Long-Term (43 years) Fate of Soil Phosphorus as Related to Cropping Systems and Fertilization.
T.Q. Zhang, C.S. Tan, C.F. Drury, and D.W. Reynolds. Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, 2585 County Road 20 E, Harrow, ON N0R 1G0, Canada
Agricultural practices affect soil P status, which is the key factor affecting P bio-availability and susceptibility to loss. Previous studies mostly focused on short-term effects and were limited to assessments within agricultural ecosystems. We evaluated the effects of consistent long-term (43 years) cropping (Continuous Corn: CC, Continuous Bluegrass: CB, and Crop Rotation of corn-oats-alfalfa-alfalfa: CR) with and without fertilization on changes in soil P as compared to that in a natural (virgin) forest ecosystem for a Brookston clay loam soil. Soil P was fractionated into various inorganic (Pi) and organic P (Po) forms. Soil P loss was determined by analysing tile drainage water samples, which were year-round continuously collected from the entire large filed plots using an automated sampling system. Under virgin forest, soil P was predominated by stable P, followed by moderately stable Pi (HCl-Pi) and moderately labile Po (NaOH-Po). Consistent cropping without fertilization reduced all forms of soil P relative to virgin forest; i.e. labile P (NaHCO3-P) and moderately labile P were reduced by 60-76% and moderately stable and stable P were reduced by 30-39%. Consistent cropping with fertilization reduced all forms of Po, in the order of CC>CR>CB, but increased labile and moderately Pi in the order of CB>CC>CR. Moderately stable Pi increased with fertilization in CC and decreased in RC and CB. All forms of soil P are in a dynamic system, of which the transformation pathways may depend on the balance of soil P input vs. output. Cultivation of virgin soil causes losses of soil Po, which may not be remedied by fertilization. Unfertilized CC and CB had similar flow-weighted mean P concentrations of 0.043, 0.035 and 0.19 mg P L-1 for Dissolved Inorganic P (DIP), Dissolved Organic P (DOP) and Particulate P (PP), respectively, while the corresponding concentrations under unfertilized RC were greater by 6 %, 25 % and 14 % , respectively. Fertilization increased flow-weighted means of DIP, DOP and PP in tile water by an average of 375 %, 393 % and 75 %, respectively, relative to no fertilization. Either with or without fertilizations, flow-weighted mean DIP and DOP followed the order CB>RC>CC, while the reverse order occurred for PP. The total annual soil P losses in tile water were 226-1253 g P ha-1 for the unfertilized treatments, and 96-611 % greater for the fertilized treatments. Total annual P loss and total tile discharge volume followed the order CB>RC>CC regardless of fertilization. The leaching of soil P into tile drainage can consequently be substantial, and strongly influenced by cropping system.