Valorisation of Fish Bones: Effects of Preservation by Formic Acid on AL-extractable calcium, Phosphorus, Magnesium, and Potassium

Fishbones contain significant amounts of plant nutrients. Fish residues may be preserved by acidification to pH < 4, which may affect the extractability and plant availability of nutrients when applied as fertilisers. Grinded bone material from cod ( Gadus morhua ) heads was mixed with formic acid to investigate if acidification would increase the concentrations of ammonium acetate-lactate (AL)-extractable nutrients. Two degrees of fineness of fishbones (coarse 2-4 mm; fine < 0.71 mm) were compared at pH 3.0 and 4.0 plus a water control in a laboratory study over 55 days. Samples for extraction of AL-soluble P, Ca, Mg and K were taken on day 2, 15, 34 and 55. Whereas acidification clearly increased the concentrations of AL-extractable calcium (Ca-AL) and magnesium (Mg-AL), extractable phosphorus (P-AL) was only significantly increased in finely grinded bones at pH 3. After 34 days, 6% of the total content of P was extracted by AL in fine fishbones at pH 3. Without acidification about 1% of the P was extracted, possibly from phospholipids. This P-AL concentration was well above P-AL extracted from acidified coarse fishbones and from fine fishbones acidified to pH 4. With acidification, about 30% of total Ca and 10% of total Mg were extracted by AL, and the Ca-AL and Mg-AL concentrations were closely correlated. The reason for lower P-AL in coarse fishbones at pH 3 and 4, and fine fishbones at pH 4 than in water controls may be a precipitation of apatite from phospholipids and dissolved calcium.

Changes in availability of Ca, K, Mg, P and S in sewage sludge as affected by pyrolysis temperature

Pyrolysis is a promising technology for sewage sludge (SS) treatment providing several improvements of SS properties for soil application. However, information on the influence of pyrolytic temperature on the availability of nutrients in resulting biochar (BC) is limited. In this study, anaerobically stabilised SS was pyrolysed in a laboratory fixed-bed reactor at 220, 320, 420, 520, and 620 °C for 30 min in the N₂ atmosphere. Pyrolysis resulted in a higher total content of all studied nutrients in BCs. Aromaticity and hydrophobicity of BCs increased with increasing temperatures while solubility decreased. Relative availability (% from total content) of nutrients in BCs was in order: Ca &gt; Mg ~ K &gt; S &gt; P. Pyrolysis at 220 °C produced acidic BC with a higher content of acetic acid-extractable nutrients compared to non-pyrolysed control. An increment in pH and a significant drop in the content of available Ca, Mg, K and S were found at temperature 320 °C. Pyrolysis at 320 °C increased the content of available P by 28 % compared to non-pyrolysed SS. At the temperature of 420 °C and higher, available contents of all studied nutrients were lower than in non-pyrolysed SS.

Soil mineral nitrogen and the rating of CaCl2 extractable nutrients

It was assumed that the determination of the mineral nitrogen (N_min) content in the 0.01 mol/L CaCl₂ could rely on measurements of single form NO₃^–-N, NH₄⁺-N or both, and even including other extractable nutrients. This hypothesis was verified based on some primary data from 17 fields: ten with oilseed rape and seven with maize as indicatory crops during three consecutive seasons in a production farm in Górzno, Poland. The contents of NO₃^–-N, NH₄⁺-N, P, K, Mg and pH were measured in soil prior to the spring vegetation start and after a crop harvest (autumn). Phosphorus in spring and NH₄⁺-N in autumn, were variables discriminating against the number of clusters. It was higher in cropping sequences (CSs) with maize than with oilseed rape. The reliability of N_min determination and distribution between clusters in spring based only on NO₃^–-N was fully corroborated for maize CSs. In autumn, irrespective of the CS, the decisive factor in N_min prediction and distribution over clusters was the NH₄⁺-N pool. This study resulted in the rating of CaCl₂ extractable nutrients, indicating their availability status, shortage or excess, on the background of the N_min temporary rating.