土壤重金属污染的酶学诊断:紫色土Cd、Cu、Pb和As对水稻根系脱氢酶的影响

本文通过酸性紫色土和石灰性紫色土中不同浓度Cd、Cu、Pb、As对水稻根系脱氢酶的影响研究,揭示了脱氢酶受抑制与产生抗性的过程。在低浓度时,土壤Cd、Cu、Pb、As对脱氢酶的影响较敏感,能因元素的不同性质反映土壤类型影响的差别。最后,以脱氢酶受抑制与初始抗性峰出现的转折点相应的土壤浓度为依据,确定了两种紫色土Cd、Cu、Pb、As的临界浓度。     

Dynamics of Belonolaimus longicaudatus Parasitism on a Susceptible St. Augustinegrass Host

St. Augustinegrass (Stenotaphrum secundatum) cv FX-313 was used as a model laboratory host for monitoring population growth of the sting nematode, Belonolaimus longicaudatus, and for quantifying the effects of sting nematode parasitism on host performance in two samples of autoclaved native Margate fine sand with contrasting amounts of organic matter (OM = 7.9% and 3.8%). Following inoculation with 50 Belonolaimus longicaudatus per pot, nematodes peaked at a mean of 2,139 nematodes per pot 84 days after inoculation, remained stable through 168 days at 2,064 nematodes per pot, and declined at 210 days. The relative numbers of juveniles and adults demonstrated senescence after 84 days. Root dry weight of nematode-inoculated plants increased briefly to an apparent equilibrium 84 days after inoculation, whereas root weights of uninoculated controls continued to increase, exceeding those of inoculated plants from 84 to 210 days (P < 0.01). At 210 days, uninoculated plants had 227% the root dry weight of inoculated plants. Transpiration of FX-313 was reduced by nematodes (P < 0.0001) at 84 and 126 days after inoculation; reduction was first observed at 42 days and last observed 168 days after inoculation (P < 0.05). OM content affected all plant performance variables at multiple dates, and generally there were no inoculation x OM content interactions. OM content had no effect on nematode numbers per pot, although there was a slight (P < 0.05) increase in the number of nematodes per gram root dry weight in the low-OM soil compared with the high-OM soil.     

Survival and Infectivity of Bursaphelenchus xylophilus in Wood Chip-Soil Mixtures

To determine the effect of soil environment on the life stages and total numbers of Bursaphelenchus xylophilus, nematode-infested wood chips alone and mixed with soil were incubated at 12 and 20 C. Nematodes were extracted at 2-week intervals for 12 weeks. Numbers of nematodes and percentage of third-stage dispersal larvae were greater at 12 C and in chips without soil. Percentage of juveniles of the propagative cycle was greater at 20 C and in chips with soil. Although B. xylophilus survived in chips with soil for 12 weeks, nematode numbers and life stage percentages changed little over time. To determine if B. xylophilus was capable of infecting wounded roots, infested and uninfested chips were mixed with soil in pots with white and Scots pine seedlings. Trees were maintained at 20 and 30 C and harvested at mortality or after 12 weeks. Only seedlings treated with infested chips contained nematodes. In field experiments, planted seedlings were mulched with infested chips to determine if nematodes would invade basal stem wounds. Among these trees, Scots pine was more susceptible than white or red pines to infection and mortality.     

Effect of excess boron on summer and winter squash

The boron tolerance of two summer squash cultivars (Cucurbita pepo L. Aristocrat Zucchini and Peter Pan Scallop) and one winter squash cultivar (Cucurbita moschata Poir. Butter boy) was determined in large, outdoor sand cultures. Boron treatments were imposed by irrigation with culture solutions that contained 1.0, 3.0, 6.0, 9.0, 12.0, or 15.0 mg B L^-1. Relative fruit yields of Zucchini, Scallop, and Butter boy were reduced 5.2%, 9.8%, and 4.3% with each unit (mg L^-1) increase in soil solution B (B_sw)>2.7, 4.9, and 1.0 mg B L^-1, respectively. Reduced yields of all cultivars were attributed to a reduction in fruit number and not fruit size. Boron concentrations in leaves and fruit were directly correlated to B_sw.     

13C isotope discrimination correlates with biological nitrogen fixation in soybean (Glycine max (L.) Merrill)

Ammonium sulphate was applied at the rate of 300 kg N ha^–1 with or without the nitrification inhibitor 1-carbamoyl-3(5)-methylpyrazol (4 kg ha^–1) to plots measuring 1.5 × 1.5 m. The fertilizer and the inhibitor were washed into the top 15-cm layer of the soil, which was highly calcareous (55% CaCO₃), and the plots were kept bare. The process of nitrification was monitored by regular soil sampling. In the absence of the inhibitor, nitrification was completed in three weeks. In the presence of the inhibitor only 10% of applied N was nitrified by the end of the third week and 42% by the end of the eighth week. Average soil temperature at 5–, 10– and 20-cm depth over the first six weeks was 26.0, 24.8 and 24.2°C, respectively.     

Ammonia volatilization after injection of anhydrous ammonia into arable soils of different moisture levels

Laboratory experiments have shown appreciable losses of ammonia after injection of anhydrous ammonia into dry and wet soils. In this study losses of ammonia injected into a moist (tension 10 kPa), dry (tension 160 kPa) and a wet (tension 1.6 kPa) sandy loam were measured under field conditions using wind tunnels. Losses were insignificant from a moist soil. However losses from a dry and a wet soil were 20% and 50% of injected ammonia, respectively. From the dry soil, losses of gaseous ammonia took place within the first hours after injection, which indicates a rapid transport through cracks and voids. From the wet soil, 20% of the injected ammonia was lost more gradually between 6 h and 6 d. This indicates that upward movement of water due to evaporation may be the cause of these ammonia losses which proceeded for longer periods.     

Influence of root diameter on the penetration of seminal roots into a compacted subsoil

A field experiment was conducted to evaluate the influence of root diameter on the ability of roots of eight plant species to penetrate a compacted subsoil below a tilled layer. The soil was a fine sandy loam red-brown earth with a soil strength of about 3.0 MPa (at water content of 0.13 kg kg^-1, corresponding to 0.81 plastic limit) at the base of a tilled layer. Relative root diameter (RRD), which was calculated as the ratio of the mean diameters of roots of plants grown in compacted soil to the mean diameters of those from uncompacted soil, was used to compare the sensitivity of roots to thicken under mechanical stress.Diameters of root tips of plants grown in soil with a compacted layer were consistently larger than those from uncompacted soil. Tap-rooted species generally had bigger diameters and RRDs than fibrous-rooted species. A higher proportion of thicker roots penetrated the strong layer at the interface than thinner roots. There were differences between plant species in the extent to which root diameter increased in response to the compaction. The roots which had larger RRD also tended to have higher penetration percentage.The results suggest that the size of a root has a significant influence on its ability to penetrate strong soil layers. It is suggested that this could be related to the effects which root diameter may have on root growth pressure and on the mode of soil deformation during penetration.     

Relationships between take-all,soil conduciveness to the disease,populations of fluorescent pseudomonads and nitrogen fertilizers

Take-all of wheat, caused by Gaeumannomyces graminis var tritici (Ggt), is reduced by ammoniacal fertilizers as compared to nitrate sources. This influence of nitrogen on the disease is only observed on nodal roots at flowering. But soil conduciveness to take-all, as measured in a soil bioassay, is modified earlier. Forty days after nitrogen application at early tillering, the NH₄-treated soil became less conducive than the NO₃-treated one. When nitrogen applications are done at sowing and at tillering, differences in disease propagation between the two soils are enhanced. Results from four years of experimentation show that when the level of natural soil inoculum is high, disease severity is reduced by ammonium, showing an effect on the parasitic phase of Ggt. At a low level of natural inoculum the effect of the source of nitrogen is mainly observed on the percent of infected plants, indicating that the saprophytic and preparasitic phases are affected. Rhizospheric bacterial populations increase from sowing to tillering, but differences on take-all conduciveness after tillering are not correlated with differences in the amounts of aerobic bacteria or fluorescent pseudomonads isolated from soils treated with different sources of nitrogen. Qualitative changes in fluorescent Pseudomonas spp. populations, like in vitro antagonism, are more likely to explain differences in soil conduciveness to take-all than are quantitative changes in this group. Nevertheless, the introduction of Ggt in a cropped soil leads to a greater increase in fluorescent pseudomonads populations than in total aerobic bacteria.The delay between reducing soil conduciveness and reducing disease in the field with ammonium nitrogen fertilization, the qualitative change of fluorescent pseudomonads populations and the role of necroses in rhizobacteria multiplication, provide information leading to our representation of a dynamic model based on the differentiation of the wheat root system into seminal and nodal roots.     

Effects of legumes on soil physical quality in a maize crop

The effect of intercropped legumes and three N fertilizer rates in a continuous maize (Zea mays L.) cropping system on the physical properties of two soils were investigated for three years. The legumes, being a mixture of alfalfa, clover and hairy vetch, had a significant cumulative effect on some physical properties of both soil. The lowest stability and smallest mean weight diameter of soil aggregates were associated with monoculture maize plots. Aggregate size and stability were not affected by N fertilization at any of the rates of 0, 70, and 140 kg ha^-1 in intercropped plots, except that aggregate stability was actually reduced by N fertilization in one soil, the Ste. Rosalie clay. In maize plots in both soils, stability and size of soil aggregates were significantly increased with increased added N. Intercropped legumes significantly decreased dry bulk density and soil penetration resistance. Added N had no measurable influence on these compaction factors. Soil water properties were not significantly affected by either intercropping or N fertilization. Positive effects noted on soil aggregation and other physical properties in intercropped plots are the result of enhanced root activity, or incorporation of legumes as green manure, or both. Improvement of soil structure in maize plots associated with increasing N application was the result of increased maize-root residues.     

Soil acidification as caused by the nitrogen uptake pattern of Scots pine (Pinus sylvestris)

Three-year-old Scots pine (Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH₄/NO₃ uptake ratio and proton efflux. N was supplied in three NH₄-N/NO₃-N ratios, 3:1, 1:1 and 1:3, either as ¹⁵NH₄+¹⁴NO₃ or as ¹⁴NH₄+¹⁵NO₃. Total N and ¹⁵N acquisition of different plant parts were measured. Averaged over the whole tree, the NH₄/NO₃ uptake ratios throughout the growing season were found to be 4.2, 2.5, and 1.5 for the three application ratios, respectively. The excess cation-over-anion uptake value (C_a-A_a) appeared to be linearly related to the natural logarithm of the NH₄/NO₃ uptake ratio. Further, this uptake ratio was related to the NH₄/NO₃ ratio of the soil solution. From these relationship it was estimated that Scots pine exhibits an acidifying uptake pattern as long as the contribution of nitrate to the N nutrition is lower than 70%. Under field circumstances root uptake may cause soil acidification in the topsoil, containing the largest part of the root system, and soil alkalization in deeper soil layers.