Phosphorus forms and retention in a sandy soil receiving dairy waste effluent.

摘要

Currently there are major concerns about the potential negative effects of nutrient losses from the waste of dairy farms on surface and ground water quality. In many confined livestock production systems, manures are normally applied at a rate designed to meet crop N requirements. However, this often results in a buildup of soil P above amounts required for optimal crop yield and increases the chances for P losses from source areas to water bodies.;This research, conducted at a dairy farm in north Florida, investigates the status of soil P under two main treatments of dairy waste effluent and two cropping systems. The N application rates were 448 and 896 kg/ha/yr which correspond to P loading of 112 and 224 kg/ha/yr. The cropping systems were perennial peanut-rye (P-R) and corn-forage sorghum-rye (C-FS-R). The objectives were to: (1) examine the accumulation of P in the soil profile, (2) quantify and characterize P forms in the soil profile, (3) quantify and characterize P retention in the soil profile, (4) determine P uptake by the cropping systems, and (5) assess the downward movement of P.;The study site, mapped as Kershaw sand, appears to have been heavily loaded with animal waste (47 mg/kg Mehlich I-extractable P (MI-P) in the native area vs. 283 mg/kg in the study site surface soils). The MI-P increased significantly with high effluent rate application, particularly under the P-R cropping system, which suggests that the C-FS-R cropping system may be more effective in P removal than the P-R cropping system. Total P (TP) increased from 343 mg/kg in 1996 to 689 mg/kg in 1998. Water soluble P (WSP) increased but primarily in the lower depths of the soil profile under both treatments.;Al- and Fe-associated P constituted the major proportion (up to 60%) of the TP in the soil profile. Labile-P accounted for 18 to 30%, and Ca- and Mg-associated P accounted for about 10% of TP. Water soluble- and labile-P concentrations from 1996 and 1998 indicated a downward movement of P in the soil profile. These same data coincided with a decrease in retention capacity as determined by "Relative Phosphorus Adsorption" (RPA). Degree of P Saturation (DPS) data indicated that the surface horizon is more likely to release P than the deeper depths. The conclusions drawn from DPS were in agreement with the conclusions arrived at from the soil adsorption capacity and equilibrium phosphorus concentration (EPC0).;Phosphorus removal was higher for the C-FS-R than for the P-R cropping system. The removal values agreed with published P uptake for such crops, but crop uptake did not alter the high level of soil P that was already present before application. When soil test P levels in the soil exceed optimum values for crop production, the application of dairy waste based on estimated N requirement may not be appropriate on heavily P loaded sandy soil such as the soil at the study site.