连续免耕对不同质地稻田土壤理化性质的影响

以我国南方地区典型的单季水稻生产大田为研究对象,按土壤质地分为壤质和粘质两个系列,探讨不同质地稻田土壤理化性质变化对连续免耕的响应规律。结果表明,在无秸秆覆盖条件下,随着免耕年限的增加,壤质和粘质稻田土壤的耕层均有紧实度提高的趋势,特别是粘质土壤,导致耕层变浅。与常年翻耕土壤相比,免耕6a后壤质水稻土0—20 cm土层的紧实度值平均增加了32%,而粘质的平均增加了90%。在相同免耕年限条件下粘质稻田土壤容重的增加也比壤质土壤的明显。壤质土壤0—10 cm土层有机质和碱解氮含量随免耕年限延长而提高,而在粘质土壤则显著降低。无论是壤质还是粘质土壤,连续免耕多年后土壤速效磷均在耕层(0—20 cm)富集,而速效钾则相反。总体而言,壤质水稻土对免耕的适宜性要优于粘质土壤;应根据土壤质地的不同选择性地实施免耕技术,并结合秸秆覆盖,以实现免耕稻田土壤的可持续利用。     

土壤砷植物暴露途径的土壤因子模拟

确定土壤中重金属生物有效性是评价土壤重金属污染生态风险的关键,然而在生态风险评价实际应用中,测定土壤重金属生物有效性耗时且高成本,限制了生物有效性评价法的应用。通过对模式植物的文献搜集,总结了影响植物富集土壤砷的7个土壤因子,揭示了土壤砷植物暴露的影响机制。运用SPSS 18.0对文献数据集各个土壤因子进行了回归分析、相关性分析和共线性分析。单因子回归分析表明营养盐类对植物富集砷影响很小,可以忽略不计;相关性分析和共线性分析结果表明土壤因子之间存在多元共线性;主成分分析结果表明,植物富集砷浓度主要受土壤质地的物理综合指标以及土壤总砷含量和土壤酸碱度等化学综合指标影响。其中植物富集浓度受土壤中总砷浓度影响最大,受土壤质地中砂粒含量影响其次;土壤粉粒、粘粒、有机质含量和阳离子交换量对富集也有一定影响;而土壤酸碱度对植物富集浓度的影响相对较小。最终通过数学模型解释植物富集砷浓度随土壤性质变化的规律,建立了土壤性质与土壤砷植物暴露的相关关系,为土壤重金属生物有效性风险评价法提供理论依据。     

黄土高原不同土壤质地农田土壤碳、氮、磷及团聚体分布特征

结合野外观测和室内分析,研究了黄土高原不同土壤质地农田土壤碳、氮、磷含量及其生态计量学特征,以及土壤团聚体分布状况,以揭示土壤质地对区域农田土壤肥力的影响,以及土壤团聚体对肥力的调控作用.结果表明: 黄土高原农田土壤大团聚体含量、主要养分含量及其生态计量比值均随土壤质地由细变粗(壤质黏土→黏壤土→砂质壤土)逐渐降低;土壤pH值和微团聚体含量则呈现出相反的变化趋势. 随土壤黏粒含量增加,大团聚体含量、有机碳、全氮和全磷含量,以及C/P和N/P显著增加,土壤pH和微团聚体含量显著降低. 土壤有机碳、全氮和全磷含量,以及C/P和N/P随大团聚体含量的增加显著增加.表明区域尺度上农田土壤肥力状况取决于土壤质地,并受土壤大团聚体的调节.     

Temporal water stress effects on nodulation,nitrogen accumulation and growth of soybean

The objective of this study, carried out over 2 years, was to evaluate the effect of soil properties on the response of maize (Zea mays L.) to zinc applications and relate these properties to soil test Zn for predicting the Zn status of soils considering the effect of environmental conditions. The relative yield, expressed as an index of crop response, was related through multiple regression to CEC (or clay), electrical conductivity (or exchangeable Na), and bulk density consistently throughout the two year period that included one relatively wet, cool and cloudy growing season when variations in relative yield were explained also by 0.5M NaHCO₃-extractable-P and organic C. A procedure is presented to establish limits for the soil propeties and soil-test-extractable-Zn and to meaningfully combine them into a model to predict soil Zn status. A model that combined soil test Zn, texture and electrical conductivity was satisfactory for the purpose of prediction and for adoption for soil testing on a routine basis. The suggested approach may be suitable for designing models with soil properties associated with crop responses to micronutrients in other situations. Deceased 22 September 1988     

不同冬小麦品种在3种质地土壤中氮代谢特征及利用效率分析

为明确不同土壤质地条件下不同品种冬小麦(Triticum aestivum)的氮代谢和利用特征, 筛选与土壤质地相适宜的高产和氮高效利用的优质小麦品种, 采用大田试验的方法, 在同一生态类型区砂土、壤土和黏土3种质地土壤上, 以当地生产上大面积应用的强筋小麦‘郑麦366’ (‘ZM366’)和中筋小麦‘矮抗58’ (‘AK58’)、‘周麦22’ (‘ZM22’)为材料, 系统地研究了土壤质地对不同冬小麦品种主要生育时期叶片氨同化关键酶谷氨酰胺合成酶(GS)活性、游离氨基酸含量、花前和花后不同器官氮素积累和分配、氮素再分配等氮代谢过程及产量、品质和氮素利用效率等的影响。结果表明: 在这3种土壤质地上, 不同品种冬小麦旗叶GS活性和游离氨基酸含量均呈倒“V”型变化特征。各品种小麦旗叶GS活性、游离氨基酸含量大小及达到最大值的时期不一样, 砂土条件下峰值早于壤土10天左右出现, 且在5月22日已检测不到GS活性和游离氨基酸含量。花前和花后小麦地上部及各器官氮积累量(NA)、氮再分配量(NR)、成熟期籽粒产量和氮素当季利用率(NUE)均以壤土上为最高。氮素转运率(NRE)、花前再分配氮素对籽粒氮素的贡献率(NRC)、氮素生理效率(NPE)、氮收获指数(NHI)以砂土上为最高。其中, 砂土上NRC达82.46%-95.84%, 是花后的7倍左右; 壤土和黏土条件下花后吸收的氮素在籽粒氮素的积累中占有较大的比例, 贡献率分别为36.6%和29.2%。同一土壤质地上3个品种比较, 在砂土上, GS活性、游离氨基酸含量、籽粒产量、蛋白质含量及NUE和NPE以‘郑麦366’最高, 而壤土上以‘矮抗58’最高, 黏土上则以‘周麦22’最高。因此, 在生产上应培育和选择与土壤质地相适应的小麦品种, 砂土地种植‘郑麦366’, 壤土条件下种植‘矮抗58’, 黏土条件下种植‘周麦22’, 可以在获得较高产量和品质的同时, 提高氮素利用效率。     

Comparison of nifH Gene Pools in Soils and Soil Microenvironments with Contrasting Properties

The similarities and differences in the structures of the nifH gene pools of six different soils (Montrond, LCSA-p, Vernon, Dombes, LCSA-c, and Thysse Kaymor) and five soil fractions extracted from LCSA-c were studied. Bacterial DNA was directly extracted from the soils, and a region of the nifH gene was amplified by PCR and analyzed by restriction. Soils were selected on the basis of differences in soil management, plant cover, and major physicochemical properties. Microenvironments differed on the basis of the sizes of the constituent particles and the organic carbon and clay contents. Restriction profiles were subjected to principal-component analysis. We showed that the composition of the diazotrophic communities varied both on a large scale (among soils) and on a microscale (among microenvironments in LCSA-c soil). Soil management seemed to be the major parameter influencing differences in the nifH gene pool structure among soils by controlling inorganic nitrogen content and its variation. However, physicochemical parameters (texture and total C and N contents) were found to correlate with differences among nifH gene pools on a microscale. We hypothesize that the observed nifH genetic structures resulted from the adaptation to fluctuating conditions (cultivated soil, forest soil, coarse fractions) or constant conditions (permanent pasture soil, fine fractions). We attempted to identify a specific band within the profile of the clay fraction by cloning and sequencing it and comparing it with the gene databases. Unexpectedly, the nifH sequences of the dominant bacteria were most similar to sequences of unidentified marine eubacteria.     

Analysis of estuarine soil gradients in salt marshes of the southwestern Netherlands with special reference to the Scheldt estuary

Macro- and microgradients in salt-marsh soils of the Scheldt estuary have been studied. The soil parameters used are clay, carbonate, organic matter, pH, nitrogen, potassium and phosphate. The hypothesis that estuarine soil gradients can be reflections of similar aquatic gradients is affirmed for two gradients. Soil gradients of upstream increasing organic matter and phosphate contents have been found, relating significantly to corresponding aquatic gradients caused by river pollution. In the same direction gradients of decreasing nitrogen and potassium contents occur. A macrogradient in carbonate content was not found. Some microgradients in clay and carbonate contents are described, and relevant mutual relationships between the soil parameters are characterized. Communication nr. 149 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

南四湖区农田土壤有机质和微量元素空间分布特征及影响因素

基于地统计学和GIS技术相结合的方法,研究了南四湖区农田土壤有机质和微量元素的空间分布特征及其影响因素。结果表明,土壤有机质和微量元素均属中等变异程度,除硼符合正态分布外,其余土壤属性均符合对数正态分布。结构分析表明,除硼为纯块金效应外,土壤有机质和其它微量元素空间自相关性较强,其中结构性因素起主导作用。克里格插值结果表明,土壤有机质分布总体趋势为由北向南逐渐降低,锰、铜、锌分布总体趋势为中部高,南北两端低。影响因素分析表明,土壤类型、耕层质地、坡度、土地利用类型和地貌类型对土壤有机质均有显著影响。土壤类型主要是由于成土母质的差异影响土壤有机质的高低与分布,随质地由砂变粘、坡度由低变高,土壤有机质含量逐步升高,田间管理水平的差异是造成不同土地利用类型下土壤有机质含量差异的主要原因。微量元素中,除硼不受影响外,铁、锰、铜和锌与土壤类型、耕层质地、坡度、土地利用类型和地貌类型密切相关。     

Associations between vegetation patterns and soil texture in the shortgrass steppe

A recent conceptual model of controls on vegetation structure in semiarid regions includes the hypothesis that the balance between the dominance of woody and herbaceous species is partly controlled by soil texture. The model predicts that the dominance of woody plants is associated with coarse textured soils, and that ecotones between woody and herbaceous plant functional types are associated with soil textural changes. We analyzed vegetation and soil data (from US Soil Conservation Service maps) for an area of shortgrass steppe in Northern Weld County, Colorado, in a canonical correlation procedure to test the hypothesis at a regional scale. In support of the model, we found significant correlations between (a) a canonical vegetation variable correlated with C₃ grass biomass and shrub biomass, and a canonical soil variable correlated with sandy topsoils, and (b) a canonical vegetation variable correlated with succulent biomass, and a canonical soil variable correlated with clay soils. Relatively sharp transitions between shrub- and grass-dominated vegetation types occur in a number of areas in the shortgrass steppe of northeastern Colorado and southeastern Wyoming, and we selected four sites to test the above hypothesis at a local scale. We gathered data on vegetation cover and soil texture from transects (50 m long) positioned across the transition zones from grassland to shrubland. We conducted a further canonical correlation analysis of the vegetation and soil data to test for the relationships between vegetation structure and soil texture, and a performed regression analyses on individual site data to describe site-specific relationships between vegetation and soil texture variables. Vegetation structure along the transects, at the level of plant functional types, was similar at all four sites. The transition from grassland to shrubland encompassed a change from a C₄ grass/half-shrub complex to a shrub/C₃ grass/succulent complex. At two of the sites these transitions were associated with a change to coarser-textured soils in the shrubland zone. Within the context of the shortgrass steppe, our overall findings support the predictions of the conceptual at a regional scale, but indicate that soil texture is only one factor that can influence vegetation structure at the local scale.     

Microbial characteristics of soils on a latitudinal transect in Siberia

Soil microbial properties were studied from localities on a transect along the Yenisei River, Central Siberia. The 1000 km‐long transect, from 56°N to 68°N, passed through tundra, taiga and pine forest characteristic of Northern Russia. Soil microbial properties were characterized by dehydrogenase activity, microbial biomass, composition of microbial community (PLFAs), respiration rates, denitrification and N mineralization rates. Relationships between vegetation, latitude, soil quality (pH, texture), soil organic carbon (SOC) and the microbial properties were examined using multivariate analysis. In addition, the temperature responses of microbial growth (net growth rate) and activity (soil respiration rate) were tested by laboratory experiments. The major conclusions of the study are as follows: 1. Multivariate analysis of the data revealed significant differences in microbial activity. SOC clay content was positively related to clay content. Soil texture and SOC exhibited the dominant effect on soil microbial parameters, while the vegetation and climatic effects (expressed as a function of latitude) were weaker but still significant. The effect of vegetation cover is linked to SOC quality, which can control soil microbial activity. 2. When compared to fine‐textured soils, coarse‐textured soils have (i) proportionally more SOC bound in microbial biomass, which might result in higher susceptibility of SOC transformation to fluctuation of environmental factors, and (ii) low mineralization potential, but with a substantial part of the consumed C being transformed to microbial products. 3. The soil microbial community from the northernmost study region located within the permafrost zone appears to be adapted to cold conditions. As a result, microbial net growth rate became negative when temperature rose above 5 °C and C mineralization then exceeded C accumulation.     

The Effect of Soil Texture and Irrigation on Rotylenchulus reniformis and Cotton

The reniform nematode, Rotylenchulus reniformis, is the most damaging nematode pathogen of cotton in Alabama. Soil texture is currently being explored as a basis for the development of economic thresholds and management zones within a field. Trials to determine the reproductive potential of R. reniformis as influenced by soil type were conducted in microplot and greenhouse settings during 2008 to 2010. Population density of R. reniformis was significantly influenced by soil texture and exhibited a general decrease with increasing median soil particle size (MSPS). As the MSPS of a soil increased from 0.04 mm in clay soil to > 0.30 mm in very fine sandy loam and sandy loam soils, R. reniformis numbers decreased. The R. reniformis population densities on all soil types were also greater with irrigation. Early season cotton development was significantly affected by increasing R. reniformis Pi, with plant shoot-weight-to-root-weight ratios increasing at low R. reniformis Pi and declining with increasing R. reniformis Pi. Plant height was increased by irrigation throughout the growing season. The results suggests that R. reniformis will reach higher population densities in soils with smaller MSPS; however, the reduction in yield or plant growth very well may be no greater than in a soil that is less preferential to the nematode.     

Study on the Relationship between Soil Selenium and Plant Selenium Uptake

Various extraction methods have been used to determine selenium (Se) concentrations in soils and plants in the second seleniferous regions of China. Our results show tea Se contents in the study area range from 1.009 to 2.6 mg/kg, which reveal that the tea areas in Ziyang County are in seleniferous regions. The four extraction methods evaluated in this study provide different information concerning soil and plant Se levels. The quality control/quality assurance program for this project indicated there is excellent agreement between total soil Se and extractable Se. For example, phosphate extractable Se results from the field investigation and greenhouse study were found to be highly correlated (R ² > 0.91) by linear regression analyses. Results from rye seedling experiments further show phosphate extractable Se has significant correlations with plant Se uptake and that a 0.1 M solution of KH₂PO₄ can be used as the extractant of soil available Se. In the acid soil, the Brassica campestris yield could be significantly reduced when the content of Se⁶⁺–Se ≥ 0.5 mg/kg, and the influence on the yield was not as obvious when the content of Se⁶⁺–Se reached up to 2.0 mg/kg. The uptake by Brassica campestris of Se⁶⁺–Se is higher than that of Se⁴⁺–Se. The main factors influencing the biological availability of soil Se, in order of their importance are CaCO₃, the presence of silt grains, organic matter and the presence of clay grains. pH could affect KH₂PO₄ extractable Se through CaCO₃.     

The role of plant species and soil condition in the structural development of the rhizosphere

Roots naturally exert axial and radial pressures during growth, which alter the structural arrangement of soil at the root–soil interface. However, empirical models suggest soil densification, which can have negative impacts on water and nutrient uptake, occurs at the immediate root surface with decreasing distance from the root. Here, we spatially map structural gradients in the soil surrounding roots using non‐invasive imaging, to ascertain the role of root growth in early stage formation of soil structure. X‐ray computed tomography provided a means not only to visualize a root system in situ and in 3‐D but also to assess the precise root‐induced alterations to soil structure close to, and at selected distances away from the root–soil interface. We spatially quantified the changes in soil structure generated by three common but contrasting plant species (pea, tomato, and wheat) under different soil texture and compaction treatments. Across the three plant types, significant increases in porosity at the immediate root surface were found in both clay loam and loamy sand soils and not soil densification, the currently assumed norm. Densification of the soil was recorded, at some distance away from the root, dependent on soil texture and plant type. There was a significant soil texture × bulk density × plant species interaction for the root convex hull, a measure of the extent to which root systems explore the soil, which suggested pea and wheat grew better in the clay soil when at a high bulk density, compared with tomato, which preferred lower bulk density soils. These results, only revealed by high resolution non‐destructive imagery, show that although the root penetration mechanisms can lead to soil densification (which could have a negative impact on growth), the immediate root–soil interface is actually a zone of high porosity, which is very important for several key rhizosphere processes occurring at this scale including water and nutrient uptake and gaseous diffusion.     

黄土高原盆地土壤有机质与影响因子的空间多尺度关系

不同影响因子对土壤有机质含量的影响存在尺度依赖性。以太原盆地土壤有机质为研究对象,于盆地上、中、下部分别设置采样带,应用多元经验模态分解(multivariate empirical mode decomposition,MEMD),分析了盆地内不同部位土壤有机质与影响因子(高程、坡度、地形湿度指数、土壤容重、砂粒、壤粒、黏粒和光谱主份等)在表征尺度的相关性,并预测了采样尺度上土壤有机质含量,旨在研究黄土高原盆地区内土壤有机质与相关因子的空间多尺度关系。研究结果表明:(1)利用MEMD法可将盆地内不同部位处的土壤有机质空间序列分解为不同表征尺度,盆地上、中和下部的表征尺度分别为6、8和7个。研究区域内,尺度约1000 m处是土壤有机质的主要表征尺度,且盆地内垂直河流方向的有机质序列主要表征尺度沿河流方向表现分散。(2)土壤有机质和影响因子的空间多尺度关系表明,高程与土壤有机质的关系主要表现在大尺度,而坡度、地形湿度指数与盆地中、下部土壤有机质的关系较明显。土壤容重与有机质在不同位置的不同表征尺度存在显著差异。土壤质地中,壤粒含量与有机质的多尺度关系最为明显。光谱主份1在全部样带中所有表征尺度上均与有机质显著相关。(3)采用MEMD法对有机质的预测精度高于基于原始数据的逐步多元回归结果。综上,研究结果可为黄土高原盆地区内土壤数字制图、土壤田块的合理设计与有机质的精确预测提供理论依据。     

Effects of plant hedgerows on soil erosion and soil fertility on sloping farmland in the purple soil area

Effect of using plant hedgerows on controlling soil and water losses has received wide recognition and this technology has been applied in many areas in the world. Yet, studies on the effect of using plant hedgerows on soil fertility on sloping lands are rare. Carrying out an eight-year fixed field experiment, the authors investigated the effect of two different hedgerows against the control treatment on soil fertility. Results showed that clay particles tended to accumulate in front of the plant hedgerows and began to erode downward below the hedgerows along the contour lines across the field. Distribution of soil organic matter and all plant nutrients except potassium (K) showed the same pattern as the clay particles. Potassium, however, was evenly distributed in the field without any noticeable influence from the hedgerows. Since the fixed experiment started, soil phosphorus (P) kept accumulating, while soil organic matter and K were in depletion. The results accordingly suggested better nutrient management practices on the sloping lands by using properly reduced rates of P and increased rates of farm manure and K. Taking the sloping field as a whole, special attention in nutrient management should be given to the soil strips —the portions below the plant hedgerows suffering from more serious soil erosion.     

Distribution of entomopathogenic nematodes in Southern Cameroon

A first survey of entomopathogenic nematodes (EPN) was conducted in three agro-ecological zones of Southern Cameroon in 2007 and 2008. Entomopathogenic nematodes were recovered from 26 of 251 soil samples (10.4%). Three species, Heterorhabditis baujardi, Steinernema sp. A and Steinernema sp. B were found. The two steinernematids were considered unidentified species. Among the positive samples, 23 samples contained only H. baujardi (88.5%), two contained Steinernema sp. A co-occurring with H. baujardi (7.7%), and one sample contained Steinernema sp. B (3.9%). H. baujardi was frequent in forest and fruit crop (cocoa and oil palm plantations). Steinernema sp. A was found in a tree plantation of teak, Steinernema sp. B in a forest habitat. Nematodes were mostly present in acidic soils with pH ranging from 3.7 to 7.0. The highest EPN presence was recorded in sandy loam, sandy clay loam, sandy clay and clay soils. EPNs were not recovered in sand, loamy sand and clay loam soils. Using principal component analysis for elucidating the major variation patterns among sampling sites, four factors explaining for 73.64% of the overall variance were extracted. Factors were a combination of geographical (latitude, longitude, altitude), soil (pH, contents of sand, silt and clay, organic carbon, texture), and moisture (wilting point, field capacity) parameters as well as climatic parameters (mean annual rainfall, mean air temperature). Logistic regression and redundancy analyses (RDA) revealed that soil pH, longitude, available water and altitude were associated with presence and absence of EPN. Both logistic regression and RDA indicated that, increasing soil pH and longitude, associated with decreasing altitude, led to higher percentages of samples containing entomopathogenic nematodes.     

Useful surrogates of soil texture for plant ecologists from airborne gamma‐ray detection

Plant ecologists require spatial information on functional soil properties but are often faced with soil classifications that are not directly interpretable or useful for statistical models. Sand and clay content are important soil properties because they indicate soil water‐holding capacity and nutrient content, yet these data are not available for much of the landscape. Remotely sensed soil radiometric data offer promise for developing statistical models of functional soil properties applicable over large areas. Here, we build models linking radiometric data for an area of 40,000 km² with soil physicochemical data collected over a period of 30 years and demonstrate a strong relationship between gamma radiometric potassium (⁴⁰K), thorium (²³²Th), and soil sand and clay content. Our models showed predictive performance of 43% with internal cross‐validation (to held‐out data) and ~30% for external validation to an independent test dataset. This work contributes to broader availability and uptake of remote sensing products for explaining patterns in plant distribution and performance across landscapes.     

不同改良措施对太子参根际土壤酚酸含量及特异菌群的影响

太子参为石竹科多年生草本植物,以根部入药,药用价值高,市场需求大,但其栽培过程中存在严重的连作障碍问题,连作导致产量品质严重下降,病虫害猖獗,探索太子参连作障碍形成的机制及其消减措施意义重大.本研究以广泛栽培的“柘参2号”为试验材料,采用土壤农化分析、高效液相色谱(HPLC)分析、荧光定量PCR(qPCR)等技术对不同连作年限或不同消减措施下太子参根际土壤的主要养分、酚酸含量动态及特异菌群变化进行分析.结果显示: 太子参连作导致产量显著下降,与正茬相比产量下降达43.5%,而水旱轮作和微生物菌肥处理可不同程度缓解其障碍效应.太子参连作土壤主要养分(如速效氮、速效磷、速效钾、全钾)未降反升,但导致土壤酸化.HPLC分析表明,太子参连作下或者随生长时期的推进,土壤酚酸类物质并未表现出明显的积累效应,膨大中期和收获期重茬土壤中大部分酚酸含量甚至低于正茬土壤.进一步荧光定量PCR分析发现,太子参连作导致前期分离筛选到的几类病原菌(如尖孢镰刀菌、踝节霉菌、Kosakonia sacchari)的绝对含量显著上升,而水旱轮作和微生物菌肥处理可有效降低3类病原菌的含量,改善微生态结构.综上认为,太子参连作障碍的发生并非由土壤养分匮乏或者酚酸持续积累直接造成,可能主要与连作下土传病原微生物的大量繁殖爆发有关,本研究结果为深入揭示太子参连作障碍的形成机制及消减修复机制提供了理论依据.     

Seasonal variations in the diversity and abundance of diazotrophic communities across soils

The nitrogen (N)-fixing community is a key functional community in soil, as it replenishes the pool of biologically available N that is lost to the atmosphere via anaerobic ammonium oxidation and denitrification. We characterized the structure and dynamic changes in diazotrophic communities, based on the nifH gene, across eight different representative Dutch soils during one complete growing season, to evaluate the amplitude of the natural variation in abundance and diversity, and identify possible relationships with abiotic factors. Overall, our results indicate that soil type is the main factor influencing the N-fixing communities, which were more abundant and diverse in the clay soils (n=4) than in the sandy soils (n=4). On average, the amplitude of variation in community size as well as the range-weighted richness were also found to be higher in the clay soils. These results indicate that N-fixing communities associated with sandy and clay soil show a distinct amplitude of variation under field conditions, and suggest that the diazotrophic communities associated with clay soil might be more sensitive to fluctuations associated with the season and agricultural practices. Moreover, soil characteristics such as ammonium content, pH and texture most strongly correlated with the variations observed in the diversity, size and structure of N-fixing communities, whose relative importance was determined across a temporal and spatial scale.     

Effect of long-term application of animal manure on physical properties of three soils

Surface soil samples to 15 cm depth were taken from replicated plots in an ongoing long-term field experiment involving application of animal manure on three soils in Virginia. The sampled plots had received either no manure or the equivalent of 289,000 kg ha^–1 of manure as dry weight. The manure was applied annually at the beginning of each spring for 15 years from 1978 through 1992. The plots were cropped similarly since 1978. Soil textures were a fine sandy loam at Holland in the Atlantic Coastal Plain region, a silt loam at Blacksburg in the Appalachian region, and a clay loam at Orange in the Piedmont region of Virginia. The following measurements were made on subsamples: liquid and plastic limits, wet aggregate stability, aggregate size distribution, dispersible clay percentage, water retention at 0. 03, 0.1, 0.3, 0.5, 1.0, and 1.5 MPa tension, and modulus of rupture of moulded briquettes at a water content corresponding to 0.1 MPa tension. Organic matter content by the Walkley-Black method was significantly higher in the manure-treated soils at all three locations. Increases were 3% for the sandy loam and 25% for the silt loam and clay loam. From these values it was estimated that at least 95% of the total applied manure had been degraded over the 15 years. Results showed that the liquid and plastic limits for all three soils were higher (p<0.05) for the manure-treated samples. However, the differences in the limits were only 2 to 3%. The modulus of rupture values were lowered by addition of the animal manure. Decreases (p<0.05) occurred for the silt loam and clay loam samples. The wet aggregate stability increased and the dispersible clay decreased in the manure-treated soils. Increases (p<0.05) in wet aggregate stability occurred for the sandy loam and silt loam samples. Decreases (p<0.05) in dispersible clay were measured for the sandy loam and clay loam samples. Water retention was consistently, but only slightly, increased by manure addition. The increases, in the order of sample texture, were clay loam > sandy loam silt loam. Increases tended to be higher at the lower values of tension. Manure addition consistently increased the weight percentages of aggregates passing a given mesh size. Increases, in order of sample texture, were silt loam > clay loam > sandy loam. In their entirety, these results show that the manure produced measurable changes in the soil physical properties. The magnitude of the changes, in most cases, were small and depended on the soil texture. Given the high total amount of manure applied, the results indicate that manure-induced physical changes in the soil were small and evidently did not accumulate over time. Rapid microbial degradation of the manure could be responsible for the lack of marked changes in the soil physical properties.