Quantifying the contribution of soil organic matter turnover to forest soil respiration, using natural abundance δ13C

Quantifying the loss of soil carbon through respiration has proved difficult, due to the challenge of measuring the losses associated with the turnover of soil organic matter (SOM) as distinct from autotrophic components. In forest ecosystems the delta C-13 value of respiration from turnover of SOM (delta(CRSOM)-C-13) is typically 2-4 parts per thousand enriched compared with that from roots and associated microbes (delta(CRROOTS)-C-13), with that from the litter (delta(CRLITTER)-C-13) lying between the two. We measured soil respiration at 50 locations in a forest soil and then used differences in isotopic signatures to quantify the proportion of soil respiration arising from the turnover of SOM (fR(SOM)) at a subset of 30 locations, chosen randomly. The soil surface CO2 efflux was collected using an open chamber system supplied with CO2-free air and the delta C-13 signature (delta(CRS)-C-13) measured, giving a mean (+/- SD) value across the site of -26.1 +/- 0.58 parts per thousand. The values of delta(CRROOTS)-C-13, delta(CRLITTER)-C-13 and delta(CRSOM)-C-13 were measured at each location by incubation of roots, litter and root-free soil and collection of the CO2 for isotopic analysis. delta(CRSOM)-C-13 became progressively depleted with length of incubation (1.5 parts per thousand, after 8 h), so CO2 was collected after 20 min. The mean value of delta 13CR(LITTER) was -27.2 +/- 0.68 parts per thousand, which was indistinguishable from delta(CRROOTS)-C-13 of -27.6 +/- 0.51 parts per thousand while delta(CRSOM)-C-13 was -25.1 +/- 0.88 parts per thousand, delta(CRROOTS)-C-13 and delta(CRSOM)-C-13 measured at each location were used as the end points of a two component mixing model to calculate fR(SOM), giving a mean value for fR(SOM) of 0.61 +/- 0.28. It was not possible to estimate fR(SOM) using the total C contents of the roots and soil which were significantly depleted in C-13 in comparison to their respired CO2. However, at seven locations the delta(CRS)-C-13 was slightly enriched compared with delta(CRSOM)-C-13 (mean 0.3 parts per thousand), which was not considered significantly different so fR(SOM) was constrained to 1.0. If these seven rings were excluded mean fR(SOM) was 0.49 +/- 0.20. We have shown the possibility of using natural abundance C-13 discrimination to quantify fR(SOM) in a forest soil with an input of carbon only from C-3 photosynthesis. (C) 2010 Elsevier Ltd. All rights reserved.