Effects Of Agricultural Activities On Wetland Soil Seed Banks In Sanjiang Plain

Seed bank are all the detached viable seeds of a species at a specific time, andincludes seeds present both above and below the soil surface. As an important sourceof regenerative material in wetland, seed bank plays an important role in vegetationsuccession, maintenance of biodiversity and wetland restoration. Withthe deepening of cognition about wetland in China, The characteristics of the wetlandseed banks and the new technique to use seed bank for wetland restorationin the context of agricultural development get more and more attention. In this paper,vegetation survey, seed bank germination and control experiment in greenhouse wereperformed to explore the characteristics of wetland seed banks in Sanjiang Plain, andthe effects of agricultural activities (e.g. wetland cultivation, agriculture irrigationrunoff, fertilizer using) on wetland seed banks. The restoration potential andrestoration strategies were also discussed. Knowledge gained from this study willprovide important insights into protection and utilization of wetland resources andprovide reference for maintenance of biodiversity and wetland restoration. The mainresults were drawn as follows:(1) Species richness of seed banks is high in Sanjiang Plain, and many wetlandspecies could retain in wetland seed banks, most of which are perennial herb.Important wetland species (e.g. Calamogrostis angustifolia and Potamogeton crispus)taking up a large percentage while other key structural species (Carex spp.) werealmost missing in the seed bank. The seed banks of both the continuous wetlands andisolated wetlands have high diversity and the similarity between them was high in46.9%, which means both of them play an important role in the maintenance ofbiodiversity. More species germinated under non-flooded condition than flooded andthe life forms differed. Emerged species such as C. angustifolia were the dominantspecies under non-flooded conditions and submerged species such as Potamogetoncrispus and Vallisneria spiralis dominated under flooded conditions. Seed density varied with depth so that seed density was higher in0–5cm than in5–10cm.(2) Farming could significantly affect the structure and density of seed banks.The number of species germinated from seed banks decreased gradually with theincrease of the farm years, which were31,21,21and8in farmed wetlands for1yr,3yrs,10yrs and20yrs respectively. The seed density of the wetland farmed for1yrwere much higher than that in wetlands farmed for3yrs and10yrs. The lowest of seeddensity occurred in wetland farmed for20yrs. Calamagrostis angustifolia, thedominant species in the vegetation of the Sanjiang Plain had1192individuals·m-2germinating in the seed bank from the natural wetland, this species was absent in seedbanks after20years of farming.(3) The discharge of agriculture irrigation runoff containing large amounts ofsuspended particles resulted in a high sediment accumulation rate (0.3to1.0cm yr-1)in the receiving wetland. Results revealed significant differences in seed germinationrates among the three plant communities and significant effects of sediment load onthe germination rates. Species richness and seedling emergence decreasedsignificantly at0.5-0.75cm of sediment addition. Species responded differently to theaddition of sediment. The number of seedlings of Pycreus korshinskyi, Sagittariatrifolia, Alisma orientale, Monochoria vaginalis, Carpesium macrocephalumdecreased gradually as the sediment addition increased from0to2cm, while thenumber of seedlings of Fimbristylis dichotoma, Eleocharis ovata, Bidens bipinnatadecreased to zero at0.5cm of sediment addition. Despite low similarity, the numberof species germinated from seed banks was higher than the original number of speciespresent in each plant community.(4) The natural wetlands have received a large amount of exogenous N and Pinput from the adjacent agricultural land because of fertilization. Low level of Nadditions (less than10g/m2) did not significantly affect the species richness andseedling density, while the seedling biomass at5g/m2of N addition was higher thanother nutrient conditions. But species richness, seedling emergence and biomassdecreased significantly at high level of N additions (20-40g/m2). The responses werespecies-specific. High level of N additions had negative impacts on seed germination, seedling growth and biomass of dominant species Eleocharis ovata, Calamagrostisangustifolia, Juncus effusus in the seed bank. P additions did not significantly affectthe species richness, seedling density and seedling biomass, but species respondeddifferently to the P additions. Seed density of Eleocharis ovata increased with theincrease of the P additions, and it was significantly higher at high level of P addition（9.6-19.2g/m2） than at low level (0-4.8g/m2）, while the seed densities of Typhalatifolia, Calamagrostis angustifolia, Juncus effusus was significantly lower at highlevel of P addition than at low level. In a word, the effects of the N additions on seedbank germination were more sensitive than P additions in Sanjiang Plain.(5) Seed banks are an important source of regenerative material in wetlands. Westudied characteristics of the soil seed banks and relationships with the vegetation indifferent restoration successional stages. With increased the restoration time, seedbank composition and size increased, and the vegetation community showedsuccession from a xerophyte-dominated community to a wetland communitydominated by Calamagrostis angustifolia. Number of species germinated from theseed banks was24and29, while21,25species were recorded in the vegetation of in7-year restored wetland,14-year restored wetland and natural wetland, respectively.The similarity between seed bank and vegetation in different restoration successionalstages was always <30%.(6) The CCA analysis indicated that, seed bank germination in top layer wassignificantly related with water regime, water depth and restoration period（sp<0.005）,and seed bank germination in bottom layer was only significantly related with waterregime（p<0.005）. Seed density had significant relations with environmental factors.Alisma orientale, Potamogeton crispus, P. malaianus, Sagittaria trifolia, Vallisneriaspiralis mainly germinated under flooded condition, while other wetland species suchas Calamagrostis angustifolia mainly germinated under non-flooded condition. Seedgerminations of Juncus effuses, Salix spp., Saussurea amurensis was more relatedwith restoration periods, and seed density of weed species Chenopodium glaucum,Capsella bursa-pastoris increased with the increase of the farming time.(7) The first step to use the seed bank for wetland restoration is to know the seed bank composition. Site selection of the wetland restoration is important and it is betterto first restore the wetlands which are near the natural wetlands. When using vestigialseed bank for wetland restoration in farming land, farming time could be the mainlimiting factor. The possibility is larger when the wetland is farmed for less time. Andduring the restoration period, more attention should be paid to make sure that theenvironmental conditions (e.g. topography, water regime, nutrient, light, temperature)were suitable, and among which the suitability of water regime in the restored sitewas the most important factor to decide the type of the vegetation and the finalsuccess of the wetland restoration. Propagule transplanting and seedling transplantingare also important methods to restore native target wetland species (e.g. Carexspecies).