Sub-humid Environment Research Articles

Impact of Tillage and Residue Burning on Carbon Dioxide Flux in a Wheat–Soybean Production System

Burning of wheat (Triticum aestivum L.) residue followed by plowing is a common management practice in wheat–soybean [Glycine Max (L.) Merrill] production systems in the mid‐southern USA. However, this residue management practice is not environmentally friendly and may not be sustainable. The objectives of this study were to (i) evaluate the effects of N fertilization of wheat, residue burning, and tillage on soil surface carbon dioxide (CO2) flux in a wheat–soybean double‐crop production system, and (ii) evaluate the role of soil temperature and soil moisture in controlling CO2 flux in a relatively warm, subhumid environment. Soil surface CO2 flux was measured nine times between June 2002 and October 2003 during the soybean growing season under all combinations of conventional‐ (CT) and no‐tillage (NT) at high and low N fertilization levels with and without residue burning at two locations in the Mississippi River delta region of eastern Arkansas. Soil surface CO2 flux was 37.6% higher (P < 0.01) from CT than from NT and 6.1% higher (P < 0.05) from the low than the high N rate treatment at one location, but not at the other. Burning did not affect soil surface CO2 flux except for a significant burning × N rate (P = 0.016) and burning × time interaction (P = 0.032) at one location, but not at the other. Both soil temperature and moisture parameters were significantly negatively correlated with temperature‐normalized soil surface CO2 flux, but soil temperature, particularly at the 10‐cm depth, explained more of the variation than did soil moisture parameters. The results of this study indicate that tillage and N fertilization of prior wheat, but not residue burning, affect the loss of C as CO2 from the soil and that there are additional soil and/or environmental factors, other than near‐surface soil moisture and temperature fluctuations, perhaps microbial biomass, that contribute to controlling soil surface CO2 flux in wheat‐soybean production systems in the subhumid region of southern USA.

Inheritance of useful traits in cassava grown in subhumid conditions

AbstractA diallel study among nine parental clones of cassava was conducted in the subhumid environment on the northern coast of Colombia. Analysis of variance suggested significant effects for the six variables analysed: fresh‐root yield, harvest index, root dry matter content, height of first branching, reaction to thrips and plant‐type scores. General and specific combining ability effects and their interaction with the environment were significant for most of the variables as well. Results suggested that dominance plays a particularly important role in the cases of fresh‐root yield and harvest index but had relatively little importance in the reaction to thrips, dry matter content or height of first branching. Specific breeding approaches are suggested for these traits, depending on the relative importance of additive or non‐additive effects in their inheritance. The correlations among different traits were also analysed and in several cases their magnitude reached statistical and biological significance.

Deficit irrigation of sunflower (Helianthus annuus L.) in a sub-humid climate

The response of sunflower (Helianthus annuus L.) to 14 irrigation treatments in a sub-humid environment (Bursa, Turkey) was studied in the field for two seasons. A rainfed (non-irrigated) treatment as the control and 13 irrigation treatments with full and 12 different deficit irrigations were applied to the hybrid Sanbro (Novartis Seed Company) planted on clay soil, at three critical development stages: heading (H), flowering (F) and milk ripening (M). The yield increased with irrigation water amount, and the highest seed yield (3.95 t ha−1) and oil yield (1.78 t ha−1) were obtained from the HFM treatment (full irrigation at three stages); 82.9 and 85.4% increases, respectively, compared to the control. Evapotranspiration (ET) increased with increased amounts of irrigation water supplied. The highest seasonal ET (average of 652 mm) was estimated at the HFM treatment. Additionally, yield response factor (k y) was separately calculated for each, two and total growth stages, and k y was found to be 0.8382, 0.9159 (the highest value) and 0.7708 (the lowest value) for the total growing season, heading, and flowering-milk ripening combination stages, respectively. It is concluded that HFM irrigation is the best choice for maximum yield under the local conditions, but these irrigation schemes must be re-considered in areas where water resources are more limited. In the case of more restricted irrigation, the limitation of irrigation water at the flowering period should be avoided; as the highest water use efficiency (WUE) (7.80 kg ha−1 mm−1) and irrigation water use efficiency (IWUE) (10.19 kg ha−1 mm−1) were obtained from the F treatment.

Uprooting of Vetiver Uprooting Resistance of Vetiver Grass (Vetiveria zizanioides)

Vetiver grass (Vetiveria zizanioides), also known as Chrysopogon zizanioides, is a graminaceous plant native to tropical and subtropical India. The southern cultivar is sterile; it flowers but sets no seeds. It is a densely tufted, perennial grass that is considered sterile outside its natural habitat. It grows 0.5–1.5 m high, stiff stems in large clumps from a much branched root stock. The roots of vetiver grass are fibrous and reported to reach depths up to 3 m thus being able to stabilize the soil and its use for this purpose is promoted by the World Bank. Uprooting tests were carried out on vetiver grass in Spain in order to ascertain the resistance the root system can provide when torrential runoffs and sediments are trying to uproot the plant. Uprooting resistance of each plant was correlated to the shoot and root morphological characteristics. In order to investigate any differences between root morphology of vetiver grass in its native habitat reported in the literature, and the one planted in a sub-humid environment in Spain, excavation techniques were used to show root distribution in the soil. Results show that vetiver grass possesses the root strength to withstand torrential runoff. Planted in rows along the contours, it may act as a barrier to the movement of both water and soil. However, the establishment of the vetiver lags behind the reported rates in its native tropical environment due to adverse climatic conditions in the Mediterranean. This arrested development is the main limitation to the use of vetiver in these environments although its root strength is more than sufficient.

Epistasis in the Expression of Relevant Traits in Cassava (Manihot esculenta Crantz) for Subhumid Conditions

There is limited knowledge on the inheritance of agronomic traits in cassava and the importance of epistasis for most crops. A nine-parent diallel study was conducted in subhumid environments. Thirty clones were obtained from each F1 cross. Each clone was represented by six plants, which were distributed in three replications at two locations. Therefore the same 30 genotypes of each F1 cross were planted in the three replications at the two locations. Analysis of variance suggested significant genetic effects for all variables analyzed (reaction to thrips, fresh root and foliage yields, harvest index, dry matter content, and root dry matter yield). Significant epistatic effects were observed for all variables, except harvest index. Dominance variance was always significant, except for dry matter content and dry matter yield. Additive variance was significant only for reaction to thrips. Results suggested that dominance plays an important role in complex traits such as root yield. The significance of epistasis can help us understand the difficulties of quantitative genetics models and QTLs in satisfactorily explaining phenotypic variation in traits with complex inheritance. Significant epistasis would justify the production of inbred parental lines to fix favorable allele combinations in the production of hybrid cassava cultivars.

RESPONSE OF DIAPREPES ABBREVIATUS (COLEOPTERA: CURCULIONIDAE) TO APPLICATION CONCENTRATIONS OF A PARTICLE FILM

Kaolin-based particle films were developed for horticultural applications as an environmentally benign method to deter arthropod pests and plant diseases (Glenn et al. 1999). Since the commercialization of a wettable powder formulation (Surround® WP, Engelhard Corp., Iselin, NJ), this product has been examined for applications against pests of temperate fruit trees (Puterka et al. 2000; Knight et al. 2000; Unruh et al. 2000); cotton (Showler 2002); whitefly (Liang & Liu 2002); thrips (Kerns & Wright 2000); and other pests. The value of the film includes deterrence of feeding and oviposition, and beneficial effects on carbon assimilation, leaf temperature, and fruit yield (Glenn et al. 2001). Particle film technology seems especially well suited for use in areas of low rainfall where leaf residues of the product can be maintained without frequent re-application. Use of particle films in the humid subtropical environment of Florida citrus groves may be limited by removal of residues by seasonally heavy rain (Lapointe 2000). In addition to deterring pests (Puterka et al. 2000; Unruh et al. 2000; Knight et al. 2000; Lapointe 2000), particle films have been shown to increase fruit tree productivity in semiarid and subhumid environments by reducing heat stress (Glenn et al. 2001). The humid subtropical environment experienced by citrus trees in Florida presents a different set of challenges for successful use of particle films. Periods of high rainfall are interspersed with dry periods accentuated by highly porous soils and intense solar radiation. Particle films, while effective in the laboratory (Lapointe 2000), may not adhere sufficiently to citrus leaves in the field in Florida to maintain deterrence to Diaprepes root weevils after heavy rains over the entire rainy season (May-October) when adults are active. Prior to establishment of field trials (reported elsewhere), I investigated the feeding and oviposition response of Diaprepes root weevil to varying concentrations of Surround WP. A hand-held sprayer was used to apply the product at three concentrations to bouquets of citrus leaves harvested from Citrus macrophylla Wester seedlings grown in a greenhouse. Methods were similar to those reported by Lapointe (2000). Foliage bouquets were sprayed to runoff with the manufacturer’s recommended concentration (x = 3% wt/ vol), 0.5x, or 0.1x, or with water alone. Foliage was allowed to dry and then placed in screened cages (30 × 30 × 30 cm) containing 5 male and 5 female Diaprepes root weevils. Weevils were obtained from a colony maintained at the U.S. Horticultural Laboratory, Ft. Pierce, FL (for rearing conditions, see Lapointe & Shapiro 1999). Each of the 4 treatments was replicated 3 times for a total of 12 cages and 120 weevils. Cages were randomly arranged on open shelves in a temperature-controlled greenhouse. Each cage was provided with wax paper strips as substrates for oviposition (Wolcott 1933). Bouquets and wax paper strips were removed every 2 days until 17 days and examined for egg masses. Leaf area consumed was assessed by tracing the leaf notches. Tracings were digitally scanned and the resulting files were imported into an image analysis computer program as described by Lapointe (2000). Leaf area consumed and total number of eggs oviposited were analyzed by ANOVA with the type III sum of squares for cage as the error term. Means were compared by the Tukey honestly significant differences (HSD) test (Abacus Concepts 1996). Linear regression was used to calculate the deposition of particle film required to achieve 50% reduction in leaf consumption and oviposition. The deterrent effect of Surround against feeding and oviposition fell off quickly as coverage was reduced in a greenhouse trial. Adult weevils fed untreated citrus leaves over 17 d consumed approximately 3 times as much leaf area as weevils

Carbon Storage in Soils of the North American Great Plains: Effect of Cropping Frequency

Summer fallow (fallow) is still widely used on the North American Great Plains to replenish soil moisture between crops. Our objective was to examine how fallowing affects soil organic carbon (SOC) in various agronomic and climate settings by reviewing long‐term studies in the midwestern USA (five sites) and the Canadian prairies (17 sites). In most soils, SOC increased with cropping frequency though not usually in a linear fashion. In the Canadian studies, SOC response to tillage and cropping frequency varied with climate—in semiarid conditions, SOC gains under no‐till were about 250 kg ha−1 yr−1 greater than for tilled systems regardless of cropping frequency; in subhumid environments, the advantage was about 50 kg ha−1 yr−1 for rotations with fallow but 250 kg ha−1 yr−1 with continuous cropping. Specific crops also influenced SOC: Replacing wheat (Triticum aestivum L.) with lentil (Lens culinaris Medikus) had little effect; replacing wheat with lower‐yielding flax (Linum usitatismum L.) reduced SOC gains; and replacing wheat with erosion‐preventing fall rye (Secale cereale L.) increased SOC gains. In unfertilized systems, cropping frequency did not affect SOC gains, but in fertilized systems, SOC gains often increased with cropping frequency. In a Colorado study (three sites each with three slope positions), SOC gains increased with cropping frequency, but the response tended to be highest at the lowest potential evaporation site (where residue C inputs were greatest) and least in the toeslope positions (despite their high residue C inputs). The Century and the Campbell et al. SOC models satisfactorily simulated the relative responses of SOC although they underestimated gains by about one‐third.

Genotype by environment interactions for productivity and resistance to gastro-intestinal nematode parasites in Red Maasai and Dorper sheep

AbstractRed Maasai and Dorper sheep were evaluated for their resistance to gastro-intestinal (GI) nematode parasites (predominantly Haemonchus contortus), productivity and productive efficiency (assessed on a metabolizable energy basis) in experiments undertaken at the Kenyan coast (sub-humid environment) and the Kenya highlands (semi-arid environment). In both ewes and lambs there were few significant genotype by environment (G X E) interactions for either resistance (assessed by faecal egg counts-FEC) or resilience (assessed by blood packed red cell volume-PCV) to GI nematodes. Red Maasai sheep were consistently more resistant (low FEC) and more resilient (high PCV) than Dorper sheep. However, there were significant G X £ interactions for ewe reproductive performance and for ewe and lamb mortality rates and live weights. These interactions were due to very poor performance of the Dorper compared to the Red Maasai in the sub-humid coastal environment and to the much improved performance of the Dorper in the semi-arid environment. When these component traits were combined into estimates of flock productivity and productive efficiency there were highly significant GXE interactions with the Red Maasai sheep being considerably more efficient than Dorper sheep in the sub-humid environment, while in the semi-arid environment there was a negligible breed difference in productive efficiency. The results are discussed in terms of breeding strategies for smallholder farmers and pastoralists managing sheep in low-input systems in sub-humid and semi-arid environments.

Terminal Lake Flooding and Wetland Expansion in Nelson County, North Dakota

The Devils Lake Basin of North Dakota, an interior drainage basin located within a dry, subhumid environment, has experienced pervasive flood conditions since the 1993 onset of a wet spell of unprecedented magnitude and duration. This unique natural-hazard environment has resulted in flooding from both the expansion of the surface area of the basin's terminal lakes (Devils Lake and Stump Lake) and increases in the number and size of rural wetlands. To assess the relative extent of both terminal lake and rural wetland flooding, we focused on Nelson County, which contains Stump Lake and is representative of other counties in the basin. Remotely sensed data acquired by Landsat Thematic Mapper was used to map open-water extent in 2001, and results were compared to 1992 land-cover data provided by the United States Geological Survey (USGS). Our analysis indicates a 53% increase in the size of Stump Lake and a 426% increase in the area of rural wetland ponds. Stump Lake flooding is spatially restricted and has had limited impact upon the surrounding lakeshore environment. Rural wetland flooding is pervasive and has a deleterious effect upon the region's agricultural economic base.

Effects of Kaolin Particle Film on Olive Fruit Yeld, Oil Content and Quality

The response of olive trees (Olea europaea L.) to a kaolin-based particle film formulation was evaluated in the warm sub-humid Mediterranean environment of northwest Syria for its effect on fruit yield, oil content and quality. Kaolin-sprayed olive trees significantly outperformed their unsprayed counterparts in terms of fruit yield and size, percentages of dry matter and oil content. Oil samples obtained from kaolin-sprayed trees contained less peroxides after 70 days of storage compared with that extracted from untreated control trees. Consistent with previous findings on apple and grapefruit trees, the kaolin particle film proved to be an efficient growth enhancer for olive trees in the sub-humid Mediterranean area.

Importance of rapid canopy closure for wheat production in a temperate sub-humid environment: experimentation and simulation

Rapid canopy closure (RCC) is one of the physiological attributes that may enhance genetic yield potential of wheat ( Triticum aestivum L.), particularly in a short growing season. The objectives of this study were (1) examination of the interrelationships among RCC-related traits and grain yield under rainfed conditions in a temperate sub-humid environment, and (2) quantitative evaluation of the value of RCC to wheat production in the environment. Canopy development before closing is described by an exponential equation, y=α L exp ( β L t), where y is the crop leaf area index (LAI), t the thermal time, α L the LAI at the beginning of the exponential growth and β L the relative growth rate of LAI (m 2 m −2 °C d −1). Field experiment results, using 13 wheat cultivars, showed that exponential phase of canopy development can be divided into two sections; an early section during which canopy dry weight and LAI are highly related to α L, and a second section where crop dry weight and LAI are highly dependent on β L. Grain weight had a positive effect on α L parameter and canopy dry weight and LAI during early section of exponential phase. In cultivars with high grain yield, canopy dry weight and LAI were also higher during exponential phase of canopy development. High grain yield was related to higher β L, but not with α L. There was genotypic difference for RCC parameters, α L and β L, indicating the possibility of genetic improvement for these traits. However, a negative correlation (−0.69, P<0.01) observed between the parameters may reduce gain from a higher β L. Simulation analysis was performed for four hypothetical and standard (Zagros) cultivars using a modified SUCROS1-Wheat model to explore possible long-term yield consequences of RCC. These four cultivars were created based on observed variation and association between the parameters in the field experiment. Results showed that attempts to increase RCC and thereby yield through greater β L would be more beneficial than α L. Twenty percent increase in β L caused yield gain of 7–10% for irrigation conditions and 13–18% for rainfed conditions, depending to the values of α L compared to yield gain of 3 and 5% resulted from a 20% greater α L under irrigated and rainfed conditions, respectively. Sensitivity tests with decreased α L and β L showed that a large decrease in grain yield is obtained for 20 and 40% lower α L and β L. Hence, it was concluded that in locations with non-optimum agronomic management affecting RCC, yield increase has to be obtained firstly by improvement of agronomic measures. The implications for other environments and future research needs are discussed.

Effects of improved drainage and nitrogen source on yields, nutrient uptake and utilization efficiencies by maize (Zea mays L.) on Vertisols in sub-humid environments

Nitrogen is the most limiting plant nutrient in Vertisols in Kenya. Soil properties, climatic conditions and management factors as well as fertilizer characteristics can influence fertilizer nitrogen (N) use efficiency by crops. Vertisols, characterized by low-basic water infiltration rate, are prone to waterlogging under sub-humid and humid conditions. We determined effects of drainage, N source and time of application on yields, nutrient uptake and utilization efficiencies by maize grown on Vertisols in sub-humid environments. Treatments comprised two furrows (40 cm and 60 cm deep) and a check (i.e., no furrow), calcium nitrate to furnish NO3-N, ammonium sulphate to supply NH4-N at 100 kg N ha−1, a control (i.e., no fertilizer N), and fertilizer N application at sowing, 40 days after sowing, and split (i.e., half the rate at sowing and half 40 days after sowing). A split-plot design was used in which drainage formed the main plots and N source × time of N application formed the sub-plots. Higher grain and total dry matter yields, harvest index, leaf N content, uptake of N, P and K, as well as N agronomic (NAE) and recovery (NRE) efficiencies were obtained from drained compared to undrained plots. The increase ingrain yields as a result of drainage varied from 31 to 45% for control, 35 to 43% for NO3-N, and 16 to 21% for NH4-N treatments. Drainage resulted in total N uptake increases from 50 to 80 kg N ha−1 in control plots, 80 to 130 kg N ha−1 in NO3-N treated plots, and 90 to 130kg N ha−1 in NH4-N treated plots. Ammonium-N source was superior to NO3-N source in terms of higher yields, NAE, and NRE in undrained plots, but the two N sources behaved similarly in drained plots. Delayed or split NO3-N application gave higher yields, NAE and NRE than when all N was applied at sowing in undrained plots. There was no difference between 40 cm and 60 cm deep furrows in terms of crop yields and nutrient use efficiencies. Thus, draining excess water with furrows at least 40 cm deep is essential for successful crop production in these Vertisols under sub-humid conditions.

The evaluation of dual-purpose groundnut (Arachis hypogaea) varieties for fodder and seed production at Shika, Nigeria

At Shika in a subhumid environment of Nigeria, a 3-year study was carried out to select newly developed groundnut varieties for use in crop–livestock production systems. The study examined 11 groundnut varieties. Emergence time, plant stands at full emergence, forage and seed yields and yield components were examined. Whole plant samples were analysed for crude protein (CP) content. Varieties ICGV 87123 gave the lowest forage yield and cultivar M517-80I, the highest, with seven varieties recording forage yields above 5 t/ha. The CP content of forage was lowest (14·8%) for variety M576-80I and highest (21·6%) for variety M554-76. Mean seed yield (over 3 years) varied significantly from 0·73 to 1·68 t/ha. Only two varieties had mean seed yield &gt;1 t/ha. The relationship between seed and forage yields was positive and significant (r = 0·529, P &lt; 0·006). Varieties RMP 12, 88-80I and M517–80 were most promising for both forage and seed production.

The performance of selected soil and water conservation measures—case studies from Ethiopia and Eritrea

This paper investigates the performance of selected soil and water conservation measures in the highlands of Ethiopia and Eritrea, namely Fanya Juu, soil/stone bund, grass strips and double ditches. The impact of these techniques on runoff, soil loss, crop yield and biomass production is measured at on-farm experimental sites in seven research sites under different agro-ecological conditions. On one hand, most measures brought about a considerable reduction in soil loss and runoff. The latter, however, increases the waterlogging hazard, particularly in sub-humid environments. On the other hand, crop yield and biomass production did not increase, as it would have been necessary to compensate for the high costs of soil conservation. Thus, the measures tested only partially fulfil their requirements. To enhance their adaptability to local conditions, they must be supplemented with biological and agronomic measures that help improve production.

A model for simulating rock–water interactions in a weathering profile subjected to frequent alternations of wetting and drying

Physically based equations for unsaturated groundwater flow and solute transport have been coupled with kinetic rate laws for mineral dissolution–precipitation, and mass balance/mass action equations for aqueous species, in a numerical model that is capable of simulating rock–water interactions in a weathering profile subjected to fluctuating boundary conditions. A numerical experiment was conducted to demonstrate how incipient soil development may proceed in a warm subhumid environment. The simulation involved a hypothetical coarse-textured parent material that was subjected to frequent wetting and drying during an annual water cycle. The hypothetical weathering profile evolved rapidly; dissolution of primary minerals (enstatite, forsterite, and diopside) and precipitation of secondary clay–minerals (kaolinite and Ca-montmorillonite) occurred monotonically despite the abrupt fluctuations in soil-moisture content. In contrast, the activities of aqueous species and dissolution–precipitation rates of calcite were very sensitive to the changing moisture conditions in the upper part of the profile. Although the simulation involved numerous simplifying assumptions, reasonable results were achieved and the calculated (from the model) rate of chemical denudation fell within the range of contemporary denudation rates determined from the dissolved loads of rivers.

Herbage yields and quality in four woody forage plants in a subhumid environment in Ghana

There is keen interest in feeding small ruminants with herbage from forage shrubs and trees in Ghana because of the general perception about these plants being perennial and remaining green for lengthy periods. In this study, plant growth rate, herbage production and quality of two indigenous woody plants, Griffonia simplicifolia and Baphia nitida were compared to those of two introduced shrubs, Leucaena leucocephala and Gliricidia sepium. Gliricidia sepium achieved the greatest height and produced the highest herbage dry matter (2t/ha) compared to average production of 0.15 t/ha by the three other species. Gliricidia sepium herbage also had high contents of crude protein, calcium and ash. In regrowth herbage production at cutting intervals of 6 and 12 weeks, Gliricidia sepium again outperformed the three other species, however, there was little difference in herbage yield among the three other species. In all species one harvest at 12 weeks regrowth interval produced more herbage than the combination of yield of two six week interval harvests.

Effect of age and sex of egg strain chickens and of two stocking densities of broilers on voluntary water intake and performance in a sub-humid environment.

Voluntary water intake and performance of egg-strain chickens of different age groups and sex were compared. Cockerels grew faster and consumed 41% more feed and 43% more water than pullets of the same strain and age. Cockerels of nineteen weeks consumed significantly more feed (P < 0.05) than layers of 30 weeks of the same strain, but there was no significant difference in their daily voluntary water intake. The physiological activities associated with egg laying in chickens, apparently require more water to feed ratio than for the active growth of cockerels. In a second experiment, the effect of 2 stocking densities on voluntary water intake and performance were compared. Birds on higher stocking density (0.09 m2/bird) performed similarly to those on low stocking density (0.18 m2/bird), but the higher stocking density birds recorded slightly higher mortality. Voluntary water consumption was not significantly affected (P > 0.05), although birds on higher stocking density recorded slightly higher average daily voluntary water consumption. The higher stocking density birds recorded slightly higher per cent mortality during the 8 week period, but higher per cent mortality was recorded with the low stocking density birds during the 0 to 4 week brooding period.

Adaptation and complementarity of Digitaria eriantha and Medicago sativa on a solodic soil in a subhumid environment with summer and winter rainfall

Summary. A cutting experiment was conducted over 4 years on a solodic soil on the Far North-West Slopes of New South Wales to compare the environmental adaptation of Digitaria eriantha ssp. eriantha cv. Premier and Medicago sativa cv. Hunter River. Adaptation was tested by comparing seasonal yield responses of the monocultures and the mixture at high and low levels of nitrogen (N); monoculture yields were also compared with those predicted by a simple climate-based model. When N was non-limiting, digitaria yields were highest in summer while lucerne yields were highest in spring. Lucerne yields in summer were often much lower than those predicted by the model but the plant recovered well during periods of milder temperature, until decimated by flooding. Yields of the mixture were similar to those of digitaria in summer when it was grass dominant, and similar to lucerne in spring when it was legume dominant. Overall, at high N the mixture did not outyield digitaria. In the absence of fertiliser N, digitaria and lucerne monocultures gave similar dry matter yields. However, over the 3 warm seasons, the mixture produced 72% more dry matter than the monocultures and 263% (325 kg/ha) more N than digitaria. The complementary seasonal responses of digitaria and lucerne, and high yields of the mixture suggest a role for the mixture on solodic soils on the Far North-West Slopes of New South Wales provided the effects of irregular flooding on lucerne can be minimised by management or more tolerant cultivars.

Influence of long-term tillage, straw and N fertilizer on barley yield, plant-N uptake and soil-N balance

Long-term influence of N fertilizer, tillage and straw on crop production and soil properties are not well known in central Alberta. Field experiments were established in autumn 1979, on a Black Chernozemic soil and on a Gray Luvisolic soil in north-central Alberta to determine the long-term effect of tillage, straw and N fertilizer on yield and N uptake of barley ( Hordeum vulgare L.). Fertilizer N was applied annually at 56 kg ha −1. The 11 year averages of barley yields and N uptake under zero tillage were lower than under conventional tillage. Retention rather than removal of straw tended to reduce barley yield for the initial 6 years and 2 year at Site 1 and Site 2, respectively. A simple mathematical model of average annual plant N uptake and grain yield could account for most of the variation in the data observed at both sites ( R 2 = 0.907; P < 0.01). Final values of soil N, calculated using a mass balance approach, agree closely with values measured at the end of the eleventh year. Conventional tillage and zero tillage, with addition of fertilizer N and retention of straw, were the only treatments with apparent but small net addition of N to soil at Site 1 (40 kg ha −1 and 117 kg ha −1, respectively). At Site 2, only the zero tillage treatment with addition of fertilizer and retention of straw gained soil N (29 kg ha −1). In conclusion, soil ecosystems functioning in subhumid environments with slight to moderate heat limitations such as those in central Alberta can adapt, within a few years, to zero tillage practices with full retention of straw.

Source approach for estimating soil and vegetation energy fluxes in observations of directional radiometric surface temperature

A two-layer model of turbulent exchange that includes the view geometry associated with directional radiometric surface temperature is developed and evaluated by comparison of model predictions with field measurements. Required model inputs are directional brightness temperature and its angle of view, fractional vegetation cover or leaf area index, vegetation height and approximate leaf size, net radiation, and air temperature and wind speed. One advantage of the approach described in this paper is that directional brightness temperatures are considered so that the model should have wider applicability than single-layer models, and it opens the possibility of a simple solution if directional measurements are available from two substantially different view angles. Comparisons with several hundred measurements from two large-scale field experiments were performed. One study was conducted in a semiarid rangeland environment in Southern Arizona (Monsoon '90) while the other was conducted in a subhumid environment, namely the tall grass prairie in Eastern Kansas (FIFE). For the Monsoon '90 site, root-mean-square-differences (RMSD) between model predictions and measurements were between 35 and 60 W m −2 for soil, sensible and latent heat flux. With the FIFE site data RMSD values were between 50 and 60 W m −2. The larger scatter with the FIFE data was mainly caused by the model having difficulty reproducing the fluxes for the observation period with dormant vegetation. Considerations of the expected variability associated with flux measurements over complex surfaces suggests that model-derived fluxes were in acceptable agreement with the observations. However refinements in formulations of soil heat flux probably would improve agreement between model predictions and measurements.