Biodegradation of mixed litter-derived dissolved organic matter with varying evenness in a temperate freshwater wetland.

Litter-derived dissolved organic matter (DOM) plays an essential role in biogeochemical cycles. In wetlands, species relative abundance and its change have great influences on input features of litter-derived DOM, including chemical characteristics per se and functional diversity of chemical characteristics. Functional diversity is an important factor controlling organic matter biodegradation, but little is known in terms of the DOM. We mixed litter leachates of four macrophytes with a constant concentration (20 mg DOC L-1) but varying dominant species and volume ratios, i.e. 15:1:1:1 (low-evenness), 5:1:1:1 (mid-evenness), and 2:1:1:1 (high-evenness), generating a gradient of chemical characteristics and functional diversity (represented by functional dispersion index FDis). Based on a 42-d incubation, we measured degradation dynamics of these DOM mixtures, and analyzed potential determinants. After 42 days of incubation, the high-evenness treatments, along with mid-evenness treatments sometimes, had most degradation, while the low-evenness treatments always had least degradation. The degradation of mixtures related significantly to not only the volume-weighted mean chemical characteristics but also FDis. Furthermore, the FDis even explained more variation of degradation. The non-additive mixing effects, synergistic effects (faster degradation than predicted) in particular, on degradation of DOM mixtures were rather common, especially in the high- and mid-evenness treatments. Remarkably, the mixing effects increased linearly with the FDis values (r2adj. = 0.426). This study highlights the critical role of functional diversity in regulating degradation of mixed litter-derived DOM. Resulting changes in chemistry and composition of litter leachates due to plant community succession may exert substantial influences on biogeochemical cycling.

How does exotic Spartina alterniflora affect the contribution of iron-bound organic carbon to soil organic carbon in salt marshes?

Iron-bound organic carbon (OC-FeR) is important for the stability of soil organic carbon (SOC) in salt marshes, and the Spartina alterniflora invasion reshaped local salt marshes and changed the SOC pool. To evaluate the effects of S. alterniflora invasion on the contribution of OC-FeR to SOC, we determined the OC-FeR content and soil characteristics in the 0-50 cm soil profile along the vegetation sequence, including mudflats (MF), S. alterniflora marshes established in 2003 (SA03) and 1989 (SA89), the ecotone of S. alterniflora and Phragmites australis (SE), S. salsa marsh (SS), and P. australis marsh (PA). The SOC content was 6.55-17.5 mg g-1 in the S. alterniflora marshes. Reactive iron oxides (Fed, Feo, Fep) accumulated significantly in the S. alterniflora and P. australis salt marshes. PA and S. alterniflora marshes had higher DOC contents of 0.28-0.77 mg g-1. The OC-FeR content in the 0-50 cm soil profile in these ecosystems ranged from 0.3 to 3.29 mg g-1, with a contribution to the SOC content (fOC-FeR) of approximately 11 %, which was highest in SA03 (16.3 % ~ 18.8 %), followed by SA89, SE, and PA. In addition, the molar ratios of OC-FeR to Fed were <1, indicating that the iron oxides were associated with SOC through sorption more than coprecipitation. According to the structural equation model, SOC, DOC and iron oxides were the direct driving factors of OC-FeR formation, while the vegetation zone indirectly functioned by regulating organic C inputs, iron oxide formation, and pH. This study suggested that S. alterniflora invasion promotes iron-bound organic carbon accumulation by increasing organic C inputs and regulating iron oxide formation in salt marshes, but such promotion will degenerate with development duration.

Morphological and transcriptomic responses/acclimations of erect-type submerged macrophyte Hydrilla verticillata both at low-light exposure and light recovery phases.

Light intensity is a determinant for submerged macrophytes. Little is known about their molecular responses to low-light exposure, despite more informative and responsive than morphological traits. For erect-type submerged macrophytes, the stem is more crucial relative to the leaf in acclimation to low-light stress, but receives less attention. We determined morphological and stem transcriptomic responses/acclimations of Hydrilla verticillata to extremely and mildly low light (7.2 and 36 µmol photons m-2 s-1, respectively), that is, EL and ML, with the radiation intensity of 180 µmol photons m-2 s-1 as the control. Low-light exposure continued for 9 days, followed by a 7-day recovery phase (180 µmol photons m-2 s-1). At the exposure phase, the low-light treatments, in particular the EL, decreased the relative growth ratio, but induced greater height and longer stem internode distance and epidermal cell. Such responses/acclimations continued into the recovery phase, despite more or less changes in the magnitude. Transcriptome showed that the photosynthetic system was inhibited at the exposure phase, but the macrophyte adjusted hormone synthesis relating to cell division and elongation. Moreover, the EL activated cell stress responses such as DNA repair. Following light recovery, the macrophyte exhibited a strong-light response, although energy metabolism enhanced. Especially, the EL enriched the pathways relating to anthocyanin synthesis at such phase, indicating an activation of photoprotective mechanism. Our findings suggest that negative influences of low light occur at both low-light exposure and recovery phases, but submerged macrophytes would acclimate to light environments. Transcriptome can show molecular basis of plant responses/acclimations, including but not limited to morphology. This study establishes a bridge connecting morphological and molecular responses/acclimations.

Vertical patterns of leaf physiology and biofilm characteristics for Hydrilla verticillata in both single and mixed communities.

Little is known about how community composition affects vertical patterns of leaf characteristics for submerged macrophytes in freshwater lakes. Here, after sampling Hydrilla verticillata in both single and mixed communities in shallow and deep areas in a shallow lake, we measured vertical patterns of leaf biofilm and physiology characteristics. Upper leaves of H. verticillata always had more attached abiotic biofilm matters, and all biofilm characteristics exhibited declining trends from top to bottom segments in deep areas. Moreover, the amount of attached biofilm matter in the mixed community was less than in the single community in shallow areas, but the reverse was true in deep areas. The vertical pattern of leaf physiology characteristics was obvious in the mixed community. In the shallow area, leaf pigment concentrations showed increasing trends with an increasing water depth, but the enzymatic specific activity of peroxidase (POD-ESA) was precisely the opposite. In the deep area, leaf chlorophyll concentrations were greatest in the leaves of bottom segments and lowest in top segments, while carotenoids and POD-ESA were greatest in the leaves of the middle segment-II. Light intensity and biofilm were found to play an important role in regulating the vertical patterns of photosynthetic pigments and POD-ESA. Our study highlighted the effect of community composition on the vertical pattern of leaf physiology and biofilm characteristics. HIGHLIGHTS: Biofilm characteristics always showed increasing trends with increasing water depth. Community composition changed the amount of attached biofilm matter. The vertical pattern of leaf physiology was more obvious in mixed communities. Light intensity and biofilm regulated the vertical pattern of leaf physiology.

Conversion of drylands to paddy fields on former wetlands restores soil organic carbon by accumulating labile carbon fractions in the Sanjiang Plain, northeast China.

BACKGROUND: Since the 1990s, drylands have been extensively converted to rice paddy fields on the former wetlands in the Sanjiang Plain of northeast China. However, the influence of this successiveland-use change from native wetlands to drylands to rice paddy fields on soil organic carbon (C) dynamics remains unexplored. Here, we compared the difference in soil organic C stock among native wetlands, drylands, and paddy fields, and then used a two-step acid hydrolysis approach to examine the effect of this land-use change on labile C I (LPI-C), labile C II (LPII-C), and recalcitrant C (RP-C) fractions at depths of 0-15 cm, 15-30 cm, and 30-50 cm. RESULTS: Soil organic C stock at a depth of 0-50 cm was reduced by 79% after the conversion of wetlands to drylands but increased by 24% when drylands were converted to paddy fields. Compared with wetlands, paddy fields had 74% lower soil organic C stock at a depth of 0-50 cm. The conversion of wetlands to drylands reduced the concentrations of LPI-C, LPII-C, and RP-C fractions at each soil depth. However, land-use change from drylands to paddy fields only increased the concentrations of LPI-C and LPII-C fractions at the 0-15 cm and 30-50 cm depths. CONCLUSION: The conversion of drylands to paddy lands on former wetlands enhances the soil organic C stock by promoting labile C fraction accumulation, and labile C fractions are more sensitive to this successive land-use change than recalcitrant C fractions in the Sanjiang Plain of northeast China. © 2022 Society of Chemical Industry.

Similarities and differences among the responses to three chlorinated organophosphate esters in earthworm: Evidences from biomarkers, transcriptomics and metabolomics.

The wide use of chlorinated organophosphate esters (Cl-OPEs) as additive flame retardants has aroused concern about their potential risks on ecosystem and human health. However, knowledge about the toxicity of Cl-OPEs on soil organisms remains limited. In this study, earthworms, Eisenia fetida, were exposed to three representative Cl-OPEs, i.e., tris(2-chloroethyl) phosphate (TCEP), tris(2-chloro-1-methylethyl) phosphate (TCPP), and tris(1,3-dichloro-2-propyl) phosphate (TDCPP) in artificial soil. Using a combination of biochemical indicators (biomarkers), transcriptomics, and metabolomics, we compared the Cl-OPE-induced toxicity to E. fetida and provide new insight into the related molecular mechanism. All three Cl-OPEs elicited immune defense by the earthworms, as evidenced by increased acid phosphatase and alkaline phosphatase activities, and the genes involved in immune-related pathways (e.g., lysosomal and interleukin-17 signaling pathways). Furthermore, no effects on acetylcholinesterase activity were observed among the three Cl-OPEs. However, the TCPP and TDCPP treatments significantly decreased the neurotransmitter serotonin, suggesting the potential neurotoxicity of Cl-OPEs. Although TCEP affected the genes involved in carbohydrate and amino acid metabolism, the changes in the corresponding metabolites were not statistically significant. In contrast, exposure to TCPP and TDCPP induced oxidative stress, and affected xenobiotic metabolism and energy metabolism, leading to the decreased body weight in E. fetida. Based on these toxic effects, TCPP and TDCPP were more severely toxic than TCEP, despite their structural similarity. Given that the use of TCEP has been tightly regulated, our results suggest the potentially toxic effects of TCPP and TDCPP should not be ignored in future risk assessments of flame retardants.

Rapid adaptive responses of rosette-type macrophyte Vallisneria natans juveniles to varying water depths: The role of leaf trait plasticity.

Rosette-type submerged macrophytes are widely distributed across a range of water depths in shallow lakes and play a key role in maintaining ecosystem structures and functions. However, little is known about the rapid adaptive responses of such macrophytes to variations in water depth, especially at the juvenile stage. Here, we conducted a short-term in situ mesocosm experiment, in which the juveniles of Vallisneria natans were exposed to a water depth gradient ranging from 20 to 360 cm. Twenty-two leaf-related traits were examined after 4 weeks of growth in a shallow lake. Most (18) traits of V. natans generally showed high plasticity in relation to water depth. Specifically, juveniles allocated more biomass to leaves and had higher specific leaf area, leaf length-to-width ratio, chlorophyll content, and carotenoids content in deep waters, displaying trait syndrome associated with high resource acquisition. In contrast, V. natans juveniles in shallow waters had higher leaf dry matter content, leaf soluble carbohydrate content, carotenoids per unit chlorophyll, and peroxidase activity, pertaining to resource conservation. Notably, underwater light intensity was found to be the key factor explaining the trait plasticity along the water depth gradient, and 1.30 mol photons m-2 d-1 (at 270 cm) could be the optimal irradiance level based on the total biomass of V. natans juveniles. The present study highlights the significance of leaf trait plasticity for rosette-type macrophytes in response to variations in water depth and sheds new light on the differences between trade-offs in deep- and shallow-water areas.

Effects of long-term nitrogen addition on dissolved organic matter characteristics in a temperate wetland of Northeast China.

Dissolved organic matter (DOM) plays an indispensable role in ecosystem services and functions in wetlands. While most wetlands have undergone increased nitrogen (N) loading due to intensive human activities, the response of DOM characteristics to long-term N addition remains unexplored. In this study, we assessed the changes in dissolved organic carbon (DOC), NH4+, NO3-, dissolved organic N (DON), dissolved total N (DTN), and dissolved total phosphorus (DTP) in surface water and soil pore water at 15 cm depth after 10 years of N addition at four levels (0, 60, 120, and 240 kg N hm-2 year-1) in a freshwater marsh of Northeast China. We also examined the effect of N addition on DOM aromaticity and humification by measuring the specific UV absorbance at 254 nm (SUVA254), the color per C unit (C/C ratio), and the fulvic acid/humic acid ratio (E4/E6 ratio). Our results showed that N addition significantly altered DOM properties, but the direction and magnitude of these changes generally did not vary with the N addition level. During the growing season, DOC, NH4+, NO3-, DON, and DTN concentrations in both surface water and soil pore water were increased by N addition. Accordingly, N addition increased the DOC/DTP and DTN/DTP ratios but decreased the DOC/DTN ratio in surface water and soil pore water. In addition, the SUVA254 value and C/C ratio increased, while the E4/E6 ratio reduced after N addition in surface water and soil pore water, indicating increases in DOM aromaticity and humification. These observations suggest that long-term N addition changes DOM characteristics by causing stoichiometric imbalances and increasing recalcitrant compounds in temperate freshwater wetlands, which may then deteriorate water quality, alter microbial-mediated ecological processes, and impact downstream aquatic ecosystem structures.

Warming enhances the cadmium toxicity on macrophyte Myriophyllum aquaticum (Vell.) Verd. seedlings.

Due to a close contact with water column, submerged macrophytes are easily disturbed by environment change in freshwater ecosystems, especially at the seedling stage. In recent decades, freshwater ecosystems have been subject to severe cadmium (Cd) pollution, which can cause toxic effects on the growth of submerged macrophytes. Moreover, the temperature rise resulting from climate warming and water level decline may further aggravate such effect, especially in shallow lakes. Here, we investigated the independent and interaction effects of Cd exposure levels (0, 0.5, 1, and 2.5 mg L-1) and temperature (15, 25, and 30 °C) on morphological and physiological traits of Myriophyllum aquaticum (Vell.) Verd. Seedlings generated from propagules and seeds. The temperature rise and Cd exposure generally resulted in a significant increase of Cd concentrations and antioxidant enzyme activities in leaves, as well as a decrease of chlorophyll a and b concentrations. The number and length of leaves generated from propagules always show a downward trend with the increase of Cd exposure, regardless of the temperature. Moreover, the lowest leaf number and length always occurred at high temperature (i.e. 30 °C) when the Cd exposure level increased to 1 and 2.5 mg L-1. For the seedlings generated from seeds, the temperature rise caused an increase of leaf emergence rate under low Cd exposure levels, but resulted in a significant decrease with the Cd exposure level. This study indicates the negative effects of Cd exposure and temperature rise on submerged macrophytes at the seedling stage, and highlights that temperature rise would enhance Cd toxicity.

Potamogeton crispus responses to varying water depth in morphological plasticity and physiological traits.

Submerged macrophytes, important primary producers in shallow lakes, play a crucial role in maintaining ecosystem structure and function. By altering a series of environmental factors, especially light intensity, water depth has great influences on growth of submerged macrophytes. Here, by hanging pots statically at water depths of 40, 60, 80, 100, 120, 140, 160, 180, 200, and 220 cm, respectively, we investigated effects of water depths on morphological plasticity and physiological traits of Potamogeton crispus. At 40 and 60 cm water depths versus other water depths, P. crispus showed lower plant height, larger stem diameter, thicker leaves, and smaller leaf area, leaf length, and specific leaf area. With water depth increasing, the plant height, leaf area, and leaf length gradually increased until 160 cm water depth, while the stem diameter and leaf thickness gradually decreased until 200 cm water depth. In comparison, the plant height, leaf length, and leaf number significantly decreased when the water depth further increased to 180-220 cm. The leaves contained lower concentrations of superoxide dismutase and peroxidase at 100-160 cm water depth, and lower catalase concentrations at 40-140 cm water depth, especially at 80-100 cm. In shallow waters, the concentration of chlorophyll a and b in leaves were both lower, while the ratio of chlorophyll a to b was relatively higher. As the water depth of 40-220 cm, the chlorophyll a and b concentrations increased significantly with increasing water depth, while their ratio gradually decreased. The present study provides new insights into the adaptation strategies of submerged macrophytes to the variation in water levels, and our findings are beneficial for ecosystem construction and management.

Microplastics with cadmium inhibit the growth of Vallisneria natans (Lour.) Hara rather than reduce cadmium toxicity.

Microplastics and heavy metals are discharged into a freshwater environment either directly or via surface runoff and are largely deposited in sediments, posing risks to aquatic organisms. Few studies have thus far been devoted to the interaction of microplastics and heavy metals in sediments. Whether microplastics can affect the toxicity and accumulation of heavy metals in submerged macrophytes remains unclear. We evaluated the effects of polyvinyl chloride microplastics (PVC-MPs) and cadmium (Cd) exposure levels (0, 5, 15, and 25 mg) on Vallisneria natans (Lour.) Hara grown in sediment in a microcosm experiment for 14 d. In this study, PVC-MPs decreased the fresh weights of V. natans in the absence of Cd and markedly reduced the fresh weights at 5 and 15 mg Cd exposure levels. Moreover, PVC-MPs substantially increased the malondialdehyde (MDA) content of V. natans leaves at a Cd exposure of 25 mg. However, the PVC-MPs neither reduced the Cd concentration nor independently increased the antioxidant enzyme activities of the plants. These findings indicate that microplastics can independently, or jointly with a Cd contaminant, inhibit the growth of submerged macrophytes rather than reduce Cd toxicity. To our knowledge, this study is the first to evaluate the effects of microplastics and Cd exposure in sediments on the growth and physiological traits of submerged macrophytes, which could provide important implications for the interaction and future risk assessment of microplastics and heavy metals in sediments of freshwater ecosystems.

Chronic nitrogen addition promotes dissolved organic carbon accumulation in a temperate freshwater wetland.

Temperate wetlands have been undergoing increased nitrogen (N) inputs in the past decades, yet its influence on dissolved organic carbon (DOC) dynamics is still elusive in these ecosystems. Here, using a field multi-level N addition (0, 6, 12, and 24 g N m-2 year-1) experiment, we investigated the changes in aboveground plant biomass, DOC production from plant litters, DOC biodegradation, and DOC concentration in surface water and soil pore water (0-15 cm depth) following 10 years of N addition in a freshwater marsh of Northeast China. We observed that, irrespective of N addition levels, N addition caused an increase in DOC production from plant litters under both non-flooded and flooded conditions. Conversely, DOC biodegradation was inhibited by N addition in both surface water and soil pore water. Because of enhanced DOC production from plant litters and declined DOC biodegradation, N addition elevated DOC concentration in surface water and soil pore water across the growing season. In addition, long-term N addition increased aboveground plant biomass, but decreased species richness. Our results suggest that long-term N enrichment promotes DOC accumulation through the contrasting effects on litter-derived DOC production and microbial decomposition of DOC in temperate wetlands.

[Spatial Distribution and Pollution Evaluation of Carbon, Nitrogen, and Phosphorus in Sediments of Zhushan Bay at Taihu Lake].

To reveal the distribution characteristics of carbon, nitrogen and phosphorus in the sediments of Zhushan Bay at Taihu Lake, sedimentary columns were collected and sliced by 2 cm vertically from ten sampling points in three sections of Zhushan Bay. The content of total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) were determined for each slice to reveal their horizontal and vertical distribution. The results showed that:spatially, the content of TN, TP, and TOC increased in the surface sediments of Zhushan Bay from the open lake area to the bay, and inside the bay these indexes were significantly higher than in the open lake area (P<0.01). The content of TN, TP, and TOC in the surface sediments in the bay (section A) were 1.53 mg·g-1, 1.55 mg·g-1, and 11.31 mg·g-1, respectively, while in the surface sediments near the open lake (section C) they were only 0.75 mg·g-1, 0.57 mg·g-1, and 6.70 mg·g-1, respectively. Vertically, a feature of surficial enrichment was shown, and the contents of carbon, nitrogen and phosphorus in all three sections showed a decreasing trend with increase of depth. The contents of TN, TP and TOC in the surface sediments are 2-3 times, 2-5 times, and 2-3 times those in the bottom sediments, respectively. Generally, the average TP content in the sediment of Zhushan Bay is 0.93 mg·g-1, suggesting an apparent heavy pollution, while the average TN content is 1.11 mg·g-1 as slight pollution. According to the organic nitrogen index and comprehensive pollution index, the northern part of Zhushan Bay is suffering from heavy pollution, where the organic pollution is relatively strong. The TP pollution index is between 1.03 and 3.87, indicating heavy pollution in Zhushan Bay.

Responses of foliar phosphorus fractions to soil age are diverse along a 2 Myr dune chronosequence.

Plants respond to soil phosphorus (P) availability by adjusting leaf P among inorganic P (Pi) and organic P fractions (nucleic acids, phospholipids, small metabolites and a residual fraction). We tested whether phylogenetically divergent plants in a biodiversity hotspot similarly adjust leaf P allocation in response to P limitation by sampling along a 2 Myr chronosequence in southwestern Australia where nitrogen (N) limitation transitions to P limitation with increasing soil age. Total P and N, and P allocated to five chemical fractions were determined for photosynthetic organs from Melaleuca systena (Myrtaceae), Acacia rostellifera (Fabaceae) and Hakea prostrata (Proteaceae). Soil characteristics were also determined. Acacia rostellifera maintained phyllode total P and N concentrations at c. 0.5 and 16 mg g-1 DW, respectively, with a constant P-allocation pattern along the chronosequence. H. prostrata leaves allocated less P to Pi, phospholipids and nucleic acids with increasing soil age, while leaf N concentration was constant. M. systena had the greatest variation in allocating leaf P, whereas leaf N concentration decreased 20% along the chronosequence. Variation in P-allocation patterns was only partially conserved among species along the chronosequence. Such variation could have an impact on species distribution and contribute to species richness in P-limited environments.

Plant functional group controls litter decomposition rate and its temperature sensitivity: An incubation experiment on litters from a boreal peatland in northeast China.

In boreal peatlands, litter decomposition plays an important role in modulating ecosystem carbon (C) cycling and nutrient turnover. However, how climate warming and plant functional group interact to affect litter decomposition is still unclear in these ecosystems. Here, we collected fresh litters of six plant functional groups (nitrogen (N)-fixing species, deciduous tree, deciduous shrub, evergreen shrub, graminoid, and Sphagnum moss) from a boreal peatland located in northeast China. A laboratory incubation experiment was used to determine the effect of temperature (10 °C vs. 20 °C) on microbial respiration and mass loss during decomposition. Among the six functional groups, the litters of N-fixing species and deciduous shrub, followed by deciduous tree, generally had the greatest mass losses and microbial respiration rates, whereas the Sphagnum moss decomposed with the slowest rate at both incubation temperatures. Increasing incubation temperature from 10 °C to 20 °C, microbial respiration rate and mass loss increased slightly for Sphagnum moss litters (25% and 19%, respectively), but increased dramatically for vascular plant litters (84-135% and 49-85%, respectively). For litters from vascular plants, both decomposition rate and temperature sensitivity showed a tight linear correlation with the initial C:N and C:phosphorus ratios. Considering that climate warming will cause increased dominance of woody plant species coupled with decreased cover by Sphagnum mosses, this study provides clear evidence that climate warming and the associated changes to vegetation community composition can synergistically accelerate plant litter decomposition in boreal peatlands.

Long-term phosphorus addition enhances the biodegradability of dissolved organic carbon in a nitrogen-limited temperate freshwater wetland.

Phosphorus (P) enrichment is expected to strongly influence dissolved organic carbon (DOC) biodegradation. However, the relationship between P availability and DOC biodegradation is largely unknown in nitrogen (N)-limited ecosystems. Here, we investigated the changes in the ratio of DOC to dissolved total nitrogen (DTN), specific UV absorbance at 254nm (SUVA254), and DOC biodegradation in surface water and soil pore water (0-15cm depth) following eight years of multi-level P addition (0, 1.2, 4.8, and 9.6gPm-2year-1) in an N-limited freshwater marsh in Northeast China. We found that P addition caused an increase in DOC biodegradation in surface water and soil pore water, irrespective of the P addition levels. Compared with the control treatment, the P addition rates of 1.2, 4.8, and 9.6gPm-2year-1 increased DOC biodegradation by 20.7%, 15.2%, and 14.5% in surface waters, and 11.3%, 9.4%, and 12.0% in soil pore waters, respectively. The DOC biodegradation was separately negatively correlated with the DOC:DTN ratio and SUVA254, indicating that the positive effect of P addition on DOC biodegradation was caused by the elevated N concentration and the reduced DOC aromaticity. Our findings suggest that P enrichment enhances the biodegradability of DOC through increased N availability and altered DOC chemical composition, which would accelerate DOC loss from the waters and alter ecosystem C balance in N-limited temperate wetlands.

Effect of long-term phosphorus addition on the quantity and quality of dissolved organic carbon in a freshwater wetland of Northeast China.

Understanding how P enrichment alters the quantity and quality of dissolved organic carbon (DOC) is important, because of their role in regulating the C cycle. Here, we established a four-level P addition experiment (0, 1.2, 4.8, and 9.6gPm-2year-1) in a N-limited freshwater wetland in the Sanjiang Plain, Northeast China. The aim of this study was to examine the effects of eight years of P addition on DOC concentration, SUVA254 (Abs254/DOC concentration, indicating the aromaticity of DOC), C:C ratio (Abs400/DOC concentration, indicating the proportion of colored humic substances in DOC), and E4:E6 ratio (Abs465/Abs665, indicating the molecular size of humic substances) in surface water and soil pore water (0-15cm depth) during the growing season (June through September). Our results showed similar changing trends in concentration and optical properties of DOC following eight years of P addition in the both surface water and soil pore water across the sampling dates. Generally, P addition decreased DOC concentration, SUVA254, and C:C ratio, and increased E4:E6 ratio, irrespective of P addition levels. These altered optical properties of DOC indicated that P addition decreased the molecular weight and aromaticity of DOC, and thus increased the quality of DOC. These results suggest P enrichment substantially reduces the quantity of DOC in N-limited temperate freshwater wetlands, and imply that increased DOC quality following P addition can further provide a positive feedback to decreased DOC pool.

Effects of P addition on plant C:N:P stoichiometry in an N-limited temperate wetland of Northeast China.

Phosphorus (P) enrichment induced by anthropogenic activities results in modified plant nutrient status, which potentially alters the stoichiometry of carbon (C), nitrogen (N), and P in plants. However, how increased P availability changes plant C:N:P stoichiometry at different hierarchical scales is unclear in N-limited ecosystems. In this study, we conducted a four-level P addition experiment (0, 1.2, 4.8, and 9.6gPm(-)(2)year(-1)) to elucidate the effect of P enrichment on plant C:N:P stoichiometric ratios at both the species and community levels in a freshwater wetland in the Sanjiang Plain, Northeast China. We found that species- and community-level plant C:N:P stoichiometry responded consistently to six years of P addition, although there was a shift in species dominance. Phosphorus addition increased plant N and P concentrations and thus decreased C:N, C:P, and N:P ratios irrespective of the P addition levels. These similar change trends at different scales resulted from the identical responses of plant N and P concentrations in different species to P addition. Moreover, plant N concentration exhibited an increasing trend with increasing P addition levels, whereas plant C:N ratio showed a declining trend. At the community level, P addition at the rates of 1.2, 4.8, and 9.6gPm(-2)year(-1) decreased the C:N ratio by 24%, 27%, and 34%; decreased the C:P ratio by 33%, 35%, and 38%; and decreased the N:P ratio by 12%, 10%, and 6%, respectively. Our results indicate that the stoichiometric responses to P addition are scale-independent, and suggest that altered plant C:N:P stoichiometry induced by P enrichment would stimulate organic matter decomposition and accelerate nutrient cycles in N-limited temperate freshwater wetlands.

Responses of plant nutrient resorption to phosphorus addition in freshwater marsh of Northeast China.

Anthropogenic activities have increased phosphorus (P) inputs to most aquatic and terrestrial ecosystems. However, the relationship between plant nutrient resorption and P availability is still unclear, and much less is known about the underlying mechanisms. Here, we used a multi-level P addition experiment (0, 1.2, 4.8, and 9.6 g P m(-2) year(-1)) to assess the effect of P enrichment on nutrient resorption at plant organ, species, and community levels in a freshwater marsh of Northeast China. The response of nutrient resorption to P addition generally did not vary with addition rates. Moreover, nutrient resorption exhibited similar responses to P addition across the three hierarchical levels. Specifically, P addition decreased nitrogen (N) resorption proficiency, P resorption efficiency and proficiency, but did not impact N resorption efficiency. In addition, P resorption efficiency and proficiency were linearly related to the ratio of inorganic P to organic P and organic P fraction in mature plant organs, respectively. Our findings suggest that the allocation pattern of plant P between inorganic and organic P fractions is an underlying mechanism controlling P resorption processes, and that P enrichment could strongly influence plant-mediated biogeochemical cycles through altered nutrient resorption in the freshwater wetlands of Northeast China.