Catchment-wide variations and biogeochemical time lags in soil fatty acid carbon isotope composition for different land uses: Implications for sediment source classification. (August 2020)

• Record Type: Journal Article
• Authors: Upadhayay, Hari Ram; Griepentrog, Marco; Bodé, Samuel; Bajracharya, Roshan Man; Cornelis, Wim; Collins, Adrian L.; Boeckx, Pascal
• Is Part Of: Organic geochemistry. Volume 146(2020)
• Publication Date: 2020-08
• Subjects: Long-chain fatty acids -- Cluster analysis -- Land terraces -- Headwater -- Sediment fingerprinting; Organic geochemistry -- Periodicals; Biogeochemistry -- Periodicals; Géochimie organique -- Périodiques; 553.205
• DOI: 10.1016/j.orggeochem.2020.104048
• Languages: English
• ISSNs: 0146-6380

Abstract

Highlights: Variations in fatty acid δ 13 C values for land uses were assessed at catchment scale. Highly variable δ 13 C-FAs values reflected vegetation composition and change. Paired δ 13 C values of fatty acids and bulk carbon detected C4 crops legacy effects. A priori land use classification for sediment sources was questionable. Sediment source classification required combining functionally similar land uses. Abstract: Stable carbon isotope values of fatty acids (>20 C-atoms) (δ 13 C-FAs) have been increasingly used to apportion sediment sources using isotope mixing models. Understanding the variation of δ 13 C-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 δ 13 C values of FAs (C22–32 ) in soils under land uses within a catchment (23 km 2 ) in the mid-hills of Nepal. High δ 13 C-FA variability in agricultural terraces (−32.8‰ to −20.2‰) and mixed forest (−37.3‰ to −22.3‰) was observed. This is due to differences in inputs of FAs of different chain-lengths with variable δ 13 C-FAs values by C3 and C4 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 δ 13 C-FA values in a mixed forest. A pine forest transect that had undergone land cover change about 40 years ago showed high variability of δ 13 C-FA values strongly linked to legacy effects of previous C4 crops. Importantly, soils from the border of a land use and fresh surface deposits mostly increased the within-land use variability of δ 13 C-FAs values. Overall, δ 13 C-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.