Investigating soil nitrogen dynamics in natural, invaded and cleared fynbos riparaian ecotones and implications for riparian functioning

Fourie, Marno J. (2014-04)

Thesis (MScConsEcol)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Riparian ecotones provide a variety of ecosystems services, however they are understudied in Mediterranean-type ecosystems compared to other types of ecosystems. Riparian ecotones occupy the space between aquatic ecosystems and terrestrial environments, and they are seen as a transitional area between these two ecosystems, which allow movement of species and resources. Riparian ecotones are threatened by invasive alien plant species (IAPs) that establish after disturbance, whether it is human-induced or natural. In South Africa, particularly the fynbos region, comprising much of the Mediterranean-type ecosystem, the most prominent invasive alien plants are from Australia and belong to the genus Acacia. They are nitrogen-fixers that can have a great influence on riparian ecotone functioning if left unmanaged due to the addition of nitrogen to the system; they are also profligate users of water when growing in riparian environments, and are recognised as transformer species. The South African government started a clearing program, the “Working for Water Program” in 1995, in order to clear IAPs from riparian ecotones. Recent studies produced substantial knowledge on the effect that clearance of alien woody species have on restoration of indigenous species. However, the effect of invasion and removal of invasive species on soil processes, and potential changes in water quality in fynbos riparian ecotones have not been investigated in great detail. Even though there have been numerous investigations carried out globally in this field, we still lack relevant information for Mediterranean-type ecosystems, specifically in the Western Cape where fynbos riparian ecotones are poorly studied. This study evaluated whether invasion by Acacia spp. caused changes to net nitrogen mineralization, available nitrogen, nitrous oxide (N2O) emissions and denitrification potential and whether soil properties were changed irreversibly. The study also evaluated whether the clearing of invasive species, which is widely applied as a management practice in the Western Cape, led to restoration of riparian functioning in terms of nitrogen cycling (considering that restoration includes both recovery of structure and function) or whether the invasion led to irreversible changes in riparian ecosystems. The results from the study showed that there was no difference in the nitrogen mineralization activity from Acacia spp. invasion compared to natural (“reference”) sites and clearing the IAPs did not lead to changes in nitrogen mineralization. Nitrogen mineralization also did not differ between riparian zones (wet and dry banks) and upper terrestrial areas. Low C:N ratios in invaded and cleared sites implied that carbon was limiting and were responsible for the lack of differences in the nitrogen mineralization activity. There was more NH4 + and NO3 - available for uptake by plants and/or soil micro-organisms in the invaded sites than natural and cleared sites. There was also more NH4 + and NO3 - available in the dry banks than the wet banks and the terrestrial sites and NH4 + became more dominant over time. Clearing of invasive alien Acacia spp. led to a change in the availability of nitrogen towards preinvasion conditions as similar levels of total available nitrogen for the cleared sites (9.65μg.g-1) compared to the natural sites (7.87μg.g-1) were measured. The field N2O emissions were similar for the invaded and cleared sites compared to the natural sites and were most likely the result of the sandy soil that limited denitrification. The maximum denitrification potential was similar across different landscape positions and under different invasions statuses and the N2O emissions from the field and laboratory incubations were in the same order of magnitude, which implied that N2O emissions from the field is not only the product of anaerobic denitrification, but also a product of aerobic soil respiration processes such as nitrogen mineralization. Results from the study were used to discuss implications for the “Working for Water Program” and for restoration of riparian functioning and the effects of invasion in general. The effects of invasive alien Acacia spp. were visible in areas where clearing efforts has not yet taken place. It is possible that soil recovery processes have taken place in the areas that were cleared from IAP’s compared to natural and invaded sites. It is very important that clearing efforts be monitored and that follow-up clearing takes place in order to prevent further negative impacts of enhanced nitrogen inputs to the soil underneath the Acacias. It is important for “Working for Water” to keep on clearing IAP’s from riparian ecotones, especially when there is the potential for restoring riparian ecotones through the natural process of denitrification.

AFRIKAANSE OPSOMMING: Rivieroewers verskaf ‘n verskeidenheid ekosisteem dienste, maar hulle is swak bestudeer in Mediterreense ekosisteme in vergelyking met ander tipes ekosisteme. Rivieroewers kom voor in die ruimte tussen akwatiese ekosisteme en terrestriële omgewings, en word gesien as ‘n oorgangsone tussen die twee ekosisteme, wat die beweging van spesies en hulpbronne moontlik maak. Rivier oorgangsones word deur eksotiese indringer plante bedreig wat hulleself na versteruring vestig, ongeag of die versteuring menslik of natuurlik is. In Suid-Afrika, veral die fynbos streek, wat ‘n groot deel van die Mediterreense ekosisteem uitmaak, kom die mees prominente eksotiese indringer plante in die algemeen van Australië af en behoort tot die genus Akasië. Hulle is stikstof-fikseerders wat ‘n groter impak kan hê op die funksionering van rivier oorgangsones as gevolg van die toevoeging van stikstof tot die ekosisteem indien hulle nie aktief bestuur word nie; hulle is ook buitengewoon groot gebruikers van water as hulle langs die rivieroewers groei en word ook herken as omskakeling spesies. Die Suid-Afrikaanse regering het ‘n program, die “Working for Water Program”, in 1995 geloods om eksotiese indringer plante langs riveroewers te verwyder. Baie inligting is gegenereer deur onlangse studies oor die effek van die verwydering van houtagtige eksotiese spesies op die herstel van inheemse plantegroei. Die effek wat die verwydering van eksotiese indringer spesies op grondprosesse en potensiële veranderinge in water kwaliteit van fynbos rivier oorgangsones kan hê was nog nie deur vorige studies in diepte ondersoek nie. Alhoewel daar ‘n verskeidenheid van ondersoeke op ‘n globale skaal in dié veld uitgevoer word, is daar steeds ‘n tekort aan ondersoeke in Mediterreense ekosisteme, veral in die Wes-Kaap waar fynbos rivier oorgangsones nog swak bestudeer is. Hierdie studie het bepaal of die indringing deur Akasië spp. veranderinge aan die stikstof mineralisasie, N2O gas emissies en denitrifikasie potensiaal gebring het en of dit onherstelbaar verander was. Hierdie studie het ook die verwydering van eksotiese indringer spesies, wat ‘n algemeen toegepaste bestuurspraktyk in die Wes-Kaap is, ge-evalueer om te bepaal of dit tot die herstel van rivieroewer funksionering lei in terme van nutriënt siklering (in gedagte dat die herstel beide strukturele en funksionele herstel insluit) en of die indringing tot onomkeerbare veranderinge in rivier ekosisteme lei. Die resultate van die studie het bewys dat die stikstof mineralisasie aktiwiteit van die areas wat deur die indringer plante besmet is in vergelyking met natuurlike areas nie verander het nie en dat daar geen verskil is as die indringer plante verwyder word nie. Daar was ook geen verskil in die stikstof mineralisasie aktiwiteit van rivieroewers en die hoërliggende terrestriële areas nie. Lae C:N verhoudings in besmette en skoongemaakte areas impliseer dat loging van NO3- geskied het en dat dit die soortgelyke stikstof mineralisasie aktiwiteit kan verduidelik. Daar was meer NH4+ en NO3- beskikbaar vir opname deur plante en/of grondmikrobes in die besmetteareas as die natuurlike en skoongemaakte areas. Daar was ook meer NH4+ en NO3- beskikbaar in die droër dele van die rivieroewers as die nat dele en die hoërliggende terrestriële areas en NH4+ het meer dominant geraak met die verloop van tyd. Die verwydering van die eksotiese indringer Akasië spp. het moontlik gelei tot ‘n verandering in die beskikbaarheid van stikstof met soortgelykke vlakke as die natuurlike areas wat ondersteun word deur soortgelyke vlakke in beskikbare stikstof in skoongemaakte areas (9.65μg.g-1) en natuurlike areas (7.87μg.g-1). Die werklike veld N2O emissies was dieselfde in die besmetteareas as die areas waar indringers verwyder was en kan heel moontlik wees as gevolg van die sanderige aard van die rivieroewers wat nie voordelig vir die denitrifikasie proses is nie. Die maksimum denitrifikasie potensiaal was soortgelyk in al die verskillende landskapposisies en indringer status en die N2O emissies van die veld en laboratorium inkubasies was in dieselfde graad van orde, wat impliseer dat veld N2O emissies nie net die produk van anaerobiese denitrifikasie is nie, maar ook van aerobiese grond respirasie prosesse soos stikstof mineralisasie. Resultate van die studie was gebruik om implikasies vir die “Working for Water Program” en herstel van rivieroewer funksionering en die algemene impak van eksotiese indringer plante te bespreek. Die impak van eksotiese indringer Akasië spp. was sigbaar in areas waar verwydering van dié plante nog nie plaasgevind het nie. Dit is moontlik dat grond herstel prosesse alreeds plaasgevind het in die areas waar eksotiese indringer plante verwyder was in vergelyking met natuurlike en besmette areas. Dit is baie belangrik dat die verwyderingsproses gemoniteer word en dat opvolg verwydering van indringer plante plaasvind om verdere negatiewe impakte van stikstof insette tot die grond onder die Akasiës te voorkom. Dit is belangrik vir “Working for Water” om aan te hou om indringerplante van rivieroewers te verwyder, veral wanneer daar potensiaal is om die areas te herstel deur die natuurlike proses van denitrifikasie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/86766
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