Temperature-Driven Trade-Offs Between Carbon Stability and DTPA-Extractable Micronutrients in Vineyard-Pruning Biochars (NW Spain)

Sustainable management of vineyard residues through biochar production requires balancing carbon stability with agronomically relevant nutrient functionality. Pyrolysis temperature controls this trade-off by affecting carbon condensation and micronutrient availability. This study aimed to determine how pyrolysis temperatures (300 and 600 °C) govern this trade-off in vineyard-trimming biochars. The motivation focuses on optimizing carbon storage while maintaining micronutrient availability. Biochars were produced by slow pyrolysis at 300 and 600 °C for 1 h and characterized using proximate and elemental analyses, total macro- and micronutrient determination, and DTPA extraction to evaluate potentially bioavailable trace elements. The results showed that increasing temperature from 300 to 600 °C reduced yield (45.15 to 32.30%) and volatile matter (40.33 to 16.50%), while increasing fixed carbon from 55.37 to 77.33% and total carbon from 66.49 to 77.89%. Atomic ratios (H/C: 0.67 to 0.31; O/C: 0.32 to 0.18) confirmed enhanced carbon condensation at 600 °C. Regarding nutrients, although total Mn, Fe, Cu, and Zn concentrations declined at higher temperatures, their potentially bioavailable fractions (operationally defined as extractable with the chelating agent DTPA showed element-specific redistribution; Fe, Cu, and Zn extractability increased, while Mn decreased. These findings reveal a temperature-driven trade-off between carbon sequestration and micronutrient release.