Nitrogen Removal Pathways in Plant Combination Wetlands Based on ¹⁵N Stable Isotope Tracer Technique

Constructed wetlands (CWs) are an important method for the ecological remediation of polluted water bodies. Their removal efficiency of nitrogen (N) is remarkable. The N removal efficiency varies across CWs with different plant species. The questions are whether this efficiency is improved in CWs with plant combinations, and how is the removal path worthy of further study. In this study, Myriophyllum elatinoides and Pontederia cordata were used as research objects and examined through controlled laboratory experiments. The pathways for ammonia nitrogen (NH₄⁺-N) removal were investigated using the ¹⁵N stable isotope tracer technique in M. elatinoides, P. cordata, and M. elatinoides + P. cordata combination. Compared with single plant, M. elatinoides and P. cordata, N removal amount of M. elatinoides + P. cordata combination was increased by 1.8% and 3.6%, respectively. According to the ¹⁵N mass balance, the contributions to N removal by M. elatinoides, P. cordata, and their combination through sediment adsorption were 8.30%, 7.96%, and 6.94%, respectively, through plant uptake were 41.76%, 35.34%, and 48.67%, respectively, and through microbial action were 49.94%, 56.71%, and 44.39%, respectively. Microbial action played a dominant role in N removal process in CWs. Compared with single plant CWs, plant combination CWs could improve N removal efficiency and enhance NH₄⁺-N uptake by plants from water. This study is helpful to better understand the removal pathway of NH₄⁺-N in plant combinations CWs.