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

[摘要] ENGLISH ABSTRACT: Riparian ecotones provide a variety of ecosystems services, however they are understudied inMediterranean-type ecosystems compared to other types of ecosystems. Riparian ecotones occupythe space between aquatic ecosystems and terrestrial environments, and they are seen as atransitional area between these two ecosystems, which allow movement of species and resources.Riparian ecotones are threatened by invasive alien plant species (IAPs) that establish afterdisturbance, 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 arefrom Australia and belong to the genus Acacia. They are nitrogen-fixers that can have a greatinfluence on riparian ecotone functioning if left unmanaged due to the addition of nitrogen to thesystem; they are also profligate users of water when growing in riparian environments, and arerecognised 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 theeffect that clearance of alien woody species have on restoration of indigenous species. However, theeffect of invasion and removal of invasive species on soil processes, and potential changes in waterquality in fynbos riparian ecotones have not been investigated in great detail. Even though there havebeen numerous investigations carried out globally in this field, we still lack relevant information forMediterranean-type ecosystems, specifically in the Western Cape where fynbos riparian ecotones arepoorly studied. This study evaluated whether invasion by Acacia spp. caused changes to net nitrogenmineralization, available nitrogen, nitrous oxide (N2O) emissions and denitrification potential andwhether soil properties were changed irreversibly. The study also evaluated whether the clearing ofinvasive species, which is widely applied as a management practice in the Western Cape, led torestoration of riparian functioning in terms of nitrogen cycling (considering that restoration includesboth recovery of structure and function) or whether the invasion led to irreversible changes in riparianecosystems.The results from the study showed that there was no difference in the nitrogen mineralization activityfrom Acacia spp. invasion compared to natural ('reference) sites and clearing the IAPs did not lead tochanges 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 impliedthat carbon was limiting and were responsible for the lack of differences in the nitrogen mineralizationactivity. There was more NH4+ and NO3- available for uptake by plants and/or soil micro-organisms inthe invaded sites than natural and cleared sites. There was also more NH4+ and NO3- available in thedry 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 preinvasionconditions 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 sitesand were most likely the result of the sandy soil that limited denitrification. The maximumdenitrification potential was similar across different landscape positions and under different invasionsstatuses and the N2O emissions from the field and laboratory incubations were in the same order ofmagnitude, which implied that N2O emissions from the field is not only the product of anaerobicdenitrification, 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 forrestoration of riparian functioning and the effects of invasion in general. The effects of invasive alienAcacia spp. were visible in areas where clearing efforts has not yet taken place. It is possible that soilrecovery processes have taken place in the areas that were cleared from IAP's compared to naturaland invaded sites. It is very important that clearing efforts be monitored and that follow-up clearingtakes place in order to prevent further negative impacts of enhanced nitrogen inputs to the soilunderneath the Acacias. It is important for 'Working for Water to keep on clearing IAP's from riparianecotones, especially when there is the potential for restoring riparian ecotones through the naturalprocess of denitrification.

[发布机构] Stellenbosch University