Human appropriation of biogenic silicon - the increasing role of agriculture

Terrestrial plants create roughly 84Tmol biogenic silicon (BSi) each year. This substantial uptake of dissolved silicon (DSi) by land plants represents an important, yet often overlooked, pathway of Si on the global scale. Agricultural crops account for similar to 35% of BSi created by land plants globally due to a combination of large biomass and relatively high Si concentrations in the tissue of many crops. In this review, we investigate the global importance of terrestrial BSi fixation by agricultural crops. Using annual data from 1961 to 2012 of the 10 most important crops (ranked by mass produced globally), we quantify how agriculture has altered the amount of BSi created on land. Over this time period, average annual BSi production by the top 10 crops tripled globally, increasing by an additional 39GmolSi each year. We refer to this BSi in agricultural crops as human-appropriated biogenic Si (HABSi). Based on projected increases in human populations and land cover change, HABSi is projected to increase by 22-35% by 2050. Human agricultural practices are increasing the size of relatively labile plant BSi pool on land. Considering its bioavailable nature, we suggest that increasing plant BSi on land has the capacity to alter Si availability in downstream receiving waters, although this depends on the fate of harvesting and threshing losses, which varies substantially across the globe. We highlight similarities between the N, P and Si cycles in agricultural settings, such as the often unidirectional flow of Si away from croplands (similar to N) and soil BSi depletion (similar to P). We then address how changing human diets will continue to alter plant BSi cycling in future decades.