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Abstract
Soil phosphorus (P) biogeochemistry is fundamental to how terrestrial ecosystems function and cycling of soil P can modulate P bioavailability as can human actions. The Calhoun Critical Zone Observatory is an ancient landscape that has a modern (1750-1950) history of forest clearing for agriculture that included P fertilization and severe surface erosion followed by reforestation. I hypothesized that historic P fertilization impacts current P biogeochemistry and that soil P supply for afforestation is buffered by fertilizer P. My investigations of P pool changes with topography and with agricultural land use found that historically farmed hillslopes have more extractable P relative to forests without farming down through 2m depth. Further, in toe slope locations, organic P (Po) is elevated compared to ridges. Phosphorus in soil solution and on ion exchange resins were higher in reference areas but were low in [P] similar to local streams. My analysis of soil P bioavailability under afforestation from 1957 to 2017, cataloged changes in 0-60 cm mineral soil P fractions in plots uncut since planting and plots cut in 2007. I found extractable P decreased in uncut and cut plots in 0-7.5 cm. Further, between 2005 to 2017 the slowly cycling Po did not increase in the uncut plots but increased in 0-7.5 cm soil of cut plots. Changes in soil P fractions relative to tree P-demand suggest Ca-P and Po pools are supplying soil P. To investigate the effect of different land uses on P losses over time I combined multiple years of soil, soil solution, and stream water data from two small watersheds with the soil and water assessment tool (SWAT). I found annual total P loss under 100% agriculture was six times greater than under 100% forest cover while intermediate conditions were three times less than under agriculture. Further, the model predicted rates of soil P leaching that were consistent with the observed increases in extractable soil P. Simulation indicated high rainfall years or events (i.e., hurricanes) may have historically facilitated P movement. After 60+ years since agricultural abandonment the Calhoun Critical Zone Observatory retains a legacy on soil P biogeochemistry.