Twelve-year conversion of rice paddy to wetland does not alter SOC content but decreases C decomposition and N mineralization in Japan.

Land-use change worldwide has been driven by anthropogenic activities, which profoundly regulates terrestrial C and N cycles. However, it remains unclear how the dynamics and decomposition of soil organic C (SOC) and N respond to long-term conversion of rice paddy to wetland. Here, soil samples from five soil depths (0-25 cm, 5 cm/depth) were collected from a continuous rice paddy and an adjacent wetland (a rice paddy abandoned for 12 years) on Shonai Plain in northeastern Japan. A four-week anaerobic incubation experiment was conducted to investigate soil C decomposition and N mineralization. Our results showed that SOC in the wetland and rice paddy decreased with soil depth, from 31.02 to 19.66 g kg-1 and from 30.26 to 18.86 g kg-1, respectively. There was no significant difference in SOC content between wetland and rice paddy at any depth. Soil total nitrogen (TN) content in the wetland (2.61-1.49 g kg-1) and rice paddy (2.91-1.78 g kg-1) showed decreasing trend with depth; TN was significantly greater in the rice paddy than in the wetland at all depths except 20-25 cm. Paddy soil had significantly lower C/N ratios but significantly larger decomposed C (Dec-C, CO2 and CH4 production) and mineralized N (Min-N, net NH4+-N production) than wetland soil across all depths. Moreover, the Dec-C/Min-N ratio was significantly larger in wetland than in rice paddy across all depths. Rice paddy had higher exponential correlation between Dec-C and SOC, Min-N and TN than wetland. Although SOC did not change, TN decreased by 14.1% after the land-use conversion. The Dec-C and Min-N were decreased by 32.7% and 42.2%, respectively, after the12-year abandonment of rice paddy. Conclusively, long-term conversion of rice paddy to wetland did not distinctly alter SOC content but increased C/N ratio, and decreased C decomposition and N mineralization in 0-25 cm soil depth.

Phosphorus deficiency alters root length, acid phosphatase activity, organic acids, and metabolites in root exudates of soybean cultivars.

Phosphorus (P) deficiency alters the root morphological and physiological traits of plants. This study investigates how soybean cultivars with varying low-P tolerance values respond to different P levels in hydroponic culture by assessing alterations in root length, acid phosphatase activity, organic acid exudation, and metabolites in root exudates. Three low-P-tolerant cultivars ('Maetsue,' 'Kurotome,' and 'Fukuyutaka') and three low-P-sensitive cultivars ('Ihhon,' 'Chizuka,' and 'Komuta') were grown under 0 (P0) and 258 µM P (P8) for 7 and 14 days after transplantation (DAT). Low-P-tolerant cultivars increased root length by 31% and 119%, which was lower than the 62% and 144% increases in sensitive cultivars under P0 compared to P8 at 7 and 14 DAT, respectively. Acid phosphatase activity in low-P-tolerant cultivars exceeded that in sensitive cultivars by 5.2-fold and 2.0-fold at 7 and 14 DAT. Root exudates from each cultivar revealed 177 metabolites, with higher organic acid exudation in low-P-tolerant than sensitive cultivars under P0. Low-P-tolerant cultivars increased concentrations of specific metabolites (oxalate, GABA, quinate, citrate, AMP, 4-pyridoxate, and CMP), distinguishing them from low-P-sensitive cultivars under P0. The top five metabolomic pathways (purine metabolism, arginine and proline metabolism, TCA cycle, glyoxylate and dicarboxylate metabolism, alanine, aspartate, and glutamate metabolism) were more pronounced in low-P-tolerant cultivars at 14 DAT. These findings indicate that increasing root length was not an adaptation strategy under P deficiency; instead, tolerant cultivars exhibit enhanced root physiological traits, including increased acid phosphatase activity, organic acid exudation, specific metabolite release, and accelerated metabolic pathways under P deficiency.

Nitrogen and phosphorus concentrations in growth media affect the relationship between root endophytic fungi and host plant.

Endophytic fungi (EPF) colonize plant roots and enhance their growth. The relationship between host plant and EPF can be affected by several factors, such as growth media, host species, and fungal species. The objective of this study was to clarify the effect of nutrient concentration in growth media on the relationship between host plant and root EPF. Brassica campestris was grown in 1/100 Murashige and Skoog (MS), 1/10 MS, 1/100 MS and 1/10 nitrogen (high N), and 1/100 MS and 1/10 phosphorus (high P) media. B. campestris was inoculated with four root EPFs isolated from forest soils in Indonesia and harvested 28 days after transplant. Shoot dry weight (SDW) and colonization in roots were measured. All the isolates colonized roots of B. campestris. Two isolates increased the SDW of B. campestris grown on 1/100 MS media. The shoot growth response of B. campestris to EPF colonization on 1/100 MS was higher than that on 1/100 high N and 1/100 high P MS media. These results suggest that concentration of nitrogen and phosphorus in growth media determine the relationship between B. campestris and root EPF.

Multiple analysis of root exudates and microbiome in rice (Oryza sativa) under low P conditions.

Plants release various metabolites from roots and root exudates contribute to differences in stress tolerance among plant species. Plant and soil microbes have complex interactions that are affected by biotic and abiotic factors. The purpose of this study was to examine the differences in metabolites in root exudates of rice (Oryza sativa) cultivars and their correlation with bacterial populations in the rhizosphere. Two rice cultivars (O. sativa cv. Akamai and O. sativa cv. Koshihikari) were grown in soils fertilized with 0 g P kg-1 (- P) or 4.8 g P kg-1 (+ P). Root exudates and root-attached soil were collected at 13 and 20 days after transplanting (DAT) and their metabolites and bacterial community structure were determined. The exudation of proline, serine, threonine, valine and 4-coumarate were increased under low P conditions in both cultivars. There was a positive correlation between the concentration of pantothenate in root exudates and the representation of members of the genera Clostridium and Sporosarcina, which were negatively correlated with root dry weight. Gracilibacter, Opitutus, Pelotomaculum, Phenylobacterium and Oxobacter were positively correlated with root dry weight and presence of allantoin, 2-aminobtyrate and GlcNac. This study provides new information about the response of plants and rhizosphere soil bacteria to low P conditions.

Metabolite profiling of the hyphal exudates of Rhizophagus clarus and Rhizophagus irregularis under phosphorus deficiency.

Arbuscular mycorrhizal (AM) fungal extraradical hyphae exude their metabolites into the soil. Root exudate metabolites are affected by plant species and P status. However, the effect of P status on AM hyphal exudate metabolites has been unknown. This study aimed to examine hyphal exudate metabolite composition of two AM fungal species and their response to P deficiency through metabolite profiling. Rhizophagus clarus and R. irregularis were grown in a two-compartment in vitro culture system of Linum usitatissimum roots on solid modified Strullu-Romand medium in combination with two P levels (3 µM (P3) and 30 µM (P30)). Hyphal exudates were collected from the hyphal compartment at 118 days after inoculation (DAI). The metabolite composition of the hyphal exudates was determined by capillary electrophoresis/time-of-flight mass spectrometry, resulting in the identification of a total of 141 metabolites at 118 DAI. In the hyphal exudates of R. clarus, the concentrations of 18 metabolites, including sugars, amino acids, and organic acids, were significantly higher (p < 0.05) under P3 than under P30 conditions. In contrast, the concentrations of 10 metabolites, including sugar and amino acids, in the hyphal exudates of R. irregularis were significantly lower (p < 0.05) under P3 than under P30 conditions. These findings suggest that the extraradical hyphae of AM fungi exude diverse metabolites of which concentrations are affected by P conditions and differ between AM fungal species.

Secretion of acid phosphatase from extraradical hyphae of the arbuscular mycorrhizal fungus Rhizophagus clarus is regulated in response to phosphate availability.

Arbuscular mycorrhizal (AM) fungi increase phosphate (P) uptake by plants. Organic phosphate comprises 30-80% of total P in most agricultural soils. Some plants can utilize organic phosphate by secreting acid phosphatase (ACP) from their roots, especially under low P conditions. Although secretion of ACP from extraradical hyphae of AM fungi has been reported, the specific factors that affect the secretion of ACP are unknown. The objective of the present study was to investigate whether secretion of ACP from extraradical hyphae is induced by low P conditions. First, specimens of Allium fistulosum were either inoculated with the AM fungus Rhizophagus clarus strain CK001 or remained uninoculated and were grown in soil with 0.5 g P2O5 kg-1 soil or without P fertilization using two-compartment pots. Soil solution was collected using mullite ceramic tubes 45 days after sowing. The soil solution was analyzed for ACP activity by using p-nitrophenylphosphate. Second, Ri T-DNA transformed roots (i.e., hairy roots) of Linum usitatissimum inoculated with R. clarus were grown on solid minimal media with two P levels applied (3 and 30 µM P) using two-compartment Petri dishes under in vitro conditions. Hyphal exudates, extraradical hyphae, and hairy roots were collected and analyzed for ACP activity. ACP activity in the soil solution of the hyphal compartment in the A. fistulosum inoculation treatment was higher without P fertilization than with P fertilization. AM colonization also was higher without P fertilization than with P fertilization. In the in vitro two-compartment culture, ACP activity of hyphal exudates and extraradical hyphae were higher under the 3-µM treatment than under the 30-µM treatment. These findings suggest that the secretion of ACP from the extraradical hyphae of R. clarus into the hyphosphere is promoted under low P conditions.

Dark conditions enhance aluminum tolerance in several rice cultivars via multiple modulations of membrane sterols.

Aluminum-sensitive rice (Oryza sativa L.) cultivars showed increased Al tolerance under dark conditions, because less Al accumulated in the root tips (1 cm) under dark than under light conditions. Under dark conditions, the root tip concentration of total sterols, which generally reduce plasma membrane permeabilization, was higher in the most Al-sensitive japonica cultivar, Koshihikari (Ko), than in the most Al-tolerant cultivar, Rikuu-132 (R132), but the phospholipid content did not differ between the two. The Al treatment increased the proportion of stigmasterol (which has no ability to reduce membrane permeabilization) out of total sterols similarly in both cultivars under light conditions, but it decreased more in Ko under dark conditions. The carotenoid content in the root tip of Al-treated Ko was significantly lower under dark than under light conditions, indicating that isopentenyl diphosphate transport from the cytosol to plastids was decreased under dark conditions. HMG2 and HMG3 (encoding the key sterol biosynthetic enzyme 3-hydroxy-3-methylglutaryl CoA reductase) transcript levels in the root tips were enhanced under dark conditions. We suggest that the following mechanisms contribute to the increase in Al tolerance under dark conditions: inhibition of stigmasterol formation to retain membrane integrity; greater partitioning of isopentenyl diphosphate for sterol biosynthesis; and enhanced expression of HMGs to increase sterol biosynthesis.

Ancient rice cultivar extensively replaces phospholipids with non-phosphorus glycolipid under phosphorus deficiency.

Recycling of phosphorus (P) from P-containing metabolites is an adaptive strategy of plants to overcome soil P deficiency. This study was aimed at demonstrating differences in lipid remodelling between low-P-tolerant and -sensitive rice cultivars using lipidome profiling. The rice cultivars Akamai (low-P-tolerant) and Koshihikari (low-P-sensitive) were grown in a culture solution with [2 mg l-1 (+P)] or without (-P) phosphate for 21 and 28 days after transplantation. Upper and lower leaves were collected. Lipids were extracted from the leaves and their composition was analysed by liquid chromatography/mass spectrometry (LC-MS). Phospholipids, namely phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and phosphatidylinositol (PI), lysophosphatidylcholine (lysoPC), diacylglycerol (DAG), triacylglycerol (TAG) and glycolipids, namely sulfoquinovosyl diacylglycerol (SQDG), digalactosyldiacylglycerol (DGDG), monogalactosyldiacylglycerol (MGDG) and 1,2-diacyl-3-O-alpha-glucuronosyl glycerol (GlcADG), were detected. GlcADG level was higher in both cultivars grown in -P than in +P and the increase was larger in Akamai than in Koshihikari. DGDG, MGDG and SQDG levels were higher in Akamai grown in -P than in +P and the increase was larger in the upper leaves than in the lower leaves. PC, PE, PG and PI levels were lower in both cultivars grown in -P than in +P and the decrease was larger in the lower leaves than in the upper leaves and in Akamai than in Koshihikari. Akamai catabolised more phospholipids in older leaves and synthesised glycolipids in younger leaves. These results suggested that extensive phospholipid replacement with non-phosphorus glycolipids is a mechanism underlying low-P-tolerance in rice cultivars.

Higher sterol content regulated by CYP51 with concomitant lower phospholipid content in membranes is a common strategy for aluminium tolerance in several plant species.

Several studies have shown that differences in lipid composition and in the lipid biosynthetic pathway affect the aluminium (Al) tolerance of plants, but little is known about the molecular mechanisms underlying these differences. Phospholipids create a negative charge at the surface of the plasma membrane and enhance Al sensitivity as a result of the accumulation of positively charged Al(3+) ions. The phospholipids will be balanced by other electrically neutral lipids, such as sterols. In the present research, Al tolerance was compared among pea (Pisum sativum) genotypes. Compared with Al-tolerant genotypes, the Al-sensitive genotype accumulated more Al in the root tip, had a less intact plasma membrane, and showed a lower expression level of PsCYP51, which encodes obtusifoliol-14α-demethylase (OBT 14DM), a key sterol biosynthetic enzyme. The ratio of phospholipids to sterols was higher in the sensitive genotype than in the tolerant genotypes, suggesting that the sterol biosynthetic pathway plays an important role in Al tolerance. Consistent with this idea, a transgenic Arabidopsis thaliana line with knocked-down AtCYP51 expression showed an Al-sensitive phenotype. Uniconazole-P, an inhibitor of OBT 14DM, suppressed the Al tolerance of Al-tolerant genotypes of maize (Zea mays), sorghum (Sorghum bicolor), rice (Oryza sativa), wheat (Triticum aestivum), and triticale (×Triticosecale Wittmark cv. Currency). These results suggest that increased sterol content, regulated by CYP51, with concomitant lower phospholipid content in the root tip, results in lower negativity of the plasma membrane. This appears to be a common strategy for Al tolerance among several plant species.

Five-year vegetation conversion from pasture to C3 and C4 plants affects dynamics of SOC and TN and their natural stable C and N isotopes via mediating C input and N leaching.

Understanding the effects of land-use change on stock and composition of soil organic carbon (SOC) and nitrogen (N) is pivotal for sustainable agriculture and climate change adaption. However, previous studies have often overlooked the specific vegetation type in land-use changes. Therefore, a five-year lysimeter block experiment was conducted, involving non-vegetation, eulalia (C4 plant), and clover (C3 plant) to investigate the impacts of vegetation conversion from pasture on SOC and N dynamics and their natural stable isotopes. Non-vegetation caused 26.21 % and 25.88 % decreases in SOC and total N (TN) contents. Five-year eulalia and clover cultivation maintained stable SOC content, with clover exhibiting higher soil TN content. Eulalia-derived soil C was 1.64-7.58 g C kg-1 and SOC loss in eulalia treatment was 1.86-7.90 g C kg-1. Soil δ13C in eulalia increased at a rate of 0.90 ‰ year-1, significantly surpassing clover and non-vegetation treatments. Conversely, soil δ15N decreased over time, showing insignificant difference among all treatments. Eulalia exhibited significantly higher dry weight and δ13C but lower TN content compared with clover. However, no significant differences were observed in total C and δ15N between the two vegetation treatments. Non-vegetation exhibited higher dissolved organic C concentration than two vegetation treatments in 2017, decreasing over time. Dissolved TN and nitrate concentrations in leachate followed the order clover> non-vegetation> eulalia, with nitrate being the predominant form of N leaching from leachate. Our findings reveal that vegetation conversion affects soil C and N contents, and alters their natural isotopes as well as the leaching of labile soluble nutrients. Notably, non-vegetation consistently reduced SOC and TN contents, whereas eulalia cultivation maintained SOC content, improved C/N ratio and δ13C, and reduced N leaching compared with clover cultivation. These results highlight the potential of eulalia as a candidate plant for enhancing C sequestration and reducing N leaching in cold regions of Japan.

Characterization of yield and cumulative nitrous oxide emission of maize varieties in responses to different nitrogen application rates.

The nitrogen use efficiency (NUE) of maize is usually below 60%. Considering future food supply and climate change, selective breeding of maize with high nitrogen (N)-efficient varieties, covering genetic diversities, is an effective strategy for identifying specific elements for controlling NUE and productivity per arable farming unit while reducing environmental damage. This study evaluated the yield and nitrous oxide (N2O) emission of 30 maize varieties under two different N doses of 57.5 kg N ha-1 (N1, N-sufficient) and 173 kg N ha-1 (N3, N-high) applied in two equal splits on 2 and 4 weeks after germination (WAG). Then, the tested maize varieties were categorized into four groups based on the grain yield and cumulative N2O, that is, efficient-efficient (EE) under both N1 and N3, high-N efficient (HNE) under N3 only, low-N efficient (LNE) under N1 only, and nonefficient-nonefficient (NN) under neither N1 nor N3. Maize yield was significantly positively correlated with shoot biomass, N-accumulation, and kernel-number under N1 and with N2O-flux at 5 WAG, NH4+, shoot biomass, and all of yield components under N3, whereas cumulative N2O showed a significant positive correlation with NO3- under N3 only and with N2O flux at 3 WAG under both N levels. The EE generally showed higher grain yield, yield components, N-accumulation, dry matter accumulation, root volume, and NH4+ in soil and lower cumulative N2O and NO3- in soil relative to NN maize varieties. The EE variety groups of maize can be a feasible strategy for increasing N fertilizer efficiency without reducing maize production as well as decrease the negative impact of N lost in agricultural system.

Structurally distinct mitoviruses: are they an ancestral lineage of the Mitoviridae exclusive to arbuscular mycorrhizal fungi (Glomeromycotina)?

Mitoviruses in the family Mitoviridae are the mitochondria-replicating "naked RNA viruses" with genomes encoding only the replicase RNA-dependent RNA polymerase (RdRp) and prevalent across fungi, plants, and invertebrates. Arbuscular mycorrhizal fungi in the subphylum Glomeromycotina are obligate plant symbionts that deliver water and nutrients to the host. We discovered distinct mitoviruses in glomeromycotinian fungi, namely "large duamitovirus," encoding unusually large RdRp with a unique N-terminal motif that is endogenized in some host genomes. More than 400 viral sequences similar to the large duamitoviruses are present in metatranscriptome databases. They are globally distributed in soil ecosystems, consistent with the cosmopolitan distribution of glomeromycotinian fungi, and formed the most basal clade of the Mitoviridae in phylogenetic analysis. Given that glomeromycotinian fungi are the only confirmed hosts of these viruses, we propose the hypothesis that large duamitoviruses are the most ancestral lineage of the Mitoviridae that have been maintained exclusively in glomeromycotinian fungi.

Lipidome Profiling of Phosphorus Deficiency-Tolerant Rice Cultivars Reveals Remodeling of Membrane Lipids as a Mechanism of Low P Tolerance.

Plants have evolved various mechanisms for low P tolerance, one of which is changing their membrane lipid composition by remodeling phospholipids with non-phospholipids. The objective of this study was to investigate the remodeling of membrane lipids among rice cultivars under P deficiency. Rice (Oryza sativa L.) cultivars (Akamai, Kiyonishiki, Akitakomachi, Norin No. 1, Hiyadateine, Koshihikari, and Netaro) were grown in 0 (-P) and 8 (+P) mg P L-1 solution cultures. Shoots and roots were collected 5 and 10 days after transplanting (DAT) in solution culture and subjected to lipidome profiling using liquid chromatography-mass spectrometry. Phosphatidylcholine (PC)34, PC36, phosphatidylethanolamine (PE)34, PE36, phosphatidylglycerol (PG)34, phosphatidylinositol (PI)34 were the major phospholipids and digalactosyldiacylglycerol (DGDG)34, DGDG36, 1,2-diacyl-3-O-alpha-glucuronosylglycerol (GlcADG)34, GlcADG36, monogalactosyldiacylglycerol (MGDG)34, MGDG36, sulfoquinovosyldiacylglycerol (SQDG)34 and SQDG36 were the major non-phospholipids. Phospholipids were lower in the plants that were grown under -P conditions than that in the plants that were grown under +P for all cultivars at 5 and 10 DAT. The levels of non-phospholipids were higher in -P plants than that in +P plants of all cultivars at 5 and 10 DAT. Decomposition of phospholipids in roots at 5 DAT correlated with low P tolerance. These results suggest that rice cultivars remodel membrane lipids under P deficiency, and the ability of remodeling partly contributes to low P tolerance.

Winter nocturnal warming affects the freeze-thaw frequency, soil aggregate distribution, and the contents and decomposability of C and N in paddy fields.

Climate warming is expected to cause greater increases in nocturnal temperatures than daytime temperatures, thereby altering freeze-thaw cycles. Although the importance of freeze-thaw cycles in regulating soil aggregate stability and nutrient availability has attracted increasing attention, little is known about how winter nocturnal warming modulates freeze-thaw frequency, soil aggregate distribution, or the contents and mineralization of soil organic carbon (SOC) and total nitrogen (TN) in paddy fields. The nocturnal soil temperature in the upper 0-2 cm layer in a paddy field was elevated by approximately 2 °C using a passive nocturnal warming method during winter. An anaerobic experiment with a first-order reaction model was conducted to measure the C decomposition (C0) and N mineralization (N0) potentials in bulk soil and four soil aggregate fractions. Winter nocturnal warming significantly decreased freeze-thaw frequency and affected soil aggregate distribution and SOC and TN contents in <0.25 mm aggregate. Both SOC and TN fractions were significantly increased in the 0.25-1 mm aggregate but decreased in the >2 mm aggregate due to winter nocturnal warming. Winter nocturnal warming did not affect C0, N0, C0/SOC, and N0/TN in bulk soil. However, it decreased C0 and C0/SOC in all aggregates except the 0.25-1 mm aggregate, and increased N0 and N0/TN in all aggregates except the >2 mm aggregate. In the nocturnal warming treatment, the highest C0 and N0 values were found in the <0.25 mm aggregate, but only the N0 in the <0.25 mm aggregate was significantly larger than that in the other three soil aggregates. Our study indicated that winter nocturnal warming would reduce the freeze-thaw frequency and change C and N distributions in soil aggregates, resulting in increased soil N availability in the subsequent rice growth season.

Solid-phase extraction for metabolomic analysis of high-salinity samples by capillary electrophoresis-mass spectrometry.

Environmental samples such as soil solutions contain inorganic ions such as NH4(+), K(+), Na(+), NO3(-), and PO4(3-) in high concentrations, which must be removed prior to capillary electrophoresis-mass spectrometry analysis to obtain accurate results. However, the separation of these inorganic ions from ionic metabolites, which are the target compounds in capillary electrophoresis-mass spectrometry analysis, is difficult because the physicochemical properties of the inorganic ions are similar to those of the ionic metabolites. In this study, we used various solid-phase extraction (SPE) columns for the purification of the samples containing inorganic ions in high concentrations. We found that cation-exchange SPE columns successfully filtered out the inorganic ions while retaining most of the organic compounds, which were easily collected with high recovery rates. In addition, 17 cationic metabolites in the soil solution were quantified by CE-MS analysis following the SPE purification process. The results suggest that our method can be used to analyze other environmental samples containing inorganic ions in high concentrations.

Five-year soil warming changes soil C and N dynamics in a single rice paddy field in Japan.

Soil temperature is an important determinant of carbon (C) and nitrogen (N) cycling in terrestrial ecosystems, but its effects on soil organic carbon (SOC) and total nitrogen (TN) dynamics as well as rice biomass in rice paddy ecosystems are not fully understood. We conducted a five-year soil warming experiment in a single-cropping paddy field in Japan. Soil temperatures were elevated by approximate 2 °C with heating wires during the rice growing season and by approximate 1 °C with nighttime thermal blankets during the fallow season. Soil samples were collected in autumn after rice harvest and in spring after fallow each year, and anaerobically incubated at 30 °C for four weeks to determine soil C decomposition and N mineralization potentials. The SOC and TN contents, rice biomass, dissolved organic carbon (DOC) and microbial biomass carbon (MBC) concentrations were measured in the study. Soil warming did not significantly enhance rice aboveground and root biomasses, but it significantly decreased SOC and TN contents and thus decreased soil C decomposition and N mineralization potentials due to depletion of available C and N. Moreover, soil warming significantly decreased DOC concentration but significantly increased MBC concentration. The ratios of C decomposition potential to N mineralization potential, decomposition potential to SOC, and N mineralization to TN were not affected by soil warming. There were significant seasonal and annual variations in SOC, C decomposition and N mineralization potentials, soil DOC and MBC under each temperature treatments. Our study implied that soil warming can decrease soil C and N stocks in paddy ecosystem probably via stimulating microbial activities and accelerating the depletion of DOC. This study further highlights the importance of long-term in situ observation of C and N dynamics and their availabilities in rice paddy ecosystems under increasing global warming scenarios.

Petroleum hydrocarbons degradation in contaminated soil using the plants of the Aster family.

Oil extraction is one of the causes of soil contamination with the total petroleum hydrocarbons. The objective of this study was to clarify the effect of Asteraceae plants on the degradation of petroleum hydrocarbon in contaminated soil. Initial soils with 40 and 90 g kg-1 of total petroleum hydrocarbon (TPH) were prepared. There were three treatments: (1) no addition, (2) addition of FeCl3 and nitrilotriacetic acid (NTA) solution, and (3) addition of FeCl3 + NTA and the cultivation of nine Asteraceae plants. The concentration of TPH was measured using infrared spectrophotometer, 2 and 3 months after transplanting (MAT). Shoot and root dry weights were measured 3 MAT. The concentration of TPH in soil cultivated with Cosmos caudatus was lower than that of the initial soil (40 g kg-1 TPH), 2 MAT. The concentrations of TPH in soils cultivated with Calendula officinalis, Callistephus chinensis, C. caudatus, and Tagetes sp. were also lower than that in the initial soil, 3 MAT. The concentrations of TPH in soils cultivated with Achillea filipendulina, Anthemis tinctoria, Tagetes erecta, Chrysanthemum coronarium, C. officinalis, C. chinensis, and C. caudatus were lower than that in the initial soil (90 g kg-1 TPH), 2 MAT. The concentrations of TPH in soils cultivated with T. erecta, A. tinctoria, Zinnia elegans, C. chinensis, C. caudatus, and Tagetes sp. were lower than that in the initial soil, 3 MAT. A. filipendulina and C. coronarium died at both 40 and 90 kg-1 TPH soils. These results suggest that the roots of Asteraceae plants degrade petroleum hydrocarbon in contaminated soil and C. chinensis and Z. elegans are more suitable for using TPH remediation. Plant survival and extensive root system are important factors for the remediation of TPH in contaminated soil.

Co-application of poultry-litter biochar with Azolla has synergistic effects on CH4 and N2O emissions from rice paddy soils.

Poultry-litter biochar and Azolla as green manure amendments are reported to enhance paddy soil fertility and rice yields. However, whether their co-application in lowland rice paddies has synergistic effects and whether those benefits are accompanied by greenhouse gas (GHG) emissions remains unknown. The objective of this study was to determine the effects of poultry-litter biochar (hereafter: biochar) and its co-application with Azolla as green manure (hereafter: Azolla), on the simultaneous methane (CH4) and nitrous oxide (N2O) emissions from a lowland paddy soil planted with rice during a single rice growing season in Tsuruoka, Yamagata, Japan. Biochar and Azolla amendments were applied once before rice was transplanted at a density of 20 t ha-1 and 133.9 kg N ha-1, respectively. Compared with NPK, NPK + biochar, and Azolla only treatments, Azolla and biochar co-application (i.e., Azolla + biochar) significantly increased CH4 emissions by 33%-197.6% in the early stages of rice growth (before 63 days after transplanting, DAT), but did not significantly influence CH4 emissions at both late rice growth stages (after 63 DAT,) and whole rice growth period (112 DAT). Conversely, Azolla + biochar significantly reduced N2O emissions by 83.0%-97.1% before 63 DAT, and by 76.4%-95.9% during the whole rice growth period at 112 DAT, with a significantly high interaction between biochar and fertilizer amendments. There were no significant N2O emission differences among all treatments after 63 DAT. Additionally, Azolla + biochar significantly increased rice grain yield by 27.3%-75.0%, and consequently, decreased both yield-equivalent CH4 emissions by 24.7%-25.0% and N2O emissions by 81.8%-97.7%. Our findings suggest that the co-application of poultry-litter biochar and Azolla as green manure offers a novel approach to increase rice yield while reducing the emissions of non-carbon dioxide greenhouse gases.

Impact of Introduction of Arbuscular Mycorrhizal Fungi on the Root Microbial Community in Agricultural Fields.

Arbuscular mycorrhizal (AM) fungi are important members of the root microbiome and may be used as biofertilizers for sustainable agriculture. To elucidate the impact of AM fungal inoculation on indigenous root microbial communities, we used high-throughput sequencing and an analytical pipeline providing fixed operational taxonomic units (OTUs) as an output to investigate the bacterial and fungal communities of roots treated with a commercial AM fungal inoculum in six agricultural fields. AM fungal inoculation significantly influenced the root microbial community structure in all fields. Inoculation changed the abundance of indigenous AM fungi and other fungal members in a field-dependent manner. Inoculation consistently enriched several bacterial OTUs by changing the abundance of indigenous bacteria and introducing new bacteria. Some inoculum-associated bacteria closely interacted with the introduced AM fungi, some of which belonged to the genera Burkholderia, Cellulomonas, Microbacterium, Sphingomonas, and Streptomyces and may be candidate mycorrhizospheric bacteria that contribute to the establishment and/or function of the introduced AM fungi. Inoculated AM fungi also co-occurred with several indigenous bacteria with putative beneficial traits, suggesting that inoculated AM fungi may recruit specific taxa to confer better plant performance. The bacterial families Methylobacteriaceae, Acetobacteraceae, Armatimonadaceae, and Alicyclobacillaceae were consistently reduced by the inoculation, possibly due to changes in the host plant status caused by the inoculum. To the best of our knowledge, this is the first large-scale study to investigate interactions between AM fungal inoculation and indigenous root microbial communities in agricultural fields.

Dissection of niche competition between introduced and indigenous arbuscular mycorrhizal fungi with respect to soybean yield responses.

Arbuscular mycorrhizal (AM) fungi associate with most land plants and deliver phosphorus to the host. Identification of biotic/abiotic factors that determine crop responses to AM fungal inoculation is an essential step for successful application of the fungi in sustainable agriculture. We conducted three field trials on soybean with a commercial inoculum and developed a new molecular tool to dissect interactions between the inoculum and indigenous fungi on the MiSeq sequencing platform. Regression analysis indicated that sequence read abundance of the inoculum fungus was the most significant factor that determined soybean yield responses to the inoculation, suggesting that dominance of the inoculum fungus is a necessary condition for positive yield responses. Agricultural practices (fallow/cropping in the previous year) greatly affected the colonization levels (i.e. read abundances) of the inoculum fungus via altering the propagule density of indigenous AM fungi. Analysis of niche competition revealed that the inoculum fungus competed mainly with the indigenous fungi that are commonly distributed in the trial sites, probably because their life-history strategy is the same as that of the inoculum fungus. In conclusion, we provide a new framework for evaluating the significance of environmental factors towards successful application of AM fungi in agriculture.