Context- and density-dependent effects of introduced oysters on biodiversity

Pacific oysters, Crassostrea gigas, have been introduced throughout much of the world, become invasive in many locations and can alter native assemblage structure, biodiversity and the distribution and abundance of other species. It is not known, however, to what extent their effects on biodiversity change as their cover increases, and how these effects may differ depending on the environmental context. Experimental plots with increasing cover of oysters were established within two estuaries in two different habitats commonly inhabited by C. gigas, (mussel-beds and mud-flats) and were sampled after 4 and 15 months. Within mud-flat habitats, macroscopic species living on or in the substratum increased in richness, Shannon–Wiener diversity and number of individuals with oyster cover. In mussel-bed habitats, however, these indices were unaffected by the cover of oysters except at one estuary after 15 months when species richness was significantly lower in plots with the greatest cover of oysters. Assemblage structure differed with oyster cover in mud-flats but not in mussel-beds, except at 100 % cover in one location and at one time. Within mud-flats at one location and time (of four total tests), assemblages became more homogenous with increasing cover of oysters leading to a significant decrease in β-diversity. These responses were primarily underpinned by the facilitation of several taxa including a grazing gastropod (Littorina littorea), an invasive barnacle (Austrominius modestus) and a primary producer (Fucus vesiculosus) with increasing cover of oysters. Although there were consistent positive effects of C. gigas on mud-flat biodiversity, effects were weak or negative at higher cover on mussel-beds. This highlights the need for the impacts of invasive species to be investigated at a range of invader abundances within different environmental contexts.     

Changes in Ecosystem Function Across Sedimentary Gradients in Estuaries

The input of terrestrial silt and clay (hereafter mud) into coastal environments can alter sediment grain size distribution affecting the structure and functioning of benthic communities. The relationship between sediment mud content and macrofaunal community structure has been well documented, but not the effects on ecosystem function. In 143 plots from the mid-intertidal sites in 9 estuaries, we measured sediment properties, macrofaunal community composition and fluxes of O₂ and NH₄ ⁺ across the sediment–water interface to derive process-based measures of ecosystem function across the sand–mud gradient. We observed reductions in measures of macrofaunal diversity and decreases in the maximum density of key bioturbating bivalves (Austrovenus stutchburyi and Macomona liliana) with increased mud content. Concurrently, the maximum rates of sediment oxygen consumption (SOC), NH₄ ⁺ efflux (NH₄ ⁺) and biomass standardized gross primary production (GPP_Chl-a ) also decreased with increasing mud content. Environmental predictors explained 34–39% (P = 0.005–0.01) of the total variation in ecosystem function in distance-based linear models. After partitioning out the effect of mud, A. stutchburyi abundance was positively correlated and explained 25 and 23% (P = 0.0001) of the variation of SOC and NH₄ ⁺, respectively. Also, mud content (negatively correlated) and temperature (positively correlated) explained 26% of variability in GPP_Chl-a (P = 0.0001). Our results highlight the importance of increased mud content and the associated reduction in the abundance of strongly interacting key species on the loss of ecosystem function in intertidal sand flats.     

Environment-Dependent Distribution of the Sediment nifH-Harboring Microbiota in the Northern South China Sea

The South China Sea (SCS), the largest marginal sea in the Western Pacific Ocean, is a huge oligotrophic water body with very limited influx of nitrogenous nutrients. This suggests that sediment microbial N₂ fixation plays an important role in the production of bioavailable nitrogen. To test the molecular underpinning of this hypothesis, the diversity, abundance, biogeographical distribution, and community structure of the sediment diazotrophic microbiota were investigated at 12 sampling sites, including estuarine, coastal, offshore, deep-sea, and methane hydrate reservoirs or their prospective areas by targeting nifH and some other functional biomarker genes. Diverse and novel nifH sequences were obtained, significantly extending the evolutionary complexity of extant nifH genes. Statistical analyses indicate that sediment in situ temperature is the most significant environmental factor influencing the abundance, community structure, and spatial distribution of the sediment nifH-harboring microbial assemblages in the northern SCS (nSCS). The significantly positive correlation of the sediment pore water NH₄⁺ concentration with the nifH gene abundance suggests that the nSCS sediment nifH-harboring microbiota is active in N₂ fixation and NH₄⁺ production. Several other environmental factors, including sediment pore water PO₄³⁻ concentration, sediment organic carbon, nitrogen and phosphorus levels, etc., are also important in influencing the community structure, spatial distribution, or abundance of the nifH-harboring microbial assemblages. We also confirmed that the nifH genes encoded by archaeal diazotrophs in the ANME-2c subgroup occur exclusively in the deep-sea methane seep areas, providing for the possibility to develop ANME-2c nifH genes as a diagnostic tool for deep-sea methane hydrate reservoir discovery.     

Land cover change effects on soil chemical and biological properties after planting Mongolian pine (Pinus sylvestris var. mongolica) in sandy lands in Keerqin, northeastern China

We compared soil moisture content, pH, total organic carbon (C _org), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH₄ ⁺–N, NO₃ ^?–N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (C _mic), soil metabolic quotient (qCO₂), soil microbial quotient (C _mic/C _org) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)–grass savanna in southeastern Keerqin, in northeast China. Soil C _org, TN and TP concentrations (0–10, 10–20, 20–40 and 40–60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH₄ ⁺–N and NO₃ ^?–N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0–10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on C _mic and qCO₂ of surface soil were not significant; but C _mic/C _org were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type?×?sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.     

Elevated CO2 increases microbial carbon substrate use and nitrogen cycling in Mojave Desert soils

Identifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. We assessed the effects of elevated atmospheric CO₂ on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community‐level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata) during the 2004–2005 growing season, an above‐average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO₂, and sampling date. Microbial C and N activities were lower in interspace soils compared with soils under Larrea, and responses to date and CO₂ treatments were cover specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO₂, but increased under elevated CO₂. Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO₂ positively affected both. Elevated CO₂ also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils under Larrea, and was lower in soils exposed to elevated CO₂. Gross rates of NH₄⁺ transformations increased over the growing season, and late‐season NH₄⁺ fluxes were negatively affected by elevated CO₂. Gross NO₃⁻ fluxes decreased over time, with early season interspace soils positively affected by elevated CO₂. General increases in microbial activities under elevated CO₂ are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO₂, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO₂ shift the spatial distribution of Mojave Desert plant communities.     

Glacier ecosystem response to episodic nitrogen enrichment in Svalbard, European High Arctic

We describe the climatology, hydrology and biogeochemistry of an extreme nitrogen deposition event that occurred in the highly glacierised environment of the European High Arctic during June 1999. Meteorological analysis, three-dimensional air mass trajectories and a 3D transport model show that blocking high pressures over Scandinavia and the rapid advection of western European pollution toward Svalbard were sufficient to cause the most concentrated (1.15 ppm NO₃–N and 1.20 ppm NH₄–N), high magnitude (total 26 mm and up to 2.4 mm h^?1 at 30 m above sea level) nitrogen deposition event on record in this sensitive, high Arctic environment (78.91° N, 11.93° E). Since the event occurred when much of the catchment remained frozen or under snow cover, microbial utilisation of nitrogen within snowpacks and perennially unfrozen subglacial sediments, rather than soils, were mostly responsible for reducing N export. The rainfall event occurred long before the annual subglacial outburst flood and so prolonged (ca. 10 day) water storage at the glacier bed further enhanced the microbial assimilation. When the subglacial outburst eventually occurred, high runoff and concentrations of NO₃ ^? (but not NH₄ ⁺) returned in the downstream rivers. Assimilation accounted for between 53 and 72% of the total inorganic nitrogen deposited during the event, but the annual NO₃ ^? and NH₄ ⁺ runoff yields were still enhanced by up to 5 and 40 times respectively. Episodic atmospheric inputs of reactive nitrogen can therefore directly influence the biogeochemical functioning of High Arctic catchments, even when microbial activity takes place beneath a glacier at a time when terrestrial soil ecosystems remain frozen and unresponsive.     

Context-Specific Bioturbation Mediates Changes to Ecosystem Functioning

Species are often grouped according to their biological or functional traits to better understand their contribution to ecosystem functioning. However, it is becoming clear that a single species can perform different roles in different habitats. Austrohelice crassa, a burrow-building mud crab shifts its primary bioturbational role to that of a vertical mixer in non-cohesive sediments as frequent burrow collapse greatly enhances sediment reworking. We conducted in situ crab density manipulations in two sediment environments (a non-cohesive sand and a cohesive muddy-sand) to examine if the context-specific functional roles were linked to changes in solute fluxes across the sediment–water interface. Across both habitats, we show that A. crassa regulated nutrient cycling, creating strong density driven effects on solute exchanges. Increasing crab density increased sediment O₂ demand and the flux of NH₄ ⁺ from the sediment, indicating much of the response was physiologically driven. Clear interactions between A. crassa and microphytobenthos were also detected in both habitats. Despite lowering microphyte standing stock through deposit feeding, A. crassa increased benthic primary production per unit of chlorophyll a. Our experiment also revealed important context-specific differences, most notably for NH₄ ⁺ fluxes, which were higher where burrows and their associated microbial communities were most stable (muddy-sand). This study highlights the need to integrate interactions between organism behavior and habitat type into functional group studies to broaden conceptual frameworks and avoid oversimplification of highly complex organism–sediment interactions.     

Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA

Forest fires often result in a series of biogeochemical processes that increase soil nitrate (NO₃ ^?) concentrations for several years; however, the dynamic nature of inorganic nitrogen (N) cycling in the plant–microbe–soil complex makes it challenging to determine the direct causes of increased soil NO₃ ^?. We measured gross inorganic N transformation rates in mineral soils 2 years after wildfires in three central Idaho coniferous forests to determine the causes of the elevated soil NO₃ ^?. We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and carbon (C) availability. We found no significant differences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO₃ ^? uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO₃ ^? caused slightly elevated NO₃ ^? concentrations in the burned soils. C availability was positively correlated with microbial NO₃ ^? uptake rates. Despite reduced microbial NO₃ ^? uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO₃ ^? concentrations 2 years post fire. Soil NH₄ ⁺ concentrations between the treatments were not significantly different (P = 0.673). However, gross NH₄ ⁺ production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively correlated with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region.     

Benthic decomposition of Zostera marina roots: a controlled laboratory experiment

The initial benthic decomposition of Zostera marina roots was studied in a controlled flow-through chamber experiment for 23 days. Sediment chambers without added roots served as controls. The inflowing and outflowing artificial seawater (ASW) was analyzed for O₂, ΣCO₂, urea-N, NH₄⁺ and NO₂⁻+NO₃⁻. Sediment profiles of Eh, particulate organic carbon (POC) and nitrogen, dissolved organic nitrogen (DON), dissolved free amino acids (DFAA), urea-N, NH₄⁺, DFAA and urea turnover rates, sulfate reduction and counts of total anaerobic heterotrophic bacteria and different functional groups were determined. Fluxes of O₂, ΣCO₂, urea-N and NH₄⁺ were stimulated during root decomposition compared to the unamended control. There were indications of stimulated bacterial growth based on counts of total anaerobic heterotrophic bacteria, anaerobic phosphatase utilizers, ammonifyers and sulfate reducers. Independent estimates of nitrogen and carbon incorporation into bacterial biomass during root decomposition indicate that a major fraction of the nitrogen for microbial growth was mobilized from the indigenous particulate organic nitrogen (PON) pool, whereas the energy source for bacterial growth was mainly obtained from the added eelgrass roots. Most of the nitrogen mineralized during root decomposition was incorporated into the bacterial biomass resulting in a low efflux of urea-N and inorganic nitrogen from the sediment to the water column.     

A meta‐analysis of soil salinization effects on nitrogen pools,cycles and fluxes in coastal ecosystems

Salinity intrusion caused by land subsidence resulting from increasing groundwater abstraction, decreasing river sediment loads and increasing sea level because of climate change has caused widespread soil salinization in coastal ecosystems. Soil salinization may greatly alter nitrogen (N) cycling in coastal ecosystems. However, a comprehensive understanding of the effects of soil salinization on ecosystem N pools, cycling processes and fluxes is not available for coastal ecosystems. Therefore, we compiled data from 551 observations from 21 peer‐reviewed papers and conducted a meta‐analysis of experimental soil salinization effects on 19 variables related to N pools, cycling processes and fluxes in coastal ecosystems. Our results showed that the effects of soil salinization varied across different ecosystem types and salinity levels. Soil salinization increased plant N content (18%), soil NH₄⁺ (12%) and soil total N (210%), although it decreased soil NO₃^? (2%) and soil microbial biomass N (74%). Increasing soil salinity stimulated soil N₂O fluxes as well as hydrological NH₄⁺ and NO₂^? fluxes more than threefold, although it decreased the hydrological dissolved organic nitrogen (DON) flux (59%). Soil salinization also increased the net N mineralization by 70%, although salinization effects were not observed on the net nitrification, denitrification and dissimilatory nitrate reduction to ammonium in this meta‐analysis. Overall, this meta‐analysis improves our understanding of the responses of ecosystem N cycling to soil salinization, identifies knowledge gaps and highlights the urgent need for studies on the effects of soil salinization on coastal agro‐ecosystem and microbial N immobilization. Additional increases in knowledge are critical for designing sustainable adaptation measures to the predicted intrusion of salinity intrusion so that the productivity of coastal agro‐ecosystems can be maintained or improved and the N losses and pollution of the natural environment can be minimized.     

Effects of Bromus tectorum invasion on microbial carbon and nitrogen cycling in two adjacent undisturbed arid grassland communities

Soil nitrogen (N) is an important component in maintaining ecosystem stability, and the introduction of non-native plants can alter N cycling by changing litter quality and quantity, nutrient uptake patterns, and soil food webs. Our goal was to determine the effects of Bromus tectorum (C₃) invasion on soil microbial N cycling in adjacent non-invaded and invaded C₃ and C₄ native arid grasslands. We monitored resin-extractable N, plant and soil δ¹³C and δ¹⁵N, gross rates of inorganic N mineralization and consumption, and the quantity and isotopic composition of microbial phospholipid biomarkers. In invaded C₃ communities, labile soil organic N and gross and net rates of soil N transformations increased, indicating an increase in overall microbial N cycling. In invaded C₄ communities labile soil N stayed constant, but gross N flux rates increased. The δ¹³C of phospholipid biomarkers in invaded C₄ communities showed that some portion of the soil bacterial population preferentially decomposed invader C₃-derived litter over that from the native C₄ species. Invasion in C₄ grasslands also significantly decreased the proportion of fungal to bacterial phospholipid biomarkers. Different processes are occurring in response to B. tectorum invasion in each of these two native grasslands that: 1) alter the size of soil N pools, and/or 2) the activity of the microbial community. Both processes provide mechanisms for altering long-term N dynamics in these ecosystems and highlight how multiple mechanisms can lead to similar effects on ecosystem function, which may be important for the construction of future biogeochemical process models.     

Effects of ammonium on the activity and community of methanotrophs in landfill biocover soils

The influence of NH₄⁺ on microbial CH₄ oxidation is still poorly understood in landfill cover soils. In this study, effects of NH₄⁺ addition on the activity and community structure of methanotrophs were investigated in waste biocover soil (WBS) treated by a series of NH₄⁺-N contents (0, 100, 300, 600 and 1200 mg kg⁻¹). The results showed that the addition of NH₄⁺-N ranging from 100 to 300 mg kg⁻¹ could stimulate CH₄ oxidation in the WBS samples at the first stage of activity, while the addition of an NH₄⁺-N content of 600 mg kg⁻¹ had an inhibitory effect on CH₄ oxidation in the first 4 days. The decrease of CH₄ oxidation rate observed in the last stage of activity could be caused by nitrogen limitation and/or exopolymeric substance accumulation. Type I methanotrophs Methylocaldum and Methylobacter, and type II methanotrophs (Methylocystis and Methylosinus) were abundant in the WBS samples. Of these, Methylocaldum was the main methanotroph in the original WBS. With incubation, a higher abundance of Methylobacter was observed in the treatments with NH₄⁺-N contents greater than 300 mg kg⁻¹, which suggested that NH₄⁺-N addition might lead to the dominance of Methylobacter in the WBS samples. Compared to type I methanotrophs, the abundance of type II methanotrophs Methylocystis and/or Methylosinus was lower in the original WBS sample. An increase in the abundance of Methylocystis and/or Methylosinus occurred in the last stage of activity, and was likely due to a nitrogen limitation condition. Redundancy analysis showed that NH₄⁺-N and the C/N ratio had a significant influence on the methanotrophic community in the WBS sample.     

Effect of water level fluctuation on nitrogen removal from constructed wetland mesocosms

Nitrogen removal processes were investigated at three frequencies of water level fluctuation, static, low and high (0, 2 and 6 d⁻¹), in duplicate gravel-bed constructed wetland mesocosms (0.145 m³) with and without plants (Schoenoplectus tabernaemontani). Fluctuation was achieved by temporarily pumping wastewater into a separate tank (total drain time ∼35 min). Intensive sampling of the mesocosms, batch-fed weekly with ammonium-rich (∼100 g m⁻³ NH₄-N) farm dairy wastewaters, showed rates of chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal increased markedly with fluctuation frequency and in the presence of plants. Nearly complete removal of NH₄-N was recorded over the 7 day batch period at the highest level of fluctuation, with minimal enhancement by plants. Redox potentials (Eh) at 100 mm depth rose from initial levels of around −100 to >350 mV and oxidised forms of N (NO₂ and NO₃) increased to ∼40 g m⁻³, suggesting conditions were conducive to microbial nitrification at this level of fluctuation. In the unplanted mesocosms with low or zero fluctuation, mean NH₄-N removals were only 28 and 10%, respectively, and redox potentials in the media remained low for a substantial part of the batch periods (mid-batch Eh ∼+100 and −100 mV, respectively). In the presence of wetland plants, mean NH₄-N removal in the mesocosms with low or zero fluctuation rose to 71 and 54%, respectively, and COD removal (>70%) and redox potential (mid-batch Eh>200 mV) were markedly higher than in the unplanted mesocosms. Negligible increases in oxidised N were recorded at these fluctuation frequencies, but total nitrogen levels declined at mean rates of 2.4 and 1.8 g m⁻² d⁻¹, respectively. NH₄-N removal from the bulk water in the mesocosms was well described (R²=0.97–0.99) by a sorption-plant uptake-microbial model. First-order volumetric removal rate constants (k_v) rose with increasing fluctuation frequency from 0.026 to 0.46 d⁻¹ without plants and from 0.042 to 0.62 d⁻¹ with plants. As fluctuation frequency increased, reversible sorption of NH₄-N to the media, and associated biofilms and organic matter, became an increasingly important moderator of bulk water concentrations during the batch periods. TN mass balances for the full batch periods suggested that measured plant uptake estimates of between 0.52 and 1.07 g N m⁻² d⁻¹ (inversely related to fluctuation frequency) could fully account for the increased overall removal of TN recorded in the planted systems. By difference, microbial nitrification-denitrification losses were therefore estimated to be approximately doubled by low-level fluctuation from 0.7 to 1.4 g N m⁻² d⁻¹ (both with and without plants), rising to a maximum rate of 2.1 g N m⁻² d⁻¹ at high fluctuation, in the absence of competitive uptake by plants.     

A closeup study of early beech litter decomposition: potential drivers and microbial interactions on a changing substrate

Aims

Litter decomposition and subsequent nutrient release play a major role in forest carbon and nutrient cycling. To elucidate how soluble or bulk nutrient ratios affect the decomposition process of beech (Fagus sylvatica L.) litter, we conducted a microcosm experiment over an 8 week period. Specifically, we investigated leaf-litter from four Austrian forested sites, which varied in elemental composition (C:N:P ratio). Our aim was to gain a mechanistic understanding of early decomposition processes and to determine microbial community changes.

Methods

We measured initial litter chemistry, microbial activity in terms of respiration (CO₂), litter mass loss, microbial biomass C and N (C_mic and N_mic), non purgeable organic carbon (NPOC), total dissolved nitrogen (TDN), NH₄ ⁺, NO₃ ^- and microbial community composition (phospholipid fatty acids – PLFAs).

Results

At the beginning of the experiment microbial biomass increased and pools of inorganic nitrogen (N) decreased, followed by an increase in fungal PLFAs. Sites higher in NPOC:TDN (C:N of non purgeable organic C and total dissolved N), K and Mn showed higher respiration.

Conclusions

The C:N ratio of the dissolved pool, rather than the quantity of N, was the major driver of decomposition rates. We saw dynamic changes in the microbial community from the beginning through the termination of the experiment.     

Stream acidification increases nitrogen uptake by leaf biofilms: implications at the ecosystem scale

1. While anthropogenic stream acidification is known to lower species diversity and impair decomposition, its effects on nutrient cycling remain unclear. The influence of acid‐stress on microbial physiology can have implications for carbon (C) and nitrogen (N) cycles, linking environmental conditions to ecosystem processes. 2. We collected leaf biofilms from streams spanning a gradient of pH (5.1–6.7), related to chronic acidification, to investigate the relationship between qCO₂ (biomass‐specific respiration; mg CO₂‐C g^?1 fungal C h^?1), a known indicator of stress, and biomass‐specific N uptake (μg NH₄‐N mg^?1 fungal biomass h^?1) at two levels of N availability (25 and 100 μg NH₄‐N L^?1) in experimental microcosms. 3. Strong patterns of increasing qCO₂ (i.e. increasing stress) and increasing microbial N uptake were observed with a decrease in ambient (i.e. chronic) stream pH at both levels of N availability. However, fungal biomass was lower on leaves from more acidic streams, resulting in lower overall respiration and N uptake when rates were standardized by leaf biomass. 4. Results suggest that chronic acidification decreases fungal metabolic efficiency because, under acid conditions, these organisms allocate more resources to maintenance and survival and increase their removal of N, possibly via increased exoenzyme production. At the same time, greater N availability enhanced N uptake without influencing CO₂ production, implying increased growth efficiency. 5. At the ecosystem level, reductions in growth because of chronic acidification reduce microbial biomass and may impair decomposition and N uptake; however, in systems where N is initially scarce, increased N availability may alleviate these effects. Ecosystem response to chronic stressors may be better understood by a greater focus on microbial physiology, coupled elemental cycling, and responses across several scales of investigation.     

Belowground heathland responses after 2 years of combined warming, elevated CO2 and summer drought

Terrestrial ecosystems are exposed to atmospheric and climatic changes including increases in atmospheric CO₂ concentration, temperature and alterations of precipitation patterns, which are predicted to continue with consequences for ecosystem services and functioning in the future. In a field scale experiment on temperate heathland, manipulation of precipitation and temperature was performed with retractable curtains, and atmospheric CO₂ concentration was increased by FACE. The combination of elevated CO₂ and warming was expected to affect belowground processes additively, through increased belowground sequestration of labile carbohydrates due to elevated CO₂ in combination with temperature increased process rates. Together, these changes might increase microbial activity and availability of plant nutrients. Two years after the start of the experiment, belowground processes responded significantly to the treatments. In the combined temperature and CO₂ treatment the dissolved organic nitrogen concentration decreased and the ammonium concentration increased, but this release of nutrients was not mirrored by plant parameters. Microbial biomass carbon and microbial enrichment with ¹³C and ¹⁵N (1 year after ¹³C ₂ ¹⁵ N-glycine was injected into the soil) increased in warmed plots and in elevated CO₂ plots, but not when these treatments were combined. Furthermore, drought led to an increase in Calluna biomass and total plant nitrogen pool. The full combination of warming, elevated CO₂ and periodic drought did not unambiguously express the ecosystem responses of single factors additively, which complicates predictions of ecosystem responses to multifactor climate change.     

Responses of soil chemical and biological properties to nitrogen addition in a Dahurian larch plantation in Northeast China

Soil microbial properties play a key role in belowground ecosystem functioning, but are not well understood in forest ecosystems under nitrogen (N) enrichment. In this study, soil samples from 0–10 cm and 10–20 cm layers were collected from a Dahurian larch (Larix gmelinii Rupr.) plantation in Northeast China after six consecutive years of N addition to examine changes in soil pH, nutrient concentrations, and microbial biomass and activities. Nitrogen addition significantly decreased soil pH and total phosphorus, but had little effect on soil total organic carbon (TOC) and total N (TN) concentrations. The NO ₃ ^? -N concentrations in the two soil layers under N addition were significantly higher than that in the control, while NH ₄ ⁺ -N concentrations were not different. After six years of N addition, potential net N mineralization and nitrification rates were dramatically increased. Nitrogen addition decreased microbial biomass C (MBC) and N (MBN), and MBC/TOC and MBN/TN in the 0–10 cm soil layer, but MBC/MBN was increased by 67% in the 0–10 cm soil layer. Soil basal respiration, microbial metabolic quotient (qCO₂), and β-glucosidase, urease, acid phosphomonoesterase and nitrate reductase activities in the two soil layers showed little change after six years of N addition. However, soil protease and dehydrogenase activities in the 0–10 cm layer were 41% and 54% lower in the N addition treatment than in the control, respectively. Collectively, our results suggest that in the mid-term N addition leads to a decline in soil quality in larch plantations, and that different soil enzymes show differentiated responses to N addition.     

Sedimentary nitrogen uptake and assimilation in the temperate zooxanthellate sea anemone Anthopleura aureoradiata

The sea anemone Anthopleura aureoradiata (Carlgren), which harbours symbiotic dinoflagellates (zooxanthellae), is abundant on mudflats and rocky shores around New Zealand. We measured the potential for particulate nitrogen uptake from sediment by A. aureoradiata and the subsequent consequences of this uptake on the nitrogen status of its zooxanthellae. Sediment was rinsed, labelled with (¹⁵NH₄)₂SO₄, and provided to anemones at low (0.23 g ml^− 1) and high (1.33 g ml^− 1) sediment loads for 6 h. Both anemone tissues and zooxanthellae became enriched with ¹⁵N. Enrichment of anemone tissues was similar at both high and low sediment loads, but the zooxanthellae became more enriched at the lower load. This was presumably because the uptake of ammonium, arising from host catabolism, by zooxanthellae is light driven and because the anemones at the lower load were able to extend their tentacles into the light while those at the higher load were not. The influence of sediment uptake on the nitrogen status of the zooxanthellae was determined by measuring the extent to which 20 μM NH₄⁺ enhanced the rate of zooxanthellar dark carbon fixation above that seen in filtered seawater (FSW) alone; the ammonium enhancement ratio (AER) was expressed as [dark NH₄⁺ rate/dark FSW rate], where ‘rate’ refers to C fixation and a ratio of 1.0 or less indicates nitrogen sufficiency. When anemones were starved with and without rinsed sediment in nitrogen-free artificial seawater for 8 weeks, zooxanthellar nitrogen deficiency became apparent at 2-4 weeks and reached similar levels in both treatments (AER = ~ 2). In contrast, anemones fed 5 times per week for 8 weeks with Artemia nauplii were nitrogen sufficient (AER = 1.03). In the field, zooxanthellae from mudflat anemones were largely nitrogen sufficient (AER = 1.26), while nitrogen deficient zooxanthellae were present in anemones from a rocky intertidal site (AER = 2.93). These results suggest that, while there was evidence for particulate nitrogen uptake, dissolved inorganic nitrogen (especially ammonium) in interstitial pore water may be a more important source of nitrogen for the zooxanthellae in mudflat anemones, and may explain the marked difference in nitrogen status between the mudflat and rocky shore populations.     

Phylogenetic diversity and activity of anaerobic microorganisms of high-temperature horizons of the Dagang oil field (P. R. China)

The number of microorganisms of major metabolic groups and the rates of sulfate reduction and methanogenesis processes in the formation waters of the high-temperature horizons of Dagang oil field have been determined. Using cultural methods, it was shown that the microbial community contained aerobic bacteria oxidizing crude oil, anaerobic fermentative bacteria, sulfate-reducing bacteria, and methanogens. Using cultural methods, the possibility of methane production from a mixture of hydrogen and carbon dioxide (H₂ + CO₂) and from acetate was established, and this result was confirmed by radioisotope methods involving NaH¹⁴CO₃ and ¹⁴CH₃COONa. Analysis of enrichment cultures 16S rDNA of methanogens demonstrated that these microorganisms belong to Methanothermobacter sp. (M. thermautotrophicus), which consumes hydrogen and carbon dioxide as basic substrates. The genes of acetate-utilizing bacteria were not revealed. Phylotypes of the representatives of Thermococcus spp. were found among archaeal 16S rDNA. 16S rRNA genes of bacterial clones belong to the orders Thermoanaerobacteriales (Thermoanaerobacter, Thermovenabulum, Thermacetogenium, and Coprothermobacter spp.), Thermotogales, Nitrospirales (Thermodesulfovibrio sp.) and Planctomycetales. 16S rDNA of a bacterium capable of oxidizing acetate in the course of syntrophic growth with H₂-utilizing methanogens was found in high-temperature petroleum reservoirs for the first time. These results provide further insight into the composition of microbial communities of high-temperature petroleum reservoirs, indicating that syntrophic processes play an important part in acetate degradation accompanied by methane production.     

Microbial Decomposition of Wood in Streams: Distribution of Microflora and Factors Affecting [14C]Lignocellulose Mineralization

The distribution and lignocellulolytic activity of the microbial community was determined on a large log of Douglas fir (Pseudotsuga menziesii) in a Pacific Northwest stream. Scanning electron microscopy, plate counts, and degradation of [¹⁴C]lignocelluloses prepared from Douglas fir and incubated with samples of wood taken from the surface and within the log revealed that most of the microbial colonization and lignocellulose-degrading activity occurred on the surface. Labeled lignocellulose and surface wood samples were incubated in vitro with nutrient supplements to determine potential limiting factors of [¹⁴C]lignocellulose degradation. Incubations carried out in a nitrogenless mineral salts and trace elements solution were no more favorable to degradation than those carried out in distilled water alone. Incubations supplemented with either (NH₄)₂SO₄ or organic nitrogen sources showed large increases in the rates of mineralization over incubations with mineral salts and trace elements alone, with the greatest effect being observed from an addition of (NH₄)₂SO₄. Subsequent incubations with (NH₄)₂SO₄, KNO₃, and NH₄NO₃ revealed that KNO₃ was the most favorable for lignin degradation, whereas all three supplements were equally favorable for cellulose degradation. Supplementation with glucose repressed both lignin and cellulose mineralization. The results reported in this study indicate that nitrogen limitation of wood decomposition may exist in streams of the Pacific Northwest. The radiotracer technique was shown to be a sensitive and useful tool for assessing relative patterns of lignocellulose decay and microbial activity in wood, along with the importance of thoroughly characterizing the experimental system before its general acceptance.