Relationship and determinants of water and carbon dioxide (CO2) exchange in desert ecosystem, China

Knowledge on relationship and determinants of water and carbon dioxide (CO2) exchange is crucial to land managers and policy makers especially for the desertified land restoration. However, it remains highly uncertain in terms of water use and carbon sequestration for artificial plantation in desert. Here, the continuous water and carbon fluxes were measured using eddy covariance (EC) in conjunction with hydrometeorological measurements over an artificial C4 shrub, Haloxylon ammodendron (C. A. Mey.) Bunge, from July 2020 to 2021 in Tengger Desert, China. In the entirely 2021, evapotranspiration (ET) was 189.5 mm, of which 85% (150 mm) occurred during growing season, that was comparable with precipitation (132.2 mm) plus dew (33.5 mm) and potential other sources (e.g. deep subsoil water). This ecosystem was a strong carbon sink with net ecosystem production (NEP) up to 446.4 g C m-2 yr-1, which was much higher than surrounding sites. Gross primary production (GPP, 598.7 g C m-2 yr-1) was comparable with other shrubs but ecosystem respiration (Re, 152.3 g C m-2 yr-1) was lower. Random Forest showed that environmental factors can explain 71.56% and 80.07% variation of GPP and ET, respectively. However, environmental factors have divergent effect on water and carbon exchange, of which soil hydrothermic factors (e. g. soil moisture content and soil temperature) determine the magnitude and seasonal pattern of ET and Re, while aerodynamics factors (e.g., net radiation, atmospheric temperature and wind speed) determine GPP and NEP. As such, divergent response of abiotic factors resulted in the decoupling of water and carbon exchange. Our results suggest that H. ammodendron was a suitable species for large-scale afforestation in desert or desertification-prone region given its low water use but high carbon sequestration. Therefore, we concluded that artificial planting H. ammodendron in dryland could provide an opportunity for climate change mitigation, however, the long-term time series data would be needed to confirm it.