Effects of nutrient enrichment on mangrove leaf litter decomposition

Effects of catchment land use on temperate mangrove forests

Human land use changes are threatening the integrity and health of coastal ecosystems worldwide. Intensified land use for anthropogenic purposes increases sedimentation rates, pollutants, and nutrient concentrations into adjacent coastal areas, often with detrimental effects on marine life and ecosystem functioning. However, how these factors interact to influence ecosystem health in mangrove forests is poorly understood. This study investigates the effects of catchment human land use on mangrove forest architecture and sedimentary attributes at a landscape-scale. Thirty sites were selected along a gradient of human land use within a narrow latitudinal range, to minimise the effects of varying climatic conditions. Land use was quantified using spatial analysis tools with existing land use databases (LCDB5). Twenty-six forest architectural and sedimentary variables were collected from each site. The results revealed a significant effect of human land use on ten out of 26 environmental variables. Eutrophication, characterised by changes in redox potential, pH, and sediment nutrient concentrations, was strongly associated with increasing human land use. The δ15N values of sediments and leaves also indicated increased anthropogenic nitrogen input. Furthermore, the study identified a positive correlation between human land use and tree density, indicating that increased nutrient delivery from catchments contributes to enhanced mangrove growth. Propagule and seedling densities were also positively correlated with human land use, suggesting potential recruitment success mechanisms. This research underpins the complex interactions between human land use and mangrove ecosystems, revealing changes in carbon dynamics, potential alterations in ecosystem services, and a need for holistic management approaches that consider the interconnectedness of species and their environment. These findings provide essential insights for regional ecosystem models, coastal management, and restoration strategies to address the impacts of human pressures on temperate mangrove forests, even in estuaries that may be relatively healthy.

Human activity has increasingly affected recent carbon accumulation in Zhanjiang mangrove wetland, South China

Mangrove wetlands are an important component of blue carbon (C) ecosystems, although the anthropogenic impact on organic C accumulation rate (OCAR) in mangrove wetlands is not yet clear. Three sediment cores were collected from Zhanjiang Gaoqiao Mangrove Reserve in Southern China, dated by 210Pb and 137Cs, and physico-chemical parameters measured. Results show that the OCARs in mangroves and grasslands have significantly increased by 4.4 and 1.3 times, respectively, since 1950, which is consistent with the transformation of organic C sources and the increase of sedimentation rate. This increment is due to increased soil erosion and nutrient enrichment caused by land use change and the discharge of fertilizer runoff and aquaculture wastewater. This study provides clear evidence for understanding the changes in organic C accumulation processes during the Anthropocene and is conducive to promoting the realization of C peak and neutrality targets.

The influence of tide-brought nutrients on microbial carbon metabolic profiles of mangrove sediments

Mangrove ecosystems in the intertidal zone are continually affected by tidal inundation, but the impact of tidal-driven nutrient inputs upon bacterial communities and carbon metabolic features in mangrove surface sediments remains underexplored, and the differences in such impacts across backgrounds are not known. Here, two mangrove habitats with contrasting nutrient backgrounds in Shenzhen Bay and Daya Bay in Shenzhen City, China, respectively, were studied to investigate the effects of varying tidal nutrient inputs (especially dissolved inorganic nitrogen and PO43--P) on bacterial community composition and functioning in sediment via field sampling, 16S rDNA amplicon sequencing, and the quantitative potential of microbial element cycling. Results showed that tidal input increased Shenzhen Bay mangrove's eutrophication level whereas it maintained the Daya Bay mangrove's relatively oligotrophic status. Dissolved inorganic nitrogen and PO43--P levels in Shenzhen Bay were respectively 12.6-39.6 and 7.3-29.1 times higher than those in Daya Bay (p < 0.05). In terms of microbial features, Desulfobacteraceae was the dominant family in Shenzhen Bay, while the Anaerolineaceae family dominated in Daya Bay. Co-occurrence network analysis revealed more interconnected and complex microbial networks in Shenzhen Bay. The quantitative gene-chip analysis uncovered more carbon-related functional genes (including carbon degradation and fixation) enriched in Shenzhen Bay's sediment microbial communities than Daya Bay's. Partial least squares path modeling indicated that tidal behavior directly affected mangrove sediments' physicochemical characteristics, with cascading effects shaping microbial diversity and C-cycling function. Altogether, these findings demonstrate that how tides influence the microbial carbon cycle in mangrove sediments is co-correlated with the concentration of nutrient inputs and background status of sediment. This work offers an insightful lens for better understanding bacterial community structure and carbon metabolic features in mangrove sediments under their tidal influences. It provides a theoretical basis to better evaluate and protect mangroves in the context of global change.

Response of macrophyte litter decomposition in global blue carbon ecosystems to climate change

Blue carbon ecosystems (BCEs) are important nature-based solutions for climate change-mitigation. However, current debates question the reliability and contribution of BCEs under future climatic-scenarios. The answer to this question depends on ecosystem processes driving carbon-sequestration and -storage, such as primary production and decomposition, and their future rates. We performed a global meta-analysis on litter decomposition rate constants (k) in BCEs and predicted changes in carbon release from 309 studies. The relationships between k and climatic factors were examined by extracting remote-sensing data on air temperature, sea-surface temperature, and precipitation aligning to the decomposition time of each experiment. We constructed global numerical models of litter decomposition to forecast k and carbon release under different scenarios. The current k averages at 27 ± 3 × 10-2  day-1 for macroalgae were higher than for seagrasses (1.7 ± 0.2 × 10-2  day-1 ), mangroves (1.6 ± 0.1 × 10-2  day-1 ) and tidal marshes (5.9 ± 0.5 × 10-3  day-1 ). Macrophyte k increased with both air temperature and precipitation in intertidal BCEs and with sea surface temperature for subtidal seagrasses. Above a temperature threshold for vascular plant litter at ~25°C and ~20°C for macroalgae, k drastically increased with increasing temperature. However, the direct effect of high temperatures on k are obscured by other factors in field experiments compared with laboratory experiments. We defined "fundamental" and "realized" temperature response to explain this effect. Based on relationships for realized temperature response, we predict that proportions of decomposed litter will increase by 0.9%-5% and 4.7%-28.8% by 2100 under low- (2°C) and high-warming conditions (4°C) compared to 2020, respectively. Net litter carbon sinks in BCEs will increase due to higher increase in litter C production than in decomposition by 2100 compared to 2020 under RCP 8.5. We highlight that BCEs will play an increasingly important role in future climate change-mitigation. Our findings can be leveraged for blue carbon accounting under future climate change scenarios.

The accumulation and release characteristics of heavy metals on the cultivation environment in Gracilaria litters during decay process

Gracilaria bioremediates heavy metals (Cd, Cr, Pb, Ni, Cu, Zn, Fe, and Mn) and improves water quality in mariculture zones. However, Gracilaria litter produced during the growth and harvest process has become a critical bottleneck problem that limits the sustainable development of the Gracilaria cultivation industry. Experiments of decaying dried (dead) and frozen fresh (falling and dying) G. lemaneiformis and G. lichenosdies were carried out using the litterbag technique under laboratory-controlled and in situ conditions. The results showed that decay rates (k), decomposed time in 50 % (t50) and in 95 % (t95) varied between dried and frozen fresh Gracilaria and were different between G. lemaneiformis and G. lichenosdies. All Gracilaria samples showed an 80 %-90 % weight loss in 15-45 d. The variation in MAIs (accumulation index of metals) between the dried and frozen fresh Gracilaria litters differed significantly and provided evidence that metals could be imported or exported from litter to the environment. Based on our estimates from the 15-45 d experiment, the decay of Gracilaria can release and adsorb heavy metals. The enrichment of Fe, Pb, and Mn was more significant than the release, but the release of Cr, Zn, Cd, Pb, Cu, and Ni was more significant than the enrichment. Heavy metals in Gracilaria litters were accumulated and released simultaneously during decay. The present study simulated and underscores that Gracilaria cultivation intensely influences heavy metals recycled in marine environments It provides a theoretical basis for seaweed management for the sustainable development of the seaweed industry in the mariculture zone.

The ecological effect of large-scale coastal natural and cultivated seaweed litter decay processes: An overview and perspective

Seaweeds are important components of marine ecosystems and can form a large biomass in a few months. The decomposition of seaweed litter provides energy and material for primary producers and consumers and is an important link between material circulation and energy flow in the ecosystem. However, during the growth process, part of the seaweed is deposited on the sediment surface in the form of litter. Under the joint action of the environment and organisms, elements enriched in seaweed can be released back into the environment in a short time, causing pollution problems. The cultivation yield of seaweed worldwide reached 34.7 million tons in 2019, but the litter produced during the growth and harvest process has become a vital bottleneck that restricts the further improvement of production and sustainable development of the seaweed cultivation industry. Seaweed outbreaks worldwide occur frequently, producing a mass of litter and resulting in environmental pollution on coasts and economic losses, which have negative effects on coastal ecosystems. The objective of this review is to discuss the decomposition process and ecological environmental effects of seaweed litter from the aspects of the research progress on seaweed litter; the impact of seaweed litter on the environment; and its interaction with organisms. Understanding the decomposition process and environmental impact of seaweed litter can provide theoretical support for coastal environmental protection, seaweed resource conservation and sustainable development of the seaweed cultivation industry worldwide. This review suggests that in the process of large-scale seaweed cultivation and seaweed outbreaks, ageing or falling litter should be cleared in a timely manner, mature seaweed should be harvested in stages, and dried seaweed produced after harvest and washed up on shore should be handled properly to ensure the benefits of environmental protection provided by seaweed growth and sustainable seaweed resource development.

Coordinated pattern of multiple element variability in Aegiceras corniculatum propagule in shrimp aquaculture effluent habitats

Human activities and environment changes have changed river estuary ecosystems, which impacts element changes in coastal sediments and mangroves. Mangrove propagule chemical traits showed a systematic shift along environmental gradients. But knowledge about how the pattern of multi-element variability is coordinated in propagule remains limited, and the conservation of macro and trace elements in propagules is also unknown. In this study, the concentrations, variability and coordinated pattern variation of 13 elements in Aegiceras corniculatum propagule across shrimp aquaculture effluent habitats, as well as the relationship between propagule element and environment factors were explored. We used CV to quantify the variability of each element, and then explore the pattern of multi-element variability. The results showed that: (1) in the habitats affected by shrimp aquaculture, the elements content shows: C > K > Cl > N > Na > P > S > Mg > Ca > Fe > Mn > Zn > Cu, and the coefficient variation shows: Mn > Cu > Fe > Zn > S > N > P > Cl > Na > K > Mg > Ca > C, which means that the element concentration are negatively correlated with the element variability and the variability of macro-elements was more conservative than micro-elements in these habitats; (2) pH, OM, C:P, and SiO₃^2- were the four important environmental factors explaining the A. corniculatum propagule variation. In conclusion, effluent from shrimp aquaculture does affect the coordinated pattern of multiple element variability in A. corniculatum propagules. These results provide a strong evidence for assessing the impact of shrimp aquaculture effluent discharges on mangrove and provide an important theoretical basis for mangrove conservation and restoration.

Potential impacts of siltation by Spartina alterniflora on nutrient dynamics in its decomposing litters in coastal marsh of the Min River estuary, southeast China

The Spartina alterniflora started to invade the Min River estuary (Southeast China) in 2002 and, thereafter, its invasion area showed an increasing trend. Since the siltation depths caused by S. alterniflora in the Min River estuary were much higher (4.8-7.2 cm yr^-1) than the values reported in other coastal regions of China (3.5-6.5 cm yr^-1), the impacts of siltation on nutrient cycle processes in this region might be more evident. In order to explore the potential effects of siltation by S. alterniflora on nutrient ((carbon (C), nitrogen (N), phosphorous (P) and sulfur (S)) variations in its decaying litters, three one-off siltation treatments (no siltation scenario (0 cm yr^-1, NSS), current siltation scenario (5 cm yr^-1, CSS) and strong siltation scenario (10 cm yr^-1, SSS)) were designed in coastal marsh of the Min River estuary and the in-situ decomposition experiment was conducted from February 2016 to February 2017 by litterbag technique. Results showed that the siltation caused by S. alterniflora showed significant impact on its decomposition rate, following the sequence of NSS (0.005638 d^-1) > SSS (0.003005 d^-1) > CSS (0.002478 d^-1) (p < 0.05). The total carbon (TC) contents in decomposing litters in the three siltation treatments showed dissimilar fluctuations and significantly higher values were observed in the CSS and SSS treatments compared to the NSS treatment. The contents of total nitrogen (TN) and total sulfur (TS) in decomposing detritus in the three siltation treatments generally showed increasing trend during the whole decomposition, while those of total phosphorus (TP) showed increasing trend after decomposing for 30 days. The differences in nutrient variations among the three siltation treatments, to a great extent, rested with the alterations of substrate quality in detritus during the experiment. Although the stocks of C, N, P and S in detritus in the three siltation treatments evidenced the release from litters to the surroundings during decomposition, the release amounts of these nutrients in some periods were at a lower level. With increasing siltation depths, the release of C, N and P from detritus was generally restrained during the whole decomposition, while that of S from decaying litters was inhibited only at the late stage of decomposition. This paper found that the siltation caused by S. alterniflora reduced the nutrient return (particularly for C, N and P) from its detritus, which, in turn, might greatly alter the nutrient cycle in S. alterniflora marsh.

Linking eutrophication to carbon dioxide and methane emissions from exposed mangrove soils along an urban gradient

Mangroves are one of the most important but threatened blue carbon ecosystems globally. Rapid urban growth has resulted in nutrient inputs and subsequent coastal eutrophication, associated with an enrichment in organic matter (OM) from algal and sewage sources and substantial changes in greenhouse gas (GHG) emissions. However, the effects of nitrogen (N) and phosphorus (P) enrichment on mangrove soil OM composition and GHG emissions, such as methane (CH₄) and carbon dioxide (CO₂), are still poorly understood. Here, we aim to evaluate the relationships between CO₂ and CH₄ efflux with OM composition in exposed soils from three mangrove areas along watersheds with different urbanization levels (Rio de Janeiro State, Brazil). To assess spatial (lower vs. upper intertidal zones) and seasonal (summer vs. winter) variability, we measured soil-air CO₂ and CH₄ fluxes at low spring tide, analyzing elementary (C, N, and P), isotopic (δ¹³C and δ¹⁵N), and the molecular (n-alkanes and sterols) composition of surface soil OM. A general trend of OM composition was found with increasing urban influence, with higher δ¹⁵N (proxy of anthropogenic N enrichment), less negative δ¹³C, more short-chain n-alkanes, lower C:N ratio (proxies of algal biomass), and higher epicoprostanol content (proxies of sewage-derived OM). The CO₂ efflux from exposed soils increased greatly in median (25/75 % interquartile range) from 4.6 (2.9/8.3) to 24.0 (21.5/32.7) mmol m^-2 h^-1 from more pristine to more urbanized watersheds, independent of intertidal zone and seasonality. The CO₂ fluxes at the most eutrophicated site were among the highest reported worldwide for mangrove soils. Conversely, CH₄ emissions were relatively low (three orders of magnitude lower than CO₂ fluxes), with high peaks in the lower intertidal zone during the rainy summer. Thus, our findings demonstrate the influence of coastal eutrophication on global warming potentials related to enhanced heterotrophic remineralization of blue carbon within mangrove soils.

Biomass decomposition and heavy metal release from seaweed litter, Gracilaria lemaneiformis, and secondary pollution evaluation

The seaweed Gracilaria lemaneiformis can bioremediate heavy metals and improve the environmental quality of mariculture zones. However, the seaweed litter that is produced in the growth and harvest processes becomes one of the important bottlenecks and causes secondary pollution that restricts the development of sustainable seaweed cultivation. Seaweeds exist widely in the coastal areas of the world and are cultivated on a large scale in Asia, but their decomposition process is rarely studied. Experiments that compared decaying dry (dead) and fresh (falling and dying) Gracilaria were conducted to quantify the differences in decomposition rates and heavy metal release in different physiological states. The heavy metals in the seawater and sediment were investigated. The litterbag technique under controlled laboratory conditions was used. The results indicated that the decomposition rates (k) and decay times in 50% (t50%) and 95% (t95%) values varied between dry and fresh Gracilaria. Fresh Gracilaria exhibited a weight loss rate of 15%, and the dry weight loss was 44%. The variations in MAIs (accumulation index of metals) and M_R (release rate of metals) between the dry and fresh Gracilaria litters differed significantly, which provides evidence that metals are released back into the environment from Gracilaria litters. The contacted sediments could accelerate the heavy metal release from Gracilaria. Based on our estimates obtained from a 45 d experiment, at least 27.5％ of Cd, 16％ of Cu, 60.1％ of Pb, 72.3％ of Zn, 49.4％ of Fe, 38.6％ of Mn, 68.1％ of Cr, and 67.5％ of Ni present in the fresh Gracilaria and 37.4％ of Cd, 46.2％ of Cu, 77.7％ of Pb, 53.7％ of Zn, 42.7％ of Fe, 67.2％ of Mn, 75.1％ of Cr, and 73.5％ of Ni present in the dried Gracilaria were released back into the water when the biomass was left to decay. This study simulates and underscores that Gracilaria has an strong effect on the heavy metal cycles in marine environments and offers a theoretical basis for the development of sustainable seaweed industries in mariculture zones.

Coastal Sediment Nutrient Enrichment Alters Seagrass Blue Carbon Sink Capacity

The seagrass ecosystem is among the most efficient natural carbon sinks that can contribute to climate change mitigation. However, little is known about the effects of coastal nutrient enrichment caused by anthropogenic activities and/or climate change on the capacity of the seagrass blue carbon sink. Our experimental manipulations of sediment nutrient enrichment shifted the blue carbon sink capabilities of seagrass meadows. Sediment nutrient enrichment significantly increased the nutrient content of seagrass litter, stimulating the decomposition of rhizome + root litter by ∼10% while retarding the decomposition of leaf litter by ∼5%. Sediment N + P enrichment increased seagrass growth and litter production, while enrichment of N or P alone did not. Organic carbon (C_org) stocks in the surface sediments (0-5 cm) were 34% higher than those in the control with N + P enrichment due to high litter production and the low decomposition rate of nutrient-enriched leaf litter. However, C_org stocks in the subsurface sediments (5-20 cm) did not increase with sediment nutrient enrichment, which is likely due to accelerated decomposition of rhizome + root litter. Our findings suggest that nutrient loading in coastal sediments alters the blue carbon sink and storage capacities in seagrass meadows by changing the rates of carbon sequestration and decomposition.

Human disturbance drives loss of soil organic matter and changes its stability and sources in mangroves

Mangrove soils with high organic carbon (C_org) content are likely to contain C_org that is vulnerable to remineralization during land use changes. Mangrove conversion to different land uses might deplete soil C_org stocks causing variable carbon dioxide emissions, but the extent of these emissions and the fraction of soil C_org (i.e., labile or stable/recalcitrant) that is mostly lost is poorly understood. Here, we study mangrove soil C_org degradability and its susceptibility to mineralization after mangrove disturbance. We measured changes in soil properties, organic matter (OM) stability and C_org pools and sources across a mangrove disturbance gradient (i.e., pristine forests, degraded mangroves receiving domestic sewage and shrimp farm effluents, and shrimp ponds). Results showed that the conversion of mangroves to shrimp ponds caused the most severe changes in soil properties, OM and C_org characteristics. Shrimp pond soils contained the lowest OM-C_org pools, consisted mostly of stable OM (i.e., recalcitrant and refractory; 36.0 ± 5.7% of the total OM) and enriched δ¹³C_org (-22.6 ± 2.7‰). Conversely, control mangrove soils had the largest OM-C_org pools consisting of a large unstable OM fraction (i.e., labile; 46.4 ± 4.2%) and lighter δ¹³C_org (-26.8 ± 0.4‰) being characteristic of C_org from a mangrove origin. Conversion of mangroves to shrimp ponds and its degradation by shrimp farm and domestic sewage effluents caused a loss of 97%, 61%, and 35% of soil C_org stocks in the upper meter, representing potential emissions of ~1200, 800, and 400 Mg CO₂ ha^-1, respectively. These losses were explained by enhanced OM mineralization of unstable fractions driven by the loss of the physico-chemical protection provided by fine-grained soils and vegetation cover. The differences in C_org stability among sites can be used to predict potential carbon dioxide produced during mineralization, hence aid at prioritizing areas for conservation, restoration or management.

Effects of a nutrient enrichment pulse on blue carbon ecosystems

Coastal ecosystems are under increasing pressure from land-derived eutrophication in most developed coastlines worldwide. Here, we tested for 277 days the effects of a nutrient pulse on blue carbon retention and cycling within an Australian temperate coastal system. After 56 days of exposure, saltmarsh and mangrove plots subject to a high-nutrient treatment (~20 g N m^-2 yr^-1 and ~2 g P m^-2 yr^-1) had ~23% lower superficial soil carbon stocks. Mangrove plots also experienced a ~33% reduction in the microbe Amplicon Sequence Variant richness and a shift in community structure linked to elevated ammonium concentrations. Live plant cover, tea litter decomposition, and soil carbon fluxes (CO₂ and CH₄) were not significantly affected by the pulse. Before the end of the experiment, soil carbon- and nitrogen-cycling had returned to control levels, highlighting the significant but short-lived impact that a nutrient pulse can have on the carbon sink capacity of coastal wetlands.

Effects of exogenous nitrogen import on variations of nutrient in decomposing litters of Suaeda salsa in coastal marsh of the Yellow River estuary, China

To explore the effects of exogenous nitrogen (N) import on decomposition of Suaeda salsa in coastal marsh of the Yellow River estuary, the decomposition rates and the variations of nutrient (C, N, P, and S) in decomposing litters were investigated from May 2014 to November 2015. The in situ experiment included four N addition treatments: N0 (no N import treatment, 0 g N·m-2·year-1), Nlow (low N import treatment, 3.0 g N·m-2·year-1), Nmedium (medium N import treatment, 6.0 g N·m-2·year-1), and Nhigh (high N import treatment, 12.0 g N·m-2·year-1). Results showed that N enrichment showed significant influence on the decomposition rate of S. salsa, in the order of Nmedium (0.00112 d-1) > Nhigh (0.00096 d-1) > Nlow(0.00092 d-1) > N0 (0.00075 d-1) (p < 0.05). Compared to the N0 treatment, the decomposition rates of S. salsa in the Nlow, Nmedium, and Nhigh treatments increased by 22.76%, 49.33%, and 28.00%, respectively. The contents of TC in decomposing litters in the four N import treatments generally showed a decreasing trend, while those of TN and TP showed an increasing trend. By comparison, dissimilar variations of TS contents in decomposing litters were observed among the four treatments. The differences in decomposition rate and nutrient variation among the four N addition treatments might be dependent on the alterations of substrate quality in decomposing litters. With a few exceptions, stocks of C and S in decomposing litters generally evidenced the export from litters to the environment, while those of N and P showed net accumulation. As N addition reached Nmedium level, although the C released from decomposing litters to the surroundings was stimulated, the S return was restrained. Moreover, N additions generally promoted the accumulation of N and P in decomposing litters. This paper found that, with increasing N addition, the decomposition rates and the nutrient variations in S. salsa would be altered greatly and the alteration was particularly evident in the Nmedium treatment. From the perspective of nutrient return, as N enrichment reached or exceeded Nmedium level in future, the deficient status of P and S in S. salsa marsh might be serious, which would affect the stability and health of ecosystems.

Nitrogen mineralization and eutrophication risks in mangroves receiving shrimp farming effluents

Nitrogen (N) inputs originated from shrimp farming effluents were evaluated for potential changes in the net N mineralization for mangrove soils from Northeastern Brazil. Our study provides notable information and assessment for the potential enhancement of N mineralization in preserved and shrimp-impacted semi-arid mangrove soils of the Jaguaribe River estuary, which is one of the largest shrimp producers of Brazil, using an analytical and daily tidal variation experimental approach. Nitrogen-rich effluents promoted a significant (p value < 0.001) increase of the total soil N content (1998 ± 201 mg kg^-1 on average) compared with the preserved sites (average: 1446 ± 295 mg kg^-1). The effluents also increased the N mineralization in the shrimp-impacted sites (N-min: 86.6 ± 37.5 mg kg^-1), when compared with preserved mangroves (N-min: 56.5 ± 23.8 mg kg^-1). Over a daily tidal variation experiment, we found that just 30% (36.2 ± 20.6 mg kg^-1) of mineralized N remains stored in the soil, whereas 70% (102.9 ± 38.8 mg kg^-1) was solubilized in tidal waters. Therefore, the N mineralization process may trigger eutrophication by increasing N inorganic bioavailability in mangrove soils receiving N-rich effluents from shrimp ponds, which in turn might increase primary producers' activity. This approach has not been studied so far in semi-arid mangroves, where the shrimp farming activity is one of the most important economic activities.

Effects of spatial expansion between Phragmites australis and Cyperus malaccensis on variations of arsenic and heavy metals in decomposing litters in a typical subtropical estuary (Min River), China

To investigate the effects of spatial expansion between native invasive species (Phragmites australis) and commom native species (Cyperus malaccensis) on variations of micro-elements (Pb, Cr, Cu, Zn, Ni, Cd and As) in decomposing litters in the Min River estuary, in situ filed decomposition experiment was conducted in P. australis (PA) community (before expansion, BE), C. malaccensis (CM) community (before expansion, BE) and P. australis-C. malaccensis (PA'-CM') community (during expansion, DE) from February 2016 to February 2017 by space-for- time substitution method. Results showed that the decomposition of C. malaccensis were faster than those of P. australis whether at BE stage or at DE stage. The decomposition rate of PA' increased by 24.40% compared to PA whereas the value of CM' decreased by 15.67% compared with CM. The concentrations of Pb, Cu, Zn, Ni, Cd and As in decomposing litters of P. australis (PA and PA') and C. malaccensis (CM and CM') generally showed increasing tendency and the values in the former were significantly lower than those in the latter (p < 0.05). The physicochemical sorption onto recalcitrant organic fractions and the substrate quality (C/N and M/C ratios) of decomposing litters were two important factors affecting the differences in As/metals variations between species. The levels of Cr in decaying litters increased initially and decreased afterward, and the values in P. australis were significantly higher than those in C. malaccensis (p < 0.05). Whether at BE stage or at DE stage, stocks of As/metals in decomposing litters of P. australis (PA and PA') were generally higher than those of C. malaccensis (CM and CM'). The lower stocks of As/metals in CM or CM' might be more dependent on its lower mass remaining. Compared with PA at BE stage, the accumulation of As/metals in decomposing litters of PA' at DE stage decreased greatly, which might be ascribed to the lower precipitation of metal sulfides in PA'. Stocks of Zn, Ni, Cd and Cr in CM' and stocks of Cr in PA' generally evidenced the export of metals from decomposing litter to environment, indicating that the potential exposure risk of Zn, Ni, Cd and Cr might be increased as CM was invading by PA. This study found that the spatial expansion between P. australis and C. malaccensis not only altered the stocks of As/metals in decomposing litters but also increased the exposure risk of Zn, Ni, Cd and Cr in ecotone. In future, as the ecological functions of ecotone was precisely evaluated during the expansion of the two plants in the Min River estuary, the alterations of litter decomposition rates and the exposure risks of Zn, Ni and Cd caused by CM' should be emphasized.

Influence of fungi and bag mesh size on litter decomposition and water quality

Litter decomposition is a complex process that is influenced by many different physical, chemical, and biological processes. Environmental variables and leaf litter quality (e.g., nutrient content) are important factors that play a significant role in regulating litter decomposition. In this study, the effects of adding fungi and using different mesh size litter bags on litter (Populus tomentosa Carr. and Salix matsudana Koidz.) decomposition rates and water quality were investigated, and investigate the combination of these factors influences leaf litter decomposition. Dissolved oxygen (DO), chemical oxygen demand (COD), total phosphorus (TP), and ammonia-nitrogen (NH₃-N) were measured during the 112-day experiment. The salix leaf litter (k = 0.045) displayed faster decomposition rates than those of populous leaf litter (k = 0.026). Litter decomposition was initially slow and then accelerated; and by the end of the experiment, the decomposition rate was significantly higher (p = 0.012, p < 0.05) when fungi were added to the treatment process compared to the blank, and litter bags with different mesh sizes did not influence the decomposition rate. The variations in the decomposition rates and nutrient content were influenced by litter quality and a number of environmental factors. The decomposition rate was most influenced by internal factors related to litter quality, including the N/P and C/P ratios of the litter. By quantifying the interact effect of environment and litter nutrient dynamic, to figure out the revetment plant litter decomposition process in a wetland system in biological physical and chemical aspects, which can help us in making the variables that determine decomposition rates important for assessing wetland function.

Impact of Global Change on Nutrient Dynamics in Mangrove Forests

The cycling of essential nutrients is central to mangrove productivity. A mass balance shows that mangroves rely on soil ammonification, nitrification, and dissimilatory reduction to ammonium for available nitrogen. Mangroves are often nutrient limited and show tight coupling between nutrient availability and tree photosynthesis. This relationship and, thus, forest productivity can be disrupted by various disturbances such as deforestation, changes in hydrology due to impoundments, land-use change, increasing frequency and intensity of storms, increasing temperatures, increasing atmospheric CO2 concentrations, and a rising sea-level. Deforestation and hydrological changes are the most devastating to soil nutrient-plant relations and mangrove productivity. Land-use changes can result in positive and negative impacts on mangroves and can also results in increasing frequency of storms and intensity of storms. Increasing temperatures and atmospheric CO2 levels have an initially enhanced effect on mangroves and microbial transformation rates of nitrogen and phosphorus. The effects of rising seas are complex and depend on the local rate of sea-level rise, the soil accretion rate, the subsidence or uplift rate, and the tidal position. If mangroves cannot keep pace with a sea-level rise, seaward mangroves will likely drown but landward mangroves will expand and show enhanced growth and more rapid nutrient cycling if space permits.

Litter decomposition in hyper-arid deserts: Photodegradation is still important

Photodegradation due to litter exposure to solar UV radiation is presumed to contribute to the surprisingly fast decomposition in some arid and semi-arid regions; however, few studies have directly examined photodegradation effects in hyper-arid regions (annual precipitation <150mm) and its dependence on precipitation. Three litters with different initial qualities (low vs high C:N) were decomposed under full spectrum sunlight (UV radiation) and UV filtering from solar radiation at three sites with contrasting precipitation amounts (144mm, 76mm and 16mm) for 2.5years. UV radiation increased mass loss and litter decomposition rates by 23-70%. UV photodegradation effects (UVE) on litter decomposition rate differed among experimental sites, with significantly stronger effects in less arid sites (144mm and 76mm) than more arid site (16mm). High-quality litter (low C:N ratio) showed the fastest decomposition rate, and UVE was also affected by litter quality, but no consistent trend was observed. Litter N loss was greatest in full sunlight and the linear relationships between C and N contents was not changed by UV filtering over time. UV radiation increased C loss of all fractions, and hemicellulose and cell solubles showed significant contributions to litter mass loss. Our findings suggest that UV photodegradation can increase mass loss and nutrient release by the positive priming effects on microbial decomposition in hyper-arid regions, although UVE differed among three sites with contrasting precipitation amounts.

Impacts of burial by sediment on decomposition and heavy metal concentrations of Suaeda salsa in intertidal zone of the Yellow River estuary, China

Three one-off burial treatments were designed in intertidal zone of the Yellow River estuary to determine the effects of sediment burial on decomposition and heavy metal levels of Suaeda salsa. Sediment burial showed significant effect on decomposition rate of S. salsa. With increasing burial depth, Cu, Zn, Cd and Co levels generally increased, while Cr and Mn levels decreased. Except for Zn, Mn, Cd and Co, stocks of Pb, Cr, Cu, Ni and V in S. salsa among burials were greatly different. The S. salsa in three burials was particular efficient in binding V and Co and releasing Pb, Zn and Cd, and, with increasing burial depth, stocks of Cr, Cu, Ni and Mn shifted from accumulation to release. In future, the eco-toxic risk of Pb, Cr, Cu, Zn, Ni, Mn and Cd exposure might be serious as the strong burial episodes occurred in S. salsa marsh.

Potential for Sulfate Reduction in Mangrove Forest Soils: Comparison between Two Dominant Species of the Americas

Avicennia and Rhizophora are globally occurring mangrove genera with different traits that place them in different parts of the intertidal zone. It is generally accepted that the oxidizing capacity of Avicennia roots is larger than that of Rhizophora roots, which initiates more reduced conditions in the soil below the latter genus. We hypothesize that the more reduced conditions beneath Rhizophora stands lead to more active sulfate-reducing microbial communities compared to Avicennia stands. To test this hypothesis, we measured sulfate reduction traits in soil samples collected from neighboring Avicennia germinans and Rhizophora mangle stands at three different locations in southern Florida. The traits measured were sulfate reduction rates (SRR) in flow-through reactors containing undisturbed soil layers in the absence and presence of easily degradable carbon compounds, copy numbers of the dsrB gene, which is specific for sulfate-reducing microorganisms, and numbers of sulfate-reducing cells that are able to grow in liquid medium on a mixture of acetate, propionate and lactate as electron donors. At the tidal locations Port of the Islands and South Hutchinson Islands, steady state SRR, dsrB gene copy numbers and numbers of culturable cells were higher at the A. germinans than at the R. mangle stands, although not significantly for the numbers at Port of the Islands. At the non-tidal location North Hutchinson Island, results are mixed with respect to these sulfate reduction traits. At all locations, the fraction of culturable cells were significantly higher at the R. mangle than at the A. germinans stands. The dynamics of the initial SRR implied a more in situ active sulfate-reducing community at the intertidal R. mangle stands. It was concluded that in agreement with our hypothesis R. mangle stands accommodate a more active sulfate-reducing community than A. germinans stands, but only at the tidal locations. The differences between R. mangle and A. germinans stands were absent at the non-tidal, impounded location.

Effects of sediment burial disturbance on macro and microelement dynamics in decomposing litter of Phragmites australis in the coastal marsh of the Yellow River estuary, China

From April 2008 to November 2009, a field decomposition experiment was conducted to investigate the effects of sediment burial on macro (C, N) and microelement (Pb, Cr, Cu, Zn, Ni, and Mn) variations in decomposing litter of Phragmites australis in the coastal marsh of the Yellow River estuary. Three one-off sediment burial treatments [no sediment burial (0 mm year(-1), S0), current sediment burial (100 mm year(-1), S10), and strong sediment burial (200 mm year(-1), S20)] were laid in different decomposition sites. Results showed that sediment burials showed significant influence on the decomposition rate of P. australis, in the order of S10 (0.001990 day(-1)) ≈ S20 (0.001710 day(-1)) > S0 (0.000768 day(-1)) (p < 0.05). The macro and microelement in decomposing litters of the three burial depths exhibited different temporal variations except for Cu, Zn, and Ni. No significant differences in C, N, Pb, Cr, Zn, and Mn concentrations were observed among the three burial treatments except for Cu and Ni (p > 0.05). With increasing burial depth, N, Cr, Cu, Ni, and Mn concentrations generally increased, while C, Pb, and Zn concentrations varied insignificantly. Sediment burial was favorable for C and N release from P. australis, and, with increasing burial depth, the C release from litter significantly increased, and the N in litter shifted from accumulation to release. With a few exceptions, Pb, Cr, Zn, and Mn stocks in P. australis in the three treatments evidenced the export of metals from litter to environment, and, with increasing burial depth, the export amounts increased greatly. Stocks of Cu and Ni in P. australis in the S10 and S20 treatments were generally positive, evidencing incorporation of the two metals in most sampling times. Except for Ni, the variations of C, N, Pb, Cr, Cu, Zn, and Mn stocks in P. australis in the S10 and S20 treatments were approximated, indicating that the strong burial episodes (S20) occurred in P. australis marsh in the future would have little influence on the stocks of these elements. With increasing burial depths, the P. australis was particularly efficient in binding Cu and Ni and releasing C, N, Pb, Cr, Zn, and Mn, implying that the potential eco-toxic risk of Pb, Cr, Zn, and Mn exposure might be very serious. This study emphasized the effects of different burials on nutrient and metal cycling and mass balance in the P. australis marsh of the Yellow River estuary.

Decomposition and heavy metal variations of the typical halophyte litters in coastal marshes of the Yellow River estuary, China

The concentrations of C, Pb, Cr, Cu, Zn, Ni and Mn were determined in decomposing litters of Phragmites australis, Suaeda salsa and Suaeda glauca in three plots of the Yellow River estuary to investigate the variations of metal stocks. Results showed that the decomposition rates significantly differed among species (p < 0.05), in the order of S. glauca (0.002010 d(-1)) > S. salsa (0.000814 d(-1)) > P. australis (0.000766 d(-1)). The concentrations of Cu and Zn in the three litters (particularly S. glauca) generally showed increasing tendency, while those of Pb, Cr, Ni and Mn exhibited different temporal variations. Compared to P. australis and S. salsa, the key mechanisms affecting the variation of metals in S. glauca might be more complex. In most periods, Pb stocks in P. australis, S. salsa and S. glauca, Zn stocks in S. salsa and S. glauca, and Cr, Ni and Mn stocks in P. australis and S. glauca were lower than the initial ones, implying that release exceeded incorporation. Comparatively, Zn stocks in P. australis, Cr, Ni and Mn stocks in S. salsa and in particular Cu stocks in the three litters were generally positive, evidencing incorporation of these metals in most sampling times. The three halophytes were particular efficient in binding Cu and releasing Pb, indicating that the potential eco-toxic risk of Pb exposure might be serious. This study emphasized the strong influences of key biotic (litter types, carbon/metal ratios and activities of microbial organisms) and abiotic variables (salinity, sediment resuspension induced by tidal inundation and passive sorption onto recalcitrant organic fractions) on metal cycling in coastal marshes of the Yellow River estuary.