Annual accounting of net greenhouse gas balance response to biochar addition in a coastal saline bioenergy cropping system in China

• Biochar use increased soil RH without reducing NPP over the cropping season.
• Biochar suppression effect on soil CH4 fluxes occurred when combined with N input.
• The effectiveness of biochar for reducing N2O emissions was weakened by N input.
• Biochar use improving ecosystem CO2 sequestration and GHGs sink strength.

The potential of biochar for mitigating climatic impacts of coastal saline bioenergy production is not well established. A full accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by biochar amendment combined with or without nitrogen (N) fertilizer application was examined in an annual coastal reclaimed Jerusalem artichoke bioenergy cropping system. The net ecosystem exchange of CO2 (NEE) was determined by the difference between soil heterotrophic respiration (RH) and net primary production (NPP) using static chamber method. Biochar amendment raised the seasonal RH but without suppressing the NPP during the Jerusalem artichoke cropping season. Soil CH4 emissions were 72% and 80% lower in the biochar amended than unamended plots when combined with N fertilizer application during the Jerusalem artichoke cropping and non-cropping seasons, respectively. The biochar-induced soil N2O mitigation efficiency was weakened by N fertilizer input over the annual cycle. Annual NGHGB and GHGI were negative for all the field treatments and were significantly lower in biochar amended than in unamended soils, suggesting that Jerusalem artichoke cropping system served as a net sink of GHGs due to net ecosystem CO2 and biochar-induced C sequestration exceeding CO2-equivalents released as CH4 and N2O emissions. On average, biochar amendment significantly enhanced GHGs sink capacity by resulting in almost 4–5 folds decrease in annual NGHGB or GHGI when combined with N fertilizer application or not. Therefore, higher biomass gain as potential alternative source of biofuels but lower climatic impacts of bioenergy production would be reconciled by biochar use in southeast coastal China.