Abstract
Abstract. Soils represent the largest reservoir of organic carbon (OC) on land. Upon mobilization, this OC is either returned to the atmosphere as carbon dioxide (CO2) or transported and ultimately locked into (marine) sediments, where it will act as a long-term sink of atmospheric CO2. These fluxes of soil OC are, however, difficult to evaluate, mostly due to the lack of a soil-specific tracer. In this study, a suite of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane lipids of soil bacteria, is tested as specific tracers for soil OC from source (soils under arable land, ley, grassland, and woodland) to sink (Loe Pool sediments) in a small catchment located in southwest England (i.e. Carminowe Creek draining into Loe Pool). The analysis of brGDGTs in catchment soils reveals that their distribution is not significantly different across different land use types (p>0.05) and thus does not allow land-use-specific soil contributions to Loe Pool sediments to be traced. Furthermore, the significantly higher contribution of 6-methyl brGDGT isomers in creek sediments (isomerization ratio (IR) = 0.48±0.10, mean ± standard deviation (SD); p<0.05) compared to that in catchment soils (IR = 0.28±0.11) indicates that the initial soil signal is substantially altered by brGDGT produced in situ. Similarly, the riverine brGDGT signal appears to be overwritten by lacustrine brGDGTs in the lake sedimentary record, indicated by remarkably lower methylation of branched tetraethers (MBT5ME′=0.46±0.02 in creek bed sediments and 0.38±0.01 in lake core sediments; p<0.05) and a higher degree of cyclization (DC = 0.23±0.02 in creek bed sediments and 0.32±0.08 in lake core sediments). Thus, in this small catchment, brGDGTs do not allow us to trace soil OC transport. Nevertheless, the downcore changes in the degree of cyclization and the abundance of isoprenoid GDGTs produced by methanogens in the Loe Pool sediment do reflect local environmental conditions over the past 100 years and have recorded the eutrophication history of the lake.
Highlights
Around 1500–2000 Pg of carbon is stored in soils in the form of organic matter, which is about 2 times the amount of carbon in the atmosphere and 3 times the amount of carbon in vegetation (Janzen, 2004; Smith, 2008)
BrGDGTs were tested as a tracer for the transport of soil organic carbon (OC) from different vegetation and land use types from source to sink (Loe Pool) in the Carminowe Creek catchment with the aim of reconstructing the provenance of the soil OC in Loe Pool sediments over time
Temperature and precipitation can be considered equal for all soils due to the small size of the catchment, changes in branched and isoprenoid tetraether (BIT) index values and the relative contribution of 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) along a part of the hilltop transects indicate that soil water content (SWC) may exert a control on brGDGT signals, assuming that SWC increases downslope
Summary
Around 1500–2000 Pg of carbon is stored in soils in the form of organic matter, which is about 2 times the amount of carbon in the atmosphere and 3 times the amount of carbon in vegetation (Janzen, 2004; Smith, 2008). It is hard to determine the exact amount of soil OC that is transported to the ocean, as the dynamic processes that soil OC undergoes during transport, such as degradation and sequestration, are elusive. This is mostly due to the lack of a specific tracer to distinguish soil OC from the total pool of OC that is comprised of plant-derived OC, aquatically produced OC, and fossil OC from rock erosion (Blair et al, 2004; Aufdenkampe et al, 2011)
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