Heterotrophic Microbial Activity Research Articles

Empirical evidence that soil carbon formation from plant inputs is positively related to microbial growth

Plant-carbon inputs to soils in the form of dissolved sugars, organic acids and amino acids fuel much of heterotrophic microbial activity belowground. Initial residence times of these compounds in the soil solution are on the order of hours, with microbial uptake a primary removal mechanism. Through microbial biosynthesis, the dissolved compounds become dominant precursors for formation of stable soil organic carbon. How the chemical class (e.g. sugar) of a dissolved compound influences stabilization in field soils is unknown and predictions from our understanding of microbial metabolism, turnover and identity are contradictory. We show that soil carbon formation, from chronic amendments of dissolved compounds to fertilized and unfertilized grasslands, is 2.4-times greater from a sugar than an amino acid. Formation rates are negatively correlated with respiration rates of the compounds, and positively correlated with their recovery in microbial biomass. These relationships suggest that the efficiency of microbial growth on a compound is positively related to formation rates of soil organic carbon. Fertilization does not alter these findings, but together nitrogen and phosphorus additions reduce soil carbon formation. Our results highlight the need to consider both nutrient enrichment and global-change induced shifts in the form of dissolved root inputs to soils to predict future soil carbon stocks and hence phenomena such as climate warming and food security to which these stock sizes are intimately tied.

The role of microbial adaptation and biodegradable dissolved organic carbon on the attenuation of trace organic chemicals during groundwater recharge

It is widely recognized that efficient biological attenuation of bulk organic matter and trace organic chemicals (TOrC) can occur in managed aquifer recharge (MAR) systems receiving reclaimed water. The heterotrophic microbial activity in these subsurface systems is a function of the availability of biodegradable dissolved organic carbon (BDOC) present in reclaimed water. This study examined the influence of environmental factors, such as BDOC-rich (>1.6mg/L) and BDOC-starving (<1mg/L) conditions and microbial adaptation, on the attenuation of TOrC, including clofibric acid, dichlorprop, gemfibrozil, ibuprofen, ketoprofen, mecoprop, and naproxen, within soil-columns mimicking groundwater recharge. Under conditions that were characterized by a lack of BDOC and a biocommunity that was not yet adapted to these conditions, attenuation of biodegradable TOrC was less than 15%. After a three-month adaptation period, biotransformation increased to more than 80% for the biodegradable TOrC. This suggests that adaptation likely initiates enzyme expressions that eventually results in TOrC transformations even under seemingly less favorable conditions (i.e., lack of biodegradable carbon). For both non-adapted (stressed) and adapted conditions in the presence of higher concentrations of BDOC and travel times of 7days, the degree of biotransformation was variable across compounds but generally exceeded 25%. This suggests that BDOC above a minimum level (>1.6mg/L) can provide favorable microbial conditions resulting in TOrC removal, even for non-adapted systems. However, it is noteworthy that adapted MAR systems that were fed with low BDOC levels performed similarly or better with respect to TOrC biotransformation than systems that received BDOC levels above 1.6mg/L. These findings are important for field-scale applications. They suggest that MAR facilities that are microbiologically active and are fed with highly treated water with effluent concentrations of less than 1mg/L (i.e., nanofiltration permeate) can still attenuate biodegradable TOrC.

Microbial activity in deep marine sediments: does pressure make the difference?

We attempted to evaluate the effects of high hydrostatic pressure on microbial heterotrophic activity in deep marine sediments from the Atlantic Ocean. We investigated the potential respiration rates (acetate/glucose oxidation to CO2) in oxic sediments recovered from up to ∼4500 m water depth. Incubations were performed at ambient pressure and at near in situ pressure (∼40-45 MPa) with sediments stored at ambient pressure and at in situ pressure. Potential respiration rates in sediments stored at ambient pressure were lower when measured at in situ pressure than when measured at ambient pressure, independently of the substrate used. It appears that the pressure of storage is critical since potential respiration rates of sediments stored at in situ pressure were higher than in the counterpart sediments stored at ambient pressure.

Effect of bacterial mineralization of phytoplankton-derived phytodetritus on the release of arsenic, cobalt and manganese from muddy sediments in the Southern North Sea. A microcosm study

Muddy sediments of the Belgian Continental Zone (BCZ) are contaminated by metals such as Co, As, Cd, Pb, and Ni. Previous studies have suggested that mineralization of phytodetritus accumulating each year on sediments might cause secondary contaminations of the overlying seawater (metal effluxes). The aim of the present research was to investigate these effluxes using a microcosm approach. Muddy sediments were placed in microcosms (diameter: 15cm) and overlaid by phytodetritus (a mix of Phaeocystis globosa with the diatom Skeletonema costatum). The final suspension was 130.6mgL−1 (dw) and the final chlorophyll a content was 750±35μgL−1 (mean±SD). Natural seawater was used for controls. Microcosms were then incubated in the dark at 15°C during 7days. Metals were monitored in overlying waters and microbial communities were followed using bacterial and nanoflagellate DAPI counts, thymidine incorporation, community level physiological profiling (CLPP) and fluorescein diacetate analysis (FDA). Benthic effluxes observed in sediments exposed to phytodetritus were always more elevated than those observed in controls. Large effluxes were observed for Mn, Co and As, reaching 1084nmolm−2day−1 (As), 512nmolm−2day−1 (Co), and 755μmolm−2day−1 (Mn). A clear link was established between heterotrophic microbial activity and metal effluxes. The onset of mineralization was very fast and started within 2h of deposition as revealed by CLPP. An increased bacterial production was observed after two days (8.7mgCm−2day−2) and the bacterial biomass appeared controlled by heterotrophic nanoflagellates. Calculations suggest that during phytoplankton blooms the microbial activity alone may release substantial amounts of dissolved arsenic in areas of the BCZ covered by muddy sediments.

Nutrient speciation and hydrography in two anchialine caves in Croatia: tools to understand iodine speciation

Despite iodine being one of the most abundant of the minor elements in oxic seawater, the principal processes controlling its interconversion from iodate to iodide and vice versa, are still either elusive or largely unknown. The two major hypotheses for iodate reduction involve either phytoplankton growth in primary production, or bacteria during regeneration. An earlier study intended to exploit the unusual nature of anchialine environments revealed that iodide is oxidised to iodate in the bottom of such caves, whereas reduction of iodate occurs in the shallower parts of the water column. This investigation was made on the hypothesis that study of the nitrogen and phosphorus nutrient systems within the caves might offer a bridge between the iodine chemistry and the marine bacteria which are assumed to be the agent of change of the iodine in the caves. Accordingly, the hydrography, the nutrient chemistry, and some further iodine studies were made of two anchialine caves on the east coast of the Adriatic Sea in Croatia. Iodate and iodide were determined by differential pulse voltammetry and cathodic stripping square-wave voltammetry, respectively. Total iodine was determined indirectly, as iodate, after oxidation of reduced iodine species with UV irradiation and strong chemical oxidants. Nutrient concentrations were measured by spectrophotometry. Nutrient profiles within the well stratified water columns indicate a relatively short-lived surface source of nitrate and phosphate to the caves, with a more conventional, mid-water, nutrient regeneration system. The latter involves nitrite and ammonium at the bottom of the halocline, suggestive of both autotrophic and heterotrophic microbial activity. High iodate/low iodide deep water, and conservative behaviour of total inorganic iodine were confirmed in both systems. Iodate is reduced to iodide in the hypoxic region where nutrient regeneration occurs. The concentrations of organic iodine were surprisingly high in both systems, generally increasing toward the surface, where it comprised almost 80% of total iodine. As with alkalinity and silica, the results suggest that this refractive iodine component is liberated during dissolution of the surrounding karst rock. A major, natural flushing of one of the caves with fresh water was confirmed, showing that the cave systems offer the opportunity to re-start investigations periodically.

Linking microbial heterotrophic activity and sediment lithology in oxic, oligotrophic sub-seafloor sediments of the North Atlantic Ocean

Microbial heterotrophic activity was investigated in oxic sub-seafloor sediments at North Pond, a sediment pond situated at 23°N on the western flank of the Mid-Atlantic Ridge. The North Pond sediments underlie the oligotrophic North Atlantic Gyre at 4580-m water depth and cover a 7–8 million-year-old basaltic crust aquifer through which seawater flows. Discrete samples for experimentation were obtained from up to ~9 m-long gravity cores taken at 14 stations in the North Pond area. Potential respiration rates were determined in sediment slurries incubated under aerobic conditions with 14C-acetate. Microbial heterotrophic activity, as defined by oxidation of acetate to CO2 (with O2 as electron acceptor), was detected in all 14 stations and all depths sampled. Potential respiration rates were generally low (<0.2 nmol of respired acetate cm−3 d−1) in the sediment, but indicate that microbial heterotrophic activity occurs in deep-sea, oxic, sub-seafloor sediments. Furthermore, discernable differences in activity existed between sites and within given depth profiles. At seven stations, activity was increased by several orders of magnitude at depth (up to ~12 nmol of acetate respired cm−3 d−1). We attempted to correlate the measures of activity with high-resolution color and element stratigraphy. Increased activities at certain depths may be correlated to variations in the sediment geology, i.e., to the presence of dark clay-rich layers, of sandy layers, or within clay-rich horizons presumably overlying basalts. This would suggest that the distribution of microbial heterotrophic activity in deeply buried sediments may be linked to specific lithologies. Nevertheless, high-resolution microbial examination at the level currently enjoyed by sedimentologists will be required to fully explore this link.

Both catabolic and anabolic heterotrophic microbial activity proceed in frozen soils.

A large proportion of the global soil carbon pool is stored in soils of high-latitude ecosystems in which microbial processes and production of greenhouse gases proceed during the winter months. It has been suggested that microorganisms have limited ability to sequester substrates at temperatures around and below 0 °C and that a metabolic shift to dominance of catabolic processes occurs around these temperatures. However, there are contrary indications that anabolic processes can proceed, because microbial growth has been observed at far lower temperatures. Therefore, we investigated the utilization of the microbial substrate under unfrozen and frozen conditions in a boreal forest soil across a temperature range from -9 °C to +9 °C, by using gas chromatography-isotopic ratio mass spectrometry and (13)C magic-angle spinning NMR spectroscopy to determine microbial turnover and incorporation of (13)C-labeled glucose. Our results conclusively demonstrate that the soil microorganisms maintain both catabolic (CO(2) production) and anabolic (biomass synthesis) processes under frozen conditions and that no significant differences in carbon allocation from [(13)C]glucose into [(13)C]CO(2) and cell organic (13)C-compounds occurred between +9 °C and -4 °C. The only significant metabolic changes detected were increased fluidity of the cell membranes synthesized at frozen conditions and increased production of glycerol in the frozen samples. The finding that the processes in frozen soil are similar to those in unfrozen soil has important implications for our general understanding and conceptualization of soil carbon dynamics in high-latitude ecosystems.

Heterotrophic microbial activity in lake sediments: effects of organic electron donors

Allochthonous and autochthonous organic matter deposited in benthic sediments are mineralized by microbial communities, resulting in release of nutrients to the water column. Lakes with different trophic states may have sediments with different carbon and nutrient concentration with consequently different microbial communities. Microbial diversity of surface sediments of three subtropical lakes of different trophic state was investigated by measuring catabolic response to a wide variety of carbon-substrates. Basal carbon dioxide and methane production rates were highest in Lake Apopka (hypereutrophic), followed by Lake Annie (oligo-mesotrophic) and Lake Okeechobee (eutrophic) sediments. The oligo-mesotrophic Lake Annie showed the highest metabolic quotient (qCO2; proportion of basal respiration per unit of microbial biomass, 0.008 ± 0.001) indicating inefficient use of energy. The low qCO2 found in Lake Apopka sediment indicated higher efficiency in using energy. Lake Okeechobee sediments had intermediary values of qCO2 (M9 0.005 ± 0.001; M17 0.006 ± 0.0003; KR 0.004 ± 0.001) as compared with other lakes (lake Apopka 0.004 ± 0.14). Lake Apopka’s sediment catabolic diversity was higher than that observed in other sediments. Addition of organic electron donors to sediment samples from all lakes stimulated heterotrophic activity; however, the extent of the response varied greatly and was related to microbial biomass. The hypereutrophic Lake Apopka sediments had the highest respiration per unit of microbial biomass with the addition of electron donors indicating that these sediments respired most of the C added. These results showed that sediments with different biogeochemical properties had microbial communities with distinct catabolic responses to addition of the C sources.

Heterotrophic microbial activities and nutritional status of microbial communities in tropical marsh sediments of different salinities: the effects of phosphorus addition and plant species

Oligotrophic, phosphorus (P) limited herbaceous wetlands of northern Belize are being impacted by P loading from fertilizer runoff. P enrichment causes a shift in autotroph communities from a microphyte (cyanobacterial mats, CBM) to macrophyte (Eleocharis spp., Typha domingensis) dominated system. To document potential effects of P, salinity, and macrophyte species on the heterotrophic microbial community nutritional status (represented especially by specific phospholipids fatty acids and specific respiration rate), biomass and activities, we took soil samples from established P enrichment plots in replicated marshes of two salinity levels. P addition increased microbial biomass carbon (C), nitrogen (N) and P, as well as soil nutrient transformation rates (nitrogenase activity, N mineralization and immobilization, methanogenesis). The effect of plant species (Eleocharis vs Typha sites) was generally lower than the effect of P addition (CBM vs Eleocharis sites) and was most evident at the low salinity sites, where Eleocharis dominated plots had enhanced nitrogenase activity and P microbial immobilization. Salinity reduced the overall rates of microbial processes; it also weakened the positive effect of both P addition and plant species on microbial activities. Lastly, the amount of N stored in microbial cells, likely in form of osmoprotective compounds, was enhanced by salinity.

Rare earth element signatures in the Messinian pre-evaporitic Calcare di Base formation (Northern Calabria, Italy): evidence of normal seawater deposition

The Calcare di Base (CdB), a carbonate unit deposited at the base of the evaporitic succession, is a key to understanding the paleoenvironmental conditions before the Messinian Salinity Crisis. The concentration of rare earth elements and yttrium (REE + Y) in the CdB, cropping out in the Rossano Basin, were measured with inductively coupled mass spectrometry (ICP-MS). The CdB shows coherent and reliable REE + Y patterns with typical characteristics of microbial carbonates: (1) enrichment in heavy REE (NdSN/YbSN = 0.52, SD = 0.09), (2) negative Ce and positive La anomalies, (3) marine Y/Ho ratios (57.08, SD = 3.07), (4) slightly positive Gd anomalies, and (5) tetrad effect especially in the La–Ce–Pr–Nd, and [Pm]–Sm–Eu–Gd tetrads. These features suggest that the CdB incorporated REEs in equilibrium with oxygenated seawater. They also corroborate the depositional model of Guido et al. in Palaeogeogr Palaeoclimatol Palaeoecol 255:265–283, (2007) that maintains carbonatogenesis occurred via heterotrophic microbial activity. The REE + Y signatures of the CdB support the interpretation that these sediments were deposited in normal marine condition and cannot belong to the evaporite succession.

Dynamics and recovery of fertilizer 15N in soil and winter wheat crop under minimum versus conventional tillage

The development of reduced tillage in cropping systems raises the question of adaptation of N fertilization of crops in response to changes in the chemical, physical and biological soil caused by the cessation of ploughing. The purpose of the work was to investigate the fate of fertilizer 15N applied in spring to a winter wheat crop ( Triticum aestivum L.) under minimum tillage (MT) and conventional tillage (CT) and to examine the hypothesis that in MT systems plants face stronger microbial competition for N. The experiment was conducted in Northern France on a silt loam in two different rotations [sugar beet ( Beta vulgaris L.)–winter wheat–maize ( Zea mays L.)–winter wheat and seed flax ( Linum usitatissimum L.)–winter wheat–pea ( Pisum sativum L.)–winter wheat] from which we selected two crop sequences, pea–wheat and maize–wheat. At the beginning of stem elongation, 15N labelled fertilizer was applied at the rates of 100 kg N ha −1 for wheat after pea and 120 kg N ha −1 for wheat after maize. The dynamics of fertilizer 15N in soil and plant was assessed at six dates between N application and wheat harvest. In addition, soil respiration was monitored continuously on the maize–wheat CT and MT plots as a way of assessing heterotrophic microbial activity. The type of tillage (MT vs CT) had little effect on fertilizer N dynamics in the soil. The amount of labelled N immobilized was low with values ranging from 3.5 to 11.9% of applied 15N. Plant 15N recovery (above ground biomass + roots) amounted to 59–63% at flowering and harvest. The 15N balance at harvest time (i.e. the sum of 15N recovered from all plant and soil compartments) indicates that approximately 29% of the fertilizer N applied was lost whichever the tillage considered. The type of rotation (i.e. the nature of the preceding crop) had more influence on wheat N uptake than the type of tillage. The lack of observed effect of tillage on the dynamics of nitrogen fertilizer and plant recovery is in agreement with most published results obtained in other soil and climate conditions.

Major contribution of autotrophy to microbial carbon cycling in the deep North Atlantic’s interior

Abstract Current estimates point to a mismatch of particulate organic carbon supply derived from the surface ocean and the microbial organic carbon demand in the meso- and bathypelagic realm. Based on recent findings that chemoautotrophic Crenarchaeota are abundant in the mesopelagic zone, we quantified dissolved inorganic carbon (DIC) fixation in the meso- and bathypelagic North Atlantic and compared it with heterotrophic microbial activity. Measuring 14C-bicarbonate fixation and 3H-leucine incorporation revealed that microbial DIC fixation is substantial in the mesopelagic water masses, ranging from 0.1 to 56.7 μmol C m−3 d−1, and is within the same order of magnitude as heterotrophic microbial activity. Integrated over the dark ocean’s water column, DIC fixation ranged from 1–2.5 mmol C m−2 d−1, indicating that chemoautotrophy in the dark ocean represents a significant source of autochthonously produced ‘new organic carbon’ in the ocean’s interior amounting to about 15–53% of the phytoplankton export production. Hence, chemoautotrophic DIC fixation in the oxygenated meso- and bathypelagic water column of the North Atlantic might substantially contribute to the organic carbon demand of the deep-water microbial food web.

Effects of soil organic matter composition on unfrozen water content and heterotrophic CO 2 production of frozen soils

Several recent studies have highlighted the importance of soil organic matter (SOM) mineralization at high latitudes during winter for ecosystem carbon (C) balances, and the ability of the soil to retain unfrozen water at sub-zero temperatures has been shown to be a major determinant of C mineralization rates. Further, SOM is believed to strongly influence the liquid water contents in frozen surface layers of boreal forest soils and tundra, but the mechanisms and specific factors involved are currently unknown. Here we evaluate the effects of the chemical composition of SOM on the amount of unfrozen water, the pore size equivalents in which unfrozen water can exist, and the microbial heterotrophic activity at sub-zero temperatures in boreal forest soils. To do this, we have characterized the chemical composition of SOM in forest soil samples (surface O-horizons) using solid state CP-MAS (cross polarization magic angle spinning) NMR spectroscopy. The acquired information was then used to elucidate the extent to which different fractions of SOM can explain the observed variations in unfrozen water content, pore size equivalents, and biogenic CO 2 production rates in the examined soil samples under frozen conditions (−4 °C). The data evaluation was done by the use of principal component analysis (PCA) and projections to latent structures by means of partial least square (PLS). We conclude that aromatic, O-aromatic, methoxy/ N-alkyl and alkyl C are the major SOM components affecting frozen boreal forest soil’s ability to retain unfrozen water and sustain heterotrophic activity (95% confidence level). Our results reveal that solid carbohydrates have a significant negative impact (95% confidence level) on CO 2 production in frozen boreal spruce forest soils, in contrast to the positive effects of carbohydrate polymers during unfrozen conditions. We conclude that the hierarchy of environmental factors controlling SOM mineralization changes as soils freeze. The effect of SOM composition on pore size distribution and unfrozen water content has a superior influence on SOM mineralization and hence on heterotrophic CO 2 production of frozen soils.

Effect of unsteady flow on nitrate loss in an oligotrophic, glacial meltwater stream

The McMurdo Dry Valleys of Antarctica are among the coldest, driest ecosystems on Earth. During the austral summer, glacial meltwater supports cyanobacterial mat communities in some streams, but they are not ubiquitous. We conducted a nitrate (NO3−) enrichment tracer injection in Huey Creek to quantify NO3− loss in a Dry Valley stream where algal mats would not obscure hyporheic microbial processes. Unsteady streamflow led to diel variability in the tracer concentration and in surface/subsurface water and solute exchange. Subsequently, concentrations of NO3−, nitrite (NO2−), ammonium (NH4+), and dissolved organic carbon (DOC) varied significantly during the injection, with a net loss of NO3−, NO2−, and DOC, and production of nitrous oxide. These mass changes within a reach were often coincident with high streamflows. Reactivity also coincided with the highest DOC concentrations, suggesting that DOC is the primary limitation to heterotrophic microbial activity in the stream. Together, streamflow and DOC availability create the hot spots and hot moments that dominate NO3− reactivity and removal in this polar desert ecosystem. The combination of spatially and temporally variable hyporheic dynamics and solute availability underscore the limitations of common nutrient uptake metrics and transient storage models when unsteady flow conditions exist.

Carbon‐nitrogen interactions in fertility island soil from a tropical semi‐arid ecosystem

Summary 1. Biological nitrogen (N) fixation by symbiotic and free‐living organisms is considered the main pathway for N soil enrichment in desert and semi‐desert ecosystems. This fact is more noticeable in tropical ecosystems where legume species have a high relative abundance. However, this biological fixation pathway does not guarantee the maintenance of soil N pools, and N conservation pathways are important in understanding controls over soil N cycling. 2. In dryland ecosystems, desert plants can form a ‘fertility island’ (FI) as soils beneath plants show higher concentrations of N and organic matter. 3. Here we assess how carbon (C) and N may interact to conserve soil N within the FI soil of two legume species (Prosopis laevigata and Parkinsonia praecox), one a known N‐fixer and the other believed not to fix N, as well as within adjacent bare ground soil. In a semi‐arid tropical ecosystem in central Mexico, we examined spatial patterns in C and N pools and transformation rates, and we investigated seasonal variations in these relationships. 4. Results show that FI soil C and N could be linked to total N storage through net C and N immobilization in microbial biomass and heterotrophic microbial activity. Soil under P. laevigata canopy had greater total N as well as N accumulated in microbial biomass than soil under P. praecox and bare ground soil. Nevertheless, inorganic N and potential net N mineralization rates were similar under soils of both species, although we expected higher inorganic N and N‐mineralization values in N‐fixer species to explain the greater total N. Higher total N concentrations under P. laevigata probably result from greater inputs of organic C and a higher potential net C mineralization rate in comparison to P. praecox and bare ground soil. 5. Even though N input and output values were not measured, the results highlight the importance of assessing the role of organic C, heterotrophic microbial activity, and N storage in microbial biomass in order to understand controls over N retention in soil N cycling. Thus, soil C‐N interactions could be a control factor of N soil conservation in this tropical semi‐arid ecosystem.

Aerobic heterotrophic microbial activity associated with seagrass roots: effects of plant type and nutrient amendment

The oxygen dynamics of seagrass roots implicitly influences the associated microbial activity. The oxic zone surrounding roots creates a distinct electrochemical gradient where the con- centration of oxygen is highest closest to the root tip and decreases sharply with distance away from the root. Microbes within each micro-niche utilise progressively less energy-favourable electron acceptors (e.g. oxygen, nitrate, sulphate). It is probable that oxygen-consuming bacteria would have greatest activity at or near the root tips of seagrass. This study assessed the potential of a community- level physiological profiling assay, based on a high throughput, microtitre-based oxygen sensor, to evaluate aerobic heterotrophs in seagrass ecosystems. Seagrasses were collected from shallow water within the Banana River Estuary, Kennedy Space Center. The sensitivity of the method for character- izing microbial communities associated with seagrasses was tested both spatially (i.e. rhizoplane, rhi- zosphere) and following imposed light stress (i.e. shading). Results showed that oxygen availability and competition for nutrients defined the microbial response to carbon substrates, with the fastest response and largest peaks for N-containing carbon sources (e.g. asparagine, natural extracts). Aer- obic microbial activity was higher for Halodule wrightii than Syringodium filiforme. Primary nitrogen and secondary phosphorus limitation was observed for microbial activity from the rhizoplane and rhi- zosphere of H. wrightii. Shading appeared to reduce nutrient limitation for H. wrightii and reduced aerobic microbial activity. The microbial activity and functional response varied between the rhizos- phere and rhizoplane, which suggests that microbial activity on this fine spatial scale needs to be considered when evaluating biogeochemistry associated with seagrass sediments.

Emissions of Nitrous Oxide, Ammonia, and Carbon Dioxide from a Cambisol at Two Contrasting Soil Water Regimes and Urea Granular Sizes

The superiority of mixing and deep placement of prilled urea (PU) or urea supergranules (USG) over surface‐broadcast application for reducing nitrogen (N) loss from lowland rice is well established. In upland agricultural systems, rainfall and/or the application and loss of irrigation water from soil systems may regulate urea N transformations and gaseous losses, depending on the method of fertilizer application and the particle size. To develop further insights into these processes, experiments were carried out in a silt loam soil mixed with PU or amended with point‐placed USG at a depth of 7.5 cm. Two soil water regimes were used: around field capacity (AFC) with low evaporative conditions (depletion: 77 to 69% water‐filled pore space, WFPS) and below field capacity (BFC) with high evaporative conditions following two irrigations (depletion: 70 to 55% WFPS). The nitrous oxide (N2O) emission was greater at AFC than at BFC, where nitrification was more rapid. The N2O peaks appeared mostly after the disappearance of nitrite (NO2 −), presumably dominated by nitrifier and/or chemodenitrification and the degree of emissions probably depended on the stability period and the reduction of NO2 − induced by the soil water regimes. The relative N2O losses from the added N were small (⩽0.20%) for all treatments after 21 days. The point at which 50% of its emissions (t½) occurred was delayed up to 6 days longer than found from the application of PU. The differences between PU and USG application were likely linked with the concentrations of ammonium (NH4 +), NO2 −, and pH. These high concentrations continued longer at AFC than at BFC and were limited to a distance of <5.0 cm from the application zone. Similarly, the relative losses of the added N ranged from 0.19 to 0.56% at AFC and 0.08 to 0.37% at BFC, the highest being with USG application. Based on the areas receiving equal N, the N2O and ammonia (NH3) emissions from USG differed marginally with PU. Carbon dioxide (CO2) release was higher at AFC than BFC, in which the USG application probably limited microbial respiration preferentially to methane oxidation. A correlation study showed that the N2O flux was best explained together with CO2, nitrate (NO3 −), NO2 −, and WFPS (R 2 = 0.67***). This indicates the influence of both auto‐ and heterotrophic microbial activities toward N2O emission, with soil water being an important regulatory factor.

Improved Enrichment and Isolation of Polycyclic Aromatic Hydrocarbons (PAH)-Degrading Microorganisms in Soil Using Anthracene as a Model PAH

Lack of attention to soil and microbial characteristics that influence PAHs degradation has been a leading cause of failures in isolation of efficient PAH degraders and bioaugumentation processes with microbial consortia. This study compared the classic method of isolation of PAHs-degraders with a modified method employing a pre-enrichment respirometric analysis. The modified enrichment of PAH degrading microorganisms using in vitro microcosm resulted to reduced enrichment period and more efficient PAH-degrading microbial consortia. Results indicate that natural soils with strong heterotrophic microbial activity determined through pre-enrichment analysis, are better suited for the isolation of efficient PAH degrading microorganisms with significant reduction of the enrichment period.

N 2O and NO emissions from an Andisol field as influenced by pelleted poultry manure

Animal manures from intensive livestock operations can be pelleted to improve handlings and recyclings of embodied nutrients. The aim of this study was to evaluate the influence of pelleted poultry manure on N 2O and NO fluxes from an Andisol field. In autumn 2006 and summer 2007, poultry manure (PM), pelleted poultry manure (PP), and chemical fertilizer (CF) were applied at a rate of 120 kg N ha −1 in each cultivation period to Komatsuna ( Brassica rapa var. peruviridis). Nitrous oxide and NO fluxes were measured using an automated monitoring system. A soil incubation experiment was also conducted to determine the influence of intact and ground pelleted manure on N 2O, NO, and CO 2 production with a water-filled pore space (WFPS) of 30 or 50%. In the field measurements, N 2O emission rates from the organic fertilizer treatments were larger than that from the CF treatment, possibly because organic C stimulated denitrification. The highest N 2O flux was observed from the PP treatment after a rainfall following fertilization, and the cumulative emission rate (2.72 ± 0.22 kg N ha −1 y −1) was 3.9 and 7.1 times that from the PM and CF treatments, respectively. In contrast, NO emission rates were highest from the CF treatment. The NO/N 2O flux ratio indicated that nitrification was the dominant process for NO and N 2O production from the CF treatment. Cumulative N 2O emission rates from all treatments were generally higher during the wetter cultivation period (autumn 2006) than during the drier cultivation period (summer 2007). In contrast, NO emission rates were higher in the drier than in the wetter cultivation period. The incubation experiment results showed a synergistic effect of soil moisture and the pelleted manure form on N 2O emission rates. The intact pelleted manure with the 50% WFPS treatment produced the highest N 2O and CO 2 fluxes and resulted in the lowest soil NO 3 − content after the incubation. These results indicate that anaerobic conditions inside the pellets, caused by rainfall and heterotrophic microbial activities, led to denitrification, resulting in high N 2O fluxes. Controlling the timing of N application by avoiding wet conditions might be one mitigation option to reduce N 2O emission rates from the PP treatment in this study field.

Patterns of heterotrophic microbial activity in eutrophic and oligotrophic peatlands

Nutrient enrichment of peatlands may alter patterns of heterotrophic microbial activity (HMA) and organic matter cycling. The utilization of C-substrates by heterotrophic microbial communities in response to changes in environmental conditions may serve as sensitive indicators of changes in the trophic state of wetlands. The objective of this study was to measure the response of heterotrophic microbial communities to added C-substrates in the plant detritus layer and underlying soil (0–10 cm) for eutrophic, transitional, and oligotrophic sites along a nutrient enrichment gradient in the Florida Everglades, USA. The short-term response to C-substrates (alcohols, amino acids, carboxylic acids, and polysaccharides) was measured as CO 2 production. The nutrient gradient was characterized by decreasing P concentrations and microbial biomass from eutrophic to oligotrophic sites. Basal respiration was 73% higher at eutrophic than oligotrophic sites, and 41% higher in detritus than underlying soil. Heterotrophic microbial activity varied along the gradient with greater C-substrate utilization at the eutrophic site resulting from higher levels of microbial biomass and inorganic nutrients compared to the oligotrophic site. The C-substrates enhanced CO 2 production at all sites suggesting that labile organic C was a limiting factor to HMA in these peatlands. Substrate-induced respiration (SIR) of detritus was 25, 45, and 42% greater for polysaccharides than other C-substrates at the eutrophic, transitional, and oligotrophic sites, respectively. Likewise, SIR at the eutrophic, transitional, and oligotrophic sites was 0, 40, and 39%, respectively, greater for detritus amended with carboxylic acids than with amino acids and alcohols. Polysaccharides dominated HMA profiles at all sites along the nutrient gradient in detritus. The transitional site was characterized by carboxylic acids and alcohols, while the HMA profile at the oligotrophic site was dominated by carboxylic acids. Patterns of HMA along the nutrient gradient provided insight into the microbial response to changes in tropic status, indicating the heterotrophic microbial community was more sensitive with increasing eutrophication.