Showing papers on "Soil organic matter published in 1989"

Minerals In Soil Environments

Abstract: Minerals in soil environments , Minerals in soil environments , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Effects of heavy metals in soil on microbial processes and populations (a review)

Abstract: The effects of Cd, Cu, Zn, and Pb on soil microorganisms and microbially mediated soil processes are reviewed. The emphasis is placed on temperate forest soils. The sensitivity of different measurements is discussed, and data compiled to compare relative toxicity of different metals. On the whole the relative toxicity of the metals (on a μg g−1 soil basis) decreased in the order Cd > Cu > Zn > Pb, but differences between different investigations were found. The influence of abiotic factors on metal toxicity is briefly discussed and especially examplified by different soil organic matter content. Evidence of tolerance and adaptation in the soil environment and the time scale involved in the evolution of a metal-tolerant microbial community after metal exposure are also considered.

Texture, climate, and cultivation effects on soil organic matter content in U.S. grassland soils

Abstract: Soil organic C content, a major source of system stability in agroecosystems, is controlled by many factors that have complex interactions. The purpose of this study was to evaluate the major controls over soil organic carbon content, and to predict regional patterns of carbon in range and cultivated soils. We obtained pedon and climate data for 500 rangeland and 300 cultivated soils in the U.S. Central Plains Grasslands, and statistically analyzed relationships between C and soil texture and climate. Regression models of the regional soils database indicated that organic C increased with precipitation and clay content, and decreased with temperature. Analysis of cultivated and rangeland soils indicated that C losses due to cultivation increased with precipitation, and that relative organic C losses are lowest in clay soils. Application of the regression models to a regional climate database showed potential soil organic matter losses to be highest in the northeastern section of the Central Plains Grasslands, decreasing generally from east to west. These statistical data analyses can be combined with more mechanistic models to evaluate controls of soil organic matter formation and turnover, and the implications for regional management. S ORGANIC MATTER is a major component of biogeochemical cycles of the major nutrient elements, and the quantity and quality of soil organic matter both reflect and control primary productivity. The amount of soil organic matter represents the balance of primary productivity and decomposition and as such is a sensitive and integrated measure of change in ecosystem function. Understanding the processes that control soil organic matter dynamics and their I.C. Burke, CM. Yonker, W.J. Parton, C.V. Cole and D.S. Schimel, Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523; and K. Flach, Agronomy Dep., Colorado State Univ., Fort Collins, CO 80523. Received 20 June 1988. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 53:800-805 (1989). response to management is essential for informed use of agricultural land. Jenny (1980) describes four sets of state factors responsible for the formation of soil organic matter, and illustrates the influence of parent material, time, climate, and biota as individual controls over soil properties. Controls over soil organic matter properties may have complex interactions; separate analysis of such controls may limit useful predictions. Parton et al. (1988) illustrate the use of a mechanistic model in evaluating simultaneously changing controls. Although such models can be highly successful, field data are necessary to validate predictions across complex gradients. It is widely recognized that cultivation of grassland soils leads to depletion of soil organic matter (Alway, 1909; Russel, 1929; Hide and Metzger, 1939; Haas et al., 1957; and many others). Soil organic C losses of as much as 50% have been documented in the U.S. Central Plains Grasslands (Haas et al., 1957), with losses strongly dependent on management regime and regional location. The extent of soil organic matter depletion has been shown to depend upon the same variables as those controlling soil organic matter formation: climate (Haas et al., 1957; Honeycutt, 1986; Cole et al., 1989), soil texture (Tiessen et al., 1982; Schimel et al., 1985a), landscape position (Schimel et al., 1985a,b; Honeycutt, 1986; Yonker et al., 1988), and management regime (Janzen, 1987; Cole et al., 1988). An integrated assessment of soil organic matter losses across the U.S. Central Grasslands requires analysis of soils with varying temperature, precipitation, and soil physical properties. The objectives of this paper were threefold: (i) to establish quantitative relationships between native soil organic matter levels in the Central Plains Grasslands and key driving variables: precipitation, temperature, and soil texture; (ii) to develop predictions of cultivation induced soil organic carbon loss as a function BURKE ET AL.: TEXTURE, CLIMATE, AND CULTIVATION EFFECTS ON U.S. GRASSLAND SOILS 801 of climate and soil texture; and (iii) to use these predictions to map potential soil organic C depletion.

Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter

Abstract: Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope analyses of C and N.

Microbially Mediated Increases in Plant-Available Phosphorus

Abstract: Publisher Summary This chapter elaborates the microbially mediated increases in plant-available phosphorus (P). The importance of microorganisms in soil nutrient cycling and their role in plant nutrition has been realized for a long time. Their active part in the decomposition and mineralization of organic matter and release of nutrients is crucial to sustaining the plant productivity. The concentration of total P in soils ranges from 0.02 to 0.5% and averages approximately 0.05%, the variation being largely because of differences in weathering intensity and parent material composition. The uptake of P from relatively insoluble sources can be affected by the type of plant growing in the soil. The effect of mycorrhizae on plant P uptake and the effect of soil P on mycorrhizae were among the first aspects of these symbioses studied. Under soil conditions, potential benefits of adding P-solubilizing (PS) organisms would depend on several factors, one of the most important being the activity of the PS microbial population already in the soil. In almost all cases, the major sources of PS isolates have been soils. The mechanism of action of PS microorganisms is also elaborated.

Carbon isotopes in soils and palaeosols as ecology and palaeoecology indicators

Abstract: THERE are few quantitative techniques in use today for palaeoecological reconstruction in terrestrial depositional systems. One approach to such reconstructions is to estimate the proportion of C3 to C4 plants once present at a site using carbon isotopes from palaeosol carbonates1–3. Until now, this has been hampered by an inadequate understanding of the relationship between the carbon isotopic composition of modern soil carbonate and coexisting organic matter. Here we have found that the two systematically differ by 14–16% in undisturbed modern soils. This difference is compatible with isotopic equilibrium between gaseous CO2, and aqueous and solid carbonate species in a soil system controlled by diffusive mass transfer of soil CO2 derived from irreversible oxidation of soil organic matter. Organic matter and pedogenic carbonate from palaeosols of Pleistocene to late Miocene age in Pakistan also differ by 14–16%,. This indicates that diagenesis has not altered the original isotopic composition of either phase, thus confirming their use in palaeoecological reconstruction.

Dynamics of Soil Organic Matter in Tropical Ecosystems

Abstract: Organic inputs to the soil must be considered separate from soil organic matter (SOM) to understand properly the physical, chemical, and biologi­ cal processes involved in the sustainability of tropical agroecosystems. Present management of organic inputs is largely empirical and qualita­ tive. A better understanding of the role ot organic inputs on the processes involved in litter decomposition, nutrient cycling, and soil aggregation is needed. The management of organic inputs should be as predictable as that of inorganic inputs, such as fertilizers and lime. The application of or­ ganic inputs should promote the synchrony of nutrient release w;th plant growth demands. Predictive parameters are needed to approach synchrony. Research on decomposition of organic inputs should be con­ ducted in various agroecosystems. Very little is known about the role of root litter in tropical agroecosystems. Six research imperatives are iden­ tified for tropical agroecosystems: (1)quantify the biomass and nutrient content of aboveground organic inputs: (2)develop predictive parameters for nutrient release patterns, or quality, of organic inputs; (3) investigate the effect of piacement of organic inputs on nutrient Organic Input Management in Tropical Agroecosystems 125 availability; (4) quantify production and nutrient releaie by roots, identify resource quality parameters (see Chapter 4) that describe root decom­ position, and determine the relative importance of roots as sources of nutrients: (5) investigate the effects of quality and placement of organic in­ puts on soil physical properties; and (6) understand how organic inputs are transformed into functional SOM pools in soils differing in texture, mineralogy, and moisture and temperature regimes. We need reliable field methods ior measuring root production and decomposition, transfer from organic inputs into SOM fractions, and transfer processes among SOM pools. The diversity of tropical species and farming systems provides ample combinations of crops and organic inputs for improving tropical agroecosystems. Agroecosystems differ from natural ecosystems in that large amounts of biomass and nutrients are removed as crops. Nutrient outputs via crop harvests far exceed the other nutrient loss pathways combined. This is shown in Table 1 where asummary of annual crop nutrient budget studies from temperate-region countries is presented. A long-standing principle of sound agriculture is to replace the nutrienis lost via crop harvests by adding either inorganic or organic fertilizers. Aside from adding nutrients, organic additions improve soil physical properties and help maintain soil organic matter content, all of which positively affect plant productivity (Allison, 1973). Organic inputs to the soil consist of aboveand belowground litter, crop residues, mulches, green manures, animal manures, and sewage. Some of these inputs are grown and recycled on site " Tablo 1. Summary of nutrient inputs and outputs (kg hayr") of 18 crop production systems from the United Statos, United Kingdom, Netherlands, France, Israel, and Japan. (Mean and standard deviations calculated from data of Frissel, 1978.) Nutrient inouts or outouts Nitrogen Phosohorus Potassium Inputs Inorganic fertilizer 156 t 189 39 "t38 119 = 188 Nfixation 19 ±: 39 Other 13 ±: 13 0.4 t 0.8 9 13 Total 188 ± 75 39 "t39 127 t 176 Outputs -arvest removal 103 t 86 16 ±16 91 ±t 13 Denitrification 20 224a Leacflng 38b nd nd Total 127 ±: 95 16 = 6 91 ± 13 Balance 60 "93 24 ±30 37 = 77 a. Mean of 10 out ot 18 data sets reportir,j denitrification. b. Mean of 3 out of 18 data sets reporting leaching. nd = not determined. 126 Dynamics of SOM in Tropical Ecosystems (internal to the system), while others are brought in from other sites, thu, constituting an cetrnal source of nutrients and orezanic C. Organic inputs are distinct from soil organic matter (SONM), which can be defincd as organic material of biological oricin that has undergone partial or complete transformation in the soil and is located below the soil surface. In much of the literature, organi.: 'puts and SOM arc lumped together as "organic matter," creating considerable con-fusion %4hcnattempting to evaluate and manage the role of organic :nputs c mpared to that of sod organic matter. We proposc, therefore, to discourai'c using the term organicmatterand encourage the use of organic inputs and soti ,rgantc matter ISOM). Although early acriculturc depcnded heavily on organic inputs, the emphasis shiftcd to inorganic fertilizcrs as they became abundant and economi­ cally practical. Today, because agricultural development in much of the tropics is largely limited by economic constraints, emphasis must again be placed on use of organic inputs along 'ith chcmic:;l fertilizers. This paper briefly reviews Figure 1. Conceptual model cf ra;cr -ccls and transfers of soil organic matter. Numbers indicate focus of research 'mperatives (see text conclusions).

Carbon Sources for Bacterial Denitrification

Abstract: There are many factors that affect denitrification and these have been reviewed by Delwiche and Bryan (1976), Payne (1981), and Knowles (1982). None of these authors have provided an in-depth review of the critical role of organic carbon in the denitrification process. Organic carbon availability is one of the most important factors that affects denitrifying activity in soil, yet the chemistry of soil organic matter is only partially understood and its complexity has probably inhibited studies. A readily available C substrate is often added to denitrifying systems in the laboratory to ensure an electron supply when other variables are studied.

Winter Legume Effects on Soil Properties and Nitrogen Fertilizer Requirements

Abstract: Winter legume cover crops have been shown to provide significant amounts of N to subsequent nonleguminous crops, but benefits beyond those directly attributed to N are rarely cited. A 3 yr field study at two Georgia locations utilizing a randomized, complete-block, split-plot design with four replications was begun in 1985 to measure the equivalent fertilizer N supplied by winter annual legumes and to monitor changes in soil physical and chemical properties. Corn, Zea mays L. was grown on a Rome gravelly clay loam soil (fine-loamy, mixed, thermic Typic Hapludult) at the Limestone Valley location, and grain sorghum [Sorghum bicolor (L) Moench] on a Greenville sandy clay loam soil (clayey, kaolinitic, thermic Rhodic Paleudult) in the Coastal Plain. Main plots were cover crops of hairy vetch (Vicia villosa Roth.), crimson clover (Trifolium incarnatum L), berseem clover (Trifolium alexandrinum L), winter pea [Pisum sativum subsp. arvense (L) poir], wheat (Triticum aestivum L), and fallow. Subplots were broadcast NH₄NO₃ fertilizer of varying rates. Hairy vetch and crimson clover replaced the greatest amount of fertilizer N averaging 123 and 99 kg N ha⁻¹, respectively. More water-stable aggregates were found following cover crops than fallow in the 0- to 0.025-m depth at the Coastal Plain location. Higher infiltration rates were found following cover crops than fallow at both locations and infiltration rates were greater following hairy vetch than following wheat at the Coastal Plain site. An adapted winter legume cover crop can replace all of the fertilizer N necessary for optimum rain-fed grain sorghum and up to two-thirds of that required for corn production, and improve soil physical properties.

Organic contaminants in Welsh soils: polynuclear aromatic hydrocarbons

Abstract: Soil samples from 49 locations in Wales have been analyzed for 14 polynuclear aromatic hydrocarbons (PAHs) by HPLC with UV and fluorescence detection to define the normal or typical range of PAHs in surface soils (0–5 cm). The ΣPAH (defined as the sum of 14 PAH compounds sought) values vary by over 3 orders of magnitude. In general, soils show a constant qualitative mixture of PAH components but marked differences in absolute values along the remote-rural-urban gradient. Soil organic matter content has some controlling influence over soil ΣPAH values, but other soil properties (percent clay, bulk density) do not. In general, soils naturally rich in organic matter, such as peat bogs or deciduous and coniferous woodland soils, contain higher ΣPAH concentrations. SIMCA pattern recognition studies have been used to examine patterns in the samples and the variables. Principal component analysis supported the classification of “remote/rural” and “urban” soil samples based on the pattern of PAH; the distinction between remote/rural and urban is made at 600 μg of ΣPAH kg-1. The data are discussed further in the context of factors controlling soil PAH patterns.

Influence of sorghum residues and tillage on soil organic matter and soil microbial biomass in an australian vertisol

Abstract: Short- and medium-term changes in soil organic matter content following a change in soil management or land use are often difficult to measure because they occur slowly against a large background of soil organic matter which can have considerable spatial variability. Results from an experiment with grain sorghum (Sorghum bicolor L.) on a vertisol in sub-tropical Australia demonstrate the usefulness of two techniques for detecting trends in surface soil organic matter before they can be assessed by conventional methods. Firstly, using soil microbial biomass C as a sensitive indicator of changes in soil organic matter. Secondly, by using initial values of soil organic C or total N, measured before imposition of treatments, as a covariate in an analysis of variance. The combination of these techniques provided the most sensitive approach for detecting changes. The above-ground residues of sorghum (41 dry matter ha−1) were either retained or removed from plots that received conventional or zero tillage for 6 yr. Averaged over tillage treatments, soil organic C in the surface 0–10 cm layer was 8% greater in the residue-retained than in the residue-removed treatment, a difference equivalent to 16% of the C added as residues. The trend to increased soil total N was not significant. Residue retention caused larger percentage increases in microbial biomass C, measured by the chloroform fumigation-incubation method, than in total organic C and total N. The increase in biomass C was 12%, biomass N 23% and biomass P 45%, equivalent to 0.7% of the C, 7% of the N and 32% of the P added in residues. Residue retention decreased the biomass C-to-P ratio from 48 to 35, but these values were still much wider than those previously measured in U.K. soils. Residue retention increased respiration by about 45% (measured by CO2 evolution during a 30-day incubation) but had little effect on biomass C-to-N ratio or mineralization of N. Averaged over the two residue management treatments, soil organic C in the surface 10 cm layer was 7% greater under zero tillage than under conventional tillage. The corresponding increase in biomass C was 14–21%, but there were no differences in biomass N or biomass P. CO2 evolution and specific respiration by the biomass (μ g CO2-C evolved g−1 biomass C day−1) were less in zero-tilled than in conventionally tilled soils. The combined effects of residue retention and zero tillage caused increases of 15% in surface soil organic C, 18% in soil total N and 31% in biomass C.

Priming effect of some organic additions to 14C-labelled soil.

Abstract: Small amounts of organic substances or finely-ground plant material were added to soil containing 14C-labelled organic matter. The rates of CO2 and 14CO2 production were measured with an i.r. spectrometer and a liquid scintillation counter, respectively. The extra 14CO2 (primed C) that evolved during decomposition most probably originated from conversions in the (microbial) biomass. We suggest that priming according to the original definition, viz., an acceleration of the decomposition of (stable) soil organic matter (humus), did not exist or was negligible in our experiments. Extra 14CO2 was produced after unlabelled glutamate was added to washed suspensions of 14C-labelled biomass in perlite. The amount of 14CO2 varied with the amount of biomass. The difference between the priming (extra 14CO2 production) caused by glucose (small and lasting only 2–3 days) and that caused by glutamate and aspartate (relatively large and lasting 30–40 days) was very pronounced. After decomposition, glucose, cellulose, wheat straw and sewage sludge left more carbon in the soil than was lost due to priming, so that there was a positive net effect. The two amino acids, glutamate and aspartate, however, had slightly negative net effects. The phenomena observed are probably the result of an accelerated turnover of biomass-carbon. The extra loss of 14C during the priming period was less than the original amount of 14C present in the biomass.

The role of Ca-organic interactions in soil aggregate stability .III. Mechanisms and models

Abstract: Serial additions of glucose after drying-wetting cycles led to substantial aggregation of the surface soil from a red-brown earth. Addition of glucose and Ca compounds resulted in up to 80% of the soil bound in particles >2000µm with only a small portion of the soil in particles 4 0 µm. Treatment of the soils with Na4P2O7 caused dispersion of most of the clay but did not disrupt larger aggregates. More disaggregation was caused by treatments with HCI (0.02 M and 0.1 M), indicating the presence of pyrophosphate-resistant binding mechanisms which were presumed to be due to Ca bridging. Addition of Ca to the soil reduced the efficiency of extraction of organic materials by NaOH. This effect was eliminated by a pretreatment with 1 M HCl, indicating a 'bridging' effect of Ca between polycarboxylic macromolecules (humic acids) and clays. The role of Ca in clay and organic matter flocculation, the bridging of clay aggregates to organic materials and interactions of these colloidal aggregates with the biologic cycle is illustrated in some pictorial models.

Effects of acidification and liming on carbon and nitrogen mineralization and soil organisms in mor humus

Abstract: The aim was to determine if changes in C and N mineralization after acidification and liming could be explained by changes in the soil organism biomass. Intact soil cores from F/H layers in a Norway spruce (C:N=31) and a Scots pine (C:N=44) stand in central Sweden were treated in the laboratory for 55 days with deionized water (control), weak H2SO4 (successively applied as 72 mm of acid rain of pH 3.1), strong H2SO4 (applied as a single high dose of pH 1), and lime CaCO3. Strong acidification reduced C mineralization and increased net N mineralization in both soils. Weak acidification resulted in similar but less pronounced effects. Liming initially stimulated C mineralization rate, but the rates declined, indicating that an easily available C source was successively used up by the microorganisms. Liming also increased net N mineralization in the C:N=31 humus, but not significantly in the C:N--44 humus. Strong acidification generally affected the amounts of FDA-active fungal hyphae, nematodes and enchytraeids more than the other treatments did. The increases in net N mineralization after acidification and liming could only partly be explained by the decreases in biomass N in soil organisms. Mineralization of biomass N from killed soil organisms could at the most explain up to about 30% of the increase in net N mineralization after strong acidification. Most of the effects on N mineralization seemed to depend on the fact that acidification reduced and liming increased the availability of C and N to the microorganisms. Furthermore, acidification seemed to reduce the incorporation of N from dead organisms into the soil organic matter and, thereby, make the N compounds more readily available to microbial decomposition and mineralization.

Plant-and soil-related controls of the flow of carbon from roots through the soil microbial biomass

Abstract: The flow of carbon from plant roots through the microbial biomass is one of the key processes in terrestrial ecosystems. Roots release considerable amounts of organic materials which are utilized by microbes as substrate for biosynthesis and energy supply. The fate of photosynthates and other organic material in the soil-root environment under different conditions was studied using 14C-tracers. Soil structure and texture had a large effect on the turnover of the 14C-labelled materials through the microbial biomass. Finer, clayey soils tended to be more ‘preservative’ than coarser, sandy soils, i.e. larger amounts of 14C were incorporated in microbial biomass and soil organic matter fractions in clayey soils than in sandy soils.

Soil nutrient availability

Abstract: Many methods have been developed for assessing the availability of soil nutrients, but for a variety of reasons none are universally applicable. In this chapter, we discuss the conceptual basis for measuring nutrient availability and describe the strengths and limitations of some of the methods for assessing nonagricultural soils. We also discuss methods for characterizing soil acidity, salinity and redox potential because they often control nutrient cycling and availability.

Long-Term Effects of No-Tillage, Crop Residue, and Nitrogen Application on Properties of a Vertisol

Abstract: The objective was to compare the effects of 13 yr of conventional tillage vs. no-tillage, crop residue retained vs. burned, and no fertilizer N vs. application of 23 and 69 kg N ha yr on organic C content, total N, mineralizable N, pH, electrical conductivity, chloride, exchangeable sodium percentage (ESP), and aggregation index (undispersed fraction <20 μm silt + clay) in a fine-textured Vertisol (650 g clay kg soil). Tillage and crop residue management can substantially affect soil organic matter and microbial activity in the surface layers, and water relations and salt movement to at least 1.2-m depth, even in a fine-textured Vertisol. -from Author

Control of Nitrogen Mineralization a Sagebrush Steppe Landscape

Abstract: Factors controlling N turnover in sagebrush ecosystems are separable into two groups. The first group consists of properties that exhibit strong spatial patterning at a landscape scale but are temporally static at time scales of years or tens of years. These static properties include plant species assemblages, total soil nutrient pools, and soil texture. A second group includes properties that vary across the landscape over shorter time scales, i.e., annually, seasonally, and diurnally. These dynamic properties include soil moisture, temperature, and amount of available nutrients. This paper evaluates the landscape vari- ability of properties in both of these groups, and examines the extent to which these factors control N turnover. Static ecosystem properties were entered into a principal components analysis resulting in four axes of landscape variability. A statistical analysis of the relationship of net N mineralization with the principal components and with soil temperature and moisture suggested that soil microclimate and organic matter quality both control in situ N turnover. Soil microclimate limited N mineralization to a short season in early spring and summer; only during this time did soil organic matter exert control. In landscape positions where soil organic matter pools were low, improved soil microclimate conditions did not increase N mineralization rates. A similar approach may be useful in evaluating control over eco- system processes in other systems that are characterized by strong seasonal and spatial variability.

AMS 14C measurements of fractionated soil organic-matter - an approach to deciphering the soil carbon-cycle

Abstract: [RADIOCARBON, VOL 31, No. 3, 1989, P 644-654] AMS 14C MEASUREMENTS OF FRACTIONATED SOIL ORGANIC MATTER: AN APPROACH TO DECIPHERING THE SOIL CARBON CYCLE S E TRUMBORE Department of Geological Sciences, Columbia University and Lamont-Doherty Geological Observatory, Palisades, New York 10964 J S VOGEL and J R SOUTHON Department of Archaeology, Simon Fraser University, Burnaby, British Columbia, V5A IS6 Canada ABSTRACT. 14C measurements are reported for fractionated soil organic matter from a genetic soil sequence which was sampled several times during the period of atmospheric nuclear weapons testing. Fractionation of the soils by density followed by acid hydrolysis was successful in separating the organic matter into components with mean residence times for carbon ranging from 5 to 20 years (low density fraction) to several thousand years (residue after acid hydrolysis). Comparison of the infiltration of bomb 14C into the vertical soil profile with the distribution of 137Cs, gives clues as to the mechanism (most probably dissolved transport) for importing carbon into deeper soil layers. INTRODUCTION An estimated 1300 to 1500 x 1015g of carbon is sequestered as organic matter in soils (Schlesinger, 1977; Post et a1, 1982). This is roughly twice the amount of carbon present either in the atmosphere as CO2 or as land bios- phere (Whittaker & Likens, 1973), and 200 times the amount of CO2 added to the atmosphere annually by fossil fuel burning (Rotty, 1977). To under- stand the evolution of soil humic material, and to better incorporate the organic matter in soils into models of the global carbon cycle, it is necessary to quantify the relative amounts and turnover times of labile and refractory carbon in soil organic matter, and how these vary with parameters such as climate, vegetation and geography. 14C dating of bulk soil organic matter gives 14 C ages which range from over-modern (containing 14C produced by atmospheric nuclear weapons testing) to > 10,000 yr. Because many of the published 14C measurements of soil organic material were made since the 1960s, the reported ages reflect varying degrees of bomb 14C contamination. To correctly interpret the radiocarbon age of soil organic matter as a mean residence time, 14C mea- surements must be made of pre-bomb soils. The rate of infiltration of the 14C produced by atmospheric nuclear weapons testing into the soil carbon reservoir can be a useful tool for deciphering carbon turnover in soils on time scales of decades to hundreds of years. O'Brien & Stout (1978), O'Brien (1984,1986) and Harkness, Har- rison & Bacon (1986) have shown that the increase in 14C content in bulk soil organic matter can only be explained if soil organic matter is a mixture of components that accumulate and decay at different rates. Fractionation of soil organic matter by chemical or physical means separates soil organic matter into components with different ages (eg, Campbell et al, 1967; Scharpenseel, Ronzani & Pietig,1968; Martel & Paul,

The impact of protozoa on the availability of bacterial nitrogen to plants

Abstract: Microbial N from 15N-labelled bacterial biomass was investigated in a microcosm experiment, in order to determine its availability to wheat plants. Sterilized soil was inoculated with either bacteria (Pseudomonas aeruginosa alone or with a suspension of a natural bacterial population from the soil) or bacteria and protozoa to examine the impact of protozoa. Plant biomass, plant N, soil inorganic N and bacterial and protozoan numbers were determined after 14 and 35 days of incubation. The protozoa reduced bacterial numbers in soil by a factor of 8, and higher contents of soil inorganic N were found in their presence. Plant uptake of N increased by 20010 in the presence of protozoa. Even though the total plant biomass production was not affected, the shoot: root ratios increased in the presence of protozoa, which is considered to indicate an improved plant nutrient supply. The presence of protozoa resulted in a 65010 increase in mineralization and uptake of bacterial 15N by plants. This effect was more pronounced than the protozoan effect on N derived from soil organic matter. It is concluded that grazing by protozoa strongly stimulates the mineralization and turnover of bacterial N. The mineralization of soil organic N was also shown to be promoted by protozoa.

Cadmium soil sorption at low concentrations: VIII. correlation with soil parameters

Abstract: Cadmium distribution coefficients, K d were determined at low Cd concentrations (solute: 02 to 30 μg Cd dm−3, soil: 0044 to 11 mg Cd kg−1) for 63 Danish agricultural soils The K d values ranged from 15 to 2450 L kg−1 About 40% of the soils had K d values below 200 L kg−1 The observed K d values correlated very well with soil pH (r 2 = 072) Introducing soil organic matter content as a second parameter improved the correlation some (r 2 = 079) No further improvements were obtained by introducing traditional soil parameters as clay, silt, fine sand, coarse sand and CEC or ‘reactive’ parameters as oxyhydroxides of Mn, Fe and Al The identified regression equation for predicting K d values indicates that K d approximately doubles for each 05 unit increase in pH or 2% increase (weight basis) in organic matter content

A comparative study on nutrient cycling in wet heathland ecosystems : II. Litter decomposition and nutrient mineralization.

Abstract: The concept of the relative nutrient requirement (L n) that was introduced in the first paper of this series is used to analyse the effects of the dominant plant population on nutrient cycling and nutrient mineralization in wet heathland ecosystems. A distinction is made between the effect that the dominant plant species has on (1) the distribution of nutrients over the plant biomass and the soil compartment of the ecosystem and (2) the recirculation rate of nutrients. The first effect of the dominant plant species can be calculated on the basis of the δ/k ratio (which is the ratio of the relative mortality to the decomposition constant). The second effect can be analysed using the relative nutrient requirement (L n). The mass loss and the changes in the amounts of N and P in decomposing above-ground and below-ground litter produced by Erica tetralix and Molinia caerulea were measured over three years. The rates of mass loss from both above-ground and below-ground litter of Molinia were higher than those from Erica litter. After an initial leaching phase, litter showed either a net release or a net immobilization of nitrogen or phosphorus that depended on the initial concentrations of these nutrients. At the same sites, mineralization of nitrogen and phosphorus were measured for two years both in communities dominated by Molinia and in communities dominated by Erica. There were no clear differences in the nitrogen mineralization, but in one of the two years, phosphate mineralization in the Molinia-community was significantly higher. On the basis of the theory that was developed, mineralization rates and ratios between amounts of nutrients in plant biomass and in the soil were calculated on the basis of parameters that were independently measured. There was a reasonable agreement between predicted and measured values in the Erica-communities. In the Molinia-communities there were large differences between calculated and measured values, which was explained by the observation that the soil organic matter in these ecosystems still predominantly consisted of Erica-remains.

Rates of Soil Chemical Processes

Abstract: Rates of soil chemical processes , Rates of soil chemical processes , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Effect of Whole-Tree Harvesting on the Sulfur Dynamics of a Forest Soil

Abstract: Sulfur constituents in the soil at the Hubbard Brook Experimental Forest are dominated by C-bonded S and ester-sulfate with smaller concentrations of adsorbed sulfate and ZnHCl-reducible S forms. The relative proportions of these constituents are similar to those found in other Spodosols. Two years after a whole-tree harvest of Watershed 5 there was a significant increase in adsorbed sulfate in the E and Bh horizons from 0.70 to 3.16 and from 2.73 to 4.96 kg ha⁻¹, respectively, while organic S pools remained relatively constant except for some change in ester-sulfate pools. This increase in sulfate in these horizons is consistent with patterns of soil solution and stream chemistry, which indicate increased sulfate adsorption due to acidification following mineralization of N, nitrification, and nitric acid leaching. Mass balance calculations comparing SO₄ flux in stream water from watershed 5 with Watershed 6 (reference) accounted for 88% of the sulfate adsorbed in the E and Bh horizons of the former watershed. Organic S ranged 0.7 to 9.5 µmol S L⁻¹ in stream and soil solutions. When this organic S is transported through the soil and deposited in the B horizon, it may serve as a large, stable pool of soil S, less subject to change than that of adsorbed sulfate.

The role of Ca-organic interactions in soil aggregate stability. I: Laboratory studies with 14C-glucose, CaCO3 and CaSO4-2H2O

Abstract: 14C-glucose with or without CaCO3 or CaS04.2H2O was incubated in a leached fine sandy loam under controlled temperature and humidity for 120 days. The addition of CaSO4.2H2O decreased the release of 14CO2, compared with soil with 14C-glucose and CaCO3 and 14C-glucose only. Addition of CaCO3 initially stimulated release of 14CO2 but subsequently the release of 14CO2 was inhibited compared with treatments with no calcium additions. Addition of both CaSO4.2H20 and CaCO3 resulted in more residual 14C in the soil at the end of the incubation period than in the control soil to which no calcium was added. The addition of CaSO4.2H2O and CaCO3 led to increases in water-stable aggregates 50-250 µm diameter, and decreased the amount of dispersible clay. In the presence of calcium and glucose, the stabilization of aggregates >1000µm occurred and they persisted for a longer time than when no additions of calcium were made.

Management of earthworm populations in agro-ecosystems: A possible way to maintain soil quality?

Abstract: Earthworm activities in natural ecosystems are very diverse and vary markedly according to soil and climate conditions. Within their drilosphere, i.e. the soil and microflora which they influence, they affect the physical properties of soils through their burrowing activities or by producing above and below-ground casts which are generally resistant macroaggregate structures. They affect the decomposition of organic matter in a number of ways, e.g., by incorporating leaf litter into the soil and activating both mineralization and humification of the soil organic matter. Their overall effect appears to favour short and rapid, rather than long-term turnover of organic matter and nutrients. Finally, their effects on nutrient release and physical properties of soils are generally assumed to be in synchrony with plant demand and regulated so as to promote conservation of the soil structure and organic matter reserves.

Abiotic controls on the functional structure of soil food webs

Abstract: The hypothesis that the trophic structure of soil food webs changes as a result of the abiotic environment was examined by reviewing studies of soil biota. In dry soils with a water potential below −1.5 MPa, most bacteria, protozoans, and many species of nematodes are not active. These taxa persist in the soil in a state of anhydrobiosis. Because soil fungi grow at soil water potentials of −6.0 to −8.0 MPa, soil food webs in dry environments appear to be fungal-based and fungal grazers in dry environments appear to be predominantly fungiphagous mites. There is indirect evidence that some species of fungiphagous mites remain inactive in dry soils in a state of “cryptobiosis”. In habitats where there is insufficient vegetative cover to shade and modify the soil surface, the functional soil food web consists of fungi and a few taxa of soil acari for extended periods of time.