Jason B. West

Nitrogen limitation constrains sustainability of ecosystem response to CO2

Enhanced plant biomass accumulation in response to elevated atmospheric CO2 concentration could dampen the future rate of increase in CO2 levels and associated climate warming. However, it is unknown whether CO2-induced stimulation of plant growth and biomass accumulation will be sustained or whether limited nitrogen (N) availability constrains greater plant growth in a CO2-enriched world. Here we show, after a six-year field study of perennial grassland species grown under ambient and elevated levels of CO2 and N, that low availability of N progressively suppresses the positive response of plant biomass to elevated CO2. Initially, the stimulation of total plant biomass by elevated CO2 was no greater at enriched than at ambient N supply. After four to six years, however, elevated CO2 stimulated plant biomass much less under ambient than enriched N supply. This response was consistent with the temporally divergent effects of elevated CO2 on soil and plant N dynamics at differing levels of N supply. Our results indicate that variability in availability of soil N and deposition of atmospheric N are both likely to influence the response of plant biomass accumulation to elevated atmospheric CO2. Given that limitations to productivity resulting from the insufficient availability of N are widespread in both unmanaged and managed vegetation, soil N supply is probably an important constraint on global terrestrial responses to elevated CO2.

Tamarisk biocontrol in the western United States: ecological and societal implications

Tamarisk species (genus Tamarix), also commonly known as saltcedar, are among the most successful plant invaders in the western United States. At the same time, tamarisk has been cited as having enormous economic costs. Accordingly, local, state, and federal agencies have undertaken considerable efforts to eradicate this invasive plant and restore riparian habitats to pre-invasion status. Traditional eradication methods, including herbicide treatments, are now considered undesirable, because they are costly and often have unintended negative impacts on native species. A new biological control agent, the saltcedar leaf beetle (Diorhabda elongata), has been released along many watersheds in the western US, to reduce the extent of tamarisk cover in riparian areas. However, the use of this insect as a biological control agent may have unintended ecological, hydrological, and socioeconomic consequences that need to be anticipated by land managers and stakeholders undertaking restoration efforts. Here, we examine the possible ramifications of tamarisk control and offer recommendations to reduce potential negative impacts on valued riparian systems in the western US.

A simplified GIS approach to modeling global leaf water isoscapes

The stable hydrogen (δ2H) and oxygen (δ18O) isotope ratios of organic and inorganic materials record biological and physical processes through the effects of substrate isotopic composition and fractionations that occur as reactions proceed. At large scales, these processes can exhibit spatial predictability because of the effects of coherent climatic patterns over the Earths surface. Attempts to model spatial variation in the stable isotope ratios of water have been made for decades. Leaf water has a particular importance for some applications, including plant organic materials that record spatial and temporal climate variability and that may be a source of food for migrating animals. It is also an important source of the variability in the isotopic composition of atmospheric gases. Although efforts to model global-scale leaf water isotope ratio spatial variation have been made (especially of δ18O), significant uncertainty remains in models and their execution across spatial domains. We introduce here a Geographic Information System (GIS) approach to the generation of global, spatially-explicit isotope landscapes ( = isoscapes) of “climate normal” leaf water isotope ratios. We evaluate the approach and the resulting products by comparison with simulation model outputs and point measurements, where obtainable, over the Earths surface. The isoscapes were generated using biophysical models of isotope fractionation and spatially continuous precipitation isotope and climate layers as input model drivers. Leaf water δ18O isoscapes produced here generally agreed with latitudinal averages from GCM/biophysical model products, as well as mean values from point measurements. These results show global-scale spatial coherence in leaf water isotope ratios, similar to that observed for precipitation and validate the GIS approach to modeling leaf water isotopes. These results demonstrate that relatively simple models of leaf water enrichment combined with spatially continuous precipitation isotope ratio and climate data layers yield accurate global leaf water estimates applicable to important questions in ecology and atmospheric science.

Plant diversity, CO2, and n influence inorganic and organic n leaching in grasslands

In nitrogen (N)-limited systems, the potential to sequester carbon depends on the balance between N inputs and losses as well as on how efficiently N is used, yet little is known about responses of these processes to changes in plant species richness, atmospheric CO2 concentration ([CO2]), and N deposition. We examined how plant species richness (1 or 16 species), elevated [CO2] (ambient or 560 ppm), and inorganic N addition (0 or 4 g x m(-2) x yr(-1)) affected ecosystem N losses, specifically leaching of dissolved inorganic N (DIN) and organic N (DON) in a grassland field experiment in Minnesota, USA. We observed greater DIN leaching below 60 cm soil depth in the monoculture plots (on average 1.8 and 3.1 g N x m(-2) x yr(-1) for ambient N and N-fertilized plots respectively) than in the 16-species plots (0.2 g N x m(-2) x yr(-1) for both ambient N and N-fertilized plots), particularly when inorganic N was added. Most likely, loss of complementary resource use and reduced biological N demand in the monoculture plots caused the increase in DIN leaching relative to the high-diversity plots. Elevated [CO2] reduced DIN concentrations under conditions when DIN concentrations were high (i.e., in N-fertilized and monoculture plots). Contrary to the results for DIN, DON leaching was greater in the 16-species plots than in the monoculture plots (on average 0.4 g N x m(-2) x yr(-1) in 16-species plots and 0.2 g N x m(-2) x yr(-1) in monoculture plots). In fact, DON dominated N leaching in the 16-species plots (64% of total N leaching as DON), suggesting that, even with high biological demand for N, substantial amounts of N can be lost as DON. We found no significant main effects of elevated [CO2] on DIN or DON leaching; however, elevated [CO2] reduced the positive effect of inorganic N addition on DON leaching, especially during the second year of observation. Our results suggest that plant species richness, elevated [CO2], and N deposition alter DIN loss primarily through changes in biological N demand. DON losses can be as large as DIN loss but are more sensitive to organic matter production and turnover.

Stable isotope ratios of marijuana. I. Carbon and nitrogen stable isotopes describe growth conditions.

Abstract:  There remains significant uncertainty in illicit marijuana cultivation. We analyzed the δ13C and δ15N of 508 domestic samples from known U.S.A. counties, 31 seized from a single location, 5 samples grown in Mexico and Colombia, and 10 northwest border seizures. For a subset, inflorescences and leaves were analyzed separately. These data revealed a strong correspondence, with inflorescences having slightly higher δ13C and δ15N values than leaves. A framework for interpreting these results is introduced and evaluated. Samples identified as outdoor‐grown by δ13C were generally recorded as such by the Drug Enforcement Administration (DEA). DEA‐classified indoor‐grown samples had the most negative δ13C values, consistent with indoor cultivation, although many were also in the outdoor‐grown domain. δ15N indicated a wide range of fertilizers across the dataset. Samples seized at the single location suggested multiple sources. Northwest border δ13C values suggested indoor growth, whereas for the Mexican and Colombian samples they indicated outdoor growth.

Isoscapes to address large-scale Earth science challenges

Sugar cane cropping for biofuel production reduces water discharge from a northern Indian basin and threatens downstream communities. Regulators want to partition blame between climate change—induced declines in mountain snowpack and excessive evaporation from poorly managed fields. In the same basin, a tiger is found shot. Is it the nuisance animal that has been tormenting local communities, or is it a different animal poached from the upland forests?

Isotope Landscapes for Terrestrial Migration Research

Publisher Summary Isotope tracking of migratory terrestrial animals (for example, birds, bats, and insects) relies on the assimilation and fixation of intrinsic isotopic markers from the environment into animal body tissue. The power of the isotopic markers relates to the extent and pattern of spatial isotope ratio variations in the environmental substrates from which they are assimilated (primarily food, water, and air). This chapter introduces these patterns of variation for the commonly applied or applicable stable isotope systems and describes methods by which the spatial landscapes of environmental isotopic variation (“isoscapes”) are modeled and predicted at scales relative to the study of migratory behavior and ecology. Examples of isoscapes for some isotopic systems are presented along with discussion of challenges and cautionary notes related to the creation and interpretation of isoscapes. A discussion of opportunities and future directions in isoscape modeling is also presented. In addition, the principles and methodology underlying the development of these products and relevant to their informed use are described.

Applications of Stable Isotope Measurements for Early‐Warning Detection of Ecological Change

Publisher Summary This chapter describes the rationale and framework for stable isotope monitoring to assess ecological condition and change at ecosystem to global scales. The isotope ratios of compounds in aquatic, terrestrial, and atmospheric environments are very sensitive to changes in ecological processes. As such, isotope measurements may serve as an early-warning signal of ecological changes related to ecosystem functions. This unique application and role for stable isotope monitoring can greatly assist management efforts and inform environmental policy. This chapter goes on to propose a specific framework for developing an isotope-monitoring network and the spatial modeling necessary to detect and understand ecological change at a continental scale. The framework is based on isotopic measurements of atmospheric inputs, ecosystem outputs, changes between inputs and outputs as elements are cycled within ecosystems, and sentinel organisms as integrators and indicators of ecological change. Such a framework has the capacity to provide unique insight into how climate and land-use changes and associated biotic and abiotic disturbances impact ecological functioning and connectivity across large regions to continents.

Future Directions and Challenges for Using Stable Isotopes in Advancing Terrestrial Animal Migration Research

Publisher Summary Animal migration is an exciting field that will provide many years of research for scientists in numerous disciplines. The true potential of the isotope techniques will only be realized in cases where the researcher has been careful to first choose the species and migratory system that shows promise isotopically, and then consider the sources of variance in the model used to infer origins. The path ahead involves great emphasis on understanding the mechanisms that can influence isotopic variation spatially and within organisms of interest. It also involves a careful consideration of how to statistically infer origins or establish the probability of assignment. Use of more refined isoscape models that involve several elements and the careful use of new remote sensing geographic information system (GIS) layers will also be a fruitful area of research. These areas of research and development are well beyond the scope of any single researcher or laboratory, and hence this field is sure to emerge as one of the best examples of multidisciplinary collaborative research.

Forensic applications of light-element stable isotope ratios of Ricinus communis seeds and ricin preparations.

Seeds of the castor plant Ricinus communis are of forensic interest because they are the source of the poison ricin. We tested whether stable isotope ratios of castor seeds and ricin preparations can be used as a forensic signature. We collected over 300 castor seed samples worldwide and measured the C, N, O, and H isotope ratios of the whole seeds and oil. We prepared ricin by three different procedures, acetone extraction, salt precipitation, and affinity chromatography, and compared their isotope ratios to those of the source seeds. The N isotope ratios of the ricin samples and source seeds were virtually identical. Therefore, N isotope ratios can be used to correlate ricin prepared by any of these methods to source seeds. Further, stable isotope ratios distinguished >99% of crude and purified ricin protein samples in pairwise comparison tests. Stable isotope ratios therefore constitute a valuable forensic signature for ricin preparations.