Organic acid behaviour in a calcareous soil implications for rhizosphere nutrient cycling

Calcareous soils are frequently characterized by the low bioavailability of plant nutrients. Consequently, many vascular plant species are unable to successfully colonize calcareous sites and the floristic composition of calcareous and acid silicate soils has been shown to differ markedly. The root exudation of oxalate and citrate has been suggested to play a pivotal role in same nutrient acquisition mechanisms operating in calcareous soils. The aim of this study was therefore to investigate the nutrient extraction efficiency of three individual organic acids commonly identified in root exudates, i.e. citric, malic and oxalic acid. Our results clearly demonstrate the context dependent nature of nutrient release by organic acids. The degree of P extraction was highly dependent on which organic acid was added, their concentration and pH, and their contact time with the soil. P is generally more efficiently extracted by organic acids at a high pH and follows the series oxalate>citrate>malate. The opposite relationship between pH and extraction efficiency was apparent for most other cations examined (e.g. Zn, Fe), which are more efficiently extracted by organic acids at low pH. A serious constraint to the ecological importance of organic acid exudation in response to P deficiency is, however, their very low P mobilization efficiency. For every mol of soil P mobilized, 1000 mol of organic acid has to be added. It can, however, be speculated that in a calcareous soil with extremely low P concentrations it is still beneficial to the plants to exude organic acids in spite of the seemingly high costs in terms of carbon.     

The impact of trees, ectomycorrhiza and potassium availability on simple organic compounds and dissolved organic carbon in soil

The presence of tree roots and symbiotic mycorrhizal fungi is recognized to have a substantial impact on carbon dynamics in soils. In this study the effect of Pinus sylvestris seedlings and the ectomycorrhizal fungus Hebeloma crustuliniforme on a number of biogeochemical variables, mainly related to labile carbon pools was investigated. The impact of K limitation as a potential regulatory factor was also examined. Columns filled with E horizon ±plants and ±mycorrhizal fungi were incubated for 18.5 months. The results demonstrate that plants, as well as mycorrhizal fungi, significantly increased the concentrations of some simple organic acids, including oxalate, in soil solution. Observations for dissolved organic carbon were slightly contradictory but the cumulative amount found in drainage water was ∼20% higher in planted versus non-planted columns. Soil from planted treatments also showed more rapid mineralisation kinetics for oxalate. However carbon utilization (mineralisation vs. biomass) of oxalate and glucose by the soil microbial biomass was less influenced by plants. At harvest a component integration study of soil autotrophic and heterotrophic respiration was performed which revealed that both plant and mycorrhiza had a positive effect on the heterotrophic respiration. Potassium omission had little effect on the variables studied with the exception of the maximum mineralisation rate for oxalate, which increased when K was withdrawn. The results are discussed in the context of the dynamics of labile soil carbon pools and ecosystem C fluxes.     

Effect of active roots on the decomposition of soil organic materials

Summary The effect of one form of soil organic matter, such as living roots or root exudates on another form of soil organic matter, such as dead roots or incorporated litter and litter leachates, has been studied from various perspectives over the last 25 years. The effect seems to be either positive (priming) or negative (conserving). The present review concentrates on the conserving effect, measured as a decrease in ¹⁴CO₂ released, in both field and greenhouse/growth chamber studies. The field experiments suggested that certain physical conditions in the soil, such as less available moisture or restricted aeration which led to lower microbial activity, explained the conserving effect of living roots on soil organic matter. Although more detailed greenhouse/growth chamber studies confirmed the conserving effect per se, it appears that biological rather than physical factors could better explain the reduction in the rate of decomposition of ¹⁴C-labelled plant residues in the presence of roots. However, a complex picture has emerged through a variety of postulates, all proposed in attempts to explain the conserving effect. Finally, the most recent studies have argued that the decrease in decomposition of labelled organic matter in planted soil is probably more apparent than real. A decrease in respired ¹⁴CO₂ could be explained by an incorporation of ¹⁴C derived from old roots into the rhizosphere microbial populations of the living roots. To make any further progress on the fundamental question of how soil organic matter moves along its continuum from a living to a refractory state, the microenvironment needs to be examined at periodic intervals. New developments in improved histochemical and electron-probe microanalyses look promising.LRS Contribution no. 3878970     

Plant capture of free amino acids is maximized under high soil amino acid concentrations

Free amino acids (AA's) represent a significant source of available N for some plants and soil microorganisms. It can be expected, however, that significant competition will exist between plants and microorganisms for this organic N resource. Our study indicated that microbial capture and utilization of glycine was very rapid at a range of soil solution concentrations (0.1 μM to 10 mM) indicating that significant competition will exist between roots and soil microorganisms. Plant capture of free AA's was maximal at high soil solution concentrations where microbial utilization was slowest. Our results suggest that plant capture of soil dissolved organic N may primarily occur in organic rich patches in soil where concentrations of free AA's are high.     

Organic acids in root exudates and soil solution of Norway spruce and silver birch

Here we report on low molecular weight organic acids in root exudates and soil solutions of Norway spruce and silver birch grown in rhizoboxes, sterile microcosms and the field. Monocarboxylic acids dominated in all three experimental systems. Formic, shikimic and oxalic acids were found in both spruce and birch microcosms. Fumaric acid was exclusive for spruce, while lactic, malonic, butyric and phthalic acids were only found in the birch microcosms. In spruce rhizoboxes oxalic, lactic, formic, butyric and pthalic acids were found. In addition, citric, adipic, propionic, succinic and acetic acids were observed in the rhizosphere of birch. Behind root windows in the field, only oxalic and lactic acids were found in the rhizosphere of spruce fine roots, whereas also formic and phthalic were observed close to birch fine roots, all at low concentrations. The rhizosphere of mycorrhizal short roots of birch contained butyric acid along with the acids observed for birch fine roots. Our results emphasise that characteristics of both the trees e.g. species, developmental stage, root density, mycorrhizal status, and the experimental system, i.e. growth conditions are important for the composition and the amount of organic acids. We conclude that the rhizosphere of birch contains more organic acids at higher concentrations than spruce.     

Stimulation of root exudation of rice seedlings by Azospirillum strains: carbon budget under gnotobiotic conditions

Summary The association of rice seedlings (cv. Delta) with different strains of Azospirillum was studied under monoxenic conditions in the dark. Axenic 3-day-old seedlings were obtained on a C- and N-free medium and inoculated with 6 · 10⁷ bacteria per plant in a closed vial. Seven days later, different components of a carbon budget were evaluated on them and on sterile controls: respired CO₂, carbon of shoot and roots, bacterial and soluble carbon in the medium. Two strains (A. lipoferum 4B and A. brasilense A95) isolated from the rhizosphere of rice caused an increase in exudation, + 36% and + 17% respectively compared with sterile control. Shoot carbon incorporation and respiration were reduced by inoculation. A third strain (A. brasilense R07) caused no significant change in exudation. A. lipoferum B7C isolated from maize did not stimulate rice exudation either. We further investigated a possible effect of nitrogen fixation on this phenomenon: inhibition of nitrogen fixation by 10% C₂H₂ did not modify the extent of C exudation by rice associated with A. lipoferum 4B or with the non-motile A. lipoferum 4T.     

不同树龄苹果园土壤微生物生态特征研究

为了评价苹果园种植年限对土壤微生物和土壤肥力的影响,选择不同种植年限的果园,利用平板培养和脂肪酸甲酯(FAME)图谱法研究不同树龄苹果园土壤微生物生态特征变化。结果表明,壮龄果园的土壤微生物活性最高,是幼龄果园的1.23倍,老龄果园的1.37倍;幼龄和壮龄果园微生物生物量差异不显著,但均远大于老龄果园,约为老龄果园的3倍;随着树龄增大,细菌/真菌的比例下降,放线菌的含量也随树龄增大而降低;土壤微生物的多样性随树龄增大先增高后降低,以壮龄果园的微生物多样性为最好。果树的长期种植引起了土壤理化性质及土壤微生物数量、活性、多样性及种群结构的改变。通过研究不同树龄苹果园的土壤微生物生态特征,以期能为根据不同树龄果园土壤状况进行合理管理和解决重茬障碍提供一定的依据。     

Organic manure stimulates biological activity and barley growth in soil subject to secondary salinization

A pot experiment was performed to compare the impact of organic manure on soil enzymatic activity, respiration rate and the growth of two barley cultivars (Hordeum vulgare L.) differing in their salt tolerance under a simulated salinized environment. A plastic pot with a hole (2 cm in diameter) in the center of bottom was filled with an anthropogenic (paddy) soil and placed in a porcelain container containing NaCl solution (3.0 g L⁻¹) such that a secondary salinization process was simulated via upward capillary water movement along the soil profile. A treatment with neither organic manure nor simulated soil salinization was taken as a control (CK1). The organic manure was applied either inside or outside rhizobag made of nylon cloth (40 μm of pore size). The soil was treated with: 20 g kg⁻¹ rice straw (RS), 20 g kg⁻¹ pig manure (PM), or 10 g kg⁻¹ rice straw plus 10 g kg⁻¹ pig manure (RS+PM). No organic manure was added in an additional control treatment (CK2). The results indicated that the placement of organic manure both inside and outside rihzobags significantly increased the activity of urease, alkaline phosphatase and dehydrogenase, as well as respiration rate in both rhizosphere and bulk soils. Also, nutrient uptake by barley plants was enhanced in the treatments with organic manure amended either inside or outside rhizobags. The activity of these enzymes along with the respiration rate was higher in rhizosphere than in non-rhizosphere when organic manure was supplied inside rhizobags, while the opposite was found in the case of manure incorporated outside rhizobags. Among all the treatments, RS+PM treatment had most significant stimulating effects on enzymatic and microbial activity and shoot dry weight of barley, followed by PM and RS. Moreover, more significant stimulating effects on both enzyme activity and plant growth were achieved in the treatments with manure amended inside rhizobags than outside rhizobags. The results of the present study confirmed the view that incorporation of organic manure especially into soil-root zones is an effective low-input agro-technological approach to enhancing soil fertility and minimizing phytotoxicity induced by secondary salinization.     

Soil solution and extractable soil nitrogen response to climate change in two boreal forest ecosystems

Several studies show that increases in soil temperature result in higher N mineralization rates in soils. It is, however, unclear if additional N is taken up by the vegetation or accumulates in the soil. To address this question two small, forested catchments in southern Norway were experimentally manipulated by increasing air temperature (+3°C in summer to +5°C in winter) and CO₂ concentrations (+200 ppmv) in one catchment (CO₂T-T) and soil temperature (+3°C in summer to +5°C in winter) using heating cables in a second catchment (T-T). During the first treatment year, the climate treatments caused significant increases in soil extractable NH₄ under Vaccinium in CO₂T-T. In the second treatment year extractable NH₄ in CO₂T-T and NO₃ in T-T significantly increased. Soil solution NH₄ concentrations did not follow patterns in extractable NH₄ but changes in soil NO₃ pools were reflected by changes in dissolved NO₃. The anomalous behavior of soil solution NH₄ compared to NO₃ was most likely due to the higher NH₄ adsorption capacity of the soil. The data from this study showed that after 2 years of treatment soil inorganic N pools increased indicating that increases in mineralization, as observed in previous studies, exceeded plant demand and leaching losses.     

有机污染物根际胁迫及根际修复研究进展

根际环境及根际微生物是植物降解有毒有害有机污染物的基础。污染土壤植物修复的纵深研究产生了根际修复新技术。通过总结近20年来有机污染物胁迫的根际效应的研究,探讨了有机物污染士壤根际修复的可能性,为加强有机污染物在环境中的迁移、调控研究及土壤有机物污染的原位修复提供有利信息。     

Interactions of Azospirillum spp. in soils: a review

 This review summarizes and discusses the current knowledge and the, as yet, unanswered questions on the interactions of Azospirillum spp. in bulk soil (but not in the rhizosphere). It contains sections on the isolation of these bacteria from tropical to temperate soils, and on their short- and long-term persistence in bulk soil. The interactions of these bacteria with soil particles and minerals such as clay, sand and Ca, and the effect of soil pH, soil redox potential, and the cation exchange capacity of the soil on them is demonstrated. Data is presented on the distribution of Azospirillum spp. in soils, on their production of fibrillar material essential for anchoring the cells to soil particles, on the effects of soil irrigation, and of external soil treatments, and on the effect of soil C and C used in bacterial inoculants on the cells. It shows that root exudates possibly govern bacterial motility in the soil. Finally, the effect of pesticide applications, the relationships with other soil microorganisms such as Bdelovibrio spp., Bradyrhizobium spp., and phages, and the potential use of a community-control model of Azospirillum spp. in soil and in the rhizosphere is suggested. Received: 11 November 1998     

Highlights and perspectives of soil biology and ecology research in China

As seen for the publications in several distinguished soil related journals, soil biology and ecology is booming in China in recent years. This review highlights the major findings of the soil biology and ecology projects conducted in China during the past two decades. Special attention is paid on the responses of soil biota to environmental change, and the roles of soil functional groups in C transformation, nutrient cycling and pollution remediation. We also point out the future challenges facing the Chinese soil biologists and soil ecologists. In the future, more systematic studies rather than scattered case studies are needed, more controlled field experiments rather than short-term laboratory studies should be encouraged. Besides, we need to focus more on the linkage between aboveground and belowground organisms, the interactions between different groups of soil food web, and the coupling of observation with modeling. It is essential to employ the state-of-the-art technology in research of soil biology and ecology because to answer the emerging scientific questions relies heavily on the development of new technology. Our ultimate goals are to push forward the research on soil biology and ecology in China and to encourage the interaction and collaboration between the international community and research groups in China.     

Rates of in situ carbon mineralization in relation to land-use, microbial community and edaphic characteristics

Plant-derived carbon compounds enter soils in a number of forms; two of the most abundant being leaf litter and rhizodeposition. Our knowledge concerning the predominant controls on the cycling of leaf litter far outweighs that for rhizodeposition even though the constituents of rhizodeposits includes a cocktail of low molecular weight organic compounds which represent a rapidly cycling source of carbon, readily available to soil microbes. We determined the mineralization dynamics of a major rhizodeposit, glucose, and its relationship to land-use, microbial community and edaphic characteristics across a landscape in the southeastern United States. The landscape consists of cultivated, pasture, pine plantation, and hardwood forest sites (n = 3). Mineralization dynamics were resolved in both winter and summer using an in situ¹³C-glucose pulse-chase approach. Mineralization rates of the labeled glucose decline exponentially across the 72 h measurement periods. This pattern and absolute mineralization rates are consistent across seasons. An information-theoretic approach reveals that land-use is a moderately strong predictor of cumulative glucose mineralization. Measures assessing the size, activity, and/or composition of the microbial community were poor predictors of glucose mineralization. The strongest predictor of glucose mineralization was soil-extractable phosphorus. It was positively related to glucose mineralization across seasons and explained 60% and 48% of variation in cumulative glucose mineralization in the summer and winter, respectively. We discuss potential mechanisms underlying the relationship between soil phosphorus and glucose mineralization. Our results suggest that specific soil characteristics often related to land-use and/or land-management decisions may be strong predictors of glucose mineralization rates across a landscape. We emphasize the need for future research into the role of soil phosphorus availability and land-use history in determining soil organic carbon dynamics.     

Soil properties are key determinants for the development of exudate gradients in a rhizosphere simulation model

The rhizosphere is increasingly recognised as a key compartment of the soil. The aim of this work is to explore the role of soil, plant and solute properties on the development of exudate gradients in the rhizosphere. To this end, a numerical model of solute movements around a single root was developed and analysed for its sensitivity to several parameters including soil water content, lifetime of exudates, root exudation rates, among others. To keep the model as simple as possible, water content is assumed to be homogenously distributed around the root. In the sensitivity analysis, the thickness of the simulated rhizosphere (e.g. the maximum distance to which exudates moved) ranged between 0.2 and 61 mm with an average of about 7 mm, which is of the same order of magnitude as published values. Similarly, the time needed to reach steady state between exudation and degradation of exudates ranged from a few hours to more than 100 days with an average of 62 days. In the sensitivity analysis, adsorption properties solute's lifetime and soil water content appears to be key determinants of both the extent of the rhizosphere and the time to steady state and therefore should have a fundamental role in determining the degree of interactions between neighbouring roots and with other soil organisms.     

Organic and inorganic soil phosphates and acid phosphatase activity in the rhizosphere of 80-year-old Norway spruce [Picea abies (L.) Karst.] trees

Summary Inorganic and organic phosphates (P) were measured in bulk soil, rhizosphere soil and mycorrhizal rhizoplane soil of Norway spruce. Various methods of P extraction and estimation were compared. In addition, acid phosphatase activity and mycelial hyphae length were determined. In soil solutions from various locations, about 50% (range 35%–65%) of the total P was present as organic P. Compared to the bulk soil, the concentrations of readily hydrolysable organic P were lower in the rhizosphere soil and in the rhizoplane soil; this difference was particularly marked in the humus layer. In contrast, the concentrations of inorganic P either remained unaffected or increased. A 2- to 2.5-fold increase was found in the activity of acid phosphatase in the rhizoplane soil in comparison to the bulk soil. There was a positive correlation (r = 0.83^***) between phosphatase activity and the length of mycelial hyphae. The results stress the role of organic P and of acid phosphatase in the rhizosphere in the P uptake by mycorrhizal roots of spruce trees grown on acid soils.     

Tillage and wind effects on soil CO2 concentrations in muck soils

Rising atmospheric carbon dioxide (CO₂) concentrations from agricultural activities prompted the need to quantify greenhouse gas emissions to better understand carbon (C) cycling and its role in environmental quality. The specific objective of this work was to determine the effect of no-tillage, deep plowing and wind speeds on the soil CO₂ concentration in muck (organic) soils of the Florida Everglades. Miniature infrared gas analyzers were installed at 30 cm and recorded every 15 min in muck soil plowed with the Harrell Switch Plow (HSP) to 41 cm and in soil Not Tilled (NT), i.e., not plowed in last 9 months. The soil CO₂ concentration exhibited temporal dynamics independent of barometric pressure fluctuations. Loosening the soil resulted in a very rapid decline in CO₂ concentration as a result of “wind-induced” gas exchange from the soil surface. Higher wind speeds during mid-day resulted in a more rapid loss of CO₂ from the HSP than from the NT plots. The subtle trend in the NT plots was similar, but lower in magnitude. Tillage-induced change in soil air porosity enabled wind speed to affect the gas exchange and soil CO₂ concentration at 30 cm, literally drawing the CO₂ out of the soil resulting in a rapid decline in the CO₂ concentration, indicating more rapid soil carbon loss with tillage. At the end of the study, CO₂ concentrations in the NT plots averaged about 3.3% while that in the plowed plots was about 1.4%. Wind and associated aerodynamic pressure fluctuations affect gas exchange from soils, especially tilled muck soils with low bulk densities and high soil air porosity following tillage.     

A sensitive method for measuring cyanide and cyanogenic glucosides in sand culture and soil

Summary Sensitive methods for measuring cyanide and cyanogenic glucosides in soil and sand culture have been developed. A microdiffusion technique is described which depends on the enzymic conversion of linamarin and lotaustralin to HCN, its release following acidification and incubation, and its detection in NaOH. Conditions for hydrolysis and HCN recovery have been optimised. The cyanide content of a silt loam soil (under non-cyanogenic wheat) was 5.47 nmol cyanide g^–1 air-dried soil whilst that in an organic soil under the cyanogenic bracken, Pteridium aqgilinum, was 12.2 nmol g^–1. Exudation of cyanogenic glucosides by linseed, Linum usitatissimum, was measured in plant growth tubes containing sand and a nutrient medium. Sterile plants exuded an average of 6.88 nmol glucosides plant^–1 week^–1 whilst, in contaminated tubes, the level fell to 4.72 nmol. Analysis of plant roots on each sampling occasion showed that 6.88 nmol was, on average, equivalent to 16.15% of the total root content of cyanogenic glucosides. There was a low but positive correlation between fresh weight of plant roots and the level of exuded glucosides. There was no evidence that plant roots produced free HCN.     

Fitting plants to soil through mycorrhizal fungi: Mycorrhiza effects on plant growth and soil organic matter

Summary Vesicular-arbuscular mycorrhizal (VAM) fungi affect diverse aspects of plant form and function. Since mycorrhiza-mediated changes in host-plant responses to root colonization by different VAM fungi vary widely, it is important to assess each endophyte for each specific effect it can elicit from its host as part of the screening process for effectiveness. Three species of VAM fungi and a mixture of species were compared with non-VAM controls for their effects on soil organic matter contents and on nutrition and morphology in two varieties (native and hybrid) of corn (Zea mays L.) and one of sunflower (Helianthus annuus L.) in P-sufficient and N-deficient soil in pot cultures. Differences in soil organic matter due to the fungal applications were highly significant with all host plants. Native corn responded more to VAM colonization than the hybrid did; differences in treatments were significant in leaf area, plant biomass, and root: shoot ratio in the former, but not in the latter. Responses in the sunflower were similar to those in the native corn. Significant VAM treatment-related differences in shoot N and P contents were not reflected in shoot biomass, which was invariant. Correlations between plant or soil parameters and the intensity of VAM colonization were found only in soil organic matter with the native corn, in specific leaf area in the hybrid corn, and in plant biomass in the sunflower. The presence of the different endophytes and not the intensity of colonization apparently elicited different host responses.