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1.
Night radiative frost is a highly limiting factor for agriculture in Andean highlands. Nevertheless, a diversity of crop species have been domesticated there, commonly showing high heterogeneity in plant growth at the field level. The possible protective effect of crop canopy heterogeneity against nocturnal radiative frost is examined using a dual approach, combining a field experiment and a simplified energy balance model at the leaf level. Leaf and air temperatures were registered over an entire quinoa crop cycle in the Andean highlands of Bolivia, comparing two cultivars: Blanca de Yanamuyu, a traditional landrace with high plant height heterogeneity, and Surumi, a more homogeneous selected variety. In both cultivars, inverted air density profiles during calm and clear nights result in air temperature changes up to 3 °C over 0.5 m height, with minimum air temperature concentrated at the upper part of the canopy. In these conditions, leaf temperature gradients of up to 2.6 °C m−1 develop within the canopy of the traditional landrace, with minimum leaf temperature significantly higher (P < 0.001) in shaded plants of the landrace than in the selected cultivar. A dynamic model of leaf temperature based on canopy parameters and climatic records at screen level adequately simulates leaf temperature differences in the case of a vertically heterogeneous quinoa canopy. A sensitivity analysis of the model reveals that canopy height, leaf area index, and sky cloudiness have the most important influences on the development of the sheltering effect, while air temperature and air humidity play a minor role under typical radiative frost conditions. As for wind speed, its actual influence remains unclear due to experimental and modelling limitations at low wind speeds. The significance of these results is discussed in terms of the trade-off between stress adaptation and biomass productivity.  相似文献   

2.
Effect of Vent Arrangement on Windward Ventilation of a Tunnel Greenhouse   总被引:8,自引:4,他引:8  
The effect of ventilation configuration of a tunnel greenhouse with crop on airflow and temperature patterns was numerically investigated using a commercial computational fluid dynamics (CFD) code. The numerical model was firstly validated against experimental data collected in a tunnel greenhouse identical with the one used in simulations. The airflow patterns were measured and collected using a three-dimensional sonic anemometer and the greenhouse ventilation rate was deduced using a tracer gas technique. A good qualitative and quantitative agreement was found between the numerical results and the experimental measurements. After its validation, the CFD model was used to study the consequences of four different ventilator configurations on the natural ventilation system. The ventilation configuration affects the ventilation rate of the greenhouse and the airflow and air temperature distributions as well. For the different configurations, computed ventilation rates varied from 10 to 58 air changes per hour for an outside wind speed of 3 m s−1 and for a wind direction perpendicular to the openings. Likewise, the simulations highlight that while the mean air temperature at the middle of the tunnels varied from 28·2 to 29·8°C, for an outside air temperature of 28°C, there are regions inside tunnels 6°C warmer than outside air. Average air velocity in the crop cover varied according to the arrangement of the vents from 0·2 to 0·7 m s−1. The consequences of the marked climate heterogeneity on plant activity through the variation of crop aerodynamic resistance as well as the influence of the vent configurations on the efficiencies of ventilation on flow rate and air temperature differences between inside and outside, are also discussed.  相似文献   

3.
A biogas production assessment method based on the visual monitoring of biogas evolution events in an anaerobic waste stabilisation pond was developed and applied to an anaerobic pond treating farm dairy wastewater in New Zealand. Major biogas-induced perturbations at the pond surface were classified as either type 1 or 2 events and other observed biogas activities as small bubble events. Mean counts of types 1 and 2 events varied from 7·3 to 30·0 per hour and 4·3–34·0 per hour, respectively, over the pond surface and the frequency of events decreased as both organic loading and temperature increased. Preliminary estimates of areal gas production rates, obtained using the observational method, ranged from 0·002 to 0·015 m3 m−2 day−1 for major eruptions and 0·0004–0·024 m3 m−2 day−1 for small bubble events, giving a total range of 0·002–0·039 m3 m−2 day−1. Pond temperatures at 2·75 m depth showed relatively minor fluctuations on a diurnal basis and ranged between 13 and 15°C from days 1–60, reaching a maximum of 24°C at day 190. Refinements proposed for future method development include an increased number and range of event categories, the automatic recording of events and the use of an improved cover. Further work is required to assess the general applicability of the method to anaerobic ponds.  相似文献   

4.
Extraction of protein from the leaves of green plants is very important because of the high cost of conventional forms of protein such as meat, milk and fish. In order to design machinery for this extraction, and also to embark on leaf protein concentrate extraction, it is necessary to measure and analyse the energy requirements to carry out each process involved in the extraction, using different plant species.Experiments were carried out to determine the amount of crude protein, and the thermal energy required to extract leaf protein concentrate, from juices obtained from the leaves of some plant species. Leaves from the following plants were selected: cassava (Manihot esculanta), Siam weed (Chromolaena odorata), bitter leaf (Vernonia amygdalina), gliricidia (Gliricidia maculata) and thorny tree (Hura crepetans). The leaves from the plant species were macerated in a laboratory pulper. Juice was obtained from the samples using perforated cylinders and a hydraulic press. The specific heat capacity of the juices was determined using the cooling curve method. The values of the heat capacities were used to calculate the amount of thermal energy required to raise the temperature of each juice from its normal temperature of about 25°C to a total protein coagulation temperature of about 80°C. The crude protein content of the extract was determined using the Kjeldal method.Results indicate that the green coagulum extracted from all the juices all have a protein content of at least 37%. The thermal energy required to coagulate protein from the juices ranges from 1·59 kJ kg−1 for Hura crepetans to 2·7 kJ kg−1 for Vernonia amygdalina. The energy requirement to obtain crude protein (CP) ranges from 8 kJ kg−1 [CP] with Hura crepetans to 182 kJ kg−1 [CP] with Vernonia amygdalina. Both results are statistically significant at the 0·01 confidence interval. It is concluded that the choice of plant species can significantly lower the thermal energy requirement for the extraction of leaf protein concentrate.  相似文献   

5.
This paper summarizes results from 8 years (1996–2003) of eddy covariance-based ecosystem CO2 exchange measurements at the Borden Forest Research Station (44°19′N, 79°56′W). The site represents a mid-latitude, 100-year-old, mixed deciduous and coniferous forest dominated by red maple, aspen and white pine. The years 1996 and 1997 were relatively cold, had a late spring and received below average photosynthetic photon flux density (PPFD). This contrasts with an early spring, warmer soil and air temperatures during 1998–1999, and with distinctly wet year of 2000 and dry years of 2001–2003. The combination of early spring, warmer air and soil temperature and relatively high level of PPFD was associated with higher net ecosystem productivity (NEP) that peaked during 1999. Photosynthetic capacity was reduced and NEP showed a mid-growing season depression during the dry years of 2001–2003. Annual average ecosystem respiration (R) determined from a light response model was 30% less than R derived from a logistic respiration equation, relating night time CO2 flux and soil temperature. However these independently determined R values were well correlated indicating that the site is unaffected by fetch and spatial heterogeneity problems. Based on the combined 8 years of growing season daytime data, an air temperature of 20–25 °C and a vapor pressure deficit (VPD) of 1.3 kPa were found to be the optimal conditions for CO2 uptake by the canopy. Over the 1996–2003 period, the forest sequestered carbon at an average rate of 140 ± 111 gC m−2 y−1. The corresponding gross ecosystem photosynthesis (GEP) and R over this period were 1116 ± 93 gC m−2 y−1 and 976 ± 68 gC m−2 y−1, respectively. The annual carbon sequestration ranged from 19 gC m−2 in 1996 to 281 gC m−2 in 1999. However, these estimates were sensitive to frictional velocity threshold () used for screening data associated with poor turbulent mixing at night. Increasing from 0.2 m s−1 (based on the inflection point in the nighttime CO2 flux vs. u* relationship) to 0.35 m s−1 (determined using a selection algorithm based on change-point detection) modified the 8-year mean NEP estimate from 140 ± 111 gC m−2 y−1 to 65 ± 120 gC m−2 y−1. Both approaches show that the Borden forest was a low to moderate sink of carbon over the 8-year period.  相似文献   

6.
Soil modification via biopedturbation by burrow-building seabirds was examined in a Mediterranean, island ecosystem. Physical and chemical soil properties were compared between a colony of Wedge-tailed Shearwaters (Puffinus pacificus) and adjacent heath across a 14-month period. When compared to heath soil, the biopedturbated soil was 28% drier (6.04±5.40 vol%), had increased bulk density (by 29% to 1.30±0.11 g cm−3, 51% porosity), wetting capacity (by 83% to 0.55±0.83 molarity of ethanol droplet), hydraulic conductivity (by 266% to 398.91±252.04 mm h−1), and a greater range in soil surface temperature (31.7±6.2 °C diurnally to 18.3±3.2 °C nocturnally). Soil penetration resistance was reduced by 26% at a depth of 0–100 mm (326.5±122.4 kPa) and by 55% at 500–600 mm (1116.8±465.0 kPa). Colony soil also had increased levels of nitrate (by 470%), phosphorous (118%), ammonium (102%), sulphur (69%), and potassium (34%), decreased levels of iron (by 50%) and organic carbon (61%), was more alkaline, and had a 78% greater conductivity. Shearwaters deposited guano at a rate of 234.4 kg ha−1 yr−1 (dry mass). Chemical analysis of guano equated this to 50.9, 5.7, 5.5, and 3.6 kg ha−1 yr−1 of nitrogen, potassium, sulphur, and phosphorous, respectively. Experimentally constructed burrows demonstrated that digging alone can alter physical and chemical soil factors, but that changes in the nutrient profile of colony soils are predominantly guano-driven. We argue that the physical impact of seabirds on soil should not be overlooked as a soil-forming and ecosystem-shaping factor in island ecosystems, and that biopedturbation can exert major bottom-up influences on insular plant and animal communities.  相似文献   

7.
Several newly developed capacitance sensors have simplified real-time determination of soil water content.Previous work has shown that salinity and temperature can affect these sensors,but relatively little has been done to correct these effects.The objectives of this study were to evaluate the effect of media temperature and salinity on the apparent water content measured with a single capacitance sensor (SCS),and to mitigate this effect using a temperature dependent scaled voltage technique under laboratory conditions.A column study was conducted containing two media:pure deionized water and quartz sand under varying water contents (0.05 to 0.30 cm3 cm-3) and salinity (0 to 80 mmol L-1).Media temperature was varied between 5 and 45℃ using an incubator.The SCS probes and thermocouples were placed in the middle of the columns and were logged at an interval of 1 minute.There was strong negative correlation between sensor reading and temperature of deionized water with a rate of -0.779 mV ℃-1.Rates of SCS apparent output were 0.454 and 0.535 mV ℃-1 for air in heating and cooling cycles,respectively.A similar positive correlation with temperature was observed in sand at different water contents.The SCS probe was less sensitive to temperature as salinity and water content increased.Using a temperature-corrected voltage calibration model,the effect of temperature was reduced by 98%.An analytical model for salinity correction was able to minimize the error as low as ± 2% over the salinity level tested.  相似文献   

8.
RZ-SHAW is a hybrid model, comprised of modules from the Simultaneous Heat and Water (SHAW) model integrated into the Root Zone Water Quality Model (RZWQM) that allows more detailed simulation of different residue types and architectures that affect heat and water transfer at the soil surface. RZ-SHAW allows different methods of surface energy flux evaluation to be used: (1) the SHAW module, where evapotranspiration (ET) and soil heat flux are computed in concert with a detailed surface energy balance; (2) the Shuttleworth–Wallace (S–W) module for ET in which soil surface temperature is assumed equal air temperature; and (3) the PENFLUX module, which uses a Penman transformation for a soil slab under incomplete residue cover. The objective of this study was to compare the predictive accuracy of the three RZ-SHAW modules to simulate effects of residue architecture on net radiation, soil temperature, and water dynamics near the soil surface. The model was tested in Akron, Colorado in a wheat residue-covered (both standing and flat) no-till (NT) plot, and a reduced till (RT) plot where wheat residue was incorporated into the soil. Temperature difference between the soil surface and ambient air frequently exceeded 17 °C under RT and NT conditions, invalidating the isothermal assumption employed in the S–W module. The S–W module overestimated net radiation (Rn) by an average of 69 Wm−2 and underestimated the 3-cm soil temperature (Ts3) by 2.7 °C for the RT plot, attributed to consequences of the isothermal assumption. Both SHAW and PENFLUX modules overestimated midday Ts3 for RT conditions but underestimated Ts3 for NT conditions. Better performances of the SHAW and PENFLUX surface energy evaluations are to be expected as both approaches are more detailed and consider a more discretized domain than the S–W module. PENFLUX simulated net radiation slightly better than the SHAW module for both plots, while Ts3 was simulated the best by SHAW, with a mean bias error of +0.1 °C for NT and +2.7 °C for RT. Simulation results for soil water content in the surface 30 cm (θv30) were mixed. The NT conditions were simulated best by SHAW, with mean bias error for θv30 within 0.006 m3 m−3; RT conditions were simulated best by the PENFLUX module, which was within 0.010 m3 m−3.  相似文献   

9.
The abundance, activity, and temperature response of aerobic methane-oxidizing bacteria were studied in permafrost-affected tundra soils of northeast Siberia. The soils were characterized by both a high accumulation of organic matter at the surface and high methane concentrations in the water-saturated soils. The methane oxidation rates of up to 835 nmol CH4 h−1 g−1 in the surface soils were similar to the highest values reported so far for natural wetland soils worldwide. The temperature response of methane oxidation was measured during short incubations and revealed maximum rates between 22 °C and 28 °C. The active methanotrophic community was characterized by its phospholipid fatty acid (PLFA) concentrations and with stable isotope probing (SIP). Concentrations of 16:1ω8 and 18:1ω8 PLFAs, specific to methanotrophic bacteria, correlated significantly with the potential methane oxidation rates. In all soils, distinct 16:1 PLFAs were dominant, indicating a predominance of type I methanotrophs. However, long-term incubation of soil samples at 0 °C and 22 °C demonstrated a shift in the composition of the active community with rising temperatures. At 0 °C, only the concentrations of 16:1 PLFAs increased and those of 18:1 PLFAs decreased, whereas the opposite was true at 22 °C. Similarly, SIP with 13CH4 showed a temperature-dependent pattern. When the soils were incubated at 0 °C, most of the incorporated label (83%) was found in 16:1 PLFAs and only 2% in 18:1 PLFAs. In soils incubated at 22 °C, almost equal amounts of 13C label were incorporated into 16:1 PLFAs and 18:1 PLFAs (33% and 36%, respectively). We concluded that the highly active methane-oxidizing community in cold permafrost-affected soils was dominated by type I methanotrophs under in situ conditions. However, rising temperatures, as predicted for the future, seem to increase the importance of type II methanotrophs, which may affect methane cycling in northern wetlands.  相似文献   

10.
Earthworm growth is affected by fluctuations in soil temperature and moisture and hence, may be used as an indicator of earthworm activity under field conditions. There is no standard methodology for measuring earthworm growth and results obtained in the laboratory with a variety of food sources, soil quantities and container shapes cannot easily be compared or used to estimate earthworm growth in the field. The objective of this experiment was to determine growth rates of the endogeic earthworm Aporrectodea caliginosa (Savigny) over a range of temperatures (5–20 °C) and soil water potentials (−5 to−54 kPa) in disturbed and undisturbed soil columns in the laboratory. We used PVC cores (6 cm diameter, 15 cm height) containing undisturbed and disturbed soil, and 1 l cylindrical pots (11 cm diameter, 14 cm height) with disturbed soil. All containers contained about 500 g of moist soil. The growth rates of juvenile A. caliginosa were determined after 14–28 days. The instantaneous growth rate (IGR) was affected significantly by soil moisture, temperature, and the temperature×moisture interaction, ranging from −0.092 to 0.037 d−1. Optimum growth conditions for A. caliginosa were at 20 °C and −5 kPa water potential, and they lost weight when the soil water potential was −54 kPa for all temperatures and also when the temperature was 5 °C for all water potentials. Growth rates were significantly greater in pots than in cores, but the growth rates of earthworms in cores with undisturbed or disturbed soil did not differ significantly. The feeding and burrowing habits of earthworms should be considered when choosing the container for growth experiments in order to improve our ability to extrapolate earthworm growth rates from the laboratory to the field.  相似文献   

11.
A water hyacinth (Eichhornia crassipes) chopper cum crusher was developed at College of Technology and Engineering, Udaipur, India to solve the problem of the bulk of freshly harvested water hyacinth during transportation. The performance of the chopper cum crusher was evaluated on the basis of its ability to reduce volume and weight of fresh water hyacinth. Two variables namely feed rate and knife speeds were studied. Relationships were developed between changes in specific volume, knife speed; percent weight loss and feed rates. Weight reduction studies showed that, with the increase in feed rate and knife speed, there was a decrease in weight loss. Maximum weight loss of 33·77% was achieved with the minimum feed rate of 1·0 t h−1 and knife speed of 3·14 m s−1. Regression models were developed to fit the data. The developed machine reduced the specific volume and weight of fresh water hyacinth by up to 64 and 31·54%, respectively, at the recommended feed rate of 1·0 t h−1 and knife speed of 4·71 m s−1.  相似文献   

12.
This study sought to assess the influence of compost and earthworms (Dendrobaena veneta) upon the level of hydrocarbon catabolism in petroleum contaminated forecourt soil (extractable petroleum hydrocarbons (EPH) 10 + 1.8 g kg−1 and total 16 United States Environment Protection Agency (USEPA) polycyclic aromatic hydrocarbons (PAH) 1.62 ± 0.5 g kg−1). The catabolic activity of the indigenous microorganisms within uncombined materials (soil and compost) and within the combined treatments (soil plus compost; either with or without earthworms) was assessed by 14C-radiorespirometry (14C-hexadecane, 14C-toluene and 14C-phenanthrene). Maximum levels of catabolic activity were observed (at the end of the incubation period; 84 d) for all three compounds in the combined contaminated soil, compost and earthworm mixtures. Significant (p < 0.05) enhancement factors (relative to the soil only control) in catabolic activity in the combined treatments (soil:compost (1:0.5)) of 3.6 times, 1.5 times and 3.5 times were observed for 14C-hexadecane, 14C-phenanthrene and 14C-toluene, respectively; with maximum levels of catabolic activity for these substrates being 68.6 ± 1.7%, 37.9 ± 5.3% and 85.9 ± 1.3%.  相似文献   

13.
The effect of different residual bulking agents (paper, cardboard, grass clippings, pine needles, sawdust and food wastes) in mixtures with sewage sludge (1:1 dry weight) on the growth and reproduction of Eisenia andrei, Bouché 1972 was studied in smallscale laboratory experiments with batches of sixty earthworms. The maximum weight achieved and the highest growth rate were attained in the mixture with food waste (755±18 mg and 18.6±0.6 mg day−1 respectively) whereas the smallest size and the lowest growth rate was achieved in the mixture of sewage sludge with sawdust (572±18 mg and 11±0.7 mg day−1 respectively). The earthworms showed much higher reproductive rates in the paper and cardboard mixtures (2.82±0.39 and 3.19±0.30 cocoons earthworm−1 week−1 respectively) compared to the control with sewage sludge alone (0.05±0.01 cocoons earthworm−1 week−1).  相似文献   

14.
Physical Properties of Vetch Seed   总被引:1,自引:2,他引:1  
The physical properties of vetch seed were evaluated as a function of moisture content. The average length, width and thickness were 5·19, 4·33 and 3·63 mm, respectively, at 10·57% dry basis (d.b.) moisture content. In the moisture range from 10·57 to 20·63% d.b., studies on rewetted vetch seed showed that the thousand seed mass increased from 55·47 to 59·03 g, the projected area from 23·52 to 29·05 mm2, the sphericity from 0·837 to 0·859, the true density from 1286·2 to 1369·9 kg m−3, the porosity from 33·08% to 39·68% and the terminal velocity from 9·94 to 10·33 m s−1. The static coefficient of friction of vetch seed increased the linearly against surfaces of four structural materials, namely, rubber (0·350–0·387), aluminium (0·319–0·367), stainless steel (0·202–0·328) and galvanised iron (0·312–0·361) as the moisture content increased from 10·57 to 20·63% d.b. The bulk density and the shelling resistance decreased from 860·8 to 826·2 kg m−3 and from 148·73 to 62·68 N, respectively, with an increase in moisture content from 10·57 to 20·63% d.b.  相似文献   

15.
Experimental data on the sorption isotherms of sesame seed were used to determine the thermodynamic functions (heat of vaporisation, spreading pressure, net integral enthalpy and entropy). The heat of vaporisation decreased with increase in moisture content and approached the latent heat of pure water at moisture content between 18 and 21% dry basis. The spreading pressure increased with increase in water activity and was not significantly affected by temperature. Net integral enthalpy decreased with increase in moisture content, and became asymptotic as the moisture content of 12% was approached. Net integral entropy decreased with increase in moisture content to a minimum value of 0·138 J kg−1K−1 at moisture content of about 3·7%. It then increased with moisture content to a maximum of about 0·63 J kg−1K−1 at about 12% moisture content and thereafter, remained nearly constant.  相似文献   

16.
Lycopodium spores were released steadily into the air during 20–30 min from a line source positioned within a wheat crop. The spores were trapped on sticky strips held at angles, π, of 0, 30, 60 and 90° with respect to the horizontal and oriented to face the mean wind direction and on sticky, vertical glass rods. The aerial spore concentration, C, was measured by small suction traps. Deposits of the spores on wheat leaves were obtained from sections of leaves whose posture in the canopy was nearly horizontal, nearly vertical, or at angles between 30 and 60°. Number of spores per m2 for all trapping surfaces were obtained by counting under a microscope. Experiments were conducted on seven different days, encompassing friction velocities, u*, of 0.27–0.50 m s−1. The rate of deposition on angled surfaces, D(π), was given approximately by D(θ) = D(0) cos (θ) + D(90) sin (θ), where D(0) and D(90) were the observed rate of deposit on horizontal and on vertical surfaces, respectively. Below mid-canopy height, inertial impaction of spores was negligible, so that D(90) = 0 for all the trap surfaces. There, D(0) was mainly due to sedimentation and was very nearly equal to vs·C, where vs is the settling speed of the spore in still air. Near the top of the canopy, deposition on sticky surfaces was enhanced by inertial impaction and turbulent deposition, so that D(0) was about twice that expected from sedimentation and D(90) was about five times larger than expected from inertial impaction at the mean wind speed. Nevertheless, considering the vertical distribution of leaf area and the angles of leaves in a wheat canopy, the rate of deposition of spores for the entire depth of a wheat canopy can be calculated with a probable underestimation of only 20% by simply assuming sedimentation on horizontally projected area and impaction on vertically projected area.  相似文献   

17.
Chamber measurements of total ecosystem respiration (TER) in a native Canadian grassland ecosystem were made during two study years with different precipitation. The growing season (April–September) precipitation during 2001 was less than one-half of the 30-year mean (1971–2000), while 2002 received almost double the normal growing season precipitation. As a consequence soil moisture remained higher in 2002 than 2001 during most of the growing season and peak aboveground biomass production (253.9 g m−2) in 2002 was 60% higher than in 2001. Maximum respiration rates were approximately 9 μmol m−2 s−1 in 2002 while only approximately 5 μmol m−2 s−1 in 2001. Large diurnal variation in TER, which occurred during times of peak biomass and adequate soil moisture, was primarily controlled by changes in temperature. The temperature sensitivity coefficient (Q10) for ecosystem respiration was on average 1.83 ± 0.08, and it declined in association with reductions in soil moisture. Approximately 94% of the seasonal and interannual variation in R10 (standardized rate of respiration at 10 °C) data was explained by the interaction of changes in soil moisture and aboveground biomass, which suggested that plant aboveground biomass was good proxy for accounting for variations in both autotrophic and heterotrophic capacity for respiration. Soil moisture was the dominant environmental factor that controlled seasonal and interannual variation in TER in this grassland, when variation in temperature was held constant. We compared respiration rates measured with chambers and that determined from nighttime eddy covariance (EC) measurements. Respiration rates measured by both techniques showed very similar seasonal patterns of variation in both years. When TER was integrated over the entire growing season period, the chamber method produced slightly higher values than the EC method by approximately 4.5% and 13.6% during 2001 and 2002, respectively, much less than the estimated uncertainty for both measurement techniques. The two methods for calculating respiration had only minor effects on the seasonal-integrated estimates of net ecosystem CO2 exchange and ecosystem gross photosynthesis.  相似文献   

18.
Mathematical Modelling of Vacuum Pressure on a Precision Seeder   总被引:12,自引:0,他引:12  
The purpose of this research was to determine the optimum vacuum pressure of a precision vacuum seeder and to develop mathematical models by using some physical properties of seeds such as one thousand kernel mass, projected area, sphericity and kernel density. Maize, cotton, soya bean, watermelon, melon, cucumber, sugarbeet and onion seeds were used in laboratory tests. One thousand kernel mass, projected area, sphericity and kernel density of seeds varied from 4·3 to 372·5 g, 5–77 mm2, 38·4–85·8% and 440–1310 kg m−3, respectively. The optimum vacuum pressure was determined as 4·0 kPa for maize I and II; 3·0 kPa for cotton, soya bean and watermelon I; 2·5 kPa for watermelon II, melon and cucumber; 2·0 kPa for sugarbeet; and 1·5 kPa for onion seeds.The vacuum pressure was predicted by mathematical models. According to the results, the final model could satisfactorily describe the vacuum pressure of the precision vacuum seeder with a chi-square of 2·51×10−3, root mean square error of 2·74×10−2 and modelling efficiency of 0·99.  相似文献   

19.
Time of mulching can influence the growth environment and performance of white yam (Dioscorea rotundata Poir). An on-farm trial was conducted during the 1988–1989 and 1989–1990 seasons (October–August) in Nigeria to determine the effect of time of mulching (October–February) on the hydrothermal regime and emergence, growth and tuber yield of white yam. Application of 12.5 mg ha−1 of dry Eupatorium odoratum L. (Syn. Chromolaena odorata L.) mulch on top of the mounds significantly improved soil moisture content of the 15 cm surface layer by 50–120 g kg−1 and decreased the maximum soil temperature by 2–7°C at 15 cm depth in the early growing season (March–April). The emergence and development of yam seedlings were significantly lower in unmulched plots than in mulch-treated plots. Mulching significantly increased tuber yield by about 10–15 mg ha−1 season−1. Plots mulched in October–December were more moist by 20–60 g kg−1 and cooler by 1–3°C, and had 27–44% greater emergence than those mulched in January or February. The number of leaves per plant, vine diameter and leaf area index were also significantly greater in plots mulched in October–December than plots mulched in January or February. Consequently, shoot dry weight was about 28–36% greater in yam mulched in October–December than in yam mulched in February. However, the time of mulching had no effect on soil moisture of the surface layer at the beginning of the rainy season (April), on yam emergence in May and on tuber yield and yield components. Although time of mulching did not significantly affect tuber yield, the increase (10–15%) in the tuber yield of yam mulched in December–February compared to the yam mulched in October or November was considerable. It was concluded that yam planted in October, just before the rain stops, can be mulched in January or February without detrimental effect on emergence, growth and tuber yield.  相似文献   

20.
W.D. Nettleton  M.D. Mays 《CATENA》2007,69(3):220-229
There is much interest in predicting future carbon-soil degradation and that occurring today. We have National Soil Survey Laboratory data to assess some of the soil carbon degradation in the Great Basin and western Utah. For this we included data on 32 Nevada and Utah soils on Pleistocene geomorphic surfaces at elevations of 973 to 3172 m. Their mean annual precipitation (MAP) ranges from 20 to 55 cm and mean annual soil temperatures (MAST) from 5 to 12 °C. The MAP and MAST closely correlate with elevation (E) (r = 0.96 and − 0.97 respectively). Mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) dominates vegetation at the higher, colder elevations. Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle and Young) and juniper (Juniperus L.) dominate at intermediate elevations. Little sagebrush (Artemisia arbuscula Nutt.) and related desert species dominate at the lower, warmer elevations. We used acid dichromate digestion and FeSO4 titration to analyze for soil organic carbon (SOC) and bulk density and coarse fragments in the soils to put the data on a volume basis. The soils are well drained and uncultivated. Accumulation of organic carbon in each pedon (OCp) is correlated to MAP and MAST (r = 0.81, and − 0.78 respectively). We predicted OCp from the relationship,
(1)
r2 = 0.64, S.E. = 1.30, n = 32. The soil OC degradation that may have occurred through the Holocene ranges from 35% at sites of the present Aridisols and Vertisols to 22% for the sites of the Mollisols and Alfisols. Eq. (1) shows that today, MAST rises of 1 to 3 °C would produce further OCp degradation from today's levels of 1% to 13% in Aridisols and Vertisols and 12% to 25% in the Mollisols and Alfisols respectively. It also shows that if the MAST drop of 6 °C predicted for the Pleistocene occurred, many of the Aridisols and Vertisols likely would have been Mollisols or Alfisols during the Pleistocene. A temperature rise of 1 °C in a century would likely move the Mollisol–Aridisol boundary from its present 2300 m elevation to an elevation of about 2900 m. A temperature rise of 3 °C in a century would likely move the Mollisol–Aridisol boundary from its present 2300 m elevation to elevations of the highest elevations in Nevada and to the middle of Idaho. Increasing the temperature by 3 °C will likely also increase the area affected by severe desertification in the southern Great Basin north by about 20%.  相似文献   

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