Legacy Phosphorus After 45 Years With Consistent Cropping Systems and Fertilization Compared to Native Soils

Phosphorus (P) is an essential nutrient to plant growth and is present in soils as species of either organic (Po) or inorganic (Pi) variety, each with its own biogeochemical properties and thus availability to plants and vulnerability of loss to water resource. The overall status of soil P is heavily impacted by agricultural practices and historically additive, and thus, this long-term study by Zhang et al. was conducted to assess the status of P in both agricultural and native soil. The soils were classified as Brookston clay loam, and P status was assessed in terms of P bioavailability, P transformations, and susceptibility of losses to water resources. The agricultural practices assessed within this 45-year study include consistent cropping [continuous corn (CC), corn-oats-alfalfa-alfalfa rotation (CR), and continuous bluegrass sod (CB)], with and without P fertilization, and were compared to adjacent forest native soil. Results showed that, compared to the native soil, consistent cropping without P fertilization significantly decreased all P fractions except for water-extractable Po, with the largest decrease in labile Pi (water-Pi + NaHCO3 -Pi) and moderately labile Po. Additionally, consistent cropping with fertilization retained comparable amounts of total P in CC and RC, but increased total P in CB, relative to the native soil. The increase of total P in CB treatments was attributed to the return of grass residues which increase total P and total Po. This study revealed that long-term cropping practices significantly enhance the rate of Po mineralization regardless of whether they were fertilized. Further, NaOH-Pi and -Po played important roles by acting as a sink of legacy P in this soil. Lastly, P accumulation in grass fields is a concern as a risk of P pollution to adjacent fresh waters. This study is important in filling knowledge gaps about P status within agroecosystems as compared to native conditions, and build up theoretical bases for soil legacy P agronomic use and for soil P loss mitigation.