Assessing the effects of short-term Spartina alterniflora invasion on labile and recalcitrant C and N pools by means of soil fractionation and stable C and N isotopes

An exotic grass Spartina alterniflora was intentionally introduced to Jiuduansha wetlands in Yangtze River estuary in 1997, and since then it had rapidly replaced native plant Scirpus mariqueter that used to dominate the estuarine salt marshes. We investigated consequences of S. alterniflora invasion to soil labile and recalcitrant C and N compared to the native S. mariqueter using soil fractionation and stable C and N isotopes. Results showed that S. alterniflora increased soil labile carbon (LC), recalcitrant carbon (RC), and soil recalcitrant nitrogen (RN) contents significantly (P < 0.05) in the upper soil layers (0-60 cm) compared to the S. mariqueter soil. Soil labile nitrogen (LN) in the S. alterniflora soil, however, remained lower than that in the S. mariqueter soil (P < 0.01), except for the surface soil layer (0-20 cm). The LC accounted for, on average, 36-38% of soil organic matter (SOM) in both communities, while labile N accounted for 32% of SOM in S. alterniflora soil and 48% in S. mariqueter soil. The δ13C values in S. alterniflora soil showed that S. alterniflora contributed on average 8.6% and 3.3% to the LC and RC pools, respectively, within the 0-100-cm soil layer. The greatest labile C contribution derived from S. alterniflora was found at the 40-cm soil whereas the proportion of recalcitrant C originating from S. alterniflora showed a decreasing trend with soil depth. These changes appeared to be associated with vertical distributions of roots and rhizodeposition. We also found that the δ15N values of SOM were more enriched in S. alterniflora soil compared to S. mariqueter soil, suggesting that greater SOM input by S. alterniflora residues would stimulate microbial activity rates that could lead to increased N turnover rates in S. alterniflora soil.