The effect of invasive Spartina alterniflora on diazotrph communities as regulated by native crabs

Background/Question/Methods

Nitrogen fixed by diazotrophs is an important source of nitrogen supply to ecosystems. Soil diazotrophs have a tight, mutualistic relationship with Spartina alterniflora in salt marshes along the North American Atlantic coast. It is unclear, however, whether this relationship holds for the west Pacific coast, where S. alterniflora is invasive. Moreover, the activity and composition of diazotroph communities are strongly affected by soil disturbance. Native crabs are one of the most important soil reworkers in salt marshes. Therefore, they may influence the nitrogen fixation by diazotrophs as well as their interaction with higher plants. We carried out a field experiment in the Yangtze River estuary, China to test the hypotheses that (1) alien S. alterniflora could facilitate soil diazotrophs, and (2) native crabs could regulate the effect of S. alterniflora on diazotrophs. The experimental design consisted of paired crab present-absent plots in monotypic stands of S. alterniflora characterized by different times since introduction (1, 3, and 6 years). Soil samples were collected to determine the potential rate of nitrogen fixation (R_N), and edaphic factors. The total microbial DNAs extracted from soils were used for determining the abundance and composition of dinitrogenase reductase (nifH) gene.

Results/Conclusions

The R_N increased with the time since introduction from 3.60±1.09 (standard error) mg N m^-2 day^-1 in 1 year-old stands to 29.65±9.69 mg N m^-2 day^-1 in 6 year-old stands. Furthermore, R_N increased significantly faster in crab-present than crab-absent plots. The Shannon diversity index of diazotroph communities was positively correlated with R_N. The number of nifH genes found for 6 year-old stands was larger than that for younger stands by 1 to >2 orders of magnitude. The δ-proteobacteria, γ-proteobacteria and Clostridia together accounted for over 80% of the total abundance of diazotrophs. The composition of diazotroph communities in 6 year-old stands differed markedly from that in younger stands. Crab activities determined the diazotroph composition in youger stands, while stand age had no effect. In addition, soil abiotic conditions (e.g.,concentrations of dissolved carbon, nitrogen and phorsporous, and redox potential), which varied between crab treatments, significantly affected R_N and the diversity of diazotrophs. We concluded that S. alterniflora could facilitate the diazotroph community as the time since introduction increased. Moreover, crabs could regulate the effect of S. alterniflora on diazotrophs, possibly by creating heterogeneous microhabitats.