Saturday, 15 July 2006

Temporal Change of Soil Phosphorus Fractions Observed by Field Microplot Cylinder Experiments.

Zhongqi He, C. Wayne Honeycutt, Timothy Griffin, and Ann-Marie Fortuna. USDA-ARS, New England Plant, Soil, and Water Laboratory, Orono, ME 04469

Phosphorus (P) is an essential nutrient for plant growth. Application of animal manure can benefit crop production by providing N, P, and other nutrients and by increasing soil organic matter. On the other side, however, agricultural runoff and erosion from high P soils are considered as major contributing factors to surface water eutrophication. Thus, the behaviors of P in soils impacted by manure application should be under scrutiny to provide insight in developing strategy to synchronize the utilization of the nutrient values while minimizing their adverse environmental impacts. Laboratory incubation has been used to evaluate the dynamics and bioavailability of P in soils amended with animal manure. Studies, however, need to be conducted to determine if similar results can be found under field conditions. Thus, this study was conducted to assess the temporal change of P fractions in soils amended with animal manure by packing these soils in microplot cylinders that were installed in fallow fields. Cores of unfertilized soils were collected from two field sites (Newport and Presque Isle, Maine) by driving microplot cylinders into the soils using a slide drop hammer. In the laboratory, the surface 4 cm of soil from each cylinder was taken out and mixed with animal manure that provided an application rate of 350 kg N ha-1 applied to a 15 cm depth. The soil was then gently packed back into the cylinder. Control (unamended) soils were handled in the same way. Those cylinders were then installed in fields where the soils had been collected. Six separate cylinders for each amendment (i.e. six replicates) were removed from the fields at 3, 7, 14, 21, 28, 49, and 70 days after application, respectively. A sequential extraction procedure (H2O, 0.5 M NaHCO3, 0.1 M NaOH, and 1 M HCl) was applied to evaluate the P changes over the monitoring period. In all four fractions, the pattern and the amplitude of either inorganic P (Pi) or total extractable P (Pt) change during the monitoring period were basically same in dairy manureľamended (roughly 92 mg P kg-1 of dry soil) soils and control soils. This observation indicates that soil properties still played a major role in controlling P dynamics in dairy manure amended soils as we observed in laboratory incubation experiments. As observed in the laboratory incubation, concentrations of P species fluctuated in complementary patterns among NaHCO3, NaOH, and HCl fractions of the field soils during the monitoring period, implying the active interchange of P species in the field soils. However, unlike results from laboratory incubation that water extractable P was basically unchanged over time, there were significant changes, indeed increases in most times, of both water extractable Pi and Pt in the field soils. We attributed these changes to the impacts by inconstant environmental factors in field experiments (such as temperature and rainfall). This observation implies that practical field conditions would have yielded more labile P for plant uptake or runoff than that predicted from laboratory incubation. For comparison, swine manure was also applied to the Newport soil with the same application rate of 350 kg N ha-1. Although less P (roughly 26 mg P kg-1 of dry soil) was applied to the soil, the changes of Pi and Pt in the swine manure-amended soil were not always same to those of the control soil. It seems that the physicochemical properties of swine manure partly altered the properties of the amended soil. Thus, the impacts of swine manure on soil and its surrounding environment should be more closely monitored.

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