Showing papers on "Soil organic matter published in 1993"

Tropical soil biology and fertility: a handbook of methods.

Abstract: Based on the work of the Tropical Soil Biology and Fertility (TSBF) Programme, this is a handbook of recommended and validated methods for the characterization and analysis of tropical soils, with the aim of achieving sustainable use of soil resources. The objectives of the programme revolve around five main themes: synchrony of nutrient release and plant growth demands; management of soil organic matter; soil water balance; effects and management of soil fauna; and integration of biological processes into the maintenance of soil fertility. The methods given are endorsed by the International Soil Science Society and are part of the International Union of Biological Sciences and the UNESCO Man and the Biosphere Programme.

Terrestrial ecosystem production: A process model based on global satellite and surface data

Abstract: This paper presents a modeling approach aimed at seasonal resolution of global climatic and edaphic controls on patterns of terrestrial ecosystem production and soil microbial respiration. We use satellite imagery (Advanced Very High Resolution Radiometer and International Satellite Cloud Climatology Project solar radiation), along with historical climate (monthly temperature and precipitation) and soil attributes (texture, C and N contents) from global (1°) data sets as model inputs. The Carnegie-Ames-Stanford approach (CASA) Biosphere model runs on a monthly time interval to simulate seasonal patterns in net plant carbon fixation, biomass and nutrient allocation, litterfall, soil nitrogen mineralization, and microbial CO2 production. The model estimate of global terrestrial net primary production is 48 Pg C yr−1 with a maximum light use efficiency of 0.39 g C MJ−1PAR. Over 70% of terrestrial net production takes place between 30°N and 30°S latitude. Steady state pools of standing litter represent global storage of around 174 Pg C (94 and 80 Pg C in nonwoody and woody pools, respectively), whereas the pool of soil C in the top 0.3 m that is turning over on decadal time scales comprises 300 Pg C. Seasonal variations in atmospheric CO2 concentrations from three stations in the Geophysical Monitoring for Climate Change Flask Sampling Network correlate significantly with estimated net ecosystem production values averaged over 50°–80° N, 10°–30° N, and 0°–10° N.

Organic Carbon in Soils of the World

Abstract: The C stored in soils is nearly three times that in the aboveground biomass and approximately double that in the atmosphere. Reliable estimates have been difficult to obtain due to a lack of global data on kinds of soils and the amount of C in each soil. With new data bases, our study is able to provide more reliable data than previous estimates. Globally, 1576 Pg of C is stored in soils, with ≈ 506 Pg (32%) of this in soils of the tropics. It is also estimated that ≈ 40% of the C in soils of the tropics is in forest soils. Other studies have shown that deforestation can result in 20 to 50% loss of this stored C, largely through erosion.

Observations and modeling of biomass and soil organic matter dynamics for the grassland biome worldwide

Abstract: Century is a model of terrestrial biogeochemistry based on relationships between climate, human management (fire, grazing), soil properties, plant productivity, and decomposition. The grassland version of the Century model was tested using observed data from 11 temperate and tropical grasslands around the world. The results show that soil C and N levels can be simulated to within ±25% of the observed values (100 and 75% of the time, respectively) for a diverse set of soils. Peak live biomass and plant production can be simulated within ± 25% of the observed values (57 and 60% of the time, respectively) for burned, fertilized, and irrigated grassland sites where precipitation ranged from 22 to over 150 cm. Live biomass can be generally predicted to within ±50% of the observed values (57% of the time). The model underestimated the live biomass in extremely high plant production years at two of the Russian sites. A comparison of Century model results with statistical models showed that the Century model had slightly higher r2 values than the statistical models. Data and calibrated model results from this study are useful for analysis and description of grassland carbon dynamics, and as a reference point for testing more physiologically based models prediction's of net primary production and biomass. Results indicate that prediction of plant and soil organic matter (C and N) dynamics requires knowledge of climate, soil texture, and N inputs.

Principles of soil chemistry

Abstract: Review of the basic chemical principles electrochemical cells and chemical potentials soil composition, soil air and soil solution colloidal chemistry of organic soil constituents colloidal chemistry of inorganic soil constituents adsorption in soils cation exchange anion exchange soil reaction soil chemistry of soil-organic matter interaction.

Nutrient cycling in forests.

Abstract: SUMMARY Studies of nutrient cycling in forests span more than 100 yr. In earlier years, most attention was given to the measurement of the pools of nutrients in plants and soil and of the return of nutrients from plant to soil in litterfall. The past 20 yr or so have seen a major concentration on the processes of nutrient cycling, with particular emphasis on those processes by which the supply of nutrients to the growing forest is sustained. In the more highly productive forests, up to 10 tonnes of litter of low nutritional quality is deposited annually on the forest floor. The decomposition of this litter, the mineralization of the nutrients it holds, and the uptake of nutrients by tree roots in the carbon-rich environment which results are the themes of this review. Studies of decomposition of litter in forests have been dominated by the role of nitrogen as a limiting factor, a domination which reflects the preponderance of studies of temperate forests in the Northern Hemisphere. For many forests of the world growing on soils of considerable age, it seems more probable that growth and nutrient cycling are limited by phosphorus (or some other element). There is increasing evidence for a number of forests that phosphorus is immobilized in the first stages of decomposition to a significantly greater extent than is nitrogen. Advances in research will depend, as with studies of soil organic matter, in denning and developing analytical techniques for studying biologically active forms of potentially limiting nutrients, rather than total elemental concentrations. The availability of phosphorus in forests is sustained by phosphorus cycling. More than 50% of the total phosphorus in the surface soils is in organic forms and much of the more labile phosphorus is in the form of diesters. Phosphorus availability is determined by competition between biological and geochemical sinks, and it is clear that the sinks in the rhizosphere (plant roots, microorganisms, soil mineral and organic components) are extensively modified by active processes (e.g. production of exudates, nutrient storage in a variety of organic or polymeric forms and nutrient transport away from sites of uptake). There is abundant evidence that roots of many species exude compounds which have the ability to solubilize sources of phosphorus of otherwise low availability. The significance of root exudates (for example, phosphatases, organic acids) in the functioning of perennial ecosystems has yet to be quantified and there are conflicting reports as to the effects of simple organic acids on phosphorus availability. The distribution of phosphorus sinks and their relative competitiveness and their modification are topics of fundamental importance for future research. In contrast to the mineralization of phosphorus, our knowledge of transformations and availability of nitrogen in forest soils is well-developed. Net nitrogen mineralization rates approximate rates of nitrogen return in litterfall but the contribution of nitrification is variable. Nitrification is not inhibited by the low pH of many forest soils and there is increasing evidence of nitrate immobilization by microorganisms and of increased diversity and better competitiveness for NH4+ of nitrifying microorganisms than has previously been accepted. Variability in rates of nitrification is often interpreted as being due to allelopathy. Hypotheses invoking allelopathy are more or less untestable, and it seems likely that new techniques using 15N in situ will lead to a more fundamental understanding of nitrogen transformations in forest soils. Recent studies in coniferous forest soils have highlighted the short (< 1 d) turnover time of NH4+. Finally, it seems that forest soils are resistant to major changes in patterns of nitrogen mineralization (and certainly, because of the large number of sinks, in patterns of phosphorus mineralization) following disturbance by natural events such as wind-throw and fire, and by man-made events such as logging and fertilizing. The long-term disturbance by acid rain is a more complex matter since forest ecosystems are not adequate buffers for nitrate.

Cadmium, Lead, Zinc, Copper, and Nickel in Agricultural Soils of the United States of America

Abstract: Three thousand forty-five surface soil samples from 307 different soil series were analyzed for Pb, Cd, Zn, Cu, Ni, cation exchange capacity (CEC), organic C, and pH in the course of a study of trace element uptake by major agricultural crops. The soil data from this study are summarized here statistically and in map form to show their interactions and generalized geographic distribution patterns. Amounts of all five metal elements are generally low in the Southeast. A regional high of about 15 mg/kg Pb covers the Mississipi, Ohio, and Missouri River valleys. Higher values for other elements are generally concentrated in the West and in the lower Mississippi River Valley. Maximum Cd levels were found in soils of the coast ranges of central and southern California. Copper levels are noticeably higher in organic soil areas of Florida, Oregon, and the Great Lakes. Nickel and Cu concentrations are high in serpentine soil areas of California. Nickel levels are also somewhat higher in the glaciated areas of the northern great plains and in northern Maine. For the entire dataset, the values of the minimum-maximum, 5th, 50th, and 95th percentiles are as follows: (mg/kg dry soil) Cd, <0.005 to 2.0, 0.036, 0.20, 0.78; Pb, 0.5 to 135, 4.0, 11, 23; Zn, 1.5 to 264, 8.0, 53, 126; Cu, 0.3 to 495, 3.8, 18.5, 95; Ni, 0.7 to 269, 4.1, 18.2, 57; pH (pH units) 3.9–8.9, 4.7, 6.1, 8.1; CEC (cmol/kg) 0.6 to 204, 2.4, 14.0, 135; and organic C % 0.09 to 63, 0.36, 1.05, 33.3. Metal levels generally increased with increasing clay concentration. Contribution from the National Soil Survey Lab. Midwest Technical Center, Soil Conservation Service, Lincoln, NE.

A Hierarchical Model for Decomposition in Terrestrial Ecosystems: Application to Soils of the Humid Tropics

Abstract: A general model is presented in which the dynamics of decomposition in terrestrial ecosystems are determined by a set of hierarchically organized factors which regulate microbial activity at decreasing scales of time and space in the following order: climate - clay mineralogy + nutrient status of soil - quality of decomposing resources - effect of macroorganisms (i.e., roots and invertebrates). At the lower scale of determination, biological systems of regulation based on mutualistic relationships between macro- and microorganisms ultimately determine the rates and pathways of decomposition. Four such systems are defined, i.e., the litter and surface roots system, the rhizosphere, the drilosphere and the termitosphere in which the regulating macroorganisms are respectively litter arthropods and surface roots, live subterranean roots, endogeic earthworms, and termites. In the humid tropics, this general model is often altered because climatic and edaphic constraints are in many cases not important and because high temperature and moisture conditions greatly enhance the activity of mutualistic biological systems of regulation which exert a much stronger control on litter and soil organic matter dynamics. This general hypothesis is considered in the light of available information from tropical rain forests and humid savannas. Theoretical and practical implications regarding the biodiversity issue and management practices are further discussed. It is concluded that biodiversity is probably determined, at least partly, by soil biological processes as a consequence of enhanced mutualistic interactions, which enlarge the resource base available to plants. It is also concluded that any effort to restore or rehabilitate degraded soils in the humid tropics is promised to fail unless optimum levels of root and invertebrate activities are promoted and the resulting regulation effects operate in the four abovedescribed biological systems of regulation. Research required to substantiate and adequately test the present set of concepts and hypotheses are expressed.

The Scale of Nutrient Heterogeneity Around Individual Plants and Its Quantification with Geostatistics

Abstract: Heterogeneity in the soil is regularly invoked as important for competitive interactions among plants (Chapin 1980), but surprisingly few attempts have been made to examine in situ soil heterogeneity in the context of individual plants (Snaydon 1962). Variability around individuals is of fundamental importance because current theories of plant competition (e.g., Grace and Tilman 1990) differ in their treatments of heterogeneity and scale. Although quantifying scale has been historically problematic, statistical advances of recent decades (Matheron 1963, Burgess and Webster 1980) now provide tools for ecologists to specifically address scale and heterogeneity in their experiments (Robertson et al. 1988, Rossi et al. 1992). In this study we use geostatistical techniques to quantify the scale and variability of soil nutrients at distances from 10 cm to 10 m in the field, with emphasis on the variability around individual perennial plants. The study area is 30 km south of Logan, Utah in a native sagebrush steppe (41029' N, 1 1047' W, 1575 m elevation); the soil is a silt loam formed from noncalcareous alluvial material. Average soil organic matter and pH are -2.7% (dry mass basis) and 6.3%, respectively. Prevalent at the site are the native shrub Artemisia tridentata (Rydb.) Beetle and the native tussock grass Pseudoroegneria spicata (Pursh) A. L6ve., both common Great Basin perennials. The sagebrush (Artemisia) plants tend to be relatively small (usually < 0.5 m tall with a fairly sparse canopy) for this species, but growth ring analysis of five plants showed several to be between 15 and 20 yr old. Neighbors of mature plants are sometimes closely spaced, in some cases within 0.5 m. Other plant genera present include Balsamorhiza, Zigadenus, Viola, Poa, and Lomatium.

Comparison of carbon dynamics in tropical and temperate soils using radiocarbon measurements

Abstract: The magnitude and timing of the response of the soil carbon reservoir to changes in land use or climate is a large source of uncertainty in global carbon cycle models. One method of assessing soil carbon dynamics, based on modeling the observed increase of 14C in organic matter pools during the 30 years since atmospheric weapons testing ended, is described in this paper. Differences in the inventory and residence time of carbon are observed in organic matter from soils representing tropical (Amazon Basin, Brazil) and temperate (western slope of the Sierra Nevada mountains, California) forest ecosystems. The majority of the organic carbon in the upper 22 cm of the tropical soil (7.1 kgC m−2) has residence times of 10 years or less, with a minor component of very refractory carbon. The estimated annual flux of carbon into and out of the soil organic matter in this horizon of the mineral soil, based on modeling of the 14C data, is between 1.9 and 5.5 kgC m−2 yr−1. In contrast, organic matter in the temperate soil over a similar depth interval (0-23 cm; 5.2 kgC m−2), is made up of approximately equal amounts of carbon with residence times of 10, 100, and 1000 years. The estimated annual flux of carbon into and out of this soil is 0.22 to 0.45 kgC m−2 yr−1. Rapid turnover of organic matter with density <1.6 - 2.0 g cm−3contributes a major component of the annual flux of carbon into and out of both soil types. Hydrolysis of mineral soil organic matter of density < 1.6-2.0 g cm−3 removed 14C-enriched components from the temperate soil but had no effect on the 14C content of the residue in 0 - 22 cm layer of the tropical soil. The results presented here show that carbon cycle models which treat soil carbon dynamics as a single reservoir with a turnover rate based on radiocarbon measurements of bulk soil organic matter underestimate the annual fluxes of organic matter through the soil organic matter pool, particularly in tropical regions.

Amounts, dynamics and sequestering of carbon in tropical and subtropical soils

Abstract: The organic carbon pool in the upper 1 m of the world's soils contains 1220 Gt organic carbon, 1.5 times the total for the standing biomass. In the widespread deep soils in the tropics the carbon stored below 1 m may add about 50 Gt C. The contributions of charcoal, roots and soil fauna should be added to these totals. The much less dynamic carbonate-carbon pool amounts to 720 Gt C. Changes in land use, particularly by clearing of forests, reduce organic carbon by 20 to 50% in the upper soil layers, but little in deeper layers. On the other hand, there are indications that a human-induced enrichment of soil organic matter can be maintained over centuries. Research on the causative soil processes should be supported, because an improved understanding of this phenomenon might lead to better management strategies and sound programs to stimulate organic carbon storage and fertility levels in tropical and subtropical soils. Recent research data on the CO[sub 2] fertilization effect and the associated antitranspiration effect due to an increase of CO[sub 2] in the atmosphere indicate that a positive influence on soil organic carbon levels can be expected. 64 refs, 5 figs, 5 tabs

Site-Related ^(13)C of Tree Leaves and Soil Organic Matter in a Temperate Forest

Abstract: In order to relate the isotopic composition of soil organic matter to parent vegetation and soil type, we measured the carbon stable isotope ratios of tree leaves and soil C at 14 locations in a temperate forest. Sites were selected over a wide variety of soil types and related vegetation associations, but within a single regional climate. The 613C of the bulk leaf material falling on the soil varied among sites, ranging from -29.5 to - 26'oo. Within this range, differences up to 1.5%oo were attributable to differences in tree species. Further differences were related to site. The 613C values of soil organic matter cover the range -29.8 to -24.30%oo and varied with both parent vegetation 613C and depth. Trans- formation and decay of organic C within the litter layers led to no isotope enrichment, irrespective of vegetation and humus type: mull, hydromull, moder, or anmoor. The dif- ference between woody and leaf material in litter was low, 0.23 ? 0.35 Too (mean ? 1 SD). On the other hand 613C in mineral horizons always increased with depth, reaching values richer than litter by 1.0 ? 0.5%oo at 20 cm and up to +1.5%oo at 1 m. The trend was independent of the soil physico-chemical characteristics, whether aerobic, anaerobic, eu- trophic, oligotrophic, or podzolic. Isotope ratios of soil organic C in forests appear to provide useful estimators of tree isotopic composition. When compared to the measure- ments of the vegetation, they offer the advantage of an easily obtained value that naturally integrates over plant material, time and, eventually, local area.

Long-Term Nutrient Accumulation Rates in the Everglades

Abstract: Anthropogenic nutrient inputs to the northern Everglades of Florida during the last three decades have resulted in alteration of vegetation and soil nutrient storage. Due to the nutrient-limited status of this ecosystem, increased loading may have altered the capacity for long-term nutrient accumulation. Our study was conducted to determine the potential long-term nutrient accumulation rates for this ecosystem along a gradient of nutrient loading. Accumulation rates were calculated using the vertical peat accretion rates, as determined by Cs dating, and nutrient concentration profiles. Intact soil cores were obtained along a 15-km transect and evaluated as a function of distance from the inflow structure. Soil cores were sectioned into 1-cmdepth increments and analyzed for '-"Cs, P, N, C, and selected cations. Vertical accretion rates of peat decreased logarithmically with distance from the inflow, with rates of 1.1 cm yr~' at 0.3 km from the inflow to about 0.25 cm yr' in unimpacted sawgrass (Cladium jamaicense Crantz)-dominated areas. Phosphorus, N, and C accumulation rates in soil and floodwater total P concentrations also showed similar relationships. The P accumulation rates ranged from 0.54 to 1.14 g P myr~' in cattail (Typha spp.)-dominated areas, and 0.11 to 0.25 g P myr~' in sawgrass-dominated areas. The C/P and N/P accumulation ratios increased with distance from the inflow, suggesting that a greater proportion of P accumulated in the system, compared with C and N. Similar P retention coefficients were obtained when calculated using either changes in surface water total P concentration, or the long-term P accretion rates. These findings suggest that P was either directly adsorbed by soil or precipitated with Ca in the water column and deposited on the soil surface. This hypothesis was further supported by a highly significant correlation between P and Ca accretion rates, suggesting that Ca-bound P controls equilibrium concentrations in this ecosystem. L NUTRIENT ACCUMULATION in wetland ecosystems is determined by the balance between inputs and outputs. Nutrients in wetlands undergo several biogeochemical transformations, some resulting in the loss of certain nutrients as gaseous end products or through leaching and discharge to outflow, while K.R. Reddy and W.F. DeBusk, Soil and Water Science Dep., Institute of Food and Agricultural Sciences, 106 Newell Hall, Univ. of Florida, Gainesville, FL 32611; R.D. DeLaune, Laboratory for Wetland Soils and Sediments, Louisiana State Univ., Baton Rouge, LA 70803; and M.S. Koch, Dep. of Everglades Systems Research, South Florida Water Management District, P.O. Box 24680, West Palm Beach, FL 33416. Florida Agric. Exp. Stn. Journal Series no. R-02740. Received 13 May 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:1147-1155 (1993). others result in nutrient accumulation within the ecosystem. Nutrient accumulation can occur through sedimentation or organic matter accumulation. In peatdominated wetlands, a major portion of the nutrients is stored in live and detrital plant tissue, microbial biomass, and stabilized soil organic matter. Nutrients stored in vegetation and microbial biomass can be readily released through natural die-off and decomposition (Davis, 1991). In herbaceous wetlands, nutrient storage in vegetation is usually short term (Reddy and DeBusk, 1987), while in forested wetlands incorporation of nutrients into woody tissue of trees can result in long-term storage (Richardson and Davis, 1987). As C and N are cycled through a wetland, a portion can be lost as gaseous end products. For example, organic C is converted to CO2 and CH4, and is lost from the system. This process is influenced by the hydrologic regime of the system, with frequent wet and dry cycles increasing decomposition rates and loss of C (Reddy and Patrick, 1975). In the EAA, oxidation of organic matter under drained conditions accounted for soil loss of about 3 cm yr (Snyder et al., 1978). However, CO2 fixation by vegetation and accumulation of detrital material in many wetlands usually offsets decomposition, resulting in a net C accumulation. Similarly, organic N is mineralized to NH4-N, which is subsequently lost through nitrification-denitrification and NH3 volatilization reactions (Reddy and Patrick, 1984). However, P released during decomposition is usually retained by the wetland through sorption and precipitation reactions (HowardWilliams, 1985). Nutrients added to a wetland are rapidly incorporated into living and detrital plant material, and eventually incorporated into soil organic matter (Puriveth, 1980; Day, 1982; Davis and van der Valk, 1983; DeBusk and Reddy, 1987). Long-term nutrient retention by soil organic matter is affected by environmental factors such as temperature, hydroperiod and fire. Nutrient accumulation rates have been estimated for many wetland ecosystems using Cs as a marker (DeLaune et al., 1978; Hatton et al., 1983; Kadlec and Robbins, 1984; Patrick and DeLaune, 1990). Peat Abbreviations: EAA, Everglades Agricultural Area; WCA, Water Conservation Area; ENP, Everglades National Park; SRP, soluble reactive P; TKN, total Kjeldahl N. 1148 SOIL SCI. SOC. AM. J., VOL. 57, JULY-AUGUST 1993

Management of tropical soils as sinks or sources of atmospheric carbon

Abstract: The prevailing paradigm for anticipating changes in soil organic carbon (SOC) with changes in land use postulates reductions in SOC in managed systems (agriculture and tree plantations) relative to mature tropical forests. Variations of this notion are used in carbon models to predict the role of tropical soils in the global carbon cycle. Invariably these models show tropical soils as sources of atmospheric carbon. We present data from a variety of studies that show that SOC in managed systems can be lower, the same as, or greater than mature tropical forests and that SOC can increase rapidly after the abandonment of agricultural fields. History of land use affects the comparison of SOC in managed and natural ecosystems. Our review of the literature also highlights the need for greater precautions when comparing SOC in mature tropical forests with that of managed ecosystems. Information on previous land use, bulk density, and consistency in sampling depth are some of the most common omissions in published studies. From comparable SOC data from a variety of tropical land uses we estimate that tropical soils can accumulate between 168 and 553 Tg C/yr. The greatest potential for carbon sequestration in tropical soils is in the forest fallows which cover some 250 million hectares. Increased attention to SOC by land managers can result in greater rates of carbon sequestration than predicted by current SOC models.

Chapter 7 Physical Characteristics of Volcanic Ash Soils

Abstract: Publisher Summary Volcanic ash soils have many unique physical properties that are attributable directly to the properties of the parent material, the noncrystalline materials formed by weathering, and the soil organic matter accumulated during soil formation. These properties include dark soil color, difficult clay dispersion, unique consistence, low bulk density, and high water holding capacity. Soil color is the most striking feature observed for volcanic ash soils, especially for their A horizons. Andisols typically display a large difference in texture when comparing field and laboratory determination methods. Noncrystalline materials play an important role as cementing agents and react with an excess amount of sodium hexametaphosphate. Furthermore, each of the inorganic colloids shows a different point of zero net charge, so that complete dispersion of mineral particles is virtually impossible. Andisols generally have low bulk densities that are attributable to the development of highly porous soil structure. Andisols usually show low degrees of stickiness, plasticity, and hardness that result from the abundance of noncrystalline materials and/or soil organic matter. These physical properties also provide an excellent environment for root growth. Andisols have well-developed soil structure resulting in high porosity with a range of pore sizes that retain a large amount of water with varying tensions.

Formation of carbon monoxide from the photodegradation of terrestrial dissolved organic carbon in natural waters

Abstract: The photochemical formation of carbon monoxide (CO) in water samples obtained from wetlands, lakes, and near-coastal/shelf areas and in aqueous solutions of soil organic matter was investigated. All of these samples contained dissolved organic matter that was largely derived from terrestrial sources. The studies show that, although the water samples had widely varying optical properties and CO photoproduction rates, the efficiencies for photochemical CO formation were remarkably similar in all waters examined. Model calculations further indicated that photodegradation of terrestrial dissolved organic matter (e.g., in wetland and near-coastal environments) may be an important global source of carbon monoxide and a key process in cycling of dissolved organic matter in these environments.

Ion interactions and constraints to plant nutrition in Australian sodic soils

Abstract: Many of the arable soils in Australia are affected by salinity and/or sodicity. Nutrient deficiency and ion toxicity may occur in both saline and sodic soils. Ho-ever, the mechanism for these constraints on plant growth in sodic soils differs from that of saline soils. Fertility of sodic soils with low nutrient reserves is compounded by the low supply of water and oxygen to roots in profiles with dispersive clays. Nutrient constraints in sodic soils are created by the electron and proton activities (pE and pH) in an environment of degraded soil structure. Australian sodic soils accumulate relatively low levels of organic matter. High sodium, high pH and low biological activity, commonly found in these soils, are not conducive for both the accumulation of organic matter and its mineralization. As a result, these soils are deficient in N and S. Australian soils are highly weathered and have moderate to low reserves of many plant nutrients such as Cu, Mn, Mo, Zn and P. Solubility of phosphorus is generally increased in sodic soils. Poor leaching conditions accumulate boron in soil layers. Higher concentrations of sodium than of calcium in these soils are the major cause of both physical and nutritional problems. Therefore, amelioration of sodicity is the logical first step in improving the chemical fertility of sodic soils. However, fertilizer application and improvement of soil organic matter are essential to increase yields to match the potential yield predictable from climate.

Calculation of nitrogen mineralization in soil food webs

Abstract: In agricultural practices in which the use of inorganic fertilizer is being reduced in favour of the use of organic manure, the availability of nitrogen (N) in soil for plant growth depends increasingly on N mineralization. In simulation models, N mineralization is frequently described in relation to the decomposition of organic matter, making a distinction in the quality of the chemical components available as substrate for soil microbes. A different way to model N mineralization is to derive N mineralization from the trophic interactions among the groups of organisms constituting the soil food web. In the present study a food web model was applied to a set of food webs from different sites and from different arable farming systems. The results showed that the model could simulate N mineralization rates close to the rates obtained from in situ measurements, from nitrogen budget analyses, or from a decomposition based model. The outcome of the model suggested that the contribution of the various groups of organisms to N mineralization varied strongly among the different sites and farming systems.

Soil organic matter, CEC, and moisture sensing with a portable NIR spectrophotometer

Abstract: Soil reflectance data were collected with a portable near infrared (NIR) spectrophotometer and were correlated with soil organic matter content in laboratory and field tests. Laboratory calibrations yielded an r2 of 0.89 and a standard error of prediction of 0.40% organic matter with 30 representative Illinois soils at 1.5 MPa and 0.033 MPa moisture tension levels. Limited in-furrow field operation produced much higher errors, due to the movement of soil past the sensor during data acquisition. Estimation of cation exchange capacity (CEC) and soil moisture content was also accomplished in the laboratory.

Soils and the environment : an introduction

Abstract: Preface Units, symbols and general information 1. Introduction: Soil in a natural and man-made environment Part I. Soil Properties and Processes: 2. The soil components 3. Development of soils 4. Sorptive properties of soil 5. Organisms and soil processes 6. Movement of water, air, solutes and heat in soil Part II. Soils in Relation to the Environment: 7. Soil as a medium for plant growth 8. Soil conditions and crop production 9. Soil acidification 10. Heavy metals and radionuclides in soil 11. Soils, the atmosphere, global warming and ozone depletion 12. Soil erosion and conservation 13. Soils in the environment: problems and solutions Subject index.

Soil microbial biomass and activity of a disturbed and undisturbed shrub-steppe ecosystem

Abstract: Disturbance of shrub-steppe soils and alterations in plant cover may affect the distribution, size and activity of soil microorganisms and their ability to biogeochemically cycle essential nutrients Therefore, the soil microbial biomass and activity and selected soil enzyme activities were determined for two arid ecosystems, an undisturbed perennial shrub-steppe and annual grassland, which was initially shrub-steppe and has been an annual grassland since the disturbance caused by farming ceased in the 1940s Soils were sampled at 0–5 and 5–15 cm depths beneath sagebrush ( Artemisia tridentata Mutt), bluebunch wheatgrass [ Elytrigia spicata (Pursh) DR Dewey] and cryptogamic soil lichen crust at the perennial site and beneath downy brome ( Bromus tectorum L) at the annual grassland site Soils were analyzed for physical properties, inorganic N, microbial biomass C and N, respiration and several enzymes The soil pH and bulk density usually increased, while inorganic N, total N and total C decreased as a function of soil depth Soil microbial biomass C and N, soil respiration and soil dehydrogenase activity were 2–15 times higher in the top 5 cm of soil than at the 5–15 cm depth regardless of plant type Loss of this surface soil would therefore be detrimental to microbially-mediated cycling of nutrients Surface soil (0–5 cm depth) microbial biomass C and N and soil respiration, dehydrogenase and phosphatase activity were influenced by plant type and decreased in the order B tectorum A tridentata = E spicata soil crust Spatial distribution of plant species at the shrub-steppe site resulted in “islands” of enhanced microbial biomass and activity underneath the shrubs and grasses when compared to the interplant areas covered with soil crust When plant cover was used to compute a landscape estimate of soil microbial biomass C and N for the perennial shrub-steppe and the annual grassland, similar values were obtained This indicates that while the distribution of microorganisms may be more heterogeneous in the shrub-steppe, the average across the landscape is the same as the more homogeneous annual grassland

Changes in soil organic matter with cropping as measured by organic carbon fractions and 13C natural isotope abundance

Abstract: The decline in soil organic matter with cropping is a major factor affecting the sustainability of cropping systems. Changes in total C levels are relatively insensitive as a sustainability measure. Oxidation with different strength KMnO4 has been shown to be a more sensitive indicator of change. The relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping, whilst the relative size of the unoxidised fraction increased. Changes in δ13 C ratio have been used to measure C turnover in systems which include C3 and C4 species.

Size of the soil microbial biomass in a long-term field experiment as affected by different n-fertilizers and organic manures

Abstract: The size of the soil microbial biomass was measured in a more than 30 yr old field experiment, whose treatments included different N fertilizers and organic manures. The size of the microbial biomass was measured as biomass C and N by the chloroform fumigation-incubation technique, as K2SO4 extractable ninhydrin-reactive N released upon fumigation and as the soil's ATP content. There was a high degree of correlation (r > 0.88) between the fumigation-based methods and the ATP determinations. Compared with the biomass estimate by ATP, biomass C was underestimated in the ammonium sulphate fertilized soil (pH 4.4), the peat-amended soils, and the sewage sludge amended soil. Biomass N was only underestimated in the ammonium sulphate and peat-amended soil, whereas there was a good correlation between the ninhydrin assay and the ATP assay for all soils. Between three successive years biomass C showed larger, statistically significant, variations than the size of the biomass measured by the ninhydrin assay. There was a high degree of correlation (r > 0.90) between both the rate of base respiration and the size of the microbial biomass and the soil's carbon content. These relationships generally held independent of whether carbon was derived from stabilized soil organic matter (in the fallow soil), from crop residues, or from organic manures such as straw, green manure, farmyard manure, or sawdust. Relative to the soil's carbon content the microbial biomass was smaller than expected in the peat amended-soils, the ammonium sulphate fertilized, and the sewage sludge-amended soil. The rate of base respiration was only lower than expected in the sewage sludge treated soil. The size of the biomass was negatively affected by a low soil pH, but the rate of base respiration was not. Liming some of the soils indicated that other factors than low pH restricted the size of the biomass in the peat and sewage-sludge amended soils, but not in the ammonium sulphate fertilized soils.

Regulators of denitrification in an organic riparian soil

Abstract: We investigated microbial denitrification in an organic riparian zone and identified factors which regulated its rate. The riparian zone received nitrate from incoming groundwater draining an upslope forest which was spray irrigated with treated effluent. Soil cores were taken from the riparian zone and the following variables were measured: KCl-extracted nitrate, water soluble carbon concentration, organic matter content, moisture content, denitrifying enzyme activity, on-site denitrification rates and natural N 2 O production. Five sampling surveys were made at a range of field temperatures (12–21°C). The riparian soil was continually water-saturated and contained an average organic matter content of 26%. Nitrate concentration in groundwater entering the upslope edge of the riparian zone was generally greater than 5 mg N l −1 . In combination, these factors resulted in an ideal environment for denitrification. Mean and median denitrification rates were found to be 1.12 and 0.95g N m −2 day −1 ; while mean and median N 2 O production rates were 73 and 84 mg N m −2 day −1 These rates were 1–3 orders of magnitude greater than those reported in previous studies of upland soils. Up to 77% of the variation in on-site denitrification rate could be explained by nitrate concentration and denitrifying enzyme activity. Temperature may also have regulated the rate of denitrification; however, insufficient observations at different temperatures were made to fully establish a temperature effect. N 2 O production was found to be most highly correlated to on-site denitrification rate. Rates of denitrifying enzyme activity were also greater than those generally found in upland soils, the mean and median rates were 810 and 740 ng N g −1 h −1

Nutrient limitations on microbial respiration in peat soils with different total phosphorus content

Abstract: The effects of phosphate and ammonium additions on microbial respiration were determined using three peat soils with similar total N content (25–38 g kg−1) and different total P (TP) content: low (231 mg P kg−1), intermediate (385 mg P kg−1), and high (1.473 g P kg−1). These soils, obtained from Everglades National Park, Florida, had been formed predominantly from the partial decomposition of sawgrass (Cladium jamaicense). Soil respiration was measured by gas chromatography. Amendment of the soils with 0–100 mm PO4 or NH4 resulted in changes in the C-to-nutrient ratio that ranged from 4 to 2042 for C:P and 11.5 to 14.1 for C:N. Kinetic parameters for soil respiration were obtained by fitting the data to a kinetic model that describes soil C mineralization as the sum of an exponential decay function for readily mineralizable C and a zero-order decay function for stable C. Estimates of the zero-order rate constant for endogenous respiration in unamended soil samples increased with TP content of the soil (in mmol CO2 kg−1 d−1): low TP (36–40) < intermediate TP (49–52) < high TP (107–114). Addition of phosphate stimulated the respiration rate of the low and intermediate TP soil, but had no effect on the rate of soil respiration of the high TP soil. Ammonium additions inhibited soil respiration in the low and intermediate TP soils at most concentrations tested; the rate of respiration was inversely proportional to the amount of NH4 added. Addition of ammonium to the high TP soil stimulated respiration. Microbial respiration in the low and intermediate TP soils is limited by P availability, whereas N appears to limit respiration in the high TP soil. These results suggest that P pollution may have a marked, long-term effect on microbial respiration in organic soils with low TP content.

The chemical nature of nitrogen in native soil organic matter

Abstract: H. Knicker, R. Frtind and H.-D. Ltidemann Institut for Biophysik und physikalische Biochemie der Universit~it, W-8400 Regensburg, FRG Fossil fuels and soil organic matter (SOM) together contain approximately five times more carbon than the biota and the atmosphere. Of this, soil organic matter accounts for about 30 % of the carbon present. In addition, SOM has an average carbon/nitrogen ratio of 10/1 and contains a huge fraction of the total ni- trogen available for plant growth [1]. Taking into account that the abundance of nitrogen in the earth's crust is much lower than that of carbon, this is a signif- icant fraction of the total nitrogen acces- sible to the biosphere. Under natural soil conditions, without the addition of miner- al fertilizers, SOM provides the majority of the nitrogen necessary for plant growth. It is also thought to be respon- sible for the interaction between agricul- tural biocides and the soil [5-7]. The chemical structure of this ubiquitous material, SOM, and especially the chem- ical nature of the nitrogen are thus of great and general importance. The mo- lecular structure of the nitrogen- containing fraction is, however, still a matter of controversy [2-4]. Structural models based on partial chem- ical analysis claim that a significant part of the nitrogen is present in the form of heteroaromatic structures, while NMR- spectroscopic studies on lSN-enriched composts and recent humic material found approximately 85 % of the signal intensity in the amide/peptide region of chemical shift and no signals in the range typical for heteroaromatic nitrogen. A major fraction of the native soil organic matter has been in the soil for several hundred to several thousand years [8, 9]. Compared to these time spans, laboratory-produced material has been fermented for at most 1 year, and it could be argued that heteroaromatic structures are only produced after much longer fermentation periods. This criti- cism may be overcome by the study of lsN-CPMAS spectra of soil organic mat- ter with natural lSN levels. This has not been achieved hitherto, because the low natural abundance (0.4 %) and the small gyromagnetic ratio of the 15N nucleus and therefore its low sensitivity in NMR experiments appeared to make this experiment an impossible one. The most abundant 14N-isotope (99.6 %) cannot be studied by high-resolution NMR because its large nuclear quadrupole moment leads to very broad and unresolved signals, especially in solid-state NMR [101. In previous systematic studies on 15N- enriched composts and organic soil extracts [11] our group optimized all spectral parameters for the ~sN-CPMAS experiment. A crude estimate showed that it should be possible to obtain 15N spectra with a tolerable signal-to-noise ratio after the accumulation of approximately one million transients. In the present paper we report on the first successful results of such experiments. Six German soils were studied as detailed in Table 1. In Figs. 1 and 2 some of the spectra obtained are shown. They fully corrobo- rate the conclusions drawn from the stud- ies of short-term composting exper-