Transformations and movement of P in the rhizosphere

Summary Wheat plants labelled with³³P were grown in thin layers of soil amended with³²P-labelled fertiliser. Roots were separated from the soil during plant growth by a porous membrane to overcome difficulties in measuring microbial P in rhizosphere soil. Over the 22 day growth period, net movement of³³P out of healthy growing roots varied from 0.9–4.9% of the total³³P translocated to the root. Over the same period the plants took up 12.0% and the microbial biomass 14.1% of the fertiliser³²P. On drying and rewetting of the soil after the plants were harvested, a large proportion of root P moved into soil fractions while³²P appeared to accumulate in the biomass and stable P forms.     

Nutrient acquisition from different soil depths by pedunculate oak

Eight oak trees (Quercus robur L.) received ³²P at a soil depth of 50 cm and ³³P at a soil depth of 15 cm at the end of June 2002 through plastic tubes inserted into the mineral soil. The phosphorus uptake from different soil depths was estimated by analysing the concentration of ³²P and ³³P in the foliage of oak growing in a mixed stand in southern Sweden. ³²P and ³³P were recovered in the leaves/needles after 21 and 39 days. The recovery of labelled P in oak was higher from 15 cm soil depth than from 50 cm, however, more than 4% of the total amount of labelled P was taken up from 50 cm. This indicates that oak can utilize deep soil layers for nutrient uptake. A study on the uptake of Cs (as an analogue to K) and ¹⁵N into the leaves was performed on the same trees and detectable amounts of ¹⁵N and Cs were recovered in leaves and buds. This indicates that ¹⁵N and Cs can be used to study nutrient uptake of mature trees from the mineral soil.     

Fine-root morphology and uptake of 32P and 35S in a Norway spruce (Picea abies (L.) Karst.) stand subjected to various nutrient and water supplies

An investigation of fine (< 1 mm in diameter) and small (1–2 mm in diameter) roots in the organic soil layer was carried out in a Norway spruce forest stand with different treatments of water and nutrients, including control (C); ammonium sulphate application (NS); nitrogen-free fertilization (V); irrigation with liquid fertilization (a complete nutrient solution) (IF); NS followed by artificial drought (ND); V followed by artificial drought (VD). In order to evaluate the vitality and function of the fine roots, the following approaches were used: i) classification of fine roots, based on morphological characteristics; ii) nutrient uptake bioassay, using ³²P-phosphate and ³⁵S-sulphate; iii) nutrient concentration in fine roots and its relation to nutrient uptake. The NS treatment showed effects on the fine and small roots, with a decrease in amount of living roots, and a decrease in the total amount of fine and small roots. The VD treatment resulted in increased amounts of living small roots, while the ND treatment showed the opposite, as compared with the V and NS treatments, respectively. The uptake of P was negatively related to the P supply, with a higher P uptake for C and NS fine roots than for IF and V fine roots. The specific root length (SRL, m g^-1 DW) decreased for NS fine roots and increased for IF fine roots, indicating a further increase in uptake for NS roots and a decreased uptake for IF roots if calculated on a root length basis. So far, the NS and IF treatments maintain a considerable increase in above-ground biomass with a significantly reduced root biomass and standing crop.     

Amino acids as a nitrogen source for tomato seedlings: The use of dual-labeled (C, N) glycine to test for direct uptake by tomato seedlings

Direct uptake of organic nitrogen (ON) compounds, rather than inorganic N, by plant roots has been hypothesized to constitute a significant pathway for plant nutrition. The aim of this study was to test whether tomatoes (Solanum lycopersicum cv. Huying932) can take up ON directly from the soil by using ¹⁵NH₄Cl, K¹⁵NO₃, 1, 2-¹³C₂¹⁵N-glycine labeling techniques. The ¹³C and ¹⁵N in the plants increased significantly indicating that a portion of the glycine-N was taken up in the form of intact amino acids by the tomatoes within 48 h after injection into the soil. Regression analysis of excess ¹³C against excess ¹⁵N showed that approximately 21% of the supplied glycine-N was taken up intact by the tomatoes. Atom% excesses of ¹⁵N and ¹³C in the roots were higher than in any shoots. Results also indicated rapid turnover of amino acids (e.g., glycine) by soil microorganisms, and the poor competitive ability of tomatoes in absorbing amino acids from the soil solution. This implies that tomatoes can take up ON in an intact form from the soil despite the rapid turnover of organic N usually found under such conditions. Given the influence of climatic change and N pollution, further studies investigating the functional ecological implications of ON in horticultural ecosystems are warranted.     

Transformations between organic and inorganic sediment phosphorus in Lake Balaton

In order to estimate microbial P content and biological P uptake in sediments, the tungstate precipitation method of Orrett & Karl (1987) was used in sediment extracts. This method allows a simple and rapid separation of organic and inorganic ³²P radioactivity. Either inorganic ³²P (as carrierfree H₃ ³²PO₄) or organic ³²P (as ³²P-labelled algal material) was added to surface sediment suspensions of shallow Lake Balaton. Inorganic ³²P was rapidly transformed into organic ³²P, and this process was completely inhibited by formaline. P content of living benthic microorganisms was estimated from steady state distribution of the radioactivity. Transformation of algal organic P into inorganic P could also be detected.In extremely P limited Lake Balaton benthic microorganisms were shown to supplement their high P requirements by inorganic P uptake. The velocity of the inorganic into organic P transformation, i.e. the rate of microbial P uptake, was comparable to P uptake in the water column. Microbial P uptake contributed significantly to total P fixation by sediments, particularly at low ( 100 µg P l^–1) phosphate additions.     

Effects of variations in soil water potential,depth of N placement,and cultivar on postanthesis N uptake by wheat

This study was conducted to investigate the influence of soil water potential, depth of N placement, timing, and cultivar on uptake of a small dose of labeled N applied after anthesis by wheat (Triticum aestivum L.) Understanding postanthesis N accumulation should allow better control of grain protein concentration through proper manipulation of inputs. Two hard, red spring-wheat cultivars were planted in early and late fall each yr of a 2-yr field experiment. Less than 1 kg N ha^–1 as K ¹⁵NO₃ was injected into the soil at two depths: shallow (0.05 to 0.08 m) and deep (0.15 to 0.18 m). In both years an irrigation was applied at anthesis, and injections of labeled N were timed 4, 12, and 20 days after anthesis (DAA). Soil water potential was estimated at the time of injection. Mean recovery of ¹⁵N in grain and straw was 57% of the ¹⁵N applied. Recovery did not differ between the high-protein (Yecora Rojo) and the low-protein (Anza or Yolo) cultivars. Mean recovery from deep placement was 60% versus only 54% from shallow placement (p < 0.01). Delaying the time of injection decreased mean recovery significantly from 58% at 4 DAA to 54% at 20 DAA. This decrease was most pronounced in the shallow placement, where soil drying was most severe. Regressions of recovery on soil water potential of individual cultivar x yr x planting x depth treatments were significant only under the driest conditions. Stepwise regression of ¹⁵N recovery on soil water potential and yield parameters using data from all treatments of both years resulted in an equation including soil water potential and N yield, with a multiple correlation coefficient of 0.64. The translocation of ¹⁵N to grain was higher (0.89) than the nitrogen harvest index (0.69), and showed a highly significant increase with increase in DAA. This experiment indicates that the N uptake capacity of wheat remains reasonably constant between 4 and 20 DAA unless soil drying is severe.     

Fine Roots vs. Needles: A Comparison of 13C and 15N Dynamics in a Ponderosa Pine Forest Soil

Plant allocation patterns may affect soil C and N storage due to differences in litter quality and the depth of plant C and N inputs into the soil. We studied the dynamics of dual-labeled (¹³C/¹⁵N) Pinus ponderosa needles and fine roots placed at two soil depths (O and A horizon) in a temperate conifer forest soil during 2 y. Input of C as fine roots resulted in much more C retained in soil (70.5 ± 2.2% of applied) compared with needle C (42.9 ± 1.3% of applied) after 1.5 y. Needles showed faster mass loss, rates of soil ¹³CO₂ efflux, and more ¹⁵N immobilized into microbial biomass than did fine roots. The larger proportion of labile C compounds initially present in needles (17% more needle C was water soluble than in fine roots) likely contributed to its shorter C residence time and greater degree of transformation in the soil. A double exponential decay function best described the rate of ¹³C loss, with a smaller initial pulse of C loss from fine roots (S₁k₁) and a slower decay rate of the recalcitrant C pool for fine roots (0.03 y⁻¹) compared with (0.19 y⁻¹) for needles. Soil ¹³C respiration, representing heterotrophic respiration of litter C, was much more seasonal from the O horizon than from the A. However, offsetting seasonal patterns in ¹³C dynamics in the O horizon resulted in no net effect of soil depth on total ¹³C retention in the soil after 1.5 y for either litter. Almost 90% of applied litter N was retained in the soil after 1.5 y, independent of litter quality or soil depth. Very small amounts of ¹³C or ¹⁵N (<3% of applied) moved to the horizon above or below the placement depth (i.e., O to A or A to O). Our results suggest that plant allocation belowground to fine roots results in more C retained and less N mineralized compared with allocation aboveground to needles, primarily due to litter quality differences.     

Biodegradation of Trichloroacetic Acid in Norway Spruce/Soil System

Trichloroacetic acid (TCA) belongs to secondary atmospheric pollutants affecting the forest health. Distribution of [1,2-¹⁴C]TCA-residues and TCA biodegradation were investigated in 4-year-old nursery-grown trees of Norway spruce [Picea abies (L.) Karst.] in the whole plant/soil system. Radioactivity was monitored in needles, wood, roots and soil as well as in the air. During two weeks of exposure TCA was continuously degraded, especially in the soil. Estimates of radioactivity balance showed loss of radioactivity into the atmosphere in the form of ¹⁴CO₂; unincorporated [1,2-¹⁴C]TCA, chloroform, carbon monoxide and methane were not detected at all. TCA degradation to CO₂ was indicated also in the spruce needles. Moreover, it was found that soil litter contained [1,2-¹⁴C]TCA unavailable to microorganisms.     

Seed source variation in 32P uptake in inus taeda L. seedlings: A possible regulation by efflux

Short-term ³²P uptake experiments were conducted with intact seedlings of loblolly pine (Pinus taeda L.) to examine possible seed source variation in net accumulation of ³²P in roots and shoots, and in rates of unidirectional influx. Seed source had a highly significant effect on biomass and P concentrations of shoots and roots. Seedlings from two seed sources representing fast-growing populations (a broadly-adapted and wet-site seed source) accumulated over 60% more total seedling P than smaller seedlings from a drought-hardy seed source, reflecting higher biomass and root P concentrations. Rates of unidirectional ³²P influx in seedlings from the drought-hardy seed source were more than twice the rates of the seedlings from the broadly-adapted seed source. However, after 24 h in labeled uptake solution, net accumulation of ³²P was similar, suggesting that rates of unidirectional efflux from roots of the drought-hardy seed source were also high. Although there were no significant differences in biomass and tissue P concentrations between the two fast-growing seed sources, rates of unidirectional influx in seedlings from the broadly-adapted seed source were 42% lower than rates in seedlings from the wet-site source. Yet, after 24 h in labeled uptake solution, net accumulation of ³²P in seedlings from the broadly-adapted seed source was 50% higher. Unidirectional efflux out of the root may regulate net uptake of P as much, if not more, than influx in loblolly pine seedlings-at least under high-P growth conditions. The results in this study do not support previous studies with herbaceous plants suggesting that fast-growing species typically exhibit higher rates of nutrient uptake than slow-growing species.     

The fate of18O labelled phosphate in soil/plant systems

KH₂PO₄ labelled with¹⁸O and³²P was mixed with soil that was placed in pots in which grass seed was sown. Grass samples were taken after 5, 7, and 12 weeks. The dilution factor (DF) for¹⁸O in the first cut was much greater than the DF for³²P, indicating that the bulk of the¹⁸O in the absorbed phosphate was lost. The DFs for¹⁸O and³²P determined in phosphate extracted from the soil at the end of the pot experiment indicated that half the¹⁸O excess in the applied phosphate was lost.A succeeding experiment showed no loss of¹⁸O when the treated soil was shaken for 3 months with water to which a germicide was added. Thus, the loss of¹⁸O was presumably caused by biochemical processes which brought about the replacement of¹⁸O by¹⁶O. We suggest that the loss of¹⁸O from applied labelled phosphate may be used as a measure of biological activity in soil.     

Nitrogen-15 labeling effectiveness of two tropical legumes

Nitrogen-15 foliar applications for the production of field-labeled plant tissues may achieve more effective labeling of plant shoot and root tissues and minimize directly labeling the soil N fraction as occurs when¹⁵ N is soil applied. Consequently, foliar-labeled plant tissues should be better suited for subsequent ¹⁵N mineralization studies. A field experiment was conducted to determine the effectiveness of ¹⁵N-labeling and the accumulation of ¹⁵N in various plant parts of two tropical legumes. Desmodium ovalifolium Guillemin and Perrottet and Pueraria phaseoloides (Roxb.) Benth., grown in 0.5 m² microplots, were labeled with foliar-applied urea containing 99 atom% ¹⁵N. Plants in each microplot received a total of 0.1698 g ¹⁵N that was applied all at once or split equally into two, three or four applications. Legume shoots and roots and soil were destructively harvested and analyzed for total ¹⁵N content. Averaged over both legumes and foliar application rates, total plant (shoots, flowers, leaf litter, and roots) recovery was approximately 79% of the ¹⁵N applied. The soil contained 3% of the ¹⁵N applied, of which 2.5 and 0.5% were in the inorganic and organic fractions, respectively. Nitrogen-15 recovery in shoots (76%) was sixty-five fold greater than in roots (1%) and about nineteen fold greater than the sum of roots and soil (4.1%), a much greater percent recovery than observed in other foliar labeling studies. Averaged over all four foliar split-application rates, ¹⁵N recovery by Desmodium shoots was greater than Pueraria. Results demonstrate that ¹⁵N foliar application to legumes is an effective method for labeling, resulting in atom% excess ¹⁵N levels and ¹⁵N recoveries comparable to those reported with the more traditional soil-labeling approach. Another advantage of this method is a nondestructive, in situ labeling method that permits separation of shoot and root residual N contribution to subsequent crops in N tracer studies.     

Properties of potassium uptake by seedling roots of grape cultivars

Uptake rates of (⁸⁶Rb)K⁺ by seedling roots of six cultivars were measured and compared with K⁺ content of the root, K⁺ leakage, H⁺ efflux, and K⁺-ATPase activity of a partially purified plasmalemma fraction.Different cultivars showed significantly different rates of (⁸⁶Rb)K⁺ uptake. The uptake rates of the first (0–5 min) period did not correlate with K⁺ content of the seedling roots.The rates of uptake in the 10 to 30 min period, supposed to be active, were negatively correlated with K⁺ content of the root. Roots consistently leaked K⁺ during the first 5 min. This leakage was positively correlated with the endogenous K⁺ content of the tissue.H⁺ efflux was significantly different among the cultivars and correlated with the K⁺-ATPase activity of a microsomal fraction partially purified on discontinuous (18/34%) sucrose gradient. The relationships among transport parameters are discussed.Part of the present research was carried out at the Department of Agricultural Biotechnology of University of Padova, Italy.Part of the present research was carried out at the Department of Agricultural Biotechnology of University of Padova, Italy.     

Cytoplasmic streaming in the root cortex and its role in the delivery of potassium to the shoot

The influence of cytochalasin B (CB), a potent inhibitor of cytoplasmic streaming, on ⁸⁶Rb-labelled K⁺ translocation by detopped Lycopersicon esculentum Mill., Cucumis sativus L. and Zea mays L. plants was examined by measuring the radioactivity in xylem exudate before and after the addition of CB to the medium bathing the roots. CB caused complete cessation of cytoplasmic streaming in root segments within 15 min but was without effect on either total ⁸⁶Rb uptake or exudation. Thus factors other than cytoplasmic streaming limit the movement of K⁺ across the symplast of the root of higher plants.     

Do oaks have different strategies for uptake of N, K and P depending on soil depth?

The uptake of nutrients from deep soil layers has been shown to be important for the long-term nutrient sustainability of forest soils. When modelling nutrient uptake in forest ecosystems, the nutrient uptake capacity of trees is usually defined by the root distribution. However, this leads to the assumption that roots at different soil depths have the same capacity to take up nutrients. To investigate if roots located at different soil depths differ in their nutrient uptake capacity, here defined as the nutrient uptake rate under standardized conditions, a bioassay was performed on excised roots (<1 mm) of eight oak trees (Quercus robur L.). The results showed that the root uptake rate of ⁸⁶Rb⁺ (used as an analogue for K⁺) declined with increasing soil depth, and the same trend was found for . The root uptake rate of , on the other hand, did not decrease with soil depth. These different physiological responses in relation to soil depth indicate differences in the oak roots, and suggest that fine roots in shallow soil layers may be specialized in taking up nutrients such as K⁺ and which have a high availability in these layers, while oak roots in deep soil layers are specialized in taking up other resources, such as P, which may have a high availability in deep soil layers. Regardless of the cause of the difference in uptake trends for the various nutrients, these differences have consequences for the modelling of the soil nutrient pool beneath oak trees and raise the question of whether roots can be treated uniformly, as has previously been done in forest ecosystem models. Responsible Editor: Herbert Johannes Kronzucker.     

Native soil and fresh fertilizer phosphorus uptake as affected by rate of application and P fertilizers

³²P labelled fertilizers were used to measure native soil and fresh fertilizer phosphorus uptake byLolium perenne L. in greenhouse experiments. The P source evaluation was carried out for multiple rates of application for a standard P fertilizer (DAP) on low and medium soil P levels and for North Carolina rock phosphate (RP) at the medium soil P level only. At the low soil P level, the native P uptake increase was independent of P-DAP applied, and represented 19% of the nil P uptake. At the medium soil P level, the variability of the native soil as a nutrient P source depended on the phosphate fertilizer applied, and the rate of application. Consequently the amount of total P uptake could conceal differences in P fertilizer evaluations as the nutrient P source. Fresh P uptake increased linearly with the rates of P applied as standard or tested P fertilizer. The comparison of various P sources by means of fresh P uptake ratio (i.e. fresh P uptake from tested phosphate divided by fresh P uptake from standard phosphate) was independent of the rate of application. It was therefore suggested that various phosphorus sources be evaluated by measuring the fresh P uptake for a single rate of application.     

Der basipetale [2-14C] Abscisinsäuretransport in Wurzeln intakter Bohnenkeimlinge und seine Bedeutung für den Wurzelgeotropismus

Summary One h and 3 h after point source application of [2-¹⁴C] ABA to the upper side of geotropically stimulated root tips of intact seedlings of Phaseolus coccineus L. a weak downward lateral transport of radioactivity was observed. A clear upward lateral translocation, however, occurred when ABA was applied to the lower side of the root tip. Thus a uniform distribution of radioactivity was established in horizontal roots regardless of the site of the application point, while in upright roots the distribution was asymmetric. Six h after ABA injection radioactivity was uniformly distributed in both the vertical and horizontal root tips.

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Abkürzungen ABA Abscisinsäure - dpm disintegrations perminute - FG Frischgewicht - GA Gibberellinsäure     

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).     

Relationship between intracellular pH and N metabolism in maize (Zea mays L.) roots

In vivo ³¹P nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO₃ vs. NH₄) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0–5 mm) and root segments (5–30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo ¹⁵N NMR was used to characterize the pathway of primary ammonium assimilation in the ammonium-grown roots and to compare the activity of the apical and more-basal root parts. The kinetics of ¹⁵NH₄ ⁺ incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammonium-treated root tips.     

Short-term effects of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide on perennial ryegrass

The current study investigated the short-term physiological implications of plant nitrogen uptake of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) under both greenhouse and field conditions. ¹⁵N labelled urea amended with 0.0, 0.01, 0.1 and 0.5% nBTPT (w/w) was surface applied at a rate equivalent to 100 kg N ha^–1 to perennial ryegrass in a greenhouse pot experiment. Root, shoot and soil fractions were destructively harvested 0.75, 1.75, 4, 7 and 10 days after fertilizer application. Urease activity was determined in each fraction together with ¹⁵N recovery and a range of chemical analyses. The effect of nBTPT amended urea on leaf tip scorch was evaluated together with the effect of the inhibitor applied on its own on plant urease activity.nBTPT-amended urea dramatically reduced shoot urease activity for the first few days after application compared to unamended urea. The higher the nBTPT concentration the longer the time required for shoot activity to return to that in the unamended treatment. At the highest inhibitor concentration of 0.5% shoot urease activity had returned to that of unamended urea by 10 days. Root urease activity was unaffected by nBTPT in the presence of urea but was affected by nBTPT in the absence of urea.Transient leaf tip scorch was observed approximately 7–15 days after nBTPT + urea application and was greatest with high concentrations of nBTPT and high urea-N application rates. New developing leaves showed no visual sign of tip necrosis.Urea hydrolysis of unamended urea was rapid with only 1.3% urea-N remaining in the soil after 1.75 days. N uptake and metabolism by ryegrass was rapid with ¹⁵N recovery from unamended urea, in the plant (shoot + root) being 33% after 1.75 days. Most of the ¹⁵N in the soil following the urea+0.5% nBTPT application was still as urea after 1.75 days, yet ¹⁵N plant recovery at this time was 25% (root+shoot). This together with other evidence, suggests that if urea hydrolysis in soil is delayed by nBTPT then urea can be taken up by ryegrass as the intact molecule, albeit at a significantly slower initial rate of uptake than NH₄ ⁺-N. Protein and water soluble carbohydrate content of the plant were not significantly affected by amending urea with nBTPT however, there was a significant effect on the composition of amino acids in the roots and shoots, suggesting a difference in metabolism.Although nBTPT-amended urea affected plant urease activity and caused some leaf-tip scorch the effects were transient and short-lived. The previously reported benefit of nBTPT in reducing NH₃ volatilization of urea would appear to far outweigh any of the observed short-term effects, as dry-matter production of ryegrass is increased.     

Carbon distribution and nitrogen partitioning in a soil-plant system with barley (Hordeum vulgare L.), ryegrass (Lolium perenne) and rape (Brassica napus L.) grown in a14CO2-atmosphere

To examine the influence of plant-microorganism interactions on soil-N transformations (e.g. net mineralization, net immobilization) a pot experiment was conducted in a¹⁴C-labelled atmosphere by using different (two annuals, one perennial) plants species. It was assumed that variation in below-ground, microorganism-available C would influence N transformations in soil. Plant species were fertilized (low rate) with¹⁵N-labelled nitrogen and grown, during days 13 and 62 after germination, in a growth chamber with a¹⁴C-labelled atmosphere. Nitrification was inhibited by using nitrapyrin (N-Serve). During the chamber period, shoots were harvested, and associated roots and soil were collected on two sampling occasionm, e.g. after 4 and 7 weeks in the growth chamber.The distribution of net (%) assimilated¹⁴C was significantly affected by both plant and time factors, and there was a significant plant × time interaction. There were significant differences between plants in all plant-soil compartments examined as well as in the degree of the plant × time interaction.Differences in the¹⁴C distribution between plants were due to both interspecific and developmental variation. In general, when comparing¹⁵N and¹⁴C quantities between species, many of the differences found between plants can be explained by the differences determined in the weight of shoot or root parts. Despite the fact that amounts of C released were greater in ryegrass than in the other plant-treatments no unequivocal evidence was found to show that the effects of plant-microorganism interactions on soil-N mineralization were greater under ryegrass. Possible mechanisms accounting for the partitioning of N found among plant biomass, soil biomass and soil residues are discussed.