However, ectomycorrhizae have also been shown to provide their host plants with significant amounts of the mineral derived nutrients calcium , potassium , magnesium , and phosphorous . Studies have shown that ectomycorrhizal fungi may also play a role in the weathering of soil minerals, enhancing mineral nutrient uptake from these otherwise highly recalcitrant nutrient pools . Ectomycorrhizal communities are species rich, with well over a hundred ECM species having been documented in monodominant forests , and dozens or more on individual trees . Our knowledge of the respective ecological niches of ECM fungi is poor, but there is ample evidence that suggests discreet, nonSoverlapping niches of habitat preference and nutrient acquisition exist for some species. As atmospheric CO2 concentrations rise, forest growth and tree’s nutrient demands may increase. The Earth’s atmospheric concentrations of CO2 are increasing due to fossil fuel combustion, agriculture, and deforestation, and are predicted to continue to rise, even if we arrest the increasing rate of anthropogenic CO2 emissions. A number of studies have shown that plants grow faster and fix more CO2 when CO2 concentrations are increased above ambient levels . This increased growth however, is dependant on increased nutrient uptake to support increased standing plant biomass There is evidence that forests are responding to this increased nutrient demand caused by CO2SstimulatedSgrowthSenhancement by increasing root growth and developing a deeper distribution of roots.
Anthropogenic nitrogen pollution threatens to alter the productivity and carbon storage of temperate and boreal forests. Anthropogenic nitrogen pollution from energy generation, transport,growing strawberries vertically and agriculture has more than doubled the inputs of nitrogen to terrestrial ecosystems . Emissions of the other major component of acid rain, sulfur, were successfully reduced in the early 1990’s, and public attention to acid rain has since diminished greatly. ANP however, has either remained steady or increased somewhat in the developed world, and has risen sharply, and is predicted to rise even more sharply in the 21st century in the developing world . Nitrogen put into the atmosphere by transport and energy generation returns to earth as HNO3 and can fall as wet or dry deposition many hundreds of miles from pollution sources . Soil nitrogen status is, for temperate and boreal forests, the dominant edaphic factor controlling forest productivity and shaping species composition . Anthropogenic nitrogen pollution has facilitated invasive species establishment in many forests of the temperate and boreal zone and contributed to widespread species loss . There is ample evidence that moderate levels of ANP may significantly increase the net primary productivity of temperate forests . Beyond a certain amount of accumulated ANP forest productivity may drop sharply as a result of soil acidification and excess N inputs leaching out other essential nutrients, which then become limiting to forest productivity. This shift from nitrogen limitation to limitation or coSlimitation by phosphorous , potassium , or calcium due to prolonged ANP has already been observed in a number of forests in Eastern North America. Thus, the continued productivity of forests sustaining heavy nitrogen deposition will become dependant on the uptake of these mineral derived nutrients. Mineral weathering increases the supply of these nutrients and neutralizes the acidifying effects of nitrogen deposition.Decreased below ground carbon allocation equates to decreased inputs of carbon into deep soil; carbon inputs which may lead to longer Sterm soil carbon retention than above ground litter inputs . This decreased below ground carbon allocation also has profound effects on mycorrhizal relations. Understanding how nitrogen deposition and elevated atmospheric CO2 concentrations will affect forest productivity and soil carbon storage is essential to predicting how future anthropogenic emissions of carbon will affect the Earth’s climate.
At present, an amount of carbon equivalent to 600 % of our annual CO2 emissions is taken up by the planet’s terrestrial biota each year, the majority in forests . Even small increases in forest productivity could be a major negative feedback to greenhouse gasS induced climate change. There is five times as much carbon stored in soils as there is in living plant biomass, a change of just 1 % in soil carbon pools is equal to 3 years of anthropogenic carbon emissions . There is a wide variety of effects of anthropogenic emissions of N and C that may affect these huge stocks of soil carbon, with the net effect, increase or decrease, very much unclear. Ectomycorrhizal communities may play a major role in determining how forest productivity and soil carbon stocks are affected by anthropogenic carbon and nitrogen emissions. Elevated CO2 has been found to alter ectomycorrhizal community composition and increase mycorrhizal colonization . Anthropogenic nitrogen pollution has been found to alter ectomycorrhizal community composition , decrease ECM diversity and decrease colonization intensity . The potential loss of ECM species from nitrogen deposition reduces forest biodiversity and may represent a reduction in forests’ resiliency to future environmental change. ECM represent a very large sink for fixed carbon; studies have found more than 60% of recent carbon assimilation and net primary production may be allocated to ectomycorrhizal symbionts, though most estimates are closer to 15%.Reductions in C allocation to ECM may significantly reduce soil C storage and serve as a positive feedback to global change. The reductions in carbon allocation to ectomycorrhizae observed under nitrogen limitation may continue as more N deposition occurs or may level off as other nutrients become limiting to forest growth. The increase in below ground carbon allocation observed with elevated CO2 may continue as global CO2levels increase, or may level off or reverse if plants become sufficiently nutrient limited that carbon fixation rates are reduced.
If ectomycorrhizal community shifts observed under elevated CO2 and nitrogen inputs represent a shift towards ectomycorrhizal species that are better able to provide the nutrients most limiting to plant growth, then forests may likely adapt to their shifting nutrient demands and continue to increase in productivity in response to increasing CO2 levels and nitrogen deposition. If, on the other hand, these shifts in community composition, and reductions in mycorrhizal colonization reflect temperate and boreal forests’ adaptation to limitation by nitrogen and only nitrogen,best vetical garden system then increasing amounts of forest may experience reduced productivity in response to continued nitrogen deposition and the fertilization effect from CO2 enrichment will likely decrease as forests become more severely nutrient limited. My dissertation attempts to shed light on which of these two scenarios is likely to unfold over the coming decades of continued anthropogenic global change. In chapter one, I investigated the effects of nitrogen addition on ectomycorrhizal community composition and colonization in a deciduous forest. Very high levels of nitrogen fertilization significantly changed ectomycorrhizal community composition, decreased ectomycorrhizal diversity, and shifted the ectomycorrhizal distribution more towards the mineral soil. These results suggest that the ectomycorrhizal community may be shifting to meet the changing nutrient demands of the forest and outline a potential mechanism for increased soil carbon storage under anthropogenic nitrogen deposition. Based on the high fungal diversity found in the mineral soil, and the fact that the ectomycorrhizal abundance in the mineral soil increased in response to nitrogen deposition I decided to investigate how the heterogeneous distribution of nutrients in mineral soil determines fungal species distribution. In chapter two, I tried to assess which soil properties determine fungal species distribution. Our sampling design prevented us from assessing the role of some of the chemical properties examined, but carbon content and depth emerged as the most influential soil properties determining fungal community composition.
Calcium content also appeared to be important in determining fungal community composition. While pure culture studies have shown that ectomycorrhizal fungi vary in their ability to stimulate mineral weathering and take up mineral nutrients, the observed species shifts in ectomycorrhizal communities can only reflect shifting nutrient demands from host plants if plants can allocate carbon to the mycorrhizal fungi that are providing the most mineral nutrients. In Chapter 3, I present a literature review on the current state of knowledge on how plant nutrient demand drives fungal weathering. Within the plant physiology literature there is evidence that plants may be able to respond to phosphorous and potassium limitation with increased carbon allocation to mycorrhizal fungi providing those nutrients. Magnesium limitation reduces below ground carbon allocation, and the effects of calcium limitation on carbon allocation are unclear. In studies of ectomycorrhizal weathering there is a distinct lack of explicit testing of the role of host nutrient status in driving fungal weathering. I conclude by making a number of recommendations for how future studies can address this important question. For the fourth chapter of this dissertation I investigated how elevated CO2 affects plant growth, biotic weathering, and organic acid exudation, as well as the roles of plants, their ectomycorrhizal symbionts, and organic acids in stimulating mineral weathering. Elevated CO2 increased plant growth but did not increase mineral weathering. Pine seedlings but not ectomycorrhizae significantly increased mineral weathering, though there was some indication that one of the two ectomycorrhizal species examined, Piloderma&fallax, may have stimulated mineral weathering. These results do not support the hypothesis that increased nutrient demand by plants, caused by increased CO2 availability, will stimulate weathering, though our system’s departures from forest soil conditions hamper our ability to directly relate our results to processes in forested ecosystems. In my doctoral research I set out to determine whether the ecological, chemical, and physiological nature of the ectomycorrhizal symbiosis will allow for ectomycorrhizal communities to shift their functioning in accordance with the changing nutrient demands of forests experiencing global change. My research indicates that ectomycorrhizal communities may shift to accommodate the shifting nutrient demands of forest undergoing sustained heavy nitrogen deposition, but failed to find an effect of elevated CO2 on mycorrhizal fungi or biotic weathering. I also identified a number of ways in which future studies could address these questions in a more targeted, verifiable manner.Anthropogenic nitrogen pollution is a global problem that threatens the ecological integrity of many terrestrial and aquatic ecosystems. Anthropogenic nitrogen pollution from energy generation, transport, and agriculture has more than doubled the inputs of nitrogen to terrestrial ecosystems . Emissions of the other major component of acid rain, sulfur, were successfully reduced in the early 1990’s and public attention to acid rain has since diminished greatly. Anthropogenic nitrogen pollution however, has either remained steady or increased somewhat in the developed world, and has risen sharply, and is predicted to rise even more sharply in the 21st century in the developing world . Nitrogen pollution from agriculture either leaches into streams, lakes, and groundwater as nitrate or volatilizes off of fields and manure deposits to return to the earth in the form of ammonium. Nitrogen put into the atmosphere by transport and energy generation returns to earth as HNO3 and can fall as wet or dry deposition many hundreds of miles from pollution sources. As a result of both these processes, but primarily due to the more far reaching HNO3 , large forested regions are receiving inputs of HNO3 that threaten to dramatically alter their ecology and species composition , and, when prolonged severe deposition occurs, reduce their productivity.Ectomycorrhizal fungi are essential to forest health and are particularly important to the nitrogen nutrition of temperate and boreal forests. They form intimate associations with tree roots, providing the roots with nutrients and receiving fixed carbon from their plant hosts. Ectomycorrhizae form symbiosis with less than 3% of the world’s plant species but with many of the dominant trees of temperate and boreal forests, particularly the plant families Pinaceae and Fagaceae, . Ectomycorrhizal communities are species rich, with well over a hundred ECM species having been documented in monodominant forests , and dozens or more on individual trees . Ecomycorrhizae have been shown to transfer significant amounts of P , Mg , Ca , and K to their plant hosts, but their provision of nitrogen is generally considered their primary contribution to plant health.Studies have shown that ECM may provide up to 80% of a host plant’s total N uptake . The extraradical mycelia of ECM greatly increase the volume of soil that roots can exploit . Through the use of a diverse suite of enzymes ECM may be able to solubilize and take up nitrogen from organic N pools that roots cannot utilize .