Interactions between biological processes,cultivation and soil structure

Summary An inherent (autochthonous) biomass is characteristic of a soil while the input of substrates for plant roots or crop residues promotes the transient (zymogenous) biomass. However successful micro-organisms will show aspects of both types of ecological strategy. The biomass generated from plant residue substrates can include toxin-producing and pathogenic species but also beneficial organisms such as N-fixers and polysaccharide-producers. Rhizosphere activity can, depending on soil, plant and microbial species, stabilize or destabilize soils. Microbial activity should be considered in soil management and it may be possible to manipulate the soil population balance towards beneficial organisms.Keynote address     

Changes in soil properties and vegetation following livestock grazing exclusion in degraded arid environments of South Tunisia

In this study, characteristics of vegetation and soil properties under continued grazing and exclusion of livestock for 6 and 12 years were examined in a degraded Stipa tenacissima steppe in South Tunisia.     

Effects of grazing and soil micro-climate on decomposition rates in a spatio-temporally heterogeneous grassland

Grazing and seasonal variation in precipitation and temperature are important controls of soil and plant processes in grasslands. As these ecosystems store up to 30% of the world’s belowground carbon (C), it is important to understand how this variability affects mineral soil C pools/fluxes, and how C cycling might be affected by changes in precipitation and temperature, due to climate change. The aim of this study was to investigate the effects of grazing and differences in soil temperature and moisture on standard organic matter (OM) decomposition rates (cotton cloth) incubated in the top 10 cm soil of grasslands with variable topography in Yellowstone National Park (YNP) during the 2004 growing season. Grazing did not affect soil temperature, moisture, cotton cloth decomposition rates, soil bulk density, soil C and N concentrations, or soil C:N ratios. However, a large spatio-temporal variability in decomposition was observed: cotton cloth decomposition was positively related to soil moisture and soil C and N concentrations, and negatively to soil temperature. Highest decomposition rates were found in wetter slope bottom soils [season averages of decomposition given as rate of decomposition (cotton rotting rate = CRR) = 23–26%] and lower rates in drier, hill-top soils (season averages, CRR = 20%). Significantly higher decomposition rates were recorded in spring, early summer and early fall when soils were moist and cool (spring, CRR = 25%; early summer, CRR = 26%; fall, CRR = 20%) compared to mid-summer (CRR = 18%) when soils were dry and warm. Our findings suggest that climate-change related decreases in precipitation and increases in temperature predicted for North American grasslands would decrease soil OM decomposition in YNP, which contrasts the general assumption that increases in temperature would accelerate OM decomposition rates.     

栽培模式对旱地小麦产量和土壤肥力的影响

通过田间试验,研究地膜覆盖、秸秆还田和种植绿肥对冬小麦籽粒产量和土壤肥力的影响.结果表明: 与传统模式相比,地膜覆盖并不总能提高旱地小麦产量,3年平均产量无显著变化,但降低20～40 cm土壤全氮、有效磷、速效钾、有效硫、有效锌和有效锰含量,对土壤有机质、硝态氮、有效铁和有效铜含量无显著影响.秸秆还田的小麦产量3年平均降低12.1%,收获期0～20 cm土层全氮提高5.8%,20～40 cm土层有效铜含量提高6.2%,而有效磷和有效锰分别降低36.1%和10.2%,对开花期和收获期土壤有机质、硝态氮、速效钾、有效硫、有效锌和有效铁无显著影响.种植绿肥的籽粒产量降低12.1%,同时土壤pH、有效磷和有效硫含量降低,有机质、全氮、硝态氮、有效锌和有效锰均增加,对土壤速效钾、有效铁和有效铜均无显著影响.综上,在旱地土壤肥力较低的条件下,地膜覆盖和秸秆还田不利于土壤肥力的提升,使小麦增产受到限制;种植绿肥培肥效果最好,但应考虑区域降水情况,注意其带来的减产问题.     

Effects of grazing on the abundance and diversity of annual plants in Chihuahuan desert scrub habitat

We assess the impact of release from cattle grazing on the abundance and diversity of both winter and summer annual plant communities at an upper Chihuahuan Desert scrub site in south-eastern Arizona. In contrast to previous studies, we found that removal of herbivores (cattle) had little impact on ephemeral plant assemblages at our site. The total number of summer annual individuals per quadrat did not differ significantly, but there were significantly more winter annual plants on ungrazed quadrats. The number of species per quadrat, however, did not differ significantly between sites exposed to, or protected from, grazing in either season. Of 79 annual species recorded (34 in winter, 45 in summer), only 2 species, 1 in each season, responded significantly to the removal of cattle: Stephanomeria exigua and Polygala tweedyi were more abundant on ungrazed plots. Three additional species, Eriastrum diffusum and Cryptantha micrantha in winter, and Mollugo cerviana (summer), approached statistical significance. Differences in the effect of cattle grazing on annual plants between our results and those at other sites in the arid southwest most likely reflect differences in the speed of response by annuals in different areas. Comparisons of this with other studies underscores recent calls for studies at broader spatial and greater temporal scales.     

Effects of pesticides and heavy metals on biological processes in soil

Summary The side-effects of pesticides and metal cations are reviewed with regard to input, distribution, persistence and toxicity. The sensitivity of microbial processes is discussed in relation to adaptability and resistance of microbial populations. Procedures for an ecological assessment of toxic side-effects are discussed.Keynote address     

Indirect effects of habitat disturbance on invasion: nutritious litter from a grazing resistant plant favors alien over native Collembola

Biological invasions are major threats to biodiversity, with impacts that may be compounded by other forms of environmental change. Observations of high density of the invasive springtail (Collembola), Hypogastrura manubrialis in heavily grazed renosterveld vegetation in the Western Cape, South Africa, raised the question of whether the invasion was favored by changes in plant litter quality associated with habitat disturbance in this vegetation type. To examine the likely mechanisms underlying the high abundance of H. manubrialis, cages with three types of naturally occurring litter with different nutrient content were placed out in the area and collected after different periods of time. Hypogastrura manubrialis was mainly found in the nutrient‐rich litter of the yellowbush (Galenia africana), which responds positively to disturbance in the form of overgrazing. This suggests that invasion may have been facilitated by a positive interaction with this grazing resistant plant. By contrast, indigenous Collembola were least abundant in yellowbush litter. Negative correlations between high abundance of H. manubrialis and the abundance and diversity of other species suggest that competitive interactions might underlie low abundance of these other species at the patch level. Group behavior enables H. manubrialis to utilize efficiently this ephemeral, high quality resource, and might improve its competitive ability. The results suggest that interactions among environmental change drivers may lead to unforeseen invasion effects. H. manubrialis is not likely to be very successful in un‐grazed renosterveld, but in combination with grazing, favoring the nutrient‐rich yellowbush, it may become highly invasive. Field manipulations are required to fully verify these conclusions.     

Effects of roots and litter on mineralization processes in forest soil

Summary Leaf litter breakdown and fine root production, including exudation, are two major influences upon carbon and nitrogen mineralization rates in forest soil. Sieving and root removal experiments were used to examine their effects. Although carbon mineralization rates declined in smaller particle size fractions of forest litter, this trend largely disappeared when results were calculated on an ash-free basis. Nitrogen mineralization by contrast, was greatest in smaller fractions.Much of the variation in carbon mineralization rates appeared to be associated with fine roots. A rapid initial exponential decay phase noted in laboratory respiration studies was probably associated with disappearance of available carbon in the form of root exudates and/or the microorganisms dependent on them. Clear cutting caused a marked reduction in the size of available carbon pools, reflecting decreased root exudation and rhizosphere activity. A model of mineralization is proposed which represents the available and humified carbon pools.Deceased     

不同森林恢复类型对土壤微生物群落的影响

为了评价不同森林恢复类型与方式对南方红壤丘陵区退化生态系统土壤微生物群落的影响,借助氯仿熏蒸法、平板涂抹法和BIOLOG检测法,比较研究了4种森林恢复类型土壤微生物的群落特征．结果表明,4种森林恢复类型土壤微生物生物量碳、细菌数量差异显著,2项指标均以天然次生林土壤最高,人工林次之,荒地最差;碳源平均颜色变化率(AWCD法)和微生物代谢多样性指数(丰富度和多样性)在5种植被类型的土壤中也有明显差异,其趋势与微生物量碳、细菌数量基本相同;天然次生林土壤微生物群落利用碳源的整体能力和功能多样性比人工林和荒地强．相关分析表明,0～20和20～40cm土壤微生物的代谢多样性与根系生物量紧密相关(r=0．933,P<0．05;r=0．925,P<0．05)．自然恢复更有利于改善土壤微生物的结构和功能．     

Relationship between soil-stored seed banks and degradation in eastern Nama Karoo rangelands (South Africa)

Landscape, local scale and seasonal composition of soil-stored seed banks of a southern African semi-arid rangeland (Nama Karoo) were investigated through assessment of seed germinated from field-collected soil. Variables included percentage above-ground canopy cover, numbers of seeds germinated, and selected physical and chemical soil characteristics. Organic matter content of soil sampled from under plant canopies (closed-canopy) is significantly higher than that of inter-canopy samples (open-canopy). Significantly more seeds germinated from closed-canopy soil samples are compared with those from open-canopy microsites. It is argued that reduction in rangeland canopy cover due to overgrazing (and resultant erosion through wind and water movement) leads to reduced seed retention by the leaf litter layer and thus by the soil. This degraded resource appears to have gradually lost an inherent buffer capacity, undermining attempts at ecological restoration and necessitating concerted and directed efforts to restore these fragile systems.     

季节变化对沙漠生态系统土壤中自由生活线虫和原生动物间营养关系的影响

选择沙漠常绿灌木丛枝霸王(Zygophyllum dumosum)群落及群落间的开阔地带采样,研究土壤线虫和原生动物的种群大小、物种组成及营养结构.本研究共获线虫29属,裸变形虫33属90种;在所获各类原生动物中,鞭毛虫和纤毛虫均为食细菌类群,而鞭毛虫个体数量最多;在沙漠生态系统中,土壤湿度被认为是影响生物活性的最重要因素之一.然而,本研究显示:土壤湿度对线虫和原生动物是否重要取决于这些动物对土壤湿度的生理生态适应能力;土壤线虫和原生动物的体型大小、生活史策略和活动能力与土壤湿度的有效性相关不显著,但它们对微生物和微小土壤动物区系间的相互作用有重要影响,在干旱生态系统中,这种影响能够显著改变土壤生态系统短期的养分循环.本研究还表明,土壤线虫和原生动物种群间的关系主要取决于两者营养类群组成的变化.     

放牧对贝加尔针茅草原植被根系分布和土壤理化特征的影响

以内蒙古贝加尔针茅草原为研究对象,分析了放牧对植被根系分布特征的影响及其与土壤理化性状的相互关系。结果表明:放牧导致植被根系分布发生显著变化;0～10cm土层根系生物量占0～40cm根系总生物量的65.9%～82.6%,直径≥2mm根系和直径1mm的根系对放牧影响较为敏感;植被根系分布与土壤容重、含水量、全氮、全磷、有机碳含量以及阳离子交换量有一定程度的相关性,放牧主要通过改变土壤理化特征影响不同径级根系生物量的分布,同时各径级根系分布特征对土壤理化性状有一定的反馈作用。     

沼泽湿地垦殖前后土壤温度变化及其对土壤热状况的影响

沼泽湿地具有重要的生态与环境功能,其对全球气候变化较为敏感,土壤温度变化能够很好地指示气候的波动.沼泽湿地土壤温度呈明显的"正弦曲线"型年、季动态,不同深度土壤年均温度呈"U"型特征.5～9月份沼泽湿地10 cm土壤平均温度为11.69±3.04℃,垦殖后农田土壤为1.80±3.41℃.沼泽湿地土壤8、9月份土壤呼吸分别为156.41±76.91 mg·m^-2·h^-1和116.75±57.43 m^-2·h^-1,是同时期农田土壤呼吸通量的14.6%和13.1%,土壤温度与土壤呼吸通量呈显著正相关关系.     

Form and function of grass ring patterns in arid grasslands: the role of abiotic controls

Ring-shaped growth patterns commonly occur in resource-limited arid and semi-arid environments. The spatial distribution, geometry, and scale of vegetation growth patterns result from interactions between biotic and abiotic processes, and, in turn, affect the spatial patterns of soil moisture, sediment transport, and nutrient dynamics in aridland ecosystems. Even though grass ring patterns are observed worldwide, a comprehensive understanding of the biotic and abiotic processes that lead to the formation, growth and breakup of these rings is still lacking. Our studies on patterns of infiltration and soil properties of blue grama (Bouteloua gracilis) grass rings in the northern Chihuahuan desert indicate that ring patterns result from the interaction between clonal growth mechanisms and abiotic factors such as hydrological and aeolian processes. These processes result in a negative feedback between sediment deposition and vegetation growth inside the bunch grass, which leads to grass die back at the center of the grass clump. We summarize these interactions in a simple theoretical and conceptual model that integrates key biotic and abiotic processes in ring formation, growth and decline.     

放牧对草地土壤氮素循环关键过程的影响与机制研究进展

放牧是人类管理利用草地生态系统的最主要途径之一.食草动物的采食、践踏、卧息和排泄物归还等干扰不仅会改变草地地上植物群落,也会对土壤养分循环产生显著的影响.随着人类活动的加剧,放牧强度和频率也在逐渐增加,从而对草地土壤氮素循环关键过程产生重要影响.放牧主要通过改变土壤的物理性质、土壤氮库以及微生物的组成和结构,进而影响氮素在土壤中的迁移与转化.适度放牧会促进土壤氮素的矿化过程和硝化过程,加快氮素的周转,有利于植物吸收可利用氮素,而对于反硝化的影响与草地的水热条件和土壤类型等密切相关.目前,关于放牧强度对土壤氮素循环关键过程影响的研究结果不一致,其影响机制尚不明晰,尤其对于不同类型的草原仍存在较大的差异.本研究在大量查阅国内外已有研究结果的基础上,论述了放牧对土壤氮素循环关键过程的影响效应,总结了放牧对土壤氮素循环的影响机制,指出了目前研究过程中存在的不足,并对未来研究中值得重点关注和深入研究的科学问题进行了探讨与展望,为进一步理解放牧对草地土壤氮素循环的影响提供参考.     

绿洲农业不同种植方式防止土壤风蚀和保持土壤水分的比较

2001年～2002年,在河西走廊中断黑河中游,研究了不同方式对春季就地起沙、风速梯度、水分保持等方面的效应．结果表明,冬灌免耕、粮草间作和早春覆膜措施都不同程度地遏制了就地起沙,减少了土壤风蚀．早春覆膜在减少土壤风蚀量的同时,提高了土壤蓄水量,比春耕裸露农田蓄水量增加了35．6％．目前．春耕播种裸地是形成沙尘暴和浮尘的粉尘含量的主要来源之一,黑河中、上游农田耕地每年产生的粉尘输移量达4．8×10^6～6．0×10^6t,高于同区域沙漠粉尘输移量．河西走廊粉尘输移量1hm^2农田相当于1．5hm^2沙漠．     

Plant N capture from pulses: effects of pulse size,growth rate,and other soil resources

In arid ecosystems, the ability to rapidly capture nitrogen (N) from brief pulses is expected to influence plant growth, survival, and competitive ability. Theory and data suggest that N capture from pulses should depend on plant growth rate and availability of other limiting resources. Theory also predicts trade-offs in plant stress tolerance and ability to capture N from different size pulses. We injected K¹⁵NO₃, to simulate small and large N pulses at three different times during the growing season into soil around the co-dominant Great Basin species Sarcobatus vermiculatus, Chrysothamnus nauseosus ssp. consimilis, and Distichlis spicata. Soils were amended with water and P in a partial factorial design. As predicted, all study species showed a comparable decline in N capture from large pulses through the season as growth rates slowed. Surprisingly, however, water and P availability differentially influenced the ability of these species to capture N from pulses. Distichlis N capture increased up to tenfold with water addition while Chrysothamnus N capture increased up to threefold with P addition. Sarcobatus N capture was not affected by water or P availability. Opposite to our prediction, Sarcobatus, the most stress tolerant species, captured less N from small pulses but more N from large pulses relative to the other species. These observations suggest that variation in N pulse timing and size can interact with variable soil water and P supply to determine how N is partitioned among co-existing Great Basin species.     

菠萝-甘蔗轮作的土壤生态效应

通过2000～2002年进行菠萝-甘蔗轮作田间试验,研究了菠萝、甘蔗轮作对甘蔗土壤生态的影响。结果表明：菠萝甘蔗轮作田与甘蔗连作田比较,能明显地改善土壤的通气和蓄水性,加速速效性养分的释放;增强土壤酶活性;增加土壤微生物总量(一般好气性细菌、真菌、放线菌);有益的氨化细菌和硝化细菌成倍增加,而无益的厌氧性细菌、反硝化细菌则受到抑制;固氮菌数量小且变化不大。菠萝-甘蔗轮作对甘蔗土壤生态环境的改善其实质在于：菠萝、甘蔗轮作后,由于其作物种类的不同导致土壤结构的改善和土壤微生物种类和数量增多,土壤生物活性增强,并导致土壤速效养分的增加。     

长期施肥对土壤微生物的影响及其在养分调控中的作用

为揭示长期施肥条件下土壤微生物类群及其与土壤养分的关系,对北京褐潮土定位试验田第12年的土壤微生物以及土壤养分因子进行了测定和分析．结果表明,NPK与有机肥长期配合施用能明显提高土壤各养分含量,增加土壤微生物的数量．猪粪比玉米秸秆能更好地培肥地力,创造有利于土壤微生物生长繁育的土壤生态化学环境．土壤微生物与土壤养分之间具有很好的相关性．其中,细菌、固氮菌数量与有机质、全氮、碱解氮、全磷、速效磷含量呈显著正相关;本研究未观察到真菌、放线菌数量与土壤养分因子之间有明显的相关性．