Scale and uncertainty in modeled soil organic carbon stock changes for US croplands using a process‐based model

Process‐based model analyses are often used to estimate changes in soil organic carbon (SOC), particularly at regional to continental scales. However, uncertainties are rarely evaluated, and so it is difficult to determine how much confidence can be placed in the results. Our objective was to quantify uncertainties across multiple scales in a process‐based model analysis, and provide 95% confidence intervals for the estimates. Specifically, we used the Century ecosystem model to estimate changes in SOC stocks for US croplands during the 1990s, addressing uncertainties in model inputs, structure and scaling of results from point locations to regions and the entire country. Overall, SOC stocks increased in US croplands by 14.6 Tg C yr^?1 from 1990 to 1995 and 17.5 Tg C yr^?1 during 1995 to 2000, and uncertainties were ±22% and ±16% for the two time periods, respectively. Uncertainties were inversely related to spatial scale, with median uncertainties at the regional scale estimated at ±118% and ±114% during the early and latter part of 1990s, and even higher at the site scale with estimates at ±739% and ±674% for the time periods, respectively. This relationship appeared to be driven by the amount of the SOC stock change; changes in stocks that exceeded 200 Gg C yr^?1 represented a threshold where uncertainties were always lower than ±100%. Consequently, the amount of uncertainty in estimates derived from process‐based models will partly depend on the level of SOC accumulation or loss. In general, the majority of uncertainty was associated with model structure in this application, and so attaining higher levels of precision in the estimates will largely depend on improving the model algorithms and parameterization, as well as increasing the number of measurement sites used to evaluate the structural uncertainty.     

Assessment of soil calcium status in red spruce forests in the northeastern United States

Long-term changes in concentrations of available Ca in soils of redspruce forests have been documented, but remaining questions aboutthe magnitude and regional extent of these changes have precluded anassessment of the current and future status of soil Ca. To addressthis problem, soil samples were collected in 1992—93from 12 sites in New York, Vermont, New Hampshire, and Maine toprovide additional data necessary to synthesize all availableresearch results on soil Ca in red spruce forests. Sites werechosen to encompass the range of environmental conditionsexperienced by red spruce. Concentrations of exchangeableCa ranged from 2.13 to 21.6 cmol_ckg^–1 in the Oa horizon, and from 0.11 to 0.68cmol_c kg^–1 in the upper 10 cm of theB horizon. These measurements expanded the range of exchangeable Ca reported in the literature for both horizons in northeastern redspruce forests. Exchangeable Ca was the largest Ca fraction in theforest floor at most sites (92% ofacid-extractableCa), but mineral Ca was the largest fraction at the three sites that also had the highest mineral-matter concentrations. Theprimary factor causing variability in Ca concentrations among siteswas the mineralogy of parent material, but exchangeable concentrationsin the B horizon of all sites were probably reduced by acidicdeposition. Because the majority of Ca in the forest floor isin a readily leachable form, and Ca inputs to the forest floor from the mineral soil and atmospheric deposition have beendecreasing in recent decades, the previously documented decreasesin Ca concentrations in the forest floor over previous decades mayextend into the future.     

Chemosensory Adaptation to Lysozyme and GTP Involves Independently Regulated Receptors in Tetrahymena thermophila

ABSTRACT. Chemosensory adaptation is seen in Tetrahymena thermophila following prolonged exposure (ten minutes) to micromolar concentrations of the chemorepellents lysozyme or guanosine triphosphate (GTP). Since these cells initially show repeated backward swimming episodes (avoidance reactions) in these repellents, behavioral adaptation is seen as a decrease in this repellent-induced behavior. The time course of this behavioral adaptation is paralleled by decreases in the extents of surface binding of either [³²P]GTP or [³H]lysozyme in vivo. Scatchard plot analyses of repellent binding in adapted cells suggests the behavioral adaptation is due to a dramatic decrease in the number of surface binding sites, as represented by decreased Bmax values. The estimated K_D values for nonadapted cells are 6.6 μM and 8.4 μM for lysozyme and GTP binding, respectively. Behavioral adaptation and decreased surface receptor binding are specific for each repellent. The GTP adapted cells (20 μM for ten minutes) still respond behaviorally to 50 μM lysozyme and bind [³H]lysozyme normally. Lysozyme adapted cells (50 μM for ten minutes) still bind [³²P]GTP and respond behaviorally to GTP. All the behavioral and binding changes seen are also reversible (deadaptation). Neomycin was shown to be a competitive inhibitor of [³H]lysozyme binding and lysozyme-induced avoidance reactions, but it had no effect on either [³²P]GTP binding or GTP-induced or avoidance reactions. These results are consistent with the hypothesis that there are two separate repellent receptors, one for GTP and the other for lysozyme, that are independently downregulated during adaptation to cause specific receptor desensitization and consequent behavioral adaptation.