Managing soil fertility for restoration of native grasslands.
Competition from annual species may be a significant factor limiting the re-establishment of native perennial species in degraded grasslands. In arid environments this may occur in post-drought situations when prolific annual growth results from relatively high levels of available nutrients. In environments with more rainfall, the growth of annuals may be promoted by increased soil fertility resulting from oversowing of native grasslands with legumes and application of superphosphate. Although growth of some native species responds favourably to increased fertility, their regeneration in degraded communities could be compromised if increased fertility promotes stronger competition from annuals.
We conducted an experiment at Trangie Agricultural Research Centre, in the semi-arid aseasonal rainfall zone of NSW, in which the fertility of the soil supporting degraded native grassland was either enhanced by addition of fertiliser, reduced by application of sugar as a readily assimilable source of carbon for soil micro-organisms, or left untreated. After two years, the yield of annuals was significantly lower, and yield of perennials significantly higher, in the reduced fertility treatment. Density of the two major native perennial grasses (wallaby grass and curly windmill grass) increased in the reduced fertility treatment but declined in the other two treatments. Basal area of these species declined in all treatments, but the decline was least with reduced fertility.
Our results support the hypothesis that competition from annual species can limit regeneration of native perennial grasses and that manipulation of fertility, and consequently the extent of this competition, may be important in facilitating grassland restoration. Where feasible, management techniques (e.g., pasture cropping) that achieve at least a temporary reduction in fertility may prove to be an important component of restoration management. Elsewhere, management of grazing may be the only tool available to manipulate the competitive balance or to ensure that infrequent regeneration opportunities are realised.