Prevalent and highly mobile antibiotic resistance genes in commercial organic fertilizers.

Compost-based organic fertilizers made from animal manures may contain high levels of antibiotic resistance genes (ARGs). However, the factors affecting the abundance and profile of ARGs in organic fertilizers remain unclear. We conducted a national-wide survey in China to investigate the effect of material type and composting process on ARG abundance in commercial organic fertilizers and quantified the contributions of bacterial composition and mobile genetic elements (MGEs) to the structuring of ARGs, using quantitative PCR and Illumina sequencing of 16S rRNA gene amplicons. The tetracycline, sulfonamide, aminoglycoside and macrolide resistance genes were present at high levels in all organic fertilizers. Seven ARGs that confer resistance to clinically important antibiotics, including three ß-lactam resistance genes, three quinolone resistance genes and the colistin resistance gene mcr-1, were detected in 8 - 50% the compost samples, whereas the vancomycin resistance gene vanC was not detected. Raw material type had a significant (p < 0.001) effect on the ARG abundance, with composts made from animal feces except some cattle feces generally having higher loads of ARGs than those from non-animal raw materials. Composting process type showed no significant (p > 0.05) effect on ARG abundance in the organic fertilizers. MGEs exerted a greater influence on ARG composition than bacterial community, suggesting a strong mobility of ARGs in the organic fertilizers. Our study highlights the need to manage the risk of ARG dissemination from agricultural wastes.

Root exudate chemistry affects soil carbon mobilization via microbial community reassembly.

Plant roots are one of the major mediators that allocate carbon captured from the atmosphere to soils as rhizodeposits, including root exudates. Although rhizodeposition regulates both microbial activity and the biogeochemical cycling of nutrients, the effects of particular exudate species on soil carbon fluxes and key rhizosphere microorganisms remain unclear. By combining high-throughput sequencing, q-PCR, and NanoSIMS analyses, we characterized the bacterial community structure, quantified total bacteria depending on root exudate chemistry, and analyzed the consequences on the mobility of mineral-protected carbon. Using well-controlled incubation experiments, we showed that the three most abundant groups of root exudates (amino acids, carboxylic acids, and sugars) have contrasting effects on the release of dissolved organic carbon (DOC) and bioavailable Fe in an Ultisol through the disruption of organo-mineral associations and the alteration of bacterial communities, thus priming organic matter decomposition in the rhizosphere. High resolution (down to 50 nm) NanoSIMS images of mineral particles indicated that iron and silicon co-localized significantly more organic carbon following amino acid inputs than treatments without exudates or with carboxylic acids. The application of sugar strongly reduced microbial diversity without impacting soil carbon mobilization. Carboxylic acids increased the prevalence of Actinobacteria and facilitated carbon mobilization, whereas amino acid addition increased the abundances of Proteobacteria that prevented DOC release. In summary, root exudate functions are defined by their chemical composition that regulates bacterial community composition and, consequently, the biogeochemical cycling of carbon in the rhizosphere.

[Prediction of yield and yield components in hybrid rice by using molecular markers].

Yield and yield components in hybrid rice were investigated using AFLP, RAPD and SSR markers. Ten restorer and five male-sterile lines were crossed in all possible pairs resulting in 50 crosses. Positive loci, effect-increasing loci, effect-decreasing loci and non-environmental loci were selected from the 931 marker loci surveyed in the 15 parental lines and their correlation with yield and yield components were analyzed. The results indicated as follows (1) The correlation between genetic difference and yield and yield components calculated on all three molecular loci failed to reach significant level for most of the traits investigated and can not be used to predict yield and yield components directly. (2) Positive loci were of limited usefulness in the prediction of yield and yield components for their variation with different traits investigated despite that they can improve the correlation coefficient in some degree. (3) Effect-increasing and effect-decreasing loci can greatly improve correlation coefficient and may be used to predict the yield and yield components for their consistence with the environment. (4) The coefficient based on non-environmental loci was high though it was a bit lower than that based on effect-increasing and effect-decreasing loci. It indicated that environment had great effect on yield and yield components in rice.