Susan Trumbore

N deposition levels in the San Bernardino Mountains are amongst the highest in the country. Increased N levels can have considerable effects on decomposition and other components of the C cycle, however, these effects are not well understood, particularly, in xeric ecosystems. Two sites along an anthropogenic N deposition gradient were used for this study. At each site, there were N addition treatments of 50 and 150 kg N/ha/yr. Heterotrophic respiration rates from microcosm incubations showed that N addition levels of 50 kg/ha/yr increased respiration in the O horizon at both sites. However, the 150 kg/ha/yr treatment exhibited decreased respiration, suggesting that at high amendment levels, N depressed microbial decomposition. Radiocarbon signatures of the CO2 respired from the Oa decreased significantly with N amendment at both sites, suggesting that the isotopic composition of the tissue produced, and decomposed, differed from that of background N deposition. Beta-glucosidase activity in the Oa horizon increased with N amendment at the highly polluted site, but not at the less polluted site. Polyphenol oxidase activity decreased with N addition at the low pollution site, but not at the highly polluted site. Both of these changes suggest that the source of respiration shifted from older more recalcitrant substrates (i.e. lignin), to younger more labile substrates (i.e. cellulose) in response to N amendment. Therefore, the material being decomposed was younger, explaining the decrease in 14C values.

ABSTRACT Turfgrass is the single largest irrigated crop in the U.S., yet turfgrass land cover has been largely neglected in ecological studies. The purpose of this study was to quantify biogeochemical cycling of a turfgrass lawn in southern California in response to varying fertilization and temperature in a controlled experiment. Replicated plots of mixed fescue/bermudagrass were subjected to an average of 4°C nighttime warming with infrared lamps and were fertilized with two levels of N (48 and 196 kg ha-1yr-1 in a factorial experiment. The fertilization events cause pulses of nitrous oxide emissions. Thus far, emissions ranged from 0.27 mg N m-2d- 1 in unheated and unfertilized plots to 0.54 mg N mg N m-2d-1 in plots treated with both high N and high temperature, and are comparable to emissions from fertilized agricultural soil. Observed nitrous oxide emissions suggest that urban soils likely play an important role in local greenhouse gas budgets, and that more intensive management may be required at elevated temperatures to sustain high turfgrass productivity.

The accumulation of terrestrial carbon in boreal soils happens predominately in the organic layers and is determined by biological processes. It is, however, strongly influenced by environmental conditions such as perennially cold temperatures and high moisture content. Net primary productivity (NPP) and decomposition, along with fire regimes and the chemical composition of the organic matter, also play a role in

Carbon (C) enters terrestrial ecosystems through photosynthesis, and returns to the atmosphere by numerous pathways (above- and belowground, auto- and heterotrophic), collectively called ecosystem respiration. Incomplete understanding of how new photosynthetic products partition among these pathways, particularly belowground, is a major source of uncertainty with modeling ecosystem respiration in space and time. The allocation of newly assimilated C initially constrains

A unique, large release of radiocarbon occurred near the Oak Ridge Reservation (ORR), Oak Ridge, TN in July/August 1999. Measurements of 14C in tree ring cellulose throughout the ORR area demonstrate that the 1999 release was unprecedented in its uptake by vegetation. We are taking advantage of the whole-ecosystem isotopic label generated by this release to address five outstanding issues

ABSTRACT Automated measurements provide the high-resolution information that enables us to analyze potential causes for diel variability in soil respiration. These diel patterns are the complex result of biological and physical processes that determine the production and diffusion of CO2 through the soil. We examined the diel patterns of soil respiration from four arid California ecosystems: (1) a pinon-juniper woodland in at the Burns Pinon Ridge Reserve near Joshua Tree National Park, (2) a cold desert shrub community and (3) a perennial grassland near the city of Bishop in the Owens Valley, and (4) a mixed oak-pine forest at the James Reserve in the San Jacinto Mountains. In addition to automated chamber and environmental measurements at these sites, we used isotopic (14C) partitioning techniques to separate the plant and microbial sources contributing to soil respiration at certain time points. Here we present the diel cycles of soil respiration and environmental variables, the source partitioning results, and hypotheses about what processes determine these diel patterns that both span, and are specific to the studied ecosystems. In these systems dominated by Mediterranean or desert climates, we observed that factors like relative humidity can dominate the diel variations in soil respiration for sites with very dry surface litter. At other sites and times of year, diel variation in soil respiration reflects photosynthetic and VPD influence on root respiration. The combination of automated chamber measurements with isotopes provides information useful for separating the plant and heterotrophic control on diel and seasonal soil respiration fluxes.

We are using a combination of method to quantify controls on plant and microbial sources of soil respiration over the course of a year at the San Jacinto Mountains James Reserve in Southern California. High frequency autochamber flux measurements are combined with monitoring of air and soil temperature and moisture conditions to monitor diel and seasonal variations in flux. A

Irrigated turfgrass ecosystems sequester carbon in soil organic matter, but they may also release nitrous oxide, due to fertilization associated with intensive management practices. Nitrous oxide is an important green house gas with a global warming potential (GWP) of 300 times that of carbon dioxide on a 100 yr time horizon. Although regular irrigation and fertilization of turfgrass create favorable

This work seeks to improve our ability to quantify C cycling rates in fine roots of trees in mature deciduous and coniferous forests. We use two different types of atmospheric 14CO2 enrichment to trace the time elapsed since C in plant tissues was fixed from the atmosphere by photosynthesis. The first uses a local enrichment of 14CO2 which occurred in

... Authors: Townsend-Small, A.; Pataki, D.; Tyler, S.; Trumbore, S. Affiliation: AA(Department of Earth System Science, University of California, Irvine, CA 92697, United States ; ), AB(Department of Earth System Science, University of California, Irvine, CA 92697, United States ...

While terrestrial climate models view C budgets through time-slices of environmental reconstructions, yet during climate transitions, pervasive geomorphic response to climate likely dictate the direction and magnitude of C exchange. Geomorphic processes therefore act ...

Marine sediments comprise a large methane reservoir (~10^19 g carbon), yet little of this potent greenhouse gas makes its way to the atmosphere. This is because microbially mediated methane oxidation in the shallow sediments and water column effectively consumes methane and limits the ocean's annual emissions to 2.5% of the total methane flux to the atmosphere. Despite the key role

Abstract We used the variations in parent material, climate and topography found in Kruger National Park, South Africa, to investigate the most important factors controlling carbon dynamics in savannah soils. Carbon dynamics were investigated using radiocarbon signatures of carbon in density-separated organic matter and in CO2 respired during incubation of surface soils. Carbon respired in incubations of 0-2cm and 2-8 cm depth intervals had a narrow range of 14C signatures regardless of the parent material, ...

ABSTRACT The radiocarbon signature of organic matter stored in, or respired from, ecosystems provides a strong constraint for testing the timescales of organic matter cycling in ecosystems, and for testing ideas about the processes controlling stabilization and destabilization of C in soil. Radiocarbon integrates the total time C spends in an ecosystem since originally being fixed from the atmosphere, which includes storage time in plants before being used to grow tissues, the lifetime of the tissues themselves, the time it takes for them to decompose. For example, in forested ecosystems, much of C that has ages of a decade or more can be derived from roots that have themselves lived up to a decade and this influences the 14C age of light fraction soil organic matter and respired CO2 from these ecosystems. Another example is the link between depth profiles of C and radiocarbon, which reflect stabilization mechanisms on at least two timescales. Finally, the changes in radiocarbon over time, tracked by repeated sampling, provide a clear constraint on C dynamics and indicate that traditional soil fractionation methods are not successful because of the mix of ages represented in the C that is isolated. This talk explores some of these patterns in terms of their use for answering questions important for improving models of soil organic matter cycling.