Search

Soil carbohydrates, such as glucose and fructose, are essential components of soil organic matter (SOM). They serve as the primary carbon and energy sources for soil microorganisms and play a crucial role in regulating SOM stability, both directly and indirectly. Despite their ecological importance, the transformation processes and forms of soil carbohydrates are still not well-understood, despite centuries of extensive study. The use of stable carbon isotopes as natural tracers offers distinct advantages over radioisotope techniques, including safety, cleanliness, and ease of control. As a result, stable carbon isotope analysis has been widely employed in soil carbon cycling research. The examination of carbon isotopic variation in carbohydrates using stable carbon isotope tracers contributes to our understanding of carbohydrate transformation and accumulation processes within the plant-soil-microorganism system. It also enhances our comprehension of soil carbon metabolism. This paper provides an introduction to the types, sources, and pretreatment methods of soil carbohydrates. Furthermore, it reviews the theory and methods of stable carbon isotope analysis as applied to carbohydrates and their relevance to the study of soil carbon cycling. Lastly, it offers a comprehensive summary of current research challenges and prospects for future advancements in soil carbon cycling research.

Microorganisms regulate soil organic matter (SOM) formation through accumulation and decomposition of microbial necromass, which is directly and indirectly affected by elevated CO2 and N fertilization. We investigated the role of microorganisms in SOM formation by analyzing 13C recovery in microorganisms and carbon pools in paddy soil under two CO2 levels, with and without N fertilization, after continuous 13CO2 labelling was stopped. Microbial turnover transferred 13C from living microbial biomass (determined by the decrease in phospholipid fatty acids) to necromass (determined by the increase in amino sugars). 13C incorporation in fungal living biomass and necromass was higher than that in bacteria. Bacterial turnover was faster than necromass decomposition, resulting in net necromass accumulation over time; fungal necromass remained stable. Elevated CO2 and N fertilization increased the net accumulation of bacterial, but not fungal, necromass. CO2 levels, but not N fertilization, significantly affected 13C incorporation in SOM pools. Elevated CO2 increased 13C in particulate organic matter via the roots, and in the mineral-associated organic matter (MAOM) via bacterial, but not fungal, necromass. Overall, bacterial necromass plays a dominant role in the MAOM formation response to elevated CO2 because bacteria are sensitive to elevated CO2. [ABSTRACT FROM AUTHOR]

Rationale: Soil microbial heterotrophic C‐CO2 respiration is important for C cycling. Soil CO2 differentiation and quantification are vital for understanding soil C cycling and CO2 emission mitigation. Presently, soil microbial respiration (SR) quantification models are based on native soil organic matter (SOM) and require consistent monitoring of δ13C and CO2. Methods: We present a new apparatus for achieving in situ soil static chamber incubation and simultaneous CO2 and δ13C monitoring by cavity ring‐down spectroscopy (CRDS) coupled with a soil culture and gas introduction module (SCGIM) with multi‐channel. After a meticulous five‐point inter‐calibration, the repeatability of CO2 and δ13C values by using CRDS‐SCGIM were determined, and compared with those obtained using gas chromatography (GC) and isotope ratio mass spectrometry (IRMS), respectively. We examined the method regarding quantifying SR with various concentrations and enrichment of glucose and then applied it to investigate the responses of SR to the addition of different exogenous organic materials (glucose and rice residues) into paddy soils during a 21‐day incubation. Results: The CRDS‐SCGIM CO2 and δ13C measurements were conducted with high precision (< 1.0 µmol/mol and 1‰, respectively). The optimal sampling interval and the amount added were not exceeded 4 h and 200 mg C/100 g dry soil in a 1 L incubation bottle, respectively; the 13C‐enrichment of 3%–7% was appropriate. The total SR rates observed were 0.6–4.2 µL/h/g and the exogenous organic materials induced ‐49%–28% of priming effects in native SOM mineralisation. Conclusions: Our results show that CRDS‐SCGIM is a method suitable for the quantification of soil microbial CO2 respiration, requiring less extensive lab resources than GC/IRMS. [ABSTRACT FROM AUTHOR]

Root exudates can significantly modify microbial activity and soil organic matter (SOM) mineralization. However, how root exudates and their C/N stoichiometric ratios control rice field (paddy) soil C mineralization is poorly understood. This study used a mixture of glucose, oxalic acid, and alanine as root exudate mimics for three C/N stoichiometric ratios (CN6, CN10, and CN80) to explore the underlying mechanisms involved in SOM mineralization. The input of root exudates enhanced CO2 emissions by 1.8–2.3‐fold that of soil with only C additions (C‐only). Artificial root exudates with low C/N ratios (CN6 and CN10) increased the metabolic quotient (qCO2) by 12% over those with higher stoichiometric ratios (CN80 and C‐only), suggesting a relatively high energy demand for microorganisms to acquire organic N from SOM by increasing N‐hydrolase production. The increase of stoichiometric ratios of C‐ to N‐hydrolase [β‐1,4‐glucosidase to β‐1,4‐N‐acetyl glucosaminidase (NAG)] promoted SOM degradation compared to those involved in organic C‐ and N‐degradation, which had a significant positive correlation with qCO2. The stoichiometric ratios of microbial biomass were positively correlated with C use efficiency, indicating root exudates with higher C/N ratios provide an undersupply of N for microorganisms that trigger the release of N‐degrading extracellular enzymes. Our findings showed that the C/N stoichiometry of root exudates controlled SOM mineralization by affecting the specific response of the microbial biomass through the activity of C‐ and N‐releasing extracellular enzymes to adjust the microbial C/N ratio. [ABSTRACT FROM AUTHOR]