The relative importance of grassed alleys in C dynamics of open-canopy vineyards

Vineyards have the potential to act as potential carbon sinks due to various characteristics such as low soil disturbance, high biodiversity and long-term carbon reservoirs. In mountainous regions where soil stability and erosion are priorities, grassed alleys are essential to vineyard management. Cover cropping is also frequently employed to improve soil quality and provide various ecosystem services. Among them, these practices are generally considered to have a positive impact on carbon sequestration, although there is still debate over the extent of this. Disentangling the carbon fluxes of grapevines and resident herbaceous vegetation as well as the in-plant allocation is essential for understanding the effects of management decisions and environmental conditions on the fate of sequestered carbon.

To this end, we conducted continuous carbon flux measurements over the growing season of 2021 (15 April – 15 November) using an eddy covariance tower mounted in a grassed and irrigated hillside vineyard in Alto Adige, Italy. The cultivars present were Chardonnay and Sauvingon blanc on SO4 rootstock (average density: 6500 vines ha ^-1) and the vines were trained in a vertical shoot position manner with Guyot pruning. Eddy covariance measurements were complemented with surveys using soil respiration chambers and biometric measurements of net primary production (NPP).

Results showed that the seasonal gross primary production (GPP) of the vineyard was very high (2409 ± 35 g C m^-2) relative to other studies, but was closely matched by the carbon lost as respiration (Reco; 2163 ± 88 g C m^-2), of which the majority originated from the soil. The resulting carbon accumulation during the season (NEP, net ecosystem production; 246 ± 54 g C m^-2) was moderate, and a large portion was accounted for by the berries exported after harvest. The grassed alleys played an important role carbon assimilation, accounting for roughly half of the above-ground vegetative growth for the season. 25% of the final carbon storage was attributed to the growth of permanent grapevine organs. Periods of summer heat in combination with relatively long absences of rain occurred, during which the NEP decreased and drought stress was observed in the grass cover but not the grapevines.

In comparison with other studies reported in literature, the patterns of observed ecosystem fluxes of our site more closely resembled managed grasslands in the area than forests or other vineyards, possibly due to the open structure of the canopy, which may differ between vineyard training systems. However, literature suggests that the biomass produced by the grasses is more easily decomposed than the grapevine leaf litter and pruning material, so although the ground cover accounts for a large portion of the carbon accumulated, we speculate that it contributes proportionally less to long-term storage by increasing soil organic carbon. Therefore, while changing climate conditions may adversely affect the short-term carbon sequestration of vineyards, they are likely to have less of an impact on long-term accumulation attributable to grapevines.