Showing papers on "Soil organic matter published in 1998"

Organic acids in the rhizosphere: a critical review

Abstract: Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Soil Fertility and Fertilizers: An Introduction to Nutrient Management

Abstract: 1. Introduction. 2. Basic Soil-Plant Relationships. 3. Soil Acidity and Alkalinity. 4. Nitrogen. 5. Phosphorus. 6. Potassium. 7. Sulfur, Calcium, and Magnesium. 8. Micronutrients. 9. Soil Fertility Evaluation. 10. Basics of Nutrient Management. 11. Nutrients, Water Use, and Other Interactions. 12. Economics of Plant-Nutrient Use. 13. Agricultural Productivity and Environmental Quality. Index.

Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest

Abstract: Variation in soil temperature can account for most of the seasonal and diel variation in soil CO2 efflux, but the temperature effect is not always consistent, and other factors such as soil water content are known to influence soil respiration. The objectives of this research were to study the spatial and temporal variation in soil respiration in a temperate forested landscape and to evaluate temperature and soil water functions as predictors of soil respiration. Soil CO2 fluxes were measured with chambers throughout an annual cycle in six study areas at the Harvard Forest in central Massachusetts that include soil drainage classes from well drained to very poorly drained. The mean annual estimate of soil CO2 efflux was 7.2 Mg ha–1, but ranged from 5.3 in the swamp site to 8.5 in a well-drained site, indicating that landscape heterogeneity is related to soil drainage class. An exponential function relating CO2 fluxes to soil temperature accounted for 80% of the seasonal variation in fluxes across all sites (Q10 = 3.9), but the Q10 ranged from 3.4 to 5.6 for the individual study sites. A significant drought in 1995 caused rapid declines in soil respiration rates in August and September in five of the six sites (a swamp site was the exception). This decline in CO2 fluxes correlated exponentially with decreasing soil matric potential, indicating a mechanistic effect of drought stress. At moderate to high water contents, however, soil water content was negatively correlated with soil temperature, which precluded distinguishing between the effects of these two confounded factors on CO2 flux. Occurrence of high Q10 values and variation in Q10 values among sites may be related to: (i) confounding effects of high soil water content; (ii) seasonal and diel patterns in root respiration and turnover of fine roots that are linked to above ground phenology and metabolism; and (iii) variation in the depth where CO2 is produced. The Q10 function can yield reasonably good predictions of annual fluxes of CO2, but it is a simplification that masks responses of root and microbial processes to variation in temperature and water content throughout the soil.

Influence of lime, fertilizer and manure applications on soil organic matter content and soil physical conditions: A review

Abstract: The effects of lime, fertilizer and manure applications on soil organic matter status and soil physical properties are of importance to agricultural sustainability. Their effects are complex and many interactions can occur. In the short-term, liming can result in dispersion of clay colloids and formation of surface crusts. As pH is increased the surface negative charge on clay colloids increases and repulsive forces between particles dominate. However, at higher lime rates, Ca2+ concentrations and ionic strength in soil solution increase causing compression of the electrical double layer and renewed flocculation. When present in sufficient quantities, both lime and hydroxy-Al polymers formed by precipitation of exchangeable Al, can act as cementing agents bonding soil particles together and improving soil structure. Liming often causes a temporary flush of soil microbial activity but the effect of this on soil aggregation is unclear. It is suggested that, in the long-term, liming will increase crop yields, organic matter returns, soil organic matter content and thus soil aggregation. There is a need to study these relationships on existing long-term liming trials. Fertilizers are applied to soils in order to maintain or improve crop yields. In the long-term, increased crop yields and organic matter returns with regular fertilizer applications result in a higher soil organic matter content and biological activity being attained than where no fertilizers are applied. As a result, long-term fertilizer applications have been reported, in a number of cases, to cause increases in water stable aggregation, porosity, infiltration capacity and hydraulic conductivity and decreases in bulk density. Fertilizer additions can also have physico-chemical effects which influence soil aggregation. Phosphatic fertilizers and phosphoric acid can favour aggregation by the formation of Al or Ca phosphate binding agents whilst where fertilizer NH4 + accumulates in the soil at high concentrations, dispersion of clay colloids can be favoured. Additions of organic manures result in increased soil organic matter content. Many reports have shown that this results in increased water holding capacity, porosity, infiltration capacity, hydraulic conductivity and water stable aggregation and decreased bulk density and surface crusting. Problems associated with large applications of manure include dispersion caused by accumulated K+, Na+ and NH4 + in the soil and production of water-repellant substances by decomposer fungi.

Legume-based cropping systems have reduced carbon and nitrogen losses

Abstract: In agricultural systems, optimization of carbon and nitrogen cycling through soil organic matter can improve soil fertility and yields while reducing negative environmental impact. A basic tenet that has guided the management of soil organic matter for decades has been that equilibrium levels of carbon and nitrogen are controlled by their net input and that qualitative differences in these inputs are relatively unimportant1,2,3. This contrasts with natural ecosystems in which there are significant effects of species composition and litter quality on carbon and nitrogen cycling4,5. Here we report the net balances of carbon and nitrogen from a 15-year study in which three distinct maize/soybean agroecosystems are compared. Quantitative differences in net primary productivity and nitrogen balance across agroecosystems do not account for the observed changes in soil carbon and nitrogen. We suggest that the use of low carbon-to-nitrogen organic residues to maintain soil fertility, combined with greater temporal diversity in cropping sequences, significantly increases the retention of soil carbon and nitrogen, which has important implications for regional and global carbon and nitrogen budgets, sustained production, and environmental quality.

Determinants of Soil Microbial Communities: Effects of Agricultural Management, Season, and Soil Type on Phospholipid Fatty Acid Profiles

Abstract: Phospholipid fatty acid (PLFA) profiles were measured in soils from organic, low-input, and conventional farming systems that are part of the long term Sustainable Agriculture Farming Systems (SAFS) Project. The farming systems differ in whether their source of fertilizer is mineral or organic, and in whether a winter cover crop is grown. Sustained increases in microbial biomass resulting from high organic matter inputs have been observed in the organic and low-input systems. PLFA profiles were compared to ascertain whether previously observed changes in biomass were accompanied by a change in the composition of the microbial community. In addition, the relative importance of environmental variables on PLFA profiles was determined. Redundancy analysis ordination showed that PLFA profiles from organic and conventional systems were significantly different from April to July. On ordination plots, PLFA profiles from the low-input system fell between organic and conventional systems on most sample dates. A group of fatty acids (i14:0, a15:0, 16:1ω7c, 16:1ω5c, 14:0, and 18:2ω6c) was enriched in the organic plots throughout the sampling period, and another group (10Me16:0, 2OH 16:1 and 10Me17:0) was consistently lower in relative abundance in the organic system. In addition, another group (15:0, a17:0, i16:0, 17:0, and 10Me18:0) was enriched over the short term in the organic plots after compost incorporation. The relative importance of various environmental variables in governing the composition of microbial communities could be ranked in the order: soil type > time > specific farming operation (e.g., cover crop incorporation or sidedressing with mineral fertilizer) > management system > spatial variation in the field. Measures of the microbial community and soil properties (including microbial biomass carbon and nitrogen, substrate induced respiration, basal respiration, potentially mineralizable nitrogen, soil nitrate and ammonium, and soil moisture) were seldom associated with the variation in the PLFA profiles.

A model linking organic matter decomposition, chemistry and aggregate dynamics

Abstract: Soil Processes and C Cycles Pedospheric Processes and the Carbon Cycle, R. Lal, J. Kimble, and R. Follett Carbon Pools in Different Biomes Stocks and Dynamics of Soil Carbon Following Deforestation for Pasture in Rondonia, C. Neill, C. Cerri, J.M. Melillo, B.J. Feigl, P.A. Steudler, J.F.L. Moraes, and M.C. Piccolo Spatial Patterns in Soil Organic Carbon Pool Size in the Northwestern United States, J.S. Kern, D.P. Turner, and R.F. Dodson Organic Carbon in Deep Alluvium in Southeast Nebraska and Northeast Kansas, R.B. Grossman, D.H. Harms, M.S. Kuzila, S.A. Glaum, S.L. Hartung, and J.R. Fortner Soil Carbon Dynamics in Canadian Agroecosystems, H.H. Janzen, C.A. Campbell, E.G. Gregorich, and B.H. Ellert The Amount of Organic Carbon in Various Soil Orders and Ecological Provinces in Canada, D. Tarnocai Canada's Soil Organic Carbon Database, B. Lacelle Rate of Humus (Organic Carbon) Accumulation in Soils of Different Ecosystems, A. Gennadiyev Land Use and Soil Management Effects on Soil Organic Carbon Dynamics on Alfisols in Western Nigeria, R. Lal Arctic Paleoecology and Soil Processes: Developing New Perspectives for Understanding Global Change, W.R. Eisner Soil Carbon Distribution in Nonacidic and Acidic Tundra of Arctic Alaska, J.G. Bockheim, D.A. Walker, and L.R. Everett Characteristics of Soil Organic Matter in Arctic Ecosystems of Alaska, C.L. Ping, G.J. Michaelson, W.M. Loya, R.J. Chandler, and R.L. Malcolm Soil Structure and Other Physical Processes for C Sequestration Soil Structure and Organic Carbon: A Review, B.D. Kay Dynamics of Soil Aggregation and C Sequestration, D.A. Angers and C. Chenu Soil Aggregate Stabilization and Carbon Sequestration: Feedbacks Through Organo-Mineral Associations, J.D. Jastrow and R.M Miller Impact of Variations in Granular Structures on C Sequestration in Two Alberta Mollisols, Z. Chen, S. Pawluk, and N.G. Juma A Model Linking Organic Matter Decomposition, Chemistry, and Aggregate Dynamics, J.A. Golchin, J.A. Baldock, and J.M. Oades Soil Organic Carbon Dynamics and Land Use in the Colombian Savannas: Aggregate Size Distribution, W. Trujillo, E. Amezquita, M.J. Fisher, and R. Lal Soil Chemical Processes Dissolved Organic Carbon: Sources, Sinks, and Fluxes and Role in the Soil Carbon Cycle, T.R. Moore Geochemical History of Carbon on the Planet: Implications for Soil Carbon Studies, D.Ye. Konyushkov Nitrogen, Sulfur, and Phosphorus and the Sequestering of Carbon, F.L. Himes Soil Biological Properties Management of Soil C by Manipulation of Microbial Metabolism: Daily vs. Pulsed C Additions, D.C. Jans-Hammermeister, W.B. McGill, and R.C. Izaurralde Investigations to the Carbon and Nitrogen Dynamics of Different Long-Term Experiments by Means of Biological Soil Properties, A. Weigel, E.-M. Klimanek, M. Korschens, and St. Mercik Effect of Corn and Soybean Residues on Earthworm Cast Carbon Content and Natural Abundance Isotope Signature, D.R. Linden and C.E. Clapp Soil Erosion and C Dynamics Soil Organic Carbon Distribution in Aggregates and Primary Particle Fractions as Influenced by Erosion Phases and Landscape Position, R.M. Bajracharya, R. Lal, and J.M. Kimble Carbon Storage in Eroded Soils after Five Years of Reclamation Techniques, R.C. Izaurralde, M. Nyborg, E.D. Solberg, H.H Janzen, M.A. Arshad, S.S. Malhi, and M. Molina-Ayala Soil Quality and C Sequestration Quantification of Soil Quality, C.A. Seybold, M.J. Mausbach, D.L. Karlen, and H.H. Rogers Relationships Between Soil Organic Carbon and Soil Quality in Cropped and Rangeland Soils: The Importance of Distribution, Composition, and Soil Biological Activity, J.E. Herrick and M.M. Wander Soil Quality Indices of Piedmont Sites under Different Management Systems, B.F. McQuaid and G.L. Olson Impact of Carbon Sequestration on Functional Indicators of Soil Quality as Influenced by Management in Sustainable Agriculture, C.M. Monreal, H. Dinel, M. Schnitzer, D.S. Gamble, and V.O. Biederbeck Modeling C Dynamics Modeling Soil Carbon in Relation to Management and Climate Change in Some Agroecosystems in Central North America, K. Paustian, E.T. Elliott, and K. Killian Predicting Soil Carbon in Mollisols Using Neural Networks, E.R. Levine and D. Kimes A Retrospective Modeling Assessment of Historical Changes in Soil Carbon and Impacts of Agricultural Development in Central USA, 1900 to 1990, A.S. Patwardhan, A.S. Donigian, Jr., R.V. Chinnaswamy, and T.O. Barnwell Modeling Soil Carbon and Agicultural Practices in the Central U.S.: An Update of Preliminary Study Results, A.S. Donigian, A.S. Patwardhan, R.V. Chinnaswamy, and T.O. Barnwell Experimental Verification of Simulated Soil Organic Matter Pools, C.A. Cambardella Modeling Tillage and Surface Residue Effects on C Storage under Ambient vs. Elevated CO2 and Temperature in Ecosys, R.F. Grant, R.C. Izaurralde, M. Nyborg, S.S. Malhi, E.D. Solberg, and D.J. Hammermeister Methods of SOC Determination Using Bulk Radiocarbon Measurements to Estimate Soil Organic Matter Turnover Times, K.G. Harrison Impacts of Climatic Change on Carbon Storage Variations in African and Asian Deserts, E. Lioubimtseva Carbon Turnover in Different Climates and Environments, H.W. Scharpenseel and E.M. Pfeiffer Impact of Climate on C Dynamics Carbon Sequestration in Soil: Knowledge Gaps Indicated by the Symposium Presentations, D.J. Greenland Knowledge Gaps and Researchable Priorities, R. Lal, J. Kimble, and R. Follett Index

Plant-induced changes in soil structure: Processes and feedbacks

Abstract: Soil structure influences the growth and activity of organisms living in soil. In return, microbes, fauna, and plants affect structure. The objective of this paper is to review the role of plants in modifying soil structure. Vegetation affects structural form and stability at different scales and through various direct and indirect mechanisms. By penetrating the soil, roots form macropores which favour fluid transport. They also create zones of failure which contribute to fragment the soil and form aggregates. This phenomenon is enhanced by the wetting and drying cycles associated with plant growth. Drying also causes shrinkage and strengthening of the soil. Anchorage of roots and the exudation of cementing material stabilizes soil structure. Finally, as a source of C, roots and plant residues provide a food source to the microflora and fauna which contribute to structure formation and stabilization. In return, plant-induced changes in structure will affect plant growth mostly by modifying the root physical environment, and the water and nutrient cycles.

Carbon distribution and losses: erosion and deposition effects

Abstract: Because of concerns about the eventual impact of atmospheric CO2 accumulations, there is growing interest in reducing net CO2 emissions from soil and increasing C storage in soil. This review presents a framework to assess soil erosion and deposition processes on the distribution and loss of C in soils. The physical processes of erosion and deposition affect soil C distribution in two main ways and should be considered when evaluating the impact of agriculture on C storage. First, these processes redistribute considerable amounts of soil C, within a toposequence or a field, or to a distant site. Accurate estimates of soil redistribution in the landscape or field are needed to quantify the relative magnitude of soil lost by erosion and accumulated by deposition. Secondly, erosion and deposition drastically alter the biological process of C mineralization in soil landscapes. Whereas erosion and deposition only redistribute soil and organic C, mineralization results in a net loss of C from the soil system to the atmosphere. Little is known about the magnitude of organic C losses by mineralization and those due to erosion, but the limited data available suggest that mineralization predominates in the first years after the initial cultivation of the soil, and that erosion becomes a major factor in later years. Soils in depositional sites usually contain a larger proportion of the total organic C in labile fractions of soil C because this material can be easily transported. If the accumulation of soil in depositional areas is extensive, the net result of the burial (and subsequent reduction in decomposition) of this active soil organic matter would be increased C storage. Soil erosion is the most widespread form of soil degradation. At regional or global levels its greatest impact on C storage may be in affecting soil productivity. Erosion usually results in decreased primary productivity, which in turn adversely affects C storage in soil because of the reduced quantity of organic C returned to the soil as plant residues. Thus the use of management practices that prevent or reduce soil erosion may be the best strategy to maintain, or possibly increase, the worlds soil C storage.

Evaluating environmental consequences of producing herbaceous crops for bioenergy.

Abstract: The environmental costs and benefits of producing bioenergy crops can be measured both in terms of the relative effects on soil, water and wildlife habitat quality of replacing alternate cropping systems with the designated bioenergy system, and in terms of the quality and amount of energy that is produced per unit of energy expended. While many forms of herbaceous and woody energy crops will likely contribute to future biofuels systems, The Department of Energy's Bioenegy Feedstock Development Program (BFDP), has chosen to focus its primary herbaceous crops research emphasis on a perennial grass species, switchgrass ( Panicum virgatum) . The choice of switchgrass as a model bioenergy species was based on its high yields, high nutrient use efficiency and wide geographic distribution. Another important consideration was its positive environmental attributes. The latter include its positive effects on soil quality and stability, its cover value for wildlife, and relatively low inputs of energy, water and agrochemicals required per unit of energy produced. A comparison of the energy budgets for corn, which is the primary current source of bioethanol, and switchgrass reveals that the efficiency of energy production for a perennial grass system can exceed that for an energy intensive annual row crop by as much as 15 times. In addition potential reductions in CO 2 emissions, tied to the energetic efficiency of producing transportation fuels and replacing non-renewable petrochemical fuels with ethanol derived from grasses are very promising. Calculated carbon sequestration rates may exceed those of annual crops by as much as 20–30 times, due in part to carbon storage in the soil. These differences have major implications for both the rate and efficiency with which fossil energy sources can be replaced with cleaner burning biofuels. Current research is emphasizing quantification of changes in soil nutrients and soil organic matter to provide improved understanding of the long term changes in soil quality associated with annual removal of high yields of herbaceous energy crops.

Ecology of Soil Erosion in Ecosystems

Abstract: Each year, about 75 billion tons of soil are eroded from the world's terrestrial ecosystems. Most agricultural land in the world is losing soil at rates ranging from 13 tons/ha/year to 40 tons/ha/year. Because soil is formed very slowly, this means that soil is being lost 13–40 times faster than the rate of renewal and sustainability. Rain and wind energy are the two prime causes of erosion from tilled or bare land. Erosion occurs when the soil lacks protective vegetative cover. Soil erosion reduces the productivity of the land by loss of water, soil organic matter, nutrients, biota, and depth of soil. The greatest threat to providing food for a rapidly growing human population is soil erosion. Abandoned, eroded agricultural land is replaced by clearing forested ecosystems.

Changes in Soil Chemical Properties Resulting from Organic and Low-Input Farming Practices

Abstract: levels in the organic system indicate that animal manures did not increase salinity. Overall, our findings indicate that organic and lowSoil chemical properties during the transition from conventional input farming in the Sacramento Valley result in small but important to organic and low-input farming practices were studied over 8 yr in increases in soil organic C and larger pools of stored nutrients, which California’s Sacramento Valley to document changes in soil fertility are critical for long-term fertility maintenance. status and nutrient storage. Four farming systems differing in crop rotation and external inputs were established on land previously managed conventionally. Fertility in the organic system depended on animal manure applications and winter cover crops; the two conventional

Relationship between soil chemical factors and grassland diversity

Abstract: Many studies carried out during these last few years have focused on the factors influencing plant diversity in species-rich grasslands. This is due to the fact that these ecosystems, among the most diversified in temperate climates, are extremely threatened; in some areas, they have almost disappeared. The re-establishment of these habitats implies to know the living conditions of the associations to be recreated. Very often, the typical species of these communities have become so rarefied that the seed bank or the seed rain are not sufficient to recreate the plant community. Most of the time, to achieve the restoration of these communities, they have to be totally recreated by sowing. For the restoration or the maintenance of the community, the soil chemical characteristics have also to be appropriate or if not modified. This research tends to establish a relation between some soil chemical factors and the plant diversity of a great number of stations. This research has illuminated the relationship between soil extractable phosphorus and potassium and plant diversity. Over 5 mg of phosphorus per 100 g of dry soil (acetate + EDTA extraction), no station containing more than 20 species per 100 m(2) has been found. The highest number of species is found below the optimum content of the soil for plant nutrition (5-8 mg P/100 g). Concerning the potassium, the highest number of species is found at 20 mg/100, a value corresponcing to an optimum content of the soil for plant nutrition. High potassium contents, in opposition to phosphorus contents, are thus compatible with high values of diversity. Other factors (i.e. pH, organic matter, total nitrogen and calcium) do not show so clearly a relation with plant diversity. Excess of N-NO3 is known for its negative effect on the diversity of plant communities. In these environments, apart from the atmospheric deposits which can be important in some areas, N-NO3 is derived mainly from the symbiotic fixation of atmospheric nitrogen by legumes as well as from the mineralization of the organic matter of the soil. It is possible that, when in small quantities, the available soil phosphorus could be a limiting factor of the N-NO3 supply by these two sources. In this hypothesis, nitrogen would remain the main element limitating plant diversity but its availability would be controlled by phosphorus.

Phytoremediation of Uranium-Contaminated Soils: Role of Organic Acids in Triggering Uranium Hyperaccumulation in Plants

Abstract: Uranium phytoextraction, the use of plants to extract U from contaminated soils, is an emerging technology. We report on the development of this technology for the cleanup of U-contaminated soils. In this research, we investigated the effects of various soil amendments on U desorption from soil to soil solution, studied the physi ological characteristics of U uptake and accumulation in plants, and developed techniques to trigger U hyperac cumulation in plants. A key to the success of U phytoextraction is to increase soil U availability to plants. We have found that some organic acids can be added to soils to increase U desorption from soil to soil solution and to trigger a rapid U accumulation in plants. Of the organic acids (acetic acid, citric acid, and malic acid) tested, citric acid was the most effective in enhancing U accumulation in plants. Shoot U concentrations of Brassica juncea and Brassica chinensis grown in a U-contaminated soil (total soil U, 750 mg kg-1) increased from less than 5 mg kg-1 t...

Microbial Diversity and Community Structure in Two Different Agricultural Soil Communities

Abstract: In this study, two different agricultural soils were investigated: one organic soil and one sandy soil, from Stend (south of Bergen), Norway. The sandy soil was a field frequently tilled and subjected to crop rotations. The organic soil was permanent grazing land, infrequently tilled. Our objective was to compare the diversity of the cultivable bacteria with the diversity of the total bacterial population in soil. About 200 bacteria, randomly isolated by standard procedures, were investigated. The diversity of the cultivable bacteria was described at phenotypic, phylogenetic, and genetic levels by applying phenotypical testing (Biolog) and molecular methods, such as amplified rDNA restriction analysis (ARDRA); hybridization to oligonucleotide probes; and REP-PCR. The total bacterial diversity was determined by reassociation analysis of DNA isolated from the bacterial fraction of environmental samples, combined with ARDRA and DGGE analysis. The relationship between the diversity of cultivated bacteria and the total bacteria was elucidated. Organic soil exhibited a higher diversity for all analyses performed than the sandy soil. Analysis of cultivable bacteria resulted in different resolution levels and revealed a high biodiversity within the population of cultured isolates. The difference between the two agricultural soils was significantly higher when the total bacterial population was analyzed than when the cultivable population was. Thus, analysis of microbial diversity must ultimately embrace the entire microbial community DNA, rather than DNA from cultivable bacteria.

Effects of tree species, stand age and soil type on soil microbial biomass and its activity in a southern boreal forest

Abstract: Microbial C (Cmic) and N (Nmic), the Cmic-to-organic C (Corg) and Nmic-to-total N (Nt) ratios and the specific respiration of microbial biomass were investigated in a southern boreal mixed forest. The forest stands were 50 and 124 years old and consisted of trembling aspen, paper birch and mixed conifers comprising white spruce and balsam fir. Stands were growing on soils derived either from clay (89% average clay content) or till (46% average clay content) deposits in the clay belt region of northern Quebec. In the forest floors the relative concentrations of microbial C and N and the Cmic-to-Corg and Nmic-to-Nt ratios, regarded as measures of organic matter quality, declined with stand age whereas the specific microbial respiration increased, indicating decreasing C assimilation efficiency. In the mineral soils, in contrast, Cmic-to-Corg and Nmic-to-Nt ratios increased with stand age. The Cmic-to-Nmic ratio widened with stand age in both the forest floors and mineral soils, suggesting that the proportion of fungi had increased. Concentrations of microbial C and N were on average lower in forest floor beneath conifers (Cmic-to-Corg 1.9%, Nmic-to-Nt 7.5%) than beneath the deciduous species birch (Cmic-to-Corg 2.2%, Nmic-to-Nt 8.6%) and aspen (Cmic-to-Corg 2.4%, Nmic-to-Nt 9.2%). Average Cmic-to-Nmic ratios were only slightly different in the forest floors beneath the different tree species (Cmic-to-Nmic: conifers 8.9, birch 7.2, and aspen 8.3). In both forest floors and mineral soils, average concentrations of Cmic and Nmic were generally higher in the clay than in the till soils, but the Cmic-to-Corg ratios were similar in both soil types. The average Nmic-to-Nt ratios were lower in till than in clay soils only beneath conifers. The average specific microbial respiration (qCO2=μg CO2-C mg Cmic−1 d−1) in clay soils (22) was approximately half that in till soils (41). Since the microbial parameters measured were sensitive to the factors stand age, tree species and soil type, they may have the potential to be used as indicators of the influence of forest management on soil organic matter quality.

Vascular plant 15N natural abundance in heath and forest tundra ecosystems is closely correlated with presence and type of mycorrhizal fungi in roots.

Abstract: In this study we show that the natural abundance of the nitrogen isotope 15, δ15N, of plants in heath tundra and at the tundra-forest ecocline is closely correlated with the presence and type of mycorrhizal association in the plant roots. A total of 56 vascular plant species, 7 moss species, 2 lichens and 6 species of fungi from four heath and forest tundra sites in Greenland, Siberia and Sweden were analysed for δ15N and N concentration. Roots of vascular plants were examined for mycorrhizal colonization, and the soil organic matter was analysed for δ15N, N concentration and soil inorganic, dissolved organic and microbial N. No arbuscular mycorrhizal (AM) colonizations were found although potential host plants were present in all sites. The dominant species were either ectomycorrhizal (ECM) or ericoid mycorrhizal (ERI). The δ15N of ECM or ERI plants was 3.5-7.7‰ lower than that of non-mycorrhizal (NON) species in three of the four sites. This corresponds to the results in our earlier study of mycorrhiza and plant δ15N which was limited to one heath and one fellfield in N Sweden. Hence, our data suggest that the δ15N pattern: NON/AM plants > ECM plants ≥ ERI plants is a general phenomenon in ecosystems with nutrient-deficient organogenic soils. In the fourth site, a␣birch forest with a lush herb/shrub understorey, the differences between functional groups were considerably smaller, and only the ERI species differed (by 1.1‰) from the NON species. Plants of all functional groups from this site had nearly twice the leaf N concentration as that found in the same species at the other three sites. It is likely that low inorganic N availability is a prerequisite for strong δ15N separation among functional groups. Both ECM roots and fruitbodies were 15N enriched compared to leaves which suggests that the difference in δ15N between plants with different kinds of mycorrhiza could be due to isotopic fractionation at the␣fungal-plant interface. However, differences in δ15N between soil N forms absorbed by the plants could also contribute to the wide differences in plant δ15N found in most heath and forest tundra ecosystems. We hypothesize that during microbial immobilization of soil ammonium the microbial N pool could become 15N-depleted and the remaining, plant-available soil ammonium 15N-enriched. The latter could be a main source of N for NON/AM plants which usually have high δ15N. In contrast, amino acids and other soil organic N compounds presumably are 15N-depleted, similar to plant litter, and ECM and ERI plants with high uptake of these N forms hence have low leaf δ15N. Further indications come from the δ15N of mosses and lichens which was similar to that of ECM plants. Tundra cryptogams (and ECM and ERI plants) have previously been shown to have higher uptake of amino acid than ammonium N; their low δ15N might therefore reflect the δ15N of free amino acids in the soil. The concentration of dissolved organic N was 3-16 times higher than that of inorganic N in the sites. Organic nitrogen could be an important N source for ECM and, in particular, ERI plants in heath and forest tundra ecosystems with low release rate of inorganic N from the soil organic matter.

Environmental Soil and Water Chemistry: Principles and Applications

Abstract: WATER CHEMISTRY AND MINERAL SOLUBILITY. Physical Chemistry of Water and Some of Its Constituents. Solution/Mineral-Salt Chemistry. SOIL MINERALS AND SURFACE CHEMICAL PROPERTIES. Soil Minerals and Their Surface Properties. Sorption and Exchange Reactions. ELECTROCHEMISTRY AND KINETICS. Redox Chemistry. Pyrite Oxidation Chemistry. Reaction Kinetics in Soil-Water Systems. SOIL DYNAMICS AND AGRICULTURAL-ORGANIC CHEMICALS. Organic Matter, Nitrogen, Phosphorus and Synthetic Organics. COLLOIDS AND TRANSPORT PROCESSES IN SOILS. Soil Colloids and Water-Suspended Solids. Water and Solute Transport Processes. The Chemistry and Management of Salt-Affected Soils and Brackish Waters. LAND-DISTURBANCE POLLUTION AND ITS CONTROL. Acid Drainage Prevention and Heavy Metal Removal Technologies. SOIL AND WATER: QUALITY AND TREATMENT TECHNOLOGIES. Water Quality. Soil and Water Decontamination Technologies. Appendix. Suggested and Cited References. Index.

EFFECTS OF ACIDITY ON MINERALIZATION: pH- DEPENDENCE OF ORGANIC MATTER MINERALIZATION IN WEAKLY ACIDIC SOILS

Abstract: The literature is ambiguous regarding the influence of acidity on mineralization of soil organic matter. Although mineralization is often regarded as being relatively insensitive to acidity, reports of agronomically-significant increases in N mineralization after liming of acid soils are common. We analyzed 61 soils (pH 5.1–7.9), representing all agro-ecological zones of Saskatchewan, Canada, to determine the pH-dependence of N mineralization. Mineralization was measured by aerobic incubation. There was no statistical relationship between the parameters of the first-order kinetic equation [i.e. the rate constant (k) and potentially mineralizable N (N0)] used to describe the incubation data and soil pH. However, when pH of two slightly acid (pH 5.7 and 5.8) soils was raised using Ca(OH)2, mineralization of N and C was stimulated. Initially, the rate of CO2 evolution from soils treated with Ca(OH)2, to raise pH to 7.3–7.4, was 2–3 times that from the unamended soils. Rate of CO2 evolution from Ca(OH)2-treated soil declined rapidly after about 7–10 d. During the entire 100-d incubation, Ca(OH)2-treated soils at pH 7.3–7.4 produced 37% and 67% more CO2C than their untreated counterparts. We observed comparable increases in N mineralization. The effect of Ca(OH)2 was attributed to release of labile organic matter when pH was increased. Dissolved organic matter in saturated paste extracts was well correlated with C and N mineralized. A model consisting of two simultaneous first-order equations was needed to describe mineralization in Ca(OH)2-treated soil. Application of Ca(OH)2 increased the labile pool of mineralizable C from 18 to 157 mg kg−1 in one soil and from 45 to 301 mg kg−1 in the other. We showed that the phosphate-borate buffer test for mineralizable N is pH-dependent because of the effect of pH on organic N solubility. In contact with the buffer, soil pH is raised to 11.2 (i.e. buffer pH), resulting in release of organic N, which is then susceptible to hydrolysis. Organic N extracted using an unbuffered extractant, hot 2 m KCl, was independent of soil pH.

Plant-Soil interactions in temperate grasslands

Abstract: We present a conceptual model in which plant-soil interactions in grasslands are characterized by the extent to which water is limiting. Plant-soil interactions in dry grasslands, those dominated by water limitation (‘belowground-dominance’), are fundamentally different from plant-soil interactions in subhumid grasslands, where resource limitations vary in time and space among water, nitrogen, and light (‘indeterminate dominance’). In the belowground-dominance grasslands, the strong limitation of soil water leads to complete (though uneven) occupation of the soil by roots, but insufficient resources to support continuous aboveground plant cover. Discontinuous aboveground plant cover leads to strong biological and physical forces that result in the accumulation of soil materials beneath individual plants in resource islands. The degree of accumulation in these resource islands is strongly influenced by plant functional type (lifespan, growth form, root:shoot ratio, photosynthetic pathway), with the largest resource islands accumulating under perennial bunchgrasses. Resource islands develop over decadal time scales, but may be reduced to the level of bare ground following death of an individual plant in as little as 3 years. These resource islands may have a great deal of significance as an index of recovery from disturbance, an indicator of ecosystem stability or harbinger of desertification, or may be significant because of possible feedbacks to plant establishment. In the grasslands in which the dominant resource limiting plant community dynamics is indeterminate, plant cover is relatively continuous, and thus the major force in plant-soil interactions is related to the feedbacks among plant biomass production, litter quality and nutrient availability. With increasing precipitation, the over-riding importance of water as a limiting factor diminishes, and four other factors become important in determining plant community and ecosystem dynamics: soil nitrogen, herbivory, fire, and light. Thus, several different strategies for competing for resources are present in this portion of the gradient. These strategies are represented by different plant traits, for example root:shoot allocation, height and photosynthetic pathway type (C3 vs. C4) and nitrogen fixation, each of which has a different influence on litter quality and thus nutrient availability. Recent work has indicated that there are strong feedbacks between plant community structure, diversity, and soil attributes including nitrogen availability and carbon storage. Across both types of grasslands, there is strong evidence that human forces that alter plant community structure, such as invasions by nonnative annual plants or changes in grazing or fire regime, alters the pattern, quantity, and quality of soil organic matter in grassland ecosystems. The reverse influence of soils on plant communities is also strong; in turn, alterations of soil nutrient supply in grasslands can have major influences on plant species composition, plant diversity, and primary productivity.

Soil Structure and Organic Carbon: A Review

Abstract: This chapter explores the relations between soil structure, organic carbon, and land use practices. The structure of soils can be considered from four different and fundamental aspects: form, stability, resiliency, and vulnerability. Each aspect of soil structure can be considered across a range of scales. Soil texture has a major influence on the form, stability, and resiliency of soil structure as well as the response of soil structure to weather, biological factors, and management. Organic carbon in soil represents materials of plant, animal, and microbial origin that are in various stages of decomposition and are associated with the mineral fraction with different degrees of intimacy. Pores may be created and stabilized by soil fauna. The total porosity that is measured at any given time in nonswelling soils is strongly influenced by soil characteristics such as texture and organic carbon content and by management. Macropores can represent as much as a third of the total porosity of soils.

Soil microbial biomass — what do the numbers really mean?

Abstract: Summary. Soil microbial biomass comprises less than 5% of organic matter in soil. However, it performs at least 3 critical functions in soil and the environment. It is a labile source of carbon, nitrogen, phosphorus, and sulfur; it is an immediate sink of carbon, nitrogen, phosphorus and sulfur; and it is an agent of nutrient transformation and pesticide degradation. In addition, microorganisms form symbiotic associations with roots, act as biological agents against plant pathogens, contribute towards soil aggregation, and participate in soil formation. Critical evaluation of the significance of soil microbial biomass is hampered by the reliable measurement of microbial biomass, and simultaneous partitioning of its 3 major functions in soil. For comparative purposes, soil microbial biomass and its derived indices have been successfully used to measure early changes induced by land use practices, zero tillage, crop rotations and other cultural practices, nutrient cycling, land disposal of sewage sludge, and applications of herbicides and insecticides. However, as a routine analytical tool, it is limited by the cumbersome and time consuming measurements, lack of benchmarking values and interpretation, ambiguous relationship with productivity, and cost-effectiveness. With increasing demand to monitor soil quality and protection of the environment, improved and rapid techniques, including molecular biotechniques, will be required to measure soil microbial biomass for its size of sink, source and rates of turnover. Eventually, agricultural science will benefit and utilise soil microbial biomass as an analytical tool to produce abundant, economical, and clean food and fibre.

Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance

Abstract: Terrestrial ecosystems are thought to be a major sink for carbon at the present time. The endeavor to find this terrestrial sink and to determine the mechanisms responsible has dominated terrestrial research on the global carbon cycle for years. Some of the mechanisms advanced to explain the “missing sink” are also negative feedbacks to a global warming. Here we distinguish between mechanisms likely to act as feedbacks to a global warming and other mechanisms consistent with a terrestrial sink that are not feedbacks to a global warming. One of the postulated negative feedback mechanisms that also helps explain the current “missing sink” is based on the theory that carbon should accumulate in vegetation as a result of a warming-enhanced mineralization of nitrogen in soil organic matter. The theory assumes that mineralized N is neither retained in the soil (through reimmobilization by microbial biomass) nor lost from the ecosystem, but rather becomes available for plant growth. None of these assumptions is supported yet by field data. In contrast, trends across existing climatic gradients suggest that warmer temperatures will lead to a decrease in the C:N ratio of soils (i.e., the mineralized N remains in soil). Data pertaining to temporal variability in the global carbon balance are conflicting with respect to the question of whether increasing temperatures cause a release or storage of terrestrial carbon. The answer seems to depend in part on time scale. Most likely, multiple mechanisms, including some that release carbon and others that accumulate it, account for the present net accumulation of carbon on land. However, a positive feedback between temperature and the release of CO2 to the atmosphere by terrestrial respiration seems likely to grow in importance and could change significantly the role that terrestrial ecosystems play in the global carbon balance.

Decomposition and nitrogen release patterns of tree prunings and litter

Abstract: Many studies have shown that agroforestry tree prunings can supply sufficient nutrients to meet crop demand, with the exception of phosphorus. The potential of these organic inputs to supply nutrients depends on their resource quality. Various indices have been developed to predict decomposition and nitrogen release patterns of tree prunings. To date the (lignin + polyphenol):N ratio seems to be the most robust ratio for predicting mass loss and nitrogen release. However, no critical value can be given because of the different methods used to analyze polyphenols. Suggested areas of future research include development of robust indices for predicting plant litter quality, decomposition patterns of belowground litter (roots), residual effects of tree biomass additions, and effects of adding mixtures of organic materials of contrasting quality. The overall challenge is to develop ways of managing organic matter decomposition to optimize short- and long-term release of nutrients and the maintenance of soil organic matter.

Soil solution speciation of lead(II): effects of organic matter and pH

Abstract: The effects of adjusting the levels of soil organic matter in a Pb-contaminated soil on the solubility and free Pb 2+ speciation were studied within the pH range 3 to 8. A contaminated orchard soil containing 284 mg Pb kg -1 was treated with leaf compost to increase soil organic matter and with H 2 O 2 to decrease it, yielding six soil organic matter levels between 25.6 and 83.7 g C kg -1 . The equilibrated solutions were then analyzed for total dissolved Pb by graphite furnace atomic absorption spectrometry and for labile Pb by differential pulse anodic stripping voltammetry (DPASV). The labile Pb values were used to calculate the free Pb 2+ activity based on the assumption that organo-Pb complexes are not DPASV-labile. The data showed that 30 to 50% of dissolved Pb is present as soluble OM complexes at low pH and up to 80 to 99% at near-neutral pH. The solubility of Pb shows a linear decrease from pH 3 to 6.5 and is independent of soil organic matter in that pH range. From pH 6.5 to 8, higher pH promotes the formation and dissolution of organo-Pb complexes, which increase Pb solubility. In this pH range, higher organic matter content results in higher concentrations of dissolved and labile Pb.