Sucrose application is ineffectual as a restoration aid in a transformed southern African lowland fynbos ecosystem

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 (200 g m^− 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.