John G. McColl

Polyphenols as regulators of plant-litter-soil interactions in northern California’s pygmy forest: A positive feedback?

The convergent evolution of polyphenol-rich plant communities has occurred on highly acidic and infertile soils throughout the world. The pygmy forest in coastal northern California is an example of an ecosystem on an extremely infertile soil that has exceptionally high concentrations of polyphenols. Many ‘negative feedbacks’ have been identified whereby plants degrade fertile soils through production of polyphenol-rich litter, sequestering soil nutrients into unavailable form and creating unfavorable conditions for seed germination, root growth, and nutrient uptake. But in the context of plant-litter-soil interactions in ecosystems adapted to soils that are inherently acidic and infertile (such as the pygmy forest), there are also many ‘positive feedbacks’ that result from polyphenol production. By inhibiting decomposition, polyphenols regulate the formation of a mor-humus litter layer, conserving nutrients and creating a more favorable medium for root growth. Polyphenols shift the dominant pathway of nitrogen cycling from mineral to organic forms to minimize potential N losses from the ecosystem and maximize litter-N recovery by mycorrhizal symbionts. Polyphenol complexation of Al, Mn and Fe reduce potential Al toxicity and P fixation in soil. Polyphenols regulate organic matter dynamics, leading to the accumulation of organic matter with cation exchange capacity to minimize leaching of nutrient cations. Humic substances derived from polyphenolic precursors coat rhizosphere soil surfaces, improving physical and chemical conditions for root growth and nutrient cycling. Although their long-accepted adaptive value for antiherbivore defense is now in doubt, polyphenol alteration of soil conditions and regulation of nutrient cycling illustrate how fitness can be influenced by the ‘extended’ phenotype in plant-litter-soil interactions.

Linkages between phosphorus transformations and carbon decomposition in a forest soil

Phosphorus mineralization is chemically coupled with organic matter (OM) decomposition in surface horizons of a mixed-conifer forest soil from the Sierra Nevada, California, and is also affected by the disturbance caused by forest harvesting. Solution13C nuclear magnetic resonance (NMR) spectroscopy of NaOH extracts revealed a decrease of O-alkyl and alkyl-C fractions with increasing degree of decomposition and depth in the soil profile, while carbonyl and aromatic C increased. Solid-state13C-NMR analysis of whole soil samples showed similar trends, except that alkyl C increased with depth. Solution31P-NMR indicated that inorganic P (P1) increased with increasing depth, while organic-P (Po) fractions decreased. Close relationships between P mineralization and litter decomposition were suggested by correlations between P1 and C fractions (r = 0.82, 0.81, −0.87, and −0.76 for carbonyl, aromatic, alkyl and O-alkyl fractions, respectively). Correlations for diester-P and pyrophosphate with O-alkyl (r = 0.63 and 0.84) and inverse correlations with aromatics (r = −0.74 and −0.72) suggest that mineralization of these P fractions coincides with availability of C substrate. A correlation between monoester P and alkyl C (r = 0.63) suggests mineralization is linked to breakdown of structural components of the plant litter. NMR analyses, combined with Hedley-P fractionation, suggest that post-harvest buildup of labile P in decomposed litter increases the potential for leaching of P during the first post-harvest season, but also indicates reduced biological activity that transports P from litter to the mineral soil. Thus, P is temporarily stored in decomposed litter, preventing its fixation by mineral oxides. In the mineral horizons,31P-NMR provides evidence of decline in biologically-available P during the first post-harvest season.

Forest Fire: Effects on Phosphorus Movement to Lakes

After a wildfire in the virgin forest of a lake-watershed region in northeastern Minnesota, the phosphorus concentration in the runoff was elevated for 2 years and decreased in the third year. However, there was no increase in the phosphorus concentrations of a lake and its input stream. This indicates that, under similar circumstances, controlled burning will not damage streams or lakes by elevating phosphorus levels.

Effects of simulated acid rain on germination and early growth of Douglas-fir and ponderosa pine

SummaryGermination percentage of Douglas-fir (Pseudotsuga menziesii) was reduced by 30% by severe acid-rain treatment (pH 2.0), and seedlings that germinated at pH 2.0 soon died from fungal attack. Less acidic treatments did not affect germination.Two-year-old seedlings of Douglas-fir and ponderosa pine (Pinus ponderosa) planted in native granitic soil and sprayed for 12 weeks with acid (2∶3, sulfuric: nitric) at pH 5.6, 4.0, 3.0 and 2.0, also showed little effects except at pH 2.0. There were no significant differences (atp<0.05) between acid treatments for length and for dry weight of needles that developed during acid treatment. However, at pH 2.0, needles exhibited white acid burns, brown tips, and seedlings became limp and wilted; symptoms worsened with duration of treatment. Both new and old needles eventually died at pH 2.0. Implications of this study are discussed.

Effects of forest management on soil nitrogen in Pinus radiata stands in the Australian Capital Territory

Abstract Field ammonium and nitrate levels, and nitrogen mineralization and nitrification rates, were determined in the 0–20-cm soil layer in Pinus radiata (D. Don) stands (recently clearcut, young (5 years old) and mature (45 years old) near Canberra (A.C.T.) from 17 November 1981 to 3 May, 1982. Total nitrogen contents in the yellow podzolic soils were low ( 4 -N were

Seasonal nutrient variation in trembling aspen.

SummaryVariation in concentrations of N, K, Mg, and Ca in vegetative-buds, leaves, twigs, and male catkins of trembling aspen (Populus tremuloides Michx.), determined throughout a growing season in central Minnesota, reflected the different physiological roles of the nutrients. Practical implications are discussed.

Nitrogen fixation in the rhizosphere and rhizoplane of barley

SummaryAerobic and anaerobic N2-fixing bacteria developed in the rhizosphere of barley seedlings and exhibited N2ase activity when seedlings were grown in sterilized sand-nutrient cultures containing low levels of combined nitrogen. The source of the N2-fixing bacteria appeared to be the seed. Average daily rates up to 0.9 μmoles C2H4 h−1 g−1 dry root tissue were measured, but the intensity of the activity was affected by moisture levels and concentration of combined N in the rhizosphere. Removal and washing of the roots did not remove the activity, and roots remained active even after surface-sterilization. An unidentified aerobic N2-fixing bacterium was isolated from the rhizoplane of active barley roots. Inoculation of barley seedlings with the aerobic N2-fixing bacterium enhanced N2ase activity of excised roots 10-fold, with average rates of 0.9, 1.1 and 1.3 μmoles h−1 g−1 dry root assayed under pO2 of 0.01, 0.02 and 0.04 atm respectively. The aerobic N2-fixing bacterium also exhibited N2ase activity when inoculated into the rhizosphere of oat, rice and wheat seedlings. Microscopic observations of sterilized live and stained barley roots suggest that the aerobic N2-fixing bacterium is an endophyte which infects root tissue and metamorphoses into vesicle-like structures.

Soil chemical and microbial effects of simulated acid rain on clover and soft chess

Effects of simulated acid rain, comprised of HNO3 and H2SO4 in the mole ratio of 3:1, at pH 5.6, 4.5, 4.0 and 3.0, were tested on the grass, soft chess (Bromus mollis L.) and on clover (Trifolium subterraneum L. var. Woogenellup) in a sandy soil of granodiorite parent material. Soft chess was grown in unfertilized soil, whereas clover was grown in both unfertilized soil and soil fertilized with NH4NO3 and CaSO4·2H2O at the rates of 224 kg ha−1 N and 78 kg ha−1 S. Two acid-spray irrigation periods of 31 and 26 weeks duration, each delivering 400 mm and separated by a dry period of 23 weeks, simulated typical rainfall of northern California rangeland. Plants were harvested after each of the two spray periods. There were very few deleterious effects of acid rain on plant growth or soil and microbial processes. No significant (p<0.05) effects were shown by soil microbial biomass, CO2 production, nodules per unit weight of clover root, acetylene reduction, denitrification and nitrification potentials, or for soft chess plant weights, and N and P uptake. Mineralizable-N was unaffected also, except in one case. However, pH of soil to 10 mm depths was significantly lower in the pH 3.0 treatment after the first spray period, with a corresponding decrease in exchangeable soil Ca; these effects became significant at greater soil depth only after the second spray period. There were significant effects of acid treatments shown by clover, some of which may be advantageous. Treatments of intermediate acidity generally provided added N and S, which acted as fertilizers, and compensated for possible decreases in plant productivity attributable to acidity per se. There was also evidence of decreased P uptake in unfertilized soil at pH 3. In conclusion, effects of simulated acid rain were minimal, and in some cases were advantageous because of the added N and S having a fertilizer effect on plant nutrition and growth.

Acetylene reduction byDaviesia mimosoides under Eucalyptus

SummaryDaviesia mimosoides is a common understorey legume in Eucalyptus forests of the Brindabella Range in southeastern Australia, capable of fixing atmospheric nitrogen. Rates of N fixation were measured by the acetylene-reduction technique over a growing season in the field. Pot trials under controlled conditions were also carried out to elucidate effects of soil moisture, temperature, and light.Average rates in the field varied from about 1–5 μ mol C2H4/g/h (wet weight of nodule), but rates up to 14 μ mol C2H4/g/h were measured in optimum controlled conditions. Annual N-fixation rates approximate 4.5–7.0 kg/ha.In pot trials, rate of acetylene reduction decreased with soil moisture to about−10 MPa tension, with a marked depression at about−6 MPa, but within the normal field range of soil moisture there was little correlation of moisture with average acetylene reduction rate. Rates were similar in the temperature range of 20–30°C, but were depressed by either low or high temperature (<10 or >30°C). Diurnal fluctuations in acetylene reduction rates were not correlated with solar radiation, but rates were limited by high mid-day temperatures.

El Nino effects on hydrogen ion concentration of a California reservoir

Hydrogen ion concentration, [H+], of discharge water from Pardee reservoir in the central Sierra Nevada, California was greater than expected in years of El Nino occurrence over the period 1954–86. This pattern is in addition to the general increase in [H+] over the same period attributed to acidic atmospheric deposition. Monthly means of [H+] also show differences between El Nino and non-El Nino years. Total annual runoff does not seem to be a controlling factor; the source and timing of storms are probably more important. Storms are usually from the west or northwest, but during El Nino years tropical-like storms from a more-southerly direction appear to carry acidic pollutants to the central Sierra Nevada.