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Microbial communities of soil aggregate-size fractions were explored with molecular and networking assays for topsoil samples from a clayey rice paddy under long-term fertilization treatments. The treatments included no fertilizer (NF) as control, chemical fertilizer only (CF), chemical fertilizer with swine manure (CFM), and chemical fertilizer with rice straw return (CFS). Following a wet-sieving protocol, water-stable aggregates were separated into size fractions of large macroaggregates (L-MacA, >2,000 μm), macroaggregates (MacA, 2,000–250 μm), microaggregates (MicA, 250–53 μm), fine microaggregates (F-MicA, 53–2 μm), and fine clay (F-Clay, <2 μm). Mass proportion was 32.3–38.2% for F-MicA, 23.0–31.5% for MacA, 19.0–23.1% for MicA, 9.1–12.0% for L-MacA, and 4.9–7.5% for F-Clay, respectively. The proportion of MacA was increased, but F-Clay was reduced by fertilization, whereas the mean weight diameter was increased by 8.0–16.2% from 534.8 μm under NF to 621.5 μm under CFM. Fertilization affected bacterial 16S rRNA and fungal 18S rRNA gene abundance in F-MicA and F-Clay but not in aggregates in size larger than 53 μm. However, bacterial and fungal community α-diversities and community structures were quite more divergent among the fertilization treatments in all size fractions. Organic carbon and gene abundance of bacteria and fungi were enriched in both L-MacA and MacA but depleted in F-Clay, whereas microbial Shannon diversity was rarely changed by fraction size under the four treatments. L-MacA and MacA contained more bacteria of r-strategists and copiotrophs, whereas F-MicA and F-Clay were demonstrated with a higher abundance of K-strategists and oligotrophs. Guilds of parasitic and litter saprotrophic fungi were enriched in F-MicA but depleted in L-MacA. Furthermore, most of bacterial and fungal operational taxonomic units were strongly interacted in L-MacA and MacA rather than in MicA and F-Clay. Thus, MacA acted as micro-hotspots enriched with functional and networked microbial communities, which were enhanced with organic/inorganic fertilization in the rice paddy. [ABSTRACT FROM AUTHOR]

A field experiment was conducted in a Cd-contaminated rice paddy field to evaluate the effect of inorganic and organic metal stabilizers on Cd mobility and rice uptake. A dose of inorganic stabilizer of calcium hydroxide (CH), silicon slag (SS), and wheat straw biochar (BC) was amended respectively to topsoil before rice transplanting. Rice production was managed with the same water regime and fertilization practices consistently between treatments including a control without amendment. Samples of topsoil and rice plant were collected at rice harvest to analyze the Cd mobility and uptake by rice. Without affecting rice grain yield, the stabilizers significantly decreased CaCl-extractable Cd in a range of 44 to 75 % compared to the control, corresponding to soil pH changes under the different treatments. Accordingly, Cd concentrations both in rice tissue and in rice grain were very significantly decreased under these treatments. The decrease in rice Cd uptake was correlated to the decrease in extractable Cd, which was again correlated to soil pH change under the different treatments, indicating a prevalent role of liming effect by the amendments. While applied at a large amount in a single year, organic stabilizer of BC decreased Cd extractability by up to 43 % and Cd rice uptake by up to 61 %, being the most effective on Cd immobilization. However, the long-term effect on soil health and potential tradeoff effects with different stabilizers deserve further field monitoring studies. [ABSTRACT FROM AUTHOR]

Soil amendment of biochar (BSA) had been shown effective for mitigating greenhouse gas (GHG) emission and alleviating metal stress to plants and microbes in soil. It has not yet been addressed if biochar exerts synergy effects on crop production, GHG emission, and microbial activity in metal-polluted soils. In a field experiment, biochar was amended at sequential rates at 0, 10, 20, and 40 t ha, respectively, in a cadmium- and lead-contaminated rice paddy from the Tai lake Plain, China, before rice cropping in 2010. Fluxes of soil carbon dioxide (CO), methane (CH), and nitrous oxide (NO) were monitored using a static chamber method during the whole rice growing season (WRGS) of 2011. BSA significantly reduced soil CaCl extractable pool of Cd, and DTPA extractable pool of Cd and Pb. As compared to control, soil CO emission under BSA was observed to have no change at 10 t ha but decreased by 16-24 % at 20 and 40 t ha. In a similar trend, BSA at 20 and 40 t ha increased rice yield by 25-26 % and thus enhanced ecosystem CO sequestration by 47-55 % over the control. Seasonal total NO emission was reduced by 7.1, 30.7, and 48.6 % under BSA at 10, 20, and 40 t ha, respectively. Overall, a net reduction in greenhouse gas balance (NGHGB) by 53.9-62.8 % and in greenhouse gas intensity (GHGI) by 14.3-28.6 % was observed following BSA at 20 and 40 t ha. The present study suggested a great potential of biochar to enhancing grain yield while reducing carbon emission in metal-polluted rice paddies. [ABSTRACT FROM AUTHOR]

The effect of fertilization practices on crop productivity and C storage of cropland soils has been a key focus of research into sustainable agriculture and global change. We present results from a long-term fertilization trial in a rice paddy in the Tai Lake region, China and report variation of rice yields and N efficiency with organic carbon accumulation under different fertilization regimes. The fertilization treatments were no fertilizer application (NF), application of chemical fertilizers only (CF), combined application of chemical fertilizers and pig manure (CFM), and straw return (CFS), respectively since 1987. The rice paddy had been consistently cultivated with double cropping of rice (Oryza sativa) and rape (Brassica campestris) under minimum tillage. The yields of rice grain and rape seeds were recorded each year. Topsoil samples from 0 to 5cm and 5 to 15cm were collected after rape harvest in 2005 and soil organic carbon (SOC) contents and properties of microbial activity were determined. Significant differences in average rice yield, but not rape yield, were observed between the fertilization treatments. A higher and more stable yield of rice was found under CFM and CFS than under CF. Since 1987, there has been a prominent topsoil C accumulation in a range of 0.1–0.4tha−1 yr−1, being greater under CFS and CFM than under CF. Comparing between the fertilized plots, grain productivity and C accumulation was enhanced by 21% and 24%, and 72% and 103% under CFM and CFS compared to CF, respectively. Increased rice productivity was coincident with an increased organic C accumulation rate under fertilization. The coupled effect of increased rice yield and C accumulation may be attributable to the enhanced microbial activity, which was found much higher under combined fertilization. N use efficiency was higher under combined fertilization (by 12.6% and 39.0% for CFM and CFS, respectively) compared to inorganic-only fertilization, meaning that less inorganic N fertilizer would be required for the same level of production, thereby potentially saving C emissions from fertilizer manufacture. This study suggests a win–win effect of combined inorganic/organic fertilization on soil organic carbon accumulation and crop productivity in rice fields through increasing N efficiency possibly by enhanced microbial activity. Well-managed, combined organic/inorganic fertilization could both enhance C storage in soils, and reduce emissions from N fertilizer use, while contributing to high crop productivity in agriculture. [Copyright &y& Elsevier]

Quantifying methane (CH 4) emissions from paddy fields is essential for evaluating the environmental risks of the paddy rice production system, and improving the accuracy of CH 4 modeling is a key issue that needs to be addressed. Based on a database containing 835 field measurements, both single national and region-specific models were established to estimate CH 4 emissions from paddy fields considering different environmental factors and management patterns using 70% of the measurements. The remaining 30% of the measurements were then used for model evaluation. The performance of the region-specific model was better than that of the single national model. The region-specific model could simulate CH 4 emissions in an unbiased manner with R2 values of 0.15–0.70 and efficiency values of 11–60%. The paddy rice type, water regime, organic amendment, latitude, and soil characteristics (pH and bulk density) were identified as the main drivers in the models. By inputting the high-resolution spatial data of these drivers into the established model, the CH 4 emissions from China's paddy fields were estimated to be 4.75 Tg in 2015, with a 95% confidence interval of 4.19–5.61 Tg. The results indicated that establishing and driving a region-specific model with high-resolution data can improve the estimation accuracy of CH 4 emissions from paddy fields. Image 1 • The region-specific methane emission model performed better than the single national model. • Using high-spatial-resolution database can reduce the estimation uncertainty. • The methane emissions from China's cropland were estimated to be 4.75 Tg in 2015. • The uncertainty in methane emissions estimates was as low as −11.8%∼18.1%. The performance of the region-specific model was better than that of the single national model, and inputting high-resolution data lowered the estimation uncertainty of methane emissions from paddy fields. [ABSTRACT FROM AUTHOR]

While the impacts of farm management practices such as fertilization, tillage and straw return on soil organic carbon dynamics in croplands have been widely studied, the effects of irrigation management in irrigated rice paddies have not yet been widely assessed. Changes in plow layer thickness and soil organic carbon content of rice paddies were analyzed using data obtained in a county-level survey of soil fertility conducted in 2005 and 2006 in Guichi County, Anhui Province, China. Both soil thickness and organic carbon content of plow layer showed skewed normal distributions, with their averages of 14.58 ± 3.92 cm, and 16.45 ± 6.02 g/kg, respectively. The irrigation method was found to have significant influences on both plow layer thickness and soil organic carbon content, as the plow layer thickness and soil organic carbon content had an inverse response to the irrigation intensity derived from different irrigation methods. The land-level performance of irrigation/drainage infrastructure and the irrigation water sources were detected to have significant effect on plow layer thickness, but little influence on soil organic carbon content. While the capacity of irrigation/drainage infrastructure had a remarkable effect on soil organic carbon content but little impact on plow layer thickness. However, the irrigation condition for surveyed fields was detected to have little effect on both plow layer thickness and soil organic carbon content. These results indicated that irrigation management should keep the balance between surface erosion on plow layer thickness and soil organic carbon accumulation. Hence, developing new technique for good irrigation infrastructure and water management in future will help soil organic carbon accumulation as well as improve the soil for enhanced crop growth in rice agriculture. [ABSTRACT FROM AUTHOR]

Biochar soil amendment has been increasingly recommended for enhancing soil fertility and crop productivity while reducing greenhouse gas emissions in agricultural soils. However, a clear understanding of the cost benefits and longevity of the positive effects over long term would be a prerequisite for large scale biochar production and application in agriculture. In this study, the long-term effects of a single biochar amendment on soil fertility, crop yield and greenhouse gas emissions were assessed in a six-year field experiment of a fertile rice paddy from Southwest China. The field trial was established in 2010 and was managed under a rice-winter wheat rotation system throughout six years until 2016. The experiment employed a nested design with biochar soil amendment at application rates of 0, 20 and 40 t ha−1 without N fertilization, and 150 kg ha−1 with N fertilization, respectively. Soil properties and crop yields were measured and the emissions of CH 4 and N 2 O were monitored during the rice cultivation period in the 1st, 2nd and 5th year since 2010. Soil pH, organic carbon, total N and available potassium content were all increased under biochar amendment and persisted throughout the six years. However, grain yields across the years were generally not affected by biochar amendment except a yield increase in 2015. This grain yield increase (18.3%) was reported in the wheat season of 2015, when biochar had been applied at 40 t ha−1 in plots along with N fertilizer. Biochar amendment reduced N 2 O emissions from the rice paddies only for the first two seasons following the single amendment. In conclusion, a single but high rate of biochar amendment provided limited and temporary benefits for improving grain yields and reduction of greenhouse gases in the fertile paddy soil. • Long-term lasting effects of a single biochar amendment were investigated; • Biochar provided limited and temporary benefits for grain production; • Biochar addition improved soil fertility and this effect could last at least for 6 years. • Soil fertility improvement with biochar addition did not lead to rice yield increase. • Reduction effect of N 2 O emission with biochar only lasted two rice seasons. [ABSTRACT FROM AUTHOR]

Abstract Abundance and allocation of organic matter, microbial community and their extracellular enzyme activities in soil aggregates had been increasingly concerned for mediating biogeochemical cycle and biodiversity provided by soil. In this study, the variations of soil organic carbon (SOC) accumulation and extracellular enzyme activities in aggregate size fractions of a rice paddy with long-term fertilization treatments were analyzed. These treatments included no fertilizer application (NF) as a control, chemical fertilizer only (CF), chemical fertilizer combined with swine manure (CFM), and with straw residue return (CFS). Hereby, size fractions of soil aggregates respectively of >2000 μm (large macroaggregates), 250–2000 μm (macroaggregates), 53–250 μm (microaggregates), 2–53 μm (fine aggregates), and <2 μm (fine clay) were obtained from undisturbed soil cores following water-dispersion and subsequent wet-sieving using a low energy separation protocol. Of these aggregate fractions, SOC and total nitrogen (N) contents were determined with wet digestion and soil enzyme activities were analyzed with substrate-induction incubation assays. Contents of SOC and total N were both highest in macroaggregates, followed by large macroaggregates and microaggregates, but depleted in fine microaggregates. Activities of carbon-degrading enzymes and N -acetyl-glucosaminidase were markedly enriched in soil aggregate fractions in size >53 μm. Whereas, activity of phenol oxidase was generally concentrated in soil aggregate fractions in size between 2 and 2000 μm while that of peroxidase in size between 53 and 2000 μm. Activity of β -glucosidase and cellobiohydrolase was both significantly elevated under CFS and that of peroxidase under CF, CFM and CFS treatments, but phenol oxidase activity was suppressed under CFS treatment. The enzyme activities of β -xylosidase, β -glucosidase, cellobiohydrolase and N -acetyl-glucosaminidase varied greatly (CVs up to 50%), rather with size fractions than with treatments, while those of α-Glucosidase and phenol oxidase varied moderately (CVs up to 35%), rather with treatments than with size fractions. However, those of acid phosphatase and peroxidase changed slightly (CVs up to 20%), regardless of both size fractions and fertilization treatments. Thus, abundance and enrichment of all the soil enzyme activities analyzed depended not on SOC although the overall extracellular enzyme activity was generally well correlated to SOC abundance, among aggregate size fractions. Only activities of carbon-degrading enzymes were greatly enriched in macroaggregates, which were seen reduced under combined organo/mineral fertilization. This study highlighted macroaggregates as biochemically active micro- hotspots and lower activities of carbon-degrading enzymes with SOC accumulation under long-term rational fertilization in the rice paddy. Graphical abstract Unlabelled Image Highlights • Water stable aggregates separated from a rice paddy under fertilization treatment • Enzyme activities changed inconsistently with C and N levels among size fractions. • Aggregate enzyme activities varied rather with size classes than with fertilization. • Much increased carbon degrading enzyme activities in macro-aggregates • Macroaggregates as biochemical hotpots, sensitive to fertilization practice [ABSTRACT FROM AUTHOR]

Highlights • Mid-season drainage decreased CH 4 emission while increased N 2 O emission. • Mid-season drainage decreased the global warming potential (GWP). • Increasing drainage times did not affect the response of GWP to mid-season drainage. • Organic matter amendment did not affect the response of GWP to mid-season drainage. • GWP decreased under mid-season drainage when N fertilizer rate increases. Abstract Paddy rice cultivation is an important source of global anthropogenic methane emissions. Drainage the flooded soils can reduce methane substantially, but N 2 O emission occur concurrently, which would offset the reduction of methane emission. It remains unclear how mid-season drainage affects the global warming potential (GWP) of CH 4 and N 2 O emissions. In this study, a meta-analysis was conducted to investigate the effect of mid-season drainage on GWP and the factors that control the response of GWP to mid-season drainage. Results showed that mid-season drainage decreased CH 4 emission by 52% while increased N 2 O emission by 242%. The GWP under mid-season drainage decreased by 47% compared to continuously flooding. The yield-scaled GWP under mid-season drainage decreased by 48%. Mid-season drainage had no effect on rice grain yield. Although soil drainage times and organic matter amendment are important factors affecting CH 4 and N 2 O emissions in rice paddy field, the study showed that neither of them had effect on the response of GWP to mid-season drainage. The reduction rate of the GWP under mid-season drainage increased when N fertilization application rate increases from 50 kg ha−1 to >200 kg ha−1. This study demonstrated that CH 4 is still a dominant greenhouse gas in rice paddies under water management with mid-season drainage. Nitrogen fertilization is an important factor that regulates the response of GWP to mid-season drainage. High nitrogen fertilization rate would decrease the overall emission of CH 4 and N 2 O under mid-season drainage. However, increasing drainage times or applying organic fertilizer under mid-season does not change the overall emission rate of CH 4 and N 2 O. [ABSTRACT FROM AUTHOR]

Both elevated atmospheric carbon dioxide (CO 2 ) and rising temperature can alter soil methane (CH 4 ) fluxes, leading to a feedback to climate change. However, predicting this feedback needs to understand the microbial mechanisms involved in CH 4 emissions driven by climate change. A 3-year field measurement of CH 4 fluxes from rice paddies was taken in 2012–2014 to examine their responses to elevated CO 2 (enriched up to 500 μmol mol −1 ) and rising canopy air temperature (above ambient 1.5–2.0 °C) using a free-air CO 2 enrichment (FACE) system. Using real-time PCR and Illumina MiSeq sequencing of 16S rRNA genes, we measured the abundance and composition of methanogenic community in rhizosphere soil of rice paddies in 2014. Elevated CO 2 and rising temperature showed additive effects on CH 4 fluxes and methanogen abundances, where CH 4 fluxes were correlated with methanogen abundances. Elevated CO 2 , rising temperature and their combination increased seasonal CH 4 emissions by 28–120%, 38–74% and 82–143%, respectively. Either elevated CO 2 or rising temperature did not significantly alter the diversity of methanogenic community, and methanogenic genera Methanosaeta , Methanosarcina , Methanobacterium , Methanocella and Methanoregula dominated in rhizosphere soils for all treatments. However, elevated CO 2 induced a shift from acetoclastic to hydrogenotrophic methanogens in their relative abundances. Rising temperature stimulated CH 4 emissions by increasing CH 4 production per individual predominant methanogen genus. Besides the enhancement of soil C substrates and rhizosphere methanogen abundances as previously reported, an additive effect of elevated CO 2 and canopy warming on CH 4 emissions is also associated with elevated CO 2 -induced changes in the composition of methanogenic archaea and warming-stimulated the activity of methanogenic archaea in rice paddies. [ABSTRACT FROM AUTHOR]

Biochar is becoming an environmentally friendly material for remediation of heavy metal contaminated soils and improving food safety. A field trial over four rice seasons was conducted to investigate the use of biochar and low Cd accumulating cultivars on Cd uptake in a heavy metal contaminated soil. Wheat straw derived biochar was applied at 0, 20 and 40 t ha − 1 . Two rice cultivars with differing Cd accumulation abilities were selected in each season. The results showed that both biochar and low Cd affinity cultivars significantly reduced rice grain Cd accumulation. Biochar had no significant effect the first season but thereafter consistently reduced rice grain Cd by a maximum of 61, 86 and 57% over the next three seasons. Zn accumulation in the rice grains was not decreased by biochar application, although available soil Zn was sharply reduced (35–91%). Indica conventional rice cultivars had much lower Cd, but higher Zn and lower Cd/Zn ratios in the grain than indica hybrid cultivars. Biochar was more effective for mitigating grain Cd accumulation in low Cd affinity cultivars than in high affinity cultivars. Soil pH was sustainably increased (up to nearly 1 unit) while available Cd significantly decreased by a maximum of 85% after biochar addition. The translocation of Cd from rice roots to shoots was reduced from 20 to 80% by biochar. Low uptake affinity cultivars combined with biochar reduced late rice grain Cd concentration and Cd/Zn ratios by 69–80% and 72–80%, respectively. It indicated that the management of combining biochar and low Cd affinity cultivars should be an efficient way to remediate Cd contaminated rice paddies and reduce health risk associated with consuming rice from these soils. [ABSTRACT FROM AUTHOR]

Biochar functionality related to soil microbial community changes has not yet been fully understood. In this study, we present a cross site field experiment on bacterial community changes of rice paddies among three sites (Jiangxi province, JX; Hunan province, HN; and Sichuan province, SC) from South China with biochar amended (BSA) at 0, 20 and 40 t ha −1 before rice plantation in 2010. Changes in bacterial abundance and diversity of topsoil (0-15 cm) sampled at rice harvest were assessed. Increases in soil pH, soil organic carbon, total N, soil microbial biomass, as well as bacterial gene copy numbers and diversity indices (phylogenetic diversity, Shannon, Chao1 and OTU richness) were consistently observed under BSA at 40 t ha −1 , though generally insignificant at 20 t ha −1 across the sites. Cluster analysis of both terminal restriction fragment length polymorphism (T-RFLP) profiles and pyrosequencing of the 16S gene indicated a strong impact of biochar on bacterial community composition, though the changes were variable across the sites. In particular, BSA at 20 and 40 t ha −1 greatly increased the relative abundance of Betaproteobacteria (by 54% and 80%) and Deltaproteobacteria (by 164% and 151%) in JX while decreased Betaproteobacteria (by 46% and 52%) and increased Chloroflexi (by 27% and 61%) in SC site, respectively. However, no significant changes were detected in HN site. In addition, some significant but variable changes were observed in the abundance of nitrifying, denitrifying and N-fixing bacteria groups with biochar addition among sites. This study suggested a potential role of biochar in enhancing bacterial abundance, community diversity and modifying the community compositions, particularly of the bacteria involved in N cycling. However, changes in soil microbial structure and functioning related to biochar treatment deserve further studies. [ABSTRACT FROM AUTHOR]

Highlights: [•] Biochar significantly increased soil pH, organic matter and immobilized soil Cd and Pb. [•] Biochar treatment consistently reduced rice Cd and Pb content in three years. [•] Contaminated biochar from the study field contained much higher heavy metals than fresh biochar. [•] Biochar caused metal immobilization primarily due to the precipitation and surface adsorption. [ABSTRACT FROM AUTHOR]

Highlights: [•] Biochar treatment at 20–40tha−1 reduced rice Cd by 20%–90% from metal polluted rice fields. [•] Grain Cd under biochar treatment fell in a safe level by 0.4mgkg−1 in soils with Cd in range 0.16–4.83mgkg−1. [•] Biochar amendment caused Cd immobilization primarily due to the liming effect by biochar. [Copyright &y& Elsevier]

Highlights: [•] A methanogenesis sub-model was developed in the DAYCENT ecosystem model. [•] Methane emissions in rice paddies of China are accurately simulated by the model. [•] DAYCENT accounted for 83% of the variation in methane emissions from 72 sites. [•] DAYCENT is most sensitive to the amount of C substrate available for methanogenesis. [Copyright &y& Elsevier]

• Aggregate size fractions separated of a maize residue amended paddy topsoil. • Macroaggregate pore system investigated with SR-μCT tomography. • A bimodal pore size distribution proved macroaggregation from microaggregates. • Maize manure and biochar enhanced macroaggregate C storage and microporosity. • Maize biochar boosted pore throats and micro-pore networking in macroaggregates. Soil organic carbon (SOC) sequestration in croplands was well known through physical protection of particulae organic carbon (POC) in macroaggregates, particularly with crop residue amendment. How different forms of crop residue could affect development of macroaggregates and their micro-pore system remained unclear. In a field experiment, a rice paddy was amended respectively with fresh (CS), manured (CM) and pyrolyzed (CB) maize residue at 10 t ha−1 in carbon equivalent, in comparison to no residue amendment (CK). In 3 years following amendment, undisturbed topsoil (0–15 cm) cores were collected under the treatments respectively. Following size fractionation of water-stable aggregates and SOC pool analysis, macroaggregate (MAC) samples were characterized with SR-µCT tomography. Compared to CK, bulk SOC storage increased by 13.9 % and 31.3 %, POC storage by 32.6 % and 63.3 % while MAC mass proportion by 7.4% and 24.5%, under CM and CB, respectively. Coincidently, total micro-porosity of MAC was increased by ca 30 % but porosity ratio of connected pores to isolated pores increased almost 1-fold though fractal dimension (FD) increased slightly, both under CM and CB. However, indices of pore throats were seen higher under CB than other treatments though micro-pore networking promoted under both CM and CB. In addition, micro-porosity and pore structure as well as POC were all observed similar between fresh residue and no residue treatments. Thus, crop residue amendment in different forms shifted micro-pore structure and potential functionality of MAC while changed their mass proportion and carbon storage of the paddy topsoil. Overall, crop residue recycled as biochar to paddy could ensure SOC sequestration through enhanced physical protection of POC in MAC, which could further potentially contribute to soil function improvement. How the short-term changes in macroaggregation and the pore system could contribute to the soil fertility and functioning deserve further studies. [ABSTRACT FROM AUTHOR]

To understand how climate change impacts soil fertility and ecosystem functioning, it is necessary to explore how different pools of soil organic matter (SOM) at aggregate scale are functionally affected in their molecular composition. Here, we collected rhizospheric soil at rice harvest from a paddy field in which a simulated climate change experiment was conducted for 6 years, including CO 2 enrichment up to 500 μmol mol−1 (CE), air warming by 2 °C (WA), combination of CO 2 enrichment and air warming (CW), and the ambient environment as control. The soil samples were separated in three aggregate fractions (macroaggregates, 2000-250 μm; microaggregates, 250-53 μm; clay & silt, <53 μm) by wet sieving procedure, their molecular composition was detected by off-line pyrolysis gas chromatography mass spectrometry (GC/MS), and the analysis of microbial communities was conducted by phospholipid fatty acids (PLFAs). The mass proportion of macroaggregates increased by 32%, 55%, and 109%, while that of microaggregates decreased by 30%, 14% and 54%, compared to control under CE, WA, and CW treatments, respectively. The mass proportion of macroaggregates was significantly positively correlated with root biomass, while it was significantly negatively correlated in microaggregates, which suggested that the formation of macroaggregates was derived from microaggregates due to root entanglement, and/or mucilage. The molecular composition of SOM depleted in phenolic compounds (phenols and phenolic compounds) while accumulated in lipids (alcohols, alkanes/alkenes/alkynes and fatty acids) with decreasing aggregate size. The increased yields of lipids and phenolic compounds in macro-and microaggregates under CE treatment were likely related to the enhanced root litter, whereas they were reduced under WA treatment due to increase of fungal dominance. A lesser increment of those compounds was noticed under CW treatment and it was attributed to the antagonistic effect, in which the increment effect of CO 2 elevation can be counteracted by warming. The molecular composition hardly changed in clay & silt fraction under all climatic treatments, thus suggesting that the effect of elevated CO 2 and/or warming on SOM molecular composition faded with decreasing aggregate size. Shared by control and a climatic treatment, changes of common molecules in the whole soils depended on their distribution among aggregates. Moreover, the differences in common molecules distribution induced by climatic treatments were significantly correlated to those in mass proportion of aggregates. Considering that common molecules dominated the molecular abundance across all aggregate fractions, our findings indicate that the alteration of SOM molecular composition in the whole soils under climatic treatments appears to be modified by the variation in mass proportion of aggregates. • Elevated CO 2 and/or temperature increased macro- but lowered micro-aggregation • With decreasing aggregate size phenolic compounds depleted while lipids accumulated • More SOM molecular changes detected in macroaggregates than in fine size fractions • Aggregate alterations by climate change regulated entire soil OM molecular content [ABSTRACT FROM AUTHOR]

While biochar enhanced carbon sequestration and stability of soil organic matter (SOM), changes in organic molecular composition in biochar-amended soils had been poorly addressed. In this study, molecular composition changes of a paddy topsoil 2 years following amendments at 10 t ha−1 OC equivalent with untreated (CS), manured (CM) and charred (CB) maize straw were compared to no amendment (CK). Topsoil SOM was sequentially extracted with ultrapure water (UWE), solvent (TSE), base hydrolysis (BHY) and CuO oxidation (CUO) and molecular compounds in these extracted fractions were detected quantitatively by GC/MS. Compared to CK, SOC content was increased respectively by 12% under CS and CM, and by 36% under CB. Fraction abundance both of UWE and CUO was increased but that of TSE unchanged under CS and CM, while that of BHY unchanged under CS but increased by > 60% under CM and CB, respectively. Under CB relative to CS and CM, abundance of TSE and BHY fractions was greatly increased but that of UWE and CUO unchanged. Specifically, abundances of water-soluble monosaccharides, low molecular weight organic acids and lignin-derived phenols, especially cinnamyl-based monomers, were all enhanced under CS, but abundances of n-alkanols, fatty acids in free lipids and di-acids and hydroxyl fatty acids in bound lipids were decreased under both CS and CM. In contrast, CB amendment increased abundances of n-alkanols, alkanes and sterols in free lipids while enhanced molecular and functional group diversity of UWE and TSE fractions. Overall, short-term crop residue amendment altered the abundance and molecular diversity of OM mainly associated with short-lived UWE and labile TSE fractions and biochar enhanced preservation of plant derived molecules mainly in lipids. Thus, returning crop residue as biochar could be a sustainable approach to enhance not only SOM pool but molecular diversity also in agricultural soils. [Display omitted] • Amendment of untreated, manured and charred maize straw to a rice paddy at carbon equivalent compared in a field experiment. • Molecular changes of topsoil organic matter following amendment explored using sequential extraction - GC/MS detection. • Positive changes in diversity of both single molecules and functional groups of labile organic matter with charred straw. • Relative abundance of labile fraction and its composition indicative of OC dynamics with straw treatments. • Plant-derived biomarkers decreased under fresh straw but preserved under charred straw. [ABSTRACT FROM AUTHOR]

• Rice fields of a clayey paddy without and with biochar 6 years compared in a farm. • Soil carbon increased and soil aggregation enhanced in biochar field. • Microbial growth and enzyme activities promoted greatly in biochar field. • Beneficiary by microbial community composition changes in biochar field. • Legacy of improved soil health for better production over years in biochar field. • Mechanistic link between C increase, aggregation and microbial promotion unraveled. "4 per 1000 Initiative" launched at COP21 has strongly recommended that increasing soil organic carbon (SOC) is a global imperative. However, the strategies to improve agricultural soil functioning and health by an increase in soil C have been poorly addressed. In a rice farm trail, topsoil samples were collected in 2015 respectively in a field amended with biochar (BA) (equivalent to 10 t ha−1) in 2009 and an adjacent field with no biochar (NB). Soil aggregation, biochemical activities and microbial communities of the samples were analyzed with microscopic, micro-biochemical and molecular assays respectively. Compared to NB field, SOC storage increased by 45%, total and available nitrogen pool enhanced by approximately 30% while the bulk density decreased and soil pH unchanged in BA field. A 25% increase in mean weight diameter of water stable aggregates was observed in the BA compared to NB field. Correspondingly, microbial biomass nitrogen and enzyme activities in BA field was enhanced both by approximately 30% compared to NB field. Furthermore, with community structure altered, a moderate (32%) increase in total bacterial abundance and a significant decrease in microbial abundance of amino acid metabolism and fungal pathotrophs were observed in BA field compared to NB field. This could link to the higher (10%) grain yield with lower yield inter-annual variability in BA field compared to NB field, reported previously. All these changes clearly demonstrated a legacy of paddy soil health improvement over years following one time biochar amendment. Overall, addition of biochar to the clayey nutrient rich paddy soil could sustain an increase in SOC, soil aggregation and soil health functioning, with positive changes in microbial community over years. Thus, carbon increase as per required by the "4 per 1000 Initiative" could be a mechanistic driver to enhance soil fertility and improve soil-(plant) health for ensuring food production in world rice agriculture facing land degradation and climate change impacts, which could be assisted by biochar from crop residue in an approach of circular economy. [ABSTRACT FROM AUTHOR]

While crop productivity and carbon flux/partitioning in agroecosystem have been widely addressed, potential changes in organic matter composition through root deposition are still unknown under climate change. In this study, root exudates of rice and rhizospheric topsoil were collected in a paddy field under 6 years of simulated climate changes respective of CO 2 enrichment, canopy air warming, and their combination as compared to the control. The molecular composition of root exudates was analyzed with liquid chromatography/mass spectrometry (LC/MS) while soil organic matter (SOM) was analyzed with pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), along with a phospholipid fatty acid analysis (PLFA) for characterizing soil microbial community. Compared to control, root exudation under elevated CO 2 was enhanced but the exudate composition remained unchanged. On the other hand, root exudation rate under warming was unaffected, but the exudate composition was changed. Relative abundance of phenolic compounds in root exudates under warming, was decreased by 26% but that of amino acids was increased by 53%. However, neither root exudation nor the exudate composition was altered under combined CO 2 and temperature elevation. Furthermore, increases in abundance of heterocyclic nitrogen compounds, phenolic acids and phenols (by 37%, 65%, and 26%, respectively) in SOM were relevant to the increase in bacterial/fungal (B/F) ratio under elevated CO 2. This accumulation was possibly attributed to preferential utilization of the increased root exudates by bacteria. However, the overall reduction of some highly hydrophobic SOM compounds (fatty acids, phenols, and phenolic acids) by 14%, could be induced by accelerated decomposition in line with significant decrease in B/F ratio under warming. Indeed, the decreased relative abundance of phenolic compounds such as p -coumaric acid (by 49%) in root exudates lead to fungal increase, which accelerated SOM decomposition. Yet, the molecular composition of SOM was hardly changed under the combination of elevated CO 2 and temperature. Overall, our findings suggested that in a rice paddy CO 2 enrichment increased root exudation and B/F ratio, while air warming altered the root exudate composition and decreased B/F ratio, resulting in changes of SOM composition. These results indicate that root exudates are a key component for the regulation of SOM dynamics under climate change scenarios. Image 1 • Elevated CO2 increased root exudates rate, while warming altered their composition. • The combined CO2 and temperature elevation did not alter the root exudates. • Exudate utilization by bacteria may accumulate stable SOM compounds at elevated CO2. • Increased fungi accelerated the decomposition of recalcitrant compounds at warming. • The counteraction of CO2 elevation and warming maintained the stable SOM compounds. [ABSTRACT FROM AUTHOR]