Spatial Distribution of Soil Organic Carbon and Total Nitrogen in a Ramsar Wetland, Dafeng Milu National Nature Reserve

The invasion and expansion of Spartina alterniflora in coastal salt marsh wetlands have greatly affected the material cycle of the ecosystem. A total of 372 topsoil samples were collected from 124 sites representing two land-cover types by implementing an unprecedented high sampling density study in the Dafeng Milu National Nature Reserve. Classical statistics and geostatistics were used to quantify soil organic carbon (SOC) and total nitrogen (TN) spatial distribution. Redundancy analysis (RDA) was used to detect correlations between environmental factors, SOC, and TN. The results showed that SOC and TN have moderate variability. The spatial distributions of SOC and TN were similar, and the highest values were observed in the southwest of the study area. In different land cover types, the SOC and TN in the vegetation coverage areas with Spartina alterniflora as the dominant species were significantly higher than those in bare land. RDA showed that TN and aboveground biomass significantly affected the spatial distribution of SOC, while SOC and AGB dominated the spatial distribution of TN.

Population attributes of Littoraria angulifera (Gastropoda: Littorinidae) in mangroves in Bahia State, northeastern Brazil

Littoraria angulifera (Lamarck, 1822) is an estuarine gastropod of tropical occurrence, which lives mainly on trunks of mangrove tree species. This study aimed to evaluate the population attributes of this species, such as abundance, space-time distribution, sex ratio and recruitment in mangroves in Bahia State, northeastern Brazil. The specimens were collected monthly throughout 2018, on trunks of the red mangrove Rhizophora mangle L. at two heights and on two horizontal levels. Specimens that were on the marine grass Spartina alterniflora Loisel were also collected. After counting, biometrics and sexing, the specimens were returned alive to the environment. The average size and weight of L. angulifera was higher (p<0.05) in places with taller and less sparse trees and the vertical distribution on the trunks showed a preference for strata close to the soil. Both results are related to shading and protection against desiccation. Females were more abundant than males, at an approximate sex ratio F: M of 1.4: 1. Recruits were observed throughout the period, showing continuous reproduction of the species with a recruitment peak in spring (September to November). The study revealed the importance of keep the mangroves intact to allow the maintenance of the natural stocks of the species.

Lateral detrital C transfer across a Spartina alterniflora invaded estuarine wetland

Background The lateral movements of mass and energy across the terrestrial-aquatic interface are being increasingly recognized for their importance in the carbon (C) balance of coastal/estuarine wetlands. We quantified the lateral flux of detrital C in the Yangtze estuary where invasive Spartina alterniflora has substantially and extensively altered the ecosystem structure and functions. Our overall objective was to close the C budget of estuarine wetlands through field sampling, tower-based measurements, and modeling. Methods A lateral detrital C exchange evaluation platform was established in a case study of the Yangtze River Estuary to investigate the effect of ecosystem structural changes on lateral detrital C transfer processes. This study estimated the lateral detrital C exchange based on the gross primary production (GPP) by performing coupled modeling and field sampling. Tower-based measurements and MODIS time series and CH4 outgassing and biomass simultaneously measured the lateral detrital C flux to characterize the relative contributions of lateral (i.e., detritus) C fluxes to the annual marsh C budget. Results The C pools in the plants and soil of Spartina marshes were significantly higher than those of the native community dominated by Phragmites australis. The GPP based on MODIS (GPPMODIS) was 472.6 g C m−2 year−1 and accounted for 73.0% of the GPP estimated from eddy covariance towers (GPPEC) (646.9 ± 70.7 g C m−2 year−1). We also detected a higher GPPMODIS during the pre-growing season, which exhibited a similar lateral detrital C flux magnitude. On average, 25.8% of the net primary production (NPP), which ranged from 0.21 to 0.30 kg C m−2 year−1, was exported during lateral exchange. The annual C loss as CH4 was estimated to be 17.9 ± 3.7 g C m−2 year−1, accounting for 2.8% of the GPPEC. The net positive detrital C flux (i.e., more detritus leaving the wetlands), which could exceed 0.16 kg C m−2 day−1, was related to daily tides. However, the observed lateral detrital C flux based on monthly sampling was 73.5% higher than that based on daily sampling (i.e., the sum of daily sampling), particularly in March and October. In addition, spatiotemporal granularities were responsible for most of the uncertainty in the lateral detrital C exchange. Conclusion This research demonstrated that an integrated framework incorporating modeling and field sampling can quantitatively assess lateral detrital C transport processes across the terrestrial-aquatic interface in estuarine wetlands. However, we note some limitations in the application of the light-use efficiency model to tidal wetlands. Spartina invasion can turn the lateral C balance from a C source (209.0 g C m−2 year−1) of Phragmites-dominated marshes into a small C sink (-31.0 g C m−2 year−1). Sampling over a more extended period and continuous measurements are essential for determining the contribution of different lateral detrital C flux processes to closing the ecosystem C budgets. The sampling spatiotemporal granularities can be key to assessing lateral detrital C transfer.

Effects of Imazapyr on Spartina alterniflora and Soil Bacterial Communities in a Mangrove Wetland

The invasion of Spartina alterniflora (S. alterniflora) has caused serious damage to coastal wetland ecosystems in China, especially the mangrove wetlands in South China. This study aimed to validate the effect of imazapyr on S. alterniflora and soil. The controlled experiment was conducted in May 2021 at the Zhangjiangkou Mangrove Wetland Reserve. In the experiment, 25% (W) imazapyr was used, and six treatments were set up: 3035, 6070, and 9105 mL/acre 25% imazapyr and 1299, 2604, and 5202 mL/acre of AGE 809 + 6070 mL/acre 25% imazapyr. The results showed no side effects on mangrove plants in the spraying area. The highest control efficiency (95.9%) was given by 2604 mL/acre of AGE 809 + 6070 mL/acre 25% imazapyr. The residues of imazapyr in different soils were reduced to 0.10–0.59 mg/kg. The sequencing results showed no significant difference in the overall bacterial communities under different treatments (p > 0.05). The soil bacterial diversity in the samples with adjuvant was higher than that in the samples without adjuvant, while the abundance values were the opposite. There were 10 main communities (>0.3%) at phylum level in all soil samples, among which Proteobacteria, Bacteroidetes, Acidobacteria, Chloflexi, and Actinobacteria were the dominant communities, and the latter four’s abundance changed significantly (p < 0.05). There were significant abundance differences between the groups of oligotrophic and eutrophic bacteria. The redundancy analysis and Monte Carlo tests showed that the total organic carbon (TOC), total phosphorus (TP), available phosphorus (AP), ammonia nitrogen, and total nitrogen were the main factors affecting soil bacterial diversity. At the same time, TOC, AP, and TP were the most critical factors affecting the overall characteristics of soil bacterial communities in different treatments, while soil residues had no significant effect on bacteria. This might be due to the addition and degradation of imazapyr and the coverage of S. alterniflora. The best recommendation is 2604 mL/acre of AGE 809 + 6070 mL/acre 25% imazapyr to be applied in China’s mangrove wetland reserves and coastal wetlands.

The Impact of Sea Embankment Reclamation on Greenhouse Gas GHG Fluxes and Stocks in Invasive Spartina alterniflora and Native Phragmites australis Wetland Marshes of East China

The introduction of embankment seawalls to limit the expansion of the exotic C4 perennial grass Spartina alteniflora Loisel in eastern China’s coastal wetlands has more than doubled in the past decades. Previous research focused on the impact of sea embankment reclamation on the soil organic carbon (C) and nitrogen (N) stocks in salt marshes, whereas no study attempted to assess the impact of sea embankment reclamation on greenhouse gas (GHG) fluxes in such marshes. Here we examined the impact of sea embankment reclamation on GHG stocks and fluxes of an invasive Spartina alterniflora and native Phragmites australis dominated salt marsh in the Dongtai wetlands of China’s Jiangsu province. Sea embankment reclamation significantly decreased soil total organic C by 54.0% and total organic N by 73.2%, decreasing plant biomass, soil moisture, and soil salinity in both plants’ marsh. It increased CO2 emissions by 38.2% and 13.5%, and reduced CH4 emissions by 34.5% and 37.1%, respectively, in the Spartina alterniflora and Phragmites australis marshes. The coastal embankment wall also significantly increased N2O emission by 48.9% in the Phragmites australis salt marsh and reduced emissions by 17.2% in the Spartina alterniflora marsh. The fluxes of methane CH4 and carbon dioxide CO2 were similar in both restored and unrestored sections, whereas the fluxes of nitrous oxide N2O were substantially different owing to increased nitrate as a result of N-loading. Our findings show that sea embankment reclamation significantly alters coastal marsh potential to sequester C and N, particularly in native Phragmites australis salt marshes. As a result, sea embankment reclamation essentially weakens native and invasive saltmarshes’ C and N sinks, potentially depleting C and N sinks in coastal China’s wetlands. Stakeholders and policymakers can utilize this scientific evidence to strike a balance between seawall reclamation and invasive plant expansion in coastal wetlands.