Concomitant rock phosphate dissolution and lead immobilization by phosphate solubilizing bacteria (Enterobacter sp.)

This paper examines the potential value of phosphate solubilizing bacteria (Enterobacter cloacae) in the dissolution of rock phosphate (RP) and subsequent immobilization of lead (Pb) in both bacterial growth medium and soils. Enterobacter sp. showed resistance to Pb and the bacterium solubilized 17.5% of RP in the growth medium. Enterobacter sp. did not enhance Pb immobilization in solution because of acidification of bacterial medium, thereby inhibiting the formation of P-induced Pb precipitation. However, in the case of soil, Enterobacter sp. increased Pb immobilization by 6.98, 25.6 and 32.0% with the RP level of 200, 800 and 1600 mg P/kg, respectively. The immobilization of Pb in Pb-spiked soils was attributed to pyromorphite formation as indicated by XRD analysis. Inoculation of phosphate solubilizing bacteria with RP in soil can be used as an alternative technique to soluble P compounds which can cause eutrophication of surface water.     

The effect of flow path and mixing layer on phosphorus release: physical mechanisms and temperature effects

Soil phosphorus (P) concentrations typically are greater in surface soils compared with subsurface soils. Surface soils have a greater chance to interact with runoff leading to P transport to streams. The thin surface layer where P concentrates is referred to as the mixing layer denoting where water and chemicals mix during transport. The objective of this study was to evaluate the effect of hydrologic flow paths on soluble reactive phosphorus (SRP) loss at two temperatures. Laboratory flumes were built to simulate infiltration, return flow, saturation excess, and interflow, and subsequent interaction with the mixing layer. The sandy loam soil in the flumes was kept at saturation throughout all experiments, so that biochemical effects were normalized. Flow through the flumes was maintained at 3.6 mm/h for 24 to 99 h (at 6 and 25 degrees C) with water entering and exiting the flumes at different ports (to simulate different flow paths) or as low intensity rainfall. Experiments were performed with and without an artificially created P-enriched surface layer (5 mm thick, total P increased from 1010 mg/kg in the original soil to 2310 mg/kg by addition of dissolved phosphate). Results indicated that (i) SRP release was greater in soil with a mixing layer than in soil without a mixing layer; (ii) SRP release was greater during experiments at 25 degrees C than at 6 degrees C; (iii) at 25 degrees C, SRP release was greatest when water traversed the mixing layer in the upward direction (i.e., in return flow), and by flow parallel to the mixing layer (i.e., surface runoff); and (iv) at 6 degrees C, SRP release in subsurface flow following rainfall was slightly greater than in return flow and infiltration. Our results confirmed the presence of a variable, temperature-dependent desorption process when runoff water interacted with the mixing layer. Our findings have important implications for how different water flow paths in and over the soil interact with P in the soil, and what the ultimate concentration will be in runoff and interflow.     

The effects of soil carbon on phosphorus and sediment loss from soil trays by overland flow

Soil chemical constituents influence soil structure and erosion potential. We investigated manure and inorganic fertilizer applications on soil chemistry (carbon [C] quality and exchangeable cations), aggregation, and phosphorus (P) loss in overland flow. Surface samples (0-5 cm) of a Hagerstown (fine, mixed, semiactive, mesic Typic Hapludalf) soil, to which either dairy or poultry manure or triple superphosphate had been applied (0-200 kg P ha(-1) yr(-1) for 5 yr), were packed in boxes (1 m long, 0.15 m wide, and 0.10 m deep) to field bulk density (1.2 g cm(-3)). Rainfall was applied (65 mm h(-1)), overland flow collected, and sediment and P loss determined. All amendments increased Mehlich 3-extractable P (19-177 mg kg(-1)) and exchangeable Ca (4.2-11.5 cmol kg(-1)) compared with untreated soil. For all treatments, sediment transport was inversely related to the degree of soil aggregation (determined as ratio of dispersed and undispersed clay; r = 0.51), exchangeable Ca (r = 0.59), and hydrolyzable carbohydrate (r = 0.62). The loss of particulate P and total P in overland flow from soil treated with up to 50 kg P ha(-1) dairy manure (9.9 mg particulate phosphorus [PPI, 15.1 mg total phosphorus [TP]) was lower than untreated soil (13.3 mg PP, 18.1 mg TP), due to increased aggregation and decreased surface soil slaking attributed to added C in manure. Manure application at low rates (<50 kg P ha(-1)) imparts physical benefits to surface soil, which decrease P loss potential. However, at greater application rates, P transport is appreciably greater (26.9 mg PP, 29.5 mg TP) than from untreated soil (13.3 mg PP, 18.1 mg TP).