Biochar mineralization and priming effect in a poplar short rotation coppice from a 3-year field experiment

The present study assesses the stability of biochar, its effect on original soil organic matter (SOM) decomposition, and the interactions with plant roots over a 3-year period in a short rotation coppice plantation in Northern Italy. Biochar produced from gasification of maize silage (δ13C of biochar ≈ - 13.8‰) was applied into the soil of the plantation (δ13C of SOM ≈ -23.5‰). Total and heterotrophic respirations were measured in control and biochar-treated plots, and the amount of biochar-derived carbon dioxide (CO2) was calculated using an isotopic mass balance method. Biochar mean residence time (MRT) was assessed using exponential decay models. The remaining amount of biochar at the end of the experiment was estimated by a soil isotopic mass balance. In the absence of plant roots, MRT of the more recalcitrant biochar fraction (96% of the total) was 24.3 years, significantly lower than expected from previous shorter-term studies or laboratory incubations. In the presence of plant roots, MRT decreased to 12.6 years, confirming the previously observed positive effect of roots on biochar decomposition. However, in the absence of roots, the biochar decreased the decomposition of original SOM by 16%, indicating long-lasting protective effect on SOM (negative priming effect). The soil isotopic mass balance suggested that part of the applied biochar could have been lost by downward migration. This study provides the first estimation of the biochar stability and priming effect on SOM in field conditions in the medium term0.