This paper presents a study of the microbial abundance in post-bauxite-mining land soil from Zece Hotare, Bihor county, Romania. The soil samples were collected from 12 soil variants, in the year 2020, after 15 years of long-term restoration. Some chemical parameters and bacterial numbers of six groups of microorganisms were determined in the restored mining land, and these characteristics were compared with those of the soil from a beech forest situated in an adjacent area unaffected by bauxite exploitation. On the basis of the total number of microorganisms belonging to each group studied, the bacterial potential of the soil quality was assessed, calculating the bacterial soil quality index (BSQI), while the Shannon diversity index and the Jaccard distance were applied to show the level of bacterial diversity. The characteristics of the studied chemical and microbiological parameters determined in the beech adjacent area were very similar to those observed in the high-level plateau, low-level plateau, and Black locust areas, indicating similar soil conditions; therefore, the ecological reconstruction 15 years ago, had a very favorable impact on restoration in some affected areas.

Forest soils belong to the major carbon sinks on Earth because of their high organic matter content. Forest soils from Europe store approximately 1.5 times more carbon than trees (EC/UN-ECE 2003). As dystric cambisol (2,292,385 ha) and eutric cambisol (869,909 ha) are the most widespread forest soils in Romania, we studied 5958 dystric cambisol pedogenetic horizons and 6784 eutric cambisol pedogenetic horizons. A series of correlations was made between soil organic matter and elevation, but also with tree age and stand production class. The differences between stratified soil organic matter in terms of slope aspect categories were tested, and multiple linear regression was used to determine the influences of some relief (elevation) and stand (age) characteristics on the soil organic matter content. Overall, the soil organic matter content increased with increasing elevation. Based on all 12,742 soil samples over a period of 33 years, the soil organic matter content is influenced by elevation and tree age, especially on shaded and partially shaded slope aspects.