Changes in nitrogen cycling and greenhouse gas emissions following invasion by kudzu (Pueraria montana)

Kudzu (Pueraria montana), a leguminous vine native to Asia, covers more than 3 million ha in the southeastern United States and is expanding its range northward. With its high rates of nitrogen fixation in its native range and high degree of nodulation and nitrogenase activity in the United States, it seems likely that kudzu invasion presents a substantial new source of nitrogen to these ecosystems and nitrogen oxide emissions to the atmosphere. To date, however, the impacts of kudzu invasion on nitrogen cycling and trace gas fluxes in the eastern United States have not been investigated. We examine kudzu's effect on nitrogen inputs to soil and nitrogen cycling at 3 pairs of invaded and uninvaded sites in Maryland. Newly senesced litter from kudzu contains significantly higher concentrations of nitrogen than that of co-occurring tree species, suggesting that kudzu represents a new source of organic nitrogen in these sites. Inorganic nitrogen in soils bears out this suggestion: nitrate levels were 4 times higher in sites invaded by kudzu in April, 2006, and remained higher throughout the growing season. We also found increases and trends toward increases in rates of nitrification, nitrogen mineralization, and denitrification enzyme activity. In spring, 2007, we started measurements of NO and N₂O fluxes from invaded and uninvaded soils. Our data strongly suggest that kudzu is having significant impacts on the nitrogen cycling of invaded ecosystems, and potentially on regional air quality as well.