## 2 Materials and Methods
### Study Area  
#### 2.1.1 Bio-Based Fertiliser Products  
Three BBFs were investigated in this study and compared with mineral fertiliser (MF): liquid fraction of digestate (LFD); potassium concentrate (KC); and ammonium sulphate solution (AS). The BBFs were obtained from a mesophilic (38 \({}^{\circ}\)C) anaerobic co-digestion (AD) plant located at the premises of a pig farmer in Oirschot (the Netherlands). The biogas installation produces 1000 m\({}^{3}\) gas h\({}^{-1}\), which corresponds to 9.8 MWh. The AD plant has a hydraulic retention time of 30 days, and an input feed consisting of 60% pig slurry and 40% plant-based products (i.e., crop residues, food waste). LFD was obtained after mechanical separation of raw digestate by means of a belt filter press system. The product was further refined for the production of AS and KC by inducing ammonia volatilisation from LFD in a 4-stage thermal vacuum evaporation system during increased temperature regime. The ammonia rich gas stream was washed by H\({}_{2}\)SO\({}_{4}\) solution to dissolve ammonia and form AS. After the evaporation of LFD, most of the N was removed and the remaining product is characterised by a high concentration of suspended solids and potassium, referred to as KC.  
The chemical composition of the BBFs is reported in Table 1. Each BBF was analysed by a certified laboratory that uses standardized methods, and therefore it was assumed that the analysis of a single sample was representative for the entire BBF batch. The same applies to the other analyses carried out in this study. The dry matter (DM) content was determined as the residual weight after 48 h of drying at 105 \({}^{\circ}\)C. Organic matter (OM) of the applied products was determined by ashing the samples at 550 \({}^{\circ}\)C for 5 h. Total N was determined using Kjeldahl destruction, whereas NH\({}_{4}\)+-N and NO\({}_{3}\)-N (excluded from Table 1) were measured colourimetrically by a continuous flow auto-analyser (Chemlab System 4, Skalar, the Netherlands) after subsequent extraction in 1 M KCl, conform ISO 14256. The organic N was further calculated by subtracting mineral N (i.e., NH\({}_{4}\)+-N and NO\({}_{3}\)-N) from total N. The total carbon was analysed by using dumas dry combustion method. The total organic carbon (TOC) was measured using the Macherey-Nagel 985093 method and measuring the solution in a P-PRO-32 spectrophotometer (Macherey-Nagel, Duren, Germany). To analyse the P and K content, the BBFs were pre-treated according to NEN7431, P and K were extracted according to NEN7433, and analysed according to NEN7435. During pre-treatment, the products were homogenised, dried, and crushed. For the extraction, sulphuric acid, hydrogen peroxide, and copper sulphate were used. The P content was analysed using the continuous flow auto-analyser (Chemlab System 4, Skalar, the Netherlands), and the K content was analysed using inductively coupled plasma optical emission spectrometry (ICP-OES) (Varian Vista MPX, Santa Clara, California, USA). The sulphur (S) content was measured by DIN EN ISO 11885: 2009-09, EG2003/2003. The pH and electrical conductivity (EC) were determined on the fresh sample using an Orion-520A pH meter (USA) and Orion-star A212 conductivity meter, respectively. All three experiments used the BBFs from the same batch to avoid potential differences in chemical composition of the tested products.