Methods for testing short- and long-term phosphorus fertilizing efficiency of products with varying solubility

Phosphorus (P) recovery from nutrient-rich side streams (NRSS) and derived products is crucial to ensure sustainable food production in the future and to enhance the circular economy, but the agronomic efficiency of these products needs to be validated to reach these targets. In this study, we used a Hedley fractionation scheme and the diffusive gradient in thin film (DGT) method to determine P availability in 83 NRSS and derived products originating from Finland, Sweden, and Germany. Furthermore, two independent short- and long-term growth experiments with barley (Hordeum vulgare L.) and ryegrass (Lolium perenne L.), respectively, were conducted to evaluate P availability in 15 selected NRSS. In addition to the DGT soil test, different fertilizer extractants, 2 % formic acid (FA), 2 % citric acid, and neutral ammonium citrate, were tested for predicting P availability in growth experiments. Livestock manures and slurries were found to contain a notable portion of labile P and were comparable to superphosphate (SP). Despite the low shares of labile P in struvite (7.2 %) and AshDec® (1.3 %), they exhibited P availability comparable to SP fertilizer, as indicated by DGT (99 % and 238 % of SP equivalence, respectively). This suggests that factors other than solubility influenced P availability in these side streams. The DGT method as a promising soil test predicted both short- and long-term P availability better than the selected conventional chemical extraction methods did. The 2 % FA extract exhibited the poorest performance, overestimating P availability in some nutrient sources while underestimating others in long-term. These findings enhance our understanding of P availability in potential raw materials for fertilizers, facilitating more effective P management strategies in the circular economy.

Considering inorganic P binding in bio-based products improves prediction of their P fertiliser value

Prediction of the relative phosphorus (P) fertiliser value of bio-based fertiliser products is agronomically important, but previous attempts to develop prediction models have often failed due to the high chemical complexity of bio-based fertilisers and the limited number of products included in analyses. In this study, regression models for prediction were developed using independently produced data from 10 different studies on crop growth responses to P applied with bio-based fertiliser products, resulting in a dataset with 69 products. The 69 fertiliser products were organised into four sub-groups, based on the inorganic P compounds most likely to be present in each product. Within each product group, multiple regression was conducted using mineral fertiliser equivalents (MFE) as response variable and three potential explanatory variables derived from chemical analysis, all reflecting inorganic P binding in the fertiliser products: i) NaHCO₃-soluble P, ii) molar ratio of calcium (Ca):P and iii) molar ratio of aluminium + iron (Al + Fe):P. The best regression model fit was achieved for sewage sludges with Al-/Fe-bound P (n = 20; R² = 79.2%), followed by sewage sludges with Ca-bound P (n = 11; R² = 71.1%); fertiliser products with Ca-bound P (n = 29; R² = 58.2%); and thermally treated sewage sludge products (n = 9; R² = 44.9%). Even though external factors influencing P fertiliser values (e.g. fertiliser shape, application form, soil characteristics) differed between the underlying studies and were not considered, the suggested prediction models provide potential for more efficient P recycling in practice.

Closing global P cycles: The effect of dewatered fish sludge and manure solids as P fertiliser

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 0.5 M 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.

Recycling potential of secondary phosphorus resources as assessed by integrating substance flow analysis and plant-availability

The plant-availability of phosphorus (P) plays a central role in the ability of secondary P resources to replace mineral fertilizer. This is because secondary P plant-availability varies, often with large fractions of residual P that has no immediate fertilization effect. Therefore, if low quality secondary P fertilizers are applied, they will accumulate in soils that, in the long run, may increase the risk of P runoff and eutrophication. Substance flow analyses (SFA), used to identify potentials for improved P management, have not considered this well-known quality barrier. We, therefore, argue that traditional SFA over-estimates the fertilizer potential of secondary P resources. Using Norway as a case, we present a plant-availability extended SFA methodology that integrates SFA and the concept of relative agronomic efficiency. To account for the plant-available soil P stock and long-term soil interactions, we adjust the Norwegian P fertilization demand based on soil P values. We found that, while the method has uncertainties particularly for long-term estimations, it more realistically estimates secondary P fertilizer potentials and is adaptable to other countries. For Norway, we found the overall secondary P fertilizer potential reduced by 6-55% when considering plant-availability. The most important secondary resource was manure, which had the highest P plant-availability and quantities large enough (10.9kt plant-available P/yr) to meet Norway's entire P fertilization demand (5.8kt plant-available P/yr). However, barriers related to its transportability need to be overcome to efficiently use this resource. Fish sludge was also an important product, with 6.1kt plant-available P/yr but with uncertain plant-availability data. We argue that high quality secondary P resources can theoretically meet Norway's P fertilization demand and, therefore, make Norway mineral P independent. However, it is important that their use is carefully regulated based on plant-availability to eliminate the soil accumulation of both available and residual P.