Temperature sensitivity and priming of organic matter with different stabilities in a Vertisol with aged biochar

Understanding the temperature sensitivity (Q10) of carbon (C) mineralization and priming of organic matter with different stabilities in a soil with aged biochar is required to enable better forecasting of biochar C sequestration potential under a warming climate. Here, we quantified the Q10 and priming of C mineralization in a Vertisol from: (i) newly added labile organic matter (LOM) in the presence of “aged biochars”; and (ii) stable (“aged”) native soil organic matter in the presence of aged biochars or new LOM. We also quantified the Q10 of aged biochar-C (BC) or aged soil organic carbon (SOC)+BC mineralization. Leaf litter from Eucalyptus saligna (a source of LOM) was applied at 4% w/w (δ13C −38‰) to a Vertisol (δ13C −14‰), containing either wood, leaf or poultry litter biochar (δ13C −25 to −28‰), and nil biochar (control soil), previously incubated for 4 years. These biochar−soil mixtures and the control soil, with or without LOM, were re-incubated at 10, 20, 30 and 40 °C for 252 days. The results showed that 22–39% of LOM-C, 0.10–2.81% of aged BC and 2.4–77.0% of “aged SOC” mineralized across all temperatures over 252 days. The Q10 of C mineralization increased with decreasing quality of C substrates in the soil, that is, LOM (1.17–1.21) < SOC (1.23–1.66), SOC + BC (1.23–1.60) < aged BC (1.92–2.26). Positive priming of SOC mineralization was greater by LOM (cf. aged biochar), causing a significant decrease in the SOC Q10 at all temperatures. The aged biochars resulted in negative priming of LOM-C mineralization, mainly at 10 °C, with no impact on the LOM Q10. The results suggest that global warming and tropical climates may lower the C sequestration potential of biochar, by reducing its capacity to slow the mineralization of LOM-C, while increasing the mineralization of native SOC.