Turnover rates of hydrolysable aliphatic lipids in Duke Forest soils determined by compound specific 13C isotopic analysis

Ester bound, long chain fatty acids (e-LFAs), hydroxy fatty acids (FAs) and di-acids are key components of the hydrolysable aliphatic lipids in soil organic matter (SOM). Cutin and suberin are important sources of these SOM components, which may contribute to the recalcitrant soil carbon pool. However, their carbon isotopic composition and turnover rates are largely unknown. This study represents the first compound specific isotopic analysis (CSIA) of major hydrolysable lipids in the surface mineral soil horizons from the Duke Forest, North Carolina, USA. The investigated lipids include hydroxy FAs and di-acids that only originate from cutin and suberin (i.e., cutin- and suberin-derived compounds) and e-LFAs that originate from plant waxes as well as cutin and suberin. These compounds were 2–7‰ more depleted in 13C as compared to the bulk organic matter in plant tissues as well as in soils. Furthermore, they were more depleted in 13C in plant tissues produced under the elevated versus ambient CO2 concentration in the free air CO2 enrichment (FACE) experiment. Under a steady-state condition, this isotopic signature was incorporated into the hydrolysable lipids in the surface soil at a rate of 0.029–0.031 year−1. This turnover rate corresponds to a mean residence time (MRT) of 32–34 year, which is longer than that of bulk SOM but much shorter than that expected for recalcitrant SOM components. We suggest that a significant fraction of cutin and suberin is transformed into non-hydrolysable SOM, which may not be accounted for in the hydrolysable fraction of SOM.