Gully erosion reduces carbon and nitrogen storage and mineralization fluxes in a headwater catchment of south‐eastern Queensland, Australia

Abstract

Increased erosion associated with land use change often alters the flux of sediments and nutrients, but few studies have looked at the interaction between these disrupted cycles. We studied the effects of gully erosion on carbon and nitrogen storage in surface soil/sediment and herbaceous vegetation and on C and N mineralization in a headwater catchment used for cattle grazing. We found significantly lower C and N stored in an incising gully compared with an intact valley. This storage was significantly higher in an adjacent stabilizing gully, although not to the levels found in the intact valley. The intact valley had two to four times higher soil/sediment concentrations of total organic C, total N and Colwell extractable P than the incising gully. Lower storage was not explained by differences in vegetation biomass density or silt and clay content. Vegetation accounted for only 8% of C and 2% of N storage. Although not a significant store in itself, vegetation has an important indirect role in restoring and maintaining soil/sediment C and N stocks in eroding areas. We found significant linear relationships between C and N mineralization rates and soil/sediment C and N content, with lower rates occurring in the eroded sediment. These findings support our initial hypothesis that gully erosion reduces C and N storage and mineralization rates in eroding catchments. The implications of this study include a change to the quality of eroded sediments in headwater catchments, causing C‐poorer and N‐poorer sediments to be exported but overall loads to increase. Copyright © 2013 John Wiley & Sons, Ltd.

Controls on carbon and nitrogen export in an eroding catchment of south‐eastern Queensland, Australia

“…For smaller flows, there is evidence that vegetation litter deposited within gullies and the stream system is being mobilised (Figure ), in that N content of particles transported for small flows is similar to vegetation litter N content found by Garzon‐Garcia et al . () in this catchment (in light grey in Figure b). The C sediment content for small flows (Figure a) was much smaller than C vegetation litter content (Garzon‐Garcia et al ., ), which could be due to C solubilisation/mineralisation.…”

“…() in this catchment (in light grey in Figure b). The C sediment content for small flows (Figure a) was much smaller than C vegetation litter content (Garzon‐Garcia et al ., ), which could be due to C solubilisation/mineralisation. As flow increases, mineral fractions would mobilise starting with the richer smallest fractions previously deposited within gullies and ending with heavier bank subsoil particles <63 µm, which are the poorest of the fine fraction (Figure ).…”

“…These are considered to be the most likely sources contributing to the sediment exported from the gullied sub-catchments. Bulk TOC and TN content for these same sediment sources had been previously quantified (Garzon-Garcia et al, 2013). Six soil/sediment samples (sieved to <5 mm) from each source were composited.…”

“…In each case, the six samples were chosen randomly from a statistically representative group from each source (samples with TOC and TN % content lying within the first and third quartiles, respectively). Details of sampling methods can be found in the study of Garzon-Garcia et al (2013).…”

“…The effect of erosion on C and nutrient storage and export is less well understood. In a recent paper, it was shown that gully erosion reduced the storage of both C and N in headwater catchments in south-eastern Queensland (Garzon-Garcia et al, 2013). Sediment and associated nutrients pose a threat to high-value coastal systems and increase the risk of eutrophication in receiving waters (Mosisch et al, 2001;Abal et al, 2005).…”