Impacts of Multiple Environmental Changes on Long- Term Nitrogen Loading From the Chesapeake Bay Watershed

Excessive nitrogen can enter estuarine and coastal areas from land, disturbing coastal ecosystems and causing serious environmental problems. The Chesapeake Bay is one of the regions that have experienced hypoxia and harmful algal blooms in recent decades. This study estimated nitrogen export from the Chesapeake Bay watershed (CBW) to the estuary from 1900 to 2015 by applying a state-of-the-art numerical model. Nitrogen loading from the CBW continually increased from the 1900s to the 1990s and has declined since then. The key contributors to nitrogen export have shifted from atmospheric nitrogen deposition (before the 1960s) to synthetic nitrogen fertilizer (after the 1980s). Antipollution policies and implementation measures have played critical roles in the decrease of nitrogen export since the 1980s, and further reduction in riverine nitrogen export will likely require regulation on the application of nitrogen fertilizer

Synergistic Effects of Climate Change and Grazing on Net Primary Production of Mongolian Grasslands

In arid and semi-arid regions, grassland degradation has become a major environmental and economic problem, but little information is available on the response of grassland productivity to both climate change and grazing intensity. By developing a grazing module in a process-based ecosystem model, the dynamic land ecosystem model (DLEM), we explore the roles of climate change, elevated CO2, and varying grazing intensities in affecting aboveground net primary productivity (ANPP) across different grassland sites in Mongolia. Our results show that both growing season precipitation totals and average temperature exert important controls on annual ANPP across six sites over a precipitation gradient, explaining 65% and 45% of the interannual variations, respectively. Interannual variation in ANPP, measured as the ratio of standard deviation among years to long-term mean, increased from 9.5 to 18.9% to 23.9–32.5% along a gradient of high to low precipitation. Historical grazing resulted in a net reduction in ANPP across all sites ranging from 2% to 15.4%. Our results further show that grassland ANPP can be maintained at a grazing intensity of 1.0 and 0.5 sheep/ha at wet and dry sites, respectively, indicating that dry sites are more vulnerable to grazing compared to wet sites. In addition, precipitation use efficiency (PUE) decreased while nitrogen use efficiency (NUE) increased across a gradient of low to high precipitation. However, grazing resulted in a net reduction in both PUE and NUE by 47% and 67% across all sites. Our results indicate that seasonal precipitation totals, average temperatures and grazing are important regulators of grassland ANPP in Mongolia. These results have important implications for grassland productivity in semi-arid regions in Central Asia and beyond

Riverine Carbon Cycling Over the Past Century in the Mid-Atlantic Region of the United States

Rivers are an important component of the terrestrial-aquatic ocean continuum as they serve as a conduit for transporting carbon from the land to the coastal ocean. It is essential to track the fate of this carbon, including how much carbon is buried in the riverbed, outgassed to the atmosphere, and exported to the ocean. However, it is often difficult to quantify these carbon transport processes on the watershed scale because observational data obtained by field surveys can only be used to estimate the magnitude of these processes at distinct points. In this study, we used a coupled terrestrial-aquatic ecosystem model to assess the century-long full carbon budget of the riverine ecosystem across the watersheds of Chesapeake Bay and Delaware Bay. In addition, we examined the individual and combined impacts of climate change and anthropogenic activities on these terrestrial ecosystems and the resultant CO2emissions of their associated rivers. We found that climate variability and land conversion (from cropland to impervious surfaces and forest) are the most important factors governing the long-term change in riverine carbon dynamics. We also highlighted the importance of riverine CO2 emissions in the overall regional carbon budget

Preindustrial nitrous oxide emissions from the land biosphere estimated by using a global biogeochemistry model

To accurately assess how increased global nitrous oxide (N2O) emission has affected the climate system requires a robust estimation of the preindustrial N2O emissions since only the difference between current and preindustrial emissions represents net drivers of anthropogenic climate change. However, large uncertainty exists in previous estimates of preindustrial N2O emissions from the land biosphere, while preindustrial N2O emissions on the finer scales, such as regional, biome, or sector scales, have not been well quantified yet. In this study, we applied a process-based Dynamic Land Ecosystem Model (DLEM) to estimate the magnitude and spatial patterns of preindustrial N2O fluxes at the biome, continental, and global level as driven by multiple environmental factors. Uncertainties associated with key parameters were also evaluated. Our study indicates that the mean of the preindustrial N2O emission was approximately 6.20TgNyr−1, with an uncertainty range of 4.76 to 8.13TgNyr−1. The estimated N2O emission varied significantly at spatial and biome levels. South America, Africa, and Southern Asia accounted for 34.12, 23.85, and 18.93%, respectively, together contributing 76.90% of global total emission. The tropics were identified as the major source of N2O released into the atmosphere, accounting for 64.66% of the total emission. Our multi-scale estimates provide a robust reference for assessing the climate forcing of anthropogenic N2O emission from the land biospher

Effects of tropospheric ozone pollution on net primary productivity and carbon storage in terrestrial ecosystems of China

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): D22S09, doi:10.1029/2007JD008521.We investigated the potential effects of elevated ozone (O3) along with climate variability, increasing CO2, and land use change on net primary productivity (NPP) and carbon storage in China's terrestrial ecosystems for the period 1961–2000 with a process-based Dynamic Land Ecosystem Model (DLEM) forced by the gridded data of historical tropospheric O3 and other environmental factors. The simulated results showed that elevated O3 could result in a mean 4.5% reduction in NPP and 0.9% reduction in total carbon storage nationwide from 1961 to 2000. The reduction of carbon storage varied from 0.1 Tg C to 312 Tg C (a decreased rate ranging from 0.2% to 6.9%) among plant functional types. The effects of tropospheric O3 on NPP were strongest in east-central China. Significant reductions in NPP occurred in northeastern and central China where a large proportion of cropland is distributed. The O3 effects on carbon fluxes and storage are dependent upon other environmental factors. Therefore direct and indirect effects of O3, as well as interactive effects with other environmental factors, should be taken into account in order to accurately assess the regional carbon budget in China. The results showed that the adverse influences of increasing O3 concentration across China on NPP could be an important disturbance factor on carbon storage in the near future, and the improvement of air quality in China could enhance the capability of China's terrestrial ecosystems to sequester more atmospheric CO2. Our estimation of O3 impacts on NPP and carbon storage in China, however, must be used with caution because of the limitation of historical tropospheric O3 data and other uncertainties associated with model parameters and field experiments.This research is funded by NASA Interdisciplinary Science Program (NNG04GM39C)

Modeling and Monitoring Terrestrial Primary Production in a Changing Global Environment: Toward a Multiscale Synthesis of Observation and Simulation

There is a critical need to monitor and predict terrestrial primary production, the key indicator of ecosystem functioning, in a changing global environment. Here we provide a brief review of three major approaches to monitoring and predicting terrestrial primary production: (1) ground-based field measurements, (2) satellite-based observations, and (3) process-based ecosystem modelling. Much uncertainty exists in the multi-approach estimations of terrestrial gross primary production (GPP) and net primary production (NPP). To improve the capacity of model simulation and prediction, it is essential to evaluate ecosystem models against ground and satellite-based measurements and observations. As a case, we have shown the performance of the dynamic land ecosystem model (DLEM) at various scales from site to region to global. We also discuss how terrestrial primary production might respond to climate change and increasing atmospheric CO2 and uncertainties associated with model and data. Further progress in monitoring and predicting terrestrial primary production requires a multiscale synthesis of observations and model simulations. In the Anthropocene era in which human activity has indeed changed the Earth’s biosphere, therefore, it is essential to incorporate the socioeconomic component into terrestrial ecosystem models for accurately estimating and predicting terrestrial primary production in a changing global environment

Integrating Herbivore Population Dynamics Into a Global Land Biosphere Model: Plugging Animals Into the Earth System

Mammalian herbivores are an essential component of grassland and savanna ecosystems, and with feedbacks to the climate system. To date, the response and feedbacks of mammalian herbivores to changes in both abiotic and biotic factors are poorly quantified and not adequately represented in the current global land surface modeling framework. In this study, we coupled herbivore population dynamics in a global land model (the Dynamic Land Ecosystem Model, DLEM 3.0) to simulate populations of horses, cattle, sheep, and goats, and their responses to changes in multiple environmental factors at the site level across different continents during 1980–2010. Simulated results show that the model is capable of reproducing observed herbivore population dynamics across all sites for these animal groups. Our simulation results also indicate that during this period, climate extremes led to a maximum mortality of 27% of the total herbivores in Mongolia. Across all sites, herbivores reduced aboveground net primary productivity (ANPP) and heterotrophic respiration (Rh) by 14% and 15%, respectively (p \u3c 0.05). With adequate parameterization, the model can be used for historical assessment and future prediction of mammalian herbivore populations and their relevant impacts on biogeochemical cycles. Our simulation results demonstrate a strong coupling between primary producers and consumers, indicating that inclusion of herbivores into the global land modeling framework is essential to better understand the potentially large effect of herbivores on carbon cycles in grassland and savanna ecosystems