Exotic purple loosestrife invasion of native cattail freshwater wetlands: Effects on organic matter distribution and soil nitrogen cycling

The ecological consequences of exotic plant invasion have focused largely on interactions with native plants or animals, whereas ecosystem level consequences, such as organic matter accumulation, soil nitrogen (N) transformations, hydrological fluctuation and changes in surface water chemistry remain largely unknown. We studied the invasion of exotic emergent plant purple loosestrife (Lythrum salicaria) in a freshwater wetland dominated by cattail (Typha latifolia). Transects were set in Montezuma National Wildlife Refuge, NY that originated in a pure Typha area, crossed over mixed zone, and extended into a pure Lythrum area. Comparing monotypic plots, there was significantly more standing-dead biomass of Lythrum than Typha (1.88 kg m−2 versus 0.59 kg m−2); while collapsed dead Typha shoots formed a layer of litter which was absent in the Lythrum area. Soil organic matter content was significantly higher in Lythrum sediment than in Typha sediment (35.2 kg m−2 versus 27.5 kg m−2, down to 20 cm deep). Average monthly N mineralization rates were significantly higher in the Lythrum plots than in Typha plots (911 mg N m−2 versus 638 mg N m−2). N transformations were also controlled by local hydrology. Net N mineralization rates were more than doubled when water table dropped to expose the sediment to the atmosphere, and ammonium production was changed to nitrate production. Surface water NH4+ and NO3− concentrations remained low in all sites, suggesting strong plant uptake by both species. In a concurrent greenhouse study, we found Lythrum plants transpired about twice the amount of water as Typha, and had above- and below-ground biomass that more than doubled that of Typha. The combined field and greenhouse studies suggest that Lythrum invasion may cause changes in organic matter distribution, N cycling and water chemistry in freshwater wetlands.