Simultaneous nutrients and carbon removal during pretreated swine slurry degradation in a tubular biofilm photobioreactor

The biodegradation potential of an innovative enclosed tubular biofilm photobioreactor inoculated with a Chlorella sorokiniana strain and an acclimated activated sludge consortium was evaluated under continuous illumination and increasing pretreated (centrifuged) swine slurry loading rates. This photobioreactor configuration provided simultaneous and efficient carbon, nitrogen, and phosphorous treatment in a single-stage process at sustained nitrogen and phosphorous removals efficiencies ranging from 94% to 100% and 70–90%, respectively. Maximum total organic carbon (TOC), NH₄ ⁺, and PO₄ ³⁻ removal rates of 80 ± 5 g C m_r ⁻³ day⁻¹, 89 ± 5 g N m_r ⁻³ day⁻¹, and 13 ± 3 g P m_r ⁻³ day⁻¹, respectively, were recorded at the highest swine slurry loadings (TOC of 1,247 ± 62 mg L⁻¹, N–NH₄ ⁺ of 656 ± 37 mg L⁻¹, P–PO₄ ³⁺ of 117 ± 19 mg L⁻¹, and 7 days of hydraulic retention time). The unusual substrates diffusional pathways established within the phototrophic biofilm (photosynthetic O₂ and TOC/NH₄ ⁺ diffusing from opposite sides of the biofilm) allowed both the occurrence of a simultaneous denitrification/nitrification process at the highest swine slurry loading rate and the protection of microalgae from any potential inhibitory effect mediated by the combination of high pH and high NH₃ concentrations. In addition, this biofilm-based photobioreactor supported efficient biomass retention (>92% of the biomass generated during the pretreated swine slurry biodegradation).     

Stem respiration of ponderosa pines grown in contrasting climates: implications for global climate change

We examined the effects of climate and allocation patterns on stem respiration in ponderosa pine (Pinus ponderosa) growing on identical substrate in the cool, moist Sierra Nevada mountains and the warm, dry, Great Basin Desert. These environments are representative of current climatic conditions and those predicted to accompany a doubling of atmospheric CO₂, respectively, throughout the range of many western north American conifers. A previous study found that trees growing in the desert allocate proportionally more biomass to sapwood and less to leaf area than montane trees. We tested the hypothesis that respiration rates of sapwood are lower in desert trees than in montane trees due to reduced stem maintenance respiration (physiological acclimation) or reduced construction cost of stem tissue (structural acclimation). Maintenance respiration per unit sapwood volume at 15°C did not differ between populations (desert: 6.39 ± 1.14 SE μmol m⁻³ s⁻¹, montane: 6.54 ± 1.13 SE μmol m⁻³ s⁻¹, P = 0.71) and declined with increasing stem diameter (P = 0.001). The temperature coefficient of respiration (Q ₁₀) varied seasonally within both environments (P = 0.05). Construction cost of stem sapwood was the same in both environments (desert: 1.46 ± 0.009 SE g glucose g⁻¹ sapwood, montane: 1.48 ± 0.009 SE glucose g⁻¹ sapwood, P = 0.14). Annual construction respiration calculated from construction cost, percent carbon and relative growth rate was greater in montane populations due to higher growth rates. These data provide no evidence of respiratory acclimation by desert trees. Estimated yearly stem maintenance respiration was greater in large desert trees than in large montane trees because of higher temperatures in the desert and because of increased allocation of biomass to sapwood. By analogy, these data suggest that under predicted increases in temperature and aridity, potential increases in aboveground carbon gain due to enhanced photosynthetic rates may be partially offset by increases in maintenance respiration in large trees growing in CO₂-enriched atmospheres. Received: 4 November 1996 / Accepted: 23 January 1997     

Growth Efficiency and Carbon Balance for the Sponge Haliclona oculata

To obtain more knowledge about carbon requirements for growth by sponges, the growth rate, respiration rate, and clearance rate was measured in situ in Haliclona oculata. We found that only 34% of the particulate carbon pumped through the sponge was used for both respiration and growth. The net growth efficiency, being the ratio of carbon incorporated in biomass and the total carbon used by the sponge for respiration and growth, was found to be 0.099 ± 0.013. Thus, about 10% of the total used carbon was fixed in biomass, and over 90% was used for generating energy for growth, maintenance, reproduction, and pumping. H. oculata had 2.5 μmol C available for every micromole O₂ consumed. A value of 0.75 for respiratory quotient (RQ in micromole CO₂ micromole O₂⁻¹) was used for H. oculata, which is the average value reported in literature for different marine invertebrates. Thus, carbon was available in excess to meet the respiratory demand. Oxygen was found not to be the limiting factor for growth, since only 3.3% of the oxygen pumped through the sponge body was used. Our results indicate that both oxygen and carbon availability are not limiting. The low growth efficiency agrees with the low growth rates found for the species used in this study.     

Biodegradation kinetics of monoterpenes in liquid and soil-slurry systems

Batch experiments were conducted to evaluate the biodegradation rates of limonene, α-pinene, γ-terpinene, terpinolene and α-terpineol at 23 °C under aerobic conditions. Biodegradation was demonstrated by the depletion of monoterpene mass, CO₂ production and a corresponding increase in biomass. Monoterpene degradation in liquid cultures devoid of soil followed Monod kinetics. The maximum specific growth rate (μ_max) was 0.02 h⁻¹ and 0.06 h⁻¹ and the half-velocity constant (K _s ) varied from 32 mg/l to 3 mg/l for the limonene and α-terpineol respectively. The recovery of monoterpenes by solvent extraction from autoclaved and azide-amended soil-slurry samples decreased over time and ranged from 69% to 73% for 120 h of incubation period. Although a significant fraction of monoterpene hydrocarbon could not be extracted, mineralization of these compounds in the soil-slurry systems took place, as shown by CO₂ production. The soil-normalized degradation rates for the hydrocarbon monoterpenes ranged from 0.6 μg g⁻¹ h⁻¹ to 2.1 μg g⁻¹ h⁻¹. A kinetic model – which combined monoterpene biodegradation in the liquid phase and net desorption – was developed and applied to data obtained from soil-slurry assays. Received: 10 September 1996 / Received revision: 16 December 1996 / Accepted: 10 January 1997     

An extractive membrane biofilm reactor for degradation of 1,3-dichloropropene in industrial waste water

A bacterial biofilm, capable of mineralising a technical mixture of cis- and trans-1,3-dichloropropene (DCPE), was enriched on the biomedium side of an extractive membrane biofilm reactor (EMBR). The membrane separates the biomedium from the industrial waste water, in terms of pH, ionic strength and the concentration of toxic chemicals. The biofilm, attached to a silicone membrane, is able to mineralise DCPE after its diffusion through the membrane. Five bacterial strains with degradation capabilities were isolated from the metabolically active biofilm and further investigated in batch experiments. Two of them, Rhodococcus erythropolis strains EK2 and EK5, can grow with DCPE as the sole carbon source. Pseudomonas sp. EK1 utilises cis-3-chloroallylalcohol and cis-3-chloroacrylic acid, whereas the metabolite trans-3-chloroacrylic acid represents a dead-end product of the pathway of this strain. The other two strains, Delftia sp. EK3 and EK4, although unable to grow with DCPE as the carbon source, can transform DCPE and its upper-pathway intermediates at reasonable conversion rates. They may represent helper functions of the biofilm consortium, which mineralised up to 12.5 mmol DCPE per hour per gram of biomass protein. Higher feed rates in the EMBR (up to 15 mmol per hour per 100-l bioreactor volume) and shock loads corresponding to concentrations up to 1.8 mmol l⁻¹ led to a significant increase in the freely floating bacterial biomass in the reactor medium (OD₅₄₆= 0.2). At the standard operating feed rate of 1.8 mmol h⁻¹, the free biomass concentration was very low (OD₅₄₆= 0.04). Received: 23 April 1999 / Received revision: 1 July 1999 / Accepted: 5 July 1999     

Nitrate removal from drinking water using a membrane-fixed biofilm reactor

Biological treatment of drinking water is a cost-effective alternative to conventional physico/chemical processes. A new concept was tested to overcome the main disadvantage of biological denitrification, the intensive post-treatment process to remove microorganisms and remnant carbon source. The biological reaction zone and carbon supply were separated from the raw water stream by a nitrate-permeable membrane. Denitrification takes place in a biofilm, which is immobilized at the membrane. In a series of bench-scale runs, different types of membranes and reactor configurations were investigated. The best denitrification rates achieved were 1230 mg NO₃ ⁻-N m⁻² day⁻¹. In one run, raw water containing 100 mg NO₃ ⁻ l⁻¹ was completely freed from nitrate. The membrane and the attached biofilm also represent a barrier against the passage of the C source and nutrients into the raw water. At concentrations of 20 mg l⁻¹ ethanol and 15 mg l⁻¹ phosphate in the bioreactor no diffusion through the membrane into the treated water was observed. Without any post-treatment, the effluent met nearly all the relevant criteria for drinking water; only the colony count was slightly increased. Received: 18 December 1996 / Received last revision: 14 April 1997 / Accepted: 19 April 1997     

CO2 exchange of the endolithic lichen Verrucaria baldensis from karst habitats in northern Italy

CO₂ exchange of the endolithic lichen Verrucaria baldensis was measured in the laboratory under different conditions of water content, temperature, light, and CO₂ concentration. The species had low CO₂ exchange rates (maximum net photosynthesis: c. 0.45 μmol CO₂ m⁻² s⁻¹; maximum dark respiration: c. 0.3 μmol CO₂ m⁻² s⁻¹) and a very low light compensation point (7 μmol photons m⁻² s⁻¹ at 8°C). The net photosynthesis/respiration quotient reached a maximum at 9–15°C. Photosynthetic activity was affected only after very severe desiccation, when high resaturation respiratory rates were measured. Microclimatic data were recorded under different weather conditions in an abyss of the Trieste Karst (northeast Italy), where the species was particularly abundant. Low photosynthetically active radiation (normally below 40 μmol photons m⁻² s⁻¹), very high humidities (over 80%), and low, constant temperatures were measured. Thallus water contents sufficient for CO₂ assimilation were often measured in the absence of condensation phenomena. Received: 22 September 1996 / Accepted: 26 April 1997     

Exchanges of oxygen, carbon dioxide, nitrogen and water in the caecilian Dermophis mexicanus

Oxygen consumption was measured in five Dermophis mexicanus and averaged (±SEM) 0.047 ± 0.004 ml O₂ g⁻¹ h⁻¹. Carbon dioxide production averaged 0.053 ± 0.005 ml CO₂ g⁻¹ h⁻¹ in the same five animals 1 week later. This metabolic rate is similar to metabolic rates of other Gymnophionans but lower than metabolic rates reported for Anurans and Urodeles. Total nitrogen excretion averaged 1.37 μmol N g⁻¹ h⁻¹ which is higher than that found for other amphibians. Of this, 82.5% (1.13 μmol N g⁻¹ h⁻¹) was in the form of urea while 17.5% (0.24 μmol N g⁻¹ h⁻¹) was in the form of NH₃ + NH⁺ ₄. Such ureotelism is typical of terrestrial amphibians like D. mexicanus. Osmotic water flux averaged 0.0193 ml g⁻¹ h⁻¹ in control (sham injected) animals and was not significantly altered by injection of either arginine vasotocin or mesotocin. This osmotic flux is similar to osmotic fluxes found for other terrestrial amphibians. The combined data suggest that metabolism in D. mexicanus is, like most other Gymnophionans, lower than other amphibians. The high rates of nitrogen (especially urea) excretion suggests that this fossorial animal accumulates urea like other burrowing amphibians. Accepted: 27 June 2000     

Biodegradation of the pesticide 4,6-dinitro-ortho-cresol by microorganisms in batch cultures and in fixed-bed column reactors

A mixed culture of microorganisms able to utilize 4,6-dinitro-ortho-cresol (DNOC) as the sole source of carbon, nitrogen and energy was isolated from soil contaminated with pesticides and from activated sludge. DNOC was decomposed aerobically in batch cultures as well as in fixed-bed column reactors. Between 65% and 84% of the substrate nitrogen was released as nitrate into the medium, and 61% of the carbon from uniformly ¹⁴C-labelled DNOC was recovered as ¹⁴CO₂. The mixed microbial culture also decomposed 4-nitrophenol and 2,4-dinitrophenol but not 2,3-dinitrophenol, 2,6-dinitrophenol, 2,4-dinitrotoluene, 2,4-dinitrobenzoic acid or 2-sec-butyl-4,6-dinitrophenol (Dinoseb). Maximal degradation rates for DNOC by the bacterial biofilm immobilized on glass beads in fixed-bed column reactors were 30 mmol day⁻¹ (l reactor volume)⁻¹, leaving an effluent concentration of less than 5 μg l⁻¹ DNOC in the outflowing medium. The apparent K _s value of the immobilized mixed culture for DNOC was 17 μM. Degradation was inhibited at DNOC concentrations above 30 μM and it ceased at 340 μM, possibly because of the uncoupling action of the nitroaromatic compound on the cellular energy-transducing mechanism. Received: 27 March 1997 / Received revision: 5 June 1997 / Accepted: 7 June 1997     

Production of curdlan using sucrose or sugar cane molasses by two-step fed-batch cultivation of Agrobacterium species

Maltose and sucrose were efficient carbon sources for the production of curdlan by a strain of Agrobacterium sp. A two-step, fed-batch operation was designed in which biomass was first produced, followed by curdlan production which was stimulated by nitrogen limitation. There exists an optimal timing for nitrogen limitation for curdlan production in the two-step, fed-batch operation. Maximum curdlan production (60 g L⁻¹) was obtained from sucrose with a productivity of 0.2 g L⁻¹ h⁻¹ when nitrogen was limited at a cell concentration of 16.0 g L⁻¹. It was also noted that the curdlan yield from sucrose was as high as 0.45 g curdlan g⁻¹ sucrose, and the highest specific production rate was 1.0 g curdlan g⁻¹ cells h⁻¹ right after nitrogen limitation. Of particular importance was the use of molasses as a cheap carbon source to produce curdlan in the two-step, fed-batch cultivation. As high as 42 g L⁻¹ of curdlan with a yield of 0.35 g curdlan g⁻¹ total sugar was obtained after 120 h of fed-batch cultivation. Received 20 August 1996/ Accepted in revised form 26 November 1996     

Treatment of cyanide-containing wastewater from the food industry in a laboratory-scale fixed-bed methanogenic reactor

During the process of producing cassava starch from Manihot esculenta roots, large amounts of cyanoglycosides were released, which rapidly decayed to CN⁻ following enzymatic hydrolysis. Depending on the varying cyanoglycoside content of the cassava varieties, the cyanide concentration in the wastewater was as high as 200 mg/l. To simulate anaerobic stabilization, a wastewater with a chemical oxygen demand (COD) of about 20 g/l was prepared from cassava roots and was fermented in a fixed-bed methanogenic reactor. The start-up phase for a 99% degradation of low concentrations of cyanide (10 mg/l) required about 6 months. After establishment of the biofilm, a cyanide concentration of up to 150 mg CN⁻/l in the fresh wastewater was degraded during anaerobic treatment at a hydraulic retention time of 3 days. All nitrogen from the degraded cyanide was converted to organic nitrogen by the biomass of the effluent. The cyanide-degrading biocoenosis of the anaerobic reactor could tolerate shock concentrations of cyanide up to 240 mg CN⁻/l for a short time. Up to 5 mmol/l NH₄Cl (i.e. 70 mg N/l = 265 mg NH₄Cl/l) in the fresh wastewater did not affect cyanide degradation. The bleaching agent sulphite, however, had a negative effect on COD and cyanide removal. For anaerobic treatment, the maximum COD space loading was 12 g l⁻¹ day⁻¹, equivalent to a hydraulic retention time of 1.8 days. The COD removal efficiency was around 90%. The maximum permanent cyanide space loading was 50 mg CN⁻ l⁻¹ day⁻¹, with tolerable shock loadings up to 75 mg CN⁻ l⁻¹day⁻¹. Under steady-state conditions, the cyanide concentration of the effluent was lower than 0.5 mg/l. Received: 15 August 1997 / Received revision: 10 October 1997 / Accepted: 14 October 1997     

Nitrification, denitrification, and dechlorination in bleached kraft pulp mill wastewater

This study deals with combining the biologi cal removal of organic halogens with the removal of nitrogen from bleached kraft pulp mill wastewater in fluidized-bed reactors under nitrifying and denitrifying conditions. Untreated and biotreated bleached kraft pulp mill wastewaters had no detrimental effect on nitrification or denitrification. The nitrifying biofilm reactor, pregrown on synthetic inorganic feed with ammonia, removed without a lag phase adsorbable organic halogens [7.2 mg Cl (g biomass volatile solids)⁻¹day⁻¹] from bleached kraft pulp mill wastewater and selected chlorophenols from synthetic wastewater. Electron microscopical examination of the biofilm showed that bacteria, morphologically similar to the nitrifying species Nitrosomonas or Nitrobacter, and Nitrosospira were dominant. The denitrifying fluidized-bed reactor, pregrown on nitrate and methanol, denitrified without a lag phase bleached kraft pulp mill wastewater. Under denitrifying conditions, 35% of the total organic carbon content of untreated bleached kraft pulp mill waste water was removed. The reducing power delivered by untreated bleached kraft pulp mill wastewater for denitrification was 2 mmol electrons/mmol carbon mineralized. Dechlorination under denitrifying conditions was negligible. Received: 21 November 1996 / Received revision: 27 January 1997 / Accepted: 1 February 1997     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Biogeochemistry of inter-reef sediments on the northern and central Great Barrier Reef

The deposition and cycling of carbon and nitrogen in carbonate sediments located between coral reefs on the northern and central sections of the Great Barrier Reef were examined. Rates of mass sediment accumulation ranged from 1.9 kg m⁻² year⁻¹ (inshore reefs) to 2.1–4.9 kg m⁻² year⁻¹ (between mid-shelf reefs); sedimentation was minimal off outer-shelf reefs. Rates of total organic carbon decomposition ranged from 1.7 to 11.4 mol C m⁻² year⁻¹ and total nitrogen mineralization ranged from 77 to 438 mmol N m⁻² year⁻¹, declining significantly with distance from land. Sediment organic matter was highly reactive, with mineralization efficiencies ranging from 81 to 99% for organic carbon and 64–100% for nitrogen, with little C and N burial. There was no evidence of carbonate dissolution/precipitation in short-term incubation experiments. Rates of sulfate reduction (range 0–3.4 mmol S m⁻² day⁻¹) and methane release (range 0–12.8 μmol CH₄ m⁻² day⁻¹) were minor or modest pathways of carbon decomposition. Aerobic respiration, estimated by difference between total O₂ consumption and the sum of the other pathways, accounted for 55–98% of total carbon mineralization. Rates of ammonification ranged from 150 to 1,725 μmol NH₄ m⁻² day⁻¹, sufficient to support high rates of denitrification (range 30–2,235 μmol N₂ m⁻² day⁻¹). N₂O release was not detected and rates of NH₄ ⁺ and NO₂ ⁻ + NO₃ ⁻ efflux were low, indicating that most mineralized N was denitrified. The percentage of total N input removed via denitrification averaged ≈75% (range 28–100%) with little regenerated N available for primary producers. Inter-reef environments are therefore significant sites of energy and nutrient flow, especially in spatially complex reef matrices such as the Great Barrier Reef.     

Energy metabolism of underwater swimming in river-otters (Lutra lutra L.)

We used a still-water swim channel in conjunction with open-flow oxygen and carbon dioxide respirometry to examine the energy requirements of river-otters (Lutra lutra L.) swimming voluntarily underwater in Neumünster Zoo (Germany). While at rest on land (5 °C), river-otters had a respiratory quotient of 0.77 and a resting metabolic rate of 4.1 W kg⁻¹. This increased to an estimated 6.4 W kg⁻¹ during rest in water (11–15 °C) and to 12.3 W kg⁻¹ when the animals were feeding in the channel. River-otters swimming under water preferred a mean speed of 0.89 m s⁻¹, and their energy requirements attained 11.6 W kg⁻¹. Cost of transport, however, was minimal at 1.3 m s⁻¹ and amounted to 0.95 J N⁻¹ m⁻¹. Accepted: 3 November 1997     

Biomass and metabolism of zooplankton in the Bransfield Strait (Antarctic Peninsula) during austral spring

Zooplankton biomass (as dry weight), respiration and ammonia excretion were studied in three different size classes (200–500, 500–1000 and >1000 μm) in the Bransfield Strait during December 1991. Average mesozooplankton biomass was 86.45 ± 56.74 mg · dry weight · m⁻², which is in the lower range of the values cited in the literature for polar waters. Higher biomass was observed in the Weddell water. The small size fraction accounted for about 50% of total biomass while the largest one represented 35%. Rather high metabolic rates were found, irrespective of whether the organisms were incubated in the presence of food. No significant differences were observed in mass specific respiration and ammonia excretion rates between different temperatures of incubation (0.2–2.3°C) and between the size classes studied. Because of the very low biomass values observed, the metabolic requirements of mesozooplankton during December represented a small fraction of the primary production. Accepted: 5 September 1998     

Toluene degradation in the recycle liquid of biotrickling filters for air pollution control

Pollutant degradation in biotrickling filters for waste air treatment is generally thought to occur only in the biofilm. In two experiments with toluene degrading biotrickling filters, we show that suspended microorganisms in the recycle liquid may substantially contribute to the overall pollutant removal. Two days after reactor start up, the overall toluene elimination capacity reached a maximum of 125 g m⁻³ h⁻¹, which was twice that found during prolonged operation. High biodegradation activity in the recycle liquid fully accounted for this short-term peak of pollutant elimination. During steady-state operation, the toluene degradation in the recycle liquid was 21% of the overall elimination capacity, although the amount of suspended biomass was only 1% of the amount of immobilized biomass. The results suggest that biotrickling filter performance may be improved by selecting operating conditions allowing for the development of an actively growing suspended culture. Received: 16 June 1999 / Received revision: 17 November 1999 / Accepted: 15 December 1999     

Quantification of water and biomass in small colony variant PAO1 biofilms by confocal Raman microspectroscopy

The Raman spectra, water content, and biomass density of wild-type (WT) Pseudomonas aeruginosa PAO1, small colony variant (SCV) PAO1, and Pseudoalteromonas sp. NCIMB 2021 biofilms were compared in order to determine their variation with strain and species. Living, fully submerged biofilms were analyzed in situ by confocal Raman microspectroscopy for up to 2 weeks. Water to biomass ratios (W/BRs), which are the ratios of the O–H stretching vibration of water at 3,450 cm⁻¹ to the C–H stretching band characteristic of biomass at 2,950 cm⁻¹, were used to estimate the biomass density and cell density by comparison with W/BRs of protein solutions and bacterial suspensions, respectively, on calibration curves. The hydration within SCV biofilm colonies was extremely heterogeneous whereas W/BRs were generally constant in young WT biofilm colonies. The mean biomass in biofilm colonies of WT or colony cores of SCV was typically equivalent to 16% to 27% protein (w/v), but was 10% or less for NCIMB 2021. The corresponding cell densities were 7.5 to >10 × 10¹⁰ cfu mL⁻¹ for SCV, while the maximum cell density for NCIMB biofilms was 2.8 × 10¹⁰ cfu mL⁻¹.     

Contribution of root to soil respiration and carbon balance in disturbed and undisturbed grassland communities, northeast China

Changes in the composition of plant species induced by grassland degradation may alter soil respiration rates and decrease carbon sequestration; however, few studies in this area have been conducted. We used net primary productivity (NPP), microbial biomass carbon (MBC), and soil organic carbon (SOC) to examine the changes in soil respiration and carbon balance in two Chinese temperate grassland communities dominated by Leymus chinensis (undisturbed community; Community 1) and Puccinellia tenuiflora (degraded community; Community 2), respectively. Soil respiration varied from 2.5 to 11.9 g CO₂ m⁻² d⁻¹ and from 1.5 to 9.3 g CO₂ m⁻² d⁻¹, and the contribution of root respiration to total soil respiration from 38% to 76% and from 25% to 72% in Communities 1 and 2, respectively. During the growing season (May–September), soil respiration, shoot biomass, live root biomass, MBC and SOC in Community 2 decreased by 28%, 39%, 45%, 55% and 29%, respectively, compared to those in Community 1. The considerably lower net ecosystem productivity in Community 2 than in Community 1 (104.56 vs. 224.73 g C m⁻² yr⁻¹) suggests that the degradation has significantly decreased carbon sequestration of the ecosystems.     

Seasonal variation in nutrient limitation of microbial biofilms colonizing organic and inorganic substrata in streams

Humans have increased the availability of nutrients including nitrogen and phosphorus worldwide; therefore, understanding how microbes process nutrients is critical for environmental conservation. We examined nutrient limitation of biofilms colonizing inorganic (fritted glass) and organic (cellulose sponge) substrata in spring, summer, and autumn in three streams in Michigan, USA. Biofilms were enriched with nitrate (NO₃ ⁻), phosphate (PO₄ ³⁻), ammonium (NH₄ ⁺), NO₃ ⁻ + PO₄ ³⁻, NH₄ ⁺ + PO₄ ³⁻, or none (control). We quantified biofilm structure and function as chlorophyll a (i.e., primary producer biomass) and community respiration on all substrata. In one stream, we characterized bacterial and fungal communities on cellulose in autumn using clone library sequencing and denaturing gradient gel electrophoresis to determine if community structure was linked to nutrient limitation status. Despite oligotrophic conditions, primary producer biomass was infrequently nutrient limited. In contrast, respiration on organic substrata was frequently limited by N + P combinations. We found no difference between biofilm response to NH₄ ⁺ versus NO₃ ⁻ enrichment, although the response to both N-species was positively related to water column PO₄ ³⁻ concentrations and temperature. Molecular analysis for fungal community composition suggested no relationship to nutrient limitation, but the dominant members of the bacterial community on cellulose were different on NO₃ ⁻, PO₄³, and NO₃ ⁻ + PO₄ ³⁻ treatments relative to control, NH₄ ⁺, and NH₄ ⁺ + PO₄ ³⁻ treatments, which matched patterns for biofilm respiration rates from each treatment. Our results show discrete patterns of nutrient limitation dependent upon substratum type and season, and imply changes in bacterial community structure and function may be linked following nutrient enrichment in streams.