Priming effects are strong short-term changes in the turnover of soil organic matter caused by comparatively moderate treatments of the soil. In the course of priming effects large amounts of C, N and other nutrients can be released or immobilized in soil in a very short time. These effects have been measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of such phenomena. The aim of this overview is to reveal possible causes and processes leading to priming actions using the references on agricultural ecosystems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative real and apparent priming effects induced after the addition of different organic and mineral substances to the soil. Soil microbial biomass plays the key role in the processes leading to the real priming effects. The most important mechanisms for the real priming effects are the acceleration or retardation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and different uncontrolled losses of mineralized N from the soil are responsible for the apparent N priming effects. Other multiple mechanisms (predation, competition for nutrients between roots and microorganisms, preferred uptake, inhibition, etc.) in response to addition of different substances are also discussed. These mechanisms can be distinguished from each other by the simultaneous monitoring of C and N release dynamics; its comparison with the course of microbial activity; and by the labelling of different pools with 14 C or 13 C and 15 N. Quantitative methods for describing priming effects and their dynamics using 14 C and 15 N isotopes, as well as for non-isotopic studies are proposed.

Review of mechanisms and quantification of priming effects.

http://files.aprendiendoquimica7.webnode.com.co/200000006-1247d13423/Limousin,%20G%20et%20al.%20Sorption%20Isotherms%20A%20review%20on%20Physical%20Bases,%20modeling%20and%20measurement.pdf

Sorption isotherms: A review on physical bases, modeling and measurement

Simulations of N2O and CO2 emissions from soils were conducted with a rain-event driven, process-oriented model (DNDC) of nitrogen and carbon cycling processes in soils. The magnitude and trends of simulated N2O (or N2O + N2) and CO2 emissions were consistent with the results obtained in field experiments. The successful simulation of these emissions from the range of soil types examined demonstrates that the DNDC will be a useful tool for the study of linkages among climate, soil-atmosphere interactions, land use, and trace gas fluxes.

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A model of nitrous oxide evolution from soil driven by rainfall events: 1. Model structure and sensitivity

https://pubag.nal.usda.gov/download/25475/PDF

Salinity–mineral nutrient relations in horticultural crops

Although the decomposition of plant material in soil is an extremely complex process, relatively simple models can give good fits to the decay process. Thus a two-compartment model gives a close representation, over the first few years, of the decay of 14 C-labelled plant material in soil. A model containing a single homogeneous humus compartment decomposing by a first-order process is surprisingly useful for soil organic nitrogen over periods measured in decades. More sophisticated multicompartmental models are now widely used to represent turnover in soil. One of these, the Rothamsted turnover model, is described in detail and shown to give a useful representation of data from the Rothamsted long-term field experiments.

The turnover of organic carbon and nitrogen in soil.

Potassium uptake by plants can be affected by high salinity and the Na concentration in the soil solution. There is abundant evidence that Na and the Na/Ca ratio affects K uptake and accumulation within plant cells and organs and that salt tolerance is correlated with selectivity for K uptake over Na. This provides the basis for hypothesis which exists in the literature and was examined in this study, that K application can reduce salinity damage to plants. The main objectives of this study were to: (i) study the effects of salinity and K fertilization interactions on corn yield and nutrient uptake; (ii) test the possibility that salinity damage can be reduced by elevating K fertilization rate; and (iii) study K dynamics in soil as a function of the salinity of the irrigation water, in soils with high and low indigenous potassium. The response of corn (Zea mays (L.) cv. ‘Jubilee’) to K fertilization under saline and non-saline conditions was studied by growing corn in two soil types in a pot experiment. Rates of K application to a 3 kg pot were: 0, 15 and 30 mmol K to the Gilat soil and 7.5, 15 and 30 mmol K to the Nordiya soil as KCl. The desired quantity of K was applied in one dose after seedling emergence. The salinity levels of the irrigation water were 4, 20 and 40 mmol charge 1−1. The irrigation was applied at least every second day and in excess to avoid water stress and to ensure drainage. Increased salinity in the irrigation water significantly decreased yield in both soils. Potassium significantly increased yield at all salinity levels only in the sandy soil which had a low natural level of K, but there was no difference in the relative yield decrease with salinity increase between the lowest and highest K application rates. Potassium fertilization did not eliminate the deleterious effects of salinity on corn yield despite its beneficial effect of increasing K content and reducing the Na∶K ratio in plant tissue. Potassium uptake by plants was the major factor in K dynamic processes. Potassium adsorption, release and fixation were secondary factors while leaching was an insignificant factor in overall K balance under cropping conditions.

Potassium-salinity interactions in irrigated corn

The efficiency and balance of nitrogen from one year's application was studied in a long-term fertigation experiment. Enriched nitrogen fertilizer, K15NO3, was applied to a 22-year-old Shamouti orange tree with a history of high N applications (N3) and to an N-starved tree (N1). The distribution of N in the different parts of the trees and in the soil was determined after the experimental trees were excavated. Similar total recovery of the labeled fertilizer N was found in the trees and soil in both treatments (N1−61.7% N3−56%). However, the distribution between tree and soil was different. The amount of recovered residual fertilizer in the soil was much larger in the N3 treatment than in N1. The highest percentage of fertilizer N was found in the new organs,i.e. fruits, twigs and leaves. The roots and branches took up only 6–14% from the labeled fertilizer. Only 20.9% of the leaf N and 23.4% of the fruit N in the N3 tree originated in the labeled fertilizer, indicating translocation of N from older parts of the tree to new growth. Evidence was found of storage of N in the wooded branches, while the roots contained a surprisingly small part of labeled fertilizer.

The fate of15N labeled nitrogen applied to mature citrus trees

The study of ion-exchange kinetics on soils and soil constituents has attracted much interest throughout the last decade. The introduction of a stirred-flow method to study these reactions, as well as other adsorption-desorption phenomena, has stimulated this interest. However, the manner in which rate data are interpreted using this technique and other flow methods has been questioned. Accordingly, a numerical solution for various instantaneous equilibrium and kinetic models is presented. It was found that rate data that could be described using an instantaneous equilibrium model could also be interpreted using a kinetic model. Therefore, the validity of kinetic rate coefficients obtained by flow methods is questionable. An analytical approach was developed to outline experimental methods that could be used to distinguish between instantaneous equilibrium and time-dependent reactions and to differentiate between solution-concentration-dependent kinetic models vs. those that are independent of solution concentration. By using this approach, and employing various flow rates and influent concentrations, as well as using a stopped-flow technique, it is easy to decide whether rates of reactions can be measured using the stirred-flow method for specific experimental conditions. H the study of ion-exchange kinetics on soils has received less attention than chemical-equilibrium studies. During the last decade, however, much interest has been generated concerning ionexchange kinetics. However, the problems of correctly interpreting the experimental results and making comparisons between different kinetic methods have not been solved (Sparks, 1989). For example, in comparing the magnitude of rate coefficients for K-Ca exchange on two clay minerals and a soil, Ogwada and Sparks (1986) found that the rate coefficients were greatly affected by the type of kinetic method. To enable measurements of rapid reactions and to minimize transport phenomena, a new method, reA. Bar-Tal and S. Feigenbaum, Inst. of Soils and Water, Agric. Res. Organization, The Volcani Center, Bet Dagan 50250, Israel; D.L. Sparks, Dep. of Plant and Soil Sciences, and J.D. Pesek, College of Agricultural Sciences, Univ. of Delaware, Newark, DE 19717-1303. Contribution no. 2514-E, 1989 series, from the Agric. Res. Organization, The Volcani Center, Bet Dagan, Israel. Published with the approval of the director of the Delaware Agric. Exp. Stn. as Miscellaneous Paper no. 1322. Contribution no. 254 of the Dep. of Plant and Soil Sciences, Univ. of Delaware. Received 16 Feb. 1989. *Corresponding author. Published in Soil Sci. Soc. Am. J. 54:1273-1278 (1990). ferred to as a stirred-flow technique, was developed by Carski and Sparks (1985). This method combines some of the favorable aspects of both batch and miscible-displacement techniques. However, the data analyses employed by Carski and Sparks (1985) were based solely on the calculated adsorbed quantities, which are not measured directly. Consequently, this could lead to experimental and mathematical errors, and data analyses should be based on effluent solution concentration (Schnabel and Fitting, 1989). Fundamentally, all chemical reactions are time dependent but, if the reaction is too rapid to be determined by the employed method or under the specific conditions used in the measurements, the process can be denned as an instantaneous one. The flow methods used in soil chemistry for determination of kinetic rate coefficients are based on measurements of solutionconcentration change vs. time (Sparks, 1986, 1989). Consequently, the adsorbed amount as a function of time is also a function of solution concentration. These techniques have been used for studying adsorption isotherms, assuming instantaneous equilibrium (Egozy, 1980; Isaacson and Hayes, 1984; Miller et al., 1989). In a previous study (Seyfried et al., 1989), we posed the possibility that the cation-exchange reaction on soils is very rapid and, if the stirred-flow method is used under experimental conditions such as those described by Carski and Sparks (1985), a kinetic process is not measured. Rather, what occurs is a process of solution dilution in the chamber combined with an instantaneous equilibrium exchange reaction. Therefore, there is a need to establish systematic tests to distinguish between instantaneous and timedependent reactions for a range of experimental conditions, and between different types of kinetic models. The objectives of this paper are to develop experimental techniques to distinguish between: (i) instantaneous and time-dependent reactions; and (ii) kinetic reactions that are dependent on solution concentration vs. those that are independent using the stirred-flow chamber. MATERIALS AND METHODS Theoretical Considerations

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Analyses of adsorption kinetics using a stirred-flow chamber: I. Theory and critical tests.

The variation of nitrification rates among soils and within soil profiles has received little attention compared with the well-established relations with soil moisture and temperature. In this study, the rate of nitrification was determined in profiles of various soil types and under different agricultural management. Verhulst's equation was used to express the accumulation of nitrate (NO3) with time and modified to describe the rate of ammonium (NH4) decrease. The maximal rate of nitrification (Kmx) and the delay period (t') were derived from the equation and used to characterize quantitatively the nitrification process in various soil samples. The Kmx and t' of surface soil samples were from 5 to 70 mg kg d and from 0.2 to 8 d, respectively. The Kmx decreased and t' increased with soil depth. Soil factors mostly affecting the rate parameters were: pH decreasing from 7.8 to 6.6 or HCO3 decreasing below 1 mol m, previous agricultural management, and soil depth. Data obtained from N studies indicated that mineralization rate of soil organic N in NH4 treated soils was not always negligible relative to nitrification. Additional Index Words: soil depth, lag period, priming effect, N. View complete article To view this complete article, insert Disc 4 then click button8

Nitrification Rates in Profiles of Differently Managed Soil Types1

The computer model NCSOIL computes the dynamics of C and N transformations in soil. In its original version, the microbial biomass assimilated organic N from the decay of organic pools, and inorganic N was immobilized only when decomposed organics were deficient in N with respect to biomass needs for growth (NCSOIL—direct version). A new version was presented in which it was assumed that microbial biomass immobilized N only from the inorganic pool, while the decay of organic N always resulted in N mineralization (NCSOIL-MIT version). NCSOIL-MIT was calibrated and tested for the incorporation of inorganic ¹⁵N in the organic soil N against experimental data and compared with the direct version. Net N mineralization was highly sensitive to the microbial efficiency of soil decomposable organic C incorporation. An efficiency of 0.2 gave a good account of net N mineralization for soils unamended with organic substances. NCSOIL-MIT simulated the incorporation of ¹⁵N, added as NH₄, into the organic soil pools. The kinetics of ¹⁵N immobilization was sensitive to microbial efficiency as well as to other parameters that control soil N turnover. NCSOIL—direct version required the decomposition of polysaccharide-like material in order to obtain ¹⁵N incorporation in the soil organic pools, and the kinetics of ¹⁵N immobilization depended on the rate of polysaccharide decomposition.

Sala Feigenbaum

Papers

Potassium-salinity interactions in irrigated corn

The fate of15N labeled nitrogen applied to mature citrus trees

Analyses of adsorption kinetics using a stirred-flow chamber: I. Theory and critical tests.

Nitrification Rates in Profiles of Differently Managed Soil Types1

Simulation of nitrogen-15 immobilization by the model NCSOIL