Effect of Azospirillum brasilense inoculation on root morphology and respiration in tomato seedlings

Summary The level of Azospirillum brasilense strain Cd colonization in the rhizosphere of some vegetables was 10⁴–10⁵ colony-forming units (CFU) per root of one plant in 2-week-old plants inoculated with 5 × 10⁸ Azospirillum cells. Significant increases in root length (35%) and in top (90%) and root (50%) dry weight and total leaf area (90%) were observed in 18-day-old inoculated tomato plants compared with non-inoculated controls. An inoculum concentration of 1 × 10⁸ to 5 × 10⁸ CFU/ml stimulated the appearance of root hairs. Large numbers of bacteria (1 × 10⁹ CFU/ml) caused asymmetrical growth of the root tip. In a petri dish system, Azospirillum (1 × 10⁸ CFU/ml) increased root dry weight (150%), protein content (20%), respiration rate per root (70%) and the specific activity of malate dehydrogenase (45%–65%) over non-inoculated controls. The specific respiration rate, expressed as micromol of O₂ per minute per milligram of dry weight of roots, was significantly lower in inoculated roots, suggesting that less energy was spent for accumulation of more dry material.     

Ten years of precipitation chemistry in Haifa,Israel

Rain event samples have been collected in Haifa, Israel, for nine hydrological years 1981 to 1990. Precipitation amount, pH, SO₄ ⁼, NO₃ ^?, Cl^?, NH₄ ⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺ and alkalinity of rainwater samples were recorded. The sampling and analysis program was based on WMO recommendations for background networks. The sampling was performed manually, and the analysis was based on wet chemistry for ions and atomic absorptions for metals. Data of 187 rain samples showed that the average pH was 5.3±1.1∶ 26% of the rain events were below pH of 5.6 and 23% above pH of 7.0. Some simple chemical mass-balance considerations indicate that natural sources, sea salt and soil carbonates are the main contributors to rain chemistry. However, the presence of low pH events observed over the years suggests that the impact of anthropogenic emissions may overwhelm the buffering capacity of the alkaline aerosol.     

Aerosol composition of urban and desert origin in the eastern Mediterranean

Monthly dustfall samples collected in 16 sites in the Tel-Aviv urban areas, as well as those collected in 10 rural sites in Israel during several years of measurements, were analyzed for their physical properties and chemical content. Although dustfall measurements represent a very simple and crude technique the analysis of the data provides some very interesting conclusions: (a) The southwest part of Israel, which is scarcely populated, suffers from high dustfall values due to dust storm blowing from the Sahara and other arid lands to the east Mediterranean during the spring. (b) On an annual basis the natural dustfall of dust storms origin amounts to 25 to 30% of the Tel-Aviv residential area deposition or 3 to 4 tonne km^?2 mo^?1. (c) Mapping of dustfall data within an urban region reflects fairly well the main industrial and commercial activities within the city. (d) In residential areas the inorganic water insoluble fraction of the dust is contributed by natural sources, while the organic fraction is mostly of anthropogenic origin. The inorganic water soluble fraction is spread in a uniform pattern within the city.     

The effect of various residue mulch-tillage combinations on soil physical conditions and performance of irrigated cotton

Cotton is a major irrigated summer field crop in Israel, and is commonly grown in the same field for 3–5 years in succession. There is only a narrow time window between harvest and early rains for pest control by means of clearing the land surface of residues, and to perform preparatory tillage. The time available may be insufficient to achieve this with the conventional deep plowing tillage system, and some operations may have to be carried out on moist soil between rainfall events. In response to indications of decreasing yield due to compaction, various limited-tillage systems in permanent traffic lanes have been developed, culminating in a machine that performs all residue-disposal and tillage operations in a single pass through the field. A comparison of several limited-traffic and conventional practices was carried out for 2 years on a loessial silt loam (Calcic Haploxeralf). It was found that both soil condition and yield were worst in the two treatments commonly used by farmers: deep plowing and deep incorporation of residues with the combination machine. Tillage effects were dominant, masking any effect of residue amount and disposal method. Large differences were found between the zones of the permanent wheel track treatments, as were cyclic changes in soil condition reflecting the seasonal sequence of tillage operations. Some cumulative compaction occurred, due mainly to a gradual widening of the wheel tracks rather than to repeated passes in the original rut. The findings of this work show that the optimal choice is to replace the previously preferred field practices by shallow slot-mulching with simultaneous subsoiling by the combination machine, which meets the sanitation requirements, maintains satisfactory yields and saves energy and labor.     

Aerosol composition of urban and desert origin in the Eastern Mediterranean. I

Aerosol samples were collected in Tel-Aviv, Israel, during two different atmospheric situations, clear days and dust storms, and were analyzed for shape, size distribution, elemental composition, and presence of sulfate in individual particles. Results of two cases are presented in this paper. In both cases about 80% of the 0.2 to 2.0 µm particles contained sulfate. On the clear day over 50% of the total were pure sulfates while 27% were mixed sulfates; on the dusty day 63% were mixed sulfates — desert particles coated with sulfate — and only 20% were pure sulfates. The sulfate content of the desert particles was fairly high, on the order of 0.1 g sulfate g^?1 of desert particles. A possible explanation of this phenomenon may be heterogeneous nucleation of SO₂ on the surfaces of insoluble desert particles during their passage over the Mediterranean.     

Rates of decomposition of plant residues and available nitrogen in soil, related to residue composition through simulation of carbon and nitrogen turnover

The dynamics of inorganic N in soil following the application of plant residues depends on their composition. We assumed that all plant materials are composed of similar components, each decomposing at a specific rate, but differ in the proportions of the various components. The NCSOIL model that simulates C and N turnover in soil was used to link the rates of residue decomposition to their composition, defined as soluble, cellulose-like and lignin-like C and N, and thereby integrate short and long-term effects of residues on available N dynamics in soil. Five plant residues in a wide range of C:N ratios were incubated in soil for 24 weeks at 30 °C, during which C and N mineralization were measured. The materials with large C:N ratios (corn, rice hulls and wheat straw) were also incubated with NH₄⁺-N to avoid N deficiency. The residues were analyzed for total and soluble C and N. The partitioning of insoluble C and N between cellulose- and lignin-like pools was optimized by best fit of simulated C and N mineralization to measured results. The decomposition rate constants of the soluble and lignin-like pools were assumed to be 1.0 and 10⁻⁵ d⁻¹, respectively, and that of the cellulose-like pool, obtained by model optimization against mineralization of cellulose with NH₄⁺-N in soil, was 0.051 d⁻¹. The optimized, kinetically defined lignin-like pool of all residues was considerably larger than lignin contents normally found in plant residues by the Van Soest procedure. Gross N mineralization of tobacco and rape residues was similar, but N recovery from tobacco was larger, because a larger fraction of its C was in the lignin-like pool. N in rice hulls, corn and wheat residues was mostly recalcitrant, yet rice hulls did not cause N deficiency, because most of its C was recalcitrant too. The soluble components of the residues had strong short-term effects on available N in soil, but the cellulose-like pool was equally important for short and medium-term effects. Soluble and cellulose-like C were 29 and 42% of total C, respectively, in corn and 7 and 50% in wheat. Maximal net inorganic N losses, measured in both residue treatments after 2 weeks, were 42 mg g⁻¹ C applied as corn and 31 mg g⁻¹ C applied as wheat, or 84 and 110 mg g⁻¹ decomposed C of corn and wheat, respectively. Rice hulls immobilized N slowly, but by the end of 24 weeks all three residues immobilized 26-27 mg N kg⁻¹ C applied. The different dynamics of N immobilization demonstrated the need to determine the decomposability of C and N rather than their total contents in plant residues.     

Advances in modeling machine-soil-plant interactions

Modern agriculture involves mechanized operations which affect crop growth and yields through changes in the soil environment. Field research to study the phenomena involved in the soil-machine-crop system is usually site-specific with respect to edaphic, climatic and management factors. Field experiments are often expensive to conduct, and generalization to other sites or practices is usually tenuous at best. Computer-based techniques to simulate machine-soil-plant (or crop) systems can test many combinations of site characteristics and management practices, and screen the most promising combinations. Once these promising combinations are validated, the simulation techniques, together with stochastically-supported data bases can be used to predict tillage management systems suitable for specific sites and crops.Current efforts to model plant response to soil conditions created by tillage and machines have serious shortcomings, because the link between tool action, subsequent soil structure, and the parameters that describe fluxes of water, heat and gas need further development for deterministic modeling. Machine-soil models focus on efficiency of the tillage operation, while soil-plant models focus on soil structure and related parameters influencing plant growth. While current models for simulating the machine-soil-crop system have limitations, an approach to bridge between the two model applications offers great promise and should be a high research priority. When used with improved measurements of tool action and of soil structural properties before and after tillage and traffic, including position or placement of the crop residues, current models can be doubly effective for improving tillage management.