Effects of amendment of different biochars on soil carbon mineralisation and sequestration

The aim of this study was to determine the impact of addition of different biochars on soil carbon mineralisation and sequestration. Different biochars were produced from two types of feedstock, fresh dairy manure and pine tree woodchip, each of which was pyrolysed at 300, 500, and 700°C. Each biochar was mixed at 5% (w/w) with a forest loamy soil and the mixture was incubated at 25°C for 180 days, during which soil physicochemical properties and soil carbon mineralisation were measured. Results showed that the biochar addition increased soil carbon mineralisation at the early stage (within the first 15 days) because biochar brought available organic carbon to the soil and changed associated soil properties, such increasing soil pH and microbial activity. The largest increase in soil carbon mineralisation at the beginning of incubation was induced by the dairy manure biochar pyrolysed at 300°C. Soil carbon mineralisation was enhanced more significantly by the dairy manure biochars than by the woodchip biochars, and the enhancement effect decreased with increasing pyrolysis temperature. Although the biochar addition induced increased soil carbon mineralisation at the beginning of the incubation, soil carbon mineralisation rates decreased sharply within a short time (within 15 days) and then remained very low afterwards. Carbon mineralisation kinetic modelling indicated that the stable organic matter in biochars could be sequestrated in soil for a long time and resulted in high levels of carbon sequestration, especially for the woodchip biochars pyrolysed from higher temperatures.