soil mass was applied at two subsoil depths (15–25 cm and 30–45 cm). Soil samples were collected by depth (0–15, 15–30, 30–60 cm) to analyze soil aggregates, pore distribution, and root biomass. Especially, additional soil samples for synchrotron-based X-ray computed microtomography (μCT) analysis were taken from a section (25–30 cm) between the two biochar layers. The volumetric soil water contents (SWC) by depth, tree defect rates, and mortality were continuously monitored.

Our results showed that biochar treatment halved both the tree defect rate in the first growing season and the final tree mortality after four years compared to the control. As plants require abundant water for initial establishment, the initial tree defect rate in the biochar treatment could be lower due to biochar maintaining SWC within an adequate range, particularly during the dry season (March to May). This is supported by a larger plant-available water (PAW) range in the biochar treatment, driven by an increased proportion of micro-aggregates (53–250 µm) and the corresponding rise in micro-pore volume (0.2–9.1 µm). We expect that μCT images will support our findings. The long-term tree mortality is likely influenced by the resilience and rehabilitation ability to extreme climate events. As our field site was affected by the monsoon climate zone, it experienced periodic extreme flooding during observation. The days when SWC exceeded field capacity were 5.6 times higher in the control than in the biochar treatment. This is reflected in the marginally higher dry biomass observed in both coarse roots (>2 mm) and fine roots (<2 mm) in the biochar treatment compared to the control. This indicates the biochar treatment could enhance root resilience, alleviating flood-induced water stress. Further analysis of nutrients, including inorganic nitrogen and phosphorus, will help determine whether nutrient dynamics also contributed to improved tree survival and root development. Our results provided the evidence that subsoil biochar application improves early tree establishment and resilience to extreme climate events, thereby enhancing the long-term survival rate for newly planted trees.