How Coastal Plants Adapt To A New Environment Created By Embankment Reclamation

Coastal wetlands have important and enormous ecosystem services with the unique hydrological characteristics and rich biodiversity,while large areas of coastal wetlands have been reclaimed to provide land resources for agriculture,aquaculture etc.or reclaimed as land banking all over the world.The implementation of reclamation projects has caused the huge destruction of coastal ecosystem structure and function.Although embankment reclamation is a recurring activity in the coastal wetlands of China,the effect of embankment construction(hereafter referred to as "reclamation")on plant growth has attracted little attention.Leaf carbon,nitrogen and phosphorus stoichiometry(C-N-P stoichiometry)of a plant can be used to reflect plant adaptation to new environments created by reclamation.In the present study,with three typical coastal plants(i.e.Spartina alterniflora Loisel.,Phragmites australis(Cav.)Trin.ex Steud.and Suaeda salsa(L.)Pall.)as the research target species,we investigated the plant biomass,plant tissue C-N-P stoichiometry,soil C-N-P stoichiometry,and soil moisture,salinity,bulk density and pH and estimated C,N and P pools(g/m2)of plant and soil in both embankment-reclaimed and natural salt marshes in eastern China.Then the correlations between the alterations of plant C-N-P stoichiometry and the changes in soil parameters due to reclamation were analyzed,and the linkage between leaf CN-P stoichiometric pattern and plant biomass production were established in order to clear:how coastal plants adapt to a new environment created by embankment reclamation.In addition,we compared the similarities and differences in C-N-P stoichiometry among S.alterniflora,S,salsa and P.australis,and elucidated how C-NP stoichiometry reflect their own nutrient utilization strategy.The main conclusions are as follows:(1)In S.alterniflora salt marsh,reclamation significantly decreased plant biomass production,soil organic C and N content,soil salinity and soil moisture.LeafN content was not significantly changed,but P content significantly decreased which generated a higher biomass C:P and N:P ratios in the reclaimed marsh.We speculate that the decrease in leaf P content is possibly attributed to decreases in soil salinity,soil moisture and soil organic C and N content following reclamation,which caused a lower P availability in the soil.Biomass of S.alterniflora was positively(or negatively)correlated with foliar P content(or N:P ratio).The results indicate that the decreased aboveground biomass in the reclaimed marsh is likely to be correlated with an increase in the leaf N:P ratio,in accordance with the "growth rate hypothesis".The decreased foliar P content and increased N:P ratio indicate that P is likely to be a limiting nutrient for the growth of S.alterniflora in the reclaimed marsh.However,previously published threshold values of the N:P ratio as indicators of N or P limitation of plant growth may not be applicable to S.alterniflora at our study site.(2)In S.salsa salt marsh,reclamation slightly reduced plant biomass,soil moisture and soil salinity.The lower foliar N content level and N:P ratio,together with the lower soil N content level compared with those in other sites of previous studies,indicated that the growth of S.salsa might be limited by N in both marshes,while reclamation had probably worsened the N limitation in light of the lower leaf N:P ratio in the reclaimed marsh.The decreased soil moisture and salinity might decrease the availability of N in the soil,which probably caused the lower total amount of N assimilated by the plant body in the reclaimed marsh and contribute to the slightly decreased biomass production of S.salsa.However,phosphorus might be in a "luxury absorption" pattern in the reclaimed marsh in light of the lower biomass N:P and C:P ratios.The superfluous P can be stored in fruits in the reclaimed marsh,confirmed by the increased biomass allocation rate of fruit and remarkably increasing in both content and storage of P in the fruits at the mature stage.(3)In P.australis salt marsh,reclamation significantly boosted biomass production and increased soil organic C and N content and soil moisture but decreasing soil bulk density and salinity.Leaf N content and C:N ratio were not significantly influenced by reclamation during the vegetative growth stage;while leaf and spike N content significantly increased(C:N ratio decreased)at the mature stage,together with an increased biomass allocation rate of spike in the reclaimed marsh,indicating that reclamation probably facilitated the seed production of P.australis by the increase of N absorption at the mature stage.Leaf and stalk P content increased and C:P and N:P ratio decreased distinctly in the reclaimed marsh.The changes in soil organic C and N content,soil moisture and redox potential and soil bulk density probably increased the availability of soil P as total P content did not change.In addition,the increase in root biomass allocation rate and the decrease in soil salinity is likely to facilitate root absorption of P.The significantly increased biomass production of P.australis is probably related to the lower foliar N:P ratio,which generated a more rapid growth rate in the reclaimed marsh.Phosphorus was probably a limiting nutrient to the growth of P.australis in the natural marsh,while reclamation enhanced the P uptake amount of P.australis and alleviated(or changed)this limitation.(4)Root N and P content in S.salsa showed different seasonal patterns from those in S.alterniflora and P.australis.Difference in N and P content between leaf and stem(or root)of S.salsa was different from that of two other species.It indicated that nutrient allocation patterns(especially N)among different organs in S.salsa were different from those in S.alterniflora and P.australis.The latter two had significantly higher foliar C content than S.salsa,suggesting they possess an obviously stronger carbon fixation ability.Their N and P use efficiency is also higher than S.salsa,deduced from their higher foliar C:N and C:P ratios.Aboveground biomass was significantly positively correlated with leaf P content and negatively with leaf N:P in S.alterniflora and P.australis,implying that P is likely to be a crucial factor during their growth.The natural S.alterniflora marsh had the highest aboveground biomass and C,N and P storages,and soil C and N pools,while the reclaimed P.australis marsh came next.In contrast,soil P pool was lowest in the natural S.alterniflora marsh.Plant biomass were closely related to soil C and N sequestration in coastal wetlands,but not to soil P.The high P content and storage of natural S.alterniflora indicated its strong P absorption and retention capacity and its potential to remove superfluous P in water and soil especially in the saline environment.Reclamation produced opposite consequences on plant C and N sequestration and soil C and N sink in S.alterniflora and P.australis,suggesting that the simultaneous reclamation in both may maintain the steady C and N sinks of coastal wetlands.S.alterniflora,S.salsa,and P.australis adopted different response strategies of growth by changing tissue C-N-P stoichiometry and biomass allocation pattern to adapt to the new environment created by reclamation.The variations in tissue C-N-P stoichiometric patterns could well reflect the growth of coastal plants in response to reclamation,particularly for S.alterniflora and P.australis.It is deduced that the reclamation-induced changes in soil salinity and moisture may play a crucial role in the changing process of plant C-N-P stoichiometry.Our results are expected to provide important scientific evidence for the impact evaluation of reclamation and the sustainable development of coastal wetlands in eastern China.