Xudong Zhu

Carbon Consequences and Agricultural Implications of Growing Biofuel Crops on Marginal Agricultural Lands in China

Using marginal agricultural lands to grow energy crops for biofuel feedstocks is a promising option to meet the biofuel needs in populous China without causing further food shortages or environmental problems. Here we quantify the effects of growing switchgrass and Miscanthus on Chinese marginal agricultural lands on biomass production and carbon emissions with a global-scale biogeochemical model. We find that the national net primary production (NPP) of these two biofuel crops are 622 and 1546 g C m(-2) yr(-1), respectively, whereas the NPP of food crops is about 600 g C m(-2) yr(-1) in China. The net carbon sink over the 47 Mha of marginal agricultural lands across China is 2.1 Tg C yr(-1) for switchgrass and 5.0 Tg C yr(-1) for Miscanthus. Soil organic carbon is estimated to be 10 kg C m(-2) in both biofuel ecosystems, which is equal to the soil carbon levels of grasslands in China. In order to reach the goal of 12.5 billion liters of bioethanol in 2020 using crop biomass as biofuel feedstocks, 7.9-8.0 Mha corn grain, 4.3-6.1 Mha switchgrass, or 1.4-2.0 Mha Miscanthus will be needed. Miscanthus has tremendous potential to meet future biofuel needs, and to benefit CO(2) mitigation in China.

Rising methane emissions in response to climate change in Northern Eurasia during the 21st century

We used a biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to examine the methane (CH4) exchanges between terrestrial ecosystems and the atmosphere in Northern Eurasia from 1971 to 2100. Multiple model simulations using various wetland extent datasets and climate change scenarios were conducted to assess the uncertainty of CH4 fluxes, including emissions and consumption. On the basis of these simulations we estimate the current net emissions in the region to be 20–24 Tg CH4 yr − 1 (1 Tg = 1012 g), two-thirds of which are emitted during the summer. In response to climate change over the 21st century, the annual CH4 emissions in the region are projected to increase at a rate of 0.06 Tg CH4 yr − 1, which is an order of magnitude greater than that of annual CH4 consumption. Further, the annual net CH4 emissions are projected to increase by 6–51% under various wetland extent datasets and climate scenarios by the end of the 21st century, relative to present conditions. Spatial patterns of net CH4 emissions were determined by wetland extent. Net CH4 emissions were dominated by wetlands within boreal forests, grasslands and wet tundra areas in the region. Correlation analyses indicated that water table depth and soil temperature were the two most important environmental controls on both CH4 emissions and consumption in the region. Our uncertainty analyses indicated that the uncertainty in wetland extent had a larger effect on future CH4 emissions than the uncertainty in future climate. This study suggests that better characterization of the spatial distribution and the natural diversity of wetlands should be a research priority for quantifying CH4 fluxes in this region.

Carbon and nitrogen dynamics in bioenergy ecosystems: 2. Potential greenhouse gas emissions and global warming intensity in the conterminous United States.

This study estimated the potential emissions of greenhouse gases (GHG) from bioenergy ecosystems with a biogeochemical model AgTEM, assuming maize (Zea mays L.), switchgrass (Panicum virgatum L.), and Miscanthus (Miscanthus × giganteus) will be grown on the current maize‐producing areas in the conterminous United States. We found that the maize ecosystem acts as a mild net carbon source while cellulosic ecosystems (i.e., switchgrass and Miscanthus) act as mild sinks. Nitrogen fertilizer use is an important factor affecting biomass production and N2O emissions, especially in the maize ecosystem. To maintain high biomass productivity, the maize ecosystem emits much more GHG, including CO2 and N2O, than switchgrass and Miscanthus ecosystems, when high‐rate nitrogen fertilizers are applied. For maize, the global warming potential (GWP) amounts to 1–2 Mg CO2eq ha−1 yr−1, with a dominant contribution of over 90% from N2O emissions. Cellulosic crops contribute to the GWP of less than 0.3 Mg CO2eq ha−1 yr−1. Among all three bioenergy crops, Miscanthus is the most biofuel productive and the least GHG intensive at a given cropland. Regional model simulations suggested that substituting Miscanthus for maize to produce biofuel could potentially save land and reduce GHG emissions.

Influence of changes in wetland inundation extent on net fluxes of carbon dioxide and methane in northern high latitudes from 1993 to 2004

United States. National Aeronautics and Space Administration. Land Use and Land Cover Change program (NASA-NNX09AI26G)

Pan-Arctic land?atmospheric fluxes of methane and carbon dioxide in response to climate change over the 21st century

United States. Dept. of Energy. SciDAC Institute on Quantum Simulation of Materials and Nanostructures

Net exchanges of methane and carbon dioxide on the Qinghai-Tibetan Plateau from 1979 to 2100

Methane (CH4) is a potent greenhouse gas (GHG) that affects the global climate system. Knowledge about land–atmospheric CH4 exchanges on the Qinghai-Tibetan Plateau (QTP) is insufficient. Using a coupled biogeochemistry model, this study analyzes the net exchanges of CH4 and CO2 over the QTP for the period of 1979–2100. Our simulations show that the region currently acts as a net CH4 source with 0.95 Tg CH4 y−1 emissions and 0.19 Tg CH4 y−1 soil uptake, and a photosynthesis C sink of 14.1 Tg C y−1. By accounting for the net CH4 emission and the net CO2 sequestration since 1979, the region was found to be initially a warming source until the 2010s with a positive instantaneous radiative forcing peak in the 1990s. In response to future climate change projected by multiple global climate models (GCMs) under four representative concentration pathway (RCP) scenarios, the regional source of CH4 to the atmosphere will increase by 15–77% at the end of this century. Net ecosystem production (NEP) will continually increase from the near neutral state to around 40 Tg C y−1 under all RCPs except RCP8.5. Spatially, CH4 emission or uptake will be noticeably enhanced under all RCPs over most of the QTP, while statistically significant NEP changes over a large-scale will only appear under RCP4.5 and RCP4.6 scenarios. The cumulative GHG fluxes since 1979 will exert a slight warming effect on the climate system until the 2030s, and will switch to a cooling effect thereafter. Overall, the total radiative forcing at the end of the 21st century is 0.25–0.35 W m−2, depending on the RCP scenario. Our study highlights the importance of accounting for both CH4 and CO2 in quantifying the regional GHG budget.

Rising methane emissions from northern wetlands associated with sea ice decline

Abstract The Arctic is rapidly transitioning toward a seasonal sea ice‐free state, perhaps one of the most apparent examples of climate change in the world. This dramatic change has numerous consequences, including a large increase in air temperatures, which in turn may affect terrestrial methane emissions. Nonetheless, terrestrial and marine environments are seldom jointly analyzed. By comparing satellite observations of Arctic sea ice concentrations to methane emissions simulated by three process‐based biogeochemical models, this study shows that rising wetland methane emissions are associated with sea ice retreat. Our analyses indicate that simulated high‐latitude emissions for 2005–2010 were, on average, 1.7 Tg CH4 yr−1 higher compared to 1981–1990 due to a sea ice‐induced, autumn‐focused, warming. Since these results suggest a continued rise in methane emissions with future sea ice decline, observation programs need to include measurements during the autumn to further investigate the impact of this spatial connection on terrestrial methane emissions.

Carbon and nitrogen dynamics in bioenergy ecosystems: 1. Model development, validation and sensitivity analysis.

Biofuel made from conventional (e.g., maize (Zea mays L.)) and cellulosic crops (e.g., switchgrass (Panicum virgatum L.) and Miscanthus (Miscanthus × giganteus)) provides alternative energy to fossil fuels and has been considered to mitigate greenhouse gas emissions. To estimate the large‐scale carbon and nitrogen dynamics of these biofuel ecosystems, process‐based models are needed. Here, we developed an agroecosystem model (AgTEM) based on the Terrestrial Ecosystem Model for these ecosystems. The model was incorporated with biogeochemical and ecophysiological processes including crop phenology, biomass allocation, nitrification, and denitrification, as well as agronomic management of irrigation and fertilization. It was used to estimate crop yield, biomass, net carbon exchange, and nitrous oxide emissions at an ecosystem level. The model was first parameterized for maize, switchgrass, and Miscanthus ecosystems and then validated with field observation data. We found that AgTEM well reproduces the annual net primary production and nitrous oxide fluxes of most sites, with over 85% of total variation explained by the model. Local sensitivity analysis indicated that the model sensitivity varies among different ecosystems. Net primary production of maize is sensitive to temperature, precipitation, cloudiness, fertilizer, and irrigation and less sensitive to atmospheric CO2 concentrations. In contrast, the net primary production of switchgrass and Miscanthus is most sensitive to temperature among all factors. Nitrous oxide fluxes are sensitive to management in maize ecosystems, and sensitive to climate factors in cellulosic ecosystems. The developed model should help advance our understanding of carbon and nitrogen dynamics of these biofuel ecosystems at both site and regional levels.

Estimation of the North–South Transect of Eastern China forest biomass using remote sensing and forest inventory data

The assessment of forest biomass is required for the estimation of carbon sinks and a myriad other ecological and environmental factors. In this article, we combined satellite data (Thematic Mapper (TM) and Moderate Resolution Imaging Spectrometer (MODIS)), forest inventory data, and meteorological data to estimate forest biomass across the North–South Transect of Eastern China (NSTEC). We estimate that the total regional forest biomass was 2.306 × 109 Megagrams (Mg) in 2007, with a mean coniferous forest biomass density of 132.78 Mg ha−1 and a mean broadleaved forest biomass density of 142.32 Mg ha−1. The mean biomass density of the entire NSTEC was 129 Mg ha−1. Furthermore, we analysed the spatial distribution pattern of the forest biomass and the distribution of biomass along the latitudinal and longitudinal gradients. The biomass was higher in the south and east and lower in the north and west of the transect. In the northern part of the NSTEC, the forest biomass was positively correlated with longitude. However, in the southern part of the transect, the forest biomass was negatively correlated with latitude but positively correlated with longitude. The biomass had an increasing trend with increases in precipitation and temperature. The results of the study can provide useful information for future studies, including quantifying the regional carbon budget.

Adaptive characteristics of grassland community structure and leaf traits along an altitudinal gradient on a subtropical mountain in Chongqing, China

Community structure and leaf traits are important elements of terrestrial ecosystems. Changes of community structure and leaf traits are of particular use in the study of the influence of climate change on terrestrial ecosystems. Patterns of community structure (including species richness, above- and below-ground biomass) and leaf traits (including leaf mass per area (LMA), nitrogen content both on mass and area bases (Nmass and Narea), and foliar δ13C) from 19 grassland plots along an altitudinal transect at Hongchiba in Chongqing, China, were analyzed. Species richness along the altitudinal transect had a hump-shaped pattern. Above-ground biomass had a quadratic decrease along the altitudinal gradient whereas below-ground biomass had the opposite pattern. Change of above-ground biomass of various taxonomic groups with altitude was also studied. Poaceae showed strong negative relationships and Asteraceae showed a hump-shaped relationship with increase of altitude. Five common species of the grassland, Trifolium pratense, Geranium wilfordii,Aster tataricus, Leontopodium leontopodioides, and Spiraea prunifolia, were particularly studied for variation of leaf traits along the altitudinal gradient. Averaged for all species, LMA, Narea and foliar δ13C had positive correlations with altitude. Nmass did not change significantly as altitude increased. LMA and Narea showed significant positive relationships with foliar δ13C. The adaptive features of leaf traits among different species were not consistent. The study highlights specific adaptation patterns in relation to altitude for different plant species, provides further insights into adaptive trends of community structure and leaf traits in a specific ecological region filling a gap in the definition of global patterns, and adds to the understanding of how adaptive patterns of plants may respond to global climate change.