Nutritional Contribution Of Atmospheric Deposition To The Strandveld Vegetation Of West Coast South Africa

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.