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Rice ( I Oryza sativa i L.) is the world's second most important cereal crop and a staple food for approximately 50% of the global population. In their study on C allocation, Zang et al. ([16]) observed no immediate effects on C allocation following C assimilation but found that all administered C remained below ground. Tracking the photosynthesized carbon input into soil organic carbon pools in a rice soil fertilized with nitrogen. [Extracted from the article]

Aims: The rhizosphere has ecological importance as a microbial hotspot to understand the 'real' processing rates of element cycles without being misled by inactive bulk soil. It is thus essential to estimate the rhizosphere size and its response to anthropogenic distribution during crop growth. Methods: In situ β-glucosidase, cellobiohydrolase (C-acquiring), and acid and alkaline phosphatase (P-acquiring) activity was examined using soil zymography in rice rhizosphere. Temporal dynamics of enzymatic rhizosphere size under phosphate and cellulose fertilization were calculated based on the expansion of enzyme activity hotspot. Results: After 35 days of root development, radial expansion of cellobiohydrolase and acid phosphatase from the root centre to bulk soil was further than that of β-glucosidase and alkaline phosphatase, respectively. Root development expanded β-glucosidase hotspot in rhizosphere, but inhibited rhizosphere expansion of phosphatase activities. P fertilization caused strong N competition between plants and microorganisms, reducing rhizosphere expansion of all tested enzyme activities. Cellulose had no significant effects on C-acquiring enzyme activity expansion, but led to extended acid and alkaline phosphatase hotspots in the rice rhizosphere under P fertilization. Conclusion: i) Plant growth stage affects the rice enzymatic rhizosphere size; ii) P fertilization in P-limited soil enhances rhizosphere enzyme activities but reduces the radial expansion; iii) non-labile C application affects enzymatic rhizosphere expansion in an enzyme-specific manner interactively with P fertilization. [ABSTRACT FROM AUTHOR]

Background and aims: Water and nutrient management influences the allocation and stabilisation of newly assimilated carbon (C) in paddy soils. This study aimed to determine the belowground allocation of C assimilated by rice and the subsequent C stabilisation in soil aggregates and as mineral-organic associates depending on combined alternate wetting and drying (AWD) versus continuous flooding (CF) and P fertilisation. Methods: We continuously labelled rice plants in 13CO2 atmosphere under AWD versus CF water management, and at two P fertilisation levels (0 or 80 mg P kg−1 soil). The 13C allocation to soil and its incorporation into the wet-sieved aggregate size classes and density fractions of the rhizosphere and bulk soils were analysed 6, 14, and 22 days after the labelling was started (D6, D14, and D22, respectively). Results: Under both water regimes and P fertilisation levels, the proportion of photoassimilates was the highest in the silt- and clay-size aggregate classes and in the mineral-associated fraction. On D6 and D14, P fertilization resulted in smaller 13C incorporation into soil, independent of water management. In the rhizosphere soil, at D22, P fertilisation increased 13C incorporation over no P amendment in macroaggregates (>250 μm) by 32% (AWD) and 42% (CF), in microaggregates (250–53 μm) by 97% (CF), and in the silt + clay size class (<53 μm) by 83% (CF). Further, P fertilisation led to larger 13C incorporation into the rhizosphere soil light fraction (75% at AWD and 90% at CF) and dense fraction (38% and 45%, respectively), and into the bulk soil macroaggregates (71% and 78%, respectively). Conclusions: Phosphorus fertilisation increased the contents of recent photoassimilates in soil aggregate classes with longer residence time as well as of the particulate organic matter with the continuation of plant growth. This positive response of the stabilisation of recent plant photosynthates in soil to P fertilisation can increase the potential of paddy soil for C sequestration. This potential is not limited by the introduction of alternate wetting and drying water-saving technique. [ABSTRACT FROM AUTHOR]

Background and aims: Carbon (C) and nitrogen (N) availability in soil change microbial community composition and activity and so, might affect soil organic matter (SOM) decomposition as well as allocation of plant assimilates. The study was focused on interactions between C and N availability and consequences for rhizodeposition and microbial community structure in paddy soil. Methods: Rice continuously labeled in a 13CO2 atmosphere was fertilized with either carboxymethyl cellulose (CMC) (+C), ammonium sulfate (+N), or their combination (+CN), and unfertilized soil was used as a control. 13C was traced in aboveground and belowground plant biomass, soil organic matter, and microbial biomass. Microbial community composition was analyzed by phospholipid fatty acids (PLFAs). Results: +CN application led to a higher yield and lower root C and N content: 13C assimilated in shoots increased by 1.39-fold and that in roots decreased by 0.75-fold. Correspondingly, after +CN addition, 13C from rhizodeposits incorporated into SOM and microorganisms decreased by 0.68-fold and 0.53-fold, respectively, as compared with that in the unfertilized soil. The application of +C or + N alone resulted in smaller changes. CMC led to a 3% of total N mobilized from SOM and resulted in a positive priming effect. Both fertilizations (+C, +N, or + CN) and plant growth stages affected soil microbial community composition. With decreasing microbial biomass C and N, and PLFA content under +CN amendment, +CN fertilization decreased Gram-positive (G+)/ Gram-negative (G-) ratios, and resulted in lower G+ bacteria and fungi abundance, whereas G- and actinomycetes were stimulated by N fertilization. Conclusions: Organic C fertilization led to a positive N priming effect. Organic C and mineral N application decreased C input by rhizodeposition associated with lower 13C recovery in SOM and microbial incorporation. C and N addition also altered microbial community composition, as +CN decreased content of microbial groups, such as G+ bacteria and fungi, but +N stimulated G- bacteria and actinomycetes. [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]

Soil organic matter (SOM) mineralization is affected by various abiotic and biotic factors, as well as the input of exogenous organic substances. Our previous studies have shown that SOM mineralization in flooded rice paddies is lower than that in adjacent upland soils in subtropical agro-ecosystems. However, the main factors contributing to the differences in SOM mineralization remain unclear. To compare the effects of biotic and abiotic factors on SOM mineralization between upland and paddy soils, we incubated upland and paddy (flooded) soils with three low molecular weight organic substances (LMWOS, i.e., glucose, acetic acid, or oxalic acid) for 30 days under field conditions. Generally, the average CO 2 efflux from upland soil was higher than that in paddy soil with the same LMWOS addition. The total content of phospholipid fatty acids (PLFAs) in paddy soil was 2–5 times higher than that of upland soil, irrespective of the LMWOS added. Redundancy analyses indicated that microbial community composition was influenced mainly by the low redox potential (Eh) and dissolved organic carbon in paddy soil. Structural equation modeling revealed that, among abiotic factors, temperature exerted indirect effects on SOM mineralization by influencing biotic factors in both soils; Eh has a positive and direct effect on SOM mineralization in paddy soil. In terms of biotic factors, SOM mineralization in upland soil was mainly regulated by the quantity of bacteria. In paddy soil, SOM mineralization was largely influenced by the ratio of fungal to bacterial PLFAs and peroxidase activity. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO 2 ) and methane (CH 4 ) production in paddy soils play a crucial role in the global carbon (C) cycle and greenhouse gas emissions. A rhizosphere priming effect (RPE) may change these emissions, but the relationships between RPE, CH 4 emission, and the effect of N fertilization are unknown. We investigated the RPE on CO 2 and CH 4 emissions and their dependence from N fertilization in a 13 CO 2 continuous labelling experiment by partitioning total CO 2 and CH 4 derived from roots and soil organic matter (SOM). Because of plant-derived CO 2 , rice plants strongly increased total CO 2 emission compared to that from unplanted soil. SOM-derived CO 2 and CH 4 increased in the presence of roots but decreased after N fertilization. The RPE for CO 2 at an early growth stage (≤40 days) was negative: −1.3 and −1.9 mg C day −1 kg −1 soil without and with N fertilization, respectively. However, 52 days after transplanting, RPE for CO 2 got to positive. The RPE for CH 4 increased gradually up to 1.6 and 0.5 mg C day −1 kg −1 soil at the end of the experiment without and with N fertilization, respectively. Moreover, the RPE for CH 4 got half of the RPE for CO 2 after 64 days showing the relevance of CH 4 emissions for greenhouse gases balance and C cycling in paddy ecosystems. The RPE for CO 2 and CH 4 emissions increased with microbial biomass content and activities of xylanase and N -acetylglucosaminidase. Supporting the results to RPE, the enzyme activities decreased with N fertilization, suggesting that reduced N limitation decreased microbial potential to mine N from SOM. In conclusion, for the first time we showed that root-microbial interactions stimulated SOM mineralization in rice paddies through rhizosphere priming effects not only for CO 2 but also for CH 4 , but the RPE decreased with N fertilization. [ABSTRACT FROM AUTHOR]

In this study, we assessed the interactive effects of phosphorus (P) application and irrigation methods on the abundances of marker genes (cbbL, cbbM, accA and aclB) of CO2-fixing autotrophs. We conducted rice-microcosm experiments using a P-limited paddy soil, with and without the addition of P fertiliser (P-treated-pot (P) versus control pot (CK)), and using two irrigation methods, namely alternate wetting and drying (AWD) and continuous flooding (CF). The abundances of bacterial 16S rRNA, archaeal 16S rRNA, cbbL, cbbM, accA and aclB genes in the rhizosphere soil (RS) and bulk soil (BS) were quantified. The application of P significantly altered the soil properties and stimulated the abundances of Bacteria, Archaea and CO2-fixation genes under CF treatment, but negatively influenced the abundances of Bacteria and marker genes of CO2-fixing autotrophs in BS soils under AWD treatment. The response of CO2-fixing autotrophs to P fertiliser depended on the irrigation management method. The redundancy analysis revealed that 54% of the variation in the functional marker gene abundances could be explained by the irrigation method, P fertiliser and the Olsen-P content; however, the rhizosphere effect did not have any significant influence. P fertiliser application under CF was more beneficial in improving the abundance of CO2-fixing autotrophs compared to the AWD treatment; thus, it is an ideal irrigation management method to increase soil carbon fixation. [ABSTRACT FROM AUTHOR]

The rhizosphere is the most dynamic hotspot of microbial activity in the soil. Despite these dynamics, the spatial pattern of many rhizosphere properties may remain stable because they are continuously reproduced in the changing environment. Low substrate concentration can strongly reduce the rate response of an enzymatic reaction subjected to increased temperature and is recognized as a canceling effect on enzyme temperature sensitivity. Carbon input from rhizodeposits affects C availability in the rhizosphere, and thus the enzyme activities responsible for organic matter decomposition, and their temperature sensitivities, upset the dynamics and stability of biochemical processes in the rhizosphere. However, it is unclear whether a canceling effect occurs in the rhizosphere. We studied temperature effects on chitinase and phosphatase during rice ( Oryza sativa L.) growth at 18 and 25 °C. The spatial distribution of enzyme activities was imaged using soil zymography and showed that the overall activities of these enzymes increased with temperature but decreased with rice growth. The temporal dynamics of hotspot areas were enzyme-specific. During growing days 14–30, hotspot areas decreased from 2-2.5% to 0.3–0.5% for chitinase, but increased from 2% to 6–7% for phosphatase. The distribution pattern of both enzymes shifted from being dispersed throughout the soil to being associated with the roots. For the first time, we showed the extent of rhizosphere enzyme activity in paddy soil and demonstrated that it is temporally stationary and independent of temperature. However, the temperature sensitivity of enzyme activities declined radically ( Q 10 ∼1.3–1.4) at the root surface compared to that of bulk soil ( Q 10 ∼1). We conclude that the spatio-temporal pattern of rhizosphere enzymatic hotspots is mainly affected by plant growth. High temperature sensitivity ( Q 10 > 1) at the root-soil interface for the tested enzymes revealed that warming will lead to faster nutrient mobilization in the rhizosphere than in root-free soil. [ABSTRACT FROM AUTHOR]

Many soil functions are modulated by processes at soil biogeochemical interfaces (BGIs). However, characterizing the elemental dynamics at BGIs is hampered by the heterogeneity of soil microenvironments. In order to investigate the processes of BGI formation in an upland soil (Mollisol) and a paddy soil (Oxisol), we developed a SoilChip method by assembling dispersed soil particles onto homogeneous 800-μm-diameter microarray chips and then submerging them in a solution that contained dissolved organic matter (OM) extracted from one of the two soils. The chips with Mollisol particles were incubated at 95–100% humidity, whereas the chips with Oxisol particles were incubated at 100% humidity. Dynamics of individual elements at the soils' BGIs were quantitatively determined using X-ray photoelectron spectroscopy (XPS). Distinct differences in the soil-microbe complexes and elemental dynamics between the Mollisol and Oxisol BGIs suggested that the formation of specific BGIs resulted from the complex interaction of physical, chemical, and microbial processes. By integrating the SoilChip and XPS, it was possible to elucidate the dynamic formation of the two different soil BGIs under standardized conditions. Therefore, the SoilChip method is a promising tool for investigating micro-ecological processes in soil. [ABSTRACT FROM AUTHOR]

Carbon monoxide (CO)-oxidizing bacteria are active consumers of atmospheric CO, undergoing a CO oxidation process that is catalyzed by carbon monoxide dehydrogenase (CODH). Little is known about the phylogeny of a group of bacterial CO-oxidizers known as the BMS clade, which possess putative CODH genes, or their abundances in soils. In this study coxL genes indicative of bacteria belonging to the BMS clade were amplified from DNA directly extracted from four vegetated soils: paddy rice (PR) soil, maize cropland (ML) soil, tea plantation (TP) soil, and natural forest (NF) soil. Quantitative PCR (qPCR) analysis showed that BMS coxL gene copy numbers ranged from 1.47 × 10 7 to 7.40 × 10 8 copies g −1 dry soil and differed significantly among PR, ML, TP, and NF soils (P < 0.05). CoxL gene abundance appeared to be linked to total P and Olsen P concentrations, as revealed by correlation analysis (P < 0.05). Phylogenetic analysis of BMS coxL sequences indicated a diverse clade of BMS CO oxidizers closely related to Bradyrhizobium sp. ARR65, Rhizobiales GAS188, Paraburkholderia phytofirmans, and Tistlia consotensis. However, the community compositions of BMS coxL -containing bacteria differed among soils. Collectively, these results indicate a high abundance and diversity of BMS clade CO oxidizers in a wide variety of vegetated soils. [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]

This study aimed to better understand the stabilisation of rice rhizodeposition in paddy soil under the interactive effects of different N fertilisation and water regimes. We continuously labelled rice ('Zhongzao 39') with CO under a combination of different water regimes (alternating flooding-drying vs. continuous flooding) and N addition (250 mg N kg urea vs. no addition) and then followed C incorporation into plant parts as well as soil fractions. N addition increased rice shoot biomass, rhizodeposition, and formation of C (new plant-derived C) in the rhizosphere soils under both water regimes. By day 22, the interaction of alternating flooding-drying and N fertilisation significantly increased shoot and root C allocations by 17 and 22%, respectively, over the continuous flooding condition. The interaction effect also led to a 46% higher C allocation to the rhizosphere soil. Alone, alternating water management increased C deposition by 43%. In contrast, N addition increased C deposition in rhizosphere soil macroaggregates under both water regimes, but did not foster macroaggregation itself. N treatment also increased C deposition and percentage in microaggregates and in the silt and clay-size fractions of the rhizosphere soil, a pattern that was higher under the alternating condition. Overall, our data indicated that combined N application and a flooding-drying treatment stabilised rhizodeposited C in soil more effectively than other tested conditions. Thus, they are desirable practices for improving rice cropping, capable of reducing cost, increasing water use efficiency, and raising C sequestration. [ABSTRACT FROM AUTHOR]

CO assimilation by autotrophic microbes is an important process in soil carbon cycling, and our understanding of the community composition of autotrophs in natural soils and their role in carbon sequestration of these soils is still limited. Here, we investigated the autotrophic C incorporation in soils from three natural ecosystems, i.e., wetland (WL), grassland (GR), and forest (FO) based on the incorporation of labeled C into the microbial biomass. Microbial assimilation of C (C-MBC) differed among the soils from three ecosystems, accounting for 14.2-20.2% of C-labeled soil organic carbon (C-SOC). We observed a positive correlation between the cbbL (ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) large-subunit gene) abundance, C-SOC level, and C-MBC concentration confirming the role of autotrophic bacteria in soil carbon sequestration. Distinct cbbL-bearing bacterial communities were present in each soil type; form IA and form IC RubisCO-bearing bacteria were most abundant in WL, followed by GR soils, with sequences from FO soils exclusively derived from the form IC clade. Phylogenetically, the diversity of CO-fixing autotrophs and CO oxidizers differed significantly with soil type, whereas cbbL-bearing bacterial communities were similar when assessed using coxL. We demonstrate that local edaphic factors such as pH and salinity affect the C-fixation rate as well as cbbL and coxL gene abundance and diversity. Such insights into the effect of soil type on the autotrophic bacterial capacity and subsequent carbon cycling of natural ecosystems will provide information to enhance the sustainable management of these important natural ecosystems. [ABSTRACT FROM AUTHOR]

Although it has been separately reported that biochar primes the decomposition of soil organic matter (SOM) or fresh organic matter, little is known about the simultaneous effects of biochar on SOM versus plant litter mineralization. We applied dual 13 C/ 14 C isotopic labels to partition soil CO 2 efflux and C pools into three sources: SOM, litter and biochar. Biochar made by slow pyrolysis (400 °C) of 14 C labeled residues of rice ( Oryza sativa , C3) and maize ( Zea mays , C4) litter were added separately or in combination to a silty Fluvisol with a C3 isotopic signature and incubated at 25 °C over a period of 6 months. Biochar decomposition was very slow, with a mean rate of 0.017% d −1 . Approximately 63% of biochar-derived CO 2 was produced in the first month. Mixing with litter reduced biochar mineralization by 14%. Addition of biochar alone to soil induced a cumulative positive priming effect (0.24 mg C g −1 soil) on SOM decomposition over 183 days, a much smaller effect than litter-induced priming (1.05 mg CO 2 -C g −1 soil). Compared to soils with only litter amended, biochar and litter added in combination decreased SOM mineralization by 19% while increasing litter mineralization by 6.9%, with no net changes in total CO 2 release. Increased litter- but not SOM-derived C in microbial biomass in the presence of biochar suggested that biochar caused preferential microbial utilization of litter over SOM. Given that immobilization of mineral N in the soil-litter mixture was markedly enhanced following the addition of biochar, we proposed that the biochar-induced preferential microbial utilization of litter over SOM was due primarily to alterations in N cycling. In conclusion, the priming effects of litter on SOM are changed by the presence of biochar. [ABSTRACT FROM AUTHOR]

Background and aims: The cycling of photosynthate carbon (C) released in the rhizosphere has significant implications for C sequestration, microbial activities, and nutrient availability in the soil. It is known that the soil organic matter (SOM) content affects the nutrient status, root growth, rhizodeposition, and microbial composition and activity; however, the effects of SOM and consequently of soil fertility on the belowground allocation and dynamics of photosynthetic C remain unknown. Methods: To examine the effects of SOM on the allocation and dynamics of photosynthetically fixed C, rice plants grown on soils with low (0.5 %), moderate (1.4 %), or high (3.4 %) C content were labeled with CO and harvested six times in one month. Results: The highest C amount was released from the roots into the soil with high SOC content, whereas the opposite pattern was observed for CO losses. Microbial C increased with C in SOM, when soil C content was low or moderate, but decreased when C content was high. At 30 d after labeling, rice plants allocated 2560 kg C ha, 3030, kg C ha, and 4580 kg C ha in the soil with low, moderate, and high SOC content, respectively, accounting for a rhizodeposition of approximately 13 %, 15 %, and 30 %, respectively. Most of the root-derived C in low SOM soil was mineralized quickly. In contrast, high and moderate SOM content led to higher incorporation of rhizodeposits into SOM and higher belowground C protection against microbial decomposition. Conclusions: We concluded that SOM content and consequently, soil fertility play a crucial role in the amount of photosynthates allocated by the plant into the soil and C stabilization. A high SOM level is maintained by the high C input and has longer stability. [ABSTRACT FROM AUTHOR]

Rhizodeposits have received considerable attention, as they play an important role in the regulation of soil carbon (C) sequestration and global C cycling and represent an important C and energy source for soil microorganisms. However, the utilization of rhizodeposits by microbial groups, their role in the turnover of soil organic matter (SOM) pools in rice paddies, and the effects of nitrogen (N) fertilization on rhizodeposition are nearly unknown. Rice ( Oryza sativa L.) plants were grown in soil at five N fertilization rates (0, 10, 20, 40, or 60 mg N kg soil) and continuously labeled in a CO atmosphere for 18 days during tillering. The utilization of root-derived C by microbial groups was assessed by C incorporation into phospholipid fatty acids. Rice shoot and root biomass strongly increased with N fertilization. Rhizodeposition increased with N fertilization, whereas the total C incorporation into microorganisms, as indicated by the percentage of C recovered in microbial biomass, decreased. The contribution of root-derived C to SOM formation increased with root biomass. The ratio of C in soil pools (SOM and microbial biomass) to C in roots decreased with N fertilization showing less incorporation and faster turnover with N. The C incorporation into fungi (18:2ω6,9c and 18:1ω9c), arbuscular mycorrhizal fungi (16:1ω5c), and actinomycetes (10Me 16:0 and 10Me 18:0) increased with N fertilization, whereas the C incorporation into gram-positive (i14:0, i15:0, a15:0, i16:0, i17:0, and a17:0) and gram-negative (16:1ω7c, 18:1ω7c, cy17:0, and cy19:0) bacteria decreased with N fertilization. Thus, the uptake and microbial processing of root-derived C was affected by N availability in soil. Compared with the unfertilized soil, the contribution of rhizodeposits to SOM and microorganisms increased at low to intermediate N fertilization rates but decreased at the maximum N input. We conclude that belowground C allocation and rhizodeposition by rice, microbial utilization of rhizodeposited C, and its stabilization within SOM pools are strongly affected by N availability: N fertilization adequate to the plant demand increases C incorporation in all these polls, but excessive N fertilization has negative effects not only on environmental pollution but also on C sequestration in soil. [ABSTRACT FROM AUTHOR]

Elucidating the biodiversity of CO-assimilating bacterial and algal communities in soils is important for obtaining a mechanistic view of terrestrial carbon sinks operating at global scales. 'Red' acidic soils (Orthic Acrisols) cover large geographic areas and are subject to a range of management practices, which may alter the balance between carbon dioxide production and assimilation through changes in microbial CO-assimilating populations. Here, we determined the abundance and diversity of CO-assimilating bacteria and algae in acidic soils using quantitative PCR and terminal restriction fragment length polymorphism (T-RFLP) of the cbbL gene, which encodes the key CO assimilation enzyme (ribulose-1,5-bisphosphate carboxylase/oxygenase) in the Calvin cycle. Within the framework of a long-term experiment (Taoyuan Agro-ecosystem, subtropical China), paddy rice fields were converted in 1995 to four alternative land management regimes: natural forest (NF), paddy rice (PR), maize crops (CL), and tea plantations (TP). In 2012 (17 years after land use transformation), we collected and analyzed the soils from fields under the original and converted land management regimes. Our results indicated that fields under the PR soil management system harbored the greatest abundance of cbbL copies (4.33 × 10 copies g soil). More than a decade after converting PR soils to natural, rotation, and perennial management systems, a decline in both the diversity and abundance of cbbL-harboring bacteria and algae was recorded. The lowest abundance of bacteria (0.98 × 10 copies g soil) and algae (0.23 × 10 copies g soil) was observed for TP soils. When converting PR soil management to alternative management systems (i.e., NF, CL, and TP), soil edaphic factors (soil organic carbon and total nitrogen content) were the major determinants of bacterial autotrophic cbbL gene diversity. In contrast, soil phosphorus concentration was the major regulator of algal cbbL community composition. Our results provide new insights into the diversity, abundance, and modulation of organisms responsible for microbial autotrophic CO fixation in red acidic soils subjected to changing management regimes. [ABSTRACT FROM AUTHOR]

Aims: Replenishment of soils with carbon (C) produced during photosynthesis plays an important role in global C cycling. Nitrogen (N) fertilization is critical for rice production, but its effects on the deposition of photosynthesis-derived C into soil C pools is poorly understood. To address this, we used continuous C-labeling to quantify the deposition of photosynthesis-derived C into various soil organic pools in a rice-soil system. Methods: Rice ( Oryza sativa L.) was continuously supplied with C-labeled CO (C-CO) for 36 days, with increasing N fertilizer rates (0 [N], 10 [N], 20 [N], or 40 mg N kg soil [N], respectively). Results: Rice shoot and root biomass significantly increased following N fertilization. The amount of photosynthesis-derived C converted into soil organic carbon (C-SOC) was proportional to the soil N concentration, and accounted for 8.0-19.3 % of rice biomass C. The C-SOC content was positively correlated with the rice root biomass, suggesting that N increased root exudation of photosynthesis-derived C. The amounts of C-labeled C in the dissolved organic carbon (C-DOC) and in the microbial biomass carbon (C-MBC), as proportions of C-SOC, were 3.9-7.8 and 6.6-24.0 %, respectively. The C-DOC, C-MBC, and C-SOC as proportions of total DOC, MBC, and SOC were 9.7-11.6, 6.9-10.6, and 0.37-1.71 %, respectively. Conclusions: Nitrogen fertilization promotes deposition of photosynthesis-derived C into SOC pools in a rate-dependent manner. However, the C-MBC as a proportion of both C-SOC (C-MBC/C-SOC) and MBC (C-MBC/MBC) increase during rice growth at lower N concentrations. [ABSTRACT FROM AUTHOR]

Assimilating atmospheric carbon (C) into terrestrial ecosystems is recognized as a primary measure to mitigate global warming. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is the dominant enzyme by which terrestrial autotrophic bacteria and plants fix CO 2 . To investigate the possibility of using RubisCO activity as an indicator of microbial CO 2 fixation potential, a valid and efficient method for extracting soil proteins is needed. We examined three methods commonly used for total soil protein extraction. A simple sonication method for extracting soil protein was more efficient than bead beating or freeze–thaw methods. Total soil protein, RubisCO activity, and microbial fixation of CO 2 in different agricultural soils were quantified in an incubation experiment using 14 C-CO 2 as a tracer. The soil samples showed significant differences in protein content and RubisCO activity, defined as nmol CO 2 fixed g −1 soil min −1 . RubisCO activities ranged from 10.68 to 68.07 nmol CO 2 kg −1 soil min −1 , which were closely related to the abundance of cbbL genes ( r = 0.900, P = 0.0140) and the rates of microbial CO 2 assimilation ( r = 0.949, P = 0.0038). This suggests that RubisCO activity can be used as an indicator of soil microbial assimilation of atmospheric CO 2 . [ABSTRACT FROM AUTHOR]

Many studies have shown that the microbial biomass content in paddy soils is much higher than that in upland soils, but a comprehensive review of the underlying mechanisms and processes is lacking. We conducted a meta-analysis of published literature on the microbial biomass content in continuous paddy soils (>1700 data pairs) and paddy-upland rotation soils (>1100 data pairs) as compared to that in adjacent upland soils (>360 data pairs), measured by the fumigation extraction or fumigation incubation method. The microbial biomass carbon (MBC) content in paddy soils was double that in upland soils. This MBC surplus in paddy soils compared to upland soils was explained by (1) higher input of root C and rhizodeposits by rice plants compared with upland crops; (2) lower oxygen availability and consequently slower microbial turnover; (3) higher microbial C assimilation efficiency in paddy soils; and (4) additional C stabilization on iron (oxyhydr)oxides in paddy soils. The proportion of MBC in total soil organic C in paddy-upland rotation, paddy, and upland soils was 3.5%, 2.5%, and 2.1%, respectively. The higher microbial biomass C/N ratio in paddy soils (12.4 ± 0.11) compared to upland soils (9.9 ± 0.21) reflects greater N losses (through nitrate leaching and denitrification) in relation to slower C losses under anoxic conditions. Despite higher temperature and better water availability, microbial biomass turnover was 1.1–1.6 times slower in paddy soils than in upland soils because of oxygen limitation. Multiple stepwise regression and redundancy analyses showed that microbial biomass in continuous paddy and paddy-upland rotation soils was affected by similar soil factors (such as total N and organic C), whereas microbial biomass in upland soils was mainly affected by pH and the organic C content. Paddy-upland rotation soils undergo oxic–anoxic cycles and consequently can absorb and coprecipitate organic compounds with iron (oxyhydr)oxides as an additional advantage for C stabilization. We conclude that the reduced microbial activity and slower microbial turnover under oxygen-limited conditions lead to nearly two times higher microbial biomass content in paddy than in upland soils. • Microbial biomass (MB) content in paddy soils is twice more than that in upland soils. • Lower O 2 availability leads to slow decomposition of organic matter and MB turnover. • Fe and Mn oxidation-reduction dynamics stabilize C in paddy and paddy-rotation soils. • High MB content in paddy soils is because of high microbial substrate use efficiency. [ABSTRACT FROM AUTHOR]

Soils were incubated continuously in an atmosphere of CO and the distribution of labeled C into soil organic carbon (C-SOC) was determined at 0-1, 1-5, and 5-17 cm down the profile. Significant amounts of C-SOC were measured in paddy soils with a mean of 1,180.6 ± 105.2 mg kg at 0-1 cm and 135.3 ± 47.1 mg kg at 1-5 cm. This accounted for 5.9 ± 0.7 % and 0.7 ± 0.2 %, respectively, of the total soil organic carbon at these depths. In the upland soils, the mean C-SOC concentrations were 43 times (0-1 cm) and 11 times (1-5 cm) lower, respectively, than those in the paddy soils. The amounts of C incorporated into the microbial biomass (MBC) were also much lower in upland soils (5.0 ± 3.6 % and 2.9 ± 1.9 % at 0-1 and 1-5 cm, respectively) than in paddy soils (34.1 ± 12.4 % and 10.2 ± 2.1 % at 0-1 and 1-5 cm, respectively). Similarly, the amount of C incorporated into the dissolved organic carbon (DOC) was considerably higher in paddy soils (26.1 ± 6.9 % and 6.9 ± 1.3 % at 0-1 and 1-5 cm, respectively) than in upland soils (6.0 ± 2.7 % and 4.3 ± 2.2 %, respectively). The observation that the majority of the fixed C-SOC, RubisCO activity and cbbL gene abundance were concentrated at 0-1 cm depth and the fact that light is restricted to the top few millimeters of the soil profiles highlighted the importance of phototrophs in CO fixation in surface soils. Phylogenetic analysis of the cbbL genes showed that the potential for CO fixation was evident throughout the profile and distributed between both photoautotrophic and chemoautotrophic bacteria such as Rhodopseudomonas palustris, Bradyrhizobium japonicum, Rubrivivax gelatinosus and Ralstonia eutropha. [ABSTRACT FROM AUTHOR]

Abstract: Maintaining a diverse functional and taxonomic microbial community in central to preserving soil quality and for ensuring food security. Growing evidence suggests that organic farming systems possess higher quality soils with robust microbial activity in comparison to conventionally managed systems. Although plastic tunnel greenhouses are widely used, their effects on microbial communities are largely unknown. We examined how four treatments impacted soils and their microbial communities: (1) organic management in greenhouses (Or-Gr) and (2) open fields (Or-Op), and (3) conventional management in greenhouses (Co-Gr) and (4) open fields (Co-Op). We measured physicochemical and microbiological parameters, community-level physiological profiles, and phospholipid fatty acid (PLFAs) contents of soils (0–20 cm depth). Both organic and greenhouse management significantly increased total organic C (SOC), total N, microbial biomass C (MBC) and N (MBN), and basal- and substrate-induced respiration (P < 0.05). Or-Gr had significantly higher total, bacterial (both Gram-positive and -negative), and fungal PLFA concentrations (P < 0.05) than the other treatments. Generally, soil quality followed the series Or-Gr > Or-Op > Co-Gr > Co-Op. MBC, MBN, and PLFA concentrations were positively correlated (r > 0.90, P < 0.01) with SOC, total N, and cation exchange capacity and negatively with soil pH. Organic and greenhouse management had a significant interaction effect. Our findings suggest that greenhouse management should be promoted for food security. [Copyright &y& Elsevier]

Autotrophic microorganisms, which can fix atmospheric CO 2 to synthesize organic carbon, are numerous and widespread in soils. However, the extent and the mechanism of CO 2 fixation in soils remain poorly understood. We incubated five upland and five paddy soils from subtropical China in an enclosed, continuously 14 CO 2 -labeled, atmosphere and measured 14 CO 2 incorporated into soil organic matter (SOC 14 ) and microbial biomass (MBC 14 ) after 110 days. The five upland soils supported dominant crops soils (maize, wheat, sweet potato, and rapeseed) in the region, while all paddy soils were cultivated in a regime consisting of permanently-flooded double-cropping rice cultivation. The upland and paddy soils represented typical soil types (fluvisols and ultisols) and three landforms (upland, hill, and low mountain), ranging in total carbon from low (<10 g kg −1 soil organic carbon) to medium (10–20 g kg −1 ) to high (>20 g kg −1 ). Substantial amounts of 14 CO 2 were fixed into SOC 14 (mean 20.1 ± 7.1 mg C kg −1 in upland soil, 121.1 ± 6.4 mg C kg −1 in paddy soil) in illuminated soils (12 h light/12 h dark), whereas no 14 C was fixed in soils incubated in continuous darkness. We concluded that the microbial CO 2 fixation was almost entirely phototrophic rather than chemotrophic. The rate of SOC 14 synthesis was significantly higher in paddy soils than in upland soils. The SOC 14 comprised means of 0.15 ± 0.01% (upland) and 0.65 ± 0.03% (paddy) of SOC. The extent of 14 C immobilized as MBC 14 and that present as dissolved organic C (DOC 14 ) differed between soil types, accounting for 15.69–38.76% and 5.54–18.37% in upland soils and 15.57–40.03% and 3.67–7.17% of SOC 14 in paddy soils, respectively. The MBC 14 /MBC and DOC 14 /DOC were 1.76–5.70% and 1.69–5.17% in the upland soils and 4.23–28.73% and 5.65–14.30% in the paddy soils, respectively. Thus, the newly-incorporated C stimulated the dynamics of DOC and MBC more than the dynamics of SOC. The SOC 14 and MBC 14 concentrations were highly significantly correlated ( r = 0.946; P < 0.0001). We conclude that CO 2 uptake by phototrophic soil microorganisms can contribute significantly to carbon assimilation in soil, and so warrants further future study. [ABSTRACT FROM AUTHOR]

Elucidating the biodiversity of CO-assimilating bacterial communities under different land uses is critical for establishing an integrated view of the carbon sequestration in agricultural systems. We therefore determined the abundance and diversity of CO assimilating bacteria using terminal restriction fragment length polymorphism and quantitative PCR of the cbbL gene (which encodes ribulose-1,5-biphosphate carboxylase/oxygenase). These analyses used agricultural soils collected from a long-term experiment (Pantang Agroecosystem) in subtropical China. Soils under three typical land uses, i.e., rice-rice (RR), upland crop (UC), and paddy rice-upland crop rotation (PU), were selected. The abundance of bacterial cbbL (0.04 to 1.25 × 10 copies g soil) and 16S rDNA genes (0.05-3.00 × 10 copies g soil) were determined in these soils. They generally followed the trend RR > PU > UC. The cbbL-containing bacterial communities were dominated by facultative autotrophic bacteria such as Mycobacterium sp., Rhodopseudomonas palustris, Bradyrhizobium japonicum, Ralstonia eutropha, and Alcaligenes eutrophus. Additionally, the cbbL-containing bacterial community composition in RR soil differed from that in upland crop and paddy rice-upland crop rotations soils. Soil organic matter was the most highly statistically significant factor which positively influenced the size of the cbbL-containing population. The RR management produced the greatest abundance and diversity of cbbL-containing bacteria. These results offer new insights into the importance of microbial autotrophic CO fixation in soil C cycling. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO) assimilation by autotrophic bacteria is an important process in the soil carbon cycle with major environmental implications. The long-term impact of fertilizer on CO assimilation in the bacterial community of paddy soils remains poorly understood. To narrow this knowledge gap, the composition and abundance of CO-assimilating bacteria were investigated using terminal restriction fragment length polymorphism and quantitative PCR of the cbbL gene [that encodes ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO)] in paddy soils. Soils from three stations in subtropical China were used. Each station is part of a long-term fertilization experiment with three treatments: no fertilizer (CK), chemical fertilizers (NPK), and NPK combined with rice straw (NPKM). At all of the stations, the cbbL-containing bacterial communities were dominated by facultative autotrophic bacteria such as Rhodopseudomonas palustris, Bradyrhizobium japonicum, and Ralstonia eutropha. The community composition in the fertilized soil (NPK and NPKM) was distinct from that in unfertilized soil (CK). The bacterial cbbL abundance (3-8 × 10 copies g soil) and RubisCO activity (0.40-1.76 nmol CO g soil min) in paddy soils were significantly positively correlated, and both increased with the addition of fertilizer. Among the measured soil parameters, soil organic carbon and pH were the most significant factors influencing the community composition, abundance, and activity of the cbbL-containing bacteria. These results suggest that long-term fertilization has a strong impact on the activity and community of cbbL-containing bacterial populations in paddy soils, especially when straw is combined with chemical fertilizers. [ABSTRACT FROM AUTHOR]

Abstract: Information on the input, distribution and fate of photosynthesized carbon (C) in plant–soil systems is essential for understanding their nutrient and C dynamics. Our objectives were to: 1) quantify the input to, and distribution of, photosynthesized C by rice into selected soil C pools by using a C14 continuous labelling technique and 2) determine the influence of the photosynthesized C input on the decomposition of native soil organic carbon (SOC) under laboratory conditions. The amounts of C14 in soil organic C (SOC14) were soil dependent, and ranged from 114.3 to 348.2 mg C kg−1, accounting for 0.73%–1.99% of total SOC after continuous labelling for 80 days. However, the mean SOC14 concentrations in unplanted soils (31.9–64.6 mg kg−1) were accounted for 21.5% of the rice-planted soils. The amounts of C14 in the dissolved organic C (DOC14) and in the microbial biomass C (MBC14), as percentages of SOC14, were 2.21%–3.54% and 9.72%–17.97%, respectively. The DOC14 and MBC14 were 6.72%–14.64% and 1.70%–7.67% of total DOC and MBC respectively after 80-d of rice growth. At 80-d of labelling, the SOC14 concentration was positively correlated with the MBC14 concentration and rice root biomass. Rice growth promotes more photosynthesized (newly-derived) C into soil C pools compared to unplanted soils, reflecting the release of root exudates from rice roots. Laboratory incubation of photosynthesized (plant-derived) C in soil decreased the decomposition of native SOC (i.e. a negative priming effect), in some, but not all cases. If this negative priming effect of the new C on native SOC also occurs in the field in the longer term, paddy soils will probably sequester more C from the atmosphere if more photosynthesized C enters them. [Copyright &y& Elsevier]

Soils were incubated for 80 days in a continuously labeled 14CO2 atmosphere to measure the amount of labeled C incorporated into the microbial biomass. Microbial assimilation of 14C differed between soils and accounted for 0.12% to 0.59% of soil organic carbon (SOC). Assuming a terrestrial area of 1.4 X 108 km2, this represents a potential global sequestration of 0.6 to 4.9 Pg C year-1. Estimated global C sequestration rates suggest a "missing sink" for carbon of between 2 and 3 Pg C year-1. To determine whether 14CO2 incorporation was mediated by autotrophic microorganisms, the diversity and abundance of CO2-fixing bacteria and algae were investigated using clone library sequencing, terminal restriction fragment length polymorphism (TRFLP), and quantitative PCR (qPCR) of the ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) gene (cbbL). Phylogenetic analysis showed that the dominant cbbL-containing bacteria were Azospirillum lipoferum, Rhodopseudomonas palustris, Bradyrhizobium japonicum, Raistonia eutropha, and cbbL-containing chromophytic algae of the genera Xanthophyta and Bad!lariophyta. Multivariate analyses of T-RFLP profiles revealed significant differences in cbbL-containing microbial communities between soils. Differences in cbbL gene diversity were shown to be correlated with differences in SOC content. Bacterial and algal cbbL gene abundances were between 106 and 108 and 103 to 105 copies g-1 soil, respectively. Bacterial cbbL abundance was shown to be positively correlated with RubisCO activity (r = 0.853; P < 0.05), and both cbbL abundance and RubisCO activity were significantly related to the synthesis rates of [14C]SOC (r = 0.967 and 0.946, respectively; P < 0.01). These data offer new insights into the importance of microbial autotrophy in terrestrial C cycling. [ABSTRACT FROM AUTHOR]

Crop-assimilated carbon (C), an important source of soil organic carbon (SOC), represents a key linked component of the C cycle in the soil-plant-atmosphere continuum. In the farmland ecosystem, however, the quantitative characterization and mechanism involved in the distribution and transformation of the assimilated C in soil over the plant's life cycle are problems relatively easily ignored. Research in this area is therefore indispensable for a thorough understanding of the process and characteristics of the organic C cycle in farmland soil. This paper provides an overview on: (1) the distribution, transformation rules, and structural features of crop-assimilated C in soil and its contribution to SOC and the function of microorganisms in the transformation of assimilated C. (2) The chemical compositions and structural features of the assimilated C after its entry into soil organic matter. (3) The relationship between assimilated C, rhizosphere deposition, and C isotope technology. Based on the findings, we consider that further research on the distribution of crop-assimilated C in the soil-crop system and the quantitative relations of several C-transformation steps such as crop input, transformation, protection, and stabilization in different ecosystems should be conducted. Moreover, the component and structure of 'new C' input into the soil by rhizosphere deposition in C assimilation and its relationship with oxidation and mineralized stability should also be accounted for. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this study was to investigate the influence of four different horticultural management practices in open field and in greenhouse conditions under organic and conventional cultivation on the amount of soluble organic nitrogen (SON) present in the soil. Soils used in greenhouses and open field cultivation were sampled in Shanghai, China, where organic farming has been conducted for 3 years or conventional faming has been continued in the same area. The amounts of SON, nitrate (NO3 −) and ammonium (NH4 +) were greater in the greenhouse soils than those under open field cultivation, which indicated a higher degree of soil management was imposed under greenhouse conditions. Greenhouse cultivation is also known to accelerate the turnover of SON in the soils, which may explain the significantly higher amounts of SON present in these soils. Organic farming, which does not use artificial fertilizers and pesticides, also resulted in significantly higher amounts of SON (average 42.10 mg kg−1) compared with soils under conventional faming (24.59 mg kg−1). The reasons for the observed differences in pool sizes of soluble inorganic nitrogen (SIN) and NO3 − in the greenhouse soils and the open fields include (a) the heavy application of both complex fertilizer and organic fertilizer that exceeded crop requirements and (b) warmer temperatures and moist soils in the greenhouses, which are likely to lead to greater rates of N cycling compared with the open field soils. These results suggest that SON may be an important source of N in all horticultural systems, representing a pool of labile N readily available for plant growth. However, its concentration is less sensitive to different management practices than SIN. In contrast to SON, the total soluble nitrogen and inorganic N (SIN) pools varied widely with the different management practices although they were dominated by NO3 − in all treatments. Soil organic N was positively related to dissolved organic carbon and NO3 − contents. This relationship indicates that NO3 − and dissolved organic matter play a key role in the retention of SON in soil. [Copyright &y& Elsevier]

Soil soluble organic nitrogen plays a significantly important role in nitrogen biogeochemical cycling. The aim of this study was to investigate the amount of soluble organic and inorganic nitrogen pools extracted by water, salt solutions, and centrifugal-drainage technique from different horticultural management system soils. Approximately 5.4-16.6, 4.4-46.5, 7.1-18.2, 8.8-27.8, 1.8-5.6, and 1.7-4.4 mg kg - 1 soluble organic nitrogen were obtained by 1M KCl, 0.5M K2SO4, 10 mM CaCl2, 1/15 mM phosphate buffer (pH 7.0), water, and centrifugal-drainage technique, respectively, in the 0-20 cm horticultural soils. Large variation in soluble organic nitrogen pools was observed across the sites in the present study and the volumes of soluble organic nitrogen pools generally followed the order: organic production system soil > conventional production system soil > transitional production system soil. In relative contrast to soluble organic nitrogen, organic management, known primarily for the absence of artificial fertilizers and pesticides, resulted in significantly lower levels of both soluble inorganic nitrogen and total soluble nitrogen in the soil. The degree of soluble inorganic nitrogen recovery appears highly dependent on the different chemical extractants and generally followed the series: phosphate buffer (pH 7.0) > KCl≈K2SO4 > CaCl2 > water > centrifugal-drainage technique. The soluble organic carbon and nitrogen in all soil extracts were all positively related to soil total carbon, total nitrogen, and electric conductivity value. This suggested that soil total carbon and nitrogen played a central role in the retention of soluble organic nitrogen in soils. Further studies on investigating the soluble organic nitrogen degradation pathway and its bottleneck are warranted. [ABSTRACT FROM AUTHOR]

Abstract: Direct uptake of organic nitrogen (ON) compounds, rather than inorganic N, by plant roots has been hypothesized to constitute a significant pathway for plant nutrition. The aim of this study was to test whether tomatoes (Solanum lycopersicum cv. Huying932) can take up ON directly from the soil by using 15NH4Cl, K15NO3, 1, 2-13C2 15N-glycine labeling techniques. The 13C and 15N in the plants increased significantly indicating that a portion of the glycine-N was taken up in the form of intact amino acids by the tomatoes within 48h after injection into the soil. Regression analysis of excess 13C against excess 15N showed that approximately 21% of the supplied glycine-N was taken up intact by the tomatoes. Atom% excesses of 15N and 13C in the roots were higher than in any shoots. Results also indicated rapid turnover of amino acids (e.g., glycine) by soil microorganisms, and the poor competitive ability of tomatoes in absorbing amino acids from the soil solution. This implies that tomatoes can take up ON in an intact form from the soil despite the rapid turnover of organic N usually found under such conditions. Given the influence of climatic change and N pollution, further studies investigating the functional ecological implications of ON in horticultural ecosystems are warranted. [Copyright &y& Elsevier]

[Display omitted] • Mechanisms of higher C and N stocks in paddies than upland soils are reviewed. • Climate effects on stocks are weakened by management (puddling and flooding). • Larger organic C input in paddies compared to most upland cereals was found. • Lower O 2 availability leads to slow decomposition of organic matter in paddies. • Fe and Mn oxidation–reduction dynamics stabilise C in paddy soils. Paddy soils, a type of Hydragric Anthrosol, have much greater soil organic C (SOC) and total N (TN) contents than that in upland soils. However, this fact has never been generalized or mechanistically explained. We conducted a global meta-analysis on the organic C and total N contents and their stocks in continuous paddy soils (578 sites) and compared them with those in adjacent upland soils. Average C stocks up to depths of 35 cm in upland and paddy soils were 31 and 47 Mg C ha−1, respectively. The N stocks in upland and paddy soils were 2.2 and 3.2 Mg N ha−1, respectively. The combined effects of mean annual temperature and precipitation showed that C and N stocks in paddy and upland soils are generally the largest under cool and humid conditions and the smallest in warm and dry climates. Quantitative analysis of climatic, and soil physical and chemical factors showed that 1) climate effects are weakened by management such as puddling and flooding, thereby increasing the importance of soil physico-chemical properties, which control soil organic matter (SOM) stabilization, and 2) climate (e.g., mean annual precipitation) mainly affects C and N stocks in upland soils; the chemical properties (such as pH), on the other hand, primarily affect C and N stocks in paddy soils. Greater C and N stocks in paddy soils are the result of 1) a larger input of organic C by rice than by most upland cereals, 2) slower decomposition of plant residues and SOM under anoxic conditions, and 3) a greater importance of sesquioxides in the biochemical stabilization of SOM. We conclude that these man-made paddy soils store more organic C and N than their upland neighbors despite long-term and intensive management. [ABSTRACT FROM AUTHOR]

As the carbon (C) credit market evolves, incorporating organic matter into soils has emerged as a key strategy in C farming. Soil heterotrophic respiration (RH) plays a pivotal role in maintaining the C balance in terrestrial ecosystems, yet the contrasting impacts of fresh and pyrogenic organic matter applications on soil RH, and associated underlying mechanisms, have not been fully investigated. Through a 2-year field experiment, we investigated how applying maize straw and its derived biochar affect the physical, chemical, and microbial properties of soil in a subtropical Moso bamboo forest. Results showed that straw application increased soil RH, while biochar application suppressed it. Soil RH was correlated positively with β-glucosidase and cellobiohydrolase activities but negatively with RubisCO enzyme activity. Increased soil RH under straw application was linked to the increased β-glucosidase/cellobiohydrolase activities driven by elevated water-soluble organic C and O-alkyl C levels as well as GH48 and cbhI gene abundances, and the decreased RubisCO enzyme activity caused by reduced cbbL gene abundance. Conversely, reduced soil RH under biochar application was linked to reductions in β-glucosidase and cellobiohydrolase activities induced by increased aromatic C and decreased GH48 and cbhI gene levels, and increases in RubisCO enzyme activity driven by higher cbbL gene abundance. More importantly, changes in soil RH were clearly linked to microbial dynamics. Specifically, increases in the relative abundances of Alphaproteobacteria and Sordariomycetes and decreases in AD3 and Tremellomycetes contributed to the enhanced soil RH under straw application. With biochar application, the reverse effect occurred, ultimately contributing to the reduced soil RH. Our study demonstrates that maize straw application increases while biochar application decreases soil RH in the subtropical forest. These findings reveal that biochar reduced soil RH through changing microbial activity in subtropical forests, providing insight into complex dynamics of soil C cycling in response to diverse interventions. [ABSTRACT FROM AUTHOR]

Low-molecular-weight organic carbon (LMWOC) from root exudate influences soil organic carbon cycling via priming of microbial activity. However, the mechanisms underlying the uptake and utilization of specific exudates by microorganisms in soils remain unclear. To address this gap in knowledge, a one-month 13C (0.1 mg C﹒g soil) tracer incubation study was conducted to investigate the fate of the most abundant root exudate groups (using 13C-labeled glucose, acetic acid, and oxalic acid) in paddy vs. upland soil. After 2 days of incubation, the microbial substrate use efficiency (SUE) was >80% in paddy soil, which was approximately 1.9, 2.9, and 1.3 times that in uplands with glucose, acetic acid, and oxalic acid addition, respectively. The SUE in paddy soil with glucose or acetic acid addition was always higher than that in uplands over time (P < 0.05). In both soils, the SUE of glucose was 1–4 times that of carboxylic acids (P < 0.05). The recovery of 13C-labeled total phospholipid fatty acids (PLFAs) in paddy soils was 1.5–2 times that in uplands (P < 0.05). In both soils, bacteria preferred to utilize glucose and acetic acid to synthesize cellular components. Throughout the incubation period, bacteria dominated over fungi in terms of LMWOC consumption. Gram-positive and -negative bacteria were dominant in upland and paddy soils, respectively. From days 11–30, the contribution of fungi and actinomycetes to LMWOC utilization began to appear. Temperature positively regulated 13C distribution in microbial groups (P < 0.05), and increased dissolved organic carbon in upland soil accelerated microbial SUE. The results of this study clarify microbial effects on the high soil carbon sequestration capacity of paddy soil as compared with upland in subtropical areas. [ABSTRACT FROM AUTHOR]

Continuous nitrogen (N) loading alters plant growth and subsequently has the potential to impact soil organic carbon (SOC) accumulation in salt marshes. However, the knowledge gap of photosynthesized carbon (C) allocation in plant-soil-microbial systems hampers the quantification of C fluxes and the clarification of the mechanisms controlling the C budget under N loading in salt marsh ecosystems. To address this, we conducted an N fertilization field observation combined with a 5 h 13C-pulse labeling experiment in a salt marsh dominated by Suaeda. salsa (S. salsa) in the Yellow River Delta (YRD), China. N fertilization increased net 13C assimilation of S. Salsa by 277.97%, which was primarily allocated to aboveground biomass and SOC. However, N fertilization had little effect on 13C allocation to belowground biomass. Correlation analysis showed that 13C incorporation in soil was significantly and linearly correlated with 13C incorporation in shoots rather than in roots both in a 0 N (0 g N m−2 yr−1) and +N (20 g N m−2 yr−1) group. The results suggested that SOC increase under N fertilization was mainly due to an increased C assimilation rate and more efficient downward transfer of photosynthesized C. In addition, N fertilization strongly improved the 13C amounts in the chloroform-labile SOC component by 295.26%. However, the absolute increment of newly fix 13C mainly existed in the form of residual SOC, which had more tendency for burial in the soil. Thus, N fertilization enhanced SOC accumulation although C loss increased via belowground respiration. These results have important implications for predicting the carbon budget under further human-induced N loading. • Newly fixed carbon allocation and turnover in a salt marsh were quantified in situ. • Nitrogen fertilization enhanced photosynthetic carbon fixation in salt marshes. • Increased newly fixed carbon in +N group mainly allocated in shoot and soil organic carbon. • Soil organic carbon accumulation was mainly benefited by greater shoot growth. • Nitrogen fertilization improved residual soil organic carbon and is favor for carbon sequestration. [ABSTRACT FROM AUTHOR]

Microorganisms regulate soil organic matter (SOM) formation through accumulation and decomposition of microbial necromass, which is directly and indirectly affected by elevated CO2 and N fertilization. We investigated the role of microorganisms in SOM formation by analyzing 13C recovery in microorganisms and carbon pools in paddy soil under two CO2 levels, with and without N fertilization, after continuous 13CO2 labelling was stopped. Microbial turnover transferred 13C from living microbial biomass (determined by the decrease in phospholipid fatty acids) to necromass (determined by the increase in amino sugars). 13C incorporation in fungal living biomass and necromass was higher than that in bacteria. Bacterial turnover was faster than necromass decomposition, resulting in net necromass accumulation over time; fungal necromass remained stable. Elevated CO2 and N fertilization increased the net accumulation of bacterial, but not fungal, necromass. CO2 levels, but not N fertilization, significantly affected 13C incorporation in SOM pools. Elevated CO2 increased 13C in particulate organic matter via the roots, and in the mineral-associated organic matter (MAOM) via bacterial, but not fungal, necromass. Overall, bacterial necromass plays a dominant role in the MAOM formation response to elevated CO2 because bacteria are sensitive to elevated CO2. [ABSTRACT FROM AUTHOR]

Combined straw and straw-derived biochar input is commonly applied by farmland management in low-fertility soils. Although straw return increases soil organic matter (SOM) contents, it also primes SOM mineralization. The mechanisms by which active microorganisms mineralize SOM and the underlying factors remain unclear for such soils. To address these issues, paddy soil was amended with 13C-labeled straw, with and without biochar (BC) or ferrihydrite (Fh), and incubated for 70 days under flooded conditions. Compound-specific 13C analysis of phospholipid fatty acids (13C-PLFAs) allowed us to identify active microbial communities utilizing the 13C-labeled straw and specific groups involved in SOM mineralization. Cumulative SOM mineralization increased by 61% and 27% in soils amended with Straw + BC and Straw + Fh + BC, respectively, compared to that with straw only. The total PLFA content was independent of the straw and biochar input. However, 13C-PLFAs contents increased by 35–82% after biochar addition, reflecting accelerated microbial turnover. Compared to that in soils without biochar addition, those with biochar had an altered microbial community composition-increased amounts of 13C-labeled gram-positive bacteria (13C-Gram +) and fungi, which were the main active microorganisms mineralizing SOM. Microbial reproduction and growth were susceptible to nutrient availability. 13C-Gram + and 13C-fungi increased with Olsen P but decreased with dissolved organic carbon and NO 3 - contents. In conclusion, biochar acts as an electron shuttle, stimulates iron reduction, and releases organic carbon from soil minerals, which in turn increases SOM mineralization. Gram + and fungi were involved in straw decomposition in response to biochar application and responsible for SOM mineralization. Highlights: Straw return accelerates microbial mineralization of SOM. Combined biochar and straw input raises SOM mineralization by 60%. Biochar addition increases 13C incorporation from straw into Gram + bacteria and fungi. 13C-Gram + bacteria and 13C-fungi are the dominant active microorganisms that mineralize SOM. [ABSTRACT FROM AUTHOR]

The anaerobic oxidation of methane (AOM) in marine ecosystems is ubiquitous and coupledlargely to sulfate reduction. In comparison, little is known about the role of AOM interrestrial environments, and the dominant mechanisms for terrestrial AOM remain elusive.Submerged agricultural fields, such as rice paddies, with intensive CH4 turnover, mayprovide a high potential for AOM; however, the AOM rate, electron acceptors andmechanisms in these environments are largely unexplored. Here, we used 13CH4isotope tracers to quantify the AOM rate in paddy soils under organic (i.e. pigmanure, biochar) and mineral fertilizers (i.e. NPK), and examine the potential ofalternative electron acceptors (AEAs) including Fe3+, NO3−, SO42− and humicacids.During 84 days of incubation, the cumulative AOM (13CH4-derived CO2) reached 0.15-1.3μg C g−1 soil depending on fertilization. There was a linear correlation (R2 = 0.55 ∼ 0.93, p&#x003C; 0.05) between the amount of gross produced and net oxidized CH4. NO3− was the mostpotent AEA with an AOM rate reaching 0.80 ng C g−1 soil h−1 under pig manurefertilization. The role of Fe3+ on AOM remained unclear, whereas SO42− inhibitedAOM but strongly stimulated CO2 production indicating intensive sulfate-inducedanaerobic organic matter oxidation. Humic acids were the second most potent AEA,especially when without mineral fertilizers and after biochar addition. Humic acidsaddition increased methanogenesis for 5-6-times in all paddy soils as compared withcontrol and other AEAs. We demonstrated for the first time that organic AEAs (e.g.humic acids) are important drivers of AOM – this along with the proven nitrate(nitrite)-dependent AOM mechanisms in paddy soils, whereas Fe3+ and SO42− arepreferential electron donors for mineralization of native soil organic matter. Consequently, ina broader ecological view, the organic and mineral fertilization controls an importantmethane sink under anaerobic conditions in submerged agricultural paddy ecosystems. [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]

Background and aims: The stoichiometric ratio of carbon (C): phosphorus (P) acquisition is strongly correlated with soil available C:P ratio. However how the stoichiometric relationship between acquiring C and P through microbial metabolism affects bioavailable P is poorly understood in semi-arid agricultural ecosystems. Methods: Our objective was to investigate the underlying mechanisms of the P availability in typical P-limited steppe soil from Kazakhstan in response to mineral nutrient (Na2HPO4) with and without Dactylis glomerata L. leaves addition in a 38-day incubation experiment. Results: Four bioavailable P fractions content (CaCl2-P, Citrate-P, Enzyme-P, and HCl-P) were improved. Sole application of P fertilizer decreased the maximal velocity (Vmax) of P acquisition enzyme (phosphomonoesterase) but increased microbial C limitation, resulting in increasing the ratio of C to P acquisition but decreasing the ratio of available dissolved organic C: Olsen-P. In contrast, plant residues returning (the application of sole D. glomerata leaves and the combined application of D.glomerata and mineral P) increased Vmax of C (β-1, 4-glucosidase, β-D-cellobiosidase, β-1, 4-xylosidase) and P acquisition enzymes, however decreasing microbial C and P limitation through improving microbial metabolism. Furthermore, the spearman correlation and piecewiseSEM analysis suggested that microbial C limitation and EEAC:P had a negative effect on P availability, illustrating that the decreasing of microbial C limitation can improve soil bioavailable P. Conclusion: The decomposition of organic residues eliminated microbial P limitation and increased P availability by allocating C and P acquisition enzymes to balance the stoichiometric ratio of microbial C and P demand. [ABSTRACT FROM AUTHOR]

Viruses are extremely abundant in soils and have various important ecological implications therein. The quantification and identification of soil viruses are critical for a comprehensive understanding of their abundance, diversity and ecological functions. Herein, we compared three physical dispersion methods (shaking, vortexing and sonicating) and 10 extraction buffers to determine the best protocol for virus extraction from a broad range of soils in the (sub)tropics. A 5-min sonication combined with potassium citrate (AKC) buffer (1% potassium citrate amended with 10% PBS, 5 mM EDTA and 150 mM MgSO4) treatment yielded virus-like particles (VLPs) at a yield one to two orders of magnitude greater than that achieved via other methods, as determined through epifluorescence microscopy (EFM). Using the most successful method, viral abundance in red soils ranged from 2.3 × 107 VLPs g−1 soil to 1.3 × 109 VLPs g−1 soil, with approximately two times more VLPs in paddy soils than in adjacent upland soils. VLP abundance increased with soil water content (R2 = 0.60), soil organic carbon (R2 = 0.70) and bacterial abundance (R2 = 0.69), suggesting that land use strongly affects viral abundance. This comparison clearly showed that extraction methods determine the measured viral abundance to a great extent. Based on our tests of a broad range of physical dispersion methods and chemical solutions, we concluded that a 5-min sonication combined with potassium citrate buffer treatment is the best approach for extracting VLPs from red subtropical soils. [ABSTRACT FROM AUTHOR]

Long-term soil chronosequences are valuable model systems for investigating pedogenesis and investigating the process of element coupling. Here, we assessed the coupling relationships among C, Fe, and Fe-reducing bacteria (Anaeromyxobacter, Geobacter, and Shewanella) in a paddy soil chronosequence of approximately 50 to 1000 years. Soils of the chronosequence originated from tidal marsh under nearly identical landscape and climate conditions. During 1000 years of rice cultivation, soil organic carbon (SOC) contents in surface horizons (0–20 cm) increased from 10.4 to 21.8 g kg−1. In contrast, total Fe contents declined from 59.6 to 45.1 g kg−1 during the initial 50 years of paddy rice cultivation and then further decreased at a low rate of 0.004 g kg−1 soil year−1 (equivalent to 10 kg ha−1 soil year−1). Organically complexed Fe oxides (Fep) increased from 219 to 642 mg g−1 with increasing time of pedogenesis, but free total Fe oxides (Fed) and amorphous Fe oxides (Feo) declined at early stage of soil development, followed by a slow accumulation at later stages of the chronosequence. Gene copy numbers of Anaeromyxobacter and Geobacter increased from 4.6 × 105 and 3.6 × 106 copies g−1 to 3.8 × 107 and 3.6 × 107 copies g−1 dry soil with continuous paddy rice cultivation, while concurrently Shewanella gene abundance decreased gradually from 4.5 × 105 to 9.3 × 104 copies g−1 dry soil. Using structural equation modeling (SEM), different coupling relationships were observed among C, Fe, and Fe-reducing bacteria for the first 300 years of paddy chronosequence and thereafter. Overall, all Fe-reducing bacteria did not show consistent variation. With the stable microbial community and iron oxide fractions, the microbially mediated dissimilatory coupling relationship between C and Fe becomes simple during 1000 years of paddy soil development. [ABSTRACT FROM AUTHOR]

The factors regulating potential acquisition of sulphur (S)-containing amino acids by plant roots from the rhizosphere remain poorly understood. Using radio tracer (14C and 35S), we studied the competition for two S-containing amino acids (i.e., cysteine (Cys) and methionine (Met)) within 24 hours (h), by the rhizosphere microbial community and maize plants (Zea mays L.). Our results showed that the capture of Cys and Met by the maize plants was much lower, with only <10% of the added amino acid-14C or 35S captured by the plant, compared to the rhizosphere microbial community (76.9%) on average. We suggest that this could be a result of relatively high availability of inorganic N and S in soil solution, the lack of transmembrane for amino acids on maize root cells, as well as the rapid turnover of Cys and Met by soil microbes in the rhizosphere. The addition of inorganic S, significantly reduced plant capture of Cys and Met-14C by maize plants in the rhizosphere but had little effect on the capture of Cys and Met-35S (p<0.05). Overall, our results imply that (1) Cys and Met are available carbon (C), nitrogen (N) and S sources for maize plants, potentially contributing to total plant N and S demand under certain conditions; (2) Utilization of Cys and Met by maize roots in the rhizosphere is independent of inorganic S availability; (3) Increased amino acid concentration led to higher capture by both plants and soil microbes, but had little effect on the competition success on either side. [ABSTRACT FROM AUTHOR]

Application of biochar to soil may stabilize soil organic carbon (SOC), concomitantly increasing nutrient retention. However, the interactive effect of biochar and nutrients on SOC and the underlying microbial mechanisms remain poorly understood, particularly in intensively managed forests where decarbonization is substantial after converting from natural forests. This 80-day incubation experiment aimed to quantify native SOC mineralization as affected by biochar (B) and nutrients [nitrogen (N) or phosphorus (P)], linking to the chemical composition of SOC, soil microbial community composition, and enzyme activities within a subtropical Moso bamboo (Phyllostachys edulis) forest soil. Results presented that compared to the control (nil-nutrient), nutrients (N, P, and NP) significantly destabilized native SOC [positive priming effect (PE); 20–98% increase in SOC mineralization], whereas such destabilization effect was significantly reduced by biochar (6.0–19%). The positive PE by nutrient was due to the increases in O-alkyl C, microbial biomass C, available mineral N, soil pH, β-glucosidase, and invertase activities. Meanwhile, the greater PE by N than P could be attributed to (i) decreases in diversity of bacterial and fungal communities; and (ii) increases in the relative abundances of microbial taxa such as Bacilli, Planctomycetes, and Alphaproteobacteria. Importantly, biochar's stabilization effect was because biochar not only lowered NH4+-N and NO3−-N and β-glucosidase activity, but also increased the activity of C-fixing enzyme (RubisCO) and polyphenol oxidase activity. Furthermore, biochar significantly decreased soil O-alkyl C that possibly resulted in less labile SOC mineralization, but increased aromatic C resulting in lower fungal diversity. We conclude that the biochar significantly reduces the destabilization effects of nutrients on SOC, highlighting that the biochar application is an effective approach to mitigate soil CO2 emissions within subtropical forest. [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 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]