Carbon flow in plant microbial associations

Includes bibliographical references (page 474).Measurement of the distribution of the photosynthesis product in the symbiotic association of a legume, a mycorrhizal fungus, and nitrogen-fixing bacteria showed that the fungus incorporated 1 percent of the photosynthesis product and respired 3 percent. The nodules of a 5-week-old plant utilized 7 to 12 percent of the photosynthesis product. The legume compensated in part for the needs of its microbial partners through increased rates of photosynthesis.Publisher version: http://www.jstor.org/stable/1686536

Response of mycorrhizal and P-fertilized soybeans to nodulation by Bradyrhizobium or ammonium nitrate

Includes bibliographical references (page 150).Management of N2-fixing bacteria or P-scavenging endomycorrhizae may lead to decreased fertilizer use on extensively cropped lands. To measure the effectiveness of these microsymbionts, soybean [Glycine max (L.) Merr. cv. Amsoy 71] plants were grown in a growth chamber in a soil [Josephine silty clay loam (mesic Typic Haploxerult)] low in plant-available N and P. Plants were inoculated with different Bradyrhizobium strains or received nutrient solutions of different N concentrations (0.0, 1.0, 2.0, 4.0 mM N) and P adequate for maximum plant growth under these conditions. Other plants were infected with a vesicular-arbuscular mycorrhizal (VAM) fungus and a Bradyrhizobium strain and received no N or P in the nutrient solution. The purpose of this study was to determine the growth response of soybean to N fertilization or nodulation by B. japonicum under conditions of high P availability or V AM-assisted P uptake. Nodulated non-VAM soybean plants had dry weights and development similar to that of the 4.0 mM N fertilizer treatment. Total N and Mn, leaf area, and leaf P of nodulated plants were higher than in the comparable N-fertilized plants in the absence of P stress. Soybeans infected with both the VAM fungus and Bradyrhizobium were similar in total dry weight, leaf area, and development to plants that received 1.0 or 2.0 mM N. They, however, contained more leaf N, more root Cu and Zn, and less Mn and P than the 2.0 mM N treatment. It is concluded that a number of host characteristics of nodulated plants are due to the altered functional aspects of the symbiosis and not N input alone. The presence of the VAM fungus can decrease nutrient stress in environments limited in P, Zn and Cu, elements essential in N2 fixation

Greenhouse gases in intensive agriculture: contributions of individual gases to the radiative forcing of the atmosphere

Includes bibliographical references (pages 1924-1925).Agriculture plays a major role in the global fluxes of the greenhouse gases carbon dioxide, nitrous oxide, and methane. From 1991 to 1999, we measured gas fluxes and other sources of global warming potential (GWP) in cropped and nearby unmanaged ecosystems. Net GWP (grams of carbon dioxide equivalents per square meter per year) ranged from 110 in our conventional tillage systems to -211 in early successional communities. None of the annual cropping systems provided net mitigation, although soil carbon accumulation in no-till systems came closest to mitigating all other sources of GWP. In all but one ecosystem, nitrous oxide production was the single greatest source of GWP. In the late successional system, GWP was neutral because of significant methane oxidation. These results suggest additional opportunities for lessening the GWP of agronomic systems.Publisher version: http://www.jstor.org/stable/3077685

Comparisons between P-fertilized and mycorrhizal plants

Includes bibliographical references (pages 155-156).In experimentation with vesicular-arbuscular mycorrhizal (VAM) fungi, the availability of non-VAM control plants of equal size to VAM plants is a fundamental requirement. The purpose of this work was to determine nutrient regimes needed to achieve growth equivalence between VAM and non-VAM plants. Soybean [Glycine max (L.)Merr.] cv. Amsoy 71 and sorghum [Sorghum bicolor (L.) Moench] cv. Bok 8 plants were grown under controlled conditions in a soil (Josephine silty clay loam, mesic Typic Haploxerult) low in plant-available P. Soybeans were inoculated with one of four species and sorghum with one of two species of VAM fungi. Non-inoculated control plants received nutrient solutions that contained 0.0, 0.2, 0.4, or 1.0 mM P. while the growth of P-supplemented controls may be equivalent to VAM plants, an important question remains: Are these plants also equivalent in terms of such functional parameters as leaf development, dry matter partitioning, and nutrient assimilation? The objective of this experiment was to answer these questions. The response to VAM colonization was similar in both hosts, although less extensive colonization was observed in sorghum. Dry weight, leaf area, and P content increased exponentially with nutrient solution P level. Plants colonized with VAM fungi grew 3 to 6 times larger than the P-free controls but attained only 35 to 65% of maximum growth possible with high fertilizer P input. Host response to VAM colonization was equivalent to that of plants receiving between 0.12 and 0.22 mM P for phytomass, leaf area, and N content. Mycorrhizal plants contained less P, Mn, and root Fe but more Zn and Cu than comparable plants fertilized with P. It was concluded that P-treated, non-VAM plants differed physiologically and anatomically from VAM plants of equivalent size grown under P stress. It may therefore be necessary to establish the comparability of VAM plants and of "VAM-equivalent controls" separately for each plant parameter of interest. Even then, differential growth responses in VAM-host associations may prevent complete comparability between VAM and P-fertilized plants

Injection of nitrogen-15 into trees to study nitrogen cycling in soil

Includes bibliographical references (page 319).Most 15N dilution techniques disturb either the soil or N-pool size. The objective of this study was to develop a method of labeling the roots of Populus trees with 15N without physically disturbing the soil. Such a method would enable the direct measurement of the flux of 15N from dead roots into the soil organic matter. Leaf and root biomass were labeled by injection of 15N directly into the vessel elements of hybrid Populus trees during their second growing season. The 15N was uniformly distributed throughout the canopy and root system. The rate and amount of 15N turnover from plant tissue can be determined by pool transfer or through differences in plant 15N concentrations. The 15N was detected in the dead-root pool 8 wk after injection, indicating root turnover. Results demonstrate the ability to measure the contribution of fine-root litter to N-cycling processes without disturbing the soil environment

Biological and molecular structure analyses of the controls on soil organic matter dynamics

Includes bibliographical references (page 170).The dynamics of soil organic carbon (SOC) are controlled by the interaction of biological, physical, and chemical parameters. These are best measured by a combination of techniques such as long-term field sites with a C3↔C4 plant switch. Acid hydrolysis and 14C- dating measure the mean residence time (MRT) of the resistant fraction. Long-term incubation allows the in situ biota to identify and decompose the labile SOC components. Statistical analysis (curve fitting) of the CO2 release curves, determines the pool size and of the two labile fractions (1). The effect of chemical structure is measured with pyrolysismolecular beam mass spectrometry (py-MBMS). The dynamics of charcoal, clay and silt are measured with both 13C and 14C

Microbial effects

Includes bibliographical references (pages 167-176).The recommendations for microbiological research needs are based on the following general scenario: a) increased atmospheric CO2 will increase crop productivities by 10-40%, depending on crop and geographic area, which in turn will increase biomass and soil organic matter by 5-40%; b) additional root-derived materials and crop residue in the soil will increase soil microbial activities, producing a greater flux in most major cycles and possibly some changes in pool sizes of -10% to +30%; c) these effects will increase biological Nz fixation, and the increased demand for N will place significant limitations on phosphorus and other mineral nutrients; d) no significant changes will occur in soil O2 or CO2. The postulated doubling of atmospheric CO2 is not likely to have a direct effect on soil microbial activity because during the growing season, the concentration of CO2 in the soil atmosphere is already ten to fifty times higher than existing atmospheric CO2. Based on all available experimental information, it is estimated that a doubling of atmospheric CO2 will cause an increase in primary productivity of ten to forty percent, depending on locale. The increase in biomass will, in turn, produce a limitation of available soil nutrients, especially nitrogen and phosphorus. Increased organic carbon together with nitrogen and/or phosphorus limitation will result in a preferential increase in nitrogen fixation and mycorrhizal activities as the expedient means for supplying required nutrients to sustain the predicted increase in primary productivity. Therefore, increased emphasis should be placed on fundamental research related to soil microbiology with special reference to nitrogen-fixing, nitrifying and denitrifying bacteria, and to the mycorrhizal fungi

Adhesion, Spreading and Fragmentation of Human Megakaryocytes Exposed to Subendothelial Extracellular Matrix: A Scanning Electron Microscopy Study

Platelet agonists and subendothelial extra-cellular matrix (ECM) induce morphological and biochemical changes in animal megakaryocytes, reminiscent of the response of platelets to the same substances. We have examined the behavior of human megakaryocytes exposed for up to 36 hours to the ECM produced by cultured bovine corneal endothelial cells. By phase contrast and scanning electron microscopy these megakaryocytes demonstrated non-reversible adherence and flattening with formation of long filopodia, thus confirming that human megakaryocytes acquire platelet functional capacities. In addition, megakaryocyte fragmentation into prospective platelets was apparently induced by the ECM. Up to 50% of the adherent megakaryocytes underwent spontaneous fragmentation into small particles which individually reacted like platelets on the ECM. The interaction of the megakaryocytes with the ECM was specific since no adherence, flattening or fragmentation occured upon incubation of the megakaryocytes on regular tissue culture plastic or glutaraldehyde fixed ECM. Thus we have demonstrated platelet like behaviour of human megakaryocytes in response to this physiological basement membrane and a possible role of the subendothelium in platelet production which may occur in vivo as megakaryocytes cross the sinusoid walls and enter the blood stream

Fate of legume and fertilizer nitrogen-15 in a long-term cropping systems experiment

Includes bibliographical references (pages 914-915).Relying more on biological N2 fixation has been suggested as a way to meet one of the major challenges of agricultural sustainability. A 15N study was conducted to compare the fate of applied legume and fertilizer N in a long-term cropping systems experiment. Nitrogen-15-1abeled red clover (Trifolium pratense L.) and (NH4)2SO4 ere applied microplots within the low-input and conventional cropping systems of the Farming Systems Trial at the Rodale Institute Research Center in Pennsylvania. The 15SN was applied to soil and traced into corn (Zea mays L.) in 1987 and 1988. Residual 15SN was also traced into second-year spring barley (Hordeum vulgare L.). Legume and fertilizer 15SN remaining in soil was measured and loss of N was calculated by difference. More fertilizer than legume N was recovered by crops (40 vs. 17% of input), more legume than fertilizer N was retained in soil (47 vs. 17% of input), and similar amounts of N from both sources were lost from the cropping systems (39% of input) over the 2-yr period. More fertilizer than legume N was lost during the year of application (38 vs. 18% of input), but more legume than fertilizer N was lost the year after application (17 vs. 4% of input). Residual fertilizer and legume 15SN was distributed similarly among soil fractions. Soil microbial biomass was larger in the legume-based system. A larger, but not necessarily more active, soil microbial biomass was probably responsible for the greater soil N supplying capacity in the legume-based compared with fertilizer-based system

Changes in ecosystem carbon following afforestation of native sand prairie

Includes bibliographical references (pages 1622-1624).Determining the dynamics of carbon (C) as a function of vegetation and residue inputs is important for predicting changes in ecosystem functions and the global C cycle. Litter and soil samples were analyzed from plantations of eastern red cedar (Juniperous virginiana) and ponderosa pine (Pinus ponderosa) and native prairie at the Nebraska National Forest to evaluate the impact of different types of land management on soil C contents and turnover rates. Total soil C to a depth of 1 m was greatest in the cedar stands. Pine ecosystems stored more C in the tree biomass and litter but lost more native prairie C from the soil. The soil 13C content showed 82% of the original, and prairie C remained under cedars compared with ∼45% under pine. Soil cation contents were greatest overall in cedar soils and lowest in pine. The C content in cedar soils was strongly related to Ca content. Differences in microbial community fatty acid profiles were related to vegetation type, and nutrients explained ∼60% of the variation in profiles. Our research indicates that changes in soil C and nutrient content following conversion from prairie to forest are dependent on tree species planted, characteristics of the plant litter, and cation cycling in the plant–soil system