The response of ecosystem drying and rewetting in a gravel bed river
Soil and sediment respiration in habitats within the floodplain of the Fiume Tagliamento in north-eastern Italy were studied. The observed habitats (terrestrial: forest, floodplain forest, islands, large wood accumulation and gravel, aquatic: pond and channel) showed significant (p<0.05) differences in in situ respiration rates of soils or sediments. Comparing the terrestrial habitats gravel sediment was found to have the lowest mean respiration rate (0.24 μmol CO2 m-2 s-1), whereas floodplain forest soils (3.10 μmol CO2 m-2 s-1) and islands soils (3.13 μmol CO2 m-2 s-1) had the highest mean respiration. Comparing the two aquatic habitats channel was found to have significantly (p<0.05) higher sediment respiration than pond.
The in situ measurements covered only a small temperature range. Therefore no correlation between respiration rate and temperature could be found. Moisture showed no correlation with terrestrial in situ respiration either.
Four factors explained about 62% of the variation in respiration rate at a reference temperature of 15°C in the 4 terrestrial habitats within the active floodplain: organic matter content of the soil fraction > 2 mm, total organic matter content, grain size fractions > 8 mm and < 0.063 mm.
Floodplain forest soils and sediments of large wood accumulation showed larger within-habitat variation in respiration than their diurnal variability, indicating respiration to be relatively stable at each site and the habitat to be very heterogeneous.
The influence of inundation and rewetting events on respiration was experimentally tested on soils and sediments from floodplain forest, gravel, islands and large wood accumulation. Soils and sediments were wetted and inundated at two temperatures (12°C & 20°C). Generally rewetted soils showed higher respiration rates compared to post inundated ones. A correlation for respiration rate and temperature was found in the laboratory measurements. Respiration under inundated conditions was significantly (p<0.05) lower
compared to the in situ respiration for floodplain forest soils, island soils and large wood accumulation sediments.
Soil and sediment respirations in different habitats within a floodplain were affected by a combination of soil and sediment characteristics and hydrological conditions.