Variations of Soil Nutrient and Organic Matter Content in a Subtropical Mangrove Ecosystem

Variations of soil nutrient and organic matter content along tidal gradients of a small mangrove forest in Hong Kong were investigated. Effects of sampling months on nutrient status were also analysed. Two transects, A and B, were established perpendicular to the shoreline. Transect A transverse a wider band of vegetation and was less disturbed by human activities; while Transect B was close to human settlement and all mangrove plants were more frequently flooded by incoming and outgoing tides. Surface soils at 5 and 10 m intervals along Transects A and B, respectively, were collected in December 1989, March 1990, July 1990 and September 1990. Concentrations of soil organic matter, total and extractable nitrogen, phosphorus and potassium were high in landward sites and decreased gradually along tidal gradients; while pH and salinity increased with distance from landward to seaward sites. These trends along tidal gradients indicate the importance of tidal frequency and amplitude on soil properties. Significant differences in soil properties were also found between two transects. Transect A had significantly higher organic matter and nutrient content than Transect B. In both transects, there were sites with extraordinarily high levels of nutrients and organic matter (5–10 times higher than the average values) suggesting local contamination. Most of these contaminated sites were located at the landward edges. Positive correlations between organic matter, N, P and K content were found suggesting that these nutrients were from similar input sources. Variations along tidal gradients and local contaminated sites at the landward edges suggest that human discharges, litter deposition and surface runoff were major nutrient inputs. In both transects, no definite seasonal periodicity was found and temporal variations existed in a more random manner than variations along tidal gradients.     

Effects of Landfill Gas on Growth and Nitrogen Fixation of Two Leguminous Trees (Acacia Confusa,Leucaena Leucocephala)

A study was made on the effects of landfill gas on ARA (acetylene reducing activity) of nodules of two woody legumes (Acacia confusa and Leucaena leucocephala) widespread on landfill sites in Hong Kong. The effects of the three main components of landfill gas, O₂, CO₂ and CH4, were first measured separately over a 1-hr period. Maximum ARA was found at 20% O₂ (close to atmospheric partial pressure) and ARA decreased as the O₂ decreased in the range of 16–1%. Acacia confusa nodular ARA was significantly inhibited at 30–50% CO₂, but not Leucaena leucocephala nodular ARA. CH₄ had no significant effect on ARA of either species. As the landfill gas concentrations in the landfill topsoil were mostly > 10% O₂ and < 10% CO₂, root nodules should fix N₂ effectively over these ranges of gases. A four-week test was conducted to assess the long-term influence of landfill gas on seedlings of the two legumes. Landfill gas and elevated CO₂ both suppressed their growth and their nodular ARA. Even under the influence of the gases, however, seedlings with nodules formed a higher biomass than seedlings lacking nodules. The growth of the two legumes under actual landfill conditions was investigated by transplanting non-inoculated and pre-inoculated seedlings to two landfill sites in Hong Kong: Junk Bay and Shuen Wan Landfill. After six months, most of the non-inoculated seedlings became infected: Acacia confusa 63 and 70%, Leucaena leucocephala 17 and 89%, respectively, at the test sites. The results indicate that there were free rhizobia at these landfill sites to infect the legumes and they had formed effective nodules to fix N₂ under landfill conditions.     

Mechanisms of stabilization of earthworm casts and artificial casts

Summary Fresh casts were collected from the earthworm species Aporrectodea caliginosa, and artificial casts were also made. The casts were subjected to ageing, drying-rewetting, and sterilization by hexanol vapour. Clay dispersion was determined, as a measure of the lack of stability of the casts. Two soils were used, the topsoil of a recently reclaimed polder soil in the Netherlands and the topsoil from a South Australian duplex soil. For both soils the fresh worm casts had higher dispersible clay than the artificial casts. During ageing, both types of casts became more stable. There are strong indications that this was mainly due to changes on the surface of the casts. Fungi developed on the surface of 6-day-old worm casts made of Australian soil. This gave a higher stability to the casts compared to artificial casts of the same age without fungal growth. With both types of casts, hexanol inhibited fungal growth on the surface of the casts, reducing the stabilizing effect of ageing. The fungus did not develop on Dutch soil casts until after 42 days, and the development of a higher stability with age was also less marked than in the Australian soil. When the casts were subjected to a drying and rewetting cycle before analysis, they became much more stable than the casts that were analyzed wet. The drying-rewetting cycle removed most of the differences between the worm casts and the artificial casts, and also removed any effects of ageing.Dedicated to the late Prof. Dr. W. Kühnelt     

Determination of phenol in honey by liquid chromatography with amperometric detection

A sensitive, selective analytical method has been developed for determination of phenol in honey by liquid chromotography (LC) with amperometric detection (AMD). Phenol is extracted with benzene from the distillate of honey. The benzene extract is washed with 1% sodium bicarbonate solution and then reextracted with 0.1N sodium hydroxide followed by cleanup on a C18 cartridge. Phenol is determined by reverse-phase LC with amperometric detection. An Inertsil ODS column (150 X 4.6 mm, 5 microns) is used in the determination. The mobile phase is a mixture (20 + 80 v/v) of acetonitrile and 0.01M sodium dihydrogen phosphate containing 2mM ethylenediaminetetraacetic acid, disodium salt (EDTA) with the pH adjusted to 5.0. The flow rate is 1 mL/min under ambient conditions. The applied potential of the AMD using a glassy carbon electrode is 0.7 V vs an Ag/AgCl reference electrode. Average recoveries of phenol added to honey were 79.8% at 0.01 ppm spiking level, 90.4% at 0.1 ppm, and 91.0% at 1.0 ppm. Repeatabilities were 3.4, 1.3, and 1.8%, respectively. The detection limit of phenol in honey was 0.002 ppm. For analysis of 112 commercial honey samples, the range and average values of 32 detected samples were 0.05-5.88 ppm and 0.71 ppm, respectively.     

Assessment of tillage translocation and tillage erosion by hoeing on the steep land in hilly areas of Sichuan, China

Most of the tillage erosion studies have focused on the effect of tractor-plough tillage on soil translocation and soil loss. Only recently, have a few studies contributed to the understanding of tillage erosion by manual tillage. Furthermore, little is known about the impact of tillage erosion in hilly areas of the humid sub-tropics. This study on tillage erosion by hoeing was conducted on a purple soil (Regosols) of the steep land, in Jianyang County, Sichuan Province, southwestern China (30°24′N and 104°35′E) using the physical tracer method.

The effects of hoeing tillage on soil translocation on hillslopes are quite evident. The tillage transport coefficients were 26–38 kg m⁻¹ per tillage pass and 121–175 kg m⁻¹ per tillage pass respectively for k₃- and k₄-values. Given that there was a typical downslope parcel length of 15 m and two times of tillage per year in this area, the tillage erosion rates on the 4–43% hillslopes reached 48–151 Mg ha⁻¹ per year. The downslope soil translocation is closely related to slope gradient. Lateral soil translocation by such tillage is also obvious though it is lower than downslope soil translocation. Strong downslope translocation accounts for thin soil layers and the exposure of parent materials/rocks at the ridge tops and on convexities in the hilly areas. Deterioration in soil quality and therefore reduction in plant productivity due to tillage-induced erosion would be evident at the ridge tops and convex shoulders.     

Physical protection of mineralizable C in aggregates from long-term pasture and arable soil

Depending on agricultural management, soil aggregation can provide physical protection of organic matter against rapid decomposition. Within a given soil series, farm management affects the quality and quantity of organic inputs, soil disturbance and biological activity, and thereby the processes of aggregate formation (biogenic vs. physicogenic). We determined the physical protection of readily mineralizable organic matter against mineralization in undisturbed aggregates from a conventional arable field and a permanent pasture (>70 years). Soil samples from the two fields were incubated at constant temperature and moisture content, corresponding to field capacity. The increase in CO₂ evolution due to crushing (<250 μm) of the aggregates was used to estimate the macroaggregate-protected C fraction. The fraction of C protected at the microstructural level was estimated from the increase in CO₂ evolution after ball-mill grinding. In addition, aggregate size distribution and bulk density and porosity of undisturbed soil and macroaggregates were determined. Unprotected C fractions were not significantly different between the management systems and ranged from 1.9% to 2.4% of total organic C. In the arable soil, 1.4% of total soil organic C was physically protected in macroaggregates. Crushing of macroaggregates did not significantly increase C mineralization in the pasture soil. The results indicate that mineralization was considerably suppressed in the dominantly large and dense physicogenic macroaggregates from the arable field, but not in the dominantly porous, biogenic macroaggregates of the pasture soil. However, the protection in macroaggregates from the arable soil is not likely to be effective on the long-term because of the low water stability and the disrupting forces of cultivation under field conditions. A relatively high additional C mineralization from ground compared to crushed soil material, especially in the upper layer of the pasture soil, suggests a more important C protection at the microstructural level. Higher C protection in microaggregates from the pasture soil was supported by a previous micromorphological study of soil microstructures in thin sections of the considered management systems.     

Purification and characterization of an isoflavone-conjugates-hydrolyzing beta-glucosidase from endophytic bacterium

An isoflavone conjugates hydrolyzing beta-glucosidase (ICHG) from endophytic bacterium, Pseudomonas ZD-8 was purified to homogeneity by successive ammonium sulfate precipitation, gel filtration on SephadexG-100, DEAE-sephrose CL-6B and DEAE-Sephacel chromatography. The enzyme was a monomeric protein with an apparent molecular mass of 33 kDa as determined by SDS-PAGE and gel filtration. It was optimally active at pH 6.0 and 40 degrees C and had a specific activity of 1485 U mg of protein(-1) against genistin. The ICHG readily hydrolyzed rho-nitrophenyl-beta-glucoside, rho-nitrophenyl-beta-galactoside, genistin, daidzin, with Km values of 1.64, 1.87, 0.012, 0.014 mM, respectively. The ICHG showed a pronounced specificity for glucose in the 7-position of isoflavone and flavone conjugates and hydrolyzed effectively malonyl isoflavone glucosides as well as isoflavone glucosides with similar kinetics. Glucose and glucono-delta-lactone inhibited the enzyme competitively with Ki values of 84 mM and 23 mM, respectively. The enzyme did not require divalent cations for activity, and its activity was strongly inhibited by Hg2+, Ag+, rho-chloromercuribenzoate, iodoacetic acid, and N-ethylmaleimide while reducing agents such as beta-mercaptoethanol, dithiothreitol, dithioerythritol, glutathione slightly activated the enzyme.     

自控变频调速式灌溉水注肥装置的研究

为现代农业节水灌溉与精准施肥所需的配套注肥装置的开发,研究采用输液泵将肥料原液和酸碱调节液强行注入灌溉管网,并与灌溉水按比例混合获得一定浓度和酸碱度(EC & pH)的灌溉液。灌溉液浓度和酸碱度通过EC传感器和pH传感器检测并反馈PLC控制系统,通过变频调速进行EC值与pH值动态调节,实现节水灌溉系统的精准施肥灌溉。采用自控变频调速技术的灌溉水注肥装置经过实际运行,完全满足设计要求,可实现灌溉液的正常输送和EC值与pH的动态调节。特别是射流自吸泵变频调速技术的实现,不仅有效实现了灌溉液浓度（EC值）的动态调节,同时降低了设备成本,完全可用于节水灌溉注肥装置的开发。