Root-induced nitrogen mineralisation: A nitrogen balance model

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C:N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.     

根分泌物及其生态效应

根分泌物是个古老而年轻的研究领域。早在18,19世纪,人们(Plenk,1795;Decandolle,1830)就观察到根分泌物对邻近植株的促生和抑制作用,1904年Hilter提出“根际”的概念,标志着人们对根分泌物及其生态效应的进一步认识。此后人们对根分泌物研究逐步展开。Lyon和Willson(1920)发现,生长于无菌水培液中的植物能释放有机物,为深入研究奠定了基础。但很长一段时间这个研究领域一     

根系分泌物生态学研究

在植物生长过程中 ,由根系的不同部位分泌或溢泌一些无机离子及有机化合物 ,这些物质统称为根系分泌物。植物在其生长过程中不断地分泌无机离子及有机化合物 ,这是植物长期适应外界环境而形成的一种适应机制。早在 2 0世纪 5 0年代就有人对植物根系分泌物进行了研究 ,Ｒｏｖｉｒａ等[4 3] 和Ｖａｎｃｕｒａ等[4 7] 对根土界面根系分泌物进行了系统的研究 ,切尔诺布里维卡[2 6 ] 研究了植物根系分泌物的生物学作用 ,揭示了其在间作中的作用 ,直到 70年代对根系分泌物的研究才出现了蓬勃发展的趋势。近年来的研究表明 ,根系分泌物是保持根际微…     

The prediction of C and N content and their potential mineralisation in heterogeneous soil samples using Vis–NIR spectroscopy and comparative methods

The development of a rapid, accurate and cost-effective method for the prediction of constituents related to soil nitrogen (N) supply is considered important. The potential of using visible (Vis) and near infrared reflectance (NIR) spectroscopy (400–2500 nm) as such a method was investigated. Vis–NIR calibrations were performed for organic carbon (C_org) and total N (N_tot) content and their potential mineralisation using 80 grassland soil samples of rather heterogeneous origin. Prediction accuracy was tested using a 'take-out-four' validation strategy (48 samples). Within investigated variables a ratio of standard deviation of reference data to standard error of bias corrected prediction (RPD) within 1.7 (r²=0.65) and 2.7 (r²=0.87) were achieved. Apparent differences in Vis–NIR prediction accuracy among the variables were partly due to errors in the reference values. Thawed moist samples tend to be more accurately predicted than dried samples, and no benefit was derived from the grinding of sieved (4 mm) and dried samples. Prediction accuracy did not differ using two different systems for sample presentation to the Vis–NIR analyses. Comparative predictions of C_org and N_tot and their potential mineralisations were performed using the take-out-four validation strategy and simple linear regression to loss on ignition (LOI) values and hot KCl extracted NH₄-N (N_hotKCl) values as predictors. Likewise, the reference values of C_org and N_tot were also used as predictors for each other and for the potential C and N mineralisation constituents. Accuracy obtained for the Vis–NIR predictions of investigated constituents was in general equal or better than prediction accuracy obtained by these comparative methods. The Vis–NIR method provided promising predictions of variables important for the soil N supply.     

Loss of exudates from the roots of perennial ryegrass inoculated with a range of micro-organisms

To determine the effect of microbial metabolites on the release of root exudates from perennial ryegrass, seedlings were pulse labelled with [¹⁴C]-CO₂ in the presence of a range of soil micro-organisms. Microbial inoculants were spatially separated from roots by Millipore membranes so that root infection did not occur. Using this technique, only microbial metabolites affected root exudation. The effect of microbial metabolites on carbon assimilation and distribution and root exudation was determined for 15 microbial species. Assimilation of a pulse label varied by over 3.5 fold, dependent on inoculant. Distribution of the label between roots and shoots also varied with inoculant, but the carbon pool that was most sensitive to inoculation was root exudation. In the absence of a microbial inoculant only 1% of assimilated label was exuded. Inoculation of the microcosms always caused an increase in exudation but the percentage exuded varied greatly, within the range of 3–34%.     

A novel method of quantifying root exudation in the presence of soil microflora

A microcosm is described in which root exudation may be estimated in the presence of microorganisms. Ryegrass seedlings are grown in microcosms in which roots were spatially separated from a microbial inoculant by a Millipore membrane. Seedlings grown in the microcosms were labelled with [¹⁴C]-CO₂, and the fate of the label within the plant and rhizosphere was determined. Inoculation of the microcosms with Cladosporium resinae increased net fixation of the [¹⁴C] label compared to plants grown under sterile conditions. Inoculation also increased root exudation. The use of the microcosm was illustrated and its applications discussed.     

Ectomycorrhizal fungi in culture respond differently to increased carbon availability

Carbon (C) availability to ectomycorrhizal fungi is likely to increase at elevated atmospheric CO(2). To determine whether there are any broad patterns in species' responses that relate to their ecology, we compared growth, respiration, N uptake and C exudation of 17 fungal isolates in liquid culture. As a surrogate for increased C availability we used three different C:N ratios (10:1, 20:1 and 40:1), moving from conditions of C limitation to conditions of N limitation. Responses were species-specific, and suilloid fungi were the most responsive in terms of growth and respiration. In contrast, a group of eight isolates showed no growth increase above C:N 20:1. This inability to respond was not due to N limitation, although there were marked differences in N uptake between isolates. At higher C availability isolates generally became more efficient in converting C into biomass. Six isolates showed net release of exudates into the culture medium (up to 40% of the C in biomass and respiration). We conclude that the findings were in agreement with field observations, and suggest that pure culture observations can yield ecologically relevant information on how ectomycorrhizal fungi may respond under conditions of elevated CO(2).     

Organic acids in the rhizosphere – a critical review

Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.     

Effects of carbon and nitrate additions to soil upon leaching of nitrate,microbial predators and nitrogen uptake by plants

Amendments with glucose significantly reduced the amount of nitrate leached from a sandy soil amended with nitrate. The decrease was most likely caused by immobilisation of the nitrate into microbial cells. Populations of ciliates and flagellates and amoebae, but not nematodes, increased 7–14 days following glucose amendments. Mineralisation of the immobilised nitrate occurred during this period. Some of the mineralised nitrogen appeared to be available to ryegrass plants only if the roots exploited most of the soil during the period of maximum predator activity. After 28 days, 44% of the organic N remaining in the soil after leaching was taken up by the plants. The difference developed over the last 2 weeks when amoebal populations were large.     

Carbon input from plant to soil through root exudation inDigitaria adscendens andAmbrosia artemisiifolia

The amount of carbon released into soil through root exudation byDigitaria adscendens Herrm. andAmbrosia artemisiifolia L. var.elatior Desc., which often predominate at the early stages of secondary succession, was evaluated in a laboratory experiment conducted over a 60-day period. Differences in the amount of exuded carbon between these species and between the developmental stages were examined. The amount of carbon exuded increased with growth in both species. The percentage of exuded carbon to photosynthetically net fixed carbon, which was higher at younger stages (13%) inD. adscendens, decreased to 3.1% with time. On the other hand, no reduction in the amount of carbon exuded was observed inA. artemisiifolia (4.7–8.1% range). The total amount of carbon released through root exudation inD. adscendens andA. artemisiifolia was estimated at 3.1% and 6.9% of photosynthetically net fixed carbon, respectively. These results suggest the possibility that wild plants exude a considerable amount of carbon from their roots to the soil, and emphasizes the necessity for considering root exudation in the carbon cycle.     

Phosphorus (P) uptake and growth of a root hairless barley mutant (bald root barley, brb) and wild type in low- and high-P soils

The recently isolated root‐hairless mutant of barley (Hordeum vulgare L), bald root barley, brb offers a unique possibility to quantify the importance of root hairs in phosphorus (P) uptake from soil. In the present study the ability of brb and the wild‐type, barley genotype Pallas producing normal root hairs to deplete P in the rhizosphere soil was investigated and the theory of diffusion and mass flow applied to compare the predicted and measured depletion profiles of diffusible P. Pallas depleted twice as much P from the rhizosphere soil as brb. The P depletion profile of Pallas uniformly extended to 0.8 mm from the root surface, which was equal to the root hair length (RHL). The model based on the theory of diffusion and mass flow explained the observed P‐depletion profile of brb, and the P depletion outside the root‐hair zone of Pallas, suggesting that the model is valid only for P movement in rhizosphere soil outside the root‐hair zone. In low‐P soil (P in soil solution 3 µm ) brb did not survive after 30 d, whereas Pallas continued to grow, confirming the importance of root hairs in plant growth in a P‐limiting environment. In high‐P soil (P in soil solution 10 µm ) both brb and Pallas maintained their growth, and they were able to produce seeds. At the high‐P concentration, RHL of the Pallas was reduced from 0.80 ± 0.2 to 0.68 ± 0.14 mm. In low‐P soil, P‐uptake rate into the roots of Pallas was 4.0 × 10^?7 g mm^?1 d^?1 and that of brb was 1.9 × 10^?7 g mm^?1 d^?1, which agreed well with the double amount of P depleted from the rhizosphere soil of Pallas in comparison with that of brb. In high‐P soil, the P uptake rates into the roots of brb and Pallas were 3.3 and 5.5 × 10^?7 g mm^?1 d^?1, respectively. The results unequivocally confirmed that in a low‐P environment, root hairs are of immense importance in P acquisition and plants survival, but under high‐P conditions they may be dispensable. The characterization of phenotypes brb and Pallas and the ability to reproduce seeds offers a unique possibility of molecular mapping of QTLs and candidate genes conferring root‐hair formation and growth of barley.     

根系氮吸收过程及其主要调节因子

氮（N）是植物根系吸收最多的矿质元素之一．全球变化将使土壤中N的有效性发生改变,影响陆地生态系统碳分配格局与过程．研究根系N吸收及其调控对预测生态系统结构和功能具有重要理论意义．由于土壤中存在多种形态的N源,长期的生物进化和环境适应导致植物根系对不同形态N的吸收部位、机理及调控有较大差别．因此,植物长期生长在以某一形态N源为主的土壤上就形成了不同的N吸收机制和策略．本文简述了近年来在植物根系N吸收和调控方面的最新研究进展,重点评述了不同形态N在土壤中的生物有效性,根系N吸收部位,N在木质部中的装载和运输,不同形态N（NO3^-、NH4^＋和有机氮）的吸收机制,以及根系N吸收的自身信号调控和环境因子对根系N吸收的影响．在此基础上,提出了目前根系N吸收研究中存在的几个问题．     

Soil mineral nitrogen availability was unaffected by elevated atmospheric pCO2 in a four year old field experiment (Swiss FACE)

The effect of elevated (60 Pa) atmospheric carbon dioxide partial pressure (p_CO2) and N fertilisation on the availability of mineral N and on N transformation in the soil of a Lolium perenne L. monoculture was investigated in the Swiss FACE (Free Air Carbon dioxide Enrichment) experiment. The apparent availability of nitrate and ammonium for plants was estimated during a representative, vegetative re-growth period at weekly intervals from the sorption of the minerals to mixed-bed ion-exchange resin bags at a soil depth of 5 cm. N mineralisation was measured using sequential coring and in situ exposure of soil cores in the top 10 cm of the soil before and after the first cut in spring 1997. High amounts of mineral N were bound to the ion exchange resin during the first week of re-growth. This was probably the combined result of the fertiliser application, the weak demand for N by the newly cut sward and presumably high rates of root decay and exudation after cutting the sward. During the first 2 weeks after the application of fertiliser N at the first cut, there was a dramatic reduction in available N; N remained low during the subsequent weeks of re-growth in all treatments. Overall, nitrate was the predominant form of mineral N that bound to the resin for the duration of the experiment. Apparently, there was always more nitrate than ammonium available to the plants in the high N fertilisation treatment for the whole re-growth period. Apparent N availability was affected significantly by elevated p_CO2 only in the first week after the cut; under high N fertilisation, elevated p_CO2 increased the amount of mineral N that was apparently available to the plants. Elevated p_CO2 did not affect apparent net transformation of N, loss of N or uptake of N by plants. The present data are consistent with earlier results and suggest that the amount of N available to plants from soil resources does not generally increase under elevated atmospheric p_CO2. Thus, a possible limiting effect of N on primary production could become more stringent under elevated atmospheric p_CO2 as the demand of the plant for N increases. This revised version was published online in June 2006 with corrections to the Cover Date.     

Growth,Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen （N） uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root ： shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.     

Growth, Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen (N) uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root: shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.