Unexpected Carbon Accumulation in Degraded Soils of Central Brazil: Ecological and Biogeochemical Drivers.

A 14-year restoration chronosequence was used to investigate how resource input from biosolid application and subsequent plant colonization promoted the reconstruction of mined soils in central Brazil. A combination of vegetation surveys with the analysis of plant and soil isotopic abundances (δ¹³C and delta δ¹⁵N) in multiple chemical and physical fractionation revealed that: (i) after several decades without vegetation cover, the input of nutrient-rich biosolids into exposed regoliths prompted (year 0.5 to 3) the establishment of a diverse plant community (> 30 species); (ii) the synergistic effect of resource input and plant colonization yielded unprecedented increases in soil carbon (year 3 to 6), accumulating as chemically stable compounds in occluded physical fractions and reaching much higher levels than observed in undisturbed ecosystems; and (iii) invasive grasses progressively excluded native species (year 6 to 9), limiting nutrient availability, but contributing more than 65% (year 14) of the total accumulated soil organic carbon. Consistent with hierarchical theory, a gradual formation of macroaggregates was observed shortly after the incorporation of biosolids, followed by a preferential accumulation of plant-derived carbon in microbially-formed microaggregates. A clear association between pyrophosphate-extractable iron and carbon in occluded soil fractions revealed dynamic links between iron-coordinated compounds, decomposition, and aggregation. Iron coordination triggered rapid soil carbon accumulation, stabilizing new inputs and increasing soil development, in contrast to previous but unsuccessful efforts to restore mined areas through nutrient application or introduction of plant species alone.