Non-additive effects of bamboo-derived biochar and dicyandiamide on soil greenhouse gas emissions, enzyme activity and bacterial community

Carbon dioxide (CO2) and nitrous oxide (N2O) are important greenhouse gases, with large amounts released from agriculture and forest ecosystems. Bamboo-derived biochar and dicyandiamide (DCD) have the potential to reduce soil CO2 and/or N2O fluxes. However, the interactive effects of bamboo-derived biochar and DCD on soil gas emissions, enzyme activities, and bacterial communities remain unclear. To reduce this knowledge gap, four treatments were arranged in this laboratory incubation experiment: (1) blank control (CK); (2) 3% (w/w) bamboo-derived biochar addition (BA); (3) dicyandiamide addition (DA); and (4) combined bamboo-derived biochar and dicyandiamide addition (CBDA). During the 63-day incubation, the bamboo-derived biochar and DCD applications decreased cumulative CO2 emissions by 30% and 43% and cumulative N2O emissions by 69% and 70%, respectively. However, the combined application of bamboo-derived biochar and DCD had antagonistic effects on the inhibition of N2O emissions. Compared with CK, bamboo-derived biochar or DCD application significantly reduced the soil protease activity and the nodes and edges of bacterial community co-occurrence networks. The combined applications of bamboo-derived biochar and DCD also had antagonistic effects on the inhibitions of invertase and protease activities and the complexities of bacterial co-occurrence networks. The Simpson and ACE indices of bacterial communities increased following bamboo-derived biochar application, alone or with DCD. The bamboo-derived biochar or DCD application generated "double-edge sword" effects of decreasing cumulative CO2 and N2O emissions but damaging the bacterial network complexity. The feasibility of applying bamboo-derived biochar and DCD for the mitigation of greenhouse gas emissions from soils should weigh the relative importance between the reduction of greenhouse emissions and the protection of soil bacterial communities.