Modelling the diurnal and seasonal dynamics of soil CO₂ exchange in a semiarid ecosystem with high plant–interspace heterogeneity

Abstract. We used process-based modelling to investigate the roles of carbon-flux (C-flux) components and plant–interspace heterogeneities in regulating soil CO₂ exchanges (F_S) in a dryland ecosystem with sparse vegetation. To simulate the diurnal and seasonal dynamics of F_S, the modelling considered simultaneously the CO₂ production, transport and surface exchanges (e.g. biocrust photosynthesis, respiration and photodegradation). The model was parameterized and validated with multivariate data measured during the years 2013–2014 in a semiarid shrubland ecosystem in Yanchi, northwestern China. The model simulation showed that soil rewetting could enhance CO₂ dissolution and delay the emission of CO₂ produced from rooting zone. In addition, an ineligible fraction of respired CO₂ might be removed from soil volumes under respiration chambers by lateral water flows and root uptakes. During rewetting, the lichen-crusted soil could shift temporally from net CO₂ source to sink due to the activated photosynthesis of biocrust but the restricted CO₂ emissions from subsoil. The presence of plant cover could decrease the root-zone CO₂ production and biocrust C sequestration but increase the temperature sensitivities of these fluxes. On the other hand, the sensitivities of root-zone emissions to water content were lower under canopy, which may be due to the advection of water flows from the interspace to canopy. To conclude, the complexity and plant–interspace heterogeneities of soil C processes should be carefully considered to extrapolate findings from chamber to ecosystem scales and to predict the ecosystem responses to climate change and extreme climatic events. Our model can serve as a useful tool to simulate the soil CO₂ efflux dynamics in dryland ecosystems.