Rice Functional Responses to Biochar and Straw Return in the Lower Yangtze Region Revealed by UAV-based Sun-Induced Chlorophyll Fluorescence

Biochar application and straw return are widely promoted as sustainable fertilization practices to enhance crop production, yet their impacts on growth processes, structural traits and physiological functioning remain insufficiently quantified, particularly from a canopy-scale perspective. During the 2025 rice-growing season, we conducted a field experiment in Yixing, located in the lower Yangtze River region, to investigate rice functional responses to biochar and straw return and to evaluate the capability of sun-induced chlorophyll fluorescence (SIF) to detect these responses.

The experiment included conventional fertilization as a control, three biochar application rates (0.10%, 0.50%, and 1.00%), and partial straw return. Biochar and straw return substantially enhanced root biomass (by 25–37%) and leaf area index (by 10–31%) across key growth stages, indicating improved resource acquisition capacity and canopy development. These belowground-driven changes translated into increased aboveground biomass accumulation, particularly before heading, and higher panicle density, contributing to yield formation. At the same time, biochar application increased canopy temperature and reduced leaf chlorophyll content, suggesting altered nitrogen distribution and canopy energy balance under intensified growth conditions. High biochar application reduced grain filling percentage, indicating that productivity gains are constrained by physiological regulation during reproductive stages.

To characterize canopy-scale functional dynamics, unmanned aerial vehicle (UAV) campaigns were conducted at jointing, heading, and grain-filling stages to acquire SIF observations. SIF showed strong sensitivity to management-induced differences in canopy structure, biomass accumulation, and phenological progression, consistently reflecting treatment effects across growth stages. Importantly, SIF captured both enhanced canopy function under moderate biochar and straw return and constrained physiological performance under excessive application, demonstrating its ability to integrate multiple plant functional responses.

Our results show that biochar and straw return regulate rice productivity through coordinated changes in root development, canopy structure, and physiological functioning. UAV-based SIF provides an effective, non-destructive approach to monitor these management-driven functional responses, offering new opportunities to link field experiments with larger-scale assessments of sustainable agricultural practices.