Decomposition Of Fresh Organic Matter Research Articles

Nonlinear Effects Induced by Interactions among Functional Groups of Bacteria and Fungi Regulate the Priming Effect in Malagasy Soils.

The priming effect (PE) occurs when fresh organic matter (FOM) supplied to soil alters the rate of decomposition of older soil organic matter (SOM). The PE can be generated by different mechanisms driven by interactions between microorganisms with different live strategies and decomposition abilities. Among those, stoichiometric decomposition results from FOM decomposition, which induces the decomposition of SOM by the release of exoenzymes by FOM-decomposers. Nutrient mining results from the co-metabolism of energy-rich FOM with nutrient-rich SOM by SOM-decomposers. While existing statistical approaches enable measurement of the effect of community composition (linear effect) on the PE, the effect of interactions among co-occurring populations (non-linear effect) is more difficult to grasp. We compare a non-linear, clustering approach with a strictly linear approach to separately and comprehensively capture all linear and non-linear effects induced by soil microbial populations on the PE and to identify the species involved. We used an already published data set, acquired from two climatic transects of Madagascar Highlands, in which the high-throughput sequencing of soil samples was applied parallel to the analysis of the potential capacity of microbial communities to generate PE following a 13C-labeled wheat straw input. The linear and clustering approaches highlight two different aspects of the effects of microbial biodiversity on SOM decomposition. The comparison of the results enabled identification of bacterial and fungal families, and combinations of families, inducing either a linear, a non-linear, or no effect on PE after incubation. Bacterial families mainly favoured a PE proportional to their relative abundances in soil (linear effect). Inversely, fungal families induced strong non-linear effects resulting from interactions among them and with bacteria. Our findings suggest that bacteria support stoichiometric decomposition in the first days of incubation, while fungi support mainly the nutrient mining of soil's organic matter several weeks after the beginning of incubation. Used together, the clustering and linear approaches therefore enable the estimation of the relative importance of linear effects related to microbial relative abundances, and non-linear effects related to interactions among microbial populations on soil properties. Both approaches also enable the identification of key microbial families that mainly regulate soil properties.

Deforestation for agriculture leads to soil warming and enhanced litter decomposition in subarctic soils

Abstract. The climate-change-induced poleward shift of agriculture could lead to enforced deforestation of subarctic forest. Deforestation alters the microclimate and, thus, soil temperature, which is an important driver of decomposition. The consequences of land-use change on soil temperature and decomposition in temperature-limited ecosystems are not well understood. In this study, we buried tea bags together with soil temperature loggers at two depths (10 and 50 cm) in native subarctic forest soils and adjacent agricultural land in the Yukon Territory, Canada. A total of 37 plots was established on a wide range of different soils and resampled after 2 years to quantify the land-use effect on soil temperature and decomposition of fresh organic matter. Average soil temperature over the whole soil profile was 2.1 ± 1.0 and 2.0 ± 0.8 ∘C higher in cropland and grassland soils compared to forest soils. Cumulative degree days (the annual sum of daily mean temperatures > 0 ∘C) increased significantly by 773 ± 243 (cropland) and 670 ± 285 (grassland). Litter decomposition was enhanced by 2.0 ± 10.4 % and 7.5 ± 8.6 % in cropland topsoil and subsoil compared to forest soils, but no significant difference in decomposition was found between grassland and forest soils. Increased litter decomposition may be attributed not only to increased temperature but also to management effects, such as irrigation of croplands. The results suggest that deforestation-driven temperature changes exceed the soil temperature increase that has already been observed in Canada due to climate change. Deforestation thus amplifies the climate–carbon feedback by increasing soil warming and organic matter decomposition.

Contrasting effects of maize litter and litter-derived biochar on the temperature sensitivity of paddy soil organic matter decomposition.

Organic matter input regulates the rate and temperature sensitivity (expressed as Q10) of soil organic matter (SOM) decomposition by changing microbial composition and activities. It remains unclear how the incorporation of litter-made biochar instead of litter affects the Q10 of SOM decomposition. Using a unique combination of two-and three-source partitioning methods (isotopic discrimination between C3/C4 pathways and 14C labeling), we investigated: (1) how maize litter versus litter-made biochar (of C4 origin) addition influenced the Q10 of SOM (C3 origin) under 10°C warming, and (2) how the litter or biochar amendments affected the Q10 of 14C-labeled fresh organic matter (FOM) after long-term incubation. Compared with biochar addition, litter increased the rates and Q10 of mass-specific respiration, SOM and FOM decomposition, as well as the contents of SOM-derived dissolved organic C (DOC) and total phospholipid fatty acids (PLFA). Litter-amended soils have much higher activities (Vmax) of β-glucosidase, N-acetyl-β-glucosaminidase, and leucine aminopeptidase, suggesting larger enzyme pools than in soils with biochar. The Q10 of enzyme Vmax (1.6–2.0) and Km (1.2–1.4) were similar between litter-and biochar-amended soils, and remained stable with warming. However, warming reduced microbial biomass (PLFA) and enzyme activity (Vmax), suggesting decreased enzyme production associated with smaller microbial biomass or faster enzyme turnover at higher temperatures. Reductions in PLFA content and enzyme Vmax due to warming were larger in litter-amended soils (by 31%) than in the control and biochar-amended soils (by 4–11%), implying the active litter-feeding microorganisms have a smaller degree of heat tolerance than the inactive microorganisms under biochar amendments. The reduction in enzyme activity (Vmax) by warming was lower in soils with biochar than in the control soil. Our modeling suggested that the higher Q10 in litter-amended soils was mainly caused by faster C loss under warming, linked to reductions in microbial biomass and growth efficiency, rather than the slightly increased SOM-originated substrate availability (DOC). Overall, using straw-made biochar instead of straw per se as a soil amendment lowers the Q10 of SOM and FOM by making microbial communities and enzyme pools more temperature-tolerant, and consequently reduces SOM losses under warming.

Мікробіологічна деструкція органічної речовини в агроценозах

Goal. To study the microbiological processes of straw transformation during introduction into agrocenoses of microorganisms — producers of cellulase enzyme complexes, which play an important role in the biodegradation of fresh organic matter. Methods. Field — to carry out field experiment, microbiological — to determine the quantity of cellulosolytic bacteria, biochemical — to determine the cellulase, peroxidase, and polyphenol oxidase activity of the soil, statistical — to identify differences between relevant indicators and assessment of the reliability of such differences. Results. In the conditions of a long-term field experiment on leached chornozem (short-term crop rotation: winter wheat — buckwheat — lupine), microbiological processes that occur during the mineralization of 4 t/ha of straw were studied. It was found that during the growing season buckwheat in the soil (the 1-st year after the introduction of winter wheat straw) and lupine (the 2-nd year after the introduction of organic residues) increased the number of cellulosolytic microorganisms. In buckwheat agrocenoses in the variant with Microbacterium sp. 6634 their quantity amounted to 25.6 million/g of soil with a value of 10.9 million/g of soil under control. The studied soil samples from lupine agrocenoses were also characterized by a significant number of cellulose-destructive microorganisms (17.7 million/g of soil under the action of Bacillus sp. 6658) compared to the control variant (9 million/g of soil). Under the action of bacterial destructors of organic matter, they observed increased enzymatic activity of soil (cellulase, peroxidase, and polyphenol oxidase). Cellulase activity at the use of Bacillus sp. 6658 and Pseudomonas sp. 6650 increased 1.5 times. Under the influence of microorganisms introduced into agrocenoses, an increase in the biochemical coefficient of humus accumulation according to Muromtsev’s methodology was observed. Conclusions. Introduction into the agrocenosis of bacteria — members of the genera Bacillus, Microbacterium, Pseudomonas promoted the development of leached chornozem microorganisms with cellulosolytic properties and increased cellulase, peroxidase, and polyphenol oxidase activity, which was a prerequisite for strengthening the mineralization of straw. The increase in the intensity of decomposition of fresh organic matter with the introduction of plant residues was confirmed by indicators of the biochemical coefficient of humus accumulation according to Muromtsev’s methodology. The studied bacteria are promising as bioagents of microbial preparations to optimize the processes of mineralization-immobilization using plant by-products.

Organic carbon composition and microbial transformation of seagrass meadow and its responses to eutrophication

<p indent="0mm">Carbon neutrality is an important concept in global climate change governance. Seagrass beds rank among the most productive autotrophic ecosystems on the planet, and are receiving increasing attention as globally-significant hotspots of organic carbon (“blue carbon”) sequestration. The blue carbon stored in seagrass beds has important implications for climate change mitigation. The protection of seagrass beds will also protect their organic carbon sequestration potential, consequently helping to achieve the goal of carbon neutrality. Unfortunately, about 29% of the world’s seagrass beds have been destroyed, with a loss rate of 7% a⁻¹ since 1990, mostly due to eutrophication. The organic carbon sequestration potential of seagrass beds could be influenced by the subsequent changes in organic carbon composition, and microbial transformation processes. We systematically reviewed the relevant local and international research to summarize the composition and microbial transformation of organic carbon and its responses to eutrophication. Organic carbon is present in various forms in seagrass beds, and can be roughly divided into primary producer living biomass carbon, litter organic carbon, water organic carbon, and sediment organic carbon according to its storage area. Organic carbon can also be divided into labile organic carbon (LOC), which is susceptible to microbial decomposition, and recalcitrant organic carbon (ROC), which is resistant to microbial decomposition. The total ROC (cellulose and lignin) content in seagrass is present in the range of <sc>153−561 mg/g,</sc> and accounts for more than 28.7% of the organic carbon in the seagrass plants. The ROC content of seagrass plants is affected by the species, morphology, tissue and organ types, and climatic zones. In the water column, color dissolved organic matter (CDOM) and neutral sugars are indicators of LOC, while the lignin phenol and humic-like fractions of fluorescent dissolved organic matter (FDOM) are potential tracers of ROC. The DOM in water can be divided into dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). The DOC/DON and DOC/DOP ratios are used to reflect DOC lability and microbial bioavailability, with high DON and DOP concentrations indicating high DOC lability. In sediments, extractable organic carbon and microbial biomass carbon are important indicators of LOC, while aromatic hydrocarbons and alkanes are indicative of recalcitrant components. There are significant differences in the sediment organic carbon contents of temperate and tropical regions. Microorganisms play a vital role in the transformation of organic carbon. There are geographic differences in microbial composition, which are mainly influenced by the available habitats and climate zones, but there are no differences between seagrass species within a fine-scale seagrass bed. Different microorganisms have different decomposition capacities. <italic>R</italic>-strategist microbes, including Clostridiales<italic> </italic>and<italic> </italic>Gammaproteobacteria, are able to rapidly decompose LOC, while <italic>k</italic>-strategists, such as Verrucomicrobiae<italic> </italic>and Anaerolinae, can break down the more recalcitrant compounds. There is a succession of <italic>r</italic>- to <italic>k</italic>-strategists microbes during the decomposition of fresh organic matter (including fresh seagrass litter, DOC, and SOC). Eutrophication may lead to the transformation of the community structure of primary producers in seagrass beds from seagrass to macroalgae, which will increase the input of labile carbon. This consequently causes an increase in LOC in the plant biomass, water column, and sediments, but a decrease in ROC. The nutrient loading can affect the microbial community structure and enzyme activity, which will affect the microbial transformation of organic carbon. Furthermore, nutrient and LOC inputs may induce a “priming effect” that promotes the remineralization of stored organic carbon. Several future research directions for determining the transformation of seagrass beds and their organic carbon were proposed. (1) Extensive studies of the organic carbon components (LOC and ROC) in seagrass beds are required. (2) There is a need to better understand the composition, function, and gene expression of microbial communities in seagrass beds. (3) The eutrophication mechanisms in microbial communities and the related extracellular enzyme activities need to be determined. (4) The response of the organic matter decomposition process to nutrient loading and the interaction of nutrients and algae need to be studied. There is also a need to strengthen local research on the carbon storage potential of seagrass beds to mitigate global climate change and provide theoretical support for achieving carbon neutrality.

MICROBIOLOGICAL PROCESSES OF TRANSFORMATION OF CORN PLANT RESIDUES UNDER INTRODUCTION OF DESTRUCTING MICROORGANISMS IN THE AGROCENOSES

Objective. Investigate the microbiological processes of transformation of plant residues in corn under introduction of microorganisms — producers of cellulase enzyme complexes in the agrocenoses, which play an important role in the biodegradation of fresh organic matter. Methods. Field, microbiological, biochemical, statistical. Results. In the conditions of long-term field experiment on leached chernozem (short-rotation crop rotation: corn for grain – buckwheat – soybean), the microbiological processes at mineralization of 8 t/ha of leaf-stem weight of corn have been investigated. It was found that during the growing season of buckwheat in the soil (the first year after the introduction of corn residues) and soybeans (the second year after the introduction of residues), the number of cellulolytic microorganisms increases. Their number was the highest in buckwheat agrocenoses in the variant using Microbacterium sp. 6633 and amounted to 17.7 million/g of soil, whilst a value in the control variant was 11.3 million/g of soil. The second year after the application of corn residues, the highest rates of the number of these microorganisms were reported in the variant with Bacillus sp. 6605 — 15.6 million/g of soil with control parameters of 11.1 million/g of soil. Under the action of destruction bacteria of organic matter, the enzymatic activity of the soil increased, namely: cellulase, peroxidase and polyphenol oxidase ones. Cellulase activity when using Pseudomonas sp. 6650 and Microbacterium sp. 6633 increased 1.9 times. An increase in the biochemical coefficient of humus accumulation by Muromtsev under the influence of microorganisms introduced into agrocenoses was reported. Conclusion. Under the action of representatives of the genera Bacillus, Microbacterium, Pseudomonas, the processes of mineralization of corn leaf mass in leached chernozem are activated: the number of cellulolytic microorganisms, cellulase, peroxidase and polyphenol oxidase activity increases. Parameters of the biochemical coefficient of accumulation of humus according to Muromtsev indicate an increase in the intensity of decomposition of fresh organic matter under the introduction of plant residues. These microorganisms are promising for the development of a microbial preparation designed to optimize the processes of mineralization-immobilization using crop by-products.

The Dynamics of C and N by Combination of Composted Fresh Organic Waste as Soil Amendment in the Soil Thickness at Pineapple Plantation, Lampung Indonesia

The purpose of the research was to study the effect of decomposition of fresh organic waste on improvement of soil nutrients, especially the presence carbon avalability at soil surface layer. The study was conducted to evaluate the decomposition progress of fresh organic waste and canned pineapple waste on Ultisol Lampung. The experiment was designed with a completely randomized factorial design with two factors: the first was fresh organic waste (200 ton.ha-1 chopper pineapple crops, 40 ton.ha-1 cattle manure, 40 ton.ha-1 cassava waste, 40 ton.ha-1 waste pump pineapple, 2 ton.ha-1 mill juice pineapple), and the second was thickness on the top layer (i.e. 0-15 cm, 0-30 cm and 0-45 cm), the each repeated 3 replication so total treatments were 24 plot pots. The results showed that the fresh organic waste and canning pineapple waste application can increase the resistance carbon and nitrogen on the soil. The more thickness of soil indicate to the more decrease of C, N, C/N ratio. The thickness of the soil 0-15 cm of C loss above 60%, 0-30 cm loss of C is less than 30 % and less than 20 % loos C-organic for 0-45 cm. The N loss less than 50 % on 0-15 cm, and less than 40% for 0-30 and 0-45 cm. Fresh organic matter decomposition rate in the near of top soil more faster than subsoil. The thickness of 0-45 cm it showed very low total carbon whiches affected to the the value of the C/N, It smaller than C/N ratio on the soil thickness above. Its because the process of decomposition of fresh organic matter mixed with mineral soil becomes slower. The innovation of technology that applied to combination on soil thickness with organic waste and canning pineapples can increase avalability of carbon and soil nutrients at Ultisol Lampung. It is an important aspect to improve soil fertility.

Linking short-term soil carbon and nitrogen dynamics: Environmental and stoichiometric controls on fresh organic matter decomposition in agroecosystems

We developed a continuous, nonlinear model (FLOG-CN) linking carbon mineralization and nitrogen mineralization–immobilization with respect to time that successfully reproduced the complex CO2-C and SMN dynamics for a collection of 70 paired C and N soil datasets. Application of the model to diverse C and N datasets showed that incorporating latency into the model of C mineralization, and using C to drive N dynamics, allows heterogeneous data from many different soil amendments to be described by the same model. We successfully modeled complex CO2-C and SMN dynamics of widely different shapes and from a variety of soil amendments containing plant and animal residues. The re-interpretation of these datasets with the FLOG-CN models improved the quantitative analysis of C and N dynamics, yielding new insights into how amendment characteristics and experimental conditions influence the timing and quantity of C and N mineralized. Model parameters were responsive to varying soil characteristics (pH, C, N, C:N), amendment N:C, amendment rate, incubation temperature, and N additions. Stepwise regression was used to predict model parameters using metadata available for 56 of these datasets. Significant relationships were developed to estimate model parameters independently using measured system properties or other model parameters that could be independently estimated. Estimates of C and N dynamics both fell along a 1:1 line indicating that the model parameters could be adequately described by the measured properties, but the available metadata was not able to describe C dynamics with high precision. Nitrogen mineralization–immobilization was strongly related to amendment N:C, and switched between the two processes at an amendment N:C between 0.077 and 0.085 (C:N between 11.7 and 12.9). We believe that the modeling approach described here will allow quantitative and objective comparisons of diverse C and N datasets that have been hindered by subjective descriptions of the past.

High clay content accelerates the decomposition of fresh organic matter in artificial soils

Clay is generally considered an important stabiliser that reduces the rate of decomposition of organic matter (OM) in soils. However, several recent studies have shown trends contradicting this widely held view, emphasising our poor understanding of the mechanisms underlying the clay effects on OM decomposition. Here, an incubation experiment was conducted using artificial soils differing in clay content (0, 5, and 50%) at different temperatures (5, 15, and 25 °C) to determine the effects of clay content, temperature and their interaction on fresh OM decomposition. CO2 efflux was measured throughout the experiment. Phospholipid fatty acids (PLFAs), enzyme activities, microbial biomass carbon (MBC), and dissolved organic carbon (DOC) were also measured at the end of the pre-incubation and incubation periods in order to follow changes in microbial community structure, functioning, and substrate availability. The results showed that higher clay contents promoted OM decomposition probably by increasing substrate availability and by sustaining a greater microbial biomass, albeit with a different community structure and with higher activities of most of the extracellular enzymes assayed. Higher clay content induced increases in the PLFA contents of all bacterial functional groups relative to fungal PLFA content. However, clay content did not change the temperature sensitivity (Q10) of OM decomposition. The higher substrate availability in the high clay artificial soils sustained more soil microbial biomass, resulting in a different community structure and different functioning. The higher microbial biomass, as well as the changed community structure and functions, accelerated OM decomposition. From these observations, an alternative pathway to understanding the effects of clay on OM decomposition is proposed, in which clay may not only accelerate the decomposition of organic materials in soils but also facilitate the SOM accumulation as microbial products in the long term. Our results highlight the importance of clay content as a control over OM decomposition and greater attention is required to elucidate the underlying mechanisms.

Dynamics of component carbon fluxes in a semi‐arid Acacia woodland, central Australia

Vast areas in the interior of Australia are exposed to regular but infrequent periods of heavy rainfall, interspersed with long periods at high temperatures, but little is known of the carbon budget of these remote areas or how they respond to extreme precipitation. In this study, we applied three methods to partition net ecosystem photosynthesis into gross primary production (GPP) and ecosystem respiration (Re) during two years of contrasting rainfall. The first year was wet (&gt;250 mm above average rainfall), while little precipitation fell during the second year (&gt;100 mm below average). During the first year of study, rates of GPP were large (793 g C m−2 yr−1) in this semi‐arid Mulga (Acacia aneura) and grass savanna due to complementary photosynthetic responses by the canopy and C4 understorey to cycles of heavy rainfall. Patterns in GPP during the summer and autumn matched those in leaf area index (LAI), photosynthetic activity, and autotrophic respiration. During the dry year, small but positive photosynthetic uptake by Mulga contributed to the neutral carbon budget (GPP / Re = 1.06 ± 0.03). Small rates of photosynthesis by evergreen Mulga when dry were supported by storage of soil moisture above a relatively shallow hardpan. Little soil organic matter (1.1%) was available to support heterotrophic respiration (Rh) without input of fresh substrate. The two largest sources of Re in this study were autotrophic respiration by the seasonal understorey and Rh through decomposition of fresh organic matter supplied by the senescent understorey.

Effect of carbonates on the hierarchical model of aggregation in calcareous semi-arid Mediterranean soils

Lithogenic and/or secondary carbonates are present in many semi-arid Mediterranean soils. Their role in aggregation dynamics is not completely known, and can determine the sensitivity of these soils to land-use changes and soil management. The objective of this work was to test the hierarchical model of aggregation, and to determine the relationship between aggregates formation and organic matter decomposition in calcareous soils of semi-arid Mediterranean regions, by comparing macroaggregation in the upper horizon of a calcareous soil ( Typic Calcixerept) with a non-calcareous soil ( Typic Haploxerept) and a decarbonated terra rosa ( Calcic Haploxerept) following fresh straw addition under controlled laboratory conditions during 105 days. We hypothesized that aggregation would work according to the hierarchical model in the non-calcareous soil (NONCALC) and in the decarbonated terra rosa (DECALC), but not in the calcareous soil (CALC). The three soils had similar organic C concentrations and C/N ratio, but differed in their carbonates and clay minerals content: NONCALC had a similar proportion of clay-size silicates than CALC (~ 10%), and the DECALC had a similar proportion of total clay-sized particles than CALC (including carbonates, ~ 20%). An increase in the amount of macroaggregates (> 250 μm) and microaggregates (50–250 μm) held within macroaggregates associated to an increase in the fungal activity when maize straw was added, a decline of macroaggregates with time associated to organic matter decomposition, a higher concentration of organic C and maize-derived C in microaggregates within macroaggregates than in free microaggregates, and a transfer of maize-derived C from macroaggregates to free microaggregates with time confirmed that aggregation worked according to the hierarchical model in NONCALC and DECALC. In CALC, however, the hierarchical model worked only partially. Macroaggregates formation was also stimulated by organic residues addition according to the model, but stable macroaggregates showed longer turnover rates and no relationship between straw decomposition and their decline was observed. From this, we postulate that carbonates can help stabilizing macroaggregates formed from fresh organic matter decomposition through abiotic processes, such as dissolution and re-precipitation. These results help to understand field data obtained in semi-arid calcareous agricultural soils, where it has been observed that aggregation is not always directly correlated to organic matter concentration, but that increases in soil organic matter can promote aggregation. They also indicate the need for considering the composition of the mineral fraction when modelling soil structure in semi-arid Mediterranean soils.

Internal phosphorus loading from bottom sediments of a shallow preliminary reservoir

Abstract The aim of the studies done in a shallow preliminary reservoir (western Poland) was to determine the intensity and seasonal variability of phosphorus release from bottom sediments. Ex situ studies were done using intact sediment cores taken in succeeding seasons at 3 research stations. The highest phosphorus loading was observed in spring (May and April), both in 2005 and 2006. The range of loading was between 23.7 and 66.6 mgP m-2 d-1. More intensive phosphorus release during warmer months was caused by microbiological decomposition of fresh organic matter, comprised of decaying filamentous algae from the previous vegetation season. Spatial variability resulted from differences in water depth between stations and in the biomass of filamentous green algae in 2005.

The priming effect of organic matter: a question of microbial competition?

It is generally accepted that the low quality of soil carbon limits the amount of energy available for soil microorganisms, and in turn the rate of soil carbon mineralization. The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity due to the higher availability of energy released from the decomposition of fresh organic matter. Work to date, however, suggests that supply of available energy induces no effect on SOM mineralization. The mechanisms of the priming effect are much more complex than commonly believed. The objective of this review was to build a conceptual model of the priming effect based on the contradictory results available in the literature adopting the concept of nutritional competition. After fresh organic matter input to soils, many specialized microorganisms grow quickly and only decompose the fresh organic matter. We postulated that the priming effect results from the competition for energy and nutrient acquisition between the microorganisms specialized in the decomposition of fresh organic matter and those feeding on polymerised SOM.

Increased sequestration of organic carbon in soil by hydrophobic protection

The impact of soil organic carbon dynamics on the global carbon cycle is still largely uncertain despite studies of agricultural activities and control emissions of greenhouse gases to the earth's atmosphere. Improved knowledge of organic matter dynamics should lead to reduction in CO(2 )emissions. We used stable carbon isotope analysis to detect small changes in organic carbon storage and turnover upon soil treatments with a (13)C-labeled aliphatic alcohol previously partitioned into soluble humic substances of varying hydrophobicity. We found that labeled organic carbon is increasingly protected from mineralization with increased hydrophobic character of humic matter. The stabilization of organic carbon by hydrophobic protection significantly reduced decomposition during incubation time in soil. Hydrophobic protection can become an useful tool to limit decomposition of fresh organic matter in soil and thus reduce CO(2) emission from agricultural soils on a global scale.http://link. springer.de/link/service/journals/00114/bibs/9086010/90860496. htm</HEA

Phosphate mobilization in iron-rich anaerobic sediments: microbial Fe (III) oxide reduction versus iron-sulfide formation

Mechanisms of phosphate (PO4 3- ) mobilization and retention were examined in iron-rich anaerobic freshwater wetland, lake, and coastal marine sediments. Direct microbial Fe(III) oxide reduction solubilized only 3-25% of initial solid-phase PO 4 3- during sulfate-free sediment incubation experiments. Experiments with reduced, non-sulfidic solid-phase Fe(II)-rich sediment demonstrated PO4 3- sorption by the solid-phase, and chemical equilibrium calculations indicated that conditions were favorable for precipitation of Fe(II)-P04 minerals [e.g. Fe 3 (PO 4 ) 2 ] in such sediments. These results suggested that much of the PO 4 3- released from Fe(III) oxides during microbial Fe(III) reduction was captured by solid-phase reduced iron compounds (Fe(II) hydroxide-PO4 complexes and/or Fe(II)-PO4 minerals). Enhanced liberation of PO 4 3- to sediment porewaters (33-100% of initial solid-phase PO 4 3- ) occurred during anaerobic incubation in the presence of abundant sulfate and was directly correlated with sulfate reduction and iron-sulfide mineral formation. Incubation of PO 4 3- -amended sediment with different amounts of sulfate demonstrated a linear correlation between PO 4 3- release and sulfate reduction. Release of PO 4 3- to sediment porewaters during decomposition of fresh organic matter (freeze-dried cyanobacteria) was more extensive in sulfate-amended (67% of added organic P) than in sulfate-free sediment (17% of added organic P), and the ratio of dissolved PO 4 3- released to organic carbon oxidized was seven-fold higher in sulfate-amended sediment despite a common level of overall organic C and P mineralization in the two treatments. Our results demonstrate that iron-rich anaerobic sediments can immobilize substantial amounts of PO 4 3- under Fe(III) oxide-reducing conditions, but that extensive PO 4 3- release will take place if sediment Fe compounds are converted to iron-sulfides via bacterial sulfate reduction.

Notes on Types of Bacteria Associated with Plant Roots

The occurrence of greater numbers of micro6rganisms in the vicinity of plant roots than in soil free from root penetration has become generally established (1-5). The magnitude of the increase in micropopulations on root surfaces or in the rhizosphere has been shown to be influenced by age of plant or stage of plant development, type of plant, disease resistance or region of the rhizosphere (3, 6-7), but to remain relatively uninfluenced by soil fertilization procedures (7-10). With the exception of detailed information concerning the association of symbiotic nitrogen-fixing bacteria with the leguminous plants, knowledge of types of bacteria associa.ted with root development is still limited, although there has been a number of valuable contributions to this subject. From cultural studies Starkey has concluded that all groups of bacteria do not respond equa.lly to the presence of plant roots, and from direct microscopic examinations he concluded that coccoid cells predominate, whereas spore-forming rods were seldom found (3, 11, 12). Lohnis (13) and Smith (14) have noted increased numbers of B. radiobacter in soil in the presence of plant roots. Lochhead (15) has reported Corynebacteria and Mycobacteria, common in soil generally, to be depressed in the rhizosphere, whereas Mycoplana is increased. Krassilnikov (16, 17) also noted that different groups of micro6rganisms responded differently to plant development, with cellulose-decomposing types active, and the predominating types nonspore forms. Azotobacter was found poorly distributed in the rhizosphere of corn, and absent in that of wheat; B. mnycoides a.nd B. megatherium were found in neither. But Poschenreider (18) found Azotobacter distributed in the rhizosphercs of a great variety of plants, and Truffaut and Vladykov (19) found both B. mycoides and B. megatherium and other Bacillus spp., and also Azotobacter and other special nutritional types generally prevalent in the rhizosphere of wheat. Verona and Luchetti (20), Isakora (21) and others have recognized that qualitative differences exist between soil microfloras and root microfloras, and Thom (22), in a broader discussion of the localization of microorganisms, noted that most of the organisms in contact with root surfaces belong to species active in the decomposition of fresh organic matter, and not with species associated with the breakdown of humus residues in the soil. These citations indicated a field which will require extensive study before the broader agreements between different root microfloras and the valid specific differences or peculiarities of each rhizosphere can become established. Our investigations conducted during the course of the last two years upon the