Plant Diversity and Ecosystem Function are Linked by Microbial Communities in Soil

Human activity has decreased plant diversity on a global scale, yet the interconnection between aboveground plant diversity and belowground microbial community composition and function has been largely ignored. This interconnection is especially important because many ecosystem processes are controlled by microbial communities in soil. We have found that gross N mineralization and fungal abundance increase with plant species richness in experimental grasslands. These two observations suggest that plant diversity, in some manner, has altered microbial community composition and function in a way that impacts ecosystem-level processes. We hypothesize that high plant diversity results in a greater range and concentration of plant secondary compounds, which provide relatively small amounts of energy for microbial growth, thus fostering greater fungal abundance and N availability. Our is conducted at the Cedar Creek Natural History area in east Central Minnesota. The experiment consists of 7 species richness treatments (0, 1, 2, 4, 8, 16 species) composed of native grasses, forbs and trees growing on an organic-matter-poor, sandy soil. We are measuring above and below-ground plant litter chemistry, the compositional and functional diversity of soil microbial communities, and we will trace the flow of 13C-labeled plant compounds through the soil food web to test our hypotheses.

Chronic Atmospheric Nitrate Deposition and the Consequences of Altered Ecosystem Metabolism

Collaborators - Kurt S. Pregitzer, Andrew Burton & Erik Lilliskov, School of Forest Resources & Environmental Science, Michigan Techno
logical University, Houghton, Michigan 49931
Nitrogen (N) saturation of terrestrial ecosystems is one of the most important contemporary ecological issues. Researchers at Michigan Technological University and the University of Michigan initiated a long-term, replicated (3 replicate plots x 2 treatments: N-amended and control, 4 study sites) field experiment in 1994. The purpose of this field experiment is to understand the mechanisms controlling carbon (C) and N cycling in the face of chronic N deposition and the long-term consequences of N saturation. Moderate levels of N addition (30 kg NO3--N ha-1 y-1 for seven years) have quickly caused advanced stages of N saturation to occur in this very common northern hardwood ecosystem that sp
ans an entire biome in the Great Lakes region of the USA. 
In this project, a series of hypotheses are aimed at elucidating the mechanisms and consequences of altered ecosystem metabolism. Specifically, increased N availability is hypothesized to induce an “enzymatic latch”, which decreases decomposition and enhances C storage in the soil. It is predicted that nitrate (NO3-), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) will continue to leach from these ecosystems, as they have done thus far in response to the NO3- deposition treatment. These forests may eventually become totally N saturated, with leaching outputs that equal simulated atmospheric inputs. A series of alternative hypotheses is also presented to test the theoretical predictions concerning the mechanisms regulating C and N cycling in this ecologically and economically important Great Lakes ecosystem. The study is one of few long-term experiments world-wide where predictions regarding long-term responses to N deposition, and the mechanisms controlling them, can be tested in the field, strengthening the scientific community’s ability to predict real-world ecosystem responses to global change. The experimental approach encompasses molecular analyses of microbial community composition and function as well as measurements of the ecosystem-level fluxes of C and N through these northern hardwood ecosystems. It represents a unique opportunity to integrate across levels of ecological organization to understand ecosystem-level processes and responses.

Mechanisms Controlling Soil Carbon Sequestration Under Atmospheric Nitrogen Deposition  

Collaborators - Robert L. Sinsabaugh, University of New Mexico, and Daryl L. Moorhead, University of Toledo
We initiated a landscape-level study to better understand the biochemical mechanisms by which atmospheric nitrogen (N) deposition could alter soil C storage. Our rationale for this work was based on the observation that elevated levels of inorganic N in soil solution repress the physiological capacity white rot basidiomycetes to degrade lignin, a common constituent of plant cell wall that fosters soil organic matter formation. We predicted that atmospheric N deposition will foster ecosystem-specific increases and decreases in soil C storage, based on the biochemical constituents of plant litter and the physiological suppression of white rot fungi to high levels of soil N. Our central hypothesis was, that atmospheric N deposition will alter decomposition by directly suppressing the fungal lignin oxidation, thereby shifting the rate-limiting process of oxidative litter degradation to less efficient prokaryotes. Such a response should foster an accumulation of soil organic matter in ecosystems dominated by highly lignified litter, whereas greater soil N availability would accelerate the decomposition in ecosystems with low-lignin plant litter in which cellulose degradation is initially N limited. We tested our hypothesis using a series of field and laboratory experiments conducted at six well-characterized sites, representing three types of northern temperate forest that occur across the Upper Lake States region (Fig. 1). These ecosystem range from 100% oak in the overstory (black oak-white oak ecosystem; BOWO) to 0% overstory oak (sugar maple-basswood; SMBW); the sugar maple-red oak ecosystem has intermediate oak abundance. Thus, these ecosystems span a range of leaf litter biochemistry from highly lignified litter (BOWO) to litter with very low lignin content (SMBW). We have demonstrated that atmospheric N deposition can increase or decrease the soil C storage by modifying the lignolytic capacity of microbial communities in soil.

Ecosystem Response to Elevated Tropospheric Carbon Dioxide and Ozone Regulated by Plant-Microbe Interactions in Soil

Collaborator - Kurt S. Pregitzer, School of Forest Resources & Environmental Science, Michigan Technological University, Houghton, Michigan 49931
A major uncertainty in predicting ecosystem response to a changing environment is the extent to which increases in plant and ecosystem productivity will be sustained as CO2 continues to accumulate in the Earth’s atmosphere. For example, there is uncertainty regarding the specific ways in which higher levels of tropospheric O3 will interact with rising CO2 to alter plant growth, and how th

ey might cascade through terrestrial ecosystems to impact decomposer communities in soil, which further control the flow of energy and nutrients in forest ecosystems. This project seeks to understand the mechanisms controlling ecosystem response to elevated CO2 and O3, and it is based on the premise that plant-microbe interactions in soil mediate key feedback mechanisms controlling ecosystem response to environmental change. The project centers on a conceptual model and coordinated series of hypotheses that link production and biochemical changes in plant litter to physiological changes in soil microbial communities. Each hypothesis is being tested at the FACTS II FACE experiment in Rhinelander, Wisconsin, a free-air release experiment capable of addressing how CO2 and O3 will interact to alter the structure and function of forest ecosystems. Previous results demonstrate that increases in plant growth under elevated CO2, and its diminishment by elevated O3, have altered fine root production and biochemistry, substrate availability for microbial metabolism, the composition of soil fungal communities and the formation of soil organic matter. This current project addresses key questions that remain unanswered: Why has elevated O3 accelerated fine root mortality in aspen, while net primary productivity has declined? Will this response be consistent across ecologically distinct communities, or will it diverge among community types? Why has greater belowground litter production under elevated CO2 elicited a disproportionately greater increase in cellulose degradation by soil fungi, when there is no change in fine root biochemistry? How has fine root and microbial metabolism changed such that the greatest soil CO2 flux is now occurring in the CO2 and O3 combination treatment? Will plants growing under elevated CO2 accumulate more N over time? Will O3 negate this response? This project will answer these questions and will provide insight into the mechanisms by which altered plant growth and tissue biochemistry under elevated CO2 and O3 will modify soil microbial communities and the ecosystem-level cycling of C and N in ecologically and economically forests in the Upper Great Lakes region.