Increased Soil Nitrogen Availability Research Articles

Fertilization effects on soil microbial composition and nutrient availability in integrated rice-livestock production systems

Integrated crop-livestock systems (ICLS) in paddies are an important strategy to reduce fertilizer requirements of rice (Oryza sativa). Legume crop rotations and fertilization increase soil carbon (C) and nitrogen (N) availability compared to rice monocropping. Our objectives were to evaluate the effects of fertilization, namely P and K, in no-till ICLS of continuously flooded rice and rice-soybean rotation in subtropical paddy fields on: (i) soil microbial community, (ii) labile C and N pools, (iii) microbial groups, and (iv) rice yield. Both ICLS were annual ryegrass pastures under beef cattle grazing during the winter season without tillage. Although flooded rice yield did not respond to P and K fertilization in either ICLS, P and K fertilization increased microbial biomass C and N content, and permanganate-oxidizable C (POxC) by 37%, 36%, and 42%, respectively, in both production systems, due to alleviation of nutrient limitation for microbial growth. ICLS under soybean-rice rotation had higher C availability and 22.4% higher rice yield than the rice monocropping. Positive relationships were observed between microbial biomass N and Gram-positive (G+; R2 = 0.61), Gram-negative (G-; R2 = 0.54) bacteria, and saprophytic fungi (R2 = 0.45). However, total fungi was almost 10 times lower than the PLFA of total bacteria on average of cropping systems. G+ bacteria were the only microbial group correlated with N availability under flooded rice. We conclude that P and K fertilization has a direct impact on the microbial community and biomass and consequently on the C and N availability, but does not increase rice yield under no-till ICLS. Soybean-rice rotation yields more than rice monocropping system under ICLS and soil bacteria are more sensitive than saprophytic fungi to changes in soil nutrient status during flooding period in paddy fields.

Legume-Based Mobile Green Manure Can Increase Soil Nitrogen Availability and Yield of Organic Greenhouse Tomatoes

Legume-Based Mobile Green Manure Can Increase Soil Nitrogen Availability and Yield of Organic Greenhouse Tomatoes

Legume-Based Mobile Green Manure Can Increase Soil Nitrogen Availability and Yield of Organic Greenhouse Tomatoes.

Information about the availability of soil mineral nitrogen (N) in organic greenhouse tomatoes after the application of mobile green manure (MGM), and its impact on plant nutrient status and yield is scarce. Considering this knowledge gap, the effects of legume biomass from faba beans that are cultivated outdoors (FAB), or from feed-grade alfalfa pellets at two different doses (AAL = 330 g m−2; AAH = 660 g m−2) that were applied as MGM on the nutrition and yield of an organic greenhouse crop of tomatoes were evaluated. All of the MGM treatments increased the mineral N concentrations in the soil throughout the cropping period, and the total N concentration in tomato leaves when compared to the untreated control. FAB and AAH treatments had a stronger impact than AAL in all of the measured parameters. In addition, AAL, AAH, and FAB treatments increased the yield compared to the control by 19%, 33%, and 36%, respectively. The application of MGM, either as faba bean fresh biomass or as alfalfa dry pellets, in organic greenhouse tomatoes significantly increased the plant available soil N, improved N nutrition, and enhanced the fruit yield. However, the N mineralization rates after the MGM application were excessive during the initial cropping stages, followed by a marked decrease thereafter. This may impose an N deficiency during the late cropping period.

Invertebrate functional traits and terrestrial nutrient cycling: Insights from a global meta-analysis.

Functional traits are useful for characterizing variation in community and ecosystem dynamics. Most advances in trait-based ecology to date centre on plant functional traits, although there is an increasing recognition that animal traits are also key contributors to processes operating at the community or ecosystem scale. Terrestrial invertebrates are incredibly diverse and ubiquitous animals with important roles in nutrient cycling. Despite their widespread influence on ecosystem processes, we currently lack a synthetic understanding of how invertebrate functional traits affect terrestrial nutrient cycling. We present a meta-analysis of 511 paired observations from 122 papers that examined how invertebrate functional traits affected litter decomposition rates, nitrogen pools and litter C:N ratios. Based on the available data, we specifically assessed the effects of feeding mode (bioturbation, detritus shredding, detritus grazing, leaf chewing, leaf piercing, ambush predators, active hunting predators) and body size (macro- and micro-invertebrates) on nutrient cycling. The effects of invertebrates on terrestrial nutrient cycling varied according to functional trait. The inclusion of both macro- (≥2mm) and micro-invertebrates (<2mm) increased litter decomposition by 20% and 19%, respectively. All detritivorous feeding modes enhanced litter decomposition rates, with bioturbators, detritus shredders and detritus grazers increasing decomposition by 28%, 22% and 15%, respectively. Neither herbivore feeding mode (e.g. leaf chewers and leaf piercers) nor predator hunting mode (ambush and active hunting) affected decomposition. We also revealed that bioturbators and detritus grazers increased soil nitrogen availability by 99% and 70%, respectively, and that leaf-chewing herbivores had a weak effect on litterfall stoichiometry via reducing C:N ratios by 11%. Although functional traits might be useful predictors of ecosystem processes, our findings suggest context-dependent effects of invertebrate traits on terrestrial nutrient cycling. Detritivore functional traits (i.e. bioturbators, detritus shredders and detritus grazers) are more consistent with increased rates of nutrient cycling, whereas our currently characterized predator and herbivore traits are less predictive. Future research is needed to identify, standardize and deliberately study the impacts of invertebrate functional traits on nutrient cycling in hopes of revealing the key functional traits governing ecosystem functioning worldwide.

White-tailed Deer Fecal Matter Distribution and Nutrient Contribution in Tallgrass Prairie

Herbivores can have nonconsumptive effects on ecosystems, including the redistribution of nutrients in their waste. In tallgrass prairie bison (Bison bison) historically increased soil nitrogen availability via labile waste deposits. White-tailed deer (Odocoileus virginianus) are now the dominant native large herbivore in prairie and have been shown to consume higher nutrient-content plants than those preferred by bison. Deer are also edge-dwelling species that reuse the trails they make as they move and browse throughout the prairie. Therefore, deer may differ from bison in their spatial patterns of nutrient content and redistribution of fecal matter. We examined the nutrient content and spatial distribution of deer pellets by measuring the number of deer pellet piles along deer trails and transects that were systematically placed at the forest border and in open prairie. We also measured the nitrogen content of deer pellets. White-tailed deer pellets had twofold greater nitrogen concentrations (3.43% N and 40.8% C) compared to values reported for bison fecal matter. Deer pellet piles were more concentrated on deer trails compared to the transects, resulting in fourfold greater N inputs on deer trails compared to areas off of trails. As a result, White-tailed deer have the potential to create patches of increased nutrient availability through their clumped distribution of nutrient rich fecal matter, with potential consequences for prairie plant communities.

White-tailed Deer Fecal Matter Distribution and Nutrient Contribution in Tallgrass Prairie

Herbivores can have nonconsumptive effects on ecosystems, including the redistribution of nutrients in their waste. In tallgrass prairie bison (Bison bison) historically increased soil nitrogen availability via labile waste deposits. White-tailed deer (Odocoileus virginianus) are now the dominant native large herbivore in prairie and have been shown to consume higher nutrient-content plants than those preferred by bison. Deer are also edge-dwelling species that reuse the trails they make as they move and browse throughout the prairie. Therefore, deer may differ from bison in their spatial patterns of nutrient content and redistribution of fecal matter. We examined the nutrient content and spatial distribution of deer pellets by measuring the number of deer pellet piles along deer trails and transects that were systematically placed at the forest border and in open prairie. We also measured the nitrogen content of deer pellets. White-tailed deer pellets had twofold greater nitrogen concentrations (3.43% N and 40.8% C) compared to values reported for bison fecal matter. Deer pellet piles were more concentrated on deer trails compared to the transects, resulting in fourfold greater N inputs on deer trails compared to areas off of trails. As a result, White-tailed deer have the potential to create patches of increased nutrient availability through their clumped distribution of nutrient rich fecal matter, with potential consequences for prairie plant communities.

Role of Horses as Potential Vectors of Non-Native Plant Invasion: An Overview

Non-native plant invasions can lead to staggering ecological and economic costs. Thus, land managers are concerned about vectors of seed and propagule introduction onto public lands. Because horses are one of several potential vectors of non-native seed dispersal, we summarize and interpret existing literature and identify potential impacts of horse use on non-native plant spread in natural areas. Several studies indicate non-native seeds can germinate after digestion by horses. In addition to their ability to carry viable non-native seeds in their feces, the literature indicates horses can trample native vegetation, cause soil disturbance, and increase soil nitrogen availability in some habitats. The combination of disturbance, which is known to increase invasibility, and introduction of seeds in horse feces could promote non-native plant invasions in wildlands. However, in situ studies have not demonstrated direct causal links between horses and plant invasions. More research is required to determine where, and to what extent, establishment and spread of non-natives occurs in situ as a direct result of disturbances and seed introduction by horses. Guidelines or regulations regarding horse use already exist in some recreational areas. Where none exist, we recommend development of best management practices, such as weed education programs for equestrians, use of Certified Weed Free Feed, and placement of manure bunkers at horse camps and trailheads.Lauren D. Quinn, Adda Quinn, Mietek Kolipinski, Bonnie Davis, Connie Berto, Mark Orcholski, and Sibdas Ghosh. 2010. “Role of Horses as Potential Vectors of Non-Native Plant Invasion: An Overview.” Natural Areas Journal 30(4):408-416

Plant Microbiome Innovation: M-trophs.

Human population is increasing from 7.6 billion today to an estimated 9.5–10 billion by 2050.1 How can billions more people, with increasing demands for protein and other food sources, be adequately fed without destroying the environment? Utilizing more land for agricultural purposes is not a solution, given that much of the arable land on the planet is already being used for food, fiber, chemical, or fuel production.2 One answer to this complex challenge will lie in the ability to produce more food within the current footprint in sustainable and affordable ways. By one definition, “an agricultural biologicals system is an ecologically, socially, and economically sustainable agricultural production system that promotes safe products by minimizing environmental adverse consequences and reducing the use of non-renewable natural resources.”3 In this article, we discuss opportunities that capitalize on twenty-first century technological innovation and the central role played by the microbial life intimately associated with plants: the plant and soil microbiomes. We examine the potential of Ag biologicals within the agricultural ecosystem. We elucidate their functionality as a means of increasing plant health and crop yields while maintaining environmental sustainability. Finally we illustrate the role that NewLeaf Symbiotics (St. Louis, MO) plays in the introduction of a range of Ag biologicals based on methylobacteria, or M-trophs—bio-complementary products that infuse plants with microbes that are tailored to fit specific crop genetics and environmental conditions.4 Most major agricultural input suppliers and seed companies recognize that biologicals represent a largely untapped opportunity to meet demand for novel and sustainable complements to the existing roster of agricultural chemicals, fertilizers, and genetically modified (GM) traits. Recent government, industrial, and private investments support the belief that significant improvements in crop productivity and environmental sustainability are achievable using complementary biological inputs. Between 2012 and 2015, over $2 billion was invested by Ag multinationals in agreements, mergers, and the acquisition of small biologicals companies.5 These investments are driving the development of the next generation of Ag biologicals. The broad recognition of the agricultural significance of the plant and soil microbiome is supported in large part by the reduced cost, increased speed, and improved accuracy of genomic sequencing. Simultaneous increases in computing power and analytics are driving the rapid evolution of new molecular biology tools directed at solving previously intractable problems. The recognition of the phyto-microbiome as a second genome, with the potential to supplement and interact with the plant genome, offers significant opportunity to improve crop health and yield.6 The persistent evolution of pest resistance to chemical pesticides and traits, combined with consumer demand for sustainable farming and food traceability, have created a strong and growing interest in new Ag biologicals.7 In the US there are 350-plus registered biocontrol agents based on 50-plus species. The market is growing at an average compounded annual growth rate (CAGR) greater than 14%. Every major international and domestic supplier of agronomic inputs is investing in this area. Since most Ag biological products are based on naturally occurring microbes, consumer health and safety concerns about these products are reasonably mitigated. Growers are aware and ready to try new biologicals, in part because biological products like rhizobia and Bacillus thuringiensis (Bt) have been used and accepted for decades. There are, however, significant obstacles to the introduction of novel and efficacious Ag biologicals. Microbes are part of extremely complex soil and plant systems; discovery and delivery of effective products are no simple tasks. Historically, some Ag biological market offerings failed to deliver consistent efficacy equal to conventional inputs, causing the industry to regard Ag biologicals with some caution. Improved strains, advanced formulations, and recognition of the unique value Ag biologicals can provide has led to an increase in their global market share over the past decade from less than 1% to more than 5%. Two of the major strategic challenges to increased use of Ag biologicals are: ▪ Optimization of discovery, screening, and derivation of candidate microbes, and ▪ Alignment of product development with technical and economic drivers of contemporary production farming.

The responses of shoot-root-rhizosphere continuum to simultaneous fertilizer addition, warming, ozone and herbivory in young Scots pine seedlings in a high latitude field experiment

It is not clear how climate change in combination with increasing soil nitrogen availability and herbivory affects boreal forests, the largest terrestrial biome in the world. In this study, Scots pine (Pinus sylvestris) seedlings were exposed to moderate warming (ca. 1 °C), 1.5 × ambient ozone (O3) concentration, fertilizer addition (120 kg N ha−1 yr−1) and shoot herbivory by pine sawfly (Acantholyda posticalis) alone and in combination. We measured fine root morphology, mycorrhizal colonization level, root fungal biomass (ergosterol), rhizosphere emission of biogenic volatile organic compounds (BVOCs), and microbial biomass (PLFAs) in the rhizosphere soil as well as seedling above- and below-ground growth. Warming and fertilization effects on fine root proportions or root fungal biomass occurred in combination with other factors, combination effects being usually negative on the studied variables, or then warming and fertilization in combination cancelled some other factors’ effects. O3 effects on needle growth, root fungal biomass and BVOCs were more often seen after the third exposure year, and sometimes only in combination with other studied factors. Of abiotic factors, fertilizer addition had reducing effect on rhizosphere BVOCs. Though increased nitrogen availability and warming increased both shoot and root dry masses, growth allocation to above- and below-ground parts was not equally increased in the combined exposures. Thus, we conclude that climate change factors together with increased nitrogen availability and herbivory are likely to affect the below-ground compartments negatively, more often than shoots, and ultimately change in growth allocation pattern which may affect overall seedling growth and survival in later years.

Latitudinal variation of life-history traits of an exotic and a native impatiens species in Europe

Understanding the responses of invasive and native populations to environmental change is crucial for reliable predictions of invasions in the face of global change. While comparisons of responses across invasive species with different life histories have been performed before, comparing functional traits of congeneric native and invasive species may help to reveal driving factors associated with invasion. Here we compared morphological functional trait patterns of an invasive species (Impatiens parviflora) with its congeneric native species (I. noli-tangere) along an approximately 1600 km European latitudinal gradient from France (49°34′N) to Norway (63°40′N). Soil nitrogen was recorded during six weeks of the growing season, and light, soil moisture, and nutrient availability were estimated for each sampled population using community weighted means of indicator values for co-occurring species. Temperature data were gathered from nearby weather stations.Both the native and invasive species are taller at higher latitudes and this response is strongest in the invasive species. Seed mass and number of seeds per capsule increase in I. noli-tangere but decrease in I. parviflora towards higher latitudes. Surprisingly, plant height in the invasive I. parviflora decreases with increasing soil nitrogen availability. The latitudinal pattern in seed mass is positively related to temperature in I. noli-tangere and negatively in I. parviflora. Leaf area of both species decreases with increasing Ellenberg indicator values for nitrogen and light but increases with increasing soil moisture. Soil nitrogen concentrations and Ellenberg indicator values for nitrogen have significant positive (I. noli-tangere) and negative (I. parviflora) effects on the number of seeds per capsule. Our results show that the native I. noli-tangere has efficient reproduction at its range edge while the invasive I. parviflora shows a marked decrease in seed size and seed number per capsule. These patterns are unrelated to the growth and obtained size of the plants: even low soil nitrogen availability in the north seemed not to limit plant growth and size. Our results suggest that the invasive I. parviflora tends to become more invasive at lower latitudes by producing heavier seeds and more seeds per capsule.

Nitrogen manipulation affects leaf senescence during late seed filling in soybean

It is known that soybean plants store nitrogen during vegetative growth stage, and then remobilize it to the seeds during seed filling. This nitrogen remobilization is assumed to induce leaf senescence. The objective of this study was to investigate whether the manipulation of nitrogen availability could affect leaf senescence during seed filling. Soybean variety Fukuyutaka was grown in the side-opened vinyl house in 2014 and 2015 in Saga University, Japan. The plants were sufficiently watered with the solution containing 100 mg L−1 nitrogen and other nutrients before starting the treatments. The nutrient solution with different nitrogen concentrations (5–800 mg L−1) was applied to the plants from R6 (full-seed) to R7 (physiological maturity) stages. Parallel to the progress of plant senescence after R6 stage, the leaf-SPAD value, leaf nitrogen and soluble protein contents decreased gradually in control plants (100 mg L−1 nitrogen); however, these parameters did not change in higher nitrogen concentration treatments, whereas decreased quickly in lower nitrogen treatments. Therefore, the leaves and stems in higher nitrogen treatments remained green even at harvest time when the whole plant senesced in control and lower nitrogen treatments. The relative expression of an autophagy gene, GmATG8c, which is associated with nitrogen remobilization, showed the lower the nitrogen availability the earlier the up-regulation. The present results revealed that the shortage of nitrogen could stimulate the leaf senescence, while increasing soil nitrogen availability could delay even stop the leaf senescence, indicating that the nitrogen availability could be a key regulating factor of monocarpic senescence in soybean.

Soil microbiological properties and available nitrogen for corn in monoculture and intercropped with forage

Abstract The objective of this work was to evaluate the effects of corn (Zea mays) in monoculture and intercropped with forage on soil microbiological properties and nitrogen availability under no-tillage in the Cerrado (Brazilian savanna). The experiment was carried out from the 2007/2008 to the 2010/2011 crop season, in a Latossolo Vermelho-Amarelo distrófico (Typic Haplustox). A randomized complete block design with three replicates was used, with the following treatments: corn in monoculture; corn intercropped with Panicum maximum; corn intercropped with Urochloa humidicola; P. maximum in monoculture; and U. humidicola in monoculture. Soil samples were taken at 0.00-0.05, 0.05-0.10, 0.10-0.20, and 0.20-0.30-m soil depths, in the begining and in the end of the last crop season. The intercropping systems of corn increased soil nitrogen availability, but did not alter total nitrogen and organic carbon contents in the soil, when compared to corn in monoculture. Corn intercropped with P. maximum increased soil microbial biomass nitrogen and microbial nitrogen quotient, in comparison to corn in monoculture, as well soil as microbial biomass carbon in the surface soil layer, when compared to corn intercropped with U. humidicola.

Plant biodiversity effects in reducing fluvial erosion are limited to low species richness.

It has been proposed that plant biodiversity may increase the erosion resistance of soils, yet direct evidence for any such relationship is lacking. We conducted a mesocosm experiment with eight species of riparian herbaceous plants, and found evidence that plant biodiversity significantly reduced fluvial erosion rates, with the eight-species polyculture decreasing erosion by 23% relative to monocultures. Species richness effects were largest at low levels of species richness, with little increase between four and eight species. Our results suggest that plant biodiversity reduced erosion rates indirectly through positive effects on root length and number of root tips, and that interactions between legumes and non-legumes were particularly important in producing biodiversity effects. Presumably, legumes increased root production of non-legumes by increasing soil nitrogen availability due to their ability to fix atmospheric nitrogen. Our data suggest that a restoration project using species from different functional groups might provide the best insurance to maintain long-term erosion resistance.

Experimental warming studies on tree species and forest ecosystems: a literature review

Temperature affects a cascade of ecological processes and functions of forests. With future higher global temperatures being inevitable it is critical to understand and predict how forest ecosystems and tree species will respond. This paper reviews experimental warming studies in boreal and temperate forests or tree species beyond the direct effects of higher temperature on plant ecophysiology by scaling up to forest level responses and considering the indirect effects of higher temperature. In direct response to higher temperature (1) leaves emerged earlier and senesced later, resulting in a longer growing season (2) the abundance of herbivorous insects increased and their performance was enhanced and (3) soil nitrogen mineralization and leaf litter decomposition were accelerated. Besides these generalizations across species, plant ecophysiological traits were highly species-specific. Moreover, we showed that the effect of temperature on photosynthesis is strongly dependent on the position of the leaf or plant within the forest (canopy or understory) and the time of the year. Indirect effects of higher temperature included among others higher carbon storage in trees due to increased soil nitrogen availability and changes in insect performance due to alterations in plant ecophysiological traits. Unfortunately only a few studies extrapolated results to forest ecosystem level and considered the indirect effects of higher temperature. Thus more intensive, long-term studies are needed to further confirm the emerging trends shown in this review. Experimental warming studies provide us with a useful tool to examine the cascade of ecological processes in forest ecosystems that will change with future higher temperature.

Soil nitrogen availability and herbivore attack influence the chemical defenses of an invasive plant (Linaria dalmatica; Plantaginaceae)

Chemical defenses are thought to contribute to the invasion success and impacts of many introduced plants; however, for most of these species, little is known about these compounds and how they vary in natural environments. Plant allelochemical concentrations may be affected by a variety of abiotic and biotic factors, including soil nutrients and herbivores. Moreover, such quantitative variation is likely to play an important role in species interactions involving these invasive plants. The purpose of this study was to examine patterns of variation in iridoid glycoside concentrations of the invasive plant Linaria dalmatica (Plantaginaceae). We conducted a greenhouse experiment to investigate the effect of soil nitrogen availability on iridoid glycoside concentrations. Results from this experiment showed that plant iridoid glycoside concentrations decreased with increased nitrogen availability. Additionally, plants were collected from multiple field sites in order to characterize the influence of population, soil nitrogen availability, and herbivore attack on iridoid glycoside variation. Results from field studies indicated that plants demonstrated considerable seasonal variation, as well as variation within and among populations, with iridoid glycoside concentrations ranging from approximately 1 to 15% dry weight. The relationship between soil nitrogen and plant iridoid glycosides varied among populations, with a strong negative correlation in one population, a marginally significant negative relationship in a second population, and no relationship in the remaining two populations. Additionally, we found a negative relationship between iridoid glycoside concentrations and plant injury by an introduced biocontrol agent, the stem-mining weevil Mecinus janthinus (Cucurlionidae). These results show that plant allelochemical concentrations can vary widely in natural environments and suggest that levels of plant defense may be reduced by increased soil nitrogen availability and herbivore attack in this invasive plant species.

Host plant quality, spatial heterogeneity, and the stability of mite predator–prey dynamics

Population dynamics models suggest that both the over-all level of resource productivity and spatial variability in productivity can play important roles in community dynamics. Higher productivity environments are predicted to destabilize consumer–resource dynamics. Conversely, greater heterogeneity in resource productivity is expected to contribute to stability. Yet the importance of these two factors for the dynamics of arthropod communities has been largely overlooked. I manipulated nutrient availability for strawberry plants in a multi-patch experiment, and measured effects of overall plant quality and heterogeneity in plant quality on the stability of interactions between the phytophagous mite Tetranychus urticae and its predator Phytoseiulus persimilis. Plant size, leaf N content and T. urticae population growth increased monotonically with increasing soil nitrogen availability. This gradient in plant quality affected two correlates of mite population stability, population variability over time (i.e., coefficient of variation) and population persistence (i.e., proportion of plant patches colonized). However, the highest level of plant quality did not produce the least stable dynamics, which is inconsistent with the “paradox of enrichment”. Heterogeneity in plant productivity had modest effects on stability, with the only significant difference being less variable T. urticae densities in the heterogeneous compared to the corresponding homogeneous treatment. These results are generally congruent with metapopulation theory and other models for spatially segregated populations, which predict that stability should be governed largely by relative movement rates of predators and prey—rather than patch quality.

Role of Horses as Potential Vectors of Non-Native Plant Invasion: An Overview

ABSTRACT: Non-native plant invasions can lead to staggering ecological and economic costs. Thus, land managers are concerned about vectors of seed and propagule introduction onto public lands. Because horses are one of several potential vectors of non-native seed dispersal, we summarize and interpret existing literature and identify potential impacts of horse use on non-native plant spread in natural areas. Several studies indicate non-native seeds can germinate after digestion by horses. In addition to their ability to carry viable non-native seeds in their feces, the literature indicates horses can trample native vegetation, cause soil disturbance, and increase soil nitrogen availability in some habitats. The combination of disturbance, which is known to increase invasibility, and introduction of seeds in horse feces could promote non-native plant invasions in wildlands. However, in situ studies have not demonstrated direct causal links between horses and plant invasions. More research is required to de...

The “soil microbial loop” is not always needed to explain protozoan stimulation of plants

Protozoa stimulate plant growth, but we do not completely understand the underlying mechanisms, and different hypotheses seek to explain this phenomenon. To test these hypotheses, we grew the grass Yorkshire fog ( Holcus lanatus) in pots with soil, which contained either (1) no organisms but bacteria – or (2) bacteria and protozoa. Half of the pots received a glucose treatment so as to mimic an additional root exudation. We measured plant growth and plant nitrogen uptake, along with various microbial pools and processes that support plant growth. Protozoan presence significantly enhanced soil nitrogen mineralization, plant nitrogen uptake from organic nitrogen sources, plant nitrogen content, and plant growth. By contrast, we found no evidence that glucose addition, mimicking root exudation, increased soil nitrogen availability and plant nitrogen uptake. Moreover, although protozoan presence affected bacterial community structure, it did not affect the proportion of IAA-producing bacteria in the community or plant root morphology. These results refute the “soil microbial loop” hypotheses, which suggest that protozoan stimulation of plant growth results from complex interactions between plant roots, bacteria and protozoa. Our experiment thus favours the simple explanation that increased nitrogen availability is the key factor behind the positive protozoan effect on plant growth. To exploit natural resources in an efficient and environmentally friendly way, we need to understand in detail the functioning of ecosystems. This study stresses that to achieve this, it is still urgent, besides investigating intricate food-web and signal compound interactions, also to focus on the basic stoichiometric and energetic aspects of organisms.

Secondary Invasion Following the Reduction of Coronilla varia (Crownvetch) in Sand Prairie

Abstract I investigated the effect of Coronilla varia invasion and subsequent reduction on the plant community and soil nitrogen availability in a degraded Illinois sand prairie. The presence of C. varia significantly increased soil nitrogen availability and significantly decreased native species richness and cover, but neither helped nor hindered a common non-native grass, Poa pratensis. One year after the drastic reduction of C. varia, soil nitrogen remained somewhat elevated and native species richness and cover low, but P. pratensis cover increased nearly six-fold. These results suggest that C. varia's impact might be continued through altered soil nitrogen. More important, however, was the strong response of P. pratensis, which could hinder restoration efforts by competing with native species.

Changes in light and nitrogen availability under pioneer trees may indirectly facilitate tree invasions of grasslands

Summary 1 The first trees establishing in grasslands may indirectly favour their seedlings in competition with neighbouring herbaceous vegetation by increasing soil fertility with nitrogen-rich litter and by reducing light levels under their canopies. It is predicted that increasing soil nitrogen availability will accelerate invasion of trees by stimulating their growth more than that of herbaceous species. Decreasing light availability is predicted to increase tree invasion by limiting the growth of herbaceous vegetation more than that of trees (competitive release). 2 We tested these predictions using Chinese tallow tree ( Sapium sebiferum ), which is an aggressive alien invader of grasslands in the southern USA, and Hackberry ( Celtis laevigata ), which is a native tree common in these grasslands. 3 Nitrogen and light were manipulated in two factorial field experiments in grasslands in Texas, USA. In the first, nitrogen was increased and light was decreased in plots with planted Celtis or Sapium seedlings. In the second experiment, light availability to planted Celtis or Sapium seedlings was increased by holding back prairie vegetation. 4 In the first experiment, growth of Celtis and Sapium seedlings increased with nitrogen fertilization while the above-ground biomass of prairie vegetation did not change. Prairie vegetation biomass decreased and tree seedling growth increased under shading. Sapium ’s growth increased dramatically in the treatment with combined nitrogen and shade. Sapium survival decreased when shade was applied. 5 In the second experiment, Sapium growth increased with increased light. Thus, increased Sapium growth at low light levels in the shade experiment was probably a consequence of decreased competitive interference from prairie vegetation, rather than better absolute performance of Sapium in low light levels. 6 These results provide evidence for facilitation as a mechanism involved in tree invasions of grasslands. Changes in resource levels, perhaps in combination with other factors, may explain rapid conversion of grassland communities to woodlands after the first pioneer trees are established. The marked response of Sapium to the combination of nitrogen and shade suggests that these positive feedbacks may be particularly strong for this alien plant species.