Inorganic carbon is overlooked in global soil carbon research: A bibliometric analysis.

Soils are a major player in the global carbon (C) cycle and climate change by functioning as a sink or a source of atmospheric carbon dioxide (CO2). The largest terrestrial C reservoir in soils comprises two main pools: organic (SOC) and inorganic C (SIC), each having distinct fates and functions but with a large disparity in global research attention. This study quantified global soil C research trends and the proportional focus on SOC and SIC pools based on a bibliometric analysis and raise the importance of SIC pools fully underrepresented in research, applications, and modeling. Studies on soil C pools started in 1905 and has produced over 47,000 publications (>1.7 million citations). Although the global C stocks down to 2 m depth are nearly the same for SOC and SIC, the research has dominantly examined SOC (>96 % of publications and citations) with a minimal share on SIC (<4%). Approximately 40 % of the soil C research was related to climate change. Despite poor coverage and publications, the climate change-related research impact (citations per document) of SIC studies was higher than that of SOC. Mineral associated organic carbon, machine learning, soil health, and biochar were the recent top trend topics for SOC research (2020-2023), whereas digital soil mapping, soil properties, soil acidification, and calcite were recent top trend topics for SIC. SOC research was contributed by 151 countries compared to 88 for SIC. As assessed by publications, soil C research was mainly concentrated in a few countries, with only 9 countries accounting for 70 % of the research. China and the USA were the major producers (45 %), collaborators (37 %), and funders of soil C research. SIC is a long-lived soil C pool with a turnover rate (leaching and recrystallization) of more than 1000 years in natural ecosystems, but intensive agricultural practices have accelerated SIC losses, making SIC an important player in global C cycle and climate change. The lack of attention and investment towards SIC research could jeopardize the ongoing efforts to mitigate climate change impacts to meet the 1.5-2.0 °C targets under the Paris Climate Agreement of 2015. This bibliographic study calls to expand the research focus on SIC and including SIC fluxes in C budgets and models, without which the representation of the global C cycle is incomplete.

Glycoproteins of arbuscular mycorrhiza for soil carbon sequestration: Review of mechanisms and controls.

Glycoproteins, e.g., glomalin related soil proteins (GRSP), are sticky organic substances produced by arbuscular mycorrhizal fungi (AMF). This review summarizes the information on i) the biochemical nature, physical state and origin of GRSP, ii) GRSP decomposition and residence time in soil, iii) GRSP functions, in particular the physical, chemical, and biochemical roles for soil aggregation and carbon (C) sequestration, and finally iv) how land use and agricultural management affect GRSP production and subsequently, organic C sequestration. GRSP augment soil quality by increasing water holding capacity, nutrient storage and availability, microbial and enzymatic activities, and microbial production of extracellular polysaccharides. After release into the soil, GRSP become prone to microbial decomposition due to stabilization with organic matter and sesquioxides, and thereby increasing the residence time between 6 and 42 years. Temperate soils contain 2-15 mg GRSP g-1, whereas arid and semiarid grasslands amount for 0.87-1.7 mg g-1, and GRSP are lower in desert soils. GRSP content is highest in acidic soils as compared to neutral and calcareous soils. Conservation tillage, organic fertilizers and AMF inhabiting crops (e.g. maize, sorghum, soybean, and wheat) increase GRSP production and transform C into stable forms, thereby sustaining soil health and reducing CO2 emissions. Crop rotations with non-mycorrhizal species (e.g. rapeseed) and fallow soils reduce AMF growth and consequently, the GRSP production. The GRSP production increases under nutrient and water deficiency, soil warming and elevated CO2. In the context of global climate change, increased C sequestration through GRSP induced aggregate formation and organic matter stabilization prolong the mean residence time of soil C. Protecting soils against degradation under intensive land use, stable aggregate formation, and prolonging the residence time of C calls for strategies that maximize GRSP production and functions based on reduced tillage, AMF-relevant crop rotations and organic farming.

Temperature sensitivity of SOM decomposition is linked with a K-selected microbial community.

Temperature sensitivity (Q10 ) of soil organic matter (SOM) decomposition is a crucial parameter to predict the fate of soil carbon (C) under global warming. Nonetheless, the response pattern of Q10 to continuous warming and the underlying mechanisms are still under debate, especially considering the complex interactions between Q10 , SOM quality, and soil microorganisms. We examined the Q10 of SOM decomposition across a mean annual temperature (MAT) gradient from -1.9 to 5.1°C in temperate mixed forest ecosystems in parallel with SOM quality and bioavailability, microbial taxonomic composition, and functional genes responsible for organic carbon decomposition. Within this temperature gradient of 7.0°C, the Q10 values increased with MAT, but decreased with SOM bioavailability. The Q10 values increased with the prevalence of K-strategy of soil microbial community, which was characterized by: (i) high ratios of oligotrophic to copiotrophic taxa, (ii) ectomycorrhizal to saprotrophic fungi, (iii) functional genes responsible for degradation of recalcitrant to that of labile C, and (iv) low average 16S rRNA operon copy number. Because the recalcitrant organic matter was mainly utilized by the K-strategists, these findings independently support the carbon quality-temperature theory from the perspective of microbial taxonomic composition and functions. A year-long incubation experiment was performed to determine the response of labile and recalcitrant C pools to warming based on the two-pool model. The decomposition of recalcitrant SOM was more sensitive to increased temperature in southern warm regions, which might attribute to the dominance of K-selected microbial communities. It implies that climate warming would mobilize the larger recalcitrant pools in warm regions, exacerbating the positive feedback between increased MAT and CO2 efflux. This is the first attempt to link temperature sensitivity of SOM decomposition with microbial eco-strategies by incorporating the genetic information and disentangling the complex relationship between Q10 and soil microorganisms.

Conversion of cropland to natural vegetation boosts microbial and enzyme activities in soil.

The trends of enzyme activities and litter chemistry after abandonment of arable soil and succession of natural vegetation were studied in a deciduous forest zone (Moscow region, Russia). The Luvic Phaeozem chronosequence included an arable field, 3 fields with increasing abandonment periods (7, 11, 35 years), and the soil under natural forest as a reference site (never used for cropland). The activities of four hydrolytic enzymes (ß-glucosidase, cellobiohydrolase, ß-galactosidase, chitinase) in the topsoil (0-5 cm) were compared with chemical functional groups of plant litter identified by 13C NMR spectra. The total enzyme activity increased 5-fold during 35 years of postagricultural restoration of arable soil. The share of C-cycle enzymes with "narrower" substrate specificity (cellobiohydrolase and chitinase) increased during 35 years from 19% to 42% of the total enzyme pool. We explain this gain by the increase in the diversity and recalcitrance of organic compounds in the plant litter from cropland to natural forest. Aromaticity index of plant litter built up remarkably: from 0.11 in the young abandoned land to 0.14-0.15 in the 35-year abandoned land and the reference site. The share of enzymes with a "narrower" substrate specificity correlated closely with the portions of aryl C, O-aryl C, and carbonyl C functional groups in plant litter. Accordingly, the succession of natural vegetation on abandoned cropland leads to strong diversification of the plant litter composition and a corresponding production of soil enzymes with "narrower" substrate specificity.

Carbon cost of collective farming collapse in Russia.

The collapse of collective farming in Russia after 1990 and the subsequent economic crisis led to the abandonment of more than 45 million ha of arable lands (23% of the agricultural area). This was the most widespread and abrupt land use change in the 20th century in the northern hemisphere. The withdrawal of land area from cultivation led to several benefits including carbon (C) sequestration. Here, we provide a geographically complete and spatially detailed analysis of C sequestered in these abandoned lands. The average C accumulation rate in the upper 20 cm of mineral soil was 0.96 ± 0.08 Mg C ha(-1)  yr(-1) for the first 20 years after abandonment and 0.19 ± 0.10 Mg C ha(-1)  yr(-1) during the next 30 years of postagrogenic evolution and natural vegetation establishment. The amount of C sequestered over the period 1990-2009 accounts to 42.6 ± 3.8 Tg C per year. This C sequestration rate is equivalent to ca. 10% of the annual C sink in all Russian forests. Furthermore, it compensates all fire and postfire CO2 emissions in Russia and covers about 4% of the global CO2 release due to deforestation and other land use changes. Our assessment shows a significant mitigation of increasing atmospheric CO2 by prolonged C accumulation in Russian soils caused by collective farming collapse.

Influence of freeze-thaw events on carbon dioxide emission from soils at different moisture and land use.

BACKGROUND: The repeated freeze-thaw events during cold season, freezing of soils in autumn and thawing in spring are typical for the tundra, boreal, and temperate soils. The thawing of soils during winter-summer transitions induces the release of decomposable organic carbon and acceleration of soil respiration. The winter-spring fluxes of CO2 from permanently and seasonally frozen soils are essential part of annual carbon budget varying from 5 to 50%. The mechanisms of the freeze-thaw activation are not absolutely clear and need clarifying. We investigated the effect of repeated freezing-thawing events on CO2 emission from intact arable and forest soils (Luvisols, loamy silt; Central Germany) at different moisture (65% and 100% of WHC). RESULTS: Due to the measurement of the CO2 flux in two hours intervals, the dynamics of CO2 emission during freezing-thawing events was described in a detailed way. At +10 degrees C (initial level) in soils investigated, carbon dioxide emission varied between 7.4 to 43.8 mg C m-2h-1 depending on land use and moisture. CO2 flux from the totally frozen soil never reached zero and amounted to 5 to 20% of the initial level, indicating that microbial community was still active at -5 degrees C. Significant burst of CO2 emission (1.2-1.7-fold increase depending on moisture and land use) was observed during thawing. There was close linear correlation between CO2 emission and soil temperature (R2 = 0.86-0.97, P < 0.001). CONCLUSION: Our investigations showed that soil moisture and land use governed the initial rate of soil respiration, duration of freezing and thawing of soil, pattern of CO2 dynamics and extra CO2 fluxes. As a rule, the emissions of CO2 induced by freezing-thawing were more significant in dry soils and during the first freezing-thawing cycle (FTC). The acceleration of CO2 emission was caused by different processes: the liberation of nutrients upon the soil freezing, biological activity occurring in unfrozen water films, and respiration of cold-adapted microflora.