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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]

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]

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]

The photosynthate carbon (C) released in the rhizosphere plays a crucial role in C sequestration, microbial activities and nutrient availability in soil. Nitrogen (N) fertilization modifies the allocation and dynamics of photosynthates in paddy rice systems, but these effects depend on plant growth stages. Rice (Oryza sativa L.) plants were pulse labelled with 13CO2 at the tillering, elongation, heading and grain‐filling stages with 0 and 225 kg N ha−1 fertilizer. The plants and soil were sampled shortly after each pulse labelling and at harvest. Relative 13C (as % of assimilated C) in the roots and rhizosphere soil was largest at the early growth stage (tillering) and subsequently decreased. At harvest, 68% of the rhizodeposited C remained in bulk soil without N fertilizer, which corresponded to 6.2% of the net assimilated 13C. The absolute amount of net belowground C input (root + rhizodeposition) by rice was 268 and 468 kg C ha−1 under 0 and 225 kg N ha−1 fertilizer, of which rhizodeposition accounted for 60 and 40%, respectively. We concluded that N fertilization raised the belowground C input by rice mainly by increasing root biomass rather than by rhizodeposition. Highlights: Rice photosynthesis‐derived carbon (C) was quantified in soil by multiple pulse labelling with 13CO2Young rice plants allocated more assimilates into the soil compared to mature plantsNitrogen deficiency led to greater C retention in bulk soil than in the rhizosphereNitrogen fertilization increased the net belowground C input mainly with larger root biomass [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]

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]

Carbon dioxide (CO 2 ) and methane (CH 4 ) production in paddy soils play a crucial role in the global carbon (C) cycle and greenhouse gas emissions. A rhizosphere priming effect (RPE) may change these emissions, but the relationships between RPE, CH 4 emission, and the effect of N fertilization are unknown. We investigated the RPE on CO 2 and CH 4 emissions and their dependence from N fertilization in a 13 CO 2 continuous labelling experiment by partitioning total CO 2 and CH 4 derived from roots and soil organic matter (SOM). Because of plant-derived CO 2 , rice plants strongly increased total CO 2 emission compared to that from unplanted soil. SOM-derived CO 2 and CH 4 increased in the presence of roots but decreased after N fertilization. The RPE for CO 2 at an early growth stage (≤40 days) was negative: −1.3 and −1.9 mg C day −1 kg −1 soil without and with N fertilization, respectively. However, 52 days after transplanting, RPE for CO 2 got to positive. The RPE for CH 4 increased gradually up to 1.6 and 0.5 mg C day −1 kg −1 soil at the end of the experiment without and with N fertilization, respectively. Moreover, the RPE for CH 4 got half of the RPE for CO 2 after 64 days showing the relevance of CH 4 emissions for greenhouse gases balance and C cycling in paddy ecosystems. The RPE for CO 2 and CH 4 emissions increased with microbial biomass content and activities of xylanase and N -acetylglucosaminidase. Supporting the results to RPE, the enzyme activities decreased with N fertilization, suggesting that reduced N limitation decreased microbial potential to mine N from SOM. In conclusion, for the first time we showed that root-microbial interactions stimulated SOM mineralization in rice paddies through rhizosphere priming effects not only for CO 2 but also for CH 4 , but the RPE decreased with N fertilization. [ABSTRACT FROM AUTHOR]

Microplastics (MPs) can affect soil organic carbon (C) cycling in paddy soil by changing microbial function and soil properties. In this study, a laboratory incubation experiment was applied to explore the effect of polyethylene (PE) MP on methane (CH 4) and carbon dioxide (CO 2) emissions from paddy soils after rice straw addition. Moreover, the microbial functional genes encoding CH 4 oxidation and production and organic C degradation were measured. MP addition significantly (p < 0.05) affected the release of CH 4 , but this effect varied with soil type. The amendment of PE MPs significantly (p < 0.05) reduced CH 4 emission by 16.9 % for the acidic soil (pH = 5.0) and 16.1 % for the alkaline soil (pH = 8.2), which was probably due to these plastic particles inhibiting CH 4 production by reducing the abundance of the mcrA gene or stimulating CH 4 oxidation with the increasing number of pmoA gene copies, respectively. For the neutral soils, the presence of PE MPs did not significantly (p > 0.05) alter CH 4 emissions and their associated microbial functional genes. Across the experimental paddy soils, the response of CH 4 emissions to MPs was negatively and positively correlated with those of soil ammonium (NH 4 +) content and mcrA gene abundance, respectively, suggesting that the change in these variables may control the release of CH 4 in the presence of MPs. In addition, PE MPs had a negligible effect on soil CO 2 emissions and the abundances of all functional genes encoding starch, hemicellulose, and lignin degradation. Our results demonstrated that the presence of MPs affects the release of CH 4 , probably by regulating NH 4 + substrates and methanogens, and this emerging environmental factor should be considered when estimating CH 4 emissions from paddy fields. • The impact of microplastics (MPs) on CH 4 emissions varied with paddy soils. • MPs reduced CH 4 emission and mcrA gene abundance in acidic soil. • MPs decreased CH 4 emission but increased pmoA gene copy in alkaline soil. • MPs affected CH 4 emission by regulating ammonium substrates and methanogens. [ABSTRACT FROM AUTHOR]

The deterioration of the microbiome within the rhizosphere is the main reason behind continuous cropping obstacles and is affected by fertilization regimes. However, the potential associations among fertilization regimes, rhizosphere microbial communities, and the severity of continuous cropping obstacles are poorly understood. Here, we investigated how three-year fertilization regimes (including no fertilizer, chemical fertilizer, organic fertilizer, combination of chemical and organic fertilizer, and bio-organic fertilizer application) affect the rhizosphere microbiome and its associations with continuous cropping growth constraints during the whole growing period of watermelon, using a rhizobox system under greenhouse conditions. The results showed that different fertilization treatments induced drastic shifts in the richness, structure, and composition of rhizosphere microbial communities, whereas the growth stages induced few changes. Compared with no fertilizer treatment, the continuous application of chemical fertilizer inhibited the growth of watermelon. This was strongly associated with the reduced abundance of key ecological clusters within the bacterial co-occurrence network, which contains beneficial bacterial taxa, such as Lysobacter sp. and Haliangium sp. In contrast, continuous application of organic and bio-organic fertilizers promoted the growth of watermelon and was associated with the decreased abundance of key ecological clusters within the fungal co-occurrence network, which contains pathogenic fungal taxa, such as Fusarium sp., Cladosporium sp., and Acremonium sp. Overall, continuous chemical fertilizer application can induce severe continuous cropping obstacles by inhibiting the growth of beneficial bacteria. Continuous application of organic and bio-organic fertilizers can help alleviate such obstacles by suppressing pathogenic fungi. These findings reveal the microbial mechanisms underlying the effects of different fertilization regimes on continuous cropping growth constraints of watermelon. [Display omitted] • Chemical fertilization aggravates watermelon continuous-cropping obstacle. • Organic and bio-organic fertilization alleviates continuous cropping obstacles. • Watermelon growth is associated with the key ecological clusters in rhizosphere. • Chemical fertilization inhibits the growth of beneficial bacterial taxa. • Organic and bio-organic fertilization inhibits the growth of pathogenic fungal taxa. [ABSTRACT FROM AUTHOR]

Reclamation has been widely used to alleviate the degradation of cultivated upland and support the increasing grain demand. However, the response of soil ecosystem functioning and soil health to the reclamation of coastal wetlands remains unclear. A reclaimed soil chronosequence over 1000 years in Hangzhou Bay, China, was analyzed to assess two key approaches to evaluate soil health. We used the minimum data set along with the soil quality index (SQI) area and the sensitivity–resistance approaches. The physicochemical properties of the reclaimed soils changed drastically at the initial stage (during the first 60 years) but only marginally thereafter. Owing to continuous freshwater irrigation, plant cultivation, fertilization, and desalination, from natural tidal flats converted to vegetable fields, the SQI and soil multifunctional index increased along the reclamation chronosequence. The soil properties sensitive to the reclamation of coastal wetlands (electrical conductivity, exchangeable potassium, and enzyme activities) explained most of the variation in the SQI area, followed by the resistance indicators. This suggests that small changes in the sensitivity indicators might have considerable impacts on the improvement of soil quality. The most resistant properties with the slowest changes included pH and physical characteristics—water content, bulk density, and aggregate size classes. The quality indicators identified for reclaimed soils in Hangzhou Bay based on the SQI area and sensitivity–resistance approaches can be useful for soil health evaluation for soils affected by natural and anthropogenic factors. These approaches and indicators can be effectively used to evaluate soil quality and develop sustainable agriculture. [Display omitted] • Soil quality indexes (SQI) were used and analyzed in a chronosequence of tidal flat reclamation. • SQI area and sensitivity–resistance approaches were useful for soil health evaluation. • Tidal flat reclamation increased soil quality and multifunctionality in Hangzhou Bay. • Sensitive parameters explained more variability of SQI area index than resistant soil properties. [ABSTRACT FROM AUTHOR]

The response of stable and labile C pools to global warming is uncertain, especially in paddy soils with very low oxygen availability and the dominance of electron acceptors with low efficiency. To clarify the response of organic matter decomposition to warming, flooded paddy soil was incubated at four temperatures (5, 15, 25, and 35 °C) for 75 days. The 13C-labelled Na-acetate was used as an analogue for root exudates and as a methane (CH 4) source. Soil with acetate had higher C availability to microorganisms leading to 2–2.7 times and 2–153 times higher emission of carbon dioxide (CO 2) and CH 4 on day 75 than from soil without acetate, respectively. Incubation temperature explained >40% of the variance of CO 2 and CH 4 effluxes. Acetate stimulated microbial activities and turnover and so, increased soil organic matter (SOM) mineralisation in the first week, especially at low temperatures (<15 °C) with slow acetate consumption and longer oxygen (O 2) availability. The priming effects measured as CH 4 emissions were especially sensitive to temperatures from 5 to 15 °C. The high Q 10 value of primed CH 4 (Q 10 > 10) at low temperature indicates that flooded paddy fields will contribute greatly to the greenhouse effect in warm winters, which have become common from 1970s. Caution is necessary for interpretations of previous estimates of the temperature sensitivity of SOM decomposition because the priming effect was ignored, especially that of CH 4 under the condition of limited O 2 availability in paddy and other wetland soils. [ABSTRACT FROM AUTHOR]

Previous studies on upland soils showed that 13C natural abundance can successfully reveal C stabilization pathways between aggregates and soil organic matter (SOM) density fractions. The direction of C stabilization in paddies can, however, deviate from that in upland soils owing to i) periodic drying–rewetting cycles, with oxygen pulses under oxic conditions, and thus, shifts in microbial processing of organic residues, and ii) intensive organic and mineral fertilization. To trace C stabilization in paddies, soil was sampled from a long-term field experiment under an unfertilized Control and NPK, NPK + straw, and NPK + manure fertilizer regimes. Soil was analyzed for total C, microbial biomass (MB), and dissolved organic C, and separated into three classes based on aggregate size (>250 μm, 53–250 μm, and <53 μm) followed by density fractionation of each class to obtain free and occluded light fractions as well as dense and mineral heavy fractions. Pathways of C were determined based on C content and δ13C in all pools. The highest increase in total C (69%) was in NPK + manure, whereas the MBC increased with fertilization by at least 29% compared with the Control. All fertilizers increased macro-aggregation by at least 111% compared with the Control. The highest C content in the aggregates was in the mineral fractions of macroaggregates. Fertilization decreased the δ13C of total SOM compared with the Control, indicating suppressed decomposition of organic compounds. Aggregate size classes showed a typical δ13C enrichment trend from macro-to microaggregates, reflecting similarities between paddy and upland soils. A detailed scheme of C flows within aggregates and SOM fractions based on the δ13C natural abundance revealed the following general sequence: mineral → dense → free light → occluded light fractions. This trend, which is partly opposite to that observed in upland soils, reflects the anoxic and variable redox conditions of paddies. This facilitates the predominant stabilization of recent C input in the mineral fraction with Fe oxides due to Fe2+/Fe3+ dynamics, whereas light fractions are processed by microorganisms mainly in periods without overflooding. The C pathways in the two heavy fractions were separate from those in the two light fractions, which also indicates differences in the C stabilization processes between paddy and upland soils. Thus, the present study provides further detailed insights into the C stabilizing mechanisms in paddy soils which depend on management. Image 1 • Fertilization of paddy soils increased carbon (C) stabilization in macroaggregates and mineral fraction. • C flow is ongoing from macro-to microaggregates. • C flow in paddy soils is ongoing from mineral to free light fraction, which is opposite to upland soils. • Pathways of C between light fractions are separate from those in heavy fractions. [ABSTRACT FROM AUTHOR]