Is microbial community composition in boreal forest soils determined by pH,C-to-N ratio,the trees,or all three?

In Fennoscandian boreal forests, soil pH and N supply generally increase downhill as a result of water transport of base cations and N, respectively. Simultaneously, forest productivity increases, the understory changes from ericaceous dwarf shrubs to tall herbs; in the soil, fungi decrease whereas bacteria increase. The composition of the soil microbial community is mainly thought to be controlled by the pH and C-to-N ratio of the substrate. However, the latter also determines the N supply to plants, the plant community composition, and should also affect plant allocation of C below ground to roots and a major functional group of microbes, mycorrhizal fungi. We used phospholipid fatty acids (PLFAs) to analyze the potential importance of mycorrhizal fungi by comparing the microbial community composition in a tree-girdling experiment, where tree belowground C allocation was terminated, and in a long-term (34 years) N loading experiment, with the shifts across a natural pH and N supply gradient. Both tree girdling and N loading caused a decline of ca. 45% of the fungal biomarker PLFA 18:2ω6,9, suggesting a common mechanism, i.e., that N loading caused a decrease in the C supply to ectomycorrhizal fungi just as tree girdling did. The total abundance of bacterial PLFAs did not respond to tree girdling or to N loading, in which cases the pH (of the mor layer) did not change appreciably, but bacterial PLFAs increased considerably when pH increased across the natural gradient. Fungal biomass was high only in acid soil (pH < 4.1) with a high C-to-N ratio (>38). According to a principal component analysis, the soil C-to-N ratio was as good as predictor of microbial community structure as pH. Our study thus indicated the soil C-to-N ratio, and the response of trees to this ratio, as important factors that together with soil pH influence soil microbial community composition.     

Wide geographical and ecological distribution of nitrogen and carbon gains from fungi in pyroloids and monotropoids (Ericaceae) and in orchids

* Stable isotope abundance analyses recently revealed that some European green orchids and pyroloids (Ericaceae) are partially myco-heterotrophic, exploiting mycorrhizal fungi for organic carbon and nitrogen. Here we investigate related species to assess their nutritional mode across various forest and climate types in Germany and California. * C- and N-isotope signatures of five green pyroloids, three green orchids and several obligate myco-heterotrophic species (including the putatively fully myco-heterotrophic Pyrola aphylla) were analysed to quantify the green plants' nutrient gain from their fungal partners and to investigate the constancy of enrichment in (13)C and (15)N of fully myco-heterotrophic plants from diverse taxa and locations relative to neighbouring autotrophic plants. * All green pyroloid and one orchid species showed significant (15)N enrichment, confirming incorporation of fungi-derived N compounds while heterotrophic C gain was detected only under low irradiance in Orthilia secunda. Pyrola aphylla had an isotope signature equivalent to those of fully myco-heterotrophic plants. * It is demonstrated that primarily N gain from mycorrhizal fungi occurred in all taxonomic groups investigated across a wide range of geographical and ecological contexts. The (13)C and (15)N enrichment of obligate myco-heterotrophic plants relative to accompanying autotrophic plants turned out as a fairly constant parameter.     

Litterfall and fine root biomass contribution to nutrient dynamics in second- and old-growth Douglas-fir ecosystems

Litterfall and fine root production were measured for three years as part of a carbon balance study of three forest stands in the Pacific Northwest of the United States. A young second-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stand, a second-growth Douglas-fir with red alder (Alnus rubra Bong.) stand, and an old-growth (∼550 years) Douglas-fir stand were monitored for inputs of carbon and nitrogen into the soil from litterfall and fine root production, as well as changes in soil C and N. Fine root production and soil nutrient changes were measured through the use of soil ingrowth bags containing homogenized soil from the respective stands. Litterfall biomass was greatest in the Douglas-fir-alder stand (527 g m⁻² yr⁻¹) that annually returned nearly three times the amount of N as the other stands. Mean residence time for forest floor material was also shortest at this site averaging 4.6 years and 5.5 years for C an N, respectively. Fine root production in the upper 20 cm ranged from 584 g m⁻² in the N rich Douglas-fir-alder stand to 836 g m⁻² in the old-growth stand. Fine root production (down to one meter) was always greater than litterfall with a below:above ratio ranging from 3.73 for the young Douglas-fir stand to 1.62 for the Douglas-fir-alder stand. The below:above N ratios for all three stands closely approximate those for biomass. Soil changes in both C and N differed by site, but the soil C changes in the old-growth stand mirrored those obtained in an ongoing CO₂ flux study. Results from the soil ingrowth bags strongly suggest that this method provides a simple, but sufficient device for measuring potential fine root biomass production as well as soil chemical changes.     

Dissolved organic carbon concentrations in four Norway spruce stands of different ages

Boreal forests are increasing in age partly due to reduced logging and efficient wildfire control. As a result, they also stock more carbon. Whether increased forest C stock causes greater production of dissolved organic carbon (DOC) is uncertain. DOC in bulk precipitation, throughfall and soil water was studied in 10-, 30-, 60- and 120-year-old stands of Norway spruce (Picea abies (L.) Karst.) DOC concentrations in throughfall and O horizon soil water followed the order 10 < 30 < 60 = 120 and 10 = 30 < 120 < 60, respectively. DOC fluxes followed the order 10 = 30 < 60 = 120 in throughfall, while no significant difference between stands was found for O horizon soil water. Above-ground tree litter varied according to 10 < 30 < 60 = 120, a pattern identical to that for DOC concentrations in throughfall and resembling but not identical to that for DOC concentrations in O horizon soil water. This indicates additional sources for DOC in soil water. Seasonality in DOC concentrations was observed at the base of the O horizon, and seasonality in DOC fluxes in both throughfall and O horizon soil water. Our results suggest differences in the polarity of DOC between the 10-year stand and the others, which we interpret as reflecting the lack of grown trees and possibly the different vegetation on the 10-year stand.     

Factors affecting the phytoaccumulation of weathered, soil-borne organic contaminants: analyses at the ex Planta and in Planta sides of the plant root

Persistent Organic Pollutants (POPs) in the soil–plant system were tracked from their origin in the bulk soil, into the rhizosphere soil pore water, to the xylem sap, and up to the aerial plant tissue. Specifically, the profiles of both chiral and achiral components of technical chlordane along this continuum were examined in detail for members of the Cucurbitaceae family: Cucurbita pepo L. subsp. pepo (“Black Beauty” true zucchini), Cucurbita pepo L. intersubspecific cross (“Zephyr” summer squash), and Cucumis sativus (“Marketmore” cucumber). The experiments were based on the use of mini-rhizotrons for collection and analysis of rhizosphere soil pore water for organic pollutants, as well as for low molecular weight organic acids (LMWOAs). In addition, the xylem sap and aerial plant tissue for intact, homografted, and heterografted C. pepo “Black Beauty” and C. sativus “Marketmore” plants were compared. The data indicate that profiles of the chlordane components in the pore water show no alteration in chiral patterns from those in the bulk soil and may be interpreted by physicochemical partitioning coefficients. Low molecular weight organic acids (LMWOAs) in the rhizosphere were observed to have a minor impact on bioavailability of the pollutants. However, once the pollutants cross the root membrane, major distinctive uptake and enantioselective patterns are apparent in the xylem sap, which are maintained in the aerial tissue. These in planta patterns are based on plant genotype. Specifically, grafting experiments with compatible heterografts of C. pepo and C. sativus establish that the chiral patterns are fully dependent on the plant root. The genotypic dependence of the data suggests possible mechanisms for phytoaccumulation.     

Organic and inorganic nutrient effects on growth rate–irradiance relationships in the Texas brown‐tide alga Aureoumbra lagunensis (Pelagophyceae)1

Pelagophyte species in the genera Aureococcus and Auroumbra form brown tides in coastal bays that cause food‐web disruption and extensive shading of benthic primary producers. Organic nutrients have been suggested as key factors in the origination and persistence of the East Coast (USA) brown‐tide alga Aureococcus anophagefferens Hargraves et Sieburth. To evaluate this finding for the Texas brown‐tide alga Aureoumbra lagunensis D. A. Stockw., DeYoe, Hargraves et P. W. Johnson, we grew strain TBA‐2 with dissolved inorganic nitrogen (DIN; or ) or dissolved organic nitrogen (DON; urea or glutamate) as the nitrogen (N) source under eight light intensities. Maximum growth rates decreased with N source from (1.0 div · d^?1) to (0.48 div · d^?1). Neither growth rate efficiency (α) nor I_k varied significantly between N treatments. Both inorganic phosphorus (P) and β‐glycerophosphate supported growth. Aureoumbra lagunensis can utilize at least some forms of organic N and P and can use them to persist or grow when inorganic forms become limiting. We found no evidence to support the hypothesis that organic utilization enhances or supplements growth at low light levels.     

The use of antibiotics to reduce bacterioplankton uptake of phytoplankton extracellular organic carbon (EOC) in the Potomac River estuary

Phytoplankton production and accumulation of extracellular organic carbon (EOC) was tracked during diel intervals in microcosms by inhibiting bacterioplankton assimilation of EOC with streptomycin and kanamycin. Bacterioplankton production (³H-thymidine incorporation) and metabolism (¹⁴C-glucose incorporation) were monitored in samples collected from the Potomac River estuary to determine the effect of the antibiotics. Particulate (i.e., raw water) primary production and EOC (i.e., water passing through 1.0 μm glass fiber filter) production rates were monitored to determine the impact of antibiotics on phytoplankton. In preliminary experiments, neither streptomycin nor kanamycin alone significantly inhibited bacterioplankton activity compared to controls, but when both were present secondary production and metabolism were reduced up to 90%, and remained as such for 45 h. During field evaluations using a streptomycin and kanamycin mixture (50 μM each) particulate primary production and EOC production were not statistically different in control and antibiotic treated samples indicating that the antibiotics did not negatively influence phytoplankton production rates. In the presence of antibiotics dissolved free amino acids (DFAA) and, to a lesser extent, monosaccharides were significantly elevated compared to controls. This study demonstrates that streptomycin and kanamycin are capable of inhibiting bacterioplankton metabolism and uptake of dissolved organic carbon (DOC) in the samples tested so that the contribution of EOC to the DOC pool and to bacterioplankton metabolism could be measured and assessed.     

Synergistic effects of high temperature and sulfide on tropical seagrass

To examine the synergism of high temperature and sulfide on two dominant tropical seagrass species, a large-scale mesocosm experiment was conducted in which sulfide accumulation rates (SAR) were increased by adding labile carbon (glucose) to intact seagrass sediment cores across a range of temperatures. During the initial 10 d of the 38 d experiment, porewater SAR in cores increased 2- to 3-fold from 44 and 136 μmol L^− 1 d^− 1 at 28-29 °C to 80 and 308 μmol L^− 1 d^− 1 at 34-35 °C in Halodule wrightii and Thalassia testudinum cores, respectively. Labile C additions to the sediment resulted in SAR of 443 and 601 μmol L^− 1 d^− 1 at 28-29 °C and 758 to 1,557 μmol L^− 1 d^− 1 at 34-35 °C in H. wrightii and T. testudinum cores, respectively. Both T. testudinum and H. wrightii were highly thermal tolerant, demonstrating their tropical affinities and potential to adapt to high temperatures. While plants survived the 38 d temperature treatments, there was a clear thermal threshold above 33 °C where T. testudinum growth declined and leaf quantum efficiencies (Fv/Fm) fell below 0.7. At this threshold temperature, H. wrightii maintained shoot densities and leaf quantum efficiencies. Although H. wrightii showed a greater tolerance to high temperature, T. testudinum had a greater capacity to sustain biomass and short shoots under thermal stress with labile C enrichment, regardless of the fact that sulfide levels in the T. testudinum cores were 2 times higher than in the H. wrightii cores. Tropical seagrass tolerance to elevated temperatures, predicted in the future with global warming, should be considered in the context of the sediment-plant complex which incorporates the synergism of plant physiological responses and shifts in sulfur biogeochemistry leading to increased plant exposure to sulfides, a known toxin.