Priming effects in biochar enriched soils using a three-source-partitioning approach: 14C labelling and 13C natural abundance

The changes to soil properties due to biochar addition may affect both the direction and magnitude of priming effects. However, the mechanisms involved in biochar induced priming effects still remain largely unknown due to the limitation of methods to separate more than two carbon (C) sources (e.g. soil, biochar, substrate). We combined 14C labeling with 13C natural abundance to separate the total CO2 from i) native soil organic C (SOC, C3 signature), ii) added glucose (14C labelled) and iii) biochar (C4 signature). The primed soil CO2 emissions following a large addition of glucose (1000 mg glucose kg−1 soil) to one Chinese and one German Luvisol soil were much larger (140% and 53% respectively) in a soil recently amended with maize derived biochar (pyrolyzed at 400 °C), compared to non amended soil. Glucose addition at a lower rate (100 mg C kg−1 soil) produced no significant differences in priming effects of native soil organic matter between the biochar amended and non-amended soils. Glucose also caused priming of biochar decomposition, with an additional C4 biochar loss of between 270 μg CO2-C g−1 and 540 μg CO2-C g−1 depending on soils and glucose concentrations. Approaches using two stable isotopes (13C and12C) have previously been limited to partitioning two sources (biochar C and soil organic C). Here, for the first time, 14C labeling was combined with 13C natural abundance to partition three C sources in a biochar amended soil. By partitioning soil CO2 emissions derived from SOC, added biochar and glucose decomposition, this study provides a better understanding of the priming effects following addition of substrates to biochar amended soil, to approximate to the true complexity of biochar enriched soils.