Role of Soil Organisms in the Maintenance of Species-Rich Seminatural Grasslands through Mowing

To preserve species-rich grasslands, management practices such as mowing are often required. Mowing is known to promote aboveground conditions that help to maintain plant species richness, but whether belowground effects are important as well is not known. We hypothesized that if mowing decreases belowground carbon transfer by reducing root mass, this will reduce the abundance and activity of soil decomposers and lead to diminished nutrient availability in soil. In grasslands, this would provide a means to mitigate the negative effects of nitrogen enrichment on plant species richness. We established experimental plots on grassland with one-third of plots growing untouched, one-third mowed once a summer, and one-third mowed twice a summer for three growing seasons. Root and soil attributes were measured at each plot 1 month after each mowing. Mowing decreased root mass but had few effects on belowground biota and soil NH₄-N, NO₃-N, and PO₄-P concentrations. Mowing did not affect root N concentrations. Our results show that despite reducing root mass, mowing may have few effects on those soil biota that control nutrient supply and, as a result, have no clear-cut effects on nutrient availability in soil. This suggests that the effects of mowing on soil decomposers and soil nutrient availability may not provide an effective means to mitigate N enrichment and enhance plant species richness in grasslands in a timescale of a few years. Whether such effects could, however, be achieved with longer lasting mowing remains open.     

Simultaneous feeding by aboveground and belowground herbivores attenuates plant-mediated attraction of their respective natural enemies

Herbivore-damaged plants emit volatile organic compounds that attract natural enemies of the herbivores. This form of indirect plant defence occurs aboveground as well as belowground, but it remains unclear how simultaneous feeding by different herbivores attacking leaves and roots may affect the production of the respective defence signals. We employed a setup that combines trapping of volatile organic signals and simultaneous measurements of the attractiveness of these signals to above and belowground natural enemies. Young maize plants were infested with either the foliar herbivore Spodoptera littoralis , the root herbivore Diabrotica virgifera virgifera , or with both these important pest insects. The parasitic wasp Cotesia marginiventris and the entomopathogenic nematode Heterorhabditis megidis were strongly attracted if their respective host was feeding on a plant, but this attraction was significantly reduced if both herbivores were on a plant. The emission of the principal root attractant was indeed reduced due to double infestation, but this was not evident for the leaf volatiles. The parasitoid showed an ability to learn the differences in odour emissions and increased its response to the odour of a doubly infested plant after experiencing this odour during an encounter with hosts. This first study to measure effects of belowground herbivory on aboveground tritrophic signalling and vice-versa reemphasizes the important role of plants in bridging interactions between spatially distinct components of the ecosystem.     

Above‐ and belowground insect herbivores mediate the impact of nitrogen eutrophication on the soil food web in a grassland ecosystem

Insect herbivores are important drivers of ecosystem processes in grasslands, and can mediate the grassland's response to environmental change. For example, recent evidence shows that above‐ and belowground herbivory, individually and in combination, can modify how a plant community responds to nitrogen (N) eutrophication, an important driver of global change. However, knowledge about how such effects extend to the associated soil food web is lacking. In a mesocosm experiment, we investigated how communities of soil nematodes – an abundant and functionally important group of soil organisms – responded to above‐ and belowground insect herbivory at contrasting N levels. We found that the strongest influence of above‐ and belowground herbivory on the nematode community appeared at elevated N. The abundance of root‐feeding nematodes increased when either above‐ or belowground insect herbivores were present at elevated N, but when applied together the two herbivore types cancelled out one another's effect. Additionally, at elevated N aboveground herbivory increased the abundance of fungal‐feeders relative to bacterial‐feeders, which indicates changes in decomposition pathways induced by N and herbivory. Belowground herbivory increased the abundance of omnivorous nematodes. The shifts in both the herbivorous and detrital parts of the soil food web demonstrate that above‐ and belowground herbivory does not only mediate the response of the plant community to N eutrophication, but in extension also the soil food web sustained by the plant community. We conclude that feedbacks between effects of above‐ and belowground herbivory mediate the response of the grassland ecosystem to N eutrophication.     

荒漠草原典型群落土壤粒径和养分的分布特征及其关系研究

为了明确荒漠草原区土壤机械组成与养分的关系,以宁夏盐池荒漠草原4种典型群落为研究对象,通过对不同群落(柠条、沙蒿、蒙古冰草、短花针茅)表层(0~5cm)、亚表层(5~10cm)和深层(10~15cm)土壤粒径分布分形(PSD)、养分含量的动态变化分析,揭示荒漠草原区土壤结构与土壤养分的相关性。结果表明:(1)4种典型群落土壤PSD均呈正态分布,不同群落间的土壤PSD差异显著,粒径100~500μm颗粒含量对PSD影响最大,不同群落间的差异大于不同生境间或不同土层间。(2)4种典型群落除全磷(TP)外,其余土壤全肥均随土壤深度增加呈降低趋势,且冠下大于丛间,表现出荒漠草原区特殊的"肥岛"聚集效应,不同群落间分布特征均表现为:柠条短花针茅蒙古冰草沙蒿,速效养分含量相对较高,各群落均达到适宜水平。(3)土壤养分与土壤PSD显著相关,除速效磷(AP)外,其余土壤养分与土壤分形维数(D)均呈正相关关系,粒径100~250μm、250~500μm颗粒与土壤养分呈显著或极显著负相关关系,土壤中的黏粒、粉粒在有机无机胶结过程及土壤良好的结构维持中起主要作用。     

Soil biota composition and the performance of a noxious weed across its invaded range

The success of invasive plant species is driven, in part, by feedback with soil ecosystems. Yet, how variation in belowground communities across latitudinal gradients affects invader distributions remains poorly understood. To determine the effect of soil communities on the performance of the noxious weed Cirsium arvense across its invaded range, we grew seedlings for 40 days in soils collected across a 699 km linear distance from both inside and outside established populations. We also described the mesofaunal and bacterial communities across all soil samples. We found that C. arvense typically performed better when grown in soils sourced from northern populations than from southern locations where it has a longer invasion history. We also found evidence that C. arvense performed best in soils sourced from outside invaded patches, although this was not consistent across all sites. The bacterial community showed a significant increase in the magnitude of compositional change in invaded sites at higher latitudes, while the mesofaunal community showed the opposite pattern. Bacterial community composition was significantly correlated with C. arvense performance, although mesofaunal community composition was not. Our results demonstrate that the interactions between an invasive plant and associated soil communities change across the invaded range, and the bacterial community in particular may affect variation in plant performance. Observed patterns may be caused by C.arvense presence and time since invasion allowing for an accumulation of species‐specific pathogens in southern soils, while the naïveté of northern soils to invasion results in a more responsive bacterial community. Although these interactions are difficult to predict, such effects could possibly facilitate the establishment of this exotic species to novel locations.     

Global change belowground: impacts of elevated CO2, nitrogen,and summer drought on soil food webs and biodiversity

The world's ecosystems are subjected to various anthropogenic global change agents, such as enrichment of atmospheric CO₂ concentrations, nitrogen (N) deposition, and changes in precipitation regimes. Despite the increasing appreciation that the consequences of impending global change can be better understood if varying agents are studied in concert, there is a paucity of multi‐factor long‐term studies, particularly on belowground processes. Herein, we address this gap by examining the responses of soil food webs and biodiversity to enrichment of CO₂, elevated N, and summer drought in a long‐term grassland study at Cedar Creek, Minnesota, USA (BioCON experiment). We use structural equation modeling (SEM), various abiotic and biotic explanatory variables, and data on soil microorganisms, protozoa, nematodes, and soil microarthropods to identify the impacts of multiple global change effects on drivers belowground. We found that long‐term (13‐year) changes in CO₂ and N availability resulted in modest alterations of soil biotic food webs and biodiversity via several mechanisms, encompassing soil water availability, plant productivity, and – most importantly – changes in rhizodeposition. Four years of manipulation of summer drought exerted surprisingly minor effects, only detrimentally affecting belowground herbivores and ciliate protists at elevated N. Elevated CO₂ increased microbial biomass and the density of ciliates, microarthropod detritivores, and gamasid mites, most likely by fueling soil food webs with labile C. Moreover, beneficial bottom‐up effects of elevated CO₂ compensated for detrimental elevated N effects on soil microarthropod taxa richness. In contrast, nematode taxa richness was lowest at elevated CO₂ and elevated N. Thus, enrichment of atmospheric CO₂ concentrations and N deposition may result in taxonomically and functionally altered, potentially simplified, soil communities. Detrimental effects of N deposition on soil biodiversity underscore recent reports on plant community simplification. This is of particular concern, as soils house a considerable fraction of global biodiversity and ecosystem functions.     

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO₂ to the atmosphere. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the rates of CO₂ production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO₂ in the soil air, but the sources of this CO₂ and the spatiotemporal dynamics of CO₂ production throughout the soil profile are poorly understood. We studied CO₂ dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO₂ concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO₂ production with soil profile depth. The CO₂ diffusion constants were checked for accuracy using measured soil air ²²²Rn activities. The modeled net CO₂ production rates were compared with CO₂ fluxes measured at the soil surface. In general, the modeled and measured net CO₂ fluxes were very similar although the model consistently underestimated CO₂ production rates in the surficial soil horizons when the soils were moist. Profile CO₂ production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO₂ production in both the surface and subsurface soil horizons. Whole profile CO₂ production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO₂ flux from the soil profiles are relatively low (4–11 mg C– CO₂ m⁻² h⁻¹), 20%–50% of the CO₂ diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance ¹⁴C signatures of the CO₂ and soil organic C suggest that the subsurface CO₂ is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.     

Plant and Soil Responses to Created Microtopography and Soil Treatments in Bottomland Hardwood Forest Restoration

Although microtopographic heterogeneity is common in bottomland hardwood forests, it is rarely considered in bottomland restoration efforts. The objective of this study was to determine the responses of hydrologic condition, soil physiochemical properties, and introduced and colonizing vegetation to created microtopography and soil treatments at a landfill borrow pit in northern Texas. A series of mounds and pools were created and planted with fast‐growing pioneer species as well as more desirable, later‐successional species. Erosion control mats were installed on half the plots as a source of organic matter. Erosion control mats had little influence on introduced seedling survival or colonizing species abundance, but microtopography strongly influenced hydrologic condition, soil properties, seedling survival and growth, and colonizing species abundance and distribution. Pools were flooded during much of the summer months and had significantly higher nitrate and total nitrogen concentrations than mounds. Topographic position had little effect on survival of pioneer species, but mortality of most later‐successional species was highest in pools. Colonizing species distribution and abundance were also strongly related to topographic position. Despite differences in soil nutrient concentration among topographic zones, hydrologic condition likely had the strongest influence on growth and survival of planted species and distribution of colonizing species. Creating microtopography resulted in a spatially heterogeneous system that reflected variations in natural bottomlands, and introducing a mix of species (pioneer and later‐successional) across topographic and hydrologic gradients may improve the establishment and survival of a diverse community when hydrologic condition is highly variable or difficult to predict.     

Comparison of Soil Bacterial Communities in Rhizospheres of Three Plant Species and the Interspaces in an Arid Grassland

Soil bacteria are important contributors to primary productivity and nutrient cycling in arid land ecosystems, and their populations may be greatly affected by changes in environmental conditions. In parallel studies, the composition of the total bacterial community and of members of the Acidobacterium division were assessed in arid grassland soils using terminal restriction fragment length polymorphism (TRF, also known as T-RFLP) analysis of 16S rRNA genes amplified from soil DNA. Bacterial communities associated with the rhizospheres of the native bunchgrasses Stipa hymenoides and Hilaria jamesii, the invading annual grass Bromus tectorum, and the interspaces colonized by cyanobacterial soil crusts were compared at three depths. When used in a replicated field-scale study, TRF analysis was useful for identifying broad-scale, consistent differences in the bacterial communities in different soil locations, over the natural microscale heterogeneity of the soil. The compositions of the total bacterial community and Acidobacterium division in the soil crust interspaces were significantly different from those of the plant rhizospheres. Major differences were also observed in the rhizospheres of the three plant species and were most apparent with analysis of the Acidobacterium division. The total bacterial community and the Acidobacterium division bacteria were affected by soil depth in both the interspaces and plant rhizospheres. This study provides a baseline for monitoring bacterial community structure and dynamics with changes in plant cover and environmental conditions in the arid grasslands.     

Soil legacy effects of plants and drought on aboveground insects in native and range-expanding plant communities

Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought.     

荒漠草原3种典型群落类型下土壤粒径分布与微生物量碳氮的关系

为了探讨不同群落类型的土壤粒径分布（PSD）与土壤微生物生物量（MB）的关系,以宁夏盐池荒漠草原3种典型群落类型（冰草、沙蒿、短花针茅）为研究对象,测定了不同群落2种生境（冠下、丛间）0~5、5~10和10~15 cm表土层土壤PSD和微生物量碳（MBC）、微生物量氮（MBN）含量变化,并分析了土壤颗粒组成中砂粒、粉粒和黏粒体积分数变化与土壤有机碳（SOC）、全氮（TN）和MBC、MBN间的关系。结果表明：（1）不同群落类型土壤粒径都呈“倒V”型分布趋势,但土壤退化最严重的沙蒿群落中100~500 μm粒径颗粒含量相对较多,与其他两种群落形成显著差异。（2）不同群落类型SOC、MBC、MBN含量均随土层深度的增加而降低,同土层SOC、MBN含量均表现为冠下明显大于丛间,表现出“肥岛效应”,且0~5 cm土层差异显著（P＜0.05）。（3）对土壤粒径组成与土壤SOC、MBC、MBN间相关性研究表明,在土壤SOC、MBC和MBN含量较高的冰草、短花针茅群落类型中,0.01~2、2~50和50~100 μm土壤粒径的颗粒含量也高,SOC、MBC和MBN含量与＜100 μm的粉粒含量呈正相关关系;在沙蒿群落类型中粒径为100~250和250~500 μm的土粒含量增高,导致其SOC、MBC和MBN含量较低,表明不同群落类型对土壤理化结构产生影响的同时,对微生物生物量也有显著的影响。     

Effects of Native and Non-Native Grassland Plant Communities on Breeding Passerine Birds: Implications for Restoration of Northwest Bunchgrass Prairie

One common problem encountered when restoring grasslands is the prominence of non-native plant species. It is unclear what effect non-native plants have on habitat quality of grassland passerines, which are among the most imperiled groups of birds. In 2004 and 2005, we compared patterns of avian reproduction and the mechanisms that might influence those patterns across a gradient of 13 grasslands in the Zumwalt Prairie in northeastern Oregon that vary in the degree of non-native plant cover (0.9–53.4%). We monitored the fate of 201 nests of all the breeding species in these pastures and found no association of percent non-native cover with nest densities, clutch size, productivity, nest survival, and nestling size. Regardless of the degree of non-native cover, birds primarily fed on Coleoptera, Orthoptera, and Araneae. But as percent non-native cover in the pastures increased, Orthoptera made up a greater proportion of diet and Coleoptera made up a smaller proportion. These diet switches were not the result of changes in terrestrial invertebrate abundance but may be related to decreases in percent bare ground associated with increasing cover of non-native vegetation. Measures of nest crypticity were not associated with cover of non-native vegetation, suggesting that predation risk may not increase with increased cover of non-native vegetation. Thus, the study results show that increased non-native cover is not associated with reduced food supplies or increased predation risk for nesting birds, supporting the growing body of evidence that grasslands with a mix of native and non-native vegetation can provide suitable habitat for native grassland breeding birds.     

Effects of Soil Nutrient Heterogeneity and Earthworms on Aboveground Biomass of Experimental Plant Communities

Soil nutrients are commonly heterogeneously distributed and earthworms are one of the most common soil organisms. While effects of both soil nutrient heterogeneity and earthworms have been well studied, their interactive effect on plant community productivity has rarely been tested. In a greenhouse experiment, we constructed experimental plant communities by sowing seed mixtures of four grasses, two legumes and two forbs in either a heterogeneous soil consisting of low and high nutrient soil patches or a homogeneous soil where the low and high nutrient soil patches were evenly mixed. The earthworm Eisenia fetida was either added to these soils or not. Aboveground biomass of the whole communities, grasses and legumes did not differ between the homogeneous and heterogeneous soils or between the soils with and without earthworms. However, soil nutrient heterogeneity reduced aboveground biomass of forbs, and such an effect did not interact with earthworms. In response to soil heterogeneity and earthworms, biomass ratio of the three functional groups showed similar patterns as that of their biomass. At the patch level, aboveground biomass of the whole community, grasses and legumes were greater in the high than in the low nutrient soil patches within the heterogeneous soil. A similar pattern was found for the forbs, but this was only true in the absence of earthworms. Our results suggest that soil nutrient heterogeneity and earthworms may not influence aboveground biomass of plant communities, despite the fact that they may modify the growth of certain plant functional groups within the community.     

荒漠草原4种常见植物群落土壤酶活性比较

该研究以宁夏盐池县沙边子地区4种常见的植物群落苦豆子(Sophora alopecuroides)、芨芨草(Achnatherum splendens)、油蒿(Artermisia ordosica)和盐爪爪(Kalidium foliatum)为研究对象,通过对群落组成和土壤基本理化性质的研究,以及对脲酶、过氧化氢酶、磷酸酶、蔗糖酶4种常见土壤酶活性的检测,分析了荒漠草原不同植物群落的土壤微环境。结果表明:(1)不同植物群落物种组成不同,且在研究区禾本科、菊科、藜科植物出现的频率相对较高。(2)不同植物群落土壤理化性质存在差异。苦豆子群落土壤容重较低,土壤全氮含量相对较高;芨芨草群落土壤pH明显较高,土壤有机碳含量相对较高;油蒿群落土壤水分含量较低,土壤全磷含量较低;盐爪爪群落土壤盐分含量显著高于其它植物群落。(3)不同植物群落土壤酶活性存在差异,其中芨芨草和油蒿群落的表层土(0~10cm)的土壤脲酶活性较高;油蒿和盐爪爪群落的土壤过氧化氢酶随着土层加深酶活性反而有升高趋势;苦豆子和芨芨草群落的土壤磷酸酶活性较高,盐爪爪群落各土层间磷酸酶活性无显著差异;4种群落土壤蔗糖酶活性普遍较低,且各土层间差异也不大。(4)不同植物群落的同种土壤酶活性间相关性不同。研究认为,根据不同植物群落特征及土壤特性,尤其是不同植物群落间土壤酶活性的相关性,可预测荒漠草原地区植物群落演替趋势,通过适度的人为调控,可使群落向进展方向演替。     

The Influence of Soil Inoculum and Nitrogen Availability on Restoration of High-Elevation Steppe Communities Invaded by Bromus tectorum

Cheatgrass ( Bromus tectorum L.) is an exotic annual grass that has invaded approximately 40,000,000 ha of rangelands in the United States, including montane ecosystems that are important habitats for wildlife and livestock. In addition to well-understood mechanisms by which Cheatgrass gains competitive advantage, recent studies have shown that Cheatgrass may also change the associated soil microbial community to impact native perennial plants and promote the persistence of Cheatgrass. Furthermore, reducing plant-available N represents a tool for initiating conditions that accelerate successional change from annual- to perennial-dominated communities. At a montane, mixed shrub–grassland Cheatgrass-dominated site in Colorado, we applied sucrose to reduce available N, and we added soil from a native plant community in order to reestablish the microbial community. This approach tested the idea that intact native soil microbial communities may enhance the beneficial effect of reducing soil N availability in a restoration setting. By the end of the experiment, reduced N availability decreased Cheatgrass by 9.8%, non-native annual/biennial plant cover by 15.0%, and increased relative perennial plant cover by 13.4%; soil inoculation reduced Cheatgrass by 7.6% and increased perennial abundance by 11.3%. Soil inoculum additions and reduced N availability both contributed toward restoring a perennial-dominated community and demonstrates that addition of native soil inoculum may be a useful tool for restoration efforts.     

荒漠草原3种典型群落类型的土壤微生物量碳氮研究

采用氯仿熏蒸-浸提法,以宁夏盐池荒漠草原3种典型群落(柠条、沙蒿、短花针茅)类型为研究对象,分析了不同生境(冠下、丛间)和不同土层间(0~5、5~10、10~15cm)土壤理化性质及微生物量——微生物量碳(MBC)和微生物量氮(MBN)的变化特征。结果表明:(1)3种群落土壤微生物量变化差异较大,柠条、沙蒿和短花针茅群落土壤MBC含量分别为77.00~393.18、17.27~221.71和81.05~173.37mg/kg,MBN含量分别为7.59~64.81、1.43~13.95和4.25~22.13mg/kg,MBC和MBN含量均表现为:冠下丛间,且随土层深度的增加而降低,有明显的"沃岛效应"。(2)群落类型对土壤微生物量碳氮含量的变化有显著影响,3种典型群落类型下土壤微生物量熵(qMB)、碳氮比(C/N)、微生物量碳氮比(MBC/MBN)分别在0.76~4.10、15.02~52.50、5.34~23.07范围内变化,其比值在不同生境和不同土层深度的分布特征有明显差异。(3)3种典型群落类型的土壤MBC与SOC、MBN、qMB具有显著相关关系,土壤C/N与MBC/MBN呈显著正相关关系,表明土壤MBC、MBN具有一定的生物学指示特性,可以作为评价土壤质量的生物学指标。