Relationship of Leaf Stable Isotopes (δ¹³C and δ¹⁵N) to Biomass Production in Two Fertilized Merlot Vineyards

Abstract

Grapevines (Vitis vinifera L.) are often cultivated under suboptimum conditions for both water and nutrients in order to enhance the quality of grapes for making wine. Since water supply and nitrogen nutrition are major determinants of photosynthetic activity, this study investigated the potential of leaf δ¹³C and δ¹⁵N to explain spatial variations in biomass production as a function of water use efficiency and fertilizer N uptake, respectively. The combined isotopic signal of the leaves, together with pruning weight, leaf nutrients, and soil properties, were measured in randomly selected field positions over two growing seasons in two fertilized, but different, Merlot vineyards. Significant correlations of surface soil properties and leaf nutrients with pruning weight were not great enough to explain spatial patterns in biomass production. In contrast, leaf δ¹³C and δ¹⁵N, when used as independent variables in multiple regression, explained 71% of the spatial variation of pruning weight across both fields and growing seasons. Most of the explained model variability was attributed to leaf δ¹⁵N (r² = 0.54), but within single years leaf δ¹³C was better correlated to biomass. The negative correlation between these two isotopes and pruning weight was high only within the vineyard that had steep topographic features and spatially variable growth patterns. It indicated a response of vines to adverse soil conditions by a progressive reduction of water use efficiency and an increase in fertilizer N uptake downslope from the low- to the high-biomass areas of this field. If these relationships persist in other fertilized and water-stressed fields, the isotopic signature of the leaves may be an important tool for application of site-specific management practices within single vineyards.