Source identification and budget evaluation of eroded organic carbon in an intensive agricultural catchment
Graphical abstract
Introduction
Quantifying the carbon (C) fluxes of terrestrial ecosystems is vital for accurately evaluating the global C budget and predicting the effects of the C sink/source in the soil on the Earth's climate system (Falkowski et al., 2000, Mccorkle et al., 2016). The global soil C pool is 2.3 times larger than the atmospheric pool (760 Pg) and 3.5 times larger than the biotic pool (560 Pg) (Dungait et al., 2012, Lal, 2004a). Soil organic carbon (SOC), the primary component of the largest terrestrial C pool, plays a vital role in global C cycling (Lal, 2004a, Xin et al., 2016). Consequently, SOC redistribution by soil erosion processes and subsequent transport into depressional landforms plays an important role in C biogeochemical cycling (Battin et al., 2009, Berhe et al., 2007, Stallard, 1998).
Soil erosion, especially water erosion, facilitates the translocation of soil materials and SOC dynamics (Lal, 2003, Ran et al., 2014). Soil erosion redistributes approximately 75 Pg of soil and 1–5 Pg of SOC annually (Berhe et al., 2007, Stallard, 1998). The SOC redistribution caused by soil erosion removes C-rich topsoil from eroding uplands and buries the soil at low elevation sites. Approximately 70–90% of eroded SOC is redistributed downhill or downstream, whereas the reminder is decomposed during transport or exported through source watersheds (Doetterl et al., 2012, Lal, 2003). Soil erosion and subsequent deposition have led to a net terrestrial sink of 0.12-1.5 Pg C yr−1 globally, particularly when assessed at a watershed scale (Berhe et al., 2007, Berhe et al., 2008, Stallard, 1998, Van Oost et al., 2007). Although previous studies have shown that soil erosion can result in a C sink, the studies conducted by Lal and colleagues (Jacinthe et al., 2004, Lal, 2003, Lal, 2004a, Lal, 2004b, Starr et al., 2000) suggest that soil erosion acts as a C source in the global budget. These works indicate that soil erosion transfers up to 1.14 Pg of C per year to the atmosphere because of aggregate breakdown via rain splash and runoff turbulence (Starr et al., 2000). The difference in sink/source terms primarily depends on the form of erosion, the various approaches used in different studies and an incomplete understanding of the interactions between soil erosion and the C cycle at the catchment scale, especially the uncertainty of SOC exported from eroding catchments (Berhe et al., 2007, Doetterl et al., 2016, Liu et al., 2003). Thus, additional studies are required to further our understanding of the SOC dynamics caused by soil erosion.
The amount and composition of SOC found at low-lying sites reflect the balance among SOC inputs, outputs, and transformations processes, including decomposition and leaching (Berhe et al., 2007). The identification of sediment sources (from different landscape units) is critical because the concentration and composition of SOC in the mobilized sediment (Collins et al., 2013, Mccorkle et al., 2016) and the persistence of the eroded SOC depend on the characteristics of the source material. Therefore, tracing the movement of SOC from eroding uplands is essential for quantifying the fate of laterally transported C in different landscapes (Berhe and Kleber, 2013, Mccorkle et al., 2016).
The stable carbon isotope (13C) is of particular interest for catchment-scale assessments of SOC erosion, deposition and replacement. The stable isotope composition of C (δ13C) varies significantly among ecosystem pools due to isotopic fractionation during C cycling (O'Leary, 1988). Differences in the δ13C values of SOC pools in upland soils and sediments have been used to investigate the transport of soil materials mobilized by precipitation events in catchments and deposited in accumulated sediments (Bellanger et al., 2004, Fox and Papanicolaou, 2007). Consequently, using δ13C is a common and effective method of tracing erosion pathways (Meusburger et al., 2013).
The Chinese government has constructed a large number of check dams in small agricultural catchments to control soil and water losses in the Loess Plateau area, which has suffered from severe soil erosion (Fu et al., 2011). Check dams play an important role in carbon sequestration in the Loess Plateau ecosystems (Cao et al., 2009). By 2005, 122,028 check dams had been constructed on the Loess Plateau, and they stored more than 21 billion m3 of sediment and 95.2 Tg of eroded SOC (Ministry of Water Resources of the People’s Republic of China, 2010, Wang et al., 2011, Wang et al., 2014, Xu et al., 2004). The sediments intercepted by check dams exhibit a clear sedimentary sequence, with the thickness of a couplet varying from a few centimeters to tens of centimeters (Chen et al., 2016, Zhang et al., 2006). These characteristics are rare in natural environments (Fang et al., 2014, McConnachie and Petticrew, 2006). Consequently, the sediments trapped by check dams can serve as natural archives for reconstructing the environmental history of soil erosion in these small agricultural catchments. The main objectives of this study were to (i) identify the sources and fate of SOC affected by soil erosion in a small agricultural catchment and (ii)clarify the impacts of check dams on the carbon flux in regional ecosystems.
Section snippets
Study area
The study was conducted in the Nianyangou catchment (37°35′33′′ N to 37°35′54′′ N, 110°22′4′′ E to 110°22′28′′ E), which is located in Suide County of Shaanxi Province on the Loess Plateau. The elevation of the catchment ranges from 1027 m to 1118 m, and the mean slope gradient is 12.5°. The drainage basin exhibits terrain fragmentation and complex topography characteristics, which is vulnerable to erosion. The watershed is characterized by a temperate continental monsoon climate with a mean
Sediment couplets of the depositional profile and chronology
Field surveys suggest that the check dam began to acquire silt in 1960 and was filled in by 1990. Consequently, the bottom couplet in the profile had developed by approximately 1960, and the top couplet had developed by approximately 1990 (without considering the upper cultivated layer; Fig. 2). The 137Cs activity and precipitation data were used to date the flood couplets in the sediment sequence.
The profile distribution of the 137Cs activity (Fig. 2) included one distinct peak (4.898 Bq kg−1 in
Utility of δ13C as a tracer of eroded SOC
We found that the use of bulk organic δ13C as a tracer combined with the two-end- member mixing model was an effective method for quantitatively tracing eroded SOC back to the different landscape units in this small agricultural catchment. The vertical distributions of δ13C did not exhibit obvious increasing/decreasing trends with sediment age (Fig. 3d), indicating that negligible early diagenetic changes occurred during the accumulation period (Tareq et al., 2005), and δ13C was resistant to
Conclusions
Based on the combined use of 137Cs activity and extreme rainfall events as dating methods and δ13C as a source indicator, the sources of eroded SOC at different siltation stages were identified. The δ13C values of the bulk organic material preserved in an 11.3-m depositional profile from a check dam in the Nianyangou catchment suggested that the sedimentary SOC was mostly derived from sloping cropland surface soil. This conclusion was based on the limited fractionation of the δ13C values during
Acknowledgements
Financial support for this research was provided by the National Natural Science Foundation of China (41671282 and 41525003) and the National Key Research and Development Program of China (2016YFC0402401). We thank Dr. Isaac N. from the USA for improving the English of the manuscript.
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