Soil Bacterial Diversity Responds to Long-Term Establishment of Perennial Legumes in Warm-Season Grassland at Two Soil Depths.

The introduction of rhizoma peanut (RP Arachis glabrata Benth) into bahiagrass (Paspalum notatum Flüggé) may require time to develop stable plant-soil microbe interactions as the microbial legacy of the previous plant community may be long-lasting. A previous study showed that <2 years of introducing rhizoma peanut into bahiagrass pastures minimally affected soil bacterial diversity and community composition. In this study, we compared the effects of the long-term inclusion of rhizoma peanut (>8 years) into bahiagrass on soil bacterial diversity and community composition against their monocultures at 0 to 15 and 15 to 30 cm soil depths using next-generation sequencing to target bacterial 16S V3-V4 regions. We observed that a well-established RP-bahiagrass mixed stand led to a 36% increase in bacterial alpha diversity compared to the bahiagrass monoculture. There was a shift from a soil bacterial community dominated by Proteobacteria (~26%) reported in other bahiagrass and rhizoma peanut studies to a soil bacterial community dominated by Firmicutes (39%) in our study. The relative abundance of the bacterial genus Crossiella, known for its antimicrobial traits, was enhanced in the presence of RP. Differences in soil bacterial diversity and community composition were substantial between 0 to 15 and 15 to 30 cm soil layers, with N2-fixing bacteria belonging to the phylum Proteobacteria concentrated in 0 to 15 cm. Introducing RP into bahiagrass pastures is a highly sustainable alternative to mineral N fertilizer inputs. Our results provide evidence that this system also promotes greater soil microbial diversity and is associated with unique taxa that require further study to better understand their contributions to healthy pastures.

Integrated crop-livestock systems result in less nitrate leaching than ungrazed crop systems in North Florida.

Integrated crop-livestock systems provide an array of benefits to agricultural systems, including a reduction in nitrogen (N) leaching. A farm approach to integrate crops and livestock is the adoption of grazed cover crops. Moreover, the addition of perennial grasses into crop rotations may improve soil organic matter and decrease N leaching. However, the effect of grazing intensity in such systems is not fully understood. This 3-year study investigated short-term effects of cover crop planting (cover and no cover), cropping system (no grazing, integrated crop-livestock [ICL], and sod-based rotation [SBR]), grazing intensity (heavy, moderate, and light grazing), and cool-season N fertilization (0, 34, and 90 kg N ha-1 ) on NO3 -N and NH4 -N concentration in leachate, and cumulative N leaching by using 1.5-m deep drain gauges. The ICL was a cool-season cover crop-cotton (Gossypium hirsutum L.) rotation, whereas SBR was a cool-season cover crop-bahiagrass (Paspalum notatum Flüggé) rotation. There was a treatment × year × season for cumulative N leaching (p = 0.035). Further contrast analysis indicated that cover crops decreased cumulative N leaching compared to no cover (18 vs. 32 kg N ha-1 season-1 ). Nitrogen leaching was lesser for grazed compared to nongrazed systems (14 vs. 30 kg N ha-1 season-1 ). Treatments containing bahiagrass had lesser NO3 -N concentration in leachate (7 vs. 11 mg L-1 ) and cumulative N leaching (8 vs. 20 kg N ha-1 season-1 ) compared to ICL systems. Adding cover crops can reduce cumulative N leaching in crop-livestock systems; moreover, warm-season perennial forages can further enhance this benefit.

Soil bacterial community response to rhizoma peanut incorporation into Florida pastures.

Incorporating legumes is one option for improving pasture fertility, sustainability, and biodiversity. Diazotrophic microorganisms, including rhizobia that form symbioses with legumes, represent a small fraction of the total soil microbial community. Yet, they can offset nitrogen (N) fertilizer inputs through their ability to convert atmospheric N2 into plant-usable N via biological N2 fixation (BNF). This study used amplicon sequencing of 16S rRNA genes to investigate soil bacterial community composition and diversity in grazed 'Argentine' bahiagrass (Paspalum notatum Flügge) pastures where N fertilizer was supplanted with legume-derived N from BNF in some treatments. Treatments consisted of bahiagrass fertilized with (a) mineral N (224 kg N ha-1  yr-1 ), (b) combination mineral N (34 kg N ha-1  yr-1 ) and legume-derived N via cool-season clover (CSC) (Trifolium spp.) mix, or (c) combination mineral N (34 kg N ha-1  yr-1 ) and legume-derived N via CSC mix and strips of Ecoturf rhizoma peanut (Arachis glabrata Benth.). Bradyrhizobium spp. relative abundance was 44% greater in the mixed pasture. Other bacterial genera with BNF or denitrification potentials were greater in pastures with legumes, whereas sequences assigned to genera associated with high litter turnover were greater in bahiagrass pastures receiving only mineral N. Soil bacteria alpha diversity was greater in pastures receiving 34 kg ha-1  yr-1 N fertilizer application and the CSC mix than in pastures with the CSC mix and rhizoma peanut strips. Our results demonstrate soil microbial community shifts that may affect soil C and N cycling in pastures common to the southeastern United States.

Mitigating rural WWTP impacts: System dynamics modeling of downstream nutrient outputs.

A system dynamics modeling approach was used to assess the potential impact of intentional struvite crystallization recovery on wastewater treatment plant (WWTP) allocation of N and P in effluent and biosolids outputs. Struvite crystallization has been used to recover wastewater N and P and produce valuable fertilizer. However, it is often overlooked whether additional benefits may be realized by diverting N and P from other fates. A system dynamics model was used with operational data from three activated-sludge WWTPs in North Florida. Incorporating struvite crystallization reduced the effluent P load by 37 to 100%, dependent upon the WWTP. This may translate into substantial savings for systems facing severe restrictions in effluent P release outside the plant. Additionally, biosolids P load reductions ranged from 17 to 46%. The model also predicted a 37% average increase in the biosolids N:P ratio. Increasing the N:P ratio may allow for greater biosolids land-application rates where P fertilizer restrictions exist. In comparison, the N load reductions were much less dramatic, i.e. below 10% reduction from the effluent and 14% from the biosolids. Most N inputs into an activated-sludge type WWTP are likely lost through denitrification during wastewater processing and struvite does not appear to be a significant means of recovering N from small WWTPs. However, incorporating struvite recovery into even the simplest WWTPs reduces effluent post-treatment needs and results in a more useful biosolids product.

Removal of nitrogen and phosphorus by Eucalyptus and Populus at a tertiary treated municipal wastewater sprayfield.

Various progenies of Eucalyptus grandis and E. amplifolia, and clones of Populus deltoides, were evaluated for plant removal of nitrogen (N) and phosphorus (P) for 26 months at a municipal waste spray field in north Florida. Tertiary treated wastewater containing 2.73 mg L(-1) nitrate N and 0.30 mg L(-1) total P was applied using sprinkler irrigation (93.8 m3 ha(-1) d(-1)) to fast growing trees utilized for bioenergy. Eucalyptus amplifolia and E. grandis survived and grew very poorly as the result of severe winter injury in two successive years and were not evaluated for nutrient removal. Survival and growth of P. deltoides demonstrated suitability for phytoremediation, and selected clones were evaluated for biomass and nutrient content. Removals of total N (TN) and total P (TP) were greatest for main stem (36% and 44%, respectively) and foliage (44% and 36%, respectively). Low biomass producing clones generally had higher nutrient concentrations, but high biomass producing clones removed more TN and TP. Approximately 789 kg ha(-1) TN and 103 kg ha(-1) TP were removed by the highest biomass producing P. deltoides clone, representing 215% of N and 615% of P inputs.