The Use of Grape Pomace as Soil Fertilizer: A Microcosm Study on Available C and N and Consequences for Microbial Biomass, Structure, and Catabolic Responses

Grape pomace (GP) is used as a fertilizer in viticulture due to its carbon (C) and nitrogen (N) richness. Its application follows as biologically treated (i.e., vermicompost) or as untreated (fresh), with different nutritional inputs for the soil. But constraints exist regarding the amount and mobility of nutrients as a function of the soil type and the short-term effects on the soil microbiota. In a 6 week microcosm study, we analyzed the C and N dynamics in two agricultural soils (loamy sand and silt loam) after fresh red or white GP application. Microbial responses including biomass, respiration, and ecophysiological indices were recorded at the end of the experiment. White GP increased the available C (dissolved organic carbon) in the soils compared to the control, with a greater availability in sandy loam compared to silt loam soil. Dissolved (available) N in the treated soils did not differ by GP variety or soil type, but values were lower than those in the controls, suggesting a rapid N assimilation. Red GP in the sandy loam soils accounted for the highest total phenolic content (TPC) compared to the white GP. Independently of the GP variety or treatment, values reached control levels after 6 weeks. In the GP treated soils, microbial C/N ratios were narrower compared to the controls. The ergosterol to microbial carbon ratio indicated a higher fungal fraction in the GP treated soils; in particular, in the sandy loam soil, that fits with the availability of nutrients in the respective soils. The GP treatment increased the ratios MBC:TC (microbial C:total C) and MBN:TN (microbial N:total N), independently of the GP variety but with larger ratios in the silt loam soil, indicative of nutrient immobilization. q_CO2 metabolic quotients were, in general, higher in the GP treated soils compared to the controls, with the highest values in sandy loam and red GP. The highest substrate utilization (respiration) rate was observed in the silty sandy soil, attributed mainly to the microbial biomass fraction compared to the sandy loam soil. But the functional diversity was not affected by the soil or by the GP treatment. We observed significant correlations between single chemical parameters and microbial indices apart from q_CO2, suggesting that the response of the microbiome is multifactorial but driven mainly by the composition of the GP and by the availability of nutrients which in turn depends on the soil properties. This study enables a broader understanding on the consequences of the application of fresh GP varieties in soils with different properties, which is necessary for calculations of optimal nutritional inputs.