Nitrogen fertilization and cropping system impacts on soil properties and their relationship to crop yield in the central Corn Belt, USA

Evaluating the effects of management practices on soil physical and chemical properties would be valuable to explain field-level variability in crop production. A 23-year-old experiment on a Muscatune soil (fine-silty, mixed, superactive, mesic, Aquic Argiudolls) in Illinois with five N rates [0 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) kg N ha⁻¹] and two cropping systems [continuous corn (Zea mays L.) (CC), and corn–soybean (Glycine max (L.) Merr.) rotation (CS)] was evaluated. Specific objectives were to: (i) evaluate the effects of long-term N fertilization and cropping systems on field level changes in soil physical and chemical properties and crop yield, (ii) identify the most responsive soil physical and chemical properties to N fertilizer and crop management, and (iii) investigate the relationship between the selected soil properties and crop yield. Soil was collected in May 2004 to 30 cm depth and 20 soil physical and chemical properties were measured. The univariate analysis indicated that 14 soil properties were significantly influenced by at least one treatment effect (crops, N or crops × N). Due to multicollinearity among soil properties, principal component analysis (PCA) was used to group correlated properties, resulting in five soil properties such as soil organic carbon stock (OC stock), mean weight diameter (MWD), soil C:N ratio, exchangeable potassium (K⁺) and gravimetric moisture content (ω). Finally, the multiple regression analysis performed between PCA derived soil properties and corn and soybean yields retained all the representative soil properties from PCA except ω as yield predictors for corn (P < 0.001, R² = 0.39) from CC system, whereas none of the soil properties were significantly related to corn and soybean yields from CS system. The soil properties most influenced by long-term N fertilization of continuous corn were successfully identified with PCA and multiple regression. The insignificant relationship between corn and soybean yields from CS system and PCA derived soil properties might be due to the lack of response of soybean to N fertilization. This study shows the integrated use of multivariate and regression analyses in identifying yield determining soil properties by eliminating the multicollinearity among soil properties.     

Adsorption and Desorption of Arsenate in Different Soils and Gold Mining Substrates of Minas Gerais State,Brazil

Background  Arsenic (As) availability in natural environment is related to the element’s adsorption and desorption processes in soils. Total As is better related to available As in temperate soils than in tropical soils. In tropical soils, total As is not very significant in terms of availability, therefore justifying the necessity for studies into As dynamics. Knowledge of As dynamics in soil as well as development of new analytical methodologies involving tropical soils are insufficient and necessary for future mitigation projects. Objective  The objectives of this study were: (1) To adjust methodologies which may assist in understanding arsenate dynamics in tropical soils and substrates; (2) To evaluate the adsorption and desorption of arsenate in soils and substrate samples, and to find a minimum value of arsenate available in soil which is lethal to sorghum plants. Material and Methods  Samples of three soils from Minas Gerais State (YL, RYL, and CS) and two sulfide substrates of gold mining (B1 and B2) were used in the assays. All the material was physically and chemically characterized. Remaining As (As-rem) and remaining P (P-rem) of each material, along with MAC_P and MAC_As (using the Langmuir isotherms), were obtained. After agitation to obtain MAC_P and MAC_As, arsenate was extracted by anionic resin and Mehlich-III to evaluate arsenate desorption of the material retained on the filter paper. Subsequently, arsenate desorption curves for the different materials were obtained, and arsenate availability was determined through a bioassay with sorghum plants. Samples of soils and substrate B1 were incubated with six levels of As doses. Plants were grown under greenhouse conditions for 30 days. The plants were then harvested, dried and weighed. Available As in the soils and substrate was determined by Mehlich-III. Results and Discussions  As-rem level decreased from YL (sandy) to RYL (clayey) soil samples, which always showed lower values than P-rem. Among the soils and substrates evaluated, RYL showed the highest MAC_As and MAC_P, followed by CS, YL and Bl. The results were in accordance with the values observed for As-rem and P-rem and confirm the idea that the ability of the assayed materials to remove As from the soil/substrate solution is higher than the ability to remove P. On the other hand, the binding energy (a) between soil/substrate and As is weaker than the binding energy of P. Given the fact that the studied soils present a real ability to remove As from the solution, only a small part of As would be unavailable considering MAC_As as a reference. As-Mehlich-III values were higher than As-resin for substrate Bl. Mehlich-III seemed to be more appropriate to extract labile forms of arsenate in substrate B1 as well as in the soils. Available As by Mehlich-III (26.9 mg/dm³) was considered a reference of As LCL to sorghum plants. CC50 was sensitive to the buffering capacity of each soil, showing values varying from 1.34 mg/dm³ As (clay soil with lower As-rem) to 12.31 mg/dm³ As (sandy soil with higher As-rem). Conclusions  The adaptation of the As-rem and MAC_As methodologies was satisfactory and of great value in the study of adsorption, desorption and As availability for soils and mining substrate. Mehlich-III was also satisfactory to estimate available As and was sensitive to soil buffering capacity. Nevertheless, resin can also be used as an alternative. MAC_As varied among soils and was higher than MAC_p. However, As showed higher lability than P. Using Mehlich-III, we determined the value corresponding to CC50 that showed a good reference of toxicity to available As. Outlook  The environmental implications of the As behavior are quite serious. Beside the fact that arsenate is removed very fast from the soil solution, an anthropogenic input of the element, being part of the soil quantity factor, may remain in a reversible form for a long time. As may therefore return to the soil solution and becomes available to plants, animals and the entire environment. Considering that CC50 is the maximum contents of available As the environment can tolerate to allow some vegetal biomass production, the maximum capacity of As immobilization in each soil is reduced when compared to the soils’ MAC_As values. Therefore, the maximum and safe values of reference to be used in the evaluation of incidental discharge of the element in soils must be reduced.     

Partially Molten Middle Crust Beneath Southern Tibet: Synthesis of Project INDEPTH Results

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.     

A conserved p38 MAP kinase pathway in Caenorhabditis elegans innate immunity

A genetic screen for Caenorhabditis elegans mutants with enhanced susceptibility to killing by Pseudomonas aeruginosa led to the identification of two genes required for pathogen resistance: sek-1, which encodes a mitogen-activated protein (MAP) kinase kinase, and nsy-1, which encodes a MAP kinase kinase kinase. RNA interference assays and biochemical analysis established that a p38 ortholog, pmk-1, functions as the downstream MAP kinase required for pathogen defense. These data suggest that this MAP kinase signaling cassette represents an ancient feature of innate immune responses in evolutionarily diverse species.     

Motion perception: a color-contingent aftereffect

When observers who watched repeated alternations of a red contracting spiral and green expanding spiral were later shown stationary spirals, red and a green the red stationary spiral appeared to be expanding and the green stationary spiral appeared to be contracting. These color-contingent motion after effects complement reports of motion-contingent color aftereffects and suggest that both may reflect adaptation of detectors specific to color and motion.