Effects of composite materials and revegetation on soil nutrients, chemical and microbial properties in rare earth tailings

The mining of ionic rare earth elements in Ganzhou left large area of barren tailings with severe vegetation destruction in pressing needs of remediation. However, the remediating effects of soil additives combined with revegetation on the preservation of nutrients in the tailings and microbial communities were rarely studied. For this purpose, pilot experiments were implemented in a field, with the control group (CK) only cultivating plants without adding materials, and three treatments including peanut straw biochar composite (T1), phosphorus‑magnesium composite (T2) and modified zeolite composite (T3) along with the cultivation of Medicago sativa L., Paspalum vaginatum Sw. and Lolium perenne L. Soil pH and organic matter in CK significantly decreased from 4.90 to 4.17 and from 6.62 g/kg to 3.87 g/kg after six months, respectively (p ≤ 0.05), while all the treatments could effectively buffer soil acidification (over 5.74) and delay the loss of soil organic matter. Soil cation exchange capacity was still below the detection limit in all the groups except T2. The results of rainfall runoff monitoring indicated that compared with CK, only T2 could significantly reduce the runoff loss of soil NO3- and SO42- by 45.61 %-75.78 % and 64.03 %-76.12 %, respectively (p ≤ 0.05). Compared with CK, the bacterial diversity in T2 and T3 significantly increased 21.18 % and 28.15 %, respectively (p ≤ 0.05), while T1 didn't change the bacterial or fungal diversity (p > 0.05). Co-occurrence network analysis showed that compared with CK, the whole microbial communities interacted more closely in the three treatments. Functional prediction of the microbial communities revealed all the treatments were dominated by carbon transforming bacteria and saprotrophic fungi except T2. This study demonstrated that the composite materials combined with revegetation couldn't retain soil nitrogen compounds and sulfate in rare earth tailings in the long term.