Seasonal variation in CH4 emission and its 13C-isotopic signature from Spartina alterniflora and Scirpus mariqueter soils in an estuarine wetland

Although invasions by non-native species represent a major threat to biodiversity and ecosystem functioning, little attention has been paid to the potential impacts of these invasions on methane (CH₄) emission and its ¹³C-CH₄-isotope signature in salt marshes. An invasive perennial C₄ grass Spartina alterniflora has spread rapidly along the east coast of China since its introduction from North America in 1979. Since its intentional introduction to the Jiuduansha Island in the Yangtze River estuary in 1997, S. alterniflora monocultures have become the dominant component of the Jiuduansha’s vegetation, where monocultures of the native plant Scirpus mariqueter (a C₃ grass) used to dominate the vegetation for more than 30 years. We investigated seasonal variation in soil CH₄ emission and its ¹³C-CH₄-isotope signature from S. alterniflora and S. mariqueter marshes. The results obtained here show that S. alterniflora invasion increased soil CH₄ emissions compared to native S. mariqueter, possibly resulting from great belowground biomass of S. alterniflora, which might have affected soil microenvironments and /or CH₄ production pathways. CH₄ emissions from soils in both marshes followed similar seasonal patterns in CH₄ emissions that increased significantly from April to August and then decreased from August to October. CH₄ emissions were positively correlated with soil temperature, but negatively correlated with soil moisture for both S. alterniflora and S. mariqueter soils (p?<?0.05). The δ¹³C values of CH₄ from S. alterniflora, and S. mariqueter soils ranged from -39.0‰ to -45.0‰, and -37.3‰ to -45.7‰, respectively, with the lowest δ¹³C values occurring in August in both marshes. Although the leaves, roots and soil organic matter of S. alterniflora had significantly higher δ¹³C values than those of S. mariqueter, S. alterniflora invasion did not significantly change the ¹³C- isotopic signature of soil emitted CH₄ (p?>?0.05). Generally, the CH₄ emissions from both invasive S. alterniflora and native S. mariqueter soils in the salt marshes of Jiuduansha Island were very low (0.01–0.26 mg m^-2 h^-1), suggesting that S. alterniflora invasion along the east coast of China may not be a significant potential source of atmospheric CH₄.     

Invasive alien plants increase CH4 emissions from a subtropical tidal estuarine wetland

Methane (CH₄) is an important greenhouse gas whose emission from the largest source, wetlands is controlled by a number of environmental variables amongst which temperature, water-table, the availability of substrates and the CH₄ transport properties of plants are most prominent and well characterised. Coastal wetland ecosystems are vulnerable to invasion by alien plant species which can make a significant local contribution to altering their species composition. However the effect of these changes in species composition on CH₄ flux is rarely examined and so is poorly understood. Spartina alterniflora, a perennial grass native to North America, has spread rapidly along the south-east coast of China since its introduction in 1979. From 2002, this rapid invasion has extended to the tidal marshes of the Min River estuary, an area that, prior to invasion was dominated by the native plant Cyperus malaccensis. Here, we compare CH₄ flux from the exotic invasive plant S. alterniflora with measurements from the aggressive native species Phragmites australis and the native species C. malaccensis following 3-years of monitoring. CH₄ emissions were measured over entire tidal cycles. Soil CH₄ production potentials were estimated for stands of each of above plants both in situ and in laboratory incubations. Mean annual CH₄ fluxes from S. alterniflora, P. australis and C. malaccensis dominated stands over the 3 years were 95.7 (±18.7), 38.9 (±3.26) and 10.9 (±5.26) g m^?2 year^?1, respectively. Our results demonstrate that recent invasion of the exotic species S. alterniflora and the increasing presence of the native plant P. australis has significantly increased CH₄ emission from marshes that were previously dominated by the native species C. malaccensis. We also conclude that higher above ground biomass, higher CH₄ production and more effective plant CH₄ transport of S. alterniflora collectively contribute to its higher CH₄ emission in the Min River estuary.     

Seasonal Dynamics of Soil Labile Organic Carbon and Enzyme Activities in Relation to Vegetation Types in Hangzhou Bay Tidal Flat Wetland

Soil labile organic carbon and soil enzymes play important roles in the carbon cycle of coastal wetlands that have high organic carbon accumulation rates. Soils under three vegetations (Phragmites australis, Spartina alterniflora, and Scirpusm mariqueter) as well as bare mudflat in Hangzhou Bay wetland of China were collected seasonally. Seasonal dynamics and correlations of soil labile organic carbon fractions and soil enzyme activities were analyzed. The results showed that there were significant differences among vegetation types in the contents of soil organic carbon (SOC) and dissolved organic carbon (DOC), excepting for that of microbial biomass carbon (MBC). The P. australis soil was with the highest content of both SOC (7.86 g kg^-1) and DOC (306 mg kg^-1), while the S. mariqueter soil was with the lowest content of SOC (6.83 g kg^-1), and the bare mudflat was with the lowest content of DOC (270 mg kg^-1). Soil enzyme activities were significantly different among vegetation types except for urease. The P. australis had the highest annual average activity of alkaline phosphomonoesterase (21.4 mg kg^-1 h^-1), and the S. alterniflora had the highest annual average activities of β-glycosidase (4.10 mg kg^-1 h^-1) and invertase (9.81mg g^-1 24h^-1); however, the bare mudflat had the lowest activities of alkaline phosphomonoesterase (16.2 mg kg^-1 h^-1), β-glycosidase (2.87 mg kg^-1 h^-1), and invertase (8.02 mg g^-1 24h^-1). Analysis also showed that the soil labile organic carbon fractions and soil enzyme activities had distinct seasonal dynamics. In addition, the soil MBC content was significantly correlated with the activities of urease and β-glucosidase. The DOC content was significantly correlated with the activities of urease, alkaline phosphomonoesterase, and invertase. The results indicated that vegetation type is an important factor influencing the spatial-temporal variation of soil enzyme activities and labile organic carbon in coastal wetlands.     

Effects of salt marsh invasion by Spartina alterniflora on sulfate-reducing bacteria in the Yangtze River estuary,China

To investigate how plant invasion affects sulfate-reducing bacteria (SRB) responsible for sulfate reduction, we conducted a comparative study of diversity and composition of SRB in rhizosphere soils of invasive exotic species (Spartina alterniflora) and two native species (Phragmites australis and Scirpus mariqueter) on Jiuduansha Island located in the Yangtze River estuary, China. Throughout the growing season, profiles of DGGE fingerprints of SRB had distinct variations in relation to phenological stages of these three plant species. The higher richness and abundance of SRB in the rhizospheres of native plants mainly occurred when the plants were in vegetative growth and reproductive stages. However, the higher richness and abundance of SRB also occurred in the late growing season (senescent stage) of S. alterniflora rhizosphere, during which Desulfobulbus, Desulfuromonas, Desulfovibrio, and Firmicutes were dominant. Our results adding to our previous studies suggested that abundant SRB in late stage might have close relationships with decomposition of soil organic matters produced by S. alterniflora.     

Local competitive effects of introduced Spartina alterniflora on Scirpus mariqueter at Dongtan of Chongming Island, the Yangtze River estuary and their potential ecological consequences

Dongtan of Chongming Island, Shanghai, China is a wetland of international importance, in which dominant vegetation is Scirpus mariqueter community that supports a great diversity of bird species. Spartina alterniflora, native to the eastern and gulf coasts of the USA, was intentionally introduced to the tidelands of Dongtan in May 2001. Field work were conducted at Dongtan from March 2002 to May 2003 to determine how introduced S. alterniflora affects S. mariqueter population dynamics. The results obtained here show that S. alterniflorahad strong competitive effects on S. mariqueter, and that the introduction of S. alterniflora to S. mariqueter community resulted in a significant decreases of S. mariqueter's abundance, coverage, seed and fresh corm output after two growing seasons. A concomitant consequence might be the decline in bird diversity. It is suggested that further introduction of S. alterniflora should be avoided to conserve the wetlands and their associated birds. A plan for controlling further spread of existing S. alterniflora populations in the Yangtze River estuary is urgently needed.     

Trophic Dynamics of Filter Feeding Bivalves in the Yangtze Estuarine Intertidal Marsh: Stable Isotope and Fatty Acid Analyses

Benthic bivalves are important links between primary production and consumers, and are essential intermediates in the flow of energy through estuarine systems. However, information on the diet of filter feeding bivalves in estuarine ecosystems is uncertain, as estuarine waters contain particulate matter from a range of sources and as bivalves are opportunistic feeders. We surveyed bivalves at different distances from the creek mouth at the Yangtze estuarine marsh in winter and summer, and analyzed trophic dynamics using stable isotope (SI) and fatty acid (FA) techniques. Different bivalve species had different spatial distributions in the estuary. Glauconome chinensis mainly occurred in marshes near the creek mouth, while Sinonovacula constricta preferred the creek. Differences were found in the diets of different species. S. constricta consumed more diatoms and bacteria than G. chinensis, while G. chinensis assimilated more macrophyte material. FA markers showed that plants contributed the most (38.86 ± 4.25%) to particular organic matter (POM) in summer, while diatoms contributed the most (12.68 ± 1.17%) during winter. Diatoms made the largest contribution to the diet of S. constricta in both summer (24.73 ± 0.44%) and winter (25.51 ± 0.59%), and plants contributed no more than 4%. This inconsistency indicates seasonal changes in food availability and the active feeding habits of the bivalve. Similar FA profiles for S. constricta indicated that the bivalve had a similar diet composition at different sites, while different δ¹³C results suggested the diet was derived from different carbon sources (C₄ plant Spartina alterniflora and C₃ plant Phragmites australis and Scirpus mariqueter) at different sites. Species-specific and temporal and/or spatial variability in bivalve feeding may affect their ecological functions in intertidal marshes, which should be considered in the study of food webs and material flows in estuarine ecosystems.     

Soil Respiration in Semiarid Temperate Grasslands under Various Land Management

Soil respiration, a major component of the global carbon cycle, is significantly influenced by land management practices. Grasslands are potentially a major sink for carbon, but can also be a source. Here, we investigated the potential effect of land management (grazing, clipping, and ungrazed enclosures) on soil respiration in the semiarid grassland of northern China. Our results showed the mean soil respiration was significantly higher under enclosures (2.17μmol.m⁻².s⁻¹) and clipping (2.06μmol.m⁻².s⁻¹) than under grazing (1.65μmol.m⁻².s⁻¹) over the three growing seasons. The high rates of soil respiration under enclosure and clipping were associated with the higher belowground net primary productivity (BNPP). Our analyses indicated that soil respiration was primarily related to BNPP under grazing, to soil water content under clipping. Using structural equation models, we found that soil water content, aboveground net primary productivity (ANPP) and BNPP regulated soil respiration, with soil water content as the predominant factor. Our findings highlight that management-induced changes in abiotic (soil temperature and soil water content) and biotic (ANPP and BNPP) factors regulate soil respiration in the semiarid temperate grassland of northern China.     

盐度和淹水对长江口潮滩盐沼植物碳储量的影响

盐生植物是盐沼有机碳储存的"临时库",也是土壤有机碳累积的主要来源,其碳储量大小对盐沼生态系统"碳汇"功能的发挥十分重要。以长江口潮滩本地种芦苇(Phragmites australis)和海三棱藨草(Scirpus mariqueter),及入侵种互花米草(Spartina alterniflora)为研究对象,采用单因素盆栽实验,模拟分析淹水盐度(0、5、10、15、25和35)、淹水深度(0、10、20、40、60cm和80cm)和淹水频率(每天、每3天、每7天、每10天和每15天)变化对各盐生植物地上、地下和总体碳储量大小的影响。研究结果表明,随着淹水盐度增加,芦苇、互花米草和海三棱藨草地上部分与总体碳储量均显著降低。土壤盐度可分别解释其地上部分碳储量变异的47.2%、66.5%和72.7%,与总体碳储量变异的34.7%、45.0%和62.0%。随着淹水深度增加,芦苇地上部分、总体碳储量和海三棱藨草地上部分碳储量均显著降低,其变异的68.6%、28.5%和71.1%可由淹水深度变化(10—80cm)解释。互花米草在80cm淹水深度下仍有较高的地上部分碳储量和总体碳储量。3种盐生植物碳储量对淹水频率变化的响应差异均不显著,所有处理地下部分碳储量差异也未达到显著水平。总体而言,互花米草对水盐胁迫的耐受性要强于本地种芦苇和海三棱藨草。尽管互花米草和芦苇具有相对较高的碳储量,但水盐胁迫对其碳储量的显著抑制作用不容忽视。海三棱藨草碳储量本就不高,输入土壤的有机碳量较为有限,海平面上升及盐水入侵等逆境胁迫会使其对盐沼"碳汇"贡献更加微弱。     

Co-Regulations of Spartina alterniflora Invasion and Exogenous Nitrogen Loading on Soil N2O Efflux in Subtropical Mangrove Mesocosms

Both plant invasion and nitrogen (N) enrichment should have significant impact on mangrove ecosystems in coastal regions around the world. However, how N₂O efflux in mangrove wetlands responds to these environmental changes has not been well studied. Here, we conducted a mesocosm experiment with native mangrove species Kandelia obovata, invasive salt marsh species Spartina alterniflora, and their mixture in a simulated tide rotation system with or without nitrogen addition. In the treatments without N addition, the N₂O effluxes were relatively low and there were no significant variations among the three vegetation types. A pulse loading of exogenous ammonium nitrogen increased N₂O effluxes from soils but the stimulatory effect gradually diminished over time, suggesting that frequent measurements are necessary to accurately understand the behavior of N-induced response of N₂O emissions. With the N addition, the N₂O effluxes from the invasive S. alterniflora were lower than that from native K. obovata mesocosms. This result may be attributed to higher growth of S. alterniflora consuming most of the available nitrogen in soils, and thus inhibiting N₂O production. We concluded that N loading significantly increased N₂O effluxes, while the invasion of S. alterniflora reduced N₂O effluxes response to N loading in this simulated mangrove ecosystem. Thus, both plant invasion and excessive N loading can co-regulate soil N₂O emissions from mangrove wetlands, which should be considered when projecting future N₂O effluxes from this type of coastal wetland.     

Spartina alterniflora invasions in the Yangtze River estuary,China: An overview of current status and ecosystem effects

The Yangtze River estuary is an important ecoregion. However, Spartina alterniflora, native to North America, was introduced to the estuary in the 1990s through both natural dispersal and humans and now it is a dominant species in the estuarine ecosystems, with its invasions leading to multiple consequences to the estuary. S. alterniflora had great competitive effects on native species, including Scirpus mariqueter and Phragmites australis, and could potentially exclude the natives locally. The presence of S. alterniflora had little influence on the total density of soil nematodes and macrobenthonic invertebrates, but significantly altered the structure of trophic functional groups of nematode and macrobenthonic invertebrate communities. The conversion of mudflats to Spartina meadows had significant effects on birds of Charadriidae and Scolopacidae, which might be attributable to the reduction of food resources and the physical alterations of habitats for shorebirds. S. alterniflora invasions increased the primary productivity of the invaded ecosystems, and altered carbon and nitrogen cycling processes. Our studies focused mainly on the effects of S. alterniflora invasions on the structure of native ecosystems; thus further studies are clearly needed to investigate how ecosystem functioning is affected by the modification of the structure of estuarine ecosystems by S. alterniflora invasions.     

Impacts of introduced Spartina alterniflora along an elevation gradient at the Jiuduansha Shoals in the Yangtze Estuary,suburban Shanghai,China

Although much research has focused upon the negative impacts of invasive Spartina alterniflora upon salt marshes dominated by other Spartina spp., little is known about its impacts upon native Scirpus mariqueter marshes. In 1997, S. alterniflora was introduced to the Jiuduansha Shoals, Yangtze Estuary, China, to accelerate the formation of marsh habitat via accretionary processes, with the larger goal of drawing waterfowl away from wetlands near the Pudong International Airport, Shanghai, China. In 2000, a nature reserve was established on the Jiuduansha Shoals, making the impact upon the native S. mariqueter community a high priority for research. Our objective was to quantify the impacts of introduced S. alterniflora and Phragmites australis to the native S. mariqueter-dominated community at this site in four elevation zones, as compared with a nearby natural shoal. We found that species diversity was greater in the lower elevations with the engineering, through elimination of the natural dominance of S. mariqueter. We also found that diversity was lessened in the higher elevations, due to rapid growth and exclusion by the planted S. alterniflora in conjunction with the native P. australis. Moreover, we found that the growth of the native S. mariqueter was stimulated when S. alterniflora was planted nearby. It is quite likely that the net effect of these ecological processes will be to accelerate further accretion, leading to an eventual replacement of the S. mariqueter-dominated community in the long-term. Future management approaches should focus upon harvesting, grazing, and perimeter-ditching the S. alterniflora to avoid this situation.     

闽江河口春季互花米草入侵过程对短叶茳芏沼泽土壤碳氮分布特征的影响

2014年4月,选择闽江口鳝鱼滩湿地中未被入侵的短叶茳芏群落(A)、互花米草入侵斑块边缘(B)以及互花米草入侵斑块中央(C)为研究对象,基于时空互代研究方法,探讨了互花米草入侵序列下湿地土壤碳氮空间分布特征的差异。结果表明,互花米草入侵显著降低了土壤的NO_3~--N含量(P0.05),但整体增加了NH_4~+-N含量,这与其入侵后导致湿地土壤颗粒组成发生显著变化(砂砾含量增加33.81%),进而促进了土壤的矿化作用和硝化作用,并有助于硝态氮的垂直淋失有关。互花米草入侵整体增加了土壤的碳氮含量和C/N比,与入侵进程和入侵前相比,互花米草入侵后湿地土壤的碳储量分别增加了8.73%和24.37%,氮储量则分别增加了10.22%和17.87%,这主要与其对闽江口湿地植物群落格局、养分生物循环以及强促淤作用引起的土壤颗粒组成等显著改变有关。研究发现,闽江口互花米草入侵对短叶茳芏湿地土壤碳氮含量的影响相对于江苏盐城、长江口以及杭州湾湿地的影响可能更为显著,其互花米草入侵较大改变了土壤中陆源和海源有机质的来源比例,使得入侵后湿地土壤养分的自源性增强。     

Methane production potential and methanogenic archaea community dynamics along the Spartina alterniflora invasion chronosequence in a coastal salt marsh

Invasion by the exotic species Spartina alterniflora, which has high net primary productivity and superior reproductive capacity compared with native plants, has led to rapid organic carbon accumulation and increased methane (CH₄) emission in the coastal salt marsh of China. To elucidate the mechanisms underlying this effect, the methanogen community structure and CH₄ production potential as well as soil organic carbon (SOC), dissolved organic carbon, dissolved organic acids, methylated amines, aboveground biomass, and litter mass were measured during the invasion chronosequence (0–16 years). The CH₄ production potential in the S. alterniflora marsh (range, 2.94–3.95 μg kg^?1 day^?1) was significantly higher than that in the bare tidal mudflat. CH₄ production potential correlated significantly with SOC, acetate, and trimethylamine concentrations in the 0–20 cm soil layer. The abundance of methanogenic archaea also correlated significantly with SOC, and the dominant species clearly varied with S. alterniflora-driven SOC accumulation. The acetotrophic Methanosaetaceae family members comprised a substantial proportion of the methanogenic archaea in the bare tidal mudflat while Methanosarcinaceae family members utilized methylated amines as substrates in the S. alterniflora marsh. Ordination analysis indicated that trimethylamine concentration was the primary factor inducing the shift in the methanogenic archaea composition, and regressive analysis indicated that the facultative family Methanosarcinaceae increased linearly with trimethylamine concentration in the increasingly sulfate-rich salt marsh. Our results indicate that increased CH₄ production during the S. alterniflora invasion chronosequence was due to increased levels of the non-competitive substrate trimethylamine and a shift in the methanogenic archaea community.     

Crustacean metamorphosis: an omics perspective

We quantified the independent impacts of flooding salinity, flooding depth, and flooding frequency on the native species, Phragmites australis and Scirpus mariqueter, and on the invasive species Spartina alterniflora in the Yangtze River Estuary, China. Total biomass of all three species decreased significantly with increasing salinity, but S. alterniflora was less severely affected than P. australis and S. mariqueter. Elevated flooding depth significantly decreased their live aboveground biomass of P. australis and S. mariqueter, while S. alterniflora still had high live aboveground biomass and total biomass even at the highest flooding depth. These findings indicated that S. alterniflora was more tolerant to experimental conditions than the two native species, and an unavoidable suggestion is the expansion of this non-native species in relation to the native counterparts in future scenarios of increased sea-level and saltwater intrusion. Even so, environmental stresses might lead to significant decreases in total biomass and live aboveground biomass of all three species, which would potentially weaken their ability to trap sediments and accumulate organic matter. However, the relatively high belowground-to-aboveground biomass ratio indicated phenotypic plasticity in response to stressful environmental conditions, which suggest that marsh species can adapt to sea-level rise and maintain marsh elevation.     

Determinants of seed bank dynamics of two dominant helophytes in a tidal salt marsh

We investigated dynamics and spatial distribution of Scirpus mariqueter and Spartina alterniflora seed banks at Chongming Dongtan in the Yangtze River estuary, China. Five sites along an elevational gradient were chosen, one in each of the main zones (mudflat, Scirpus monoculture, Scirpus–Spartina mixture, Spartina monoculture and Spartina–Phragmites mixture). Three surveys were performed just after seed rain, before germination and after germination, respectively. During the period of November 2005 to May 2006, soil seed density of Scirpus mariqueter declined by 36%, and that of Spartina alterniflora by 58%. The spatial distributions of their seed banks were also different. Soil seed density of Scirpus mariqueter was not determined directly by seed production, but positively correlated with total aboveground biomass of the whole plant community. On the contrary, soil seed density of Spartina alterniflora just after seed rain (November) was significantly correlated with seed production, but had a poor relationship with the community's aboveground biomass. Our results indicated that other factors such as tidal movement might have had great influence on dispersal of Scirpus mariqueter, which would also affect its population dynamics. The understanding of this process can help us improve the conservation and restoration efforts.     

Postfire carbon pools and fluxes in semiarid ponderosa pine in Central Oregon

Forest fire dramatically affects the carbon storage and underlying mechanisms that control the carbon balance of recovering ecosystems. In western North America where fire extent has increased in recent years, we measured carbon pools and fluxes in moderately and severely burned forest stands 2 years after a fire to determine the controls on net ecosystem productivity (NEP) and make comparisons with unburned stands in the same region. Total ecosystem carbon in soil and live and dead pools in the burned stands was on average 66% that of unburned stands (11.0 and 16.5 kg C m⁻², respectively, P<0.01). Soil carbon accounted for 56% and 43% of the carbon pools in burned and unburned stands. NEP was significantly lower in severely burned compared with unburned stands (P<0.01) with an increasing trend from −125±44 g C m⁻² yr⁻¹ (±1 SD) in severely burned stands (stand replacing fire), to −38±96 and +50±47 g C m⁻² yr⁻¹ in moderately burned and unburned stands, respectively. Fire of moderate severity killed 82% of trees <20 cm in diameter (diameter at 1.3 m height, DBH); however, this size class only contributed 22% of prefire estimates of bole wood production. Larger trees (> 20 cm DBH) suffered only 34% mortality under moderate severity fire and contributed to 91% of postfire bole wood production. Growth rates of trees that survived the fire were comparable with their prefire rates. Net primary production NPP (g C m⁻² yr⁻¹, ±1 SD) of severely burned stands was 47% of unburned stands (167±76, 346±148, respectively, P<0.05), with forb and grass aboveground NPP accounting for 74% and 4% of total aboveground NPP, respectively. Based on continuous seasonal measurements of soil respiration in a severely burned stand, in areas kept free of ground vegetation, soil heterotrophic respiration accounted for 56% of total soil CO₂ efflux, comparable with the values of 54% and 49% previously reported for two of the unburned forest stands. Estimates of total ecosystem heterotrophic respiration (R_h) were not significantly different between stand types 2 years after fire. The ratio NPP/R_h averaged 0.55, 0.85 and 1.21 in the severely burned, moderately burned and unburned stands, respectively. Annual soil CO₂ efflux was linearly related to aboveground net primary productivity (ANPP) with an increase in soil CO₂ efflux of 1.48 g C yr⁻¹ for every 1 g increase in ANPP (P<0.01, r²= 0.76). There was no significant difference in this relationship between the recently burned and unburned stands. Contrary to expectations that the magnitude of NEP 2 years postfire would be principally driven by the sudden increase in detrital pools and increased rates of R_h, the data suggest NPP was more important in determining postfire NEP.     

The Impacts of Above- and Belowground Plant Input on Soil Microbiota: Invasive <Emphasis Type="Italic">Spartina alterniflora</Emphasis> Versus Native <Emphasis Type="Italic">Phragmites australis</Emphasis>

Invasive plants affect soil food webs through various resource inputs including shoot litter, root litter and living root input. The net impact of invasive plants on soil biota has been recognized; however, the relative contributions of different resource input pathways have not been quantified. Through a 2 × 2 × 2 factorial field experiment, a pair of invasive and native plant species (Spartina alterniflora vs. Phragmites australis) was compared to determine the relative impacts of their living roots or shoots and root litter on soil microbial and nematode communities. Living root identity affected bacteria-to-fungi PLFA ratios, abundance of total nematodes, plant-feeding nematodes and omnivorous nematodes. Specifically, the plant-feeding nematodes were 627% less abundant when living roots of invasive S. alterniflora were present than those of native P. australis. Likewise, shoot and root biomass (within soil at 0–10 cm depth) of S. alterniflora was, respectively, 300 and 100% greater than those of P. australis. These findings support the enemy release hypothesis of plant invasion. Root litter identity affected other components of soil microbiota (that is, bacterial-feeding nematodes), which were 34% more abundant in the presence of root litter of P. australis than S. alterniflora. Overall, more variation associated with nematode community structure and function was explained by differences in living roots than root or shoot litter for this pair of plant species sharing a common habitat but contrasting invasion degrees. We conclude that belowground resource input is an important mechanism used by invasive plants to affect ecosystem structure and function.     

Temporal and spatial impact of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> invasion on methanogens community in Chongming Island,China

Methane production by methanogens in wetland is recognized as a significant contributor to global warming. Spartina alterniflora (S. alterniflora), which is an invasion plant in China’s wetland, was reported to have enormous effects on methane production. But studies on shifts in the methanogen community in response to S. alterniflora invasion at temporal and spatial scales in the initial invasion years are rare. Sediments derived from the invasive species S. alterniflora and the native species Phragmites australis (P. australis) in pairwise sites and an invasion chronosequence patch (4 years) were analyzed to investigate the abundance and community structure of methanogens using quantitative real-time PCR (qPCR) and Denaturing gradient gel electrophoresis (DGGE) cloning of the methyl-coenzyme M reductase A (mcrA) gene. For the pairwise sites, the abundance of methanogens in S. alterniflora soils was lower than that of P. australis soils. For the chronosequence patch, the abundance and diversity of methanogens was highest in the soil subjected to two years invasion, in which we detected some rare groups including Methanocellales and Methanococcales. These results indicated a priming effect at the initial invasion stages of S. alterniflora for microorganisms in the soil, which was also supported by the diverse root exudates. The shifts of methanogen communities after S. alterniflora invasion were due to changes in pH, salinity and sulfate. The results indicate that root exudates from S. alterniflora have a priming effect on methanogens in the initial years after invasion, and the predominate methylotrophic groups (Methanosarcinales) may adapt to the availability of diverse substrates and reflects the potential for high methane production after invasion by S. alterniflora.     

Temperature Sensitivity and Basal Rate of Soil Respiration and Their Determinants in Temperate Forests of North China

The basal respiration rate at 10°C (R₁₀) and the temperature sensitivity of soil respiration (Q₁₀) are two premier parameters in predicting the instantaneous rate of soil respiration at a given temperature. However, the mechanisms underlying the spatial variations in R₁₀ and Q₁₀ are not quite clear. R₁₀ and Q₁₀ were calculated using an exponential function with measured soil respiration and soil temperature for 11 mixed conifer-broadleaved forest stands and nine broadleaved forest stands at a catchment scale. The mean values of R₁₀ were 1.83 µmol CO₂ m⁻² s⁻¹ and 2.01 µmol CO₂ m⁻² s⁻¹, the mean values of Q₁₀ were 3.40 and 3.79, respectively, for mixed and broadleaved forest types. Forest type did not influence the two model parameters, but determinants of R₁₀ and Q₁₀ varied between the two forest types. In mixed forest stands, R₁₀ decreased greatly with the ratio of coniferous to broadleaved tree species; whereas it sharply increased with the soil temperature range and the variations in soil organic carbon (SOC), and soil total nitrogen (TN). Q₁₀ was positively correlated with the spatial variances of herb-layer carbon stock and soil bulk density, and negatively with soil C/N ratio. In broadleaved forest stands, R₁₀ was markedly affected by basal area and the variations in shrub carbon stock and soil phosphorus (P) content; the value of Q₁₀ largely depended on soil pH and the variations of SOC and TN. 51% of variations in both R₁₀ and Q₁₀ can be accounted for jointly by five biophysical variables, of which the variation in soil bulk density played an overwhelming role in determining the amplitude of variations in soil basal respiration rates in temperate forests. Overall, it was concluded that soil respiration of temperate forests was largely dependent on soil physical properties when temperature kept quite low.     

Exotic Spartina alterniflora invasion alters ecosystem–atmosphere exchange of CH4 and N2O and carbon sequestration in a coastal salt marsh in China

Coastal salt marshes are sensitive to global climate change and may play an important role in mitigating global warming. To evaluate the impacts of Spartina alterniflora invasion on global warming potential (GWP) in Chinese coastal areas, we measured CH₄ and N₂O fluxes and soil organic carbon sequestration rates along a transect of coastal wetlands in Jiangsu province, China, including open water; bare tidal flat; and invasive S. alterniflora, native Suaeda salsa, and Phragmites australis marshes. Annual CH₄ emissions were estimated as 2.81, 4.16, 4.88, 10.79, and 16.98 kg CH₄ ha^?1 for open water, bare tidal flat, and P. australis, S. salsa, and S. alterniflora marshes, respectively, indicating that S. alterniflora invasion increased CH₄ emissions by 57–505%. In contrast, negative N₂O fluxes were found to be significantly and negatively correlated (P < 0.001) with net ecosystem CO₂ exchange during the growing season in S. alterniflora and P. australis marshes. Annual N₂O emissions were 0.24, 0.38, and 0.56 kg N₂O ha^?1 in open water, bare tidal flat and S. salsa marsh, respectively, compared with ‐0.51 kg N₂O ha^?1 for S. alterniflora marsh and ?0.25 kg N₂O ha^?1 for P. australis marsh. The carbon sequestration rate of S. alterniflora marsh amounted to 3.16 Mg C ha^?1 yr^?1 in the top 100 cm soil profile, a value that was 2.63‐ to 8.78‐fold higher than in native plant marshes. The estimated GWP was 1.78, ?0.60, ?4.09, and ?1.14 Mg CO₂eq ha^?1 yr^?1 in open water, bare tidal flat, P. australis marsh and S. salsa marsh, respectively, but dropped to ?11.30 Mg CO₂eq ha^?1 yr^?1 in S. alterniflora marsh. Our results indicate that although S. alterniflora invasion stimulates CH₄ emissions, it can efficiently mitigate increases in atmospheric CO₂ and N₂O along the coast of China.