Sioux Falls, OR, United States
Sioux Falls, OR, United States

Time filter

Source Type

Phillips C.L.,Oregon State University | Murphey V.,Oregon State University | Lajtha K.,Oregon State University | Gregg J.W.,Terrestrial Ecosystems Research Associates
Biogeochemistry | Year: 2016

The response of soil organic carbon (SOC) to increasing temperature is a critical and uncertain component of terrestrial ecosystems feedbacks to climate. To investigate warming impacts on soil and whole-ecosystem carbon cycling, we conducted a whole-ecosystem warming experiment in grassland mesocosms in Oregon, USA, with diurnal temperature profiles simulating both asymmetric and symmetric increases in daily minimum (Tmin) and maximum (Tmax) temperature. In 2011 we reported that after 3 years of warming, the ecosystem switched from a neutral C balance to a C source, with warming causing an average loss of 88 g C m−2 year−1. Because warming had no impact on aboveground net primary productivity, decomposition of SOC was initially the suspected source of these emissions. Here we report follow-up data from years 4–9 on the effects of warming on soil C stocks, litter, and soil respiration. We employed several complimentary methods to isolate the readily-mineralizable soil C fraction, but power analysis showed that only density fractionation provided sufficient power to detect warming impacts from natural variability. Collectively, the warming treatments reduced the C content in the light density fraction by 9 % after 6 years, and also dramatically reduced litter accumulation, with about 50 % less litter C in warmed chambers by year 9. Consistent with these losses, warming also increased soil respiration by an average of 27–29 %. We conclude that warming-induced losses of unprotected soil carbon and litter, but not protected C, were detectable within a decade. © 2016, Springer International Publishing Switzerland.

Andersen C.P.,U.S. Environmental Protection Agency | Ritter W.,TU Munich | Gregg J.,Terrestrial Ecosystems Research Associates | Matyssek R.,TU Munich | Grams T.E.E.,TU Munich
Environmental Pollution | Year: 2010

Canopies of adult European beech (Fagus sylvatica) and Norway spruce (Picea abies) were labeled with CO2 depleted in 13C to evaluate carbon allocation belowground. One-half the trees were exposed to elevated O3 for 6 yrs prior to and during the experiment. Soil-gas sampling wells were placed at 8 and 15 cm and soil CO2 was sampled during labeling in mid-late August, 2006. In beech, δ13CO2 at both depths decreased approximately 50 h after labeling, reflecting rapid translocation of fixed C to roots and release through respiration. In spruce, label was detected in fine-root tissue, but there was no evidence of label in δ13CO2. The results show that C fixed in the canopy rapidly reaches respiratory pools in beech roots, and suggest that spruce may allocate very little of recently-fixed carbon into root respiration during late summer. A change in carbon allocation belowground due to long-term O3 exposure was not observed. © 2010 Elsevier Ltd. All rights reserved.

Phillips C.L.,Terrestrial Ecosystems Research Associates | Phillips C.L.,Oregon State University | Nickerson N.,Dalhousie University | Nickerson N.,St. Francis Xavier University | And 2 more authors.
Global Change Biology | Year: 2011

Increasing use of automated soil respiration chambers in recent years has demonstrated complex diel relationships between soil respiration and temperature that are not apparent from less frequent measurements. Soil surface flux is often lagged from soil temperature by several hours, which results in semielliptical hysteresis loops when surface flux is plotted as a function of soil temperature. Both biological and physical explanations have been suggested for hysteresis patterns, and there is currently no consensus on their causes or how such data should be analyzed to interpret the sensitivity of respiration to temperature. We used a one-dimensional soil CO 2 and heat transport model based on physical first principles to demonstrate a theoretical basis for lags between surface flux and soil temperatures. Using numerical simulations, we demonstrated that diel phase lags between surface flux and soil temperature can result from heat and CO 2 transport processes alone. While factors other than temperature that vary on a diel basis, such as carbon substrate supply and atmospheric CO 2 concentration, can additionally alter lag times and hysteresis patterns to varying degrees, physical transport processes alone are sufficient to create hysteresis. Therefore, the existence of hysteresis does not necessarily indicate soil respiration is influenced by photosynthetic carbon supply. We also demonstrated how lags can cause errors in Q 10 values calculated from regressions of surface flux and soil temperature measured at a single depth. Furthermore, synchronizing surface flux and soil temperature to account for transport-related lags generally does not improve Q 10 estimation. In order to calculate the sensitivity of soil respiration to temperature, we suggest using approaches that account for the gradients in temperature and production existing within the soil. We conclude that consideration of heat and CO 2 transport processes is a requirement to correctly interpret diel soil respiration patterns. © 2010 Blackwell Publishing Ltd.

Pangle L.A.,Oregon State University | Gregg J.W.,Terrestrial Ecosystems Research Associates | McDonnell J.J.,University of Saskatchewan
Water Resources Research | Year: 2014

The potential impact of projected climate warming on the terrestrial hydrologic cycle is uncertain. This problem has evaded experimentalists due to the overwhelming challenge of measuring the entire water budget and introducing experimental warming treatments in open environmental systems. We present new data from a mesocosm experiment that examined the combined responses of evapotranspiration (ET), soil moisture, and potential groundwater recharge (R; lysimeter drainage) to a 3.5C temperature increase in a grassland ecosystem experiencing a Mediterranean climate. The temperature increase was applied both symmetrically throughout the day, and asymmetrically such that daily minimum temperature was 5C greater than ambient and daily maximum temperature was 2C greater than ambient. Our results span 3 water years and show that symmetric and asymmetric warming-enhanced ET during the spring. However, this increase in ET reduced soil moisture more rapidly, resulting in less ET during the summer than occurred under ambient temperature, and no difference in total ET during the combined spring and summer (March to October). Groundwater recharge was reduced during late-spring storms relative to the ambient temperature treatment, but these reductions were less than 4% of total annual R, and were offset by slightly greater R in the fall under both warming treatments. The results highlight the potential for local interactions between temperature, vegetation, and soils to moderate the hydrological response to climate warming, particularly in environments where precipitation is seasonal and out of phase with the vegetation growing season. Key Points Examined the vadose zone water balance response to imposed climate warming treatments Warming increased spring ET, but decreased summer ET, with no significant difference annually Warming had no effect on mean annual groundwater recharge in this climate © 2013. American Geophysical Union. All Rights Reserved.

Righetti T.L.,University of Guam | Dalthorp D.,Oregon State University | Lambrinos J.,Oregon State University | Strik B.,Oregon State University | And 2 more authors.
International Journal of Environmental Analytical Chemistry | Year: 2012

We demonstrate that delta values (δ) and other relative ratio-based isotopic expressions can vary with the total amount of isotopes present in the system or subject being evaluated. Although these scaling effects are routinely overlooked, interpretive errors such as noting of spurious treatment effects or not detecting significant effects may occur. Algebraic conversions of linear or log-log equations (rare isotope predicted by common or total isotope) that suggest apparently miniscule scaling will fit the observed relationship between isotopic ratios and total or common isotopes. When the ranges of scaling induced differences in isotopic ratios are converted to the equivalent discrimination expressions (Δ) or delta values (δ), differences are within the range that is generally reported in the isotopic literature. Therefore, interpreting observed differences in isotopic ratios may require an evaluation to determine whether treatments directly affect how a rare isotope is accumulated or are associated with differences in denominator size. If effects are direct, points for different treatments fall on different linear and loglog (total isotope vs. rare isotope or common isotope vs. rare isotope) regression lines. Slope differences or derivatives may be more revealing than changes in isotopic ratios and better represent system change in a scaling system. By simply recording total common isotope or total elemental content, standard statistical procedures that evaluate changes in slopes or derivatives can be combined with an ANCOVA to better evaluate isotopic data. In many cases, scaling issues will not interfere with interpretations. In other situations it may be difficult to untangle a combination of ubiquitous scaling, treatment induced scaling and direct treatment effects. © 2012 Taylor & Francis.

Phillips C.L.,Oregon State University | Phillips C.L.,Terrestrial Ecosystems Research Associates | Nickerson N.,Dalhousie University | Nickerson N.,St. Francis Xavier University | And 5 more authors.
Rapid Communications in Mass Spectrometry | Year: 2010

The carbon isotopic composition (δ13C) of recently assimilated plant carbon is known to depend on water-stress, caused either by low soil moisture or by low atmospheric humidity. Air humidity has also been shown to correlate with the δ13C of soil respiration, which suggests indirectly that recently fixed photosynthates comprise a substantial component of substrates consumed by soil respiration. However, there are other reasons why the δ13CO2 of soil efflux may change with moisture conditions, which have not received as much attention. Using a combination of greenhouse experiments and modeling, we examined whether moisture can cause changes in fractionation associated with (1) nonsteady-state soil CO2 transport, and (2) heterotrophic soil-respired δ13CO2. In a first experiment, we examined the effects of soil moisture on total respired δ13CO2 by growing Douglas fir seedlings under high and low soil moisture conditions. The measured δ13C of soil respiration was 4.7% more enriched in the low-moisture treatment; however, subsequent investigation with an isotopologue-based gas diffusion model suggested that this result was probably influenced by gas transport effects. A second experiment examined the heterotrophic component of soil respiration by incubating plant-free soils, and showed no change in microbial-respired δ13CO2 across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired δ13CO2, but they indicate that the expected impacts of photosynthetic discrimination may be similar in direction and magnitude to those from gas transport-related fractionation. Gas transport-related fractionation may operate as an alternative or an additional factor to photosynthetic discrimination to explain moisture-related variation in soil-respired δ13CO2. © 2010 John Wiley & Sons, Ltd.

Loading Terrestrial Ecosystems Research Associates collaborators
Loading Terrestrial Ecosystems Research Associates collaborators