Provisioning of bioavailable carbon between the wet and dry phases in a semi-arid floodplain

Abstract

Ecosystem functioning on arid and semi-arid floodplains may be described by two alternate traditional paradigms. The pulse-reserve model suggests that rainfall is the main driver of plant growth and subsequent carbon and energy reserve formation in the soil of arid and semi-arid regions. The flood pulse concept suggests that periodic flooding facilitates the two-way transfer of materials between a river and its adjacent floodplain, but focuses mainly on the period when the floodplain is inundated. We compared the effects of both rainfall and flooding on soil moisture and carbon in a semi-arid floodplain to determine the relative importance of each for soil moisture recharge and the generation of a bioavailable organic carbon reserve that can potentially be utilised during the dry phase. Flooding, not rainfall, made a substantial contribution to moisture in the soil profile. Furthermore, the growth of aquatic macrophytes during the wet phase produced at least an order of magnitude more organic material than rainfall-induced pulse-reserve responses during the dry phase, and remained as recognizable soil carbon for years following flood recession. These observations have led us to extend existing paradigms to encompass the reciprocal provisioning of carbon between the wet and dry phases on the floodplain, whereby, in addition to carbon fixed during the dry phase being important for driving biogeochemical transformations upon return of the next wet phase, aquatic macrophyte carbon fixed during the wet phase is recognized as an important source of energy for the dry phase. Reciprocal provisioning presents a conceptual framework on which to formulate questions about the resistance and ecosystem resilience of arid and semi-arid floodplains in the face of threats like climate change and alterations to flood regimes.

Spatial dissimilarities in plankton structure and function during flood pulses in a semi-arid floodplain wetland system

“…They have intricate channel-floodplain networks with variable and unpredictable wet and dry phases related to changes in hydrology and geomorphology (e.g. Shiel et al, 2006;Ralph & Hesse, 2010;Ralph et al, 2011;Baldwin et al, 2013a). As such, floodplain wetlands in a spatially complex and temporally dynamic landscape provide a diversity of hydro-geomorphic habitats for biota that occupy ecological niches created and maintained by river flows and floods (Bayley, 1991;Bunn et al, 2006;Rogers & Ralph, 2011).…”

Managing rain-filled wetlands for carbon sequestration: a synthesis

“…Despite macropod movement being less restricted than that of domesticated stock, stable isotope analysis of macropod scats found on floodplains suggests that faecal material is deposited locally rather than being exported elsewhere in the landscape (Iles et al 2010). Kangaroo faecal material can reach quantities up to 193 g m À2 following flooding, and may be an important C source for wetland consumers (Kobayashi et al 2009;Baldwin et al 2013). However, the quality of the C in kangaroo faecal material and its contribution to wetland C stores is unclear.…”

“…Differences in hydrology, topography, morphology, vegetation type, climate, soil moisture and soil pH affect wetland capacity to store C. Hydrology is a critical driver of wetland ecosystem function, particularly C capture and storage (Bunn and Arthington 2002;Adhikari et al 2009;Baldwin et al 2013). Wetland plant communities are shaped by the duration, timing and frequency of inundation (Casanova and Brock 2000), whereas soil dynamics and associated biogeochemical processes, such as decomposition, change with flooding (Wilson et al 2011).…”

Restoring dissolved organic carbon subsidies from floodplains to lowland river food webs: a role for environmental flows?

“…For example, from an agricultural perspective, floods can be a much more efficient way of recharging the soil-moisture profile and groundwater than is rainfall (e.g. Baldwin et al 2013), resulting in a significant increase in post-flood primary production on the floodplain compared with rainfall responses (Baldwin et al 2013); other examples have been canvassed by Opperman et al (2009). Inundation of floodplains can also result in substantial potential benefits for the supply of ecosystem services (Zedler and Kercher 2005;Opperman et al 2010).…”

“…Second, this increase in basal resources can promote higher rates of secondary production by aquatic invertebrates (including the emergence of zooplankton from a propagule bank), which fuels aquatic food webs, that is, the so-called floodpulse concept (Junk et al 1989;Winemiller 2004). Furthermore, by recharging soil moisture and rejuvenating semi-permanent wetland habitat, floods contribute to greater terrestrial primary production during the inter-flood period, which further enhances resource availability during subsequent flood events by reciprocal provisioning (Baldwin et al 2013).…”