Soil Microbial Dynamics and Biogeochemistry in Tropical Forests and Pastures, Southwestern Costa Rica

Tropical rain forest ecosystems are currently undergoing unprecedented rates of land conversion and land use change. Recent research suggests these activities profoundly influence nutrient cycling, but the principal mechanisms driving variation in nutrient status following land conversion are still not well understood. In this study, we used soils of varying fertility (oxisols and mollisols) in Costa Rica to investigate how conversion of tropical rain forest to cattle pasture affects the size and function of the microbial community, and to explore possible relationships between microbial dynamics and biogeochemistry. Our pasture sites are relatively lightly managed, and total pools of carbon (C), nitrogen (N), and phosphorus (P) were not significantly different from their forest counterparts. However, pools of available elements were different; most notably, plant available forms of P were significantly lower in the oxisol pasture than in the oxisol forest site. In addition, we found that land conversion led to fundamental changes in the size and activity of the soil microbial community. Microbial biomass was consistently higher in forests than in pastures, particularly in the oxisol sites, where it was more than twice the pasture value. Forest sites were also characterized by a microbial community that was more active, responded more rapidly to carbon substrate additions, and showed strong seasonal variation. Our results provide evidence that changes in biogeochemical cycling following land conversion observed here and elsewhere may be directly related to changes in microbial community structure and function

Seeding Techniques for Alfalfa to Improve Subirrigated Meadows

Improving quality and quantity of forage harvested from poor condition, subirrigated hay meadows in the Nebraska Sand Hills is critical to the winter forage reserve of livestock producers. Alfalfa (Medicago sativa L.) is the most commonly used legume for meadow improvement. Broadcast seeding (11.2 kg/ha) was compared to sod seeding (11.2 kg/ha) as a method to introduce alfalfa into an alkaline subirrigated meadow (Fluvaquentic Haplustolls). Before seeding, the study area received 78.5 kg/ha phosphorous. Paraquat (0.29 kg/ha) was applied to one-half of the area to suppress plant competition and provide qualitative information on treatment consistency across a range of sod competition. Lo-till sod seeding was accomplished with a power tillage seeder. Seedling density was determined in spring the following year. Broadcast alfalfa had a greater seedling density than sod-seeded alfalfa (38.4 and 19.1 plants/m2, respectively, p=.09). Apparently, paraquat had no effect on seedling establishment and tended to reduce total yield. Yields the year of seeding, using a two harvest scheme, were greater for broadcast compared to sod-seeded alfalfa (p=.02). However, there was no significant difference between yields of broadcast alfalfa and control

Soil Microbial Networks Shift Across a High-Elevation Successional Gradient.

While it is well established that microbial composition and diversity shift along environmental gradients, how interactions among microbes change is poorly understood. Here, we tested how community structure and species interactions among diverse groups of soil microbes (bacteria, fungi, non-fungal eukaryotes) change across a fundamental ecological gradient, succession. Our study system is a high-elevation alpine ecosystem that exhibits variability in successional stage due to topography and harsh environmental conditions. We used hierarchical Bayesian joint distribution modeling to remove the influence of environmental covariates on species distributions and generated interaction networks using the residual species-to-species variance-covariance matrix. We hypothesized that as ecological succession proceeds, diversity will increase, species composition will change, and soil microbial networks will become more complex. As expected, we found that diversity of most taxonomic groups increased over succession, and species composition changed considerably. Interestingly, and contrary to our hypothesis, interaction networks became less complex over succession (fewer interactions per taxon). Interactions between photosynthetic microbes and any other organism became less frequent over the gradient, whereas interactions between plants or soil microfauna and any other organism were more abundant in late succession. Results demonstrate that patterns in diversity and composition do not necessarily relate to patterns in network complexity and suggest that network analyses provide new insight into the ecology of highly diverse, microscopic communities

Insights and inferences about integron evolution from genomic data

Research articl

Effects of natural and experimental drought on soil fungi and biogeochemistry in an Amazon rain forest

Microbiota are essential components of the soil, driving biogeochemical cycles. Fungi affect decomposition and biotic interactions with plants across scales. Climate projections suggest that extended dry seasons may transform sensitive rain forests into savanna-like vegetation, with consequent changes in biogeochemistry. Here we compare the impacts of natural seasonality with 14 years of partial throughfall exclusion in an Amazonian rain forest, focussing on soil fungal functional diversity, extracellular soil enzyme activities (EEA) and their implications for nutrient dynamics. Large changes in fungal diversity and functional group composition occur in response to drought, with a conspicuous increase in the abundance of dark-septate fungi and a decrease in fungal pathogens. The high seasonality of EEA in the control (non droughted) and suppression of seasonality in the drought treatment, together with an increased implied nitrogen demand in the dry season induced by experimental drought, suggest that the changed soil microbiota activity may signal a pending shift in the biogeochemical functioning of the forest.R.C.S. was supported by grants received from the Brazilian National Research Council (CNPq; 383166/2015-2) and Higher Education Research Council.(CAPES; 1633282). J.G. received funding from the Lendület Program (96049) of the Hungarian Academy of Sciences and the Eötvös Lóránd Research Network. This work formed part of a project funded by the Natural Environment Research Council (NERC; NE/J011002/1, NE/N006852/1)

Spin-1/2 J1-J2 model on the body-centered cubic lattice

Using exact diagonalization (ED) and linear spin wave theory (LSWT) we study the influence of frustration and quantum fluctuations on the magnetic ordering in the ground state of the spin-1/2 J1-J2 Heisenberg antiferromagnet (J1-J2 model) on the body-centered cubic (bcc) lattice. Contrary to the J1-J2 model on the square lattice, we find for the bcc lattice that frustration and quantum fluctuations do not lead to a quantum disordered phase for strong frustration. The results of both approaches (ED, LSWT) suggest a first order transition at J2/J1 $\approx$ 0.7 from the two-sublattice Neel phase at low J2 to a collinear phase at large J2.Comment: 6.1 pages 7 figure