Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^–1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Soil microbial biomass and nitrogen transformations among five tree species of the Catskill Mountains, New York, USA

We measured soil microbial biomass nitrogen (MBN), microbial uptake of ¹⁵N, potential net mineralization and net nitrification in the laboratory to determine the influence of tree species on nitrogen (N) transformations in soils of the Catskills Mountains, New York, USA. Organic horizon soils were taken from single species plots of beech (Fagus grandifolia), hemlock (Tsuga canadensis), red oak (Quercus rubra), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis). ¹⁵NH₄Cl was added to the soils and N pools were sampled at 1, 3, 10 and 28 days to examine microbial uptake of ¹⁵N over time. Soil MBN was about 60% lower in red oak and sugar maple soils than in the other three species. Soil pools of NO₃⁻ and rates of net nitrification were significantly greater in soils associated with sugar maple than hemlock, red oak and yellow birch. With the exception of sugar maple soils, microbial recovery of ¹⁵N was significantly greater after 10 and 28 days compared to 60 min and 1 day following ¹⁵N tracer addition. Microbial ¹⁵N recovery declined significantly within sugar maple stands within the first 3 days of incubation. Soil carbon to nitrogen ratio (C:N) was lowest in sugar maple soils and highest in red oak soils. However, correlations between soil C:N and MBN or rates of net mineralization and nitrification were not significant. Soil moisture could account for 22% of the variation in MBN and 36% of the variation in net mineralization. Soil microbial transformations of N vary among tree species stands and may have consequences for forest N retention and loss.     

A theoretical analysis of nonlinearities and feedbacks in soil carbon and nitrogen cycles

Analytical formulations of soil carbon and nitrogen cycles are used to explore the effects of model nonlinearities and feedbacks on the resulting dynamics. Two particular aspects are addressed: (i) nonlinearities in the decomposition rate of soil organic matter and (ii) nitrogen limitation feedbacks on microbial activity and plant-microbe competition. Although linear models of decomposition are more typical in the literature, nonlinear models accounting for the coupling between microbial biomass and its substrate have also been proposed. In deterministic conditions, linear models behave like exponential decay functions, while nonlinear models may also show fluctuating behavior, with dynamic bifurcations between stable-node and stable-focus equilibria as a function of the climatic parameters (e.g., soil moisture and temperature). Both data-model comparison and linear stability analysis support the conclusion that linear models are less suited to describe the soil fluctuating dynamics that arise under certain conditions. A second strong nonlinearity appears when the nitrogen-limitation feedback on decomposition is analyzed. Nitrogen limitation is often established when the substrate of the microbes is N-poor, and/or the competition with plants for mineral N is strong. Such conditions mainly occur when a large fraction of the microbial community cannot meet its nitrogen demand through organic N assimilation, so that mineral N is used. On the contrary, when the microbial community predominantly assimilates organic nitrogen, the occurrence of nitrogen-limitation is less likely and mineralization is given by microbial N surplus. The first case is traditionally modeled by the mineralization-immobilization turnover (MIT) scheme, while the second by the direct assimilation (DIR) scheme. However, since organic N assimilation and mineral N immobilization likely occur simultaneously because of soil heterogeneity and coexistence of different microbial communities, the two schemes only represent extreme cases. Thus, we combine them in a flexible model framework (parallel scheme) and explore how different efficiencies of organic nitrogen assimilation, mineral nitrogen availability and climatic factors control the outcome of plant-microbe competition. We conclude that models accounting only for the DIR pathway implicitly assume that microbes are superior competitors over plants, while models implementing only the MIT pathway might be too sensitive to N-limitation.     

Biodegradation of leaf litter in the warm humid tropics of Kerala, India

Ex situ biodegradation of Mangifera indica L, Artocarpus heterophyllus.Lamk. and Anacardium occidentale L. leaf litter were examined in the warm humid tropics of southern Kerala adopting the standard litter bag technique. The time taken for the decay varied with the species and it followed the order Mangifera >Artocarpus>Anacardium. Weight loss accorded a linear decline and was better correlated with soil moisture than temperature. The half-life values were 3.2, 3.4 and 4.0 months for Anacardium, Artocarpus and Mangifera, respectively. Soil faunal and floral activities were monitored during the decay and the earthworms, fungi and bacteria proved the chief degraders of the intact litter. Actinomycetes were active during the final stages. The variations in decay rates of the three species are attributed to the differences in the litter quality and activity of the decomposer organisms in soil. NPK dynamics revealed temporary phases of immobilization for nitrogen and phosphorus before final release, while potassium recorded a continuous release. Decomposition apparently improved the available NPK status of soil, potassium being liable to leaching was soon lost from the surface soil.     

以尼龙6为载体的固定化高分子季铵盐杀生剂制备

[目的]制备具有杀菌功能的高分子季铵盐杀生剂。[方法]以尼龙6为载体,4-溴丁酰氯为活化剂,在溶剂中共价接枝高分子聚乙烯亚胺(PEI),用1溴-己烷进行烷基化反应,制备固定化高分子季铵盐杀生剂。[结果]傅立叶红外光谱仪对杀生剂的检测结果表明,高分子聚乙烯亚胺接枝成功。杀生剂稳定性考察试验结果表明,高分子聚乙烯亚胺具有良好的稳定性和长期效应。[结论]固定化高分子杀生剂具有良好的抗菌及重复利用性能。     

丝素蛋白膜作为葡萄糖氧化酶载体的研究

采用戊二醛共价交联法将葡萄糖氧化酶（GOD）固定在丝素膜上，探讨了酶的固定化方法，研究了固定化酶膜的动力学性质，并与包埋法制备的酶膜进行了比较。共价法可获得高活力的酶膜（534u／cm2），固定化过程中使用甲醇或戊二醛处理对酶活力有轻微影响。固定化GOD最适pH值向中性偏移（pH7．0），最适温度为50℃，热稳定性提高，表观米氏常数比溶液酶降低，酶膜的贮存稳定性良好。     

苯酚降解菌DF51的分离鉴定，降解特性及其固定化的研究

从太原市郊苯酚污染的土样中分离到一株能以苯酚、苯甲酸、萘、联苯和苯并噻吩为唯一碳源和能源生长，并且具有同时分解单环和双环芳香类物质能力的菌株，经生理生化和16S rDNA基因序列分析鉴定为红球菌DF51(Rhodococcus sp. DF51). 在本实验条件下，菌株DF51能够有效降解浓度范围为100-800mg L-1的苯酚，该菌代谢苯酚主要是通过邻苯二酚1, 2-双加氧酶催化开环途径进行，同时辅以邻苯二酚2, 3-双加氧酶催化开环，表明菌株DF51兼有混浊红球菌(Rhodoccocus opacus R7)和红球菌PNAN5(Rhodoccocus sp. strain DF51)降解苯酚的途径. 菌株DF51固定化实验表明，该菌的固定化细胞具有降解苯酚的潜在应用价值.     

Regulation of microbial carbon,nitrogen, and phosphorus transformations by temperature and moisture during decomposition of <Emphasis Type="Italic">Calluna vulgaris</Emphasis> litter

To evaluate the effect of climate change on ecosystem functioning, the temperature and moisture response of microbial C, N, and P transformations during decomposition of Calluna vulgaris (L.) Hull. litter was studied in a laboratory incubation experiment. The litter originated from a dry heathland in the Netherlands where P limited vegetation growth. Fresh litter was incubated at 5, 10, 15, or 20°C and at a moisture content of 50, 100, or 200% in a full factorial design. Microbial nutrient transformations and activity were evaluated during two successive periods: an initial period of 48 days characterized by microbial growth and a second period from 48 to 206 days in which microbial growth declined significantly. Temperature and moisture response of respiration rate, the metabolic quotient (qCO₂), C, N, and P immobilization, net N and P mineralization and nitrification rates were evaluated by performing linear regressions. Microbial nutrient transformations and microbial activity depended both on temperature and moisture. In the first period, the respiration rate, qCO₂, microbial C and N immobilization, net P mineralization, net N mineralization and net nitrification rates were more strongly affected by temperature, while the microbial P immobilization rate was more strongly affected by moisture. The respiration rate, qCO₂, P immobilization rate, net P and N mineralization rate, and nitrification rate increased with temperature and moisture, while the C and N immobilization rate decreased with increasing temperature and increased with moisture. In the second period, C, N, and P immobilization and net N and P mineralization rates were significantly lower. The respiration rate and qCO₂ continued to increase with temperature and moisture, but C and N immobilization rates increased with temperature and declined with increasing moisture. Net P mineralization rate decreased at higher temperature and moisture, and nitrification rate declined with increasing temperature and increased with moisture. It was concluded that plant growth in these P-limited systems is very sensitive to climate change as it strongly relies on the competition for P with microbes, and temperature and moisture have a large effect on the immobilization rate of available P.     

酶固定化研究进展

目前酶作为一种性能优良的生物催化剂被广泛应用.由于酶在应用过程中的缺点如稳定性差、不能重复使用等,酶在工业化利用之前都要经过处理来改善酶的性能,于是固定化技术应运而生.通过模仿人体膜作用机理,使酶固定化,并且得到迅速的发展.主要阐述了医药学、化工、食品制造等方面固定化酶的应用情况以及该技术的一些最新研究成果.     

Estimation of nitrogen and sulfur mineralization in soils amended with crop residues contributing to nitrogen and sulfur nutrition of crops in the North Central U.S

Predicting nitrogen (N) and sulfur (S) mineralization of crop residues from the preceding crop might be a useful tool for forecasting soil N and S availability. Two soils from eastern North Dakota and three crop residues – corn, spring wheat, and soybean were used in an 8-week incubation study to estimate N and S mineralization from crop residues. The cumulative N and S mineralized were fit to a first-order kinetic model. Cumulative N mineralized ranged between 0.34 and 2.15 mg kg^?1 and 0.45 to 3.41 mg kg^?1 for the Glyndon and Fargo soils, respectively. Un-amended soils showed higher N mineralization than residue treated soils. For S, the highest mineralization occurred in un-amended Glyndon soil and in spring wheat-amended Fargo soil. This study indicates that crop residue additions can have a negative impact on plant available nutrients due to immobilization of N and S during the time when crops need the nutrients most.