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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]

Rationale: The amino acid–nitrogen (AA‐N) isotope analysis of naturally abundant or isotope‐labeled samples is indispensable for tracing nitrogen transfer in soil nitrogen biogeochemical cycling processes. Despite the usefulness of AA‐N isotope analysis, the preparation methods are complex and time‐consuming, and necessitate the use of toxic reagents. Methods: We present an improved, rapid method for AA‐N isotope analysis with high precision. At a high pH, AA‐N was released and oxidized to N2O using ClO− under vacuum. Additionally, purge‐and‐trap isotope ratio mass spectrometry was used to analyze N2O. Moreover, we investigated the effect of various factors on the N2O conversion process with glycine and applied the results to seven representative single‐N AAs (alanine, serine, cysteine, aspartic acid, glutamic acid, leucine, and phenylalanine) and five poly‐N AAs (lysine, arginine, histidine, tryptophan, and asparagine), as well as side‐chain analogs, blank reagent, and other N forms. Results: The concentration of ClO− and the pH were determined to be crucial factors for achieving desirable AA‐N to N2O conversion efficiencies. Glycine‐N had the highest N2O yield of 70%, with isotopic results consistent with those of the reference values at a high precision (within 0.5‰ for natural abundance and 0.01 atom% for 15N‐enrichment) at the nanomolar N level. Additionally, the α‐NH2 AAs were labile, and the single‐N AAs were more easily converted to N2O than poly‐N AAs. With the exception of γ‐aminobutyric acid, the N2O conversion efficiencies of the side‐chain N analogs were very low (below 5%). This method was also applicable to the 15N analysis of the total free AAs in complex soil samples without interference from analytical blanks and other forms of N. Conclusions: Our method is highly selective for the α‐NH2 groups of an amino acid, and the oxidation of the side chain is difficult. In addition, the method is sensitive, rapid, and convenient, and does not require toxic reagents. [ABSTRACT FROM AUTHOR]

Paddies contain 78% higher organic carbon (C) stocks than adjacent upland soils, and iron (Fe) plaque formation on rice roots is one of the mechanisms that traps C. The process sequence, extent and global relevance of this C stabilization mechanism under oxic/anoxic conditions remains unclear. We quantified and localized the contribution of Fe plaque to organic matter stabilization in a microoxic area (rice rhizosphere) and evaluated roles of this C trap for global C sequestration in paddy soils. Visualization and localization of pH by imaging with planar optodes, enzyme activities by zymography, and root exudation by 14C imaging, as well as upscale modeling enabled linkage of three groups of rhizosphere processes that are responsible for C stabilization from the micro‐ (root) to the macro‐ (ecosystem) levels. The 14C activity in soil (reflecting stabilization of rhizodeposits) with Fe2+ addition was 1.4–1.5 times higher than that in the control and phosphate addition soils. Perfect co‐localization of the hotspots of β‐glucosidase activity (by zymography) with root exudation (14C) showed that labile C and high enzyme activities were localized within Fe plaques. Fe2+ addition to soil and its microbial oxidation to Fe3+ by radial oxygen release from rice roots increased Fe plaque (Fe3+) formation by 1.7–2.5 times. The C amounts trapped by Fe plaque increased by 1.1 times after Fe2+ addition. Therefore, Fe plaque formed from amorphous and complex Fe (oxyhydr)oxides on the root surface act as a "rusty sink" for organic matter. Considering the area of coverage of paddy soils globally, upscaling by model revealed the radial oxygen loss from roots and bacterial Fe oxidation may trap up to 130 Mg C in Fe plaques per rice season. This represents an important annual surplus of new and stable C to the existing C pool under long‐term rice cropping. [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]