Bacterial 5-nucleotidase in aquatic ecosystems: a novel mechanism of phosphorus regeneration

Zooplankton excretion and algal alkaline phosphatase are presumed to be responsible for phosphorus recycling in aquatic ecosystems; the role of bacteria has been unclear. High levels of bacterial cell-surface 5-nucleotidase were discovered in samples of picoplankton from California coastal waters. 5-Nucleotidase rapidly generated orthophosphate from 5-nucleotide added in nanomolar amounts and could supply half the orthophosphate required by plankton. Unlike alkaline phosphatase, 5-nucleotidase was not inhibited by orthophosphate at any concentration found in aquatic environments. Initial results indicate even greater 5-nucleotidase activity in fresh water (Lake Hodges, California) and brackish water (Baltic). Release and uptake of orthophosphate were tightly coupled.     

Effects of ammonium and nitrate on mineralization of nitrogen from leguminous residues

A laboratory incubation experiment was conducted to compare the effects of NH _inf4 ^sup+ and NO _inf3 ^sup- on mineralization of N from ¹⁵N-labelled vetch (Vicia villosa Rotn) in an Illinois Mollisol, and to determine the effect of a nitrification inhibitor (nitrapyrin) on mineralization of vetch N when used with NH _inf4 ^sup+ . The addition of either NH _inf4 ^sup+ or NO _inf3 ^sup- (100 and 200 mg N kg^-1 soil) significantly increased mineralization of vetch N during incubation for 40 days. The effect was greater with NH _inf4 ^sup+ than with NO _inf3 ^sup- , and a further increase occurred in the presence of nitrapyrin (10 mg kg^-1 soil). The addition of NO _inf3 ^sup- retarded the nitrification of NH _inf4 ^sup+ -N derived from vetch.     

Immobilization of ammonium and nitrate and their interaction with native N in three Illinois Mollisols

Summary Three Illinois Mollisols were incubated for 2 weeks at 25°C after treatment with different amounts of glucose and/or ¹⁵N-labelled (NH₄)₂SO₄ or ¹⁵N-labelled KNO₃. The objectives were: (1) to compare the immobilization and interaction of NH _inf4 ^sup+ –N and NO _inf3 ^sup- –N with the native soil N, and (2) to study the relationship between immobilization of applied N and the added N interaction. As determined, immobilized N refers to forms not extractable with 2 MKCl (immobilized ¹⁵N+clay-fixed ¹⁵NH _inf4 ^sup+ ). In all cases, both NH _inf4 ^sup+ –N and NO _inf3 ^sup- –N were actively immobilized and transformed into organic forms in the presence of glucose. In the absence of glucose, a higher proportion of NH _inf4 ^sup+ than NO _inf3 ^sup- was recovered in organic forms. Although the three soils differed considerably in the amounts of applied N immobilized, similar trends in N immobilization were observed. A positive added N interaction occurred with all soils, the magnitude increasing with the rate of N addition. In the absence of glucose, higher added N interactions were obtained for NH _inf4 ^sup+ than NO _inf3 ^sup- , whereas there was very little difference between NH _inf4 ^sup+ and NO _inf3 ^sup- in the presence of glucose. The results indicate that under conditions of rapid immobilization (e.g., in the presence of glucose), NH _inf4 ^sup+ and NO _inf3 ^sup- will show comparable interaction with the native soil N, whereas in unamended soil, the extent of this interaction will be greater with NH _inf4 ^sup+ than with NO _inf3 ^sup- . Significant correlations were observed between applied N immobilized and the added N interaction only in one soil having a high initial mineral N content.     

The indigenous hill-farming system of Khasia tribes in moulvibazar district of Bangladesh: Status and impacts

Rapid economic growth of the Khasia people has resulted from a most successful betel leaf farming system practiced within the fringe of reserved forests, where government policies have facilitated effective use of the local people as a labour force for production, protection and conservation of biodiversity of the surrounding forests. Khasia tribes have traditionally grown betel leaf plants on naturally occurring trees. Deforestation is a serious problem in Bangladesh, whereas the Khasia people living within forests are protecting trees for their livelihood, including selling betel leaf, collecting fuelwood and consuming and selling fruits from support trees. It is a profitable yet sustainable forest production system, maintaining soil fertility, stable production and optimal family size, and has created employment opportunities for the people living within and outside the forests. It has enhanced the supply of socially required betel leaf to the local markets, contributed to price stability, and generated some export revenue. However, the revenue of growers has been reduced by plant diseases and the capture of resource rent by middlemen. Economic benefits could be further increased through government initiatives to improve management and the marketing system. This paper is based on a presentation at the International Conference on Economics of Sustainable Forest Management, University of Toronto, Canada, May 20–22, 2004.     

Effect of some pesticides on plant growth,root nodulation and chlorophyll content of chickpea

Different concentrations, viz. 5, 10 and 25 ppm of aldicarb, carbofuran, phorate fensulfothion and fenamiphos were observed for their pesticidal effects on chickpea plant in terms of various plant growth parameters such as plant length as well as weight, pod numbers, root-nodulations and chlorophyl content. Significant improvement in plant growth was noted in lower concentrations such as 5 and 10 ppm of different pesticides but 5 ppm concentration proved highly effective and non-phytotoxic. The phytotoxic effect was noted in those plants treated with 25 ppm concentration of all the pesticides. Carbofuran was found to be most effective for improvement of over all plant growth and fensulfothion the least.     

Intercropping with garlic alleviated continuous cropping obstacle of cucumber in plastic tunnel

Abstract

Continuous cropping obstacle is prevalent for protected cultivated cucumber in China. Intercropping garlic may effectively relieve this obstacle due to its allelopathic and antimicrobial effects. A two-growing season investigation was carried out during autumn 2009 to spring 2010 in plastic tunnel to determine the effects of intercropped garlic and green garlic on the overall growth of cucumber and soil biological properties. Results showed that green garlic exhibited the inhibitory effect on the growth of cucumber in spring cultivation 2010. Garlic–cucumber intercropping system increased yield of cucumber differently depending on garlic cultivars, with only cv. G005 showing significant increase (13.4%). Intercropping systems were evaluated as a greater net benefit system as compared to monoculture cultivation. Populations of soil bacteria and actinomyces were stimulated, while fungi were inhibited under intercropping system. Activities of soil invertase, urease, and alkaline phosphatase were encouraged under intercropping system in spring cultivation 2010 compared with monoculture. The promotion effect of intercropped garlic on urease and alkaline phosphatase maintained till garlic harvest. These results suggest that intercropping system can improve soil biology environment and alleviate continuous cropping obstacle of cucumber at different levels.     

COMPARATIVE STUDY OF DIFFERENT SALTS (SODIUM CHLORIDE,SODIUM SULFATE,POTASSIUM CHLORIDE,AND POTASSIUM SULFATE) ON GROWTH OF FORAGE SPECIES

Osmotic and specific ion effects are the most frequently mentioned mechanisms by which saline substance reduces plant growth. However, the relative importance of osmotic and specific ion effect on plant growth seems to vary depending on the salt tolerance of the plant under study. Tall wheatgrass (TW), perennial ryegrass (PR), African millet (AM) and Rhodesgrass (Rh) were grown in nutrient solution with sodium chloride (NaCl), sodium sulfate (Na₂SO₄), potassium chloride (KCl), and potassium sulfate (K₂SO₄) salinity up to electrical conductivity (EC) 27 dS m^?1. Growth of all plant species decreased significantly at high level (EC 27 dS m^?1) of NaCl and Na₂SO₄ salts. However, the growth of none of the plant species was affected significantly by KCl and K₂SO₄ at any level. Even leaf and shoot fresh weights were enhanced by K₂SO₄ in all plant species, except AM. Chlorine (Cl) was taken up in similar quantities from KCl and NaCl solutions and the content of the respective cations was similar to each other. Further sensitivity to sulfate and chloride was equal when sodium concentrations in shoots were equal, regardless of the anion composition of the media. The sodium (Na) concentration of the leaves of the plant species increased with increased NaCl and Na₂SO₄ levels in the nutrient solutions. The leaf Na concentration of TW was lower than that of the other plant species. However, the root Na concentration of TW was higher than that of the other plant species. Increased NaCl and Na₂SO₄ concentrations had a marked effect on leaf water potential of all plant species, and the TW showed higher leaf water potential at all levels of salts. Tall wheatgrass adjusted osmotically by accumulating electrolytes from the nutrient solution and by accumulation of glycinebetaine. Sodium was generally found more injurious than Chloride in all the four forage species. Salt tolerance could be ascribed as greater exclusion of Na ion.     

Strategic tillage in Australian conservation agricultural systems to address soil constraints: How does it impact weed management?

In the conservation agricultural systems practised in Australia, cultivation is not commonly utilised for the purpose of weed control. However, occasional use of tillage (strategic tillage) is implemented every few years for soil amelioration, to address constraints such as acidity, water repellence or soil compaction. Depending on the tillage method, the soil amelioration process buries or disturbs the topsoil. The act of amelioration also changes the soil physical and chemical properties and affects crop growth. While these strategic tillage practices are not usually applied for weed control, they are likely to have an impact on weed seedbank burial, which will in turn affect seed dormancy and seedbank depletion. Strategic tillage impacts on seed burial and soil characteristics will also affect weed emergence, plant survival, competitive ability of weeds against the crop and efficiency of soil applied pre-emergent herbicides. If growers understand the impacts of soil amelioration on weed demography, they can more effectively plan management strategies to apply following the strategic tillage practice. Weed seed burial resulting from a full soil inversion is understood, but for many soil tillage implements, more data is needed on the extent of soil mixing, burial of topsoil and the weed seedbank, physical control of existing weeds and stimulation of emergence following the tillage event. Within the agronomic system, there is no research on optimal timing for a tillage event within the year. There are multiple studies to indicate that strategic tillage can reduce weed density, but in most studies, the weed density increases in subsequent years. This indicates that more research is required on the interaction of amelioration and weed ecology, and optimal weed management strategies following a strategic tillage event to maintain weeds at low densities. However, this review also highlights that, where the impacts of soil amelioration are understood, existing data on weed ecology can be applied to potentially determine impacts of amelioration on weed growth.     

Assessment of the oil content of the seed produced by <Emphasis Type="Italic">Salicornia</Emphasis> L., along with its ability to produce forage in saline soils

Soil salinity presents a serious threat to crop production. The relatively poor tolerance to this stress agent exhibited by conventional crops incentivizes the search for alternative producers of food and forage in salinity-affected environments. Halophytes belonging to the genus Salicornia L. have been suggested as being able to provide both forage and edible seed oil. Here, a set of 14 Salicornia europaea L. accessions was investigated for their ability to produce seed oil and forage in response to a range of salt concentrations (100, 300 and 600 mM NaCl) in the water used for irrigation. Seed of the accessions was collected from diverse sites close to saline rivers and the sea in Iran. Salinity was shown to have a major effect on biomass yield, and on seed oil yield and composition. The ratio of unsaturated to saturated fatty acids in the seed oil was remarkably high. Forage yield was highest when irrigated with 300 mM NaCl for most of the accessions, while a level of 600 mM NaCl suited the production of seed oil.     

Comparison of two versions of the acetylene inhibition/soil core method for measuring denitrification loss from an irrigated wheat field

 Two versions of the acetylene inhibition (AI)/soil core method were compared for the measurement of denitrification loss from an irrigated wheat field receiving urea-N at a rate of 100 kg ha^–1. With AI/soil core method A, the denitrification rate was measured by analysing the headspace N₂O, followed by estimation of N₂O dissolved in the solution phase using Bunsen absorption coefficients. With AI/soil core method B, N₂O entrapped in the soil was measured in addition to that released from soil cores into the headspace of incubation vessels. In addition, the two methods were also compared for measurement of the soil respiration rate. Of the total N₂O produced, 6–77% (average 40%) remained entrapped in the soil, whereas for CO₂, the corresponding figures ranged from 12–65% (average 44%). The amount of the entrapped N₂O was significantly correlated with the water-filled pore space (WFPS) and with the N₂O concentration in the headspace, whereas CO₂ entrapment was dependent on the headspace CO₂ concentration but not on the WFPS. Due to the entrapment of N₂O and CO₂ in soil, the denitrification rate on several (18 of the 41) sampling dates, and soil respiration rate on almost all (27 of the 30) sampling dates were significantly higher with method B compared to method A. Averaged across sampling dates, the denitrification rate measured with method B (0.30 kg N ha^–1 day^–1) was twice the rate measured with method A, whereas the soil respiration rate measured with method B (34.9 kg C ha^–1 day^–1) was 1.6 times the rate measured with method A. Results of this study suggest that the N₂O and CO₂ entrapped in soil should also be measured to ensure the recovery of the gaseous products of denitrification by the soil core method. Received: 12 May 1998