land use and climate change). Therefore, in this study, the anthropogenic contribution to the 53 biogenic flux of each GHG is distinguished by removing modeled pre-industrial emissions from 54 contemporary GHG estimates. To quantify the human-induced net biogenic balance of these 55 three GHGs and its impact on climate system, we use CO 2 equivalent units (CO 2 -eq) based on 56 the global warming potentials (GWP) on a 100-year time horizon 7 . This choice has been driven 57 by the policy options being considered when dealing with biogenic GHG emissions and sinks 7,11 . 4To address the changing relative importance of each gas as a function of the selected time frame, 59 a supplemental calculation based on GWP metrics for a 20-year time horizon is also provided 60 (Table 1 and Methods). 61We first examine the overall biogenic fluxes of all three gases in the terrestrial biosphere The estimates of the global terrestrial CO 2 sink in the 2000s are -1.6 ± 0.9 Pg C/yr (TD) 78 and -1.5 ± 1.2 Pg C/yr (BU). This estimate is comparable with the most recent estimates 4 , but 79 incoporates more data sources (Table S1 in SI). than its role using GWP100 metric (Table 1). Therefore, cutting CH 4 emissions is an effective 97 pathway for rapidly reducing GHG-induced radiative forcing and the rate of climate warming in 98 a short time frame 8,11 . 99On a 100-year time horizon, the cumulative radiative forcing of agricultural and waste 100 emissions alone, including CH 4 from paddy fields, manure management, ruminants, and landfill 2000s, offsetting the human-induced land CO 2 sink by 1.4 to 1.5 times, respectively. In other 104 words, agriculture represents the largest contributor to this twofold offset of the land CO 2 sink. 105We further examine the change of human-induced biogenic GHG fluxes over past three 106 decades ( Figure 2, Table 1 Europe's land ecosystem is found to play a neutral role, similar to a previous synthesis study 9 141 using both BU and TD approaches. 27 , and increased indirect emissions from biofuel production 28 . 163The future role of the biosphere as a source or sink of GHGs will depend on future land use 164 intensification pathways and on the evolution of the land CO 2 sinks 29 . If the latter continues 165 increasing as observed in the last three decades 4 , the overall biospheric GHG balance could be 166 reversed. However, the evolution of the land CO 2 sink remains uncertain, with some projections 167 showing an increasing sink in the coming decades 3 , while others showing a weakening sink due 168 to the saturation of the CO 2 fertilization effect and positive carbon-climate feedbacks 3,30 . 169Increasing land-use intensification using today's practices to meet food and energy demands will 170 likely increase anthropogenic GHG emissions 23 . However, the results of this study suggest that 171 9 adoption of best practices to reduce GHG emissions from human-impacted land ecosystems 172 could reverse the biosphere's current warming role.

No abstract

Abstract.Continental-scale estimations of terrestrial methane (CH 4 ) and nitrous oxide (N 2 O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH 4 and N 2 O fluxes in North America's terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a −1 (1 T g = 10 12 g) of CH 4 , and 1.94 ± 0.1 T g N a −1 of N 2 O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH 4 sources to the atmosphere, but Mexico mainly oxidized and consumed CH 4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH 4 source, while all other ecosystems acted as sinks for atmospheric CH 4 , of which forests accounted for 36.8%. Regarding N 2 O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N 2 O emission. The inter-annual variations of CH 4 and N 2 O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH 4 flux, both mean annual temperature and annual precipitation were significantly correlated to annual Correspondence to: H. Tian (tianhan@auburn.edu) N 2 O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH 4 and N 2 O fluxes in North America generated in this study provide useful information for global change research and policy making.

[1] Anthropogenic nitrous pollutant emissions in China significantly increased during the last decades, which contributed to the accelerated nitrogen (N) deposition. In order to characterize spatial pattern of nitrogen deposition, we employed the kriging technique to interpolate sampling data of precipitation chemistry and ambient air concentration from site-network observations over China. The estimation of wet deposition in China was limited to aqueous NO 3 À and NH 4 + , while ambient NO 2 was the only species involved in the predicted dry deposition fluxes. To obtain wet deposition fluxes, precipitation concentration was multiplied by 20-year mean precipitation amounts with a resolution of 10 Â 10 km. Dry deposition fluxes were products of the interpolated ambient NO 2 concentration and deposition velocities modeled for the main vegetation types in China. The total deposition rates of wet and dry deposition peaked over the central south China, with maximum values of 63.53 kg N ha À1 yr À1 , and an average value of 12.89 kg N ha À1 yr À1 . With ambient NO 2 concentration data spanning from the year 1990 through 2003, we detected and evaluated trends in the time series of the annual values of atmospheric NO 2 concentration. Significant upward trends at 21 of 102 sites were exhibited, with median percent change of 61.45% over the period [1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003]. In addition, spatially continuous patterns of dry deposition fluxes based on ambient NO 2 measurements in two 5-year phases, 9 years apart, were carried out. On average, there was a rise of 7.66% in NO 2 dry deposition during 9 years throughout China.Citation: Lü, C., and H. Tian (2007), Spatial and temporal patterns of nitrogen deposition in China: Synthesis of observational data,

Abstract. Terrestrial biosphere models (TBMs) have become an integral tool for extrapolating local observations and understanding of land–atmosphere carbon exchange to larger regions. The North American Carbon Program (NACP) Multi-scale synthesis and Terrestrial Model Intercomparison Project (MsTMIP) is a formal model intercomparison and evaluation effort focused on improving the diagnosis and attribution of carbon exchange at regional and global scales. MsTMIP builds upon current and past synthesis activities, and has a unique framework designed to isolate, interpret, and inform understanding of how model structural differences impact estimates of carbon uptake and release. Here we provide an overview of the MsTMIP effort and describe how the MsTMIP experimental design enables the assessment and quantification of TBM structural uncertainty. Model structure refers to the types of processes considered (e.g., nutrient cycling, disturbance, lateral transport of carbon), and how these processes are represented (e.g., photosynthetic formulation, temperature sensitivity, respiration) in the models. By prescribing a common experimental protocol with standard spin-up procedures and driver data sets, we isolate any biases and variability in TBM estimates of regional and global carbon budgets resulting from differences in the models themselves (i.e., model structure) and model-specific parameter values. An initial intercomparison of model structural differences is represented using hierarchical cluster diagrams (a.k.a. dendrograms), which highlight similarities and differences in how models account for carbon cycle, vegetation, energy, and nitrogen cycle dynamics. We show that, despite the standardized protocol used to derive initial conditions, models show a high degree of variation for GPP, total living biomass, and total soil carbon, underscoring the influence of differences in model structure and parameterization on model estimates.

[1] The magnitude, spatial, and temporal patterns of the terrestrial carbon sink and the underlying mechanisms remain uncertain and need to be investigated. China is important in determining the global carbon balance in terms of both carbon emission and carbon uptake. Of particular importance to climate-change policy and carbon management is the ability to evaluate the relative contributions of multiple environmental factors to net carbon source and sink in China's terrestrial ecosystems. Here the effects of multiple environmental factors (climate, atmospheric CO 2 , ozone pollution, nitrogen deposition, nitrogen fertilizer application, and land cover/land use change) on net carbon balance in terrestrial ecosystems of China for the period 1961-2005 were modeled with newly developed, detailed historical information of these changes. For this period, results from two models indicated a mean land sink of 0.21 Pg C per year, with a multimodel range from 0.18 to 0.24 Pg C per year. The models' results are consistent with field observations and national inventory data and provide insights into the biogeochemical mechanisms responsible for the carbon sink in China's land ecosystems. In the simulations, nitrogen deposition and fertilizer applications together accounted for 61 percent of the net carbon storage in China's land ecosystems in recent decades, with atmospheric CO 2 increases and land use also functioning to stimulate carbon storage.

Abstract. In addition to enhancing agricultural productivity, synthetic nitrogen (N) and phosphorous (P) fertilizer application in croplands dramatically alters global nutrient budget, water quality, greenhouse gas balance, and their feedback to the climate system. However, due to the lack of geospatial fertilizer input data, current Earth system and land surface modeling studies have to ignore or use oversimplified data (e.g., static, spatially uniform fertilizer use) to characterize agricultural N and P input over decadal or century-long periods. In this study, we therefore develop global time series gridded data of annual synthetic N and P fertilizer use rate in agricultural lands, matched with HYDE 3.2 historical land use maps, at a resolution of 0.5 • × 0.5 • latitude-longitude during 1961-2013. Our data indicate N and P fertilizer use rates on per unit cropland area increased by approximately 8 times and 3 times, respectively, since the year 1961 when IFA (International Fertilizer Industry Association) and FAO (Food and Agricultural Organization) surveys of country-level fertilizer input became available. Considering cropland expansion, the increase in total fertilizer consumption is even larger. Hotspots of agricultural N fertilizer application shifted from the US and western Europe in the 1960s to eastern Asia in the early 21st century. P fertilizer input shows a similar pattern with an additional current hotspot in Brazil. We found a global increase in fertilizer N / P ratio by 0.8 g N g −1 P per decade (p < 0.05) during 1961-2013, which may have an important global implication for human impacts on agroecosystem functions in the long run. Our data can serve as one of critical input drivers for regional and global models to assess the impacts of nutrient enrichment on climate system, water resources, food security, etc. Datasets available at

Abstract. Ecosystems are important and dynamic components of the global carbon cycle, and terrestrial biospheric models (TBMs) are crucial tools in further understanding of how terrestrial carbon is stored and exchanged with the atmosphere across a variety of spatial and temporal scales. Improving TBM skills, and quantifying and reducing their estimation uncertainties, pose significant challenges. The Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) is a formal multi-scale and multi-model intercomparison effort set up to tackle these challenges. The MsTMIP protocol prescribes standardized environmental driver data that are shared among model teams to facilitate model–model and model–observation comparisons. This paper describes the global and North American environmental driver data sets prepared for the MsTMIP activity to both support their use in MsTMIP and make these data, along with the processes used in selecting/processing these data, accessible to a broader audience. Based on project needs and lessons learned from past model intercomparison activities, we compiled climate, atmospheric CO2 concentrations, nitrogen deposition, land use and land cover change (LULCC), C3 / C4 grasses fractions, major crops, phenology and soil data into a standard format for global (0.5° × 0.5° resolution) and regional (North American: 0.25° × 0.25° resolution) simulations. In order to meet the needs of MsTMIP, improvements were made to several of the original environmental data sets, by improving the quality, and/or changing their spatial and temporal coverage, and resolution. The resulting standardized model driver data sets are being used by over 20 different models participating in MsTMIP. The data are archived at the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC, http://daac.ornl.gov) to provide long-term data management and distribution.