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Purpose: The fixation of atmospheric CO2 and N2 by soil bacteria is important to the terrestrial carbon and nitrogen cycles and can be greatly affected by anthropogenic disturbance. The application of mineral fertilizer combined with manure (MCM) has been used to maintain soil quality and improve soil C storage. However, the impacts of MCM on soil CO2-fixing bacteria (CFB) and N2-fixing bacteria (NFB), encoded separately by the cbbL and nifH genes, remain largely unknown. Methods: In this study, we examined how the community structure of soil CFB and NFB from a chronosequence of Moso bamboo forests (6, 10, 15, and 20 years of intensive management) responded to long-term intensive management with MCM and identified the critical determinants that regulated them. Quantitative PCR and high-throughput pyrosequencing of the cbbL and nifH genes were used to analyze the abundance and community composition of CFB and NFB, respectively. Results: Employing MCM enhanced the content of SOC and soil available N, K, although some fluctuating values were observed over the 20-year period. The abundance of cbbL gene decreased significantly (P < 0.05) after 6 years of management and then recovered after 20 years of management. However, the abundance of nifH gene consistently declined over 20 years of management. The diversity of CFB increased significantly after 6 years of management, while that of NFB decreased significantly after 15 years of management compared to the no-fertilizer control. The phylum Proteobacteria was predominant in the CFB and NFB communities. Furthermore, the sum abundance of keystone taxa Rhodopseudomonas, Aminobacter, and Rubrivivax for CFB increased after 20 years of management, while the keystone taxon Bradyrhizobium for NFB decreased after 20 years of management compared to no-fertilizer control. Conclusions: In conclusion, the application of MCM favors the abundance, diversity, and keystone taxa of soil CFB rather NFB, which may improve soil C fixation. [ABSTRACT FROM AUTHOR]

Plant growth-promoting rhizobacteria are bacteria that improve plant growth and reduce plant pathogen damages. In this study, 100 nodule bacteria were isolated from chickpea, screened for their plant growth-promoting (PGP) traits and then characterised by PCR-RFLP of 16 S rDNA. Results showed that most of the slow-growing isolates fixed nitrogen but those exhibiting fast-growth did not. Fourteen isolates solubilized inorganic phosphorus, 16 strains produced siderophores, and 17 strains produced indole acetic acid. Co-culture experiments identified three strains having an inhibitory effect against Fusarium oxysporum, the primary pathogenic fungus for chickpea in Tunisia. Rhizobia with PGP traits were assigned to Mesorhizobium ciceri, Mesorhizobium mediterraneum, Sinorhizobium meliloti and Agrobacterium tumefaciens. We noted that PGP activities were differentially distributed between M. ciceri and M. mediterraneum. The region of Mateur in northern Tunisia, with clay–silty soil, was the origin of 53% of PGP isolates. Interestingly, we found that S. meliloti and A. tumefaciens strains did not behave as parasitic nodule-bacteria but as PGP rhizobacteria useful for chickpea nutrition and health. In fact, S. meliloti strains could solubilize phosphorus, produce siderophore and auxin. The A. tumefaciens strains could perform the previous PGP traits and inhibit pathogen growth also. Finally, one candidate strain of M. ciceri (LL10)—selected for its highest symbiotic nitrogen fixation and phosphorus solubilization—was used for field experiment. The LL10 inoculation increased grain yield more than three-fold. These finding showed the potential role of rhizobia to be used as biofertilizers and biopesticides, representing low-cost and environment-friendly inputs for sustainable agriculture. [ABSTRACT FROM AUTHOR]

The role of soil organic C (SOC) quality affecting microbial community composition and function under biochar application is poorly understood. We investigated the relationship between the pool size and chemical composition of SOC; composition of main microbial groups; enzyme activities involved in C, N, and P cycling; and soil respiration in a rice paddy amended with biochar for 20 months in a laboratory experiment at 15, 25, and 35 °C. Soil labile and recalcitrant organic C pools were determined by a two-step sulfuric acid (H2SO4) hydrolysis method. The chemical composition of SOC was determined with 13C-nuclear magnetic resonance spectroscopy. The biochar amendment at 20 and 40 t ha−1 significantly decreased the soil labile C pool I (extracted by 5 N H2SO4), alkyl, and carbonyl C contents and increased the recalcitrant C pool (acid-resistant) and aromatic C contents and the aromatic C to O-alkyl C ratio. The phospholipid-fatty acid concentrations and soil enzyme activities were unchanged by biochar application at 10 and 20 t ha−1, but both were increased at 40 t ha−1. Biochar increased the ratio of gram-positive (G+) to gram-negative (G−) bacteria and decreased that of fungi to bacteria. The recalcitrant C pool and aromatic C contents were positively correlated to the G+ bacteria abundance and were important factors in shaping composition of the main microbial groups and improving enzyme activities. Biochar application at 40 t ha−1 lowered soil respiration rates at 15 and 25 °C by decreasing labile C pool and increasing C recalcitrancy while increased temperature sensitivities of soil respiration at 25/15 °C and 35/25 °C by stimulating microbial abundance and enzyme activities. Together, our results suggest that biochar soil amendment shifted microbial community composition and function through influencing the composition of SOC. [ABSTRACT FROM AUTHOR]

Increasing the amount of carbon (C) in soils is one method to reduce the concentration of carbon dixoide (CO2) in the atmosphere. We measured organic C stored in southern Idaho soils having long-term cropping histories that supported native sagebrush vegetation (NSB), irrigated moldboard plowed crops (IMP), irrigated conservation-chisel-tilled crops (ICT), and irrigated pasture systems (IP). The CO2 emitted as a result of fertilizer production, farm operations, and CO2 lost via dissolved carbonate in irrigation water, over a 30-year period, was estimated and used to calculate net C fixation. Organic C in ecosystems decreased in the order IP>ICT>IMP> NSB. In February 2001, active fungal, bacterial, and microbial biomass was greater in IP soils than all other soils. Active fungal, bacterial, and microbial biomass was least in ICT soils at the 15–30-cm depth than all other soils. In August 2001, active bacterial biomass was greater in IMP soils than IP, ICT, and NSB soils. Active fungal biomass was greater in IP soils than all other soils. Whole-soil fatty acid profiles differed among management regimes and sampling dates and, to a lesser extent, with soil depth. FAME profiles from the NSB soils were distinct from the agricultural treatments and contained greater amounts of total fatty acids than the other treatments. The IMP and ICT soils yielded fatty acid profiles that were similar to each other, although those at the 15–30-cm depth were distinct from all other treatment-depth combinations. The IP FAME profiles suggest that arbuscular mycorrhizal fungi are more common in these soils than soils from the other treatments. Differences in carbon substrate utilization patterns (BIOLOG) among treatments were more variable and less pronounced that FAME results. In general, irrigated arid soils can both increase C storage while increasing microbial biomass and changing microbial diversity. [ABSTRACT FROM AUTHOR]