Water balance creates a threshold in soil pH at the global scale.

Soil pH regulates the capacity of soils to store and supply nutrients, and thus contributes substantially to controlling productivity in terrestrial ecosystems. However, soil pH is not an independent regulator of soil fertility-rather, it is ultimately controlled by environmental forcing. In particular, small changes in water balance cause a steep transition from alkaline to acid soils across natural climate gradients. Although the processes governing this threshold in soil pH are well understood, the threshold has not been quantified at the global scale, where the influence of climate may be confounded by the effects of topography and mineralogy. Here we evaluate the global relationship between water balance and soil pH by extracting a spatially random sample (n = 20,000) from an extensive compilation of 60,291 soil pH measurements. We show that there is an abrupt transition from alkaline to acid soil pH that occurs at the point where mean annual precipitation begins to exceed mean annual potential evapotranspiration. We evaluate deviations from this global pattern, showing that they may result from seasonality, climate history, erosion and mineralogy. These results demonstrate that climate creates a nonlinear pattern in soil solution chemistry at the global scale; they also reveal conditions under which soils maintain pH out of equilibrium with modern climate.

Sinks for nitrogen inputs in terrestrial ecosystems: a meta-analysis of 15N tracer field studies.

Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (< 1 week after 15N tracer application), total ecosystem 15N recovery was negatively correlated with fine-root and soil 15N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term (3-18 months after 15N tracer application), total ecosystem 15N retention was negatively correlated with foliar natural-abundance 15N but was positively correlated with mineral soil C and N concentration and C:N, showing that plant and soil natural-abundance 15N and soil C:N are good indicators of total ecosystem N retention. Foliar N concentration was not significantly related to ecosystem 15N tracer recovery, suggesting that plant N status is not a good predictor of total ecosystem N retention. Because the largest ecosystem sinks for 15N tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg N x ha(-1) x yr(-1) above which most ecosystems showed net losses of applied 15N tracer in response to N fertilizer addition.

Role of land-surface changes in arctic summer warming.

A major challenge in predicting Earth's future climate state is to understand feedbacks that alter greenhouse-gas forcing. Here we synthesize field data from arctic Alaska, showing that terrestrial changes in summer albedo contribute substantially to recent high-latitude warming trends. Pronounced terrestrial summer warming in arctic Alaska correlates with a lengthening of the snow-free season that has increased atmospheric heating locally by about 3 watts per square meter per decade (similar in magnitude to the regional heating expected over multiple decades from a doubling of atmospheric CO2). The continuation of current trends in shrub and tree expansion could further amplify this atmospheric heating by two to seven times.

Influence of drying-rewetting frequency on soil bacterial community structure.

Soil drying and rewetting represents a common physiological stress for the microbial communities residing in surface soils. A drying-rewetting cycle may induce lysis in a significant proportion of the microbial biomass and, for a number of reasons, may directly or indirectly influence microbial community composition. Few studies have explicitly examined the role of drying-rewetting frequency in shaping soil microbial community structure. In this experiment, we manipulated soil water stress in the laboratory by exposing two different soil types to 0, 1, 2, 4, 6, 9, or 15 drying-rewetting cycles over a 2-month period. The two soils used for the experiment were both collected from the Sedgwick Ranch Natural Reserve in Santa Ynez, CA, one from an annual grassland, the other from underneath an oak canopy. The average soil moisture content over the course of the incubation was the same for all samples, compensating for the number of drying-rewetting cycles. At the end of the 2-month incubation we extracted DNA from soil samples and characterized the soil bacterial communities using the terminal restriction fragment length polymorphism (T-RFLP) method. We found that drying-rewetting regimes can influence bacterial community composition in oak but not in grass soils. The two soils have inherently different bacterial communities; only the bacteria residing in the oak soil, which are less frequently exposed to moisture stress in their natural environment, were significantly affected by drying-rewetting cycles. The community indices of taxonomic diversity and richness were relatively insensitive to drying-rewetting frequency. We hypothesize that drying-rewetting induced shifts in bacterial community composition may partly explain the changes in C mineralization rates that are commonly observed following exposure to numerous drying-rewetting cycles. Microbial community composition may influence soil processes, particularly in soils exposed to a significant level of environmental stress.

Comparison of subsurface and surface soil bacterial communities in California grassland as assessed by terminal restriction fragment length polymorphisms of PCR-amplified 16S rRNA genes.

The integrated biomass beneath the surface horizon in unsaturated soils is large and potentially important in nutrient and carbon cycling. Compared to surface soils, the ecology of these subsurface soils is weakly understood, particularly in terms of the composition of bacterial communities. We compared soil bacterial communities along two vertical transects by terminal restriction fragment length polymorphisms (TRFLPs) of PCR-amplified 16S rRNA genes to determine how surface and deep bacterial communities differ. DNA yield from soils collected from two Mediterranean grassland transects decreased exponentially from the surface to 4 m deep. Richness, as assessed by the number of peaks obtained after restriction with HhaI, MspI, RsaI, or HaeIII, and diversity, as assessed by the Shannon diversity indices, were lowest in the deepest sample. Lower diversity at depth is consistent with species-energy theory, which would predict relatively low diversity in the low organic matter horizons. Principal components analysis suggested that, in terms of HhaI and HaeIII generated TRFLPs, bacterial communities differed between depths. The most abundant amplicons cloned from the deepest sample contained sequences with restriction sites consistent with the largest peaks observed in TRFLPs generated from deep samples. These more abundant operational taxonomic units (OTUs) appeared related to Pseudomonas and Variovorax. Several OTUs were more related to each other than any previously described ribotypes. These OTUs showed similarity to bacteria from the divisions Actinobacteria and Firmicutes.

Being accepted for who we are: evidence that social validation of the intrinsic self reduces general defensiveness.

Three studies examined the possibility that being liked intrinsically by others--for who one is--reduces self-esteem defense, whereas being liked for what one has achieved does not. All 3 studies contrasted the effects on self-esteem defense of liking based on intrinsic or achievement-related aspects of self. Study 1 showed that thoughts of being liked intrinsically reduced defensive bias toward downward social comparison. Study 2 demonstrated that being liked for intrinsic aspects of self reduced participants' tendency to defensively distance themselves from a negatively portrayed other. Study 3 revealed that being liked for intrinsic aspects of self encouraged a preference for upward over downward counterfactuals for a negative event. In all 3 studies, similar reductions in defensiveness were not found when liking was based on achievements. Discussion focuses on implications for understanding the functional value of different bases of self-worth.

Running from the shadow: psychological distancing from others to deny characteristics people fear in themselves.

Four experiments tested the hypothesis that people distance themselves from others who display characteristics they fear in themselves. In Study 1, participants were given false feedback that they were high or low in repressed anger and were given information about a person who became angry and responded in a violent or nonviolent manner. High anger feedback participants distanced themselves only from the violent person. In Study 2, high anger feedback led to distancing from a violent other but not a dishonest other, whereas dishonesty feedback led to distancing from a dishonest other but not a violent other. The results of Studies 3 and 4 replicated and extended the distancing effect with an anger induction: Participants who were insulted distanced themselves from an angry/violent person, and verbalizing their emotions about being insulted eliminated this effect. Implications for understanding defenses against undesirable self-attributions are discussed.

Decomposition and carbon cycling of dead trees in tropical forests of the central Amazon.

Decomposition rate constants were measured for boles of 155 large dead trees (>10 cm diameter) in central Amazon forests. Mortality data from 21 ha of permanent inventory plots, monitored for 10-15 years, were used to select dead trees for sampling. Measured rate constants varied by over 1.5 orders of magnitude (0.015-0.67 year-1), averaging 0.19 year-1 with predicted error of 0.026 year. Wood density and bole diameter were significantly and inversely correlated with rate constants. A tree of average biomass was predicted to decompose at 0.17 year-1. Based on mortality data, an average of 7.0 trees ha-1 year-1 died producing 3.6 Mg ha-1 year-1 of coarse litter (>10 cm diameter). Mean coarse litter standing-stocks were predicted to be 21 Mg ha-1, with a mean residence time of 5.9 years, and a maximum mean carbon flux to the atmosphere of 1.8 Mg C ha-1 year-1. Total litter is estimated to be partitioned into 16% fine wood, 30% coarse wood, and 54% non-woody litter (e.g., leaves, fruits, flowers). Decomposition rate constants for coarse litter were compiled from 20 globally distributed studies. Rates were highly correlated with mean annual temperature, giving a respiration quotient (Q 10) of 2.4 (10°C-1).

Stereotypes and terror management: evidence that mortality salience enhances stereotypic thinking and preferences.

If stereotypes function to protect people against death-related concerns, then mortality salience should increase stereotypic thinking and preferences for stereotype-confirming individuals. Study 1 demonstrated that mortality salience increased stereotyping of Germans. In Study 2, it increased participants' tendency to generate more explanations for stereotype-inconsistent than stereotype-consistent gender role behavior. In Study 3, mortality salience increased participants' liking for a stereotype-consistent African American and decreased their liking for a stereotype-inconsistent African American; control participants exhibited the opposite preference. Study 4 replicated this pattern with evaluations of stereotype-confirming or stereotype-disconfirming men and women. Study 5 showed that, among participants high in need for closure, mortality salience led to decreased liking for a stereotype-inconsistent gay man.

Microbial reductive dechlorination of PCBs.

Reductive dechlorination is an advantageous process to microorganisms under anaerobic conditions because it is an electron sink, thereby allowing reoxidation of metabolic intermediates. In some organisms this has been demonstrated to support growth. Many chlorinated compounds have now been shown to be reductively dechlorinated under anaerobic conditions, including many of the congeners in commercial PCB mixtures. Anaerobic microbial communities in sediments dechlorinate Aroclor at rates of 3 micrograms Cl/g sediment x week. PCB dechlorination occurs at 12 degrees C, a temperature relevant for remediation at temperate sites, and at concentrations of 100 to 1000 ppm. The positions dechlorinated are usually meta > para > ortho. The biphenyl rings, and the mono-ortho- and diorthochlorobiphenyls were not degraded after a one year incubation. Hence subsequent aerobic treatment may be necessary to meet regulatory standards. Reductive dechlorination of Aroclors does reduce their dioxin-like toxicity as measured by bioassay and by analysis of the co-planar congeners. The most important limitation to using PCB dechlorination as a remediation technology is the slower than desired dechlorination rates and no means yet discovered to substantially enhance these rates. Long term enrichments using PCBs as the only electron acceptor resulted in an initial enhancement in dechlorination rate. This rate was sustained but did not increase in serial transfers. Bioremediation of soil contaminated with Aroclor 1254 from a transformer spill was dechlorinated by greater than 50% following mixing of the soil with dechlorinating organisms and river sediment.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in cytoplasmic carbon and nitrogen pools in a soil bacterium and a fungus in response to salt stress.

The effects of water potential on the cellular compositions of a soil bacterium and a fungus were examined by growing the organisms in media with various KCl concentrations. In media containing up to 1 M KCl, C/N ratios in Aspergillus flavus increased significantly, while those for a Pseudomonas sp. did not. For both organisms, the proportions of cellular C and N in cytoplasm increased by a factor of 10 as salinity increased from 0 to 1 M KCl. Such compositional changes have implications for microbial biomass dynamics in soils of varying water potential and for biomass measurement by chloroform fumigation.

Identification of heterotrophic nitrification in a sierran forest soil.

A potential for heterotrophic nitrification was identified in soil from a mature conifer forest and from a clear-cut site. Potential rates of NO(2) production were determined separately from those of NO(3) by using acetylene to block autotrophic NH(4) oxidation and chlorate to block NO(2) oxidation to NO(3) in soil slurries. Rates of NO(2) production were similar in soil from the forest and the clear-cut site and were strongly inhibited by acetylene. The rate of NO(3) production was much greater than that of NO(2) production, and NO(3) production was not significantly affected by acetylene or chlorate. Nitrate production was partially inhibited by cycloheximide, but was not significantly reduced by streptomycin. Neither the addition of ammonium nor the addition of peptone stimulated NO(3) production. N labeling of the NH(4) pool demonstrated that NO(3) was not coming from NH(4). The potential for heterotrophic nitrification in these forest soils was greater than that for autotrophic nitrification.