Showing papers on "Soil organic matter published in 1986"

Aggregate structure and carbon, nitrogen, and phosphorus in native and cultivated soils

Abstract: This work corroborates the hierarchical conceptual model for soil aggregate structure presented by Tisdall and Oades (1982), extends it to North American grassland soils, and elaborates on the aspects relating to the influence of cultivation upon losses of soil organic matter. Aggregate size distributions observed for our soils are very similar to those of Australian soils, indicating that the microaggregate-macroaggregate model may hold for a wide array of grassland soils from around the world. The use of two wetting treatments prior to sieving demonstrated that the native sod soil had the same general structural characteristics as cultivated soil but the macroaggregates were more stable. When slaked, native and cultivated soil present in the microaggregate size classes had less organic C, N, and P than the soil remaining as macroaggregates, even when expressed on a sand-free basis. Moreover, the C/N, C/P, and N/P ratios of microaggregates were narrower than those of macroaggregate size classes. Much more C and N was lost than P under the conditions of this study. Element-specific differences in microbial catabolic processes vs. extracellular enzyme activity and its attendant feedback controls are postulated to account for this difference. When the macroaggregates were crushed to the size of microaggregates, mineralizable C as a percent of the total organic C was generally greater for macro- than microaggregates early in the incubation for the cultivated soil and throughout most of the incubation for the native sod soil. Mineralizable N as a percent of the total organic N was greatest in macroaggregates even when the macroaggregates were not crushed. The macroaggregate-micraggregate conceptual model is applied to help explain accumulation of soil organic matter under native conditions and its loss upon cultivation.

Cycles of Soils: Carbon, Nitrogen, Phosphorus, Sulfur, Micronutrients

Abstract: The Carbon Cycle. Soil Carbon Budgets and Role of Organic Matter in Soil Fertility. Soil Organic Matter Quality and Characterization. Environmental Aspects of the Soil Carbon Cycle. The Nitrogen Cycle in Soil: Global and Ecological Aspects. The Internal Cycle of Nitrogen in Soil. Dynamics of Soil N Transformations as Revealed by ~ 15N Tracer Studies. Impact of Nitrogen on Health and the Environment. The Phosphorus Cycle. The Sulfur Cycle. Micronutrients and Toxic Metals. Index.

Cycles of soil : carbon, nitrogem, phosphorus, sulfur, micronutrients

Abstract: The Carbon Cycle Carbon Balance of the Soil and Role of Organic Matter in Soil-Fertility Environmental Aspects of the Soil Carbon Cycle The Nitrogen Cycle in Soil: Global and Ecological Aspects The Internal Cycle of Nitrogen in Soil Impact of Nitrogen on Health and the Environment The Phosphorus Cycle The Sulphur Cycle The Micronutrient Cycle.

Soil Physical Chemistry

Abstract: Electrochemistry of the Double Layer: Principles and Applications to Soils, Upendra Singh and Goro Uehara Chemical Modeling of Ion Adsorption in Soils, John M Zachara and John C Westall Thermodynamics of the Soil Solution, Donald L Suarez Kinetics and Mechanisms of Chemical reactions at the Soil Mineral/Water Interface, Donald L Sparks Precipitation/Dissolution Reactions in Soils, Wayne P Robarge The Chemistry of Soil Organic Matter, Nicola Senesi and Elisabetta Loffredo Characterizing Soil Redox Behavior, Richmond J Bartlett

Patterns of Change in the Carbon Balance of Organic Soil-Wetlands of the Temperate Zone

Abstract: (1) Organic soil-wetlands, particularly those in the temperate zone, under natural conditions, are net carbon sinks and hence are important links in the global cycling of carbon dioxide and other atmospheric gases. Human alteration of wetlands has brought about shifts in the balance of carbon movement between the wetlands and the atmosphere. Because previous analyses have not fully considered these shifts, disturbance of carbon storage in organic soil-wetlands of the temperate zone has been analysed for the last two centuries and considered in relation to other sources of atmospheric CO2 from the biosphere. (2) Storage before recent disturbance is estimated as 57 to 83 Mt of carbon per year, over two-thirds of this in boreal peatlands. The total storage rate, lower than previous estimates, reflects accumulation rates of carbon of only 0.20 t ha-' yr-1 and less in the boreal zone where 90% of temperate organic soils are found. (3) Widespread drainage of organic soil-wetlands for agriculture has significantly altered the carbon balance. A computer model was used to track the consequent changes in the carbon balance of nine wetland regions. Drainage reduced or eliminated net carbon sinks, converting some wetlands into net carbon sources. Different regions thus can function as smaller carbon sinks, or as sources, depending on the extent of drainage. In either case a shift in carbon balance can be quantified. (4) The net carbon sink in Finland and the U.S.S.R. has been reduced by 21-33%, in Western European wetlands by nearly 50%, and in Central Europe the sink has been completely lost. Overall, by 1900 the temperate zone sink was reduced 28-38% by agricultural drainage alone. (5) By 1980 the total annual shift in carbon balance attributable to agricultural drainage was 63-85 Mt of carbon, 38% in Finland and U.S.S.R. wetlands, and 37% in Europe. Twenty-five percent of the shift occurred in North American wetlands south of the boreal zone. No apparent change occurred in boreal Canada and Alaskan wetlands. (6) Peat combustion for fuel released 32-39 Mt of carbon annually, nearly all in the U.S.S.R. A total of 590-700 Mt of carbon has been released from peat combustion since 1795, compared with a release of 4140-5600 Mt from agricultural drainage. (7) The aggregate shift in the carbon balance of temperate zone wetlands, when added to a far smaller shift from tropical wetlands, equalled 150-185 Mt of carbon in 1980 and 5711-6480 Mt since 1795. Despite occupying an area equivalent to only 2% of the world's tropical forest, the wetlands have experienced an annual shift in carbon balance 15-18% as great. Wetlands thus are seen on an area-specific basis to be concentrated sources of atmospheric CO2 which respond differently from those ecosystems assumed to have no net carbon exchange before disturbance.

Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern queensland. ii* Total organic carbon and its rate of loss from the soil profile

Abstract: The kinetics of organic C loss were studied in six southern Queensland soils subjected to different periods (0-70 years) of cultivation and cereal cropping. The equation: Ct = Ce + (C0 - Ce)exp(- kt), where C0, Ce and C, are organic C contents initially, at equilibrium and at time k respectively, and k is the rate of loss of organic C from soil, was employed in the study. The parameter k was calculated both for %C (kc) and for weight of organic C/volume of soil (k,), determined by correcting for differences in sampling depth due to changes in bulk density upon cultivation. Mean annual rainfall largely determined both C, and Ce, presumably by influencing the amount of dry matter produced. Values of kc and kw varied greatly among the soils studied. For the 0-0.1 m depth, kw was 0.065, 0.080, 0.180, 0.259, 0.069 and 1.224 year-1 respectively for Waco (black earth - initially grassland), Langland-Logie (grey brown and red clays - brigalow), Cecilvale (grey, brown and red clays - poplar box), Billa Billa (grey, brown and red clays - belah), Thallon (grey, brown and red clays - coolibah) and Riverview (red earths - silver-leaved ironbark). The k values were significantly correlated with organic Chrease activity ratio (r = 0.99***) and reciprocal of clay content (r = 0.97**) of the virgin soils. In stepwise multiple regression analysis, aggregation index (for kc values) or exchangeable sodium percentage (for kw) and organic C/urease activity ratio of soils were significantly associated with the overall rate of loss of organic C. It was inferred, therefore, that the relative inaccessibility and protection of organic matter against microbial and enzymic attack resulted in reduced organic C loss. Losses of organic C from the deeper layers (0-0.2 m, 0-0.3 m) were observed in Waco, Langlands-Logie, Cecilvale and Riverview soils, although generally rate of loss decreased with depth.

Organic and inorganic nitrogen amendments to soil as nematode suppressants.

Abstract: Inorganic fertilizers containing ammoniacal nitrogen or formulations releasing this form of N in the soil are most effective for suppressing nematode populations. Anhydrous ammonia has been shown to reduce soil populations of Tylenchorhynchus claytoni, Helicotylenchus dihystera, and Heterodera glycines. The rates required to obtain significant suppression of nematode populations are generally in excess of 150 kg N/ha. Urea also suppresses several nematode species, including Meloidogyne spp., when applied at rates above 300 kg N/ha. Additional available carbon must be provided with urea to permit soil microorganisms to metabolize excess N and avoid phytotoxic effects. There is a direct relation between the amount of "protein" N in organic amendments and their effectiveness as nematode population suppressants. Most nematicidal amendments are oil cakes, or animal excrements containing 2-7% (w:w) N; these materials are effective at rates of 4-10 t/ha. Organic soil amendments containing mucopolysaccharides (e.g., mycelial wastes, chitinous matter) are also effective nematode suppressants. Key words: amendments, biological control, fertilizers, microbial ecology, nonchemical control, pest management, waste management.

Changes in Soil Carbon Storage and Associated Properties with Disturbance and Recovery

Abstract: Organic matter in the world’s soils contains about three times as much carbon as the land vegetation. Soil organic matter is labile and is likely to change as a result of human activities. Agricultural clearing, for example, results in a decline in soil organic matter. At the present time, there may be a net release of 0.85 × 1015 g C • yr−1 from soils of the world due to agricultural clearing (Houghton et al. 1983; Schlesinger 1984), or about 15% of the annual release from fossil fuels. The release of carbon may have been greater near the turn of the century as a result of more rapid agricultural expansion into virgin areas (Stuiver 1978, Wilson 1978). It is the purpose of this chapter (1) to review briefly the present estimates of the size of the pool of carbon in world soils and (2) to offer a review and analysis of what is known about the effects of agriculture on soil carbon storage.

Influence of the nature of soil organics on the sorption of toluene and trichloroethylene

Abstract: Predictive relationships that are presently employed for estimating the soil-water partitioning of nonionic organic pollutants do not account for the variable nature of soil organic matter. The capacity of selected components of soil organic matter to sorb trichloroethylene (TCE) and toluene, two nonionic volatile organic pollutants commonly encountered in contaminated groundwaters, is examined here. Sorption coefficients were determined and correlated with selected physico-chemical characteristics of the sorbents. Results indicate that the components of soil organic matter had widely varying affinities for toluene and TCE that cannot be solely explained by their organic carbon content. Multivariate regression results show that use of a sorbent's oxygen content as well as its carbon content yields a more accurate prediction of the sorptive partitioning coefficient than relationships that rely solely on the sorbent's carbon content.

Winter legumes as a nitrogen source for no-till grain sorghum

Abstract: Because winter legume cover crops may provide significant quantities of N while conserving soil and water resources, the role of legumes in conservation tillage production systems warrants renewed attention. The objectives of this study were (i) to determine the N contribution of several legumes to a subsequent crop of grain sorghum [ (L.) Moench] under no-tillage management and (ii) to determine the influence of legume cover crops on soil fertility status. A field experiment was conducted in which four winter legumes, one nonlegume, and a no cover crop treatment were studied over a 3-yr period. In addition, four fertilizer N rates (0, 28, 56, and 112 kg N ha) were applied to no-till grain sorghum. Grain sorghum did not respond to fertilizer N when following a legume cover crop but responded to as much as 99 kg N ha when following a nonlegume cover crop or no cover crop. A mean estimate of 72 kg N ha was obtained for the fertilizer N replaced by the legume. Theeffects of the legume cover crops on soil fertility status included: (i) a lower pH, (ii) a redistribution of K to the soil surface from deeper in the soil profile, and (iii) a lower C/N ratio in soil organic matter. Since fertilizer N represents a sizeable portion of the fossil fuel energy required for nonleguminous row crop production, the estimated N contribution of legume cover crops represents a significant energy savings, enhancing the conservation value of a no-tillage production system.

Land use change and the global carbon cycle: the role of tropical soils

Abstract: Millions of hectares of tropical forest are cleared annually for agriculture, pasture, shifting cultivation and timber. One result of these changes in land use is the release of CO2 from the cleared vegetation and soils. Although there is uncertainty as to the size of this release, it appears to be a major source of atmospheric CO2, second only to the release from the combustion of fossil fuels. This study estimates the release of CO2 from tropical soils using a computer model that simulates land use change in the tropics and data on (1) the carbon content of forest soils before clearing; (2) the changes in the carbon content under the various types of land use; and (3) the area of forest converted to each use. It appears that the clearing and use of tropical soils affects their carbon content to a depth of about 40 cm. Soils of tropical closed forests contain approximately 6.7 kg C · m-2; soils of tropical open forests contain approximately 5.2 kg C · m-2 to this depth. The cultivation of tropical soils reduces their carbon content by 40% 5 yr after clearing; the use of these soils for pasture reduces it by about 20%. Logging in tropical forests appears to have little effect on soil carbon. The carbon content of soils used by shifting cultivators returns to the level found under primary forest about 35 yr after abandonment. The estimated net release of carbon from tropical soils due to land use change was 0.11–0.26 × 1015 g in 1980.

Microbial biomass as an index for tillage-induced changes in soil biological properties

Abstract: Indirect measurements of soil microbial biomass carbon (C) and nitrogen (N), using the chloroform fumigation-incubation method (CFIM), were utilized as indices of the soil biological condition on a range of tillage systems under different climatic and agronomic conditions. Biological properties within the soil profile were influenced by the degree of soil mixing, incorporation of crop residues, soil moisture regime and root growth. A wheat-fallow system reduced microbial biomass C and N in the 0–15-cm soil depth by 60%, compared to a continuous wheat system. Reduction in soil tillage caused a redistribution of biological properties within the soil. For spring cereals under semi-arid—sub-humid moisture regimes, after 4 years, zero tillage increased microbial biomass C and N in the 0–5 cm soil depth by 10–23%, compared with shallow tillage; this increase was gradually offset over time by a decline in microbial biomass at lower soil depths. In a per-humid region, a similar redistribution of soil microbial biomass was evident after 2 years. After 3 years, direct drilling of Italian ryegrass increased microbial biomass C and N in the 0–5-cm soil depth by 26–28%, compared to a cultivated system. The level of microbial biomass under the direct-drilled system was similar to that found under an associated permanent pasture. The CFIM provided a sensitive measure of tillage-induced changes in soil biological properties. Both microbial biomass C and N responded rapidly to changes in tillage and soil management. In general, the CFIM detected changes in soil biological properties prior to any measurable change in soil organic C or N.

Long-term trends in fertility of soils under continuous cultivation and cereal cropping in southern queensland. I overall changes in soil properties and trends in winter cereal yields

Abstract: Changes in fertility of some southern Queensland soils resulting from extended periods of cultivation are presented, together with trends in yields of winter cereals on these soils. Six major soils of the cereal-belt, cropped for maximum periods of 20-70 years were examined. These were: Black earths, Waco soil; grey, brown and red clays (brigalow), Langlands-Logie soil; grey, brown and red clays (poplar box), Cecilvale soil; grey, brown and red clays (belah), Billa Billa soil; grey, brown and red clays (coolibah), Thallon soil; red earths, Riverview soil. Organic matter and its constituents, especially total organic C, organic C in the light fraction, total N and mineralizable N, were affected most by cultivation, showing decreases of 19-67% overall. Other soil properties probably associated with organic matter, including bulk density and DTPA (diethylenetriaminepentaacetic acid) extractable manganese, were also significantly affected by cultivation in all soils. Soil properties affected least by cultivation were concentrations of inorganic phosphorus, total and exchangeable potassium, calcium carbonate, and dithionite extractable iron and aluminium. Most other soil properties studied (organic P, total sulfur, pH, exchangeable magnesium and sodium, exchangeable sodium percentage, and oxalate-extractable iron and aluminium) were affected by cultivation in at least four soils. Four factors accounted for 70% of the total variation among the 45 soil properties considered. They appeared to represent organic matter, clay colloids, iron and aluminium oxides, and soluble salts. Dry matter yield and/or N uptake of winter cereal crops (wheat and barley) measured in 1983 showed significant decreasing trends with period of cultivation in all soils.

Soil organic matter, effects on soils and crops

Abstract: . Manurial treatments and cropping history have remained unchanged for many years in classical and long-term experiments at Rothamsted and Woburn, in some cases for more than 100 years. Soil samples taken periodically have been analysed to follow changes in organic carbon content with time and treatment. Data presented here clearly show effects of carbon input and soil texture on equilibrium organic matter content. Until recently increasing amounts of soil organic matter had little effect on yields of arable crops especially if fertilizer nitrogen dressings were chosen correctly. However the yield potential of many crops has increased and various agronomic inputs have become available to achieve that potential. Yields of many crops are now larger on soils with extra organic matter both on the sandy loam at Woburn and the silty clay loam at Rothamsted. Some of the effect appears to be related to extra water holding capacity, some to availability of nitrogen in ways which cannot be mimicked by dressings of fertilizer N, and some to improved soil physical properties. Responses to fertilizer N have been larger on soils with more organic matter.

Straw incorporation and soil organic matter in macro-aggregates and particle size separates

Abstract: SUMMARY Two field experiments in which straw has been removed or incorporated for 17 yr (loamy sand) and 10 yr (sandy clay loam) were sampled to examine the effect of straw on the C and N contents in whole soil samples, macro-aggregate fractions and primary particle-size separates. The particle size composition of the aggregate fractions was determined. Aggregates were isolated by dry sieving. Straw incorporation increased the number of 1–20 mm aggregates in the loamy sand but no effect was noted in the sandy clay loam. Straw had no effect on the particle size composition of the various aggregate fractions. After correction for loose sand that accumulated in the aggregate fractions during dry sieving, macro-aggregates appeared to be enriched in clay and silt compared with whole soil samples. Because of the possible detachment of sand particles from the exterior surface of aggregates during sieving operations, it was inferred that the particle size composition of macro-aggregates is similar to that of the bulk soil. The organic matter contents of the aggregate fractions were closely correlated with their clay + silt contents. Differences in the organic matter content of clay isolated from whole soil samples and aggregate fractions were generally small. This was also true for the silt-size separates. In both soils, straw incorporation increased the organic matter content of nearly all clay and silt separates; for silt this was generally twice that observed for clay. The amounts of soil C, derived from straw, left in the loamy sand and sandy clay loam at the time of sampling were 4.4 and 4.5 t ha−1, corresponding to 12 and 21% of the straw C added. The C/N ratios of the straw-derived soil organic matter were 11 and 12 for the loamy sand and sandy clay loam, respectively.

Soil acidification during more than 100 years under permanent grassland and woodland at Rothamsted

Abstract: Soil samples have been taken periodically from unlimed plots of the 130-year-old Park Grass Experiment and from the 100-year-old Geescroft Wilderness at Rothamsted. Changes in the pH of the samples show how acidification has progressed. The soils are now at, or are approaching, equilibrium pH values which depend on the acidifying inputs and on the buffering capacities of the soils. We have calculated the contributions to soil acidification of natural sources of acidity in the soil, atmospheric deposition, crop growth and nutrient removal, and, where applicable, additions of fertilizers. The relative importance of each source of acidification has changed as the soils have become more acid. Acid rain (wet deposited acidity) is a negligible source, but total atmospheric deposition may comprise up to 30% of acidifying inputs at near neutral soil pH values and more as soil pH decreases. Excepting fertilizers, the greatest causes of soil acidification at or near neutral pH values are the natural inputs of H+ from the dissolution of CO2 and subsequent dissociation of carbonic acid, and the mineralization of organic matter. Under grassland, single superphosphate and small amounts of sodium and magnesium sulphates have had no effect on soil pH, whilst potassium sulphate increased soil acidity slightly. All of these effects are greatly outweighed under grassland, however, by those of nitrogen fertilizers. Against a background of acidification from atmospheric, crop and natural inputs, nitrogen applied as ammonium sulphate decreased soil pH up to a maximum of 1.2 units at a rate in direct proportion to the amount added, and nitrogen applied as sodium nitrate increased soil pH by between 0.5 and 1 unit.

Land use and organic carbon content of some subtropical soils

Abstract: The assumption that the organic matter content of tropical forest soils is oxidized to atmospheric carbon dioxide when these soils are converted to agricultural use was tested using results of soil surveys in Puerto Rico (1940's, 1960's, and 1980's). Results showed that under intensive agricultural use, soil carbon in the top 18 cm of soil was about 30–37 Mg/ha, regardless of climatic conditions. Reduced intensity of agricultural use resulted in an increase of soil carbon in the order of 0.3–0.5 Mg.ha−1. yr−1 over a 40-yr period. Rates of soil carbon accumulation were inversely related to the sand content of soils. The relation between rates of soil carbon accumulation and climate or soil texture were better defined at higher soil carbon content. Soils under pasture accumulated soil carbon and often contained similar or greater amounts than adjacent mature forest soils (60–150 Mg/ha in the top 25 or 50 cm). Soils in moist climates exhibited greater variations in soil carbon content with changes in land use (both in terms of loss and recovery) than did soils in dry climates. However, in all life zones studied, the recovery of soil carbon after abandonment of agriculture was faster than generally assumed. Low carbon-to-nitrogen ratios suggested that intensively used soils may be stable in their nutrient retention capacity. The observed resiliency of these soils suggested that their role as atmospheric carbon sources has been overestimated, while their potential role as atmospheric carbon sinks has been underestimated.

Worldwide organic soil carbon and nitrogen data

Abstract: The objective of the research presented in this package was to identify data that could be used to estimate the size of the soil organic carbon pool under relatively undisturbed soil conditions. A subset of the data can be used to estimate amounts of soil carbon storage at equilibrium with natural soil-forming factors. The magnitude of soil properties so defined is a resulting nonequilibrium values for carbon storage. Variation in these values is due to differences in local and geographic soil-forming factors. Therefore, information is included on location, soil nitrogen content, climate, and vegetation along with carbon density and variation.

Long term trends in fertility of soils under continuous cultivation and cereal cropping in southern Queensland. III. Distribution and kinetics of soil organic carbon in particle size fractions

Abstract: Distribution of soil organic carbon in sand-, silt- and clay-size fractions during cultivation for periods ranging from 20 to 70 years was studied in six major soils used for cereal cropping in southern Queensland. Particle-size fractions were obtained by dispersion in water using cation exchange resin, sieving and sedimentation. In the soils' virgin state no single particle-size fraction was found to be consistently enriched as compared to the whole soil in organic C in all six soils, although the largest proportion (48%) of organic C was in the clay-size fraction; silt and sand-size fractions contained remaining organic C in equal amounts. Upon cultivation, the amounts of organic C declined from all particle-size fractions in most soils, although the loss rates differed considerably among different fractions and from the whole soil. The proportion of the sand-size fraction declined rapidly (from 26% to 12% overall), whereas that of the clay-size fraction increased from 48% to 61% overall. The proportion of silt-size organic C was least affected by cultivation in most soils. It was inferred, therefore, that the sand-size organic matter is rapidly lost from soil, through mineralization as well as disintegration into silt-size and clay-size fractions, and that the clay fraction provides protection for the soil organic matter against microbial and enzymic degradation.

Distribution of microorganisms, total biomass, and enzyme activities in different particles of brown soil.

Abstract: A soil sample from the Ap horizon of an arable brown soil was fractionated by wet sieving, and seven size fractions of organic and mineral soil particles were separated. The organic fractions formed only 2.2% of the soil dry mass, but contained 41.5 and 29.12% of the total soil content of carbon and nitrogen, respectively, and thus represented an important reservoir of readily utilizable nutrients. Organic particles also accumulated most of the soil enzyme activities, determined asβ-glucosidase,β-acetylglucosaminidase, and proteinase activity. The highest counts of bacteria, actinomycetes, and fungi per gram of the soil fractions were obtained with the organic particles, but for the most part microorganisms accumulated in the silt-clay fraction. All soil fractions except the coarsest organic particles contained higher counts of oligotrophic bacteria than copiotrophic ones. Microbial counts, ATP contents, and enzyme activities decreased significantly with decrease in size of the organic soil particles, and increased with decrease in size of the mineral soil particles. Thus, the coarse organic particles >5 mm and the silt-clay fraction <0.05 mm represent the sites with the highest concentrations of microorganisms, ATP contents, and enzyme activities in the arable brown soil under test.

Effect of fire intensity on solution chemistry of surface soil under a Eucalyptus pauciflora forest

Abstract: The chemical composition of soil solutions (field percolates collected in situ and laboratory saturation extracts) was measured at three sites subjected to widely varying fire intensity in subalpine Eucalyptus paucfiora forest near Canberra. The sites were unburnt forest, areas prescribed burnt resulting in almost complete canopy scorch, and ashbeds (intensely heated soil). Saturation extracts were obtained 1, 58, 375, 745 and 1095 days after the fire, and soil percolates were collected on 17 occasions during the initial year after burning. Large quantities of cations (Ca2+, Mg2+, K+ , NH+4) and anions (Cl-, SO24-) and soluble silica were mobilized by burning, especially under ashbeds. Mobilization resulted from deposition of water-soluble elements in ash, immediate effects of soil heating, and enhanced rates of mineralisation of soil organic matter indicated by high concentrations of NH+4 which persisted for more than one year in surface soils under the ashbeds. After burning Ca2+ became the dominant cation in saturation extracts of surface (0-5 cm) soils for the entire 3-year study period. In the 5-15 cm soil layer, firstly NH+4 and later K+ replaced some of the Na+ in the solution phase. Most of the Cl- deposited in ash was leached below 15 cm depth within one year and was probably accompanied by transport of K+, Mg2+, Na+ and NH+4, but very little transfer of Ca2+ occurred. Concentrations of NO-3 and phosphate were always low in saturation extracts and soil percolates, and levels were unaffected by burning, despite the presence of large amounts of exchangeable NH+4 in the soil and the deposition of significant amounts of phosphate in ash. Burning increased the concentrations of soluble silica and SO24- in saturation extracts for at least 3 years after the fire. Most of the changes in soil solution chemistry measured would increase nutrient availability to the vegetation during the initial year after burning, but these changes must be balanced against losses of organic matter and nutrients during and after fires.

A study of soil erosion on vertisols of the eastern Darling Downs, Queensland .I. Effects of surface conditions on soil movement within Contour Bay catchments

Abstract: Effects of soil surface conditions on runoff and soil loss were studied on two major cracking clay soils of the Darling Downs, Queensland. Techniques used to measure soil loss between field contour bays under natural rainfall are described, and the results from 10 contour bay catchments (66 plot years) are presented. Soil movement was separated into rill, interrill, suspended sediment and channel deposition. Two slope lengths were considered (60 and 35 m), and interrill erosion appeared to be the major source of soil loss. Runoff and sediment concentration were both inversely related to surface cover and total soil movement was greatly reduced by surface cover. In an annual winter-wheat, summer-fallow system, removal of stubble resulted in soil movement of 29-62 t ha-1 year-1. Retention of stubble (stubble mulching) reduced soil movement to less than 5 t ha-1 year-1. Greater than 75% of the variance in soil movement from single events was explained by surface cover and peak runoff rate. Surface cover is a measure of the surface area protected from soil detachment and entrainment. Peak runoff rate describes the amount of energy or stream power available for detachment and entrainment.

Spectroscopic investigations of cultivation effects on organic matter of vertisols

Abstract: The major mechanism responsible for the relative stability of the persistent soil organic matter is a physical association with the inorganic components of the soil, rather than an inherent chemical or biochemical inertness of the organic matter itself.-from Authors

Profiles of CO2 concentration and δ13C values in tropical rain forests of the upper Rio Negro basin, Venezuela

Abstract: Concentration of CO2, above and below the soil surface and δ13C values of plant tissues, soil litter and organic matter were measured in a caatinga forest of the upper Rio Negro basin in southern Venezuela. CO2, concentrations near the forest floor were consistently higher than in the atmosphere. CO2, gradient in the soil was very steep probably because of the poor aeration in this flood-prone forest. δ13C values of plant tissues showed a clear pattern with lower values in the ground herbaceous plants and under-canopy trees. Tree seedlings showed δ13C values similar to the upper-canopy trees indicating their dependence on reserves carried in the seed from the mother tree. Decomposing litter and soil organic matter also showed δ13C values similar to the upper-canopy trees. It is suggested that lower δ13C values of the shade flora result primarily from the assimilation of CO2, depleted in δ13C originating from soil respiration. Probable effects of low light intensity and physiological factors are discussed.

Conservation tillage effects on soil properties and yield of corn and soya beans in Indiana

Abstract: Long-term studies have been conducted on 7 soils throughout the state of Indiana, to determined the effectiveness of a range of tillage systems in producing high crop yields while reducing erosion and improving soil physical properties. The soil studied included one sandy loam, one loam, three silt loams and two silty clay loams with a range of organic matter contents and slope and drainage classes. Tillage systems studied included conventional moldboard plow, chisel plow, disking, ridge till-plant and no-till. Conservation tillage systems resulted in higher soil water contents, lower soil temperatures, more organic matter and more water-stable aggregates near the surface, and higher bulk densities than conventional tillage systems. On sloping, well-drained, low organic matter soils, conservation tillage produced corn (Zea mays L.) yields that were equal or better than yields from conventional tillage. On poorly-drained, low organic matter, poorly structured soils, the soil structure under conservation tillage tended to improve with time as soil organic matter and aggregation increased. Corn and soya bean (Glycine max) yields have also improved with time and often exceed those from conventionally tilled soils. On poorly drained soils high in organic matter that were cropped to continuous corn, conservation tillage generally yielded less than conventional systems due to low soil temperature and excess wetness in spring. Significant interactions of tillage systems and crop rotation existed on the two poorly-drained soils on which rotation was an experimental variable.

Kinetics of Calcite Precipitation in the Presence of Water‐soluble Organic Ligands

Abstract: The rate of precipitation of calcite is an important geochemical process which can influence the saturation state of natural waters with respect to pure calcite. The present study was established to determine the influence of water-soluble soil organic ligands on the rate of calcite precipitation. A reproducible seeded growth method was used to measure the rate constant for calcite precipitation at pH = 8.4 and 25°C, in the presence of varying amounts of total soluble organic carbon (CTS) added as a water-soil extract (WSE) or Seward fulvic acid (FA). The rate equation which described the rate of calcite precipitation in the present experiments was: where R = rate of calcite precipitation (mol Ls), kf = precipitation rate constant (L molms), s = surface area of calcite seeds (mL), = divalent ion activity coefficient, brackets represent concentrations (mol L), and Ksp is the solubility of pure calcite at 25°C. Rate constants decreased to zero in the presence of 0.15 mM and 0.028 mM CTS added as the WSE and FA, respectively. The organic C surface coverage on the calcite seeds corresponding to complete inhibition of calcite precipitation was 90 and 30 atoms C nm 2 for the WSE and FA, respectively. The efficacy of inhibition of calcite precipitation in the presence of soluble soil organic matter explains why many natural waters, including soil solutions of Calciaquolls, are oversaturated with respect to pure calcite. Soluble polymeric carbon constituents which adsorb onto potential calcite seeds, render these surfaces inactive as sites for crystal growth. Additional Index Words: calcium carbonate, crystal growth, fulvic acid, calcite supersaturation. View complete article To view this complete article, insert Disc 4 then click button8