The carbon cycle and global forest ecosystem

Attempts to account for the fluxes by quantifying C sources and sinks have provided evidence of a missing C sink (Detwiler and Hall, 1988), which may be located somewhere in the temperate region of the northern hemisphere (Tanset al., 1990). Until recently, most estimates have concluded that the temperate forest is a small C source. Two recent papers (Sedjo, 1992; Kauppiet al., 1992) provided evidence that the temperate forests are substantial C sinks. This paper combines these earlier findings on temeperate forest carbon sequestration with a new estimate of the annual C releases due to tropical deforestation, 1.7 Gt, which is obtained using the FAO estimates of the rate of deforestation in the tropics over the decade of the 1980s and conservative estimates of C releases associated with this deforestation. Finally, to this is added the crude estimate of C export by the global river system found in Hallet al. (1992). Applying these estimates of the C sink function of both temperate and tropical forests to Detwiler and Hall's alternative C budgets largely eliminates the “missing C” hypothesized by Detwiler and Hall, and Tanset al.     

Human enteric viruses in sewage and their discharge into the ocean

Twenty-four hourly as well as 12 two-hour composite samples of Honolulu's untreated sewage which is discharged into the ocean were assayed for human enteric viruses. Both sampling methods yielded similar results with peaks of virus discharge occurring between 11 to 14 h and again between 20 to 23 h. Over a 24 h period, 8.5 × 10¹⁰ PFU of virus based on the 24 hourly samples and 7.5 x 10¹⁰ PFU based on the 12 two-hour composite samples were determined to be discharged into the ocean via the sewage ocean outfall. Four-hour composite samples of sewage from two selected communities were also analyzed for human enteric viruses. Higher concentrations of viruses were consistently recovered from the sewage draining Kuhio Park Terrace community than from Nuuanu community and support the hypothesis that a community (Kuhio Park Terrace) characterized by a relatively high percentage of children, high density and low socio-economic conditions will excrete higher amounts of enteric viruses than a community (Nuuanu) with contrasting characteristics.     

Aerosol deposition on plant leaves

An aerosol generator and wind tunnel system designed for use in aerosol deposition are described. Gross deposition on rough pubescent leaves was nearly seven times greater than on smooth, waxy leaves. Results suggest that aerosol deposition, on a per unit area basis, for single horizontal streamlining leaves is similar to that for arrays of leaves under similar flow conditions. Wind re-entrainment of PbCl₂ particles was negligible while 2.54 cm of simulated rainfall was sufficient to remove 85% of recently applied aerosol.     

Factors Influencing texture retention of salt-free,acidified, red bell peppers during storage

Red bell peppers were stored in a salt-free, acidified cover solution with sulfite as a microbial preservative. The texture retention of stored peppers was evaluated as a function of pH, acid, calcium, blanch treatment, and growing conditions of the peppers. Field-grown peppers softened at a faster rate than greenhouse-grown peppers and exhibited more variability in their ability to maintain texture during storage. Improved firmness retention of red peppers stored at 30 degrees C was observed when the pH was adjusted to be in the range of 3.4-3.8, when at least 5 mM calcium was added to the peppers, and when red peppers were blanched at 75 degrees C for at least 1 min.     

Unexpected indirect effect of spiders on the rate of litter disappearance in a deciduous forest

Previous experiments in meadow and forest-floor communities have uncovered a negative indirect effect of spiders and other large epigeic predators on decomposition, presumably through depression of densities of microbi-detritivores. We report the results of a 17-month field experiment in a deciduous forest in which spiders exerted the opposite effect on decomposition. In autumn we collected falling leaves of oak (Quercus), maple (Acer) and hickory (Carya), and placed three bags of mixed litter, and three groups of tethered leaves, in 1-m² fenced plots. We determined the disappearance rate of these leaves through April of the second year. Spider densities were reduced by >50% in the spider-removal treatment. The rate of leaf-litter disappearance was not higher in spider-removal plots, but instead was ca. 20% lower (P=0.057). Thus, spider predation clearly was not decreasing the rates of litter decomposition in our experiment; on the contrary, the results suggest that spiders were indirectly enhancing decomposition. Possible hypotheses to explain this unexpected result are discussed.     

Habitat moisture availability and the local distribution of the Antarctic Collembola Cryptopygus antarcticus and Friesea grisea

Population densities of the Collembola Cryptopygus antarcticus and Friesea grisea were compared in two maritime Antarctic habitats with different moisture availability. C. antarcticus was absent from the drier rock platform habitat, where F. grisea was the only collembolan collected. In contrast, the sand/pebble habitat on East Beach had greater moisture availability, and C. antarcticus dominated the arthropod community, with juveniles (individuals <1 mm length) representing 58% of the population. The hygropreference characteristics of F. grisea were determined in relative humidity (RH) gradients (12-98% RH) at 10 and 20 °C. F. grisea demonstrated a stronger preference for 98% RH conditions than C. antarcticus, suggesting that the former species is less likely to vacate moist refuges when available. The movement of both species was also monitored at 10 and 15 °C under conditions of 33, 75 and 100% RH. C. antarcticus was more active than F. grisea at both temperatures, and its movement increased at a greater rate as a consequence of reduced RH. The limited desiccation tolerance of C. antarcticus, combined with the increased water loss that would result from its continued movement under declining RH conditions, suggests this species is not well suited to drought-prone environments. In contrast, the reduced movement and ‘risk averse’ behavioural strategy of F. grisea, i.e. taking advantage of moist refuges when available, facilitates water conservation between precipitation/habitat rehydration events. This study provides the first evidence that moisture availability and habitat structure are potential habitat segregation mechanisms between these two Antarctic Collembola.     

Alteration of soil microbial communities and water quality in restored wetlands

Land usage is a strong determinant of soil microbial community composition and activity, which in turn determine organic matter decomposition rates and decomposition products in soils. Microbial communities in permanently flooded wetlands, such as those created by wetland restoration on Sacramento-San Joaquin Delta islands in California, function under restricted aeration conditions that result in increasing anaerobiosis with depth. It was hypothesized that the change from agricultural management to permanently flooded wetland would alter microbial community composition, increase the amount and reactivity of dissolved organic carbon (DOC) compounds in Delta waters; and have a predominant impact on microbial communities as compared with the effects of other environmental factors including soil type and agricultural management. Based on phospholipid fatty acid (PLFA) analysis, active microbial communities of the restored wetlands were changed significantly from those of the agricultural fields, and wetland microbial communities varied widely with soil depth. The relative abundance of monounsaturated fatty acids decreased with increasing soil depth in both wetland and agricultural profiles, whereas branched fatty acids were relatively more abundant at all soil depths in wetlands as compared to agricultural fields. Decomposition conditions were linked to DOC quantity and quality using fatty acid functional groups to conclude that restricted aeration conditions found in the wetlands were strongly related to production of reactive carbon compounds. But current vegetation may have had an equally important role in determining DOC quality in restored wetlands. In a larger scale analysis, that included data from wetland and agricultural sites on Delta islands and data from two previous studies from the Sacramento Valley, an aeration gradient was defined as the predominant determinant of active microbial communities across soil types and land usage.     

Wandering spiders limit densities of a major microbi-detritivore in the forest-floor food web

In a long-term field experiment, densities of wandering spiders (i.e. species that do not build webs to capture prey) were reduced in order to determine whether or not a major group of microbi-detritivores, the Collembola (springtails), would increase in response to lowered spider predation. Thirty 4-m² fenced plots (15 spider-removal, 15 control plots) and 15 unfenced, undisturbed reference areas were established in a deciduous forest. Spiders were taken from the removal-treatment after being collected by periodic trapping with pitfall traps that contained no preservative, and by searching the litter surface. Judging from a comparison of the numbers captured during periodic censuses in spider-removal and control plots (one census period was two consecutive days of pitfall trapping), wandering spiders were rapidly reduced by ca. 50% and were maintained at levels lower than control plots for 1.3 years. The reduction in numbers was greatest for wolf spiders (Lycosidae; ≈90%). Of six families of Collembola abundant in the leaf litter during the experiment, one family, the Tomoceridae, exhibited a significant release from spider predation. Densities of tomocerids, which are the largest and most active springtails on the research site, gradually increased until they were consistently 2× higher in the spider-removal treatment than in either fenced control plots or unfenced, open reference areas.     

Morphological characterisation of humus forms in recent coastal dune ecosystems in Belgium and northern France

The morphology of humus profiles, developed on recent sandy coastal dune ecosystems, is studied on macro- and microscale. Humus and soil profiles were described, sampled and analysed. Undisturbed, oriented samples were taken from the surface horizons for micromorphological study. The soils are situated in the National Biological Reserve of the Opal Coast, Merlimont, France and near De Haan, Belgium. On both sites, the parent material consists of aeolian calcareous sand. Surface horizons, however, are decarbonated due to leaching processes.In Merlimont, the sequence consists of two profiles, one under Pinus nigra ssp. laricio and one under Populus nigra ssp. nigra, characterised by a Resimor¹/Dysmoder^{, 2} and Rhizomull^{, 1}/Mesomull ^{, 2} humus type, respectively. At the De Haan site, three profiles under Quercus robur, Populus ssp. and P. nigra var. austriaca are investigated. The humus types are respectively Mullmoder^{, 1} to Mull/Dysmull ^{, 2}, Mull ^{, 1}/ Oligomull ^{, 2}, Mormoder ^{, 1}/Dysmoder ^{, 2}.Differences among the humus profiles are related to presence/absence among the L-, F-, H- and A-horizons, structure, rooting, presence of hyphae, sclerotia and excrements.Macromorphologically, the difference between humus profiles under coniferous and deciduous forest/grassland is evident by thicker L-, F and/or H-horizons under coniferous trees. The microstructure of the L- and F-horizons shows a loose packing of subhorizontally oriented needles. The microstructure of the H-horizons varies from intergrain microaggregate to locally weak crumb. The microstructure of the surface mineral horizon of all profiles is mainly intergrain microaggregate, sometimes single grain and exceptionally bridged grain. Intimate mixing and complexation of organic matter to the mineral fraction into aggregates is very limited in the A-horizons. All A-horizons, irrespective of the humus type, are to some degree characterised by a “pepper and salt” appearance. The horizon symbol “H+E” is introduced to label this morphology.The Mull humus forms under deciduous trees are indicative of rather intense mesofauna activities. The absence of a crumb structure in the A-horizon of these soils could be due to the fast disintegration of earthworm casts in these sandy soils with no clay or silt fraction.