The introduction of nitrogen from coal into the surface watershed nitrogen cycle due to coal mining activity

Human activity has doubled the turnover rate of the terrestrial nitrogen cycle, leading to a series of environmental problems. A little-studied nitrogen source in terrestrial and aquatic environments is the nitrogen release associated with rock strata. Southwest China features the largest continuous karsts in the world, featuring a fragile ecological environment but abundant coal resources. The current study selected a typical coal mining area to evaluate the migration and transformation of nitrogen related to coal mining in surface watershed. The findings reveal that the total nitrogen in coal seams was as high as 10,162.3 mg/kg, mainly in the form of organic nitrogen, followed by NH₄⁺-N, while the content of NO₃^--N was negligible. Based on the isotope fractionation and the co-evolution between Δ¹⁵N_NO3-NH4 and δ¹⁵N-NO₃^-/δ¹⁵N-NH₄⁺, coal mining changed the coal seams' oxidation-reduction state, resulting in the mineralization of organic nitrogen to NH₄⁺-N. Next, NH₄⁺-N gradually oxidized to NO₃^--N. Various forms of coal-origin nitrogen may be leached out by acid mine drainage (AMD), potentially contributing >10 % of NO₃^--N and 90 % of NH₄⁺-N to the surface river. Another nitrogen source that requires serious consideration is the wide use of ammonium nitrate explosives in coal mining, as blasting residues may contribute about another 10 % to NO₃^--N in surface water. Since organic nitrogen accounts for >90 % of extractable nitrogen, the release of coal-origin nitrogen may contribute much more to the total nitrogen in surface water than to NO₃^--N. Based on the fractionation of nitrogen and oxygen isotopes of nitrate, low-pH AMD promotes the volatilization of nitrate in the form of nitric acid. The conversion of different forms of nitrogen in AMD will be the focus of future attention.