Phenological transition dictates the seasonal dynamics of ecosystem carbon exchange in a desert steppe

QuestionThere are large variations in ecosystem carbon (C) exchange in desert ecosystems; however, few studies have examined the effects of community phenological staging on seasonal variations of ecosystem C exchange. We asked whether factors that control temporal changes in net ecosystem C exchange (NEE) vary with an obvious community transition from spring annuals to summer annuals and perennials in a temperate desert steppe. LocationSouth margin of the Gurbantunggute Desert, northwestern China. MethodsEcosystem C and water exchange were measured regularly using closed static chambers and analysed at daily and seasonal intervals. Soil moisture and temperature, photosynthetically active radiation (PAR) and plant biomass were also investigated. ResultsSoil temperature had a dominant influence on C release into the atmosphere from the ecosystem during the snowmelting phase (mid-March to early April). In the spring annual dominant phase (mid-April to early June), the diurnal pattern of NEE was consistent with the pattern of PAR. The ecosystem became a weak C resource (0.16 +/- 0.03 mu mol CO2 m-2 center dot s-1) and NEE was positively correlated with community biomass during this phase. In the summer annual and perennial dominant phase (late June to late September), NEE showed relatively large C release (0.74 +/- 0.03 mu mol CO2 m-2 center dot s-1) and was negatively correlated with soil temperature. ConclusionOur results indicate that the primary abiotic factor controlling NEE varies throughout the year, and NEE is determined by the interaction of a plant functional group with precipitation and temperature.