Asymbiotic dinitrogen fixation by tundra in the Imnavait Creek drainage,Alaska, USA

Summary N₂ fixation by free-living microorganisms was investigated at an intensively studied low Arctic site near Toolik Lake in the northern foothills of the Brooks Range, Alaska, during July 1987. Four characteristic vegetation associations along an elevational gradient were assayed using minimally disruptive in situ acetylene reduction assay methods. The acetylene reduction rates did not differ significantly among vegetation associations. The mean rate for the site was 9.60 mol m^–2 h^–1 or 90 g N m^–2 day^–1, which is within the range of values given for other Arctic and alpine tundra studies. The complex microtopography and resulting patchy distribution of free-living and phycobiont diazotrophs is the most likely cause of the high spatial variability in acetylene reduction activity. Rates were most variable among samples from the lowest position, a riparian site. The potential contribution of heterotrophic diazotrophs was examined through a laboratory enrichment study. Soils from the two lower slope positions showed dramatic responses to added C, suggesting that heterotrophs may contribute fixed N₂ to this system.     

Dynamics of soil microbial biomass C, N and P in disturbed and undisturbed stands of a tropical wet-evergreen forest

To understand the spatial and temporal dynamics of soil microbial biomass and its role in soil organic matter and nutrient flux in disturbed tropical wet-evergreen forests, we determined soil microbial biomass C, N and P at two soil depths (0–15 and 15–30 cm), along a disturbance gradient in Arunachal Pradesh, northeastern India. Disturbance resulted in considerable increase in air temperature and light intensity in the forest and decline in the soil nutrients concentration, which affected the growth of microbial populations and soil microbial biomass. There were significant correlations between bacterial and fungal populations and microbial biomass C, N and P. Soil microbial population was higher in the undisturbed (UD) forest stand than the disturbed forest stands during post-monsoon and less during rainy season due to heavy rainfall. Greater demand for nutrients by plants during rainy season limited the availability of nutrients to soil microbes and therefore, low microbial biomass C, N and P. Microbial biomass was negatively correlated with soil temperature and pH in all the forest stands. However, there were significant positive relationships among microbial biomass C, N and P. Percentage contribution of microbial C to soil organic C was higher in UD forest, whereas percentage contribution of microbial biomass N and P to total N and total P was higher in the moderately disturbed site than in the highly disturbed (HD) site. These results reveal that the nutrient retention by soil microbial biomass was greater in the selective logged stand and would help in the regeneration of the forest upon protection. On the other hand, the cultivated site (HD) that had the lowest labile fractions of soil organic matter may recover at a slower phase. Further, minimum and maximum microbial biomass C, N and P during rainy and winter seasons suggest the synchronization between nutrient demand for plant growth and nutrient retention in microbial biomass that would help in ecosystem recovery following disturbance.     

Geographic variation for spike and grain characteristics in durum wheat germplasm adapted to dryland conditions

Summary A set of 109 durum wheat accessions, selected from a world collection for their adaptation to dry environments, were further evaluated under moisture-limiting conditions. The entries were subdivided into 9 gene pools based on their country of origin. Data are presented to assess the phenotypic variation and distinctiveness of these pools for some spike and grain features. Analysis of variance revealed highly significant differences among materials of diverse origin for all traits except protein content. Within-country variation was generally wide as regards spike length and 1000-kernel weight. The Ethiopian germplasm presented a remarkable variability and distinctiveness. Entries with interesting values for breeding purposes were detected in most country gene pools. In general, selection of germplasm adapted to harsh environments did not result in a loss of variability for the considered traits, with the possible exception of protein content. Ethiopian entries presented the highest number of significant pairwise character correlations. The two most important traits in a discriminant analysis by country of origin were spike density and 1000-kernel weight. Diversity among gene pools was further assessed by a canonical analysis. The first three canonical variables explained 90% of the total variance. The gene pools of Ethiopia, Greece, Egypt and Jordan appeared clearly distinct from the other germplasm subsets studied. Geographically distinct patterns emerging from this study could be the result of the combined effect of natural and human selection.     

不同管理措施对滨海盐渍农田土壤CO2排放及碳平衡的影响

为探讨不同管理措施对滨海盐渍农田碳平衡的影响,本文通过玉米–小麦轮作试验,研究农田土壤的CO_2释放规律,及其农田碳收支状况。试验设计6个处理:1常规对照(CK);2有机肥常量(OF);3氮肥增施(NF);4秸秆还田(S);5有机肥加秸秆(OF+S);6免耕(NT)。研究表明,秸秆还田和有机肥的施用增加了土壤呼吸的强度,而免耕处理的CO_2平均释放量最低,不同处理下土壤呼吸总体表现为OF+SSOMNFCKNT。各处理土壤有机碳含量随着作物的收获逐渐升高,其中OF与NT增加最多,而增施氮肥处理并没有显著提高土壤的有机碳水平。各处理间的有机碳含量没有显著性差异。在两季作物种植结束后,各处理的碳输入均高于碳输出,均为碳净输入,表现出较强的碳汇特征。秸秆还田和单施有机肥的碳净输入均显著高于对照,可有效减缓因农田土壤CO_2排放而造成的全球气候变化问题。     

Salt-marsh restoration: evaluating the success of de-embankments in north-west Europe

De-embankment of historically reclaimed salt marshes has become a widespread option for re-creating salt marshes, but to date little information exists on the success of de-embankments. One reason is the absence of pre-defined targets, impeding the measurement of success. In this review, success has been measured as a saturation index, where the presence of target plant species in a restoration site is expressed as a percentage of a regional target species pool. This review is intended to evaluate and compare success of many different sites on an idealistic concept where all regional target species have the potential to establish in a site, but may not actually do so because the site is unsuitable or inaccessible. Factors affecting suitability and accessibility and management options to increase regional species diversity are discussed. The results show that many sites contain less than 50% of the regional target species, especially when sites are smaller than 30 ha. Higher species diversity is observed for sites exceeding 100 ha and for sites with the largest elevational range within mean high water neap to mean high water spring tide. Most sites younger than 20 years contain more target species than older sites. For future de-embankments it is recommended that clear targets are set from the start. This brings along the need for monitoring. Only 37 out of 70 sites with de-embankment were monitored for plant species assemblages. Setting targets will also allow adaptive management of the site. Management options that are likely to result in higher species diversity are the construction and maintenance of drainage structures and the implementation of a grazing or mowing regime.     

Microbial immobilisation and turnover of C labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass C immobilisation and turnover were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas within an agricultural field. We compared the size and activity of soil microbial biomass (SMB) in the soils of the different yield areas under field and laboratory conditions. Soils were amended with ¹³C labelled mustard (Sinapis alba) residues (both experiments) and labelled glucose (laboratory only) at 500 μg C g⁻¹ dry soil. SMB-C, dissolved organic carbon (DOC) and total C content were monitored in the field and the laboratory. CO₂-efflux was also measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, but other variables only in the laboratory treatments. A positive priming effect was measured in three of four laboratory treatments. Priming was induced after a significant increase of soil derived C in the microbial biomass. Thereafter, the total C loss through priming was always smaller than or equal to the decline in microbial biomass C. In field and laboratory experiments SMB in the HY soil immobilised less of the added substrate C than LY soil SMB. Calculated turnover times in the laboratory glucose amendment were 0.24 (HY) and 0.31 y (LY), in the laboratory mustard treatment 0.58 (HY) and 0.44 y (LY) and in the field mustard amendments 1.09 (HY) and 1.25 y (LY). In both the field mustard and laboratory glucose treatments turnover in the HY soil tended to exceed that in the LY soil. These turnover times as well as the reaction of SMB-C to drying-rewetting and substrate addition, indicated that the HY soil possessed a more active microbial community with a more rapid C turnover than the LY soil. As C turnover is considered to be closely linked to nutrient cycles, faster turnover in the HY soil may involve a better nutrient supply for crops resulting in higher agricultural yield.     

土壤无机阴离子的毛细管电泳分析

采用以铬酸钠为背景电解质、十六烷基三甲基溴化铵为电渗流改性剂的间接紫外检测法电泳系统 ,研究了分离电压、缓冲液的酸碱性等毛细管电泳测定氯离子、硫酸根、硝酸根和磷酸根的基本条件 ,可在 5min内对以上 4个离子实现准确定量。测定了 5个采自全国不同地区的土壤样品水提取液中的无机阴离子。结果显示 ,毛细管电泳可对提取液中的氯离子、硫酸根和硝酸根实现较准确的定量 ,测定结果与离子色谱比较一致 ;对硝态氮的测定结果与KCl提取 流动分析结果相关性达到p <0 10水平 ,表明测定结果可在一定程度上反映土壤氮素养分状况。但土壤用水静止提取 2 4h ,提取液中的磷未达到毛细管电泳和离子色谱法的检测限 ,毛细管电泳的测定精确度不及离子色谱     

The effect of suppression treatments on the uptake of 15N by intercropped corn from labeled alfalfa (Medicago sativa)

In greenhouse experiments, we examined the N transferred to intercropped corn from ¹⁵N-labeled alfalfa shoot residue and intact roots in an undisturbed soil system in response to two different suppression treatments and complete killing of alfalfa. The alfalfa treatments included complete killing (glyphosate only), glyphosate injury + cutting, and cutting only, with alfalfa shoot residue returned to the soil surface in all three treatments. Corn was planted in each pot following application of the treatments. When alfalfa was suppressed by glyphosate injury + cutting, corn had recovered 12% of the alfalfa N by 8 weeks of growth, but with cutting only, N recovery by corn was reduced to 4.0%. The completekill treatment resulted in 8% recovery by corn of alfalfa N. In all treatments, most of the alfalfa-N remained in the soil organic pool. A second experiment tested a cutting only treatment with ¹⁵N-labeled alfalfa residue returned to the soil surface. The ¹⁵N-labeled alfalfa residue contributed 4.1% of N to corn during the 8-week growth cycle. Twice as much ¹⁵N was found in the active microbial biomass pool in the two treatments with live intereropped plants compared to the monoculture treatments with complete killing (non-intercropped) and the control treatment of alfalfa regrowth only. An analysis of the change in the ¹⁵N content of the undisturbed alfalfa roots from just before the suppression until 8 weeks later suggested that approximately 80% of the root ¹⁵N was lost from the plant suppressed by cutting. This corresponds to 28% of the total N released from the alfalfa. The results suggest that the degree of legume suppression was a key factor in the availability of legume N to the second crop. When the two species were intercropped, more of the N available from legume residues went to plant uptake and microbial biomass and was not stabilized as quickly in the soil organic pool. Appropriate management schemes must be designed to increase N availability to the second crop without yield reduction. These studies suggest severe suppression is necessary; if successful, more of the N can be maintained in active pools.     

川西北几种常绿阔叶林土壤碳库和养分库的比较

通过对原始青冈栎 栲树 薯豆成熟常绿阔叶林、次生润楠 桤木 楠木成熟常绿阔叶林和次生绿叶石栎 光皮桦 细齿柃木幼年常绿阔叶林3种四川西北部典型常绿阔叶林下土壤碳库、全量养分库的调查和分析,揭示了该区域常绿阔叶林顶级群落下土壤碳库和养分库的整体情况及变异规律。结果表明:源于林分起源、成熟阶段和树种特性不同,3种天然林分土壤碳库差异较大。3种林分土壤碳库分别为160.35 t/hm2,138.96 t/hm2,62.14 t/hm2,平均为120.48 t/hm2。不同养分库在3种林分间表现的规律性不同,但它们都受到林分类型及特征的重要影响。有机碳和各养分的存贮随土壤深度增加均呈减小的趋势,有机碳和总氮表现尤其明显。     

长期不同施肥黑土碳氮库的组分特征

土壤有机质因组成和稳定机制等差异,具有很大的分解异质性,研究其分组特征,对于了解土壤有机质的质量具有十分重要的意义。本文利用物理分组法,对于土壤有机质组分性质影响少的优点,比较了长期不同施肥黑土不同组分碳氮库的数量变化。结果发现,23年后化肥(NPK)处理和化肥配施秸秆(NPKS)处理土壤有机碳、全氮含量波动不大。化肥配施有机肥(NPKM)处理土壤有机碳、全氮含量显著增加(P0.01)。进一步碳氮分组发现,有机无机配施(NPKS和NPKM)显著降低了有机质与矿质结合组分(150μm MF)的含量;NPKM处理显著增加了黑土粗有机质组分(150μm MOM)的比例。将大于150μm组分中有机质与矿质结合部分去除作为易分解库,剩余组分作为耐分解库,比较3种处理发现,与NPK处理相比,NPKS和NPKM处理易分解有机碳比例增加0.9和5.9个百分点;耐分解有机碳比例分别为下降14.2和18.8个百分点。易分解有机氮比例增加1.8和9.9个百分点;耐分解有机氮下降2.7和5.3个百分点。上述研究表明,NPKS和NPKM可以改善黑土土壤有机氮和土壤有机碳的数量和品质,且NPKM的效果显著优于NPKS处理。