The preservation of photosynthetic and hydrological signals in the carbon and hydrogen isotope compositions of n-fatty acids in the seasonal wetland soils of the Okavango Delta (Botswana)

Abstract

The Okavango wetland (Botswana) is the world’s largest inland delta. A strong seasonality in water input leads to the contraction and extension of wetlands in the floodplains. The extreme evapotranspiration and little precipitation lead to a difference in the hydrogen isotope signature of rain, soil and river water. Biomarkers, such as plant waxes, are stored in the soils and preserved on geological timescales. To understand which signal is preserved in the stable isotope signatures of plant waxes, soils along a 250 m-long transect spanning waterlogged to dry soils were collected over several seasons and three years. In addition, plants, and plant and soil water were collected along this transect. First, carbon isotope ratios (δ¹³C) of plant waxes (i.e, n-fatty acids) were used to classify their metabolism. δ¹³C of bulk organic matter and individual n-fatty acids analyzed in the soils show a strong dependance on the type of vegetation found along the transect (C₃ versus C₄ plants). Hydrogen isotope ratios (δ²H) of water present in soil showed that shallow-rooted C₄ grasses use superficial soil water, whereas the xylem water δ²H content in trees growing near the flooded channel indicated the use of river water. In addition, plant hydrogen fractionation between lipids and rain showed a strong influence of carbon metabolisms with larger fractionation for C₃ plants compared with C₄ grasses. n-fatty acid δ²H ratios in surface soils followed the hydrological variation in the Delta with its floods and dry periods. Hence δ²H of long-chain fatty acids seems to track the river-level variation rather than precipitation.