Mineralization Rate Of Organic Matter Research Articles

Influence of sediment permeability and mineral composition on organic matter degradation in three sediments from the Gulf of Aqaba, Red Sea

In order to investigate the influence of sediment physical and chemical characteristics on the degradation of deposited organic matter, decomposition in three sediments from the Gulf of Aqaba (Red Sea) that differ in permeability and mineral composition were compared. Freeze-dried Spirulina was added to coarse carbonate and silicate sands from a shallow nearshore region and silt-clay sediment from the deeper center region of the Gulf incubated in laboratory chambers. The stirring in the chambers caused higher solute exchange in the coarse permeable sands relative to the fine less permeable silt due to the generation of advective fluid exchange between the sediment and overlying water. This enhanced exchange increased the decomposition rates of organic matter in the incubated sands. The decomposition rates of total organic carbon in the permeable carbonate (3.0 mg C m −2 d −1) and silicate sands (2.0 mg C m −2 d −1) exceeded that in the fine-grained sediment (1.4 mg C m −2 d −1). Oxygen consumption in the coarse sands was 3-fold higher than in the silt-clay sediment, with highest rates in the carbonate sand. In carbonate and silicate sands of the same grain size, the carbonate sediment was more permeable than the silicate, resulting in 1.4-fold higher fluid exchange rates and 1.4-fold larger sedimentary organic matter mineralization rates. An in situ experiment comparing trapping efficiencies in carbonate and silicate sands showed that the higher fluid exchange rate in the carbonate sand results in larger filtration rates and a faster accumulation of particulate organic matter from the boundary layer. These experiments demonstrate that with respect to sedimentary mineralization rates, higher transport rates in permeable coarse sediments can outweigh the effect of a higher specific surface area in fine-grained silt sediments. In permeable sands, however, the higher specific surface area and fluid exchange in biogenic carbonate sands result in higher mineralization rates than in silicate sands of the same grain size.

Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O 2, pH, pCO 2 and Ca 2+ (in situ), pore-water concentration profiles of NO 3 −, NH 4 +, Fe 2+, and Mn 2+ and SO 4 2− (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O 2, NO 3 −, Fe(OH) 3 and SO 4 2−, reoxidation reactions involving Fe 2+ and Mn 2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm −2 yr −1, of which 77% were due to O 2, 17% to NO 3 − and 3% to Fe(OH) 3 and 3% to SO 4 2−. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d −1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO 3 flux to the sediment was estimated to occur at a rate of 42 g m −2 yr −1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m −2 yr −1, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.

Influence of urease and nitrification inhibitors on N losses from soils fertilized with urea

The aim of this study was to evaluate how the N losses through volatilization and leaching from soils fertilized with urea can be affected by the application of a urease inhibitor or a urease plus a nitrification inhibitor. The experiment was carried out using lysimeters with 15N-labelled urea and N-(n-butyl) thiophosphoric triamide (NBPT) as urease inhibitor and dicyandiamide (DCD) as nitrification inhibitor, comparing three different treatments: urea alone (U), urea + NBPT (UN) and urea + NBPT + DCD (UND). Both volatilization and leaching were significantly different in the soils used, according to their physico-chemical characteristics. However, the pattern of the loss was similar: the volatilization was significantly reduced by NBPT (UN), but the presence of DCD (UND) significantly increased the loss, with respect to UN. Considering leaching, the highest amount of NO3 – was lost with UND, the lowest with U. The greatest amount of N lost by leaching was soil-derived N produced by the N mineralization-immobilization turnover. We suggest that, by maintaining the NH4 + in the soils, the inhibitors, in particular DCD, caused a priming effect with a subsequent increase in the rate of soil organic matter mineralization and an extra release of soil organic N. The priming effect was real in the sandy loam (SL) soil where a net N release was observed, whereas in the clay loam (CL) soil the effect of the inhibitors was less pronounced and an apparent priming effect was observed; however, a real priming effect also cannot be excluded in this soil.

Impact of several common tree species of European temperate forests on soil fertility

The aim of the present work was to provide a synopsis of the scientific literature concerning the effects of different tree spe- cies on soil and to quantify the effect of common European temperate forest species on soil fertility. The scientific literature dealing with the tree species effect on soil has been reviewed. The composition of forest overstory has an impact on the chemical, physical and biolo- gical characteristics of soil. This impact was highest in the topsoil. Different tree species had significantly different effects on water ba- lance and microclimate. The physical characteristics of soils also were modified depending on the overstory species, probably through modifications of the soil fauna. The rates of organic matter mineralization and nitrification seem to be dependent on tree species. A coni- ferous species, Picea abies, had negative input-output budgets for some nutrients, such as Ca and Mg. This species promoted a higher soil acidification and a decrease in pH. Thus, it should not be planted in very poor soils in areas affected by acidic atmospheric deposi- tions. Nevertheless, the effect of the canopy species on soil fertility was rarely significant enough to promote forest decline. The impact of a tree species on soil fertility varied depending on the type of bedrock, climate and forest management. forest soils / tree species / fertility / sustainability / resiliency

Soil carbon dynamics and potential carbon sequestration by rangelands

The USA has about 336 Mha of grazing lands of which rangelands account for 48%. Changes in rangeland soil C can occur in response to a wide range of management and environmental factors. Grazing, fire, and fertilization have been shown to affect soil C storage in rangelands, as has converting marginal croplands into grasslands. Carbon losses due to soil erosion can influence soil C storage on rangelands both by reducing soil productivity in source areas and potentially increasing it in depositional areas, and by redistributing the C to areas where soil organic matter mineralization rates are different. Proper grazing management has been estimated to increase soil C storage on US rangelands from 0.1 to 0.3 Mg C ha −1year −1 and new grasslands have been shown to store as much as 0.6 Mg C ha −1year −1. Grazing lands are estimated to contain 10–30% of the world’s soil organic carbon. Given the size of the C pool in grazing lands we need to better understand the current and potential effects of management on soil C storage.

A conceptual approach to the biomonitoring of freshwater: the Ecological Ambience System

The concept of ecological ambience (Ecological Ambience System, EASY) is based on the idea that biocenoses (BIO) are not only related to the input of organic and mineral substances (IN) but also to the way they are stored and processed by the ecosystem. Storage, assimilation and self-purification processes ("ecosystem defences": ED) are likely to vary among the different functional units (FUs) of the ecosystem. The functional units have been defined on the basis of a simple physical description of sites in an ecosystem, because the physical structure of these units is considered as being of prime importance in the ED processes. For example, mineral and organic substances may be preferentially stored in fine-sediment units, whereas the mineralization rate of organic matter is more likely to be highest in coarse permeable sediments. If the stream ecosystem is viewed as a mosaic, its overall ecological defences will depend upon: 1) the self-purification capacity of the different functional units; 2) their relative proportion within the ecosystem. The EASY concept is now used for ecological researches and also has several applications in the biomonitoring of running waters, illustrated by the study of the River Dore. Biological compartments, specific for each functional unit, are related to operational bio-indicators to build up a global harmonisation system for biomonitoring indices. Four main biological compartments were defined with their related bio-indicators: 1) general biological quality (IBGN biotic index), 2) biological sediment quality (IOBS oligochaete index), 3) biological water quality (diatom index IBD) and 4) biological fish quality (study of fish communities). The selected bio-indicators were adjusted to an ecological classification model (Typic concept). A weighting system of the general ecological quality at a site based on the percentage cover of fine sediments is proposed. This approach can be developed using several other compartments. Major difficulties and potential improvements are discussed.

A sensitivity analysis of the prediction of the nitrogen fertilizer requirement of cauliflower crops using the HRI WELL_N computer model

HRI WELL_N is an easy to use computer model, which has been used by farmers and growers since 1994 to predict crop nitrogen (N) requirements for a wide range of agricultural and horticultural crops.A sensitivity analysis was carried out to investigate the model predictions of the N fertilizer requirement of cauliflower crops, and, at that rate, the yield achieved, yield response to the fertilizer applied, N uptake, NO3-N leaching below 30 and 90 cm and mineral N at harvest. The sensitivity to four input factors – soil mineral N before planting, mineralization rate of soil organic matter, expected yield and duration of growth – was assessed. Values of these were chosen to cover ranges between 40% and 160% of values typical for field crops of cauliflowers grown in East Anglia. The assessments were made for three soils – sand, sandy loam and silt – and three rainfall scenarios – an average year and years with 144% or 56% of average rainfall during the growing season. The sensitivity of each output variable to each of the input factors (and interactions between them) was assessed using a unique ‘sequential' analysis of variance approach developed as part of this research project.The most significant factors affecting N fertilizer requirement across all soil types/rainfall amounts were soil mineral N before planting and expected yield. N requirement increased with increasing yield expectation, and decreased with increasing amounts of soil mineral N before planting. The responses to soil mineral N were much greater when higher yields were expected. Retention of N in the rooting zone was predicted to be poor on light soils in the wettest conditions suggesting that to maximize N use, plants needed to grow rapidly and have reasonable yield potential.Assessment of the potential impacts of errors in the values of the input factors indicated that poor estimation of, in particular, yield expectation and soil mineral N before planting could lead to either yield loss or an increased level of potentially leachable soil mineral N at harvest.The research demonstrates the benefits of using computer simulation models to quantify the main factors for which information is needed in order to provide robust N fertilizer recommendations.

Evolution of organic matter and nitrogen during co-composting of olive mill wastewater with solid organic wastes

Four olive mill wastewater (OMW) composts, prepared with three N-rich organic wastes and two different bulking agents, were studied in a pilot plant using the Rutgers system. Organic matter (OM) losses during composting followed a first-order kinetic equation in all the piles, the slowest being the OM mineralisation rate in the pile using maize straw (MS). The highest N losses through NH3 volatilisation occurred in the mixtures which had a low initial C/N ratio and high pH values during the process. Such losses were reduced considerably when MS was used as the bulking agent instead of cotton waste (CW). N fixation activity increased during the bio-oxidative phase before falling during maturation. This N fixation capacity was higher in piles with a lower NH4+-N concentration. Only the composts prepared with OMW, CW and poultry manure or sewage sludge reached water-soluble organic C (CW) and NH4+-N concentrations and CW/Norg and NH4+/NO3– ratios within the established limits which indicate a good degree of compost maturity. Increases in the cation-exchange capacity, the percentage of humic acid-like C and the polymerisation ratio revealed that the OM had been humified during composting. The germination index indicated the reduction of phytotoxicity during composting.

Effect of No-Tillage and Conventional Tillage Systems on the Chemical Composition of Soil Solid Phase and Soil Solution of Brazilian Savanna Oxisols

No-tillage (NT) cropping systems are becoming increasingly important in the Brazilian savanna. To evaluate their sustainability we compared soil chemical properties in 1- to 3-year-old NT systems following 9 to 11 years of conventional tillage (CT) with systems where CT was continuously in place for 12 years. In the rainy season 1997/98, NT was cropped with soybean and CT with corn while in the rainy season 1998/99 both systems were cropped with soybean. Soil solid phase samples were taken from the 0-0.15, 0.15-0.3, 0.3-0.8, 0.8-1.2, and 1.2-2 m layers on three spatially separated plots under each of NT and CT. Soil solution samples were collected weekly at 0.15, 0.3, 0.8, 1.2, and 2 m soil depth during two rainy seasons (14 October to 28 April 1997/98 and 1998/99). We determined soil moisture contents, pH, the concentrations of exchangeable cations, the electrical conductivity (EC) of the soil solution, and the concentrations of Al, C, Ca, Cl - , K, Mg, Mn, Na, NH 4 + , NO 3 - , P, S, and Zn in solid soil and soil solution samples. Differences in soil solid phase properties and moisture content between NT and CT were small, few were significant. Under NT, the average solution pH was significantly lower (5.5), Al (26 tag 1 -1 ), Mn (17 μg 1 -1 ) and total organic C concentrations (TOC, 6.5 mg 1 -1 ) were higher than under CT (pH: 6.0, Al: 14μg 1 -1 , Mn: 14μg 1 -1 , TOC: 5.5 mg 1 -1 ). Irrespective of the different crops in the first rainy season, under NT, the EC (205 μS cm -1 ), Ca (17 mg 1 -1 ), and Mg (2.9 mg 1 -1 ) concentrations at 0-0.3 m depth were lower than under CT (EC: 224 μS cm -1 , Ca: 25 mg 1 -1 , Mg: 5.6 mg 1 -1 ). At 1.2-2 m depth, the reverse order was observed (EC: 124 μS cm -1 under NT and 84 μS cm -1 under CT, Ca: 11 mg 1 -1 under NT and 7.5 mg 1 -1 under CT, Mg: 3.1 mg 1 -1 under NT and 1.8 mg 1 -1 under CT). Our results indicate that enhanced soil acidification because of higher rates of organic matter mineralization and a more pronounced nutrient leaching because of increased pore continuity may limit the sustainability of NT.

Stochastic use of the LEACHN model to forecast nitrate leaching in different maize cropping systems

This paper proposes a stochastic application of a deterministic model (LEACHN) with the aim of forecasting the probability of exceeding given nitrate leaching levels for different cropping systems and soil hydrological characteristics. The understanding of the level of probability associated to the prediction of leaching is an important criteria for the judgement of cropping systems. After calibration of organic matter mineralization and nitrification rates in both a sandy-loam and a loamy soil of the Western Po river valley (Northern Italy), LEACHN was used as a stochastic tool to evaluate the meteorological variability and the spatial variability of hydrological parameters of a soil. Meteorological variability was generated using series of measured air temperature, rainfall and global radiation for a period of at least 15 years, and these were then expanded to 100 years using the climate simulator CLIMGEN. Soil variability was simulated using the scale factor approach. The scale factor mean and standard deviation were obtained in eight locations within a 1000-m 2 area. The stochastic scale factors were applied to parameters a and b in Campbell’s water retention function and to hydraulic conductivity. The following combinations of crops were simulated in the two soils: (a) continuous maize for silage (MM); (b) continuous maize for grain (MG); (c) a combination of late harvested Italian ryegrass and short cycle silage maize (LRM); and (d) early harvested Italian ryegrass and late maturing silage maize (ERM). The crops were fertilized with 200 or 300 kg N ha −1 year −1 (or 450 kg for MM) and submitted to three water regimes: no irrigation, irrigation on the basis of water balance and conventional irrigation, which resulted in the highest volume. The simulated leaching was higher when fertilization and irrigation inputs were higher. It was further reduced by the introduction of cover-crop and was higher in the sandy soil. All these factors interacted, creating different levels of nitrate loss risk, that ranged from a minimum leaching of 4 kg N ha −1 year −1, with a 10% breakthrough probability of 16 kg N ha −1 year −1 (low fertilized and irrigated ERM in the sandy-loam soil) to a maximum average leaching of 146 kg N ha −1 year −1 with a 10% breakthrough probability of 235 kg N ha −1 year −1 (high fertilized, conventionally irrigated MM in the sandy soil). The breakthrough probability curves associated to nitrate leaching are skewed, showing that lower than average values are more frequent than higher ones. The standard deviations of yearly leaching were closely correlated to the means and were frequently greater than the means, particularly in the sandy soil. The stochastic simulation results offered the possibility of ranking cropping systems into classes of probability of exceeding a given value of leaching and the possibility of deriving suggestions for improved crop management.

Organic carbon variations of the eastern Mediterranean Holocene sapropel: a key for understanding formation processes

This work reports on the organic carbon content of the S1 Holocene sapropel, the youngest and most extensively researched of the Eastern Mediterranean sapropels. The peak organic carbon percentage of the S1 sapropel varies between 1.2% and 3.4% at varying locations. These variations could not be correlated with sapropel thickness, or with spatial location. In contrast, there was a close linear correlation with water depth, the deeper samples containing the highest values of organic carbon. Within the S1 layer, the highest values were found in the top of the sapropel. Even if primary productivity increased during the S1 sapropelic period, the main factors responsible for sapropel deposition were changes in the degradation and preservation of organic matter in the water column and at the sediment–water interface. Water column stratification created an oxygenated surficial layer, 0–350/400 m thick, and a stagnant deep water layer. The majority of the organic material escaping the surficial layer was remineralised either at the sediment–water interface or just below it. Organic matter preserved in sapropelic sediments can be assumed to have remained for a significantly longer time (∼1000 years) within the benthic boundary layer than in the water column. Organic flux decreased with depth through the stagnant deep water layer. When stagnation was established, water exchanges either did not exist or were limited. Vertical oxidant transport depended on eddy diffusion, while oxidant fluxes at the sediment–water interface decreased with depth. The organic matter mineralisation rate also decreased and the burial efficiency increased. The linear relationship between sapropel organic content and depth was reflected in a linear decrease in oxidant flux. Regardless of location, the same organic carbon percentage was preserved at the same depth. In contrast, in the same area but at different depths, organic carbon content ranges between 1.2 and 3.4%. This observation suggests that primary productivity was of the same order of magnitude basinwide. The organic carbon concentration of sediment should be used with caution as a quantitative record of palaeoproductivity.

A combined simulation model of Scots pine, Norway spruce and Silver birch ecosystems in the European boreal zone

A combined (hybrid) spatially distributed individual-based model of a forest ecosystem (EFIMOD) has been elaborated for a Scots pine, Norway spruce and Silver birch stand of the European boreal zone. This model represents a realization of the concept of a `single plant ecosystem' ( Chertov, 1983a). EFIMOD simulates the biological cycle of every tree with an expedient soil pot in a stand. The tree models calculate the total leaf, fine root and litter mass using a set of the characteristic ecological tree parameters. The soil sub-model simultaneously allows for a calculation of the rate of tree litter and soil organic matter mineralization with the corresponding carbon dioxide and nitrogen release for plant growth. The validation of the model, with various initial stand densities on different soils, shows a relatively realistic picture of tree growth, stand self-thinning and soil changes. Convergence of leaf/needle mass has been observed at various initial stand densities. An application of EFIMOD for a qualitative evaluation of climate change effects shows fairly different reactions of the mentioned ecosystems in relation to productivity, leaf mass and soil organic matter pool. There is a shift in the timing of the first intensive self-thinning in young stands. A strong oscillation of leaf/needle mass has also been observed.

Effect of Crude Oil and Chemical Additives on Metabolic Activity of Mixed Microbial Populations in Fresh Marsh Soils.

> Abstract Hydrocarbons increase abundance of hydrocarbon-degrading microorganisms, but also decrease microbial diversity. This could disrupt ecosystem dynamics by altering soil organic matter mineralization and resultant nutrient remineralization rates. Crude oil, which is known to contain toxins and reduce microbial diversity, was hypothesized to reduce gross metabolic activity of mixed microbial populations in wetland soils. Soil respiration and Eh were compared, for 6 months, among microcosms containing marsh soils that differed in soil organic matter (Panicum hemitomon Shult. or Sagittaria lancifolia L. dominated marshes), crude oil (Arabian crude, Louisiana crude, or no oil), and additives (a cleaner, a dispersant, fertilizer, or no additive). No treatment slowed activity; instead, Louisiana plus fertilizer and all Arabian treatments temporarily accelerated activity. Additional C respired from oiled microcosms exceeded C added as crude oil by 1.4 to 3.5 times. Thus, much additional C originated from soil organic matter rather than crude oil. Crude oils temporarily lowered soil Eh, which is consistent with accelerated metabolism and demand for electron acceptors. The lack of inhibition observed at the community level does not necessarily indicate an absence of toxicity. Instead, tolerant species with metabolic versatility probably maintained activity. Stimulation probably resulted from removal of micronutrient limitation, rather than removal of grazing pressure or macronutrient limitation. Regardless, accelerated soil organic matter mineralization surely accelerated nutrient remineralization. This might explain some reports of crude oil stimulating plant growth. These results are not inconsistent with theoretical and experimental conclusions regarding effects of biodiversity on ecosystem stability and productivity, nor are they inconsistent with conclusions that crude oils contain components that are toxic to microbes, vegetation, and fauna. However, these data do indicate that crude oils also contain components that temporarily stimulate metabolic activity of surviving microbes.

Isoproturon Sorption and Degradation in a Soil from Grassed Buffer Strip

AbstractThe fate of the herbicide isoproturon [3‐(4‐isopropylphenyl)‐l,1‐dimethylurea] was investigated in soil from a grassed buffer strip. Compared to a cropped soil originating from the same experimental site (Kd = 1.8 L kg−1), sorption of isoproturon was enhanced in the grassed soil and especially in the surface layer (0‐2 cm) containing high proportion of nondecomposed plant residues (Kd = 5.0 L kg−1). Nonhumified organic fractions isolated from the surface soil layer and corresponding to above and bdow‐ground plant residues derived from the grass exhibited high sorption coefficients Kd and Koc compared to the rest of the soil. Reversibility of sorption was lower in the grassed soil than in the cropped soil and decreased rapidly with time. A rapid degradation of isoproturon was observed at different depths of the grassed soil whereas most of the herbicide remained nondegraded in the cultivated soil: half‐lives were respectively 72 d in the cultivated soil, and only 8 d in the superficial layer (0–2 cm) of the grassed soil. The highest mineralization rate of the isoproturon ring (20% after 35 d) was observed in the top layer (0–2 cm) having the highest mineralization rates of organic matter. In relation with this fast degradation, a large proportion of isoproturon residues became nonavailable to water and methanol extractions (54% of the initial applied isoproturon found as nonextractable (bound) residues). Thus the grassed strip surface soil had a high potential to dissipate isoproturon trapped from run‐off.

Tillage system effects on 15-year carbon-based and simulated N budgets in a tile-drained Iowa field

Tillage influences N fate and transport by changing soil structure, aeration, macropore continuity, plant-residue placement, and organic-matter mineralization rates. Our objective was to use 15-year N budgets to compare four primary tillage treatments for continuous corn ( Zea mays L.) production on tile-drained Aquic Hapludolls (FAO: Haplic Phaeozems) in northeastern Iowa, USA. A carbon-based N budget used annual grain yield, grain-N concentrations measured in 1992, changes in surface-soil C content between 1977 and 1988 or 1992, surface-soil C : N ratios, and measurements of NO 3–N lost in tile-drainage water. It accounted for 98, 104, 99, and 99% of the fertilizer N applied to moldboard-, chisel-, ridge-, and no-tillage-treatments, respectively. Averaged for 1977 through 1992, increased soil organic matter, harvested grain, and tile drainage accounted for ≈42, 45, and 13% of the N budget, respectively. Simulated N budgets were computed using version 3.25 of the root-zone water quality model (RZWQM). The best grain-yield predictions for 13 of the 15 years were 9% higher than the measured values, and if extreme outliers were eliminated, the predicted values were correlated (r 2 =0.75) with the average measured yield for the four tillage treatments. Simulations for 1988 and 1989 failed completely because RZWQM could not accurately describe hydrology associated with low rainfall seasons. Predicted total N accumulation was much higher than measured in 1990, 1991, or 1992. Estimates of profile NO 3–N, mineralization, seepage loss, and denitrification were not satisfactory, presumably because the model failed to simulate an accurate hydrology for the different tillage practices. We conclude that the simulation results were not suitable for predicting the fate of fertilizer N. However, both approaches for computing N budgets suggested that adopting ridge tillage, without changing N rates and other management practices related to N application technology and crop sequencing, will not reduce the potential for off-site water quality degradation.

Effects of a transient oxic period on mineralization of organic matter to CH4 and CO2 in anoxic peat incubations

Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH4 and CO2, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH4 and CO2 to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH4 production, oxic conditions during period 2 did not result in a lower potential CH4 production rate during period 3, although production was delayed ∼1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH4, oxic conditions during period 2 resulted in zero production of CH4 during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH4 production, the oxic period had a negligible effect on anoxic CO2 production during period 3, in surface as well as deep peat. In both surface and deep peat, CO2 production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO2 production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.

Organic matter mineralization rates in sediments: A within‐ and among‐lake study

Organic matter mineralization rates were measured by the accumulation of DIC + CH4 in the water overlying intact cores taken from littoral and protimdal sediments of nine Quebec lakes. The variability in areal carbon mineralization is much greater within lakes than among lakes varying in trophic richness. Organic matter mineralization in littoral sediments is more variable and, on average, threefold higher than in the profundal sediments. Sixty percent of the variation in mean summer mineralization rates is explained by site depth, a surrogate variable that incorporates the effect of temperature and may also be reflecting substrate quality and(or) supply. The lakes‐pecific characteristics most strongly correlated to the residuals of the regression with depth are catchment area‐to‐lake area ratio (CA:LA) and water residence time. In lakes with a larger CA:LA and a shorter residence time, the amount and(or) the quality of organic matter settling to the sediments at a given depth may be reduced, resulting in the lower observed mineralization rates. Total mineralization in the sediments is, not surprisingly, greater in larger lakes but the rate per unit area is smaller, reflecting the decreased importance of the littoral zone. More than half (54–100%) of the DIC + CH4 produced in the sediments is from the littoral zone. Yet, because of the large biomass in epilinmetic waters, the littoral sediments account for <20% of the sum of metabolism in the epilimnetic water column and underlying sediments. The relative importance of the sediments in metabolism in lakes is a function of both the trophy and lake morphometry. In deep lakes a smaller proportion of total respiration occurs in the sediments than in shallow lakes, and in eutrophic lakes the sediments account for a smaller proportion of total respiration than in oligotrophic lakes.

Assessing organic matter mineralization, degradability and mixing rate in an ocean margin sediment (Northeast Atlantic) by diagenetic modeling

We test whether organic matter degradability, mixing activity, and total sediment mineralization can be estimated by inversion of a coupled nonlinear numerical steady-state diagenetic model. We use a single data set comprising oxygen, nitrate, ammonium and organic carbon versus depth profiles from a slope station in the Goban Spur area (1034 m, Northeast Atlantic). Based on an extensive sensitivity analysis, it appears that (1) when using all data, the total mineralization rates can be determined with reasonable precision; bioturbation and degradability are less well constrained and (2) total mineralization rates can be determined based on nitrate and oxygen profiles only; estimates of organic matter mixing rates and degradability are refined when including the solid phase organic carbon profile. The bulk mixing rates obtained for organic carbon are one order of magnitude higher than mixing rates previously estimated from Pb-210 profiles, lending validity to the hypothesis that organic particles are mixed faster than inert particles. The degradability of the organic carbon prior to its incorporation in the sediment is in the order of 10-30 yr(-1), indicating that mineralization in this slope station of the Goban Spur area is fueled mainly by freshly deposited organic matter. [KEYWORDS: Benthic oxygen-demand; deep-sea sediments; marine-sediments; hemipelagic sediments; nutrient diagenesis; equatorial pacific; water interface; carbon; bioturbation; nitrogen]

The dynamics of carbon in particle-size fractions of soil in a forest-cultivation sequence

Cultivation of forest and grassland soils induces heavy changes in soil organic matter (SOM) dynamics. To better predict the effect of cultivation, there is a need to describe which organic pools are affected and to which extent. We used a chronosequence of thick humic forest soils converted to maize cultivation for 40 yr in southwest France. The dynamics of soil carbon was investigated through particle-size fractionation and the use of 13C allowed to distinguish forest-derived organic matter and new crop-derived organic matter. This partitioning of soil carbon by size on one hand and by age on the other provided a precise description of carbon turnover. The level towards which tend the organic pools under cultivation showed that the decay rates of soil carbon were one order of magnitude higher under cultivation than under forest. SOM can thus be considered as deprotected under cultivation. All size fractions appeared to be deprotected to the same extent. A progressive transfer of silt-sized C to clay-sized C was nevertheless suspected and attributed to the decreasing stability of fine silt-sized microaggregates with cultivation. SOM furthermore contained some very stable C present as silt-sized and possibly clay-sized particles. The turnover times of maize-derived organic matter was the same as that observed in similar soils cultivated for centuries. This indicated that the new conditions induced by cultivation were reached in the very first years after forest clearing and that the high initial SOM content and high mineralization rate of initial organic matter did not affect the dynamics of newly incorporated carbon.

Changes in the organic matter mineralization rates of an arid soil after amendment with organic wastes

This study examined the changes in organic matter mineralization when six amendment rates of municipal solid waste, sewage sludge, and compost were added to an arid soil. Short‐time incubation assays were carried out in which the amount of CO2 emitted was measured. The kinetic and mineralization constants of the organic matter were studied, as was the influence, of this amendment on the soil organic matter content. Potentially mineralizable C (C0) in the municipal solid waste‐amended soil was significantly higher than in the soil amended by sewage sludge and compost and it increased as the amendment rate of compost and sewage sludge increased. There was no direct relation between the increase in organic matter decomposition rate and the amount of organic amendments that were applied to the soil. The CO2 loss/total organic carbon (CO2 loss/TOC) ratio in the soils amended with fresh organic waste was significantly higher than in that amended with compost. The CO2 loss/TOC ratio also differed with amendment rate between fresh and composted wastes. The ratio rose slightly as the amendment rate of fresh wastes increased up to 2%, after which it stabilized, whereas it decreased as the amendment rate increased in the soils amended with compost. Linear fittings were made of the CO2 evolved as a function of dose for the different sampling times. The carbon mineralized increased in all treatments, more so in the soils amended with fresh wastes than in those amended with compost. The organic amendments resulted in a priming effect that was more pronounced during the first days of incubation and differs according to the nature of the material added.