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

Biocrusts enhance soil organic carbon stability and regulate the fate of new-input carbon in semiarid desert ecosystems

Given their global prevalence, dryland (including hyperarid, arid, semiarid, and dry subhumid regions) ecosystems are critical for supporting soil organic carbon (SOC) stocks, with even small changes in such SOC pools affecting the global carbon (C) cycling. Biocrusts play an essential role in supporting C cycling in semiarid ecosystems. However, the influence of biocrusts and their successional stages on SOC and its fraction contents, as well as their role in regulating new input C into SOC fractions remain largely unknown. In this study, we collected continuous samples of bare soil (BS) and three successional stages of biocrust soils (cyanobacterial (CC), low-cover moss (LM), and high-cover moss (HM)) at 0-5 cm depth every month for one year in a semiarid desert ecosystem. We analyzed SOC changes among the samples and their fraction contents including: labile organic C (LOC) (composed of microbial biomass C (MBC), dissolved organic C (DOC), and easily oxidized organic C (EOC)) and recalcitrant organic C (ROC) fractions, soil nutrient content including: ammonium (NH4+-N), nitrate (NO3--N), and available phosphorus (AP), and soil temperature and moisture. We also conducted a 13C pulse-labelling experiment in the field to accurately quantify the effects of biocrust successional stage on exogenous C allocation to SOC fractions. Our results showed that the three successional stages of biocrust (CC-LM-HM) increased SOC and ROC contents by an average of 5.3 ± 3.6 g kg-1 and 4.0 ± 3.0 g kg-1, respectively; and the MBC, DOC, and EOC contents increased by an average of 41.7 ± 24.8 mg kg-1, 28.7 ± 12.6 mg kg-1, and 1.2 ± 0.6 g kg-1, respectively, compared to that of BS. These increases were attributed to an increase in photosynthetic pigment content, higher nutrient levels, and more suitable microclimates (e.g., higher moisture and more moderate temperature) during biocrust succession. More importantly, SOC stability was greatly improved with biocrust succession from cyanobacteria to moss, as evidenced by the reduction in soil EOC:SOC and EOC:ROC ratios by an average of 50 ± 34 % and 99 ± 67 %, respectively, while the ROC:SOC ratio increased by 33 ± 16 % with biocrust succession compared to those of BS. The biocrust SOC, DOC, and MBC 13C contents at different stages were on average 0.096 ± 0.034 mg kg-1, 0.010 ± 0.005 mg kg-1, and 0.014 ± 0.005 mg kg-1 higher than those of BS. Similarly, the allocation of new-input C among the DOC and MBC at different biocrust stages (19 ± 10 %) was significantly higher than that of BS (9 ± 6 %). New-input C into the biocrusts was fixed by microbes (43 ± 18 %) within ∼10 days and converted into other forms of C (85 ± 5 %) after 80 days. Our study provides a new perspective on how biocrusts support C cycling in semiarid desert ecosystems by mediating new C inputs into diverse fractional contents, and highlights the significance of biocrust successional stages in maintaining soil C stocks and stability in the dryland soil system.

Characteristics of soil organic carbon fractions in four vegetation communities of an inland salt marsh

BACKGROUND

The study of soil organic carbon characteristics and its relationship with soil environment and vegetation types is of great significance to the evaluation of soil carbon sink provided by inland salt marshes. This paper reports the characteristics of soil organic carbon fractions in 0-50 cm soil layers at four vegetation communities of the Qinwangchuan salt marsh.

RESULTS

(1) The soil organic carbon content of Phragmites australis community (9.60 ± 0.32 g/kg) was found to be higher than that of Salicornia europae (7.75 ± 0.18 g/kg) and Tamarix ramosissima (4.96 ± 0.18 g/kg) and Suaeda corniculata community (4.55 ± 0.11 g/kg). (2) The soil dissolved organic carbon, particulate organic carbon and soil microbial biomass carbon in 0-50 cm soil layer of Phragmites australis community were higher, which were 0.46 ± 0.01 g/kg, 2.81 ± 0.06 g/kg and 0.31 ± 0.01 g/kg, respectively. (3) Soil organic carbon was positively correlated with dissolved organic carbon, particulate organic carbon, and microbial biomass carbon, and negatively correlated with easily oxidized organic carbon. (4) Above-ground biomass has a strong direct positive effect on soil organic carbon, total nitrogen and pH have a strong direct positive effect on microbial biomass carbon content, pH and average density have a strong direct negative effect on easily oxidized organic carbon, and particulate organic carbon.

CONCLUSIONS

The interaction between plant community characteristics and soil factors is an important driving factor for soil organic carbon accumulation in inland salt marshes.

Effects of rainfall amount and frequencies on soil net nitrogen mineralization in Gahai wet meadow in the Qinghai-Tibetan Plateau

Global climate change has led to a significant increase in the frequency of extreme rainfall events in the Qinghai-Tibetan Plateau (QTP), thus potentially increasing the annual rainfall amounts and, consequently, affecting the net soil nitrogen (N) mineralization process. However, few studies on the responses of the soil net N mineralization rates to the increases in rainfall amounts and frequencies in alpine wet meadows have been carried out. Therefore, the present study aims to assess the effects of rainfall frequency and amount changes on the N fixation capacity of wet meadow soils by varying the rainfall frequency and amount in the Gahai wet meadow in the northeastern margin of the QTP during the plant-growing season in 2019. The treatment scenarios consisted of ambient rain (CK) and supplementary irrigation at a rate of 25 mm, with different irrigation frequencies, namely weekly (DF1), biweekly (DF2), every three weeks (DF3), and every four weeks (DF4). According to the obtained results, the increased rainfall frequency and amount decreased the soil mineral N stock and increased the aboveground vegetation biomass (AB) amounts and soil water contents in the wet meadows of the QTP. Ammonium (NH4+-N) and nitrate N (NO3--N) contributed similarly to the mineral N contents. However, the ammonification process played a major role in the soil mineralization process. The effects of increasing rainfall amount and frequency on N mineralization showed seasonal variations. The N mineralization rate showed a single-peaked curve with increasing soil temperature during the rapid vegetation growth phase, reaching the highest value in August. In addition, the N mineralization rates showed significant positive correlations with soil temperatures and NH4+-N contents and a significant negative correlation with AB (P < 0.05). The results of this study demonstrated the key role of low extreme rainfall event frequencies in increasing the net soil N mineralization rates in the vegetation growing season, which is detrimental to soil N accumulation, thereby affecting the effectiveness of soil N contents.

Microbial communities in tree root-compartment niches under Cd and Zn pollution: Structure, assembly process and co-occurrence relationship

Woody plants have showed great potential in remediating severely contaminated soils by heavy metals (HMs) due to their cost-efficient and ecologically friendly trait. It is believed the root-associated microbiota plays a vital role in phytoremediation for HMs. However, the ecological process controlling the assembly and composition of tree root-associated microbial communities under HMs stress remains poorly understood. Herein, we profiled the bulk soil, rhizosphere and endosphere microbial communities of trees growing in heavily Cd and Zn polluted soil. The microbiota was gradually filtered from bulk soil to the tree roots and was selectively enriched in roots with specific taxa, such as Proteobacteria and Ascomycota. The microbial community assembly along the soil-root continuum was mainly controlled by deterministic processes from bulk soil to the endosphere, with the normalized stochasticity ratio (NST) indices of 67.16-31.05 % and 30.37-15.02 % for bacteria and fungi, respectively. Plant selection pressure sequentially increased from bulk soil to rhizosphere to endosphere, with the reduced bacterial alpha diversity accompanying the consequently reduced complexity of the co-occurrence network. Together, the findings provide new evidence for horizontal transmission of endophytic microbiome from soil to the host, which can shed light on the future screening and application of microbial-assisted phytoremediation.

Nutrient loading decreases blue carbon by mediating fungi activities within seagrass meadows

Coastal pollution, including nutrient loading, can negatively impact seagrass health and cover and may consequently alter soil organic carbon (SOC) accumulation and preservation. Key to understanding how eutrophication impacts SOC cycling in seagrass ecosystems is how nutrient loading changes the sources of carbon being deposited and how these changes in resources, both nutrients and carbon availability, influence soil microbiota community and activity. Currently, the direction and magnitude of nutrient loading impacts on seagrass SOC dynamics are poorly understood at a meadow scale, limiting our ability to reveal the driving mechanisms of SOC remineralisation. The purpose of this study was to assess the response of surface SOC and soil microbiomes to nutrient loading within tropical seagrass meadows. To achieve this, we quantified both total SOC and recalcitrant soil organic carbon (RSOC) concentrations and sources, in addition to the composition of bacterial and fungal communities and soil extracellular enzyme activities. We found that nutrient loading elevated SOC and RSOC content, mainly facilitated by enhanced algal growth. There was no nutrient effect on the soil prokaryotic communities, however, saprotrophic fungi groups (i.e. Trapeliales, Sordaridales, Saccharomycetales and Polyporales) and fungal activities were elevated under high nutrient conditions, including extracellular enzyme activities linked to seagrass-based cellulose and lignin decomposition. This relative increase in RSOC transformation may decrease the relative contribution of seagrass carbon to RSOC pools. Additionally, significantly different fungal communities were observed between adjacent T. hemprichii and E. acoroides areas, which coincided with elevated RSOC-decomposing enzyme activity in T. hemprichii meadows, even though the mixed seagrass meadow received allochthonous SOC and RSOC from the same sources. These results suggest that nutrient loading stimulated fungal activity and community shifts specific to the local seagrass species, thereby causing fine-scale (within-meadow) variability in SOC cycling in response to nutrient loading. This study provides evidence that fungal composition and activity, mediated by human activities (e.g. nutrient loading), can be an important influence on seagrass blue carbon accumulation and remineralisation.

Profile Soil Carbon and Nitrogen Dynamics in Typical Chernozem under Long-Term Tillage Use

For the first time in research literature, this report presents the seasonal changes of total organic carbon (TOC), total nitrogen (TN), and TOC:TN ratio in Chernozem solum (0–100 cm) as effected by 14 years of application of conventional tillage (CTu), deep reduced tillage (DRTu), and reduced tillage (RTu) under barley growing. During the season, TOC content drastically declined in the spring, increased in the summer, decreased in the middle of August, and recovered in October. TN content was gradually decreased during a crop growing season and renewed in the autumn. A trend of TOC:TN changes (vertical peak curve) in 0–30 cm soil layer varied from TOC (S-shaped curve) and TN (unsymmetrical decayed curve). The amplitude of seasonal TOC and TN changes in deeper layers was far fewer related to the upper horizons. The highest amplitude in 0–30, 30–60 and 60–100 cm layers was under: DRTu, CTu, DRTu—for TOC and DRTu, CTu, RTu—for TN correspondently. Tillage practices differently stratified the content of organic carbon and nitrogen in Chernozem profile. Minimum tillage benefited TOC sequestration in 0–5 and 5–10 cm layers: 24.83 ± 0.64- and 24.65 ± 0.57 g kg−1—under RTu, 24.49 ± 0.62- and 24.71 ± 0.47 g kg−1—under DRTu, while CT—deeper than 20 cm: 22.49–15.03 g kg−1. The vertical distribution of TN content repeated TOC trend. TOC:TN ratio upraised from 12.60 in 0–5 to 14.33 in 80–100 cm layer and was the highest in summertime. A total (0–100 cm) profile was much greater under RTu and DRTu—for TN, and CTu, DRTu—for TOC. The correlation coefficient (r) was almost negligible between TOC and: T (air temperature), P (precipitation) and W (soil moisture). The strong and very strong r was found for TN—W, and P—W pairs. The negative r was between: TOC–P, TN–P, TOC:TN-W, TOC:TN–T and P–W pairs.

Improvements in Soil Physical, Chemical and Biological Properties at Natural Saline and Non-Saline Sites Under Different Management Practices

Soil salinity is known to be a significant threat to food security for the increasing population, which is further aggravated under the climate change scenario. Indo-Gangetic plain (IGP) is one of the most productive in the world and is most affected by salinity. To understand the modifications in soil characteristics under different management practices followed to reclaim salinity affected land, the present study was conducted at variously reclaimed saline areas of three districts of Uttar Pradesh situated in IGP. Soil from six sites (electrical conductivity (EC) ranging from 0.89 to 10.28 mS) following different management practices, RJT (Rajatalab, rice-wheat +organic), BBN (Beerbhanpur, rice-wheat +inorganic), MZM (Mirzamurad, rice-mustard +organic), BRP (Baraipur, rice-wheat +organic), DHR (Dharahara, rice-fallow +organic) and SLM (Salempur, rice-wheat +inorganic) were assessed for physical, chemical and biological properties during the vegetative stage and after harvest of crops. Soil quality index (SQI) based on representative parameters obtained by principal component analysis and yield of crops were also calculated at saline and non-saline sites. The SLM site showed highest salinity followed by BRP, DHR, MZM, while BBN and RJT were non-saline. Total organic carbon, total nitrogen, microbial activity, and microbial biomass were low at saline compared to non-saline sites but were higher under organic matter amendment compared to inorganic. Activities of soil enzymes were negatively influenced while β-glucosidase and alkaline phosphatase activities were enhanced under higher salinity. Organic amendments were more efficient in improving the soil properties along with SQI at saline soil resulting into a better yield in all crop combinations compared to inorganic amendments.

An emerging chemical fumigant: two-sided effects of dazomet on soil microbial environment and plant response

Methyl bromide has been banned worldwide because it causes damage to the ozone layer and the environment. To find a substitute for methyl bromide, the relationships among fumigation, plant growth, and the microbial community in replant soil require further study. We performed pot and field experiments to investigate the effects of dazomet fumigation on soil properties and plant performance. Changes in soil microbial community structure and diversity were assessed using high-throughput sequencing, and plant physiological performance and soil physicochemical properties were also measured. Dazomet fumigation enhanced photosynthesis and promoted plant growth in replant soil; it altered soil physical and chemical properties and reduced soil enzyme activities, although these parameters gradually recovered over time. After dazomet fumigation, the dominant soil phyla changed, microbial diversity decreased significantly, the relative abundance of biocontrol bacteria such as Mortierella increased, and the relative abundance of pathogenic bacteria such as Fusarium decreased. Over the course of the experiment, the soil microbial flora changed dynamically, and soil enzyme activities and other physical and chemical properties also recovered to a certain extent. This result suggested that the effect of dazomet on soil microorganisms was temporary. However, fumigation also led to an increase in some resistant pathogens, such as Trichosporon, that affect soil function and health. Therefore, it is necessary to consider potential negative impacts of dazomet on the soil environment and to perform active environmental risk management in China.

Effect of Emerging Soil Chemical Amendments on the Replant Soil Environment and Growth of Malus hupehensis Rehd. Seedlings

The effects of different soil chemical amendments (T1, 1‰ quicklime + 1‰ superphosphate; T2, 1‰ quicklime; T3, 1‰ superphosphate) on the soil environment and growth of Malus hupehensis Rehd. seedlings in aged apple orchard soil were studied to provide new insight into the prevention and control of apple replant disease. The amendments differed in their ability to ameliorate the soil environment; nevertheless, they all promoted the growth of M. hupehensis Rehd. seedlings, and the greatest enhancement of growth was observed in T1. On August 15, 2018, soil urease, sucrase, phosphatase, and catalase activities were 1.67 times, 1.32 times, 1.62 times, and 1.35 times higher in T1 compared with CK, respectively. The soil pH increased, which alleviated soil acidification. T1 also promoted the renewal of the community structure and the diversity of soil microorganisms. The copy numbers of Fusarium solani and Fusarium oxysporum were 71.96 and 70.30% lower in T1 compared with CK, respectively. The seedling height and root length of M. hupehensis Rehd. seedlings increased by 40.97 and 289.69% in T1 compared with CK, respectively. Therefore, soil replanting obstacles can be overcome with the application of quicklime and superphosphate; these soil chemical amendments also improve the soil microbial ecological environment and promote the growth of M. hupehensis Rehd. seedlings.

Terricolous mosses impact soil microbial biomass carbon and enzymatic activity under temperate forest types of the Garhwal Himalayas

Estimates of enzymatic activity are used as indices for soil quality, microbial nutrient demand, microbial growth, and activity. Mosses trap soil moisture, influence soil temperature, and create a microenvironment promoting an overall higher level of microbial activity, thus making the decomposition of organic matter more favorable. This study determines the role of mosses in influencing soil biochemical properties in three temperate forest types of the Garhwal Himalayas, Uttarakhand, viz., moist temperate deciduous forest, Ban oak forest, and moist deodar forest. Activities of major soil enzymes (dehydrogenase, β-glucosidase, acid phosphatase, aryl sulfatase, phenol oxidase, and urease) and soil microbial biomass carbon (SMBC) were determined under two different substrates, i.e., with and without moss cover in two different seasons, viz., monsoon and winter. The Pearson correlation of enzymes with specific soil nutrients they act upon has also been shown. The SMBC and on average activities of all the enzymes were predominantly higher in soil with moss cover during monsoon season and without moss cover in the winter season. SMBC in the three study sites ranged from 280.55 to 1707.64 µg C/g. Statistically significant differences (p < 0.01) were observed for all the properties within the substrates among all the three sites and across the two seasons. Our results suggest that mosses play a significant role in positively influencing soil biochemical properties in both seasons by creating a microscale mosaic that offers a high degree of heterogeneity in soil function. Our study emphasizes that mosses strongly affect soil enzyme activities and microbial biomass, thus improving soil health.

Vertical and seasonal changes in soil carbon pools to vegetation degradation in a wet meadow on the Qinghai-Tibet Plateau

Wet meadows provide opportunities to decrease carbon dioxide (CO₂) and methane (CH₄) released into the atmosphere by increasing the soil organic carbon (SOC) stored in wetland systems. Although wet meadows serve as the most important and stable C sinks, there has been very few investigations on the seasonal distributions of SOC fractions in high-altitude wet meadows. Here, we studied the effects of four vegetation degradation levels, non-degraded (ND), lightly degraded (LD), moderately degraded (MD), and heavily degraded (HD), on the measured vertical and seasonal changes of SOC and its different fractions. Among these vegetation degradation levels, 0-10 and 10-20 cm soil depths in ND plots had significantly higher SOC contents than the other degradation levels had throughout the year. This is attributed to the relatively greater inputs of aboveground plant litter and richer fine-root biomass in ND plots. Particulate organic carbon (POC) and light fraction organic carbon (LFOC) showed similar vertical and seasonal variations in autumn, reaching a minimum. Moreover, microbial biomass (MBC) and easily oxidizable organic carbon (EOC) contents were highest in summer and the smallest in winter, while dissolved organic carbon (DOC) content was highest in spring and lowest in summer, and were mainly concentrated in the 0-20 cm layer. Pearson correlation analysis indicated that soil properties and aboveground biomass were significantly related to different SOC fractions. The results indicate that vegetation degradation reduces the accumulation of total SOC and its different fractions, which may reduce carbon sink capacity and soil quality of alpine wet meadows, and increase atmospheric environmental pressure. In addition, vegetation biomass and soil characteristics play a key role in the formation and transformation of soil carbon. These results strengthen our understanding of soil C dynamics, specifically related to the different C fractions as affected by vegetation degradation levels and soil depth, in wet meadow systems.

Macroalgae bloom decay decreases the sediment organic carbon sequestration potential in tropical seagrass meadows of the South China Sea

Seagrass meadows are experiencing worldwide declines mainly because of nutrient enrichment, which always result in macroalgae bloom and consequently periodic collapse and decomposition. However, effects of macroalgae decay on the sediment organic carbon (SOC) sequestration capacity remain unknown. Depending on the macroalgae biomass in eutrophic seagrass meadows of South China Sea, we carried out a laboratory chamber experiment to investigate the sediment labile organic carbon (OC) compositions and the influencing SOC transformation enzyme activity variations of seagrass meadows in response to common macroalgae bloom species (Cladophora spp.) decomposition. Although the dehydrogenase and β-glucosidase activities were not affected by macroalgae decomposition, the macroalgae decomposition significantly elevated the salt-extractable carbon (SEC) content, SEC/SOC, levels of invertase and polyphenol oxidase activities, and the CO₂ release. Overall, this study indicates that macroalgae decomposition stimulates the SOC transformation, and therefore, it is not benefit for SOC sequestration within seagrass meadows of the South China Sea.

Exotic Spartina alterniflora invasion increases CH4 while reduces CO2 emissions from mangrove wetland soils in southeastern China

Mangroves are critical in global carbon budget while vulnerable to exotic plant invasion. Spartina alterniflora, one of typical salt marsh plant grows forcefully along the coast of China, has invaded the native mangrove habitats in Zhangjiang Estuary. However, the effects of S. alterniflora invasion on soil carbon gases (CH₄ and CO₂) emission from mangroves are not fully understood. Accordingly, we conducted a field experiment to investigate the soil CH₄ and CO₂ emission during growing seasons in 2016 and 2017 at four adjacent wetlands, namely bare mudflat (Mud), Kandelia obovata (KO), Avicennia marina (AM) and S. alterniflora (SA). Potential methane production (PMP), potential methane oxidation (PMO), functional microbial abundance and soil biogeochemical properties were measured simultaneously. Our results indicate that S. alterniflora invasion could dramatically increase soil CH₄ emissions mainly due to the enhancement in PMP which facilitated by soil EC, MBC, TOC and mcrA gene abundance. Additionally, S. alterniflora invasion decreases soil CO₂ emission. Both heterotrophic microbial respiration (16S rRNA) and methane oxidation (pmoA and ANME-pmoA) are responsible for CO₂ emission reduction. Furthermore, S. alterniflora invasion greatly increases GWP by stimulating CH₄ emissions. Thus, comparing with mangroves, invasive S. alterniflora significantly (p < 0.001) increases CH₄ emission while reduces CO₂ emission.

Response of soil physicochemical properties and enzyme activities to long-term reclamation of coastal saline soil, Eastern China

Soil enzyme activity during different years of reclamation and land use patterns could indicate changes in soil quality. The objective of this research is to explore the dynamics of 5 soil enzyme activities (dehydrogenase, amylase, urease, acid phosphatase and alkaline phosphatase) involved in C, N, and P cycling and their responses to changes in soil physicochemical properties resulting from long-term reclamation of coastal saline soil. Soil samples from a total of 55 sites were collected from a coastal reclamation area with different years of reclamation (0, 7, 32, 40, 63a) in this study. The results showed that both long-term reclamation and land use patterns have significant effects on soil physicochemical properties and enzyme activities. Compared with the bare flat, soil water content, soil bulk density, pH and electrical conductivity showed a decreasing trend after reclamation, whereas soil organic carbon, total nitrogen and total phosphorus tended to increase. Dehydrogenase, amylase and acid phosphatase activities initially increased and then decreased with increasing years of reclamation, whereas urease and alkaline phosphatase activities were characterized by an increase-decrease-increase trend. Moreover, urease, acid phosphatase and alkaline phosphatase activities exhibited significant differences between coastal saline soil with 63years of reclamation and bare flat, whereas dehydrogenase and amylase activities remained unchanged. Aquaculture ponds showed higher soil water content, pH and EC but lower soil organic carbon, total nitrogen and total phosphorus than rapeseed, broad bean and wheat fields. Rapeseed, broad bean and wheat fields displayed higher urease and alkaline phosphatase activities and lower dehydrogenase, amylase and acid phosphatase activities compared with aquaculture ponds. Redundancy analysis revealed that the soil physicochemical properties explained 74.5% of the variation in soil enzyme activities and that an obvious relationship existed between soil nutrients and soil enzyme activities. These results will assist governmental evaluation of the quality of reclaimed coastal soil.

Sediment microbes mediate the impact of nutrient loading on blue carbon sequestration by mixed seagrass meadows

Recent studies have reported significant variability in sediment organic carbon (SOC) storage capacity among seagrass species, but the factors driving this variability are poorly understood, limiting our ability to make informed decisions about which seagrass types are optimal for carbon offsetting and why. Here we show that differences in SOC storage capacity among species within the same geomorphic environment can be explained (in part) by below-ground processes in response to nutrient load; specifically, differences in the activity of microbes harboured by morphologically-different seagrass species. We found that increasing nutrient load enhanced the relative contribution of seagrass and algal sources to SOC pools, boosting sediment microbial biomass and extracellular enzyme activity within mixed seagrass meadows composed of Thalassia hemprichii and Enhalus acoroides, and thus possibly weaken the seagrass blue carbon sequestration capacity. The relative contribution of seagrass plant material to sediment bacterial organic carbon (BOC) and the influencing SOC-decomposing enzymes in E. acoroides meadows were half that of T. hemprichii meadows living side-by-side, even though the mixed seagrass meadows received SOC from the same sources. Overall this research suggests that microbial activity can vary significantly among seagrass species, thereby causing fine-scale (within-meadow) variability in SOC sequestration capacity in response to nutrient load.

Methane production potential and emission at different water levels in the restored reed wetland of Hangzhou Bay

Changes in the hydrological conditions of coastal wetlands may potentially affect the role of wetlands in the methane (CH₄) cycle. In this study, the CH₄ production potential and emissions from restored coastal reed wetlands at different water levels were examined in eastern China at a field scale in two phenological seasons. Results showed that the total CH₄ flux from reeds at various water levels were positive, indicating that they were “sources” of CH₄. During the peak growing season, CH₄ flux from reeds was greater than that during the spring thaw. CH₄ flux from reeds in inundated conditions was greater than that in non-inundated conditions. The CH₄ production potential during the peak growing season was far greater than that during the spring thaw. However, the effect of water level on wetland CH₄ production potential differed among seasons. The correlations among CH₄ production potential, soil properties and CH₄ flux change at different water level. These results demonstrate that water level was related to CH₄ production and CH₄ flux. The growing season also plays a role in CH₄ fluxes. Controlling the hydrological environment in restored wetlands has important implications for the maintenance of their function as carbon sinks.

Effects of nutrient load on microbial activities within a seagrass-dominated ecosystem: Implications of changes in seagrass blue carbon

Nutrient loading is a leading cause of global seagrass decline, triggering shifts from seagrass- to macroalgal-dominance. Within seagrass meadows of Xincun Bay (South China Sea), we found that nutrient loading (due to fish farming) increased sediment microbial biomass and extracellular enzyme activity associated with carbon cycling (polyphenol oxidase, invertase and cellulase), with a corresponding decrease in percent sediment organic carbon (SOC), suggesting that nutrients primed microorganism and stimulated SOC remineralization. Surpisingly, however, the relative contribution of seagrass-derived carbon to bacteria (δ¹³C_bacteria) increased with nutrient loading, despite popular theory being that microbes switch to consuming macroalgae which are assumed to provide a more labile carbon source. Organic carbon sources of fungi were unaffected by nutrient loading. Overall, this study suggests that nutrient loading changes the relative contribution of seagrass and algal sources to SOC pools, boosting sediment microbial biomass and extracellular enzyme activity, thereby possibly changing seagrass blue carbon.