Water-carbon relationships and variations from the canopy to ecosystem scale in a sparse vineyard in the northwest China

Abstract

Ecosystem water and carbon fluxes are inherently coupled through stomata. A deep understanding of the water-carbon relationship contributes to model development, water and carbon management, and decision-making. This study investigated the pattern of the water-carbon relationship, the difference in water-carbon relationship at both the ecosystem and canopy scales in different growth stages in a vineyard in arid northwest China for three consecutive years (2017–2019) via an eddy covariance system combined with sap flow sensors and microlysimeters. The results showed that the significant increase in soil evaporation (E) controlled by high surface soil water content (SWC, 10 cm) led to a transformation from a linear to nonlinear relationship between daily water and carbon fluxes from the canopy to ecosystem scale. Moreover, the water-carbon relationship varied with growth stages at both the ecosystem and canopy scales. The water-carbon coupling strength (the determination coefficient of the water-carbon correlation) was higher in the early and late growing seasons than in the middle growing season. Additionally, water use efficiency (WUE) was more dependent on the carbon flux in the early and late growing seasons. However, WUE was mainly determined by water flux in the middle growing season, and the control of E surpassed that of transpiration (T) in ecosystem WUE. The results of the structural equation model (SEM) indicated that canopy development in the early and late growing seasons together with the water and radiation conditions in the middle growing season played vital roles in controlling the water-carbon relationship. This study highlighted the critical role of SWC in controlling the water-carbon relationship and the differences in the coupling in various growth stages of an irrigated vineyard in the arid northwest China.