Archaeological evidence from coastal southwest Florida suggests this region and its local inhabitants (the Calusa) were affected by drought and cooling during the Vandal Minimum climate episode (ad 500–800). To test this hypothesis, we reconstructed seasonal-scale climate conditions using stable oxygen and carbon isotope ratios (δ 18 O and δ 13 C) preserved in Ariopsis felis otoliths and Mercenaria campechiensis shells. Comparing δ 18 O records from both species distinguishes between cool versus warm and wet versus dry conditions. δ 18 O values from four otoliths indicate cooling of winter temperatures occurred in the early Vandal Minimum (ad 500–600) and late half of the middle Vandal Minimum (ad 650–700). Persistent dry summers punctuated by occasional years with wet summers throughout the Vandal Minimum were detected from δ 18 O values of eight archaeological shells. Our climate reconstructions are in good agreement with archaeological observations and with the cool and dry conditions documented in Europe.
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Complex Drivers of Riparian Soil Oxygen Variability Revealed Using Self‐Organizing Maps
Oxygen (O2) regulates soil reduction‐oxidation processes and therefore modulates biogeochemical cycles. The difficulties associated with accurately characterizing soil O2variability have prompted the use of soil moisture as a proxy for O2, as O2diffusion into soil water is much slower than in soil air. The use of soil moisture alone as a proxy measurement for O2could result in inaccurate O2estimations. For example, O2may remain high during cool months when soil respiration rates are low. We analyzed high‐frequency sensor data (e.g., soil moisture, CO2, gas‐phase soil pore O2) with a machine learning technique, the Self‐Organizing Map, to pinpoint suites of soil conditions associated with contrasting O2regimes. At two riparian sites in northern Vermont, we found that O2levels varied seasonally, and with soil moisture. For example, 47% of low O2levels were associated with wet and cool soil conditions, whereas 32% were associated with dry and warm conditions. Contrastingly, the majority (62%) of high O2conditions occurred under dry and warm conditions. High soil moisture levels did not always lead to low O2, as 38% of high O2values occurred under wet and cool conditions. Our results highlight challenges with predicting soil O2solely based on water content, as variable combinations of soil and hydrologic conditions can complicate the relationship between water content and O2. This indicates that process‐based ecosystem and denitrification models that rely solely on soil moisture to estimate O2may need to incorporate other site and climate‐specific drivers to accurately predict soil O2.
more » « less- Award ID(s):
- 2012123
- PAR ID:
- 10422826
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 59
- Issue:
- 6
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Microbial biomass is known to decrease with soil drying and to increase after rewetting due to physiological assimilation and substrate limitation under fluctuating moisture conditions, but how the effects of moisture changes vary between dry and wet environments is unclear. Here, we conducted a meta‐analysis to assess the effects of elevated and reduced soil moisture on microbial biomass carbon (MBC) and nitrogen (MBN) across a broad range of forest sites between dry and wet regions. We found that the influence of both elevated and reduced soil moisture on MBC and MBN concentrations in forest soils was greater in dry than in wet regions. The influence of altered soil moisture on MBC and MBN concentrations increased significantly with the manipulation intensity but decreased with the length of experimental period, with a dramatic increase observed under a very short‐term precipitation pulse. Moisture effect did not differ between coarse‐ and fine‐textured soils. Precipitation intensity, experimental duration, and site standardized precipitation index (dry or wet climate) were more important than edaphic factors (i.e., initial water content, bulk density, clay content) in determining microbial biomass in response to altered moisture in forest soils. Different responses of microbial biomass in forest soils between dry and wet regions should be incorporated into models to evaluate how changes in the amount, timing and intensity of precipitation affect soil biogeochemical processes.more » « less
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Abstract The US Southwest is in a drought crisis that has been developing over the past two decades, contributing to marked increases in burned forest areas and unprecedented efforts to reduce water consumption. Climate change has contributed to this ongoing decadal drought via warming that has increased evaporative demand and reduced snowpack and streamflows. However, on the supply side, precipitation has been low during the 21st century. Here, using simulations with an atmosphere model forced by imposed sea surface temperatures, we show that the 21st century shift to cooler tropical Pacific sea surface temperatures forced a decline in cool season precipitation that in turn drove a decline in spring to summer soil moisture in the southwest. We then project the near-term future out to 2040, accounting for plausible and realistic natural decadal variability of the Pacific and Atlantic Oceans and radiatively-forced change. The future evolution of decadal variability in the Pacific and Atlantic will strongly influence how wet or dry the southwest is in coming decades as a result of the influence on cool season precipitation. The worst-case scenario involves a continued cold state of the tropical Pacific and the development of a warm state of the Atlantic while the best case scenario would be a transition to a warm state of the tropical Pacific and the development of a cold state of the Atlantic. Radiatively-forced cool season precipitation reduction is strongest if future forced SST change continues the observed pattern of no warming in the equatorial Pacific cold tongue. Although this is a weaker influence on summer soil moisture than natural decadal variability, no combination of natural decadal variability and forced change ensures a return to winter precipitation or summer soil moisture levels as high as those in the final two decades of the 20th century.
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Premise The impact of elevated CO2concentration ([CO2]) and climate warming on plant productivity in dryland ecosystems is influenced strongly by soil moisture availability. We predicted that the influence of warming on the stimulation of photosynthesis by elevated [CO2] in prairie plants would operate primarily through direct and indirect effects on soil water.
Methods We measured light‐saturated photosynthesis (
A net), stomatal conductance (g s), maximum Rubisco carboxylation rate (V cmax), maximum electron transport capacity (J max) and related variables in four C3plant species in the Prairie Heating and CO2Enrichment (PHACE) experiment in southeastern Wyoming. Measurements were conducted over two growing seasons that differed in the amount of precipitation and soil moisture content.Results A netin the C3subshrubArtemisia frigida and the C3forbSphaeralcea coccinea was stimulated by elevated [CO2] under ambient and warmed temperature treatments. Warming by itself reducedA netin all species during the dry year, but stimulated photosynthesis inS. coccinea in the wet year. In contrast,A netin the C3grassPascopyrum smithii was not stimulated by elevated [CO2] or warming under wet or dry conditions. Photosynthetic downregulation under elevated [CO2] in this species countered the potential stimulatory effect under improved water relations. Warming also reduced the magnitude of CO2‐induced down‐regulation in this grass, possibly by sustaining high levels of carbon utilization.Conclusions Direct and indirect effects of elevated [CO2] and warming on soil water was an overriding factor influencing patterns of
A netin this semi‐arid temperate grassland, emphasizing the important role of water relations in driving grassland responses to global change.
