Interactions between biochar and soil organic carbon decomposition: Effects of nitrogen and low molecular weight carbon compound addition

Conversion of plant residues to biochar is an attractive strategy for atmospheric CO2 emission mitigation and soil carbon (C) storage enhancement. However, our understanding of the factors controlling biochar persistence in soil is still limited, and generally based on biochar chemical recalcitrance. In addition to its chemical properties, biochar C decomposition might be limited by other factors, such as N and low molecular weight C compounds (LMW-C) availability. We presented results from a laboratory incubation experiment, where 4 atom% 13C enriched biochar was added at a rate of 10% by weight to two different soils, with or without N and LMW-C additions. Sucrose was used as the LMW-C, and its 13C natural abundance composition was manipulated, by mixing sucrose from sugar beet (C3) and from sugar cane (C4), to make it equal or dissimilar to the native soil organic carbon (SOC). This approach allowed partitioning the biochar-C from the 'soil + LMW-C' in the biochar amendment units and the LMW-C from SOC in the non-biochar amended units. Biochar-C, SOC and LMW-C were traced into CO2 throughout the 112 day incubation and into microbial biomass C (MBC) and dissolved organic C (DOC) at the end of the incubation. Compared to LMW-C addition, N was a more significant factor stimulating biochar-C mineralization and biochar-C incorporation in microbial biomass. Biochar significantly decreased SOC mineralization and this negative effect was not influenced by N and LMW-C additions. Biochar addition promoted formation of SOC derived DOC; however, DOC accumulated in soil rather than being mineralized to CO2. According to our study, biochar was confirmed to be highly resistant to decomposition, and N or LMW-C availability did not affect the overall soil C sequestration potential of biochar. Biochar effects on soil DOC dynamics will need to be farther investigated in the field to better constrain the overall C sequestration potential of biochar.