Organic matter in density fractions of water-stable aggregates in silty soils: Effect of land use

The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm⁻³ (free POM_<1.6), occluded particulate organic matter with densities <1.6 g cm⁻³ (occluded POM_<1.6) and 1.6-2.0 g cm⁻³ (occluded POM_1.6-2.0) and mineral-associated SOM (>2.0 g cm⁻³) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 μm) and in the sieve fraction <53 μm from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the ¹³C/¹²C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM_<1.6>occluded POM_<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 μm mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 μm, 76 yr in the 53-250 μm aggregate class, and 102 yr in the sieve fraction <53 μm. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 μm to 66 yr in aggregates <53 μm. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 μm to 42 yr in the fraction 53-250 μm. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.     

Interaction between a peat carrier and bacterial seed treatments evaluated for biological control of the take-all diseases of wheat (Triticum aestivum L.)

Summary Field plots were established in Indiana, Oregon, and Montana to evaluate the potential for biological control of various strains of bacteria as seed treatments to reduce the severity of take-all root, crown, and foot rot of wheat. The bacteria were grown in liquid broth Cas-amino acid broth media, mixed with finely ground peat, and applied to seed with methyl cellulose as a glue just before planting in field soils conducive for severe take-all. Autoclave-sterilized peat (minus bacteria) seed treatments increased take-all, immobilized Mn, and reduced plant vigor and grain yields. These effects were intensified when the pH of the natural peat was adjusted from 5.2 to 7.0 with CaCO₃. The ability of the bacterial strains to counteract this peat-induced predisposition to take-all varied, and was influenced by planting site, genetic tolerance of the cultivar, and N treatment. Although the strains differed in their ability to suppress the peat-induced take-all, none of the isolates fully nullified the deleterious effects of the peat carrier. It is clear from this study that the carrier used with potential biological-contol agents may have a greater influence on disease than the biological agent.     

Naturschutz aus der Sicht der Waldbesitzer

Ohne Zusammenfassung     

Simulating logging scenarios in secondary forest embedded in a fragmented neotropical landscape

In publications on tropical forest fragmentation, regrowing secondary forests (SF) on abandoned agricultural land are often overlooked. These forests represent highly fragmented components in the landscape matrix which are generally less than 100 ha in size. SF are found on privately owned land where they are contributing to on-farm income through logging and/or use of non-timber products. Hence, defining pure conservation goals for this forest type seems to be unrealistic. In this paper, we used the process-based model F 3-Q to simulate successional processes and logging scenarios in SF exposed to different degrees of fragmentation (either facing non-forest land on only one side or totally surrounded by agricultural land) in the subtropical eastern part of Paraguay. We compared results with primary forest embedded in a similar matrix. Under light fragmentation, bole volume of SF approached primary forest values after about 50 years of succession. Species composition, however, was clearly distinct from primary forest over the first 200 years of succession. The development of bole volume and species composition in severely fragmented SF was similar to the less fragmented SF over the initial 50 years. However, limited seed input, largely confined to on-site seed sources in the severely fragmented SF, led to a decline in bole volume by about one-third compared to the reference value over the simulation period of 400 years. By applying a minimum felling diameter (MFD) of 35 cm in the lightly fragmented SF, first logging was only possible after 30 years, resulting in a mere 3.7 m³ ha⁻¹ harvest. Highest timber yields were obtained with 10 yr cutting cycles and a maximum removal of 20 stems ha⁻¹, though sustainable yields only resulted from lowering the MFD to 30 cm. Logging in severely fragmented SF accelerated the decline of bole volume. Overall, results suggest that timber production in SF without silvicultural treatments is low and hence does not constitute a viable option to diversify farm income. Only managed SF may serve this function. Research focusing on the elaboration of management options for SF, integrating as much as possible of the original vegetation, is needed.     

Crystal structure of a carbon monoxide dehydrogenase reveals a [Ni-4Fe-5S] cluster

The homodimeric nickel-containing CO dehydrogenase from the anaerobic bacterium Carboxydothermus hydrogenoformans catalyzes the oxidation of CO to CO2. A crystal structure of the reduced enzyme has been solved at 1.6 angstrom resolution. This structure represents the prototype for Ni-containing CO dehydrogenases from anaerobic bacteria and archaea. It contains five metal clusters of which clusters B, B', and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters. The active-site clusters C and C' are novel, asymmetric [Ni-4Fe-5S] clusters. Their integral Ni ion, which is the likely site of CO oxidation, is coordinated by four sulfur ligands with square planar geometry.     

Deep and Bottom Water of the Weddell Sea's Western Rim

Oceanographic observations from the Ice Station Weddell 1 show that the western rim of the Weddell Gyre contributes to Weddell Sea Bottom Water. A thin (< 300 meters), highly oxygenated benthic layer is composed of a low-salinity type of bottom water overlying a high-salinity component. This complex layering disappears near 66 degrees S because of vertical mixing and further inflow from the continental margin. The bottom water flowing out of the western rim is a blend of the two types. Additionally, the data show that a narrow band of warmer Weddell Deep Water hugged the continental margin as it flowed into the western rim, providing the continental margin with the salt required for bottom-water production.     

Three-dimensional structure of cholera toxin penetrating a lipid membrane

Two-dimensional crystals of cholera toxin bound to receptors in a lipid membrane give diffraction extending to 15 A resolution. Three-dimensional structure determination reveals a ring of five B subunits on the membrane surface, with one-third of the A subunit occupying the center of the ring. The remaining mass of the A subunit appears to penetrate the hydrophobic interior of the membrane. Cleavage of a disulfide bond in the A subunit, which activates the toxin, causes a major conformational change, with the A subunit mostly exiting from the B ring.     

Climate modification by atmospheric aerosols

Theoretical considerations and empirical evidence indicate that atmospheric turbidity, a function of aerosol loading, is an important factor in the heat balance of the earth-atmosphere system. Turbidity increase over the past few decades may be primarily responsible for the decrease in worldwide air temperatures since the 1940's.     

Mass-Spectrometric 230Th-234U-238U Dating of the Devils Hole Calcite Vein

The Devils Hole calcite vein contains a long-term climatic record, but requires accurate chronologic control for its interpretation. Mass-spectrometric U-series ages for samples from core DH-11 yielded (230)Th ages with precisions ranging from less than 1,000 years (2sigma) for samples younger than approximately 140 ka (thousands of years ago) to less than 50,000 years for the oldest samples ( approximately 566 ka). The (234)U/(238)U ages could be determined to a precision of approximately 20,000 years for all ages. Calcite accumulated continuously from 566 ka until approximately 60 ka at an average rate of 0.7 millimeter per 10(3) years. The precise agreement between replicte analyses and the concordance of the (230)Th/(238)U (234)U/(238)U ages for the oldest samples indicate that the DH-11 samples were closed systems and validate the dating technique in general.