Tracing the source of soil organic matter eroded from temperate forest catchments using carbon and nitrogen isotopes

Abstract

Soil erosion continuously redistributes soil and associated soil organic matter (SOM) on the Earth's surface, with important implications for biogeochemical cycling of essential elements and terrestrial carbon sequestration. Despite the importance of soil erosion, surprisingly few studies have evaluated the sources of eroded carbon (C). We used natural abundance levels of the stable and radioactive isotopes of C (13 C and 14 C) and stable isotope of nitrogen (15 N) to elucidate the origins of SOM eroded from low-order catchments along the western slopes of the Sierra Nevada of California, USA. Our work was conducted in two relatively undisturbed catchments (low elevation = 1800 m, and high elevation = 2300 m) of the Kings River Experimental Watersheds (KREW) in the Sierra National Forest. Sediment captured in basins at the outlet of each gauged watershed were compared to possible source materials, which included: upland surficial organic horizons (i.e., forest floor) and mineral soils (0-0.6 m) from three landform positions (i.e., crest, backslope, and toeslope), stream bank soils (0-0.6 m), and stream-bed materials (0-0.05 m). We found that most of the organic matter (OM) in the captured sediments was composed of O-horizon material that had high C concentrations. Radiocarbon analyses also showed that the captured OM is composed of modern (post-1950) C, with fraction modern values at or above 1.0. Our results suggest that surface (sheet) erosion, as opposed to channeling through established streams and episodic mass wasting events, is likely the largest source of sediment exported out of these minimally disturbed, headwater catchments. The erosional export of sediment with a high concentration of C, especially in the form of relatively undecomposed litter from the O horizon, suggests that a large fraction of the exported C is likely to be decomposed during or after erosion; hence, it is unlikely that soil erosion acts as a significant net sink for atmospheric CO 2 in these low-order, temperate forest catchments.

Quantifying nutrient uptake as driver of rock weathering in forest ecosystems by magnesium stable isotopes

“…In contrast, in lowland supply-limited regimes and floodplains the low bioelement concentrations in plant debris, the low particulate organic matter sediment yield (0.1-1 % of total sediment yield) (Galy et al, 2015;Hilton, 2017) and the low amorphous opal flux (0.6 % of total sediment yield) result in nutrient export to occur predominantly in the dissolved form. The postulated fast weathering and rapid nutrient erosion coupling is significant only in geologically active mountains where CWD and bio-opal erosion are high (Galy et al, 2015;McCorkle et al, 2016), outpace nutrient recycling and might constitute a significant solid export flux of elements released by weathering and hence not accounted for in weathering flux estimates based on dissolved river loads.…”

“…Our study sites are underlain by granodiorite bedrock (Bateman and Wones, 1972) and mantled by weakly developed soils comprising entisols and inceptisols (Bales et al, 2011). The main vegetation cover is Sierran mixed conifer comprising Pinus ponderosa, Pinus lambertiana, Abies concolor and Libocedrus decurrens (McCorkle et al, 2016).…”

Thermal alteration of soil organic matter properties: a systematic study to infer response of Sierra Nevada climosequence soils to forest fires

“…Moreover, in systems such as the Sierra Nevada, which are dominated by steep slopes, movement of the rain-snow transition zone upward is likely to increase proportion of precipitation that occurs as rain. The kinetic energy of raindrops and the observed increase in hydrophobicity of soils after fires (Johnson et al, , 2004 can lead to higher rates of erosional redistribution, especially for the free light fraction or particulate C that is not associated with soil minerals (Berhe et al, 2012a;Berhe and Kleber, 2013;McCorkle et al, 2016;Stacy et al, 2015).…”

Pyrogenic Carbon Erosion: Implications for Stock and Persistence of Pyrogenic Carbon in Soil

“…Generally, low intensity precipitation events drive preferential transport of light carbonaceous material (higher enrichment ratios, higher concentration of C) (Schiettecatte et al, 2008;Wang et al, 2010). However, during high intensity or longer duration rainfall events, a mixture of mineral material along with SOM, including pyrogenic carbon, is mobilized from the soil surface or even deeper soil horizons (large rainfall events lead to scouring of the surface or river banks or creation of deep rills and gullies), as was observed by Stacy et al (2015) and McCorkle et al (2016).…”

“…In particular, research has focused on the potential for erosion to constitute a net sink for atmospheric CO 2 , on the order of 0.12-1.5 Gt C y −1 (Stallard, 1998;Lal, 2003;Berhe et al, 2007;Battin et al, 2009;Regnier et al, 2013;Doetterl et al, 2016). In upland, eroding landform positions (shoulder positions), erosion leads to losses of SOM through direct removal of soil mass Harden et al, 2008;Berhe, 2012;Nadeu et al, 2012;Stacy et al, 2015;McCorkle et al, 2016). About 70-90% of the eroded topsoil material is redistributed downhill or downstream, and this material is not exported out of the source watersheds but instead is deposited in toeslope and footslope landform positions (Gregorich et al, 1998;Stallard, 1998;Lal, 2003).…”