Root proliferation,nitrate inflow and their carbon costs during nitrogen capture by competing plants in patchy soil

The responses of roots to nitrogen- and phosphorus-rich patches of soil include proliferation of laterals and stimulation of nutrient inflow (uptake rate per unit root length) within the patch. Nitrate uptake from an N-rich patch is thereby maximised and, perhaps, compensates for an uneven supply of nitrate to the whole root system. Paradoxically, the often weak correlation between root length density and N uptake found in experiments on single plants and crop monocultures suggests that root proliferation in patches has only a minor compensatory influence on N capture. This paradox was resolved when it was realised that localised root proliferation during inter-specific competition for nitrate can lead to a strong association between root length density and nitrate capture. Here, a simple model of inter-specific competition is used to estimate the stimulation in inflow required in one plant to match the N capture of a competitor that responds only by root proliferation, and to estimate associated carbon costs. The model predicts that nitrate inflow must increase proportionally more than root length density to achieve the same N capture. For example, the N capture possible with a 10% increase in root length density can be matched by increasing N inflow by anything from 20% to 20-fold, depending on the initial conditions: the faster the rate of change in root length density, the greater the required relative increase in inflow. In those terms, proliferation would seem the better option, but one that may be more costly in terms of its carbon requirement.     

N capture by Plantago lanceolata and Brassica napus from organic material: the influence of spatial dispersion, plant competition and an arbuscular mycorrhizal fungus

This study investigated N capture by Plantago lanceolata L. and Brassica napus L. from complex organic material (dual-labelled with 15N/13C) added either as a thin concentrated layer (discrete patch treatment) or dispersed uniformly with the background sand:soil mix in a 10 cm band (dispersed treatment) when grown in monoculture or in interspecific competition and in the presence or absence of a mycorrhizal inoculum (Glomus mosseae). No 13C enrichments from the organic material were detected in the plant tissues, but 15N enrichments were present. Total plant uptake of N from the organic material on a microcosm basis was not affected by the spatial placement of the organic material, but Plantago monocultures captured less N than the species in interspecific competition (i.e. 23% versus 38% of the N originally added). N capture from Brassica monocultures was no different to either Plantago monocultures or both species in mixture. However, N capture from the organic material by both individual Plantago and Brassica plants was reduced when grown with Brassica plants (by 10-fold and by more than half, respectively). N capture from the organic material was directly related to the estimated root length produced in the sections containing the organic material: the individual that produced the greatest root length captured most N. Strikingly, when the organic material was added as a discrete patch the N captured by Brassica, a non-mycorrhizal species, actually increased when the G. mosseae inoculum was present compared to when G. mosseae was absent (i.e. 35% versus 19% of the N originally added).     

Importance of soil cover box area for the determination of N2O emissions from arable soils

To obtain nutrients mineralised from organic matter in the soil, plants have to respond to its heterogeneous distribution. We measured the timing of nitrogen uptake by wheat from a localised, ¹⁵N labelled organic residue in soil, as well as the timing of changes in root length density. We calculated the rates of N uptake per unit root length (inflows) for roots growing through the residue and for the whole root system. A stimulated local inflow appeared to be the main mechanism of exploitation of the residue N during the first five days of exploitation. 8% of the N that the plants would ultimately obtain from the residue was captured in this period. Roots then proliferated in the residue. This, together with a rapidly declining N inflow, contributed to the capture, over the next seven days, of 63% of the N that the plants derived from the residue. After that time, massive root proliferation occurred in the residue, but relatively little further N was captured.     

An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient-rich patches in soil

Most work on root proliferation to a localized nutrient supply has ignored the possible role of mycorrhizal fungi, despite their key role in nutrient acquisition. Interactions between roots of Plantago lanceolata , an added arbuscular mycorrhiza (AM) inoculum and nitrogen capture from an organic patch ( Lolium perenne shoot material) dual-labelled with ¹⁵N and ¹³C were investigated, to determine whether root proliferation and nitrogen (N) capture was affected by the presence of AM fungi. Decomposition of the organic patch in the presence and absence of roots peaked in all treatments at day 3, as shown by the amounts of ¹³CO₂ detected in the soil atmosphere. Plant N concentrations were higher in the treatments with added inoculum 10 d after patch addition, but thereafter did not differ among treatments. Plant phosphorus concentrations at the end of the experiment were depressed by the addition of the organic residue in the absence of mycorrhizal inoculum. Although uninoculated plants were also colonized by mycorrhizal fungi, colonization was enhanced at all times by the added inoculum. Addition of the AM inoculum increased root production, observed in situ by the use of minirhizotron tubes, most pronouncedly within the organic patch zone. Patch N capture by the end of the experiment was c . 7.5% and was not significantly different as a result of adding an AM inoculum. Furthermore, no ¹³C enrichments were detected in the plant material in any of the treatments showing that intact organic compounds were not taken up. Thus, although the added AM fungal inoculum benefited P. lanceolata seedlings in terms of P concentrations of tissues it did not increase total N capture or affect the form in which N was captured by P. lanceolata roots.     

Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material

Nitrogen (N) capture by arbuscular mycorrhizal (AM) fungi from organic material is a recently discovered phenomenon. This study investigated the ability of two Glomus species to transfer N from organic material to host plants and examined whether the ability to capture N is related to fungal hyphal growth. Experimental microcosms had two compartments; these contained either a single plant of Plantago lanceolata inoculated with Glomus hoi or Glomus intraradices, or a patch of dried shoot material labelled with (15)N and (13)carbon (C). In one treatment, hyphae, but not roots, were allowed access to the patch; in the other treatment, access by both hyphae and roots was prevented. When allowed, fungi proliferated in the patch and captured N but not C, although G. intraradices transferred more N than G. hoi to the plant. Plants colonized with G. intraradices had a higher concentration of N than controls. Up to one-third of the patch N was captured by the AM fungi and transferred to the plant, while c. 20% of plant N may have been patch derived. These findings indicate that uptake from organic N could be important in AM symbiosis for both plant and fungal partners and that some AM fungi may acquire inorganic N from organic sources.     

Comparison of early development of three grasses: Lolium perenne, Agrostis stolonifera and Poa pratensis

BACKGROUND AND AIMS: To improve the management of grass communities, early plant development was compared in three species with contrasting growth forms, a caespitose (Lolium perenne), a rhizomatous (Poa pratensis) and a caespitose-stoloniferous species (Agrostis stolonifera). METHODS: Isolated seedlings were grown in a glasshouse without trophic constraints for 37 d (761 degrees Cd). The appearance of leaves and their location on tillers were recorded. Leaf appearance rate (LAR) on the tillers and site-filling were calculated. Tillering was modelled based on the assumption that tiller number increases with the number of leaves produced on the seedling main stem. Above- and below-ground parts were harvested to compare biomass. KEY RESULTS: Lolium perenne and A. stolonifera expressed similar bunch-type developments. However, root biomass was approx. 30 % lower in A. stolonifera than in L. perenne. Poa pratensis was rhizomatous. Nevertheless, the ratio of above-ground : below-ground biomass of P. pratensis was similar to that of L. perenne. LAR was approximately equal to 0.30 leaf d(-1) in L. perenne, and on the main stem and first primary tillers of A. stolonifera. LAR on the other tillers of A. stolonifera was 30 % higher than on L. perenne. For P. pratensis, LAR was 30 % lower than on L. perenne, but the interval between the appearance of two successive shoots from rhizomes was 30 % higher than the interval between two successive leaf stages on the main stem. Above-ground parts of P. pratensis first grew slower than in the other species to the benefit of the rhizomes, whose development enhanced tiller production. CONCLUSIONS: Lolium perenne had the fastest tiller production at the earliest stages of seedling development. Agrostis stolonifera and P. pratensis compensated almost completely for the delay due to higher LAR on tillers or ramets compared with L. perenne. This study provides a basis for modelling plant development.     

A trade-off between nitrogen uptake and use increases responsiveness to elevated CO2 in infrequently cut mixed C3 grasses

The aim of this study was to evaluate whether the responsiveness of mixed C3 grass species to elevated CO2 is related more to nitrogen uptake or to N-use efficiency. Nitrogen uptake and whole-plant N-use efficiency were investigated with two binary mixtures: Lolium perenne was mixed either with Festuca arundinacea or with Holcus lanatus. The swards were grown on sand with or without CO2 doubling, and subjected to two cutting frequencies. A C20 alcohol was used as a marker to determine species proportion in the total root mass of the mixtures. The mean residence time of N was calculated from that of 15N-labelled fertilizer. Lolium perenne took up significantly more N per unit root mass than its grass competitors, but its N-use efficiency was lower. Elevated CO2 significantly reduced the N uptake of the three grass species. A trade-off between N capture and use was found, as N-use efficiency and N-uptake rate were negatively correlated. A high N-use efficiency, and conversely low N uptake appeared to favour the responsiveness to elevated CO2 of the infrequently cut grasses.     

A proteomic and targeted metabolomic approach to investigate change in Lolium perenne roots when challenged with glycine

A combined proteomic and isotope tracer approach was used to investigate the impact of supplying N as glycine to roots of Lolium perenne. Initially, ammonium nitrate was supplied to all plants, after which half received glycine as their sole N source, while the remainder continued to receive ammonium nitrate. Plants supplied with glycine acquired less N than those receiving the mineral source, resulting in reduced root nitrate concentrations. The amino acid complement of roots was also strongly affected by the form of N supplied, and 15N labelling indicated that the biochemical fate of acquired N in roots was dependent on the form of N available for uptake. Proteomic analysis of Lolium roots indicated that 6% of 627 root proteins resolved on 2D gels changed in abundance in response to the form of N applied. Multivariate analysis of protein abundance clearly discriminated the proteomes of L. perenne roots as a function of treatment applied. Seven affected proteins were identified (mostly by protein homology with sequenced species), including methionine adenosyltransferase, an enzyme involved in glycine metabolism. Although some changes in root amino acid and protein complements were due to responses to reduced N supply, both the distinct fate of 15N tracers and the abundances of identified proteins could be attributed specifically to the form of N available to roots. The results demonstrate the potential of targeted proteomic approaches to identify functioning of plants where more traditional methods cannot resolve multiple, co-incident biological interactions and element fluxes.     

A demonstration of a 1:1 correspondence between chiasma frequency and recombination using a Lolium perenne/Festuca pratensis substitution

A single chromosome of the grass species Festuca pratensis has been introgressed into Lolium perenne to produce a diploid monosomic substitution line 2n = 2x = 14. The chromatin of F. pratensis and L. perenne can be distinguished by genomic in situ hybridization (GISH), and it is therefore possible to visualize the substituted F. pratensis chromosome in the L. perenne background and to study chiasma formation in a single marked bivalent. Recombination occurs freely in the F. pratensis/L. perenne bivalent, and chiasma frequency counts give a predicted map length for this bivalent of 76 cM. The substituted F. pratensis chromosome was also mapped with 104 EcoRI/Tru91 and HindIII/Tru91 amplified fragment length polymorphisms (AFLPs), generating a marker map of 81 cM. This map length is almost identical to the map length of 76 cM predicted from the chiasma frequency data. The work demonstrates a 1:1 correspondence between chiasma frequency and recombination and, in addition, the absence of chromatid interference across the Festuca and Lolium centromeres.     

Effects of earthworms and organic litter distribution on plant performance and aphid reproduction

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of ¹⁵N-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and ¹⁵N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more ¹⁵N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores.     

Influence of nitrate concentration at the root surface on yield and nitrate uptake of kohlrabi (Brassica oleracea gongyloides L.) and spinach (Spinacia oleracea L.)

The objective of this study was to determine if plant roots have to take up nitrate at their maximum rate for achieving maximum yield. This was investigated in a flowing-solution system which kept nutrient concentrations at constant levels. Nitrate concentrations were maintained in the range 20 to 1000 μM. Maximum uptake rate for both species was obtained at 100 μM. Concentrations below 100 μM resulted in decreases in uptake rate per cm root (inflow) for both spinach and kohlrabi by 1/3 and 2/3, respectively. However, only with kohlrabi this caused a reduction in N uptake and yield. Thus indicating that this crop has to take up nitrate at the maximum inflow. Spinach, however, compensated for lower inflows by enhancing its root absorbing surface with more and longer roots hairs. Both species increased their root length by 1/3 at low nitrate concentrations.     

Dynamic and steady-state responses of inorganic nitrogen pools and NH(3) exchange in leaves of Lolium perenne and Bromus erectus to changes in root nitrogen supply

Short- and long-term responses of inorganic N pools and plant-atmosphere NH(3) exchange to changes in external N supply were investigated in 11-week-old plants of two grass species, Lolium perenne and Bromus erectus, characteristic of N-rich and N-poor grassland ecosystems, respectively. A switch of root N source from NO(-)(3)to NH(4)(+) caused within 3 h a 3- to 6-fold increase in leaf apoplastic NH(4)(+) concentration and a simultaneous decrease in apoplastic pH of about 0.4 pH units in both species. The concentration of total extractable leaf tissue NH(4)(+) also increased two to three times within 3 h after the switch. Removal of exogenous NH(4)(+) caused the apoplastic NH(4)(+) concentration to decline back to the original level within 24 h, whereas the leaf tissue NH(4)(+)concentration decreased more slowly and did not reach the original level in 48 h. After growing for 5 weeks with a steady-state supply of NO(-)(3)or NH(4)(+), L. perenne were in all cases larger, contained more N, and utilized the absorbed N more efficiently for growth than B. erectus, whereas the two species behaved oppositely with respect to tissue concentrations of NO(-)(3), NH(4)(+), and total N. Ammonia compensation points were higher for B. erectus than for L. perenne and were in both species higher for NH(4)(+)- than for NO(-)(3)-grown plants. Steady-state levels of apoplastic NH(4)(+), tissue NH(4)(+), and NH(3) emission were significantly correlated. It is concluded that leaf apoplastic NH(4)(+) is a highly dynamic pool, closely reflecting changes in the external N supply. This rapid response may constitute a signaling system coordinating leaf N metabolism with the actual N uptake by the roots and the external N availability.     

Identification of two novel phytosiderophores secreted by perennial grasses

It has been suggested that some perennial grasses secrete phytosiderophores in response to iron (Fe) deficiency, but the compounds have not been identified. Here, we identified and characterized the phytosiderophores secreted by two perennial grasses, Lolium perenne cv. Tove and Poa pratensis cv. Baron. Root exudates were collected from the roots of Fe-deficient grasses and then purified with various chromatographies. The structure of the purified compounds was determined using both nuclear magnetic resonance and fast atom bombardment mass spectrometry. Both species secreted phytosiderophores in response to Fe deficiency, and the amount of phytosiderophores secreted increased with the development of Fe deficiency. The type of phytosiderophores secreted differed with plant species; L. perenne cv. Tove secreted 3-epihydroxy-2'-deoxymugineic acid (epiHDMA), 2'-deoxymugineic acid (DMA) and an unknown compound, whereas P. pratensis cv. Baron secreted DMA, avenic acid A (AVA) and an unknown compound. Purification and subsequent analysis with nuclear magnetic resonance and mass led to identification of the two novel phytosiderophores; 3-hydroxy-2'-deoxymugineic acid (HDMA) from L. perenne, and 2'-hydroxyavenic acid A (HAVA) from P. pratensis. Both novel phytosiderophores have similar chelating activity to known phytosiderophores.     

Nutrient inflows into apple roots

Summary The rates of uptake of nutrients from solution by apple roots were measured (a) in a root laboratory, using intact roots of mature trees growing under field conditions and (b) in controlled environment using young trees. Maximum nitrate inflows into Discovery/M.9 roots under field conditions were only slightly lower than those into roots of the same genotype in controlled environment, but up to 80 times lower than those into roots of Worcester Pearmain seedlings. At any given external P concentration, P inflows into roots of field-grown trees were about 2.5-times lower than those into the roots of young trees in controlled environment.Nitrate inflows were constant above a solution concentration of 20 mmol m^–3 in both field-grown and small trees. In both cases, phosphate inflows increased linearly with solution concentration up to 10 mmol m^–3.Among the various plant and environmental factors influencing nutrient uptake characteristics of apple roots were: the scion genotype, tissue nutrient levels, root origin, the form in which N is supplied, level of irradiance of the shoot, root temperature and the season of the year. The effects of these factors are illustrated with examples.     

Intercropping effect on root growth and nitrogen uptake at different nitrogen levels

Aims Intercropping legumes and non-legumes may affect the root growth of both components in the mixture, and the non-legume is known to be strongly favored by increasing nitrogen (N) supply. The knowledge of how root systems affect the growth of the individual species is useful for understanding the interactions in intercrops as well as for planning cover cropping strategies. The aim of this work was (i) to determine if different levels of N in the topsoil influence root depth (RD) and intensity of barley and vetch as sole crops or as an intercropped mixture and (ii) to test if the choice of a mixture or the N availability in the topsoil will influence the N uptake by deep roots.Methods In this study, we combined rhizotron studies with root extraction and species identification by microscopy with studies of growth, N uptake and 15 N uptake from deeper soil layers, for studying the root interactions of root growth and N foraging for barley (Hordeum vulgare L.) and vetch (Vicia sativa L.), frequently grown in mixtures as cover crops. N was added at 0 (N0), 50 (N1) and 150 (N2) kg N ha^-1. The roots discrimination relying on the anatomical and morphological differences observed between dicots and monocots proved to be a reliable method providing valuable data for the analysis.Important findings The intercrop and the barley attained slightly higher root intensity (RI) and RD than the vetch, with values around 150 crosses m^-1 and 1.4 m, respectively, compared to 50 crosses m^-1 and 0.9 m for the vetch. At deep soil layers, intercropping showed slightly larger RI values compared to the sole-cropped barley. The barley and the intercropping had larger root length density (RLD) values (200–600 m m ?3) than the vetch (25–130) at 0.8–1.2 m depth. The topsoil N supply did not show a clear effect on the RI, RD or RLD; however, increasing topsoil N favored the proliferation of vetch roots in the intercropping at deep soil layers, with the barley:vetch root ratio ranging from 25 at N0 to 5 at N2. The N uptake of the barley was enhanced in the intercropping at the expense of the vetch (from ~100mg plant^-1 to 200). The intercropped barley roots took up more labeled nitrogen (0.6mg 15 N plant^-1) than the sole-cropped barley roots (0.3mg 15 N plant^-1) from deep layers.     

Physical and genetic mapping in the grasses Lolium perenne and Festuca pratensis

A single chromosome of the grass species Festuca pratensis has been introgressed into Lolium perenne to produce a diploid monosomic substitution line 2n = 2x = 14. In this line recombination occurs throughout the length of the F. pratensis/L. perenne bivalent. The F. pratensis chromosome and recombinants between it and its L. perenne homeologue can be visualized using genomic in situ hybridization (GISH). GISH junctions represent the physical locations of sites of recombination, enabling a range of recombinant chromosomes to be used for physical mapping of the introgressed F. pratensis chromosome. The physical map, in conjunction with a genetic map composed of 104 F. pratensis-specific amplified fragment length polymorphisms (AFLPs), demonstrated: (1) the first large-scale analysis of the physical distribution of AFLPs; (2) variation in the relationship between genetic and physical distance from one part of the F. pratensis chromosome to another (e.g., variation was observed between and within chromosome arms); (3) that nucleolar organizer regions (NORs) and centromeres greatly reduce recombination; (4) that coding sequences are present close to the centromere and NORs in areas of low recombination in plant species with large genomes; and (5) apparent complete synteny between the F. pratensis chromosome and rice chromosome 1.     

Nitrogen uptake and root morphological responses of defoliated Lolium perenne (L.) to a heterogeneous nitrogen supply

Lolium perenne L. (c.v. Magella) plants were grown under three nutrient treatments for six weeks and then defoliated to test the hypothesis that for their regrowth they could acquire N equally well irrespective of N distribution. Two different N levels were applied; uniform level 1 N (U1), uniform level 2 (U2) and heterogeneous level 2 (H2). A system where the nutrient patch could be applied without barriers to root growth was adopted. A single defoliation to 4 cm height resulted in a reduction in tillering, biomass increment and N uptake at 3 weeks after defoliation. Root growth was reduced by defoliation under all N treatments. Defoliation was found to reduce the proportion of N in the shoots which was derived from root uptake from 7 to 14 days. At 21 days this effect was significant for the plants with a heterogeneously distributed supply only. By the end of the regrowth period, the undefoliated plants from H2 had a shoot biomass and N content equal to that of plants receiving the same total N but supplied homogeneously (U2). However, defoliation reduced the ability of the plant to acquire N from the patch. No preferential root growth was measured into the N-rich patch, but an increased root diameter within the patch was found. Root diameter was reduced by defoliation, coinciding with a reduction in concentration of N in the root tissue. As a result of the increased sink strength of the growing leaves after defoliation, the roots may become a source of carbon and also nitrogen. These responses to an N-rich patch under defoliation could alter a plant's competitive balance in a mixed sward. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparison of tall fescue (Cyperales: Gramineae) to other cool-season turfgrasses for tolerance to European chafer (Coleoptera: Scarabaeidae)

Three cultivars of tall fescue, Festuca arundinacea Schreb., were compared with three cultivars each of fine fescue (Festuca spp.), Kentucky bluegrass (Poa pratensis L.), and perennial ryegrass (Lolium perenne L.) to evaluate tolerance to root-feeding by European chafer grubs, Rhizotrogus majalis (Razoumowsky). Potted turfgrasses were infested with initial densities equivalent to 33 or 66 grubs per 0.1 m2 on 19 August 2000. More grubs were added in late September and October, bringing the total to 66 or 143 grubs per 0.1 m2. Plant growth, root loss, weight gain, and survival of grubs were measured. The experiment was repeated in fall of 2001 with an initial density of 66 grubs per 0.1 m2. The proportion of root mass lost as a result of grub feeding was a function of turf species, root growth, grub survival, and grub growth during the test. Grubs gained the most weight and consumed the most roots when feeding on fine fescue. Fine fescue suffered the greatest percentage of root loss in 2000, despite having the most rapid root growth and largest mass in control pots. Cultivars of tall fescue appeared to be the most tolerant of grub feeding, having the smallest reduction in root mass in both years. Data from fine fescue, Kentucky bluegrass, and perennial ryegrass cultivars were not as consistent as tall fescue, because for some cultivars root growth and grub survival were different between years. We also found that grubs increased in mass by 20% when the mass of available roots was doubled.     

Nitrogen capture by plants grown in N-rich organic patches of contrasting size and strength

This study investigated the response of root proliferation, plant N capture and soil microbial activity in a Lolium perenne sward to L-lysine (¹⁵N/¹³C dual-labelled) patches of contrasting size and strength. Three patch treatments provided the same quantity (c. 5.6 mg) of N, but at different concentrations (i.e. 200 mM x 1 ml; 100 mM x 2 ml; 40 mM x 5 ml), while the other two treatments provided five times (1 m x 1 ml) or 0.2 (40 mM x 1 ml) as much N. Control patches were added as 1 ml H2O. Root production and root mortality in the patch addition zone were measured in situ using mini-rhizotron tubes and were found to differ between the various treatments. Instantaneous root birth rates were significantly (P < 0.05) greater in the highest N added patch (1 M x 1 ml) at day 14 only, indicating a more marked proliferation response in this treatment compared to those containing less N. However, of the total N available, roots in the 1 M x 1 ml L-lysine patch captured significantly (P < 0.05) less (29%) than in the other four treatments (40-47%), which did not differ significantly from each other . Microbial activity was significantly (P < 0.05) greater in the highest N added patch (1 M x 1ml) than in all other treatments. The response of roots and micro-organisms in relation to N capture is discussed.Key words: Lolium perenne, L-lysine patches, N capture, microbial activity, decomposition.      

The Vertical Pattern of Rooting and Nutrient Uptake at Different Altitudes of a South Ecuadorian Montane Forest

The vertical pattern of root length densities (RLD) of fine roots (<2 mm in diameter) and nitrogen (N) uptake potential were determined at different altitudes (1,900, 2,400, and 3,000 m a.s.l.) of a tropical montane forest in order to improve our knowledge about the depth distribution of nutrient uptake in this ecosystem. At higher altitudes, precipitation rate and frequency of fog were higher than at lower altitudes while mean annual air temperature decreased with increasing altitude. Soils were always very acid with significantly lower pH at a depth of 0.0–0.3 m in mineral soil at 3,000 m (2.8–2.9) than at 1,900 and 2,400 m (3.1–3.5). The vertical distribution of RLD was very similar both during the dry and the rainy season. During the dry season the percentage of root length in the organic layer increased from 51% at 1,900 m to 61% at 2,400 m and 76% at 3,000 m. At 3,000 m, RLD was markedly higher in the upper 0.05 m than in the remaining organic layer, whereas at 1,900 m and 2,400 m RLD were similar in all depths of the organic layer. In mineral soil, RLD decreased to a greater degree with increasing soil depth at the upper two study sites than at 1,900 m. The relative N uptake potential from different soil layers (RNUP) was determined by ¹⁵N enrichment of leaves after application of ¹⁵N enriched ammonium sulphate at various soil depths. RNUP closely followed fine root distribution confirming the shallower pattern of nutrient uptake at higher altitudes. RNUP was very similar for trees, shrubs and herbs, but shallower for saplings which obtained N only from the organic layer at both altitudes. Liming and fertilizing (N, P, K, Mg) of small patches in mineral soil had no significant impact on fine root growth. We conclude that the more superficial nutrient uptake ability at higher altitudes may be partly related to increased nutrient input from canopy by leaching. However, the specific constraints for root growth in the mineral soil of tropical montane forests warrant further investigations.