Article

Chromolaena odorata (L.) K&R (Asteraceae) invasion effects on soil microbial biomass and activities in a forest-savanna mosaic

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Abstract

Plant invasion may have significant ecological and socio-economic impacts across agroecologies. Chromolaena odorata (Asteraceae) is one of the world’s most invasive plants albeit it is considered a suitable fallow plant in West Africa. However, its impacts on soil biological processes are poorly understood. This study was conducted in intermingled forest and savanna sites invaded by C. odorata in Central Côte d’Ivoire (West Africa) to bridge this knowledge gap. Invaded forest sites (COFOR) were compared to adjacent natural forest fragments (FOR) while invaded savanna sites (COSAV) were compared to adjacent natural savanna fragments (SAV). Soil (0–10 cm depth) physico-chemical variables, including soil organic C (SOC), total soil N and available N and P concentrations were measured. Additionally, soil microbial biomass (MBC), carbon mineralization (Cmin), acid phosphatase, β-glucosidase, and fluorescein diacetate were measured. Further, the MBC/SOC ratio and the metabolic quotient (qCO2) were calculated. An index of invasion effect (IE) computed as the cumulative percent change in the microbial and enzyme activities was determined for each ecosystem context. Results showed that soil MBC and MBC/SOC ratio declined in COFOR relative to FOR. In general, Cmin, enzymatic activities, qCO2 and available N and P significantly increased in the C. odorata sites relative to the respective reference ecosystems, particularly savanna, potentially due to a larger gap in the litters’ quality. As a result, the invasion effect was twice as high in savanna (IE = 292.8%) as in forest (IE = 147.5%). However, a Principal Component Analysis showed that the COSAV were close to COFOR stands without mixing, probably due to contrasting initial soil organic matter and clay contents. These results improved our knowledge on the changes in soil microbial attributes and the mechanisms of soil fertility restoration or improvement in response to C. odorata invasion in natural forests and savannas of West Africa.

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... It was introduced as an ornamental plant to the botanical garden in Calcutta, India in 1845 (CAB Reviews 2019 14, No. 009;Online ISSN 1749-8848) and became a harmful weed (Naidoo and Naidoo, 2018;Vaisakh and Pandey, 2012). Usually, they threaten subsistence and semi-commercial farming through their spread (McConnachie et al., 2011;Zuberi et al., 2014); however, they are also prominent in forest areas (Malahlela et al., 2015;Koné et al., 2021). They negatively impact endemic plants as they contain a large amount of Fusarium semitectum or Fusarium, fungi found in the soil, and a native pathogen (Mangla et al., 2008). ...
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We studied the effects of invasions by three plant species: Reynoutria japonica, Rudbeckia laciniata, and Solidago gigantea, on arbuscular mycorrhizal fungi (AMF) communities in habitats located within and outside river valleys. Arbuscular mycorrhizal colonization, AMF abundance and species richness in soils were assessed in adjacent plots with invaders and native vegetation. We also quantified the performance (expressed as shoot mass, chlorophyll fluorescence, and the concentration of elements in shoots) of two common, mycorrhizal native plants, Plantago lanceolata and Trifolium repens, grown in these soils. The invasions of R. japonica, R. laciniata, and S. gigantea influenced AMF communities compared to native vegetation, but the changes depended on the mycorrhizal status of invaders. The effects of non-mycorrhizal R. japonica were the most pronounced. Its invasion reduced AMF abundance and species richness. In the plots of both mycorrhizal plants, R. laciniata and S. gigantea, we observed decreased AMF species richness in comparison to native vegetation. The AMF community alterations could be due to (i) depletion of organic C inputs to AMF in the case of R. japonica, (ii) plant secondary metabolites that directly inhibit or selectively stimulate AMF species, or (iii) changes in soil physicochemical properties induced by invasions. The effect of invasion on AMF abundance and species richness did not generally differ between valley and outside-valley habitats. The invasions affected photosynthetic performance and the concentrations of elements in the shoots of P. lanceolata or T. repens. However, the directions and magnitude of their response depended on both species identity and the mycorrhizal status of invaders.
Article
Full-text available
We studied the effects of invasions by three plant species: Reynoutria japonica, Rudbeckia laciniata, and Solidago gigantea, on arbuscular mycorrhizal fungi (AMF) communities in habitats located within and outside river valleys. Arbuscular mycorrhizal colonization, AMF abundance and species richness in soils were assessed in adjacent plots with invaders and native vegetation. We also quantified the performance (expressed as shoot mass, chlorophyll fluorescence, and the concentration of elements in shoots) of two common, mycorrhizal native plants, Plantago lanceolata and Trifolium repens, grown in these soils. The invasions of R. japonica, R. laciniata, and S. gigantea influenced AMF communities compared to native vegetation, but the changes depended on the mycorrhizal status of invaders. The effects of non-mycorrhizal R. japonica were the most pronounced. Its invasion reduced AMF abundance and species richness. In the plots of both mycorrhizal plants, R. laciniata and S. gigantea, we observed decreased AMF species richness in comparison to native vegetation. The AMF community alterations could be due to (i) depletion of organic C inputs to AMF in the case of R. japonica, (ii) plant secondary metabolites that directly inhibit or selectively stimulate AMF species, or (iii) changes in soil physicochemical properties induced by invasions. The effect of invasion on AMF abundance and species richness did not generally differ between valley and outside-valley habitats. The invasions affected photosynthetic performance and the concentrations of elements in the shoots of P. lanceolata or T. repens. However, the directions and magnitude of their response depended on both species identity and the mycorrhizal status of invaders.
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Plant-soil feedbacks are an important aspect of invasive species success. One type of feedback is alteration of soil nutrient cycling. Cheatgrass invasion in the western USA is associated with increases in plant-available nitrogen (N), but the mechanism for this has not been elucidated. We labeled cheatgrass and crested wheatgrass, a common perennial grass in western rangelands, with 15N-urea to determine if differences in root exudates and turnover could be a mechanism for increases in soil N. Mesocosms containing plants were either kept moist, or dried out during the final 10 days to determine the role of senescence in root N release. Soil N transformation rates were determined using 15N pool dilution. After 75 days of growth, cheatgrass accumulated 30 % more total soil N and organic carbon than crested wheatgrass. Cheatgrass roots released twice as much N as crested wheatgrass roots (0.11 vs. 0.05 mg N kg−1 soil day−1) in both soil moisture treatments. This occurred despite lower root abundance (7.0 vs. 17.3 g dry root kg−1 soil) and N concentration (6.0 vs. 7.6 g N kg−1 root) in cheatgrass vs. crested wheatgrass. We propose that increases in soil N pool sizes and transformation rates under cheatgrass are caused by higher rates of root exudation or release of organic matter containing relatively large amounts of labile N. Our results provide the first evidence for the underlying mechanism by which the invasive annual cheatgrass increases N availability and establishes positive plant-soil feedbacks that promote its success in western rangelands.
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Our understanding of the link between plant functional traits and ecological impact of invasive alien plant species is fragmentary and the mechanisms leading to impacts are poorly understood. Moreover, current knowledge is heavily biased to the temperate regions of the world and we know much less about traits and impacts of invaders in tropical and subtropical ecosystems. We studied two leaf traits of the invasive alien shrub Chromolaena odorata and the impacts of its invasion on native vegetation in savannas. We compared specific leaf area (SLA) and leaf area index (LAI) between C. odorata and native species and assessed how C. odorata differentially affects canopy light interception, soil moisture, soil nutrients, and litter accumulation compared to native species. We found that C. odorata has higher SLA and LAI than native species, lower light and moisture levels below its canopy, but higher nutrient levels and a higher litter accumulation rate. Because of its higher SLA, C. odorata grows faster, resulting in more biomass, increased litter accumulation and higher nutrient availability. Due to its high SLA and LAI, C. odorata intercepts more light and reduces available moisture more than do native trees due to higher transpiration rates, reducing the biomass of native understory vegetation. This study provides empirical evidence for strong links between plant functional traits and ecological impact of invasive plant species, highlighting the importance of traits in predicting ecosystem-level impacts of invasive plant species.
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The significance of arbuscular mycorrhizal fungi (AMF) in the process of plant invasion is still poorly understood. We hypothesize that invasive plants would change local AMF community structure in a way that would benefit themselves but confer less advantages to native plants, thus influencing the extent of plant interactions. An AMF spore community composed of five morphospecies of Glomus with equal density (initial AMF spore community, I-AMF) was constructed to test this hypothesis. The results showed that the invasive species, Solidago canadensis, significantly increased the relative abundance of G. geosperum and G. etunicatum (altered AMP spore community, A-AMF) compared to G. mosseae, which was a dominant morphospecies in the monoculture of native Kummerowia striata. The shift in AMF spore community composition driven by S. canadensis generated functional variation between I-AMP and A-AMF communities. For example, I-AMP increased biomass and nutrient uptake of K. striata in both monocultures and mixtures of K. striata and S. canadensis compared to A-AMP. In contrast, A-AMP significantly enhanced root nitrogen (N) acquisition of S. canadensis grown in mixture. Moreover, mycorrhizal-mediated N-15 uptake provided direct evidence that I-AMF and A-AMF differed in their affinities with native and invading species. The non-significant effect of A-AMF on K. striata did not result from allelopathy as root exudates of S. canadensis exhibited positive effects on seed germination and biomass of K. striata under naturally occurring concentrations. When considered together, we found that A-AMF facilitated the invasion of S. canadensis through decreasing competitiveness of the native plant K. striata. The results supported our hypothesis and can be used to improve our understanding of an ecosystem-based perspective towards exotic plant invasion. (C) 2014 Published by Elsevier Masson SAS.
Article
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Very little has hitherto been known about the response of structurally and functionally diverse arbuscular mycorrhizal fungi (AMF) to plant invasions. We investigated the impact of two alien invasive species, namely Anthemis cotula and Conyza canadensis, on rhizospheric AMF at different sites in the Kashmir Himalaya, India. We compared AMF species composition, and density of their spores in the rhizospheric soils of A. cotula and C. canadensis with comparable nearby un-invaded sites. Whilst 9 and 7 AMF species were recovered from the rhizospheres of A. cotula and C. canadensis, respectively, the corresponding un-invaded sites yielded 17 and 10 AMF species. Only 3 AMF species were common between the rhizospheres of the two invasive species. The spore density of AMF, unlike species richness, was higher in the rhizospheres of both the invasive species compared with the un-invaded sites nearby. This study brings to light alien invasive species-specific shifts in AMF diversity.
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The use of exotic tree species and especially fast-growing trees (acacias, pines or eucalypts) has been frequently recommended to rehabilitate and restore in short term degraded areas resulting from natural events or human activities. The consequences on the environment of the introduction of these species, as the case invasive, are generally evaluated on their impact on plant biodiversity and soil physico-chemical characteristics but rarely on the composition of microbial communities. The soil microbiota, especially mycorhizal fungi, plays a key role vis-a-vis biological mechanisms governing the chemical soil fertility, productivity and stability of plant terrestrial ecosystems. Chosen approach was to describe the impact of exotic species on soil biological characteristics and the consequences of the recovering of vegetation cover composed of native species from the previous stand. After pointing out importance of the use of acacias worldwide, two case studies implemented in Senegal and Algeria, have shown that two Australian acacias, Acacia holosericea and Acacia mearnsii, induce deep changes in the functional diversity of soil microorganisms and in the structure of symbiotic microorganisms communities (mycorhizal fungi and rhizobia). These acacias inhibit the growth of two native tree species, Quercus suber and Faidherbia albida. These results confirm the need to identify the biological processes related to the actions of introducing exotic species in order to modulate their use. And this knowledge will prevent risks and ensure the performance of afforestation including rehabilitation of degraded land.
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In initial ecosystems concentrations of all macro- and micronutrients can be considered as extremely low. Plant litter therefore strongly influences the development of a degraders' food web and is an important source for C and N input into soil in such ecosystems. In the present study, a 13C litter decomposition field experiment was performed for 30 weeks in initial soils from a post-mining area near the city of Cottbus (Germany). Two of this regions' dominant but contrasting pioneering plant species (Lotus corniculatus L. and Calamagrostis epigejos L.) were chosen to investigate the effects of litter quality on the litter decomposing microbial food web in initially nutrient-poor substrates. The results clearly indicate the importance of litter quality, mainly the amount of N stored in the litter material and its bioavailability for the degradation process and the development of microbial communities in the detritusphere and bulk soil. Whereas the degradation process of the L. corniculatus litter which had a low C/N ratio was fast and most pronounced changes in the microbial community structure were observed 1-4 weeks after litter addition, the degradation of the C. epigejos litter material was slow and microbial community changes mainly occurred at between 4 and 30 weeks after litter addition to the soil. However for both litter materials a clear indication for the importance of fungi for the degradation process was observed both on the abundance level as well as on the level of 13C incorporation (activity).
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Counting mycorrhizal infective propagules (spores, fruitbodies, colonized roots) in forest soils was used to assess impacts of shifting cultivation and selective logging on habitat partitioning and abundance of inoculum of arbuscular mycorrhizal and ectomycorrhizal fungi in south Cameroon. Intact soil cores, disturbed soil samples and fruitbodies were collected from late successional forest stands inside and outside the crown projection of ectomycorrhizal clumps, early successional forest stands, fallows of Chromolaena odorata with and without Gnetum lianas, fields of food crops, forestry practices including skid trails and bare soil landings. Fractional mycorrhizal colonisation was assessed by the gridline intersect method. Spores of arbuscular mycorrhizal fungi were extracted by the wet-sieving and decanting method followed by sugar centrifugation and counted under microscope. Fruitbodies of ectomycorrhizal fungi were collected, described in fresh state, dried before microscopic examination and identification. Due to creation of skid trails, landings and forest roads, selective logging had a very large negative impact on both ectomycorrhizae and arbuscular mycorrhizae (AM) whereas shifting cultivation had only a slight positive effect on arbuscular mycorrhizas and a negative effect on ectomycorrhizae. Both types of mycorrhizal inocula were differently partitioned: AM inoculum was continuous throughout the landscape while ectomycorrhizal inoculum was patchy, suggesting the need for different management approaches for both arbuscular mycorrhizal and ectomycorrhizal forest patches. Ectomycorrhizal forest clumps indicated least disturbed forest stands. Owing to their specific biodiversity, they should be included as indicators of sustainable forest management; Carpophores of ectomycorrhizal fungi classify as sub-indicators in the principles, criteria and indicators of African Timber Organizatio/International Tropical Timber Organization (ATO/IITO)
Chapter
This book contains 24 chapters containing case studies and research papers on the biological control of pests by naturally occurring agents, exotic agents or by seasonal manipulation in Africa. This book will be of interest to those working in the areas of crop protection, entomology and pest management. An index of terms and species found in the book is provided.
Article
The status (weed or beneficial shrub) of Chromolaena odorata in West Africa has been a major subject of debate since its introduction in the 1930s. Its negative impacts on agriculture and biodiversity are clear. However, indigenous people have harnessed several benefits of the plant, making management strategies less straightforward. Using indigenous voices in Ghana, the current perceptions of C. odorata in West Africa were assessed. Here, 90 respondents in three regions of Ghana were interviewed, using questionnaires, on aspects relating to the abundance, perceptions and management of C. odorata in their communities. Overall, results showed that the density of C. odorata had declined. The majority of the respondents considered C. odorata as a fallow shrub, a medicinal plant or both. However, impacts on crop production and injuries incurred during physical control were also highlighted. The local uses of the plant in Ghana included treating fevers and injuries, and as a fallow shrub for soil nutrient replenishment. Although most of the respondents classified C. odorata as a useful shrub in Ghana, they also highlighted the use of local farming tools such as cutlasses for the slash of the plant. The use of herbicides for the control of C. odorata was rare and not applied specifically for C. odorata. In conclusion, local awareness programmes on the deleterious impacts of C. odorata on ecosystem functioning and livelihoods are required for its effective management in West Africa.
Article
An increase of soil organic carbon (SOC) stocks in agricultural soils does not only have positive effects on soil quality and soil resilience but may also contribute to climate change mitigation. The '4 per 1000′ (4p1000) initiative launched at the 2015 United Nations Climate Change Conference in Paris aims at increasing global SOC stocks in 0-40 cm depth by annually 4‰ in order to compensate the increase of anthropogenic CO 2 emissions. In this study we analysed the feasibility of this target for agricultural soils in Bavaria, Southeast Germany. Assuming a total organic carbon (OC) amount of 276 Tg currently stored in the upper 40 cm of agricultural soils in Bavaria (cropland and grassland), the 4p1000 goal corresponds to an annual carbon (C) sequestration of 1.1 Tg. Based on a site-specific analysis of present soil management, we developed spatially explicit C sequestration scenarios including five promising management practices (cover cropping, improved crop rotation, organic farming, agroforestry and conversion of arable land to grassland). The results revealed that the 4p1000 target is not feasible for Bavaria. The total potential of the five practices to sequester C resulted in increases in 0.3 to 0.4 Tg OC per year corresponding to around 1‰ of the present SOC stocks. Expansion of cover crops and agroforestry were identified as most promising options to increase SOC in agricultural soils. Although only around 1.5% of Bavaria's yearly GHG emissions would be compensated, this represents an essential contribution to climate change mitigation. Besides the need to develop new incentive systems (particularly for agroforestry), implementation of networks including farms and/or field trials that demonstrate improved soil management practices would be required to inform farmers and other stakeholders about the benefits of such practices. To maintain a resilient agriculture that withstands more extreme weather conditions in the future, healthy soils are needed. We therefore conclude that expected positive effects of a SOC stock increase on nutrient and water storage, soil erosion, biodiversity and food security are crucial for climate change adaptation.
Article
Chromolaena odorata (L.) R.M. King and H. Robinson (Asteraceae) is a perennial weedy shrub of neotropical origin and a serious biotic threat in its invasive range. The Asian-West Africa (AWA) biotype of C. odorata present in West Africa is both morphologically and genetically different from the southern African (SA) biotype. The AWA biotype was first introduced into Nigeria in the late 1930s and rapidly spread across West Africa. Currently, 12 of the 16 countries in West Africa have been invaded, with significant negative effects on indigenous flora and fauna. However, locals in West Africa have found several uses for the weed. As chemical, physical and other conventional methods were unsustainable, costly and largely ineffective, three biological control agents, Apion brunneonigrum (Coleoptera: Brentidae), Pareuchaetes pseudoinsulata (Lepidoptera: Erebidae) and Cecidochares connexa (Diptera: Tephritidae), have been released in West Africa between the 1970s and the early 2000s. However, only C. connexa and P. pseudoinsulata established, contributing to the control of the weed, in six and four countries in West Africa respectively. Limited research funding, the absence of post-release evaluations of the established agents, and the ‘conflict of interest’ status of C. odorata (i.e. being beneficial for local use but damaging to ecosystem services and agriculture), are serious factors deterring the overall biological control effort. Here, using historical records and field surveys, we examine the invasion history, spread, impacts, and management of C. odorata in West Africa and make recommendations for the sustainable management of C. odorata in the region.
Article
Native plants and animals can rapidly become superabundant and dominate ecosystems, leading to claims that native species are no less likely than alien species to cause environmental damage, including biodiversity loss. We compared how frequently alien and native species have been implicated as drivers of recent extinctions in a comprehensive global database, the 2017 International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Alien species were considered to be a contributing cause of 25% of plant extinctions and 33% of animal extinctions, whereas native species were implicated in less than 5% and 3% of plant and animal extinctions, respectively. When listed as a putative driver of recent extinctions, native species were more often associated with other extinction drivers than were alien species. Our results offer additional evidence that the biogeographic origin, and hence evolutionary history, of a species are determining factors of its potential to cause disruptive environmental impacts.
Book
Cambridge Core - Natural Resource Management, Agriculture, Horticulture and forestry - Properties and Management of Soils in the Tropics - by Pedro A. Sanchez
Article
Invasive alien species are a well-recognised driver of social-ecological change globally. Much research has focused on ecological impacts, but the role of invasive species for livelihoods and human well-being is less well known. Understanding the effects (benefits and costs) of invasive species on livelihoods and human well-being is important for guiding policy formulation and management. Here we review the literature on the role of invasive species in livelihoods to assess what is known, identify knowledge gaps and provide recommendations for future research. Literature was collected using key word searches and included both journal publications and grey literature. Slightly less than half (48%) of species studied had both substantial positive and negative impacts on local livelihoods (e.g. Australian Acacia spp. species; Camelus dromedaries; Lantana camara; Prosopis spp.), with 37% inducing mainly costs (Chromolaena odorata; Lissachatina fulica; Opuntia stricta) and 16% producing mainly benefits (Opuntia ficus-indica; Acacia spp.). Some species, such as Acacia dealbata, fell into different categories depending on the social-ecological context. Key benefits or services included the provision of fuelwood, fodder, timber and food products for local households communities and to a lesser extent supporting and regulating services such as soil improvement and shade. A number of species also provided cultural services such as recreation and spiritual values and provided many with an opportunity to earn a cash income. However, invasive species also harm livelihoods and increase vulnerability through encroaching on land and reducing mobility or access. They can also decrease the supply of natural resources used by households and reduce agricultural production (livestock and/or crops) which can result in losses of income and increased vulnerability. Furthermore, some invasive species were seen to have negative implications for human health and safety and reduce the cultural value of landscapes. Economic impacts on livelihoods as a result of invasive species were highly variable and very dependent on the social-ecological contexts. These negative implications can reduce resilience and adaptive capacity of households and communities thus increasing their vulnerability to change. Drawing on case studies we highlight that efforts for managing invasive species need to safeguard livelihood benefits while mitigating negative impacts. In concluding we highlight future research and policy needs on the topic of invasive species, livelihoods and human well-being.
Thesis
Der Abbau von organischer Bodensubstanz (OBS) ist ein wichtiger Prozess des globalen Kohlenstoffkreislaufes (C-Kreislaufes), der im Wesentlichen von Bodenmikroorganismen durchgeführt wird. Trotz der großen Bedeutung der mikrobiellen Aktivität für die globale C-Bilanz, ist der Einfluss von Substratqualität und -verfügbarkeit auf Bodenmikroorganismen wenig erforscht. Ein Großteil des pflanzenbürtigen C wird indirekt durch den Einbau in Mikroorganismen der OBS zugeführt, aber über das darauffolgende Schicksal von C in der mikrobiellen Gemeinschaft ist wenig bekannt. Die mikrobielle Biogeochemie ist deswegen in Erdsystemmodellen nur unzureichend implementiert. Die im 5. Kapitel präsentierte Studie nutzte ein im Jahr 2009 angelegtes Feldexperiment um den C-Eintrag in drei Bodentiefen nach einem C3-C4 Pflanzenwechsel in fünf aufeinanderfolgenden Jahren zu untersuchen. Hierbei wurde durch den Anbau von Maispflanzen wurzelbürtiger C (unterirdischer C-Kanal), sowie durch die Applikation von Maisstreu auf die Bodenoberfläche sproßbürtiger C (oberirdischer C-Kanal) in den Boden eingetragen. Der prozentuale Anteil von maisbürtigem C variierte in den Bodenpools: geringere Einträge fanden sich in der OBS und im extrahierbaren organischen C (EOC) und höhere Einträge in den mikrobiellen Gruppen. Eine Versorgung mit beiden Pflanzensubstraten zeigte im Oberboden einen additiven Effekt in fast allen untersuchten Bodenpools. Sowohl wurzel- als auch sproßbürtiger C wurde bis in eine Tiefe von 70 cm transferiert. In allen drei Tiefen assimilierten Pilze die angebotenen C-Substrate im größeren Umfang als Gram-positive und Gram-negative Bakterien. Nach der fünften Wachstumsperiode konnte ein Einbau von bis zu 78 % Mais-C in die pilzliche Biomasse nachgewiesen werden, was auf verstärkte Nutzung des Mais-C durch saprotrophe Pilze hindeutet. Die zweite Studie untersuchte den Einfluss abnehmender Substratqualität auf Bodenmikroorganismen an der Boden-Streu Grenzschicht in einem Mikrokosmenexperiment. Ein reziproker Austausch von markierter 13C und unmarkierter 12C Maisstreuauflage auf Bodenkerne erlaubte es, den C-Eintrag von Pflanzenresten in die Hauptakteure (Bakterien und Pilze) des detritivoren Nahrungsnetzes und den darauffolgenden C-Umsatz in der mikrobiellen Biomasse zu drei unterschiedlichen Zeiten während des Streuabbaus zu bestimmen. Die Qualität (d. h. das Alter) der Maisstreu beeinflusste hierbei die Aufnahme von C in Bakterien und Pilze. Einfach verfügbarer C aus kürzlich eingetragener Streu wurde von beiden Mikroorganismengruppen genutzt, während saprotrophe Pilze zusätzlich noch komplexeres C-Substrat im fortgeschrittenen Abbaustadium nutzten. Bakterien reagierten unterschiedlich auf die angebotene Streu, indem sie entweder den Streu-C in ihren Phospholipidfettsäuren (PLFAs) über die Zeit umsetzten, es dauerhaft speicherten und/oder zusätzlich noch mikrobiell freigesetzten C aufnahmen. Saprotrophe Pilze hingegen zeigten einen deutlichen Abbau von Mais-C in der pilzlichen PLFA. Die mittlere Verweildauer von C in der pilzlichen Biomasse betrug 32 bis 46 Tage und wurde genauso schnell oder sogar schneller als in den bakteriellen PLFAs umgesetzt. Für die in Kapitel 7 präsentierte Studie wurde ein weiteres Feldexperiment genutzt, um die Mitglieder der Herbivoren- und Detritivorennahrungskette über zwei aufeinanderfolgende Jahre zu untersuchen. Drei Behandlungen wurden etabliert um wurzelbürtiges, sproßbürtiges und autochthones organisches Material als verfügbares C-Substrat zur Verfügung zu stellen: Anbau von Maispflanzen, Ausbringen von Maisstreu und Brachflächen. Entgegen den Erwartungen zeigte die veränderte C-Versorgung durch Entfernen der Ackerpflanze nur wenig Einfluss auf die mikrobielle Gemeinschaftsstruktur des Bodennahrungsnetzes. Bakterien und Pilze zeigten eine ausgeprägte Anpassungsfähigkeit an die geringere C-Verfügbarkeit. In der ersten Vegetationsperiode war die Nematodenabundanz unter Pflanzenkultivierung vergleichbar mit denen der Streu- und Brachflächen. Nach der zweiten Ernte wurden hingegen die Mitglieder der Detritivorennahrungskette durch den Abbau von Wurzelbiomasse gefördert. Die Ergebnisse dieser Studie zeigen eine ausgeprägte Widerstandsfähigkeit des Bodenmikronahrungsnetzes auf Substrat- und Nährstoffverfügbarkeit und deuten darauf hin, dass älteres organisches Material genutzt wird, um C-Mangel auszugleichen. Zusammenfassend hat die vorliegende Arbeit dazu beigetragen neue Erkenntnisse zu mikrobiellen Abbauprozessen mit unterschiedlicher zeitlicher und räumlicher Auflösung zu erlangen. Die Nutzung von stabilen Isotopen in Kombination mit PLFA-Biomarkeranalysen ermöglichte es, den C-Fluss zwischen abiotischen und mikrobiellen C-Pools im Boden zu untersuchen und den Anteil von Bakterien und Pilzen am C-Umsatz im Boden zu bestimmen. Die vorliegenden Ergebnisse können als Basis für ein empirisches Modell des C-Flusses durch das gesamte Bodennahrungsnetz genutzt werden.
Article
Natural ecosystem conversion to agriculture is known to alter soil carbon dynamic. Following such conversion in many tropical areas, land undergoes fallow-cropping cycles where fallows are invaded by the pantropical weed Chromoleana odorata. This study was undertaken in the forest-savanna interface area of Côte d’Ivoire to evaluate the impact of these cycles on soil carbon stocks (SOCS) and yam yields, decades after natural ecosystem conversion. Trials involved four treatments including yam farms in forest (FOR, n = 10), in forest-derived C. odorata fallows (FoDCo, n = 7), in savanna (SAV, n = 3) and in savanna-derived C. odorata fallows (SaDCo, n = 3). Prior to turning plots to farms, soil was sampled in the 0–10, 10–20 and 20–40 cm layers for physical, chemical and microbial parameters. Since forest and savanna soils were different in granulometry, FoDCo was compared to FOR, and SaDCo to SAV. The soil organic matter and nutrient concentrations in the 0–10 cm soil layer in FoDCo and FOR were similar, except for available P which was higher in the former. SaDCo was higher than SAV in terms of SOC, available P, mineral N, and NO3-N:Mineral N ratio. With regard to SOCS, value in FoDCo equalled that in FOR regardless of soil layers (63.2 ± 4.4 and 63.7 ± 4.6 Mg ha⁻¹ in 0–40 cm layer, respectively). However, SOCS significantly increased in SaDCo relative to SAV in the 0–20 cm (31.4 ± 2.7 vs. 24.2 ± 2.1 Mg ha⁻¹). Consistently, yam yield in FoDCo was like that in FOR while it doubled in SaDCo compared to SAV, with SOC, CEC and mineral N as the controlling factors. The major finding of this study is that the predominance of C. odorata in fallow phases allows at least for maintenance of SOCS and yam yields decades after natural ecosystem conversion to farmland. Furthermore, evidence of the feasibility of the “4 per mille” was given. These results are highly useful in forest protection strategies since farmers usually cut forest for yam cropping.
Article
Environmental disturbances seem to be increasing in frequency and impact, yet we have little understanding of the belowground impacts of these events. Soil fauna, while widely acknowledged to be important drivers of biogeochemical function, soil structure and sustainability, and trophic interactions, are understudied compared to other belowground organisms such as archaea, bacteria, and fungi. In this review we summarize the current state of knowledge of soil fauna as it relates to and is influenced by various disturbances. We focus our review on three main natural and anthropogenic disturbance types: 1) natural disturbances, including damage from wind and flooding; 2) invasive species, including above and belowground flora and fauna; and 3) climate change impacts on the atmosphere and temperature. We do not address the impacts of wildfires, forestry, agricultural practices, mining, or human-caused pollution, as these topics have all been covered in other works. We highlight knowledge gaps and suggest future avenues of research, with hope that the importance of soil fauna and their influences on ecosystems will be given greater emphasis in future research.
Article
A single solution reagent is described for the determination of phosphorus in sea water. It consists of an acidified solution of ammonium molybdate containing ascorbic acid and a small amount of antimony. This reagent reacts rapidly with phosphate ion yielding a blue-purple compound which contains antimony and phosphorus in a 1:1 atomic ratio. The complex is very stable and obeys Beer's law up to a phosphate concentration of at least 2 μg/ml.The sensitivity of the procedure is comparable with that of the stannous chloride method. The salt error is less than 1 %.
Article
Habitat suitability assessment of the invasive species Chromolaena odorata (L.) King and Robinson from Himalayan forests reveal some interesting findings and conclusions. Out of the total 72 species recorded from different study sites, 29 were found exotic and invasive belonging to 21 genera and 8 families. Indigenous species accounted for 59% of the total species represented by 26 genera and 11 families. Perennials outnumbered the annuals in all the study sites. C. odorata and Lantana camara L. were the only invasive species to be found common in all the sites with high IVI values. The present work reveals that sites with high biotic pressure, maximum temperature variation, open forest canopy and free from herbivory are the most suitable habitat for the growth of C. odorata. An elevated (P), (K), (Mg), (SOM), (N) and acidic soil from all the invaded sites were found to be the possible reason for further invasion of C. odorata.
Article
Exotic plant invasion presents a serious threat to native ecosystem structure and function. Little is known about the role of soil microbial communities in facilitating or resisting the spread of invasive plants into native communities. The purpose of this research is to understand how the invasive annual plant Ambrosia artemisiifolia L. facilitates its competition capacity through changing the structure and function of soil microbial communities. The soil characteristics of different areas invaded by A. artemisiifolia were examined. Greenhouse experiments were designed to assess the effect of A. artemisiifolia invasion-induced changes of soil biota on co-occurring plant growth, and on the interactions between A. artemisiifolia and three co-occurring plant species. The results showed that the soil organic C content was the highest in heavily invaded sites, the lowest in native plant sites, and intermediate in newly invaded sites. Soil available N, P and K concentrations in heavily invaded site were 2.4, 1.9 and 1.7 times higher than those in native plant soil, respectively. Soil pH decreased as A. artemisiifolia invasion intensity increased, and was lower in invaded sites (heavily invaded and newly invaded) than in native plant sites. The soil microbial community structure was clearly separated in the three types of sites, and A. artemisiifolia invasion increased anaerobe, sulfate-reducing bacteria and actinomycete abundance. Soil biota of invaded sites inhibits growth of co-occurring plants (Galinsoga parviflora Cav., Medicago sativa L. and Setaria plicata (Lam.) T. Cooke.) compared to soil biota from un-invaded sites, but facilitates A. artemisiifolia growth and competition with co-occurring plants. A. artemisiifolia biomass was 50-130% greater when competing with three co-occurring plants, compared to single-species competition only (invasion by A. artemisiifolia alone), in heavily invaded soil. Results of the present study indicated that A. artemisiifolia invasion alters the soil microbial community in a way that favors itself while inhibiting native plant species, with measurable effects on performance of co-occurring plants.
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Soil enzyme activities are the direct expression of the soil community to metabolic requirements and available nutrients. While the diversity of soil organisms is important, the capacity of soil microbial communities to maintain functional diversity of those critical soil processes through disturbance, stress or succession could ultimately be more important to ecosystem productivity and stability than taxonomic diversity. This review examines selected papers containing soil enzyme data that could be used to distinguish enzyme sources and substrate specificity, at scales within and between major nutrient cycles. Developing approaches to assess soil enzyme functional diversity will increase our understanding of the linkages between resource availability, microbial community structure and function, and ecosystem processes.
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A method is described for the rapid and simple assay of soil β-glucosidase activity. It involves colorimetric estimation of ρ-nitrophenol released by β-glucosidase activity when soil is incubated in McIlvaine buffer (pH 4.8) with ρnitrophenyl βd-glucoside and toluene at 30°C for 1 hr. The method has been applied to three different soils. The range of β-glucosidase activity in cultivated soils was from 10.1 to 15.2 mµ mole per min per gram of dried soil. Km value for ρ-nitrophenyl β-d-glucoside was 3.3 × 10 M. Optimum pH was 4.8.
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A simple method of assaying soil phosphatase activity is described. It involves colorimetric estimation of the p-nitrophenol released by phosphatase activity when soil is incubated with buffered (pH 6·5) sodium p-nitrophenyl phosphate solution and toluene at 37° C for 1 hr. The method is rapid and precise, and it has significant advantages over methods previously proposed for assay of soil phosphatase activity.