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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]

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]

Similar to higher plants, microbial autotrophs possess photosynthetic systems that enable them to fix CO2. To measure the activity of microbial autotrophs in assimilating atmospheric CO2, five paddy soils were incubated with14C-labeled CO2for 45 days to determine the amount of14C-labeled organic C being synthesized. The results showed that a significant amount of14C-labeled CO2incorporated into microbial biomass was soil specific, accounting for 0.37%–1.18% of soil organic carbon (14C-labeled organic C range: 81.6–156.9 mg C kg−1of the soil after 45 days). Consequently, high amounts of C-labeled organic C were synthesized (the synthesis rates ranged from 86 to 166 mg C m−2d−1). The amount of atmospheric14CO2incorporated into microbial biomass (14C-labeled microbial biomass) was significantly correlated with organic C components (14C-labeled organic C) in the soil (r = 0.80,p < 0.0001). Our results indicate that the microbial assimilation of atmospheric CO2is an important process for the sequestration and cycling of terrestrial C. Our results showed that microbial assimilation of atmospheric CO2has been underestimated by researchers globally, and that it should be accounted for in global terrestrial carbon cycle models. [ABSTRACT FROM AUTHOR]

The soluble components of rhizodeposition-root exudates-are the most important sources of readily available carbon (C) for rhizosphere microorganisms. The first steps of exudate utilization by microorganisms define all further flows of root C in the soil, including recycling and stabilization. Nevertheless, most studies have traced root exudates C much later after its initial uptake by microorganisms. To understand microbial uptake and utilization of rice root exudates, we traced C incorporated into microbial groups by C profiles of phospholipid fatty acids (PLFAs) within a short time (6 h) after CO pulse labeling. Labeling was conducted five times during three growth stages: active root growth (within the 21 days after transplanting), rapid shoot growth (37 and 45 days), and rapid reproduction (53 and 63 days). C was quickly assimilated throughout the rhizosphere microorganism, and the incorporation rate increased with rice maturity. Despite low redox conditions in paddy soil, fungi outcompeted bacteria in utilizing the root exudates. At all growth stages, fungal PLFAs (18:2 w6, 9c/18:0) showed the highest C levels, whereas actinomycete PLFAs (16:0 10-methyl) showed the lowest C incorporation. Principal component analysis revealed that the rhizosphere microbial community differed among rice growth stages, whereas the whole microbial community remained stable. In conclusion, the rapid incorporation of carbon from root exudates into microorganisms in paddy soils depends on the growth stage of the rice plant and is the first step of C utilization in rice rhizosphere, further defining C utilization and stabilization. [ABSTRACT FROM AUTHOR]

As part of a long-term sloped land use experiment established in 1995 at Taoyuan Agro-ecosystem Research Station (111°26′ E, 28°55′ N) in China, soil samples were collected from three land use types, including cropland (CL), natural forest, and tea plantation. Quantitative polymerase chain reaction and terminal restriction fragment length polymorphism were used to determine the abundance and community composition of amoA-containing bacteria (AOB) and archaea (AOA). The results indicate that land use type induced significant changes in soil potential nitrification rate and community composition, diversity, and abundance of AOB and AOA. Both AOB and AOA community compositions were generally similar between upper and lower slope positions (UP and LP), except within CL. The LP soils had significantly ( p < 0.05) higher diversity and abundance of both AOB and AOA than in the UP. Potential nitrification rate was significantly correlated ( p < 0.05) with diversity and abundance of AOA, but not with AOB. Among land use types, the NO and amoA-containing AOA runoff loss was greatest in CL. Nitrate-N runoff loss was significantly correlated ( p < 0.05) with the loss of AOA amoA copies in the runoff water. Furthermore, relationships between NO-N runoff loss and abundance of AOA but not of AOB at both slope positions were significantly correlated ( p < 0.05). These findings suggest that AOA are more important than AOB in nitrification and NO-N runoff loss in acidic soils across sloped land use types. [ABSTRACT FROM AUTHOR]

Abstract: Information on the input, distribution and fate of photosynthesized carbon (C) in plant–soil systems is essential for understanding their nutrient and C dynamics. Our objectives were to: 1) quantify the input to, and distribution of, photosynthesized C by rice into selected soil C pools by using a C14 continuous labelling technique and 2) determine the influence of the photosynthesized C input on the decomposition of native soil organic carbon (SOC) under laboratory conditions. The amounts of C14 in soil organic C (SOC14) were soil dependent, and ranged from 114.3 to 348.2 mg C kg−1, accounting for 0.73%–1.99% of total SOC after continuous labelling for 80 days. However, the mean SOC14 concentrations in unplanted soils (31.9–64.6 mg kg−1) were accounted for 21.5% of the rice-planted soils. The amounts of C14 in the dissolved organic C (DOC14) and in the microbial biomass C (MBC14), as percentages of SOC14, were 2.21%–3.54% and 9.72%–17.97%, respectively. The DOC14 and MBC14 were 6.72%–14.64% and 1.70%–7.67% of total DOC and MBC respectively after 80-d of rice growth. At 80-d of labelling, the SOC14 concentration was positively correlated with the MBC14 concentration and rice root biomass. Rice growth promotes more photosynthesized (newly-derived) C into soil C pools compared to unplanted soils, reflecting the release of root exudates from rice roots. Laboratory incubation of photosynthesized (plant-derived) C in soil decreased the decomposition of native SOC (i.e. a negative priming effect), in some, but not all cases. If this negative priming effect of the new C on native SOC also occurs in the field in the longer term, paddy soils will probably sequester more C from the atmosphere if more photosynthesized C enters them. [Copyright &y& Elsevier]

Phosphorus (P) application is an indispensable practice in agricultural crop production. However, the effect of P on crop and microbial biomass stoichiometries as well as its role in soil nutrient dynamics, remain poorly understood, thereby limiting our ability to predict the ecological consequences of agricultural practices. Herein, we conducted a P application experiment in a subtropical P-limited paddy soil to examine the effect of P addition on soil nutrient availability as well as on rice and the microbial C:N:P stoichiometries. The results showed that soil dissolved organic carbon (DOC) and available P content were simultaneously affected by P application and rice growth stage. Further, addition of P served to increase rice C and N content while decreasing the C: P and N: P stoichiometric ratios, indicating that P input contributed to rice growth and N absorption. Marked increases in soil microbial biomass carbon (MBC) and phosphorus (MBP) were also observed in soil treated with P. Moreover, the microbial C:N:P stoichiometry increased with rice growth stage, suggesting that changes in microbial and plant nutrient demand throughout the rice plant growth cycle causes competition between microorganisms and plants for N and P. Pearson's correlation and redundancy analyses revealed that available nutrients of soil were intercorrelated and regulated the patterns of rice and microbial C:N:P stoichiometries. In conclusions, P enrichment altered soil nutrient composition, microbial nutrient mining strategies, and rice C:N:P stoichiometry, thereby contributing to rice growth and improving crop yield. • P addition increased rice C and N pool and decreased rice stoichiometric ratios of C: P and N: P. • P input induced C and N limitations in the soil and microbial biomass. • The soil microbial ecological C: N: P stoichiometry varied with the rice growth stage. • Altered soil C: N: P stoichiometr affect the P strategy and N fixation of plants. [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]

Assimilating atmospheric carbon (C) into terrestrial ecosystems is recognized as a primary measure to mitigate global warming. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is the dominant enzyme by which terrestrial autotrophic bacteria and plants fix CO 2 . To investigate the possibility of using RubisCO activity as an indicator of microbial CO 2 fixation potential, a valid and efficient method for extracting soil proteins is needed. We examined three methods commonly used for total soil protein extraction. A simple sonication method for extracting soil protein was more efficient than bead beating or freeze–thaw methods. Total soil protein, RubisCO activity, and microbial fixation of CO 2 in different agricultural soils were quantified in an incubation experiment using 14 C-CO 2 as a tracer. The soil samples showed significant differences in protein content and RubisCO activity, defined as nmol CO 2 fixed g −1 soil min −1 . RubisCO activities ranged from 10.68 to 68.07 nmol CO 2 kg −1 soil min −1 , which were closely related to the abundance of cbbL genes ( r = 0.900, P = 0.0140) and the rates of microbial CO 2 assimilation ( r = 0.949, P = 0.0038). This suggests that RubisCO activity can be used as an indicator of soil microbial assimilation of atmospheric CO 2 . [ABSTRACT FROM AUTHOR]

Abstract: Maintaining a diverse functional and taxonomic microbial community in central to preserving soil quality and for ensuring food security. Growing evidence suggests that organic farming systems possess higher quality soils with robust microbial activity in comparison to conventionally managed systems. Although plastic tunnel greenhouses are widely used, their effects on microbial communities are largely unknown. We examined how four treatments impacted soils and their microbial communities: (1) organic management in greenhouses (Or-Gr) and (2) open fields (Or-Op), and (3) conventional management in greenhouses (Co-Gr) and (4) open fields (Co-Op). We measured physicochemical and microbiological parameters, community-level physiological profiles, and phospholipid fatty acid (PLFAs) contents of soils (0–20 cm depth). Both organic and greenhouse management significantly increased total organic C (SOC), total N, microbial biomass C (MBC) and N (MBN), and basal- and substrate-induced respiration (P < 0.05). Or-Gr had significantly higher total, bacterial (both Gram-positive and -negative), and fungal PLFA concentrations (P < 0.05) than the other treatments. Generally, soil quality followed the series Or-Gr > Or-Op > Co-Gr > Co-Op. MBC, MBN, and PLFA concentrations were positively correlated (r > 0.90, P < 0.01) with SOC, total N, and cation exchange capacity and negatively with soil pH. Organic and greenhouse management had a significant interaction effect. Our findings suggest that greenhouse management should be promoted for food security. [Copyright &y& Elsevier]

Rice rhizodeposition plays an important role in carbon sequestration in paddy soils. However, the pathways through which rice rhizodeposits contribute to soil organic C (SOC) formation are poorly understood because of specific paddy soil conditions. Furthermore, microbial necromass has been largely ignored in studies examining the contribution of rhizodeposits to C sequestration during plant growth. To evaluate the contribution of microbial necromass to SOC formation via rhizodeposition, rice (Oryza sativa L.) plants were continuously labeled with 13CO 2 for 38 days under ambient (aCO 2 , 400 μL L−1) or elevated CO 2 (eCO 2 , 800 μL L−1) in a paddy field at two levels of N fertilization. The distributions of photosynthetic-13C in the shoots and roots, microbial communities, and SOC fractions were quantified. eCO 2 increased plant growth and, consequently, the total 13C incorporated into the shoots, roots, and SOC compared to aCO 2 , while N fertilization (100 kg N ha−1) decreased root biomass and rhizodeposits in the soil and microbial pools, including living biomass (phospholipid fatty acids, PLFA) and microbial necromass (amino sugars). Rhizodeposits were initially immobilized mainly by bacteria and preferentially recovered in fungal necromass (glucosamine). While 13C incorporation into PLFAs was slightly increased during plant growth, 13C in microbial necromass increased greatly between the tillering and booting stages. Fungal necromass, which is less decomposable compared to bacterial residues, was the largest contributor to C sequestration with rhizodeposits via the mineral-associated SOC fraction, particularly under elevated CO 2 without N fertilization. This study reveals the significance of the C pathways from rhizodeposits through fungal necromass and organo-mineral associations for the build up of SOC in paddy fields. • Contribution of rhizodeposits to soil organic C (SOC) under high CO2 is quantified. • 13C was used to trace components in the plant-soil-microbe continuum. • 13C-amino sugars were accumulated in mineral SOC fraction at the booting stage. • Rice rhizodeposits build up SOC via fungal necromass under elevated CO2. • Microbial necromass inclusion in climate models helps SOC-sequestration prediction. [ABSTRACT FROM AUTHOR]

Iron-bound organic carbon (Fe-OC) complexes are important for stabilizing soil organic carbon (SOC) against biodegradation. However, it is unclear how the stabilization of OC and its release from Fe minerals subsequently affect the priming effect on SOC mineralization. To address the knowledge gap, we incubated typical paddy soil for 60 days by adding 2-line ferrihydrite (2LFh) or 6-line Fh (6LFh)-bound glucose, each with both high and low amounts of glucose, under anaerobic conditions. Approximately 21% more CO 2 was derived from 2LFh-bound glucose than from 6LFh-bound glucose. Glucose addition alone stimulated SOC mineralization and caused a positive priming effect (0.27% of SOC). In contrast, 2LFh- and 6LFh-bound glucose inhibited SOC mineralization to both CO 2 and CH 4 and subsequently induced a negative priming effect, ranging from −0.33% to −0.55% SOC. Compared to 2LFh-bound glucose, 6LFh-bound glucose induced a lower priming effect on CO 2 emissions (2-fold lower), which was attributed to the lower Fe-reduction rate of 6LFh and OC released. In addition, the available nutrients adsorbed by 6LFh were more difficult to release than those by 2LFh, which aggravated microbial nutrient limitation, and further decreased microbial activity. The priming effect for CH 4 emissions was directly proportional to the glucose level loaded. The Fe reduction rates were higher in Fh-bound high amount of glucose than that in the Fh-bound low amount of glucose, which subsequently provided more available C sources for methanogens. Thus, Fe minerals have a high capacity for SOC accumulation, as they prevent bound OC from mineralization and decrease native SOC priming. Moreover, the protection of SOC by Fe minerals depended on its crystalline structure and the amount of OC loading. Our results show that promoting the transformation from weakly crystalline Fe oxides to more crystalline forms would increase SOC accumulation and stability over the complete rice-growing period. [Display omitted] • Ferrihydrite (Fh) addition reduced paddy soil organic carbon (SOC) mineralization. • 6-line Fh (6LFh) restricted OC release because it is difficult to reduce 6LFh. • OC bound by 6LFh had a lower mineralization rate than that by 2LFh. • Only glucose addition caused positive SOC priming. • 6LFh-bound glucose caused stronger negative SOC priming than 2LFh-bound glucose. [ABSTRACT FROM AUTHOR]

Carbon and nutrient inputs are required to stimulate the formation and mineralization of soil organic carbon (SOC) through processes related to microbial growth and priming effects (PEs). PEs are thought to affect microbial life strategies, however, the mechanisms underlying their role in SOC formation and microbial dynamics remain largely unknown, particularly in paddy soils. Here, we examined the underlying strategies and response mechanisms of microorganisms in regulating PEs and C accumulation in flooded paddy soil. Levels and stoichiometric ratios of resources were evaluated over a 60-day incubation period. Low (equivalent to 50% soil microbial biomass C [MBC]) and high (500% MBC) doses of 13C-labeled glucose were added to the soil, along with mineral N, P, and S (NPS) fertilizers at five concentrations. Glucose mineralization increased linearly with NPS concentration under both low and high glucose inputs. However, glucose addition without nutrients induced the preferential microbial utilization of the readily available C, leading to negative PEs. Under high-glucose input, the intensity of negative PEs increased with increasing NPS addition (PE: from −460 to −710 mg C kg−1 soil). In contrast, under low-glucose inputs, the intensity of positive PEs increased with increasing NPS addition (PE: 60–100 mg C kg−1 soil). High-glucose input with NPS fertilization favored high-yield microbial strategists (Y-strategists), increasing glucose-derived SOC accumulation. This phenomenon was evidenced by the large quantities of 13C detected in microbial biomass and phospholipid fatty acids (PLFAs), increasing the soil net C balance (from 0.76 to 1.2 g C kg−1). In contrast, low levels of glucose and NPS fertilization shifted the microbial community composition toward dominance of resource-acquisition strategists (A-strategists), increasing SOC mineralization. This was evidenced by 13C incorporation into the PLFAs of gram-positive bacteria, increased activity of N- and P-hydrolases, and positive PEs for acquiring C and nutrients from soil organic matter. Consequently, the soil net C balance decreased from 0.31 to 0.01 g C kg−1 soil. In conclusion, high C input (i.e., 500% MBC), particularly alongside hig NPS addition, increases SOC content via negative priming and microbial-derived C accumulation due to the shift toward Y-strategist communities which efficiently utilize resources. This study highlights the importance of mineral fertilization management when incorporating organic supplements in paddy soils to stimulate microbial turnover and C sequestration. [Display omitted] • Glucose mineralization increases linearly with added N, P, and S concentration. • Low- and high-glucose input without nutrients causes negative priming effects (PEs). • Nutrient addition causes opposite trends in PE at low and high glucose input. • Microorganisms adjust life strategies in response to resource stoichiometry. • High glucose and nutrient inputs lead to negative PEs and a positive net C balance. [ABSTRACT FROM AUTHOR]

Autotrophic microorganisms, which can fix atmospheric CO 2 to synthesize organic carbon, are numerous and widespread in soils. However, the extent and the mechanism of CO 2 fixation in soils remain poorly understood. We incubated five upland and five paddy soils from subtropical China in an enclosed, continuously 14 CO 2 -labeled, atmosphere and measured 14 CO 2 incorporated into soil organic matter (SOC 14 ) and microbial biomass (MBC 14 ) after 110 days. The five upland soils supported dominant crops soils (maize, wheat, sweet potato, and rapeseed) in the region, while all paddy soils were cultivated in a regime consisting of permanently-flooded double-cropping rice cultivation. The upland and paddy soils represented typical soil types (fluvisols and ultisols) and three landforms (upland, hill, and low mountain), ranging in total carbon from low (<10 g kg −1 soil organic carbon) to medium (10–20 g kg −1 ) to high (>20 g kg −1 ). Substantial amounts of 14 CO 2 were fixed into SOC 14 (mean 20.1 ± 7.1 mg C kg −1 in upland soil, 121.1 ± 6.4 mg C kg −1 in paddy soil) in illuminated soils (12 h light/12 h dark), whereas no 14 C was fixed in soils incubated in continuous darkness. We concluded that the microbial CO 2 fixation was almost entirely phototrophic rather than chemotrophic. The rate of SOC 14 synthesis was significantly higher in paddy soils than in upland soils. The SOC 14 comprised means of 0.15 ± 0.01% (upland) and 0.65 ± 0.03% (paddy) of SOC. The extent of 14 C immobilized as MBC 14 and that present as dissolved organic C (DOC 14 ) differed between soil types, accounting for 15.69–38.76% and 5.54–18.37% in upland soils and 15.57–40.03% and 3.67–7.17% of SOC 14 in paddy soils, respectively. The MBC 14 /MBC and DOC 14 /DOC were 1.76–5.70% and 1.69–5.17% in the upland soils and 4.23–28.73% and 5.65–14.30% in the paddy soils, respectively. Thus, the newly-incorporated C stimulated the dynamics of DOC and MBC more than the dynamics of SOC. The SOC 14 and MBC 14 concentrations were highly significantly correlated ( r = 0.946; P < 0.0001). We conclude that CO 2 uptake by phototrophic soil microorganisms can contribute significantly to carbon assimilation in soil, and so warrants further future study. [ABSTRACT FROM AUTHOR]

Labile carbon (C) inputs affect the soil carbon:nitrogen (C:N) ratio and microbial stoichiometric homeostasis, which control the intensity and direction of the priming effect (PE). Here, we clarified how soil microorganisms regulate enzyme production and PE to maintain the C:N stoichiometric balance. Specifically, we conducted an incubation experiment by adding 13C-labeled glucose to four long-term fertilized paddy soils: no fertilization; fertilization with mineral nitrogen, phosphorus, and potassium (NPK); NPK combined with straw; and NPK with manure (NPKM). After glucose addition, the dissolved organic carbon-to-ammonium (DOC:NH 4 +) ratio (24–39) initially increased, but subsequently decreased after day 2 following glucose exhaustion. In parallel, the microbial C:N imbalance [(DOC:NH 4 +):(microbial biomass C:microbial biomass N)] rapidly decreased from day 2 (4.6–7.2) to day 20 (<0.5). Thus, microorganisms became C limited after 20 days of incubation. Excess C, resulting from glucose addition, increased N-hydrolase (chitinase) production and N mining from soil organic matter (SOM) through positive PEs. However, C hydrolase (β -1,4-glucosidase and β-xylosidase) activity increased, while that of N hydrolase (chitinase) decreased, following glucose exhaustion. Consequently, the C:N microbial biomass ratio increased as the DOC:NH 4 + ratio decreased, leading to negative PEs. NPKM-fertilized soil had the largest cumulative PE (2.3% of soil organic carbon) because it had the highest microbial biomass and iron (Fe) reduction rate. Thus, this increased N mining from SOM maintained the microbial C:N stoichiometric balance. We concluded that soil microorganisms regulate C- and N-hydrolase production to control the intensity and direction of PE, maintaining the C:N stoichiometric balance in response to labile C inputs. [Display omitted] • Glucose input into paddy soil increased DOC:NH 4 + ratio and microbial biomass (MB). • High C availability increased activity of N-hydrolases and priming effect (PE). • Decreased MB and N-hydrolase activity induced negative PE with labile C exhaustion. • Highest cumulative PE observed in combined NPK and chicken manure fertilized soil. • C and N hydrolase production is key for regulating C:N stoichiometry of MB and PE. [ABSTRACT FROM AUTHOR]

Rationale: Soil microbial heterotrophic C-CO 2 respiration is important for C cycling. Soil CO 2 differentiation and quantification are vital for understanding soil C cycling and CO 2 emission mitigation. Presently, soil microbial respiration (SR) quantification models are based on native soil organic matter (SOM) and require consistent monitoring of δ 13 C and CO 2 ., Methods: We present a new apparatus for achieving in situ soil static chamber incubation and simultaneous CO 2 and δ 13 C 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 CO 2 and δ 13 C 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 CO 2 and δ 13 C 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 13 C-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 CO 2 respiration, requiring less extensive lab resources than GC/IRMS., Competing Interests: The authors declare no conflict of interest., (© 2023 The Authors. Analytical Science Advances published by Wiley‐VCH GmbH.)

Tillage is a common agricultural practice affecting soil structure and biogeochemistry. To evaluate how tillage affects soil microbial CO2 fixation, we incubated and continuously labelled samples from two paddy soils and two upland soils subjected to simulated conventional tillage (CT) and no-tillage (NT) treatments. Results showed that CO2 fixation ((14)C-SOC) in CT soils was significantly higher than in NT soils. We also observed a significant, soil type- and depth-dependent effect of tillage on the incorporation rates of labelled C to the labile carbon pool. Concentrations of labelled C in the carbon pool significantly decreased with soil depth, irrespective of tillage. Additionally, quantitative PCR assays revealed that for most soils, total bacteria and cbbL-carrying bacteria were less abundant in CT versus NT treatments, and tended to decrease in abundance with increasing depth. However, specific CO2 fixation activity was significantly higher in CT than in NT soils, suggesting that the abundance of cbbL-containing bacteria may not always reflect their functional activity. This study highlights the positive effect of tillage on soil microbial CO2 fixation, and the results can be readily applied to the development of sustainable agricultural management.

The effect of different cropping systems on CO2 fixation by soil microorganisms was studied by comparing soils from three exemplary cropping systems after 10 years of agricultural practice. Studied cropping systems included: continuous cropping of paddy rice (rice-rice), rotation of paddy rice and rapeseed (rice-rapeseed), and rotated cropping of rapeseed and corn (rapeseed-corn). Soils from different cropping systems were incubated with continuous (14)C-CO2 labeling for 110 days. The CO2-fixing bacterial communities were investigated by analyzing the cbbL gene encoding ribulose-1,5-bisphosphate carboxylase oxygenase (RubisCO). Abundance, diversity and activity of cbbL-carrying bacteria were analyzed by quantitative PCR, cbbL clone libraries and enzyme assays. After 110 days incubation, substantial amounts of (14)C-CO2 were incorporated into soil organic carbon ((14)C-SOC) and microbial biomass carbon ((14)C-MBC). Rice-rice rotated soil showed stronger incorporation rates when looking at (14)C-SOC and (14)C-MBC contents. These differences in incorporation rates were also reflected by determined RubisCO activities. (14)C-MBC, cbbL gene abundances and RubisCO activity were found to correlate significantly with (14)C-SOC, indicating cbbL-carrying bacteria to be key players for CO2 fixation in these soils. The analysis of clone libraries revealed distinct cbbL-carrying bacterial communities for the individual soils analyzed. Most of the identified operational taxonomic units (OTU) were related to Nitrobacter hamburgensis, Methylibium petroleiphilum, Rhodoblastus acidophilus, Bradyrhizobium, Cupriavidus metallidurans, Rubrivivax, Burkholderia, Stappia, and Thiobacillus thiophilus. OTUs related to Rubrivivax gelatinosus were specific for rice-rice soil. OTUs linked to Methylibium petroleiphilum were exclusively found in rice-rapeseed soil. Observed differences could be linked to differences in soil parameters such as SOC. We conclude that the long-term application of cropping systems alters underlying soil parameters, which in turn selects for distinct autotrophic communities.

Soils were incubated for 80 days in a continuously labeled (14)CO(2) atmosphere to measure the amount of labeled C incorporated into the microbial biomass. Microbial assimilation of (14)C differed between soils and accounted for 0.12% to 0.59% of soil organic carbon (SOC). Assuming a terrestrial area of 1.4 × 10(8) km(2), this represents a potential global sequestration of 0.6 to 4.9 Pg C year(-1). Estimated global C sequestration rates suggest a "missing sink" for carbon of between 2 and 3 Pg C year(-1). To determine whether (14)CO(2) incorporation was mediated by autotrophic microorganisms, the diversity and abundance of CO(2)-fixing bacteria and algae were investigated using clone library sequencing, terminal restriction fragment length polymorphism (T-RFLP), and quantitative PCR (qPCR) of the ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) gene (cbbL). Phylogenetic analysis showed that the dominant cbbL-containing bacteria were Azospirillum lipoferum, Rhodopseudomonas palustris, Bradyrhizobium japonicum, Ralstonia eutropha, and cbbL-containing chromophytic algae of the genera Xanthophyta and Bacillariophyta. Multivariate analyses of T-RFLP profiles revealed significant differences in cbbL-containing microbial communities between soils. Differences in cbbL gene diversity were shown to be correlated with differences in SOC content. Bacterial and algal cbbL gene abundances were between 10(6) and 10(8) and 10(3) to 10(5) copies g(-1) soil, respectively. Bacterial cbbL abundance was shown to be positively correlated with RubisCO activity (r = 0.853; P < 0.05), and both cbbL abundance and RubisCO activity were significantly related to the synthesis rates of [(14)C]SOC (r = 0.967 and 0.946, respectively; P < 0.01). These data offer new insights into the importance of microbial autotrophy in terrestrial C cycling.