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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]

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]

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]