Temporal and spatial patterns of N2O and CO2 emissions in a semiarid Mediterranean riparian zone, contrast between a wet and dry year

Francesc Sabater1, Susana Bernal2, Eduardo Martin1, Clara Romero2, Alexandra Serra2, Joan L. Riera1, Lidia Cañas1, Esperança Gacia2, Eugènia Martí2
1 Ecology Department, University of Barcelona
2 Center for Advanced Studies of Blanes (CEAB-CSIC). Spain

O 6.1 in Biogeochemistry of wetlands

15.07.2014, 11:00-11:20, H17

Riparian zones are shown to be hot-spots of greenhouse gas emissions, especially in temperate-humid regions. In contrast, gas emissions in riparian zones from Mediterranean regions may be constrained in time and space by water scarcity; and thus, soil water availability may dictate the spatio-temporal variability of riparian gas emissions in these regions. To explore this question we examined the temporal and spatial variation of N2O and CO2 emission rates in a Mediterranean riparian zone across a stream-riparian gradient and under contrasting hydrological conditions. Additionally, we identified the environmental factors controlling the variability in gas emissions. The study started in May 2011, just after the riparian zone was entirely inundated after a 50-year recurrence flood, and it lasted until October 2012, once the stream flow resumed after an extremely dry summer. Rates of gas emissions were measured fortnightly using in situ close chambers distributed at 18 sampling plots across the stream-riparian profile, which followed a gradient in soil texture (from sandy to clay-loam soil) and in the relative abundance of hydrophytes. At each plot, we also measured soil moisture, temperature, REDOX, pH, soil nitrate, total C and N, organic matter content, and percentage of grass coverage. The N2O emission rates ranged from 0.5 to 8 μg N m-2 h-1, being highest during the wet summer and near the stream edge and lowest during the dry summer and at uphill locations. The CO2 emission rates varied widely during the study period, ranging from 0.2 to 20 g CO2 m-2 h-1, with highest rates measured at uphill locations and lowest rates measured at the stream edge. Emission rates for the two gasses showed a remarkable seasonal pattern, with higher rates during the vegetative than during the dormant period, though the amplitude of this temporal variability was higher during the dry than during the wet year. We also observed that the spatial variability across the riparian gradient was higher during the vegetative than the dormant period. Factors controlling the spatial variation of gas emission rates differed between the wet and the dry year. During the wet year, soil moisture and REDOX were the most relevant factors of both gas emissions. During the dry year, organic matter, temperature, and soil porosity were the main drivers of gas emissions. Moreover, N2O emissions were nil until a soil moisture threshold of 20% was reached, suggesting that this gas emissions are more susceptible to water scarcity than CO2 emission. In conclusion, gas emissions from this Mediterranean riparian soil were highly responsive to hydrological conditions, though soil attributes such as organic matter and porosity contributed to their spatial variability. In this sense, the marked seasonality and the recurrence of floods and droughts of the Mediterranean regions provide a unique opportunity to examine ecosystem responses to extreme climate conditions within short-time scales.

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Letzte Änderung 03.04.2014