Many soils are deep, yet soil below 20 cm remains largely unexplored. Exotic plants can have shallower roots than native species, so their impact on microorganisms is anticipated to change with ...depth. Using environmental DNA and extracellular enzymatic activities, we studied fungal and bacterial community composition, diversity, function, and co‐occurrence networks between native and exotic grasslands at soil depths up to 1 m. We hypothesized (1) the composition and network structure of both fungal and bacterial communities will change with increasing depth, and diversity and enzymatic function will decrease; (2) microbial enzymatic function and network connectedness will be lower in exotic grasslands; and (3) irrigation will alter microbial networks, increasing the overall connectedness. Microbial diversity decreased with depth, and community composition was distinctly different between shallow and deeper soil depths with higher numbers of unknown taxa in lower soil depths. Fungal communities differed between native and exotic plant communities. Microbial community networks were strongly shaped by biotic and abiotic factors concurrently and were the only microbial measurement affected by irrigation. In general, fungal communities were more connected in native plant communities than exotic, especially below 10 cm. Fungal networks were also more connected at lower soil depths albeit with fewer nodes. Bacterial communities demonstrated higher complexity, and greater connectedness and nodes, at lower soil depths for native plant communities. Exotic plant communities’ bacterial network connectedness altered at lower soil depths dependent on irrigation treatments. Microbial extracellular enzyme activity for carbon cycling enzymes significantly declined with soil depth, but enzymes associated with nitrogen and phosphorus cycling continued to have similar activities up to 1 m deep. Our results indicate that native and exotic grasslands have significantly different fungal communities in depths up to 1 m and that both fungal and bacterial networks are strongly shaped jointly by plant communities and abiotic factors. Soil depth is independently a major determinant of both fungal and bacterial community structures, functions, and co‐occurrence networks and demonstrates further the importance of including soil itself when investigating plant–microbe interactions.
Aims Grasslands dominated by non-native (exotic) species have replaced purely native-dominated areas in many parts of the world forming 'novel' ecosystems. Altered precipitation patterns are ...predicted to exacerbate this trend. It is still poorly understood how soil microbial communities and their functions differ between high diversity native- and low diversity exotic-dominated sites and how altered precipitation will impact this difference. Methods We sampled 64 experimental grassland plots in central Texas with plant species mixtures of either all native or all exotic species; half with summer irrigation. We tested how native vs. exotic plant species mixtures and summer irrigation affected bacterial and fungal community composition and structure, the influence of niche vs. neutral processes for microbial phylotype co-occurrence (C-score analysis), and rates of phosphorus and nitrogen mineralization across an 8-year experiment. Results Native and exotic-dominated plots had significantly different fungal community composition and structure, but not diversity, throughout the length of the study, while changes in bacterial communities were limited to certain wet and cool years. Nitrogen and phosphorus mineralization rates were higher under native plant mixtures and correlated with the abundance of particular fungal species. Microbial communities were more structured in exotic than native grassland plots, especially for the fungal community. Conclusions The results indicate that conversion of native to exotic C4 dominated grasslands will more strongly impact fungal than bacterial community structure. Furthermore, these impacts can alter ecosystem functioning belowground via changes in nitrogen and phosphorus cycling.
Many grasslands have been transformed by exotic species with potentially novel ecological interactions. We hypothesized that exotic and native plant species differ, on average, in their percentage ...mycorrhizal colonization, and that mycorrhizal colonization is positively related to plant performance in the field. We compared colonization by arbuscular mycorrhizae (AM) fungi in perennial native and exotic species that were paired phylogenetically and by functional groups and grown under a common environment in field plots in Central Texas, USA. Roots were collected from plants in monoculture plots, stained, and percent colonization was assessed with a microscope. Aboveground biomass and dominance in mixture were used as measures of plant performance. Exotic species had significantly higher colonization of AM than native species, and this result was consistent across functional groups. Percent colonization was positively correlated with biomass and dominance in mixture across native species, but not across exotic species. Our results indicate that mycorrhizal dependence is a more important predictor of competitive balance among native than exotic plant species in the subhumid grasslands of the USA.
Aims Grasslands dominated by non-native (exotic) species have replaced purely native-dominated areas in many parts of the world forming ‘novel’ ecosystems. It is still poorly understood how soil ...microbial communities and their functions are affected between these native- and exotic-dominated sites. Methods We sampled 64 experimental grassland plots in central Texas with plant species mixtures of either all native or all exotic species; half with summer irrigation. We tested how native vs. exotic plant species mixtures and summer irrigation affected bacterial and fungal community composition and structure, microbial phylotype co-occurrence, and rates of mineralization across sampling years. Results Plant origin significantly affected fungal community composition and structure, but not diversity, throughout the length of the study, while changes in bacterial communities were limited to certain years. Nitrogen and phosphorus mineralization rates were higher under native plant mixtures, and correlated with changes in particular fungal species. Microbial communities were more structured in exotic than native grassland plots, especially for the fungal community. Conclusions The results indicate that conversion of native to exotic grasslands will more strongly impact fungal than bacterial communities structure. Furthermore, these impacts can alter ecosystem functioning belowground via changes in nitrogen and phosphorus cycling.
Seismic design requirements for precast concrete cladding panel connections have evolved significantly over the past fifty years. This paper summarizes the pertinent requirements from the Uniform ...Building Code from 1967 to 1997, and the International Building Code 2000. A hypothetical design illustrates how emphasis in the code has evolved for both lateral force requirements and story drift displacement requirements arriving at a balance of moderate lateral force and displacement requirements. The numerical results are based on a hypothetical case of panel connections for a ten-story moment-resisting steel frame structure built in seismic Zone 4. This historical summary is of value to designers who deal with the seismic rehabilitation of precast panel connections.
PDF
