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• Switching from continuous flooding (CF) to rainfed (TF) reduced CH 4 emission by 78 %. • Shifting CF into plastic film mulching (PM) led to a 54 % decrease in CH 4 emission. • PM reduced input and GWP costs relative to CF, thus rising NEEB by 3500 CNY ha–1. • PM relative to TF increased NEEB by 6000 CNY ha–1 because of yield gains promotion. • Combining PM with WDR enhanced NEEB again as GWP costs decline and yield gains rise. Plastic film mulching (PM) and water-saving and drought-resistant rice (WDR) are believed to increase crop yields and decrease CH 4 emissions, respectively. Here, a comprehensive estimation is prepared on how the combination of PM and WDR impacts rice yields, CH 4 and N 2 O emissions, global warming potential (GWP), greenhouse gas emission intensity (GHGI), and net economic ecosystem benefits (NEEB). Shifting continuous flooding (CF) into traditional rainfed (TF) and PM decreased CH 4 emissions (35–92 %), GWP (33–91 %), and GHGI (31–88 %). Moreover, PM considerably reduced input costs and GWP costs relative to CF whereas increasing grain yields and yield gains relative to TF, thus promoting NEEB by around 3500 and 6000 CNY ha–1, respectively. Combining PM with WDR further enhanced NEEB by 935 CNY ha–1. The findings indicate that PM with the combination of WDR would be a promising strategy for lower greenhouse gas emissions and higher grain yields and economic benefits in rice agriculture. [Display omitted] [ABSTRACT FROM AUTHOR]

Aim: Quantification of total belowground carbon (C) input and allocation to various pools in rice–soil systems depending on plant age, chase period, and nitrogen (N) availability. Methods: Rice plants with and without N fertilization were 13CO2 pulse labelled at tillering, elongation, heading, and filling stages and were destructively sampled after 6 h of labelling and at the final harvest. The allocation of C by rice was also generalized based on literature pertaining to 94 studies with respect to plant age and chase period. Results: The C allocation in roots and soil strongly decreased with plant age. The literature review showed that C allocation to roots increased within the first 10 days of labelling and remained stable thereafter. Nitrogen fertilization had no effect on C allocation immediately after assimilation, but increased C remained belowground at 1.7-times that at final harvest. The total belowground net C input by one rice crop was 630–1080 kg C ha−1, including rhizodeposition of 160–330 kg C ha−1. Conclusion: Multiple pulse labelling at various plant growth stages and taking multiple subsequent samples as well as nutrient availability should be considered for tracing C flows more accurately for precise C balance in rice paddy systems. [ABSTRACT FROM AUTHOR]

Aims: Although nitrogen (N) fertilization is widely used to increase rice yield, its impact on the distribution, transformation, and fates of photosynthetic carbon (C) in rice–soil systems is poorly understood. To address this, we quantified the C flows into various pools in a rice–soil system. Methods: Rice (Oryza sativa L.) was pulse-labeled with 13CO2 at the tillering stage. Samples were collected six times during the 26 days following labeling. We quantified the partitioned photosynthesized C into various pools using stable isotopic techniques and estimated C flows. Results: Although the net distribution of assimilated C to belowground pools did not change, N fertilization promoted C assimilation in aboveground biomass. C allocation into soil was enhanced by N fertilization during early growth, but decreased during late growth. N fertilization induced higher mass-specific rhizodeposition (per unit root dry weight) and its turnover rate compared with the unfertilized system. However, with higher microbial turnover, the daily C allocation from roots to soil was similar at both fertilization levels. Conclusions: Although total C input into soil is enhanced by N fertilization, its further fate is N fertilization independent, thus leading to a net accumulation of C input in rice paddy soil similar to that observed unfertilized soil. [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]

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]

Abstract Rhizodeposition represents a readily available C and energy source for soil microorganisms, that plays an important role in the regulation of C and nutrient cycling in ecosystems and exerts a strong influence on C sequestration. The dynamics of rice rhizo-C in soils and its allocation to microorganisms during rice growth, as well as the effects of nitrogen (N-NH 4 +) fertilization are poorly understood, particularly with respect to the initial uptake of rhizo-C by microorganisms and its utilization during the entire growth period. To assess these two processes, rice plants were grown in pots with or without N fertilization (0 and 225 kg N-NH 4 + ha−1), and 13C incorporation into microbial groups was traced by phospholipid fatty acids (PLFAs) analysis within 6 h after 13CO 2 pulse labeling. Labeling was performed at five growth stages: tillering, elongation, heading, filling, and maturation. 13C incorporated into soil microbial biomass C changed rapidly at the beginning of the study period, before elongation, but remained stable thereafter. 13C incorporation into rhizosphere and bulk soil microbial biomass was higher with than without N addition. This stimulation was likely due to the excessive increase in phytomass formation and root exudates after N fertilization and the increased assimilate C input into the soil. Structural equation modelling suggested that N fertilization strongly affected carbon transfer between rhizosphere and non-rhizosphere. Hence, N-NH 4 + application may not only increase rhizo-C flow into microorganisms but it may also increase the effect of rhizosphere on bulk-soil microorganisms and subsequent processes related to soil C-cycling. Graphical abstract Unlabelled Image Highlights • G+ and G− bacteria are the main groups initially involved in rhizodeposits uptake. • Changes in microbial rhizo-C utilization occurred mainly early during rice growth. • Succession of soil microbial community occurred mainly early during rice growth. • Effect of rhizodeposits on microbial composition was altered by N application. [ABSTRACT FROM AUTHOR]

Mean residence time (MRT) of carbon (C) in soil is the most important parameter of C sequestration and stability and crucial for CO 2 removal from the atmosphere. Climate and soil properties controls of MRT of upland soils are well known, but the drivers of C stability in paddies were never summarized. Here, we estimated MRT of paddies across monsoon Asia using the stock-over-flux method, i.e., soil organic C (SOC) stock over organic matter input considering the net primary production (NPP), and determined the main factors affecting SOC turnover. The average MRT of paddy soils in monsoon Asia ranges between 19 and 50 yr, depending on straw management. These estimates are similar to recent estimates for the global average MRT across all soils, but longer than for upland croplands. Tropical regions have the shortest MRT for rice paddies (16–42 yr), while the MRT of C in soils of temperate and subtropical regions are longer (20–56 yr). Across a wide range of environmental factors, MRT was most strongly affected by temperature. We estimate that 2 °C warming decreases MRT by 7% on average, with the strongest decreases in the western Indonesian islands and north-east China. Because C stocks per area in paddy soils are larger and the MRT is longer than in corresponding upland cropland soils, paddies play a key role in the global C cycle. Our results emphasize the need for management practices that retain stable soil C input rates to reduce possible positive feedbacks for global warming. • Mean residence time (MRT) of C in paddies ranges 19–50 yr. • MRT was shortest in tropics, while similar in temperate and subtropics. • MAT influenced C turnover more than other environmental factors. • MRT of C in paddies is declining at an average rate of 7% to 2 °C warming. [ABSTRACT FROM AUTHOR]

Background and aims: The turnover of plant- and microbial- derived carbon (C) plays a significant role in the soil organic C (SOC) cycle. However, there is limited information about the turnover of the recently photosynthesized plant- and soil microbe-derived C in paddy soil. Methods: We conducted an incubation study with four different C-labeled substrates: rice shoots (Shoot-C), rice roots (Root-C), rice rhizodeposits (Rhizo-C), and microbe-assimilated C (Micro-C). Results: Shoot- and Root-C were initially rapidly transformed into the dissolved organic C (DOC) pool, while their recovery in microbial biomass C (MBC) and SOC increased with incubation time. There were 0.05%, 9.8% and 10.0% of shoot-C, and 0.06%, 15.9% and 16.5% of root-C recovered in DOC, MBC and SOC pools, respectively at the end of incubation. The percentages of Rhizo- and Micro-C recovered in DOC, MBC, and SOC pools slowly decreased over time. Less than 0.1% of the Rhizo- and Micro-C recovered in DOC pools at the end of experiment; while 45.2% and 33.8% of Rhizo- and Micro-C recovered in SOC pools. Shoot- and Root-C greatly increased the amount of C-PLFA in the initial 50 d incubation, which concerned PLFA being indicative for fungi and actinomycetes while those assigning gram-positive bacteria decreased. The dynamic of soil microbes utilizing Rhizo- and Micro-C showed an inverse pattern than those using Shoot- and Root-C. Principal component analysis of C-PLFA showed that microbial community composition shifted obviously in the Shoot-C and Root-C treatments over time, but that composition changed little in the Rhizo-C and Micro-C treatments. Conclusions: The input C substrates drive soil microbial community structure and function with respect to carbon stabilization. Rhizodeposited and microbial assimilated C have lower input rates, however, they are better stabilized than shoot- and root-derived C, and thus are preferentially involved in the formation of stable SOC in paddy soils. [ABSTRACT FROM AUTHOR]

Background and aims: Paddy field abandonment has become a major concern in China, particularly among traditional rice-cultivation regions. Abandonment results in the alteration of many processes affecting C sequestration and turnover, but the final effects on C stocks remain unknown. Methods: To examine the effects of paddy abandonment on topsoil organic C (SOC) content and stocks, a long-term experiment was performed in subtropical China, examining an abandoned paddy field with different pre-abandonment fertilization history (from 1991 to 2006 years) and SOC gradients (from 19.2 to 22.0 g C kg). Results: Paddy field abandonment significantly reduced the topsoil SOC content and stock (0-20 cm). Eight years after cultivation ceased, SOC content and C stock had decreased by 9.9-20.9% and 10.2-20.8%, respectively, yielding a mean annual loss rate of 0.30-0.60 g C kg·yr. and 0.50-1.15 t C ha·yr., respectively. Soils with higher initial SOC content were more sensitive to abandonment than soils with low SOC levels. Dissolved organic C (DOC) was more sensitive to abandonment, as evidenced by the faster decrease of DOC than SOC. The rapid reduction of SOC content, combined with a strong decrease in DOC, indicates that post-abandonment C inputs into the soil were far lower than the concurrent SOC mineralization. The SOC content decreases was likely because of the shift from anaerobic to aerobic conditions. This change leads to faster litter decomposition and SOC mineralization, accompanied with decreasing SOC retention or stabilization by soil aggregates, mineral or Fe redox processes. Conclusion: Abandonment of paddy soils leads to switch from a C sink to a C source, resulting in high C losses. The succession of grasses in abandoned fields did not compensate for the losses of soil C stocks. [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]

The input of recently photosynthesized C has significant implications on soil organic carbon sequestration, and in paddy soils, both plants and soil microbes contribute to the overall C input. In the present study, we investigated the fate and priming effect of organic C from different sources by conducting a 300-d incubation study with four different 13C-labelled substrates: rice shoots (Shoot-C), rice roots (Root-C), rice rhizodeposits (Rhizo-C), and microbe-assimilated C (Micro-C). The efflux of both 13CO2 and 13CH4 indicated that the mineralization of C in Shoot-C-, Root-C-, Rhizo-C-, and Micro-C-treated soils rapidly increased at the beginning of the incubation and then decreased gradually afterwards. In addition, the highest level of C mineralization was observed in Root-C-treated soil (45.4%), followed by Shoot-C- (31.9%), Rhizo-C- (7.9%), and Micro-C-treated (7.7%) soils, which corresponded with mean residence times of 33.4, 46.1, 62.9, and 192 d, respectively. Furthermore, the cumulative mineralization of native soil organic carbon in Shoot-C-treated soils was 1.48- fold higher than in untreated soils, and the priming effect of Shoot-C on CO2 and CH4 emission was strongly positive over the entire incubation. However, Root-C failed to exhibit a significant priming effect, which suggests that it could potentially be used to mitigate CH4 emission. Although the total C contents of Rhizo-C- (1.89%) and Micro-C-treated soils (1.9%) were higher than those of untreated soil (1.8%), no significant differences in total C emissions were observed. However, the 13C emissions of Rhizo-C- and Micro-C-treated soils gradually increased over the entire incubation period, which indicated that soil organic C-derived emissions were lower in Rhizo-C- and Micro-C-treated soils than in untreated soil, and that rhizodeposits and microbe-assimilated C could be used to reduce the mineralization of native soil organic carbon and to effectively improve soil C sequestration. The contrasting behaviours of the different photosynthesized C substrates suggests that recycling rice roots in paddies is more beneficial than recycling shoots and reveals the importance of increasing rhizodeposits and microbe-assimilated C in paddy soils via nutrient management. [ABSTRACT FROM AUTHOR]

Red soils are the major land resource in subtropical and tropical areas and are characterized by low phosphorus (P) availability. To assess the availability of P for plants and the potential stability of P in soil, two pairs of subtropical red soil samples from a paddy field and an adjacent uncultivated upland were collected from Hunan Province, China. Analysis of total P and Olsen P and sequential extraction was used to determine the inorganic and organic P fractions in different aggregate size classes. Our results showed that the soil under paddy cultivation had lower proportions of small aggregates and higher proportions of large aggregates than those from the uncultivated upland soil. The portion of >2-mm-sized aggregates increased by 31 and 20 % at Taoyuan and Guiyang, respectively. The total P and Olsen P contents were 50-150 and 50-300 % higher, respectively, in the paddy soil than those in the upland soil. Higher inorganic and organic P fractions tended to be enriched in both the smallest and largest aggregate size classes compared to the middle size class (0.02-0.2 mm). Furthermore, the proportion of P fractions was higher in smaller aggregate sizes (<2 mm) than in the higher aggregate sizes (>2 mm). In conclusion, soils under paddy cultivation displayed improved soil aggregate structure, altered distribution patterns of P fractions in different aggregate size classes, and to some extent had enhanced labile P pools. [ABSTRACT FROM AUTHOR]

Dissolved organic nitrogen (DON) is a significant nitrogen (N) pool in most soils and isconsidered to be important for N cycling. Increasing evidence suggests that not only aminoacids but also small peptides are potential direct nutrient sources for both soil microorganismsand plants. However, current understanding of turnover and underlying mechanismsof those low molecular weight DON (LMW DON) compounds in soils remainsincomplete. In this study, we aimed to investigate mineralization of LMW DONcompounds in a paddy soil from different depths under different N fertilisationpractices; and to test the linkages between soil characteristics and LMW DONturnover. To accomplish this, we collected soils from different depths (i.e., 0-10, 1020, 20-30 and 30-40 cm) in a paddy field under long-term different fertilisationpractices (i.e., control without fertiliser, chemical fertiliser and organic fertiliseralone and their combination) in southern China. We measured soil physiochemicalproperties and abundance and structure of soil microbial communities after long-termfertilisation management. In 14C mineralization assay (7 days at 22 oC), we used theamino acid L-alanine and its peptide tri-L-alanine as model L-enantiomer substratesto investigate the rates of LMW DON mineralisation in paddy soils. Overall, adouble first-order kinetic model conformed very well to the experimental data ofamino acid and peptide mineralization. For both form of LMW DON, the poolsizes of both catabolic and anabolic processes were significantly affected by Nfertiliser, soil depth, and their interaction (P &#x003C;0.0001), thus contributing to pronounceddifferences in microbial C use efficiency among treatments. However, microbial uptakerates of peptide but not amino acid were significantly affected by N fertiliser, soildepth, and their interaction (all P &#x003C;0.05). Results of canonical correspondenceanalysis suggest that microbial C use efficiency of both LMW DON was positivelycorrelated with soil pH, while microbial uptake rates were predominately associatedwith other soil biochemistry properties (e.g. soil and microbial C and N indices). [ABSTRACT FROM AUTHOR]

Abstract The anaerobic oxidation of methane (AOM) removes most of the biologically produced methane (CH 4) from marine ecosystems before it enters the atmosphere and thus mitigates greenhouse gas emissions. As compared to marine environments, surprisingly little is known about the role of AOM in terrestrial ecosystems. Particularly, how AOM controls the CH 4 budget of paddy soils is unexplored, partly reflecting analytical difficulties in analyzing CH 4 turnover. To date, the most commonly used method to study AOM in soils is in vitro incubation of microcosms with CH 4 injection into the headspace with/without shaking of slurry. Shaking, however, introduces various errors and disturbances. Here we measured AOM in rice paddy soil using a new alternative approach that introduced 13C-labelled CH 4 directly into soil slurry via a silicone tube without shaking. The results were compared to those obtained by the classical approaches (i.e., with and without tubes and/or shaking). In all batches, 13C enrichment of CO 2 after 13CH 4 injection clearly confirmed the occurrence of AOM in paddy soil. The cumulative AOM during 84 days reached 0.16–0.24 μg C g−1 dry soil without shaking, but it was 33–80% lower with shaking. Unexpectedly, the effect of silicone tubes on AOM was insignificant either with or without shaking, suggesting that the CH 4 concentration in water (slurry) was not the main limiting factor for AOM. Without shaking, the controls without CH 4 addition revealed a steady increase of CH 4 in the headspace/tube, whereas the CH 4 concentration in jars with shaking was constantly low during 59 days. This suggests that shaking inhibited methanogenesis. There was a strong linear correlation between the amount of CH 4 oxidized and CH 4 produced with shaking (R 2 = 0.91), whereas without shaking this relationship followed a power growth regression. Based on the current and reported AOM rates, rough upscale to paddy soils in China showed recycling of ca. 2.0 Tg C of CH 4 each year, making AOM a crucial terrestrial CH 4 sink. Graphical abstract Image 1 Highlights • 13CH 4 tracer confirmed the occurrence of anaerobic oxidation of methane (AOM). • CH 4 concentration in slurry was not the main limiting factor for AOM. • Shaking inhibited methanogenesis and decreased AOM by 33–80%. • AOM in paddy soils of China could recycle ca. 2.0 Tg C CH 4 per year. [ABSTRACT FROM AUTHOR]

Viruses are ubiquitous and believed to play pivotal roles in soil ecosystems. However, variations in composition and function of viruses in adjacent paddy and upland soils remain largely unexplored. Thus, we aimed to compare the viral diversity and function in four pairs of paddy and upland soil samples by linking soil viromes with soil properties. Microviridae was the most abundant virus family in both soil samples. The proportion of Genomoviridae was larger, and that of Siphoviridae was smaller in upland soils. Moreover, upland soils harbored more virulent viruses, whereas paddy soils harbored more temperate viruses and viral genera. Available phosphorus content and pH were the dominant factors contributing to the viral community structures in upland and paddy soils, respectively. Viruses in upland soils encoded metabolic genes for degrading cell components, whereas viruses in paddy soils encoded genes for metabolizing amino acids. Additionally, viruses in paddy fields carried more carbohydrate-active enzyme genes and had broader host ranges. Hence, given the concentration of dissolved organic carbon, the difference in viral lifestyles between upland and paddy soils suggest that viral-induced cell lysis can influence nutrient availability. These findings provide a scientific basis for increasing virus-mediated microbial-derived carbon storage in soils by regulating nutrient availability. [ABSTRACT FROM AUTHOR]

The turnover of labile organic carbon (C) in the rhizosphere is rapid and affects the soil nutrient supply capacity. However, the mechanism by which rhizosphere labile organic C influences nutrient uptake is still unclear. The rhizosphere soils of summer rice among four upland-paddy rotation treatments of rice-winter fallow (RF), rice-potato with rice straw mulch (RP), rice-wheat (RW), and rice-Chinese milk vetch (RC) were characterized, and plant nitrogen (N) and phosphorus (P) uptake were quantified at the various growth stages. The results showed that labile organic C fractions differed at various growth stages. Readily oxidizable C was higher prior to than post the flowering stage; dissolved organic C was 118–151 % higher at the flowering stage than at the other stages, while microbial biomass C gradually decreased over the growth period. In addition, rotations with cover crops in winter crops and the N fertilizer input were conducive to increasing labile organic C and nutrient supply, and enzyme activities generally in the order of RP > RC > RW > RF. The RP treatment increased labile organic C fractions by 24–76 % and amount of available N and P in soil by 48–160 %. The effects of rotation and N treatments on labile organic C, N and P were associated with changes in the structure and composition of bacterial communities (RDA = 63–81 %). The structural equation model showed that the effects of rotation and N treatments on plant N and P uptake were partially regulated by labile organic C fractions (R 2 = 0.6–0.9) which in turn correlated with soil nutrient supply and enzyme activities. The concentrations of readily oxidizable C and microbial biomass C at early growth stages, as well as particulate and dissolved organic C in the late growth stages, showed promise as indicators for optimizing rotations and N application in the field. Future research should focus on validation of these indicators across a wider range of soil types and climatic conditions, and the correlation between these indicators and crop productivity should be investigated for further understanding the potential impacts on agricultural sustainability and soil management. • Dynamics of labile organic C (LOC) in rice rhizosphere is characterized. • The LOC fractions function differently at various rice growth stages. • Rotation with winter crops enhances soil nutrient availability and enzyme activity. • Nitrogen fertilization increased soil nutrient supply. • Changes of rhizosphere LOC fractions affect N and P uptake by rice plants. [ABSTRACT FROM AUTHOR]

Iron (Fe) plaque is a concentrated form of microbially available Fe oxide that coats rice plant root surfaces, representing a high density of Fe oxide and which potentially mediates paddy soil CH 4 emissions. Using a combination of methods including Fe plaque induction, isotopic labeling, pure microbial strains, and Fe oxide addition experiments, we investigated the impact of Fe plaque on methane (CH 4) emissions from paddy soils and explored the associated mechanisms underlying the influence of Fe plaque on CH 4 emissions. A 13C–CH 4 isotopic labeling experiment showed that Fe plaque did not significantly affect CH 4 oxidation and associated gene expression, whereas Fe plaque significantly enriched methanogenic archaea and their expression of genes associated with methanogenesis. Pure microbial strain and Fe oxide addition experiments showed that the enhancement of CH 4 production in the presence of Fe plaque was caused by the (semi) conductive minerals within the Fe plaque, specifically, hematite, which promoted the extracellular electron transfer between the methanogenic archaea and their syntrophic bacteria, resulting in enhancement of methanogenesis. Our results imply that the presence of Fe plaque will accelerate CH 4 emissions from paddy soils and suppressing Fe plaque has the potential to mitigate CH 4 emissions. [Display omitted] • Fe plaque increased CH 4 emissions by facilitating extracellular electron transfer. • Fe oxide promoted direct interspecies electron transfer for methanogenesis. • Limiting Fe plaque could reduce paddy soil CH 4 emissions. [ABSTRACT FROM AUTHOR]

Plant inputs and their subsequent microbial transformations drive the formation and accumulation of soil organic carbon (SOC). Rice paddy is more conducive to SOC accumulation than uplands, primarily because of predominant anaerobic conditions. However, the role of microbes and plants in the buildup of SOC under prolonged rice cultivation has not been well explored in the literature. In a millennium-scale paddy soil chronosequence, we used amino sugars (AS) and lignin phenols (LN) as tracers to investigate microbial and plant-derived necromass changes and evaluated their contributions to SOC accumulation with increasing rice cultivation duration. Across the 1000-year rice cultivation process, AS and LN contents increased with SOC accumulation. Soil pH and salinity are considered to play vital roles in regulating the retention of AS and LN. In contrast to the control of soil enzyme activity on LN accrual (e.g., peroxidase), the microbial biomass and fungi-to-bacteria ratio greatly affected AS accumulation in paddy soil. The components of AS and LN also changed with time, exhibiting a significant accumulation of galactosamine and cinnamyl phenol units in the late stage. Long-term rice cultivation is more conducive to the accumulation of bacterial residues. AS demonstrated a greater contribution to SOC than LN compounds within 100 years, whereas the contribution of LN ultimately exceeded AS in the late stage. Concurrently, we found a higher degree of oxidative lignin degradation in younger soils and reduced degradation with increasing duration of rice paddy cultivation. The accumulation of microbial necromass is more than plant necromass in the early stage, and it is the opposite in the late stage. Our results are critical to understand the formation and sequestration processes of SOC in paddy soils. [Display omitted] • Amino sugars (AS) and lignin (LN) accumulate with paddy SOC sequestration over time. • AS reached steady-state equilibrium faster than LN under prolonged rice cultivation. • Soil pH and salinity are vital roles in regulating the retention of AS and LN. • Prolonged rice cultivation can contribute to the accumulation of bacterial residues. [ABSTRACT FROM AUTHOR]

Abstract As a homologous gene encoding microbial alkaline phosphomonoesterase, the expression of phoD is critically controlled by P availability and thus contributes to the mineralization of soil organic P under P-depleted condition. However, its role in the regulation of soil P turnover is largely unknown due to the complex coupling of physiochemical and biological processes in the P cycle, especially in paddy field. We hypothesized that 1) P fertilization would decrease the abundance of phoD gene and change the composition of phoD -harboring microbial community and 2) the high abundance of phoD -harboring microorganisms in P-poor soil would stimulate the synthesis of alkaline phosphomonoesterase, thus mitigating P limitation via the mineralization of organic P. After 42 days of rice growth, the phoD abundance negatively correlated with soil P availability, and it was significantly higher in non-fertilized treatments than in P-fertilized treatments for both rhizosphere and bulk soils. A stronger competition among phoD -harboring microorganisms was detected in non-fertilized soil than in P-fertilized soil, with Bradyrhizobium , Methylobacterium, and Methylomonas being the dominant taxa in all samples. However, the high phoD gene abundance under P-poor condition was mainly due to the growth of rare operational taxonomic units (OTUs) affiliated to Actinobacteria and Cyanobacteria (relative abundance < 3%). Consistent with our hypothesis, the growth of phoD -harboring microorganisms stimulated the hydrolysis of organic P in non-fertilized soil. However, in the P-fertilized treatments, the increase in OTU abundance was accompanied by the depletion of exchangeable P and accumulation of microbial biomass P. Our findings suggest that phoD -harboring microorganisms have the potential to immobilize P in biomass when the supply is sufficient while mineralize organic P under P-poor condition, during which the rare taxa play an important role. Graphical abstract Image 1 Highlights • Competition among phoD -harboring microbes is intense under P-poor conditions. • Increase in phoD gene abundance is related to the growth of rare species. • phoD -harboring microbes store P in biomass under P-sufficient condition. • phoD -harboring microbes might reuse the intracellular P when P is depleted. [ABSTRACT FROM AUTHOR]

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]

Root morphology reflects plant adaptations to phosphorus (P) deficiency. We hypothesized that changes in rice root morphology reflect P deficiency decrease after ferric iron (Fe(III))-bound phosphate (Fe–P) dissolution in low-redox paddy soils. We developed a novel in-situ 32P phosphor-imaging approach under flooding to estimate P uptake by rice roots released from Fe–P dissolution. 32P-labeled ferrihydrite (31 mg P kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots but not microorganisms from direct Fe–P mobilization, or (2) directly mixed with soil to enable roots and microorganisms unrestricted access to the Fe–P. The establishment of low redox conditions (E h values between −176 and −224 mV) drove the reductive dissolution of Fe–P. Rice root-derived organic acids alone were unable to control Fe–P dissolution, and Fe(III) reduction is predominately a microbially-mediated process. Direct root access to Fe–P raised both the number and mean diameter of crown roots and root tips, and increased P uptake by 149–231%. Crown root elongation rate, 32P activities along roots and root tips were 5–133% higher when roots directly accessed Fe–P compared to Fe–P excluded from roots in mesh bags. Iron accumulation on roots depended on the rice growth stage, but not on their access to Fe–P. Roots' access to Fe–P increased rice crown roots elongation and branching and increased P accessibility under P deficiency. [Display omitted] • Fe–P dissolution increased rice root growth and induced root morphological changes. • Increased root tip numbers raised P uptake by roots from the soil. • Organic acids exuded by rice roots were unable to control Fe–P dissolution. • Iron accumulation on root surface was independent of root's access to Fe–P. [ABSTRACT FROM AUTHOR]

A decrease in C inputs from the return of crop residues to soil has occurred in many regions worldwide in recent years. The effects of this decline in C inputs could provide valuable information for assessing the long-term impact of litter C inputs on soil organic C (SOC) in rice paddy soils. The present study aimed to evaluate the response of rice paddy SOC accumulation to changes in actual C inputs in subtropical China, with emphasis on the response of C accumulation to declining C inputs. For this, we used a long-term field experiment on paddy soil in a rice-rice ( Oryza sativa L.) cropping system running from 1990 to 2014. The four treatments were CK (control, no fertilizer), OM (organic matter application), NPK (N, P, and K fertilizer application), and NPKOM (NPK and organic matter application). Organic matter application for the OM and NPKOM treatments included rice straw and green manure that were left in the field after harvest and chopped, along with rice residues with stubbles and roots. In all treatments, C sequestration showed an increasing trend (from 0.207 to 0.880 g kg −1 yr −1 ) in the early and middle stages of the experiment (1990–2006) followed by a decreasing trend (from −0.429 to −0.064 g kg −1 yr −1 ) in the late stage (2007–2014). The trends were more pronounced for the OM and NPKOM treatments than for their CK and NPK counterparts. The changes in SOC stocks were consistent with changes in C inputs ( p < 0.01). During the late stage, yield and litter inputs from crop residues and green manure decreased, quickly affecting SOC stock in paddy soils. This declining trend in annual rice yields was mainly caused by the decline in first rice yields, accounting for 42.3–91.5% of the decrease in annual C inputs. Insufficient P or N and K supply and unfavorable climatic factors (decreases in sunshine duration and both maximum and minimum temperatures) are possible reasons for the decline in first rice yields and green manure biomass in the late stage. Collectively, the results suggest that C stocks in high-productivity paddy soils respond very sensitively to a decline in C inputs. This raises the risk of loss of C stock in paddy soil if, in the long run, a large return of C to soil with crop residues or by other sources, e.g., green manure, cannot be achieved. [ABSTRACT FROM AUTHOR]

Understanding the effects of external organic and inorganic components on soil organic C (SOC) and fertilizer N use efficiency (NUE, in kg of grain per kg of N applied) is essential for better stewardship of domesticated soils. We collected 106 paired-treatment data points from 28 long-term field fertilization trials of subtropical paddy soils with double-rice cropping systems in China. The effects of chemical fertilizer (NPK), NPK plus straw (NPK + s), and NPK plus manure (NPK + m) on rice yield, SOC density, and NUE were assessed. The greatest SOC sequestration rate was found with the use of NPK + m (0.67 Mg ha −1 year −1 ), whereas this value was lower with NPK + s (0.48 Mg ha −1 year −1 ) and NPK (0.30 Mg ha −1 year −1 ). The soil C sequestration rate decreased with the experimental time, leading to a sequestration period of 43, 65, and 55 years to reach a new equilibrium value of SOC for NPK, NPK + s and NPK + m, respectively. Under the same N input condition, the treatment with N fertilizer proportionally replaced by manure (NPK + m) could enhance both rice yield and NUE by 28% and 27%, respectively, whereas the in situ rice straw incorporation (NPK + s) showed no distinct effect. Additional manure amendment on the basis of existing N fertilizer application rate did not have an effect on both rice yield and NUE. In contrast, additional rice straw incorporation decreased NUE by 24%, even though no distinct change of rice yield was found. Our results indicate that application of chemical fertilizer plus manure, rather than rice straw, to paddy fields is a promising practice to enhance SOC accumulation and improve rice yield, as well as the crop N use efficiency in subtropical rice production of China. [ABSTRACT FROM AUTHOR]

• Increased C mineralization in soil caused by frequent glucose additions. • Increasing frequency of glucose additions decreased soil Eh and improved rice straw mineralization. • Frequent glucose additions decreased fungi/bacteria ratio and accelerated rice straw mineralization. Crop straw amendment is a farm management approach for enhancing soil carbon sequestration. Moreover, crop straw residue decomposition rates are affected by irregular exogenous organic carbon secretion or metabolism such as that from root exudates or microbial metabolites. To quantify the mineralization rate of straw residue affected by exogenous organic carbon addition and elucidate the underlying mechanisms, in this study, a total of 0.5 g 13C- labeled dry rice straw was added into pre-incubated soil (equivalent to 200 g dry soil) and incubated for 20 days. Then,1mL of a solution with 50 mg glucose was added only once into the soil on day 21 (G(1a)) or day 35 (G(1b)), twice with 25 mg glucose every 14 days (G(2)) or four times with 12.5 mg glucose every 7 days (G(4)). Soil without glucose addition served as a control. The frequent addition of glucose induced higher soil respiration than that induced by only one addition, and stimulated the mineralization of rice straw carbon compared to that in the control in the following order: G(4) (14.6%) > G(1a) (12.7%) > G(2) (12.3%) > G(1b) (11.2%) = control (11.2%). In general, frequent addition of glucose decreased the soil redox potential (Eh) and fungi to bacteria ratio. Structural equation modeling analysis showed that the rice straw carbon mineralization rate was positively correlated with the 13C-dissolved organic carbon (13C-DOC) content and negatively regulated by Eh. Microbial bioactivity exerted indirect positive effects on rice straw carbon mineralization by influencing the 13C-DOC content. A decrease in the fungi to bacteria ratio induced by frequent addition of glucose improved the microbial bioactivity and enhanced rice straw carbon mineralization. Our results suggested that single exogenous organic carbon additions may underestimate residue or soil organic matter decomposition rates in the field. [ABSTRACT FROM AUTHOR]

Biogeochemical cycles of phosphorus (P) and iron (Fe) are tightly interlinked, especially in highly weathered acidic subtropical and tropical soils rich in iron (oxyhydr)oxides (Fe(III)). The quantitative contribution of the reductive dissolution of Fe(III)-bound inorganic P (P i) (Fe–P) in low-redox paddy soils may cover the demands of rice plants (Oryza sativa L.) and microorganisms. We hypothesized that microbially-driven Fe(III) reduction and dissolution can cover the P demand of microorganisms but not of the young rice plants when the plants' P demand is high but their root systems are not sufficiently developed. We grew pre-germinated rice plants for 33 days in flooded rhizoboxes filled with a paddy soil of low P availability. 32P-labeled ferrihydrite (30.8 mg kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots from directly mobilizing Fe–P (pellets-in-mesh bag treatment), or (2) in the same pellet form but without a mesh bag to enable roots accessing the Fe–P (pellets-no-mesh bag treatment). Without mesh bags, P i was more available leading to increases in microbial biomass carbon (MBC) by 18–55% and nitrogen (MBN) by 4–108% in rooted soil as compared to P i pellets not directly available to roots. The maximum enzyme activities (V max) of phosphomonoesterase and β-glucosidase followed this pattern. During rice root growth, day 10 to day 33, MBC and microbial biomass phosphorus (MBP) contents in both rooted and bottom bulk (15–18 cm) soil gradually decreased by 28–56% and 47–49%, respectively. In contrast to our hypothesis, the contribution of Fe–P to MBP remarkably decreased from 4.5% to almost zero from 10 to 33 days after rice transplantation, while Fe–P compensated up to 16% of the plant P uptake at 33 days after rice transplantation, thus outcompeting microorganisms. Although both plants and microorganisms obtained P i released by Fe(III) reductive dissolution, this mechanism was not sufficient for the demand of either organism groups. [Display omitted] • 32P application can assess plant-microbial competitive dynamics for P absorbed by Fe. • Contribution of Fe–P to MBP decreased from 4.5% to almost zero with rice growth. • Fe–P compensated up to 16% of rice P uptake during 33 days after rice transplantation. • Rice plants outcompeted microorganisms for P at the tillering stage. [ABSTRACT FROM AUTHOR]

Highlights: [•] Straw incorporation in paddy fields enhanced CH4 emissions. [•] Biochar amendment in paddy fields reduced CH4 emissions. [•] N2O emissions increased by biochar, but decreased by straw incorporation. [•] Biochar amendment is a candidate practice to mitigate GHG emissions in paddy fields. [ABSTRACT FROM AUTHOR]

Rice paddies are a significant source and sink of methane (CH 4). The field surface emission and laboratory turnover potential of CH 4 have been extensively studied. However, there is a knowledge gap in underlying mechanisms linking in - situ belowground CH 4 production and oxidation potentials affected by rice growth and fertilization. Here, we implemented an improved soil silicone tube approach and 13C-labeled CH 4 to measure the in-situ production and oxidation of CH 4 in the subsoil (20–30 cm) during rice growth under four long-term (7 years) fertilization treatments. Urea N (uN) fertilization decreased 3-folds CH 4 production as compared to non-fertilized control (Control). In combination with uN, biochar application (uN + Biochar) resulted in similarly low [CH 4 ], but manure fertilization (uN + Manure) stimulated CH 4 production which increased with rice growth and reached 0.6 mmol L−1 of soil tube at mature stage relative to uN. During the rice growth, δ13C–CH 4 values under all fertilizations progressively decreased as compared with the start of the experiment but remained in the range of acetoclastic methanogenesis (−75‰). Pronounced δ13C–CO 2 enrichment under uN + Manure indicated the increasing contribution of CO 2 -reduction pathway. The CH 4 oxidation was highest (20.1 μmol L−1) under uN + Manure at tillering stage, followed by uN (14.6 μmol L−1) at seedling stage. The lowest oxidation potential was under uN + Biochar along with the weakest CH 4 concentration dynamics. Water-locked conditions, low measured O 2 concentration in silicon tubes, as well as the positive relationship between CH 4 oxidation and NO 3 − and DOC suggested a possibility of anaerobic oxidation of CH 4 (AOM). In conclusion, the intensive rice growth during seedling and tillering stages controls subsoil CH 4 concentrations via methanotrophic activity (anaerobic oxidation pathway). Manure fertilization offset the inhibiting effect of mineral N fertilization on CH 4 production but stimulated CH 4 oxidation. Among all fertilization treatments biochar strongly suppressed the CH 4 turnover and can therefore be recommended for greenhouse gas mitigation strategy in cleaner rice production. [Display omitted] • Manure stimulated while urea inhibited the CH 4 production in paddy subsoil. • Biochar enhanced the acetoclastic methanogenesis pathway. • CH 4 oxidation in paddy subsoil increased with NO 3 − and DOC concentration. [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]

Rice paddy agro-ecosystems play an important role in global carbon (C) sequestration. Because of flooding management, paddy soil experience periodical changes in oxygen availability, which may make soil organic carbon (SOC) mineralisation unique as compared to upland or other wetland ecosystems. However, at present, information about the relevant mechanisms involved in paddy SOC mineralisation is limited and unclear. We selected three paddy soils with variable iron (Fe) contents, which were either fumigated with chloroform (to reduce microbial biomass) or remained un-fumigated. Soils were incubated for 78 days in one of three treatments: alternating nonflooded–flooded (NF: moist for 0–30 days (oxygen-abundant) and flooded for 31–78 days (oxygen-limited)), continuously flooded (CF: oxygen-limited), and continuously anaerobically flooded (AF: oxygen-depleted). Fumigation reduced the microbial biomass C by more than 70%. Except for the nonflooded period in the NF treatment, the SOC mineralisation rate, at the late stage of each treatment, was significantly lower in the fumigated than in the un-fumigated soil. A multiple regressions showed that a reduction in dissolved organic C contents over time contributed to the cumulative SOC mineralisation only during the nonflooded period in the NF treatment. Furthermore, the labile C pool size was smaller in the AF treatment relative to the other treatments. These imply that dissolved substrates in oxygen-depleted paddy soil were of greater recalcitrance, most likely due to thermodynamic reasons. SOC mineralisation correlated with changes in the redox potential and the Fe2+ contents in the CF and AF treatments only. This indicates that under oxygen-limited and -depleted conditions, Fe played a significant role as an electron acceptor during SOC mineralisation. Correlation and linear regression analyses also suggest that Fe influenced dissolved organic C contents, and hydrolase and oxidative activities. Our findings show that SOC bioavailability is a rate-limiting factor for SOC mineralisation, but only under oxygen-abundant conditions. However, under oxygen-limited or -depleted conditions, microbial biomass, the recalcitrance of organic C compounds, and the availability of electron acceptors are key regulators in determining the intensity and rate of SOC mineralisation. Image 1 • The 'Regulatory Gate' hypothesis is not always valid for paddy soils. • Mechanisms of SOC mineralisation vary with O 2 conditions in paddy soil. • SOC mineralisation rate is limited by C bioavailability in aerated condition. • Microbial biomass, C recalcitrance and electron acceptors are key regulators under O 2 limitation. • Fe influences soil redox, labile C pool and oxidative activity in paddy soil. [ABSTRACT FROM AUTHOR]