Effects of Spartina alterniflora invasion and exogenous nitrogen on soil nitrogen mineralization in the coastal salt marshes

•S. alterniflora invasion dramatically stimulated N mineralization and nitrification.•N addition increased net N mineralization and nitrification in the coastal soils.•N addition enhanced the sensitivity of N mineralization to temperature change.

The rapid expansion of Spartina alterniflora, exogenous nitrogen (N) input and climate change in the coastal wetlands in the east of China may interactively affect soil N mineralization in costal salt marshes. A laboratory incubation experiment was conducted with two exogenous N forms (NO3−N and NH4+N), six temperatures (5, 10, 15, 20, 25 and 30 °C) and three plant marshes (the invasive S. alterniflora and native Suaeda salsa and Phragmites australis) to investigate these interactive effects on soil net N mineralization rate and net nitrification rate in coastal wetlands. The averaged rates of net N mineralization and net nitrification over incubation temperature from 5 to 30 °C were significantly higher in the S. alterniflora marsh than in the S. salsa and P. australis marshes. The differences in N mineralization and nitrification among the tested soils were closely related with soil properties. With the addition of NO3−N, net N mineralization and nitrification were increased in the studied soils by 16-29% and 34-54%, respectively, whereas with the addition of NH4+N, they were increased by 58-69% and 65-94%, respectively. Net mineralization and nitrification in the tested soils increased with incubation temperature from 5 to 30 °C regardless of N addition. However, N addition (either NH4+N or NO3−N) not only dramatically raised the sensitivity of net N mineralization in the S. alterniflora marsh to temperature fluctuation, but also remarkably enhanced the percentage of net nitrification in net mineralization at incubation temperature from 15 to 25 °C, with peak values at 25 °C for all studied soils. These results suggest that under a changing climate, N supplying capacity of the S. alterniflora soil, as well as N pool, is notably great, which may shed light on the mechanism of rapid expansion of S. alterniflora in coastal China.