Labile carbon limits in-stream mineralization in a subtropical headwater catchment affected by gully and channel erosion

Gully and channel erosion are known to export large quantities of soil organic matter (SOM) to stream ecosystems. However, the implications for in-stream processing of SOM ultimately depend on its susceptibility to mineralization. We studied the influence of carbon (C) and nitrogen (N) lability on fine sediment organic matter mineralization following a high flow event in a small severely eroded headwater catchment in south-eastern Queensland, Australia. High-flow event stream water was incubated for 20 days under aerobic conditions in the dark, with four treatments: control, glucose enriched, glycine enriched, and glucose + glycine enriched. Destructive sampling was carried out at 0, 2, 6, 12, and 20 days to quantify different C, N, and phosphorus (P) fractions and specific UV absorbance (a proxy for aromaticity). Net C mineralization was very slow for all treatments with rates slowing markedly towards the end of the incubation. The addition of labile N did not significantly increase net C mineralization; however, net N mineralization significantly increased with the addition of labile C in the absence of labile N. We found increasingly larger net N mineralization relative to C towards the end of the incubation, even though initial substrate stoichiometry in the control indicated that N was the limiting element. Our results suggest that SOM mineralization is limited by C and not N bioavailability. One of the main implications of our study is that mineral N exported downstream in association with gully and channel eroded sediment has little influence on the in-stream processing of eroded organic C due to slow mineralization rates and fluxes in the water column. As a consequence, this mineral N would be available for generating primary productivity in downstream aquatic ecosystems.