Nitrogen limitation, 15N tracer retention, and growth response in intact and Bromus tectorum-invaded Artemisia tridentata ssp. wyomingensis communities

Annual grass invasion into shrub-dominated ecosystems is associated with changes in nutrient cycling that may alter nitrogen (N) limitation and retention. Carbon (C) applications that reduce plant-available N have been suggested to give native perennial vegetation a competitive advantage over exotic annual grasses, but plant community and N retention responses to C addition remain poorly understood in these ecosystems. The main objectives of this study were to (1) evaluate the degree of N limitation of plant biomass in intact versus B. tectorum-invaded sagebrush communities, (2) determine if plant N limitation patterns are reflected in the strength of tracer ¹⁵N retention over two growing seasons, and (3) assess if the strength of plant N limitation predicts the efficacy of carbon additions intended to reduce soil N availability and plant growth. Labile C additions reduced biomass of exotic annual species; however, growth of native A. tridentata shrubs also declined. Exotic annual and native perennial plant communities had divergent responses to added N, with B. tectorum displaying greater ability to use added N to rapidly increase aboveground biomass, and native perennials increasing their tissue N concentration but showing little growth response. Few differences in N pools between the annual and native communities were detected. In contrast to expectations, however, more ¹⁵N was retained over two growing seasons in the invaded annual grass than in the native shrub community. Our data suggest that N cycling in converted exotic annual grasslands of the northern Intermountain West, USA, may retain N more strongly than previously thought.     

Biogeographic differences in the effects of <Emphasis Type="Italic">Centaurea stoebe</Emphasis> on the soil nitrogen cycle: novel weapons and soil microbes

The success of some invasive plants may be due in part to native organisms lacking adaptation to species-specific biochemical traits of invaders—the Novel Weapons Hypothesis. We tested this hypothesis in the context of soil microbial communities by comparing the effects of Centaurea stoebe and the root exudate (±)-catechin, on ammonification and nitrification in both the non-native and native ranges of this species. In a non-native range (Montana), soil nitrate (NO₃ ⁻) concentrations were lower in invaded than uninvaded grasslands. This did not appear to be due only to higher uptake rates as both C. stoebe plants and catechin significantly reduced resin extractable NO₃ ⁻, the maximum rate of nitrification, and gross nitrification in Montana soils. Thus, reduced NO₃ ⁻ in invaded communities may be due in part to the inhibition of nitrifying bacteria by secondary metabolites produced by C. stoebe. The effects of C. stoebe on N-related processes were different in Romanian grasslands, where C. stoebe is native. In Romanian soil, C. stoebe had no effect on resin extractable NH₄ ⁺ or NO₃ ⁻ (compared to other plant species), the maximum rate of nitrification, nor gross nitrification. A relatively high concentration of catechin reduced the maximum rate of nitrification in situ, but substantially less than in Montana. In vivo, gross ammonification was lowest when treated with catechin. Our results suggest biogeographic differences in the way a plant species alters nitrogen cycling through the direct effects of root exudates and adds to a growing body of literature demonstrating the important belowground effects of invasive plants.     

Multi-scale impacts of crested wheatgrass invasion in mixed-grass prairie

Most of North America’s northern Great Plains have been cultivated for crop production, leaving remnants of natural mixed-grass prairie fragmented and threatened by alien plant invasions. The region’s most widespread alien perennial forage crop, crested wheatgrass (Agropyron cristatum sensu amplo), has invaded native grassland and raised concerns regarding its ecological impact. To evaluate impacts at multiple scales of organization, adjacent invaded and uninvaded mixed-grass prairie were sampled at eight widely separated locations. At the population level, native C₃ mid-grasses and forbs were less abundant in invaded grasslands, while native C₃ and C₄ short-grass abundance was not different. At community and landscape levels, diversity was lower in invaded grasslands largely because of lower forb species richness and cover, and crested wheatgrass dominance of both cover (14% basal cover) and seedbank (404 seeds m⁻²). At the ecosystem level, both vegetation and litter biomass were greater in invaded grasslands, however, below ground organic matter (roots and litter), soil organic carbon, total nitrogen and phosphorus were not different. Crested wheatgrass invasion of mixed-grass prairie was associated with lower diversity within and among plant communities, and appears to simplify the composition of mixed-grass prairie landscapes. Hypotheses for crested wheatgrass dominance and persistence following invasion are suggested.     

Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

Plant phenology and life span influence soil pool dynamics: Bromus tectorum invasion of perennial C3–C4 grass communities

In water-limited ecosystems, small rainfall events can have dramatic impacts on microbial activity and soil nutrient pools. Plant community phenology and life span also affect soil resources by determining the timing and quantity of plant nutrient uptake, storage, and release. Using the replacement of C₃–C₄ perennial grasses by the invasive annual grass Bromus tectorum as a case study, we investigated the influence of phenology and life span on pulse responses and sizes of soil carbon (C) and nitrogen (N) pools. We hypothesized that available and microbial C and N would respond to small rainfall events and that B. tectorum invasion would increase soil C and N pools by reducing inter-annual plant C and N storage and alter seasonal pool dynamics by changing the timing of plant uptake and litter inputs. We tested our hypotheses by simulating small rainfall events in B. tectorum and perennial grass communities three times during the growing season. Microbial pools responded strongly to soil moisture and simulated rainfall events, but labile C and N pools were affected weakly or not at all. All pools were larger beneath B. tectorum than perennial grasses. Soil C and N pools increased after senescence in both communities. Our results suggest that transforming a perennial into a B. tectorum dominated community increases the overall size of soil C and N pools by decreasing plant C and N storage and changes seasonal pool dynamics by altering dominant plant phenology. Our results indicate strong roles for water, life span and phenology in controlling soil C and N pools and begin to elucidate the biogeochemical effects of altering plant community phenology and life span.     

Using Soil 13C to Detect the Historic Presence of Schizachyrium scoparium (Little Bluestem) Grasslands on Martha's Vineyard

Abstract We used differences in soil carbon δ¹³C values between forested sites and grasslands dominated by the C₄ grass Schizachyrium scoparium (little bluestem) to detect the presence of former grasslands in the historical landscape of the coastal sand plain of Martha's Vineyard, Massachusetts, U.S.A. Soil δ¹³C was measured at (1) sites with long‐term forest or grassland vegetation and (2) sites with known histories where forest vegetation invaded grassland and where forest converted to grassland. The δ¹³C of soil under long‐term grassland was –24.1‰ at 0 to 2 cm depth and –23.4‰ at 2 to 10 cm and was enriched by 3.4‰ and 2.8‰ compared with soil under long‐term forest. In forests that invaded grasslands dominated by S. scoparium, soil δ¹³C decreased as C derived from trees replaced C from S. scoparium. This decline occurred faster in surface soils and in the light soil organic matter fraction than in the mineral soil. In forests that converted to grasslands, soil δ¹³C increased and the rate of increase was similar in surface and mineral soil and in the different soil organic matter fractions. Rates of change indicated that soil δ¹³C could be used to detect changes in vegetation involving the presence or absence of S. scoparium during the last 150 years. Application of this model to a potential grassland restoration site on Martha's Vineyard where the landscape history was not known indicated that the site was previously unoccupied by S. scoparium during this time. The δ¹³C of surface mineral soil can be useful for detecting the presence of historic S. scoparium grasslands but only in the period well after European settlement of these coastal sand plain landscapes.     

Plant Effects on Spatial and Temporal Patterns of Nitrogen Cycling in Shortgrass Steppe

Because of the water-limited nature and discontinuous plant cover of shortgrass steppe, spatial patterns in ecosystem properties are influenced more by the presence or absence of plants than by plant type. However, plant type may influence temporal patterns of nutrient cycling between plant and soil. Plants having the carbon-3 (C₃) or carbon-4 (C₄) photosynthetic pathway differ in phenology as well as other attributes that affect nitrogen (N) cycling. We estimated net N mineralization rates and traced nitrogen-15 (¹⁵N) additions among plant and soil components during May, July, and September of 1995 in native plots of C₃ plants, C₄ plants, or mixtures of C₃ and C₄. Net N mineralization was significantly greater in C₃ plots than in C₄ plots during both July and September. C₃ plots retained significantly more ¹⁵N in May than did mixed and C₄ plots; these differences in ¹⁵N retention were due to greater ¹⁵N uptake by C₃ plants than by C₄ plants during May. There were no significant differences in total ¹⁵N retention among plant communities for July and September. Soil ¹⁵N was influenced more by presence or absence of plants than by type of plant; greater quantities of ¹⁵N remained in soil interspaces between plants than in soil directly under plants for July and September. Our results indicate that plant functional type (C₃ versus C₄) can affect both the spatial and the temporal patterns of N cycling in shortgrass steppe. Further research is necessary to determine how these intraseasonal differences translate to longer-term and coarser-scale effects of plants on N cycling, retention, and storage. Received 8 December 1997; accepted 6 May 1998.     

Plant invasion alters nitrogen cycling by modifying the soil nitrifying community

Plant invasions have dramatic aboveground effects on plant community composition, but their belowground effects remain largely uncharacterized. Soil microorganisms directly interact with plants and mediate many nutrient transformations in soil. We hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling. To examine this possible link, monocultures and mixtures of exotic and native species were maintained for 4 years in a California grassland. Gross rates of nitrogen (N) mineralization and nitrification were quantified with ¹⁵N pool dilution and soil microbial communities were characterized with DNA‐based methods. Exotic grasses doubled gross nitrification rates, in part by increasing the abundance and changing the composition of ammonia‐oxidizing bacteria in soil. These changes may translate into altered ecosystem N budgets after invasion. Altered soil microbial communities and their resulting effects on ecosystem processes may be an invisible legacy of exotic plant invasions.     

氮素添加和刈割对内蒙古弃耕草地土壤氮矿化的影响

以内蒙古多伦县恢复生态学试验示范研究站弃耕10余年的草地为研究对象,于2006年起分别设置对照、氮素添加、刈割和氮素添加+刈割4种处理,每种处理6次重复,研究弃耕草地氮素添加和刈割对土壤氮矿化的影响,结合土壤理化性质和植被地上生产力的动态变化,分析弃耕草地土壤氮矿化对植被恢复的响应,为当地草地恢复与重建提供理论依据和数据支持。实验结果表明:1氮素添加显著增加了植物地上净初级生产力(ANPP)和土壤无机氮库,与对照相比分别提高115%和196%,同时显著提高了土壤总硝化速率;但是氮素添加对总氨化速率、土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳氮比(MBC/MBN)、微生物呼吸(MR)以及呼吸熵(q CO2)均无显著影响;2总氨化速率和硝化速率对刈割处理的响应均不显著,但是刈割处理显著降低了土壤MR(P0.05);3氮素添加+刈割处理5—7a后,土壤总氨化和硝化速率均无显著变化;但是氮素添加+刈割处理显著增加了ANPP、土壤无机氮库和q CO2,同时显著降低了MBC和MBC/MBN。这说明在弃耕草地适应性管理中,氮素添加可以显著提高草地生产力,但是长期的氮添加对土壤微生物氮的转化是否有利还值得我们进一步研究。     

Fire, native species, and soil resource interactions influence the spatio-temporal invasion pattern of Bromus tectorum

Bromus tectorum (cheatgrass) is an invasive annual that occupies perennial grass and shrub communities throughout the western United States. Bronus tectorum exhibits an intriguing spatio‐temporal pattern of invasion in low elevation ponderosa pine Pinus ponderosa/bunchgrass communities in western Montana where it forms dense rings beneath solitary pines following fire. This pattern provides a unique opportunity to investigate several indirect effects of native vegetation that influence the invasion pattern of B. tectorum, and specifically how native species, disturbance, and soil resources interact to influence the spatio‐temporal pattern of invasion. We established four replicate field sites, each containing burned‐tree, burned‐grass, unburned‐tree, and unburned‐grass sampling locations, and initiated a series of field sampling and greenhouse experiments utilizing these locations. The objective of our first greenhouse experiment was to identify whether belowground factors contributed to the pattern of B. tectorum biomass observed in these field locations. This experiment generated a B. tectorum biomass response that was nearly identical to the invasion pattern observed in the field, suggesting further investigation of belowground factors was necessary. We measured resin‐sorbed NH₄⁺ and NO₃⁻ during one generation of B. tectorum, and measured a suite of P fractions through a sequential extraction procedure from these soils. These data revealed that a resource island of high N and P exists beneath pine trees. Through a second greenhouse experiment, we determined that N limited B. tectorum biomass in tree soil, whereas P limited biomass in bunchgrass soil. Finally, through a germination experiment we determined that pine litter strongly inhibited B. tectorum germination. These data suggest B. tectorum is regulated by P in bunchgrass soil, and by N and inhibition by pine litter beneath trees, effects that are likely alleviated by fire. These data demonstrate the combined role of direct and indirect interactions between native and invasive species in regulating biological invasions.     

Two Invasive Plants Alter Soil Microbial Community Composition in Serpentine Grasslands

Plant invasions pose a serious threat to native ecosystem structure and function. However, little is known about the potential role that rhizosphere soil microbial communities play in facilitating or resisting the spread of invasive species into native plant communities. The objective of this study was to compare the microbial communities of invasive and native plant rhizospheres in serpentine soils. We compared rhizosphere microbial communities, of two invasive species, Centaurea solstitialis (yellow starthistle) and Aegilops triuncialis (barb goatgrass), with those of five native species that may be competitively affected by these invasive species in the field (Lotus wrangelianus, Hemizonia congesta, Holocarpha virgata, Plantago erecta, and Lasthenia californica). Phospholipid fatty acid analysis (PLFA) was used to compare the rhizosphere microbial communities of invasive and native plants. Correspondence analyses (CA) of PLFA data indicated that despite yearly variation, both starthistle and goatgrass appear to change microbial communities in areas they invade, and that invaded and native microbial communities significantly differ. Additionally, rhizosphere microbial communities in newly invaded areas are more similar to the original native soil communities than are microbial communities in areas that have been invaded for several years. Compared to native plant rhizospheres, starthistle and goatgrass rhizospheres have higher levels of PLFA biomarkers for sulfate reducing bacteria, and goatgrass rhizospheres have higher fatty acid diversity and higher levels of biomarkers for sulfur-oxidizing bacteria, and arbuscular mycorrhizal fungi. Changes in soil microbial community composition induced by plant invasion may affect native plant fitness and/or ecosystem function.     

Microbial immobilization drives nitrogen cycling differences among plant species

In many terrestrial ecosystems nitrogen (N) limits productivity and plant community composition is influenced by N availability. However, vegetation is not only controlled by N; plant species may influence ecosystem N dynamics through positive or negative effects on N cycling. We examined four potential mechanisms of plant species effects on nitrogen (N) cycling. We found no species differences in gross ammonification suggesting there are no changes in the ecosystem N cycling rate between the soil organic matter pool (SOM) and the plant/microbial pool. We also found weak differences among plant species in gross nitrification, thus plant species only marginally change the relative sizes of the NH₄⁺ and NO₃^? pools. Next, more than 90% of mineralized N was microbially immobilized, and microbial N immobilization was positively correlated with root biomass. Finally, while species differed in extractable soil NO3^? concentration, these differences were not related to root biomass suggesting that microbial immobilization drives net N mineralization and soil NO₃^? levels. Our results indicate that plant species do not cause feedbacks on the N cycling rate among the three major ecosystem N pools over nine years. However, plant carbon (C) inputs to the soil control microbial N immobilization and thereby change N partitioning between the plant and microbial N pools. Furthermore our results suggest that the SOM pool can act as a strong bottleneck for N cycling in these systems.     

Response of Microbial Community Composition and Function to Soil Climate Change

Soil microbial communities mediate critical ecosystem carbon and nutrient cycles. How microbial communities will respond to changes in vegetation and climate, however, are not well understood. We reciprocally transplanted soil cores from under oak canopies and adjacent open grasslands in a California oak–grassland ecosystem to determine how microbial communities respond to changes in the soil environment and the potential consequences for the cycling of carbon. Every 3 months for up to 2 years, we monitored microbial community composition using phospholipid fatty acid analysis (PLFA), microbial biomass, respiration rates, microbial enzyme activities, and the activity of microbial groups by quantifying ¹³C uptake from a universal substrate (pyruvate) into PLFA biomarkers. Soil in the open grassland experienced higher maximum temperatures and lower soil water content than soil under the oak canopies. Soil microbial communities in soil under oak canopies were more sensitive to environmental change than those in adjacent soil from the open grassland. Oak canopy soil communities changed rapidly when cores were transplanted into the open grassland soil environment, but grassland soil communities did not change when transplanted into the oak canopy environment. Similarly, microbial biomass, enzyme activities, and microbial respiration decreased when microbial communities were transplanted from the oak canopy soils to the grassland environment, but not when the grassland communities were transplanted to the oak canopy environment. These data support the hypothesis that microbial community composition and function is altered when microbes are exposed to new extremes in environmental conditions; that is, environmental conditions outside of their “life history” envelopes.     

Bacterial abundance and composition in marine sediments beneath the Ross Ice Shelf,Antarctica

Marine sediments of the Ross Sea, Antarctica, harbor microbial communities that play a significant role in the decomposition, mineralization, and recycling of organic carbon (OC). In this study, the cell densities within a 153‐cm sediment core from the Ross Sea were estimated based on microbial phospholipid fatty acid (PLFA) concentrations and acridine orange direct cell counts. The resulting densities were as high as 1.7 × 10⁷ cells mL^?1 in the top ten centimeters of sediments. These densities are lower than those calculated for most near‐shore sites but consistent with deep‐sea locations with comparable sedimentation rates. The δ¹³C measurements of PLFAs and sedimentary and dissolved carbon sources, in combination with ribosomal RNA (SSU rRNA) gene pyrosequencing, were used to infer microbial metabolic pathways. The δ¹³C values of dissolved inorganic carbon (DIC) in porewaters ranged downcore from ?2.5‰ to ?3.7‰, while δ¹³C values for the corresponding sedimentary particulate OC (POC) varied from ?26.2‰ to ?23.1‰. The δ¹³C values of PLFAs ranged between ?29‰ and ?35‰ throughout the sediment core, consistent with a microbial community dominated by heterotrophs. The SSU rRNA gene pyrosequencing revealed that members of this microbial community were dominated by β‐, δ‐, and γ‐Proteobacteria, Actinobacteria, Chloroflexi and Bacteroidetes. Among the sequenced organisms, many appear to be related to known heterotrophs that utilize OC sources such as amino acids, oligosaccharides, and lactose, consistent with our interpretation from δ¹³C_PLFA analysis. Integrating phospholipids analyses with porewater chemistry, δ¹³C_DIC and δ¹³C_POC values and SSU rRNA gene sequences provides a more comprehensive understanding of microbial communities and carbon cycling in marine sediments, including those of this unique ice shelf environment.     

Soil microbial responses to fire and interacting global change factors in a California annual grassland

Wildfire in California annual grasslands is an important ecological disturbance and ecosystem control. Regional and global climate changes that affect aboveground biomass will alter fire-related nutrient loading and promote increased frequency and severity of fire in these systems. This can have long-term impacts on soil microbial dynamics and nutrient cycling, particularly in N-limited systems such as annual grasslands. We examined the effects of a low-severity fire on microbial biomass and specific microbial lipid biomarkers over 3?years following a fire at the Jasper Ridge Global Change Experiment. We also examined the impact of fire on the abundance of ammonia-oxidizing bacteria (AOB), specifically Nitrosospira Cluster 3a ammonia-oxidizers, and nitrification rates 9?months post-fire. Finally, we examined the interactive effects of fire and three other global change factors (N-deposition, precipitation and CO₂) on plant biomass and soil microbial communities for three growing seasons after fire. Our results indicate that a low-severity fire is associated with earlier season primary productivity and higher soil-NH₄ ⁺ concentrations in the first growing season following fire. Belowground productivity and total microbial biomass were not influenced by fire. Diagnostic microbial lipid biomarkers, including those for Gram-positive bacteria and Gram-negative bacteria, were reduced by fire 9- and 21-months post-fire, respectively. All effects of fire were indiscernible by 33-months post-fire, suggesting that above and belowground responses to fire do not persist in the long-term and that these grassland communities are resilient to fire disturbance. While N-deposition increased soil NH₄ ⁺, and thus available NH₃, AOB abundance, nitrification rates and Cluster 3a AOB, similar increases in NH₃ in the fire plots did not affect AOB or nitrification. We hypothesize that this difference in response to N-addition involves a mediation of P-limitation as a result of fire, possibly enhanced by increased plant competition and arbuscular mycorrhizal fungi–plant associations after fire.     

Inter-Specific Competition,but Not Different Soil Microbial Communities,Affects N Chemical Forms Uptake by Competing Graminoids of Upland Grasslands

Evidence that plants differ in their ability to take up both organic (ON) and inorganic (IN) forms of nitrogen (N) has increased ecologists’ interest on resource-based plant competition. However, whether plant uptake of IN and ON responds to differences in soil microbial community composition and/or functioning has not yet been explored, despite soil microbes playing a key role in N cycling. Here, we report results from a competition experiment testing the hypothesis that soil microbial communities differing in metabolic activity as a result of long-term differences to grazing exposure could modify N uptake of Eriophorum vaginatum L. and Nardus stricta L. These graminoids co-occur on nutrient-poor, mountain grasslands where E. vaginatum decreases and N. stricta increases in response to long-term grazing. We inoculated sterilised soil with soil microbial communities from continuously grazed and ungrazed grasslands and planted soils with both E. vaginatum and N. stricta, and then tracked uptake of isotopically labelled NH₄ ⁺ (IN) and glycine (ON) into plant tissues. The metabolically different microbial communities had no effect on N uptake by either of the graminoids, which might suggest functional equivalence of soil microbes in their impacts on plant N uptake. Consistent with its dominance in soils with greater concentrations of ON relative to IN in the soluble N pool, Eriophorum vaginatum took up more glycine than N. stricta. Nardus stricta reduced the glycine proportion taken up by E. vaginatum, thus increasing niche overlap in N usage between these species. Local abundances of these species in mountain grasslands are principally controlled by grazing and soil moisture, although our results suggest that changes in the relative availability of ON to IN can also play a role. Our results also suggest that coexistence of these species in mountain grasslands is likely based on non-equilibrium mechanisms such as disturbance and/or soil heterogeneity.     

Direct Effects of Soil Amendments on Field Emergence and Growth of the Invasive Annual Grass Bromus tectorum L. and the Native Perennial Grass Hilaria jamesii (Torr.) Benth

Bromus tectorum L. is a non-native, annual grass that has invaded western North America. In SE Utah, B. tectorum generally occurs in grasslands dominated by the native perennial grass, Hilaria jamesii (Torr.) Benth. and rarely where the natives Stipa hymenoides Roem. and Schult. and S. comata Trin. & Rupr. are dominant. This patchy invasion is likely due to differences in soil chemistry. Previous laboratory experiments investigated using soil amendments that would allow B. tectorum to germinate but would reduce B. tectorum emergence without affecting H. jamesii. For this study we selected the most successful treatments (CaCl₂, MgCl₂, NaCl and zeolite) from a previous laboratory study and applied them in the field in two different years at B. tectorum-dominated field sites. All amendments except the lowest level of CaCl₂ and zeolite negatively affected B. tectorum emergence and/or biomass. No amendments negatively affected the biomass of H. jamesii but NaCl reduced emergence. Amendment effectiveness depended on year of application and the length of time since application. The medium concentration of zeolite had the strongest negative effect on B. tectorum with little effect on H. jamesii. We conducted a laboratory experiment to determine why zeolite was effective and found it released large amounts of Na⁺, adsorbed Ca²⁺, and increased Zn²⁺, Fe²⁺, Mn²⁺, Cu²⁺, exchangeable Mg²⁺, exchangeable K, and NH₄⁺ in the soil. Our results suggest several possible amendments to control B. tectorum. However, variability in effectiveness due to abiotic factors such as precipitation and soil type must be accounted for when establishing management plans.     

An invasive aster (Ageratina adenophora) invades and dominates forest understories in China: altered soil microbial communities facilitate the invader and inhibit natives

Exotic plant invasion may alter underground microbial communities, and invasion-induced changes of soil biota may also affect the interaction between invasive plants and resident native species. Increasing evidence suggests that feedback of soil biota to invasive and native plants leads to successful exotic plant invasion. To examine this possible underlying invasion mechanism, soil microbial communities were studied where Ageratina adenophora was invading a native forest community. The plant–soil biota feedback experiments were designed to assess the effect of invasion-induced changes of soil biota on plant growth, and interactions between A. adenophora and three native plant species. Soil analysis showed that nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), and available P and K content were significantly higher in a heavily invaded site than in a newly invaded site. The structure of the soil microbial community was clearly different in all four sites. Ageratina adenophora invasion strongly increased the abundance of soil VAM (vesicular-arbuscular mycorrhizal fungi) and the fungi/bacteria ratio. A greenhouse experiment indicated that the soil biota in the heavily invaded site had a greater inhibitory effect on native plant species than on A. adenophora and that soil biota in the native plant site inhibited the growth of native plant species, but not of A. adenophora. Soil biota in all four sites increased A. adenophora relative dominance compared with each of the three native plant species and soil biota in the heavily invaded site had greater beneficial effects on A. adenophora relative dominance index (20% higher on average) than soil biota in the non-invaded site. Our results suggest that A. adenophora is more positively affected by the soil community associated with native communities than are resident natives, and once the invader becomes established it further alters the soil community in a way that favors itself and inhibits natives, helping to promote the invasion. Soil biota alteration after A. adenophora establishment may be an important part of its invasion process to facilitate itself and inhibit native plants.     

Temporal evolution of biochar's impact on soil nitrogen processes – a 15N tracing study

Biochar addition to soils has been proposed as a means to increase soil fertility and carbon sequestration. However, its effect on soil nitrogen (N) cycling and N availability is poorly understood. To gain better insight into the temporal variability of the impact of biochar on gross soil N dynamics, two ¹⁵N tracing experiments, in combination with numerical data analysis, were conducted with soil from a biochar field trial, 1 day and 1 year after application of a woody biochar type. The results showed accelerated soil N cycling immediately following biochar addition, with increased gross N mineralization (+34%), nitrification (+13%) and ammonium (NH₄⁺) and nitrate (NO₃⁻) immobilization rates (+4500% and +511%, respectively). One year after biochar application, the biochar acted as an inert substance with respect to N cycling. In the short term, biochar's labile C fraction and a pH increase can explain stimulated microbial activity, while in the longer term, when the labile C fraction has been mineralized and the pH effect has faded, the accelerating effect of biochar on N cycling ceases. In conclusion, biochar accelerates soil N transformations in the short-term through stimulating soil microbial activity, thereby increasing N bio-availability. This effect is, however, temporary.