Variable source and age of different forms of carbon released from natural peatland pipes

Abstract

We used the carbon isotope composition ( 14 C and δ 13 C) to measure the source and age of DOC, POC, dissolved CO 2 and CH 4 (δ 13 C only) released from three natural peat pipes and the downstream catchment outlet of a small peatland in northern England. Sampling under different hydrological extremes (high flows associated with storm events and low flows before or after storms) was used to explore variability in C sources as flow paths change over short periods of time. The δ 13 C composition of organic C differed (δ 13 C-DOC -28.6‰ to -27.6‰; δ 13 C-POC -28.1‰ to -26.1‰) from that of the dissolved gases (δ 13 C-CO 2 -20.5‰ to +1.1‰; δ 13 C-CH 4 -67.7‰ to -42.0‰) and showed that C leaving the catchment was a mixture of shallow/deep pipe and non-pipe sources. The isotopic composition of the dissolved gases was more variable than DOC and POC, with individual pipes either showing 13 C enrichment or depletion during a storm event. The 14 C age of DOC was consistently modern at all sites; POC varied from modern to 653 years BP and evasion CO 2 from modern to 996 years BP. Differences in the isotopic composition of evasion CO 2 at pipe outlets do not explain the variability in δ 13 C and 14 C at the catchment outlet and suggest that overland flow is likely to be an important source of CO 2. Our results also show that the sources of CO 2 and CH 4 are significantly more variable and dynamic than DOC and POC and that natural pipes vent old, deep peat CO 2 and POC (but not DOC) to the atmosphere.