Residential and agricultural soils dominate soil organic matter loss in a typical agricultural watershed of subtropical China

Soil organic matter (SOM) loss in agricultural watersheds driven by hydrology threatens the aquatic environment, and increases the risk of greenhouse gas emissions from aquatic systems, and will be detrimental to both the green development of agriculture and the achievement of the national “carbon peaking and carbon neutrality” goal of China. Consequently, we explored the use of δ13C, δ15N, and the carbon to nitrogen ratio (C/N) as end-members and combined this with a Bayesian stable-isotope mixed model to quantitatively estimate the contribution of typical land-use types (i.e., forest, rice, vegetable, tea, and residential soils) to sedimentary organic matter in a typical agricultural watershed in subtropical China. The results indicated that there was significant spatial variation of δ13C, δ15N, and the C/N in the target sediment mixture and its sources. The range of sediment δ13C (−26.2‰ to −23.8‰) and δ15N (3.9–7.9‰) were within the ranges of the sources, with δ13C and δ15N ranges of − 29.3‰ to − 20‰ and − 1.2‰ to 9‰, respectively. The average C/N of sediment was 11.1 ± 0.52, which was significantly higher than all source samples. The spatial distribution of δ13C and δ15N indicated that SOM loss in this catchment may primarily be attributed to vegetable and residential soils. According to the qualitative identification from dual fingerprint mark composite graphs of the C/N and δ15N, as well as δ13C and δ15N, the sedimentary SOM in the Tuojia watershed was mainly derived from sewage. Furthermore, Bayesian stable-isotope mixed model analysis indicated that residential soil was the main contributor to the SOM loss in the watershed, with a contribution rate of 75.8 %, followed by agricultural land (rice and vegetable soils) with a combined input of 12 %. The findings highlight that anthropogenic life and agricultural production have a vital impact on the carbon and nitrogen cycle in this agricultural catchment, and improving the rural habitation environment may play a key role in the control of SOM loss in subtropical agricultural watersheds.