[Tracing the sources of sedimentary organic matter in Nanyue small watershed based on 13C, 15N and C/N]

Abstract

Losses of organic matter in agricultural watersheds result in eutrophication and land degra-dation, which not only threaten water quality and food security, but also lead to environmental problems such as the greenhouse gases emission. We used ¹³C, ¹⁵N and C/N as fingerprint markers to trace the sources of sedimentary organic matter at the outlet in the Nanyue small watershed. We analyzed the spatial distribution in watershed sedimentary organic matter and soils of typical land use types, including forest, paddy field, and vegetable fields. The Bayesian stable isotope mixing model was used to quantitatively estimate the contribution of different sources. The results showed that there was significant spatial variation of δ¹³C. The δ¹³C of sediment organic matter (-22.6‰±0.53‰) and forest soil (-23.13‰±1.71‰) was significantly higher than that of paddy soil (-25.24‰±1.4‰). The differences of δ¹⁵N among the sources were not significant, with sediment having the maximum (4.37±0.83)‰ and forest soil having the minimum (2.38±1.97)‰. Forest soil had the highest C/N of 16.66±7.18, while paddy soil had the lowest C/N of 11.95±0.92. The results of the Bayesian stable isotope mixture model showed that the contribution rates of forest land, paddy fields and vegetable fields to the organic matter deposited at the outlet in the watershed were 19.6%, 15.7%, and 64.7%, respectively. Paddy filed and vegetable field had a combined contribution rate of 80.4%. It was concluded that, soils of agricultural land were the main sources of organic matter deposited in the Nanyue small watershed, and that nutrient loss in the watershed would be effectively controlled by optimizing farmland management.

Keywords: Bayesian stable isotope mixing model; land use type; nutrient loss; sedimentary organic matter; source.