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.     

Compacted soil affects soil macrofauna populations in a semi-arid environment in central Queensland

Population densities of soil macrofauna were assessed in a field experiment with annually compacted treatments (applied to whole plots) and management treatments to repair initially compacted soil. Earthworms accounted for about half the macrofauna recovered during the experiment. Compaction of wet surface soil (water content>plastic limit) by agricultural machinery generally reduced numbers of macrofauna and earthworms. Annual compaction with a 10 Mg axle load on wet soil reduced mean macrofauna numbers from 70 to 15 m⁻² and mean earthworm numbers from 41 to 2 m⁻². Annual compaction with 6 Mg on soil drier than the plastic limit to a depth of 0.08 m had no adverse effect on the soil macrofauna. A 3-year pasture ley had more macrofauna (211 m⁻²) than a control treatment under cropping (29 m⁻²) but numbers declined when cropping was resumed.     

Soil macroinvertebrate communities and ecosystem services in deforested landscapes of Amazonia

Land use changes in the Amazon region strongly impact soil macroinvertebrate communities, which are recognized as major drivers of soil functions (Lavelle et al., 2006). To explore these relations, we tested the hypotheses that (i) soil macrofauna communities respond to landscape changes and (ii) soil macrofauna and ecosystem services are linked. We conducted a survey of macrofauna communities and indicators of ecosystem services at 270 sites in southern Colombia (department of Caqueta) and northern Brazil (state of Pará), two areas of the Amazon where family agriculture dominates. Sites represented a variety of land use types: forests, fallows, annual or perennial crops, and pastures. At each site we assessed soil macroinvertebrate density (18 taxonomic units) and the following ecosystem service indicators: soil and aboveground biomass carbon stock; water infiltration rate; aeration, drainage and water storage capacities based on pore-size distribution; soil chemical fertility; and soil aggregation. Significant covariation was observed between macrofauna communities and landscape metric data (co-inertia analysis: RV = 0.30, p < 0.01, Monte Carlo test) and between macrofauna communities and ecosystem service indicators (co-inertia analysis: RV = 0.35, p < 0.01, Monte Carlo test). Points located in pastures within 100 m of forest had greater macrofauna density and diversity than those located in pastures with no forest within 100 m (Wilcoxon rank sum test, p < 0.01). Total macroinvertebrate density was significantly correlated with macroporosity (r² = 0.42, p < 0.01), as was the density of specific taxonomic groups: Chilopoda (r² = 0.43, p < 0.01), Isoptera (r² = 0.30, p < 0.01), Diplopoda (r² = 0.31, p < 0.01), and Formicidae (r² = 0.13, p < 0.01). Total macroinvertebrate density was also significantly correlated with available soil water (r² = 0.38, p < 0.01) as well as other soil-service indicators (but with r² < 0.10). Results demonstrate that landscape dynamics and composition affect soil macrofauna communities, and that soil macrofauna density is significantly correlated with soil services in deforested Amazonia, indicating that soil macrofauna have an engineering and/or indicator function.     

Effects of tillage,organic resources and nitrogen fertiliser on soil carbon dynamics and crop nitrogen uptake in semi-arid West Africa

Tillage, organic resources and fertiliser effects on soil carbon (C) dynamics were investigated in 2000 and 2001 in Burkina Faso (West Africa). A split plot design with four replications was laid-out on a loamy-sand Ferric Lixisol with till and no-till as main treatments and fertiliser types as sub-treatments. Soil was fractionated physically into coarse (0.250–2 mm), medium (0.053–0.250 mm) and fine fractions (< 0.053 mm). Particulate organic carbon (POC) accounted for 47–53% of total soil organic carbon (SOC) concentration and particulate organic nitrogen (PON) for 30–37% of total soil nitrogen concentration. The POC decreased from 53% of total SOC in 2000 to 47% of total SOC in 2001. Tillage increased the contribution of POC to SOC. No-till led to the lowest loss in SOC in the fine fraction compared to tilled plots. Well-decomposed compost and single urea application in tilled as well as in no-till plots induced loss in POC. Crop N uptake was enhanced in tilled plots and may be up to 226 kg N ha⁻¹ against a maximum of 146 kg N ha⁻¹ in no-till plots. Combining crop residues and urea enhanced incorporation of new organic matter in the coarse fraction and the reduction of soil carbon mineralisation from the fine fraction. The PON and crop N uptake are strongly correlated in both till and no-till plots. Mineral-associated N is more correlated to N uptake by crop in tilled than in no-till plots. Combining recalcitrant organic resources and nitrogen fertiliser is the best option for sustaining crop production and reducing soil carbon decline in the more stabilised soil fraction in the semi-arid West Africa.     

Microbial mineralization of biochar and wheat straw mixture in soil: A short-term study

A short-term incubation study was carried out to investigate the effect of biochar addition to soil on CO₂ emissions, microbial biomass, soil soluble carbon (C) nitrogen (N) and nitrate–nitrogen (NO₃–N). Four soil treatments were investigated: soil only (control); soil + 5% biochar; soil + 0.5% wheat straw; soil + 5% biochar + 0.5% wheat straw. The biochar used was obtained from hardwood by pyrolysis at 500 °C. Periodic measurements of soil respiration, microbial biomass, soluble organic C, N and NO₃–N were performed throughout the experiment (84 days). Only 2.8% of the added biochar C was respired, whereas 56% of the added wheat straw C was decomposed. Total net CO₂ emitted by soil respiration suggested that wheat straw had no priming effect on biochar C decomposition. Moreover, wheat straw significantly increased microbial C and N and at the same time decreased soluble organic N. On the other hand, biochar did not influence microbial biomass nor soluble organic N. Thus it is possible to conclude that biochar was a very stable C source and could be an efficient, long-term strategy to sequester C in soils. Moreover, the addition of crop residues together with biochar could actively reduce the soil N leaching potential by means of N immobilization.     

Microbial activity,community structure and potassium dynamics in rhizosphere soil of soybean plants treated with glyphosate

With the advent of glyphosate [N-(phosphonomethyl)glycine] tolerant crops, soils have now been receiving repeated applications of the herbicide for over 10 years in the Midwestern USA. There is evidence that long-term use of glyphosate can cause micronutrient deficiency but little is known about plant potassium (K) uptake interactions with glyphosate. The repeated use of glyphosate may create a selection pressure in soil microbial communities that could affect soil K dynamics and ultimately K availability for crops. Therefore, the objectives of this study were to characterize the effect of foliar glyphosate applied to GR (glyphosate resistant) soybeans on: (1) rhizosphere microbial community profiles using ester linked fatty acid methyl ester (EL-FAME) biomarkers, (2) exchangeable, non-exchangeable, and microbial K in the rhizosphere soil, and (3) concentrations of soybean leaf K. A greenhouse study was conducted in a 2 × 2 × 3 factorial design with two soil treatments (with or without long-term field applications of glyphosate), two plant treatments (presence and absence of soybean plants), and three rates of glyphosate treatments (0×, 1× at 0.87, and 2× at 1.74 kg ae ha^?1, the recommended field rate). After each glyphosate application, rhizosphere soils were sampled and analyzed for microbial community structure using ester linked fatty acid methyl ester biomarkers (EL-FAME), and exchangeable, plant tissue and microbial biomass K. Glyphosate application caused a significant decrease in the total microbial biomass in soybean rhizosphere soil that had no previous exposure to glyphosate, at 7 days after glyphosate application. However, no significant changes were observed in the overall microbial community structure. In conclusion, the glyphosate application lowered the total microbial biomass in the GR soybean rhizosphere soil that had no previous exposure to glyphosate, at 7 days after glyphosate application; caused no changes in the microbial community structure; and did not reduce the plant available K (soil exchangeable or plant tissue K).     

Responses of soil nematode community structure to soil carbon changes due to different tillage and cover crop management practices over a nine-year period in Kanto,Japan

The response of the soil food web structure to soil quality changes during long-term anthropogenic disturbance due to farming practices has not been well studied. We evaluated the effects of three tillage systems: moldboard plow/rotary harrow (MP), rotary cultivator (RC), and no-tillage (NT), three winter cover-crop types (fallow, FL; rye, RY; and hairy vetch, HV), and two nitrogen fertilization rates (0 and 100 kg N ha⁻¹ for upland rice, and 0 and 20 kg N ha⁻¹ for soybean production) on changes in nematode community structure. Sixty-nine taxa were counted, total nematode abundance (ALL), bacterial feeders (BAC), predators (PRD), omnivores (OMN), and obligatory root feeders (ORF) were more abundant in NT than in MP and RC, but fungal feeders and facultative root feeders (FFR) were more abundant in RC than in NT and MP. Cover crop also influenced nematode community structure; rye and hairy vetch were always higher in ALL, BAC, FFR, ORF, and OMN than fallow. Seasonal changes in nematode community structure were also significant; in particular, as soil carbon increased, nematode abundance also increased. The relationship between nematode indices and soil carbon was significant only in NT, but not in MP and RC. In NT, with increasing soil carbon, enrichment index and structure index (SI) were positive and significant and channel index was negative. Bulk density was significantly negatively correlated with FFR and ORF. Seasonal difference in nematode community between summer and autumn was larger in an upland rice rotation than in a soybean rotation. Over the nine-year experiment, SI increased not only in NT but also in MP and RC, suggesting that repeated similar tillage inversions in agroecosystems may develop nematode community structures adapted to specific soil environmental conditions. Because NT showed the highest values of both SI and soil carbon, the increase of soil carbon in NT is expected to have a great impact on developing a more diverse nematode community structure.     

Mineralization dynamics and biochemical properties during initial decomposition of plant and animal residues in soil

The use of organic residues as soil amendments or fertilisers may represent a valuable recycling strategy. In this study, a series of laboratory assays was performed to study the effects of the application of organic residues on C and N mineralization and biochemical properties in a Mediterranean agricultural soil. Two crop residues (straw and cotton) and two animal by-products (meat bone meal and blood meal) were added at three rates (5, 10 and 20 mg g^?1 on dry weight basis) to a moist (40% water holding capacity) sandy soil and incubated at 20 °C for 28 days. Each residue underwent a different mineralization pattern depending on the nature and complexity of its chemical constituents. In all cases, the addition of the waste produced, after a short lag-phase, an exponential increase in the soil respiration rate, reflecting the growth of microbial biomass. The amount of total extra CO₂-C evolved after 28 days, expressed as % in respect to added C, differed significantly (P < 0.005) among application doses: 5 > 10 > 20 mg g^?1 and residue type: meat bone meal > blood meal > cotton cardings > wheat straw. Plant residues led to a rapid immobilisation of N that affected microbial size and activity and further mineralization. Animal by-products produced an immediate and remarkable increase of mineral N in the soil. However, the large amounts of NH₄⁺ released in the soil at high rates of animal residues led, in some cases, to temporary adverse effects on microbial biomass growth and nitrification. All residues produced a significant increase in soil microbial biomass size and activity, being the intensity of the response related to their chemical properties.     

Earthworm populations in relation to soil organic matter dynamics and management in California tomato cropping systems

Earthworms are key regulators of soil structure and soil organic matter (SOM) dynamics in many agroecosystems. They are greatly impacted by agricultural management, yet little is known about how these factors interact to control SOM dynamics. This study sought to explore linkages between agricultural management, earthworms and aggregate associated SOM dynamics through a survey of tomato (Solanum lycopersicum L.) cropping systems in northern California. Earthworms and soil samples were collected between February and April of 2005 from 16 fields under one of three types of residue management: (1) tomato mulch – no postharvest tillage and tomato residues left on the soil surface, (2) cover crop – tomato residues tilled in and leguminous cover crop planted, and (3) bare fallow – tomato residues tilled in and soil surface left exposed throughout the winter. Earthworms were collected via hand-sorting and identified to species, while soils were wet sieved to yield four aggregate size classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the silt and clay fraction (<53 μm). The combined large and small macroaggregate fraction was then fractionated into coarse particulate organic matter (cPOM; 250 μm), microaggregates within macroaggregates (mM; 53–250 μm) and macroaggregate occluded silt and clay (Msc; <53 μm). The earthworms identified in this survey were composed entirely of exotic species and were dominated by Aporrectodea caliginosa. Earthworm abundance was related to residue management, with the tomato mulch systems averaging 4.5 times greater fresh earthworm biomass than bare fallow (P = 0.024). Aggregate stability and total soil C and N also appeared to be influenced by residue management, such that the tomato mulch system displayed significantly greater mean weight diameters than the bare fallow system (P = 0.049), as well as more than 50% greater total soil C and N (P = 0.049 and P = 0.036; respectively). Earthworm biomass was also found to be positively correlated with total soil C (P = 0.009, R² = 0.39) and N (P = 0.010, R² = 0.039) as well as the proportion of macroaggregate C in the cPOM fraction (P = 0.028, R² = 0.30). Our findings suggest that residue handling and the associated management practices (e.g., tillage, organic vs. conventional agriculture) are important for both earthworm populations and SOM storage. Although earthworms are known to influence SOM in many ways, other factors appear to play a more prominent role in governing aggregate associated SOM dynamics.     

Effects of cover crop quality and quantity on nematode-based soil food webs and nutrient cycling

Soil food webs cycle nutrients and regulate parasites and pathogens, services essential for both agricultural productivity and ecosystem health. Nematodes provide useful indicators of soil food web dynamics. This study was conducted to determine if nematode soil food web indicators and crop yield can be enhanced by combinations of cover crops in a conservation tillage system. The effects of three cover crop treatments (vetch/pea, oat/wheat and oat/wheat/pea/vetch) with low, medium and high C:N and a bare fallow control were investigated in Davis, CA. Nematode fauna, soil properties and plant productivity were measured. Soil food web indices, including the Enrichment Index (EI), Structure Index (SI), Basal Index (BI), and Channel Index (CI), based on the composition of nematode assemblages, were calculated to infer soil food web condition. Cover cropped tomato/corn rotations had twice the number of enrichment opportunist bacterial feeding nematodes, active participants in nitrogen mineralization, than fallowed tomato/corn rotations (opportunist bacterial feeders = 163 versus 98). In winter fallowed plots food webs were basal, common in disturbed, nutrient-poor conditions (BI = 37). Total number of enrichment opportunist nematodes, soil NH₄-N levels, and inferred nitrogen mineralization, were higher in cover crop treatments with low to mid C:N ratios. Omnivore and predator nematodes were scarce, averaging less than 6 nematodes 100 g^?1 in all treatments. In year one, plant productivity was highest after fallow. In contrast, in year two productivity was highest after cover crops with high nitrogen content and productivity significantly correlated with the structure of the soil fauna. Monitoring the abundance of enrichment opportunists may provide managers with a new tool to evaluate soil food web nitrogen mineralization and plant productivity.     

Arable weeds,cover crops,and tillage drive soil microbial community composition in organic cropping systems

Cover crops have traditionally been used to reduce soil erosion and build soil quality, but more recently cover crops are being used as an effective tool in organic weed management. Many studies have demonstrated microbial community response to individual cover crop species, but the effects of mixed species cover crop communities have received less attention. Moreover, the relationship between arable weeds and soil microbial communities is not well understood. The objective of this study was to determine the relative influence of cover crop diversity, early-season weed communities, and tillage on soil microbial community structure in an organic cropping system through the extraction of fatty acid methyl esters (FAMEs). A field experiment was conducted between 2009 and 2011 near Mead, NE where spring-sown mixtures of zero (control), two, and eight cover crop species were included in a sunflower–soybean–corn crop rotation. A mixture of four weed species was planted in all experimental units (excluding the no-cover control), and also included as an individual treatment. Cover crops and weeds were planted in late-March, then terminated in late-May using a field disk or sweep plow undercutter, and main crops were planted within one week of termination. Three (2009) or four (2010–11) soil cores were taken to a depth of 20 cm in all experimental units at 45, 32, and 25 days following cover crop termination in 2009, 2010, and 2011, respectively. Total FAMEs pooled across 2009 and 2010 were greatest in the two species mixture–undercutter treatment combination (140.8 ± 3.9 nmol g⁻¹) followed by the eight species mixture–undercutter treatment combination (132.4 ± 3.9 nmol g⁻¹). Abundance of five (2009 and 2010) and seventeen (2011) FAME biomarkers was reduced in the weedy treatment relative to both cover-cropped treatments and the no-cover control. In 2009 and 2010, termination with the undercutter reduced abundance of most actinomycete biomarkers while termination with the field disk reduced abundance of C18:1(cis11) and iC16:0. Canonical discriminant analysis of the microbial community successfully segregated most cover crop mixture by termination method treatment combinations in 2009 and 2010. Microbial communities were most strongly influenced by the presence and type of early-spring plant communities, as weeds exerted a strong negative influence on abundance of many key microbial biomarkers, including the AMF markers C16:1(cis11) and C18:1(cis11). Weeds may alter soil microbial community structure as a means of increasing competitive success in arable soils, but this relationship requires further investigation.     

Evaluation of the “bait-lamina test” to assess soil microfauna feeding activity in mixed grassland

The soil fauna plays an important role in regulating nutrient cycling through predation and comminution of organic residues. The bait-lamina test was proposed as a practical mean to assess soil faunal feeding activity. The test consists of vertically inserting 16-hole-bearing plastic strips filled with a plant material preparation into the soil. We assessed the value of the bait-lamina test in the mixed grassland ecoregion of Canada, using 5-year-old seeded grass field plots. There were five treatments of grass communities, i.e. monocultures of (1) Russian wild rye, (2) switchgrass, (3) green needlegrass, or (4) western wheatgrass, and (5) a grass mixture of western wheatgrass + green needlegrass + switchgrass + little blue stem, which were distributed in four complete blocks, and six bait mix treatments, i.e. bait made with ground tissue of (1) Russian wild rye, (2) switchgrass, (3) green needlegrass, (4) western wheatgrass, (5) alfalfa, or with (6) wheat bran, which were randomized in each plot. Bait-lamina strips were used in groups of five strips inserted in the soil; they were equally spaced across the inter-row. The analysis revealed a marginally significant effect (P = 0.065) of plant community on soil faunal feeding activity with more activity in mixed grass species compared to green needlegrass plots. Bait flavour had no significant effect (P = 0.22) on feeding. More (P < 0.0005) feeding activity was detected close to the soil surface (0–5 mm deep) than at 20 mm below the surface and at deeper depths. Feeding activity was relatively low over the 65-day period of the test (13 June to 17 August 2005), with 8.3% and 2.0% of the lamina (holes filled) showing signs of feeding in the 0–5 and 5–15 mm soil layer. We conclude that the bait-lamina test is appropriate to assess the activity of surface litter processing fauna in mixed grassland, but it is not a good indicator of the decomposition occurring in the soil matrix, where most litter is produced. We recommend the use of a large number of replicated bait-lamina strips and a whole growing season incubation period in soils of the semiarid prairie ecoregion where the surface litter layer is thin and faunal saprobes are relatively scarce.     

Field margins and management affect settlement and spread of an introduced dew-worm (Lumbricus terrestris L.) population

To study the feasibility of earthworm introduction for increasing the macroporosity and permeability of arable heavy clay, deep-burrowing earthworms (Lumbricus terrestris L.) were inoculated into a tile drained experimental field in Jokioinen, S-W Finland in autumn 1996. Inoculation with the Earthworm Inoculation Unit technique was at the up-slope end of the field, in the field margins under permanent grass, and inside the four 0.46 ha plots of the field. The experiment was monitored on three occasions. In 1998 the L. terrestris population had persisted in low numbers only in field and plot margins. By 2003, when the field had been under set-aside grass for three years, density had grown in the margins and L. terrestris were also found inside the field at a very low density. The third monitoring was in autumn 2009, after a further four years as set-aside and a subsequent division of the field into no-till and ploughing management, and looked at the effects of management (margins, no-till, ploughing), distance from the inoculation and sub-drainage on L. terrestris abundance. The abundance displayed a clear gradient over the field, declining from 14 ind. and 18 g m^?2 at 5–9 m from inoculation, to 1 ind. and 2 g m^?2 at 56–60 m distance. Margins had the highest abundances (16 ind. and 32 g m^?2), followed by no-till (4 ind. and 4 g m^?2) and ploughing (1 ind. and 1 g m^?2). Abundances were significantly higher above the tiles than between them (P < 0.05). The results demonstrate the importance of no-till and sub-drain line habitats as settlement supports for the inoculated population. Field margins proved to be decisive for inoculation success, by providing bridgeheads for population establishment and later by acting as source areas for the colonisation of the field. This finding highlights the general importance of field margins in the dispersal ecology of earthworms in arable landscapes.     

Edge effects and trampling in boreal urban forest fragments – impacts on the soil microbial community

Fragmentation of forest ecosystems increases the proportion of edge habitat and is accompanied by a change in plant species composition. The recreational use of urban forests leads to decreased vegetation cover and the formation of paths, and thus, to fragmentation at small scales. We studied the impacts of forest and path edge effects on the soil microbial community structure (by using the phospholipid fatty acid (PLFA) method) and microbial activity (measured as basal respiration) in 34 mesic boreal urban forest fragments in Finland. We sampled the humus layer 1) from the forest edge into the interior (0–80 m), and 2) at different distances from paths. Microbial community structure was only slightly affected by the forest edge but differences were found between distances of 0–10 m and over 50 m from the edge. These changes correlated with changes in soil pH. Although changes in the microbial community structure were not pronounced, microbial biomass and activity were 30–45% lower at the first 20 m into the forest fragments, due to a low moisture content of the humus near the edge. The decreased microbial activity detected at forest edges implies decreased litter decomposition rates, and thus, a change in ecosystem nutrient cycling. The microbial community structure differed between paths and surrounding areas and correlated with changes in soil pH. Paths also supported approximately 25–30% higher microbial biomass with a transition zone of at least 1 m from the path edge. Path associated disturbances (mainly alterations in vegetation and soil pH) were reflected in the soil microbial community structure up to 1.5 m from the paths.     

Soil available phosphorus status determines indigenous mycorrhizal colonization of field and glasshouse-grown spring wheat from Argentina

Wheat production (Triticum aestivum L.) has increased across the world during last century with the intensification of agriculture. Phosphorus (P) fertilization is a common practice to improve wheat growth in Argentina. We investigate whether indigenous arbuscular mycorrhizal colonization (AMC) of hard red spring wheat is controlled by shoot P content (SPc) or by available soil P in an agricultural soil from the southeastern Argentine Pampas. In the field, AMC was monitored four times during two growing seasons of a conventional wheat crop. Treatments were: without P supply, annual supply of 11 and 22 kg P ha⁻¹ during the last 5 years, and 164 kg P ha⁻¹ applied once 5 years before the experiment. In the glasshouse, AMC was assessed three times in wheat growing in pots filled with the soil from unfertilized plots; treatments were: P (0 and 20 mg P pot⁻¹), and nitrogen (N) fertilization (0 and 150 mg N pot⁻¹). A range of soil P between 6 and 60 mg P kg⁻¹ was obtained and the AMC ranged from 1% to 67% of root length colonized under both field and glasshouse conditions. P supplied annually increased growth and SPc but decreased AMC. N fertilization did not affect growth or AMC. Variations in SPc did not account for AMC. Variability in AMC was best accounted for local current soil available P content (r² = 0.59). A linear-plateau relationship between soil P and indigenous AMC was established in wheat plants growing under contrasting environmental and experimental (field and glasshouse) conditions. Indigenous AMC was depressed by available soil P in the range 0–27 mg P kg⁻¹ (a decrease of 2.8% mg P⁻¹ kg⁻¹). Above 27 mg P kg soil⁻¹, AMC was stabilized at about 10%. Grain yield increased with fertilization and the highest relative shoot dry matter in field was obtained at 15.5 mg P kg soil⁻¹. The soil P range that ensures high wheat production without deterring indigenous AMC is discussed.     

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.     

Characterizing the relationships between soil organic matter components and microbial function and composition along a tillage disturbance gradient

We studied the effect of no-till (disc seeder), conventional-till (tine scarifier+disc seeder) and rotary-till (rotary hoe+disc seeder) management on soil organic matter (SOM) components, rates of carbon (C) and nitrogen (N) cycling, substrate utilization and microbial community composition. We hypothesized that labile SOM fractions are sensitive to changes in tillage techniques and, in turn mediate any tillage-induced changes in microbial function and composition. A replicated field site was established in May 1998 in the semi-arid agricultural region of Western Australia and soils were collected in September 2004. We found soil pH varied between different tillage techniques as an initial lime application was mixed to deeper soil depths in rotary-till soil than no-till and conventional-till soil. Total-C was greater in surface soil and lower in subsurface soil from no-till and conventional-till plots than from rotary-till plots, but there was no effect of tillage technique on total-C when averaged across soil depths. Light (specific density <1.0 g cm^?3) fraction organic matter (LFOM), dissolved organic matter (DOM) and microbial biomass (MB) C and N pools, and rates of C and N cycling all tended to decrease with soil depth. In general, LFOM-C and N, dissolved organic C (DOC) and microbial biomass carbon (MB-C), soil respiration, cellulase activity, gross immobilization rates were positively correlated (r>0.50) and were greater in no-till and conventional-till soil than rotary-till soil both within, and across soil depths. These soil variables generally increased (r>0.5) with increasing soil pH. Dissolved organic N and gross N mineralization were positively correlated (r>0.90) but neither was affected by tillage techniques. No-till soil had greater utilization of carboxylic acids and lower utilization of amino acids and carbohydrates than conventional-till and rotary-till soil; surface soil also had greater utilization of carboxylic acids than subsurface soil. In turn, substrate utilization differed between soil depths, and between no-till soil and conventional-till and rotary-till soil; these differences were correlated with soil pH, total-N, DOC, LFOM-N and microbial biomass nitrogen (MB-N). Bacterial and fungal biomasses generally decreased with soil depth and were greater in no-till and conventional-till soil than rotary-till soil. Microbial community composition differed between all tillage techniques and soil depths; these differences were correlated with soil textural classes, soil pH, and total, LFOM, DOM and microbial C and N pools. These results indicate that most tillage-induced changes to soil properties were associated with the greater soil disturbance under rotary-till than under no-till or conventional-till management. Our results indicate that tillage-induced changes to soil pH, and LFOM, DOM and microbial biomass pools are likely to be important regulators of the rates of C and N cycling, substrate utilization and microbial community composition in this coarse textured soil.     

Effects of reduced tillage and liming on microbial activity and soil properties in a weakly-structured soil

The effects of reduced tillage and lime on crop yield and soil physical and microbial properties were studied in a weakly-structured silty clay loam soil. Two autumn primary tillage practices were compared, mouldboard ploughing to 20–25 cm and cultivation to 12 cm. Seedbed preparation was carried out by several harrowing operations in the mouldboard ploughed treatment, and with a PTO-driven harrow in the same operation as sowing in the shallow cultivation treatment. The tillage treatments were applied alone or were combined with liming aimed at soil structural improvement. Lime was added as 6.5 Mg CaO ha⁻¹ before the start of the experiment and mixed into the top 12 cm of soil with a disc cultivator. A 4-year crop rotation was used: spring barley, spring oilseed rape, spring/winter wheat and oats, and all crops were compared each year. Crop residues were retained in the experiment and incorporated at cultivation. Aggregate stability was improved by the shallower tillage depth, probably as an effect of an increase in soil organic matter and a more active microbial biomass. Liming had little effect on soil structure variables but increased microbial activity to some extent. This was reflected in higher crop yields, especially when the shallow tillage depth was combined with liming. Penetration resistance in the seedbed subsoil was highest when mouldboard ploughing was carried out in plots without liming. Data were examined with principal component analyses, and the structures in the data were presented as scores and loading plots, which revealed groupings between samples and relationships between variables, respectively.     

The effects of fresh and stabilized pruning wastes on the biomass,structure and activity of the soil microbial community in a semiarid climate

The incorporation of organic amendments from pruning waste into soil may help to mitigate soil degradation and to improve soil fertility in semiarid ecosystems. However, the effects of pruning wastes on the biomass, structure and activity of the soil microbial community are not fully known. In this study, we evaluate the response of the microbial community of a semiarid soil to fresh and composted vegetal wastes that were added as organic amendments at different doses (150 and 300 t ha⁻¹) five years ago. The effects on the soil microbial community were evaluated through a suite of different chemical, microbiological and biochemical indicators, including enzyme activities, community-level physiological profiles (CLPPs) and phospholipid fatty acid analysis (PLFA). Our results evidenced a long-term legacy of the added materials in terms of soil microbial biomass and enzyme activity. For instance, cellulase activity reached 633 μg and 283 μg glucose g⁻¹ h⁻¹ in the soils amended with fresh and composted waste, respectively. Similarly, bacterial biomass reached 116 nmol g⁻¹ in the soil treated with a high dose of fresh waste, while it reached just 66 nmol g⁻¹ in the soil amended with a high dose of composted waste. Organic amendments produced a long-term increase in microbiological activity and a change in the structure of the microbial community, which was largely dependent on the stabilization level of the pruning waste but not on the applied dose. Ultimately, the addition of fresh pruning waste was more effective than the application of composted waste for improving the microbiological soil quality in semiarid soils.     

Effects of tillage and nitrogen management on soil chemical and physical properties after 23 years of continuous sorghum

Long-term tillage and nitrogen (N) management practices can have a profound impact on soil properties and nutrient availability. A great deal of research evaluating tillage and N applications on soil chemical properties has been conducted with continuous corn (Zea Mays L.) throughout the Midwest, but not on continuous grain sorghum (Sorghum bicolor (L.) Moench). The objective of this experiment was to examine the long-term effects of tillage and nitrogen applications on soil physical and chemical properties at different depths after 23 years of continuous sorghum under no-till (NT) and conventional till (CT) (fall chisel-field cultivation prior to planting) systems. Ammonium nitrate (AN), urea, and a slow release form of urea were surface broadcast at rates of 34, 67, and 135 kg N ha⁻¹. Soil samples were taken to a depth of 15 cm and separated into 2.5 cm increments. As a result of lime applied to the soil surface, soil pH in the NT and CT plots decreased with depth, ranging from 6.9 to 5.7 in the NT plots and from 6.5 to 5.9 in the CT plots. Bray-1 extractable P and NH₄OAc extractable K was 20 and 49 mg kg⁻¹ higher, respectively, in the surface 2.5 cm of NT compared to CT. Extractable Ca was not greatly influenced by tillage but extractable Mg was higher for CT compared to NT below 2.5 cm. Organic carbon (OC) under NT was significantly higher in the surface 7.5 cm of soil compared to CT. Averaged across N rates, NT had 2.7 Mg ha⁻¹ more C than CT in the surface 7.5 cm of soil. Bulk density (Δ_b) of the CT was lower at 1.07 g cm⁻³ while Δ_b of NT plots was 1.13 g cm⁻³. This study demonstrated the effect tillage has on the distribution and concentration of certain chemical soil properties.