Transformations of Nitrogen and Carbon in Entrenched Biosolids at a Reclaimed Mineral Sands Mining Site

Biosolids deep-row incorporation (DRI) provides high levels of nutrients to the reclamation sites; however, additions of N in excess of the vegetation requirements can potentially impair water quality. The effects of anaerobically digested (AD) and lime stabilized (LS) DRI biosolids and inorganic N fertilizer were compared on C and N transformations and transport at a reclaimed mineral sands mining site. Biosolids were applied at 213 and 426 Mg AD biosolids ha−1 and 328 and 656 Mg LS biosolids ha−1 (dry mass), and inorganic N fertilizer was applied at 0 (control) and 504 kg N ha−1 yr−1 Zero tension lysimeters were installed to collect leachate for determination of vertical N transport, and the biosolids seams were analyzed for N and C transformations after 28 mo aging. The leaching masses from the DRI biosolids treatments were 139 to 291 kg ha−1 NO3–N, 61 to 243 kg ha−1 NH4–N, and 61 to 269 kg ha−1 organic N, while the fertilizer treatment did not differ from the control. Aged biosolids analysis showed that total N lost over the course of 2 yr was 15.2 Mg ha−1 and 10.9 Mg ha−1 for LS and AD biosolids, respectively, which was roughly 50% of the N applied. Organic C losses were 81 Mg ha−1 and 33 Mg ha−1 for LS and AD biosolids, respectively. Our results indicated that entrenchment of biosolids in coarse-textured media should not be used as a mined land reclamation technique because the anaerobic conditions required to limit mineralization and nitrification cannot be maintained in such permeable soils.