Drought-induced shift from a carbon sink to a carbon source in the grasslands of Inner Mongolia, China

Precipitation has an impact on both gross ecosystem productivity (GEP) and ecosystem respiration (Reco), which ultimately influences net ecosystem productivity (NEP). A positive NEP denotes an ecosystem functioning as a carbon sink; whereas, a negative NEP denotes an ecosystem functioning as a carbon source. Therefore, drought plays an important role in the carbon balance of an ecosystem. However, little is known about the point at which the ecosystem converts from a carbon sink to a carbon source in extreme droughts. Such knowledge is crucial for predicting terrestrial carbon cycling under climate change, and consequently, was the subject of this study. We imposed two types of drought treatments on desert-grassland: (1) press-drought, in which the quantity of natural precipitation was reduced by 66% from May to August; and (2) pulse-drought, in which the quantity of natural precipitation was reduced by 100% during June and July. Reco and NEP were measured and GEP was calculated and then regression analyses were employed to determine the point at which the carbon sink shifts to a carbon source. The regression equation of NEP (μmol·m⁻²·s⁻¹) on Reco (μmol·m⁻²·s⁻¹) took the form: NEP = 0.504 Reco – 0.086 and, consequently, when Reco equaled 0.171 μmol·m⁻²·s⁻¹, there was zero change in the carbon sink and GEP also equaled 0.171 μmol·m⁻²·s⁻¹. Below this value, the ecosystem functioned as a carbon source, whereas above this value, the ecosystem functioned as a carbon sink. Structural equation models (SEM) demonstrated that coverage, standing biomass, pH, soil water content and total soil carbon were the main driving factors on desert grassland ecosystem carbon fluxes.