Soil dissimilatory nitrate reduction processes in the Spartina alterniflora invasion chronosequences of a coastal wetland of southeastern China: Dynamics and environmental implications

Aims: The invasion of Spartina alterniflora has a significant influence on soil biogeochemistry cycling in coastal wetlands. However, the roles of the S. alterniflora invasion chronosequence in regulating soil dissimilatory NO₃ ⁻ reduction processes (denitrification (DNF), anaerobic ammonium oxidation (ANA) and dissimilatory nitrate reduction to ammonium (DNRA)) remains unclear. The objective of this study was therefore to reveal the effects of S. alterniflora invasion on the soil NO₃ ⁻ reduction processes and associated gene abundance. Methods: We investigated plant biomass, soil properties, NO₃ ⁻ reduction processes and associated gene abundance of NO₃ ⁻ reduction pathways following S. alterniflora invasion chronosequences of 6, 10, and 14 years compared to Cyperus malaccensis in a coastal wetland of southeastern China. Results: The S. alterniflora invasion generally increased plant biomass, soil water content, available substrates, nirS, anammox bacterial 16S rRNA and nrfA gene abundance, but it decreased soil bulk density. Soil DNF, ANA and DNRA rates in stands of S. alterniflora ranged from 1.52 to 17.58, 0.31 to 1.27 and 0.14 to 2.01 nmol N g⁻¹ h⁻¹, respectively, which were generally higher than the values in stands of C. malaccensis. The soil NO₃ ⁻ reduction rates generally increased with the increasing chronosequence of invasion by S. alterniflora, while the changes in DNF and ANA rates were less pronounced than changes in DNRA. DNF was the dominant pathway (70.00–92.41%), and the ANA and DNRA contributed 2.49–15.27% and 5.10–20.75% to the total NO₃ ⁻ reduction, respectively. The contributions of DNF and ANA to the total NO₃ ⁻ reduction decreased slightly, while the contribution of DNRA increased remarkably after S. alterniflora invasion. Soil NO₃ ⁻ reduction processes were influenced by available substrates and associated microbial activities. It is estimated that an N loss of approximately 520.97 g N m⁻² yr.⁻¹ in C. malaccensis and 794.46 g N m⁻² yr.⁻¹ in S. alterniflora were linked to both DNF and ANA processes. Conclusions: The S. alterniflora invasion altered soil NO₃ ⁻ reduction processes by increasing soil microbial activities and available substrates and thus may further mediate the soil N availability in the coastal wetlands.