Abstract
The addition of carbon (C) to the soil as sucrose has been suggested as a countermeasure to reduce plant available nitrogen (N) and increase the competitive advantage of slower growing native perennial species over faster growing annual species. To make this approach a successful restoration tool, C addition must induce the resident soil bacteria and fungi to immobilize plant available soil nutrients. In this study, both the efficacy of sucrose applications as a restoration aid and their dependence on soil microbial activity were examined in field and greenhouse trials. Carbon as sucrose (200gm−2) was added to normal and sterilized soils containing various combinations of native perennial and annual species. Their effects on soil N levels, as well as on the photosynthetic efficiency, growth and N uptake of the introduced native species, were measured. Diminished foliar chlorophyll contents, effective quantum yields (ΔF/Fm′) of Photosystem II (PSII) and dry mass accumulation in response to sucrose applications were observed in both the annual and perennial introduced species, but were not reflected in corresponding reductions in soil N levels. These sucrose-induced inhibitory effects, as well as diminished plant N uptake, were more pronounced in normal than sterilized soils. This implied a bacterial component immobilizing soil N essential for plant photosynthesis and growth. However, this premise was partly contradicted by the unaltered total bacterial numbers following sucrose application in the normal soils, although coliform numbers did increase with sucrose application in these soils. These findings point to a likely abiotic mechanism of sucrose-induced inhibition of photosynthesis and growth in introduced native plants, which renders sucrose application ineffectual as a restoration aid in transformed lowland fynbos ecosystems.
Highlights
Southern African Mediterranean climate fynbos ecosystems, included among 34 global biodiversity hot spots (Mittermeier et al, 2004), are heavily fragmented, with up to 95% of some, e.g. renosterveld, transformed through agriculture and viticulture (Milton, 2004)
The reductions in foliar chlorophyll and ΔF/Fm′ were commonly observed among the introduced native perennials, not evident in their dry mass accumulation due to their high premature mortalities (N 90%) over the dry summer season
These findings do not conform to previous reports that sucrose applications inhibit the growth of faster growing early seral species with high nutrient demands to a greater extent than that of slower growing late seral species adapted to low nutrient environments (Eschen et al, 2006)
Summary
Southern African Mediterranean climate fynbos ecosystems, included among 34 global biodiversity hot spots (Mittermeier et al, 2004), are heavily fragmented, with up to 95% of some, e.g. renosterveld, transformed through agriculture and viticulture (Milton, 2004). Alien grasses of temperate and subtropical origin proliferate in these highly fragmented ecosystems (Steinschen et al, 1996) and are known to impact on ecosystem structure, function and resources (D'Antonio and Vitousek, 1992) Their recent increase in abundance in lowland areas (Steinschen et al, 1996) has been attributed to habitat deterioration caused by plowing, vegetation clearing and burning, soil nutrient enrichment from surrounding agricultural areas and grazing by herbivores, which disperse the grass seeds on their hides and in their dung (Milton, 2004). Various methods for restoring transformed lowland fynbos ecosystems (especially renosterveld) invaded by alien grasses have been examined These include removal of alien grasses by indigenous herbivores
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