Assessment of sources variability of riverine particulate organic matter with land use and rainfall changes using a three-indicator (d¹³C, d¹⁵N, and C/N) Bayesian mixing model

In intensive agricultural watersheds, riverine particulate organic matter (POM) may be transported from many sources such as rice paddies, crop uplands, forests, and livestock farming areas during rainy seasons. However, the impacts of land-use and rainfall changes on the POM sources are not well understood. In this study, changes in the sources of riverine POM were investigated in an agricultural area of Korea between 2014 and 2020/21. During this period, land-use and rainfall patterns changed dramatically. The delta C-13, delta N-15, and C/N of the POM sources as well as those of riverine POM were analyzed, and a stable isotope analysis in R (SIAR) model was utilized for source apportionment. There were differences in delta C-13, delta N-15, and C/N among the sources. For example, manure had higher delta C-13 (-22.6 +/- 3.3 parts per thousand) and delta C-15 (+10.6 +/- 5.9 parts per thousand) than soils (from -28.0 +/- 0.8 parts per thousand to -25.1 +/- 1.2 parts per thousand for delta C-13 and +3.6 +/- 1.7 parts per thousand to +9.8 +/- 1.4 parts per thousand for delta N-15). For soils, the delta C-13 and delta C-15 were higher for upland soils, while C/N was greater for forest soils than for others. For riverine POM, the delta C-15 marginally changed; however, the delta C-13 and C/N increased from -26.1 +/- 0.9 parts per thousand to -20.8 +/- 5.3 parts per thousand and from +7.7 +/- 1.7 to +18.8 +/- 8.3 between 2014 and 2020/21, respectively. The SIAR model showed that the contributions of paddy (from 41.0% to 14.9%) and upland fields (from 48.1% to 23.7%) to riverine POM decreased between the periods due to decreased paddy area and the implementation of best management practice on upland fields, respectively. However, the contribution of forests (from 3.5% to 28.0%) and manure (from 7.4% to 33.5%) increased probably due to improper management of forest clear-cutting sites and livestock manure storage sites. The contributions of agricultural soils to riverine POM decreased in drier years. Our study suggests that land management rather than land-use area is critical in riverine POM management, particularly in wetter years.