Long-term geothermal warming reduced stocks of carbon but not nitrogen in a subarctic forest soil

Soil organic carbon fractions in China: Spatial distribution, drivers, and future changes

Soil is the world's largest terrestrial carbon pool and plays an important role in the global carbon cycle, which may be greatly affected by global change. Recently, research frameworks have indicated that division of soil organic carbon (SOC) into two forms particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) can help us better understand SOC cycle. However, there is a lack of the use of meta-analysis combined with machine learning models to explore the spatial distribution of SOC fractions at large scales. Based on 356 studies conducted in Chinese terrestrial ecosystems, we performed a meta-analysis of extracted data and measured data combined with machine learning models to reveal the spatial distribution of soil POC density (POCD) and MAOC density (MAOCD) and the main drivers of variations in POCD and MAOCD. Our study demonstrated that POCD and MAOCD in China's soil were 3.24 and 2.61 kg m-2, with stocks of 31.10 and 25.06 Pg, respectively. Climate, soil, and vegetation properties together explained 44.9 % and 27.2 % of the variation in POCD and MAOCD, respectively. Climate was more important than other variables in controlling the changes in POCD, with mean annual temperature being specifically the main driver. Soil, however, was more important than other variables in controlling changes in MAOCD, with soil clay content being the main driver. Compared to the other climate scenarios, the rate of change in POCD and MAOCD was higher with a 1.5 °C increase in temperature. In the future, we should pay more attention to the impact of climate change on POCD, which provides a theoretical basis for achieving the "dual-carbon" target. Our study contributes to the understanding of the potential mechanisms of the changes in SOC fractions under global change and provides useful information for future prediction models to simulate the impacts of global change.

High level of winter warming aggravates soil carbon, nitrogen loss and changes greenhouse gas emission characteristics in seasonal freeze-thaw farmland soil

Changes in the soil environment caused by winter warming is affecting the carbon and nitrogen cycles of seasonal freeze-thaw farmland soil. A field experiment was conducted in a seasonal freeze-thaw farmland soil of northeast China to investigate the effects caused from different levels of warming (W1 + 1.77 °C, W2 + 0.69 °C and C + 0 °C) on soil carbon and nitrogen dynamics, microbial biomass and greenhouse gases fluxes. During the early and middle winter, the contents of all kinds of soil carbon and nitrogen (Ammonium, nitrate, total nitrogen, dissolved organic carbon, readily oxidizable organic carbon and soil organic carbon) tended to increase with the increase of warming level, while during the late winter, their contents under different temperature treatments roughly present trend of W2 ≥C > W1. Except for the late thawing period, warming increased the contents of soil microbial biomass carbon and nitrogen, during the late thawing period, with the increase of warming level, MBC and MBN decreased significantly. Warming would stimulate the release of greenhouse gases from soil. But due to the differences of soil environmental conditions in each period and soil nutrient dynamics under different treatments, which made the effects of different levels of warming on soil GHGs fluxes in different periods are different. Our study suggested that low-level warming improved the availability of soil carbon and nitrogen, increased the contents of microbial biomass and greenhouse gas emissions. However, although high-level winter warming showed a similar phenomenon in the early and middle winter to the low-level warming, during the late winter, high-level warming increased soil nutrients loss and broke the seasonal coupling relationship between crop nutrient acquisition and soil microbial nutrient supply, and even led to the adaptation of soil CO2 release to it. This is of great significance for exploring the carbon and nitrogen cycle mechanisms of global terrestrial ecosystem.

The legacy of one hundred years of climate change for organic carbon stocks in global agricultural topsoils

Soil organic carbon (SOC) of agricultural soils is observed to decline in many parts of the world. Understanding the reasons behind such losses is important for SOC accounting and formulating climate mitigation strategies. Disentangling the impact of last century's climate change from effects of preceding land use, management changes and erosion is difficult and most likely impossible to address in observations outside of warming experiments. However, the record of last century's climate change is available for every part of the globe, so the potential effect of climate change on SOC stocks can be modelled. In this study, an established and validated FAO framework was used to model global agricultural topsoil (0-30 cm) SOC stock dynamics from 1919 to 2018 as attributable to climate change. On average, global agricultural topsoils could have lost 2.5 ± 2.3 Mg C ha^-1 (3.9 ± 5.4%) with constant net primary production (NPP) or 1.6 ± 3.4 Mg C ha^-1 (2.5 ± 5.5%) when NPP was considered to be modified by temperature and precipitation. Regional variability could be explained by the complex patterns of changes in temperature and moisture, as well as initial SOC stocks. However, small average SOC losses have been an intrinsic and persistent feature of climate change in all climatic zones. This needs to be taken into consideration in reporting or accounting frameworks and halted in order to mitigate climate change and secure soil health.