The Effects Of Exotic Plant Spartina Alterniflora On Ecosystem Nitrogen Cycling In Estuarine Salt Marsh

As a component of global change, biological invasions have profound effects on the native ecosystems. One of the ecosystem-level consequences of invasive alien species caused is their impacts on carbon and nitrogen cycling. Although a growing number of studies on these impacts have been conducted over the last two decades, the underlying mechanisms are still poorly understood. Recent studies especially concentrate on the production-decomposition cycling within communities, while the system input/output exchange with environment controlled by invasive ecosystem engineer effects is left unconcerned. The first objective of this study was to explore the effects of Spartina alterniflora (SA) invasion on ecosystem N pools and cycling in the Phragmites australis(PA) communities at Dongtan estuarine wetlands, Chongming island. The second objective was to estimate the importance of invasive ecosystem engineer effects (sediment accretion) as a mechanism of invasive impact on native ecosystem nitrogen cycling.Spartina significantly increased ecosystem C and N pool. Field observation experiment was conducted from Apr 2007 to Apr 2008. The results showed that the average aboveground biomass in SA communities(1.54±0.05 kg dry weight m-2) was significantly higher than that in PA communities(0.87±0.05 kg dry weight m-2), respectively 80% greater.The average aboveground total carbon pool in SA communities (514±18g m-2) was significantly higher than that in PA communities(333±16g m-2), respectively 54% greater.The average aboveground total nitrogen pool in SA communities (14.60±0.65g m-2) was significantly higher than that in PA communities(10.63±0.54g m-2), respectively 37% greater.The average soil total carbon pool(0-20cm) in SA communities (3270±54g m-2) was significantly higher than that in PA communities(2998±49g m-2), respectively 9% greater.The average soil total nitrogen pool(0-20cm) in SA communities (175.94±4.91 g m-2) was significantly higher than that in PA communities(153.49±5.76g m-2), respectively 15% greater.The average soil inorganic nitrogen pool(0-20cm) in SA communities (2.50±0.06g m-2) was significantly higher than that in PA communities(1.97±0.05g m-2), respectively 27% greater. While the average soil NO3-nitrogen pool(0-20cm) in SA communities (1.71±0.04g m-2) was significantly higher than that in PA communities(1.22±0.02g m-2), respectively 40% greater, the average soil NH4-nitrogen pool(0-20cm) in SA communities (0.80±0.03 g m-2) and PA communities (0.75±0.05g m-2) had no significant difference. Controlled transplant experiment was conducted from Apr 2005 to Sep 2006. While the aboveground plant pools showed same patterns as in field observation experiment, all the soil pools had no significant difference between SA and PA communities. These contrast results indicated that production-decomposition cycling within communities was not the only mechanism to explain the soil carbon and nitrogen pool increase caused by SA invasion.Spartina obtained extra inorganic N subsidies during tidal exchange. Field manipulation experiment on soil columns in PVC tubes was conducted in 2007, and field observation experiment on soil-water inorganic nitrogen exchange was conducted during spring tide in 2008. Both results showed that SA communities acquired more inorganic nitrogen than PA communities during tidal exchange. In the manipulation experiment, inorganic nitrogen pool increase in soil columns caused by tidal subsidy was significantly higher in SA communities (14.84±0.67mg kg-1 month±1) than in PA communities (2.97±0.24mg kg-1 month-1), in spite of the original community which the soil column was sampled in, while inorganic nitrogen pool increase caused by net mineralization had no significant difference between SA (14.15±0.68mg kg-1 month-1)and PA (13.59±0.69mg kg-1 month-1)communities. In the observation experiment, soil inorganic nitrogen content increase was significantly higher in SA communities (9.43±1.18mg L-1) than in PA communities (4.92±0.51mg L-1), and inorganic nitrogen content decrease in tidal water was also significantly higher in SA communities (2.92±0.36mg L-1) than in PA communities (2.07±0.20mg L-1). All the results revealed that difference in tidal subsidy acquirement was a principal mechanism to explain the increase of aboveground total nitrogen pool and soil inorganic nitrogen pool after SA invasion.The C and N pool accumulated by Spartina were mainly composed of sediments. Bottles and PVC plates were buried in both communities to collect sediments from 2007 to 2008. Sediment load in both methods was significantly higher in SA communities (in bottle:6.69±0.68kg m-2 month-1; on plate:38.60±3.33kg m-2 yr-1) than in PA communities (in bottle:4.27±0.53kg m-2 month-1; on plate:21.42±2.16kg m-2 yr-1). Besides, the total carbon pool, total nitrogen pool and inorganic nitrogen pool of sediments on plate were also significantly higher in SA communities (TC:0.69±0.08kg m-2 yr-1; TN:40.05±4.50g m-2 yr-1; inorg-N:953±85mg m-2 yr-1) than in PA communities (TC:0.42±0.05kg m-2 yr-1; TN:24.56±2.97g m-2 yr-1; inorg-N:415±37mg m-2 yr-1). Standing dead litter of SA and PA was added to bare ground from 2007 to 2008. SA litter had significant higher aerial decomposition rate (K=0.0038 day-1) than PA litter (K=0.0021 day-1). In multiple comparison test, Soil total carbon pool in quadrates with SA litter addition (1.96±0.02kg m-2) was significantly higher than that in the control groups (bare ground without litter addition, 1.91±0.02kg m-2), soil total carbon pool in quadrates with P A litter addition (1.93±0.02kg m-2) had no significant difference with both. Soil total nitrogen and inorganic nitrogen pool showed no significant difference on the whole. Thus, the conclusion was that sediment accretion effect was an important mechanism for SA to raise soil total nitrogen and inorganic nitrogen pool as an invasive ecosystem engineer, while the rise of soil total carbon pool could be mainly explained by increase in NPP and decomposition rate.The impact of Spartina on N cycling also affected the expansion of this invasive plant itself. Nitrate reductase activities (NRA) in SA and PA leaves were measured in Jun 2008. The NRA in SA leaves (1.69±0.11, In value) were significantly higher than that in PA leaves (0.87±0.08, In value) on the whole. Nitrogen fertilization could significantly raise NRA in both leaves. Submerging treatment significantly increased NRA in SA leaves, and decreased NRA in PA leaves. When using multiple regressions and PCA method to analyze datasets in field observation and manipulation experiments, sediment properties were found to have significant correlation with plant community characteristics. These results suggested that the consequence of impact on nitrogen cycling in native ecosystem invaded by SA might affect the expansion of SA itself. Such interactions were likely to create potential feedbacks.All the results in this study emphasized the importance of the control on ecosystem exchange with environment by invasive engineers as an underlying mechanism to explain the impact on nitrogen cycling in native ecosystems caused by invasive plants. This conclusion provided basic information for invasive plants management, as well as an insight into the mechanisms that individual plant species (including non-invasive plants and native plants) changed ecosystem-level biogeochemical cycling.