Effects of Land Use on Microbial Indices in Tantalite Mine Soils, Western Rwanda

Rare metal mining in western Rwanda created massive degraded mine wastelands. Some of them were restored by afforestation with pine and eucalyptus forests while others have been cultivated alongside arable lands, owing to limited land availability. We assessed the effects of these land uses on microbial biomass and activity (basal C and net N mineralization) in a laboratory experiment with fresh soils (0–20 cm) of an unmined native forest, two tantalite mine soils (Technosols) afforested with pine and eucalyptus forests, an arable soil, and two cultivated Technosols. Afforestation increased soil organic C (SOC) and total N in pine and eucalyptus Technosols, reaching 34–40% and 28–30%, respectively, of that in the native forest. The tree species differed significantly in microbial activity and biomass, exhibiting a trajectory towards recovery depicted in lower metabolic quotient. Basal respiration of pine Technosol and native forest soil were similar, while that of eucalyptus Technosol was 11 and 14% less compared to its forest counterparts. Conversely, net N mineralization in pine and eucalyptus Technosols were 40 and 65% of that in native forest soil, respectively. Soil microbial biomass C in pine and eucalyptus Technosols were 33 and 48% of that in native forest soil. Furthermore, a decreased SOC availability to soil microorganisms at low soil pH promoted fungal ergosterol as an indicator for saprotrophic fungi. The arable soil and cultivated Technosols did not differ significantly in SOC, N, microbial biomass, and activity. The soil biological quality was constrained by soil acidity, dithionite-extractable Al, and low P availability.