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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]

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]

The current carbon dioxide (CO 2 ) evolution-based standard method for determining biodegradable microplastics (MPs) degradation neglects its priming effect on soil organic matter decomposition, which misestimates their biodegradability. Here, a 13 C natural abundance method was used to estimate the mineralization of poly(lactic acid) (PLA) MP in various agricultural soils, and to trace its utilization in different microbial groups. In alkaline soils, the PLA-derived CO 2 emissions increased with increasing soil carbon/nitrogen (C/N) ratios, and the mineralization of PLA MP concentrations ranged from 3-33 %, whereas the CO 2 evolution method probably over- or under-estimated the mineralization of PLA in alkaline soils with different soil C/N ratios. Low PLA mineralization (1-5 %) were found in the acidic soil, and the standard method largely overestimated the mineralization of PLA MP by 1.3- to 3.3-fold. Moreover, the hydrolysate of PLA MP was preferentially assimilated by Gram-negative bacteria, but Gram-positive bacterial decomposition mainly contributed to the release of PLA-derived CO 2 at low MP concentrations (≤ 1 %). Overall, the 13 C natural abundance method appears to be suitable for tracking the mineralization and microbial utilization of biodegradable PLA in soils, and the PLA-derived C is mainly assimilated and decomposed by bacterial groups., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)