Effects of aging under field conditions on biochar structure and composition: Implications for biochar stability in soils.

The effects of aging on biochar (BC) properties, composition and carbon sequestration are still under debate. This study aimed at illustrating the qualitative alterations of five different BCs aged during a 24-month field experiment located in Southwest Spain. To determine the recalcitrance of each BC, physical fragmentation test, scanning electron microscopy, C NMR spectroscopy and CO-respiration experiments were performed. The physical fragmentation values of all types of BC increased significantly over time at field conditions. FESEM examinations of aged BCs showed collapsed structures and the presence of entrapped soil material and microbial mats into the BC pores. The C NMR spectroscopy demonstrated an increase of the relative abundance of O-alkyl C and alkyl C at expenses of aromatic-C in aged BCs. The C losses of all BCs ranged from 27% to 11% of the initial C. In contrast, the nitrogen (N) content of wood-derived BCs significantly increased probably due to the sorption of nitrogen containing compounds into these highly-porous weathered chars. With the exception of that for the sewage sludge-BC, the pH of all aged BCs decreased from >9 to the soil pH, indicating a short lasting of the liming effect caused by BC addition. The respiration experiment revealed that BC recalcitrance was much lower than expected and, within the range of decades. Only wood-derived BCs significantly increased the mean residence time of the slow C pool of the Cambisol by factors between 3.4 and 7.7. Mediterranean climate conditions and the characteristics of the Cambisol used probably accelerated the microbial degradation of BCs.