| Bioturbation is a key factor affecting the speed of the community succession. In salt marsh ecosystems, the bioturbation influences the biogeochemical cycles and plant community succession through changing carbon (C) and nitrogen (N) fluxes and microhabitats by soil reworking. Two types of bioturbations are important in Dongtan salt marsh in Yangtze River estuary, China:plant invasion(Spartina alterniflora) and native crab bioturbation. The distribution and behavior of crabs were significantly influenced by S. alterniflora invasion, while the crabs performed different ecological functions in invasive and native communities.In this study, artificial communities were established to study the effects of crab bioturbation on the interspecific relationships between alien S. alterniflora and native Phragmites australis and Scirpus mariquete. Meanwhile, the combined effects of plant invasion and crab bioturbation on biogeochemical cycles, including free living N fixation, N transformation as well as iron and sulfur cycling, were examined in the field-manipulated quadrats in Dongtan salt marsh. It was found that the crab activities improved the superiority of t S. alterniflora vs. native species; the plant invasion was more effective than crab disturbance, and the crabs functioned differently in invasive and native communities; the bioturbation indirectly affected substrates cycles through changing the pools size of available carbon and nitrogen. The diversified pathways of carbon and nitrogen flux by bioturbantion may weaken the carbon-nitrogen coupling, while the bioturbation may strengthen the relationship between iron and sulfur cycles and carbon and nitrogen cycles. The specific findings are summarized as follows:(1) Crab disturbance affected the interspecific relationships between alien S. alterniflora and native P. australis and S. mariquete probably by changing soil propoerties. Three monocultures were all promoted by crab bioturbation. S. alterniflora competed out the native plants in mixed-cultures under crab’s bioturbation, and visa versa. The soil N availability was improved by crab bioturbation, and the competitive advantage of S. alterniflora with crabs may attribute to its higher N demand and stronger ability to exploit N.(2) The free living N fixation in Dongtan salt marsh was both affected by S. alterniflora invasion and crab bioturbation. The rates of soil N-fixing in Dongtan salt marsh ranged from1to67mg·m-2·d-1, and increased with the age of S. alterniflora communities. Crab activity decreased the N-fixing rate in1-year-old S. alterniflora communities, but increased in5-6year-old S. alterniflora communities. Belowground biomass, orthophosphate (PO43-), the ratio of available N to phosphorus (DTN/PO43-) and salinity were negatively correlated with the soil N-fixing rate, while aboveground biomass, crab burrow volume, dissolved nitrogen (DTN), redox potential, and pH positively affected the soil N-fixing rate. In addition, the soil N-fixing rate was positively correlated with the diversity of the nitrogenase gene nifH. The diazostroph community under S. alterniflora was composed mainly of sulfate-reducing bacteria (SRB), photosynthetic bacteria and plant symbiotic bacteria, which overall accounted for more than80%of the diazostroph community. The proportion of SRB declined with the age of communities, but the proportion of purple sulfur bacteria and photosynthetic bacteria increased. The proportions of Fe(III)-reducing bacteria decreased in1-year-old and2-3-year-old communities, but increased in5-6-year-old communities by crab bioturbation. Meanwhile, the proportion of photosynthetic bacteria and plant symbiotic bacteria increased in1-year-old and2-3-year-old communities. The heterogeneity of the diazostroph community structure may attribute to the changed microhabitats by S. alterniflora and crab bioturbation.(3) The N transformation process in Dongtan salt marsh was also both affected by plant invasion and crab disturbance. Plant colonization, especially S. alterniflora invasion, might accelerate the rate of soil degradation of organic matter as well as the uptake rate of dissolved organic matter by microbes, but it inhibited microbial mineralization rate. Through a growing season, the native communities slowed the decline of soil soluble organic nitrogen (DON) and ammonium nitrogen (NH4+), which was replenished by the increasing degradation of total nitrogen (TN) under vegetation. In invasive communities, a large quantity of NH4+was absorbed by&alterniflora, resulting in N limitation in soil. Furthermore, the microbial immobilization rates of NH4+were accelerated due to the N limitation in soil. Crab bioturbation may accelerate the microbial immobilizations rates of NH4+, and the decline of reaction substrate may further inhibit the nitrification and nitrate (NO3-) consumption. (4) The distribution of iron and sulfur and their contribution to N cycle were both affected by plant invasion and crab disturbance. Generally, the effects of plant invasion were more notable than crab activities, and crab disturbance was more pronounced in invasive communities. Soil depth, crab burrow volume and belowground biomass had stronger effects on the distribution of soil components in invasive (26.0%) than in native communities (11.8%). The distribution of labile components (soluble substrates) was mainly influenced by crab burrow volume and belowground biomass, while the distribution of stable components such as total carbon (TC), TN and total organic carbon (TOC) was mainly affected by the soil depth. The anaerobic respiration in invasive community was more intense than that in native community. Plant invasion and crab bioturbation both enhanced sulfate reduction but weakened the iron reduction process. Sulfate reduction pathway might be related to the N input process, while the iron reduction pathway might be associated with nitrate consumption process. The iron reduction pathway accounted for more than90%of the contribution to anaerobic carbon oxidation, while the sulfate reduction pathway contributed to only less than10%in native communities. These two pathways competed with each other and the substrate SO42-and Fe(Ⅲ) drove the reduction process. By contrast, the iron and sulfate reduction pathways shared evenly in invasive communities, whereas these two pathways coexisted and the reduction process might be driven by the supply of dissolved organic carbon (DOC). |
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