Pyrolysis temperature dependent effects of biochar on shifting fluorescence spectrum characteristics of soil dissolved organic matter under warming

Biochar generally shift the content and molecular composition of soil dissolved organic matter (DOM) which represent the reactive components and have essential roles in coupling elemental cycling in soil. However, it is not clear how the effects of biochar on soil DOM composition is shifted under warming. This causes a knowledge gap to fully understand the fate of SOM affected by biochar application in a warming climate. To fill this gap, we conducted a simulated climate warming incubation of soil to study the influence of biochar with different pyrolysis temperatures and feedstock types on soil DOM components composition. Three-dimensional fluorescence spectrum analysis combining excitation emission matrix-parallel factor analysis (EEM-PARAFAC), fluorescence region integral (FRI), UV–vis spectrometry, principal component analysis (PCA), clustering analysis, Pearson correlation and multi-factor analysis of variance on fluorescence parameters (including FRI on Region I-V, FI, HIX, BIX, H/P) and soil DOC and DON content were analyzed for this purpose. Results showed that biochar shifted soil DOM composition and enhanced soil humification, which was dominantly pyrolysis-temperature dependent. Biochar shifted the soil DOM components composition probably through mediating soil microbial processing rather than direct input of their pristine DOM, and the influence of biochar on soil microbial processing was pyrolysis-temperature dependent and highly affected by warming. Medium-temperature biochar was more efficient for enhancing soil humification, by accelerating the transformation of protein-like components into humic-like components. Soil DOM composition presented a rapid response to warming, and long-term incubation may eliminate the effects of warming on shifting soil DOM composition. By revealing the heterogeneous effects of biochar with different pyrolysis temperatures on fluorescence characteristics of soil DOM components, our study provides a hint for the essential role of biochar on enhancing soil humification, and also suggests a vulnerability of biochar for soil carbon sequestration under warming.