Net carbon sink of China was overestimated by more than 35%

As a carbon source/sink of atmospheric carbon dioxide, the net regional carbon budget (NRCB) of terrestrial ecosystems is very important to effect global warming, especially China with the largest emissions at present. However, the carbon consumption is difficult to measure accurately, which is caused by the emissions of CH4 and CO, the utilization of agriculture, forestry and grass, and the emissions from rivers and other physical processes, such as forest fires. Therefore, the spatial patterns and driving factors of NRCB are not clear. Here, we used multi-source data to estimate the NRCB of 31 provincial administrative divisions of China and to develop NRCB datasets from 2000 to 2018. We found that the average of NRCB was 669 TgC yr−1, and it significantly decreased at a rate of 2.56 TgC yr−1. The relative contribution rates of fluxes of emissions from anthropogenic (FEAD), reactive carbon and creature ingestion (FERCCI), autotrophic respiration (Ra), heterotrophic respiration (Rh) and natural disturbances (FEND) were 35.17%, 26.09%, 19.68%, 17.38% and 1.68% respectively. In addition, NRCB datasets of the different administrative regions of China were mapped. These datasets will provide support for China's carbon neutrality and the study of the global carbon cycle.

Environmental Performance of Soapberry (Sapindus Mukorossi Gaertn.) Cultivation in Southeast China based on a Life Cycle Assessment: A Potential Feedstock for Forest-based Biodiesel

Sapindus mukorossi G. has been considered as a potential feedstock for forest-based biodiesel in China. To optimize the cultivation of soapberry and ensure its sustainable supply, an environmental life cycle assessment (LCA) was conducted using a chronological approach combined with extrapolation. Soapberry plantations with two degrees of cultivation intensities were comparatively analyzed. For the studied environmental categories, nitrogen fertilization accounted for half or more of the global warming potential, primary energy demand, acidification and eutrophication potential. The main contributors to ozone depletion were pesticides and potassium fertilizer. The plantations with a relatively low cultivation intensity presented better environmental performance, mainly due to the lower input of fertilizers, but they are not a priority choice for soapberry cultivation because of low yield. Stakeholders should focus on how to reduce the environmental impacts of the plantations with a relatively high cultivation intensity in this area. Overall, classified management, increasing the yield, reducing the inputs of chemicals and decreasing the unproductive years are the key ways to improve the environmental performance of soapberry cultivation in Southeast China. Woody biomass carbon should be included in LCAs, and 3.71-5.11 t CO2 can be fixed by soapberry plantations per ha year, indicating that soapberry cultivation provides a net carbon sink.

Large and seasonally varying biospheric CO2 fluxes in the Los Angeles megacity revealed by atmospheric radiocarbon

Measurements of Δ14C and CO2 can cleanly separate biogenic and fossil contributions to CO2 enhancements above background. Our measurements of these tracers in air around Los Angeles in 2015 reveal high values of fossil CO2 and a significant and seasonally varying contribution of CO2 from the urban biosphere. The biogenic CO2 is composed of sources such as biofuel combustion and human metabolism and an urban biospheric component likely originating from urban vegetation, including turf and trees. The urban biospheric component is a source in winter and a sink in summer, with an estimated amplitude of 4.3 parts per million (ppm), equivalent to 33% of the observed annual mean fossil fuel contribution of 13 ppm. While the timing of the net carbon sink is out of phase with wintertime rainfall and the sink seasonality of Southern California Mediterranean ecosystems (which show maximum uptake in spring), it is in phase with the seasonal cycle of urban water usage, suggesting that irrigated urban vegetation drives the biospheric signal we observe. Although 2015 was very dry, the biospheric seasonality we observe is similar to the 2006–2015 mean derived from an independent Δ14C record in the Los Angeles area, indicating that 2015 biospheric exchange was not highly anomalous. The presence of a large and seasonally varying biospheric signal even in the relatively dry climate of Los Angeles implies that atmospheric estimates of fossil fuel–CO2 emissions in other, potentially wetter, urban areas will be biased in the absence of reliable methods to separate fossil and biogenic CO2.

Carbon Emissions and Removals by Forests: New Estimates 1990–2020

Trends in global, regional and national CO2 emissions and removals from forest for the period 1990–2020, are estimated for the first time using data from the Forest Resources Assessment (FRA) 2020, providing new information with respect to the previous FRA 2015. Estimates indicate significant reduction of deforestation emissions over the study period, albeit more slowly than previously assessed, from an average of 4.3 Gt CO2 yr−1 during 1991–2000, to an average of 2.9 Gt CO2 yr−1 during 2016–2020. Remaining forest land was a significant net carbon sink globally and over the entire period, albeit decreasing in strength, from −3.4 Gt CO2 yr−1 in 1991–2000 to −2.5 Gt CO2 yr−1 during 2016–2020. The overall net contribution of forests to atmospheric CO2(i.e., the combined effect of deforestation and forest emissions/removals) was an overall emission source of roughly 0.4 Gt CO2 yr−1 on average during 1991–2020, more than one-third less than previously estimated. Remarkably, the new data also suggest an overall net sink of about −0.7 Gt CO2 yr−1 during 2011–2015, never reported before. Forest emissions/removals data independently reported by countries to the United Nations Framework on Climate Change were in excellent agreement with the FAO estimates over the entire period 1990–2020, confirming a large sink on forest land estimated for 2011–2015. Data are made available as open access via the Zenodo portal (Tubiello, 2020), with DOI 10.5281/zenodo.3941973.

Low below-ground organic carbon storage in a subarctic Alpine permafrost environment

This study investigates the soil organic carbon (SOC) storage in Tarfala Valley, northern Sweden. Field inventories, upscaled based on land cover, show that this alpine permafrost environment does not store large amounts of SOC, with an estimate mean of 0.9 ± 0.2 kg C m−2 for the upper meter of soil. This is 1 to 2 orders of magnitude lower than what has been reported for lowland permafrost terrain. The SOC storage varies for different land cover classes and ranges from 0.05 kg C m−2 for stone-dominated to 8.4 kg C m−2 for grass-dominated areas. No signs of organic matter burial through cryoturbation or slope processes were found, and radiocarbon-dated SOC is generally of recent origin (<2000 cal yr BP). An inventory of permafrost distribution in Tarfala Valley, based on the bottom temperature of snow measurements and a logistic regression model, showed that at an altitude where permafrost is probable the SOC storage is very low. In the high-altitude permafrost zones (above 1500 m), soils store only ca. 0.1 kg C m−2. Under future climate warming, an upward shift of vegetation zones may lead to a net ecosystem C uptake from increased biomass and soil development. As a consequence, alpine permafrost environments could act as a net carbon sink in the future, as there is no loss of older or deeper SOC from thawing permafrost.

Low soil organic carbon storage in a subarctic alpine permafrost environment

This study investigates the soil organic carbon (SOC) storage in Tarfala Valley, Northern Sweden. Field inventories upscaled based on land cover show that this alpine permafrost environment does not store large amounts of SOC, with an estimate mean of 0.9 ± 0.2 kg C m−2 for the upper meter of soil. This is one to two orders of magnitude lower than what has been reported for lowland permafrost terrain. The SOC storage varies for different land cover classes and ranges from 0.05 kg C m−2 for stone-dominated to 8.4 kg C m−2 for grass-dominated areas. No signs of organic matter burial through cryoturbation or slope processes were found and radiocarbon dated SOC is generally of recent origin (<2000 cal yr BP). An inventory of permafrost distribution in Tarfala Valley, based on bottom temperature of snow measurements and a ogistic regression model, showed that at an altitude where permafrost is probable, the SOC storage is very low. In the high altitude permafrost zones (above 1500 m), soils store only ca 0.1 kg C m−2. Under future climate warming an upward shift of vegetation zones may lead to a net ecosystem C uptake from increased biomass and soil development. As a consequence, alpine permafrost environments could act as a net carbon sink in the future, as there is no loss of older or deeper SOC from thawing permafrost.