The impacts of historical land-use on phosphorus movement in the Calhoun Critical Zone Observatory in the southeastern US Piedmont

Human actions through land-use can alter soil phosphorus (P) distribution over time and space as vegetation is altered and added fertilizer P is translocated downslopes by runoff and erosion, or through the soil profile by leaching. In the southeastern US Piedmont, a more than 100-year period of human land-use of forest clearing and farming, which included P fertilization, caused severe surface erosion before reforestation. This history resulted in elevated surface soil P in farmed ridge tops, even after 70 years of reforestation, but little data exists on redistribution of this P downslope or down profile during these 70 years. We aimed to investigate the effect of different land-use histories on soil P losses over time. Combined with multiple years of current soils, soil solution, and stream water data from two small watersheds in the Calhoun Critical Zone Observatory (Calhoun CZO) in South Carolina, USA, we use the soil and water assessment tool (SWAT) to simulate a trajectory of seven different land-uses on P movement. Results indicated annual solution total P loss under 100% agriculture (3.7 kg ha⁻¹ year⁻¹) was six times greater than under 100% forest while mixed forest and agriculture experienced about one-third the loss observed under agricultural land. Furthermore, the model predicted P leaching that ranged from 0.036 to 0.1 kg ha⁻¹ year⁻¹ with the highest rates during early reforestation. These increased leaching rates could account for 7–20 kg ha⁻¹ of vertical P movement through the soil profile over 200 years. This content of P leaching is consistent with observed differences in the content of extractable soil P in the 2 m of soil under farmed vs. never farmed locations. Simulation under high rainfall indicated wet periods or events could augment P mobilization and better approximated observed increases in soil extractable P content. These land-use change simulations indicate reforestation reduces surface runoff and erosion but increases vertical leaching and, despite soil P retention mechanisms, may move P deep into the soil profile.