## 2 Materials and Methods
### Analysis of Mycotoxin Content  
Analytical standards for aflatoxin B1 (AFB1), deoxynivalenol (DON), fumonisin B1 and B2 (FUMs), HT-2 toxin, ochratoxin A (OTA), T-2 toxin, and zearalenone (ZEN) used for chromatographic analysis were obtained from Sigma-Aldrich (Poznan, Poland). Acetonitrile, methanol, LC-MS-purity water, and reagents used in chromatographic analysis, extraction, and eluent preparation, i.e., ammonium formate (AFOR), formic acid (FA), and ammonium fluoride (AFLU), were purchased from Merck (Warszawa, Poland).  
Prior to extraction, 50 g of the compost substrate sample was dried at 60 \({}^{\circ}\)C. To extract mycotoxins from the sample, 1 g of the solid-dried and ground material was weighed into a screw-capped falcon-type tube (15 mL), and 5 mL of the extraction solution (90% acetonitrile with 1% FA) was added. In the case of leachate samples, 1 mL of leachate was measured into a falcon tube (15 mL) and 5 mL of the extraction solution (90% acetonitrile with 10% FA) was added. The sample was then subjected to ultrasound for 30 s using a Sonics Vibra Cell apparatus, after which the samples were shaken for 30 min at 10 Hz using an Eberbach model EL.680.Q.25 shaker. After shaking, the samples were centrifuged for 10 min at 5000 rpm (4 \({}^{\circ}\)C) using an Eppendorf Centerfuge 5804 R centrifuge (Leipzig, Germany). The supernatant was filtered into the chromatography vials through a Teflon syringe filter with a pore diameter of 0.22 um prior to subjection to LC-MS/MS analysis.  
Importantly, due to the low recovery of ZEN from the compost samples, ZearaalTestWB immunoaffinity columns (Vicam, Nixa, MO, USA) were used to additionally purify the solid-dried compost extracts (dissolved in a mixture of 90% acetonitrile with 1% FA). Then, 5 mL of the extracts were purified using the columns according to the manufacturer's protocol and subjected to the LC-MS/MS analysis.  
Determination of mycotoxin content was performed using an Agilent 1260 Infinity II chromatograph (LC) coupled to an Agilent 6470 Triple Quadrupole (MS) mass detector (Agilent Technologies, Inc., Santa Clara, CA, USA). Samples were chromatographically separated at a mobile phase flow rate of 0.5 mL-min\({}^{-1}\) using a 2.1 \(\times\) 100 mm Agilent Poroshell 120, SB-C18, 2.7 um column. Mobile phase A was a solution consisting of H\({}_{2}\)O, 0.2% FA, 5 mM AFOR, and 0.5 mM AFLU. Mobile phase B consisted of methanol with 0.5% FA and 0.5 mM AFLU. The analytes were tested using gradient separation. Initially, the proportion of phase B was 10% for 1 min, then it was increased linearly to 50% after 2 min. It was then increased to 95% at the 10 th minute and was maintained at this level until the 14 th minute. After that, it was reduced to the initial proportion of 10% and the column was conditioned in this manner for 4 min before the next sample injection was performed. Sandwich injection was used, with 40 \(\mu\)L of H\({}_{2}\)O and 2.5 \(\mu\)L of the actual sample. The column temperature during chromatographic separation was 40 \({}^{\circ}\)C. The injection needle was washed with a 50% methanol solution for 3 s. The parameters of the ion source were as follows: Electrospray Ionization + Jest Stream Technology Ion Source (ESI + AJS); gas temperature, 250 \({}^{\circ}\)C; gas flow, 8 L-min\({}^{-1}\); nebulizer, 45 psi; sheath gas, 350 \({}^{\circ}\)C; heath gas flow, 11 L-min\({}^{-1}\); and capillary voltage, 3300 V. The Multiple Reaction Monitoring (MRM) parameters for each compound are shown in Supplementary Figure S1. Chromatographic data were collected and processed with Agilent MassHunter Workstation LC/MS Data Acquisition software for the 6400 Series Triple Quadrupole, Version 10.1, Build 10.1.67 (Agilent Technologies; Santa Clara, CA, USA).