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Paddy soils make up the largest anthropogenic wetlands on earth, and are characterized by a prominent potential for organic carbon (C) sequestration. By quantifying the plant‐ and microbial‐derived C in soils across four climate zones, we identified that organic C accrual is achieved via contrasting pathways in paddy and upland soils. Paddies are 39%–127% more efficient in soil organic C (SOC) sequestration than their adjacent upland counterparts, with greater differences in warmer than cooler climates. Upland soils are more replenished by microbial‐derived C, whereas paddy soils are enriched with a greater proportion of plant‐derived C, because of the retarded microbial decomposition under anaerobic conditions induced by the flooding of paddies. Under both land‐use types, the maximal contribution of plant residues to SOC is at intermediate mean annual temperature (15–20°C), neutral soil (pH~7.3), and low clay/sand ratio. By contrast, high temperature (~24°C), low soil pH (~5), and large clay/sand ratio are favorable for strengthening the contribution of microbial necromass. The greater contribution of microbial necromass to SOC in waterlogged paddies in warmer climates is likely due to the fast anabolism from bacteria, whereas fungi are unlikely to be involved as they are aerobic. In the scenario of land‐use conversion from paddy to upland, a total of 504 Tg C may be lost as CO2 from paddy soils (0–15 cm) solely in eastern China, with 90% released from the less protected plant‐derived C. Hence, preserving paddy systems and other anthropogenic wetlands and increasing their C storage through sustainable management are critical for maintaining global soil C stock and mitigating climate change. [ABSTRACT FROM AUTHOR]

Biological nitrogen (N2) fixation as a source of new N input into the soil by free-living diazotrophs is important for achieving sustainable rice agriculture. However, the dominant environmental drivers or factors influencing N2 fixation and the functional significance of the diazotroph community structure in paddy soil across a climatic gradient are not yet well understood. Thus, we characterized the diazotroph community and identified the ecological predictors of N2 fixation potential in four different climate zones (mid-temperate, warm-temperate, subtropical, and tropical paddy soils) in eastern China. Comprehensive nifH gene sequencing, functional activity detection, and correlation analysis with environmental factors were estimated. The potential nitrogenase activity (PNA) was highest in warm-temperate regions, where it was 6.2-, 2.9-, and 2.2-fold greater than in the tropical, subtropical, and mid-temperate regions, respectively; nifH gene abundance was significantly higher in warm-temperate and subtropical zones than in the tropical or mid-temperate zones. Diazotroph diversity was significantly higher in the tropical climate zone and significantly lower in the mid-temperate zone. Non-metric multidimensional scaling and canonical correlation analysis indicated that paddy soil diazotroph populations differed significantly among the four climate zones, mainly owing to differences in climate and soil pH. Structural equation models and automatic linear models revealed that climate and nutrients indirectly affected PNA by affecting soil pH and diazotroph community, respectively, while diazotroph community, C/P, and nifH gene abundance directly affected PNA. And C/P ratio, pH, and the diazotroph community structure were the main predictors of PNA in paddy soils. Collectively, the differences in diazotroph community structure have ecological significance, with important implications for the prediction of soil N2-fixing functions under climate change scenarios. [ABSTRACT FROM AUTHOR]

Hydrophilic and hydrophobic organic compounds extracted from 13C-labelled maize residues were incubated with soils to evaluate their mineralization and priming effect (PE) caused by their utilization by microbial groups. Two soils with contrasting soil properties were collected from well-drained upland and water-logged paddy. Mineralization of the 13C-labelled fractions and their PE were quantified by monitoring the CO2 efflux and 13C enrichment during a 40-day incubation. The composition of main microbial groups (bacteria and fungi) involved in the utilization of 13C-labelled fractions was determined based on phospholipid fatty acids (PLFAs) analysis. At the initial stage (6–24 h), hydrophilic fraction had faster mineralization rate (3.6–70 times) and induced 1.5–10 times stronger PE (positive in upland soil and negative in paddy soil) than those of hydrophobic fraction. The 13C-PLFAs data showed that the incorporation of hydrophilic fraction into bacteria was 11.4–16.4 times greater than that into fungi, whereas the hydrophobic fraction incorporated into fungi was 1.0–1.5 times larger than that into bacteria at day 2. This indicated greater contributions of r-strategists (fast-growing bacteria) for the uptake of hydrophilic fraction versus K-strategists (slow-growing fungi) for hydrophobic fraction. Compared with K-strategists, the r-strategists possessed a much faster metabolism and thus triggered stronger apparent PE by accelerating microbial biomass turnover, resulting in higher mineralization and stronger PE for the hydrophilic than hydrophobic fraction. The slower and less mineralization of both fractions in paddy than in upland soils is due to the suppression of microbial activity and substrate utilization under flooding. At the end of 40-day incubation, the cumulative mineralization of hydrophilic and hydrophobic fractions was similar. Consequnently, microbial mechanisms underlying the utilization of organic compounds with contrasting solubility (hydrophilic or hydrophobic) are crucial for evaluating the stabilization and destabilization (e.g., priming) processes of soil organic matter. [ABSTRACT FROM AUTHOR]

Although cumulative N2O emissions are greater in the winter fallow season than in the rice-growing period, the mechanisms by which the emissions affect fallow paddy fields remain unclear. We aimed to identify N2O flux characteristics and illustrate how key nirS-, nirK- and nosZ-containing denitrifiers affect N2O emission levels in acidic fallow paddy soil. Five water-filled pore space (WFPS) levels were set at 25%, 50%, 75%, 100% and 125%, respectively. During the 48-h-long, high-flux incubation period, the N2O flux was the highest in soil samples with 75% WFPS, followed by those with 100% WFPS. The size of nirS-containing denitrifier community was more sensitive to the shifts in soil moisture and showed a stronger correlation with N2O flux than that of nirK-containing denitrifiers, whereas higher N2O concentrations induced an increase in the levels of nosZ-containing bacteria. After incubation for 48 h, nirK- and nosZ-denitrifying bacterial composition varied remarkably under 50%, 75%, and 100% WFPS treatments. However, the composition of nirS-containing denitrifying bacterial community gradually varied with an increase in soil moisture from 25% to 100% WFPS. Certain dominant OTUs of nirK- nirS- and nosZ-containing denitrifiers were highly abundant, especially under treatments of 50%, 75% and 100% WFPS, which were closely associated with the N2O flux. Thus, nirK, nirS and nosZ-containing denitrifiers respond to soil moisture differently, and enriched species might mainly be involved in controlling N2O flux in fallow paddy soils via denitrification, while the abundance of nirS-containing denitrifiers might affect N2O emission levels more significantly than that of nirK-containing denitrifiers. [ABSTRACT FROM AUTHOR]

Microbial residues are key components of stable soil organic C (SOC). However, the accumulation patterns of fungal and bacterial residues across climate regions are largely unknown, especially in paddy soils. In this study, the amounts of microbial-derived amino sugars (AS) with their constituents, glucosamine (GlcN), galactosamine (GalN), and muramic acid (MurN, a biomarker of bacterial residues) were quantified in paddy soils, which were collected from mid-temperate, warm-temperate, subtropical, and tropical climate regions across eastern China. The contents of total AS and fungal-derived GlcN (F-GlcN, a biomarker of fungal residues) were lowest in the warm-temperate region, but not significantly different among the other three climate regions. The MurN content and its contribution to SOC accumulation were higher in the warmer and wetter regions (subtropic and tropic) than in the cooler and drier ones (mid-temperate and warm-temperate). Consequently, the ratio of F-GlcN to MurN was lower in the warmer and wetter regions (8.5–15.4) than in the cooler and drier ones (12.8–28.8). These results illustrate that the bacteria participating in SOC transformation and stabilization in paddy soils exerted more prominent activities in the warmer and wetter regions than in the cooler and drier regions. Structure equation models emphasize that the contrasting patterns of fungal and bacterial residues' contribution to SOC accumulation in paddy ecosystems along the latitudinal gradient were mainly attributed to their different responses to the climate factors of temperature and precipitation. [ABSTRACT FROM AUTHOR]

This study aimed to trace the incorporation of 13C-labeled hydrophilic and hydrophobic compounds extracted from maize straw into bacterial and fungal phospholipid fatty acids (PLFAs) in upland and paddy soils. In both soils, the amounts of bacterial 13C-PLFAs recovered from hydrophilic C were 1.2–2.8 times higher than those from hydrophobic C. By contrast, the amounts of fungal 13C-PLFAs recovered from hydrophobic C were 3.8–12.8 times greater than those from hydrophilic C. These results indicate that bacteria have a preference for the uptake of hydrophilic compounds, whereas fungi prefer hydrophobic compounds in both soils. The fungal preference for hydrophobic compounds is stronger than bacterial preference for hydrophilic compounds. This is partly owing to the efficient incorporation of hydrophobic compounds into actinomycete PLFA of 10Me18:0 in upland soil and bacterial PLFA of 18:1ω7c in paddy soil. Consequently, the specific uptake of hydrophilic and hydrophobic compounds from plant residues by bacteria and fungi will lead to divergent C pathways in soil. • Bacteria prefer to take up hydrophilic C. • Fungi prefer to take up hydrophobic C. • Fungal preference for hydrophobic C is stronger than bacterial preference for hydrophilic C. [ABSTRACT FROM AUTHOR]

[Display omitted] • We quantified microbial anabolism (growth and necromass formation) in agricultural soils. • Stronger microbial 13C incorporation and necromass formation in upland than paddy soil. • Microbial growth and necromass formation in warm temperate were the lowest in upland. • Weaker growth but stronger necromass formation in warmer than cooler climates in paddy. • Consistent pattern of necromass formation between short-term and long-term periods. Well-drained upland soils generally have stronger microbial catabolism during organic C transformation than water-logged paddy soils. However, the intensity of microbial anabolism and necromass formation processes in these contrasting agricultural soils is unclear. To quantify these processes, 40 pairs of adjacent upland and paddy soils collected from four climates (mid-temperate, warm temperate, subtropics, and tropics) across eastern China were incubated with 13C-labeled root exudates under simulated field water conditions for 50 days. Upland soil collected from warm temperate exhibited a higher 13C incorporation into living microbial biomass than other climates. In contrast, the lowest newly formed necromass was detected due to the inhibition of fungal anabolism under the high pH condition. Paddy soils collected from cooler climates (mid-temperate and warm temperate) exhibited faster microbial biomass growth than those from warmer climates (subtropics and tropics). Still an opposite trend was observed for microbial necromass accumulation, possibly because of the faster turnover rate of microbial biomass induced by the high N availability in warmer climates. Regardless of climates, 13C incorporated into living microbial biomass (phospholipid fatty acids) was 1.4–2.6 times higher in upland than paddy soils, resulting in 1.8–3.9 times greater accumulation of newly-formed microbial necromass in the former. This was mainly caused by the stronger fungal anabolism (2.5–5.6 times higher) due to the oxygen-sufficient condition of upland soil. Our findings highlighted the weaker accumulation but stronger stability of organic C stored in upland soils because of the greater microbial catabolism and anabolism during organic C transformation. [ABSTRACT FROM AUTHOR]

Regularly flooded rice paddies usually show greater soil organic C and microbial-derived C contents than adjacent upland counterparts, but the soil microbial physiological traits under these two different land uses spanning regions remain unclear. Here, we collected 40 pairs of adjacent paddy and upland soils from four different climates (mid temperate, warm temperate, subtropics, and tropics) across eastern China to determine the microbial growth, respiration, and C use efficiency using the 18O-H 2 O incubation method. Upland soils from warmer climates exhibited lower microbial growth but higher respiration normalized to microbial biomass C (q Growth and q Respiration , respectively) than those from cooler climates, since the lower soil pH and higher clay content in warmer climates induced a shift from microbial growth to respiration. Whereas, paddy soils from warmer climates had consistently lower q Growth and q Respiration than cooler climates , probably due to the long term water-logged condition decreased the sensitivity of microbial metabolism in response to lower pH. Paddy soils had higher q Growth , but lower q Respiration than upland soils, resulting in a greater C use efficiency regardless of climate zones. The difference in microbial C use efficiency between paddy and upland soils was positively correlated to their difference in soil organic C content. From the perspective of microbial C metabolism, the greater organic C accumulation in paddy than that in upland soils is attributed to the weaker microbial uptake of organic C and stronger microbial anabolism under the water-logged condition. • Paddy soils in warmer climates had lower q Growth and q Respiration than cooler climates. • Upland soils in warmer climates had lower q Growth but higher q Respiration than in cooler climates. • Paddy soils had higher C use efficiency than upland soils regardless of climate zone. [ABSTRACT FROM AUTHOR]

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

Denitrification is important for nitrogen balance in agricultural ecosystems. It is well established that the abundance and structure of denitrifier are strongly influenced by environmental factors. However, knowledge of how nirS - and nirK -harboring microbial communities vary in paddy soils of different climatic regions is lacking, along with how these microbes are associated with denitrification potential. In this study, forty paddy soils from tropical, sub-tropical, warm-temperate, and mid-temperate climate zones were collected. The results showed that soils from the sub-tropical zone had the highest denitrification rates. The abundances of nirS and nirK had a strong negative association with C/N ratio and clearly varied among the four climate zones. Variation in nirS- and nirK- containing communities existed across sampling sites from each climate zone in terms of α- and β-diversity. Soil pH and climate factors significantly affected community diversity. Network analysis revealed that different climate regions had similar keystone taxa like Azospira , and Achromobacter , which were significantly related to denitrification rates. Structural equation modeling indicated that the differences in denitrifying enzyme activity among the four climate zones were mostly explained by climate factors, soil pH, and nirS biodiversity. Specifically, the biodiversity of nirS was more important than that of nirK in regulating potential denitrification activity in paddy soil, suggesting that nirS- type denitrifiers may have high activity under anaerobic conditions. Our results allow a deeper insight into the relative contribution of nirS- and nirK- containing communities to the soil denitrification activity in paddy soils across climate zones. This study highlighted the need of manipulation experiment to explain how denitrifier biodiversity affect potential denitrification activity in the future. • Soils from the sub-tropical zone have the highest denitrification rates. • nirS and nirK gene abundances show clear variations among the four climate zones. • nirS and nirK gene abundances have strong negative associations with C/N ratio. • The pH, C/N and climate factor significantly affected the community structure. • nirS gene biodiversity is important to regulate denitrification rates in paddy soil. [ABSTRACT FROM AUTHOR]