Mobility of total carbon, nitrogen and polyphenols from grape pomace in the soil column

Grape pomace (GP) can be legally applied as an organic fertilizer in the vineyards in Germany. Some risks are associated with this common practice, since grape pomace is observed to have a high carbon to nitrogen ratio and contains bioactive secondary metabolites. Despite these concerns, up to date little is known about the mobility of substances in the vineyard soil. In this study, our goal was to investigate the mobility of the macronutrient content of GP, derived from four Rhineland palatinate grape varieties, in three different soils in a column model. We used a three-step lab-scale approach that included the analysis of total carbon (C), nitrogen (N) and polyphenolic content (TPC) to analyse the mobility in:

1) the GP, representing the maximum total amount

2) the rainwater, representing the aqueous extractable fraction of the total amount

3) the soil column, as the soil-mobile fraction, as well as the leachate

Our results showed that up to 4 % of the total polyphenolic content of the pomace is leached into the soil. The recovery in the soil strongly depends on the combination of soil type and grape variety investigated. Generally, sandy and acidic soils showed an even distribution of phenolics with a high recovery rate (up to 92 %) of the water extractable amount. Most polyphenols could be recovered from the upper soil layer (0-10 cm). Despite the low pH of GP, there was no effect on soil pH. The same holds true for the C/N ratio. These results give a first impression of the mobility of macronutrients in the soil using a column model, supporting the need for incubation experiments that aim for the effect of the application on biogeochemical processes.