Decadal carbon balance and its response to 2 degree warming among heterogenous Arctic landscapes

<p>The large amounts of carbon (C) stored in the Arctic region can strongly interact with the climate system through the exchange of carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) under the unmitigated environmental changes. Currently, there are still large uncertainties in the C exchange and the subsequent C-climate feedbacks between the land and atmosphere across the Arctic region, to which the highly heterogeneous landscapes make a key contribution. However, our knowledge on the present and future ecosystem C balance jointly considering the exchange of CO<sub>2</sub> and CH<sub>4</sub> in the Arctic region with heterogeneous landscapes is still limited. In this study, a process-based biogeochemistry model was calibrated and validated using the empirical data on concurrently measured CO<sub>2</sub> and CH<sub>4</sub> exchange observed using eddy covariance, automatic and manual chamber methods and associated climate, soil and plant data derived from several heterogeneous landscapes in the Kaamanen region. With the validated model, decadal C balance during 2005-2018 and its response to 2 <sup>o</sup>C warming were evaluated for the constituent land cover types (LCTs). Our results showed that most LCTs were a sink for atmospheric CO<sub>2</sub> and a source of CH<sub>4</sub> during 2005-2018. Under the 2 <sup>o</sup>C warming scenario, most ecosystems continued to be CO<sub>2</sub> sinks and CH<sub>4</sub> sources. Moreover, the CO<sub>2</sub> budget in most LCTs did not change significantly as the two major fluxes of gross primary productivity (GPP) and total ecosystem respiration (TER) increased simultaneously thus maintaining similar rates of net ecosystem exchange (NEE) in response to warming, while a significant increase in CH<sub>4</sub> emission from most LCTs was evident. Our results presented here provide us a better understanding and prediction of C dynamics and the inherent C-climate feedbacks in the Arctic region.</p>

14C estimation of soil C02 turnover rates in Ultisols with different land use

<p>The CO<sub>2</sub> flux from soil is a large and significant flux in most ecosystems and can account for more than 2/3 of total ecosystem respiration. In many cases, CO<sub>2</sub> flux from soil is estimated by the eddy covariance technique or by classical chamber method with measures of bulk concentration and isotopic composition of carbon dioxide. Whereas most these studies estimated CO<sub>2</sub> flux from the soil surface, we analyzed its concentration and isotope composition directly in soil profiles down to 5m depth.</p><p>This experiment was conducted in Sumter National Forest by NSF Calhoun CZO research program. A 10cm diameter auger was used to core up to 5 m depth and capped PVC pipe segments of 750 cm<sup>3</sup> volume serve as gas reservoirs, each with two gas impermeable tubes that connected the gas reservoirs. Soil gas reservoirs are installed at 5m, 3m, 1.5m, and 0.5m depths from the soil surface. On a three-week interval, soil gases were extracted with a pump and analyzed in the field for CO<sub>2</sub> and O<sub>2</sub> concentration with samples collected in Tedlar bags for analysis. The samples were collected in summer 2016 under 3 different land uses: hardwood stands that are taken to be never cultivated; old-field pine stands, which had been used for growing cotton in 19<sup>th</sup> century and then abandoned; and cultivated sites which were used growing cotton, but for the last 50-60 years for growing corn, wheat, legume, sorghum, and sunflowers.</p><p>The radiocarbon analyses in the soil CO<sub>2</sub> profile were conducted for the first time. It was discovered that concentration of 14C increased with depth and Δ<sup>14</sup>C changed from 40-60%o in the top 0.5m to about 80-140 ‰ at 5m depth depending on land use.</p><p> </p>

Global satellite-driven estimates of heterotrophic respiration

While heterotrophic respiration (Rh) makes up about a quarter of gross global terrestrial carbon fluxes, it remains among the least-observed carbon fluxes, particularly outside the midlatitudes. In situ measurements collected in the Soil Respiration Database (SRDB) number only a few hundred worldwide. Similarly, only a single data-driven wall-to-wall estimate of annual average heterotrophic respiration exists, based on bottom-up upscaling of SRDB measurements using an assumed functional form to account for climate variability. In this study, we exploit recent advances in remote sensing of terrestrial carbon fluxes to estimate global variations in heterotrophic respiration in a top-down fashion at monthly temporal resolution and 4∘×5∘ spatial resolution. We combine net ecosystem productivity estimates from atmospheric inversions of the NASA Carbon Monitoring System-Flux (CMS-Flux) with an optimally scaled gross primary productivity dataset based on satellite-observed solar-induced fluorescence variations to estimate total ecosystem respiration as a residual of the terrestrial carbon balance. The ecosystem respiration is then separated into autotrophic and heterotrophic components based on a spatially varying carbon use efficiency retrieved in a model–data fusion framework (the CARbon DAta MOdel fraMework, CARDAMOM). The resulting dataset is independent of any assumptions about how heterotrophic respiration responds to climate or substrate variations. It estimates an annual average global average heterotrophic respiration flux of 43.6±19.3 Pg C yr−1. Sensitivity and uncertainty analyses showed that the top-down Rh are more sensitive to the choice of input gross primary productivity (GPP) and net ecosystem productivity (NEP) datasets than to the assumption of a static carbon use efficiency (CUE) value, with the possible exception of the wet tropics. These top-down estimates are compared to bottom-up estimates of annual heterotrophic respiration, using new uncertainty estimates that partially account for sampling and model errors. Top-down heterotrophic respiration estimates are higher than those from bottom-up upscaling everywhere except at high latitudes and are 30 % greater overall (43.6 Pg C yr−1 vs. 33.4 Pg C yr−1). The uncertainty ranges of both methods are comparable, except poleward of 45∘ N, where bottom-up uncertainties are greater. The ratio of top-down heterotrophic to total ecosystem respiration varies seasonally by as much as 0.6 depending on season and climate, illustrating the importance of studying the drivers of autotrophic and heterotrophic respiration separately, and thus the importance of data-driven estimates of Rh such as those estimated here.

Global, Satellite-Driven Estimates of Heterotrophic Respiration

While heterotrophic respiration (Rh) makes up about a quarter of gross global terrestrial carbon fluxes, it remains among the least observed carbon fluxes, particularly outside the mid-latitudes. In situ measurements collected in the Soil Respiration Database (SRDB) number only a few hundred worldwide. Similarly, only a single data-driven wall-to-wall estimate of annual average heterotrophic respiration exists, based on bottom-up upscaling of SRDB measurements using an assumed functional form to account for climate variability. In this study, we exploit recent advances in remote sensing of terrestrial carbon fluxes to estimate global variations in heterotrophic respiration in a top-down fashion at monthly temporal resolution and 4 x 5° spatial resolution. We combine net ecosystem productivity estimates from atmospheric inversions of the NASA Carbon Monitoring System- Flux (CMS-Flux) with an optimally-scaled gross primary productivity dataset based on satellite-observed solar-induced fluorescence variations to estimate total ecosystem respiration as a residual of the terrestrial carbon balance. The ecosystem respiration is then separated into autotrophic and heterotrophic components based on a spatially-varying carbon use efficiency retrieved in a model-data fusion framework (the CARbon DAta MOdel fraMework, CARDAMOM). The resulting dataset is independent of any assumptions about how heterotrophic respiration responds to climate or substrate variations. It estimates an annual average global average heterotrophic respiration flux of 43.6 ± 19.3 Pg C/yr. These top-down estimates are compared to bottom-up estimates of annual heterotrophic respiration, with new uncertainty estimates that partially account for sampling and model errors. Top-down heterotrophic respiration estimates are higher than those from bottom-up upscaling everywhere except at high latitudes, and are 30 % greater overall (43.6 Pg C/yr vs. 33.4 Pg C/yr). The uncertainty ranges of both methods are comparable, except poleward of 45 degrees North, where bottom-up uncertainties are greater. The ratio of top-down heterotrophic to total ecosystem respiration varies seasonally by as much as 0.6 depending on season and climate, illustrating the importance of studying the drivers of autotrophic and heterotrophic respiration separately, and thus the importance of data-driven estimates of Rh such as those estimated here.

Spatial and temporal variation of CO₂ efflux along a disturbance gradient in a <i>miombo</i> woodland in Western Zambia

Carbon dioxide efflux from the soil surface was measured over a period of several weeks within a heterogeneous Brachystegia spp. dominated miombo woodland in Western Zambia. The objectives were to examine spatial and temporal variation of soil respiration along a disturbance gradient from a protected forest reserve to a cut, burned, and grazed area outside, and to relate the flux to various abiotic and biotic drivers. The highest daily mean fluxes (around 12 μmol CO2 m−2 s−1) were measured in the protected forest in the wet season and lowest daily mean fluxes (around 1 μmol CO2 m−2 s−1) in the most disturbed area during the dry season. Diurnal variation of soil respiration was closely correlated with soil temperature. The combination of soil water content and soil temperature was found to be the main driving factor at seasonal time scale. There was a 75% decrease in soil CO2 efflux during the dry season and a 20% difference in peak soil respiratory flux measured in 2008 and 2009. Spatial variation of CO2 efflux was positively related to total soil carbon content in the undisturbed area but not at the disturbed site. Coefficients of variation of efflux rates between plots decreased towards the core zone of the protected forest reserve. Normalized soil respiration values did not vary significantly along the disturbance gradient. Spatial variation of respiration did not show a clear distinction between the disturbed and undisturbed sites and could not be explained by variables such as leaf area index. In contrast, within plot variability of soil respiration was explained by soil organic carbon content. Three different approaches to calculate total ecosystem respiration (Reco) from eddy covariance measurements were compared to two bottom-up estimates of Reco obtained from chambers measurements of soil- and leaf respiration which differed in the consideration of spatial heterogeneity. The consideration of spatial variability resulted only in small changes of Reco when compared to simple averaging. Total ecosystem respiration at the plot scale, obtained by eddy covariance differed by up to 25% in relation to values calculated from the soil- and leaf chamber efflux measurements but without showing a clear trend.

Spatial and temporal variation of CO₂ efflux along a disturbance gradient in a <i>miombo</i> woodland in Western Zambia

Carbon dioxide efflux from the soil surface was measured over a period of several weeks within a heterogeneous Brachystegia spp. dominated miombo woodland in Western Zambia. The objectives were to examine spatial and temporal variation of soil respiration along a disturbance gradient from a protected forest reserve to a cut, burned, and grazed area outside, and to relate the flux to various abiotic and biotic drivers. The highest daily mean fluxes (around 12 μmol m−2 s−1) were measured in the protected forest in the wet season and lowest daily mean fluxes (around 1 μmol m−2 s−1) in the most disturbed area during the dry season. Diurnal variation of soil respiration was closely correlated with soil temperature. The combination of soil water content and soil temperature was found to be the main driving factor at seasonal time scale. There was a 75% decrease in soil CO2 efflux during the dry season and a 20% difference in peak soil respiratory flux measured in 2008 and 2009. Spatial variation of CO2 efflux was positively related to total soil carbon content in the undisturbed area but not at the disturbed site. Coefficients of variation of efflux rates between plots decreased towards the core zone of the protected forest reserve. Normalized soil respiration values did not vary significantly along the disturbance gradient. Spatial variation of respiration did not show a clear distinction between the disturbed and undisturbed sites and was neither explained by soil carbon nor leaf area index. In contrast, within plot variability of soil respiration was explained by soil organic carbon content. Three different approaches to calculate total ecosystem respiration (Reco) from eddy covariance measurements were compared to two bottom-up estimates of Reco obtained from chambers measurements of soil- and leaf respiration which differed in the consideration of spatial heterogeneity. The consideration of spatial variability resulted only in small changes of Reco when compared to simple averaging. Total ecosystem respiration at the plot scale, obtained by eddy covariance differed by up to 25% in relation to values calculated from the soil- and leaf chamber efflux measurements but without showing a clear trend.