Wetting properties of fungi mycelium alter soil infiltration and soil water repellency in a γ-sterilized wettable and repellent soil

Soil water repellency (SWR) has a drastic impact on soil quality resulting in reduced infiltration, increased runoff, increased leaching, reduced plant growth, and increased soil erosion. One of the causes of SWR is hydrophobic fungal structures and exudates that change the soil–water relationship. The objective of this study was to determine whether SWR and infiltration could be manipulated through inoculation with fungi. The effect of fungi on SWR was investigated through inoculation of three fungal strains (hydrophilic – Fusarium proliferatum, chrono-amphiphilic – Trichoderma harzianum, and hydrophobic – Alternaria sp.) on a water repellent soil (WR-soil) and a wettable soil (W-soil). The change in SWR and infiltration was assessed by the water repellency index and cumulative infiltration respectively. F. proliferatum decreased the SWR on WR-soil and slightly increased SWR in W-soil, while Alternaria sp. increased SWR in both the W-soil and the WR-soil. Conversely T. harzianum increased the SWR in the W-soil and decreased the SWR in the WR-soil. All strains showed a decrease in infiltration in W-soil, while only the F. proliferatum and T. harzianum strain showed improvement in infiltration in the WR-soil. The ability of fungi to alter the SWR and enmesh soil particles results in changes to the infiltration dynamics in soil.     

Does biochar influence soil physical properties and soil water availability?

Aims

This study aims to (i) determine the effects of incorporating 47 Mg ha^?1 acacia green waste biochar on soil physical properties and water relations, and (ii) to explore the different mechanisms by which biochar influences soil porosity.

Methods

The pore size distribution of the biochar was determined by scanning electron microscope and mercury porosimetry. Soil physical properties and water relations were determined by in situ tension infiltrometers, desorption and evaporative flux on intact cores, pressure chamber analysis at ?1,500 kPa, and wet aggregate sieving.

Results

Thirty months after incorporation, biochar application had no significant effect on soil moisture content, drainable porosity between –1.0 and ?10 kPa, field capacity, plant available water capacity, the van Genuchten soil water retention parameters, aggregate stability, nor the permanent wilting point. However, the biochar-amended soil had significantly higher near-saturated hydraulic conductivity, soil water content at ?0.1 kPa, and significantly lower bulk density than the unamended control. Differences were attributed to the formation of large macropores (>1,200 μm) resulting from greater earthworm burrowing in the biochar-amended soil.

Conclusion

We found no evidence to suggest application of biochar influenced soil porosity by either direct pore contribution, creation of accommodation pores, or improved aggregate stability.     

How do soil fauna and soil microbiota respond to beech forest growth?

The dynamics and performance of soil biota during forest rotation were studied in monoculture beech stands forming a chronosequence of four different age-classes(30,62,111,153 yr).Biomass was monitored in major groups of microflora,microfauna,mesofauna,and macrofauna.Resource availability(litter layer,soil organic mater),biomass of the two dominant decomposer groups(microflora,earthworms)as well as the biomass of mesofauna and microfauna were found to remain quite stable during forest succession.Nevertheles...     

Gradients in soil solution composition between bulk soil and rhizosphere – In situ measurement with changing soil water content

Soil solution composition changes with time and distance from the root surface as a result of mass flow, diffusion, plant nutrient uptake and root exudation. A model system was designed, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 m mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). K⁺ concentration in the rhizosphere soil solution decreased during the initial growth stage (12 days after planting, DAP). Thereafter K⁺ accumulated with time, due to mass flow as the dominating process. The extend of K⁺ accumulation depended on the initial fertiliser application. As K⁺ concentrations in soil solution increase, not only as a result of transport exceeding uptake, but also as a result of decreasing soil water content, it is hypothesised that K concentration in soil solution is not the only trigger for the activity of K transporters in membranes, but ABA accumulation in roots induced by decreasing soil matric potentials may add to the regulation. A strong decrease of rhizosphere pH with time is observed as a result of H⁺ efflux from the roots in order to maintain cation-anion balance. In addition the K⁺ to Ca²⁺ ratio was altered continuously during the growing period, which has an impact on Ca²⁺ uptake and thus firmness of cell walls, apoplast pH, membrane integrity and activity of membrane transporters. The value of osmotic potential in the rhizosphere soil solution increased with time indicating decreasing soil water availability. Modelling approaches based on the data obtained with the system might help to fill in the time gaps caused by the low temporal resolution of soil solution sampling method.     

Do mesocosms influence photosynthesis and soil respiration?

Mesocosms, enclosed outdoor experimental systems, are commonly used in terrestrial ecology. They are frequently used to study the effects of elevated CO₂ and temperature on terrestrial ecosystem processes. Despite their advantages and frequent use it is important to verify, through explicit measures, that mesocosms reliably model the larger system. In this study, fully-coupled, soil–litter–plant mesocosms were constructed in Corvallis using native soil and litter, and planted with Douglas-fir (Pseudotsuga menziesii Mirb. Franco) seedlings. Needle photosynthesis and soil respiration were measured repeatedly over a 21-month period in mesocosms and compared to measurements made at two field sites (Toad Creek and Falls Creek) planted at the same density as the mesocosms. Under the temperature and soil moisture conditions, photosynthetic and soil respiration rates in the mesocosms were not significantly different than the rates at Toad Creek, where the soil and litter in the mesocosms were collected. In contrast, the soil at Falls Creek was different than the soil in the mesocosms and at Toad Creek and photosynthetic and soil respiration rates at Falls Creek were significantly different than at the other two sites. The lack of significant differences between rates measured in the mesocosms in Corvallis and at the Toad Creek field site indicate that the mesocosms did not cause significant artifacts in the data and that the results for these rates in the mesocosms can be extrapolated to field settings with comparable edaphic conditions.     

Causal soil—plant relationships and path coefficients

Summary Data obtained from uncontrolled experiments are often fitted to regression models, in which the dependent variable is assumed to be affected by a number of independent factors. The regression coefficient then gives the rate of a change of an effect caused by unit change in the independent variable on the assumption that this change in the causal factor does not result in a change in another independent factor (partial regression coefficient). In many cases however, this assumption is not valid; this is particularly the case with investigations into the quantitative relationships of plants.The principle of path coefficients introduced by Wright and used up till now mainly in genetics, allows among other things for the possibility of making allowance for these indirect influences. For this purpose the investigator has to formulate a closed causal linear system withm primary causes (x) andn effects (y). By a closed linear system is understood a network in which each variable is a linear combination of one or more other variables of this system or is one of the variables that is determined by none of the variables in this system; the latter are the primary causes,x. The parameters which give the extents of the influences are called path coefficients. The derivation of path coefficients is demonstrated by the equations 1–5 of the example of the simple system in Figure 2.The potentialities of the method of path coefficients are illustrated by its application to an investigation into the effects of soil and other factors on the MgO and K₂O content of herbage. The conventional regression model is given in Figure 1. Figure 3 presents a more realistic model which has been constructed that the variables, proportion of weeds and crude-protein content, are treated as cause as well as effect. The path coefficients of this model are soluble and are given in Table 2. For comparison, the regression coefficients estimated according to the model in Figure 1 are given in Table 3. In the model in Figure 4 the influence of the K₂O content of the soil on the MgO content of the herbage is shown to be of a plant-physiological and not of a soil-chemical nature.The method of path coefficients has greater potentialities than the regression merhod for the solution of certain problems. In the model of Figure 5 a synthesis between soil factors, chemical and botanical composition of the herbage, and Mg content of the blood is demonstrated; this model is soluble.

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Kausale Boden—Pflanze-Zusammenhänge und Pfad-Koeffizienten

Zusammenfassung Die in einem Experiment ohne Eingriff erzielten Ergebnisse werden oft mit einer Regressionsgleichung ausgewertet. Im Model dieser Gleichung wird eine Variabele durch die sonstigen sog. unabhängigen Variabelen erklärt. Die Regressionskoeffizienten geben dann die Zunahme des Effektes an wenn eine Ursache um 1 wächst, unter Annahme dass die sonstigen erklärenden Variabelen durch die Änderung dieser Ursache selbst nicht geändert werden (partielle oder Teilregression). In vielen Fällen entspricht diese Annahme nicht der Wirklichkeit. Die von Wright entwickelte Methode mit den Pfad-Koeffizienten gibt die Möglichkeit diese Schwierigkeiten bisweilen zu beseitigen. Hierzu muss der Forscher ein geschlossen kausales, lineares System mitm primären Ursachenx undn Effekteny aufsetzen. Unter einem geschlossen kausalen System wird ein Netzwerk verstanden in dem jede Variabele entweder eine lineare Kombination einer oder mehrerer Variabelen dieses Systems oder eine der Variabelen ist, welche durch keine der Variabelen des Systems bestimmt ist. Die letzten Variabelen sind darin die primären Ursachenx. Die Grösse eines Einflusses wird durch den Pfad-Koeffizient gegeben. Ein Beispiel der Auswertung der Pfad-Koeffizienten des einfachen Systems aus Figur 2 wird durch die Gleichungen 1–5 gegeben.Die Möglichkeiten der Methode mit den Pfad-Koeffizienten werden vorgeführt an Hande einer Untersuchung nach den Einflüssen von Boden- und anderen Faktoren auf den MgO- und K₂O-Gehalt des Weidegrases in Bezug auf die Wichtigkeit dieser Zusammensetzung für das Auftreten von Hypomagnesaemie. Das Regressionsmodell dieser Untersuchung wird in Figur 1 gegeben. Figur 3 gibt ein mehr reelles Modell, worin die Variabelen Prozentsatz an Kräutern und Roheiweissgehalt des Grasses Ursache sowohl wie Effekt sind. Die Pfad-Koeffizienten dieses Modelles sind lösbar und werden in Tabelle 2 gegeben. Zur Vergleich werden in Tabelle 3 die Regressionskoeffizienten des Modelles aus Figur 1 gegeben. Im Modell von Figur 4 wird der Einfluss von Kali im Boden mit Hilfe des Kaligehaltes vom Gras physiologisch gedeutet.Die Methode mit den Pfad-Koeffizienten hat viele Vorzüge vor dem Regressionsmodell. Die Methode gibt weiter grosse Möglichkeiten für eine synthetische Auswertung der Ergebnisse. Im Modell von Figur 5 wird eine Synthese zwischen Bodenfaktoren, Zusammensetzung des Weidegrases und Mg-Gehalt des Blutes gegeben. Die Pfad-Koeffizienten dieses Modelles sind lösbar.

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Relations causales sol—plante et coefficients path

Résumé Les résultats d'un essai sans intervention sont souvent analysés par une équation de régression. Dans le modèle de cette équation une des variables est expliquée par les autres, nommées variables independantes. Les coëfficients de régression formulent alors l'accroissement de l'effet pour chaque augmentation de la cause d'une unité en admettant que les autres variables explicatives ne sont pas influencées par ce changement de la cause (régression partielle). Dans beaucoup de cas cependant cette admission est inexacte.La méthode des coëfficients path développée par Wright fournit la possibilité de surmonter ces difficultés. À cet effet le chercheur doit ébraucher un système causal linéair fermé avecm causes primairesx etn effetsy. Un système causal fermé est un réseau, dans lequel chaque variable est, soit une combination linéaire d'une ou plusieurs variables de ce système, soit une variable, qui est indépendante des variables de ce système. Ces dernières sont là-dedans les causes primairesx. L'intensité des influences est exprimée par les coefficients paths. Un exemple des calculs des coefficients path du système simple de la figure 2 est donné par les équations 1–5.Les possibilités de la méthode avec les coefficients path sont démontrées à l'aide des résultats d'une recherche sur les influences des facteurs pédologiques et autres sur la teneur en MgO et K₂O dans l'herbe de pâturage, vue l'importance de ces teneurs sur l'apparation de l'hypomagnesaemie. Le modèle de regression de cette recherche est donné dans la figure 1. La figure 3 présente un modèle plus réel, dans lequel les variables: teneur en mauvaises herbes et teneur en proteïne brute de l'herbe sont aussi bien cause qu'effet. Les coefficients path ce de modèle sont résolubles et mentionnés dans la tabelle 2. Pour comparaison la tabelle 3 mentionne les coefficients de régression du modèle de la figure 1. Le modèle de la figure 4 exprime par voie physiologique l'influence de la potasse du sol au moyen de la teneur en potasse de l'herbe.La méthode des coefficients path a plus de possibilités que le modèle de régression. Elle présente de grosses possibilités pour une analyse synthétique des résultats. Le modèle de la figure 5 donne la synthèse entre les facteurs pédologiques, composition de l'herbe et teneur en Mg du sang. Les coefficients path de ce modèle sont résolubles.

     

A disconnect between O horizon and mineral soil carbon – Implications for soil C sequestration

Changing inputs of carbon to soil is one means of potentially increasing carbon sequestration in soils for the purpose of mitigating projected increases in atmospheric CO₂ concentrations. The effect of manipulations of aboveground carbon input on soil carbon storage was tested in a temperate, deciduous forest in east Tennessee, USA. A 4.5-year experiment included exclusion of aboveground litterfall and supplemental litter additions (three times ambient) in an upland and a valley that differed in soil nitrogen availability. The estimated decomposition rate of the carbon stock in the O horizon was greater in the valley than in the upland due to higher litter quality (i.e., lower C/N ratios). Short-term litter exclusion or addition had no effect on carbon stock in the mineral soil, measured to a depth of 30 cm, or the partitioning of carbon in the mineral soil between particulate- and mineral-associated organic matter. A two-compartment model was used to interpret results from the field experiments. Field data and a sensitivity analysis of the model were consistent with little carbon transfer between the O horizon and the mineral soil. Increasing aboveground carbon input does not appear to be an effective means of promoting carbon sequestration in forest soil at the location of the present study because a disconnect exists in carbon dynamics between O horizon and mineral soil. Factors that directly increase inputs to belowground soil carbon, via roots, or reduce decomposition rates of organic matter are more likely to benefit efforts to increase carbon sequestration in forests where carbon dynamics in the O horizon are uncoupled from the mineral soil.     

Resource, biological community and soil functional stability dynamics at the soil–litter interface

The interface between decaying plant residues and soil is a focus for soil ecological processes because of resources from the residues diffusing into the soil, and microfauna that proliferate in the adjacent soil. Given that the recovery of soil function following disturbance depends on immigration, colonization and establishment of exotic organisms from adjacent un-disturbed habitats, and the availability of bio-available resources, we hypothesized that the soil–litter interface could contribute to soil functional stability. In laboratory pot trials, soil was separated into two parts by a mesh bag with the inner section amended, or not amended, with rice straw; an outer layer of unamended soil, adjacent to the litter (1.5 cm thick, either heated or not), provided a soil–litter interface. This enabled us to examine the dynamics of dissolved organic carbon (DOC), mineral nitrogen, microbial biomass carbon (MBC), nematode assemblages and functional stability during 35 days incubation. Either 1 mm or 5 μm meshes were used, which allowed nematodes to migrate (SR1) or not (SR5) through the mesh to the soil–litter interface; thus also enabling us to evaluate the role of nematodes in soil functional stability. Higher DOC and MBC but lower mineral nitrogen concentrations were found at the soil–litter interface. Heating increased the availability of soil resources such as mineral nitrogen and DOC, but decreased the MBC and total nematode abundance in the soil. The soil–litter interface was characterized by a higher abundance of nematodes, particularly microbivores, regardless of mesh aperture or disturbance. The difference in nematode abundance between SR1 and SR5 indicated that nematode propagation, due to resource diffusion and nematode migration through the mesh, contributed to the changing numbers of microbivorous nematodes depending on incubation time. The soil functional stability was calculated as a relative change in the functioning of short-term barley decomposition. Soil functional resistance, defined as the instantaneous effect of disturbance on decomposition measured on the first day, was highest in the SR5 treatment. However, soil functional resilience, defined as the recovery of soil function over the whole incubation period (35d), was highest in the SR1 treatment, which is most probably attributed to the functioning of microbivorous nematodes. Our results suggest that small-scale spatial heterogeneity, due to organic residue decomposition, can help maintain soil functions following disturbance.     

Can composition and physical protection of soil organic matter explain soil respiration temperature sensitivity?

The importance of soil organic matter (SOM) in the global carbon (C) cycle has been highlighted by many studies, but the way in which SOM stabilization processes and chemical composition affect decomposition rates under natural climatic conditions is not yet well understood. To relate the temperature sensitivity of heterotrophic soil respiration to the decomposition potential of SOM, we compared temperature sensitivities of respiration rates from a 2-year long soil translocation experiment from four elevations along a ~3000 m tropical forest gradient. We determined SOM stabilization mechanisms and the molecular structure of soil C from different horizons collected before and after the translocation. Soil samples were analysed by physical fractionation procedures, ¹³C nuclear magnetic resonance (NMR) spectroscopy, and differential scanning calorimetry (DSC). The temperature sensitivity (Q ₁₀) of heterotrophic soil respiration at the four sites along the elevation transect did not correlate with either the available amount of SOM or its chemical structure. Only the relative distribution of C into physical soil fractions correlated with Q ₁₀ values. We therefore conclude that physical fractionation of soil samples is the most appropriate way to assess the temperature sensitivity of SOM.     

urce, biological community and soil functional stability dynamics at the soil–litter interface

The interface between decaying plant residues and soil is a focus for soil ecological processes because of resources from the residues diffusing into the soil, and microfauna that proliferate in the adjacent soil. Given that the recovery of soil function following disturbance depends on immigration, colonization and establishment of exotic organisms from adjacent un-disturbed habitats, and the availability of bio-available resources, we hypothesized that the soil–litter interface could contribute to soil functional stability. In laboratory pot trials, soil was separated into two parts by a mesh bag with the inner section amended, or not amended, with rice straw; an outer layer of unamended soil, adjacent to the litter (1.5 cm thick, either heated or not), provided a soil–litter interface. This enabled us to examine the dynamics of dissolved organic carbon (DOC), mineral nitrogen, microbial biomass carbon (MBC), nematode assemblages and functional stability during 35 days incubation. Either 1 mm or 5 μm meshes were used, which allowed nematodes to migrate (SR1) or not (SR5) through the mesh to the soil–litter interface; thus also enabling us to evaluate the role of nematodes in soil functional stability. Higher DOC and MBC but lower mineral nitrogen concentrations were found at the soil–litter interface. Heating increased the availability of soil resources such as mineral nitrogen and DOC, but decreased the MBC and total nematode abundance in the soil. The soil–litter interface was characterized by a higher abundance of nematodes, particularly microbivores, regardless of mesh aperture or disturbance. The difference in nematode abundance between SR1 and SR5 indicated that nematode propagation, due to resource diffusion and nematode migration through the mesh, contributed to the changing numbers of microbivorous nematodes depending on incubation time. The soil functional stability was calculated as a relative change in the functioning of short-term barley decomposition. Soil functional resistance, defined as the instantaneous effect of disturbance on decomposition measured on the first day, was highest in the SR5 treatment. However, soil functional resilience, defined as the recovery of soil function over the whole incubation period (35d), was highest in the SR1 treatment, which is most probably attributed to the functioning of microbivorous nematodes. Our results suggest that small-scale spatial heterogeneity, due to organic residue decomposition, can help maintain soil functions following disturbance.     

DenNit – Experimental analysis and modelling of soil N2O efflux in response on changes of soil water content, soil temperature, soil pH, nutrient availability and the time after rain event

To quantify the effects of soil temperature (T_soil), and relative soil water content (RSWC) on soil N₂O emission we measured N₂O soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity, ammonium, and nitrate concentration in the field. The analysis was performed on three meadows, two bare soils and in one forest. We identified soil water content, soil temperature, soil nitrogen content, and pH as the main parameters influencing soil N₂O emission. The response of N₂O emission to soil temperature and relative soil water content was analysed for the field and climate chamber measurements. A non-linear regression model (DenNit) was developed for the field data to describe soil N₂O efflux as a function of soil temperature, soil moisture, pH value, and ammonium and nitrate concentration. The model could explain 81% of the variability in soil N₂O emission of all individual field measurements, except for data with short-term soil water changes, namely during and up to 2 h after rain stopped. We validated the model with an independent dataset. For this additional meadow site 73% of the flux variation could be explained with the model.     

Potential soil P mobilisation capacity–method development and comparison of rhizosphere soil from different crops

Background

Phosphorus (P) deficiency is wide-spread in agricultural soils. In light of increasing P fertilizer costs, it is of interest to assess the capacity of soil microbes to mobilise native soil P and added P. There is currently no method to assess P mobilisation in situ.

Methods

The soil P mobilisation potential was assessed by incubating low P soil for up to 30?days with poorly available P sources; C and N were added to increase microbial activity and ensure that only P was limiting microbial growth.

Results

The increase in microbial P from day 0 to day 15 showed that microbes were able to mobilise P from FePO₄ and phytate. The P mobilisation potential (sum of microbial and resin P) of the rhizosphere soil decreased in the following order: faba bean > chickpea and white lupin > wheat. After 10?days, up to 80% of the mobilised P was microbial P, whereas after 30?days, almost all P mobilised was resin P.

Conclusions

The method developed in this study is useful assessing not only potential of a soil to mobilise P but also, by using different poorly available P sources, the mechanisms of P mobilisation.     

Does an earthworm species acclimatize and/or adapt to soil calcium conditions? The consequences of soil nitrogen mineralization in forest soil

Calcium (Ca)-rich food can increase feeding of Lumbricidae. Earthworms can be genetically differentiated at a small spatial scale and acclimatize to the local environment during growth. Soil feeding and subsequent cast production by earthworms affects soil N mineralization. Here, we hypothesized that soil feeding and subsequent cast production by Lumbricidae species increases with high soil Ca content and that this increase is stronger in worms from high-Ca soil. We also hypothesized that changes in the soil feeding of Lumbricidae species along with the Ca content affects the soil N mineralization via changes in the cast production. Using a geophageous earthworm species (Eisenia japonica) originated from two different Ca environments (calcareous soil and sedimentary soil), we investigated cast production and soil N mineralization in three soils (sedimentary soil, sedimentary soil with Ca addition, and calcareous soil). The soil feeding of E. japonica from both origins did not always increase despite the high soil Ca content. We suggest that both the Ca content and other soil conditions (e.g., soil C:N) might be major factors in increasing soil feeding by E. japonica. Furthermore, the influence of Ca addition on cast production varied according to the earthworm origin. As expected, these differences in cast production are linked to soil N mineralization (especially nitrification). In summary, our study suggests that the acclimatization and/or adaptation of Lumbricidae species to local environmental factors not only soil Ca content explains spatially heterogeneous soil N mineralization in forest soil.     

Modeling soil–root water transport and competition for single and mixed crops

A knowledge of above and below ground plant interactions for water is essential to understand the performance of intercropped systems. In this work, root water potential dynamics and water uptake partitioning were compared between single crops and intercrops, using a simulation model. Four root maps having 498, 364, 431 and 431 soil-root contacts were used. In the first and second cases, single crops with deep and surface roots were considered, whereas in the third and fourth cases, roots of two mixed crops were simultaneously considered with different row spacing (40 cm and 60 cm). Two soils corresponding to a clay and a silty clay loam were used in the calculations. A total maximum evapotranspiration of 6 mm d^-1 for both single or mixed crops was considered, for the mixed crops however, two transpiration distributions between the crops were analyzed (3:3 mm d^-1, or 4:2 mm d^-1 for each crop, respectively). The model was based on a previous theoretical framework applied to single or intercropped plants having spatially distributed roots in a two-dimensional domain. Although water stress occurred more rapidly in the loam than in the clay, due to the rapid decrease of the soil water reserve in the loam, the role of the root arrangement appeared to be crucial for water availability. Interactions between the distribution of transpiration among mixed crops and the architecture of the root systems which were in competition led to water movements from zones with one plant to another, or vice versa, which corresponded to specific competition or facilitation effects. Decreasing the distances between roots may increase competition for water, although it may determine greater water potential gradients in the soil that increase lateral or vertical water fluxes in the soil profile. The effects of the root competition on water uptake were quite complicated, depending on both environmental conditions, soil hydrodynamic properties, and time scales. Although some biological adaptive mechanisms were disregarded in the analysis, the physically 2-D based model may be considered as a tool to study the exploitation of environmental heterogeneity at microsite scales.     

Mycorrhizal responses to biochar in soil – concepts and mechanisms

Experiments suggest that biomass-derived black carbon (biochar) affects microbial populations and soil biogeochemistry. Both biochar and mycorrhizal associations, ubiquitous symbioses in terrestrial ecosystems, are potentially important in various ecosystem services provided by soils, contributing to sustainable plant production, ecosystem restoration, and soil carbon sequestration and hence mitigation of global climate change. As both biochar and mycorrhizal associations are subject to management, understanding and exploiting interactions between them could be advantageous. Here we focus on biochar effects on mycorrhizal associations. After reviewing the experimental evidence for such effects, we critically examine hypotheses pertaining to four mechanisms by which biochar could influence mycorrhizal abundance and/or functioning. These mechanisms are (in decreasing order of currently available evidence supporting them): (a) alteration of soil physico-chemical properties; (b) indirect effects on mycorrhizae through effects on other soil microbes; (c) plant–fungus signaling interference and detoxification of allelochemicals on biochar; and (d) provision of refugia from fungal grazers. We provide a roadmap for research aimed at testing these mechanistic hypotheses.     

Protozoan grazing of bacteria in soil—impact and importance

Interactions between bacteria and protozoa in soil were studied over 2-week periods in the field and in a pot experiment. Under natural conditions the total biological activity was temporarily synchronized by a large rainfall, and in the laboratory by the addition of water to dried-out soil, with or without plants. In the field, peaks in numbers and biomass of bacteria appeared after the rain, and a peak of naked amoebae quickly followed. Of the three investigated groups—flagellates, ciliates, and amoebae—only populations of the latter were large enough and fluctuated in a way that indicated a role as bacterial regulators. The bacterial increase was transient, and the amoebae alone were calculated to be able to cause 60% of the bacterial decrease. The same development of bacteria and protozoa was observed in the pot experiment: in the presence of roots, amoebic numbers increased 20 times and became 5 times higher than in the unplanted soil. In the planted pots, the amoebic increase was large enough to cause the whole bacterial decrease observed; but in the unplanted soil, consumption by the amoebae caused only one-third of the bacterial decrease.     

Plant–soil feedbacks and the coexistence of competing plants

Plant–soil feedbacks can have important implications for the interactions among plants. Understanding these effects is a major challenge since it is inherently difficult to measure and manipulate highly diverse soil communities. Mathematical models may advance this understanding by making the interplay of the various processes affecting plant–soil interaction explicit and by quantifying the relative importance of the factors involved. The aim of this paper is to provide a complete analysis of a pioneering plant–soil feedback model developed by Bever and colleagues (J Ecol 85: 561–573, 1997; Ecol Lett 2: 52–62, 1999; New Phytol 157: 465–473, 2003) to fully understand the range of possible impacts of plant–soil feedbacks on plant communities within this framework. We analyze this model by means of a new graphical method that provides a complete classification of the potential effects of soil communities on plant competition. Due to the graphical character of the method, the results are relatively easy to obtain and understand. We show that plant diversity depends crucially on two key parameters that may be viewed as measures of the intensity of plant competition and the direction and strength of plant–soil feedback, respectively. Our analysis provides a formal underpinning of earlier claims that plant–soil feedbacks, especially when they are negative, may enhance the diversity of plant communities. In particular, negative plant–soil feedbacks can enhance the range of plant coexistence by inducing competitive oscillations. However, these oscillations can also destabilize plant coexistence, leading to low population densities and extinctions. In addition, positive feedbacks can allow locally stable forms of plant coexistence by inducing alternative stable states. Our findings highlight that the inclusion of plant–soil interactions may fundamentally alter the predictions on the structure and functioning of above-ground ecosystems. The scenarios presented in this study can be used to formulate hypotheses about the ways soil community effects may influence plant competition that can be tested with empirical studies. This will advance our understanding of the role of plant–soil feedback in ecological communities.     

Spatial and temporal dynamics of hotspots of enzyme activity in soil as affected by living and dead roots—a soil zymography analysis

Aims

Hotspots of enzyme activity in soil strongly depend on carbon inputs such as rhizodeposits and root detritus. In this study, we compare the effect of living and dead Lupinus polyphyllus L. roots on the small-scale distribution of cellulase, chitinase and phosphatase activity in soil.

Methods

Soil zymography, a novel in situ method, was used to analyze extracellular cellulase, chitinase and phosphatase activity in the presence of i. living L. polyphyllus roots prior to shoot cutting and ii. dead/dying roots 10, 20 and 30 days after shoot cutting.

Results

After shoot cutting, cellulase and chitinase activities increased and were highest at the root tips. The areas of high cellulase and phosphatase activity extend up to 55 mm away from the root. Moreover, we observed microhotspots of cellulose, chitinase, and phosphatase activity up to 60 mm away from the next living root. The number and activity of microhotspots of chitinase activity was maximal 10 days after shoot cutting.

Conclusions

The study showed that young root detritus stimulates enzyme activities stronger than living roots. Soil zymography allowed identification of microhotspots of enzyme activity up to several cm away from living and dying roots, which most likely were caused by arbuscular mycorrhizal fungi.     

Biodegradation of α- and β-endosulfan by soil bacteria

Extensive applications of persistent organochlorine pesticides like endosulfan on cotton have led to the contamination of soil and water environments at several sites in Pakistan. Microbial degradation offers an effective approach to remove such toxicants from the environment. This study reports the isolation of highly efficient endosulfan degrading bacterial strains from soil. A total of 29 bacterial strains were isolated through enrichment technique from 15 specific sites using endosulfan as sole sulfur source. The strains differed substantially in their potential to degrade endosulfan in vitro ranging from 40 to 93% of the spiked amount (100 mg l⁻¹). During the initial 3 days of incubation, there was very little degradation but it got accelerated as the incubation period proceeded. Biodegradation of endosulfan by these bacteria also resulted in substantial decrease in pH of the broth from 8.2 to 3.7 within 14 days of incubation. The utilization of endosulfan was accompanied by increased optical densities (OD₅₉₅) of the broth ranging from 0.511 to 0.890. High performance liquid chromatography analyses revealed that endosulfan diol and endosulfan ether were among the products of endosulfan metabolism by these bacterial strains while endosulfan sulfate, a persistent and toxic metabolite of endosulfan, was not detected in any case. The presence of endosulfan diol and endosulfan ether in the bacterial metabolites was further confirmed by GC-MS. Abiotic degradation contributed up to 21% of the spiked amount. The three bacterial strains, Pseudomonas spinosa, P. aeruginosa, and Burkholderia cepacia, were the most efficient degraders of both α- and β-endosulfan as they consumed more than 90% of the spiked amount (100 mg l⁻¹) in the broth within 14 days of incubation. Maximum biodegradation by these three selected efficient bacterial strains was observed at an initial pH of 8.0 and at an incubation temperature of 30°C. The results of this study may imply that these bacterial strains could be employed for bioremediation of endosulfan polluted soil and water environments.     

Epichloë gansuensis endophyte-infection alters soil enzymes activity and soil nutrients at different growth stages of Achnatherum inebrians

Plant and Soil - Epichloë endophytes are a unique model system for the study of the linkages between organisms above and belowground in ecosystems. However, the impact of Epichloë...