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Nitrogen (N) and phosphorus (P) availability plays a crucial role in carbon (C) cycling in terrestrial ecosystems. However, the C:N:P stoichiometric regulation of microbial mineralization of plant residues and its impact on the soil priming effect (PE), measured as CO 2 and CH 4 emission, in paddy soils remain unclear. In this study, the effect of soil C:N:P stoichiometry (regulated by the application of N and P fertilizers) on the mineralization of 13 C-labelled rice straw and the subsequent PE was investigated in a 100-day incubation experiment in flooded paddy soil. N and P additions increased straw mineralization by approximately 25% and 10%, respectively. Additions of both N and P led to higher CO 2 efflux, but lower CH 4 emission. With an increase in the ratios of DOC:NH 4 + -N, DOC:Olsen P, and microbial biomass C:N, 13 CO 2 efflux increased exponentially to a maximum. Compared with sole straw addition, exclusive N addition led to a weaker PE for CO 2 emission, whereas exclusive P addition induced a stronger PE for CO 2 emission. In contrast, CH 4 emitted from native soil organic matter (SOM) was reduced by 7.4% and 46.1% following P and NP application, respectively. Structural equation models suggest that available N had dominant and direct positive effects, whereas microbial biomass stoichiometry mainly exerted negative indirect effects on PE. The stoichiometry of soil enzyme activity directly down-regulated CH 4 emission from SOM. Microbes obviously regulate soil C turnover via stoichiometric flexibility to maintain an elemental stoichiometric balance between resources and microbial requirements. The addition of straw in combination with N and P fertilization in paddy soils could therefore meet microbial stoichiometric requirements and regulate microbial activity and extracellular enzyme production, resulting in co-metabolism of fresh C and native SOM. [ABSTRACT FROM AUTHOR]

Highlights: [•] Responses of crop yield and SOC stock to long-term fertilization were evaluated. [•] Rice straw incorporation increased SOC stocks by 0.24–1.00tha−1 yr−1. [•] Crop yield in upland were highly sensitive to fertilization due to its lower fertility. [•] Ex situ incorporation of rice straw in upland was recommended. [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]

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]

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]

Global warming increases belowground carbon (C) input as plant litterfall, root biomass and rhizodeposition, which influences the stocks and dynamics of soil organic matter. To clarify the effects of labile C availability (biochemical factor) and temperature (environmental factor) on enzyme activities, we incubated typical paddy soil for 75 d at four temperatures (5, 15, 25, and 35 °C) under anaerobic conditions. Acetate was used as the source of labile C and methane. The potential activities of three hydrolases (β -glucosidase, chitinase, and xylanase) were analysed on days 3, 15, and 75 after acetate addition. Activity of β -glucosidase and chitinase in soil without acetate addition was 2.1–2.7 times higher than that with acetate. Xylanase activity increased with temperature and incubation period. The enzymes involved in the C cycle were sensitive to temperature, whereas chitinase (responsible for N cycle) activity became temperature sensitive only after acetate addition (Q 10 - V max ≥ 1). Organic C mineralisation (CO 2 release) was more sensitive at low temperature with Q 10 values 1.1–3.4 times higher at 5–15 °C than at 25–35 °C. The Q 10 values for methane (CH 4) emission were 2.8–13.5 times higher at 5–15 °C than at 25–35 °C. Organic matter decomposition in paddy soil was more sensitive to temperature (Q 10 of CO 2 and CH 4 emission ≥ 1) than enzyme activities. Comparison of abiotic (temperature) and biochemical (C availability) effects indicated that warming has limited effects on hydrolase activities in paddy soil. The increase in labile C remarkably stimulated microbial activity and soil organic matter turnover. We conclude that: i) enzyme activities are more sensitive to C addition than to temperature; ii) and SOM decomposition is accelerated by both C input and warming, especially at low temperatures. Image 1 • Labile C affects the response of extracellular enzyme activities to soil warming. • The higher the labile C content, the less enzymes are produced by soil microorganisms. • Labile C addition affects microbial biomass and induces enzymatic N mining from SOM. • Acetate input increases temperature sensitivity of CO 2 and CH 4 emission below 15 °C. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of belowground C input by rice plants and its fate is essential for managing C cycling and sequestration in paddy soils. Previous reviews have summarized C input and the pathways of root-derived C in upland soils by labeling with 14C or 13C (13/14C), while rice rhizodeposition and C input in paddy soils have not been comprehensively evaluated. Here, we analyzed the results of 13/14C pulse and continuous labeling studies using 112 datasets from 13 articles on the allocation and pathways of photosynthesized C by rice plants to assess C input, budget, and amount stabilized in paddy soils. Overall, 13/14C partitioning estimated by continuous labeling was 72% to the shoots, 17% to the roots, 10% to the soil, and 1.3% was recovered in microbial biomass. Pulse-labeling studies showed a similar C partitioning: 79%, 13%, 5.5%, and 2.1%, respectively. The total belowground C input estimated based on continuous labeling was 1.6 Mg ha−1 after one rice season, of which rhizodeposition accounted for 0.4 Mg C ha−1. Carbon input assessed by pulse labeling was slightly lower (total belowground C input, 1.4 Mg ha−1; rhizodeposition, 0.3 Mg C ha−1; 14 days after labeling). Rice C input after one cropping season was lower than that by upland plants (cereals and grasses, 1.5–2.2 Mg ha−1). In contrast to upland crops, most paddy systems are located in the subtropics and tropics and have two or three cropping seasons per year. We conclude that (1) pulse labeling underestimates the total belowground C input by 15%, compared with that by continuous labeling, and (2) rhizodeposition of rice accounts for approximately 26% of the total belowground C input, regardless of the labeling method used. Based on allocation ratios, we suggest a simple and practical approach for assessment of the gross C input by rice into the soil, for partitioning among pools and for long-term C stabilization in paddies. Graphical abstract Image 1 Highlights • We reviewed the amount of C input by rice plants into paddy soils based on 13C or 14C labelling studies. • Pulse labeling underestimated the total belowground C input by 15% compared with continuous labeling. • Rhizodeposition accounted for approximately 26% of the total belowground C input by rice. • Simple method was proposed for the raw assessment of C input into the soil. [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]

Abstract: Fertilizer application has the potential to promote the sequestration of carbon (C) and nitrogen (N) in agricultural soils and thus may mitigate the effects of atmospheric greenhouse gases. In this study, the effects of fertilizer practices [i.e., no fertilizer (CK), chemical fertilizer (NPK), and chemical fertilizer plus low or high rates of organic manure (LOM or HOM)] on soil organic carbon (SOC) and total nitrogen (TN) content in the plow layer (0–20cm) of paddy soils were examined using the data from eight long-term field experimental sites (1986–2003) in Hunan Province, Southern China. The SOC and TN content with the treatments which included N fertilizer (NPK, LOM, and HOM) ranged from 16.2 to 38.6gkg−1 and from 1.07 to 3.92gkg−1, respectively. Compared with the CK treatment, the average SOC and TN content were 2.0 and 19.3%, 29.3 and 5.2%, and 19.5 and 27.1% larger, respectively, for NPK, LOM, and HOM. In addition, the average values for SOC with the four treatments (CK, NPK, LOM, and HOM) had increased by 13.1, 15.4, 35.0, and 46.3%, respectively, by 2003; for TN they had increased by 5.0, 10.5, 25.5, and 33.5%, respectively, above the values obtained in 1986. However, the increase in SOC and TN content varied substantially at the different experimental sites. Organic–chemical fertilization gradually increased SOC and TN content and then the values tended to be stable with the LOM and HOM treatments from 1986 to 2003; they also remained stable for the NPK and CK treatments. Soil TN contents were significantly correlated with SOC at each site (P <0.001). Soil C/N ratios in 2003 were generally around 10 and usually ranged from 8.5 to 12.0. The soil C/N ratios among the four treatments were not significantly different at the eight sites in 2003. Nevertheless, the average C/N ratio at the eight sites was approximately 1.08 times higher than in 1986. The results indicate that there is a coupling relationship between SOC and TN in paddy soils in the subtropical region of China, a region that could provide C and N sinks under current fertilizer practices (i.e., combined chemical and organic fertilization) and cropping conditions. However, the results do suggest that the application of organic manure is the primary contributor to C and N sequestration. [Copyright &y& Elsevier]

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]

Root exudates are a labile source of carbon (C) for microorganisms that can lead to increased CO 2 emission. Root exudates can vary in C:N stoichiometric ratio and their impact on microbially driven soil organic matter (SOM) turnover in paddy soils still remains unclear. The objective was to explore the underlying mechanisms involved in SOM decomposition due to root exudate (artificial) addition with three different C:N ratios (10, 20, and 40) during 45 days incubation. Different root exudates C:N ratios were obtained by adding mineral N and exudate components (glucose, oxalic acid, and glutamate) to paddy soil. N-only addition decreased dissolved organic C to limit CO 2 emissions, which is an indicative of C sequestration. Conversely, simulated C:N stoichiometric ratios of root exudates significantly increased both microbial activity and metabolism without altering the microbial biomass C:N ratio. However, soil available dissolved organic C to NH 4 + ratio decreased by exudates addition. The stoichiometric ratio of key C and N compound degrading enzymes activities increased only with C:N = 10 and remained unchanged with exudates C:N = 20 and 40. The q CO 2 values increased with decreasing N-containing compounds in root exudates (i.e. highest CO 2 emission was observed under C:N = 40 exudates addition). The results suggest that increasing exudates C:N ratio intensify CO 2 emission due to high microbial N demand. Overall result show that root exudates C:N ratio and soil available N co-regulate on CO 2 emission, which was controlled by microbial and potential extracellular enzyme activities. • Root exudate analogues significantly increased CO 2 emission from paddy soils. • Exudates addition increased microbial biomass and q CO 2 without altering microbial C:N ratio. • Root exudate inputs increased the accumulation of soil available NH 4 +. • N-only addition limited CO 2 emission and decreased dissolved organic C. [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]

The impact of increasing amounts of labile C input on priming effects (PE) on soil organic matter (SOM) mineralization remains unclear, particularly under anoxic conditions and under high C input common in microbial hotspots. PE and their mechanisms were investigated by a 60-day incubation of three flooded paddy soils amended with13C-labeled glucose equivalent to 50–500% of microbial biomass C (MBC). PE (14–55% of unamended soil) peaked at moderate glucose addition rates (i.e., 50–300% of MBC). Glucose addition above 300% of MBC suppressed SOM mineralization but intensified microbial N acquisition, which contradicted the common PE mechanism of accelerating SOM decomposition for N-supply (frequently termed as "N mining"). Particularly at glucose input rate higher than 3 g kg−1 (i.e., 300–500% of MBC), mineral N content dropped on day 2 close to zero (1.1–2.5 mg N kg−1) because of microbial N immobilization. To cope with the N limitation, microorganisms greatly increased N-acetyl glucosaminidase and leucine aminopeptidase activities, while SOM decomposition decreased. Several discrete peaks of glucose-derived CO 2 (contributing >80% to total CO 2) were observed between days 13–30 under high glucose input (300–500% of MBC), concurrently with CH 4 peaks. Such CO 2 dynamics was distinct from the common exponential decay pattern, implicating the recycling and mineralization of 13C-enriched microbial necromass driven by glucose addition. Therefore, N recycling from necromass was hypothesized as a major mechanism to alleviate microbial N deficiency without SOM priming under excess labile C input. Compound-specific 13C-PLFA confirmed the redistribution of glucose-derived C among microbial groups, i.e., necromass recycling. Following glucose input, more than 4/5 of total 13C-PLFA was in the gram-negative and some non-specific bacteria, suggesting these microorganisms as r -strategists capable of rapidly utilizing the most labile C. However, their 13C-PLFA content decreased by 70% after 60 days, probably as a result of death of these r -strategists. On the contrary, the 13C-PLFA in gram-positive bacteria, actinomycetes and fungi (K-strategists) was initially minimal but increased by 0.5–5 folds between days 2 and 60. Consequently, the necromass of dead r -strategists provided a high-quality C–N source to the K-strategists. We conclude that under severe C excess, N recycling from necromass is a much more efficient microbial strategy to cover the acute N demand than N acquisition from the recalcitrant SOM. • Priming effects (PEs) have a unimodal relationship to amount of added glucose. • Microbial acquisition of N at high glucose amounts was intensified without increasing PEs. • 13C-PLFA reflects recycling of necromass C between microbial groups. • 13C-enriched CO 2 peaks violating exponential decay suggest necromass decomposition. • Necromass-N recycling was necessary to overcome N deficiency without priming other SOM pools. [ABSTRACT FROM AUTHOR]