Kinetics of net nitrogen mineralisation from soil-applied grape residues

In viticulture, the long-term balance of organic matter in vineyard soil is often only ensured by the annual recycling of grape residues. The decay of soil-applied grape pomace contributes to a slow but steady input of nitrogen (N). This input path is commonly neglected in the annual fertiliser recommendation because little is known about N mineralisation from grape residues. In order to parameterise a simulation model to describe this process, a multi-temperature (4°C, 20°C, 28°C and 36°C) 14-month laboratory incubation experiment at constant moisture level was performed with residues from both hand-picked (complete clusters) and machine-harvested (destemmed) grapes applied to three different vineyard soils (7, 13 and 24% clay). 15.9 ± 5.8% of the total N of the pomace made from hand-picked grapes was released on average from a slowly decomposing pool (half-life t₅₀ = 1,119 d at 10°C), while 24.0 ± 8.2% of the total N was estimated to be released from the same pool of destemmed grape pomace (t₅₀ = 1,227 d). A rapidly decomposing pool could not be determined. Rapid decomposition was presumably inhibited due to the high content of polyphenol in the pomace, causing a temperature-dependent delay. The net N mineralisation of complete grape cluster residues was retarded for 12.4 days (10°C) on average, whereas destemmed grapes started to release N on average 5.5 days (10°C) later than the control soil. The inhibitory effect was much smaller at higher temperatures. A suitable approach that takes this effect into consideration in a model is presented.