Amount Of Exchangeable Cations Research Articles

English

In Rwanda, the traditional fired solid bricks embody high fuel consumption and quantity of clay during their molding. Traditional solid bricks are also heavy compared to perforated bricks. This study aimed at investigating the performance of perforated clay bricks made of clay soil (CS) mixed with charcoal waste (CW) as an additive. The physical, chemical and mechanical properties of CS mixed with 0, 10, 30, 40 and 50% of CW were determined. The bricks were extruded manually with the inclusion of a varied number of perforations: 4, 8, 12 and 15 number perforations. The chemical analysis included the determination of organic carbon, exchangeable acidity and chemical components (through XRD) in the soil matrix. The chemical analysis depicted an increase in CaO proportions with an increase in charcoal waste content. There was also an increase in the amounts of exchangeable cations. The research found that with an increase in perforations, weight loss is reduced. However, non-perforated bricks depicted the least abrasion resistance, followed by bricks with 4 perforations. As the CW content increased from 0 to 50%, bricks with 15 and 12 perforations displayed the highest abrasion resistance. The research concludes that an increase in charcoal waste led to an increase in the linear shrinkage of clay soil bricks due to increased calcium cations and reduced magnesium cations in the soil. An optimum of 30% of charcoal waste was identified for the linear shrinkage of clay soil bricks. Bricks having 12 perforations and below with 30% charcoal waste additive can be categorized as category I (load-bearing) bricks.

Cation exchange capacity of biochar: An urgent method modification

Biochar, produced through pyrolysis of organic matter, is negatively charged, thus contributing to electrostatic adsorption of cations. However, due to its porous structure and contents of alkaline ashes, the determination of the cation exchange capacity (CEC) is challenging. Literature values for the CEC of biochar are surprisingly variable and are often poorly reproducible, suggesting methodological problems. Here, we modify and critically assess different steps in the existing ammonium acetate (NH4OAc) method (pH 7), where ammonium (NH4+) is displaced by potassium chloride (KCl), following removal of excess NH4OAc with isopropanol, in batch mode. We used pigeon pea biochar to develop the method and conducted a test on three additional biochars with different acid neutralizing capacity. A pretreatment step of biochar was introduced, using diluted hydrochloric acid, to decrease biochar pH to near neutral, so that 1 M NH4OAc effectively buffers the biochar suspension pH at 7. This allows the CEC of all biochars to be determined at pH 7, which is crucial for biochar comparison. The dissolution of ashes may cause relatively large weight losses (e.g. for cacao shell biochar), which need to be accounted for when computing the CEC of raw biochar. The sum of NH4OAC-extractable base cations provided a smaller and better estimate of the CEC than KCl-extractable NH4+. We hypothesize that the overestimation of the CEC based on KCl-extractable NH4+ is due to the ineffectiveness of the relatively large isopropanol molecules to remove excess NH4OAc in biochars rich in micro-pores, due to size exclusion. The amount of base cations removed in the pretreatment was about three (rice husk biochar) to ten times (pigeon pea biochar) greater than the amount of exchangeable cations. The CEC values of biochar increased from 10.8 cmol/Kg carbon to 119.6 cmol/Kg carbon. These values are smaller than reported CEC values of soil organic carbon.

Using probing compounds to investigate adsorption mechanism of ciprofloxacin on montmorillonite

Ciprofloxacin (CIP) adsorption on montmorillonite was investigated using structurally related probing compounds phenylpiperazine (PP) and fluorochloroquinolone carboxylic acid (FCQCA). Desorption of the equivalent amounts of exchangeable cations accompanying PP and CIP adsorption on montmorillonite suggested that cation exchange was the major adsorption mechanism for the CIP adsorption under acidic to neutral conditions, and the piperazine amine group was responsible for the cation exchange interaction. In contrast, the adsorption of CIP and FCQCA on montmorillonite indicated that hydrogen bonding between the keto carbonyl group and coordinated water in the interlayer of montmorillonite was mainly responsible for the CIP adsorption in alkaline aqueous solution as evidenced by Fourier transform infrared analyses. X-ray diffraction analyses demonstrated that both cation exchange and hydrogen bonding between the keto carbonyl group and coordinated water lead to the intercalation of CIP in montmorillonite.

Prediction of swelling pressures of different types of bentonite in dilute solutions

A mechanistic model is developed to predict the swelling pressure of fully saturated, bentonite-based materials in distilled water or dilute saline solutions over a large range of final dry densities of bentonite. It applies a thermodynamic relationship between swelling pressure and suction to describe the contribution of crystalline swelling, while using a diffuse double-layer model to explain the behavior of osmotic swelling. In addition, it accounts for the demixing of exchangeable cations and the disintegration of the montmorillonite particles into small stacks of unit layers upon water uptake.Comparison of the model predictions with a great number of experimental results of swelling pressures of different types of bentonites and bentonite–aggregate mixtures in both distilled water and saline solutions suggests that the model works excellently in the cases tested. It is found that the water chemistry, the montmorillonite content, the type and amount of exchangeable cations in the interlayers are important in determining the extent to which the montmorillonite particles are delaminated and hence the swelling behavior of saturated, bentonite-based materials.On the other hand, the applicability of the model in predicting the water retention curves of unsaturated bentonites is also tested. The results show that the predicted curves are in good agreement with the measured data and that the montmorillonite particles are more difficult to disintegrate into small pieces in the case of unsaturated bentonites than would otherwise be possible.

Anionic RR120 dye adsorption onto raw clay: Surface properties and adsorption mechanism

Clay minerals are efficient adsorbent for dyes due to their colloidal properties. A Tunisian raw clay (Fouchana) composed of different species of clay minerals (kaolinite, smectite and minor illite) has shown efficiency for removal of anionic dye Reactive Red 120 (RR120) from aqueous solutions. In order to provide a better understanding of the role played by the different clay species in Fouchana, as well as the nature of the adsorption processes which are involved, a comparative study was conducted using standard clays, namely kaolinite KGa-2, smectite SWy-1 and illite IMt-2. Batch adsorption experiments were performed at ambient temperature and pH of the dye solutions. Acido-basic properties of the solid surfaces, electrophoretic mobility of particles, cation exchange capacity, amount of exchangeable cations and specific surface were investigated. Principal component analysis was used to highlight the influence of the physicochemical characteristics of the clay minerals in the adsorption behavior. The results show that for the conditions set in this study, kaolinite in the Fouchana raw clay plays a major role in anionic dye adsorption, mainly due to its acidic surface sites, but also to exchangeable aluminum (III) cations acting as bridges between the clay mineral and the dye molecules. Smectite and illite most likely play a secondary role by adsorption on exchangeable calcium (II) and dissociated Al (III)OH groups.

Soil Carbon and Nutrient Accumulation under Forest Plantations in Jharkhand State of India

AbstractThe decrease and degradation of the tropical forests affect not only the production of timber but also the global environment in a large scale. The ability of soil to sustain and its supply of nutrients to a growing forest are controlled by a complex of biogeochemical processes. The purpose of the present study aims to assess the degraded forest fringe areas, to promote plantations of various types and to evaluate their impacts on the soil nutrients and carbon content accumulation. The soil organic carbon (SOC) and nutrient content were evaluated and compared between plantations of mixed native species (MNS), some native tree species as Shorea robusta, Dalbergia sissoo, Dendrocalamus spp., certain agro‐forestry species and some exotic varieties. The impacts of the plantations on the SOC and the nutrients were firstly analyzed through comprehensive chemical analyses and the results were compared with the soil samples collected prior to plantation forestry. Significant changes were observed in SOC content, in nutrients, and in amounts of exchangeable cations. Soil carbon levels were highest under the MNS, Dendrocalamus and Tectona grandis stands and lowest under D. sissoo and Terminalia arjuna. Total N showed highest levels under Dendrocalamus and Pongamia pinnata and significantly higher in stands of native species; lowest total N level was observed in D. sissoo plantations. The C/N ratios of the soil varied between 9.2 and 13.5 among the exchangeable cations. Ca2+ recorded the maximum levels and Na+ showed the lowest levels.

Leachability and metal-binding capacity in ageing landfill material

In order to study the stability of landfilled heavy metals, landfill material from a combined household and industrial waste landfill was aerated for 14 months to simulate the natural ageing processes as air slowly begins to penetrate the landfill mass. During aeration, the pH of the landfill material decreased from around 8.6 to 8.1 and the carbon content also decreased. In order to investigate the possible fate of metals in ageing landfills, a four-stage sequential extraction technique was applied. The ability of the materials to bind metal ions by electrostatic attractions and to form stronger complexes was studied separately. The amount of exchangeable cations, the capacity to bind metal ions by electrostatic attraction and the capacity of the landfill material to complex copper ions were increased by the aeration process. However, results from the sequential analysis showed an increased solubility of sulphur and some metals (Cd, Co, Cu, Ni and Zn). Equilibrium speciation models (Medusa) indicated that the organic matter deposit had a significant capacity to bind metal ions provided that pH was sufficiently high. However, as carbonates are consumed over time, the risk for metal mobility increases. Therefore, the landfills can become an environmental risk, depending on variations in the solubility of metal ions due to changes in pH, redox status and the availability of organic material.

Comparison of methods for the determination of the layer charge density (LCD) of montmorillonites

Two wide spread methods exist for the determination of the layer charge density (LCD) of montmorillonites. 1) Structural formula method (SFM): calculation of the structural formula based on the chemical composition (Ross, C.S., Hendricks, S.B. (1945): Minerals of the montmorillonite group. — U. S Geol Surv Profess Paper 205-B, p. 23–79.; Stevens, R.E. (1946): A system for calculating analyses of micas and related minerals to end members. — Contributions to geochemistry, 1942–45, Geological survey bulletin, 950, p. 101 – 119.; Grim, R.E., Güven, N. (1978): Bentonites – geology, mineralogy, properties and uses. – Developments in sedimentology 46, Elsevier, New York, pp. 254.; Newman, A.C.D., Brown, G. (1987): The chemical constitution of clays. In: Chemistry of clays and clay minerals, Mineralogical Society Monograph No. 6, Newman, A.C.D. (ed.), John Wiley and sons, New York, p. 1 – 128.; Laird, D.A. (1994): Evaluation of the structural formula and alkylammonium methods of determnining layer charge. — In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 79 – 104). 2) Alkylammoniummethod (AAM): calculation of the LCD from measuring the d-spacing after intercalation of a set of n-Alkylammoniumions (Lagaly, G. (1994): Layer charge determination by alkylammonium ions.—In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 1 – 46). The SFM generally provides higher values (up to 40%, Laird, D.A. (1994): Evaluation of the structural formula and alkylammonium methods of determnining layer charge. — In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 79 – 104.). Currently, most clay mineralogists from all over the world use either SFM or AAM which leads to incomparable results. In order to unambiguously prove which method provides correct values for the LCD and to be able to calculate correct montmorillonite contents two alternative methods (method A1 and A2) are tested in this paper. These procedures are independent from the well accepted SFM and AAM. Method A1, based on the determination of CEC and inner surface area by water adsorption, failed because water adsorption depends to a higher degree on the amount of exchangeable cations than on the surface area. Method A2 is based on measuring the variable charge, the total charge (CEC) of a pure smectite fraction, and the molar mass of a formula unit. This method (A2) is applicable if fine grained impurities do not interfere with the production of the pure smectite fraction. In conclusion, by applying a fully independent method for determining the LCD the results obtained by the AAM were confirmed. This data strongly indicates that the AAM as proposed by Lagaly (1994)[Lagaly, G. (1994): Layer charge determination by alkylammonium ions.—In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., p. 1 – 46.] provides reasonable values for the LCD. Additionally, it is shown that high LCD values as commonly determined by SFM for medium to high charged smectites cannot lead to the calculation of correct montmorillonite contents. However, there is no doubt, that high charged and hence non-swellable 2:1 layer silicates cannot be investigated by the AAM (as discussed by Mermut, A.R. (1994): Problems associated with layer charge characterisation of 2:1 phyllosilicates. — In: Mermut, A.R., editor: Layer charge characteristics of 2:1 silicate clay minerals, CMS Workshop Lectures Vol. 6, Boulder, CO, Clay Miner. Soc., 79 – 104).

Diversity and dominance in palm (Arecaceae) communities interra firmeforests in the western Amazon basin

Summary 1 Current discussions about the structure and functioning of tropical plant communities revolve around whether tropical forests are dominated by sets of widespread and locally abundant species and the relative importance of environmental variation and geographical distance as determinants of variation in species composition. We explored these questions by examining diversity and dominance patterns of palm communities in lowland rain forests in the western Amazon basin (Yasuni National Park, Ecuador and Iquitos-Pebas region, Peru). 2 We used multiple regression to analyse floristic similarity between sites as a function of environmental differences (in topographic position, elevation difference, amount of exchangeable cations and soil texture) and geographical distance. We studied dominance patterns by quantifying the correlations between local abundance and landscape frequency of species within and between the regions. 3 We found floristic composition to be highly variable and more strongly related to geographical distance than to environmental differences. Together, geographical distance and environmental differences explained 70–85% of the variation in floristic similarity. 4 Species abundant in one region were also often abundant in the other, although some dominant species in Iquitos-Pebas did not occur in Yasuni. Local dominance was more pronounced in Yasuni, where the dominant species formed a larger and less variable proportion of all individuals within transects. 5 Limited dispersal and, to a lesser extent, local environmental variation appear to act as determinants of floristic variation in the western Amazon basin. Oligarchic dominance patterns were observed, particularly in Yasuni. 6 Our results lend support to the hypothesis that dispersal across broader spatial scales plays a strong role in the assembly of local ecological communities, but indicate that such limitation supplements, rather than replaces, local ecological determinism.

Relationships between Collembola, soil chemistry and humus types in forest stands (France)

Soil samples were taken under four tree species in various forest sites located in southern France. For each sampled A horizon of the soil profile, pH, organic matter content, C:N ratio and amounts of exchangeable cations (K, Ca, Mg) were measured, and collembolan fauna was extracted using Berlese-Tullgren funnels. A total of 78 species representing 11 families and 47 genera were identified. The relationships between Collembola and soil parameters were determined by co-inertia analysis which corresponds to the simultaneous Correspondence Analysis of the collembolan data table and the Multiple Correspondence Analysis of the environmental data table. The analysis revealed that the distribution of 10 species was related to pH, organic matter content via C and N and to base cations (K, Ca, Mg). Samples, linking these collembolan species and chemical parameters, were clustered according to humus forms. Collembola seem to be linked closer to the physical structure of humus than to its chemical parameters. Their specific contribution to pedogenetic processes as yet remains to be clarified.

Modelling cation exchange in columns of disturbed and undisturbed subsoil

SummaryCation exchange is often studied with disturbed and dried soils, but the applicability of the results to undisturbed soils is not straightforward. We investigated the value of exchange coefficients obtained from standard procedures for predicting cation exchange in soil. Columns of undisturbed and disturbed subsoil of a Luvisol (SBt horizon) were leached under saturated conditions with 0.4, 4, 20, 41, 102 and 205 mm BaCl2 at a Darcy velocity of 1400 mm day−1. The model PHREEQC was used to calculate one‐dimensional transport, inorganic complexation and multiple cation exchange. Two model variants were tested: m1 (exchangeable cations obtained by percolation with NH4Cl) and m2 (exchangeable cations obtained by shaking the soil with BaCl2). The exchange coefficients (Gaines–Thomas formalism) were calculated from the ion activities in solution and exchangeable cations obtained by NH4Cl percolation (m1) or shaking with BaCl2 (m2). Variant m1 predicted cation exchange of the disturbed (homogenized) soil for the entire BaCl2 concentration range, whereas variant m2 resulted in a two‐fold overestimation of desorbed K for all experiments, which was related to large amounts of K released from the soil by shaking with BaCl2. In experiments with undisturbed soil, variant m1 predicted the concentrations of Mg, Ca, K, and Na in the solution phase and the sum of cations released from exchange sites. However, variant m2 predicted changes in ion concentrations and exchangeable cations somewhat less well. This study suggests that the amounts of exchangeable cations and exchange coefficients obtained from experiments with homogenized soil by percolation are useful to predict cation concentrations in column experiments with undisturbed soils.

Acidification of Solid and Solution Phases at Greater Depths (> 150 cm) at Solling Spruce Site

The acidification of the soil and percolation water at soildepths from 150 to 500 cm was studied at the Solling spruce sitefrom 1991 to 1996. NH4Cl exchangeable cations of the fineearth and bedrock fractions were obtained from different depthsand the soil solution composition was monitored at 150, 200,300, 400 and 500 cm depths using seven suction lysimeters at each depth.In the seepage water collected from 150 and 200 cm depth, pHvalues decreased in the period 1991 to 1996, but no significantchanges were observed in solutions collected below 200 cm depth.Element budgets of Al and Mb (Na, K, Mg, Ca) cationsindicated that buffering by exchange of Al with Mb cationsoccurred mainly in surface 200 cm soil depth. High variabilities in concentrations of SO4 (at 150 cm) andMa (Al, Mn, H, Fe) cations (at 300 and 500 cm) wereobserved. High variabilities in Ma cations could beassigned to one of the lysimeters at each depththat extracted low pH solutions. The amount of exchangeablecations in the fine earth and the bedrock fractions indicatedthat the acidification front (exchangeable Mb cations < 80equivalent percent) had occurred to soil depth of more than 360cm, but the extent of acidification that might have occurred inthe preindustrial period is not known. In both fine earth andbedrock fractions, depthwise changes of exchangeable Ma andMb cations were quite similar, suggesting that rockfractions have contributed to proton buffering not only bysilicate weathering but also by cation exchange.

The distribution of solute processes on an acid hillslope and the delivery of solutes to a stream: II. Exchangeable Al3+

This paper aims to identify the spatial distribution of exchangeable base cations in soils on an acid hillslope and to investigate possible cation release processes from slope soils to the stream. The basic assumption underlying this research is that the amount of exchangeable cations in soils reflects the nutrient stores and cation leaching processes across the slope where vegetation and parent materials are similar. The distribution of exchangeable Ca2+, Mg2+, K+ and Na+ has been investigated on a three-dimensional hillslope on the Quantock Hills, Somerset, UK. A two-way ANOVA shows that soil depth is predominant in explaining the total variance of exchangeable bases, despite the steep slope gradient and clear podzolic catena development. Major nutrient base cations, such as Ca2+, Mg2+ and K+, display homogeneous topsoil storage right across the slope. This spatial pattern may indicate that the spatial distribution of major nutrient cations is tightly controlled by the soil–vegetation system in nutrient-poor heathland environments. Na+ is an exception to this vegetation-controlled spatial distribution, because of its small involvement in the soil–vegetation and soil exchangeable systems. In subsurface soils, cations liberated from the soil–vegetation system are subject to redistribution over the slope according to the hydrological flowpaths operating on the slope, with some eventually released into the stream. The saturated wedge developed at the base of the slope plays a key role in the storage and release processes of base cations from slope soils to the stream. Ca2+, Mg2+ and Na+ carried by throughflow are stored in the saturated wedge and gradually released into the stream at times of high flow. K+, however, shows an apparently different spatial behaviour, being deficient in the saturated wedge. Copyright © 1999 John Wiley & Sons, Ltd.

The distribution of solute processes on an acid hillslope and the delivery of solutes to a stream: I. Exchangeable bases

This paper aims to identify the spatial distribution of exchangeable base cations in soils on an acid hillslope and to investigate possible cation release processes from slope soils to the stream. The basic assumption underlying this research is that the amount of exchangeable cations in soils reflects the nutrient stores and cation leaching processes across the slope where vegetation and parent materials are similar. The distribution of exchangeable Ca2+, Mg2+, K+ and Na+ has been investigated on a three-dimensional hillslope on the Quantock Hills, Somerset, UK. A two-way ANOVA shows that soil depth is predominant in explaining the total variance of exchangeable bases, despite the steep slope gradient and clear podzolic catena development. Major nutrient base cations, such as Ca2+, Mg2+ and K+, display homogeneous topsoil storage right across the slope. This spatial pattern may indicate that the spatial distribution of major nutrient cations is tightly controlled by the soil–vegetation system in nutrient-poor heathland environments. Na+ is an exception to this vegetation-controlled spatial distribution, because of its small involvement in the soil–vegetation and soil exchangeable systems. In subsurface soils, cations liberated from the soil–vegetation system are subject to redistribution over the slope according to the hydrological flowpaths operating on the slope, with some eventually released into the stream. The saturated wedge developed at the base of the slope plays a key role in the storage and release processes of base cations from slope soils to the stream. Ca2+, Mg2+ and Na+ carried by throughflow are stored in the saturated wedge and gradually released into the stream at times of high flow. K+, however, shows an apparently different spatial behaviour, being deficient in the saturated wedge. Copyright © 1999 John Wiley & Sons, Ltd.

Biomass burning in windrows after clearing a tropical rainforest: Effects on soil properties, evaporation and crop yields

Burning is a widely used practice to clear fields of felled vegetation after forest clearing. Effect of vegetation-burning on soil properties and crop yields, however, has received limited attention. Two months after clearing a rainforest by the shear-blade method, dry-biomass was windrowed (4 windrows per plot) and burnt. Changes in soil properties, evaporation and crop productivity in windrows (burnt area) one year after burning the felled biomass were investigated. There were considerable increases in the amounts of exchangeable cations in the top 30-cm soil layer after burning. Consequently, soil pH increased from 5.7 to 7.0 in the 0–10-cm layer, from 5.6 to 6.5 in the 10–20-cm layer, and from 5.4 to 5.9 in the 20–30-cm layer. Burning and conversion of biomass to ash increased the availability of major nutrient elements. Bulk density was lower in the burnt area. The equilibrium infiltration rate was 106 and 13 cm h −1 in the burnt and unburnt areas, respectively. The rate of water evaporation and, thus, cumulative evaporation was higher from the burnt area. Rice (cultivar UPL-Ri-5) and plantain yielded significantly more in the burnt compared with the unburnt sites. Corn and cassava also yielded more in the burnt area, though differences were not significant. Growth and yield of cowpea was not affected. The results indicated that biomass should not be burnt in windrows in order to avoid introducing fertility variation in otherwise uniform fields.

Reactivities of 34 coals under steam gasification

The reactivities of 34 coal chars of varying rank with H 2O have been determined to examine the effect of coal rank on the gasification rate of coal char. The reactivities of chars derived from caking coals and anthracites (carbon content > 78 wt%, daf) were very small compared with those from non-caking (lower-rank) coals. The reactivities of low-rank chars do not correlate with the carbon content of the parent coals. To clarify which factor is more important in determining the reactivity, the evolution of CO and CO 2 from char, the moisture content of char and the amount of exchangeable cations were determined for these low-rank coals or their chars. These values were considered to represent the amount of active carbon sties, the porosity and the catalysis by inherent mineral matters, respectively. It was concluded that the amount of surface active sites and/or the amount of exchangeable Ca and Na control the reactivity of low-rank chars in H 2O.

Stilbite from bomeki, Miyagi Prefecture, Japan.

A calcium-rich stilbite occurs in the amygdaloidal cavity of the Upper Miocene dacite dyke at Bomeki, Miyagi Town, Miyagi Prefecture. Chemical analysis gave SiO2 56.38, Al2O3 15.89, CaO 7.94, Na2O 1.08, K2O.10, H2O+16.23 and H2O- 1.93wt%. Fe, Mn, Mg and P were not detected. This corresponds to Na1.00K.06Ca4.07Al8.96Si26.98O72•28.98H2O. Cell-parameters calculated from x-ray powder data are a0=13.43, b0=18.21, c0=11.21A and β=128.9° closing to those of stilbite. The Bomeki stilbite is nearly colorless to faintly white in color and the cleavage is perfect on (010). Mohs hardness=3-4, sp. gr.=2.22 and refractive indices α=1.492, β=1.500, γ=1.502 (γ-α=0.010), all±0.002. The amount of exchangeable cations and cations-exchangeable capacity are only 37% of the theoretical value calculated from the chemical composition.

Acid precipitation and forest soils

The impact of acidifying substances on soils is examined and related to soil properties such as the amount of exchangeable cations in the soil and the proportion of those cations which are other than hydrogen ions. The importance and influence of differential soil horizons and the transport of water through the soil profile are also discussed. Differences between how agricultural and forest ecosystems affect nutrient cycling in soil systems are outlined.