Belowground Ecosystem Recovery During Grassland Restoration: South African Highveld Compared to US Tallgrass Prairie

Abstract

Conversion of cultivated land to grassland is globally practiced to reverse soil degradation, but belowground ecosystem response to restoration has never been compared between old and new world temperate grasslands. We used a chronosequence approach to model change in root biomass and quality (indexed by C:N ratio), microbial biomass and composition [indexed by phospholipid fatty acids (PLFAs)], soil aggregate structure, and soil C and N stocks in the South African Highveld and compared recovery of these variables to a grassland restoration chronosequence in the US tallgrass prairie. We hypothesized soil C recovery, and mechanisms promoting soil C and N accrual would be convergent between these distant temperate grasslands with similar growing season precipitation, history of cultivation, and undergoing restoration with C4-grasses. Total PLFA richness and concentrations of most microbial groups rose to represent uncultivated grassland in the highveld (similar to tallgrass prairie), but in contrast to tallgrass prairie, the fungi:bacteria ratio did not increase with restoration age. In the highveld, root biomass accumulation was lower, but root quality became more representative of the never-cultivated grassland than in restorations in tallgrass prairie. Soil aggregate recovery was slightly faster in tallgrass prairie, and the pattern of macroaggregate C recovery was divergent due to less depletion in cultivated soil and higher stock of C in the uncultivated soil relative to the highveld. More rapid restoration of total soil C and N stocks in the highveld was attributed to greater soil C saturation deficit at the onset of restoration, development of higher quality root systems that promote the microbial biomass and soil aggregation, and climate conditions (distinct periodicity of rainfall and high aridity) that likely impose more limitation to decomposition relative to the tallgrass prairie ecosystem.