Effects of slow and fast pyrolysis biochar on soil C and N turnover dynamics

This study compared the effect of two principal pyrolysis methods on the chemical characteristics of biochar and the impact on C and N dynamics after soil incorporation. Biochar was produced from wheat straw that was thermally decomposed at 525 degrees C by slow pyrolysis (SP) in a nitrogen flushed oven and by fast pyrolysis (FP) using a Pyrolysis Centrifuge Reactor (PCR). After 65 days of soil incubation, 2.9% and 5.5% of the SP- and FP-biochar C, respectively, was lost as CO2, significantly less than the 53% C-loss observed when un-pyrolyzed feedstock straw was incubated. Whereas the SP-biochar appeared completely pyrolyzed, an un-pyrolyzed carbohydrate fraction (8.8% as determined by acid released C6 and C5 sugars) remained in the FP-biochar. This labile fraction possibly supported the higher CO2 emission and larger microbial biomass (SMB-C) in the FP-biochar soil. Application of fresh FP-biochar to soil immobilized mineral N (43%) during the 65 days of incubation, while application of SP-biochar led to net N mineralization (7%). In addition to the carbohydrate contents, the two pyrolysis methods resulted in different pH (10.1 and 6.8), particle sizes (113 and 23 mu m), and BET surface areas (0.6 and 1.6 m(2) g(-1)) of the SP- and FP-biochars, respectively. The study showed that independently of pyrolysis method, soil application of the biochar materials had the potential to sequester C, while the pyrolysis method did have a large influence on the mineralization-immobilization of soil N. (C) 2011 Elsevier Ltd. All rights reserved.