Decomposition of <Superscript>15</Superscript>N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

The decomposition and the fate of ¹⁵N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, ¹⁵N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and ¹⁵N concentration. Mass loss of the ¹⁵N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released ¹⁵N accumulated essentially at the soil surface. ¹⁵N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. ¹⁵N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of ¹⁵N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales. Received: 3 May 1999 / Accepted: 3 January 2000     

Neutron radiography as a tool for revealing root development in soil: capabilities and limitations

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and ¹³C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched ¹³C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.     

C-Photosynthate Partitioning and Translocation in Soybeans during Reproductive Development

Partitioning and translocation of ¹⁴C-photosynthates were examined during flowering and seed maturation in soybean (Glycine max [L.]Merr.) plants to quantify allocation to sugars, amino acids, organic acids, and starch and to study transport of C and N from leaves to reproductive sinks. The trifoliolate leaf at the eighth node was exposed to steady state levels of ¹⁴CO₂ for 2 hours, followed by immediate extraction and identification of radioactive assimilates in the fed leaf blade, tissues of the transport path (e.g. petiole and stem), and fruits if they were present. About one-third of the total ¹⁴C recovered from the leaf blades was in starch until late pod-filling, after which the proportion dropped to 16%. Sugars comprised 70% to 86% of the recovered ¹⁴C from soluble assimilates of the source leaf, with highest proportions occurring during late flowering and early pod-filling. Amino acids accounted for 8% to 17% of the ¹⁴C recovered from the soluble fraction, and were most evident during early flowering and mid to late pod-filling. The ¹⁴C-organic acids comprised from 3% to 14% of the soluble ¹⁴C-assimilates in leaves. Petioles consistently contained a higher percentage of recovered radioactivity in sugars (87-97%) and a lower percentage in amino acids (3-12%) than did leaf blades. ¹⁴C-Amino acids in petioles attained their highest levels during mid and late pod-filling, while ¹⁴C-organic acids comprised 2% or less of the recovered radioactivity after pod initiation. The distribution of ¹⁴C-assimilates in the internode below the source leaf was similar to that found in petioles. A comparison of the above data to calculated C and N requirements for seed development suggests that ¹⁴C-amino acids derived from current photosynthesis and translocated from source leaves supply at least 12% to 48% of the seed N depending on the stage of pod-filling.     

Miscanthus × giganteus leaf senescence,decomposition and C and N inputs to soil

Energy crops are currently promoted as potential sources of alternative energy that can help mitigate the climate change caused by greenhouse gases (GHGs). The perennial crop Miscanthus × giganteus is considered promising due to its high potential for biomass production under conditions of low input. However, to assess its potential for GHG mitigation, a better quantification of the crop's contribution to soil organic matter recycling under various management systems is needed. The aim of this work was to study the effect of abscised leaves on carbon (C) and nitrogen (N) recycling in a Miscanthus plantation. The dynamics of senescent leaf fall, the rate of leaf decomposition (using a litter bag approach) and the leaf accumulation at the soil surface were tracked over two 1‐year periods under field conditions in Northern France. The fallen leaves represented an average yearly input of 1.40 Mg C ha^?1 and 16 kg N ha^?1. The abscised leaves lost approximately 54% of their initial mass in 1 year due to decomposition; the remaining mass, accumulated as a mulch layer at the soil surface, was equivalent to 7 Mg dry matter (DM) ha^?1 5 years after planting. Based on the estimated annual leaf‐C recycling rate and a stabilization rate of 35% of the added C, the annual contribution of the senescent leaves to the soil C was estimated to be approximately 0.50 Mg C ha^?1yr^?1 or 10 Mg C ha^?1 total over the 20‐year lifespan of a Miscanthus crop. This finding suggested that for Miscanthus, the abscised leaves contribute more to the soil C accumulation than do the rhizomes or roots. In contrast, the recycling of the leaf N to the soil was less than for the other N fluxes, particularly for those involving the transfer of N from the tops of the plant to the rhizome.     

Nitrogen availability and susceptibility of tomato leaves to Botrytis cinerea

The aim of this study was to investigate the effect of nitrogen availability on susceptibility of tomato leaves to the fungal pathogen Botrytis cinerea. Plants with varying nitrogen availability were grown by adding N daily in exponentially increasing amounts to a nutrient solution at different rates. Leaves of plants grown at low nitrogen availability had a high leaf C/N ratio (21 g g^-1) and were about 2.5 times more susceptible to primary lesion formation by B. cinerea compared to plant grown at high nitrogen availability, which had a low leaf C/N ratio (11 g g^-1). Leaf C/N ratio accounted for 95% of variation in susceptibility. This relationship between C/N ratio and susceptibility persisted when plants were grown with exponential P addition and optimal N supply, and was thus independent of plant growth rate or related factors. We could not explain the effect of nitrogen availability by variation in the most obvious N-based resistance compound α-tomatine because more susceptible leaves with a high C/N ratio contained more α-tomatine. These leaves also contained more soluble carbohydrates. The level of soluble carbohydrates correlated positively with susceptibility, independent of the growth method. We therefore suggest that the effect of N availability on susceptibility must be explained by variation in levels of soluble carbohydrates and speculate about the role of these carbohydrates in the infection process. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mobilization of the reserve N in citrus

The mobilization of N from reserve organs (leaves, roots, branches and trunk) to developing new organs was studied at different moments of the growth cycle. Three-year-old Valencia Late orange trees (Citrus sinensis (L.) Osbeck) were grown individually outdoors in 150 L containers filled with siliceous sand. Trees were irrigated with a nutrient solution labelled with potassium nitrate with an enrichment of 4 atom ¹⁵N % excess during a complete growth cycle. At the following year, plants were irrigated with unlabelled nutrient solution, and harvested throughout the growth cycle (flowering, fruit set, second flush, third flush, and dormancy). Total N and ¹⁵N analyses were carried out in the different organs of the plants.The highest amounts of N were found in leaves and roots (33–42% and 30–38%, respectively). Distribution of ¹⁵N was similar to that obtained for total N (42 and 39% of total ¹⁵N in leaves and roots, respectively) as is expected after a long period of labelling. Old leaves were the main reserve organs, contributing a 40–50% of the total N exported. Roots and aerial woody tissues exported between 30–35% and 15–25% of total reserve N, respectively. N exported by old leaves was approximately 57% of the N accumulated during the preceding year, while roots translocated 40% and trunk plus branches 35%. More than 70% of N accumulated in new organs during spring came from N stored in old organs.     

Assimilation and translocation of nitrogen and carbon in Curcuma alismatifolia Gagnep

Curcuma or Siam tulip (Curcuma alismatifolia Gagnep.) is an ornamental flowering plant with two underground storage organs, rhizomes and storage roots. Characteristics of N and C assimilation and transport in curcuma were investigated. The plants were treated with ¹⁵NH₄⁺ + ¹⁵NO₃^? and ¹³CO₂ at 10, 13 or 21 weeks after planting. Plants were sampled at several stages up to 32 weeks. The C stored in old storage roots was used rapidly during the first 10 weeks; after which N stored in old rhizomes and old storage roots were used. The daily gain in C depending on photosynthesis was remarkably high between 10 and 21 weeks. However, the daily gain in N was relatively constant throughout the growth period. The ¹⁵N absorbed at 10 weeks was initially accumulated in leaves and roots, but some was transported to flowering organs at 13 weeks. At harvest, 41% of ¹⁵N was recovered in new rhizomes and 17% in new storage roots. After ¹³CO₂ exposure at 10 and 13 weeks, the distribution of ¹³C among organs was relatively constant in subsequent stages. When given ¹³CO₂ at 21 weeks, a large amount of labelled C was recovered in new storage roots and new rhizomes at harvest. Both new rhizomes and new storage roots stored N and C, however, rhizomes played a more important role in supplying N, while storage roots provided C.     

Mineralization of labeled N from cowpea [Vigna unguiculata (L.) Walp.] plant parts at two growth stages in sandy soil

Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N₂ fixation. The objective of this study was to measure the rate of N mineralization from cowpea plant parts harvested at onset of flowering (5 weeks) and mid pod-fill (7 weeks) under near optimum conditions. Cowpeas were grown in a greenhouse and supplied with ¹⁵NH₄ ¹⁵NO₃ to isotopically label tissue. Cowpea leaves, stems, and roots were incorporated into a sandy soil (Psammentic Paleustalf) and net N mineralized was measured several times during a 10 week incubation. The amount of N accumulated in 7-week old cowpeas was more than double that in 5-week old cowpeas. The portion of N mineralized after 10 weeks was 24% for 5-week old cowpeas and 27% for 7-week old cowpeas. The rate of N mineralization from leaves and stems increased with plant age, but decreased for roots. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation and a more rapid rate of N mineralization of the more mature cowpeas. The greatest amount of N was released from leaves, which amounted to 74 and 65% of total N mineralization from 5- and 7-week old cowpeas, respectively. The percentage of N mineralized by 10 weeks was linearly related to the tissue N concentration of the plant parts and to their C/N ratio. These relationships allow a quick estimation of the amount of N that would mineralize from cowpea residues incorporated into soil based on their N concentration or C/N ratio.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].     

Utilization of Previously Accumulated and Concurrently Absorbed Nitrogen during Reproductive Growth in Maize : Influence of Prolificacy and Nitrogen Source

A prolific maize (Zea mays L.) genotype was grown to physiological maturity under greenhouse conditions to examine the effects of reproductive sink demand on (a) the remobilization of N accumulated during vegetative growth, and (b) the partitioning of N accumulated concurrent with ear development. One- and two-eared plants were treated with either a NO₃⁻ or NH₄⁺ source of ¹⁵N-labeled N during reproductive growth. Plants with two ears enhanced grain production, N remobilization from the stalk and roots, and N translocation to the grain from concurrently assimilated N. But, remobilization of leaf-N was unaffected by ear number. In addition, N uptake and total dry matter accumulation during the reproductive period were also unaffected, although P uptake was greater in the two-eared plants. Less than 15% of the total K⁺ uptake was accumulated after silking while during this time more than 40% of the total N and more than 50% of the total P were absorbed. The data also indicate that with NO₃⁻ nutrition, internal recirculation of K⁺ between shoots and roots may play a prominent role in the transport of nitrogenous solutes during grain development. N source had no effect on dry matter production and N uptake of both one- and two-eared plants. However, slightly greater partitioning of labeled-N from the NH₄⁺ source to the grain was observed in the two-eared plants.     

Effect of nitrates supplied with the transpiration flow on assimilate translocation

Solutions of nitrates (0.5% KNO₃, 0.2% NH₄NO₃) or urea (0.15%) were fed under the pressure of 10⁴ Pa to 50–60-cm-long detached shoots of common flax (Linum usitatissimum L.). One hour after the start of supplying the solutions, an assimilation clip chamber was fastened to the middle part of the shoot (¹⁴C source area), and ¹⁴CO₂ was blown through in the light for 2.5 min. The analysis of distribution of ¹⁴C among the labeled products of photosynthesis produced by source leaves showed that nitrates reduced the incorporation of the label into sucrose. At the same time, the ratio of labeled sucrose to labeled hexoses decreased, and the incorporation of the label into serine greatly increased. Urea did not produce such effects. The pattern of distribution of ¹⁴C within the plant 3 h after the assimilation of ¹⁴CO₂ points to the suppression of assimilate efflux from the leaves of plants fed with nitrates. In plants supplied with water or urea, 17–20% of labeled carbon was found below the ¹⁴C source area of the shoot, in nitrate type of treatment, only 3–5% was found there. In plants supplied with nitrates, the cortex tissue below the source leaf contained more ¹⁴C in proteins and less in low-molecular substances. In the wood tissue, such a correlation was not observed. When the shoot was supplied with water or urea, the content of ¹⁴C in sucrose in the source leaves in 3 h declined from 55–60% to 38–42%. When the shoot was fed with nitrates, the share of label in sucrose increased from 50 to 62–73%. Autoradiography of the source leaves showed that, in plants supplied with water or urea, the label was predominantly accumulated in large vascular bundles, and in nitrate type of treatment, it was accumulated outside large bundles. Electron microscopy showed that, in nitrate plants, the companion cells of phloem endings were very much vacuolated.     

模拟氮沉降和接种外生菌根真菌对马尾松(Pinus massoniana)幼苗营养元素的影响

量化植物地上部和地下部元素含量对于理解和预测植物养分平衡如何响应大气氮沉降的变化至关重要。通过盆栽试验研究了氮沉降增加背景下外生菌根真菌对马尾松幼苗营养元素的影响。对马尾松幼苗进行了接种两种外生菌根真菌：（彩色豆马勃（Pisolithus tinctorius,Pt）与厚环乳牛肝菌（Suillus grevillei,Sg））以及4种氮素浓度添加：0 kg N hm^-2a^-1（N0）、正常氮沉降30 kg N hm^-2a^-1（N30）、中度氮沉降60 kg N hm^-2a^-1（N60）、重度氮沉降90 kg N hm^-2a^-1（N90）,共12个处理,测定了马尾松地上部和地下部大量元素和微量元素的含量。结果表明：施氮改变了营养元素在马尾松幼苗地上部和地下部的含量,马尾松幼苗磷（P）、钙（Ca）、铁（Fe）、锰（Mn）等元素均在N60时达到临界值,而当输入的量超过了马尾松对氮的需求时,氮沉降会使马尾松营养元素含量较最适浓度时降低,地上部碳（C）随施氮浓度的升高先升高后降低,N随施氮浓度的升高而升高,根系和叶片钾（K）、Ca、镁（Mg）均随施氮浓度的升高而降低,施氮也降低了根系C及微量元素的含量。但在同一施氮浓度下,接种外生菌根真菌（EMF）后能够提高大多数元素的含量,N90时接种厚环乳牛肝菌（Sg）和彩色豆马勃（Pt）的叶片N含量与对照相比分别提高112.6%和138.6%,根系N含量分别提高73.1%、71.6%;N60时接种Sg和Pt的植株叶片P含量比不施氮未接种对照分别提高了166.3%、132.9%,根系P含量分别提高了40.8%、38.5%。EMF能够维持植物养分平衡,从而降低高施氮量对植物的影响效果。这为未来气候变化情景中氮沉降增加下接种EMF可以调节植物元素含量,从而达到更适应环境的元素平衡来促进生长提供理论依据。     

Plant and microbial N acquisition under elevated atmospheric CO2 in two mesocosm experiments with annual grasses

The impact of elevated CO₂ on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO₂ on N availability to plants and microbes. We examined the effects of atmospheric CO₂ enrichment (ambient+370 μmol mol^?1) on plant and microbial N acquisition in two different mesocosm experiments, using model plant species of annual grasses of Avena barbata and A. fatua, respectively. The A. barbata experiment was conducted in a N‐poor sandy loam and the A. fatua experiment was on a N‐rich clayey loam. Plant–microbial N partitioning was examined through determining the distribution of a ¹⁵N tracer. In the A. barbata experiment, ¹⁵N tracer was introduced to a field labeling experiment in the previous year so that ¹⁵N predominantly existed in nonextractable soil pools. In the A. fatua experiment, ¹⁵N was introduced in a mineral solution [(¹⁵NH₄)₂SO₄ solution] during the growing season of A. fatua. Results of both N budget and ¹⁵N tracer analyses indicated that elevated CO₂ increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO₂ increased plant biomass N by ca. 10% but there was no corresponding decrease in soil extractable N, suggesting that plants might have obtained N from the nonextractable organic N pool because of enhanced microbial activity. In the A. fatua experiment, however, the CO₂‐led increase in plant biomass N was statistically equal to the reduction in soil extractable N. Although atmospheric CO₂ enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua, it had no significant effect on microbial biomass N in either experiment. Elevated CO₂ increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO₂ may tighten N cycling through facilitating plant N acquisition. However, it is unknown to what degree results from these short‐term microcosm experiments can be extrapolated to field conditions. Long‐term studies in less‐disturbed soils are needed to determine whether CO₂‐enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO₂ environment.     

Are carbon and nitrogen exchange between fungi and the orchid Goodyera repens affected by irradiance?

Background and Aims The green orchid Goodyera repens has been shown to transfer carbon to its mycorrhizal partner, and this flux may therefore be affected by light availability. This study aimed to test whether the C and N exchange between plant and fungus is dependent on light availability, and in addition addressed the question of whether flowering and/or fruiting individuals of G. repens compensate for changes in leaf chlorophyll concentration with changes in C and N flows from fungus to plant.Methods The natural abundances of stable isotopes of plant C and N were used to infer changes in fluxes between orchid and fungus across natural gradients of irradiance at five sites. Mycorrhizal fungi in the roots of G. repens were identified by molecular analyses. Chlorophyll concentrations in the leaves of the orchid and of reference plants were measured directly in the field.Key Results Leaf δ¹³C values of G. repens responded to changes in light availability in a similar manner to autotrophic reference plants, and different mycorrhizal fungal associations also did not affect the isotope abundance patterns of the orchid. Flowering/fruiting individuals had lower leaf total N and chlorophyll concentrations, which is most probably explained by N investments to form flowers, seeds and shoot.Conclusions The results indicate that mycorrhizal physiology is relatively fixed in G. repens, and changes in the amount and direction of C flow between plant and fungus were not observed to depend on light availability. The orchid may instead react to low-light sites through increased clonal growth. The orchid does not compensate for low leaf total N and chlorophyll concentrations by using a ¹³C- and ¹⁵N-enriched fungal source.     

Physiological studies in salinity tolerance ofSesbania aculeata POIR

A pot culture experiment was performed to evaluate salt tolerance potential ofSesbania aculeata Poir. The plant can tolerate salinity levels up to electrical conductivity (ECe), 10 mS cm^?1 and at 15 mS cm^?1 thero is about 40% reduction in dry matter production. The analysis of inorganic constituents in different plant parts revealed that the plant has the capacity to regulate sodium uptake under saline conditions and chloride uptake always exceeded that of sodium. The potassium: sodium ratio is also maintained at a fairly constant level in leaflets while it is reduced in leaf rachis, stem and roots. Salt stress caused accumulation of calcium and magnesium in all plant parts. A considerable decline in phosphorus uptake was observed due to salinity. Iron was found to be accumulated more in salt stressed roots only. Nitrogen accumulated in both roots and leaves while considerable proline accumulation was observed in leaves of salt stressed plants. The amount of soluble sugars was increased in roots and leaves due to salt stress, while starch content of roots decreased. Those changes induced by salinity are discussed in relation to salt tolerance capacity of the plant.     

Interactions between plant growth and soil nutrient cycling under elevated CO2: a meta-analysis

free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO₂ on nutrient cycling in terrestrial ecosystems. Using meta‐analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N₂ fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO₂ alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO₂ stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO₂ by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr^?1. Namely, elevated CO₂ stimulated overall above‐ and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO₂ respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr^?1) and above‐ and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO₂ in experiments receiving the high N treatments. Under low N availability, above‐ and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO₂ only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO₂ in the long‐term. Therefore, increased soil C input and soil C sequestration under elevated CO₂ can only be sustained in the long‐term when additional nutrients are supplied.     

Decomposition of leaves of huisache (Acacia tortuoso) and mesquite (Prosopis spp) in soil of the central highlands of Mexico

In the central highlands of Mexico, mesquite (Prosopis spp) and huisache (Acacia tortuoso), N₂ fixing trees or shrubs, dominate the vegetation and are used in an alley cropping system to prevent erosion and restore soil fertility. We investigated how much the leaves of both trees contribute to dynamics of carbon (C) and nitrogen (N) in soil by adding leaves of both species to soil sampled under the canopy of mesquite and huisache, outside their canopy and from fields cultivated with maize at three different sites and monitoring microbial biomass C, production of carbon dioxide (CO₂), and dynamics of inorganic N (ammonium and nitrate) in an aerobic incubation. The soluble fraction and N content of the mesquite leaves were larger than in the huisache leaves, but lignin and polyphenol content were lower. Evolution of CO₂ increased 2.7-times when mesquite and 2.4-times when huisache leaves were added to soil. During all stages of decomposition and in all treatments, C mineralization of leaves from mesquite was greater than from huisache leaves. Mesquite leaves induced an increase in mineral N of 25.6 mg N kg^–1 soil after 56 days and those of huisache 9.8 mg N kg^–1. Twenty-six percent of N from mesquite leaves and 11% of huisache was mineralized, if no priming effect was considered. Nitrogen release from the leaves was greater when the soil organic matter content was lower. It was found that soil under the canopy of mesquite and huisache effectively accumulated organic material, micro-organisms and valuable nutrients. In an alley cropping system huisache might be a better choice than mesquite as huisache grows faster than mesquite and sheds its leaves twice a year while mesquite only once, although the amount of N mineralized was larger from mesquite leaves than from those of huisache.     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.     

海岸不同生态断带植物根叶抗逆生理变化与其Na+含量的关系

以烟台海岸生态断带滨麦(Leymus mollis)和肾叶打碗花(Calystegia soldanella)为材料,在远离高潮线不同位置上取土样和植物材料,通过测定土壤Na+和两植物根叶Na+含量、丙二醛(MDA)含量、抗氧化酶(SOD、POD、CAT)活性和渗透调节物含量,以揭示滨麦和肾叶打碗花根叶中Na+在其适应海岸盐环境中的生理调控机制。结果表明,在高潮线土壤Na+含量最高,滨麦根叶Na+含量较高,两植物根叶中MDA和水分含量、抗氧化酶活力均较低,但渗透调节物含量均较高。随远离高潮线土壤Na+含量下降,滨麦根叶Na+含量下降,而肾叶打碗花根中Na+含量上升,其根叶Na+含量较滨麦分别高637%和319%。同时两植物根叶MDA含量、叶片含水量增加;两植物根中POD和SOD活力增加;两植物根叶可溶性糖和脯氨酸含量下降。但不同生态断带滨麦叶片平均含水量相对较低,MDA含量、POD和CAT和SOD活力、脯氨酸和可溶性糖含量相对较高。在盐土环境中滨麦通过降低Na+的吸收和提高抗氧化酶活力和有机渗透调节物含量维持氧自由基代谢平衡和水分平衡。而肾叶打碗花是泌盐植物,在不同生态断带其叶片Na+含量、平均含水量相对较高,叶MDA含量、POD和CAT活力、脯氨酸和可溶性糖含量均相对较低。泌盐植物的肾叶打碗花依赖根叶中积累的Na+作为无机渗透调节剂维护其离子平衡和水分平衡及正常生长。因此,积累在根叶中的Na+离子既作为无机渗透调节剂维护细胞离子平衡和水分平衡,又引发细胞生理干旱促进有机渗透调节物合成;另外还作为氧自由基诱发剂促使活性氧自由基(ROS)积累,通过积累的ROS激活抗氧化保护酶系统抑制膜脂过氧化、维护氧自由基代谢平衡。海岸沙地土壤中高浓度Na+是海滨滨麦和肾叶打碗花能长期在盐土环境中生存的依靠元素,其对植物的生理调控作用可能是滨麦和肾叶打碗花适应盐土生存的重要生理适应机理。     

The Effect of the Accumulation of Carbohydrate and Organic Nitrogen on Nitrogen Fixation (Acetylene Reduction) of Faba Bean cv. Fiord

Plants of faba bean cv. Fiord were grown under controlled conditions,without mineral N, in coarse river sand. Twenty-five days aftersowing when plants had at least eight fully opened leaves andwere nodulated and actively fixing N₂, half were topped andkept debudded for 21 d. Changes in dry weight, N₂ fixation (acetylenereduction activity), soluble carbohydrate, starch, soluble Nand total N in plants were monitored over the period. Both debudded and control plants grew and accumulated dry matter.Debudding resulted in a significant increase in the concentrationof soluble carbohydrate, starch and soluble N. but had onlya small effect on the total N concentration. A strong positivelinear relation between total plant weight and N content ofboth control and debudded plants showed that even under conditionsof excess supply of carbohydrate, faba beans have little capacityto store N. Soluble N accumulated in debudded plants presumablybecause less N was needed for the formation of new tissues thanin control plants. AR continued to increase throughout the experimentin control plants but declined in debudded plants from 6 to13 d after debudding and remained low until the end of the experiment.The decline was associated with an increase in available carbohydrateand in soluble N. The results of this experiment are consistentwith a feed back control of N₂ fixation by the soluble poolof N.Copyright 1994, 1999 Academic Press Vicia faba, faba bean, debudding, soluble N, inhibition of N₂ fixation