Abstract
The aim of this study was to contribute to closing global phosphorus (P) cycles by investigating and explaining the effect of fish sludge (feed residues and faeces of farmed fish) and manure solids as P fertiliser. Phosphorus quality in 14 filtered and/or dried, composted, separated or pyrolysed products based on fish sludge or cattle or swine manure was studied by sequential chemical fractionation and in two two-year growth trials, a pot experiment with barley (Hordeum vulgare) and a field experiment with spring wheat (Triticum aestivum). In fish sludge, P was mainly solubilised in the HCl fraction (66±10%), commonly being associated with slowly soluble calcium phosphates, and mean relative agronomic efficiency (RAE) of fish sludge products during the first year of the pot experiment was only 47±24%. Low immediate P availability was not compensated for during the second year. Thus efforts are needed to optimise the P effects if fish sludge is to be transformed from a waste into a valuable fertiliser. In manure solids, P was mainly soluble in H2O and 0.5M NaHCO3 (72±14%), commonly being associated with plant-available P, and mean RAE during the first year of the pot experiment was 77±19%. Biochars based on fish sludge or manure had low concentrations of soluble P and low P fertilisation effects, confirming that treatment processes other than pyrolysis should be chosen for P-rich waste resources to allow efficient P recycling. The field experiment supported the results of the pot experiment, but provided little additional information.
Highlights
Closing nutrient loops by recycling of organic waste sources as fer tiliser has received increased attention
The total P concentration was on average higher in products based on fish sludge (31.5 ± 10.6 g P kg− 1 Dry matter (DM)) than in products based on animal manure (13.3 ± 7.5 g P kg− 1 DM)
In products based on fish sludge, only 17 ± 2% of P was soluble in H2O and 0.5 M NaHCO3 and the sum of these fractions was significantly smaller than in products based on animal manure
Summary
Closing nutrient loops by recycling of organic waste sources as fer tiliser has received increased attention. One poorly utilised organic resource is waste from aquaculture production. Global aquaculture production is thriving, driven by the increasing global demand for farmed fish, and the amount of organic waste from aqua culture production is increasing. The compo sition of salmon feed has changed considerably over recent decades, with an increasing proportion of marine ingredients being replaced by plant-based ingredients (Aas et al, 2019). In Norwegian production, salmon feed contains on average 70% plant-based ingredients, including soya mainly imported from Brazil (Laksefakta, 2018). Agri culture and aquaculture are indirectly highly dependent on the input of phosphorus (P) fertiliser, which is often derived from mineable rock phosphate, a non-renewable and geographically limited resource
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