Assessing the Biological Value of Soluble Organic Fractions from Tomato Pomace Digestates

Anaerobic digestion is proposed for sustainable exploitation of the huge amounts of residues produced yearly by tomato processing. The aim of this study was to analyze the biological effects of water-soluble organic fractions of tomato pomace digestates applied on soil and plant, at a laboratory scale. Digestates are derived from a short-term dark fermentation batch experiment of tomato pomace and buffalo slurry at three decreasing ratios (12:1, 8:1, and 6:1, respectively Set 1, Set 2, and Set 3). Bioassays on soil and plant were carried out after addition of different amounts of dissolved organic carbon (DOC) extracted from the three sets: microbial pool content, respiration activity, root development, and cell division were considered. Digestate from Set 1 showed the highest content of volatile fatty acids (about 80 mg UL-1), suggesting an efficient fermentation process which led to lower water solubility of organic substrates. The highest carbon mineralization rate in soil (4.29%) occurred with DOC additions from Set 1 (p < 0.05). A biostimulating effect to soil microbiota was shown (p < 0.05) by the metabolic quotient at the rate of 10 ppm DOC from both Set 1 (4.28 mg CO(2)mg C mic(-1) h(-1)) and Set 2 (3.84 mg CO(2)mg C mic(-1) h(-1)). Root length reduction (p < 0.05) was observed for Set 3 (about - 30%). No significant difference (p > 0.05) was detected in the micronuclei assay. Soluble organic forms of these digestates are a nutrient source for soil microorganism metabolism and at low doses show a hormonal-like action, without toxicological or genotoxicity effects on soil biota and root cell.

Automated summary

Anaerobic digestion of tomato processing residues has potential for sustainable exploitation. The study analyzed the biological effects of water-soluble organic fractions of tomato pomace digestates on soil and plants, finding a stimulating effect on soil microbiota metabolism and root development. The soluble organic forms of these digestates act as a nutrient source for soil microorganisms and exhibit a hormonal-like action at low doses with no toxicological or genotoxicity effects on soil biota and root cells.