Tree carbon investments and soil phosphorus dividends: How do rooting systems maximize P acquisition while being thrifty with their carbon?

Food production limitations and potential global conflict associated with impending phosphorus (P) scarcity highlight the urgency of understanding factors governing P accessibility to plants. The decline in P mobility as soils develop is well accepted, but the response of plant mechanisms for accessing P bound in different forms is less clear. Rooting systems produce exudates that can liberate P bound in organic or mineral compounds, but different exudates represent distinct plant investments of fixed carbon (C). For example, organic acid exudates promote P availability via mineral dissolution and ligand exchange for a relatively small C cost, while enzymes such as phosphatases hydrolyze P from organic matter and incur double the plant C expense. The organic P recycling that phosphatases promote, however, likely is critical for vegetation where P supplies are low. Estimating P released via these exudates can help us discern the degree to which plants rely on organic- vs. mineral-bound P, thereby illuminating a potential mechanism for coping with dwindling P supplies.

We analyzed soil depth distributions of rooting system exudates and the potential P release by those same exudates in P-depleted Ultisols from the Calhoun Critical Zone Observatory, SC. By extracting soils with either acid phosphatase (APase) or oxalic acid (OA) solutions, we demonstrate that, despite requiring twice as much C to produce, APase has the potential to extract up to three orders of magnitude more P than OA per unit exudate C. Bulk soil APase activity suggests relatively high C allocation to this exudate, while organic acid quantification in these same soils hints that OA concentrations are comparatively small. However, at 100-200cm depths P released via OA sometimes surpassed that of APase, suggesting benefits for trees investing in exudates that release deep, mineral-bound P. These results indicate that rooting systems can reflect the depth distributions of P sources, and that highly developed mechanisms of organic-P recycling in these low-P soils likely confer considerable P benefit to plants despite the high C requirement. As ecosystem P content declines with soil development, and as P fertilizer becomes scarce, both natural and agricultural systems may benefit from investing in rooting systems capable of accessing recycled P.