Soil fungal population levels in cotton fields fertilized with poultry litter and their relationships to soil nutrient concentrations and plant growth parameters

Application to land of large quantities of waste materials from concentrated animal production, without causing environmental pollution, presents a major challenge to agriculture in the 21st century. Effects of land-applied animal wastes on chemical contents of soil are well documented, but less is understood of their effects on microbial populations in soil. This study was undertaken to evaluate effects of commercial application of poultry litter (PL), as a fertilizer, on soil fungal population levels and components on cotton farms in Mississippi, and to determine relationships of fungal population levels to soil nutrient contents and cotton growth and yield. On each of two farms, soil fungal population levels were estimated by dilution plating from samples of soil collected at two sampling times during 2 years from replicated plots of four fertilization treatments: 0 fertilizer, conventional mineral fertilizer (CF), low PL, and high PL. Soil fungal population levels differed significantly (P = 0.05) according to years or seasons and fertilization treatments on both farms. Population levels often were higher in soils amended with low or high PL, or with CF, than in unfertilized controls. On one farm where PL was tilled into soil, fungal population levels increased significantly during the course of the experiment in PL and CF treatments, but not in unfertilized controls, as determined by linear regression. No such increases were observed on the second farm where PL was applied no-till. Population levels of Fusarium semitectum and Penicillium purpurogenum were significantly higher in PL-treated soils than in unfertilized controls in 1 or 2 of 4 sampling events at both farms, while levels of four other species or groups of fungi usually did not differ. Fungal population levels were significantly correlated (P = 0.05) with N concentrations of soils in 1–4 sampling events on each farm and less frequently correlated with concentrations of nine other elements. In 4 of 24 instances, soil fungal population levels were significantly correlated with leaf area index, chlorophyll content, or yield of cotton, and correlation coefficients with these plant parameters were always positive. Major conclusions derived from this study are (1) no deleterious effects on population levels of total or select soil fungi were observed with use of PL as a fertilizer for commercial cotton production; and (2) soil fungal population levels may increase over time in association with greater fertility and plant growth that is induced by both mineral fertilizer and PL applications.     

A genotype dependent-response to cadmium contamination in soil is displayed by Pinus pinaster in symbiosis with different mycorrhizal fungi

Soil contamination with Cd is of primary concern and beneficial soil restoration strategies urge. The aim of this work is to evaluate the response of two different genotypes of Pinus pinaster (wild and selected) to Cd contamination and to assess how inoculation with ectomycorrhizal fungi, Suillus bovinus and Rhizopogon roseolus, influenced each genotype. Seedlings were exposed to soil contaminated at 15 and 30 mg Cd kg⁻¹. Plant growth, mycorrhizal traits and Cd accumulation in different tissues of the plant were determined at harvest. The fungal community was assessed by denaturing gradient gel electrophoresis. At 15 mg Cd kg⁻¹ S. bovinus increased aboveground development in both genotypes. At 30 mg Cd kg⁻¹ non-inoculated wild genotype accumulated more Cd in the shoots (1.7-fold) than the selected genotype; inoculation with R. roseolus decreased Cd concentration in the roots of the selected genotype whereas the opposite occurred in the wild genotype. Cd concentration in the root system was the parameter most influenced by the interaction between the three studied variables. The fungal community established was affected by the Cd concentration in the soil. Results show that different genotypes of P. pinaster react differently to Cd exposure depending on the mycorrhizal association. The importance of considering the combination between plant genotype and its symbiotic partners when aiming at the forestation of degraded land is highlighted.     

Effects of ecological restoration on microbial activity,microbial functional diversity,and soil organic matter in mixed-oak forests of southern Ohio,USA

As a result of many decades of fire suppression and atmospheric deposition the deciduous forests of eastern North America have changed significantly in stem density, basal area, tree size-frequency distribution, and community structure. Consequently, soil organic matter quality and quantity, nutrient availability, and microbial activity have likely been altered. This study evaluated the effects of four alternative forest ecosystem restoration strategies on soil microbial activity, microbial functional diversity, soil organic C, and soil N status in two mixed-oak (Quercus spp.) forests in southern Ohio, USA. The soils of these forests were sampled during the fourth growing season after application of (1) prescribed fire, (2) thinning of the understory and midstory to pre-settlement characteristics, (3) the combination of fire and thinning, and (4) an untreated control. Prescribed fire, with or without thinning, resulted in increased bacterial but not fungal activity when assessed using Biolog^®. In contrast, assays of acid phosphatase and phenol oxidase activity indicated greater microbial activity in the thinning treatment than in the other three treatments. Functional diversity of both bacteria and fungi was affected by restoration treatment, with the bacterial and fungal assemblages present in the thin + burn sites and the fungal assemblage present in the thinned sites differing significantly from those of the control and burned sites. Treatments did not result in significant differences in soil organic C content among experimental sites; however, the soil C:N ratio was significantly greater in thinned sites than in sites given the other three treatments. Similarly, there were no significant differences in dissolve inorganic N, dissolved organic N, or microbial biomass N among treatments. Bacterial and fungal functional diversity was altered significantly. Based on Biolog^® utilization treatments the bacterial assemblage in the thin-only treatment appeared to be relatively N-limited and the fungal assemblage relatively C-limited, whereas in the thin + burn treatment this was reversed. Although effects of restoration treatments on soil organic matter and overall microbial activity may not persist through the fourth post-treatment year, effects on microbial functional diversity are persistent.     

Population size of indigenous Rhizobium leguminosarum biovar trifolii in long-term field experiments with sewage sludge cake,metal-amended liquid sludge or metal salts: Effects of zinc,copper and cadmium

There is conflicting evidence, and therefore continuing concern, as to whether metals in sewage sludge are deleterious to soil microbial processes and long-term agricultural productivity. Nine field experiments with sewage sludge cakes, three with metal-amended liquid sludges and three with inorganic metal salts were set up across Britain in 1994 to give individual metal dose–response treatments to try to answer this question. This study reports on the effects of Zn, Cu and Cd on the population size of Rhizobium leguminosarum biovar trifolii, a nitrogen fixing symbiont of white clover (Trifolium repens), in soils from these experiments over 11 years. Significant (P < 0.05) reductions in indigenous rhizobial numbers occurred on the Zn metal dose–response treatments at eight of the sludge cake sites in 2005, but few consistent effects were evident on the Cu or Cd metal dose–response treatments during the 11-year monitoring period. The soil total Zn concentrations where effects occurred were near to the UK statutory limit of 300 mg kg^?1 for soils receiving sewage sludge. No significant reductions occurred in any treatments on the metal-amended liquid sludge or inorganic metal salt experiments in which the metals would be expected to be in a more bioavailable form, even after 11 years. The effects in the sludge cake experiments were related consistently with soil total Zn, with no recovery to date. The reductions in clover rhizobial numbers in the sludge cake experiments were due to Zn effects on free-living rhizobia in the soil, with gradual die-off over a long time with increasing soil total Zn concentrations. Currently, no consistent adverse effects on rhizobia have been seen at the UK limits for Cu and Cd of 135 and 3 mg kg^?1, respectively.     

Land use influences soil fungal community composition across central Victoria,south-eastern Australia

Current theory expects that fungi, on the one hand, are spatially ubiquitous but, on the other, are more susceptible than bacteria to disturbance such as land use change due to dispersal limitations. This study examined the relative importance of location and land use effects in determining soil fungal community composition in south-eastern Australia. We use terminal restriction fragment length polymorphism (T-RFLP; primer pair ITS1-F–ITS4) and multivariate statistical methods (NMDS ordinations, ANOSIM tests) to compare relative similarities of soil fungal communities from nine sites encompassing three locations (ca 50–200 km apart) and four land uses (native eucalypt forest, Pinus radiata plantation, Eucalyptus globulus plantation, and unimproved pasture). Location effects were generally weak (e.g. ANOSIM test statistic R ≤ 0.49) and were, in part, attributed to minor differences in soil texture. By contrast, we found clear and consistent evidence of land use effects on soil fungal community composition (R ≤ 0.95). That is, soils from sites of the same land use grouped together in NMDS ordinations of fungal composition despite geographic separations of up to ca 175 km (native eucalypt forests) and 215 km (P. radiata plantations). In addition, different land uses from the same location were clearly separate in NMDS ordinations, despite, in one case, being just 180 m apart and having similar land use histories (i.e. P. radiata versus E. globulus plantation both established on pasture in the previous decade). Given negligible management of all sites beyond the early establishment phase, we attribute much of the land use effects to changes in dominant plant species based on consistent evidence elsewhere of strong specificity in pine and eucalypt mycorrhizal associations. In addition, weak to moderate correlations between soil fungal community composition and soil chemical variables (e.g. Spearman rank correlation coefficients for individual variables of 0.08–0.32), indicated a minor contributing role of vegetation-mediated changes in litter and soil chemistry. Our data provide evidence of considerable plasticity in soil fungal community composition over time spans as short as 6–11 years. This suggests that – at least within geographic zones characterised by more-or-less contiguous forest cover – soil fungal community composition depends most on availability of suitable habitat because dispersal propagules are readily available for colonisation after land use change.     

Influence of organic Crotalaria juncea hay and ammonium nitrate fertilizers on soil nematode communities

Comparisons of organic and inorganic fertilizer effects on nematode communities depend on the specific organic fertilizer used. Field experiments were conducted during 2001 and 2002 in a squash (Cucurbita pepo) agroecosystem to determine if applying sunn hemp (Crotalaria juncea) hay as an organic fertilizer improved nematode communities involved in soil nutrient cycling compared to an equivalent N rate (100 kg N/ha) of ammonium nitrate. Fertilizer source had minimal effect on nematode communities in 2001 when treatments were applied after a winter cover crop of oats (Avena sativa), but differences (P ≤ 0.05) between the fertilizer sources occurred in 2002 when no winter cover cropping preceded squash. Fertilization with sunn hemp hay increased abundance of the bacterivore guilds Ba₁ and Ba₂, and increased fungivores at the end of the experiment. Compared to ammonium nitrate, fertilization with sunn hemp hay resulted in a community with lower maturity index, higher enrichment index, and lower channel index, consistent with a disturbed and nutrient-enriched soil food web undergoing bacterial decomposition. Sunn hemp hay occasionally stimulated omnivorous nematodes, but suppressed plant-parasitic nematodes relative to ammonium nitrate fertilizer. Increasing the sunn hemp hay rate to 200 kg N/ha increased the abundance of bacterivores, fungivores, and predatory nematodes, and total nematode abundance compared to hay at 100 kg N/ha. Fertilization with ammonium nitrate increased the percentage of herbivores, but reduced percentage and abundance of omnivores. In conclusion, sunn hemp fertilizer maintained greater numbers of nematodes involved in nutrient cycling as compared to ammonium nitrate.     

Responses of soil nematode community structure to different long-term fertilizer strategies in the soybean phase of a soybean–wheat–corn rotation

The impact of long-term application of fertilizers in soybean fields on soil nematode community structure was studied. The long-term application model of fertilizers lasted 13 years in a soybean–wheat–corn rotation, and included three treatments: no fertilizer (NF), chemical fertilizer (urea and ammonium phosphate, CF), and pig manure combined with chemical fertilizer (MCF). The soil nematode community structures and ecological indices were determined from soil samples taken at five soybean growth stages from May to October in the soybean phase of the rotation. Fertilizer application had significant effects on abundance of plant parasites, bacterivores and fungivores (P < 0.05), but had no significant effects on total nematodes and omnivores-predators. Abundance of plant parasites was higher in NF than in MCF and CF, and abundance of bacterivores was highest in MCF. Fertilizer application significantly affected Plant-parasitic Nematode Maturity Index (PPI) and Nematode Channel Ratio (NCR) ecological indices (P < 0.05). Shannon–Weaver Index (H′) and Species Richness (SR) indices were higher in MCF than in either NF or CF. The abundances of total nematode and plant parasites showed increasing trend with soybean growth in all three treatments. This is probably due to soil environment being more suitable for soil nematode survival with more food available for plant parasites as the soybean grows. Soybean growth stage significantly affected the H′, Free Living Nematode Maturity Index (MI) and PPI. Bacterivores significantly correlated with soil nutrient status suggesting that they could be used as a potential indicator of soil fertility.     

Improving soil structure by promoting fungal abundance with organic soil amendments

Building soil structure in agroecosystems is important because it governs soil functions such as air and water movement, soil C stabilization, nutrient availability, and root system development. This study examined, under laboratory conditions, effects of organic amendments comprised of differing proportions of labile and semi-labile C on microbial community structure and macroaggregate formation in three variously textured soils where native structure was destroyed. Three amendment treatments were imposed (in order of increasing C lability): vegetable compost, dairy manure, hairy vetch (Vicia villosa Roth). Formation of water stable macroaggregates and changes in microbial community structure were evaluated over 82 days. Regardless of soil type, formation of large macroaggregates (LMA, >2000 μm diameter) was highest in soils amended with vetch, followed by manure, non-amended control, and compost. Vetch and manure had greater microbially available C and caused an increase in fungal biomarkers in all soils. Regression analysis indicated that LMA formation was most strongly related to the relative abundance of the fungal fatty acid methyl ester (FAME) 18:2ω6c (r = 0.55, p < 0.001), fungal ergosterol (r = 0.58, p < 0.001), and microbial biomass (r = 0.57, p < 0.001). Non-metric multidimensional scaling (NMS) ordination of FAME profiles revealed that vetch and manure drove shifts toward fungal-dominated soil microbial communities and greater LMA formation in these soils. This study demonstrated that, due to their greater amounts of microbially available C, vetch or manure inputs can be used to promote fungal proliferation in order to maintain or improve soil structure.     

Accumulation of As,Pb, Zn,Cd and Cu and arbuscular mycorrhizal status in populations of Cynodon dactylon grown on metal-contaminated soils

Metal(loid) accumulation and arbuscular mycorrhizal (AM) status of the dominant plant species, Cynodon dactylon, growing at four multi-metal(loid)s-contaminated sites and an uncontaminated site of China were investigated. Up to 94.7 As mg kg^?1, 417 Pb mg kg^?1, 498 Zn mg kg^?1, 5.8 Cd mg kg^?1 and 27.7 Cu mg kg^?1 in shoots of C. dactylon were recorded. The plant was colonized consistently by AM fungi (33.0–65.5%) at both uncontaminated site and metal-contaminated sites. Based on morphological characteristics, fourteen species of AM fungi were identified in the rhizosphere of C. dactylon, with one belonging to the genus of Acaulospora and the other thirteen belonging to the genus of Glomus. Glomus etunicatum was the most common species associated with C. dactylon growing at metal-contaminated sites. Spore abundance in the rhizosphere of C. dactylon growing at the metal-contaminated soils (22–82 spores per 25 g soil) was significantly lower than that of the uncontaminated soils (371 spores per 25 g soil). However, AM fungal species diversity in the metal-contaminated soils was significantly higher than that in the uncontaminated soils. This is the first report of AM status in the rhizosphere of C. dactylon, the dominant plant survival in metal-contaminated soils. The investigation also suggests that phytorestoration of metal-contaminated sites might be facilitated using the appropriate plant with the aid of tolerant AM fungi.     

Forms of phosphorus in bacteria and fungi isolated from two Australian soils

To understand the origin of organic and condensed forms of phosphorus (P) in soils, detailed information about P forms in microorganisms is required. We isolated 7 bacteria and 8 fungi from two Australian soils and analyzed the P forms in their pure cultures by extraction with NaOH-EDTA followed by ³¹P solution nuclear magnetic (NMR) spectroscopy. The bacteria belonged to the actinobacteria and the fungi to the ascomycota, as determined by rDNA sequencing. The proportions of broad forms of P were significantly different between the bacterial and fungal isolates (analysis of similarities, p = 0.001). Ortho-, pyro- and polyphosphate were present in higher proportions in fungi, while monoester and diester P were present in higher proportions in bacteria. Spectral deconvolution of the monoester region revealed 15 distinct resonances. The three major ones, which were identified by spiking experiments as glycerol 1-phosphate, glycerol 2-phosphate and adenosine-5′-monophosphate (AMP), comprised 56–74% of P in the monoester region. Ordination by principal component analysis and testing for treatment effects using analysis of similarities showed significant separation of P distribution in the monoester region between bacterial and fungal isolates (p = 0.007). However, neither group of microorganisms had a specific single P form which might be considered characteristic. As such, it may be difficult to distinguish soil P from bacterial or fungal origins, with the possible exception of a predominantly fungal origin of pyro- and polyphosphate. The identification of three major resonances in the monoester region of microorganisms is important, since the same resonances are found in ³¹P NMR spectra of soil extracts.     

Influence of biological aggregating agents associated with microbial population on soil aggregate stability

In this study, we investigated the effects of plant residue decomposition and biological aggregating agents (microbial extracellular polysaccharides and fungal hyphae) on soil aggregate stability and determined the microbial population at different stages of soil aggregate stabilization. Experiments were conducted in a 40 days incubation period with the following six treatments: the control (soil only), soil + fungicide, soil + bactericide, soil + maize residues, soil + maize residues + fungicide, and soil + maize residues + bactericide. The maize residues treatments greatly enhanced the formation of macroaggregates. In the residue treatments, the addition of fungicide led to a significant suppression of fungal biomass and activity as well as a reduction of soil aggregate stability, which demonstrated the profound influence of fungal activity on aggregate formation. The addition of bactericide also significantly reduced soil aggregate stability, indicating that bacterial activity also played an important role in the macroaggregate formation. However, the effect of microbial extracellular polysaccharides on soil aggregate stability was not significant, which might be attributable to the fast wet sieving method used for aggregate separation. For the treatments of soil + residues and soil + residues + bactericide, the temporal variations of soil aggregate formation with two peak values suggested that other factors, such as hydrophobic compounds and phenolic acids, might be involved in the soil aggregate stabilization process.     

Arbuscular mycorrhizal abundance in contaminated soils around a zinc and lead deposit

In order to study the variations in spore abundance and root colonization parameters of arbuscular mycorrhizal (AM) fungi in a naturally heavy metals polluted site and their relationships with soil properties, 35 plots in the Anguran Zn and Pb mining region were selected along a transect from the mine to 4500 m away. Within each plot, a composite sample of root and rhizospheric soil from a dominant indigenous plant was collected. The soil samples were analyzed for their physico-chemical characteristics. Spores were extracted, counted and identified at genus level. The roots were examined for colonization, arbuscular abundance, mycorrhizal frequency and intensity. Along the transect, the total and available (DTPA-extractable) concentration of Zn decreased from 6472 to 45 mg kg⁻¹ and 75 to 5 mg kg⁻¹, respectively. For Pb the values varied from 5203 to 0 mg kg⁻¹ and 32 to 0 mg kg⁻¹, respectively. In parallel, root colonization rate in the dominant native plants (except Alyssum sp.) varied from 35% to 85% and the spore numbers from 80 to 1306 per 200 g dry soil along the transect. Spores of Glomus were abundantly found in all plots as dominant, while Acaulospora spores were observed only in some moderately polluted and in control plots. AM fungal propagules never disappeared completely even in soils with the highest rates of both heavy metals. Spore numbers were more affected by Zn and Pb concentrations than root colonization. The variations of AM fungi propagules were better related to available than to total concentration of both metals. Spore numbers were positively correlated with mycorrhizal colonization parameters, particularly with arbuscular abundance.     

How do environmental factors and different fertilizer strategies affect soil CO2 emission and carbon sequestration in the upland soils of southern China?

Upland soils have been identified as a major CO₂ source induced by human activities, such as fertilizer applications. The aim of this study is to identify the characteristics of soil CO₂ emission and carbon balance in cropland ecosystems after continuous fertilizer applications over decades. The measurements of soil surface CO₂ fluxes throughout the years of 2009 and 2010 were carried out based on a fertilization experiment (from 1990) in a double cropping system rotated with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) in upland soil in southern China. Four treatments were chosen from the experiment for this study: no-fertilizer application (SR), nitrogen–phosphorus–potassium chemical fertilizers (NPK), NPK plus pig manure (NPKM) and pig manure alone (M). Results showed that the mean value of soil CO₂ fluxes from 08:00 to 10:00 am could represent its daily mean value in summer period (June–August) and that from 09:00 am to 12:00 pm for the rest season of a year. Soil temperature and moisture combined together could explain 70–83% of variations of CO₂ emission. Annual cumulative soil CO₂ fluxes in the treatments with manure applications (8.2 ± 0.8 and 11.0 ± 1.2 t C ha⁻¹ in 2009, and 7.9 ± 0.9 and 11.1 ± 1.2 t C ha⁻¹ in 2010 in NPKM and M, respectively) were significantly higher than those in the treatments with non-manure addition (2.5 ± 0.2 and 3.4 ± 0.2 t C ha⁻¹ in 2009, and 2.1 ± 0.2 and 3.7 ± 0.3 t C ha⁻¹ in 2010 in SR and NPK, respectively). However, the treatments with manure applications represented a carbon sink in the soil (carbon output/input ratio < 1.0), which demonstrated potential for carbon sequestration.     

Assessing the tolerance to heavy metals of arbuscular mycorrhizal fungi isolated from sewage sludge-contaminated soils

Different fungal ecotypes were isolated from soils which had received long-term applications of metal-contaminated sewage sludge with the aim of studying the degree of tolerance and adaptation to heavy metals of arbuscular mycorrhizal (AM) fungi. The development and structural aspects of AM colonization produced by the different fungal isolates were studied using two host plants, Allium porrum and Sorghum bicolor, which were grown in either contaminated or non-contaminated soils. Four different AM fungi were successfully isolated from the experimental field plots: (i) Glomus claroideum, isolated from plots receiving only inorganic fertilizer; (ii) another apparently similar ecotype of Glomus claroideum, but isolated from plots with 300 m³ ha⁻¹ year⁻¹ of contaminated sludge added, (iii) an unidentified Glomus sp., present only in the less contaminated plots (100 m³ ha⁻¹ year⁻¹ of unamended sludge) and (iv) Glomus mosseae, isolated from plots receiving 100 or 300 m³ ha⁻¹ year⁻¹ of amended or unamended sludge (intermediate rates of contamination). There were consistent differences in behaviour among the four AM fungi tested with regard to the colonization levels they produced in non-contaminated and contaminated soils. Both total and arbuscular colonization were affected by heavy metal contamination. The main conclusions of this study are that Glomus sp. and G. mosseae isolates are strongly inhibited by heavy metals, which acted mainly by interfering with the growth of the external mycelium, and also by limiting the production of arbuscules. Our results suggest that G. claroideum isolates, particularly the ecotype which was isolated from the plots receiving the highest dose of metal-contaminated sludge, shows a potential adaptation to increased metal concentration in soil.     

Switch of tropical Amazon forest to pasture affects taxonomic composition but not species abundance and diversity of arbuscular mycorrhizal fungal community

Arbuscular mycorrhizal fungi (AMF) community composition and species richness are affected by several factors including soil attributes and plant host. In this paper we tested the hypothesis that conversion of tropical Amazon forest to pasture changes taxonomic composition of AMF community but not community species abundance and richness. Soil samples were obtained in 300 m × 300 m plots from forest (n = 11) and pasture (n = 13) and fungal spores extracted, counted and identified. A total of 36 species were recovered from both systems, with 83% of them pertaining to Acaulosporaceae and Glomeraceae. Only 12 species were shared between systems and spore abundance of the majority of fungal species did not differ between pasture and forest. Spore abundance was significantly higher in pasture compared to forest but both systems did not differ on mean species richness, Shannon diversity and Pielou equitability. Species abundance distribution depicted by species rank log abundance plots was not statistically different between both systems. We concluded that conversion of pristine tropical forest to pasture influences the taxonomic composition of AMF communities while not affecting species richness and abundance distribution.     

Improved soil structure and citrus growth after inoculation with three arbuscular mycorrhizal fungi under drought stress

In a controlled potted experiment, citrus (Poncirus trifoliata) seedlings were inoculated with three species of arbuscular mycorrhizal (AM) fungi, Glomus mosseae, G. versiforme or G. diaphanum. Two soil-water levels (ample water, −0.10 MPa; drought stress, −0.44 MPa) were applied to the pots 4 months after transplantation. Eighty days after water treatments, the soils and the citrus seedlings were well colonized by the three AM fungi. Mycorrhizal fungus inoculation improved plant biomass regardless of soil-water status but decreased the concentrations of hot water-extractable and hydrolyzable carbohydrates of soils. Mycorrhizal soils exhibited higher Bradford-reactive soil protein concentrations than non-mycorrhizal soils. Mycorrhizas enhanced >2 mm, 1–2 mm and >0.25 mm water-stable aggregate fractions but reduced 0.25–0.5 mm water-stable aggregates. Peroxidase activity was higher in AM than in non-AM soils whether drought stressed or not, whereas catalase activity was lower in AM than non-AM soils. Drought stress and AM fungus inoculation did not affect polyphenol oxidase activity of soils. A positive correlation between the Bradford-reactive soil protein concentrations, soil hyphal length densities, and water-stable aggregates (only >2 mm, 1–2 mm and >0.25 mm) suggests beneficial effects of the AM symbiosis on soil structure. It concluded that AM fungus colonization enhanced plant growth under drought stress indirectly through affecting the soil moisture retention via glomalin's effect on soil water-stable aggregates, although direct mineral nutritional effects could not be excluded.     

Detection and quantification of the nematophagous fungus Hirsutella minnesotensis in soil with real-time PCR

A real-time PCR assay was developed to quantify in soil the fungus Hirsutella minnesotensis, an important parasite of secondary-stage juvenile (J2) of the soybean cyst nematode. A primer pair 5′-GGGAGGCCCGGTGGA-3′ and 5′-TGATCCGAGGTCAACTTCTGAA-3′ and a TaqMan probe 5′-CGTCCGCCGTAAAACGCCCAAC-3′ were designed based on the sequence of the ITS region of the rRNA gene. The primers were highly species-specific. The PCR reaction system was very sensitive and able to detect as few as 4 conidia g^?1 soil. Regression analysis showed similar slopes and efficiency on DNA from pure culture (y = ?3.587x + 41.017, R² = 0.9971, E = 0.9055) and from Log conidia g^?1 soil (y = ?3.855x + 37.669, R² = 0.9139, E = 0.8172), indicating that the real-time PCR protocol can reliably quantify H. minnesotensis in the soil. The real-time PCR assay was applied to 20 soil samples from soybean fields, and compared with a parasitism assay. The real-time PCR assay detected H. minnesotensis in six of the soils, whereas the parasitism assay detected H. minnesotensis in the same six soils and three additional soils. The real-time PCR assay was weakly correlated (R² = 0.49) with the percentage of parasitized J2 in the six soils, indicating that different types of soil may interfere the efficiency of the real-time PCR assay, possibly due to the effect of soil types on efficacy of DNA extraction. The parasitism assay appeared to be more sensitive than real-time PCR in detecting presence of H. minnesotensis, but real-time PCR was much faster and less costly and provided a direct assessment of fungal biomass. Using the two assays in combination can obtain more complete information about the fungus in soil than either assay alone. Hirsutella parasitism was widespread and detected in 13 of the 20 field soils, indicating that these fungi may contribute to suppressiveness of soybean cyst nematode in nature and likely have high biological control potential for the nematode.     

Effects of alternative management practices on the economics,energy and GHG emissions of a wheat–pea cropping system in the Canadian prairies

In recent years alternative farming practices have received considerable attention from Canadian producers as a means to improve their net return from grain and oilseed production. Enhancing the efficiency of nitrogen fertilizer use, including a pulse crop in the rotation, reducing tillage and pesticide use are seen as viable options to reduce reliance on fossil fuel, lower input costs and decrease the risk of soil, air and water degradation. The objective of this study was to determine the effects of 16 alternative management practices for a 2-year spring wheat (Triticum aestivum L.)–field pea (Pisum sativum L.) rotation on economic returns, non-renewable energy use efficiency, and greenhouse gas emissions. The alternative management methods for wheat consisted of a factorial combination of high vs. low soil disturbance one pass seeding, four nitrogen (N) fertilizer rates (20 kg N ha^?1, 40 kg N ha^?1, 60 kg N ha^?1 and 80 kg N ha^?1), and recommended vs. reduced rates of in-crop herbicide application. Alternative management practices for field pea were high vs. low soil disturbance one pass seeding. The resulting 16 cropping systems were evaluated at the whole farm level based on 4 years (two rotation cycles) of data from field experiments conducted on two Orthic Black Chernozem soils (clay loam and loam textures) in Manitoba, Canada. The highest net returns on the clay loam soil were for the high disturbance system with 60 kg N ha^?1 applied to wheat and the recommended rates of in-crop herbicides. The lowest application rate of N, together with low disturbance seeding, provided the highest economic returns on the loam soil. Energy use efficiency was highest for the lowest rate of N application for both tillage systems. The highest rate of N fertilizer and recommended rates of in-crop herbicide produced little additional yield response, lower net returns, and higher GHG emissions. An increase in N fertilizer application from 20 kg ha^?1 to 80 kg ha^?1 increased whole farm energy requirements by about 40%, while reducing herbicide rates had negligible effects on grain yields and total energy input. Overall, as N fertilizer rate increased, the associated GHG emissions were not offset by an increase in carbon retained in the above-ground crop biomass. Moderate to high soil test NO₃-N levels at experimental sites reduced the potential for positive yield responses to N fertilizer in this study, thus minimizing the economic benefits derived from N fertilizer application.     

Vertical distribution of microbial communities under the canopy of two legume bushes in the Tehuacán Desert,Mexico

Prosopis laevigata and Parkinsonia praecox are the most abundant perennial shrubs in the Tehuacán Desert, forming ’islands of fertility’ that dominate the alluvial terraces. Both species exhibit very similar phenology, with the timing of litter foliage being the only difference between them. P. praecox litter occurs shortly after the rains, while P. laevigata maintains its leaves until the next wet season. As degradable organic matter (OM) is one of the leading factors determining soil biota composition and activity, because of the OM provided by littering, we expected that the vertical distribution of the microbial community in the vicinity of the root zone of P. praecox would be higher in comparison to P. laevigata. One soil sampling was performed; during the rainy season in August, soil samples were collected from a 0–50-cm depth at 10-cm intervals, in the vicinity of the root canopy of four individual plants of each species and the interspaces between them. Soil moisture, organic matter, and counts of bacteria and fungi under shrubs were found to decrease from the upper to deeper layers. Respiratory activity was higher in the deeper layers (p < 0.01) in all three sampling sites. Total bacterial, fungal, and heterotrophic diazotrophs were found to be significantly (p < 0.001) more numerous under shrubs than in the interspace soil. No nitrogen-fixing bacteria were isolated from interplant soils in comparison to the soil samples collected beneath the shrubs. Heterotrophic diazotrophs significantly (p < 0.01) reduced more acetylene under P. praecox (29.0 nmol/g soil) than under P. laevigata (20.1 nmol/g soil). Although the microbial numbers were unaffected by differences in plant phenology, greater nitrogenase activity under P. praecox may influence nitrogen distribution in this arid environment. Due to the fact that only one sampling was undertaken, this study elucidates the differences in the microbial community between the two shrubs, but the dynamics in the above community could not be shown.     

Evapotranspiration of irrigated and rainfed maize–soybean cropping systems

We have been making year-round measurements of mass and energy exchange in three cropping systems: (a) irrigated continuous maize, (b) irrigated maize–soybean rotation, and (c) rainfed maize–soybean rotation in eastern Nebraska since 2001. In this paper, we present results on evapotranspiration (ET) of these crops for the first 5 years of our study. Growing season ET in the irrigated and rainfed maize averaged 548 and 482 mm, respectively. In irrigated and rainfed soybean, the average growing season ET was 452 and 431 mm, respectively. On average, the maize ET was higher than the soybean ET by 18% for irrigated crops and by 11% for rainfed crops. The mid-season crop coefficient K_c (=ET/ET₀ and ET₀ is the reference ET) for irrigated maize was 1.03 ± 0.07. For rainfed maize, significant dry-down conditions prevailed and mid-season K_c was 0.84 ± 0.20. For irrigated soybean, the mid-season K_c was 0.98 ± 0.02. The mid-season dry down in rainfed soybean years was not severe and the K_c (0.90 ± 0.13) was only slightly lower than the values for the irrigated fields. Non-growing season evaporation ranged from 100 to 172 mm and contributed about 16–28% of the annual ET in irrigated/rainfed maize and 24–26% in irrigated/rainfed soybean. The amount of surface mulch biomass explained 71% of the variability in non-growing season evaporation totals. Water use efficiency (or biomass transpiration efficiency), defined as the ratio of total plant biomass (Y_DM) to growing season transpiration (T) was 5.20 ± 0.34 and 5.22 ± 0.36 g kg^?1, respectively for irrigated and rainfed maize crops. Similarly, the biomass transpiration efficiency for irrigated and rainfed soybean crops was 3.21 ± 0.35 and 2.96 ± 0.30 g kg^?1. Thus, the respective biomass transpiration efficiency of these crops was nearly constant regardless of rainfall and irrigation.