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.     

Greenhouse gas fluxes from the eutrophic Temmesjoki River and its Estuary in the Liminganlahti Bay (the Baltic Sea)

We studied concentrations of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in the eutrophic Temmesjoki River and Estuary in the Liminganlahti Bay in 2003–2004 and evaluated the atmospheric fluxes of the gases based on measured concentrations, wind speeds and water current velocities. The Temmesjoki River was a source of CO₂, CH₄ and N₂O to the atmosphere, whereas the Liminganlahti Bay was a minor source of CH₄ and a minor source or a sink of CO₂ and N₂O. The results show that the fluxes of greenhouse gases in river ecosystems are highly related to the land use in its catchment areas. The most upstream river site, surrounded by forests and drained peatlands, released significant amounts of CO₂ and CH₄, with average fluxes of 5,400 mg CO₂–C m⁻² d⁻¹ and 66 mg CH₄–C m⁻² d⁻¹, and concentrations of 210 μM and 345 nM, respectively, but N₂O concentrations, at an average of 17 nM, were close to the atmospheric equilibrium concentration. The downstream river sites surrounded by agricultural soils released significant amounts of N₂O (with an average emission of 650 μg N₂O–N m⁻² d⁻¹ and concentration of 22 nM), whereas the CO₂ and CH₄ concentrations were low compared to the upstream site (55 μM and 350 nM). In boreal regions, rivers are partly ice-covered in wintertime (approximately 5 months). A large part of the gases, i.e. 58% of CO₂, 55% of CH₄ and 36% of N₂O emissions, were found to be released during wintertime from unfrozen parts of the river.     

Production of Dissolved Organic Carbon in Canadian Forest Soils

To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC concentration in the leachate and carbon dioxide (CO₂) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils (<2 to >50 mg l⁻¹). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g⁻¹ soil C day⁻¹, falling to averages of 0.01 mg g⁻¹ soil C day⁻¹; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from less than 0.01 to 0.12 mg g⁻¹ soil C day⁻¹ (0.5–47.6 mg g⁻¹ soil C), with an average of 0.021 mg g⁻¹ soil C day⁻¹ (8.2 mg g⁻¹ soil C). DOC production rate was weakly related to temperature, equivalent to Q₁₀ values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO₂–C and DOC showed a quotient (CO₂–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO₂–C:DOC production (log₁₀ transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion of CO₂ is produced. The CO₂–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO₂–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO₂–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples.     

Abiotic reaction of nitrite with dissolved organic carbon? Testing the Ferrous Wheel Hypothesis

The Ferrous Wheel Hypothesis (Davidson et al. 2003) postulates the abiotic formation of dissolved organic N (DON) in forest floors, by the fast reaction of NO₂ ⁻ with dissolved organic C (DOC). We investigated the abiotic reaction of NO₂ ⁻ with dissolved organic matter extracted from six different forest floors under oxic conditions. Solutions differed in DOC concentrations (15–60 mg L⁻¹), NO₂ ⁻ concentrations (0, 2, 20 mg NO₂ ⁻-N L⁻¹) and DOC/DON ratio (13.4–25.4). Concentrations of added NO₂ ⁻ never decreased within 60 min, therefore, no DON formation from added NO₂ ⁻ took place in any of the samples. Our results suggest that the reaction of NO₂ ⁻ with natural DOC in forest floors is rather unlikely.     

Origin and cycling of riverine inorganic carbon in the Sava River watershed (Slovenia) inferred from major solutes and stable carbon isotopes

The Sava River and its tributaries in Slovenia represent waters strongly influenced by chemical weathering of limestone and dolomite. The carbon isotopic compositions of dissolved inorganic carbon (DIC) and suspended organic carbon (POC) fractions as well as major solute concentrations yielded insights into the origin and fluxes of carbon in the upper Sava River system. The major solute composition was dominated by carbonic acid dissolution of calcite and dolomite. Waters were generally supersaturated with respect to calcite, and dissolved CO₂ was about fivefold supersaturated relative to the atmosphere. The δ¹³C of DIC ranged from −13.5 to −3.3‰. Mass balances for riverine inorganic carbon suggest that carbonate dissolution contributes up to 26%, degradation of organic matter ∼17% and exchange with atmospheric CO₂ up to 5%. The concentration and stable isotope diffusion models indicated that atmospheric exchange of CO₂ predominates in streams draining impermeable shales and clays while in the carbonate-dominated watersheds dissolution of the Mesozoic carbonates predominates.     

Denitrification efficiency for defining critical loads of carbon in shallow coastal ecosystems

Denitrification efficiency [DE; (N₂ − N/(DIN + N₂ − N) × 100%)] as an indicator of change associated with nutrient over-enrichment was evaluated for 22 shallow coastal ecosystems in Australia. The rate of carbon decomposition (which can be considered a proxy for carbon loading) is an important control on the efficiency with which coastal sediments in depositional mud basins with low water column nitrate concentrations recycle nitrogen as N₂. The relationship between DE and carbon loading is due to changes in carbon and nitrate (NO₃) supply associated with sediment biocomplexity. At the DE optimum (500–1,000 μmol m⁻² h⁻¹), there is an overlap of aerobic and anaerobic respiration zones (caused primarily by the existence of anaerobic micro-niches within the oxic zone, and oxidized burrow structures penetrating into the anaerobic zone), which enhances denitrification by improving both the organic carbon and nitrate supply to denitrifiers. On either side of the DE optimum zone, there is a reduction in denitrification sites as the sediment loses its three-dimensional complexity. At low organic carbon loadings, a thick oxic zone with low macrofauna biomass exists, resulting in limited anoxic sites for denitrification, and at high carbon loadings, there is a thick anoxic zone and a resultant lack of oxygen for nitrification and associated NO₃ production. We propose a trophic scheme for defining critical (sustainable) carbon loading rates and possible thresholds for shallow coastal ecosystems based on the relationship between denitrification efficiency and carbon loading for 17 of the 22 Australian coastal ecosystems. The denitrification efficiency “optimum” occurs between carbon loadings of about 50 and 100 g C m⁻² year⁻¹. Coastal managers can use this simple trophic scheme to classify the current state of their shallow coastal ecosystems and for determining what carbon loading rate is necessary to achieve any future state. Guest editors: J. H. Andersen & D. J. Conley Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark     

Porewater Stoichiometry of Terminal Metabolic Products, Sulfate, and Dissolved Organic Carbon and Nitrogen in Estuarine Intertidal Creek-bank Sediments

Porewater equilibration samplers were used to obtain porewater inventories of inorganic nutrients (NH₄⁺, NO_x, PO₄³⁻), dissolved organic carbon (DOC) and nitrogen (DON), sulfate (SO₄²⁻), dissolved inorganic carbon (DIC), hydrogen sulfide (H₂S), chloride (Cl⁻), methane (CH₄) and reduced iron (Fe²⁺) in intertidal creek-bank sediments at eight sites in three estuarine systems over a range of salinities and seasons. Sulfate reduction (SR) rates and sediment particulate organic carbon (POC) and nitrogen (PON) were also determined at several of the sites. Four sites in the Okatee River estuary in South Carolina, two sites on Sapelo Island, Georgia and one site in White Oak Creek, Georgia appeared to be relatively pristine. The eighth site in Umbrella Creek, Georgia was directly adjacent to a small residential development employing septic systems to handle household waste. The large data set (>700 porewater profiles) offers an opportunity to assess system-scale patterns of porewater biogeochemical dynamics with an emphasis on DOC and DON distributions. SO₄²⁻ depletion (SO₄²⁻)_Dep was used as a proxy for SR, and (SO₄²⁻)_Dep patterns agreed with measured (³⁵S) patterns of SR. There were significant system-scale correlations between the inorganic products of terminal metabolism (DIC, NH₄⁺ and PO₄³⁻) and (SO₄²⁻)_Dep, and SR appeared to be the dominant terminal carbon oxidation pathway in these sediments. Porewater inventories of DIC and (SO₄²⁻)_Dep indicate a 2:1 stoichiometry across sites, and the C:N ratio of the organic matter undergoing mineralization was between 7.5 and 10. The data suggest that septic-derived dissolved organic matter with a C:N ratio below 6 fueled microbial metabolism and SR at a site with development in the upland. Seasonality was observed in the porewater inventories, but temperature alone did not adequately describe the patterns of (SO₄²⁻)_Dep, terminal metabolic products (DIC, NH₄⁺, PO₄³⁻), DOC and DON, and SR observed in this study. It appears that production and consumption of labile DOC are tightly coupled in these sediments, and that bulk DOC is likely a recalcitrant pool. Preferential hydrolysis of PON relative to POC when overall organic matter mineralization rates were high appears to drive the observed patterns in POC:PON, DOC:DON and DIC:DIN ratios. These data, along with the weak seasonal patterns of SR and organic and inorganic porewater inventories, suggest that the rate of hydrolysis limits organic matter mineralization in these intertidal creek-bank sediments.     

Nitrous Oxide Emission from Grazed Grassland Under Different Management Systems

Nitrous oxide (N₂O) emissions from grazed grasslands are estimated to be approximately 28% of global anthropogenic N₂O emissions. Estimating the N₂O flux from grassland soils is difficult because of its episodic nature. This study aimed to quantify the N₂O emissions, the annual N₂O flux and the emission factor (EF), and also to investigate the influence of environmental and soil variables controlling N₂O emissions from grazed grassland. Nitrous oxide emissions were measured using static chambers at eight different grasslands in the South of Ireland from September 2007 to August 2009. The instantaneous N₂O flux values ranged from -186 to 885.6 μg N₂O-N m⁻² h⁻¹ and the annual sum ranged from 2 ± 3.51 to 12.55 ± 2.83 kg N₂O-N ha⁻¹ y⁻¹ for managed sites. The emission factor ranged from 1.3 to 3.4%. The overall EF of 1.81% is about 69% higher than the Intergovernmental Panel on Climate Change (IPCC) default EF value of 1.25% which is currently used by the Irish Environmental Protection Agency (EPA) to estimate N₂O emission in Ireland. At an N applied of approximately 300 kg ha⁻¹ y⁻¹, the N₂O emissions are approximately 5.0 kg N₂O-N ha⁻¹ y⁻¹, whereas the N₂O emissions double to approximately 10 kg N ha⁻¹ for an N applied of 400 kg N ha⁻¹ y⁻¹. The sites with higher fluxes were associated with intensive N-input and frequent cattle grazing. The N₂O flux at 17°C was five times greater than that at 5°C. Similarly, the N₂O emissions increased with increasing water filled pore space (WFPS) with maximum N₂O emissions occurring at 60–80% WFPS. We conclude that N application below 300 kg ha⁻¹ y⁻¹ and restricted grazing on seasonally wet soils will reduce N₂O emissions.     

Environmental chemistry of rivers and lakes, part v: a comparative study of the chemical and physicochemical characteristics of organic carbon dissolved in river and lake waters

To determine the chemical and physicochemical characteristics of dissolved organic carbon in the Ado River and the Yasu River, the main rivers flowing into Lake Biwa, the adsorption behavior onto hydrous iron oxide (HIO) and the reactivity to KMnO₄ oxidant were investigated in parallel with measurement of the distribution profiles of dissolved organic carbon (DOC) along the rivers. In one year of observation at the mouths of the two rivers, DOC concentrations were found to vary in the Ado over the range 0.28–1.21 mg C l⁻¹ and in the Yasu over the range 1.01–2.68 mg C l⁻¹. Act-DOC, one of the fractions separated from the total DOC by its adsorption-active character onto HIO at pH 4, was thought primarily to control the variation of total DOC, as in Lake Biwa. The int-DOC, another fraction separated by its adsorption-inert or -inactive character onto HIO, remained at almost a steady value around 0.18 ± 0.07 mg C l⁻¹ in the Ado, which was lower than that (0.35 ± 0.05 mg C l⁻¹) in Lake Biwa. The act-DOC in river waters was reactive to KMnO₄ oxidant, showing a linear relation with the amount of permanganate consumed for the reaction (chemical oxygen demand: COD). In river waters, the relation can be approximated by a straight line expressed as COD (mg O₂ l⁻¹) = 0.64 × act-DOC (mg C l⁻¹) − 0.02. In contrast, in the lake water the relation was COD (mg O₂ l⁻¹) = 0.97 × act-DOC (mg C l⁻¹) − 0.50. Received: March 3, 1999 / Accepted: December 2, 1999     

Nitrogen release from surface sand of a high energy beach along the southeastern coast of North Carolina,USA

This study examined changes in dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) in coastal seawater after exposure to sand along a high energy beach face over an annual cycle between April 2004 and July 2005. Dissolved organic nitrogen, NO₃ ⁻, and NH₄ ⁺ were released from sand to seawater in laboratory incubation experiments clearly demonstrating that they are a potential source of N to underlying groundwater or coastal seawater. DON increases in seawater, after exposure to surface sands in laboratory experiments, were positively correlated with in situ water column DON concentrations measured at the same time as sand collection. Increase in NO₃ ⁻ and NH₄ ⁺ were not correlated with their in situ concentrations. This suggests that DON released from beach sands is relatively more recalcitrant while NO₃ ⁻ and NH₄ ⁺ are utilized rapidly in the coastal ocean. The release of N was seasonal with carbon to nitrogen ratios indicating that recent primary productivity was responsible for the largest fluxes in summer while more degraded humic material contributed to lower fluxes in winter. Fluxes of total dissolved nitrogen (DON and DIN) from surface sand (2.1 × 10⁻⁴ mol m⁻² h⁻¹) were similar to that of groundwater and more than an order of magnitude larger than rain deposition indicating the potential importance of surface sand derived nitrogen to the coastal zone with a corresponding impact on primary productivity.     

Factors Controlling Sediment Denitrification in Midwestern Streams of Varying Land Use

We investigated controls on stream sediment denitrification in nine headwater streams in the Kalamazoo River Watershed, Michigan, USA. Factors influencing denitrification were determined by using experimental assays based on the chloramphenicol-amended acetylene inhibition technique. Using a coring technique, we found that sediment denitrification was highest in the top 5 cm of the benthos and was positively related to sediment organic content. To determine the effect of overlying water quality on sediment denitrification, first-order stream sediments were assayed with water from second- and third-order downstream reaches, and often showed higher denitrification rates relative to assays using site-specific water from the first-order stream reach. Denitrification was positively related to nitrate (NO₃ ⁻) concentration, suggesting that sediments may have been nutrient-limited. Using stream-incubated inorganic substrata of varying size classes, we found that finer-grained sand showed higher rates of denitrification compared to large pebbles, likely due to increased surface area per volume of substratum. Denitrification was measurable on both inorganic substrata and fine particulate organic matter loosely associated with inorganic particles, and denitrification rates were related to organic content. Using nutrient-amended denitrification assays, we found that sediment denitrification was limited by NO₃ ⁻ or dissolved organic carbon (DOC, as dextrose) variably throughout the year. The frequency and type of limitation differed with land use in the watershed: forested streams were NO₃ ⁻-limited or co-limited by both NO₃ ⁻ and DOC 92% of the time, urban streams were more often NO₃ ⁻-limited than DOC-limited, whereas agricultural stream sediments were DOC-limited or co-limited but not frequently limited by NO₃ ⁻ alone.     

Stream ecosystem properties and processes along a temperature gradient

We surveyed macrophyte community structure and measured community metabolism and nutrient uptake along a temperature gradient (9.7–17.4°C) in four Icelandic streams influenced by geothermal heating. The study streams are part of the geothermal area in Hengill that is uniquely characterised by streams with comparable water chemistry despite the geothermal influence. Stream metabolism was studied applying the diurnal upstream–downstream dissolved oxygen change technique. Nutrient uptake was studied by adding solutions of nitrogen and phosphorus together with a conservative tracer. Rates of primary production (GPP) and uptake of nitrate–N and phosphate-P increased with increasing stream temperature. GPP was 20 times higher (up to 12.99 g O₂ m⁻² day⁻¹) and rates of nutrient uptake were up to 30-times higher (up to 22.99, 13.31 and 7.94 mg m⁻² h⁻¹ for ammonium, nitrate and phosphate, respectively) in the warmest streams compared with the coldest. Furthermore, macrophytes, when present, were strongly controlling ecosystem processes. Our study implies that temperature may affect stream ecosystem processes both directly (i.e. physiologically) and indirectly (i.e. by changing other structural parameters).     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Estimation of carbon storage, carbon inputs, and soil CO2 efflux of alder plantations on granite soil in central Korea: comparison with Japanese larch plantation

Alder is a typical species used for forest rehabilitation after disturbances because of its N₂-fixing activities through microbes. To investigate forest dynamics of the carbon budget, we determined the aboveground and soil carbon content, carbon input by litterfall to belowground, and soil CO₂ efflux over 2 years in 38-year-old alder plantations in central Korea. The estimated aboveground carbon storage and increment were 47.39 Mg C ha⁻¹ and 2.17 Mg C ha⁻¹ year⁻¹. Carbon storage in the organic layer and in mineral soil in the topsoil to 30 cm depth were, respectively, 3.21 and 66.85 Mg C ha⁻¹. Annual carbon input by leaves and total litter in the study stand were, respectively, 1.78 and 2.68 Mg C ha⁻¹ year⁻¹. The aboveground carbon increment at this stand was similar to the annual carbon inputs by total litterfall. The diurnal pattern of soil CO₂ efflux was significantly different in May, August, and October, typically varying approximately twofold throughout the course of a day. In the seasonally observed pattern, soil CO₂ efflux varied strongly with soil temperature; increasing trends were evident during the early growing season, with sustained high rates from mid May through late October. Soil CO₂ efflux was related exponentially to soil temperature (R ² = 0.85, P < 0.0001), but not to soil water content. The Q ₁₀ value for this plantation was 3.8, and annual soil respiration was estimated at 10.2 Mg C ha⁻¹ year⁻¹. An erratum to this article can be found at     

Reverse process of coprecipitation of dissolved organic carbon with Fe(III) precipitates in a lake

Reverse process of the coprecipitation of dissolved organic matter with Fe(III) precipitates in a lake is reported. Water containing a slight amount of dissolved oxygen from the hypolimnion of Lake Onogawa was sealed in glass bottles, and some changes in the constituents with time were followed. The water sample contained 0.1 mg l⁻¹ dissolved oxygen at the beginning of the experiment, which decreased to 0.0 mg l⁻¹ within 24 h. In response to the depletion of dissolved oxygen, there were increases in dissolved Fe from 8.4 to 11.4 mg l⁻¹ and dissolved organic carbon (DOC) from 5.1 to 6.9 mg l⁻¹ after 72 h. At the beginning of the experiment, more than 2 mg l⁻¹ of insoluble Fe, which was thought to be Fe(III) precipitates, existed in the water samples. When the water samples became anoxic, the preexisted Fe(III) must have been reduced to soluble Fe(II) over time, resulting in the increase of dissolved Fe. Simultaneous with Fe(III) reduction, coprecipitated DOC with Fe(III) must have been released. The reverse process of the coprecipitation of dissolved organic matter with Fe(III) precipitates found in the present study strongly supports in situ coprecipitation of dissolved organic matter with Fe(III) precipitates. Contribution No. 27 from the Urabandai Limnological Station, Yamagata University.     

Fluxes of greenhouse gases from Andosols under coffee in monoculture or shaded by <Emphasis Type="Italic">Inga densiflora</Emphasis> in Costa Rica

The objective of this study was to evaluate the effect of N fertilization and the presence of N₂ fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha⁻¹ year⁻¹) coffee cultivation: a monoculture (CM) and a culture shaded by the N₂ fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N₂O emissions with 84% of the annual N₂O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH₄ uptakes. The higher annual N₂O emissions from the shaded plantation (5.8 ± 0.3 kg N ha⁻¹ year⁻¹) when compared to that from the monoculture (4.3 ± 0.1 kg N ha⁻¹ year⁻¹) was related to the higher N input through litterfall (246 ± 16 kg N ha⁻¹ year⁻¹) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg⁻¹ d.w. d⁻¹) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha⁻¹ year⁻¹ and 2.2 ± 0.2 mg N kg⁻¹ d.w. d⁻¹). This confirms that the presence of N₂ fixing shade trees can increase N₂O emissions. Annual CO₂ and CH₄ fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO₂ ha⁻¹ year⁻¹, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH₄ ha⁻¹ year⁻¹, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season.     

Soil-Atmosphere Exchange of N<Subscript>2</Subscript>O and NO in Near-Natural Savanna and Agricultural Land in Burkina Faso (W. Africa)

In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N₂O and NO exchange. N₂O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental sites, that is, a natural savanna site and four agricultural sites planted with sorghum (n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N₂O exchange mostly fluctuated between −2 and 8 μg N₂O–N m⁻² h⁻¹, peak N₂O emissions of 10–35 μg N₂O–N m⁻² h⁻¹ during the second half of June 2005, and up to 150 μg N₂O–N m⁻² h⁻¹ at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event on N₂O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha⁻¹; plot B: 52.5 kg N ha⁻¹; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N₂O emissions at the two fertilized plots was detected. After 24 days, however, N₂O emission rates increased exponentially at plot A up to a mean of 80 μg N₂O–N m⁻² h⁻¹, whereas daily mean values at plot B reached only 19 μg N₂O–N m⁻² h⁻¹, whereas N₂O flux rates at plot C remained unchanged. The calculated annual N₂O emission of the nature reserve site amounted to 0.52 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and to 0.67 kg N₂O–N ha⁻¹ a⁻¹ in 2006, whereas the calculated average annual N₂O release of the crop sites was only 0.19 kg N₂O–N ha⁻¹ a⁻¹ and 0.20 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and 2006, respectively. In a laboratory study, potential N₂O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water content with subsequent drying caused the highest increase in N₂O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3–4 days. A weaker stimulation of N₂O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect of single or daily wetting to high soil water content (>67% WHC_max) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions is smaller—with the caveat that there could have been potentially higher N₂O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely increase in nitrogen fertilization in the future.     

An upflow microaerobic sludge blanket reactor operating at high organic loading and low dissolved oxygen levels

The activated sludge process (ASP) has high operational costs due to the need for aeration at dissolved O₂ (DO) levels of ≥2 mg l⁻¹ and high capital costs to construct large reactors due to a low organic loading [typically 1 kg chemical oxygen demand (COD) m⁻³ day⁻¹]. A novel method for improving the energy use and treatment efficiency of the ASP via limited oxygenation (0.4 mg DO l⁻¹) and high organic loading (6.2 kg COD m⁻³ day⁻¹) is proposed based on a laboratory-scale ASP for ammonia-rich industrial wastewaters. The sludge blanket phenomenon and granulation occurred simultaneously in the upflow microaerobic reactor.     

Studies on non-symbiotic diazotrophic bacterial populations of coastal arable saline soils of India

The effect of fluctuations of salinity in three different seasons on diazotrophic populations and N₂ fixation in six mono cropped rice field soils of the coastal region of the Gangetic delta of West Bengal, India, was studied. The average pH, ECe, organic carbon and total nitrogen of the soils ranged from 4.99–7.08, 2.02–19.58 dSm⁻¹, 4.68–12.03 g kg⁻¹ and 0.44–1.70 g kg ⁻¹, respectively. The average log colony forming units of the bacterial populations and N₂-fixation in the soils varied from 4.61 to 5.86 and 2.74 to 4.52 mg N₂ fixed 50 ml ⁻¹ culture media respectively, with the lowest value recorded in summer. Recovery of microorganisms and N₂- fixation gradually decreased with extraneous addition of NaCl in the culture media. All the eight isolates were Gram positive, spore and capsule formers. They could utilize glucose, sucrose, mannitol, starch, citrate and nitrate, and were catalase and gelatinase positive, but indole, methyl red and Vogues Proskauer reaction negative. The organisms produced alkaline reaction on TSI agar slant. The acetylene reduction assay of the isolates at 0 and 1% NaCl in the culture media were 4.51–164.52 and 1.72–100.6 nmole C₂H₄ ml⁻¹ culture media in 72 h, respectively. The isolates could fix 2.42–4.45 and 2.04–4.08 mg N₂ fixed 50 ml⁻¹ culture media at 0 and 1% NaCl in the culture media respectively. 16S rDNA sequences of the isolates were similar to the species: Bacillus sp. isolate 28A, Bacillus sp. MOLA 87, Bacillus sp. By113 (B)Ydz-dh, Bacillus sp. PN13, Bacillus licheniformis strain RH101, Bacterium Antarctica 14, Bacillus sp. PN13 and Bacillus megaterium.     

Large Loss of Dissolved Organic Nitrogen from Nitrogen-Saturated Forests in Subtropical China

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO₃⁻) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha⁻¹ y⁻¹ as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha⁻¹ y⁻¹). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha⁻¹ y⁻¹. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO₃⁻ leaching in all three forests. We hypothesize that abiotic incorporation of excess NO₃⁻ (through chemically reactive NO₂⁻) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. Dr. YT Fang performed research, analyzed data, and wrote the paper; Prof. WX Zhu participated in the initial experimental design, analyzed data, and took part in writing the paper; Prof. P Gundersen conceived the study and took part in writing; Prof. JM Mo and Prof. GY Zhou conceived study; Prof. M Yoh analyzed part of the data and contributed to the development of DON model.