Food and fuel from plant biomass - will there be enough to go around?

The ever-growing need for food and renewable energy is increasing the demand for biomass from wild and cultivated plants. The annual production of carbon in biomass - net primary production (NPP) - from terrestrial ecosystems globally is 57 Gt; of this total, humans currently appropriate 23-40%. Recent estimates suggest that the amount of plant biomass available for bioenergy is too small to significantly reduce our reliance on fossil fuels, and increasing biomass allocated to fuel would compete with the food supply. These estimates assume that maximum sustainable NPP is represented by that location's native vegetation. We invalidate this assumption by comparing NPP from native and cultivated crops at several locations globally. We also estimate the theoretical maximum biomass production (NPPmax) and the maximum biomass production that can be sustained by local water availability (NPPwater). Across six unfertilized, non-irrigated ecoregions, NPP from cultivated and non-native wild plants surpassed that of native vegetation by up to 500%. Using the rain-fed Midwestern US as an example agricultural region, we estimate NPPmax from the theoretical solar conversion efficiency of 6% to be 137 tonnes/ha, i.e. 6.8x current maize yields. This value drops to 3.8x current maize yields when constrained by local plant-available water (NPPwater) or when using an empirically observed solar conversion efficiency of 3.7%. Our analysis of terrestrial NPPwater using the highest observed solar conversion efficiency for C3 and C4 was approximately 10x greater than current estimates. These global results provide an upper bound for NPP at any given location. Crop improvement aimed at increasing solar conversion efficiency has the potential to dramatically increase NPP, and incorrect assumptions guiding current models may lead to underestimates of biomass production. However, our findings indicate that the limiting factor to plant production in rain-fed agro-ecosystems is plant-available water, and that in addition to improving photosynthetic solar conversion efficiency, efforts to improve the efficiency of water use should be a priority. While the rapidly increasing human population will continue to challenge our ability to sustainably produce enough food, fiber and fuel, incorrect assumptions in production models currently underestimate potential NPP.