Aspects of carbon and nitrogen cycling in soils of the Bornhöved Lake district II. Modelling the influence of temperature increase on soil respiration and organic carbon content in arable soils under different managements

Based on field measurements in two agriculturalecosystems, soil respiration and long-term response ofsoil organic carbon content (SOC) was modelled. Themodel predicts the influence of temperature increaseas well as the effects of land-use over a period ofthirty years in a northern German glacial morainelandscape. One of the fields carried a maizemonoculture treated with cattle slurry in addition tomineral fertilizer (maize monoculture), the otherwas managed by crop rotation and recieved organicmanure (crop rotation). The soils of both fieldswere classified as cambic Arenosols. The soilrespiration was measured in the fields by means of theopen dynamic inverted-box method and an infrared gasanalyser. The mean annual soil respiration rates were 268 (maizemonoculture) and 287 mg CO₂ m^-2 h^-1(crop rotation). Factors controlling soil respirationwere soil temperature, soil moisture, root respirationand carbon input into the soil. Q₁₀-valuesof soil respiration were generally higher in winterthan in summer. This trend is interpreted as anadaptive response of the soil microbial communities.In the model a novel mathematical approach withvariable Q₁₀-values as a result oftemperature and moisture adjustment is proposed. Withthe calibrated model soil respiration and SOC werecalculated for both fields and simulations over aperiod of thirty years were established. Simulationswere based on (1) local climatic data, 1961 until1990, and (2) a regional climate scenario for northernGermany with an average temperature increase of 2.1 K.Over the thirty years period with present climateconditions, the SOC pool under crop rotation wasnearly stable due to the higher carbon inputs, whereasabout 16 t C ha^-1 were lost under maizemonoculture. Under global warming the mean annualsoil respiration for both fields increased and SOCdecreased by ca. 10 t C ha^-1 under croprotation and by more than 20 t C ha^-1 undermaize monoculture. It was shown that overestimationof carbon losses in long-term prognoses can be avoidedby including a Q₁₀-adjustment in soilrespiration models.     

Stomatal Responses of Tradescantia albiflora to Changing Air Humidity in Light and in Darkness

Tradescantia albiflora has green variegated and white leaves.Its stomatal apparatus consists of the guard cells and two pairsof subsidiary cells. Investigations were carried out by observingthe stomata microscopically by means of a video system in situin a CO₂ exchange chamber and by simultaneously measuring thegas exchange of the leaves. In response to air humidity changes,stomatal movements in T. albiflora begin, owing to turgor changes,in the polar and lateral subsidiary cells. The stomatal responseof green leaves to changes of air humidity showed typical transientand oscillatory phases prior to steady-state reactions. In darkness,stomata closed when air humidity decreased; however, they didnot reopen when air humidity was raised again. Stomata of illuminatedwhite leaves responded like those of green leaves in darkness.With increasing soil water stress stomata responded to changingair humidity with reductions of the transient phases and a decreasingtendency to reopen when air humidity became high again. CO₂deficiency of the air caused the stomata to open in the dark,and interacted with the air humidity effect in such a way thatstomata of green leaves responded to air humidity changes indarkness in a similar way as they did in light. Key words: Stomata, humidity response, green and white leaf areas, CO₂ deficient air     

Measurements of electrical leaf surface conductance reveal re-condensation of transpired water vapour on leaf surfaces

Electrical conductance ( λ ) was measured continuously and in vivo on leaf surfaces of Vicia faba and Aegopodium podagraria . λ increased with rise and decreased with fall in humidity, exhibiting a hysteresis during an applied humidity cycle [90–20–-90% relative humidity (r.h.)]. After treatment with NaNO₃ aerosols, a sudden increase in λ was observed at 73% r.h., which is close to the deliquescence point of the salt. Transpiration and electrical conductance of untreated leaves were measured simultaneously under conditions of constant r.h., while the photosynthetic photon flux density and CO₂ concentration of the air were varied to induce changes of stomatal aperture. At 35% r.h., changes of light and CO₂ level revealed a strong correlation between stomatal conductance ( g _S) and λ for Vicia faba leaves. This was also found at 90, 75, 60, 45 and 25% r.h. on the lower but not on the astomatous, upper surface of Aegopodium podagraria . The correlation between g _S and λ for stomata-bearing leaf surfaces indicates that an equilibrium exists between the ambient water vapour phase and the liquid water phase on and within the cuticle. This is modified by transpired water vapour influencing the air humidity inside the boundary layer. Our results imply re-condensation of transpired water vapour to salts on the leaf surface and its sorption to the cuticle.     

In situ Observations of Stomatal Movements

Kappen, L., Andresen, G. and L?sch, R. 1987. In situ observationsof stomatal movements.—J. exp. Bot. 38: 126–141. A device is described by which stomatal movements in situ canbe observed and recorded continuously in light and in darkness.It is mounted in a conditioned CO₂ exchange measuring chamberso that stomatal movements can be observed whilst CO₂ exchange(photosynthesis and respiration) of the same leaf is measured.Advantages and limitations are discussed. By this method itwas shown that stomata of Vicia faba although responding inthe same direction to environmental stimuli exhibited a widerange of pore widths. Responses to changes of air humidity andof CO₂ content were clearly evident when the leaves were exposedto light. Before stomata closed due to decreasing water vapourpressure differences between leaf and air they showed a markedwidening of the pore. An inverse response occurred when watervapour pressure deficit decreased. In darkness stomata did notrespond to such changes. Key words: Stomata, leaf gas exchange, microscopic observation     

Intrathalline and size-dependent patterns of activity in Lasallia pustulata and their possible consequences for competitive interactions

1. In dense populations of the saxicolous lichen Lasallia pustulata the margins of adjacent thalli overlap each other in intraspecific competition for space and light.
2. In situ non-destructive monitoring of hydration-dependent potential photosynthetic activity by modulated fluorescence systems in different parts of the thallus shows that the activity is structured by a centre-to-margin gradient, with the centre of the thallus remaining active for substantially longer periods than the margins when the thalli dry up after being activated by wetting. The pattern reflects the water status of different parts of the thallus; the margins which are thin and exposed dry up first.
3. The activity pattern within individual lichen thalli suggests that marginal overlapping between neighbours may have a less detrimental effect on the shadowed individuals than expected from a pure consideration of the amount of area shadowed. Because the centre of the lichen thallus is active for longer periods, shadowing of this region may possibly be more harmful per area unit than an overlap at the less active margins.
4. Larger thalli are active for substantially longer periods than small ones. Even the margins of larger thalli tend to be active for a longer period than the centre of small thalli.