Quantifying Potential Nutrient Liberation Following Plant Investment in Root Exudates: Variation Across Soil Profiles and Ecosystems in the United States

Abstract Plants allocate significant amounts of carbon belowground to support water and nutrient acquisition, and a large proportion of that carbon is allocated to the production of root exudates. One utility of exudates is to aid in phosphorus acquisition, but the amount of phosphorus that can be liberated by different exudates is largely unknown. We used three common exudates—oxalic acid, malic acid, and acid phosphatase (APase)—to extract phosphate from 25 soil profiles across the United States. These extracts represent the “maximum efficiency” of specific exudates at liberating phosphorus from a wide range of soil types, providing an upper boundary to constrain biogeochemical and land‐surface models aimed at representing exudate‐related nutrient fluxes. Our findings reveal significant variability in the efficiency of exudates to liberate phosphate across locations and soil depths. Oxalic acid exhibited the highest potential extraction efficiency overall, and its efficiency often increased in deeper horizons—likely driven by higher proportions of mineral‐bound phosphorus in deeper soil layers. APase had the lowest overall potential extraction efficiency, and its efficiency typically declined with depth and had stronger associations with soil organic carbon. Malic acid potential extraction efficiency was intermediate, and depth trends were often more similar to APase than oxalic acid. Malic acid and APase showed increasing extraction efficiency at sites with higher mean annual temperature and precipitation. This study highlights the varying efficiencies of exudate compounds in liberating soil phosphorus across ecosystems and provides a framework for understanding nutrient dynamics under changing climate or land‐use conditions.