Invasive grasses increase soil carbon sequestration from near zero to forest-like levels following the restoration of mined areas in central Brazil

Background/Question/Methods

During the 1960s, surface mining for gravel resulted in the removal of large areas of the native vegetation and loss of topsoil in central Brazil. These disturbed areas are slow to recover and still lack vegetation cover. Restoration practices in this region involve the incorporation of sewage sludge as a source of organic matter and nutrients to stimulate plant establishment and growth. Several indicators support this as an effective method to improve the quality of the residual substrates following mining, however, its long-term effects on continued soil development, nutrient cycling and vegetation productivity, are yet to be evaluated. Here we use stable isotopes to assess soil organic matter and vegetation dynamics, across a chronosequence of mined sites (0, 0.5, 3, 6, 9 and 14 years since restoration) located in the vicinities of Brasilia - DF, Brazil. All sites were originally covered by savanna vegetation established over Cambisols. Restoration activities comprised scarification and incorporation of (100 dry t ha^-1) tertiary domestic sewage sludge that originated from local treatment plants. Similar climatic and topographic conditions are observed across sites and results are interpreted as function of time since restoration. For control purposes, we also characterize undisturbed soils at each study site.

Results/Conclusions

Several decades after disturbance, mined areas still lacked vegetation cover, but the incorporation of sewage sludge increased soil carbon to pre-disturbance levels (~2%) and prompted spontaneous revegetation. Few years after soil treatment (3 to 9 years), a diverse herbaceous cover (>30 species) and few woody species were observed. However, invasive grasses (Brachiaria and Melinis genera) progressively dominated the herbaceous stratum. A two-end mixing model, based on soil organic matter δ¹³C and distinct signatures of applied organic matter (-25.9‰) and invasive grasses (-12,2‰), show a monotonic increase (R²=0.95; P<0.01) in the relative contribution of invasive grasses to total soil carbon pools. Analyses of humic and fulvic acid fractions show that this increase is due to the presence of recalcitrant organic compounds derived from invasive species, which contributed more than 65% of total soil carbon 14 years after restoration. Surprisingly, total soil carbon content increased to values (>9%) that largely exceed pre-disturbance levels, and are comparable to those of tropical forests in the same region. Increased productivity combined with organo-mineral associations are possible explanation for these unexpected results.