Krumins, Janis, et al. “THE EVALUATION OF STABLE ISOTOPIC RATIOS 13C AND 15N IN HUMIC ACIDS ALONG A FEN PEAT PROFILE”. ENVIRONMENT. TECHNOLOGIES. RESOURCES. Proceedings of the International Scientific and Practical Conference, vol. 1, June 2019, pp. 123-6, https://doi.org/10.17770/etr2019vol1.4127.