Experimental evidence that phosphorus fertilization and arbuscular mycorrhizal symbiosis can reduce the carbon cost of phosphorus uptake

Plants allocate substantial amounts of carbon (C) below-ground to obtain nutrients and other resources. Increasing nutrient availability typically reduces the C investment in root growth and mycorrhizal fungi, hence reducing the C cost of nutrient acquisition. This C cost of nutrient acquisition, however, remains poorly quantified. In a P fertilization experiment with Zea mays, we examined below-ground C allocation and the C cost of phosphorus (P) uptake. In addition, we compared plants inoculated with arbuscular mycorrhizal fungi (AMF) to those growing in pasteurized soil to examine the same measures in the absence of AMF. P fertilization tended to increase above-ground plant growth more than it increased the total below-ground C flux (TBCF; root growth plus rhizosphere respiration), suggesting a reduced investment in nutrient acquisition. This was confirmed by a negative fertilization effect on the TBCF-to-total plant P ratio (similar to 25% reduction for high vs. low P fertilization). Soil pasteurization increased this proxy for the C cost of P uptake (similar to 50% increase). This novel quantification of the influence of P availability and mycorrhizal fungi on plant-soil C partitioning can guide the incorporation of these processes in vegetation models. A free Plain Language Summary can be found within the Supporting Information of this article.