Disentangling the soil and atmospheric stress on carbon sequestration in a Mediterranean pine forest

Sequestration of atmospheric CO$_2$ in a Mediterranean semi-arid Aleppo Pine Forest (Pinus halepensis) close to the border of the semi-arid timberline was characterized and quantified under field conditions. Measurements of organic and inorganic CO$_2$ sequestration with gas exchange and stock counting approaches were made in both rainfed control (approximately 12$%$ average annual soil moisture) and summer irrigated plots (approximately 24$%$ annual average soil moisture), providing the opportunity to separate the effects of atmospheric water demand from soil water stress on the atmospheric CO$_2$ sequestration responses. Measurements yield an organic carbon sequestration (OCS) rate of approximately 550 g CO$_2$ m$^{-2}$ yr$^{-1}$, two-thirds in soil and one-third in biomass. In addition, measurements yield an inorganic carbon sequestration (ICS) rate of approximately 216 g CO$_2$ m$^{-2}$ yr$^{-1}$; via calcite (CaCO$_3$) precipitation in the soil due to root exhalation of CO$_2$ (60$%$) and microbial activity (40$%$). The drip irrigated plot showed approximately 3 times higher organic CO$_2$ sequestration than the control plot. The organic sequestration is divided equally between the soil and the biomass. For the irrigated plot, the inorganic CO$_2$ was approximately 1.8 times higher than that of the control plot. However, for inorganic CO$_2$ sequestration, the soil moisture would need to be maintained lower than that of the study plot to preclude dissolving precipitated calcite. For many drylands, irrigation could be achieved by using fossil water reserves. These measured values demonstrate the relatively high potential carbon sequestration in Mediterranean drylands forests under irrigated and non-irrigated conditions.