Legacy Phosphorus After 45 Years With Consistent Cropping Systems and Fertilization Compared to Native Soils
Zhang, T.Q., Zheng, Z.M., Drury, C.F., Hu, Q.C., Tan, C.S. (2020). Legacy Phosphorus After 45 Years With Consistent Cropping Systems and Fertilization Compared to Native Soils. Frontiers in Earth Science, [online] 8 http://dx.doi.org/10.3389/feart.2020.00183
Plain language summary
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.
Agricultural practices affect the status of legacy phosphorus (P) in soils and consequently the P bioavailability and susceptibility of losses to water resources. Previous studies have primarily assessed P status within agroecosystems, and rarely have these results been compared to native conditions. We evaluated the effects of long-term (45 years) consistent cropping [continuous corn (CC), corn-oats-alfalfa-alfalfa rotation (CR), and continuous bluegrass sod (CB)] with and without P fertilization on changes in P fractions of different bioavailability in a Brookston clay loam, as compared to an adjacent forest native soil. Soil P was separated into various inorganic P (Pi) and organic P (Po) fractions using a modified sequential fractionation method. Phosphorus in native soil was predominated by moderately labile Po (NaOH-Po), 44%, followed by moderately stable Pi (HCl-P), 26%. 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 of 65 and 73 mg kg–1, respectively, over 45 years. Consistent cropping with fertilization retained comparable amount of total P in CC and RC, but increased total P in CB, relative to the native soil. Averaged over cropping systems, labile Pi, NaOH-Pi, and HCl-P increased by 129, 74, and 20 mg kg–1, respectively, whereas labile Po and moderately labile Po decreased by 8 and 60 mg kg–1, respectively, compared to the native soil. This study indicates that long-term cropping significantly enhanced the rate of moderately labile Po mineralization, irrespective of fertilization. The increases of total P and Po in the fertilized CB plots suggest that P accumulation in long-term grass fields is a concern as far as potential P contamination in surface waters.