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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.

Our understanding and quantification of global soil nitrous oxide (N2O) emissions and the underlying processes remain largely uncertain. Here, we assessed the effects of multiple anthropogenic and natural factors, including nitrogen fertilizer (N) application, atmospheric N deposition, manure N application, land cover change, climate change, and rising atmospheric CO2 concentration, on global soil N2O emissions for the period 1861–2016 using a standard simulation protocol with seven process‐based terrestrial biosphere models. Results suggest global soil N2O emissions have increased from 6.3 ± 1.1 Tg N2O‐N/year in the preindustrial period (the 1860s) to 10.0 ± 2.0 Tg N2O‐N/year in the recent decade (2007–2016). Cropland soil emissions increased from 0.3 Tg N2O‐N/year to 3.3 Tg N2O‐N/year over the same period, accounting for 82% of the total increase. Regionally, China, South Asia, and Southeast Asia underwent rapid increases in cropland N2O emissions since the 1970s. However, US cropland N2O emissions had been relatively flat in magnitude since the 1980s, and EU cropland N2O emissions appear to have decreased by 14%. Soil N2O emissions from predominantly natural ecosystems accounted for 67% of the global soil emissions in the recent decade but showed only a relatively small increase of 0.7 ± 0.5 Tg N2O‐N/year (11%) since the 1860s. In the recent decade, N fertilizer application, N deposition, manure N application, and climate change contributed 54%, 26%, 15%, and 24%, respectively, to the total increase. Rising atmospheric CO2 concentration reduced soil N2O emissions by 10% through the enhanced plant N uptake, while land cover change played a minor role. Our estimation here does not account for indirect emissions from soils and the directed emissions from excreta of grazing livestock. To address uncertainties in estimating regional and global soil N2O emissions, this study recommends several critical strategies for improving the process‐based simulations.

Soil is the largest organic carbon (C) pool of terrestrial ecosystems, and C loss from soil accounts for a large proportion of land‐atmosphere C exchange. Therefore, a small change in soil organic C (SOC) can affect atmospheric carbon dioxide (CO2) concentration and climate change. In the past decades, a wide variety of studies have been conducted to quantify global SOC stocks and soil C exchange with the atmosphere through site measurements, inventories, and empirical/process‐based modeling. However, these estimates are highly uncertain, and identifying major driving forces controlling soil C dynamics remains a key research challenge. This study has compiled century‐long (1901–2010) estimates of SOC storage and heterotrophic respiration (Rh) from 10 terrestrial biosphere models (TBMs) in the Multi‐scale Synthesis and Terrestrial Model Intercomparison Project and two observation‐based data sets. The 10 TBM ensemble shows that global SOC estimate ranges from 425 to 2111 Pg C (1 Pg = 1015 g) with a median value of 1158 Pg C in 2010. The models estimate a broad range of Rh from 35 to 69 Pg C yr−1 with a median value of 51 Pg C yr−1 during 2001–2010. The largest uncertainty in SOC stocks exists in the 40–65°N latitude whereas the largest cross‐model divergence in Rh are in the tropics. The modeled SOC change during 1901–2010 ranges from −70 Pg C to 86 Pg C, but in some models the SOC change has a different sign from the change of total C stock, implying very different contribution of vegetation and soil pools in determining the terrestrial C budget among models. The model ensemble‐estimated mean residence time of SOC shows a reduction of 3.4 years over the past century, which accelerate C cycling through the land biosphere. All the models agreed that climate and land use changes decreased SOC stocks, while elevated atmospheric CO2 and nitrogen deposition over intact ecosystems increased SOC stocks—even though the responses varied significantly among models. Model representations of temperature and moisture sensitivity, nutrient limitation, and land use partially explain the divergent estimates of global SOC stocks and soil C fluxes in this study. In addition, a major source of systematic error in model estimations relates to nonmodeled SOC storage in wetlands and peatlands, as well as to old C storage in deep soil layers.