Isotope fractionation and 13C enrichment in soil profiles during the decomposition of soil organic matter

Abstract

The mechanisms behind the (13)C enrichment of organic matter with increasing soil depth in forests are unclear. To determine if (13)C discrimination during respiration could contribute to this pattern, we compared delta(13)C signatures of respired CO(2) from sieved mineral soil, litter layer and litterfall with measurements of delta(13)C and delta(15)N of mineral soil, litter layer, litterfall, roots and fungal mycelia sampled from a 68-year-old Norway spruce forest stand planted on previously cultivated land. Because the land was subjected to ploughing before establishment of the forest stand, shifts in delta(13)C in the top 20 cm reflect processes that have been active since the beginning of the reforestation process. As (13)C-depleted organic matter accumulated in the upper soil, a 1.0 per thousand delta(13)C gradient from -28.5 per thousand in the litter layer to -27.6 per thousand at a depth of 2-6 cm was formed. This can be explained by the 1 per thousand drop in delta(13)C of atmospheric CO(2) since the beginning of reforestation together with the mixing of new C (forest) and old C (farmland). However, the isotopic change of the atmospheric CO(2) explains only a portion of the additional 1.0 per thousand increase in delta(13)C below a depth of 20 cm. The delta(13)C of the respired CO(2) was similar to that of the organic matter in the upper soil layers but became increasingly (13)C enriched with depth, up to 2.5 per thousand relative to the organic matter. We hypothesise that this (13)C enrichment of the CO(2) as well as the residual increase in delta(13)C of the organic matter below a soil depth of 20 cm results from the increased contribution of (13)C-enriched microbially derived C with depth. Our results suggest that (13)C discrimination during microbial respiration does not contribute to the (13)C enrichment of organic matter in soils. We therefore recommend that these results should be taken into consideration when natural variations in delta(13)C of respired CO(2) are used to separate different components of soil respiration or ecosystem respiration.

Carbon isotopic composition of branched tetraether membrane lipids in soils suggest a rapid turnover and a heterotrophic life style of their source organism(s)

“…In the Rowden grassland, where a shallow depth profile has been sampled, δ 13 C values of both TOC and the nalkanes increase slightly with depth, an observation that has been previously made and for which potential explanations include increasing biodegradation and incorporation of microbial biomass with depth (and time) and, though small, the Suess effect, i.e. the decrease in δ 13 C values of atmospheric CO 2 over the last 200 years (Huang et al, 1996;Ehleringer et al, 2000 and references therein; Bostrom et al, 2007). Also, carbon input via root biomass and root exudates into the deeper soil layers might have an important, though not yet well investigated, effect on carbon isotopic compositions along a soil profile (e.g.…”

Bulk organic geochemistry of sediments from Puyehue Lake and its watershed (Chile, 40°S): Implications for paleoenvironmental reconstructions

“…Even after burial of the soil, soil organic matter (SOM) frequently decomposes further, resulting in significant variations in its geochemical composition (Wynn, 2007). C/N ratios typically decrease with depth (e.g., Boström et al, 2007;Nierop et al, 2007) due to the microbial immobilization of nitrogenous material accompanied by the remineralization of carbon (Meyers and Ishiwatari, 1993). Therefore, litter has a higher C/N ratio than the humus derived from it, which has in turn a higher C/N ratio than the organic matter incorporated in soil profiles (Post et al, 1985).…”

“…The δ 13 C of SOM commonly increases with depth by 1 to 6 ‰ relative to the isotopic composition of the original biomass (Boström et al, 2007;Wynn, 2007). The mechanisms behind this process are still unclear but involve preferential decomposition of certain components, variable mobility of sorption of dissolved organic carbon with variable isotopic values, kinetic discrimination against 13 C during respiration and microbes as precursors of stable organic matter (Boström et al, 2007).…”

“…The mechanisms behind this process are still unclear but involve preferential decomposition of certain components, variable mobility of sorption of dissolved organic carbon with variable isotopic values, kinetic discrimination against 13 C during respiration and microbes as precursors of stable organic matter (Boström et al, 2007). The δ 15 N of soil organic matter similarly increases up to 10‰ with depth (Nadelhoffer and Fry, 1988).…”

Impact of a Low Severity Fire on Soil Organic Carbon and Nitrogen Characteristics in Japanese Cedar Soil, Yamagata Prefecture, Japan

“…The moving ashes on the slope can clog soil pores and thus seal the soil surface leading to a decrease in water holding capacity and eventually increasing runoff and soil erosion (Renard et al, 1997;Do Socorro da Silva, 1997;DeBano, 2000;Certini, 2005). In the B-site the distribution of soil δ 13 Calong the slope was even because of the origin of the samples; the A horizon is thicker at the bottom of the B-site probably caused by erosion leading to a higher number of samples taken from this 12 C enriched horizon (Dzurec et al, 1985;Melillo et al, 1989;Garten et al, 2000;Boström et al, 2007;Du et al, 2014). …”