Marc G. Kramer

N isotope fractionation and measures of organic matter alteration during decomposition

Most soil organic matter (SOM) derives from plant material, but there are substantial gaps in our understanding of its transformation mechanisms. Alterations that occur as SOM decays and is stabilized have proved difficult to study owing largely to its diverse initial chemical composition and stable isotope values. We examined SOM stable isotope ratios in relation to composition using solid-state 13C Cross Polarization Magic Angle Spin (CPMAS) nuclear magnetic resonance (NMR) in sequentially deeper organic and mineral horizon soil samples collected from an unpolluted ecosystem in southeast Alaska, USA, dominated by C3 vegetation. We found that, as humified carbon (C not respired as CO2 during decomposition) increased in aliphaticity (ratio of unsubstituted aliphatics to carbohydrates), it also increased in 15N but showed no clear trend in 13C. These results imply that humification (and the concomitant stabilization of soil C) at our site resulted from microbial alteration of organics rather than from accu...

Sensitivity of summer stream temperatures to climate variability in the Pacific Northwest

Estimating the thermal response of streams to a warming climate is important for prioritizing native fish conservation efforts. While there are plentiful estimates of air temperature responses to climate change, the sensitivity of streams, particularly small headwater streams, to warming temperatures is less well understood. A substantial body of literature correlates subannual scale temperature variations in air and stream temperatures driven by annual cycles in solar angle; however, these may be a low-precision proxy for climate change driven changes in the stream energy balance. We analyzed summer stream temperature records from forested streams in the Pacific Northwest for interannual correlations to air temperature and standardized annual streamflow departures. A significant pattern emerged where cold streams always had lower sensitivities to air temperature variation, while warm streams could be insensitive or sensitive depending on geological or vegetation context. A pattern where cold streams are less sensitive to direct temperature increases is important for conservation planning, although substantial questions may yet remain for secondary effects related to flow or vegetation changes induced by climate change.

Soil carbon dynamics across a windthrow disturbance sequence in southeast Alaska

Few studies have examined the influence of natural disturbances, such as windthrow, on soil organic matter formation, stabilization, and loss in soils. In shallow (<1 m) mountain forest soils, windthrow activity may result in the redistribution and mixing of mineral and organic soil horizons down to bedrock. We studied the patterns of soil carbon, the dominant constituent of soil organic matter, in watersheds along a windthrow disturbance sequence in a mountainous temperate rain forest in southeast Alaska. Our objectives were (1) to evaluate the influence of windthrow and illuviation on the accumulation of soil organic carbon in mineral horizons and (2) to compare the forms of soil organic matter that have accumulated. Soils were described, and the thickness of the major organic and mineral horizons was measured, every 5 m along transects in three watersheds with contrasting windthrow histories. A subset of the soil description sites was randomly selected and then sampled to determine the quantity and qua...

Surficial gains and subsoil losses of soil carbon and nitrogen during secondary forest development

Reforestation of formerly cultivated land is widely understood to accumulate above- and belowground detrital organic matter pools, including soil organic matter. However, during 40 years of study of reforestation in the subtropical southeastern USA, repeated observations of above- and belowground carbon documented that significant gains in soil organic matter (SOM) in surface soils (0-7.5 cm) were offset by significant SOM losses in subsoils (35-60 cm). Here, we extended the observation period in this long-term experiment by an additional decade, and used soil fractionation and stable isotopes and radioisotopes to explore changes in soil organic carbon and soil nitrogen that accompanied nearly 50 years of loblolly pine secondary forest development. We observed that accumulations of mineral soil C and N from 0 to 7.5 cm were almost entirely due to accumulations of light-fraction SOM. Meanwhile, losses of soil C and N from mineral soils at 35 to 60 cm were from SOM associated with silt and clay-sized particles. Isotopic signatures showed relatively large accumulations of forest-derived carbon in surface soils, and little to no accumulation of forest-derived carbon in subsoils. We argue that the land use change from old field to secondary forest drove biogeochemical and hydrological changes throughout the soil profile that enhanced microbial activity and SOM decomposition in subsoils. However, when the pine stands aged and began to transition to mixed pines and hardwoods, demands on soil organic matter for nutrients to support aboveground growth eased due to pine mortality, and subsoil organic matter levels stabilized. This study emphasizes the importance of long-term experiments and deep measurements when characterizing soil C and N responses to land use change and the remarkable paucity of such long-term soil data deeper than 30 cm.

Emerging land use practices rapidly increase soil organic matter

The loss of organic matter from agricultural lands constrains our ability to sustainably feed a growing population and mitigate the impacts of climate change. Addressing these challenges requires land use activities that accumulate soil carbon (C) while contributing to food production. In a region of extensive soil degradation in the southeastern United States, we evaluated soil C accumulation for 3 years across a 7-year chronosequence of three farms converted to management-intensive grazing. Here we show that these farms accumulated C at 8.0 Mg ha(-1) yr(-1), increasing cation exchange and water holding capacity by 95% and 34%, respectively. Thus, within a decade of management-intensive grazing practices soil C levels returned to those of native forest soils, and likely decreased fertilizer and irrigation demands. Emerging land uses, such as management-intensive grazing, may offer a rare win-win strategy combining profitable food production with rapid improvement of soil quality and short-term climate mitigation through soil C-accumulation.

The ecology and management of moist mixed-conifer forests in eastern Oregon and Washington: a synthesis of the relevant biophysical science and implications for future land management

Land managers in the Pacific Northwest have reported a need for updated scientific information on the ecology and management of mixed-conifer forests east of the Cascade Range in Oregon and Washington. Of particular concern are the moist mixed-conifer forests, which have become drought-stressed and vulnerable to high-severity fire after decades of human disturbances and climate warming. This synthesis responds to this need. We present a compilation of existing research across multiple natural resource issues, including disturbance regimes, the legacy effects of past management actions, wildlife habitat, watershed health, restoration concepts from a landscape perspective, and social and policy concerns. We provide considerations for management, while also emphasizing the importance of local knowledge when applying this information at the local and regional level.

New insight on Mars: A network of ancient lakes and discontinuous river segments

With an abundance of ice recently detected just below the surface of Mars [Boynton et al., 2002],the possibility that life has existed or still may exist on Mars may hinge on examining its past climate and the duration of surface water flows. Was Mars ever a warm and wet planet, or has it always been cold and dry? The study of surface depressions, in conjunction with river features, yields new insight into these questions. Together, these features provide a more complete picture of a surface water network that may have existed on what must have been a warmer early Mars.

Depth trends of soil organic matter C:N and 15N natural abundance controlled by association with minerals

Plant residues show carbon:nitrogen (C:N) decreases, 15N isotopic enrichment and preferential loss of labile substrates during microbial decay. In soil profiles, strikingly similar patterns of decreasing C:N and 15N isotopic enrichment with increasing depth are well documented. The parallel trend in organic matter composition with soil depth and during plant residue decay has been used as evidence to suggest that organic products accumulate or develop in the subsoil due to increasing intensity of microbially-driven processing, although no studies to date have verified this. Here, by applying sequential density fractionation, specific surface area, oxalate extractable Fe and Al, C:N and δ15N measures with depth to soils with relatively uniform soil mineralogy (Oxisols), climates and vegetation we show that changes in organo-mineral associations drive subsoil C:N and δ15N and C:N depth patterns more than in situ organic matter decay. Our results provide the first direct evidence that soil depth trends could be driven by mineral association instead of in situ processing.

Multiple-level defoliation assessment with hyperspectral data: integration of continuum-removed absorptions and red edges

Hyperspectral data were collected from 40 canopies of saltcedar (Tamarix ramosissima): 10 healthy canopies and 30 canopies defoliated by an introduced biological control agent, the saltcedar leaf beetle (Diorhabda carinata). These data assessed multiple-level defoliations in response to the process of biological control. Two important characteristics of the hyperspectral data – red edges and continuum-removed absorptions – were used to discriminate four defoliation categories of saltcedar (healthy plants, newly defoliated plants, completely defoliated plants and refoliating plants) at the canopy level. The red edge positions were located at ranges of 711–716 nm, 706–712 nm, 694–698 nm and 715–719 nm for the four defoliation stages described above, respectively. These red edge positions alone could not clearly judge the four defoliation categories associated with feeding by the beetles. Only the completely defoliated canopies had distinct red edge positions that could be differentiated from the other three types of canopies. While using a classification tree to integrate the red edge positions and their derivatives with the central band depths of these five continuum-removed absorptions, it was found that only two band depths of the continuum-removed absorptions were selected, which were the red absorption between 570 and 716 nm and the water absorption between 936 and 990 nm in the near-infrared region (NIR). This implied that the continuum-removed absorptions outperformed the red edges for identifying the defoliation categories. The resulting overall accuracy was 87.5%. The producer accuracy was 100%, 70%, 100% and 80% for the healthy plants, newly defoliated, completely defoliated plants and refoliating canopies, respectively. The corresponding user accuracy was 90.91%, 77.78%, 100% and 80%. Therefore, we concluded that single spectral data based variable failed to separate the four stages but a combination of the two continuum-removed absorptions located in the blue absorption and the first water absorption in the NIR improved the identification of defoliated canopies associated with the dynamic defoliation process of the biological control agent. This study developed the defoliation detection techniques of commonly used binary levels (i.e. defoliation and non-defoliation) to multiple vegetation defoliation levels. We anticipate applying these assessment techniques to wide-area collections of hyperspectral data covering the two spectral regions as described above to further evaluate the effectiveness of these biological control beetles and their impact on saltcedar management in the Western United States.

Navier-Stokes Simulation of Local Winds Over the Earth’s Topography

A numerical approach is described that simplifies and automates the CFD solution process so that Earth scientists can utilize high-resolution Navier-Stokes flow solvers as a research tool to investigate wind events on the Earth’s surface. The current approach utilizes the OVERFLOW-2 structured overset RANS code. A genetic algorithm is used to obtain an optimal multi-zone overset grid system that reduces the grid size and simulation time by maintaining high resolution over high-gradient land regions and lower resolution over low-gradient water regions. Flow simulations are presented that include flow separation and reattachment over mountainous terrain for coastal islands in Alaska (USA) and British Columbia (Canada).