Rowan Eisner, Leonie Seabrook, Clive McAlpine
Biodiversity is threatened in a post-carbon future due to the expansion of agriculture resulting from reduction in the use of petrochemical-based fertilisers. Here we prioritise alternative fertilisers based on their potential to minimise future agricultural expansion. We map the threat to biodiversity globally for the best-case scenario for replacing mineral N. To consider both biofixation and industrial nitrogen fixation, we calculated the footprint for three green manures (azolla, algae and alfalfa), and three options for mineral nitrogen production using renewable energy to power the Haber-Bosch process (wind, photovoltaics and thermal solar). Solar-powered Haber-Bosch would provide the minimum global footprint, with concentrated thermal solar power stations a particularly attractive option since they are best situated in low-rainfall areas where biodiversity is also lower. This approach would also save about 1% of global carbon emissions from the combustion of fossil fuels. Mapping the biodiversity impact of expanding the current solar power station footprint to meet the area required to replace the fossil fuel powered mineral N shows a reduction in biodiversity impact from footprint expansion to less than one ten-thousandth of that which would occur with current management practices in the absence of mineral N. A proactive approach is required in selecting alternatives to mineral N in order to limit the impact of agriculture’s post-carbon footprint on biodiversity.