Abstract. Field measurements have shown that cold-season methane (CH4) and carbon dioxide (CO2) emissions contribute a substantial portion to the annual net carbon emissions in permafrost regions. However, most earth system land models do not accurately reproduce cold-season CH4 and CO2 emissions, especially over the shoulder (i.e., thawing and freezing) seasons. Here we use the Energy Exascale Earth System Model (E3SM) land model version 1 (ELMv1-ECA) to tackle this challenge and fill the knowledge gap of how cold-season CH4 and CO2 emissions contribute to the annual totals at Alaska Arctic tundra sites. Specifically, we improved the ELMv1-ECA soil water phase-change scheme, environmental controls on microbial activity, and cold-season methane transport module. Results demonstrate that both soil temperature and the duration of zero-curtain periods (i.e., the fall period when soil temperatures linger around 0 °C) simulated by the updated ELMv1-ECA were greatly improved, e.g., the Mean Absolute Error in zero-curtain durations at 12 cm depth was reduced by 62 % on average. Furthermore, the simulated cold-season emissions at three tundra sites were improved by 84 % and 81 % on average for CH4 and CO2, respectively. Overall, CH4 and CO2 emitted during the early cold season (Sep. and Oct.), which often includes most of the zero-curtain period in Arctic tundra, accounted for more than 50 % of the total emissions throughout the entire cold season (Sep. to May). From 1950 to 2017, both CO2 emissions during the 12 cm depth zero-curtain period and during the entire cold season showed increasing trends, for example, of 0.26 g C m−2 year−1 and 0.38 g C m−2 year−1 at Atqasuk. This study highlights the importance of zero-curtain periods in facilitating CH4 and CO2 emissions from tundra ecosystems.

<p>Soil organic carbon (SOC) responses to atmospheric warming depend on soil biogeochemical and plant responses and their interactions. These processes occur over a wide range of time scales and are spatially heterogeneous, leading to difficulties in predicting emergent ecosystem carbon dynamics. Here we explore coupled soil and plant responses to warming using a mechanistic model (<em>ecosys</em>) and observations from a whole-soil warming experiment in the Sierra Nevada mountains of California (Blodgett Forest Whole Soil Warming Experiment). <em>ecosys</em> represents the coupled hydrological, thermal, soil biogeochemical, and plant processes that affect ecosystem carbon cycling, and has been applied and tested in dozens of ecosystems. We briefly describe the model components relevant to the warming study and then show that it accurately simulates observed soil moisture and temperature, SOC stocks, root biomass, and experimental warming effects on soil T and moisture. Using the simulations, we then show that the emergent short-term effect of warming on respiration losses is dominated by heterotrophic respiration vs changes in plant inputs. However, multi-season to multi-year responses strongly depend on changes in N availability and plant N uptake, leading to increased soil surface CO<sub>2</sub> emissions from heterotrophic and autotrophic respiration, increases in aboveground biomass, and relatively small changes in SOC stocks from increased litter inputs. We also explore the roles of microbial thermal adaptation and soil moisture on the SOC stock changes. Finally, we compare the short- and long-term responses to warming to evaluate how well experimental soil warming manipulations directly inform ecosystem carbon dynamics under expected long-term climate change warming. Our work highlights the need to evaluate climate change impacts on the carbon cycle for the integrated plant-soil system, and points to needed improvements in current large-scale land models.</p>

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ModifiedELM-S7 Q10=1.4(Tref=25) f(W)*f(T) 11 Standcarb Standcarb f(W)*f(T) 61 ModifiedELM-S3 Century1 f(W)*f(T) 111 ModifiedELM-S5 LloydTaylor f(W)*f(T) ModifiedELM-S7 Q10=1.5(Tref=25) f(W)*f(T) 12 Daycent Daycent1 f(W)*f(T) 62 ModifiedELM-S3 Century2 f(W)*f(T) 112 ModifiedELM-S5 Kirschbaum f(W)*f(T) ModifiedELM-S7 Q10=2(Tref=25) f(W)*f(T) 13 Daycent Daycent2 f(W)*f(T) 63 ModifiedELM-S3 Daycent1 f(W)*f(T) 113 ModifiedELM-S5 Demeter f(W)*f(T) ModifiedELM-S7 Q10=1.5(Tref=35) f(W)*f(T) 14 Demeter Demeter f(W)*f(T)*f(O) 64 ModifiedELM-S3 Daycent2 f(W)*f(T) 114 ModifiedELM-S5 Standcarb f(W)*f(T) ModifiedELM-S7 Q10=2(Tref=35) f(W)*f(T) 15 Century Century1 f(W)*f(T)*f(O) 65 ModifiedELM-S3 Q10=2.0(Tref=10) f(W)*f(T) 115 ModifiedELM-S5 Q10=1.3(Tref=25) f(W)*f(T) ModifiedELM-S7 Q10=1.6(Tref=25) f(W)*f(T) 16 Century Century2 f(W)*f(T)*f(O) 66 ModifiedELM-S3 Q10=1.4(Tref=10) f(W)*f(T) 116 ModifiedELM-S5 Q10=1.4(Tref=25) f(W)*f(T) ModifiedELM-S7 Q10=1.7(Tref=25) f(W)*f(T) 17 ModifiedELM-S1 Century1 f(W)*f(T)*f(O) 67 ModifiedELM-S3 LloydTaylor f(W)*f(T) 117 ModifiedELM-S5 Q10=1.5(Tref=25) f(W)*f(T) ModifiedELM-S7 Q10=1.8(Tref=25) f(W)*f(T) 18 ModifiedELM-S1 Century2 f(W)*f(T)*f(O) 68 ModifiedELM-S3 Kirschbaum f(W)*f(T) 118 ModifiedELM-S5 Q10=1.6(Tref=25) f(W)*f(T) ModifiedELM-S7 Q10=1.9(Tref=25) f(W)*f(T) 19 ModifiedELM-S1 Daycent1 f(W)*f(T)*f(O) 69 ModifiedELM-S3 Demeter f(W)*f(T) 119 ModifiedELM-S5 Q10=1.7(Tref=25) f(W)*f(T) ModifiedELM-S7 Q10=2.1(Tref=25) f(W)*f(T) 20 ModifiedELM-S1 Daycent2 f(W)*f(T)*f(O) 70 ModifiedELM-S3 Standcarb f(W)*f(T) 120 ModifiedELM-S5 Q10=1.8(Tref=25) f(W)*f(T) ModifiedELM-S7 Q10=2.2(Tref=25) f(W)*f(T) 21 ModifiedELM-S1 Q10=2.0(Tref=10) f(W)*f(T)*f(O) 71 ModifiedELM-S3 Q10=1.3(Tref=25) f(W)*f(T) 121 ModifiedELM-S5 Q10=1.9(Tref=25) f(W)*f(T) ModifiedELM-S8 Q10=1.3(Tref=25) f(W)*f(T) 22 ModifiedELM-S1 Q10=1.4(Tref=10) f(W)*f(T)*f(O) 72 ModifiedELM-S3 Q10=1.4(Tref=25) f(W)*f(T) 122 ModifiedELM-S5 Q10=2(Tref=25) f(W)*f(T) ModifiedELM-S8 Q10=1.4(Tref=25) f(W)*f(T) 23 ModifiedELM-S1 LloydTaylor f(W)*f(T)*f(O) 73 ModifiedELM-S3 Q10=1.5(Tref=25) f(W)*f(T) 123 ModifiedELM-S5 Q10=2.1(Tref=25) f(W)*f(T) ModifiedELM-S8 Q10=1.5(Tref=25) f(W)*f(T) 24 ModifiedELM-S1 Kirschbaum f(W)*f(T)*f(O) 74 ModifiedELM-S3 Q10=1.6(Tref=25) f(W)*f(T) 124 ModifiedELM-S5 Q10=2.2(Tref=25) f(W)*f(T) ModifiedELM-S8 Q10=1.6(Tref=25) f(W)*f(T) 25 ModifiedELM-S1 Demeter f(W)*f(T)*f(O) 75 ModifiedELM-S3 Q10=1.7(Tref=25) f(W)*f(T) 125 ModifiedELM-S5 Q10=1.5(Tref=35) f(W)*f(T) ModifiedELM-S8 Q10=1.7(Tref=25) f(W)*f(T) 26 ModifiedELM-S1 Standcarb f(W)*f(T)*f(O) 76 ModifiedELM-S3 Q10=1.8(Tref=25) f(W)*f(T) 126 ModifiedELM-S5 Q10=2(Tref=35) f(W)*f(T) ModifiedELM-S8 Q10=1.8(Tref=25) f(W)*f(T) 27 ModifiedELM-S1 Q10=1.3(Tref=25) f(W)*f(T)*f(O) 77 ModifiedELM-S3 Q10=1.9(Tref=25) f(W)*f(T) 127 ModifiedELM-S6 Century1 f(W)*f(T)*f(O) ModifiedELM-S8 Q10=1.9(Tref=25) f(W)*f(T) 28 ModifiedELM-S1 Q10=1.4(Tref=25) f(W)*f(T)*f(O) 78 ModifiedELM-S3 Q10=2(Tref=25) f(W)*f(T) 128 ModifiedELM-S6 Century2 f(W)*f(T)*f(O) ModifiedELM-S8 Q10=2(Tref=25) f(W)*f(T...

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