Catchment-wide variations and biogeochemical time lags in soil fatty acid carbon isotope composition for different land uses: Implications for sediment source classification

Stable carbon isotope values of fatty acids (>20 C-atoms) (delta C-13-FAs) have been increasingly used to apportion sediment sources using isotope mixing models. Understanding the variation of delta C-13-FAs within catchment land uses is crucial for correct classification of sediment sources but it has rarely been evaluated. Here, we assessed the variability of delta C-13 values of FAs (C22-32) in soils under land uses within a catchment (23 km(2)) in the mid-hills of Nepal. High delta C-13-FA variability in agricultural terraces (similar to 32.8% to -20.2 parts per thousand) and mixed forest (-37.3 parts per thousand to -22.3 parts per thousand) was observed. This is due to differences in inputs of FAs of different chain-lengths with variable delta C-13-FAs values by C-3 and C-4 crops as well as from farmyard manure in agricultural terraces. The pine plants succession in the forest and differences in FA preservation explained the observed variation in delta C-13-FA values in a mixed forest. A pine forest transect that had undergone land cover change about 40 years ago showed high variability of delta C-13-FA values strongly linked to legacy effects of previous C-4 crops. Importantly, soils from the border of a land use and fresh surface deposits mostly increased the within-land use variability of delta C-13-FAs values. Overall, delta C-13-FA values have low potential to function as a robust tracer for high resolution discrimination of land uses and the validity of a priori sediment source classification is most questionable in catchments that have undergone land cover change. Cluster analysis was promising for identifying and combining functionally similar land uses to define more meaningful sediment sources at the catchment scale. Crown Copyright (C) 2020 Published by Elsevier Ltd. All rights reserved.