No abstract

Phosphate-Solubilizing Bacteria (PSB) improve plant growth, yield and phosphorus content of several crops, and may be used as bioinoculant to enhance sustainable production. We evaluated the response of maize (Zea mays L.) to PSB inoculation under controlled and field conditions in Tucumán, Argentina. A pot culture experiment was conducted to investigate the effects of seven previously isolated PSB on early development of plants. Seeds were treated with each bacterial strain, and seedlings were harvested 30 days after inoculation. All strains showed a positive effect on plant growth. A significant increment in plant height (45%), shoot dry weight (40%) was determined in plants treated with Pseudomonas tolaasii IEXb, while Pseudomonas koreensis SP28 has remarkably increased P content compared to the uninoculated control. IEXb strain was selected and evaluated under field conditions in combination with triple superphosphate (TSP) as P fertilizer. The presence of IEXb strain stimulated seedling emergence (8%), shoot length (19%), grain yield (44%), 1000-grain weight (18%), total dry biomass (32%) and P content (56%) of maize plants. In general, P. tolaasii IEXb inoculation was more efficient as bioinoculant without P fertilizer than with TSP. These results provide baseline information for future studies of P. tolaasii IEXb as bioinoculant to promote an eco-friendly and sustainable agriculture.

The ability of soil microorganisms to solubilize phosphate is an important trait of plant growth-promoting bacteria leading to increased yields and smaller use of fertilizers. This study presents the isolation and characterization of phosphobacteria from Puna, northwestern Argentina and the ability to produce phosphate solubilization, alkaline phosphatase, siderophores, and indole acetic acid. The P-solubilizing activity was coincidental with a decrease in pH values of the tricalcium phosphate medium for all strains after 72 h of incubation. All the isolates showed the capacity to produce siderophores and indoles. Identification by 16S rDNA sequencing and phylogenetic analysis revealed that these strains belong to the genera Pantoea, Serratia, Enterobacter, and Pseudomonas. These isolates appear attractive for exploring their plant growth-promoting activity and potential field application.

No abstract

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia.The introduction of leucaena (Leucaena leucocephala), apart from increasing animal production, improves soil fertility through biological nitrogen (N) fixation and its deep-rooted system. There is limited information on carbon and N dynamics in hedgerow silvopastoral systems, particularly in the subsoil profile. The concentrations and vertical distribution of organic carbon (OC) and total N , and their fractions (particulate and associate forms) in the profile (0‒100 cm) of a 4-year-old leucaena stand in a Urochloa brizantha-Chloris gayana pasture were compared with those in the adjacent pure tropical grass (U. brizantha) pasture. Leucaena introduction increased the OC concentration in the subsoil (20‒100 cm) by 45%, particularly the stable form (associate OC) in the deepest horizon (50‒100 cm). This was attributed to a greater abundance of leucaena roots deeper in the profile than for grass. Leucaena also enhanced by 7.6% the N concentration (from 0.131 to 0.141%) in the topsoil (0‒20 cm) associated with an increment in the labile form (particulate organic N), due to leaf deposition, recycling of animal feces and nodule-N turnover from N fixation. Leucaena establishment has the potential to improve soil fertility and hence availability of N to companion grass growth, and can be utilized as a greenhouse gas mitigation strategy.

The Dry Chaco region is a vast plain that extends throughout north-western Argentina. This region concentrates about 13% of the total Argentine cattle national stock. Beef cattle systems in the north-west of Argentina are highly varied including extensive pastoral systems, silvopastoral systems and intensive grazing systems.

<p>In recent years deforestation of the Gran Chaco of Argentina has increased dramatically to make way for agricultural expansion.  Extensive cattle ranching in particular is widespread across the country, and in the Chaco region of the north west much of the natural vegetation has been cleared for beef and crop production.  The effects of forest clearance and grazing over time on soil carbon dynamics are unclear, with some evidence suggesting that soil carbon can to some extent recover under low intensity grazing practices, whilst others find that conversion to pasture followed by years of grazing consistently decreases soil carbon stocks.  This study investigates the effects of land use change from forest to pasture over time on soil carbon stocks in the dry Chaco of north western Argentina, through the measurement of biological, physical and chemical variables within the soil.  The hypothesis leading to this work is the key idea that livestock grazing can promote the accumulation of carbon in the soil over time through processes such as the stimulation of root growth of pasture grasses.  In turn, increased carbon inputs can lead to net carbon sequestration, with great potential to mitigate the greenhouse gas emissions of the livestock sector.  Using a chronosequence experimental design, destructive soil samples were taken from reference forest patches and pastures of 0-5, 10-15 and >20 years since deforestation and were tested for carbon, nitrogen, and phosphorus contents, root biomass, pH, electrical conductivity and texture. The research aims to investigate and explain the carbon dynamics of pastures in the years following deforestation, identify potential biotic and abiotic drivers of such dynamics, and predict potential future changes in soil carbon stocks.</p>

Deforestation of Chacoan native forests and reorientation of land use are transforming the region into agricultural use. The main purpose of this work was to evaluate the impact of different land uses on soil quality in the semi-arid Chaco (Argentina). We assessed the behaviour of soil parameters over four years of experimental conditions: 1) Exclosure pasture (EP) used as reference level, 2) Grazed pasture (GP), 3) Grazed pasture transformed to agriculture with Zero tillage (ZT) and 4) Grazed pasture transformed to agriculture under Conventional tillage (CT). Soil organic carbon, particulate and heavy organic carbon (C), total nitrogen (N), C:N ratio, pH, electric conductivity and soil respiration were measured. Soil samples were taken yearly at 0-5, 5-20 and 20-40 cm of soil depth. Differences among treatments across time were assessed by Analysis of Covariance (ANCOVA) with time (years) as covariate factor, treatments as group factor and individual scores from Principal Component Analysis (PCA) as responses. Correlated changes in the soil characteristics were detected, especially at the top soil layer. Both carbon and nitrogen contents increased in both GP and ZT systems. An opposite trend was found for CT, which also had a negative impact on salinity. Both land use change and management practices in the Chaco region represent the main human activities that modify the landscape; thus, they should be analysed by recognizing heterogeneity on farming practices and identifying their impacts on a specific site. The results of this work reinforce the utility of soil organic carbon as a single parameter for monitoring land management systems, especially for monitoring large region like Chaco that are subject to continuous transformation processes.