Spatial Heterogeneity in Soil Microbes Alters Outcomes of Plant Competition

Plant species vary greatly in their responsiveness to nutritional soil mutualists, such as mycorrhizal fungi and rhizobia, and this responsiveness is associated with a trade-off in allocation to root structures for resource uptake. As a result, the outcome of plant competition can change with the density of mutualists, with microbe-responsive plant species having high competitive ability when mutualists are abundant and non-responsive plants having high competitive ability with low densities of mutualists. When responsive plant species also allow mutualists to grow to greater densities, changes in mutualist density can generate a positive feedback, reinforcing an initial advantage to either plant type. We study a model of mutualist-mediated competition to understand outcomes of plant-plant interactions within a patchy environment. We find that a microbe-responsive plant can exclude a non-responsive plant from some initial conditions, but it must do so across the landscape including in the microbe-free areas where it is a poorer competitor. Otherwise, the non-responsive plant will persist in both mutualist-free and mutualist-rich regions. We apply our general findings to two different biological scenarios: invasion of a non-responsive plant into an established microbe-responsive native population, and successional replacement of non-responders by microbe-responsive species. We find that resistance to invasion is greatest when seed dispersal by the native plant is modest and dispersal by the invader is greater. Nonetheless, a native plant that relies on microbial mutualists for competitive dominance may be particularly vulnerable to invasion because any disturbance that temporarily reduces its density or that of the mutualist creates a window for a non-responsive invader to establish dominance. We further find that the positive feedbacks from associations with beneficial soil microbes create resistance to successional turnover. Our theoretical results constitute an important first step toward developing a general understanding of the interplay between mutualism and competition in patchy landscapes, and generate qualitative predictions that may be tested in future empirical studies

Biochar and Managed Perennial Ecosystems

Biochar is a carbon-rich material that is similar to charcoal. It is produced when biomass is burned in the absence of oxygen, a process otherwise known as pyrolysis. Pyrolysis and the production of biochar are currently being promoted as a means to both produce domestic fuel (bio-oil) while concurrently producing a co-product that increases crop yield and sequesters carbon in the soil (biochar). While there may be many potential benefits in the application of biochar to agricultural soils, such as enhanced soil fertility and improved soil water status, there are no studies of higher-order ecological and ecosystem effects of biochar and its potential synergistic interactions (either positive or negative) on complex perennial systems. The goal of this field experiment is to determine how biochar and manure addition directly affect ecosystem structure and function in perennial systems, specifically soil nutrients, water, plants, and soil organisms

Biochar and Managed Perennial Ecosystems: Testing for Synergy in Ecosystem Function and Biodiversity

Biochar is a carbon-rich material that is similar to charcoal. It is produced when biomass is burned in the absence of oxygen, a process otherwise known as pyrolysis. Pyrolysis and the production of biochar are currently being promoted as a means to both produce domestic fuel (biooil) and concurrently producing a co-product that increases crop yield and sequesters carbon in the soil (biochar). While there may be many potential benefits in the application of biochar to agricultural soils, such as enhanced soil fertility and improved soil water status, there are no studies of higher-order ecological and ecosystem effects of biochar and its potential synergistic interactions (either positive or negative) on complex perennial systems. The goal of this field experiment is to determine how biochar and manure addition directly affect ecosystem structure and function in perennial systems, specifically soil nutrients, water, plants, and soil organisms

Directing ecological restoration: impact of organic amendments on above- and belowground ecosystem characteristics

Increasing interest among restoration ecologists exists in developing strategies that stimulate biotic interactions and promote self-regulation in restored systems. These approaches should target above- and belowground organisms because they interact to regulate ecosystem pattern and process. In the following dissertation, I compare the ability of organic amendments to alter above- and belowground biological community structure and function to promote prairie establishment on Castle Drive Landfill in Garland, Dallas County, Texas. Treatments included altering the location of organic amendments in the soil profile, either applied to surface or incorporated, and varying the amount applied. Plant community composition, grass population dynamics, soil nutrient conditions, and soil biological parameters were monitored for three growing seasons. Aboveground, the surface treatments were superior for the establishment of desired and undesired plant species. Plant density patterns can be attributed to the amelioration of physical conditions and the accidental burial of seed during incorporation. Grass population dynamics suggest that surface-amended plots supported establishment, but high-volume incorporated treatments were better for enhancing survival through seasonal and long-term drought. Belowground biological responses were affected by the plant community, and not by the amendment treatments. Soil microbial biomass and carbon mineralization potential were larger in those treatments with greater plant density. The structure of the nematode community suggests that decomposition in the surface-amended plots was directed through bacterial channels while decomposition in the incorporated plots was through fungal channels. It is likely that the higher rates of plant productivity in surface treatments stimulated root exudation, thereby favoring bacteria and the nematodes that feed on them. Treatment differences in decomposition pathway were attenuated after 17 months. The soil quality indicators, Cmic/Corg, qCO2, nematode family richness and nematode density, were not affected by the restoration treatments or plant density, but did increase over time. The results of this study suggest that restoration managers should direct their energies into establishing and promoting a high-quality plant community. This can be manipulated with amendments, but care is needed not to exceed thresholds within location treatments

Spatial heterogeneity in species composition constrains plant community responses to herbivory and fertilisation

Environmental change can result in substantial shifts in community composition. The associated immigration and extinction events are likely constrained by the spatial distribution of species. Still, studies on environmental change typically quantify biotic responses at single spatial (time series within a single plot) or temporal (spatial beta diversity at single time points) scales, ignoring their potential interdependence. Here, we use data from a global network of grassland experiments to determine how turnover responses to two major forms of environmental change – fertilisation and herbivore loss – are affected by species pool size and spatial compositional heterogeneity. Fertilisation led to higher rates of local extinction, whereas turnover in herbivore exclusion plots was driven by species replacement. Overall, sites with more spatially heterogeneous composition showed significantly higher rates of annual turnover, independent of species pool size and treatment. Taking into account spatial biodiversity aspects will therefore improve our understanding of consequences of global and anthropogenic change on community dynamics

How do nutrients change flowering in prairies?

Farmers today apply more synthetic fertilizers to farm fields than ever before – but not all of these nutrients are used by crops: some fertilizer escapes through the air, soil, or water. Nitrogen, phosphorous, and potassium flow off farm fields when it rains, billow into the air when fields are plowed, and drift with the wind to other areas. Extra nutrients are also released to the air when people burn fossil fuels. We wanted to find out: what happens when these extra nutrients land on wild prairie ecosystems? How do its wild plants respond? Do they all just grow better? Or could there be any negative side effects? To answer these questions, we systematically added nutrients to experimental patches of prairie. We found that these added nutrients (specifically nitrogen) made early-season plants thrive while reducing the amount of late-season plants, but only in some prairie types. This change could have serious implications for the way prairie ecosystems function

Nematode community development early in ecological restoration: The role of organic amendments

a b s t r a c t Soil food web structure is an integral component of ecosystem function, but there are few strategies orientated towards managing its development in restoration projects. The objective of this study was to direct nematode community structure and function through the application of organic amendments to the soil of an urban landfill remediation project using native grassland vegetation. We used a 2 Â 3 factorial design in which an organic amendment was added to the soil at different locations (incorporated versus surface-applied) and amounts (none, light, heavy). Nematode and plant community structure were monitored over three growing seasons to determine the rate and direction of change. Surface application of organic amendments supported greater grass and total plant densities compared to incorporated amendment treatments, but plant density did not vary with amendment amount. Total nematode density, family diversity and family richness were not affected by the amendment treatments, but both family richness and seasonal nematode density increased over the duration of the experiment. Other descriptors of nematode community development (Structure, Maturity, and Plant Parasite Indexes) were not influenced by either amendment amount or location, but varied significantly over time. Contrary to expectations, the surface amendment treatments significantly increased bacterivorous, plant parasitic, omnivorous and predator nematode densities, but had no influence on fungi/root-tip feeding nematodes. Also contrary to our hypotheses, the surface treatments had smaller Channel Index and greater Enrichment Index values relative to the incorporated treatments during the first 15 month of the experiment. We hypothesize that the surface amendments are indirectly affecting the structure of the nematode community by promoting greater plant density, thus increasing the concentration of highquality organic matter (such as root exudates) in the soil. This promotes the development of a nematode community dominated by opportunistic groups that respond rapidly to increased resource availability. Future studies should aim to distinguish between the organic amendment's direct function as a potential food source for the soil biota versus their indirect role as an environmental variable, including their capability to alter the availability of plant-derived resources