In Australia, significant effort goes into reducing the amount of nitrogen and phosphorus
entering inland waters from point sources. However, little is known of the extent to
which riparian organic matter may act as a source of these nutrients. Also, whilst the relationships
between the nitrogen, phosphorus and carbon cycles are broadly known, there
is little quantitative data regarding the release of these elements from Australian riparian
organic matter and their subsequent microbial mineralisation within aquatic environments.
In particular, comparatively little is known of their comparative role in nutrient and organic
matter cycling within anoxic zones, and the influence that different riparian organic matter
may have on stream water quality. This lack of such data presently hampers the ability of
water managers to make educated decisions regarding the management of riparian zones
in Australia. In order to improve understanding in this area, a combination of laboratory
and in situ experiments were carried out in order to compare the abiotic release and aerobic/
anaerobic mineralisation of leaf derived dissolved organic carbon (DOC), dissolved
nitrate/nitrite (NOx) and soluble reactive phosphorus (SRP) under different environmental
conditions. Four plants common to Australian riparian zones were investigated: two native
species, Eucalyptus camaldulensis (gum) and Phragmites australis (common reed), and
two exotic species, Salix babylonica (willow) and Lolium multiflorum (rye grass). After 30
days, formaldehyde inhibited 1g willow and rye grass extracts contained the most SRP (0.7
mg/L), whilst gum extracts contained 0.3 mg/L and common reed 0.1 mg/L of SRP.Willow
and rye grass abiotically released twice as much NOx than gum and common reed, although
concentrations were only between 0.05-0.1 mg/L. Gum and common reed released the most
DOC per gram of leaf matter (14 and 12 mmol/g of leaf matter respectively), but based on
the initial carbon content of each leaf type, the largest percentage contributor of DOC under
abiotic conditions was common reed and rye grass (both 38% mass/mass), with gum (33%
mass/mass) and willow (30% mass/mass) being smaller contributors. The most bioavailable
DOC was released by rye grass and common reed, with between 83 and 94% of this
DOC microbially mineralised after 30 days in oxic conditions. When conditions were not
inhibited, microbial growth was evident almost immediately in willow, rye grass and common
reed leaf extracts. However, microbial growth was suppressed for the first 48 hours
in gum leaf extracts. After this suppression period, the rate of DOC mineralisation was
equal in willow and gum leaf extracts (0.1 day-1). Under anoxic conditions, the rate and
extent of DOC mineralisation of willow and gum leaves depended on the type of electron
acceptor provided. Added nitrate and iron III enhanced the mineralisation of both willow
and gum leaves relative to no terminal electron acceptors (from zero to 0.01-0.04 and 0.002-
0.004 moles/day respectively), but added sulphate only enhanced the mineralisation of gum
leaves (0.04 moles/day). When no additional electron acceptors were provided, particulate
leaf mineralisation was more extensive under oxic than anoxic conditions. However, the
mineralisation of leaf derived DOC were the same regardless of oxygen availability, and
after 35 days in either condition the percentage of leaf DOC mineralised for each leaf type
was of the order common reed > rye grass > willow > gum. All the leaf types tested were
able to sustain the caddis fly larvae Triplectides australis under controlled laboratory conditions,
and survival rates were high using all four leaf types as a food source. Triplectides
australis did not significantly increase the amount of DOC released from each type of leaf
matter, but they did consistently increase the proportion of simple carbohydrates present
within the DOC fraction. The results of these experiments suggest that changes to riparian
vegetation, particularly from the native to exotic species used in this study, will inherently
alter in-stream concentrations of dissolved carbon and nutrients (particularly SRP). This
potentially will affect in-stream, hyporheic and subsurface processes, particularly in areas
where surface water flow is low and riparian leaf inputs are high.
| Identifer | oai:union.ndltd.org:ADTP/218734 |
| Date | January 2005 |
| Creators | Conway, Carol Leza, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Carol Leza Conway |
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