Search

A 21-yr perspective on changes in nutrient and C pools on undisturbed upland forest sites is provided. Plots originally representing four cover types have been sampled three times. On each plot, forest biomass, forest floor, and soil, to a depth of 60 cm, were measured, sampled, and analyzed for Ca, Mg, C, N, and P. Exchangeable soil Ca and Mg have declined in most soils. Despite the low exchangeable Ca, cumulative sequestration in the biomass has exceeded the soil pool, suggesting that soil supplies below 60 cm are satisfying the biomass demand. Extractable soil P also declined, with means ranging from 4.2 to 18.2 kg [ha.sup.-1], as a result of reductions in the mineral soil and Oi horizon. The loss of extractable soil P exceeded biomass sequestration in all but one plot, suggesting abiotic soil processes as the removal mechanism. Soil C and N were either stable, although highly variable, or declined, which was unexpected in these undisturbed sites. The net C balance of these sites was controlled by aboveground sequestration, which offset changes in the soil and forest floor. Soil parent material and geomorphic setting strongly influenced the changes in soil properties during the 21-yr period, reflecting the importance of those factors in assessing soil nutrient and C cycles over that of forest cover type. The variability encountered in the periodic soil measurements highlights the difficulty in detecting temporal changes in soil chemical properties.

Forest thinning accomplished with cut-to-length and whole-tree harvesting systems, and prescribed underburning were assessed for their impacts on water relations in eastern Sierra Nevada Jeffrey pine (Pinus jeffreyi Grey. & Balf.) during a period of extended drought. Predawn and midday measurements of xylem water potential in dominant and codominant crown class trees more than a century old were made on six days spread over three growing seasons, accompanied by measurements of soil water potential completed between the predawn and midday sessions of each day. With the exception of a single predawn session, the only one of a total of 12 in which xylem water potentials did not differ among treatments, the potentials in trees of thinned stand subunits were 0.67 MPa higher on average during predawn sessions and 0.71 MPa higher during midday sessions than those in trees of the unthinned treatment. Differences between the cut-to-length and whole-tree treatments were marginal and uncommon, but when they occurred, potentials were higher in the former. Prescribed fire effects on xylem water potential were also uncommon, but when occurring generally indicated lower stress levels in the burned than in the unburned treatment. Soil water potentials largely coincided with those of xylem water, with higher potentials in either the cut-to-length or whole-tree treatments, and usually both, than in the unthinned treatment on each of the six days of measurement. Underburning influences on soil water were rare, but when evident, potentials were higher in the burned than in the unburned treatment by substantial margins. For a majority of the measurement sessions, xylem water potential was found to he negatively correlated with residual basal area but positively correlated with soil water potential. In turn, coarse fragments and organic matter in the soil profile intermittently influenced soil water potential, with the former a negative factor while the latter was positive. Overall, results of this study suggest that substantial ecophysiological advantages can be derived from density management in older, dry site forests, which at minimum are not compromised by subsequent implementation of controlled underburning. [ABSTRACT FROM AUTHOR]

Abstract Litter decomposition is a major component of nutrient cycling in forest ecosystems and is known to be sensitive to both temperature and moisture. A drought during periods of the year when moisture is normally abundant may impact decomposition more than moisture limitation during seasons that are routinely dry. We excluded precipitation for a 3-month period during each of four seasons and measured mass loss and nitrogen (N) dynamics in Acer rubrum leaf litter by using a litterbag approach. We hypothesized that rainfall exclusion would retard mass loss during the exclusion period, with the maximum effect occurring during the spring and summer months when moisture was normally adequate and temperatures optimum for decomposition, and that exclusion during these two periods would have the greatest impact on subsequent timing of decomposition events, particularly N mineralization. Mass loss in litterbags was initially reduced by precipitation exclusion in the spring treatment only. For each season, a set of bags exposed to drying in the same way and allowed to remain in the field under ambient conditions for an additional 9 months no longer showed a difference in mass loss relative to control bags. Nitrogen accumulation was immediately and significantly reduced in the exclusion treatments in all seasons; however, like mass loss, there was no difference 9 months later. We conclude that short-term effects of drought do occur, especially with regard to N accumulation, and that these effects do not persist and are unlikely to have any substantial effect on nutrient cycling. [ABSTRACT FROM AUTHOR]

The effects of changing precipitation on soil leaching in a deciduous forest were examined by experimentally manipulating throughfall fluxes in the field. In addition to an ambient treatment (AMB), throughfall fluxes were reduced by 33% (DRY treatment) and increased by 33% (WET treatment) using a system of rain gutters and sprinklers on Walker Branch Watershed, Tennessee. Soil leaching was measured with resin lysimeters in the O horizons and with ceramic cup lysimeters in the E (25 cm) and Bt (70 cm) horizons. Large and statistically significant treatment effects on N fluxes were found in the O horizons (lower N fluxes in the DRY and higher N fluxes in the WET treatment). Together with the greater O horizon N content observed in the DRY treatment, this suggested that N was being immobilized at a greater rate in the DRY treatment than in the AMB or WET treatments. No statistically significant treatment effects on soil solution were found in the E horizons with the exception of (Ca2+ + Mg2+) to K+ ratio. Statistically significant treatment effects on electrical conductivity (EC), pH, Ca2+, Mg2+, K+, Na+, SO4(2-), and Cl- were found in the Bt horizons due to differences between the DRY and other treatments. Despite this, calculated fluxes of Ca2+, Mg2+, K+, Na+, SO4(2-), and Cl- were lowest in the DRY treatment. These results suggest that lower precipitation will cause temporary N immobilization in litter and long-term enrichment in soil base cations whereas increased precipitation will cause long-term depletion of soil base cations.

Berm-isolated (0.5 ha) plots have been used since 1995 to quantify changes in soil and water quality with conversion from agricultural to bioenergy crops. Soil quality improvements, including increases in soil carbon storage, have occurred on sites planted to woody or herbaceous species, and no-till corn compared with tilled corn or cotton. Initial increases in soil carbon occurred within the upper 10 cm of the soil profile. Soil carbon on plantings of switchgrass, no-till corn, and sweetgum with a cover crop between the rows increased over the first 3 years. Soil carbon decreased by 6% on the sweetgum plantings without a cover crop and remained lower through the fifth growing season. Overall, the greatest increases in below ground carbon storage have occurred primarily within the upper 40 cm. Former land use, growth characteristics, management practices, and soil characteristics appear to be the primary factors determining the timing, depth. and extent of changes in soil carbon storage for bioenergy and no-till crops.

Quorum sensing via an N-acyl homoserine lactone (HSL) pheromone controls the biosynthesis of a carbapenem antibiotic in Erwinia carotovora. Transcription of the carbapenem biosynthetic genes is dependent on the LuxR-type activator protein, CarR. Equilibrium binding of a range of HSL molecules, which are thought to activate CarR to bind to its DNA target sequence, was examined using fluorescence quenching, DNA bandshift analysis, limited proteolysis and reporter gene assays. CarR bound the most physiologically relevant ligand, N-(3-oxohexanoyl)-L-homoserine lactone, with a stoichiometry of two molecules of ligand per dimer of protein and a dissociation constant of 1.8 microM, in good agreement with the concentration of HSL required to activate carbapenem production in vivo. In the presence of HSL, CarR formed a very high molecular weight complex with its target DNA, indicating that the ligand causes the protein to multimerize. Chemical cross-linking analysis supported this interpretation. Our data show that the ability of a given HSL to facilitate CarR binding to its target DNA sequence is directly proportional to the affinity of the HSL for the protein.