Evaluation of phosphorus in thermally converted sewage sludge: P pools and availability to wheat

Jessica E. Mackay, Timothy R. Cavagnaro, Iver Jakobsen, Lynne M. Macdonald, Mette Grønlund, Tobias Pape Thomsen, Dorette S. Müller-Stöver

Research output: Contribution to journalJournal articleResearchpeer-review


Dried sewage sludge (SS) and the by-products of four SS thermal conversion processes (pyrolysis, incineration and two types of gasification) were investigated for phosphorus (P) availability.

A sequential extraction was used to determine the distribution of P among different P pools. After mixing materials with soil, availability of the P was determined with soil P extractions and in a growth experiment with wheat.

Thermally converted SS contained a greater proportion of P within recalcitrant pools than dried SS. Despite having very different P pool distributions, the incinerated and dried SS provided similar amounts of P to plants. Plant P supply from dried and incinerated SS was lower than the comparable soluble P treatment (50 mg P kg−1), but higher than a soluble treatment at a lower rate (20 mg P kg−1). Plant P uptake in gasified and pyrolysed treatments was only marginally greater than uptake in a control (no P) treatment. Plant P uptake correlated most closely with diffusive gradients in thin films (DGT) P analysis of soil-material mixes. Phosphorus availability in the dried and incinerated SS treatments increased over time.

We propose that the dried and incinerated SS have potential as slow release P fertilisers in low pH soils.
Original languageEnglish
JournalPlant and Soil
Issue number1-2
Pages (from-to)307–317
Publication statusPublished - 2017


  • Bioash
  • Biochar
  • Diffusive gradients in thin films (DGT)
  • Sequential phosphorus extraction
  • Sewage sludge
  • Wheat


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