Consistent positive response but inconsistent microbial mechanisms of absorptive root litter-induced priming effect to warming at different decomposition stages

The fresh carbon (C) induced priming effect (PE) on soil organic C (SOC) decomposition is critical for global C cycling. Climate warming could raise absorptive roots production and turnover, and then increase the input of absorptive roots litter (ABRL). Therefore, it is urgent to understand the PE induced by ABRL under warming. We conducted a 210-day experiment by adding ABRL of Cunninghamia lanceolata into a C₄ soil and incubating them at 19 °C and 23 °C. We found that adding ABRL caused positive PE throughout the incubation. At the early stage (ES: first 30 days), labile C compounds dominated the decomposition of ABRL, significantly higher dissolved organic C (DOC), microbial biomass C (MBC), and absolute hydrolase activities were found in the ABRL treatment than in the control. These results supports that labile C inputs stimulating microbial growth, enzyme activities and cause positive PE via co-metabolic. At the later stage (LS: after 180 days), the release of structural C compounds dominated the decomposition of ABRL, a significantly lower available nitrogen (N) and a significantly higher specific potential N-acquisition (N_acq) enzymes were found in the ABRL treatment than in the control. These results suggests that microbes utilizing the C as energy to increase N_acq enzymes to decompose SOC for N mining under N limitation induced positive PE. Warming significantly increased the PEs at both stages. At the ES, warming increased the MBC at the cost of DOC, suggesting that warming intensifies the microbial co-metabolism. At the LS, warming significantly decreased the available N and increases the absolute potential oxidases activities, suggesting an increased N limitation and oxidation for N-rich recalcitrant SOC, i.e., a promoted microbial N mining. Nevertheless, we did not observe a significant effect of ABRL addition on the temperature sensitivity of SOC decomposition compared to the control. This study provides a valuable insight that warming could consistently increase the ABRL induced PE but through different microbial mechanisms along with the decomposition processing.