Effects of previous nitrogen addition on chlorine in forest soil, soil solution and biomass

There is increasing evidence that forests and forest soil contribute to the signature of chlorine composition in water bodies. However, little is known about the potential effects of land management activities on chlorine biogeochemistry. This study examines the effects of previous nitrogen addition on chlorine chemistry in a Pinus sylvestris (L.) forest located in south-central Sweden (60°00 N, 13°43 E). Repeated addition of nitrogen to study plots over a 20-year period resulted in total additions of 0, 450 and 900 kg N ha -1 . Soil samples were collected before harvesting, and soil solution and biomass were sampled following final felling. Contrary to previous findings, we found no clear evidence that previous addition of nitrogen had hampered the formation of organic chlorine in the organic soil layer. We suggest that the effects of nitrogen addition on chlorination processes are not seen in the surface soil, but are instead manifested in the migration of organic matter in the mineral soil. Soil organic matter from the E-horizon had a lower chlorination degree in the nitrogen-amended plots than in the control plots. In addition, we observed lower Cl levels in the seedling needles following high nitrogen fertilization (900N) than in 450 N or control. These results add on the importance of studying chlorine dynamics with focus on what chlorinated soil organic matter can be resistant to degradation compared to average soil organic matter and what can be leached as a result of harvesting and available for vegetation.