Characteristics of soil organic carbon and nitrogen fractions after 24 years of reclamation in a large open pit coal mine in the Loess Plateau

Abstract: Soil labile carbon and nitrogen play a key role in the sequestration and pedogenic process, thereby shaping biogeochemical nutrient cycling in a large-scale open cast coal mine. Taking the forest of Robiniapseudoacacia - Pinustabulaeformis mixed forest (RP), Robiniapseudoacacia monoculture forest (RM), PiceameyeriRehd - Piceawilsanii- Hippophaerhamnoides mixed forest (PPH), and Hippophaerhamnoides monoculture forest (HM) as research objects, this study aims to investigate the soil carbon and nitrogen fractions in the Pingshuo opencast coal mine, China. Four forests were the typical 24-year reclamation patterns in the Loess Plateau. Soil samples were collected from the soil surface of 0-20 cm in the reclaimed forests. The control samples were from the unreclaimed site and undisturbed forest. The results showed that: 1) Physical and chemical properties of reclaimed soils were significantly improved after 24 years of succession, compared with the unreclaimed soil. Several physicochemical indices of Robiniapseudoacacia- Pinustabulaeformis mixed forest exceeded those of the undisturbed forest. Nevertheless, there were significant differences in the physical and chemical properties of the 6 reclaimed plots.2) Reclamation accelerated the recovery rates of soil organic carbon fractions in reclaimed soils. Compared to the UR, soil organic carbon from RP、RM and PPH were 594.03%,158.66% and 99.25% higher, respectively; microbial biomass carbon from RP and PPH increased 174.68%, 209.34%, respectively; dissolved organic carbon from RP and HM were 110.13%、82.99% higher, respectively; Easily oxidizable carbon from RP and HM were 35.55 and 7 folds, respectively. 3) Reclamation promoted the contents of soil nitrogen fractions. Compared to the UR, soil nitrogen from RP、RM、PPH and HM were 413.24%、250.00%、100%、82.35% higher, respectively; water-soluble nitrogen from RP, PPH and HM increased 206.33%、153.02%、99.95%, respectively; microbial biomass nitrogen from PPH was 368.07% higher. 4) There was a positive correlation between carbon and nitrogen fractions (P<0.05), but a significant negative correlation (P<0.05) was found among total potassium (TK) and carbon and nitrogen fractions. Overall, the results showed that reclamation facilitated the sequestration of soil organic carbon and nitrogen fractions, and R. pseudoacacia-P. tabulaeformis mixed forest performed the best. However, a limitation of potassium may hinder the succession of reclaimed soil after 24 years of reclamation in Pingshuo opencast coal mine.