Abstract Increasing atmospheric CO 2 concentration and related climate change have stimulated much interest in the potential of soils to sequester carbon. In ‘The Jena Experiment’, a managed grassland experiment on a former agricultural field, we investigated the link between plant diversity and soil carbon storage. The biodiversity gradient ranged from one to 60 species belonging to four functional groups. Stratified soil samples were taken to 30 cm depth from 86 plots in 2002, 2004 and 2006, and organic carbon contents were determined. Soil organic carbon stocks in 0–30 cm decreased from 7.3 kg C m −2 in 2002 to 6.9 kg C m −2 in 2004, but had recovered to 7.8 kg C m −2 by 2006. During the first 2 years, carbon storage was limited to the top 5 cm of soil while below 10 cm depth, carbon was lost probably as short‐term effect of the land use change. After 4 years, carbon stocks significantly increased within the top 20 cm. More importantly, carbon storage significantly increased with sown species richness (log‐transformed) in all depth segments and even carbon losses were significantly smaller with higher species richness. Although increasing species diversity increased root biomass production, statistical analyses revealed that species diversity per se was more important than biomass production for changes in soil carbon. Below 20 cm depth, the presence of one functional group, tall herbs, significantly reduced carbon losses in the beginning of the experiment. Our analysis indicates that plant species richness and certain plant functional traits accelerate the build‐up of new carbon pools within 4 years. Additionally, higher plant diversity mitigated soil carbon losses in deeper horizons. This suggests that higher biodiversity might lead to higher soil carbon sequestration in the long‐term and therefore the conservation of biodiversity might play a role in greenhouse gas mitigation.
Plant diversity drives changes in the soil microbial community which may result in alterations in ecosystem functions. However, the governing factors between the composition of soil microbial communities and plant diversity are not well understood. We investigated the impact of plant diversity (plant species richness and functional group richness) and plant functional group identity on soil microbial biomass and soil microbial community structure in experimental grassland ecosystems. Total microbial biomass and community structure were determined by phospholipid fatty acid (PLFA) analysis. The diversity gradient covered 1, 2, 4, 8, 16 and 60 plant species and 1, 2, 3 and 4 plant functional groups (grasses, legumes, small herbs and tall herbs). In May 2007, soil samples were taken from experimental plots and from nearby fields and meadows. Beside soil texture, plant species richness was the main driver of soil microbial biomass. Structural equation modeling revealed that the positive plant diversity effect was mainly mediated by higher leaf area index resulting in higher soil moisture in the top soil layer. The fungal-to-bacterial biomass ratio was positively affected by plant functional group richness and negatively by the presence of legumes. Bacteria were more closely related to abiotic differences caused by plant diversity, while fungi were more affected by plant-derived organic matter inputs. We found diverse plant communities promoted faster transition of soil microbial communities typical for arable land towards grassland communities. Although some mechanisms underlying the plant diversity effect on soil microorganisms could be identified, future studies have to determine plant traits shaping soil microbial community structure. We suspect differences in root traits among different plant communities, such as root turnover rates and chemical composition of root exudates, to structure soil microbial communities.
Low root zone temperatures (RZT) decrease the hydraulic conductivity of roots (Lp), and thus, may affect shoot growth by insufficient water supply. In the present experiments with maize (Zea mays L.), Lp was compared after short-term (4 h) and long-term (4–5 d) treatment with low RZT (12°C) to examine if maize roots can adapt to low temperatures by increasing their Lp. At 12°C RZT, the shoot growth rate was varied, by growing the plants with their shoot base including the apical meristem either at 12°C or 24°C. In the short term, Lp was decreased at 12°C RZT to 25% of Lp at 24°C, independently of the shoot base temperature (SBT). When the roots were rewarmed to 24°C, Lp completely recovered to the values of plants which were continuously grown at 24°C RZT. In the long term, the rate of water flux through the roots at 12°C was increased by factor 2, when the SBT was increased from 12°C to 24°C. In these plants grown at 12°C RZT and 24°C SBT, Lp increased with time, but remained substantially lower than Lp of plants which were continuously grown at 24°C RZT, even when the temperature during the measurement of Lp was increased to 24°C. This indicates that the low Lp after long term treatment at 12°C is associated with structural modifications of root characteristics which influence Lp (e.g. membrane composition, suberinization, decrease in the number of lateral roots). In plants grown at 12°C RZT and 12°C SBT no increase in Lp with time was measurable, indicating that the adaptational response of the plants grown at 24°C SBT was induced by the increase in the growth-related water demand and not by the temperature perse.
ABSTRACT To assess how diurnal changes of nitrate reductase ( NIA ) expression in leaves interact with upstream and downstream processes during nitrate utilization, nitrate uptake, and nitrate and ammonium metabolism were investigated at several times during the diurnal cycle in wild‐type tobacco. Plants were grown hydroponically on 2 m M nitrate to exclude possible complications due to changes in the external availability of nitrate, and to allow nitrate uptake to be measured in the growth conditions. (a) In leaves, the NIA transcript decreases during the day and recovers at night, and NIA activity increases three‐fold during the first part and declines during the second part of the light period. Nitrate decreases during the day and recovers at night, ammonium, glutamine, glycine and serine increase during the day and decrease at night, and 2‐oxoglutarate increases three‐fold after illumination and decreases during the last part of the light period. The amplitudes of the diurnal changes are similar to or larger than in tobacco grown on high nitrate in sand. The transcript for plastid glutamine synthetase ( GLN2 ) is low at the end of the night and increases during the day, and glutamine synthetase activity increases to a peak at the end of the day and decreases at night. (b) In the roots, transcript levels for the high affinity nitrate transporter ( NRT2 ) increase in the day and decrease at night. Nitrate uptake is about 40% higher during the day than at night. (c) Comparison of the diurnal changes of the leaf metabolite pools with the rate of nitrate uptake allows diurnal changes in fluxes to be estimated. During the first part of the light, the rate of nitrate assimilation is about two‐fold higher than the rate of nitrate uptake, and also exceeds the rate at which reduced nitrogen is metabolized in the GOGAT pathway. The resulting decrease of leaf nitrate and accumulation of nitrogen in intermediates of ammonium metabolism and photorespiration represent about 40 and 15%, respectively, of the total nitrate that enters the plant in 24 h. Later in the diurnal cycle as NIA expression and activity decline, this imbalance is reversed. NRT2 expression and nitrate uptake remain relatively high, and nitrate taken up during the night is used to replenish the leaf nitrate pool. Increased GLN2 expression in leaves during the second part of the light period allows continued assimilation of ammonium released during photorespiration and remobilization of the reduced nitrogen that accumulated earlier in the diurnal cycle.
The aim of the present experiments was to study the effect of growth-related nutrient demand on Ca2+ translocation from roots to shoot of maize (Zea mays L.). The plants were grown under controlled environmental conditions in nutrient solution with constant Ca2+ supply. The growth-related demand for Ca2+ and other nutrients was modified by growing the plants with their apical shoot meristem either at air temperature (24°C/20°C day/night) or at 14°C. Reduction of the shoot meristem temperature (SMT) to 14°C decreased shoot growth without affecting root growth in the first five days, which diminished the growth-related demand of the shoot for nutrients per unit of roots. This decrease in shoot demand led to a reduction not only of Ca2+ translocation rates in intact transpiring plants but also of Ca2+ fluxes in the xylem exudate of decapitated plants. This indicates that the decrease in xylem flux of Ca2+ at low SMT was not only the result of low transpiration-related water flux, and thus possibly low apoplasmic bypass transport of Ca2+ into the stele. In decapitated plants precultured at low SMT, the water flux through the roots was diminished even more than Ca2+ flux, leading to a significant increase in the Ca2+ concentration of the exudate, and thus presumably an increase in the Ca2+ gradient between cytosol and apoplast of stelar parenchyma cells. When the osmotically driven water flux was reduced by addition of mannitol to the nutrient solution, Ca2+ concentration in the exudate markedly increased, whereas Ca2+ translocation was only slightly affected. From these results it is suggested that the decrease in Ca2+ translocation rates at low shoot demand was not related to low water flux but to direct effects on the capacity of Ca2+ transport mechanisms in the roots. Regulierung des Transportes von Ca2+ im Xylem aus den Wurzeln in den Spross von Mais durch den wachstumsbedingten Sprossbedarf Ziel der vorliegenden Versuche war es, die Auswirkungen des wachstumsbedingten Sprossbedarfes für Nährstoffe auf die Verlagerung von Ca2+ aus den Wurzeln in den Spross von Mais (Zea mays L.) zu untersuchen. Dazu wurden Pflanzen unter kontrollierten Umweltbedingungen in Nährlösung mit konstantem Ca2+-Angebot angezogen. Der wachstumsbedingte Sprossbedarf für Ca2+ und andere Nährstoffe wurde variiert, indem das apikale Sprossmeristem der Pflanzen entweder der Lufttemperatur in der Klimakammer (24°C/20°C Tag/Nacht) ausgesetzt wurde oder mithilfe einer schmalen Kühlmanschette um die Sprossbasis auf 14°C gekühlt wurde. Die Absenkung der Sprossmeristemtemperatur (SMT) auf 14°C verringerte in den ersten fünf Tagen zwar das Sprosswachstum, nicht jedoch das Wurzelwachstum. Dadurch wurde der wachstumsbedingte Bedarf des Sprosses für Nährstoffe pro Einheit Wurzeln deutlich verringert. Die Verringerung des Sprossbedarfes führte zu einer Abnahme nicht nur der Ca2+-Translokationsraten von intakten, transpirierenden Pflanzen, sondern auch der Ca2+-Flüsse im Xylemexsudat dekapitierter Pflanzen. Dies zeigt, dass die Abnahme des Ca2+-Flusses im Xylem bei tiefer SMT nicht nur auf einen geringeren transpirationsbedingten Wasserfluss und damit möglicherweise geringeren apoplastischen Ca2+-Transport in den Zentralzylinder zurückzuführen war. In dekapitierten Pflanzen, die vorher bei tiefen SMT angezogen worden waren, war der Wasserfluss durch die Wurzeln stärker vermindert als der Ca2+-Fluss. Dies war mit einem deutlichen Anstieg der Ca2+-Konzentration im Exsudat verbunden und damit vermutlich mit einer Erhöhung des Ca2+-Gradienten zwischen Cytosol und Apoplast der Xylemparenchymzellen. Eine Verringe-rung des osmotisch bedingten Wasserflusses durch Zugabe von Mannitol zur Nährlösung f¨︁hrte ebenfalls zu einer deutlichen Erhöhung der Ca2+-Konzentration im Xylemexsudat, hatte jedoch keine deutliche Verminderung der Ca2+-Translokation zur Folge. Aus diesen Ergebnissen wird gefolgert, daß die Abnahme der Ca2+-Translokation bei geringem Sprossbedarf nicht durch einen geringeren Wasserfluss durch die Wurzeln verursacht wurde, sondern auf eine veränderte Kapazität von Ca2+-Transportmechanismen in den Wurzeln zurückzuführen war.
In order to manipulate the shoot demand for mineral nutrients per unit root weight, maize ( Zea mays L.) seedlings were grown in nutrient solution with different temperatures in the root zone and at the shoot base. The aerial temperature was kept uniform at 24/20°C day/night. At a root zone temperature (RZT) of 24°C, shoot growth was reduced by decreasing the shoot base temperature (SBT) to 12°C; at a RZT of 12°C, shoot growth was increased by raising the SBT to 24°C. At both RZT root growth was not affected by the SBT. Thus, the shoot demand for nutrients per unit root was either increased by raising, or decreased by lowering the SBT. The net uptake rate of potassium (K), as determined from accumulation rates between sequential harvests, was not affected within the first 3 days after lowering the SBT, whereas net translocation rates of K into the shoot and translocation rates in the xylem exudate of decapitated plants were markedly reduced. Obviously, translocation of K into the shoot seems to be regulated independently from K uptake into the root cells. Translocation rates of K in the xylem exudate of decapitated plants were markedly reduced when the nutrient solution was replaced by CaCl 2 solution during exudation. But, depending on the SBT before decapitation, significant differences remained in the translocation rates of K even when K uptake from the nutrient solution was prevented. From the results it is suggested that xylem loading of K is regulated separately from K uptake from the external solution and that the adaptation of K translocation to shoot demand is coupled with an altered capacity of the root for xylem loading.
