ALBERT

Notizen: Fires in boreal forests frequently convert organic matter in the organic layer to black carbon, but we know little of how changing fire frequency alters the amount, composition and distribution of black carbon and organic matter within soils, or affects podzolization. We compared black carbon and organic matter (organic carbon and nitrogen) in soils of three Siberian Scots pine forests with frequent, moderately frequent and infrequent fires.Black carbon did not significantly contribute to the storage of organic matter, most likely because it is consumed by intense fires. We found 99% of black carbon in the organic layer; maximum stocks were 72 g m−2. Less intense fires consumed only parts of the organic layer and converted some organic matter to black carbon (〉 5 g m−2), whereas more intense fires consumed almost the entire organic layer. In the upper 0.25 m of the mineral soil, black carbon stocks were 0.1 g m−2 in the infrequent fire regime.After fire, organic carbon and nitrogen in the organic layer accumulated with an estimated rate of 14.4 g C m−2 year−1 or 0.241 g N m−2 year−1. Maximum stocks 140 years after fire were 2190 g organic C m−2 and 40 g N m−2, with no differences among fire regimes. With increasing fire frequency, stocks of organic carbon increased from 600 to 1100 g m−2 (0–0.25 m). Stocks of nitrogen in the mineral soil were similar among the regimes (0.04 g m−2). We found that greater intensities of fire reduce amounts of organic matter in the organic layer but that the greater frequencies may slightly increase amounts in the mineral soil.

Notizen: Abstract. Wild radish plants deprived of, and continuously supplied with solution NO−3 for 7 d following 3 weeks growth at high NO−3 supply were compared in terms of changes in dry weight, leaf area, photosynthesis and the partitioning of carbon and nitrogen (NH2-N and NO−3-N) among individual organs. Initial levels of NO−3-N accounted for 25% of total plant N. Following termination of NO−3 supply, whole plant dry weight growth was not significantly reduced for 3 d, during which time plant NH2-N concentration declined by about 25% relative to NO−3-supplied plants, and endogenous NO−3-N content was reduced to nearly zero. Older leaves lost NO−3 and NH2-N, and roots and young leaves gained NH2-N in response to N stress. Relative growth rate declined due both to decreased net assimilation rate and a decrease in leaf area ratio. A rapid increase in specific leaf weight was indicative of a greater sensitivity to N stress of leaf expansion compared to carbon gain. In response to N stress, photosynthesis per unit leaf area was more severely inhibited in older leaves, whereas weight-based rates were equally inhibited among all leaf ages. Net photosynthesis was strongly correlated with leaf NH2-N concentration, and the relationship was not significantly different for leaves of NO3−-supplied compared to NO−3-deprived plants. Simulations of the time course of NO−3 depletion for plants of various NH2-N and NO−3 compositions and relative growth rates indicated that environmental conditions may influence the importance of NO−3 accumulation as a buffer against fluctuations in the N supply to demand ratio.

Notizen: Abstract Growth-chamber cultivated Raphanus plants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m−3 (NO−3) in a balanced nutrient solution, the oldest leaves (eight-leaf stage) accumulated 2.5% NO−3-nitrogen (NO3-N) in their lamina, and almost 5% NO3-N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol−3 m−3 concentration of NO−3 in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO−3 uptake, NO−3 fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO−3 uptake by roots and mesophyll cells was greater in the light than in the dark. The NO−3 concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO−3 was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty-four per cent of the NO−3 influx was turned into organic nitrogen, with the remaining NO−3 accumulating in both the light and the dark.

Notizen: A model is presented which solves simultaneously for leaf-scale stomatal conductance, CO2 assimilation and the energy balance as a function of leaf position within canopies of well-watered vegetation. Fluxes and conductances were calculated separately for sunlit and shaded leaves. A linear dependence of photosynthetic capacity on leaf nitrogen content was assumed, while leaf nitrogen content and light intensity were assumed to decrease exponentially within canopies. Separate extinction coefficients were used for diffuse and direct beam radiation. An efficient Gaussian integration technique was used to compute fluxes and mean conductances for the canopy. The multilayer model synthesizes current knowledge of radiation penetration, leaf physiology and the physics of evaporation and provides insights into the response of whole canopies to multiple, interacting factors. The model was also used to explore sources of variation in the slopes of two simple parametric models (nitrogen- and light-use efficiency), and to set bounds on the magnitudes of the parameters.For canopies low in total N, daily assimilation rates are ∼10% lower when leaf N is distributed uniformly than when the same total N is distributed according to the exponentially decreasing profile of absorbed radiation. However, gains are negligible for plants with high N concentrations. Canopy conductance, Gc should be calculated as Gc=Aσ(fslgsl+fshgsh), where Δ is leaf area index, fsi and fsh are the fractions of sunlit and shaded leaves at each level, and gsi and gsh are the corresponding stomatal conductances. Simple addition of conductances without this weighting causes errors in transpiration calculated using the ‘big-leaf’ version of the Penman-Monteith equation. Partitioning of available energy between sensible and latent heat is very responsive to the parameter describing the sensitivity of stomata to the atmospheric humidity deficit. This parameter also affects canopy conductance, but has a relatively small impact on canopy assimilation.Simple parametric models are useful for extrapolating understanding from small to large scales, but the complexity of real ecosystems is thus subsumed in unexplained variations in parameter values. Simulations with the multilayer model show that both nitrogen- and radiation-use efficiencies depend on plant nutritional status and the diffuse component of incident radiation, causing a 2- to 3-fold variation in these efficiencies.

Notizen: Higher rates of nitrate assimilation are required to support faster growth in enhanced carbon dioxide. To investigate how this is achieved, tobacco plants were grown on high nitrate and high light in ambient and enhanced (700 μmol mol–1) carbon dioxide. Surprisingly, enhanced carbon dioxide did not increase leaf nitrate reductase (NR) activity in the middle of the photoperiod. Possible reasons for this anomalous result were investigated. (a) Measurements of biomass, nitrate, amino acids and glutamine in plants fertilized once and twice daily with 12 mol m–3 nitrate showed that enhanced carbon dioxide did not lead to a nitrate limitation in these plants. (b) Enhanced carbon dioxide modified the diurnal regulation of NR activity in source leaves. The transcript for nia declined during the light period in a similar manner in ambient and enhanced carbon dioxide. The decline of the transcript correlated with a decrease of nitrate in the leaf, and was temporarily reversed after re-irrigating with nitrate in the second part of the photoperiod. The decline of the transcript was not correlated with changes of sugars or glutamine. NR activity and protein decline in the second part of the photoperiod, and NR is inactivated in the dark in ambient carbon dioxide. The decline of NR activity was smaller and dark inactivation was partially reversed in enhanced carbon dioxide, indicating that post-transcriptional or post-translational regulation of NR has been modified. The increased activation and stability of NR in enhanced carbon dioxide was correlated with higher sugars and lower glutamine in the leaves. (c) Enhanced carbon dioxide led to increased levels of the minor amino acids in leaves. (d) Enhanced carbon dioxide led to a large decrease of glycine and a small decrease of serine in leaves of mature plants. The glycine:serine ratio decreased in source leaves of older plants and seedlings. The consequences of a lower rate of photorespiration for the levels of glutamine and the regulation of nitrogen metabolism are discussed. (e) Enhanced carbon dioxide also modified the diurnal regulation of NR in roots. The nia transcript increased after nitrate fertilization in the early and the second part of the photoperiod. The response of the transcript was not accentuated in enhanced carbon dioxide. NR activity declined slightly during the photoperiod in ambient carbon dioxide, whereas it increased 2-fold in enhanced carbon dioxide. The increase of root NR activity in enhanced carbon dioxide was preceded by a transient increase of sugars, and was followed by a decline of sugars, a faster decrease of nitrate than in ambient carbon dioxide, and an increase of nitrite in the roots. (f) To interpret the physiological significance of these changes in nitrate metabolism, they were compared with the current growth rate of the plants. (g) In 4–5-week-old plants, the current rate of growth was similar in ambient and enhanced carbon dioxide (≈ 0·4 g–1 d–1). Enhanced carbon dioxide only led to small changes of NR activity, nitrate decreased, and overall amino acids were not significantly increased. (h) Young seedlings had a high growth rate (0·5 g–1 d–1) in ambient carbon dioxide, that was increased by another 20% in enhanced carbon dioxide. Enhanced carbon dioxide led to larger increases of NR activity and NR activation, a 2–3-fold increase of glutamine, a 50% increase of glutamate, and a 2–3-fold increase in minor amino acids. It also led to a higher nitrate level. It is argued that enhanced carbon dioxide leads to a very effective stimulation of nitrate uptake, nitrate assimilation and amino acid synthesis in seedlings. This will play an important role in allowing faster growth rates in enhanced carbon dioxide at this stage.

Notizen: Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.

Notizen: We studied the effects of variations of water flux through the plant, of diurnal variation of water flux, and of variation of vapour pressure deficit at the leaf on compensation pressure in the Passioura-type pressure chamber, the composition of the xylem sap and leaf conductance in Ricinus communis. The diurnal pattern of compensation pressure showed stress relaxation during the night hours, while stress increased during the day, when water limitation increased. Thus compensation pressure was a good measure of the momentary water status of the root throughout the day and during drought. The bulk soil water content at which predawn compensation pressure and abscisic acid concentration in the xylem sap increased and leaf conductance decreased, was high when the water usage of the plant was high. For all xylem sap constituents analysed, variations in concentrations during the day were larger than changes in mean concentrations with drought. Mean concentrations of phosphate and the pH of the xylem sap declined with drought, while nitrate concentration remained constant. When the measurement leaf was exposed to a different VPD from the rest of the plant, leaf conductance declined by 400mmol m−2 s−1 when compensation pressure increased by 1 MPa in all treatments. The compensation pressure needed to keep the shoot turgid, leaf conductance and the abscisic acid concentration in the xylem were linearly related. This was also the case when the highly dynamic development of stress was taken into account.