Sources and proportions of modern versus aged organic carbon eroded from soils under different land-use within a Nepalese catchment – Insights from bulk and compound-specific 13C & 14C analysis in combination with novel isotope mixing models

Abstract

There is currently a lack of necessary measurements and data to fully capture the high degree of heterogeneity inorganic carbon (OC) exported from eroding landscapes. The respective fluxes of modern (biospheric) and aged OCare poorly constrained, and mechanisms controlling OC export have remained elusive. Recently, the age of riverineOC was found to increase with the proportion of human-dominated landscapes within a catchment, potentially dueto aged soil OC that is mobilized by human disturbance and reintroduced into the modern C cycle (Butman et al.,2015). Furthermore, most previous studies have focused on catchment outlets, whereas a greater within-catchmentfocus will undoubtedly provide further direction to pinpoint locations of aged OC export to the river, and to betterconstrain its impact on the C cycle and aquatic biogeochemistry.This study aims (1) to quantify the sources and proportions of modern versus aged OC exported from soils intoriver systems within a catchment and (2) to investigate if theses sources and proportions of OC are dependent onthe different types of land-use within a catchment. Therefore, bulk14C and compound-specific14C analyses ofvegetation markers (long-chain fatty acids, FAs) in soils under different land-use and suspended river sedimentswill be performed and the proportions of modern versus aged OC determined by application of isotope mixingmodels.The study site is located in the mid-hill region of Nepal within a catchment that exhibits different land-uses includ-ing various types of forest and agricultural land-use. Soils were sampled before and sediments during monsoonseasons. A broad data basis is already available for the content and stable C isotopes (δ13C) of bulk OC and FAs.Fatty acids will be collected in sufficient amounts by preparative gas chromatography and will be analyzed for14Cusing accelerator mass spectrometry. Isotope mixing models that use bulk and compound-specific13C and14Cdata will be applied to estimate the relative contributions of modern versus aged OC.The study will generate an extensive14C dataset of bulk OC and specific vegetation markers in soils under differentland-use and suspended river sediments to provide a within-catchment focus on the mechanisms controlling OCexport from terrestrial and aquatic ecosystems. Combination with the available complementary datasets for13Cof bulk OC and specific vegetation markers will reveal the sources and proportions of modern versus aged OCwithin the catchment. This information is needed to develop accurate constraints on fluvial transfer of modernOC, which is of importance to predict OC fluxes quantitatively as a result of anthropogenic change. The studyfurthermore aims to test whether soils under different land-use export different proportions of modern versus agedOC into river systems and thus determines the responds to human disturbance such as changes in soil management.In summary, our study will contribute to a comprehensive understanding of the factors that control the sources,transport pathways and turnover times of terrestrial OC within river systems.