BIOSOLIDS DOWN UNDER: EFFECTS OF SOIL INCORPORATION ON PHOSPHORUS AVAILABILITY
Abstract:The production of biosolids varies between Wastewater Treatment Plants with a number of different techniques used in the removal of phosphorus (P) from effluent. Often these processing methods cause differences to the total P content and P forms in the biosolids. There is often conflicting data as to the proportion of the total P available to plants following the land application of biosolids. There is limited data as to the relative effectiveness of biosolids compared with other sources of P fertilisers. A range of methods can be used to apply biosolids to agricultural land, which may result in different proportions of biosolids remaining on the soil surface. In comparison, most glasshouse pot studies fully incorporate biosolids into the soil. It is not known what effect these incorporation differences have on P availability.
Phosphorus uptake in wheat shoots (Triticum aestivum) and changes in bicarbonate extractable soil P (Colwell P) were examined following the application of biosolids to a P deficient soil. Four different application/incorporation methods were examined using equal amounts of biosolids (5 dry g/kg) to provide a total of 140mg P/kg. In addition, eight treatments using inorganic P fertiliser (monocalcium phosphate:MCP) were examined supplying from 0 to 140 mg P/kg. There were three replications in a completely randomized block design. Five wheat shoots were grown in 15cm diameter non-draining pots (1.8kg soil) in a glasshouse for 33 days. Basal nutrients were applied to all pots, which were watered to field capacity daily. The biosolids used for this study were sourced from the Beenyup Wastewater Treatment Plant (WWTP), Western Australia. These biosolids are produced from anaerobically digested secondary sludge dewatered to 15% dry matter by filter belt presses. Lime is added to the influent channel to maintain pH in the aeration system. There is no other chemical precipitation of P.
Wheat shoot dry matter (DM) was poor where no P fertiliser was applied (0.29 g/pot), reflecting the P deficiency of the soil. The highest shoot DM (3.95 g/pot) was obtained at the highest rate of MCP, which also reflected the highest improvement in available soil P (5 mg P/kg increased to 74 mg P/kg). Shoot DM with biosolids as the source of P varied from 0.76 to 3.64 g/pot dependant on the incorporation technique used. The lowest DM was the surface applied biosolids. Available soil P varied from 5mg P/kg to 52mg P/kg. There was limited movement of P out of the biosolids and into the surrounding soil, with plant roots playing a large role in sourcing P.
The proportion of biosolids remaining on the surface following soil incorporation may impact on P availability and therefore current land application rates. Other factors to be considered are the forms of P present in anaerobically digested biosolids and the ability of the soil to fix P. An under-estimation of P may lead to environmental problems, whereas over-estimation of P may result in inadequate amounts for plant growth. Field trial research to examine soil incorporation effects on soil P in conjunction with these glasshouse results is ongoing.
Document Type: Research Article
Publication date: 2002-01-01
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