Effects of Nitrogen Addition on the Growth and Physiology of Populus deltoides Seedlings under Cd and Mn Pollution

Abstract

Both nitrogen (N) deposition and heavy metal pollution are important environmental concerns that threaten ecosystem stability and ecological safety. Limited research has been conducted on the effects of N deposition on the physiological processes and allocation patterns of heavy metals (HMs) in poplars, especially under combined pollution. In our study, we used Populus deltoides as a model to investigate the effects of two levels of N addition (LN, 6 g N·m−2·a−1; HN, 12 g N·m−2·a−1) on growth, activities of antioxidant enzymes, profiles of low-molecular-weight organic acids, as well as accumulation and allocation of HMs among different organs and root orders under single Cd (30 mg kg−1) or Mn pollution (168.6 mg kg−1), and their combination. The effects of N addition depended on the dosage effects of N and the types of HMs. The combined pollution did not have more negative effects on overall growth and oxidative damage in the root tips of P. deltoides compared to single Cd or Mn pollution. Both levels of N deposition, especially LN, promoted growth in P. deltoides to varying extents under all HM pollution conditions. However, N addition only mitigated oxidative damage to the fine roots under Cd-containing pollution, which may be attributed to higher levels of low-molecular-weight organic acids such as citric acid and malic acid. In contrast, HN decreased the levels of key organic acids, such as lactic acid and pantothenic acid, potentially exacerbating Mn toxicity under Mn pollution. Both levels of N addition decreased the total amount of Cd accumulated in P. deltoides under Cd pollution alone but increased the accumulation of Cd in combined pollution (especially under LN). However, under Mn-containing pollution, the addition of N increased the accumulation of Mn and its transfer to leaves, potentially aggravating Mn toxicity. Therefore, N deposition, especially under HN, may lead to more severe HM stress for plants in soils polluted by combined Cd and Mn.