The effects of separate and combined ground rock, compost, and biochar amendments on soil C sequestration and greenhouse gas emissions from grassland soils

Mitigation strategies combining negative greenhouse gas emission pathways alongside substantial emissions reductions are needed to limit the effects of climate change. Soil ground rock, compost, and biochar amendments to grasslands have been proposed as a strategy to mitigate climate change through organic carbon (C) sequestration and inorganic C sequestration via enhanced mineral weathering. Soil amendment effects on soil biogeochemistry, C stocks, and soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions when applied concurrently remain uncertain, limiting their viability as a scalable climate change mitigation strategy. Over three growing seasons in a California rangeland, we compared the effects of combinations of compost, ground rock, and biochar amendments on soil greenhouse gas emissions, organic and inorganic C stocks, aboveground net primary productivity (NPP), and soil N cycling. Preliminary results from continuous greenhouse gas measurements suggest ground rock lowers N2O emissions up to 11 kg CO2e ha-1 yr-1 and increases the CH4 sink -10.2 to -18.4 kg CO2e ha-1 yr-1. Ground rock (-77 kg C ha-1) and compost (-107 kg C ha-1) treatments decreased inorganic C stocks in the top 20 cm (p < 0.02), suggesting an increase in leached inorganic C stocks. After two growing seasons treatments with biochar and compost amendments increased NPP relative to ground rock applications alone (p < 0.03). These results, when combined with previous findings, suggest combinations of these soil amendments may lead to increased inorganic C leaching, increasing organic soil C sequestration, and reduced greenhouse emissions.