Plant-Parasitic Nematode Communities in Dogwood,Maple, and Peach Nurseries in Tennessee

Nursery blocks (48 dogwood, 27 red maple, and 17 peach) distributed among 20 Tennessee nurseries were sampled for nematodes in March, July, and October 1981. Plant-parasitic nematodes were extracted from soil, counted by genera, and identified to species after fixation. A total of 57 species in 24 genera were found, with 1-16 species occurring in a site. The species most commonly detected were Paratylenchus projectus and Xiphinema americanum, which were found in 88% and 78% of the sites, respectively. Relationships existed between distribution and densities of some species present in more than 10% of the sites and certain soil factors (pH, bulk density, texture, and organic matter content). Plant-parasitic nematode community diversity was related to tree age, percentage of weed ground cover, and number of weed species. Site similarities in community ordinations were dependent on the individual nurseries sampled, tree age, and soil type, but clusters of sites of similar tree ages and soil types were not exclusive.     

Influence of Environmental Factors,Cultural Practices,and Herbicide Application on Seed Germination and Emergence Ecology of Ischaemum rugosum Salisb

Ischaemum rugosum Salisb. (Saramolla grass) is a noxious weed of rice that is difficult to control by chemical or mechanical means once established. A study was conducted to determine the effect of light, temperature, salt, drought, flooding, rice residue mulch, burial depth, and pre-emergence herbicides on seed germination and emergence of I. rugosum. Germination was stimulated by light and inhibited under complete darkness. Optimum temperature for germination was 30/20°C (97.5% germination). Germination reduced from 31 to 3.5% when the osmotic potential of the growing medium decreased from -0.1 to -0.6 MPa and no germination occurred at -0.8 MPa. Germination was 18 and 0.5% at 50 and 100 mM NaCl concentrations, respectively, but was completely inhibited at 150 mM or higher. Residue application at 1–6 t ha^-1 reduced weed emergence by 35–88% and shoot biomass by 55–95%. The efficacy of pre-emergence herbicides increased with increasing application rates and decreased with increasing rice residue mulching. The efficacy of herbicides was in the order of oxadiazon> pendimethalin> pretilachlor. At 6 t ha^-1, all herbicides, regardless of rates, did not differ from the control treatment. I. rugosum seeds buried at 2 cm or deeper did not emerge; however, they emerged by 4.5 and 0.5% at 0.5 and 1 cm depths, respectively, compared to the 39% germination for soil surface seeding. Flooding at 4 DAS or earlier reduced seedling emergence and shoot biomass while flooding at 8 DAS reduced only seedling emergence. The depth and timing of flooding independently reduced root biomass. Flooding at 4 and 6 cm depths reduced the root biomass. Relative to flooding on the day of sowing, flooding at 8 DAS increased root biomass by 89%. Similarly, flooding on the day of sowing and at 2 DAS reduced the root–shoot biomass ratio. Under the no-flood treatment, increasing rates of pretilachlor from 0.075 to 0.3 kg ai ha^-1 reduced weed emergence by 61–79%. At the flooding depth of 2–4 cm, pretilachlor reduced weed emergence and shoot and root biomass, but the differences across rates were non-significant. Information generated in this study will be helpful in developing integrated weed management strategies for managing this weed.     

Control of Codling Moth,Cydia pomonella (Lepidoptera: Tortricidae), with Steinernema carpocapsae: Effects of Supplemental Wetting and Pupation Site on Infection Rate

Infection of cocooned codling moth (cydia pomonella) larvae by the entomopathogenic nematode Steinernema carpocapsae was studied in three field experiments. Factors that varied within or between experiments included method of application, type of substrate containing cocooned larvae, time when nematodes were applied, seasonal effects, and supplemental wetting before or after nematode application. Conventional air-blast sprayer applications of 0.5–5.0 million infective juveniles (IJs)/tree in fall resulted in ca. 30% mortality of larvae in cardboard trap bands, whereas hand-gun application (2 million IJs/tree) produced mortality of ca. 70%. Application in the evening caused higher larval mortality than application in the morning when no supplemental wetting was used after treatments. Morning and evening applications caused equivalent larval mortality when a postwetting treatment was included. In a trial conducted in midsummer, supplemental wetting, either before or after hand-gun application of 1 million IJs/tree, enhanced nematode-produced mortality. Mortality approached 100% if both pre- and postwetting was used. Larvae in exposed cocoons on apple wood were infected at a higher rate (86%) than those on wood in less exposed positions (73%) or in nonperforated cardboard (72%). Mortality rates for larvae in perforated cardboard were intermediate (77%). Application volumes used to deliver nematodes slightly enhanced infection rate of larvae in some substrates but not others. In one trial, parasitism of codling moth by the wasp Mastrus ridibundus (Ichneumonidae) was negatively correlated with nematode infection of codling moth larvae. Dissections showed that ca. 10% of larvae infected by nematodes had been attacked by the wasp.     

Yield of glyphosate-resistant sugar beets and efficiency of weed management systems with glyphosate and conventional herbicides under German and Polish crop production

In sugar beet production, weed control is one of the most important and most expensive practices to ensure yield. Since glyphosate-resistant sugar beets are not yet approved for cultivation in the EU, little commercial experience exists with these sugar beets in Europe. Experimental field trials were conducted at five environments (Germany, Poland, 2010, 2011) to compare the effects of glyphosate with the effects of conventional weed control programs on the development of weeds, weed control efficiency and yield. The results show that the glyphosate weed control programs compared to the conventional methods decreased not only the number of herbicide applications but equally in magnitude decreased the dosage of active ingredients. The results also showed effective weed control with glyphosate when the weed covering was greater and sugar beets had a later growth stage of four true leaves. Glyphosate-resistant sugar beets applied with the glyphosate herbicide two or three times had an increase in white sugar yield from 4 to 18 % in comparison to the high dosage conventional herbicide systems. In summary, under glyphosate management sugar beets can positively contribute to the increasingly demanding requirements regarding efficient sugar beet cultivation and to the demands by society and politics to reduce the use of chemical plant protection products in the environment.     

Tropical soils degraded by slash‐and‐burn cultivation can be recultivated when amended with ashes and compost

In many tropical regions, slash‐and‐burn agriculture is considered as a driver of deforestation; the forest is converted into agricultural land by cutting and burning the trees. However, the fields are abandoned after few years because of yield decrease and weed invasion. Consequently, new surfaces are regularly cleared from the primary forest. We propose a reclamation strategy for abandoned fields allowing and sustaining re‐cultivation. In the dry region of south‐western Madagascar, we tested, according to a split‐plot design, an alternative selective slash‐and‐burn cultivation technique coupled with compost amendment on 30–year‐old abandoned fields. Corn plants (Zea mays L.) were grown on four different types of soil amendments: no amendment (control), compost, ashes (as in traditional slash‐and‐burn cultivation), and compost + ashes additions. Furthermore, two tree cover treatments were applied: 0% tree cover (as in traditional slash‐and‐burn cultivation) and 50% tree cover (selective slash‐and‐burn). Both corn growth and soil fertility parameters were monitored during the growing season 2015 up to final harvest. The amendment compost + ashes strongly increased corn yield, which was multiplied by 4–5 in comparison with ashes or compost alone, reaching 1.5 t/ha compared to 0.25 and 0.35 t/ha for ashes and compost, respectively. On control plots, yield was negligible as expected on these degraded soils. Structural equation modeling evidenced that compost and ashes were complementary fertilizing pathways promoting soil fertility through positive effects on soil moisture, pH, organic matter, and microbial activity. Concerning the tree cover treatment, yield was reduced on shaded plots (50% tree cover) compared to sunny plots (0% tree cover) for all soil amendments, except ashes. To conclude, our results provide empirical evidence on the potential of recultivating tropical degraded soils with compost and ashes. This would help mitigating deforestation of the primary forest by increasing lifespan of agricultural lands.     

Biomass yield and morphological features of the seaweed Saccharina latissima cultivated at two different sites in a coastal bay in the Atlantic coast of Spain

The kelp Saccharina latissima is an economically important edible seaweed with great potential for use in other applications. The goal of this study was to identify locations in a coastal bay with better conditions for commercial cultivation of this species. We compared the biomass yield and morphological features of S. latissima cultivated at two locations in a bay of Galicia (northwest Spain): in an external moderately exposed site and in an internal sheltered site. Biomass yield was significantly higher at the moderately exposed site than at the sheltered site (16 fresh kg vs. 12 fresh kg per linear meter of rope). Fronds were significantly larger at the moderately exposed site compared to that at the sheltered site. However, the “substantiality value” (around 40 mg cm^?2), which is an index used to assess blade quality for human consumption, did not differ significantly between the two locations. The effect of light exposure and water velocity seemed to be the most important environmental factors that explain the differences in the results obtained at both culture sites. In summary, both the moderately exposed site and the sheltered site were suitable for cultivation of S. latissima, as indicated by the high yields and similar blade quality obtained at both locations.     

Tree guards and weed mats in a dry shrubland restoration in New Zealand

Associated with restoration of 150 ha of native plant communities within a highly exposed low rainfall landscape, we evaluated whether the benefits of tree guards and weed mats justified a doubling of planting costs. One‐year‐old nursery‐grown specimens of Kānuka (Kunzea serotina) and Pomaderris (Pomaderris amoena) were planted with and without polythene and rigid plastic tree guards, and weed mats, then monitored for 12 months. Herbivory was reduced with both types of tree guard by 10–35%, but mortality was largely caused by desiccation. Polythene tree guards provided the best protection, buffering extremes of ambient and soil temperature in summer but with no effect on soil temperature in winter. Weed mat addition reduced soil temperature fluctuation further in winter; a moss turf mat was best. Both mulch and moss weed mats reduced soil temperature in summer and raised soil moisture in winter. We showed that tree guards could save at least NZ$ 70 k/ha, but this is likely to vary with plant species traits and site conditions.     

Competition by, and chemical control of, natural weed populations in spring-sown field beans (Vicia faba)

A field experiment was done in 1974 to determine the effect of the presence of weeds for different periods on the yield of field beans (Vicia faba cv. Maris Bead). A critical period of weed competition of 2 wk duration occurred from 3 to 5 wk after 50% crop emergence. When weeds were not controlled, seed yield was reduced by 46%, from 4–6 to 2–5 t/ha. In another experiment, in 1975, there was no evidence of a critical period. Seed yield was reduced by 48%, from 2–9 to 1–5 t/ha, when weeds were not controlled. Pre-emergence applications of simazine and of dimefuron, a new soil-residual herbicide, effectively controlled weeds and bean seed yields were similar to those of the regularly hand-weeded control. Dinoseb-acetate and early, but not late, post-emergence applications of dimefuron controlled weeds satisfactorily. Dimefuron is potentially useful in spring-sown field beans because it can be applied pre- or early post-emergence without damaging the crop.     

Persistence and Suppressiveness of Pasteuria penetrans to Meloidogyne arenaria Race

The long-term persistence and suppressiveness of Pasteuria penetrans against Meloidogyne arenaria race 1 were investigated in a formerly root-knot nematode suppressive site following 9 years of continuous cultivation of three treatments and 4 years of continuous peanut. The three treatments were two M. arenaria race 1 nonhost crops, bahiagrass (Paspalum notatum cv. Pensacola var. Tifton 9), rhizomal peanut (Arachis glabrata cv. Florigraze), and weed fallow. Two root-knot nematode susceptible weeds commonly observed in weed fallow plots were hairy indigo (Indigofera hirsuta) and alyce clover (Alysicarpus vaginalis). The percentage of J2 with endospores attached reached the highest level of 87% in 2000 in weed fallow, and 63% and 53% in 2002 in bahiagrass and rhizomal peanut, respectively. The percentage of endospore-filled females extracted from peanut roots grown in weed fallow plots increased from nondetectable in 1999 to 56% in 2002, whereas the percentages in bahiagrass and rhizomal peanut plots were 41% and 16%, respectively. Over 4 years, however, there was no strong evidence that endospores densities reached suppressive levels because peanut roots, pods, and pegs were heavily galled, and yields were suppressed. This might be attributed to the discovery of M. javanica infecting peanut in this field in early autumn 2001. A laboratory test confirmed that although the P. penetrans isolate specific to M. arenaria attached to M. javanica J2, no development occurred. In summary, P. penetrans increased on M. arenaria over a 4-year period, but apparently because of infection of M. javanica on peanut at the field site root-knot disease was not suppressed. This was confirmed by a suppressive soil test that showed a higher level of soil suppressiveness than occurred in the field (P ≤ 0.01).     

Faba bean grain yield, N2 fixation, and weed infestation in a long-term tillage experiment under rainfed Mediterranean conditions

Background and aims

Long-term experiments could provide valuable information to determine the effects of an agronomic practice on agro-ecosystem productivity and stability. This study evaluated the long-term (18-year) impact of different tillage systems on faba bean (Vicia faba L.) productivity, including weed and broomrape incidence, and N₂ fixation.

Methods

The experiment was carried out on a Vertisol under rainfed Mediterranean conditions. It was set up as a strip-plot design. The tillage systems were: conventional tillage (CT) with moldboard plow, reduced tillage (RT) with chisel plow, and no tillage (NT). Nitrogen fixation was estimated over 2?years in the final phase of the experiment using the ¹⁵N isotope dilution technique.

Results

On average, grain yield was 31?% and 23?% higher under NT than under CT and RT, respectively; however, the effect of tillage varied by year. The yield advantage of NT over CT was pronounced when rainfall was scarce. Average broomrape infestation was lower under NT than under CT, but a reduction in tillage intensity resulted in an increase in weed biomass. Tillage had no effect on weed richness, but the use of the NT technique progressively altered the weed composition by determining conditions conducive to the growth of a few weed species that are hard to control. Nitrogen fixation was strongly influenced by tillage, being higher under NT than under CT.

Conclusions

The results suggest that NT is a valuable option in the rainfed cereal–legume rotation systems of Mediterranean environments; however, the success of NT technique depends on the use of effective weed control strategies.     

Field host-specificity of the mile-a-minute weevil,Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae)

The safe practice of biological control relies, in part, on an accurate evaluation of a potential agent’s host-specificity via testing through a “filter of safety”. The results of laboratory tests may differ from those obtained in open field host-specificity tests, where agents are able to use their full range of host-selection behaviors. It was hypothesized that Rhinoncomimus latipes (Coleoptera: Curculionidae), the biological control agent released against mile-a-minute weed, Persicaria perfoliata (Polygonaceae), would not feed or oviposit on nontarget plants in a two-phase, open field setting. Ten weevils were placed at the base of each of 13 test plant species in a randomized complete block design with six replicates. Weevils placed at the base of mile-a-minute weed were marked with yellow fluorescent dust, and yellow weevils were subsequently found only on mile-a-minute. Weevils placed at the base of nontarget plants (marked with red fluorescent dust) rapidly colonized mile-a-minute weed. Three hours after release, the number of R. latipes found on mile-a-minute weed was significantly higher than predicted by a random distribution of weevils on all test plants. The likelihood of finding more weevils on mile-a-minute compared to nontarget plant species was 31.0% at 3 h and increased to 96.5% at 44 h after release. Whereas prerelease studies showed feeding at low levels on 9 of the 13 plant species tested here, under open field conditions R. latipes did not feed on any nontarget plant species and dispersed from these plants. In an open field setting, where the weevil was able to use its full range of host-selection behaviors, there was no observed risk of nontarget effects for any species tested.     

Assessment of plant community restoration following Tree-of-Heaven (Ailanthus altissima) control by Verticillium albo-atrum

Tree-of-Heaven (TOH) is a highly invasive woody species incurring substantial investment in control efforts across its extensive adventive range. A recently isolated strain of the fungus Verticillium albo-atrum has been found to cause near 100 % mortality of TOH in laboratory and field tests. We assessed plant communities in experimentally infected TOH stands 5–6 years post treatment and compared them to uninfected control stands. We found no statistically significant differences in introduced or native species cover between treated and control stands. Healthy TOH stands often harbor substantial populations of native and introduced invasive species, and on average, successful control of overstory TOH by V. albo-atrum did not alter vegetation cover in these communities. V. albo-atrum appears to be a promising tool in targeted control of TOH that carries a relatively low risk of opportunistic weed replacement.     

Allelopathic interference of sweet potato with cogongrass and relevant species

Cogongrass (Imperata cylindria) is an invasive weed and harmful to ecological systems and agricultural production in many countries. It was found that plant extracts and root exudates of sweet potato (Ipomoea batatas) exhibit allelopathic potential and inhibit the growth of cogongrass to a greater extent than either barnyardgrass (Echinochloa crus-galli), Indian goose-grass (Eleushine indica), or lettuce (Lactuca sativa) in bioassays. Greenhouse trials indicated that sweet potato soil reduced the emergence of the noxious weed by 50 %, yet exhibited either weaker inhibition or the promotion of barnyardgrass, Bidens (Bidens pilosa), and Leucaena (Leucaena leucocephala), while the desired growth of upland rice (Oryza sativa) was not affected. In cogongrass fields, the incorporation of 1–2 tons aboveground parts and cultivation of sweet potato provided 80–85 % weed control. On the other hand, the reduction of congograss in fields may be offset by the alternate invasion of B. pilosa which multiplied its biomass by 2–6 times with sweet potato amended soils. The findings of this study indicate that sweet potato is an effective crop in the biologic management of the invasive cogongrass in agricultural fields, thus the interactive mechanism between sweet potato and the invasive weed demands further investigation. Ecologically, this study highlights the specificity of allelopathic interactions between cogongrass and sweet potato that is helpful to minimize the disturbance from infestation of this invasive weed against native species and crops.     

Multiple introductions of <Emphasis Type="Italic">Sirex noctilio</Emphasis> (Hymenoptera: Siricidae) in northeastern North America based on microsatellite genotypes,and implications for biological control

The invasive woodwasp Sirex noctilio (Hymenoptera: Siricidae) has been moved from Eurasia into regions in the Southern Hemisphere, where extensive tree mortality has occurred in pines (Pinus spp.) introduced for forestry. More recently this woodwasp was found in northeastern North America, where pines are native, and it is a species of concern due to the economic importance of pines. Understanding the genetic diversity of North American S. noctilio points to new areas of inquiry, particularly regarding the ability of parasitic nematodes to sterilize woodwasps, which could provide control methods in the US and/or Canada. We investigated the genetic diversity of 924 S. noctilio from nine populations from New York and Pennsylvania (US), Ontario (CA), and Queensland (AU) using nine microsatellite loci. To avoid inflating the number of populations estimated by Bayesian inference, we measured the full-sibling relationships of woodwasps within 13 trees and removed all but one member of each full-sib family from the genetic analysis, resulting in a final sample size of 741 S. noctilio. Within a tree, on average 39% of woodwasps did not have a full sibling, and there were 5.6 families with at least two full-sibling members per tree. The mean family size across trees was 1.9 when single offspring (i.e., no full siblings) were included. Given the short time span since invasion, variation within North American S. noctilio is likely due to differences among founding genotypes. Genetic analyses support the hypothesis that at least two separate introductions occurred. Within North America, genetic distance measures were greatest between a site in southwestern Ontario and all other sites, suggesting that this population could represent a separate introduction event. Two methods of Bayesian clustering also support this idea; they detected 4 or 5 distinct genetic clusters with little admixture between the southwestern Ontario site and other North American populations. The wasps from Australia, where biological control with nematodes has been successful, showed low genetic diversity and clustered with the southwestern Ontario population in one out of two Bayesian analyses. Within the Ontario subset of samples, high woodwasp activity level (i.e., attack and mortality of trees) was associated with one genetic cluster more strongly than another. Population variation should be taken into account in studies of S. noctilio spread and management within North America.     

Effects of Dosage Sequence on the Efficacy of Nonfumigant Nematicides, Plantain Yields,and Nematode Seasonal Fluctuations as influenced by Rainfall

Four nonfumigant nematicides applied three times during the wet season were used to study dosage sequence and nematicide effectiveness. Control of Helicotylenchus multicinctus (Cobb) Thorne and Meloidogyne javanica (Treub) Chitwood increased plantain (Musa AAB) yields. The nematicide (aldicarb, carbofuran, oxamyl, and miral) performance and yield response varied with dosage sequences. Applications of 2, 3, and 2 g ai/tree in March, July, and October (sequence I), respectively, gave greater control of M. javanica than did applications of 3, 2, and 2 g ai/tree in March, June, and September (sequence II), respectively. However, the high initial dose sequence was effective against H. multicinctus. Persistence of the different nematicides differed over the 14-month experimental period. Miral, aldicarb, and carbofuran were the most effective treatments against either species by the end of the wet and dry seasons. Dry season residual nematode populations were significantly lower in nematicide treated than in control plots. Yield increases over controls were 96.9, 90.1, 78.4, and 70.1% for carbofuran applied by sequence II, aldicarb by II and I, and oxantyl by II, respectively. Nematode populations directly fluctuated with rainfall and dropped to low (H. multicinctus) or to undetectable (M. javanica juveniles) levels during the dry season. Of the two nematodes studied, the more serious pest to plantain was H. multicinctus; it was tolerant to drought and survived the dry season in untreated soils.     

Restoring a seasonal wetland using woven black plastic weed mat to overcome a weed threshold

Little information is available on the use of woven black polypropylene weed control mat in ecological restoration. At a 6.5‐ha area of fertile Vertosol soil ex‐farmland near Perth, Western Australia, concerted efforts to control weed using conventional methods such as herbicides, fire and cultivation proved ineffective. After 5 years, weeds still dominated the site, and native plant establishment was poor. Small‐scale preliminary trials of various weed suppression coverings were then undertaken, with plastic weed mat the most cost‐effective in overcoming the weed threshold, permitting native tree seedling establishment. In a larger‐scale trial of weed mat over the whole site, weeds were controlled and high levels of native plant establishment achieved, with a diverse range of understorey, midstorey and overstorey species providing 56% projected foliage cover. This ensured that completion criteria were finally satisfied. These results suggest that weed mat may be effective for weed control in large‐scale restoration where conventional methods have failed, as long as ultimate removal or decomposition and other issues are addressed.     

Effects of increasing weed-beet density on sugar-beet yield and quality

Weed beets are an increasing problem in many sugar-beet crops in many countries. At present about one sugar-beet field in four in England is infested with weed-beet seed. Control in other crops can be achieved using selective herbicides but in sugar beet the weed beets, many of which are of annual habit, are not easily controlled and often compete with the crop. Experiments were done to quantify the yield loss caused by weed beet in sugar-beet crops. Transects were laid out across three fields in 1985 and 1986 and plots located thereon to include the range of weed-beet densities found in the field. Weed beet did not affect the concentration of sugar (sucrose), potassium, sodium, α amino nitrogen or invert sugar in the crop beets. Root and sugar yields were progressively reduced by increasing densities of weed beet. A rectangular hyperbola described the data slightly better than an asymptotic model. There was no indication of a threshold density of weed beet below which there was no yield loss, which averaged 11.7% for each weed beet plant/m². This corresponds to an average 0.6% sugar yield loss for each 1% of bolted weed beet in the root crop up to 100%, which is similar to the reported losses resulting from bolters in the root crop.     

Integrated control of two spotted spider mite,Tetranychus urticae on cucumber grown under plastichouse conditions in egypt

Microclimatic conditions were monitored in cucumber grown under unheated plastichouses in three consecutive years, i.e. from 1990 to 1992 in Egypt. The relative humidity in the plastichouses constructed on either sandy or clay soil did not exceed 30% during mid-day time, while the average temperature did not drop below 30°C. Such conditions favoured population build up of the two spotted spider miteTetranychus urticae Koch. In 1993 and 1994, three plastichouses planted to cucumber, cultivar Rawa or Hana were selected. Relative humidity in two of them was maintained over 50% during the day using spraying nozzles, the third being kept as control. Mildew diseases induced by increased leaf wetness needed to be managed using fungicides.Phyioseiulus persimilis andP. longipes were released in the first two plastichouses, while acaricides were applied in the control. In early MayT. urticae population increase on the growing top of cucumber plants required two acaricidal applications as predatory mites did not migrate to the apical leaves which were too warm and dry. More than 80% of the maximum yield was obtained on average in plastichouses using increased relative humidity, but without any acaricidal applications.     

啮齿动物捕食压力下生境类型和覆盖处理对辽东栎种子命运的影响

在六盘山区的退化辽东栎灌丛和辽东栎次生林 (以下分别称“灌丛样地“和“次生林样地”),研究了群落生境和清除凋落物、凋落物覆盖及土壤覆盖等处理对啮齿动物取食和搬运/贮藏辽东栎种子的影响。结果表明：(1) 凋落物和土壤覆盖处理在种子释放的前期阶段有利于种子留存,但释放40天后,种子在释放点的最终留存率在灌丛样地显著高于次生林样地 (Z = -2.333, P = 0.020)。灌丛样地的自然状态的种子最终留存率显著高于凋落物覆盖处理 (Z = -0.674,P = 0.05),但其它处理间无显著差异;次生林样地的凋落物覆盖和清除凋落物处理种子的最终留存率为0%,自然状态和土壤覆盖处理均不足1%,各处理间无显著差异。(2) 种子被啮齿动物的就地取食率在不同群落生境间差异显著 (Z = -2.333,P = 0.020);在灌丛样地,凋落物覆盖处理种子被啮齿动物就地取食率最高 (45.56%),而次生林样地的土壤覆盖处理最高 (64.81%),种子被啮齿动物就地取食率在灌丛样地和次生林样地均为清除凋落物处理最低 (分别为23.70%和40.00%);生境类型和覆盖处理对种子搬运后的取食率均无显著影响。(3) 次生林样地种子被啮齿动物搬运后的埋藏率显著高于灌丛样地 (Z = -2.88,P = 0.004);在灌丛样地,土壤覆盖处理种子搬运后的埋藏率最高仅5.56%,而在次生林样地,清除凋落物处理最高达9.26%。(4) 种子被啮齿动物搬运后取食的距离在灌丛样地和次生林样地分别为3.01 m和2.13 m,差异显著 (Z = -2.080,P = 0.038),而种子搬运后的埋藏距离前者小于后者 (分别为1.35 m和2.10 m),两者间差异不显著。     

The impact of elevated CO2 on yield loss from a C3 and C4 weed in field-grown soybean

Soybean (Glycine max) was grown at ambient and enhanced carbon dioxide (CO₂, + 250 μL L^?1 above ambient) with and without the presence of a C3 weed (lambsquarters, Chenopodium album L.) and a C4 weed (redroot pigweed, Amaranthus retroflexus L.), in order to evaluate the impact of rising atmospheric carbon dioxide concentration [CO₂] on crop production losses due to weeds. Weeds of a given species were sown at a density of two per metre of row. A significant reduction in soybean seed yield was observed with either weed species relative to the weed‐free control at either [CO₂]. However, for lambsquarters the reduction in soybean seed yield relative to the weed‐free condition increased from 28 to 39% as CO₂ increased, with a 65% increase in the average dry weight of lambsquarters at enhanced [CO₂]. Conversely, for pigweed, soybean seed yield losses diminished with increasing [CO₂] from 45 to 30%, with no change in the average dry weight of pigweed. In a weed‐free environment, elevated [CO₂] resulted in a significant increase in vegetative dry weight and seed yield at maturity for soybean (33 and 24%, respectively) compared to the ambient CO₂ condition. Interestingly, the presence of either weed negated the ability of soybean to respond either vegetatively or reproductively to enhanced [CO₂]. Results from this experiment suggest: (i) that rising [CO₂] could alter current yield losses associated with competition from weeds; and (ii) that weed control will be crucial in realizing any potential increase in economic yield of agronomic crops such as soybean as atmospheric [CO₂] increases.