## 2 Materials and Methods  
### Experimental Design and Treatments  
In order to follow the changes in slurry composition and gaseous emissions after application of additives at the storage of pig slurry, a laboratory experiment was conducted in which the slurry was amended with biochar and/or clinoptilolite. The following four treatments with three replications were considered:  
1. Non-amended slurry as control (Control treatment);
2. Slurry amended with biochar at a rate of 2.5% (_uv/v_; 2.5 g of additive present in 100 g of slurry) (Biochar treatment);
3. Slurry amended with clinoptilolite at a rate of 2.5% (_uv/v_) (Clinoptilolite treatment);
4. Slurry amended with biochar and clinoptilolite and each one at a rate of 2.5% (_uv/w_) (Biochar + Clinoptilolite treatment). A single bulk sample of slurry was collected from fattening pigs in a commercial farm (Viseu, Portugal), after being subjected to mechanical separation by a screw press separator (particle size \(<\)1.0 mm, FAN model S655, BAUER, Wiener Neudorf, Austria). The total separated liquid fraction of the pig slurry was homogenized, and subsamples were retained for analysis. Slurry samples from each treatment were collected at 0 and 85 days, after being mixed thoroughly first, and then analyzed (Tables 1 and 2) by standard laboratory methods [16; 17; 18] to the dry matter content by the gravimetric method (24 h at 105 \({}^{\circ}\)C), pH value by EN 13037, total carbon by Dumas method, total nitrogen by Kjeldahl method by EN 13654-1, NH\({}_{4}\)\({}^{+}\) and NO\({}_{3}\)\({}^{-}\) by absorption spectrophotometry, biochemical oxygen demand by incubation (5 days at 20 \({}^{\circ}\)C) and _Escherichia coli_ by International Organization for Standardization (ISO) 16649-2. The commercial biochar (Piroeco Bioenergy, S.L., Malaga, Spain) was obtained from wood shavings (\(\O\) = 2 mm) pyrolyzed in a muffle furnace at 900 \({}^{\circ}\)C. The main physico-chemical properties of the biochar were: granulometry (by sieving method): 552 g kg\({}^{-1}\) with \(\O>0.30\) mm, 364 g kg\({}^{-1}\) with  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Treatments** & **pH** & **Dry Matter** & **Total C** & **Total N** & **NH\({}_{4}\)\({}^{*}\)** & **NO\({}_{3}\)\({}^{-}\)** & **NH\({}_{4}\)\({}^{+}\)/TN** & **C/N** \\ \hline \multicolumn{8}{c}{0 days of experiment} \\ \hline Control & 8.5 \(\pm\) 0.1 \({}^{4}\) & 0.7  
\(\mathcal{O}\) = 0.20-0.30 mm, 41 g kg\({}^{-1}\) with \(\mathcal{O}\) = 0.15-0.20 mm and 43 g kg\({}^{-1}\) with \(\mathcal{O}\) < 0.15 mm, bulk density (by core method): 0.1219 g cm\({}^{-3}\), pH (H\({}_{2}\)O): 10.2, humidity (by gravimetric method): 102.4 g kg\({}^{-1}\), C: 806.0 g kg\({}^{-1}\), and N: 1.9 g kg\({}^{-1}\). The commercial clinoptiolite (Zeolita Natural NUTRI-Clinoptiolita 1g568, ZeoCat Soluciones Ecologicas S.L.U., Barcelona, Spain) was of sedimentary origin, from a mine located in Turkey, with a particle size of 0-0.425 mm and with the following characteristics: mineralogical (by XRD analysis): 880-950 g kg\({}^{-1}\) clinoptiolite, 20-50 g kg\({}^{-1}\) montmorillonite, 30-50 g kg\({}^{-1}\), feldspars, 0-30 g kg\({}^{-1}\) muscovite, 0-20 g kg\({}^{-1}\) cristobalite, chemical composition: 650-720 g kg\({}^{-1}\) SiO\({}_{2}\), 100-120 g kg\({}^{-1}\) Al\({}_{2}\)O\({}_{3}\), 23-35 g kg\({}^{-1}\) K\({}_{2}\)O, 25-37 g kg\({}^{-1}\) CaO, 9-12 g kg\({}^{-1}\) MgO, 3.0-6.5 g kg\({}^{-1}\) Na\({}_{2}\)O, 0-1.0 g kg\({}^{-1}\) TiO\({}_{2}\), cation exchange capacity (by cation exchange capacity (CEC) method): 1.5-1.9 meq g\({}^{-1}\), apparent porosity: 450-500 g kg\({}^{-1}\), specific surface area: (by Brunauer, Emmett and Teller (BET) method) 70-80 m\({}^{2}\) g\({}^{-1}\), pH (H\({}_{2}\)O): 7.8, bulk density of powder: 650-850 kg m\({}^{-3}\), and humidity (by gravimetric method): 65.7 g kg\({}^{-1}\). The rates of additives were selected based on a literature survey, considering a potential decrease of environmental and microbial issues.