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A relationship that establishes a correlation between the isentropic efficiency and volumetric efficiency of a piston compressor and that takes into account the dead volume, engine pressure ratio, and the state of the working body at the start of the compression process and its adiabatic index is obtained. With a pressure ratio greater than 3–4, the isentropic efficiency will be 3–12% higher than the volumetric efficiency.

Here we show that Pythium arrhenomanes is a causal agent of root rot on maize. The pathogen was isolated from diseased root tissue collected from a conventional maize crop in Michoacan, during 2015. For taxonomic identification, the obtained isolate was used for morphological characteristics and molecular analysis (ITS and COXII genes). Pathogenicity tests confirmed that P. arrhenomanes causes root rot on maize plants.

Equations are proposed for the mechanical efficiency and losses in a piston compressor operating with various working bodies. When such a compressor operates with ammonia and freons, the mechanical efficiency is higher than on working with air.

Phytomass structural characteristics are highly related to vegetation flammability. In fire-prone species like Mediterranean gorse, which accumulate standing dead fuel, susceptibility to fire is a function of fuel load, vegetation composition and fuel cover, and these characteristics change with time. Thus, for effective fuel control management, knowledge of the vegetation structural dynamics related to fire risk is crucial for preventing future fires. This study analyses structural dynamics in the above-ground phytomass of Ulex parviflorus shrublands in relation to different stages of flammability, i.e., the amount of time elapsed since the last fire. For this, 152 plants were cut from shrublands at different stages of development (young, mature and senescent), and various dimensional measurements were taken on each. The phytomass was separated into living or dead fuel fractions as well as into twigs or branches depending on the stem diameter. Basal diameter is the variable that best predicted Ulex parviflorus total phytomass as well as that of the different fractions. Both dimensional and phytomass variables increased with plant development. In the young shrublands Ulex parviflorus constitutes 54% of total phytomass, and Ulex parviflorus's dead twigs fraction accounts for 5% of total phytomass. In the mature and senescent shrublands, this species represents 80% of total shrubland phytomass, and dead twigs reach values greater than 40%. Our results show that structural changes in the fuel over short periods of time (young and mature) reveal critical periods in shrub development. Identification of these stages is a necessary tool for planning fuel control programmes.

Fires may greatly alter the N budget of a plant community. During fire nitrogen is lost to the atmosphere. Although high light availability after fire promotes N2-fixation, the presumably high soil N availability could limit N2-fixation activity. The latter limitation might be particularly acute in legume seedlings compared with resprouts, which have immediate access to belowground stored carbon. We wished to learn whether early post-fire conditions were conducive to N2-fixation in leguminous seedlings and resprouts in two types of grassland and in a shrubland and whether seedlings and resprouts incurred different amounts of N2-fixation after fire. We set 18 experimental fires in early autumn on 6 plots, subsequently labelling 6 subplots (2 × 2 m2) in each community with 15NH4 +-N (99 atom % excess). For 9 post-fire months we measured net N mineralisation in the top 5 cm of soil and we calculated the fraction of legume N derived from the atmosphere (%Ndfa) in seedlings and resprouts. We used two independent estimates of the amounts of N derived from non-atmospheric sources in potentially N2-fixing plants: mean soil pool abundance and the 15N-enrichment of non-legumes. Despite substantial soil net N mineralisation in all burned community types (about 2.6 g Nm−2 during the first nine months after fire), the %Ndfa of various legume species was 52–99%. Legumes from both grasslands showed slightly higher N2-fixation values than shrubland legumes. As grassland legumes grew in more belowground dense communities than shrubland legumes, we suggest that higher competition for soil resources in well established grass-resprouting communities may enhance the rate of N2-fixation after fire. In contrast to our hypothesis, legume seedlings and resprouts from the three plant communities studied, had similar %Ndfa and apparently acquired most of their N from the atmosphere rather than from the soil.

Dormancy in the hard seed coats of Mediterranean species is considered a strategy that enables persistent seed banks to be formed in the soil. An important factor related to seed coat fracture and dormancy breakage in Mediterranean ecosystems is heat. Nevertheless, the effect of factors other than heat on dormancy breakage in these species has hardly been studied. To investigate the different ecological factors involved in germination, in the laboratory we applied several scarification treatments to seeds with chromatic polymorphism. We evaluated the effect of soil seed depth during experimental burns by sowing seeds at −1, −3 and −5 cm in the soil profile, and we also studied the effect of seed origin on the posterior germination of seeds from 4 and 10 year-old shrubs as well as from the soil seed bank. U. parviflorus shows clear chromatic polymorphism: its brown seeds present higher dormancy levels than its yellow seeds. The different techniques of dormancy breakage result in different degrees of germination; the highest degree of germination is generated by the mechanical treatment, followed by the acid and the heat treatments, in that order. The depth of the seeds in the soil determines the temperature thresholds and the residence times of these temperatures and whether they stimulate a massive germination at the −1 cm soil profile or only a slight germination at the −5 cm depth. Seeds recently produced by the plant show higher dormancy levels than seeds extracted from soil seed banks. Dormancy levels also depend on the shrubland age used for extracting the soil samples (3>9 years old). In effect, from the point of view of dormancy, the germination behaviour of U. parviflorus seeds seems to follow a multiresponse strategy based on different seed populations and involving both biological and abiotic processes to break dormancy.

The consequences of fire recurrence (1, 2 and 3 fires in 16 years) on plant and soil C, N, P and K from Quercus coccifera garrigues were analysed in the Valencia Region (E Spain). Plant and forest floor (L horizon) nutrient concentrations either changed weakly or showed no change with fire recurrence. At the soil surface (0-2.5 cm), soil potential mineralisable nitrogen increased and available P decreased after an initial increase, whereas exchangeable K was not affected by successive fires. However, the significance of those observed trends for N and P was site-dependent. Despite the rapid formation of the L soil horizon, fire recurrence did not permit the development of the whole forest floor profile observed in the unburned garrigues. These organic layers contained a great proportion of the total nutrient pool, especially for N. Forest-floor and aboveground plant combustion by fire may produce significant losses of N and P compared with those available in mineral soil, whereas soil exchangeable K is large enough to replace these losses. Belowground nutrient reserves may account for the quick recovery of the Quercus coccifera aboveground biomass although successive fires could deplete these reserves and produce a loss of biomass and productivity in this species.

This paper describes the application of a mechanistic model in the study of radionuclide soil–plant transfer and the obtainment of predictive estimates of radionuclide plant contamination. Soil–plant K and 134Cs transfer rates were measured and compared with those predicted by the Barber–Cushman model. The experiment was performed on pea plants grown in pots and in two different types of soil (Calcic Luvisol and Fluvisol). For K, model predictions proved valid for all development stages sampled; for 134Cs, the quality of the prediction depended on the plant stage. In both, parameter estimates varied depending on plant age and soil type. The model was also run for 134Cs using the Michaelis–Menten parameters obtained for K. In this case, the predicted values were significantly correlated with those measured, but about three times higher. Thus, a positive plant discrimination of K versus 134Cs in plant absorption is observed for the types of soil studied. As regression proved to be significant, K absorption rates may be used to estimate 134Cs absorption in determining radiocaesium plant uptake.

We studied the seasonality of total soil CO2efflux and labeled C-CO2 released from 14Clabeled straw incubated in the H horizon of asemi-arid Mediterranean forest soil. Fieldmeasurements were carried out over 520 days in aseries of reconstructed soil profiles with and withouta gravel layer below the H horizon. We monitored soilclimate and related this to soil CO2 efflux.Seasonal variations in soil CO2 efflux in asemiarid Mediterranean forest were mainly related tochanges in soil temperature. In spite of drought, highrespiration rates were observed in mid summer. Highsoil CO2 efflux in hot and dry episodes wasattributed to increases in soil biological activity.The minimum soil CO2 efflux occurred in latesummer also under dry conditions, probably related toa decrease in soil biological activity in deephorizons. Biological activity in organic layers waslimited by water potential (Ψ) in summer and bytemperature in winter. Rewetting a dry soil resultedin large increases in soil CO2 efflux only at hightemperatures. These large increases represented asignificant contribution to the decomposition oforganic matter in the uppermost horizons. Soilbiological activity in the uppermost horizons was moresensitive to changes in soil Ψ and hence tosummer rainstorms than the bulk soil microbialactivity. The presence of a layer of gravel improvedboth moisture and temperature conditions for thedecomposition of organic matter. As a result, soilCO2 efflux increased in soils containing rockfragments. These effects were especially large for theorganic layers.

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna. In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO2, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated 14C straw in the H horizon and carriedout three 40 mm rainfall simulations. Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.

Soil and wheat bromide dynamics are studied in methyl bromide-fumigated plots in a Mediterranean climate.