Nutritional contribution of atmospheric deposition to the Strandveld vegetation of West Coast South Africa
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 University of Cape Town v 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 University of Cape Town vi 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.