Evaluation of the use of a model rhizodeposition technique to separate root and microbial respiration in soil

A model rhizodeposition technique to estimate the root and microbial components of ¹⁴C soil/root respiration in pulse-labelling experiments is described. The method involves the injection of model rhizodeposits, consisting of ¹⁴C-labelled glucose, root extract or root cell wall material, into the rooted soil of an unlabelled plant, simultaneously with the pulse-labelling of a separate but similar plant with ¹⁴CO₂. In a growth chamber experiment with 30 day old wheat and barley the contribution of direct root respiration to ¹⁴C soil/root respiration over a 26 day period after labelling was estimated 89–95%. Estimates of direct root respiration in field-grown wheat and barley at different development stages in most cases accounted for at least 75% of ¹⁴C soil/root respiration over a 21 day period after labelling. The mineralization rate of injected ¹⁴C-glucose was positively correlated with the concentration of glucose-C established in soil. The use of the method in rhizosphere carbon budget estimations is evaluated. Communication No. 73 of the Dutch Programme on Soil Ecology of Arable Farming Systems. Communication No. 73 of the Dutch Programme on Soil Ecology of Arable Farming Systems.     

Low-level herbivory by root-knot nematodes (Meloidogyne incognita) modifies root hair morphology and rhizodeposition in host plants (Hordeum vulgare)

Low amounts of root infestation by plant parasitic nematodes are suggested to increase nutrient supply and in turn enhance microbial activity and net mineralization rate in the rhizosphere. These effects are generally related to “leakage” of plant-derived metabolites from damaged roots. Besides leakage, the present study examines other nematode–host interactions such as alterations in root exudation and morphology, which were almost not considered yet. This includes undamaged root parts in order to assess systemic plant response. The root-knot nematode Meloidogyne incognita (Kofoid and White 1919; Chitwood 1949) and barley (Hordeum vulgare L. cv. Europa) was used as model system. Host plants were grown in mini-rhizotrons inoculated with 0, 2,000, 4,000 or 8,000 M. incognita for 4 weeks. Root morphology, rhizodeposition (sugars, carboxylates, amino acids), and rhizosphere microbial communities (PLFAs) were assessed. In treatments with 4,000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. Generally, an enhanced release of plant metabolites (sugars, carboxylates, amino acids) from the apical root zone occurred 1 week after inoculation with 4,000 and 8,000 M. incognita, indicating root leakage. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. These systemic changes in root morphology likely contributed to the increased sugar exudation in uninfected roots in all nematode treatments at 3 weeks after inoculation. Root-knots formed a separate microhabitat within the root-system. They were characterised by decreased rhizodeposition and increased fungal to bacterial ratio in the adhering rhizosphere soil. The present study provides the first evidence that, apart from leakage, nematode root herbivory at background levels induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance.     

Immobilization and mobilization of labelled sulphur in relation to soil arylsulphatase activity in rhizosphere soil of field-grown rape,barley and fallow

During plant growth, rhizosphere soils from fallow, barley (Hordeum vulgare L. cv Esterel) and rape (Brassica napus L. cv Capitole) grown in a calcareous soil were sampled 5 times (every fortnight) from May to July 2001 at plant maturity. In order to estimate the impact of C derived from photosynthesis, the aerial parts of rape and barley in an area of 1 m² were cut off about 2 cm from the soil surface, and left a fortnight before each sampling. Both soil arylsulphatase activity and a 1-week immobilization of S fertilizer in the sampled soils were then measured. The immobilization of S fertilizer was higher in fallow, followed by barley and rape rhizosphere soil. A strong positive linear correlation (r ²=0.71, P<0.001) was found between soil arylsulphatase activity and S fertilizer immobilized. Conversely, the mobilization of endogenous organic ³⁵S (obtained after leaching free and adsorbed SO₄ ^2–-³⁵S by 0.009 M Ca(H₂PO₄)₂) in the rhizosphere soil of each plant cover pooled at the end of the 5 samplings and materialized by ryegrass (Lolium perenne L. cv Massa) ³⁵S uptake, was about 3 and 2 times higher, respectively, in rape and barley than in fallow rhizosphere soil. Accordingly, strong inverse polynomial relationships were observed between soil arylsulphatase activity and ³⁵S uptake by the whole plant (r ²=0.904, P<0.02) and roots (r ²=0.970, P<0.01) of ryegrass. Plant cuttings affected both the immobilization and mobilization of S. It is concluded that the turnover of S freshly immobilized in rape rhizosphere soil was relatively high. Therefore, rape as a preceding crop in the rotations may have a beneficial effect by increasing S availability on the succeeding crop.     

Root-colonization ability of antagonistic Streptomyces griseoviridis

Root-colonization ability of Streptomyces griseoviridis was tested on turnip rape (Brassica rapa subsp. oleifera) and carrot (Daucus carota) by the plate test and the sand-tube method. In the plate test, colonized root length of total root length was highly significantly greater for turnip rape roots (72%) from those for carrot roots (1%). In the sand-tube method, root-colonization ability was examined in nonsterile soil, and no water was added after sowing. Seeds were treated with spores of S. griseoviridis or the biofungicide Mycostop. Roots were cut into 2-cm segments, and the root segments and the rhizosphere soil were studied separately. Root-colonization frequencies and population densities of the microbe in the rhizosphere soil indicated that S. griseoviridis successfully colonized turnip rape but weakly colonized carrot. Root-colonization of turnip rape is accounted for as proliferation of S. griseoviridis in the rhizosphere of turnip rape seedlings and is not due to the movement of microbe through the rhizosphere by water infiltration.     

Bacterial inoculation of maize affects carbon allocation to roots and carbon turnover in the rhizosphere

To assess the influence of bacteria inoculation on carbon flow through maize plant and rhizosphere,¹⁴C allocation after¹⁴CO₂ application to shoots over a 5-day period was determined. Plants were grown on C- and N-free quartz sand in two-compartment pots, separating root and shoot space. While one treatment remained uninoculated, treatments two and three were inoculated withPantoea agglomerans (D5/23) andPseudomonas fluorescens (Ps I A12), respectively, five days after planting. Bacterial inoculation had profound impacts on carbon distribution within the system. Root/rhizosphere respiration was increased and more carbon was allocated to roots of plants being inoculated. After five days of¹⁴CO₂ application, more ethanol-soluble substances were found in roots of inoculated treatments and lower rhizodeposition indicated intensive C turnover in the rhizosphere. In both inoculated treatments the intensity of photosynthesis measured as net-CO₂-assimilation rates were increased when compared to the uninoculated plants. However, high C turnover in the rhizosphere reduced shoot growth of D5/23 inoculated plants, with no effect on shoot growth of Ps I A12 inoculated plants. A separation of labeled compounds in roots and rhizodeposition revealed that neutral substances (sugars) constituted the largest fraction. The relative fractions of sugars, amino acids and organic acids in roots and rhizodeposition suggest that amino acid exudation was particularly stimulated by bacterial inoculation and that turnover of this substance group is high in the rhizosphere.     

How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth?

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of ¹⁴C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of ¹⁴C to roots. In parallel, N fertilisation increased the root soluble ¹⁴C and the ¹⁴C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).     

Rates of rhizodeposition and ammonium depletion in the rhizosphere of axenic oat roots

Summary A multiple split root chamber and artificial soil were developed which allowed for maintenance of axenic conditions and for the isolation of soil from specific regions of single roots. A sterile minirhizotron was used to measure patterns and rates of root extension under sterile conditions. Carbon and nitrogen distributions in the rhizosphere of sterile oat roots were measured in combination with rates of root elongation to calculate specific rates of rhizodeposition and ammonium nitrogen uptake. The highest rates of rhizodeposition C production and N depletion occurred at the root tip (first day segment). Rhizodeposited soluble and insoluble C compounds represented up to 50% of the standing root biomass C. Within 48 hours after root entry, levels of rhizosphere ammonium-N decreased by 40–50%. The results were summarized in a simple model of root growth, rhizodeposition, and NH ₄ ⁺ −H uptake. From a dissertation by the senior author submitted to the Academic Faculty of Colorado State University in partial fulfillment of the requirements for the Ph.D. degree.     

Rhizosphere interactions, carbon allocation, and nitrogen acquisition of two perennial North American grasses in response to defoliation and elevated atmospheric CO2

Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Plant C allocation patterns and rhizosphere interactions can also be affected by rising atmospheric CO(2) concentrations, which in turn could influence plant and microbial responses to defoliation. We studied two widespread perennial grasses native to rangelands of western North America to test whether (1) defoliation-induced enhancement of rhizodeposition would stimulate rhizosphere N availability and plant N uptake, and (2) defoliation-induced enhancement of rhizodeposition, and associated effects on soil N availability, would increase under elevated CO(2). Both species were grown at ambient (400 μL L(-1)) and elevated (780 μL L(-1)) atmospheric [CO(2)] under water-limiting conditions. Plant, soil and microbial responses were measured 1 and 8 days after a defoliation treatment. Contrary to our hypotheses, we found that defoliation and elevated CO(2) both reduced carbon inputs to the rhizosphere of Bouteloua gracilis (C(4)) and Pascopyrum smithii (C(3)). However, both species also increased N allocation to shoots of defoliated versus non-defoliated plants 8 days after treatment. This response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and reduced allocation of post-defoliation assimilate to roots. In contrast, B. gracilis increased allocation of post-defoliation assimilate to roots, and did not exhibit defoliation-induced reductions in root biomass or N content. Our findings highlight key differences between these species in how post-defoliation C allocation to roots versus shoots is linked to shoot N yield, but indicate that defoliation-induced enhancement of shoot N concentration and N yield is not mediated by increased C allocation to the rhizosphere.     

Carbon flow in the rhizosphere: carbon trading at the soil–root interface

The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil–root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include: (1) indirect, fortuitous root exudate recapture as part of the root’s C and N distribution network, (2) direct re-uptake to enhance the plant’s C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for inter-root and root–microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

Moving Waves of Bacterial Populations and Total Organic Carbon along Roots of Wheat

Abstract To determine if spatial variation in soluble carbon sources along the root coincides with different trophic groups of bacteria, copiotrophic and oligotrophic bacteria were enumerated from bulk soil and rhizosphere samples at 2 cm intervals along wheat roots 2, 3, and 4 weeks after planting. There was a moderate rhizosphere effect in one experiment with soil rich in fresh plant debris, and a very pronounced rhizosphere effect in the second experiment with soil low in organic matter. We obtained wavelike patterns of both trophic groups of bacteria as well as water-soluble total organic carbon (TOC) along the whole root length (60 or 90 cm). TOC concentrations were maximal at the root tip and base and minimal in the middle part of the roots. Oscillations in populations of copiotrophic and oligotrophic bacteria had two maxima close to the root tip and at the root base, or three maxima close to the tip, in the middle section, and at the root base. The location and pattern of the waves in bacterial populations changed progressively from week to week and was not consistently correlated with TOC concentrations or the location of lateral root formation. Thus, the traditional view that patterns in bacterial numbers along the root directly reflect patterns in exudation and rhizodeposition from several fixed sources along the root may not be true. We attributed the observed wavelike patterns in bacterial populations to bacterial growth and death cycles (due to autolysis or grazing by predators). Considering the root tip as a moving nutrient source, temporal oscillations in bacterial populations at any location where the root tip passed would result in moving waves along the root. This change in concept about bacterial populations in the rhizosphere could have significant implications for plant growth promotion and bioremediation. Received: 11 May 1998; Accepted: 4 November 1998     

Decomposer biomass in the rhizosphere to assess rhizodeposition

Quantification of the organic carbon released from plant roots is a challenge. These compounds of rhizodeposition are quickly transformed into CO₂ and eventually bacterial biomass to be consumed by bacterivores (protozoa and nematodes). Microbes stimulate rhizodeposition several-fold so assays under sterile conditions give an unrealistic value. Quantifying bacterial production from ³H-thymidine incorporation falls short in the rhizosphere and the use of isotopes does not allow clear distinction between labeled CO₂ released from roots or microbes. We reduced rhizodeposition in 3–5 week old barley with a 2 week leaf aphid attack and found that biomass of bacterivores but not bacteria in the rhizosphere correlated with plant–induced respiration activity belowground. This indicated top-down control of the bacteria. Moreover, at increasing density of aphids, bacterivore biomass in the rhizosphere decreased to the level in soil unaffected by roots. This suggests that difference in bacterivore biomass directly reflects variations in rhizodeposition. Rhizodeposition is estimated from plant-induced increases in bacterial and bacterivore biomass, and yield factors, maintenance requirements, and turnover rates from the literature. We use literature values that maximize requirements for organic carbon and still estimate the total organic rhizodeposition to be as little as 4–6% of the plant-induced respiration belowground.     

Accelerated Mineralization of Two Organophosphate Insecticides in the Rhizosphere

Numerous xenobiotic compounds, including the organophosphate insecticides O, O-diethyl-O-(2-isopropyl-6-methyl-4-pyrimidinyl) phosphorothioate (diazinon) and O, O-diethyl-O-p-nitrophenyl phosphorothioate (parathion), appear to be degraded in the soil environment by an initial cometabolic attack. Comparing the mineralization rates of radiolabeled diazinon and parathion in root-free and in rhizosphere soil, we tested our hypothesis that, because of the presence of root exudates, the rhizosphere is an especially favorable environment for such co-metabolic transformations. The insecticides were added individually at 5 μg/g to sealed flasks containing either soil permeated by the root system of a bush bean plant or identical soil without roots. Periodically, the flask atmospheres were flushed through traps and the evolved ¹⁴CO₂ was quantitated. Bush bean plant roots without associated rhizosphere microorganisms failed to produce a significant amount of ¹⁴CO₂. During 1 month of incubation, rhizosphere flasks mineralized 12.9 and 17.9% of the added diazinon and parathion radiocarbon, respectively, compared to 5.0 and 7.8% by the soil without roots. The mineralization of parathion but not of diazinon was stimulated in a similar manner when soil without roots was repeatedly irrigated with a root exudate produced in aseptic solution culture. Viable counts of microorganisms on soil extract agar were not significantly altered by root permeation or by root exudate treatment of the soil, leaving population selection and/or enhanced cometabolic activity as the most plausible interpretations for the observed stimulatory effects. Rhizosphere interactions may substantially shorten the predicted half-lives of some xenobiotic compounds in soil.     

Predicted microbial biomass in the rhizosphere of barley in the field

Summary Laboratory data for the loss of root material by barley and field data for the growth of barley plants in Syria and in England have been combined to predict the amount of material lost by barley roots during a season, and to predict the resulting microbial biomass in the rhizosphere. The predicted microbial biomass C in the rhizosphere ranged from 10–34% of the total plant biomass C depending mainly upon the value used for rate of loss of root material. Total loss of root material predicted during a season in England constituted 7.7–25.4 percent of C fixed by photosynthesis. The major assumptions made in these calculations are considered, and the predicted values discussed in relation to reported values for soil microbial biomass, CO₂ fluxes from soil and associative nitrogen fixation.     

Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota

In annual crops, the partitioning of photosynthates to support root growth, respiration and rhizodeposition should be greater during early development than in later reproductive stages due to source/sink relationships in the plant. Therefore, seasonal fluctuations in carbon dioxide (CO₂) and nitrous oxide (N₂O) production from roots and root-associated soil may be related to resource partitioning by the crop. Greenhouse studies used ¹³C and ¹⁵N stable isotopes to evaluate the carbon (C) partitioning and nitrogen (N) uptake by corn and soybean. We also measured the CO₂ and N₂O production from planted pots as affected by crop phenology and N fertilization. Specific root-derived respiration was related to the ¹³C allocated to roots and was greatest during early vegetative growth. Root-derived respiration and rhizodeposition were greater for corn than soybean. The ¹⁵N uptake by corn increased between vegetative growth, tasseling and milk stages, but the ¹⁵N content in soybean was not affected by phenology. A peak in N₂O production was observed with corn at the milk stage, suggesting that the corn rhizosphere supported microbial communities that produced N₂O. Most of the ¹⁵N-NO₃ applied to soybean was not taken up by the plant and negative N₂O production during vegetative growth and floral initiation stages suggests that soybean roots supported the reduction of N₂O to dinitrogen (N₂). We conclude that crop phenology and soil N availability exert important controls on rhizosphere processes, leading to temporal variation in CO₂ and N₂O production.     

The fate of carbon in pulse-labelled crops of barley and wheat

Wheat (cv. Gutha) and barley (cv. O'Connor) were grown as field crops on a shallow duplex soil (sand over clay) in Western Australia with their root systems contained within pvc columns. At four stages during growth, the shoots were pulse-labelled for 1.5h with¹⁴CO₂; immediately prior to labelling, the soil was isolated from the shoot atmosphere by pvc sheets. After labelling, the soil atmosphere was pumped through NaOH to trap respired CO₂ and after 2.5, 5, 7.5 and 24 h from the start of labelling, columns were destructively sampled to recover¹⁴C from the roots, soil and shoot.Both species showed similar patterns of¹⁴C distribution and changes in distribution through the growing season. During early tillering, 15–25% of the¹⁴C recovered after 24 h had been respired by the roots and rhizosphere, 17–27% was retained in the roots, 0.4–1.8% was recovered as water-soluble¹⁴C in the soil and the remainder (45–67%) was present in the shoot. These percentages changed during growth so that during grain filling only 2–3% of the¹⁴C recovered after 24 h was as respired CO₂, 2–6% was in the roots, 0.2% was in the soil and over 90% was in the shoot.The distribution of¹⁴C in components of the soil-plant system changed during the 24 h after labelling with the most rapid changes occurring generally during the first 7.5 h after labelling.Using growth measurements from adjacent plots, the amounts of C added to the soil were estimated for the whole season. Carbon input to the soil was about 48 gC m^–2 for wheat and 58 gC m^–2 for barley; the crops produced total shoot dry matter of 494 (wheat) and 735 g m^–2 (barley). Of the C input to the soil, 27.8% (wheat) and 40.3% (barley) was as respired C and only 3.3 (wheat) and 4.1% (barley) was collected as exudate (water-soluble material).     

Carbon fluxes in the rhizosphere of sweet chestnut seedlings (Castanea sativa) grown under two atmospheric CO2 concentrations: 14C partitioning after pulse labelling

Partitioning of ¹⁴C was assessed in sweet chestnut seedlings (Castanea sativa Mill.) grown in ambient and elevated atmospheric [CO₂] environments during two vegetative cycles. The seedlings were exposed to ¹⁴CO₂ atmosphere in both high and low [CO₂] environments for a 6-day pulse period under controlled laboratory conditions. Six days after exposure to ¹⁴CO₂, the plants were harvested, their dry mass and the radioactivity were evaluated. ¹⁴C concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodeposition) were measured. Root production and rhizodeposition were increased in plants growing in elevated atmospheric [CO₂]. When measuring total respiration, i.e. CO₂ released from the root/soil system, it is difficult to separate CO₂ originating from roots and that coming from the rhizospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizospheric microflora and roots separately. Root activity (respiration and exudation) was increased at the higher atmospheric CO₂ concentration. The proportion attributed to root respiration accounted for 70 to 90% of the total respiration. Microbial respiration was related to the amount of organic carbon available in the rhizosphere and showed a seasonal variation dependent upon the balance of root exudation and respiration. The increased carbon assimilated by plants grown under elevated atmospheric [CO₂] stayed equally distributed between these increased root activities. ei]H Lambers     

Enhanced mineralization of [U-(14)C]2,4-dichlorophenoxyacetic acid in soil from the rhizosphere of Trifolium pratense

Enhanced biodegradation in the rhizosphere has been reported for many organic xenobiotic compounds, although the mechanisms are not fully understood. The purpose of this study was to discover whether rhizosphere-enhanced biodegradation is due to selective enrichment of degraders through growth on compounds produced by rhizodeposition. We monitored the mineralization of [U-(14)C]2,4-dichlorophenoxyacetic acid (2,4-D) in rhizosphere soil with no history of herbicide application collected over a period of 0 to 116 days after sowing of Lolium perenne and Trifolium pratense. The relationships between the mineralization kinetics, the number of 2,4-D degraders, and the diversity of genes encoding 2,4-D/alpha-ketoglutarate dioxygenase (tfdA) were investigated. The rhizosphere effect on [(14)C]2,4-D mineralization (50 microg g(-1)) was shown to be plant species and plant age specific. In comparison with nonplanted soil, there were significant (P < 0.05) reductions in the lag phase and enhancements of the maximum mineralization rate for 25- and 60-day T. pratense soil but not for 116-day T. pratense rhizosphere soil or for L. perenne rhizosphere soil of any age. Numbers of 2,4-D degraders in planted and nonplanted soil were low (most probable number, <100 g(-1)) and were not related to plant species or age. Single-strand conformational polymorphism analysis showed that plant species had no impact on the diversity of alpha-Proteobacteria tfdA-like genes, although an impact of 2,4-D application was recorded. Our results indicate that enhanced mineralization in T. pratense rhizosphere soil is not due to enrichment of 2,4-D-degrading microorganisms by rhizodeposits. We suggest an alternative mechanism in which one or more components of the rhizodeposits induce the 2,4-D pathway.     

氮沉降对杉木和枫香土壤氮磷转化及碳矿化的影响

氮沉降是全球变化的重大环境问题,根际是地下生态过程研究的前沿,但目前氮沉降对亚热带地区不同树种土壤氮、磷供应和碳矿化根际过程的影响及其机制尚不清楚。选取典型红壤区15a针叶树杉木(Cunninghamia lanceolata)和阔叶树枫香(Liquidamba formosana)为对象,野外原位开展10 g N m~(-2)a~(-1)氮沉降试验3a,于2014年8月收集杉木和枫香根际土壤和非根际土壤,测定其p H值、有效氮、速效磷、水溶性有机碳及其34 d有机碳矿化动态,并计算根际效应。结果表明:氮沉降显著降低两个树种土壤p H值和杉木根际土壤速效磷(P0.05);提高枫香非根际土壤NO~-_3-N和杉木非根际土壤水溶性有机碳含量。同时,氮沉降显著提高杉木土壤有机碳矿化速率,根际和非根际的增幅分别为71.2%和41.2%,降低枫香土壤有机碳矿化速率,根际和非根际的降幅分别为10.6%和44.1%。此外,氮沉降显著降低枫香土壤NO~-_3-N和有机碳前期矿化速率的根际效应,增强后期矿化速率的根际效应,而杉木对氮沉降响应不显著。可见,氮沉降可显著改变树木土壤养分供应和有机碳稳定性,且丘陵红壤区针叶树和阔叶树根际过程对氮沉降的响应模式有别。率先报道了亚热带不同树种根际碳、氮、磷耦合过程对氮沉降的响应格局,并较好地揭示了针叶树和阔叶树对氮沉降响应的分异机制。