Stability of desiccated rhizosphere soil aggregates of mycorrhizal Juniperus oxycedrus grown in a desertified soil amended with a composted organic residue

Adequate soil structural stability favours the establishment and viability of a stable plant cover, protecting the soil against water erosion in desertified Mediterranean environments. We studied the effect of soil drying-rewetting, inoculation with a mixture of three exotic arbuscular mycorrhizal (AM) fungi (Glomus intraradices Schenck & Smith, Glomus deserticola (Trappe, Bloss. & Menge) and Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe) and addition of a composted organic residue on aggregate stabilisation of the rhizosphere soil of Juniperus oxycedrus. The AM fungi and composted residue produced similar increases in plant growth, independently of the water conditions. Under well-watered conditions, the highest percentages of stable aggregates were recorded in the amended soil, followed by the soil inoculated with AM fungi. Excepting microbial biomass C, the soil drying increased labile C fractions (water soluble C, water soluble and total carbohydrates), whereas the rewetting decreased significantly such C fractions. Desiccation caused a significant increase in aggregate stability of the rhizosphere soil of all plants, particularly in the amended and inoculated plants. In all treatments, the aggregates formed after soil drying were unstable, since, in the rewetting, they disappear, reaching the initial levels before soil drying. Our results suggest that the aggregation mechanisms developed by rhizosphere microbial community of the amended and inoculated plants under water stress can be particularly relevant in desertified soils exposed to long desiccation periods.     

Soil phosphorus mobilization in the rhizosphere of cover crops has little effect on phosphorus cycling in California agricultural soils

Phosphorus (P) is a key limiting factor in many terrestrial ecosystems because most soil P is bound to soil minerals or organic matter. Increasing P cycling rates can increase P availability, including in agricultural soils that receive external P inputs. For example, cover crops may increase P cycling rates via plant uptake and P release during microbial decomposition. Cover crops and associated microbes may also change rhizosphere properties and stimulate soil P mobilization. We studied the potential of legume – fava bean (Vicia faba), vetches (Vicia dasycarpa, Vicia sativa, Vicia benghalensis) pea (Pisum sativum) – and cereal – rye (Secale cereale), wheat (Triticum aestivum), oat (Avena sativa) – cover crops to stimulate P cycling across management practices in two long-term systems trials in California. We measured cover crop biomass and nutrient content, P-mobilizing capacity (pH, organic acids, phosphatase activity) and soil P fraction changes in the rhizosphere. Cereals generally produced more biomass with similar P content compared to legumes, but higher C:P in cereal residues could favor microbial immobilization, delay residue mineralization and reduce P cycling rates. Legumes, especially fava bean, had the largest effect on rhizosphere properties by reducing pH and increasing organic acids concentrations and phosphatase activity. However, these changes in rhizosphere properties had a modest impact on soil P and did not increase soil P availability. Furthermore, we found no strong effect of management practices or soil P concentrations on soil P mobilization. Our results suggest that P mobilization in the rhizosphere of legumes is unlikely to increase P cycling rates in these soils, whereas P uptake and release in cereal biomass could have stronger effects.     

Microbial activity in the rhizosphere soil of six herbaceous species cultivated in a greenhouse is correlated with shoot biomass and root C concentrations

The stimulation of rhizosphere microorganisms by exudates released from roots is important for nutrient cycling and differs between plant species. The reasons for this between-species variability are poorly understood. We studied correlations between shoot biomass, soluble and non-soluble root C concentrations and rhizosphere bacterial abundance (CFU: colony forming units) and an index of microbial activity (in vitro utilization of [U-¹⁴C]glucose by soil microorganisms). We studied Briza media and Rumex acetosella (nutrient-poor habitats), Epilobium hirsutum, Eupatorium cannabinum, Rumex obtusifolius and Urtica dioica (nutrient rich habitats) cultivated in a greenhouse for 5 weeks in a forest soil. We found significant differences among species for the bacterial abundance and microbial activity in the rhizosphere. These differences poorly reflected the nutrient richness of the common habitats for these species, possibly because the soil conditions were not optimal. Nevertheless, microbial activity was positively correlated with root soluble C concentration and shoot biomass and negatively correlated with the concentration of non-soluble C in roots. These preliminary results suggest that the carbon economy could be an important control of the between-species variability of microbial activity in the rhizosphere.     

Microbial processes and community composition in the rhizosphere of European beech - The influence of plant C exudates

Plant roots strongly influence C and N availability in the rhizosphere via rhizodeposition and uptake of nutrients. This study aimed at investigating the effect of resource availability on microbial processes and community structure in the rhizosphere. We analyzed C and N availability, as well as microbial processes and microbial community composition in rhizosphere soil of European beech and compared it to the bulk soil. Additionally, we performed a girdling experiment in order to disrupt root exudation into the soil. By this novel approach we were able to demonstrate that enhanced resource availability positively affected N mineralization and hydrolytic enzyme activities in the rhizosphere, but negatively affected nitrification rates and oxidative enzyme activities, which are involved in the degradation of soil organic matter. Both rhizosphere effects on N mineralization and oxidative enzyme activities disappeared in the girdling treatment. Microbial community structure in the rhizosphere, assessed by phospholipid fatty acid analysis, differed only slightly from bulk soil but was markedly altered by the girdling treatment, indicating additional effects of the girdling treatment beyond the reduction of root exudation. Differences in oxidative enzyme activities and nitrification rates between rhizosphere soil and bulk soil, however, suggest considerable differences in the (functional) microbial community composition.     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.     

Labile,recalcitrant, microbial carbon and nitrogen and the microbial community composition at two Abies faxoniana forest elevations under elevated temperatures

We investigated the interactions of altitude and artificial warming on the soil microbial community structure in a subalpine Abies faxoniana forest in southwestern China after four years of warming. Open top chambers (OTCs) at two elevations (3000 m and 3500 m) were established, and their soil microbial characteristics, organic carbon (C) and nitrogen (N) were measured. The microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis. A two-step sulfuric acid hydrolysis was used to quantify the labile and recalcitrant C fractions in the soil organic matter. The results showed that bacterial PLFAs and gram-negative bacterial PLFAs increased and the fungal PLFAs and the fungi/bacteria ratio decreased with warming at the high altitude. By contrast, the warming effects on those parameters at low altitude were small. The higher proportion of labile easily decomposable soil C may explain the different responses of the microbial community composition at the two altitudes. An RDA analysis confirmed that the variations in the soil community structure were significantly associated with soil organic matter properties such as the sizes of the soil labile N pool (LP-N), the recalcitrant N pool (RP-N), and the labile C pool as well as dissolved organic C (DOC) and dissolved organic N concentrations (DON). Our results also showed that labile C and N pools increased with the altitude, but the microbial biomass C as measured with chloroform fumigation techniques decreased. Warming increased only the recalcitrant C pools at the high altitude. Given the longer mean residence time for recalcitrant C and the much greater size of this soil organic carbon pool, the results indicated that a rise in temperature in our case increased soil C pools at higher altitudes, at least during the early stages of experimental soil warming. Warming could also cause changes in the composition of the microbial community and enzyme activities, consequently leading to functional changes in soil ecosystem processes at the high altitude.     

Soil aggregation: Influence on microbial biomass and implications for biological processes

Our 1988 paper, describing the effects of cultivation on microbial biomass and activity in different aggregate size classes, brought together the ‘aggregate hierarchy theory’ and the ‘microbial biomass concept’. This enabled us to identify the relationships between microbial and microhabitat (aggregate) properties and organic matter distribution and explain some of their responses to disturbance. By combining biochemical and direct microscopy based quantification of microbial abundance with enzyme activities and process measurements, this study provided evidence for the role of microbial biomass (especially fungi) in macroaggregate dynamics and carbon and nutrient flush following cultivation. In the last ten years environmental genomic techniques have provided much new knowledge on bacterial composition in aggregate size fractions yet detailed information about other microbial groups (e.g. fungi, archaea and protozoa) is lacking.We now know that soil aggregates are dynamic entities – constantly changing with regard to their biological, chemical and physical properties and, in particular, their influences on plant nutrition and health. As a consequence, elucidation of the many mechanisms regulating soil C and nutrient dynamics demands a better understanding of the role of specific members of microbial communities and their metabolic capabilities as well as their location within the soil matrix (e.g. aggregates, pore spaces) and their reciprocal relationship with plant roots. In addition, the impacts of environment and soil type needs to be quantified at the microscale using, wherever possible, non-destructive ‘in situ’ techniques to predict and quantify the impacts of anthropogenic activities on soil microbial diversity and ecosystem level functions.     

Hot-water extractable carbon in soils: a sensitive measurement for determining impacts of fertilisation, grazing and cultivation

Using pre-established trial sites on allophanic soils, we investigated the impacts of long to medium-term pastoral management practices, such as fertilisation and grazing intensity, on a range of soil biological and biochemical properties; hot water-extractable C (HWC), water-soluble C (WSC), hot-water extractable total carbohydrates, microbial biomass-C and N and mineralisable N. These properties were examined for their usefulness as soil quality indicators responding to changes in the rhizosphere caused by management practices. Adjacent cropping, market garden and native bush sites located on similar soil types were included to determine the changes in soil biological and biochemical properties resulting from changes in land use. The seasonal variability of HWC and its relationship with other labile fractions of soil organic matter was also examined.Microbial biomass-C, mineralisable N and extractable total carbohydrates showed promise in differentiating treatment and land use effects. However, HWC was one of the most sensitive and consistent indicators examined at 52 different sites. The impact of different land uses on the amounts of HWC in the same soil type was far greater than that was observed for the soil organic carbon. The average values of HWC for soil under different land use were: native (4000 μg C g⁻¹ soil), sheep/beef pastures (3400), dairy pastures (3000), cropping (1000) and market gardening soils (850). HWC was also sensitive to differences within an ecosystem, e.g. effects of grazing intensities and effects of N or P fertilisers on pastures. The sheep and beef/cattle grazed pastures always had higher amounts of HWC than the intensively grazed dairy pastures. Nitrogen fertiliser application (200 and 400 kg N ha⁻¹ yr⁻¹) over the previous 5 yr had significant (P<0.001) negative impacts on HWC and other soil microbial properties. In contrast, long-term application of P fertilisers had a significant (P<0.001) positive effect on the HWC levels in pastoral soils. In the case of long-term P trials, HWC increased even though no increase in the total soil carbon pool was detected.HWC was positively correlated with soil microbial biomass-C (R²=0.84), microbial nitrogen (R²=0.72), mineralisable N (R²=0.86), and total carbohydrates (R²=0.88). All these correlations were significant at P<0.001 level of significance. The HWC was also positively correlated with WSC and total organic C. However, these correlations were poorer than those found for other soil parameters. Most of these measurements have been actively promoted as key indicators of soil quality. Given the strong correlations between HWC and other biochemical measurements, HWC could be used as an integrated measure of soil quality. When HWC is extracted, other pools of labile nutrients are also extracted along with C. Therefore it is suggested that decline in HWC would also indicate a decline in other labile organic pools of nutrients such as nitrogen, sulphur and phosphorus. About 40-50% of the C in the HWC extract was present as carbohydrates.     

Cropland soil–plant systems control production and consumption of methane and nitrous oxide and their emissions to the atmosphere

Croplands are an important source of atmospheric methane (CH₄) and nitrous oxide (N₂O), both potent greenhouse gases. Reduction of cropland CH₄ and N₂O emissions is expected to mitigate climate change. However, large uncertainty remains in the assessment and prediction of these emissions, which prevents us from establishing appropriate mitigation options and strategies. The uncertainty is attributed mainly to the high spatiotemporal variability in emissions (e.g., emission spikes of N₂O). Understanding and quantifying how hotspots of CH₄ and N₂O production in soil and then hot moments of their emissions occur would help reduce the uncertainty. This review focuses on soil–plant systems, particularly the rhizosphere, as possible hotspots of production and consumption of CH₄ and N₂O. It is well known that the rhizosphere controls CH₄ emission strongly, though each process of production and consumption remains to be quantified. On the other hand, surprisingly little attention has been paid to N₂O, besides the fact that plant roots strongly control nitrification and denitrification. We review the current knowledge of cropland CH₄ and N₂O emissions, and conclude that soil–plant interactions strongly affect cropland emissions of both gases, in which functions of plant roots affecting biogeochemical factors (e.g., availability of oxygen, labile organic carbon and inorganic nitrogen) in the rhizosphere and phenological changes are particularly important. In relation to the status of current knowledge, we discuss future research needed.     

Plant genotype strongly modifies the structure and growth of maize rhizosphere microbial communities

We studied the microbial communities in maize (Zea mays) rhizosphere to determine the extent to which their structure, biomass, activity and growth were influenced by plant genotype (su1 and sh2 genes) and the addition of standard and high doses of different types of fertilizer (inorganic, raw manure and vermicompost). For this purpose, we sampled the rhizosphere of maize plants at harvest, and analyzed the microbial community structure (PLFA analysis) and activity (basal respiration and bacterial and fungal growth rates). Discriminant analysis clearly differentiated rhizosphere microbial communities in relation to plant genotype. Although microorganisms clearly responded to dose of fertilization, the three fertilizers also contributed to differentiate rhizosphere microbial communities. Moreover, larger plants did not promoted higher biomass or microbial growth rates suggesting complex interactions between plants and fertilizers, probably as a result of the different performance of plant genotypes within fertilizer treatments, i.e. differences in the quality and/or composition of root exudates.     

Microbial colonisation of roots as a function of plant species

Fifteen plants species were grown in the greenhouse on the same soil and sampled at flowering to obtain rhizosphere soil and root material. In both fractions, the data on fungal and bacterial tissue obtained by amino sugar analysis were compared with the total microbial biomass based on fumigation-extraction and ergosterol data. The available literature on glucosamine concentrations in fungi and on muramic acid concentrations in bacteria was reviewed to prove the possibility of generating conversion values for general use in root material. All microbial properties analysed revealed strong species-specific differences in microbial colonisation of plant roots. The root material contained considerable amounts of microbial biomass C and biomass N, reaching mean levels of 10.9 and 1.4 mg g⁻¹ dry weight, respectively. However, the majority of CHCl₃ labile C and N, i.e. 89 and 55% was root derived. The average amount of ergosterol was 13 μg g⁻¹ dry weight and varied between 0.0 for Phacelia roots and 45.5 μg g⁻¹ dry weight for Vicia roots. The ergosterol content in root material of mycorrhizal and non-mycorrhizal plant species did not differ significantly. Fungal glucosamine was converted to fungal C by multiplication by 9 giving a range of 7.1-25.9 mg g⁻¹ dry weight in the root material. Fungal C and ergosterol were significantly correlated. Bacterial C was calculated by multiplying muramic acid by 45 giving a range from 1.7 to 21.6 mg g⁻¹ dry weight in the root material. In the root material of the 15 plant species, the ratio of fungal C-to-bacterial C ranged from 1.0 in mycorrhizal Trifolium roots to 9.5 in non-mycorrhizal Lupinus roots and it was on average 3.1. These figures mean that the microbial tissue in the root material consists on average of 76% fungal C and 24% bacterial C. The differences in microbial colonisation of the roots were reflected by differences in microbial indices found in the rhizosphere soil, most strongly for microbial biomass C and ergosterol, but to some extent also for glucosamine and muramic acid.     

Carbohydrates and thermal properties indicate a decrease in stable aggregate carbon following forest colonization of mountain grassland

In mountainous areas of Europe, the abandonment of grasslands followed by forest expansion is the dominant land-use change. Labile (i.e. easily decomposable) litter represents the major source for soil microbial products, which promote soil aggregation and long-term C stabilization. Our objective was to investigate changes in the content and origin of soil C components involved into aggregate stabilization (i.e. carbohydrates) following forest expansion on abandoned grassland in the Alps, where only few studies have been conducted.Changes in carbohydrates and thermally labile C were assessed along a land-use gradient in the Southern Alps (Italy) following analysis of carbohydrate monomers and thermal analysis of mineral soil and physical soil fractions. The land-use gradient comprised managed grassland, two transitional phases in which grassland abandonment led to colonization by Picea abies (L.) Karst., and an old forest dominated by Fagus sylvatica L. and P. abies.Grassland abandoned for 10 years tended to have higher levels of carbohydrate and thermally labile soil C than managed grassland and old forest, presumably caused by differences in the quality and amount of litter input. Carbohydrates and thermally labile C showed similar patterns in bulk soil, suggesting that thermal analysis can be used to complement chemical analysis although a straightforward relationship could not be established. Following forest expansion on abandoned grassland, ratios of microbially to plant-derived carbohydrates and thermally labile to resistant components decreased in bulk soil and soil fractions. Forest expansion entailed decreasing amounts of microbially derived compounds known to be important for aggregate stability, and corresponded to decreased soil C allocation to stable aggregates.The combination of carbohydrate and thermal analyses revealed a lower abundance of microbially derived C components after forest colonization on abandoned grasslands, thus resulting in lower physical protection of soil C considering that carbohydrates of microbial origin actively promote soil aggregation.     

Plant nitrogen uptake drives rhizosphere bacterial community assembly during plant growth

When plants establish in novel environments, they can modify soil microbial community structure and functional properties in ways that enhance their own success. Although soil microbial communities are influenced by abiotic environmental variability, rhizosphere microbial communities may also be affected by plant activities such as nutrient uptake during the growing season. We predicted that during the growing season, plant N uptake would explain much of the variation in rhizosphere microbial community assembly and functional traits. We grew the invasive C₃ grass Bromus tectorum and three commonly co-occurring native C₃ grasses in a controlled greenhouse environment, and examined rhizosphere bacterial community structural and functional characteristics at three different plant growth stages. We found that soil N availability and plant tissue N levels strongly correlated with shifts in rhizosphere bacterial community structure. It also appeared that the rapid drawdown of soil nutrients in the rhizosphere during the plant growing season triggered a selection event whereby only those microbes able to tolerate the changing nutrient conditions were able to persist. Plant N uptake rates inversely corresponded to microbial biomass N levels during periods of peak plant growth. Mechanisms which enable plants to influence rhizosphere bacterial community structure and function are likely to affect their competitive ability and fitness. Our study suggests that plants can alter their rhizosphere microbiomes through influencing nutrient availability. The ways in which plants establish their rhizosphere bacterial communities may now be viewed as a selection trait related to intrinsic plant species nutrient demands.     

Distribution of phosphatase activity and various bacterial phyla in the rhizosphere of Hordeum vulgare L. depending on P availability

Despite the importance of the rhizosphere for nutrient turnover, little is known about the spatial patterns of organic phosphorus mineralization by plants and by microorganisms in the rhizosphere. Therefore, the distribution of acid and alkaline phosphatase activity and the abundance of bacteria belonging to various bacterial phyla were investigated in the rhizosphere of barley (Hordeum vulgare L.) as dependent on the availability of inorganic P. For this purpose, we conducted a greenhouse experiment with barley growing in inclined boxes that can be opened to the bottom side (rhizoboxes), and applied soil zymography and fluorescence-in situ-hybridization (FISH). Acid phosphatase activity was strongly associated with the root and was highest at the root tips. Due to P fertilization, acid phosphatase activity decreased in the bulk soil, and less strongly in the rhizosphere. Alkaline phosphatase activity, i.e., microbial phosphatase activity was high throughout the soil in the control treatment and was reduced due to inorganic P fertilization especially in the rhizosphere and less strongly in the bulk soil. P-fertilization slightly increased the total number of bacteria in the rhizosphere. Moreover, P-fertilization decreased the abundance of Firmicutes and increased the abundances of Beta- and Gamma-Proteobacteria. The total number of bacterial cells was significantly higher at the root surface than at the root tip and at a distance of 30 μm from the root surface. Our results show that alkaline phosphatase activity decreased more strongly in the rhizosphere than in the bulk soil due to P fertilization, which might be because of greater C deficiency in the bulk soil compared to the rhizosphere. Furthermore, the results indicate a spatial separation between hotspots of acid phosphatase activity and hotspots of bacteria in the rhizosphere of H. vulgare. Taken together, our study shows that bacteria and phosphatase activity were very heterogeneously distributed in soil, and that the effects of P fertilization on phosphatase activity differed strongly between bulk soil and rhizosphere as well as between various zones of the rhizosphere.     

Shifts in rhizosphere microbial communities and enzyme activity of Poa alpina across an alpine chronosequence

This study quantifies the influence of Poa alpina on the soil microbial community in primary succession of alpine ecosystems, and whether these effects are controlled by the successional stage. Four successional sites representative of four stages of grassland development (initial, 4 years (non-vegetated); pioneer, 20 years; transition, 75 years; mature, 9500 years old) on the Rotmoos glacier foreland, Austria, were sampled. The size, composition and activity of the microbial community in the rhizosphere and bulk soil were characterized using the chloroform-fumigation extraction procedure, phospholipid fatty acid (PLFA) analysis and measurements of the enzymes β-glucosidase, β-xylosidase, N-acetyl-β-glucosaminidase, leucine aminopeptidase, acid phosphatase and sulfatase. The interplay between the host plant and the successional stage was quantified using principal component (PCA) and multidimensional scaling analyses. Correlation analyses were applied to evaluate the relationship between soil factors (C_org, N_t, C/N ratio, pH, ammonium, phosphorus, potassium) and microbial properties in the bulk soil. In the pioneer stage microbial colonization of the rhizosphere of P. alpina was dependent on the reservoir of microbial species in the bulk soil. As a consequence, the rhizosphere and bulk soil were similar in microbial biomass (ninhydrin-reactive nitrogen (NHR-N)), community composition (PLFA), and enzyme activity. In the transition and mature grassland stage, more benign soil conditions stimulated microbial growth (NHR-N, total amount of PLFA, bacterial PLFA, Gram-positive bacteria, Gram-negative bacteria), and microbial diversity (Shannon index H) in the rhizosphere either directly or indirectly through enhanced carbon allocation. In the same period, the rhizosphere microflora shifted from a G⁻ to a more G⁺, and from a fungal to a more bacteria-dominated community. Rhizosphere β-xylosidase, N-acetyl-β-glucosaminidase, and sulfatase activity peaked in the mature grassland soil, whereas rhizosphere leucine aminopeptidase, β-glucosidase, and phosphatase activity were highest in the transition stage, probably because of enhanced carbon and nutrient allocation into the rhizosphere due to better growth conditions. Soil organic matter appeared to be the most important driver of microbial colonization in the bulk soil. The decrease in soil pH and soil C/N ratio mediated the shifts in the soil microbial community composition (bacPLFA, bacPLFA/fungPLFA, G⁻, G⁺/G⁻). The activities of β-glucosidase, β-xylosidase and phosphatase were related to soil ammonium and phosphorus, indicating that higher decomposition rates enhanced the nutrient availability in the bulk soil. We conclude that the major determinants of the microflora vary along the successional gradient: in the pioneer stage the rhizosphere microflora was primarily determined by the harsh soil environment; under more favourable environmental conditions, however, the host plant selected for a specific microbial community that was related to the dynamic interplay between soil properties and carbon supply.     

Distribution and turnover of recently fixed photosynthate in ryegrass rhizospheres

The cycling of root-deposited photosynthate (rhizodeposition) through the soil microbial biomass can have profound influences on plant nutrient availability. Currently, our understanding of microbial dynamics associated with rhizosphere carbon (C) flow is limited. We used a ¹³C pulse-chase labeling procedure to examine the flow of photosynthetically fixed ¹³C into the microbial biomass of the bulk and rhizosphere soils of greenhouse-grown annual ryegrass (Lolium multiflorum Lam.). To assess the temporal dynamics of rhizosphere C flow through the microbial biomass, plants were labeled either during the transition between active root growth and rapid shoot growth (Labeling Period 1), or nine days later during the rapid shoot growth stage (Labeling Period 2). Although the distribution of ¹³C in the plant/soil system was similar between the two labeling periods, microbial cycling of rhizodeposition differed between labeling periods. Within 24 h of labeling, more than 10% of the ¹³C retained in the plant/soil system resided in the soil, most of which had already been incorporated into the microbial biomass. From day 1 to day 8, the proportion of ¹³C in soil as microbial biomass declined from about 90 to 35% in rhizosphere soil and from about 80 to 30% in bulk soil. Turnover of ¹³C through the microbial biomass was faster in rhizosphere soil than in bulk soil, and faster in Labeling Period 1 than Labeling Period 2. Our results demonstrate the effectiveness of using ¹³C labeling to examine microbial dynamics and fate of C associated with cycling of rhizodeposition from plants at different phenological stages of growth.     

Nitrogen and phosphorus transformations in the rhizospheres of three tree species in a nutrient-poor sandy soil

We examined acid phosphatase activity (APA), N mineralization and nitrification rates, available N and P, and microbial biomass C, N and P in rhizosphere and bulk soils of 18-year-old Siberian elm (Ulmus pumila), Simon poplar (Populus simonii) and Mongolian pine (Pinus sylvestris var. mongolica) plantations on a nutrient-poor sandy soil in Northeast China. The main objective was to compare the rhizosphere effects of different tree species on N and P cycling under nutrient-deficient conditions. All tree species had the similar pattern but considerably different magnitude of rhizosphere effects. The APA, potential net N mineralization and nitrification rates increased significantly (by 27–60%, 110–188% and 106–142% respectively across the three species) in rhizosphere soil compared to bulk soil. This led to significantly higher Olsen-P and NH₄⁺-N concentrations in rhizosphere soil, whereas NO₃⁻-N concentration was significantly lower in rhizosphere soil owing to increased microbial immobilization and root uptake. Microbial biomass C and N generally increased while microbial biomass P remained constant in rhizosphere soil relative to bulk soil, indicating the N-limited rather than P-limited microbial growth. Rhizosphere effects on P transformation were most pronounced for Siberian elm, while rhizosphere effects on N transformation were most pronounced for Mongolian pine, implying the different capacities of these species to acquire nutrients.     

根系分隔方式对玉米/大豆根际土壤碳含量及磷有效性的影响

【目的】豆科与禾本科间作体系中对磷有效性的影响主要集中在根系分泌物的活化作用,由根际沉淀引起的土壤碳含量与磷酸酶活性变化及其对红壤磷有效性的影响机制尚不清楚。【方法】本研究以间作玉米大豆为研究对象,设置根系完全分隔、尼龙网分隔、不分隔3种方式,在0、21.83、43.67、65.50和87.34 P mg kg-1（分别记为P0、P1、P2、P3和P4）磷肥施用水平下进行盆栽试验,研究根系分隔方式对间作玉米大豆根际土壤微生物量碳（MBC）、溶解性有机碳（DOC）、根际土壤有机碳（ROC）、酸性磷酸酶活性（ACP）、碱性磷酸酶活性（ALP）、速效磷和Hedley磷组分的影响。【结果】相比完全分隔,根系不分隔可提高玉米和大豆根际土壤MBC含量,显著降低玉米根际土壤DOC含量,低磷水平（P0、P1）时显著提高大豆DOC含量,显著提高玉米（仅在低磷时）和大豆根际土壤ACP活性,低磷时显著提高大豆根际土壤ALP活性。除玉米活性磷组分外,根系分隔方式对间作玉米大豆根际土壤速效磷、磷组分有显著或极显著影响。根系不分隔较完全分隔可通过降低大豆根际活性无机磷（Pi）(P0除外)和中活性Pi从而提高玉米根...     

Microbial community structure and abundance in the rhizosphere and bulk soil of a tomato cropping system that includes cover crops

Understanding microbial responses to crop rotation and legacy of cropping history can assist in determining how land use management impacts microbially mediated soil processes. In the literature, one finds mixed results when attempting to determine the major environmental and biological controls on soil microbial structure and functionality. The objectives of this research were to: (1) Qualitatively and quantitatively measure seasonal and antecedent soil management effects on the soil microbial community structure in the rhizosphere of a subsequent tomato crop (Solanum lycopersicum) and (2) Determine phylum scale differences between the rhizosphere and bulk soil microbial community as influenced by the antecedent hairy vetch (Vicia villosa), cereal rye (Secale cereale), or black plastic mulch treatments. In this report, we use terminal restriction fragment length polymorphisms in the 16s rDNA gene to characterize changes in microbial community structure in soil samples from a field replicated tomato production system experiment at USDA-ARS Beltsville Agricultural Research Center, Beltsville, MD, USA. We found season of the year had the strongest influence on the soil microbial community structure of some of the major microbial phyla. Although we monitored just a few of the major microbial phyla (four Eubacteria and Archaea), we found that the effects of the tomato plant on the structural composition of these phyla in the rhizosphere differed dependent on the antecedent cover crop. Increased understanding of how agricultural factors influence the soil microbial community structure under field conditions is critical information for farmers and land managers to make decisions when targeting soil ecosystem services that are microbially driven.     

Priming effects: Interactions between living and dead organic matter

In this re-evaluation of our 10-year old paper on priming effects, I have considered the latest studies and tried to identify the most important needs for future research. Recent publications have shown that the increase or decrease in soil organic matter mineralization (measured as changes of CO₂ efflux and N mineralization) actually results from interactions between living (microbial biomass) and dead organic matter. The priming effect (PE) is not an artifact of incubation studies, as sometimes supposed, but is a natural process sequence in the rhizosphere and detritusphere that is induced by pulses or continuous inputs of fresh organics. The intensity of turnover processes in such hotspots is at least one order of magnitude higher than in the bulk soil. Various prerequisites for high-quality, informative PE studies are outlined: calculating the budget of labeled and total C; investigating the dynamics of released CO₂ and its sources; linking C and N dynamics with microbial biomass changes and enzyme activities; evaluating apparent and real PEs; and assessing PE sources as related to soil organic matter stabilization mechanisms. Different approaches for identifying priming, based on the assessment of more than two C sources in CO₂ and microbial biomass, are proposed and methodological and statistical uncertainties in PE estimation and approaches to eliminating them are discussed. Future studies should evaluate directions and magnitude of PEs according to expected climate and land-use changes and the increased rhizodeposition under elevated CO₂ as well as clarifying the ecological significance of PEs in natural and agricultural ecosystems. The conclusion is that PEs - the interactions between living and dead organic matter - should be incorporated in models of C and N dynamics, and that microbial biomass should regarded not only as a C pool but also as an active driver of C and N turnover.