Moisture conditions trigger different response patterns of soil respiration to biochar-induced changes in soil vertical water content and temperature based on a three-year field observation study

Abstract

Biochar impacts on soil respiration (R_s) remain uncertain, particularly in dryland regions where significant CO₂ emissions result from rewetting. To examine these impacts, we conducted a three-year field experiment during the millet growing season to investigate the response patterns of R_s to biochar-induced changes in vertical soil temperature (T_s) and volumetric water content (VWC). Before R_s observation, the experimental site underwent three years of biochar amelioration (no planting) with five application rates of 1 %, 2.5 %, 4 %, 5.5 % and 7 % (BC1, BC2.5, BC4, BC5.5 and BC7, applied to the 0–20 cm soil layer). R_s was monitored within the 0–20 cm soil layer while T_s and VWC were measured simultaneously at soil depths of 5, 10 and 20 cm (T_s5, T_s10, T_s20, VWC₅, VWC₁₀ and VWC₂₀). Moisture status within the R_s measurement range was partitioned by 0.093 m³ m⁻³ (the optimal VWC for R_s) for non-biochar amended soil (control). Overall, compared with the control, BC4, BC5.5 and BC7 significantly increased R_s (33.3–63.5 %) and experienced water stress earlier at a soil depth of 5 cm. In addition, high moisture levels caused significant differences in R_s among treatments. In terms of the relationship between R_s and T_s, the Gaussian-T_s model performed better than exponential-T_s in control only at a soil depth of 5 cm under low moisture conditions. For biochar treatments, R_s did not continue to rise with increasing T_s for BC1 and BC2.5 treatments under low moisture conditions and for BC5.5 and BC7 treatments under high moisture conditions across soil depths. With biochar application rate, R_s was dominantly shaped by VWC₂₀ and T_s20 under low moisture conditions, while it was significantly influenced by T_s under high moisture conditions. These findings elucidate how R_s responds to biochar-induced changes in vertical T_s and VWC across moisture levels, providing valuable insights for comprehensively evaluating the environmental effects of biochar-amended soil in dryland areas.