Fate of grape pomace-derived plant phenols in soil during use as organic fertilizer

The grapevine is one of the most commonly cultivated plants in the world. Every year, over \(30× 10⁶\,t\) are processed into wine worldwide. After pressing the grapes, \(9-30\,%\) of the fresh weight remain in the form of grape pomace. Grape pomace is a heterogeneous mixture of grape skins, seeds and pulp. Its exact composition varies greatly depending on the vintage, grape variety and processing method. In addition to fiber, monosaccharides and proteins, grape pomace is rich in plant phenols. In Germany, the most common method of disposal is to spread it on vineyards as organic fertilizer. To date, it remains unclear what processes the phenolic compounds from the pomace undergo when introduced into the soil and what effects they have on the soil. Therefore, in this study, grape pomace from three red and three white grape varieties from the Palatinate region was examined for its properties as an organic fertilizer in vineyards. In the first step, the composition of the dry grape pomace was determined. The pomace from white grape varieties had a glucose and fructose content of \(110-180\,g/kg\) each, while the pomace from red grape varieties had a content of \(3-19\,g/kg\) each. The nitrogen content of all pomaces ranged between \(11-27\,mg/kg\). The mineral content of the pomace ranged as follows: Na: \(680-1000\,mg/kg\), K: \(12000-20000\,mg/kg\), Mg: \(560-830\,mg/kg\), Ca: \(1911-5300\,mg/kg\). The total phenolic content of the pomace from white grape varieties was \(17-23\,g GAE/kg\) and that from red grape varieties was \(45-48\,g GAE/kg\). The total tannin content of the pomace from white grape varieties was \(17-23\,g GAE/kg\), and that of red grape varieties was \(45-48\,g PCE/kg\). The main components of the phenolic profile of pomace from white grape varieties were catechin, epicatechin, and B-type procyanidins. The main components of the phenolic profile of pomace from red grape varieties were glucosides of malvidin, peonidin, petunidin, and delphinidin. Pomaces from all grape varieties contained pesticide residues. The pomace examined here was comparable in all parameters to pomace described in the literature. After applying the grape pomace to the soil as fertilizer, it is primarily subject to the influence of the weather. The leaching of plant phenols from the pomace into the soil by rain was simulated in a laboratory experiment. The experiment showed that between \(6-24\,%\) of the total phenol content and between \(2-17\,%\) of the condensed tannins were washed out of the pomace into the soil by rain. The relative amount washed out was independent of the pomace used. The relative phenol profile of the leachate is comparable to that of the remaining pomace. The absolute amount of individual compounds washed out depends on the substance and ranges from \(<1-40\,%\). Hexosides of plant phenols in particular were present in reduced concentrations after irrigation. Catechin and epicatechin, as main components of the pomace, were also among the main components in the leachate. Other main components of the profile were dimeric procyanidins and gallic acid. The concentration of plant phenols from grape pomace in soil was first investigated in a laboratory experiment. In addition to fresh grape pomace, ensiled pomace was also considered as a variant of processed grape pomace. In the laboratory experiment, fresh soil from the field was mixed with dried ensiled and untreated grape pomace and examined for phenol content and composition at three time points (\(0\,d\), \(17\,d\), \(66\,d\)). The total tannin content was significantly higher in both treatments than in untreated soil only at the first time point after \(0\,d\). The total phenolic content was significantly higher at the first time point \(0\,d\) for both silage and pomace application, and at the second time point after \(17\,d\) only for silage application. The main compounds, catechin and epicatechin, were only detectable in the system at the first measurement point (\(0\,d\)). In order to record the degradation of the compounds more accurately, various known degradation products of plant phenols were screened. The concentration of protocatechuic acid in particular was able to accurately reflect the simulated degradation kinetics and was present in significantly increased amounts in the soil after \(17\,d\) in the case of silage. After \(66\,d\), no differences in the phenol profile of the treated and untreated soil were detectable. In order to verify the data obtained in the laboratory with data obtained under real conditions, an additional field trial was conducted on a vineyard. Ensiled grape pomace was applied as organic fertilizer in a vineyard, and the soil was tested for phenolic content and composition at three time points (\(19\,d\) before fertilization, \(17\,d\) and \(52\,d\) after fertilization). As expected from the laboratory test, the total phenol and total tannin content and the content of individual plant phenols remained unchanged over the period under consideration. An exception was protocatechuic acid, which was measurable in significantly higher concentrations after \(52\,d\). During the greening of the aisles \(19\,d\) before fertilization and \(52\,d\) after fertilization, increased amounts of rutin were also measurable compared to the ungreened aisles \(17\,d\) after fertilization. This effect was observed regardless of whether the alley in question was fertilized or not. In summary, it was found that when plant phenols from grape pomace are applied to soil as organic fertilizer, most of them remain in the plant material and are not washed out. There, they are either degraded or adsorbed onto soil particles within a few weeks. To date, it has been difficult to establish a direct link between the introduction of plant phenols into the soil and their effects on soil parameters. One of the biggest challenges is to evaluate the form in which the phenols are present in the soil: adsorbed, free or catabolized. This work presents a way to monitor the complex kinetics of plant phenol degradation kinetics in soil. By measuring the protocatechuic acid content in soil, the degradation of phenols and their direct catabolism products can be tracked and a connection between them and changing soil parameters can be established.