Hideo Iwaki

Examination of four methods for measuring soil respiration

Abstract We compared the accuracy of four methods for measuring soil respiration: the open-flow infra-red gas analyzer method (OF-method); the closed chamber method (CC-method); the dynamic closed chamber method (DC-method); and the alkali absorption method (AA-method), using artificial soil medium (vermiculite with glucose and mineral nutrients) inoculated with Trichoderma sp. as respiring microorganisms. The CO 2 emission rates from the medium were measured by the four methods once a day for 10 days. We compared the estimated amounts of glucose respired by Trichoderma sp. (EGR, an integration of measured CO 2 emission rates for 10 days) with the actual amounts of glucose respired (AGR, calculated from weight loss of the medium during the same period). In the AA-method, the EGR value was 1.3 times larger than the AGR value, while in the OF, DC- and CC-methods EGR values were almost the same as the AGR (0.95, 0.95 and 0.94 times respectively). Higher CO 2 emission rates obtained by the AA-method were attributed to low CO 2 concentration (20–250 μl 1 −1 ) in the chamber, because at such low CO 2 levels the respiration rate of Trichoderma sp. was enhanced by 20–70%. These results indicate that the OF-, DC- and CC-methods are more suitable for soil respiration measurement than the conventional AA-method.

Measurement of soil respiration using closed chamber method: An IRGA technique

A closed chamber method (CC-method) using an infra-red gas analyzer (IRGA) for measuring soil respiration was examined. Two major factors which potentially cause errors: (i) volume of air sampled from the chamber; and (ii) measuring period of time, were examined in laboratory experiments. Field measurements were also conducted with both the CC-method and the open-flow IRGA method (OF-method) throughout a year. The results of laboratory experiments showed that (i) sampling volume of air should be less than 0.2% of the volume of the chamber; and (ii) the air within the chamber should be sampled several times within 20 min. Field measurements showed that soil respiration rates measured by the CC-method were not significantly different from those by the OF-method. The results of this study indicate that the CC-method is as effective for the measurement of the soil respiration rates as the OF-method.

Fluctuation of photosynthetic photon flux density within aMiscanthus sinensis canopy

Detailed measurements of diurnal variations in photosynthetic photon flux density (PPFD) were made at seven locations within the canopy of aMiscanthus sinensis grassland to evaluate the light conditions of microsites for heliophilic tree seedlings. Multiple regression analysis revealed that the short-term light fluctuation on a clear day was highly dependent on the wind speed and solar elevation angle, whereas on a cloudy day it was mainly determined by the PPFD incident from above the canopy.The relative PPFD at 40 cm aboveground varied from 0.065 to 0.252, depending on sky conditions and the sensors position in relation to clumped patches ofM. sinensis. On a clear day, the proportion of PPFD readings above 100 μmol·m−2·s−1 contributed by sunflecks ranged between 25.4% and 82.0%. Computer simulation showed that the contribution of sunflecks to the daily carbon gain ofQuercus serrata seedlings may range from 11% to 65%.The mean relative PPFD measured under diffuse light conditions was linearly related to the daily total PPFD and the daily carbon gain by single leaves ofQ. serrata seedlings. This suggests that the relative PPFD under diffuse light conditions provides an appropriate measure of site-specific light availability within a grass canopy.

Spatial heterogeneity of photosynthetic photon flux density in the canopy ofMiscanthus sinensis

Spatial variation in photosynthetic photon flux density (PPFD) was investigated in detail at different heights within the canopy of aMiscanthus sinensis grassland to evaluate the light environment of microsites for establishment of heliophilic tree seedlings. Highly heterogeneous patterns of light distribution were revealed within the apparently uniform grass canopies, especially under direct light. The frequency distribution patterns of relative PPFD (RPFD) were compared among different solar and sky conditions. With increasing height in the canopy, the mean RPFD value and standard deviation (SD) increased, while the skewness and kurtosis of the distribution decreased. The mean RPFD and SD were higher, especially at higher solar elevation angles, under direct light than those under diffuse light conditions. The frequency distribution of RPFD was more platykurtic under direct light and at higher solar elevation angles.

Impact of nutrient enrichment in a waterchestnut ecosystem at Takahama-iri Bay of Lake Kasumigaura, Japan

Waterchestnut was shown to be one of the most efficient agents to yield photosynthetic products among aquatic plants in littoral vegetation. Though the plant can be an agent to remove nutrient in agricultural and domestic wastes directly through its waterroots to some extent, waterchestnut plays a great role in removing nutrients through its roots from lake sediments to watercolumn of the lake.Waterchestnut can be choked to death when its leaves are covered completely by the heavy blue-green algal bloom typical in hypereutophic lakes.

Species diversity and primary productivity inMiscanthus sinensis grasslands

In an old field grassland dominated byMiscanthus sinensis Anderss. the community structures, phytomass, dominance ofM. sinensis and species diversity were measured. Species and life form composition of the stand were characterized by higher percentages of therophytes, woody and shrubby species, liana and alien species.In an old field grassland dominated byMiscanthus sinensis Anderss. the community structures, phytomass, dominance ofM. sinensis and species diversity were measured. Species and life form composition of the stand were characterized by higher percentages of therophytes, woody and shrubby species, liana and alien species.From May so September in 1982, all the aboveground parts were harvested from each of the four quadrats (2 m×2 m) once a month. Seasonal peak of aboveground phytomass, in September, was 1027 g d.w.m−2 to whichM. sinensis contributed as much as 96.5%. With the progress of the growing season,M. sinensis became increasingly important both in stand phytomass and in dominance, whereas species diversity based on the dry weight contributions of constituent species decreased. Our analysis of these seasonal trends showed that the diversity was largely a function of dominance of the most important species, rather than that of stand phytomass or productivity. The simultaneous measurements of 20 quadrats in late August 1983, also supported the above conclusion.

Effects of microsite light availability on the survival and growth of oak seedlings within a grassland

Effects of microsite light availability on the growth and survival of transplantedQuercus serrata Thunb. seedlings in aMiscanthus sinensis Anderss. grass canopy were investigated by a “plants eye view” approach. Diffuse site factors, i.e., the fractional transmissions of diffuse photosynthetic photon flux density, were estimated at 15 cm aboveground in 108 microsites where the seedlings grew. Microsite diffuse site factors were significantly different between surviving and dead seedlings during the experiment period from April to October (F[1,14]=10.9, P<0.01). Relative growth rate of dry weight for individual seedlings positively depended on the diffuse site factors (r2=0.482, P<0.001 in May; r2=0.312, P<0.001 in October). Only 16 seedlings produced their second stem flush within the grass canopy. The ratio of height to dry weight of the second stem flush was significantly higher for the seedlings grew in shady microsites than for those in less shady microsites (r2=0,471, P<0.01 in May). This study suggests that the microsite heterogeneity of light availability is one of the important factors affecting the establishment of tree seedlings in patchy grasslands.

Growth analysis of Quercus serrata seedlings within Miscanthus sinensis grass canopies differing in light availability.

The effects of different light regimes on the survival, growth and morphology ofQuercus serrata seedlings were studied in canopies ofMiscanthus sinensis. The seedlings of various ages (0–3 yr) were grown in three light regimes: under a denseM. sinensis canopy (TG plot) receiving 2.5%–8.7% of full sunlight, under a relatively sparse canopy (SG plot) receiving 3.8%–16.1% of light and in an adjacent open site (NG plot). There was a little difference in the survival ofQ. serrata seedlings among the three plots. Height and diameter of stem and total leaf area of the seedlings were significantly lower in the shadier plots. However, the first (bottom) flush of the stem was significantly longer in the TG plot than in the NG and SG plots. Total dry weights of individual 1- and 2-yr-oldQ. serrata seedlings in the TG plot were reduced to about one-twelfth of those in the NG plot. Although the relative proportion in dry weight of each organ did not differ significantly among the plots, leaf area ratio, specific leaf area and stem height per unit dry weight were significantly higher in shadier plots. The leaf area per unit stem height was increased considerably in the sunnier plots.

Seasonal variations of microsite light availability within aMiscanthus sinensis canopy

Photosynthetic photon flux density (PPFD) at 15 cm above the ground was measured at 20 microsites in gaps and grass patches within aMiscanthus sinensis Anderss community at 10 s intervals during 5 days every month from May to September 1989.Microsite light availability, which was characterized by daily total PPFD, sunfleck PPFD (PPFD above a threshold value of 50 or 400 μmol m−2 s−1) and the diffuse site factor, showed evident seasonal changes, with a marked reduction between June and July due to the rapid growth of the grass canopy. The monthly median value of daily total PPFD among the microsites decreased from 10.3 mol m−2 day−1 in May to 0.77 mol m−2 day−1 in September, with a reduction in the diffuse site factor from 31 to 4%. During the summer, the median value of the total time of sunflecks exceeding 50 μmol m−2 s−1 contributed 7–18% of measurement time, but the contribution of these sunflecks to daily total PPFD ranged from 29 to 59%.There was considerable microsite variation in light availability throughout the measurement period. Rank correlation analysis revealed that some microsites, such as those in gaps, consistently received more total PPFD, more sunfleck PPFD and had a higher diffuse site factor than those in grass patches. The diffuse site factor had a linearly positive relationship with daily total PPFD and total sunfleck PPFD for the 20 microsites during the measurement period, confirming that the diffuse site factor is a useful index for microsite light availability withinM. sinensis canopies.

A preliminary study on the photosynthetic induction response of Quercus serrata seedlings

To understand the sunfleck utilization of leaves in heliophilic trees within grass canopies, we studied the photosynthetic induction response ofQuercus serrata seedlings grown for 5 months in different microsites in aMiscanthus sinensis canopy. Two phases, a rapid increase in CO2 uptake and a following slow increase, were recognizabie in the time course of CO2 uptake in response to an increase of photon flux densities (PFD). When the preceding period of low light became shorter, the period of the two phases became shorter. The capacity of response to a sudden light increase was evaluated by the relative photosynthetic induction efficiency (RPIE) defined as the ratio of integrated carbon gain measured to that calculated by assuming that a steady-state assimilation would be achieved instantaneously after the light increase. RPIEs estimated were negatively related to potential sunfleck PFD of microsite. The leaves of the seedlings grown in the microsite with a lower sunfleck PFD and a shorter sunfleck duration showed a more rapid response to a sudden increase of light. These findings suggest that the leaves ofQ. serrata seedlings growing under a lower sunfleck PFD are able to increase the photosynthetic capacity more rapidly to an increase of PFD.