Decomposition Of Particulate Organic Matter Research Articles

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize-based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0-269 kg N ha(-1) yr(-1)) that created a large range in crop residue inputs (3.60-9.94 Mg dry matter ha(-1) yr(-1)), we provide the first agronomic assessment of long-term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico-chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra-aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha(-1) yr(-1)) and an excessive N rate (269 kg N ha(-1) yr(-1)), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N2 O and NO3 emissions exponentially increase when agronomic optimum N is surpassed.

Pathways of litter C by formation of aggregates and SOM density fractions: Implications from 13C natural abundance

Aggregate formation is a key process of soil development, which promotes carbon (C) stabilization by hindering decomposition of particulate organic matter (POM) and its interactions with mineral particles. C stabilization processes lead to 13C fractionation and consequently to various δ13C values of soil organic matter (SOM) fractions. Differences in δ13C within the aggregates and fractions may have two reasons: 1) preferential stabilization of organic compounds with light or heavy δ13C and/or 2) stabilization of organic materials after passing one or more microbial utilization cycles, leading to heavier δ13C in remaining C. We hypothesized that: 1) 13C enrichment between the SOM fractions corresponds to successive steps of SOM formation; 2) 13C fractionation (but not the δ13C signature) depends mainly on the transformation steps and not on the C precursors. Consequently, minimal differences between Δ13C of SOM fractions between various ecosystems correspond to maximal probability of the SOM formation pathways.We tested these hypotheses on three soils formed from cover loam during 45 years of growth of coniferous or deciduous forests or arable crops. Organic C pools in large macroaggregates, small macroaggregates, and microaggregates were fractionated sequentially for four density fractions to obtain free POM with ρ < 1.6 g cm−3, occluded POM with two densities (ρ < 1.6 and 1.6–2.0 g cm−3), and mineral fraction (ρ > 2.0 g cm−3).The density fractions were 13C enriched in the order: free POM < light occluded POM < heavy occluded POM < mineral fraction. This, as well as their C/N ratios confirmed that free POM was close to initial plant material, whereas the mineral fraction was the most microbially processed. To evaluate the successive steps of SOM formation, the Δ13C values between δ13C of SOM fractions and δ13C of bulk SOM were calculated. The Δ13C indicated that, parallel with biochemical transformations, the physical disintegration strongly contributed to the formation of free and occluded light POM. In contrast, biochemical transformations were more important than physical disintegration for formation of heavy occluded POM from light occluded POM. This was confirmed by review of 70 Δ13C values from other studies analyzed Δ13C depending on the density of SOM fractions. Accordingly, the successive steps of SOM formation were: fLF<1.6 = oLF<1.6 → oDF1.6–2.0 = MF>2.0. The obtained steps of C stabilization were independent on the initial precursors (litter of coniferous forest, deciduous forest or grasses).To test the second hypothesis, we proposed an extended scheme of C flows between the 3 aggregate size classes and 4 SOM fractions. Δ13C enrichment of the SOM fractions showed that the main direction of C flows within the aggregates and SOM fractions was from the macroaggregate-free POM to the mineral microaggregate fraction. This confirmed the earlier concept of SOM turnover in aggregates, but for the first time quantified the C flows within the aggregates and SOM density fractions based on δ13C values. So, the new 13C natural abundance approach is suitable for analysis of C pathways by SOM formation under steady state without 13C or 14C labeling.

Decomposition of particulate organic matter is more sensitive to temperature than the mineral associated organic matter

Temperature sensitivity of soil organic matter decomposition is important in determining the role of soils in future climate change. We isolated coarse and fine particulate organic matter (cPOM and fPOM) and mineral associated organic matter (MinOM) to represent labile, relatively less labile and stable pools of soil organic matter (SOM), respectively and incubated each at four different temperatures to determine temperature sensitivity of decomposition. The coarse particulate organic C, which comprised the smallest pool of soil organic C (SOC) was most decomposable and the mineral associated organic C that accounted for more than half of the SOC was least decomposable. At all the temperatures, the C mineralization rate followed the order cPOM ≥ fPOM > whole soil > MinOM. The disparity in the mineralization rates between cPOM and the other two SOM fractions and the whole soil widened with increase in temperature from 15° to 45 °C indicating that the labile pools of SOM were more sensitive to temperature than the stable pool. The Arrhenius, the Llyod and Taylor and the Gaussian models well-described the temperature dependence of organic matter decomposition, but the shape of the temperature response curve for different models varied considerably. Gaussian model yielded the highest decomposition Q10 and the Arrhenius model the lowest Q10 for different SOM fractions and whole soil. The decomposition temperature response of isolated SOM fractions mainly differed at temperatures below 25 °C beyond which the response tended to converge suggesting that the differential response of labile and stable pools to temperature will be foremost at temperatures below 25 °C beyond which the effect will be small and similar for SOM pools of different lability. The decomposition of cPOM fraction is likely to be influenced to the greatest extent and the MinOM at the least as a result of global warming.

Soil Phosphorus Dynamics of Wheat-Based Cropping Systems in the Semiarid Region of Argentina

The dynamics of soil P forms and particle size fractions was studied under three wheat-based cropping sequences in production systems of Argentina. The whole soil and its coarse (100–2000 µm) and fine (0–100 µm) fractions were analyzed to determine Bray-Kurtz extractable (Pe), organic (Po), inorganic (Pi), and total (Pte) phosphorus. The reference soil was determined at time 0 and compared to a four-year period (time 9 to 12) in three crop sequences: wheat (Triticum aestivumL.)-cattle grazing on natural grasses (WG), continuous wheat (WW), and wheat-legume (WL). Levels of Pe showed differences over time, from 10 to 16 µg g−1in WG, in line with agriculture and cattle grazing alternate sequences. In WW, P level increased with time, while in WL systems a significant decrease in P from 33.7 to 10.4 µg P g−1was found during the legume period. Soil P values varied between reference soil and soil samples in year nine and between treatments. Pi was significantly lower in WW, and its concentration increased with time. The coarse fraction of the reference plots had significantly higher levels of Po and Pi than the cultivated treatments, probably a consequence of the particulate organic matter decomposition and coarse mineral particle weathering. The observed changes in Pi content could be attributed to differences in occluded P equilibrium under different soil environments (mainly pH) and crop-tillage-climatic interaction.

Sediment flux modeling: Calibration and application for coastal systems

Benthic–pelagic coupling in shallow estuarine and coastal environments is an important mode of particle and solute exchange and can influence lag times in the recovery of eutrophic ecosystems. Links between the water column and sediments are mediated by particulate organic matter (POM) deposition to the sediment and its subsequent decomposition. Some fraction of the regenerated nutrients are returned to the water column. A critical component in modeling sediment fluxes is the organic matter flux to the sediment. A method is presented for estimating POM deposition, which is especially difficult to measure, by using a combination of long term time series of measured inorganic nutrient fluxes and a mechanistic sediment flux model (SFM). A Hooke–Jeeves pattern search algorithm is used to adjust organic matter deposition to fit ammonium (NH4+) flux at 12 stations in Chesapeake Bay for up to 17 years.POM deposition estimates matched reasonably well with sediment trap estimates on average (within 10%) and were strongly correlated with previously published estimates based upon sediment chlorophyll-a (chl-a) distribution and degradation. Model versus field data comparisons of NH4+ flux, sediment oxygen demand (SOD), sulfate (SO42−) reduction rate, porewater NH4+, and sedimentary labile carbon concentrations further validated model results and demonstrated that SFM is a powerful tool to analyze solute fluxes. Monthly model-field data comparisons clearly revealed that POM decomposition in the original SFM calibration was too rapid, which has implications for lagged ecosystem responses to nutrient management efforts. Finally, parameter adjustments have been made that significantly improve model-field data comparisons and underscore the importance of revisiting and recalibrating models as long time series (>5 years) become available.

Investigating the effect of ballasting by CaCO3 in Emiliania huxleyi, II: Decomposition of particulate organic matter

The quantitative relationship between organic carbon and mineral contents of particles sinking below 1800 m in the ocean indicates that organisms with mineral shells such as coccolithophores are of special importance for transporting carbon into the deep sea. Several hypotheses about the mechanism behind this relationship between minerals and organic matter have been raised, such as mineral protection of organic matter or enhanced sinking rates through ballast addition. We examined organic matter decomposition of calcifying and non-calcifying Emiliania huxleyi cultures in an experiment that allowed aggregation and settling in rotating tanks. Biogenic components such as particulate carbon, particulate nitrogen, particulate volume, pigments, transparent exopolymer particles (TEP), and particulate amino acids in suspended particles and aggregates were followed over a period of 30 d. The overall pattern of decrease in organic matter, the amount of recalcitrant organic matter left after 30 d, and the compositional changes within particulate organic matter indicated that cells without a shell are more subject to loss than calcified cells. It is suggested that biogenic calcite helps in the preservation of particulate organic matter (POM) by offering structural support for organic molecules. Over the course of the experiment, half the particulate organic carbon in both calcifying and non-calcifying cultures was partitioned into aggregates and remained so until the end of the experiment. The partial protection of particulate organic matter from solubilization by biominerals and by aggregation that was observed in our experiment may help explain the robustness of the relationship between organic and mineral matter fluxes in the deep ocean.

Evaluation of phosphorus mobilization potential in rewetted fens by an improved sequential chemical extraction procedure

SummaryAfter rewetting of peatlands, phosphorus (P) pore‐water concentrations were up to three orders of magnitude greater than under pristine conditions. It was hypothesized that different mobilization processes such as ion‐exchange reactions, biotic/abiotic redox reactions, acidification and ongoing anaerobic decomposition of particulate organic matter by hydrolytic cleavage and fermentation might be responsible. To identify P pools in peat samples of varying degrees of decomposition, we modified and improved a sequential chemical extraction method that allowed conclusions on potential mobilization mechanisms in rewetted peatlands. The results indicated that the earlier drainage of rewetted fens strongly increased the P mobilization potential in the upper decomposed peat layers. Accordingly, the amount of P bound to redox‐sensitive (bicarbonate/dithionite soluble) compounds (BD‐P) was, on average, one order of magnitude greater in decomposed peat of rewetted fens (5.4–14.3 μmol P g−1 dry matter or DM) than in underlying less‐decomposed peat layers (0.2–1.9 μmol P g−1 DM) or slightly decomposed peat derived from pristine fens (0.4–2.0 μmol P g−1 DM). The BD‐P fraction found in the upper very decomposed peat layers appears to be most important for P mobilization in rewetted fens and accounted for 85% of the variability of P mobilization rates. Despite uncertainties regarding P diagenetic processes in peat, as well as the development of microbial decomposition processes, in the long‐term, high pore‐water P concentrations can be expected in rewetted fens for decades to come.

NEGATIVE SELECTION EFFECTS SUPPRESS RELATIONSHIPS BETWEEN BACTERIAL DIVERSITY AND ECOSYSTEM FUNCTIONING

I assembled bacterial communities to explore the effects of bacterial diversity on multiple ecosystem functions, including bacterial community biovolume, decomposition of particulate organic matter, and biomass transfer to the next trophic level. The experiment used a two-way factorial design with four levels of bacterial diversity (one to four species) and the absence/presence of a bacterivorous ciliated protist Tetrahymena pyriformis as two main factors, and all possible combinations of the four bacterial taxa nested within each diversity level. Bacterial community biovolume increased as bacterial diversity increased, a result due largely to positive selection effects. Decomposition and consumer abundance, however, were unaffected by bacterial diversity, though both varied among bacterial composition treatments. Negative selection effects, the dominance of species that do not contribute significantly to ecosystem functioning, accounted for the lack of diversity effects on decomposition and consumer abundance. The presence of Tetrahymena reduced bacterial community biovolume but increased decomposition, without altering the diversity-functioning relationships. Decomposition was strongly linked with consumer abundance such that communities supporting larger consumer biomass exhibited higher decomposition rates. This study suggests that if the negative selection effect is common, as it might be when examining ecosystem variables other than biomass (due to the presence of keystone species that can contribute disproportionably to ecosystem functioning relative to their abundances), basic bacteria-mediated ecosystem processes, such as decomposition and energy transfer to the next trophic level, may not increase with bacterial diversity.

Consumer vs environmental productivity control of bacterial diversity and bacteria‐mediated organic matter decomposition

We experimentally investigated the determinants (consumer vs environmental productivity) and functional consequences of bacterial diversity using a simple laboratory bacterial assemblage and the bacterivorous ciliated protistColpidium striatum. A two‐way factorial design was used, with two productivity levels crossed with the presence/absence ofColpidium striatum. The effects of productivity and predation on bacterial diversity were similar to those previously reported for plant diversity in systems characterized by high productivities, with increased productivity reducing diversity and predation increasing diversity. WhenColpidiumwas present, changes in trophic level biomass and individual bacterial species abundance in response to increased productivity were largely consistent with the keystone predation model. Importantly,Colpidiumpredation also resulted in an increase in particulate organic matter decomposition, largely due to an increase in the abundance of one single bacterial species that appeared resistant to predation. These results suggest that changes in community structure as a result of trophic interactions and other factors may have profound consequences for important ecosystem functions. We suggest that future research should take an integrative approach and study causes and consequences of biodiversity simultaneously.

Emergent multiple predator effects in an experimental microbial community

AbstractStudies on multiple predator effects have typically focused on metazoans. Here we examined the effects of multiple consumers on bacteria. Specifically, we investigated the separate and combined effects of two common bacterivorous ciliates, Colpidium striatum and Paramecium caudatum, on a simple bacterial assemblage. Non‐additive multiple predator effects emerged for all bacterial species significantly affected by grazing, where bacterial responses in the two‐consumer treatment cannot be predicted by summing the separate effects of the two consumers. Some species showed risk reduction (observed responses less than expected), whereas others showed risk enhancement (observed responses larger than expected). We attributed risk alteration to the interference between the two consumers. Contradictory to theoretical predictions, total bacterial abundance in the two‐consumer treatment did not differ from single‐consumer treatments and consumer‐free controls, due largely to risk reduction and compensatory responses within the bacterial assemblage. Decomposition of particulate organic matter was greater in the two‐consumer treatment than the Paramecium single‐consumer treatment, but did not differ among other treatments. These results suggest that the presence of multiple consumers may have unexpected impacts on the structure and functioning of bacterial communities.

The Ecological Role of a Proteolytic Psychrophile, Flavobacterium limicola Belonging to the Cytophaga-Flavobacteria Cluster, in the Decomposition of Particulate Organic Nitrogen in Freshwater Sediment

Microbes of the Cytophaga-Flavobacterium (CF) group, within the phylum Bacteroidetes, have been considered contributors to the early decomposition of particulate organic matter (POM) in sediments because of their abundance in the ecosystem and their great ability to degrade macromolecules on artificial media. However, there is no report on the ability of members of this group to decompose POM in sediment. We investigated the POM-decomposition capabilities of members of the CF group, Flavobacterium limicola strains ST-82T, ST-10 and ST-92 isolated from freshwater sediment as proteolytic psychrophiles, in sterilized sediment slurries. All strains of F. limicola grew and survived at over 108 cfu per ml for more than 30 days in the sediment slurries at 5°C. Protease activity levels in the slurries inoculated with the strains were 3-5 times higher than the control level for over 30 days of incubation at 5°C. Concentrations of total dissolved nitrogen (TDN) released by the hydrolysis of particulate organic nitrogen (PON), significantly increased only in the slurries inoculated with F. limicola strains. Approximately 70-80% of the TDN released was converted to NH4+-N in these inoculated slurries. The results clearly demonstrate that F. limicola strains ST-82T, ST-10 and ST-92 are able to secrete an extracellular protease and hydrolyze PON to TDN and thereby mineralize TDN to NH4+-N in freshwater sediment. This is the first report on the decomposition and mineralization of PON by members of the CF group in benthic environments.

Enzymatic activities associated with decomposition of particulate organic matter in two shallow ponds

Particulate organic matter (POM) is an important source of energy in aquatic systems, where decomposition rates have traditionally been studied using the litter-bag technique. However, this method has limitations for estimating in situ decomposition rates, especially for fine particles. In this study the litter-bag technique is combined with measurements of microbial enzyme activities to estimate POM mass loss rates in situ. Litter-bags containing benthic POM in two size ranges, coarse (>1 mm) and fine (0.063–0.5 mm), were placed in two contrasting ponds (Toro and Oro) located in the Doñana area in southern Spain. Litter-bags were collected over a period of a year and analysed for mass loss and the activities of five extracellular enzymes (β-glucosidase, β-xylosidase, β- N-acetylglucosamidase, phenol oxidase, alkaline phosphatase). In situ sediment POM samples were also collected, sorted in the same size ranges, and assayed for the same enzyme activities. Using separate regression models relating mass loss to average cumulative enzyme activity, the mean turnover times for in situ coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) were 149 and 2778 days in Toro pond and 382 and 4347 days in Oro pond, respectively. This compared with turnover times of 578 and 2109 days for Toro and 778 and 5296 days in Oro pond for POM confined in bags. Integrating enzyme measurements and litter-bag techniques provides in situ organic matter turnover rates and may allow the assessment of global controlling factors (e.g. flooding regime, temperature and POM quality) in the decomposition process in temporally and spatially heterogeneous environments.

On the seasonal nitrogen dynamics of the Baltic proper biogeochemical reactor

During the last decade it has become increasingly obvious that the turnover of dissolved organic nitrogen DON in marine environments is quite vigorous. This paper quantie es the turnover of DON in the Baltic proper regarded as a biogeochemical reactor. In a nitrogen model for the reactor, dissolved inorganic nitrogen DIN, DON and molecular N, e xed by cyanobacteria, can be used for plant production. The decomposition of particulate organic matter is assumed to produce DON and DIN as end products in the proportions (1 2 h ) to h (0 # h # 1). The model includes two internal sink processes, denitrie cation and sequestering in the bottom sediments and accounts for external sources and sinks by import and export of DIN and DON. The annual net production in the Baltic proper is about 12.8 10 6 tonC (50gC m 2 2 ) requiring about 2.3 10 6 ton N. However only about 1.0 10 6 ton N are available as DIN and the dee cit has to be covered by an uptake of N from DON and/or e xed molecular nitrogen. The results of the model depend on the value of h . With h 5 1 the use of DON for primary production is at a minimum (0.19 10 6 ton N) while there are maxima for nitrogen e xation (1.0 10 6 ton N) and denitrie cation (1.5 10 6 ton N). However, both these values are considered unrealistically large. A more likely value of h is determined from the model in such a way that the annual rate of nitrogen e xation in the Baltic proper is in accordance with a recent estimate from the literature (0.11 10 6 ton N). This gives h 5 0.55 implying that about 0.67 10 6 ton N is denitrie ed, and 1.10 10 6 ton DON is used for net production, and 0.91 10 6 ton DON is produced by decomposition of particulate organic matter and the turnover time for DON is about 4 years. The e nding that there is a vigorous turnover of DON on the reactor level has important consequences. Firstly,earlier estimates of denitrie cation rates were based on budgets for oxygen and DIN and overlooked the DON decomposition pathway, why denitrie cation rates are severely overestimated, often by a factor of 2 or greater. Secondly, the extensive use of DON for primary production in the Baltic proper in combination with abundance of DON, challenge the widely accepted opinion that nitrogen is the production-limiting nutrient on the systems (reactor) level in the Baltic proper.

Exponential Collapse of Benthic Depurative Capacity in A Eutrophic Tropical Marine Ecosystem: A Threshold Response to Organic Nitrogen Loading

Microbial decomposition of particulate organic matter in sediments can substantially modulate eutrophication of nearshore marine environments. Flux budgets for nitrogen compounds and quantification of rates of microbial transformation can provide important information on the process of eutrophication. This survey documents sediment nitrogen budgets for a eutrophic nearshore marine environment at La Parguera on southwest coast of Puerto Rico, including consideration of the organic fraction in addition to microbial transformations. Sediments of the inshore channel at La Parguera denote the input of heavy organic loads with low redox potentials and high ammonium contents. Benthic fluxes of dissolved organic nitrogen are below those recorded for ammonium. These observations suggest that nearshore tropical sediments subject to heavy nitrogen loads act as a sink for organic nitrogen and that ammonification is a key process in the release of nitrogen from sediments to the water column. Nitrification and denitrification rates are low overall and inversely related to the redox potential. Depuration of excess nitrogen through denitrification is ineffective in these sediments. in contrast to more robust temperate environments, our work demonstrates that tropical marine systems are particularly susceptible to eutrophication given their limited capacity for depuration of excess nitrogen.

Biodiversity regulates ecosystem predictability

Links between biodiversity and ecosystem function provide compelling reasons for conserving maximal numbers of species in ecosystems1,2,3,4,5,6. Here we describe a previously unrecognized effect of biodiversity on ecosystem predictability, where predictability is inversely related to temporal and spatial variation in ecosystem properties. By manipulating biodiversity in aquatic microbial communities, we show that one process, ecosystem respiration, becomes more predictable as biodiversity increases. Analysis of similar patterns extracted from other studies2,3,6 indicates that biodiversity also enhances predictability in terrestrial ecosystems. Biodiversity can also affect average levels of ecosystem performance, but the extent to which different species make unique or redundant contributions to ecosystem processes remains controversial3,7,8,9,10. Nonlinear effects of biodiversity on the decomposition of particulate organic matter and resistance of communities to invasion indicate that different species have redundant functions in our system. The consequences of biodiversity are also not restricted to early successional situations as described in previous studies1,2,3,4,6, because strong effects persist even after ecosystems develop for periods corresponding to 40–80 generations of dominant organisms.

Formation of recalcitrant organic matter: humification dynamics of algal derived dissolved organic carbon and its hydrophobic fractions

The incorporation of radiolabeled carbon into XAD2-fractions of dissolved organic matter was followed during the growth, stationary and degradation phases of a batch culture of the Antarctic diatom Thalassiosira tumida. Changes in the fractions over the 267 day experiment were correlated with the production and decomposition of particulate organic matter. The bulk of the acid and neutral hydrophobic XAD-fractions, traditionally considered to be humic, were produced mainly during the diatom growth phase: 70% of this carbon was present in a relatively stable form by the end of the first month, representing 5% of the maximum carbon biomass. There was a progressive accumulation of a hydrophobic DOC fraction tightly bound to the XAD resin. This fraction is normally not considered following XAD2 extraction of aquatic humic material. However, it is evidently an important component of a resistant DOC pool, increasing during the experiment to levels between 40 and 50% of total DOC after 7–8 months. This fraction is thought to be mainly a product of particulate material degradation. At the end of the experiment the sum of all hydrophobic DOC fractions was 14% of the maximum particulate carbon.

Lipid biomarkers for heterotrophic alteration of suspended particulate organic matter in oxygenated and anoxic water columns of the ocean

The fatty acid, sterol and hydrocarbon composition of suspended particles from the central North Pacific VERTEX IV site and from the Black Sea were used to investigate the abundance of biomarkers produced by heterotrophic alteration of particulate organic matter (POM). At the oceanic VERTEX site, bacterial alteration of organic matter did not contribute significant amounts of diagnostic lipids to particles, whereas zooplankton did leave a marked imprint on particulate lipids. In contrast, in the anoxic zone of the Black Sea there were significant lipid indicators of bacterial decomposition of POM. Most of these were derived from anaerobic sulfate-reducing and phototrophic sulfur bacteria. A comparison of the two sites and their lipid distributions indicates that it may be difficult to use lipid biomarkers to assess the importance of bacterial decomposition of organic matter in the open ocean, as lipids from these bacteria are not readily distinguished from the more abundant planktonic lipids.

Deposition, resuspension, and decomposition of particulate organic matter in the sediments of Lake Itasca, Minnesota, USA

Sediment traps were used to investigate the settling, resuspension, and decomposition of particulate organic matter in Lake Itasca, MN (USA). Traps were deployed in the epilimnion and hypolimnion of the deepest basin during June, 1988, sampled twice during stratified conditions (August, September) and once after the lake had mixed (October). The downward flux of particulate material increased from summer to fall. The net sedimentation of organic matter ranged from 0.6 to 2.3 g m−2 d−1 at 4 m and increased to 2.1 to 3.2 g m−2 d−1 two meters above the bottom sediment indicating that resuspended sediment was at least 33% of the settling mass during all periods. The C:N ratios of captured particles (6.8–9.5) were between the ratios of plankton (5.8 to 6.8) and the sediments (9.9 to 10.2) but smaller than the ratios of terrestrial organic materials (13.5 to 222). The monosaccharide compositions of the entrapped particles were similar to plankton samples and different from the distinct composition of the sediments. Capture of rebound particles similar to the primary flux and not decomposition may have been responsible for this similarity. Total monosaccharide concentrations were lower in the sediments than in entrapped particles. Individual sugars exhibited different patterns of accumulation in the sediments. Glucose was lowest in sediments when the relative concentrations were compared to those in source materials and entrapped particles. In contrast, sediments had the highest rhamnose and fucose concentrations. Bacterial biomass could only account for small portions of these sugars in the sediment. The distinct monosaccharide composition of resuspended sediments was not strongly recorded in materials captured by the sediment traps even after the lake had mixed.

The breakdown of buried and surface-placed leaf litter in an upland stream

The breakdown of buried leaves (Eucalyptus viminalis) was investigated using surface-placed and buried leaf packs in a riffle of the Acheron River, Victoria. Leaf packs buried to a depth of 10 cm were rapidly colonized by invertebrates, with the total numbers of individuals and species exceeding those on surface leaf packs. A larger proportion of leaves in buried leaf packs was grazed in comparison with those on the surface, with the intensity of grazing also being higher for leaves in buried packs. Both surface and buried leaf packs broke down rapidly with no significant difference in weight loss with time. The high level of breakdown of buried E. viminalis leaf litter observed in this study suggests that the hyporheos of Australian headwater streams may significantly contribute to the decomposition of particulate organic matter.

Fate and effects of the insecticide Dursban® 4E in indoor Elodea-dominated and macrophyte-free freshwater model ecosystems: III. Aspects of ecosystem functioning

The effects of the insecticide Dursban® 4E (active ingredient chlorpyrifos) on aspects of water chemistry, community metabolism, and decomposition of particulate organic matter were studied in indoor freshwater microcosms intended to mimic drainage ditches. To several of these systems, a single dose (simulating aerial drift) was applied to achieve nominal chlorpyrifos concentrations of 5 or 35 μg/L. Two experiments were performed; one, in which all model ecosystems were dominated by the macrophyte Elodea nuttallii, and one, using systems devoid of macrophytes. The response of ecosystem functioning is discussed in relation to the shifts in community structure observed. Treatment effects on community metabolism, as signaled by a decrease in dissolved oxygen and pH and an increase in alkalinity and conductivity, could be observed in the high-dose Elodea-dominated microcosms only. In addition, Dursban 4E application in these systems resulted in a more persistent decrease in the decay rate of macrophyte material present in litter bags than in open water microcosms. Concentrations of inorganic nitrogen and orthophosphate in the water column of both types of model ecosystem were hardly affected by insecticide application. It is concluded that the impact of Dursban 4E on ecosystem functioning differed considerably between the two types of model ecosystems, and that functional endpoints were more robust than structural properties.