Effects of Biochar Amendment on GHG Emissions from Tropical Agricultural Soils in Hawai‘I.

Amendment of agricultural soils with biochar not only sequesters atmospheric carbon (C), but may also have potential to decrease soil greenhouse gas (GHG) emissions, thereby mitigating global warming potential (GWP) and climate change. Additionally, biochar amendment may affect crop yields by changing soil fertility. In the tropics, the effect of biochar amendment on GHG fluxes and net GWP is of high research priority and possesses notable local relevance to agriculture in Hawai‘i. To investigate effects on GHG fluxes in Hawaiian agriculture, field trials of biochar amendment in a fertile Mollisol and an infertile Oxisol were carried out with no-till management of bioenergy feedstock (napier grass, var. bana) and conventional tillage of a food crop (sweet corn and cowpea rotation). Gas flux measurements included CO₂, N₂O, and CH₄. Results show that biochar amendment affected GHG fluxes differently between soils and similarly between crop managements. In the Oxisol, biochar amendment increased cumulative fluxes of CO₂ 9%-14% and N₂O 62%-153%; in the Mollisol, biochar decreased N₂O fluxes 28% in the corn and caused no significant change in the no-till napier grass. Generally, biochar amendment reduced CH₄ sink-capacity in all systems by 31% to 55% except the Oxisol corn crop (50% increase). Cumulative annual GHG emissions will be included in a joint assessment of net GWP and economic feasibility, including crop yield effects, as an indicator of overall system sustainability. This work will assess the feasibility of biochar amendment at a scale relevant to tropical agriculture in Hawai‘i while addressing global problems of climate change and food security.