Differential effect of afforestation on nitrogen-fixing and denitrifying communities and potential implications for nitrogen cycling

Denitrification and nitrogen (N) fixation are two major processes which drive N transformation in ecosystems. We investigated the effect of afforestation of pasture in New Zealand on denitrifying and N-fixing communities using molecular techniques. We optimised and applied multiplex-terminal restriction fragment length polymorphism for simultaneous analysis of denitrifying and N-fixing communities. Soil samples from four pine (Pinus radiata) forests of various age and adjacent pastures were analysed for microbial communities, denitrification rate and some environmental variables. Analysis of variance revealed that the denitrifying community was mainly influenced by site while the N-fixing community was influenced by both site and land-use types. Among environmental variables, moisture and porosity had the strongest impact on denitrifying communities while C:N ratio explained most of the variability in the N-fixing community. Denitrification rate as measured by nitrous oxide (N₂O) flux, was significantly lower in older forest soils compared to adjacent pasture soils. Simple and step-wise regression analyses revealed that the N₂O flux rate was strongly linked to the denitrifying community composition, providing evidence for the influence of community composition on denitrification. Moreover, we explored the relationship between denitrifying and N-fixing communities using various statistical approaches. Our results for the first time showed that denitrifying and N-fixing communities are related in situ, and suggest that changes in land-use may have a significant impact on N-transformations and ecosystem processes.     

Symbiotic relationships and root nodule ultrastructure of the pasture legume Biserrula pelecinus L.—a new legume in agriculture

Biserrula pelecinus is a pasture legume species new to Australian agriculture. The potential N benefit from B. pelecinus pastures in agricultural systems may not be realised if its symbiotic interactions with Mesorhizobium spp. are not well understood. This study evaluated the symbiotic interactions of four strains of Biserrula root-nodule bacteria (WSM1271, WSM1283, WSM1284, WSM1497) with four genotypes of B. pelecinus (cv. Casbah, 93GRC4, 93ITA33, IFBI1) and with a range of related legumes, including species known to be nodulated by strains of Mesorhizobium loti and other Mesorhizobium spp. Structures of root nodules were studied using light and electron microscopy enabling the ultrastructure of effective and ineffective nodules to be compared. B. pelecinus always formed typical indeterminate, finger-like nodules. The number of bacteroids inside symbiosomes varied between host×strain combinations, however, nodules formed by ineffective associations had well developed peribacteroid membranes and abundant bacteroids. Considerable variation was found in N₂-fixing effectiveness of strains isolated from B. pelecinus on the four B. pelecinus genotypes. Strains WSM1271, WSM1284 and WSM1497 nodulated Astragalus membranaceus, only strains WSM1284 and WSM1497 nodulated Astragalus adsurgens. Strain WSM1284 also nodulated Dorycnium rectum, Dorycnium hirsutum, Glycyrrhiza uralensis, Leucaena leucocephala, Lotus edulis, Lotus glaber, Lotus maroccanus, Lotus ornithopodioides, Lotus pedunculatus, Lotus peregrinus, Lotus subbiflorus and Ornithopus sativus. The four strains from B. pelecinus did not nodulate Amorpha fruticosa, Astragalus sinicus, Cicer arietinum, Hedysarum spinosissimum, Lotus parviflorus, Macroptilium atropurpureum or Trifolium lupinaster. M. loti strain SU343 nodulated all four genotypes of B. pelecinus. However, M. loti strain CC829 only nodulated B. pelecinus genotypes 93ITA33 and IFBI1 and the nodules were ineffective. The root nodule isolates from H. spinosissimum (E13 and H4) nodulated B. pelecinus cv. Casbah whereas the commercial inoculant strain for Cicer (CC1192) could not nodulate any genotype of B. pelecinus. These results indicate that strains WSM1271, WSM1283 and WSM1497 isolated originally from B. pelecinus have a specific host range while strain WSM1284 is promiscuous in its capacity to nodulate with a broad range of related species. As B. pelecinus can be nodulated by Mesorhizobium spp. from other agricultural legumes, particularly Lotus, there is an opportunity to utilise this trait in cultivar development.     

Human land use and patterns of parasitism in tropical amphibian hosts

Landscape alterations by humans can change patterns of parasite transmission. Depending on the type alteration and the life histories of parasites and hosts, parasitism may increase or decrease. To investigate whether parasitism in tropical amphibians was associated with land use change, I studied three species of amphibians, Rana vaillanti, Eleutherodactylus fitzingeri, and Smilisca puma from the Province of Heredia, Costa Rica, in the Atlantic lowlands. Frogs were collected and examined for parasites during the rainy seasons of 2001-2003 from sites in forest or clear-cut cattle pastures. The abundances of five species of parasites and parasite species richness of R. vaillanti were significantly higher in pasture habitat. A single parasite species shared by E. fitzingeri and S. puma showed a trend of increased abundance in pasture habitats. Overall, the abundances of six parasite species (three trematodes, an intestinal nematode, an encysted nematode, and a filarial nematode) were higher in pastures, while two species (trematodes) were higher in forest. This study suggests that land use activities in tropical regions affects the abundance and richness of amphibian parasites. Converting forest to pasture may impact the abundance of amphibian parasites because subsequent water quality changes (e.g. eutrophication, higher pH) often enhance habitat and resources for intermediate hosts (e.g. snails, mosquitoes) involved in parasite life cycles. Aquatic amphibians (e.g. R. vaillanti) may be particularly prone to experiencing increases in parasitism in agricultural habitats due to the positive relationship between many aquatic intermediate hosts and water quality changes associated with agricultural land use.     

略论经济开发与生态环境的再适应——以黔西北畜牧业的衰落为例

黔西北位处长江、珠江的中上游,就自然背景而言,其主体为典型的疏林草坡生态区,生态系统十分脆弱,一旦开发不当,就会出现土地石漠化、沙化等生态灾变。历史上,生息在这一地区的氐羌系统各民族一直实施农牧混合生计模式,这样的生计模式不仅繁荣了当地的经济,维护了生态系统的稳定,而且还为长江、珠江中下游水资源的稳定做出过重要贡献。康熙＂改土归流＂后的300余年间,随着垦殖面积的扩大,无序的矿业开发,不但没有出现经济的大繁荣,反而诱发了诸种生态灾变,成了当前贵州省结构性贫困和经济性贫穷的重灾区。因此认真总结历史时期黔西北畜牧业衰落的经验教训,对于当前贵州省的经济建设、生态建设大有裨益。     

Mitigating N2O emissions from cropping systems after conversion from pasture − a modelling approach

Converting pasture to cropping is common in many of the world’s agricultural systems. This conversion results in substantial net mineralisation of soil organic matter that builds up during a phase of pasture. A few studies have shown that this mineralisation leads to increased nitrous oxide (N₂O) emissions compared to long-term pasture or long-term cropping. Understanding of interactions leading to these significant emissions is still scarce but is needed to identify mitigation options for this situation. In this study, the Agricultural Production Systems sIMulator (APSIM) was used to investigate the optimal timing of pasture termination (relative to crop planting) and management of nitrogen (N) in crops after pasture termination to maximise crop yield and limit N₂O emissions. Beforehand, APSIM’s performance in simulating yields and N₂O emissions was tested against data from field experiments conducted in the temperate high-rainfall zone of southern Australia where N₂O emissions were monitored with automatic gas collection chambers during the first year of cropping wheat after terminating long-term pasture on two adjacent sites in two consecutive years. Field experiments and simulation scenarios showed very high N₂O emissions (up to 48 kg N₂O-N ha⁻¹ yr⁻¹) in the first year of wheat after pasture termination, even without N fertiliser application. Measured cumulative N₂O emissions, crop yields and soil mineral N and water content dynamics were simulated well with APSIM. Including a routine into APSIM to account for N₂O transport through the soil profile improved the simulation of daily N₂O emissions considerably, leading to up to 67% of the measured variability in daily N₂O emissions being explained by the model. We predicted that a short fallow between termination of pasture and sowing wheat, instead of a long fallow which is the common practice, reduces N₂O emissions by more than half in the first year of cropping without a noteworthy impact on crop yield. Reducing N fertiliser applications in the first few years after pasture termination by taking available soil mineral N into account, and applying the fertiliser six to twelve weeks after sowing instead of at sowing was predicted to further reduce N₂O emissions. Since the model was calibrated against experimental data during the first year after pasture termination only, experiments determining N₂O emissions in the first two or three years after terminating pasture are needed to confirm our predictions.     

Potential contribution of lysed bacterial cells to phosphorus solubilisation in two rewetted Australian pasture soils

Soil drying renders considerable amounts of phosphorus soluble upon rewetting, which may be partly derived from lysed microbial cells. Using direct bacterial cell counting in water and tetra-sodium pyrophosphate extracts of two Australian pasture soils, we found that almost all extractable cells were lysed following the rewetting of dry soils. The amounts of phosphorus in the lysed cells corresponded closely to the increases in water-extractable phosphorus following soil drying, suggesting that bacterial cell lysis is a major source of the released phosphorus.     

棉籽粕补饲对新疆细毛羊氮代谢的影响

将２４只约１５月龄的新疆细毛母羊（平均体重２８．９ｋｇ）随机分为６个处理组（每处理４只）,以基础日粮作对照,混合精料日粮作标准,研究不同水平棉籽粕补饲的效果。结果表明,棉籽粕补饲显著提高了（Ｐ〈０．０５）绵羊的总氮采食量、氮表现消化率、氮沉积、尿囊素排出量和由尿囊素排出量估测的瘤胃微生物氮产量。补饲棉籽粕绵羊的瘤胃微生物氮产量显著低于（Ｐ〈０．０５）饲喂混合精料的绵羊,这可能与前者的能量采食量较低     

匍枝根霉和半裸镰刀菌侵染引起甜瓜采后呼吸代谢途径的变化

 采后健康甜瓜(Cucumis melo L.)果皮组织以高抗氰、高残余、无巴斯德效应和以磷酸戊糖代谢为主的呼吸方式,维持着较低的呼吸强度水平。当感染匍枝根霉(Rhizopus stolonifer(Ehrenb.) Vuill)和半裸镰刀菌(Fusarium semitectum Berk et Rav.)后,抗氰呼吸减弱,残余呼吸消失,巴斯德效应出现,磷酸戊糖途径消失,糖酵解-三羧酸循环-细胞色素电子传递呼吸系统加强,最终导致病斑形成,组织溃烂,巴斯德效应消除,乙醇高度积累,无氧呼吸高度增加。病原不同,病症不同,但引起甜瓜呼吸变化的模式是一致的。     

豆科鲜牧草破碎机的研究与试验

针对豆科鲜牧草的物理机械特性,分析了机械破碎作用的力学机理,认为合适的打击作用力是该类物料有效的破碎加工方法。在试验研究的基础上,设计研制了“93D—324型鲜牧草捣泥机”。使得在不加水的情况下,把豆科鲜牧草破碎成细碎纤维状或浆状饲料,满足喂猪的饲养要求。     

我国牧草及草坪草引种与生物入侵探讨

简述了生物入侵的概念、牧草及草坪草引种所带来的杂草入侵现状,并通过分析入侵性牧草及草坪草的形成过程、危害现状与引种利用的关系,阐明了在牧草及草坪草引种利用的同时,既要考虑经济效益,又要重视入侵物种引起的生态风险,并提出了我国牧草及草坪草引种造成生物入侵的预防对策。