| dc.description.abstract | Ecosystem nutrient availability depends on the balance between rates of nutrient
inputs and losses. Nutrients may be lost through fire and displacement of ash,
herbivory, leaching and volatilization. The main pathways through which nutrients
may be acquired are weathering of rock and atmospheric deposition. Symbiotic and
free-living diazotrophic bacteria and blue green algae also contribute N. In
ecosystems with limited occurrence of N2-fixation and occurring on low-nutrient
bedrock, atmospheric deposition is the most significant source of nutrients. Nutrients
from atmospheric deposition may be of natural or anthropogenic origin, and can be
“wet-deposited” dissolved in precipitation and “dry-deposited” when aerosols settle
out of the atmosphere onto plant and soil surfaces. Studies on nutrient cycling
around the world suggest that nutrient deposition can provide substantial amounts of
nutrients to coastal ecosystems, although mineral weathering of rocks can also a
significant source. Limited prior work on deposition in coastal areas of South Africa
suggests that nutrient deposition could be an important component of nutrient
budgets in the Cape Floristic Region. The west coast of South Africa borders a
section of the Atlantic Ocean that is highly productive and characterized by strong
seasonal winds, rough waters and strong wave action. This area is home to the
Strandveld vegetation, which grows on marine-derived soils. Based on this, I
hypothesized that marine aerosol deposition is a significant source of nutrients for
the vegetation in west coast South Africa. To test this hypothesis, I examined the
spatial and temporal characteristics of atmospheric deposition as well as the climatic
and ecological characteristics of the area. I measured deposition rates and
concentrations of essential plant nutrients (N, P, Na, Ca, Mg, and K) delivered in rain
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and horizontal precipitation. Horizontal precipitation was used to refer to all forms of
precipitation deposited horizontally and included fog, windblown aerosols, and
horizontal rainfall. I then estimated annual demand for these nutrients in 8 plant
species growing in the area and compared them to the deposition rates measured in
rain. I also compared nutrients deposited in rain water with those deposited in
horizontal precipitation, measured the amounts of NO3
-, NH4
+ and PO4
3- held in
canopies of the 8 plant species during summer, and estimated the species’ capacity
for foliar nutrient uptake.
The Strandveld vegetation was found to have relatively high soil and plant
nutrient concentrations compared to the rest of the CFR, despite its soils originating
as nutrient-poor marine derived aeolian sands. Although N and P fluxes deposited in
rain were lower than those measured in other pristine sites around the world, a large
proportion of TN (84%) and TP (51%) was organic, pointing to a strong marine
influence. The marine origin of N and P is supported by the high base cation fluxes
compared to those reported globally. The high proportion of organic N and P, and
the high base cation contents was also observed in horizontal precipitation. In this
form of deposition, base cation concentrations were highest at the coast and
contents declined with distance from the ocean, further supporting a possible marine
source. This study also suggests that dust may be an important contributor to the
deposition of some nutrients during the winter months, and both marine and
terrestrial areas could therefore be important sources of nutrient deposition to this
area. Based on leaf litter nutrient losses it was estimated that atmospheric deposition
through rain alone could potentially supply 36% and 64% of N and P annual
demand, respectively, and over 100% of the annual demand for K and Ca. This
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suggests a strong marine influence in the supply of these nutrients to the Strandveld
soils and vegetation. In addition, plants within the Strandveld vegetation intercepted
substantial amounts of moisture and nutrients in their canopies. Species with small
leaves intercepted significantly greater quantities of water and nutrients than those
with larger leaves. It was also established that all the studied Strandveld plants could
take up NO3
–, NH4
+, glycine (as a form of organic N) and Li (a proxy for K) through
their leaves.
Taken together, these results show that the Strandveld ecosystem of West
Coast National Park receives substantial inputs of nutrients from marine aerosols,
both in rain and horizontal precipitation. This deposition appears to be a critical
source of nutrients in an ecosystem with limited bedrock nutrient supplies. Over the
time scale of ecosystem development, atmospheric nutrient deposition combined
with other ecological characteristics, such as strong moisture-laden winds, may help
explain the unique biogeochemical and biogeographical characteristics of the
Strandveld. | en_US |
