冬小麦-夏玉米轮作体系中不同施氮水平对玉米生长及其根际土壤氮的影响

在河北衡水潮土上进行田间试验,以当地习惯高氮用量(小麦季施N 300 kg/hm2,玉米季施N 240 kg/hm2)为对照,研究冬小麦-夏玉米轮作体系中减少氮肥用量对玉米季植株生长、氮素吸收及根际土壤中无机氮与微生物量氮的影响。结果表明,两季作物氮肥施用量减少25%和40%,对玉米产量、生物量及植株体内氮累积量未产生明显影响,氮肥利用率提高。不同氮肥施用量对根际和非根际土壤铵态氮含量的影响不显著;减少氮肥施用量,对玉米根际土壤硝态氮含量也没有明显影响。在玉米苗期、抽雄期和成熟期,习惯高施氮量处理的非根际土壤硝态氮含量较高,其中抽雄期,非根际土壤硝态氮含量较氮肥减施40%用量处理高出近一倍,但非根际土壤微生物量氮水平含量明显降低。氮肥减施未影响根际土壤微生物量碳、氮含量,反而增加了非根际土壤微生物量碳、氮水平。在高肥力的潮土上,冬小麦/夏玉米轮作体系中适当减施氮肥并未影响玉米根际土壤氮素水平,可保证玉米稳产,实现减氮增效。     

Factors influencing the loss of organic carbon from wheat roots

Wheat plants were grown in an atmosphere containing ¹⁴CO₂ at temperatures of 10°C or 18°C for periods from 3–8 weeks. The plant roots were maintained under sterile or non-sterile conditions in soil contained in sealed pots which were flushed to displace respired ¹⁴CO₂. The ¹⁴C content of the shoots, roots and soil was measured at harvest. The loss of ¹⁴C from the roots, expressed either in terms of total ¹⁴C recovered from the pots or ¹⁴C translocated to the roots, ranged from 14.3–22.6%, mean 17.3% or 29.2–44.4%, mean 39.2%, respectively. The presence of soil microorganisms significantly increased ¹⁴CO₂ release from the rhizosphere but had no effect on the ¹⁴C content of the soil. Fractionation of 6 m HC1 hydrolysates from sterile and non-sterile soils showed the presence in all soils of material behaving as neutral sugars and amino acids, in quantities representing 5.9–9.2% and 13.4–17.2% of the soil ¹⁴C content for the sugar and amino acid fractions respectively. It is proposed that a major loss of root carbon resulted from autolysis of the root cortex. Root lysis was increased by soil microorganisms, apparently without penetration of the plant cell walls.     

THE ORIGIN OF THE PENTOSE FRACTION OF SOIL POLYSACCHARIDE

Incubation of soil with monosaccharide for 224 days resulted in the evolution of about 80 per cent of the substrate carbon as CO₂ and the transformation of 3 per cent to soil sugars whether the substrate was ¹⁴C-glucose or xylose and whether the soil was pH 7.4 or pH 5.0. There was no detectable change in the total amounts of individual sugars in the soil during incubation. ¹⁴C-glucose and xylose gave the same distribution of radioactivity among the soil sugars : hexoses and 6-deoxy-hexoses were initially well labelled, with glucose having twice the specific activity of the other sugars. As the incubation progressed some activity appeared in the pentoses (the activity in xylose became very low within the first 14 days of the ¹⁴C-xylose incubation) and that in the hexoses slowly declined, with glucose no longer predominant. Nevertheless after 448 days the hexoses were still 3–4 times more radioactive than the pentoses. The activity in rhamnose did not decline with time so that eventually it became the most strongly labelled sugar. Incubation of soil with glucose and ¹⁴C-acetate showed very little transformation of the acetate to sugars indicating that glucose is not metabolized to C₂ compounds before it is transformed to other sugars. Ammo-acids in soil incubated for 7 days with ¹⁴C-glucose had much lower levels of radioactivity than hexoses or 6-deoxy-hexoses. It is concluded that if soil pentose originates by microbial synthesis it must accumulate slowly by a long process of selective decomposition of a mixture of polysaccharides.     

Effect of plant roots on carbon metabolism of soil microbial biomass

The utilization of plant- and soil-C by the microbial biomass in the rhizosphere of maize plants was investigated as a function of root proximity. The plants were cultivated in pots with divided root chambers and their shoots supplied with ¹⁴CO₂ for 23 days. Subsequently the individual soil zones were analyzed for organic C, ¹⁴C, biomass C and biomass ¹⁴C. Plant roots induced a 197% increase in microbial biomass and a 5.4% decrease in soil organic C compared with an 1.2% decrease in the unplanted control soil. The contributions of plant- and soil-C to this increased microbial growth amounted to 68% and 32% respectively. Biomass-¹⁴C corresponded to 1.6% of the total photosynthetically fixed ¹⁴C, to about 15% of the organic ¹⁴C-input into the rhizosphere and to 58% of the plant carbon remaining in soil after the removal of roots. 20% of this biomass-¹⁴C was found outside the immediate root zone. These results demonstrate that growing roots are a significant C-source for the microbial biomass and render an additional fraction of soil-C available to microbial utilization. The efficiency of C-utilization by the rhizosphere biomass is lower than values obtained with liquid cultures in laboratory experiments. The supply of plant-C to the microbial biomass outside the immediate root vicinity indicates that the overall volume of the maize rhizosphere is greater than what has been supposed so far.     

Sugars in soil and sweets for microorganisms: Review of origin,content, composition and fate

Sugars are the most abundant organic compounds in the biosphere because they are monomers of all polysaccharides. We summarize the results of the last 40 years on the sources, content, composition and fate of sugars in soil and discuss their main functions. We especially focus on sugar uptake, utilization and recycling by microorganisms as this is by far the dominating process of sugar transformation in soil compared to sorption, leaching or plant uptake. Moreover, sugars are the most important carbon (C) and energy source for soil microorganisms.Two databases have been created. The 1st database focused on the contents of cellulose, non-cellulose, hot-water and cold-water extractable sugars in soils (348 data, 32 studies). This enabled determining the primary (plant-derived) and secondary (microbially and soil organic matter (SOM) derived) sources of carbohydrates in soil based on the galactose + mannose/arabinose + xylose (GM/AX) ratio. The 2nd database focused on the fate of sugar C in soils (734 data pairs, 32 studies using ¹³C or ¹⁴C labeled sugars). ¹³C and ¹⁴C dynamics enabled calculating the: 1) initial rate of sugar mineralization, 2) mean residence time (MRT) of C of the applied sugars, and 3) MRT of sugar C incorporated into 3a) microbial biomass and 3b) SOM.The content of hexoses was 3–4 times higher than pentoses, because hexoses originate from plants and microorganisms. The GM/AX ratio of non-cellulose sugars revealed a lower contribution of hexoses in cropland and grassland (ratio 0.7–1) compare to forest (ratio 1.5) soils.¹³C and ¹⁴C studies showed very high initial rate of glucose mineralization (1.1% min⁻¹) and much higher rate of sugars uptake by microorganisms from the soil solution. Considering this rate along with the glucose input from plants and its content in soil solution, we estimate that only about 20% of all sugars in soil originate from the primary source – decomposition of plant litter and rhizodeposits. The remaining 80% originates from the secondary source – microorganisms and their residues. The estimated MRT of sugar C in microbial biomass was about 230 days, showing intense and efficient internal recycling within microorganisms. The assessed MRT of sugar C in SOM was about 360 days, reflecting the considerable accumulation of sugar C in microbial residues and its comparatively slow external recycling.The very rapid uptake of sugars by microorganisms and intensive recycling clearly demonstrate the importance of sugars for microbes in soil. We speculate that the most important functions of sugars in soil are to maintain and stimulate microbial activities in the rhizosphere and detritusphere leading to mobilization of nutrients by accelerated SOM decomposition – priming effects. We conclude that the actual contribution of sugar C (not only whole sugar molecules, which are usually determined) to SOM is much higher than the 10 ± 5% commonly measured based on their content.     

Mineralization of 14C-labelled unripe straw in soil with and without rape (Brassica napus L.)

Soil was amended with ¹⁴C-labelled unripe straw only (C:N ratio ca. 20), with ¹⁴C-labelled unripe straw plus unlabelled ripe straw (C:N ratio ca. 100) or with ¹⁴C-labelled unripe straw plus glucose. Half the samples with ¹⁴C-labelled straw and half the samples with ¹⁴C-labelled plus unlabelled straw were cropped with rape plants. A decreased rate of mineralization of the ¹⁴C-labelled straw was found in the planted soil compared with the unplanted soil. The reduction was most profound in the soil amended with both labelled and unlabelled straw, indicating that at least part of the reduction was due to competition between plants and microorganisms for mineral N. No other explanations for the decrease in mineralization in the presence of plants were found. The soil amended with glucose which simulated the effect of root exudates showed an increased rate of mineralization. Therefore, the reduction in the presence of plants was probably not due to microbial use of the rhizodeposition in favour of the labelled straw. Only a minor part of the reduction was apparently due to uptake of labelled C by the plant, as only small amounts were found in the roots and shoots at harvest. The difference in ¹⁴C mineralization between treatments was not reflected in the number of bacteria in the soil at harvest. The number of bacteria, which was determined by plate counts and direct microscopy, was the same in all the soils, rhizosphere soils as well as bulk soils.     

水稻根际和周围土壤中微生物功能基因丰度与碳、氮状况及其通量之间的关系

Rapid nitrogen（N） transformations and losses occur in the rice rhizosphere through root uptake and microbial activities. However,the relationships between rice roots and rhizosphere microbes for N utilization are still unclear. We analyzed different N forms（NH＋4,NO-3, and dissolved organic N）, microbial biomass N and C, dissolved organic C, CH4 and N2O emissions, and abundance of microbial functional genes in both rhizosphere and bulk soils after 37-d rice growth in a greenhouse pot experiment. Results showed that the dissolved organic C was significantly higher in the rhizosphere soil than in the non-rhizosphere bulk soil, but microbial biomass C showed no significant difference. The concentrations of NH＋4, dissolved organic N, and microbial biomass N in the rhizosphere soil were significantly lower than those of the bulk soil, whereas NO-3in the rhizosphere soil was comparable to that in the bulk soil. The CH4 and N2O fluxes from the rhizosphere soil were much higher than those from the bulk soil. Real-time polymerase chain reaction analysis showed that the abundance of seven selected genes, bacterial and archaeal 16 S rRNA genes, amoA genes of ammonia-oxidizing archaea and ammonia-oxidizing bacteria, nosZ gene, mcrA gene, and pmoA gene, was lower in the rhizosphere soil than in the bulk soil, which is contrary to the results of previous studies. The lower concentration of N in the rhizosphere soil indicated that the competition for N in the rhizosphere soil was very strong, thus having a negative effect on the numbers of microbes. We concluded that when N was limiting, the growth of rhizosphere microorganisms depended on their competitive abilities with rice roots for N.     

Increased microbial activity and nitrogen mineralization coupled to changes in microbial community structure in the rhizosphere of Bt corn

The interactions between plant roots and soil microorganisms are essential for the function and stability of ecosystems, primary agricultural production and plant health. Despite the importance of soil microbes the response of these microbes to large-scale cultivation of genetically modified (GM) crops is still poorly understood. This study evaluated the potential impact of two lines of transgenic Bt maize on rhizosphere microorganisms. A time-course field experiment was conducted over a period of two years in two fields in Guadalajara (Spain) with monthly sampling from April to September. Rhizosphere soil was collected from transgenic (TG) and unmodified (WT) maize plants from each field and sampling time for the analysis of several important functional and structural soil quality parameters. Total microbial activity, as determined by H³-Thymidine and C¹⁴-Leucine incorporation, was found to be higher in the rhizospheres of the transgenic plants. Similarly, differences in potential ammonification and nitrification were observed in the second year of the study. In contrast, bacterial and fungal microbial catabolic abilities, as determined by Biolog ECO and FF plate analyses, respectively, were more influenced by sampling time than the transgenic nature of the plants. Microbial community structure was also studied by bacterial and phylum-specific PCR-DGGE and PCR cloning approaches. In general, differences were again more pronounced between sampling times, as opposed to between TG versus WT plants, although marked differences were observed within the Betaproteobacteria between plant lines. For the first time it describes the presence of Iamiaceae family in soil, specifically to TG plant rhizosphere. To summarize, the study showed that some important properties of rhizopshere microbes may be impacted by Bt maize cultivation and highlighted the fact that such potential effects need to be viewed within the context of seasonal and spatial variability.     

Differences between soil solutions obtained from rhizosphere and non-rhizosphere soils by water displacement and soil centrifugation

Soil solution was obtained from potted rhizosphere or non-rhizosphere soils by water displacement or soil centrifugation. The pH of the displaced solutions was lower than that of bulk soils when solutions were obtained from non-rhizosphere soil, although it increased as plants grew. This increase probably reflected true changes in rhizosphere pH, generated by the uptake by plants of N0₃-N. In contrast, the pH of soil centrifugates was usually close to that of the bulk soils, implying that buffering by colloids had occurred during sampling. Concentrations of elements in solutions from non-rhizosphere soil were similar for both methods when soils were incubated at ambient pCO₂. However, when non-rhizosphere soils were incubated at elevated pCO₂, displacement solutions had lower pH values, and much larger concentrations of elements, compared to soil centrifugates. Comparison of mass flow of elements versus actual plant uptake showed that Ca and Mg accumulated, while K, Zn and Cd were depleted from the rhizosphere. Displacement solutions showed this accumulation or depletion of the elements more clearly than soil centrifugates. These differences were attributed to the fact that, at constant soil moisture, the rhizosphere developed mainly in larger pores, which were sampled by displacement. With centrifugation, a mixture of pore sizes was sampled, so that rhizosphere solution was only obtained when all of the soil had become rhizosphere. Soil centrifugates obtained after 22 days of growth also contained higher concentrations of organic carbon than displacement solutions, indicating contamination due to the disruption of roots and/or micro-organisms. We conclude that water displacement is suitable for sampling solution from light to medium textured rhizosphere or non-rhizosphere soils and that soil centrifugation is only of limited suitability.     

Separation of root and microbial respiration: Comparison of three methods

In a laboratory experiment, the following methods of separating the soil CO₂ flux into the root respiration and the respiration of the rhizosphere and nonrhizosphere microorganisms were compared: (1) root exclusion, (2) component integration, and (3) ¹⁴C pulse labeling. Depending on the method used, the combined contribution of the rhizosphere microorganisms and roots varied from 18 to 40% of the total CO₂ emission; the contribution of the roots alone was 8–19%, and that of the nonrhizosphere microorganisms was 51–82%. The nonisotope methods (1 and 2) gave similar results of the separation. The pulse labeling of plants satisfactorily separated the root and microbial respiration, but it is unsuitable for determining the respiration of the nonrhizosphere microorganisms. Advantages and disadvantages of each method are discussed.     

Potassium Status in Maize Rhizosphere of Smectitic Soils

This study has been taken up to generate information on potassium status in maize rhizosphere soils differing in their clay content at different levels of added potassium. Soils with larger amounts of clay showed greater amounts of water soluble and ammonium acetate extractable K (NH₄OAc K) in both the rhizosphere as well as non-rhizosphere. In the absence of added K (control), non-rhizosphere samples showed higher water soluble and NH₄OAc K ranging from 8.0 to 28.9 mg kg^?1 and from 132.5 to 294.0 mg kg^?1, respectively compared to rhizosphere samples where water soluble K varied between 6.0 and 26.5 mg kg^?1 and NH₄OAc K from 125.0 to 262.5 mg kg^?1. The difference in K content between rhizosphere and non-rhizosphere which could have been resulted from plant K uptake was significantly related with clay content (r = 0.98**) in control whereas at 150 mg kg^?1 K addition this relationship was found to be non significant (r = 0.64^NS). Electro ultra filtration (EUF) fractions also showed similar differences in K contents in soil between rhizosphere and non-rhizosphere.     

雷公藤根际微生物特征研究

根际微生物的代谢作用, 直接促进或抑制根的营养吸收和生长, 也影响根际土壤中的物质转化, 雷公藤根系发达且多与其他树种混交栽培, 其根际微生物活性对雷公藤的生长和土壤肥力均有不可忽视的影响。以福建省泰宁县3 种不同栽培模式雷公藤林(野生雷公藤林、杉木雷公藤混交林、厚朴雷公藤混交林)为研究对象, 通过稀释平板法测定3 种不同雷公藤林分根际土壤和非根际土壤中细菌、真菌、放线菌的数量。结果表明: 根际土的微生物数量大于非根际土的微生物数量; 3 种林分, 无论是在根际土壤中, 还是在非根际土壤中, 均表现为细菌数量＞放线菌数量＞真菌数量; 根际微生物(R)比非根际微生物(S)更活跃, 3 种林分的3 大类微生物的R/S 数量比值均大于1; 3 种林分的微生物活性表现为厚朴雷公藤混交林＞杉木雷公藤混交林＞野生雷公藤林, 表明混交方式可促进雷公藤根际微生物活性。     

Root uptake of N-containing and N-free low molecular weight organic substances by maize: A C/N tracer study

Most studies showing potential organic nitrogen uptake were conducted with amino acids. They conclude that, in some ecosystems, amino acids significantly contribute to the N demand of plants and that roots have special transporters to re-uptake amino acids released into the rhizosphere. However, the relevance of the uptake of organic N compounds can only be evaluated by comparing the uptake of N-containing and N-free organic substances. We compared the uptake of alanine, glucose and acetate labelled with ¹⁴C by maize. Additionally, the N uptake was estimated by ¹⁵N labelled alanine and KNO₃. We found a similar uptake of ¹⁴C from alanine, glucose and acetate, amounting for the whole plant less than 1% of ¹⁴C input. These results show that maize did not prefer N-containing to N-free organic substances. The uptake of ¹⁵N by maize exceeded that of ¹⁴C (10- to 50-fold), irrespective of the ¹⁵N source. However, plant uptake of nitrate (23.6–35.2% of ¹⁵N input) always exceeded the uptake of N from alanine (9.6–28.8%). The uptake of organically bound N by maize growing in soil occurred mainly by transpiration flow – as dissolved organics. The contribution of specific amino acid transporters was minor.     

Role of Plasmalemma H+ ATPase in Sugar Retention by Roots of Intact Maize and Field Bean Plants

Net release and net uptake of sugars by roots of intact maize (Zea mays cv. Blizzard) and field bean (Vicia faba L. cv. Alfred) were studied at micromolar external sugar concentrations that are relevant to the rhizosphere. Besides various sugars not further characterized there was net release of glucose, fructose, sucrose, arabinose, ribose, and galactose. The net release of these sugars into the root medium (0.1 mM CaSO₄) was stimulated by the protonophore CCCP (10 μM), the sulfhydryl reagent NEM (300 μM), the specific inhibitor of plasmalemma H⁺ ATPase vanadate (0.5 mM), and by the inhibitor of the glucose carrier phlorizin (2 mM). Net uptake of glucose, fructose, and arabinose from 10 μM external concentrations was inhibited by these substances. Stimulation of net release and inhibition of net uptake was most pronounced for glucose. Sucrose added to the root medium was hydrolyzed by invertase activity leading to glucose and fructose uptake by roots. It is concluded that the retention of sugars by plant roots is not only determined by plasmalemma permeability but is also controlled by the H⁺ electrochemical gradient established by ATPase activity (retrieval mechanism). The proton gradient drives a sugar/H⁺ cotransport system that is selective for glucose but may also transport other sugars, particularly in the absence of glucose.     

Identification of organically bound iodine in soil humic substances by size exclusion chromatography / inductively coupled plasma mass spectrometry (SEC / ICP-MS)

Abstract

Sulfur transformation in riee rhizosphere was investigated. Soil enzyme arylsulfatase in rhizosphere and non-rhizosphere soil, whieh is responsible for mineralization of organic sulfur to sulfate sulfur, was studied. The Michaelis constants of arylsulfatase from Maahas c1ay and Pila c1ay loam were 3.04 × 10^-4 M and 3.97 × 10^-4 M, respectively. The arylsulfatase of rhizosphere soil showed higher activity than that of non-rhizosphere soil. Applieation of sulfate had no marked elTect on the enzyme aetivity either in rhizosphere or non-rhizosphere soil under the submerged condition. This indieates that arylsulfatase activity under the submerged condition is not inhibited by applieation of sulfate. The amount of HI-reducible sulfur in the rhizosphere and non-rhizosphere soi! inereased with time. However, rhizosphere soil had a higher amount of HI-reducible sulfur than did non-rhizosphere. Thc ditl'erence in arylsulfatase activity between the rhizosphcre and non-rhizosphere soil was not directly associated with thc number of sulfur-redueing and -oxidizing bacteria.     

Carbon Turnover in the Rhizosphere

Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as:

• –homogeneous labelling of whole plants with ¹⁴C
• –distinction between root and microbial respiration
• –separation of soil zones of known distances from the roots
• –determination of microbial soil biomass.

These methods were applied to study the following aspects:

• –release of organic plant carbon into the soil by growing roots
• –utilization of this plant carbon by the microbial biomass in the rhizosphere
• –related influence on the turnover of soil organic matter, and
• –spatial range of such root influence in the soil.

About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO₂, while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C-input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%). Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.     

Transformation of Lead Solid Fraction in the Rhizosphere of Elsholtzia splendens: The Importance of Organic Matter

The rhizosphere, enriched in organic matter, is the bottleneck of metal transfer in the soil–plant system. However, the transformation of metal fractions in the rhizosphere and the mechanisms that are involved, notably the role of organic matter, are poorly known. In this study, the solid-phase fractionation of lead (Pb) in the rhizosphere and non-rhizosphere soil of Elsholtzia splendens in a Pb-contaminated soil was investigated using a nine-step selective sequential extraction method in a pot experiment. Compared to the non-rhizosphere soil, there were measurable increases in Pb-fulvic complexes, Pb-humic complexes, organic Pb, and amorphous Pb but no significant changes in other forms of Pb in the rhizosphere soil. Pb-fulvic complexes and organic Pb, increasing from 397 to 438 mg kg^?1 and 229 to 258 mg kg^?1, respectively, showed a stronger accumulating trend than Pb-humic complexes and amorphous Pb, with an increase from 15.9 to 17.3 mg kg^?1 and 6.04 to 7.80 mg kg^?1 respectively, in the rhizosphere soil relative to non rhizosphere soil. These results may be mainly due to the enrichment of organic matter in the rhizosphere soil, resulting from root exudation and the enhanced turnover of microorganisms. The accumulation of Pb-fulvic complexes in the rhizosphere soil increases the potential phytoavailable pool, thus likely facilitating the phytoextraction of Pb in metal-contaminated soil.     

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

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

Mobilization and turnover of soil phosphorus in the rhizosphere

Recent progress in methods enables a better understanding of the turnover of P in the rhizosphere. Examples of this progress are the separation of soil layers differing in proximity to the roots, improved methods for extraction and fractionation of soil P, application of ³²P isotope dilution analysis to follow P fluxes between various fractions and direct determination of microbially bound P and of root phosphatases.

• These methods were combined to investigate the following aspects
• –labile P pools, the P fluxes between these pools and their contribution to the P supply to growing maize roots
• –the role of microbial biomass in these interactions and the partition of mobilized P between plants and microorganisms
• –modifications of sorption and transport of P in the rhizosphere
• –plant availability of native and added organic phosphates, and the relative significance of root and soil phosphatases.

There is a significant transformation of P in the rhizosphere with a corresponding redistribution among fractions of different plant availability. About 9% of the inorganic ³²P added to soil were incorporated within 2 weeks into microbial and organic fractions. The transfer of P from non-exchangeable forms exceeded the depletion of the exchangeable P by a factor of 5. About 53% of the mobilized P originated from inorganic, the remaining 47% from organic fractions. Of the mobilized P 80% was taken up by the plants and 20% was found in the microbial biomass. Up to 90% of the P in the rhizosphere soil solution was organic with a maximum just outside the root zone. Soluble inositol hexaphosphate modified the sorption of inorganic P, thus shifting its equilibrium solution concentration. The phosphatase activity of the roots is considerable. Both root phosphatase activity and the utilization of inositol hexaphosphate depend on the P supply and nutritional status of plants with regard to P. It is concluded that the rhizosphere is a key site of P transformation with a significant mobilization of P from the non-exchangeable inorganic and organic fractions. Organic P fractions not only play a significant role as a P source but also modify important soil parameters related to the sorption and transport of P in the rhizosphere.     

Crop-specific differences in the concentrations of lipids in leachates from the root zone

Although lipids are involved in diverse soil processes and affect various soil properties, the contribution of rhizodeposits and the root zone to lipid concentrations and distributions in soils is unknown. For the first time, we determined the concentrations of alkanoic acids, n-alkanes and n-alkenes in root zone leachates and roots of maize and potato using gas chromatography/mass spectrometry (GC/MS). In total, the lipid concentrations of leachates were 100 μg g^?1 (maize) and 17 μg g^?1 (potato). The saturated n-alkanoic acids, ranging from n-C₁₄ to n-C₂₈ and having the maximum at n-C₂₂ (maize) and at n-C₁₆ (potato), were more abundant than the other compounds. Maize leachates had more alkanes (20 μg g^?1) than potato leachates (3.1 μg g^?1), but the members of the homologues were nearly the same. Comparison of these distributions with data for roots, microorganisms and soil indicated that the lipids in the leachates from the root zone mainly originated from abrasion of fine roots, rhizodeposits and rhizosphere microorganisms.